InteractiveFly: GeneBrief

RNA polymerase II 215kD subunit: Biological Overview | References

The uninduced Drosophila hsp70 gene is poised for rapid activation. The rapid changes upon heat shock in levels and location of Heat shock factor (HSF), RNA polymerase II (Pol II) and its phosphorylated forms, and the Pol II kinase P-TEFb on hsp70 were examined in vivo by using both real-time PCR assays of chromatin immunoprecipitates and polytene chromosome immunofluorescence. These studies capture Pol II recruitment and progression along hsp70 and reveal distinct spatial and temporal patterns of serine 2 and serine 5 phosphorylation: in uninduced cells, the promoter-paused Pol II shows Ser5 but not Ser2 phosphorylation, and in induced cells the relative level of Ser2-P Pol II is lower at the promoter than at regions downstream. An early time point of heat shock activation captures unphosphorylated Pol II recruited to the promoter prior to P-TEFb, and during the first wave of transcription Pol II and the P-TEFb kinase can be seen tracking together across hsp70 with indistinguishable kinetics. Pol II distributions on several other genes with paused Pol II show a pattern of Ser5 and Ser2 phosphorylation similar to that of hsp70. These studies of factor choreography set important limits in modeling transcription regulatory mechanisms (Boehm, 2003).

Pol II is highly regulated both at the level of recruitment to promoters and in its progress through the stages of the transcription cycle. This regulation is executed through numerous associations with other proteins as Pol II enters the promoter, melts DNA, initiates transcription, begins early elongation, and eventually matures into a productive elongation complex. Pol II undergoes additional modifications, most notably phosphorylation of the CTD of its largest subunit as it progresses from its hypophosphorylated promoter entry form to the elongation phase, where it is highly phosphorylated at residues Ser2 and Ser5. These changes in phosphorylation are proposed to influence protein association, affecting not only Pol II's elongation properties but also its association with a variety of protein complexes that process pre-mRNA. Moreover, the pattern of phosphorylation is not stagnant during the elongation phases of the cycle and may be signaling specific associations. To define mechanisms involved in these processes in vivo, the rapidly and robustly activated hsp70 gene has been employed as a model (Boehm, 2003).

Technological advances of DNA-protein cross-linking and highly quantitative large-scale PCR assays were used to explore hsp70 activation kinetics of recruiting HSF, the critical Pol II kinase P-TEFb, and Pol II in vivo. The changes in Pol II were examined during the first and subsequent cycles of transcription that are triggered in response to an instantaneous heat shock of Drosophila cells. HSF recruitment occurs very rapidly, detectable at the earliest assay point of 5 s of heat shock, and reaches saturation within 75 s; this result is consistent with the rapid transcriptional response of heat shock genes and with previous, lower resolution assays of HSF recruitment. The recruitment of additional hypophosphorylated Pol II to the promoter occurs with rapidity similar to that of HSF recruitment but before an increase in Pol II phosphorylation at the promoter, which occurs by 75 s. All forms of Pol II achieve a maximal level on the promoter and gene by 5 min. The progress of Pol II across the gene can be observed, and its progress fits the known rate of elongation on Drosophila hsp70, 1.2 kb/min. Interestingly, total Pol II levels remain greater at the transcription start site than at the ORF, even during active transcription, consistent with the observation that promoter escape remains rate-limiting even during heat shock. P-TEFb, a major Pol II kinase, moves across the gene at a rate similar to that of Pol II during the first burst of transcription and thereafter remains distributed over the promoter and ORF during the full 20-min time course examined. This distribution supports a model where P-TEFb contributes to Pol II phosphorylation not only at the promoter but also during most or all of the elongation phase of transcription (Boehm, 2003).

The detection of Ser5 phosphorylation on a promoter-paused Pol II prior to gene induction corroborates a model where this phosphorylation is an early event involved in the transition from initiation of transcription to early phases of Pol II elongation. The mRNA associated with the paused Pol II molecule has previously been shown to be efficiently capped when long enough to allow access of the capping machinery. Since Ser5-P has been reported to enhance Pol II association with mRNA capping machinery and capping activity, the Ser5-P detected on paused Pol II might help to explain the efficient capping of paused RNAs. It is important to note that earlier analyses of the hsp70 gene, which determined that the paused Pol II CTD is hypophosphorylated prior to heat shock, were performed with antibodies different from those used in the present study. Importantly, the antibody generated to detect Pol IIo in those studies was directed against a peptide phosphorylated in vitro by CTK1, a yeast kinase thought to phosphorylate the CTD at serine 2. Thus, the previous analysis did not probe for the Ser5 phosphorylation reported here (Boehm, 2003).

While P-TEFb phosphorylates the CTD primarily at Ser2, it has also been shown to recognize Ser5 as a substrate. Present results suggest that the Ser5 detected in the uninduced state on hsp70 is not a result of P-TEFb activity, since P-TEFb is not detected prior to gene activation (as seen in this study). Ser5 is likely to be the substrate of the cdk7 component of TFIIH early in transcription. Indeed, cdk7 has been found in in vitro studies to be released earlier in the transcription cycle than P-TEFb. In vivo, Cdk7 is required very early in the transcription cycle and contributes to the generation of the paused Pol II on the promoter-proximal region of hsp70 (Boehm, 2003).

Analyses of the phosphorylation status of the CTD in other organisms have found Ser5-P levels to be higher at the promoter than at the ORF, a pattern similar to what was observed on hsp70 during active transcription. When total levels of Pol II are taken into account on hsp70, however, it appears that the level of Ser5-P remains constant along the gene. Comparatively, another study did not see a striking difference in total Pol II density along the genes analyzed. A third study detected more Ser5-P at the promoter than the 3' untranslated region of the human alpha-AT gene but also appeared to detect more total Pol II at the promoter region. Thus, it may be that in metazoans (or on some genes) the level of Ser5-P relative to Pol II is fairly constant along the gene. The possibility that the activity of a phosphatase may be system or gene specific is certainly plausible; for instance, heat shock of HeLa cells deactivates a CTD phosphatase (Boehm, 2003 and references therein).

Under non-heat shock conditions, total Pol II levels were greater at the 5' regions than at the ORFs for several genes that contain promoter-paused Pol II, while histone H1, which does not display a pause by nuclear run-on assay, shows no significant difference of 5' and ORF Pol II signals. Greater levels of Ser5-P were also detected at the 5' end of the genes containing paused Pol II, while levels on H1 were distributed evenly, indicating that this phosphorylation may be a general aspect of the regulatory status of a paused Pol II. This distribution of Ser5-P for the constitutively active genes Tub, GAP, and Actin5C is similar to the results of other studies which analyzed active transcription; however, Ser5-P levels on these genes are constant when standardized to total Pol II, similar to hsp70 in its active state. For these Drosophila genes, the higher level of Ser5-P at the promoter may be attributable to the presence and status of the paused Pol II, indicative of genes regulated at the level of elongation. Indeed, recent studies describe another constitutively active pause gene in human cells, dihydrofolate reductase, which shows a pattern similar to that of Ser5-P for these Drosophila genes (Boehm, 2003 and references therein).

Phosphorylation at Ser2 of the Pol II CTD may be important for processivity into active elongation and has been implicated in downstream events, including pre-mRNA splicing and 3' mRNA processing. Ser2-P levels are undetectable at +58 on hsp70 in the uninduced state, increase quickly at the 5' region upon heat shock, and appear constant through the gene in later time points (for example, 5 min). The increase in phosphorylation detected over time tracks with the recruitment of additional Pol II as well as the recruitment of P-TEFb. Taking into account total Pol II levels, there appears to be a slight increase in Ser2-P as Pol II progresses through the ORF. This correlates with the concomitant and approximately equivalent decrease in Pol IIa. Ser2-P patterns on additional genes containing a paused Pol II, when considered relative to levels of detectable total Pol II, are significantly higher in the ORF than are those in the 5' region. While these ratios may simply be a consequence of promoter-paused Pol II not being Ser2 phosphorylated, this result is similar to that of another study, where Ser2-P was only detected in the ORF. These observations led to speculation that an increase in Ser2-P may be important for cueing specific processes as Pol II proceeds through the gene. P-TEFb, the major kinase implicated in Ser2 phosphorylation, was detected concomitant with Pol II during active transcription on hsp70. While Pol II/P-TEFb ratios appear constant, a slight increase in Ser2-P occurs at the 3' end of the gene. As the presence of the kinase is not an indicator of its activity, work presently ongoing in the laboratory on P-TEFb kinase inactivation and hsp70 gene regulation should help to better understand this process (Boehm, 2003 and references therein).

Lastly, analysis of immunostaining of polytene chromosomes provides independent corroboration of the higher resolution and quantitative ChIP assays and provides insight into the formation and composition of the transcription puff. Paused Pol II on hsp70 was previously detected with this method, as was Pol II along hsp70 during heat shock. A modest detection of Ser5-P was observed on the promoter prior to heat shock. During the early stages of puff formation, Pol II resolves from promoter-bound HSF. Ser2-P and Ser5-P occupy the most decondensed regions of the puff forming a halo around the heat shock loci, while HSF is more concentrated at the chromosome core at one end of the puff. Taken together, these ChIP and immunofluorescence results provide a foundation for additional temporal and spatial assignments of specific factors relative to the phosphorylation events during the activation of transcription. Perturbation of the function or activity of specific factors using genetic and drug-based approaches will provide further insight into the mechanistic role of these factors in the recruitment and modification of transcriptional machinery and in the coupling of specific transcription processes and Pol II modifications to RNA processing events (Boehm, 2003).

Dynamics of heat shock factor association with native gene loci in living cells

Direct observation of transcription factor action in the living cell nucleus can provide important insights into gene regulatory mechanisms. Live-cell imaging techniques have enabled the visualization of a variety of intranuclear activities, from chromosome dynamics to gene expression. However, progress in studying transcription regulation of specific native genes has been limited, primarily as a result of difficulties in resolving individual gene loci and in detecting the small number of protein molecules functioning within active transcription units. This study reports that multiphoton microscopy imaging of polytene nuclei in living Drosophila salivary glands allows real-time analysis of transcription factor recruitment and exchange on specific native genes. After heat shock, this study has visualized the recruitment of RNA polymerase II (Pol II) to native hsp70 gene loci 87A and 87C in real time. Heat shock factor (HSF), the transcriptional activator of hsp70, is localized to the nucleus before heat shock and translocates from nucleoplasm to chromosomal loci after heat shock. Assays based on fluorescence recovery after photobleaching show a rapid exchange of HSF at chromosomal loci under non-heat-shock conditions but a very slow exchange after heat shock. However, this is not a consequence of a change of HSF diffusibility, as shown here directly by fluorescence correlation spectroscopy. The results provide strong evidence that activated HSF is stably bound to DNA in vivo and that turnover or disassembly of transcription activator is not required for rounds of hsp70 transcription. It is concluded that transcriptional activators display diverse dynamic behaviours in their associations with targeted loci in living cells. This method can be applied to study the dynamics of many factors involved in transcription and RNA processing, and in their regulation at native heat shock genes in vivo (Yao, 2006).

The rapid recovery pattern of HSF under non-heat shock (NHS) and slow recovery under heat shock (HS) corresponds in vivo to the marked difference in the DNA-binding affinity of HSF monomers (NHS) and trimers (HS). It is therefore proposed that a transcription activator's exchange dynamics on its targets may simply reflect the dissociation rate constant of the protein-promoter complex. The low affinity of some activators leads to their transient binding and has been suggested to cause the probabilistic assembly of transcriptional machinery. The high affinity of other activators leads to their stable binding, and this in turn is conducive to the formation of stable coactivator assemblies and the efficient recruitment of Pol II for repeated cycles of transcription. The exchange dynamics of some activators may involve other mechanisms; for instance, NF-kappaB, which has high affinity for DNA, was found to exchange rapidly at the tandem-repeat target gene loci. In addition, chromatin remodelling might have a function in these processes (Yao, 2006).

The slow exchange of activated HSF at the hsp70 promoter presents a sharp contrast with the rapid recruitment and elongation of RNA polymerase II at hsp70 genes during HS. During a 2-min transcription cycle (that is, the time it takes a Pol II molecule to transcribe the hsp70 gene, more than 20 Pol II molecules have begun the transcription of each hsp70 gene; however, very little new HSF has bound to the gene as shown by FRAP. Therefore, the data do not support the 'activation by destruction' hypothesis that the recruitment of new polymerase requires the ubiquitin-proteasome system (UPS) to turn over the 'spent' activator on the promoter. Moreover, more than the total amount of intracellular HSF would be degraded during a short period of heat shock if 'activation by destruction' were true for every round of heat shock gene transcription. HSF is an acidic, strong activator, like many positive regulatory factors, and hsp70 transcription resembles that of many other genes. Recent results on the yeast Gal4 activator have shown that it, too, is stably bound to its regulatory sites during gene activation. Therefore two independent and complementary approaches on the two widely studied acidic activators have revealed their stable binding to DNA during gene activation. Alternative models for activator function that propose activator recycling as a key component, such as hit and run, chaperone-assisted disassembly or UPS-mediated turnover, can apply to some but clearly not all transcriptional activators (Yao, 2006).

The stable binding of HS-activated HSF and the transient binding of ligand-activated GR collectively show the diverse 'action modes' of transcription activators: both stably bound and transiently bound activators can support gene transcription. How individual activators function in these two modes on their respective gene targets remains to be seen, with the underlying mechanisms yet to be determined. Importantly, the dynamic behaviour of coactivators, Pol II transcription and RNA-processing machinery at native mRNA genes is largely unknown in living cells, and the described experimental approach will be applicable to further investigations (Yao, 2006).

RNA polymerase is poised for activation across the genome

Regulation of gene expression is integral to the development and survival of all organisms. Transcription begins with the assembly of a pre-initiation complex at the gene promoter1, followed by initiation of RNA synthesis and the transition to productive elongation. In many cases, recruitment of RNA polymerase II (Pol II) to a promoter is necessary and sufficient for activation of genes. However, there are a few notable exceptions to this paradigm, including heat shock genes and several proto-oncogenes, whose expression is attenuated by regulated stalling of polymerase elongation within the promoter-proximal region. To determine the importance of polymerase stalling for transcription regulation, a genome-wide search was carried out for Drosophila melanogaster genes with Pol II stalled within the promoter-proximal region. The data show that stalling is widespread, occurring at hundreds of genes that respond to stimuli and developmental signals. This finding indicates a role for regulation of polymerase elongation in the transcriptional responses to dynamic environmental and developmental cues (Muse, 2007).

Promoter-proximal pausing was first described at the Drosophila heat shock genes (for example, Hsp70), where Pol II is recruited to the promoter and initiates RNA synthesis before gene activation but stalls after elongating 20-50 nucleotides into the gene. Escape of the engaged but stalled polymerase from the Hsp70 promoter region is regulated and is rate-limiting for gene expression. Subsequently, nearly a dozen Drosophila (for example, Hsp26, Hsp27 and βTub), viral (HIV), and mammalian (including Myc, Junb and Igk) promoters have been shown to possess stalled polymerase. However, stalling is currently thought to occur at only a small number of promoters, and the full spectrum of genes affected by Pol II stalling has yet to be investigated using a genome-wide approach in any organism (Muse, 2007).

Stalled Pol II is observed at the uninduced Hsp70 promoter in Drosophila S2 cells by chromatin immunoprecipitation (ChIP). Strong Pol II signal is present near the Hsp70 promoters and decreases precipitously at probes within the genes. Pol II occupancy at the Hsp70 promoter is greater than that at nearby promoters, including the aurora kinase (aur) gene, whose expression is considerably higher than that of Hsp70. Thus, ChIP analysis of uninduced Hsp70 illustrates two hallmarks of stalled Pol II: much higher Pol II signal near the promoter than within the gene, and absence of correlation between Pol II occupancy and the levels of gene expression (Muse, 2007).

To identify other genes with stalled Pol II, chromatin immunoprecipitation microarray (ChIP-chip) experiments were carried out using tiling oligonucleotide microarrays encompassing the Drosophila genome. An antibody was used against the Pol II Rpb3 subunit to detect Pol II regardless of the phosphorylation status of the Pol II Rpb1 C-terminal domain (CTD). ChIP-chip data was analyzed with previously described computational methods to identify annotated promoters occupied by polymerase. Of the unique promoters represented on both the ChIP-chip and RNA expression arrays, 5,403 promoters were bound by Pol II and 7,702 were unbound (Muse, 2007).

Among bound genes, many showed significant Pol II signals across the gene, whereas others had Pol II signal concentrated near the promoter. To identify genes with polymerase distribution consistent with stalled Pol II, namely those genes with high promoter-proximal polymerase signals accompanied by low Pol II signals within the gene, the difference was calculated between the average polymerase signals in these regions for all 5,403 bound genes. Many genes had similar average signals within the promoter and downstream regions, indicative of rather uniform Pol II binding across the gene. Although the calculated values for most genes fit within a normal Gaussian distribution, a substantial number of outliers were found that showed promoter-proximal enrichment of polymerase (PPEP) and were thus good candidates for polymerase stalling. Notably, Drosophila genes that are known to harbor stalled Pol II show PPEP (for example, Hsp26, Hsp27 and βTub (Muse, 2007).

There was no correlation between the average Pol II signal near the promoter of genes with PPEP and the RNA expression levels observed, suggesting that the amount of Pol II recruited to these promoters does not directly dictate levels of gene expression. By comparison, genes with more uniform Pol II binding showed a correlation between Pol II and expression levels. These results are in agreement with recent ChIP-chip data from human cells that identified subsets of genes at which Pol II levels did not correlate with RNA expression. However, Pol II signals in the downstream regions of both groups correlated with RNA expression (uniform Pol II binding). Transcripts from genes with PPEP were present at levels that ranged from barely detectable to substantially expressed, consistent with prior reports that promoter-proximal stalling serves not only to fully repress transcription but also to attenuate transcription of active genes (Muse, 2007).

Permanganate footprinting of a number of genes with PPEP confirmed that Pol II enrichment at these promoters resulted from stalling during early elongation. Permanganate reacts with single-stranded thymine residues, like those in an open transcription bubble, revealing both the presence and the location of a transcriptionally engaged but stalled polymerase. Permanganate hyper-reactivity was observed within the promoter-proximal region of all genes with PPEP analyzed (Muse, 2007).

To probe the mechanisms causing Pol II enrichment at candidate promoters, it was asked whether NELF, a known regulator of polymerase stalling, played a role at genes with PPEP. In support of this idea, ChIP with an antibody to NELF showed pronounced NELF occupancy of promoters with PPEP. Pol II Rpb3 ChIP-chip was carried out on partial genomic arrays (~20% of Drosophila genome) using cells that were mock-treated or depleted of NELF by RNAi. A modest duration was used of NELF-RNAi that markedly decreases NELF protein levels but does not lead to substantially altered gene expression profiles (Muse, 2007).

NELF depletion had a profound effect on polymerase signals at genes with PPEP. Moreover, the decrease in Pol II signal observed occurred only in the promoter region and not within the body of the gene. Analysis of the difference between average Pol II signals within the promoter and downstream regions for the 1,100 bound genes present on these arrays showed 200 genes with PPEP in mock-treated cells (18.2%), but only 85 genes with PPEP in the NELF-depleted sample. Thus, NELF-dependent stalling led to promoter-proximal enrichment of polymerase at nearly 60% of the candidate genes. Stalling at the remaining 85 genes may be unaffected by NELF depletion because of relatively tighter NELF retention at these genes or, alternatively, it might be NELF independent (Muse, 2007).

Querying the Gene Ontology database with a list of genes with PPEP, a significant overrepresentation was found of genes that respond to stimuli. Notably, nearly a third of are candidate genes are involved in development. Supporting a role of polymerase stalling in development, recent work has implicated stalling at the Drosophila sloppy paired 1 (slp1) gene in the regulation of cell fate specification. Furthermore, the genes involved in the processes of cell differentiation and cell communication were significantly enriched in the PPEP gene list, which also included many rapidly induced genes involved in the Toll-signaling, MAP-kinase, defense and immune-responsive pathways. Gene Ontology queries carried out with randomly selected sets of 1,000 Drosophila genes did not show significant enrichment in specific Gene Ontology categories (Muse, 2007).

To test the idea that Pol II stalled at the newly identified genes with PPEP could be released upon gene induction, advantage was taken of the fact that key players in the response to ultraviolet (UV) irradiation have PPEP. Before UV exposure, the UV-inducible genes W (also known as hid), CG12171 and Hsp70 had substantial enrichment of Pol II at their promoters compared to the downstream regions. Ultraviolet exposure activated transcription of these genes and led to a substantial decrease in stalled Pol II, as observed by permanganate mapping, as well as to a shift of Pol II signal downstream into the genes. Thus, activation of these UV-inducible genes involves the regulated release of stalled Pol II (Muse, 2007).

In conclusion, genome-wide analysis identified hundreds of Drosophila genes that possess stalled Pol II, indicating that this method of transcription regulation is much more widespread than previously appreciated. It has been shown that, in addition to heat shock-inducible promoters, a number of constitutively expressed genes have stalled Pol II, and Pol II stalling might thus be a common phenomenon. This work fully confirms that prediction and shows that NELF plays a key role in maintaining polymerase stalled near a large number of promoters. Notably, Pol II stalls near the promoters of many genes that, like Hsp70, respond to environmental or developmental stimuli, suggesting that the rapid release of stalled Pol II facilitates efficient, integrated responses to the dynamically changing environment. A stalled Pol II in the promoter-proximal region could help to establish an active chromatin structure around these genes and maintain them poised for activation. Moreover, the prevalence of promoter-proximal stalling at developmental-control genes suggests that stalling plays a fundamental role in development (Muse, 2007).

RNA polymerase stalling at developmental control genes in the Drosophila melanogaster embryo

It is widely assumed that the key rate-limiting step in gene activation is the recruitment of RNA polymerase II (Pol II) to the core promoter. Although there are well-documented examples in which Pol II is recruited to a gene but stalls, a general role for Pol II stalling in development has not been established. Comprehensive Pol II chromatin immunoprecipitation microarray (ChIP-chip) assays were carried out in Drosophila embryos and three identified distinct Pol II binding behaviors were identified: active (uniform binding across the entire transcription unit), no binding, and stalled (binding at the transcription start site). The notable feature of the ~10% genes that are stalled is that they are highly enriched for developmental control genes, which are either repressed or poised for activation during later stages of embryogenesis. It is proposed that Pol II stalling facilitates rapid temporal and spatial changes in gene activity during development (Zeitlinger, 2007).

To determine at which genes Pol II stalling occurs during development, global Pol II occupancy was analyzed in whole Drosophila embryos. Although this is one of the few systems in which genomics approaches can easily be applied to developmental questions, interpretation is complicated by the occurrence of multiple tissues. To reduce the complexity, Toll^10b embryos (2-4 h after fertilization), a well-characterized mutant that contains a homogeneous population of mesodermal precursor cells at the expense of neuronal and ectodermal cells, was used. In Toll^10b mutants, mesodermal genes are uniformly activated, whereas genes required for the development of ectodermal and neural tissues are repressed throughout the embryo. Previous whole-genome microarray experiments have identified the transcript levels of all genes in these mutants. To distinguish between stalled and active Pol II, a mixture of antibodies that recognizes both the initiating and elongating forms of Pol II was used, and whole-genome ChIP-chip assays were carried carried out (Zeitlinger, 2007).

The results show that many genes known to be repressed in Toll^10b embryos show notably high Pol II signal near the transcription start site. In some cases, the prominent Pol II peak was tightly restricted to the promoter region (for example, at the tail-up (tup) gene, whereas at other genes Pol II was also found at low abundance throughout the transcription unit (for example, the sog and brk genes. This is consistent with previous evidence that some genes, such as sog, are transiently activated but then repressed at later stages, whereas others, such as tup, are never activated in Toll^10b mutants (Zeitlinger, 2007).

The Pol II profiles of repressed genes are clearly distinct from those of active genes. For example, the stumps (also known as Hbr) gene, which encodes a fibroblast growth factor (FGF) receptor specifically expressed in mesodermal precursors show uniformly high levels of Pol II throughout the transcription unit. Furthermore, genes that are silent in the early embryo simply lack Pol II binding altogether. Thus, there appear to be three distinct classes of genes: those with Pol II distributed throughout the transcription unit, those with preferential enrichment of Pol II at the transcription site and those that lack Pol II binding altogether (Zeitlinger, 2007).

To further characterize these three groups, a principled method was developed that classifies genes on the basis of their Pol II enrichment profiles. The ratio between Pol II enrichment at the transcription start site versus internal regions of the transcription unit was calculated. It was possible to assign 76% of the protein coding genes (10,220 of 13,448 genes) to one of the three classes. At least 27% of all genes had an active Pol II profile in which Pol II was detected uniformly throughout the transcription unit. At least 12% of all genes (1,614 of 13,448) showed disproportionate accumulation of Pol II near the transcription start site. Among this group, Pol II was tightly restricted to the transcription start site at 62% of genes. At the remaining 38% of these genes, Pol II was also detected within the transcription unit, presumably because these genes -- such as sog -- are expressed at low levels in at least a subset of cells during the time frame of the analysis (2-4 h after fertilization). Finally, 37% of all genes lacked Pol II binding altogether (Zeitlinger, 2007).

Several lines of evidence confirm that the ~1,600 genes with disproportionate enrichment of Pol II at the transcription start site have a form of stalled Pol II. First, all heat shock genes, which provide the classical example of Pol II stalling, fall into this class. Second, the Pol II peaks map an average of ~50 bp downstream of the transcription start site, consistent with the location of stalled Pol II at heat shock genes. Because this is an average profile, it is possible that a fraction of Pol II occupancy comes from inactive preinitiation complexes. However, the majority of detected Pol II signal seems to come from Pol II that is stalled downstream of the transcription start site. Third, Pol II stalling at these genes is consistent with comprehensive expression analysis using whole-genome tiling arrays. Genes with Pol II tightly restricted to the transcription start site are either silent or only weakly expressed in Toll^10b mutants. In contrast, genes with similar levels of Pol II binding but uniform distribution throughout the transcription unit are expressed at substantial levels in these mutants. Finally, permanganate footprint assays were used as an independent method to confirm stalled Pol II at selected genes. For example, the rho gene showed clear permanganate sensitivity downstream of the transcription start site (+37 bp), consistent with the Pol II stalling profile seen in Toll^10b mutants (Zeitlinger, 2007).

There are considerable differences in the expression and functions of genes in the active, stalled or no Pol II classes based on in situ expression patterns (ImaGO database) and functional annotations. The set of genes with stalled Pol II is highly enriched for developmentally regulated genes, particularly those expressed in ectodermal and neuronal precursor cells. Consistent with these results, genes with stalled Pol II are highly enriched for functions in development, including neurogenesis, ectoderm development and muscle differentiation. Many of these genes encode sequence-specific transcription factors (Hox, T-box, bHLH, zinc fingers and HMG) and components of cell signaling pathways (FGF, Wnt, Notch, EGF, TGFβ, JNK and TNF (Zeitlinger, 2007; see supplemental material to article for a full list of those genes exhibiting pausing).

In contrast, the set of genes with uniform Pol II binding is highly enriched for ubiquitously expressed genes, which function mostly in metabolism and cell proliferation. The set of genes that lacks Pol II binding is highly enriched in genes that show no staining in whole-embryo in situ hybridizations, confirming that they are not expressed during early embryogenesis. Many of these genes encode proteins that have functions in adult cells, such as cuticle proteins or proteins required for vision (Zeitlinger, 2007).

Pol II stalling could reflect two nonexclusive developmental functions. It could be indicative of active transcriptional repression, or it could prepare genes for activation at later stages of embryogenesis. The second model is particularly attractive, because Pol II stalling has already been shown to prepare heat shock genes for rapid induction. Evidence was found for both models (Zeitlinger, 2007).

Pol II stalling is particularly prevalent among genes expressed in the neuroectoderm and dorsal ectoderm, which are repressed in Toll^10b embryos. To test whether Pol II stalling is specific for repressed genes, the Pol II profile was examined of these genes in mutant embryos in which they are active. For this, two well-defined mutants, Toll^rm9/rm10 and gd⁷ (2-4 h), were used in which cells adopt neurectodermal and dorsal ectodermal fates, respectively. Indeed, at these genes, Pol II is redistributed into the transcription unit in these mutants, and some genes now show the active Pol II profile. These results indicate that Pol II stalling is associated with cell-type specific repression and is subject to dynamic changes during development (Zeitlinger, 2007).

Previous studies have shown that the repression of a large set of genes in Toll^10b embryos depends on Snail, a well-studied repressor that is constitutively expressed in Toll^10b embryos but not in Toll^rm9/rm10 and gd⁷ embryos. A statistically significant association was found between repression by Snail and Pol II stalling. For example, among the 139 genes that are occupied by Snail and show reduced expression in the Toll^10b mutant, 54% have stalled Pol II, whereas only 19% of all genes with reduced expression show Pol II stalling. This suggests that Pol II stalling in Toll^10b embryos may be regulated by Snail. A role of developmental repressors in regulating Pol II stalling is also consistent with a recent study (Wang, 2007) of Drosophila segmentation (Zeitlinger, 2007).

Multiple lines of evidence suggest that Pol II stalling also occurs at genes that are poised for activation in older embryos. Genes with stalled Pol II are highly over-represented among genes that are rapidly induced within 12 h after the time frame of the analysis. Moreover, genes with stalled Pol II are enriched for genes expressed in the derivatives of the mesoderm precursors present in Toll^10b mutants, such as the developing heart and muscle cells. These genes, such as Drop (Dr) and bap, are not yet activated at the time frame of the analysis, but they nonetheless show high levels of Pol II near the transcription start site (Zeitlinger, 2007).

To confirm that muscle genes indeed show stalled Pol II before activation, permanganate assays were carried out on wild-type Drosophila embryos at 2-4 h after fertilization. Dr and lbe showed a clear permanganate footprint downstream of transcription. These footprints were specific to the early embryo stage, since S2 cells, a cell line derived from older embryos, did not show a permanganate footprint under the same conditions. These results confirm that Pol II stalling is dynamically regulated and suggest that one of its functions is to prepare genes for activation (Zeitlinger, 2007).

This genome-wide analysis showed that genes in Drosophila embryos are found in three distinct dynamic states: active, stalled or no Pol II. Stalled Pol II is particularly associated with developmental genes that are repressed and poised for activation. It is proposed that Pol II stalling prepares genes for rapid response to developmental signals during embryogenesis and thus may represent a key regulatory step for gene transcription in development (Zeitlinger, 2007).

P-TEFb kinase promotes transcription at heat shock loci

P-TEFb kinase recruitment to heat shock loci during the heat shock response and functions to stimulate promoter-paused RNA polymerase II (Pol II) to enter into productive elongation. P-TEFb is located at >200 distinct sites on Drosophila polytene chromosomes. Upon heat shock, P-TEFb, like the regulatory factor heat shock factor (HSF), is rapidly recruited to heat shock loci, and this recruitment is blocked in an HSF mutant. Yet, HSF binding to DNA is not sufficient to recruit P-TEFb in vivo, and HSF and P-TEFb immunostainings within a heat shock locus are not coincident. Insight to the function of P-TEFb is offered by experiments showing that the direct recruitment of a Gal4-binding domain P-TEFb hybrid to an hsp70 promoter in Drosophila cells is sufficient to activate transcription in the absence of heat shock. Analyses of point mutants show this P-TEFb stimulation is dependent on Cdk9 kinase activity and on Cdk9's interaction with cyclin T. These results, coupled with the frequent colocalization of P-TEFb and the hypophosphorylated form of Pol II found at promoter-pause sites, support a model in which P-TEFb acts to stimulate promoter-paused Pol II to enter into productive elongation (Lis, 2000).

P-TEFb is required to produce full-length transcripts from a variety of cellular DNA templates in an in vitro transcription system that accurately recapitulates the normal DRB-sensitive transcription seen in cells (Marshall, 1995). These results suggest that P-TEFb may have a role in transcription of many cellular genes. If so, this kinase may localize to chromosomal loci that possess genes that are the target of its activity. The chromosomal distribution of P-TEFb was examined by staining salivary gland polytene chromosomes with a highly specific antibody to the cyclin T regulatory subunit. This cyclin T subunit binds tightly to Cdk9 and is a critical component of the P-TEFb activity (Peng, 1998). Moreover, immunodepletion experiments show that the vast majority of Cdk9 is associated with a cyclin T subunit (Peng, 1998), and probing of phosphocellulose fractions from Drosophila Kc cell nuclear extracts indicates that cyclin T is present only where P-TEFb activity is found. Therefore, the cyclin T antibody provides a good means of tracking the P-TEFb complex (Lis, 2000).

Heat shock causes a rapid and dramatic activation of transcription of heat shock genes and a concomitant reduction in transcription of many normally expressed genes. Immunofluorescence analysis of polytene chromosomes reveals that Pol II relocates to heat shock loci after a brief heat shock. P-TEFb distribution also changes dramatically following heat shock. In uninduced larvae, P-TEFb is undetectable at major heat shock loci 87A and 87C, which contain the five hsp70 genes, or at 59B, which, in this strain, contains an hsp70-lacZ transgene. After a 20-min heat shock, these and all the other major heat shock loci at 63B, 67B, 93D, and 95D are the prominent sites of labeling. Loci that had high levels of P-TEFb before heat shock now have a reduced level. Therefore, P-TEFb redistributes to heat shock loci following heat shock (Lis, 2000).

The Pol II level on the 5' end of the hsp70 gene begins to be elevated in as little as 70 sec following a very rapid heat shock induction (mixing cells with warm medium), and Pol II is detected beyond the pause region and in the middle of the gene in as little as 2 min. This rapid transcriptional activation leads to a very high density of hyperphosphorylated Pol II on these genes. Could P-TEFb be playing a role in the transition of Pol II to its hyperphosphorylated, elongationally competent mode? If so, then one might expect P-TEFb to be recruited as rapidly as Pol II to these newly activated heat shock sites (Lis, 2000).

The kinetics of localization to heat shock loci at 87A and 87C and to 59B, which in this strain contains an Hsp70-lacZ transgene, were examined. No P-TEFb is detected at the native or the transgenic sites before heat shock. However, within 2 min, staining is apparent at 87A and 87C, each of which contain multiple copies of hsp70. Some staining is also detectable at the transgenic copy of Hsp70-lacZ. By 5 min of heat shock, staining at all heat shock loci is strong and this high level persists and may even increase in the 10- and 15-min time points. The level remains high during heat shock measured out to 60 min. A shift back to normal fly culture temperature (e.g., a 60-min recovery) reduces heat shock gene transcription and the normal pattern transcription is largely re-established (Lis, 2000).

The recruitment of P-TEFb to heat shock loci is completely dependent on HSF. A Drosophila temperature-sensitive mutant HSF strain, hsf4, shows a much reduced induction of heat shock gene transcription and chromosome puffing. In this strain, heat shock fails to concentrate P-TEFb at heat shock loci. Additionally, heat shock does not lead to a dramatic loss of P-TEFb at the normally active chromosomal sites in the HSF mutant strain as exemplified at 88D (Lis, 2000).

Heat shock rapidly stimulates the trimerization and binding of HSF to the heat shock elements (HSEs) located upstream of every heat shock gene. HSF acquires strong DNA-binding activity and localizes to heat shock loci on polytene chromosomes within 2 min following heat shock. Therefore, the rapid induction of HSF binding is similar to the rapid recruitment of P-TEFb seen here. Could HSF itself be sufficient to recruit P-TEFb through a stable interaction? This hypothesis was tested in vivo using a transgenic line containing a polymer of native HSF-binding sites that are unlinked to the rest of the hsp70 promoter. Following heat shock, HSF is known to localize to sites on polytene chromosomes containing this polymer. This anti-HSF staining is more than an order of magnitude stronger than that seen at the regulatory region of a single hsp70 gene, and can be compared with the 87A and 87C loci that contain two and three copies of native hsp70, respectively. The 87C signal is considerably stronger than 87A (Lis, 2000).

If HSF is sufficient to recruit P-TEFb to heat shock loci in vivo, then one would expect to see high levels of P-TEFb at the polymer site. There is detectable P-TEFb at the polymer site, but the level is less than at the native heat shock loci 87A and 87C. Moreover, the ratio of P-TEFb to HSF staining is much higher at heat shock genes than at the polymer site. These results indicate that HSF does not on its own recruit P-TEFb, and other features of the heat shock promoters are required to provide P-TEFb's strong recruitment to heat shock genes (Lis, 2000).

P-TEFb appears to resolve from HSF at the 87A locus. In most extended chromosomes examined, it is observed that the P-TEFb label separates into a doublet with HSF overlapping and falling between the peaks of the P-TEFb doublet. This can be interpreted in terms of the known arrangement of hsp70 genes at 87A. The hsp70 genes are divergently transcribed and the regulatory DNA containing the binding sites for HSEs resides in this region between the genes. HSF binds these regulatory regions as was seen from a band of fluorescence in the middle of the puff. In contrast, the centers of P-TEFb staining appear to reside downstream of the HSEs on both copies of the hsp70 gene. The partial separation of P-TEFb and HSF is also consistent with the idea that P-TEFb does not derive its stable association with heat shock genes solely through interaction with HSF (Lis, 2000).

A biochemical assay of the interaction of HSF and P-TEFb adds further support to the conclusion that these proteins do not interact strongly. Plasmids that express HSF, Cdk9-Flag, and cyclin T-6His were cotransfected into Drosophila cells. Following a standard heat shock treatment, cleared lysates were prepared from these cells, and the lysates were then chromatographed over nickel-NTA beads, which bind the 6His-tag. Portions of the lysates and nickel-bound fractions were then examined by Western blotting using HSF or Flag antibodies. Whereas Cdk9 is efficiently recovered in the Ni-bound fraction, HSF is not recovered at levels exceeding the background from cells lacking cyclin T-6His. These results and the in vivo results indicate that the high levels of P-TEFb association with heat shock loci cannot be explained by an interaction of HSF with P-TEFb (Lis, 2000).

Does the redistribution of P-TEFb to heat shock loci influence transcription of the heat shock genes? The effects of directly recruiting P-TEFb subunits, Cdk9 or cyclin T, to the hsp70 promoter were tested. A pair of Gal4-binding sites (UASgal) was introduced upstream of a Drosophila hsp70-M reporter gene. The expression of this hybrid reporter gene can be distinguished from native hsp70 genes since it is marked by fusion to a bacterial DNA sequence. This reporter construct and copper-inducible expression vectors, which express the Gal4 DNA-binding alone (G4) or G4 fused to Cdk9, cyclin T and a variety of controls, were cotransfected into Drosophila cells. The inserted UASgal sequences are upstream of the regions critical for heat shock expression, so, as anticipated, transcription of this reporter gene is heat inducible, albeit at about a twofold lower level than the control containing no UASgal insert. The reporter gene containing UASgal sites is strongly activated without heat shock when cells are cotransfected with G4 fused to the activation domain of HSF (G4-HSF). The reporter gene carrying the UASgal sites is also strongly activated without heat shock when cells are cotransfected with plasmids expressing G4-Cdk9 or G4-cyclin T. A point mutation that disrupts the activity of the kinase subunit, Cdk9/D199N (Peng, 1998), also disrupts the ability of the G4-Cdk9 hybrid protein to activate transcription from the hsp70 reporter. The levels of expression of wild-type and mutant G4-Cdk9 are similar. Also, a pair of mutations in cyclin T that disrupt its ability to interact with Cdk9, the double point mutant CycT/2XMut (Bieniasz, 1999), greatly impairs the ability of G4-cyclin T to activate transcription. These results demonstrate that artificially recruiting P-TEFb to the promoter by directly recruiting either of its two subunits is sufficient to strongly activate an hsp70 gene. A similar activation by G4-HSF, G4-Cdk9, and G4-cyclin T was observed with UASgal sequences inserted further upstream at -256, although the level of activation was reduced two to threefold (Lis, 2000).

A model is proposed in which P-TEFb acts on promoter-paused Pol II complexes to stimulate their escape into productive elongation. If P-TEFb is a major kinase that acts on the promoter-paused Pol II complex, its distribution should overlap at least some of the chromosomal sites that accumulate unphosphorylated RNA polymerase II (Pol IIa). However, the correlation need not be perfect, since the rate of formation of a promoter-paused Pol IIa is likely to be governed by mechanisms distinct from those that are responsible for recruiting P-TEFb. These mechanisms appear to be quite independent in an extreme case of heat shock genes, in which Pol IIa is present at full occupancy on the uninduced hsp70 promoter, and heat shock is needed to trigger both high levels of transcription and recruitment of P-TEFb. However, when a gene is active, Pol IIa is being continuously recruited to the promoter and maturing into a productive hyperphosphorylated polymerase II (Pol IIo) elongation complex. In the case of heat shock genes, the entry is fast enough to keep the pause region fully occupied with Pol II even when the gene is fully induced. Therefore, both the kinase responsible for phosphorylation and the Pol IIa would be expected to be present on active promoters, and their respective levels would be dictated by the relative rates of Pol entry and its maturation into a productive elongation complex (Lis, 2000 and references therein).

Chromosome were stained with antibodies to P-TEFb and Pol IIa. Most chromosomal sites in unstressed larvae that are labeled strongly by the P-TEFb (cyclin T) antibody are also labeled to various extents by the Pol IIa antibody; however, the ratio of labeling by these two antibodies varies at different sites. Therefore, the level of Pol IIa must be governed by factors that act at least somewhat independently from factors that govern the level of P-TEFb at specific sites. Nonetheless, the strong tendency of these proteins to colocalize is consistent with a model in which Pol IIa is a substrate for P-TEFb, and this phosphorylation serves to convert Pol II into a productive elongation complex (Lis, 2000).

The hyperphosphorylated form, PoI IIo, labels many more sites than does P-TEFb. Numerous sites are strongly labeled with antibody to Pol IIo, but not detectably labeled with antibody to P-TEFb. This pattern does not easily fit a model in which P-TEFb has a universal role in all Pol II transcription elongation. Presumably, there are distinct mechanisms (and other kinases) for producing Pol IIo that do not require the stable and continuous association of P-TEFb with a locus (Lis, 2000).

In contrast, there are few chromosomal sites that have P-TEFb, but no Pol IIo. A simple interpretation of this result, which is consistent with the known properties of P-TEFb, is that the recruitment of P-TEFb to a locus generally leads to efficient formation of transcription elongation complexes. These results also indicate that there is little recruitment of P-TEFb to sites that are not transcriptionally active (Lis, 2000).

P-TEFb is a kinase/cyclin heterodimer that was critical for overcoming an early block to transcriptional elongation (Marshall, 1995). Interestingly, the short transcripts of 20-40 nucleotides that are produced in the absence of P-TEFb are remarkably similar in size to those measured in vivo at genes that show promoter-associated pausing. Such pausing has been observed at a variety of genes; however, the heat shock promoters of Drosophila are perhaps the most thoroughly studied in eukaryotes. P-TEFb stimulates production of full-length transcripts in vitro (Marshall, 1995), and also from HIV templates in vivo (Mancebo, 1997). P-TEFb is normally located at >200 loci, but upon heat shock, it redistributes to native and transgenic heat shock loci with a robustness and rapidity that make it a good candidate for playing a critical role in the activation of heat shock gene transcription. The normally broad distribution of P-TEFb is simplified during heat shock, where the bulk of P-TEFb concentrates at all the major heat shock genes. The resolution of HSF and P-TEFb staining within the 87A locus is consistent with the long-held view that the DNA within this activated heat shock locus is in a very extended configuration. The two divergently transcribed hsp70 genes at this locus are separated by only 2 kb, and yet the P-TEFb staining resolves as two distinct bands. The major HSF staining resides between the two P-TEFb bands. HSF binding sites are known to reside in the region between the start sites of these genes. If the DNA in a highly decondensed puff approximates B-form DNA, that is, it has a chromatin packing ratio similar to that of highly transcribed ribosomal DNA, then the distance between the start sites of the two hsp70 genes would be ~0.7 µm. The centers of the two bands of staining are approximately twice that distance, implying that P-TEFb may be distributed over a region that extends downstream of the hsp70 start sites. Higher resolution biochemical methods will be required to precisely define the limits of the P-TEFb distribution. Nonetheless, the partial resolution of HSF and P-TEFb staining supports a view that these two components act at distinct points in the process of activating heat shock genes (Lis, 2000).

P-TEFb is not simply recruited by the hypophosphorylated Pol IIa. Pol IIa is the form of Pol II that is at the promoter pause region of hsp70 and other heat shock genes. Yet very little or no P-TEFb is detected at these sites prior the heat shock. It is speculated that a separate event must occur at these promoters to cause the association of P-TEFb, for example, another protein or proteins could recruit P-TEFb to these promoters. In the case of HIV, the Tat protein interacts with cyclin T to recruit the P-TEFb complex (Garber, 1998). For normal cellular genes, other host transcription factors may also play such a role; P-TEFb has been shown recently to be functionally recruited to MHC class II gene promoters by the CIITA activator (Kanazawa, 2000). Alternatively, transcription activation may normally allow a paused Pol IIa, a likely in vivo substrate of P-TEFb, to undergo a change or unmasking that now allows its association with P-TEFb (Lis, 2000).

P-TEFb is normally located at many chromosomal sites that are transcriptionally active. The chromosomal loci scored as positive with the cyclin T antibody may represent only a fraction of the genes that could be regulated by P-TEFb, owing to the dynamic developmental regulation of the Drosophila genome. Also, the existence of additional cyclin subunits that can couple with Cdk9 may produce a P-TEFb activity lacking cyclin T. Although whether P-TEFb activates transcription at all of the loci containing cyclin T cannot be evaluated, in the case of heat shock genes, the direct recruitment (via a Gal4 DNA-binding domain) of P-TEFb to an hsp70 promoter leads to an activation of this gene in the absence of heat shock. Although this activation is less than the very high level of activation caused by directly recruiting the activation domain of HSF, it is nonetheless clearly dependent on P-TEFb kinase activity. Interestingly, related kinases, Cdk2 and Cdk7, fail to activate this promoter and critical point mutations in the P-TEFb kinase or cyclin T disrupt the activation. The fact that Cdk7, the kinase of the TFIIH complex, fails to activate is worth noting, because it, like P-TEFb, can phosphorylate efficiently the CTD of Pol II. Perhaps these kinases have specificity for discrete steps in transcription. For example, P-TEFb may be capable of stimulating the elongation of the paused Pol II, whereas TFIIH kinase fulfills another role such as providing activity for an earlier step in transcription that does not necessarily lead to the full phosphorylation and maturation of elongationally competent Pol II. This specificity issue requires further, more focused investigation (Lis, 2000 and references therein).

Direct recruitment of P-TEFb to an HIV promoter has been shown to activate HIV transcription fully and bypasses the need for Tat (Bieniasz, 1999). Although the activation of hsp70 by directly recruiting P-TEFb that is observed in uninduced cells is strong, it is still less than that seen when the HSF activation domain is directly recruited. This fact suggests that HSF may be providing a function beyond triggering the events that lead to P-TEFb recruitment. The HSF activation domain is large enough to accommodate multiple interactions and functions (Lis, 2000 and references therein).

The colocalization of the hypophosphorylated Pol IIa with P-TEFb is intriguing, because the promoter-paused Pol II associated with all genes examined in Drosophila is hypophosphorylated. If the Pol IIa distribution is a general indicator of sites in which promoter-pausing is a part of the transcription mechanism, then P-TEFb may be stimulating the maturation of Pol II and its entry into productive elongation at a significant subset of active genes. Three of the four constitutively active genes that have been reported to have promoter-paused Pol II are at chromosomal sites that show significant P-TEFb. The fourth, Gapdh-2, is at 13F, a region that shows light P-TEFb staining. A higher resolution analysis will be required for a rigorous assignment of the P-TEFb signals to these specific genes (Lis, 2000 and references therein).

The failure to see a quantitative correlation of the intensity of staining of anti-Pol IIa and anti-P-TEFb at specific sites on polytene chromosomes is consistent with models in which the mechanism of generating paused Pol IIa is distinct from the mechanism that recruits P-TEFb. The extreme case of this is hsp70, in which, before heat shock, the promoter is fully occupied with Pol IIa, but has very little P-TEFb. Heat shock triggers the dramatic recruitment of P-TEFb, and the accumulation of Pol IIo on heat shock puffs. During heat shock, the paused Pol IIa still forms, but it escapes into productive elongation faster, once every 4 sec as compared with the uninduced level of once every 10 min. It is hypothesized that P-TEFb participates in this escape at heat shock genes and the subset of other genes that have promoter-paused Pol II (Lis, 2000).

Cdk9 is an essential kinase in Drosophila that is required for heat shock gene expression, histone methylation and elongation factor recruitment

Phosphorylation of the large RNA Polymerase II subunit C-terminal domain (CTD) is believed to be important in promoter clearance and for recruiting protein factors that function in messenger RNA synthesis and processing. P-TEFb is a protein kinase that targets the (CTD). The goal of this study was to identify chromatin modifications and associations that require P-TEFb activity in vivo. The catalytic subunit of P-TEFb, Cdk9, was knocked down in Drosophila using RNA interference. Cdk9 knockdown flies die during metamorphosis. Phosphorylation at serine 2 and serine 5 of the CTD heptad repeat were both dramatically reduced in knockdown larvae. Hsp 70 mRNA induction by heat shock was attenuated in Cdk9 knockdown larvae. Both mono- and trimethylation of histone H3 at lysine 4 were dramatically reduced, suggesting a link between CTD phosphorylation and histone methylation in transcribed chromatin in vivo. Levels of the chromo helicase protein CHD1 were reduced in Cdk9 knockdown chromosomes, suggesting that CHD1 is targeted to chromosomes through P-TEFb-dependent histone methylation. Dimethylation of histone H3 at lysine 36 was significantly reduced in knockdown larvae, implicating CTD phosphorylation in the regulation of this chromatin modification. Binding of the RNA Polymerase II elongation factor ELL was reduced in knockdown chromosomes, suggesting that ELL is recruited to active polymerase via CTD phosphorylation (Eissenberg, 2007).

Cdk9, the catalytic subunit of P-TEFb, is highly conserved among eukaryotes. The yeast kinases Ctk1 and Bur1 are both homologs of Cdk9, and both are CTD kinases in Drosophila, although loss of Bur1 has no effect on CTD phosphorylation yeast. Bur1 is essential but Ctk1 is not (Eissenberg, 2007).

RNAi knockdown of Cdk9 in transgenic flies results in lethality at the pupal stage. This is considerably later than the embryonic lethality reported for C. elegans RNAi knockdown of Cdk9. While this difference could reflect differences in the requirements for Cdk9 in these organisms, it is more likely that differences in timing or efficiency of RNAi, Cdk9 protein turnover and/or maternal Cdk9 loading accounts for the much later lethality in knockdown flies. Nevertheless, these results confirm and extend the finding that P-TEFb is essential in metazoan development (Eissenberg, 2007).

In contrast, Cdk9 homologs in fission yeast and Neurospora are not essential. Since CTD phosphorylation has been linked to promoter clearance, pre-mRNA processing and chromatin modification, it is not possible to say what aspect of P-TEFb activity is essential in metazoa. RNAi knockdown of the Drosophila Cdk9 in cultured cells causes arrest of the cell cycle at the G1-S transition, implicating this kinase in cell cycle control. It is unlikely that cell cycle arrest is causing the lethality in knockdown flies, since cell cycle mutations in Drosophila generally are associated with reduced or missing imaginal discs, and the discs in Cdk9 knockdown larvae appear overtly normal. The finding that Hsp70 transcripts are reduced in Cdk9 knockdown larvae is consistent with the reduced Hsp70 transcription previously reported in Cdk9 RNAi cultured cells. Hsp 70 is not essential in Drosophila, but the effects on Hsp70 suggest that defects in gene expression could underlie the essential requirement for Cdk9 in Drosophila development (Eissenberg, 2007).

Cdk9 knock-down flies show dramatic reductions in both serine 2 and serine 5 phosphorylation. In contrast, flavopiridol treatment of cultured cells has been found to selectively reduce serine 2 phosphorylation. The significance of this difference is unclear, but could reflect differences in experimental protocol. For example, flavopiridol treatments were limited to 15-20 min, while RNAi knockdown third instar larvae are subject to knockdown conditions for several days before assay. Longer periods of Cdk9 inactivation may be required for reduction in serine 5 phosphorylation. Alternatively, it is possible that knockdown of Cdk9 protein levels results in inhibition of TFIIH, the other known CTD kinase. Regardless of the mechanism, the RNAi knockdown clearly results in reduced phosphorylation of the CTD, enabling a test of the consequences of loss of CTD phosphorylation on chromatin modification and recruitment of RNA Polymerase II-associated factors (Eissenberg, 2007).

Loss of CTD phosphorylation in Cdk9 knockdown larvae is associated with reduced binding of the RNA Polymerase II elongation factor ELL genome-wide. ELL is broadly co-localized with phosphorylated RNA Polymerase II on polytene chromosomes, and is rapidly recruited to heat shock loci after a brief heat shock. These results suggest that the efficient recruitment of ELL to transcribed loci requires CTD phosphorylation. Whether this reflects a direct interaction of ELL with the CTD is unknown (Eissenberg, 2007).

Despite the fact that Elongin A affects the same kinetic parameter in RNA Polymerase II catalysis as ELL, Elongin A binding is not reduced by loss of CTD phosphorylation. As with ELL, the nature of Elongin A binding to RNA Polymerase II is unknown, but these observations suggest their binding can be distinguished by sensitivity to the phosphorylation state of the CTD. Since no increase of Elongin A was observed under conditions of reduced ELL binding, it seems unlikely that ELL and Elongin A compete for RNA Polymerase II binding (Eissenberg, 2007).

Spt4 and Spt5 are subunits of DSIF, which is implicated in the regulation of RNA Polymerase II elongation. Previous work suggested that reduced serine 2 phosphorylation of the RNA Polymerase II CTD has no effect on Spt5 recruitment to a heat shock gene in cultured cells (Ni, 2004). In Cdk9 knockdown flies, in which both serine 2 and 5 phosphorylation are reduced, the chromosomal distribution of Spt5 is unchanged genome-wide. This is consistent with previous reports that Spt5 interacts with both phosphorylated and unphosphorylated RNA Polymerase II (Wen, 1999; Lindstrom, 2001; Lindstrom, 2003; Eissenberg, 2007 and references therein).

The chromo domain motif is a binding site for methylated histone tails. The role of the CHD1 chromo domain in methylated histone binding is controversial. However, recent structural data determined that the double chromo domain of mammalian CHD1 binds methylated H3K4 in vitro (Flanagan, 2005). This study shows that Cdk9 knockdown leads to a loss of chromosomal CHD1. This observation is most easily interpreted as the result of loss of H3K4 methylation that also occurs in Cdk9 knockdown chromosomes. Thus, the finding reported in this study lends support to the in vitro binding data and strongly suggests that the chromo domain-methylated histone interaction plays a dominant role in targeting CHD1 to active chromatin in vivo (Eissenberg, 2007).

The observation that both H3K4 and H3K36 methylation are significantly reduced in Cdk9 knockdown chromosomes suggests a linkage between phosphorylation of the CTD and histone methylation at transcribed genes. In this respect, Cdk9 subsumes activities found in yeast Bur1/Bur2 and yeast Ctk1. Since no significant difference was observed in ASH1 protein levels on Cdk9 knockdown chromosomes, a model is favored in which Cdk9-dependent RNA Polymerase II elongation plays a mechanistic role in H3 tail methylation. In this model, RNA Polymerase II passage destabilizes histone-DNA contacts, making the histones better substrates for efficient methylation. Reduced CTD phosphorylation would lead to reduced rates of RNA Polymerase II transcription genome-wide, resulting in reduced efficiency of histone tail methylation. While the mechanism connecting CTD phosphorylation to RNA Polymerase II elongation rate is likely to be complex in vivo, the observation that reduced CTD phosphorylation is associated with reduced dELL binding suggests that loss of dELL association could be a contributing factor (Eissenberg, 2007).

Mutation in Ash1 in Drosophila results in loss of all detectable H3K4 methylation, but has no effect on H3K36 methylation. This is consistent with independent mechanisms for these two chromatin modifications. A Polymerase II passage model provides a simple mechanism to account for similar effects on both modifications based on substrate availability (Eissenberg, 2007).

P-TEFb is critical for the maturation of RNA polymerase II into productive elongation in vivo

Positive transcription elongation factor b (P-TEFb) is the major metazoan RNA polymerase II (Pol II) carboxyl-terminal domain (CTD) Ser2 kinase, and its activity is believed to promote productive elongation and coupled RNA processing. This study demonstrates that P-TEFb is critical for the transition of Pol II into a mature transcription elongation complex in vivo. Within 3 min following P-TEFb inhibition, most polymerases were restricted to within 150 bp of the transcription initiation site of the active Drosophila melanogaster Hsp70 gene, and live-cell imaging demonstrated that these polymerases were stably associated. Polymerases already productively elongating at the time of P-TEFb inhibition, however, proceeded with elongation in the absence of active P-TEFb and cleared from the Hsp70 gene. Strikingly, all transcription factors tested (P-TEFb, Spt5, Spt6, and TFIIS) and RNA-processing factor CstF50 exited the body of the gene with kinetics indistinguishable from that of Pol II. An analysis of the phosphorylation state of Pol II upon the inhibition of P-TEFb also revealed no detectable CTD Ser2 phosphatase activity upstream of the Hsp70 polyadenylation site. In the continued presence of P-TEFb inhibitor, Pol II levels across the gene eventually recovered (Ni, 2008).

This study has found that P-TEFb inhibition leads to a rapid depletion of elongating Pol II from the Hsp70 gene. Furthermore, the majority of polymerases remaining on the gene are restricted to the 5' end and are transcriptionally engaged. Together, these data indicate that P-TEFb is critical for the escape of Pol II into productive elongation in vivo. It was also found that upon P-TEFb inhibition, levels of all elongation and RNA-processing factors so far tested (P-TEFb, Spt5, TFIIS, Spt6, and CstF50) were dramatically reduced, with kinetics indistinguishable from the depletion of elongating Pol II. Therefore, at least a subset of transcription factors appear to depend on the continual presence of elongating Pol II for their association with chromatin (Ni, 2008).

The dependence of P-TEFb, Spt5, TFIIS, and CstF50 on Pol II for association with the Hsp70 gene was not surprising. P-TEFb, Spt5, and TFIIS all interact with Pol II and track with Pol II during activated transcription. CstF50 physically interacts with the CTD of Pol II, and CTD Ser2 phosphorylation is required for the association of cleavage and polyadenylation factors in S. cerevisiae. However, Spt6 was not necessarily expected to require Pol II elongation for its association with the gene. Spt6 directly interacts with histones and separates somewhat from Pol II on Hsp70: Spt6 does not colocalize with promoter-proximal Pol II but does colocalize with Pol II on the body of the gene. It is possible that Spt6 interacts cooperatively with Pol II and nucleosomes or that Spt6 interacts only with the productively elongating, Ser2-phosphorylated form of Pol II. Consistent with the latter possibility, Spt6 was recently shown to interact directly with Ser2-phosphorylated Pol II (Yoh, 2006). Interestingly, however, the interaction of Spt6 with Pol II is not required for the positive elongation activity of Spt6 in vivo. Future studies using live-cell-imaging techniques should shed light on the dynamics and mechanism of Spt6 recruitment to chromatin during transcription (Ni, 2008).

Evidence has also been obtained for the stable association of promoter-proximally stalled polymerases with the Hsp70 gene. Following a short treatment with Flavopiridol (FP), a potent and highly specific inhibitor of P-TEFb kinase activity treatment, the only transcriptionally engaged polymerases remaining on the gene were promoter proximal and the usual rapid recovery of fluorescence after photobleaching of GFP-tagged polymerase was abolished. This observation may also apply to the promoter-proximally paused polymerase on Hsp70 in uninduced cells and in the absence of FP and argues against rapid cycles of initiation, pausing, and premature termination. In further support of a stable association, the two main protein complexes believed to promote promoter-proximal pausing, DSIF and negative elongation factor, repress Pol II elongation in vitro but do not induce premature termination. Furthermore, the Spt5 subunit of DSIF was shown to cooperate with the Tat activator in preventing premature RNA dissociation from Pol II in an in vitro transcription assay. Both Spt5 and low levels of the activator HSF are present at the Hsp70 gene under noninducing conditions. It is considered unlikely, but it cannot be ruled out, that the FP prevents polymerases from prematurely terminating. It is extremely unlikely that FP prevents new polymerases from initiating, since FP is a kinase inhibitor and initiating polymerases are most favorably unphosphorylated. The definitive test of the status of paused Pol II in untreated cells awaits additional technological advances that allow rapid mapping of the paused Pol II associated with uninduced, unpuffed loci (Ni, 2008).

Consistent with previous analyses, this study has found that active P-TEFb kinase activity is not required for Pol II that is already productively elongating to continue to do so: polymerases already elongating at the time of P-TEFb inhibition cleared from the middle and downstream regions of the gene. Furthermore, these elongating polymerases still had substantial levels of Ser2 phosphorylation after FP addition, indicating that there was minimal Ser2 phosphatase activity upstream of the polyadenylation site (Ni, 2008).

In the course of this study, it was also found that Pol II levels across Hsp70 recovered with time after FP treatment. This discovery explains why only a small reduction in Pol II density on heat shock genes was detected after 20 min of FP treatment in previous work (Ni, 2004). In that work, it was concluded that the major defect in Hsp70 expression following P-TEFb inhibition was at the level of 3'-end processing of RNA. This study now shows that there is a Pol II elongation defect immediately following P-TEFb inhibition. While the persistent mRNA-processing defects likely account for a significant reduction in Hsp70 mRNA levels following a long FP treatment, the initial elongation defect demonstrated in this study may also contribute to the low mRNA levels. Previous work, in addition to demonstrating that severe mRNA-processing defects exist following P-TEFb inhibition, also showed that in the recovered phase, Ser2 phosphorylation levels are still low. Therefore, elongation and RNA processing may require different extents or patterns of phosphorylation of the CTD or other targets of P-TEFb (Ni, 2008).

There are several possible mechanisms for the recovery of Pol II levels after FP treatment. The FP may become inactivated with time, metabolized, or expelled from the cell. Alternatively, an FP-resistant kinase may compensate for reduced P-TEFb kinase activity. It is also possible that a very small residual activity of P-TEFb persists in the presence of FP and eventually enables recovery. Another intriguing possibility is that cellular levels of active P-TEFb are increased in response to FP treatment. In mammalian cells, two P-TEFb complexes exist: a large, inactive complex and a small, active complex. The treatment of mammalian cells with P-TEFb inhibitors promotes the release of P-TEFb from the large complex to increase the pool of active P-TEFb. If this same mechanism exists in Drosophila, following FP treatment, P-TEFb may be released from the complex that sequesters it in an inactive form to create a larger population of active P-TEFb molecules. Consistent with this possibility, P-TEFb levels on the 5' end of Hsp70 recovered after 3 min of FP treatment. The recovery in Pol II elongation is not yet seen at a 10-min time point, and so the recovery of P-TEFb levels at this time may indicate the beginning of the rescue of Pol II elongation ability, which then occurs gradually over the course of the next 20 min. While support for this explanation exists, a combination of the above-described possibilities contributing to the recovery of Pol II elongation ability with time after FP treatment cannot be ruled out (Ni, 2008).

The data presented in this study also show that, at least for some drugs, it is important to examine cells during very early periods of treatment to observe the immediate effect of the drug. The short FP treatments used in this study now provide evidence that P-TEFb is indeed required for the escape of Pol II into productive elongation at the Hsp70 gene. Other cases in which an elongation defect is not apparent in the presence of P-TEFb inhibitors may be so explained, or promoter-proximal pausing may not be a regulatory feature of the genes in question. In budding yeast, in which regulated promoter-proximal pausing is absent, the deletion of the Pol II CTD Ser2 kinase Ctk1 does not affect transcription elongation (Ni, 2008).

Regulation of the transcriptional activity of poised RNA polymerase II by the elongation factor ELL

Many developmentally regulated genes contain a poised RNA polymerase II (Pol II) at their promoters under conditions where full-length transcripts are undetectable. It has been proposed that the transcriptional activity of such promoters is regulated at the elongation stage of Pol II transcription. In Drosophila, the heat-shock loci expressing the Hsp70 genes have been used as a model for the regulation of the transcriptional activity of poised Pol II. Drosophila ELL (dELL) is a Pol II elongation factor capable of stimulating the rate of transcription both in vivo and in vitro. Although ELL and the elongation factor Elongin A have indistinguishable effects on RNA polymerase in vitro, the loss-of-function studies indicate that these proteins are not redundant in vivo. This study used RNAi to investigate the physiological properties of dELL and a dELL-associated factor (dEaf) in a living organism. Both ELL and Eaf are essential for fly development. dELL is recruited to heat shock loci upon induction, and its presence with Pol II at such loci is required for proper heat-shock gene expression. Consistent with a role in elongation, dELL knockdown reduces the levels of phosphorylated Pol II at heat-shock loci. This study implicates dELL in the expression of loci regulated by Pol II elongation (Smith, 2008).

Efficient transcription by RNA polymerase II (Pol II) is an intricate process that requires multiple contacts with the DNA template and nascent RNA that inevitably leads to frequent stalling during the transcription of a gene. The average rate of transcription by Pol II in vivo is an order of magnitude higher than that obtained in vitro despite additional impediments, such as traversing through nucleosomes. Using biochemical approaches, two Pol II elongation factors, Eleven nineteen lysine-rich leukemia (ELL) and Elongin A, were isolated from cell extracts as factors capable of stimulating Pol II activity by suppressing transient pausing. Despite similar in vitro activities, the Drosophila orthologs of ELL and Elongin A are each essential for development. This observation indicates that their in vivo activity is not redundant (Smith, 2008).

Recent genome-wide studies have found a large number of developmentally regulated genes that contain a paused Pol II at their promoters. Therefore, it has been proposed that the transcriptional activity of such poised Pol IIs is regulated at the level of transcription elongation. The classic model for studying genes regulated by promoter-proximal paused polymerase is Hsp70 gene induction in Drosophila. Previous studies have shown that several Pol II elongation factors are rapidly recruited to the Hsp70 genes after heat shock. Although much work has been done on the role of these factors in gene regulation in cultured cells, less is known about the role of these factors in the regulation of heat-shock gene expression in the whole organism. Although there are several mutants in the gene encoding Drosophila ELL (dELL), all of these alleles are embryonic lethal. Therefore, it was possible to use these alleles to further characterize the role of the elongation factor ELL in the regulation of the transcriptional activity of poised Pol II and Hsp70 loci. To test the role for dELL in gene expression, RNAi was used to reduce expression levels of both dELL and dELL-associated factor (dEaf) expression levels during development, and the in vivo effect of their reduction on transcription and development was examined. It was found that knockdown of dELL and dEaf results in lethality. Furthermore, knockdown of these elongation factors results in reduced Hsp70 transcript accumulation after heat shock. Immunolocalization of phosphorylated Pol II in heat-shocked dELL knockdown salivary glands demonstrates reduced levels of the elongating form of Pol II at the Hsp70 loci in the absence of dELL. These studies demonstrate that dELL is essential for full induction of heat-shock gene expression and are consistent with a role for dELL in Pol II elongation. These findings provide a role for an RNA Pol II elongation factor in the transcriptional regulation of poised Pol II (Smith, 2008).

dELL has been shown to be essential; homozygous mutant clones do not survive in the eye and homozygotes for loss-of-function alleles die at the end of embryogenesis or in early first instar. To investigate the role of dELL in transcription in flies, dELL was knocked down by RNAi, which typically reduces, but does not eliminate, the targeted gene products. A 600-bp portion of the dELL coding region was inserted into a P-element vector that drives the expression of dsRNA through two convergent Gal4 UAS promoters that flank the insert. Several transgenic lines were generated and tested for effects on viability by crossing to an Actin5C-Gal4 driver line that expresses yeast Gal4 under the cytoplasmic actin promoter. All eight dELL RNAi lines show significant loss of viability when expressed under this driver. When adult escapers were obtained, very few males were observed, indicating that males are more susceptible to loss of dELL. Greater numbers of females than males were observed at the third instar larval stage, indicating that males are dying earlier than females. A significant genome-wide reduction of dELL protein is observed by immunofluorescence analysis of dELL RNAi larval polytene chromosomes (Smith, 2008).

Through two-hybrid analysis, two interacting partners of ELL have been characterized in humans, Eaf1 and Eaf2. Eaf1 and Eaf2 are highly related and can stimulate the elongation activity of ELL in vitro. Recently, the association of Eaf with ELL was shown to be evolutionarily conserved, with the finding that Schizosaccharomyces pombe homologs SpEaf and SpELL directly interact with each other. Additionally, SpEaf enhances the stimulation by SpELL of Pol II transcription in vitro. Because Drosophila also has a single Eaf homolog, RNAi was used to knock down dEaf levels and assessed the viability of dEaf-knockdown flies in six different transgenic RNAi lines. In all lines, significant reductions were observed in the number of adult progeny of RNAi-expressing flies compared with control siblings. In addition, a consistent reduction in the male-female sex ratio was observed for dEaf RNAi, suggesting that the male-enhanced lethal phenotype (not observed for other elongation factors) is due to loss of a dELL-dEaf complex (Smith, 2008).

To test for the effectiveness of the RNAi knockdowns, dELL and dEaf mRNA levels were measured in knockdown larvae and their control siblings. Significant reductions in dELL transcripts are observed in the dELL RNAi larvae. dELL transcripts, as measured by RT-PCR, are not reduced by RNAi to the same level as dELL protein, as assessed by immunofluorescence on polytene chromosomes. Previously, it was observed that knockdown of dRTF1 by RNAi was more effective at the protein than the RNA levels presumably because the long dsRNAs produced are processed as miRNAs and interfere with translation. Because dELL is nested in an intron of the gene encoding the chromatin remodeling enzyme dMi-2, transcript levels for this gene were measured and no reduction was found of dMi-2 RNA in dELL RNAi larvae. Additionally, it was found that dEaf RNA levels are reduced in dEaf RNAi larvae. Interestingly, a significant increase in dELL levels is observed in dEaf RNAi larvae, possibly compensating for the lower dEaf levels (Smith, 2008).

dELL was previously shown to be recruited to heat-shock genes upon heat shock. To determine whether dELL is required for heat-shock gene expression, the levels of Hsp70 transcripts after heat shock were compared in dELL knockdown larvae and their control siblings. By immunofluorescence analysis, little or no dELL is seen at the Hsp70 gene after heat shock in dELL knockdown larvae, whereas the control siblings without the Gal4 driver showed the expected recruitment of dELL to the Hsp70 gene. Northern blot analysis showed reduced levels of Hsp70 mRNA levels in the dELL RNAi larvae. A similar analysis was done with dEaf RNAi larvae, and reduced Hsp70 mRNA also occurs after heat shock, although the deficit was less than observed for the dELL RNAi larvae. Similar results were observed when Hsp70 levels were measured by RT-PCR, showing greater reductions in Hsp70 RNA levels in dELL RNAi than dEaf RNAi larvae (Smith, 2008).

Chromosomal levels of dELL are markedly reduced in the absence of Cdk9, the catalytic subunit of the Pol II C-terminal domain (CTD) kinase PTEF-B. To determine whether dELL knockdown affects the recruitment of Pol II to the Hsp70 genes, dELL knockdown and control polytene chromosomes were probed with antibodies to the Ser-2-phosphorylated, elongating form of Pol II. Lower levels of Ser-2-phosphorylated Pol II were consistently observed at the Hsp70 heat-shock loci in dELL-knockdown larvae, suggesting a close link between dELL function and phosphorylation of the Pol II CTD (Smith, 2008).

ELL belongs to a class of transcription elongation factors that have been shown to stimulate the K_m and/or V_max of RNA Pol II in vitro by alleviating pausing on a purified DNA template. Another member of this class is Elongin A and its Drosophila ortholog dEloA. From the present and previous studies, it is clear that both dELL and dEloA localize to the Hsp70 gene upon heat shock, and each is required for full levels of heat-shock gene expression, suggesting that the in vivo roles of these elongation factors in Hsp70 gene transcription are not redundant. Similarly, it was observed that the knockdown phenotypes of these two proteins can be unique, such as the enhanced male lethality in dELL RNAi larvae. How could both elongation factors be redundant in vitro, yet nonredundant in vivo? The in vitro studies were performed on naked DNA templates, whereas the chromatin environment of RNA Pol II-transcribed genes can provide additional challenges to the polymerase. Each of these elongation factors has its own interaction partners and may be recruited to distinct states of the polymerase, such as initiating, elongating, or stalled polymerase. Consistent with this view, knockdown of dELL, but not dEloA, results in decreased levels of Ser-2-phosphorylated Pol II at the Hsp70 and other loci. Interestingly, the chromosomal targeting of dELL, but not dEloA, is dramatically reduced by the knockdown of CDK9, the Pol II CTD kinase, suggesting that dELL and dEloA are recruited to genes by distinct mechanisms. Fine mapping of dELL and dEloA on the well characterized Hsp70 gene at different time points after activation could clarify the distinct roles for these enzymes (Smith, 2008).

The lesser effect of dEaf knockdown on Hsp70 gene induction could be indicative of a requirement of dEaf for optimal function of dELL, whereas dELL can partially function without dEaf. Indeed, in vitro transcription studies have demonstrated that human Eaf proteins, in combination with ELL, stimulate transcription elongation by Pol II above the levels obtained with ELL alone. In dEaf RNAi larvae, it was observed that dELL levels are increased, conceivably as a cellular response to increased pausing resulting from lower dEaf levels (Smith, 2008).

Previous work on the function of dELL made use of alleles of the Su(Tpl) locus, which encodes dELL. All known Su(Tpl) alleles are embryonic lethal. In contrast, RNAi of dELL allows survival to the larval or adult stages depending on the insertion line of the dsRNA construct. Interestingly, the few 'escaper' dELL RNAi adults are overwhelmingly female. As seen with the heat-shock defect, the difference in male and female viability is less in dEaf RNAi flies than in dELL RNAi flies, consistent with dEaf enhancing, but not being absolutely required for, dELL function. A previous study showed that males express much higher levels of a dELL transcript than females, although the functional significance of this difference has not been investigated. One hypothesis is that dELL is needed in males as part of the process of X chromosome dosage compensation; Drosophila dosage compensation factors are thought to enhance transcription elongation of X-linked genes in males, and loss of any of these factors leads to male-specific lethality. In addition, reduced levels of several global chromatin regulators, including the supercoiling factor, Jil-1 H3 kinase, heterochromatin protein HP1, and the chromatin remodeler ISWI, have been reported to differentially affect the survival of males and/or the morphology of the X chromosome. However, in dELL knockdowns, MSL localization and the male polytene X chromosome morphology appears similar in dELL knockdown male larvae and their control brothers. Whether there are specific defects in dosage compensation of X-linked genes may be an interesting avenue for future investigations. Alternative explanations for a male-enhanced lethality also should be considered. For example, Drosophila males differ from females not just in having one less X chromosome, but also in carrying a Y chromosome, which comprises ~12% of the male genome. A number of genes are male-lethal due to the presence of the mostly heterochromatic Y chromosome, including modulators of position effect variegation, such as the Su(var)3-3 gene that encodes the histone demethylase LSD1, the uncharacterized Su(var)2-1, as well as the HP1-interacting protein Bonus (dTIF1), an enhancer and suppressor of position-effect variegation. For Su(var)2-1 and Bonus, the Y-lethal effect is not Y-specific but can be phenocopied by other sources of heterochromatin. A role for dELL in the regulation of heterochromatin is unknown but could conceivably be required for the expression of heterochromatin components (Smith, 2008).

NELF and DSIF cause promoter proximal pausing on the hsp70 promoter in Drosophila

Transcriptional elongation regulators NELF and DSIF collaborate to inhibit elongation by RNA polymerase IIa in extracts from human cells. A multifaceted approach was taken to investigate the potential role of these factors in promoter proximal pausing on the hsp70 gene in Drosophila. Immunodepletion of DSIF (FlyBase term: Spt5) from a Drosophila nuclear extract reduces the level of polymerase that pauses in the promoter proximal region of hsp70. Depletion of one Negative elongation factor E (NELF) subunit in salivary glands using RNA interference also reduces the level of paused polymerase. In vivo protein-DNA cross-linking shows that NELF and DSIF associate with the promoter region before heat shock. Immunofluorescence analysis of polytene chromosomes corroborates the cross-linking result and shows that NELF, DSIF, and RNA polymerase IIa colocalize at the hsp70 genes, small heat shock genes, and many other chromosomal locations. Finally, following heat shock induction, DSIF and polymerase but not NELF are strongly recruited to chromosomal puffs harboring the hsp70 genes. It is proposed that NELF and DSIF cause polymerase to pause in the promoter proximal region of hsp70. The transcriptional activator, HSF, might cause NELF to dissociate from the elongation complex. DSIF continues to associate with the elongation complex and could serve a positive role in elongation (Wu, 2003).

It is proposed that promoter proximal pausing occurs when the nascent transcript emerges from the RNA exit channel of the Pol II and is grabbed by the NELF-E subunit. Tethering of the NELF-E to the elongation complex would generate a rigid body that could restrict the movement of the Pol IIa. This model is supported by several observations. The paused polymerase is in the Pol IIa state, and NELF and DSIF only inhibit elongation by Pol IIa. In vitro transcription analysis indicates that the elongation complex is not receptive to inhibition by NELF and DSIF until the nascent transcript is ~30 nucleotides long. This length coincides approximately to the distance polymerase elongates on hsp70 before it pauses. In vitro transcription analyses indicate that DSIF and NELF associate with polymerase shortly after initiation but probably before the polymerase reaches the region of pausing. Finally, NELF-E has an RNA-binding motif that is essential for its inhibitory action (Wu, 2003 and references therein).

Although NELF and DSIF are sufficient to slow the elongation rate of purified Pol IIa, it is suspected that additional proteins are involved in stably pausing Pol II on the hsp70 promoter. In cell-free transcription reactions done with other promoters, the pausing caused by DSIF and NELF appears to be transient -- the polymerase eventually moves forward if given enough time. In contrast, several observations indicate that the Pol II on hsp70 is stably paused. The paused Pol II remains associated with the hsp70 promoter when nuclei are isolated from uninduced cells, and sarkosyl or high salt must accompany addition of nucleotides to cause the Pol II to resume elongation. In a cell-free system, Pol II remains stably paused on the hsp70 promoter for at least 30 min. GAGA factor might be involved in stabilizing the pause because mutations in the GAGA element result in a loss of paused Pol II (Wu, 2003).

Heat shock rapidly induces transcription as a result of the association of HSF with sites located upstream from the TATA element. The data suggest that HSF may activate transcription in part by causing NELF to dissociate from the Pol II. How HSF might cause the release of NELF is unclear. Phosphorylation of Pol IIa is likely to be an important step because the Pol II found in the body of the gene during heat shock is hyperphosphorylated. Phosphorylation of DSIF is another possibility as this has been observed to occur early in elongation in vitro. It is also unclear which kinase might be responsible for phosphorylating the Pol II. P-TEFb (see Cdk9) is a candidate because it associates with the hsp70 gene during heat shock induction, and HSF can be bypassed by directing a Gal4/P-TEFb fusion protein to the hsp70 promoter. No interaction, however, has been detected between P-TEFb and HSF. Recent results show that HSF associates with the mediator. Drosophila mediator contains a kinase that phosphorylates the CTD, and phosphorylation can occur synergistically with the TFIIH kinase. Perhaps HSF recruits the mediator and in turn the mediator releases the paused polymerase by phosphorylating the CTD (Wu, 2003).

The strong immunofluorescence staining observed for DSIF at heat shock loci during heat shock indicates that DSIF is associated with many of the polymerase molecules transcribing the gene. RNA polymerase initiates at a rate of once every few seconds during heat shock resulting in a train of elongation complexes traversing the gene. In the absence of NELF, DSIF might act as a positive elongation factor. Shortly after DSIF was discovered, another investigation identified DSIF as a cofactor required for reconstituting tat-dependent transcription. In this situation, DSIF appears to be stimulating elongation. DSIF has been found in a complex with another positive elongation factor called Tat-SF1. Tat-SF1 was first identified as a stimulatory factor for Tat, but subsequent results indicate that Tat-SF1 may promote elongation on cellular genes. In yeast, DSIF appears to act as either a positive or negative regulator of elongation depending on circumstances. A hypothesis that unites the positive and negative activities of DSIF considers this factor an adaptor that connects other modulators to the elongation complex. In this regard, DSIF has been shown to bind on its own to Pol II, whereas the stable association of NELF with Pol II requires the presence of DSIF (Wu, 2003 and references therein).

NELF and DSIF appear to associate with several hundred interbands in polytene chromosomes. Each interband could contain many genes. The weak staining of interbands by Hoecsht suggests that the DNA in the interbands is in a decondensed state. Residing in these decondensed regions could be genes whose primary control mechanism does not involve a disruption of chromatin structure or even assembly of the initiation complex. Instead, alleviating repression by NELF and DSIF could underlie the mechanism of activation (Wu, 2003).

Efficient release from promoter-proximal stall sites requires transcript cleavage factor TFIIS

Uninduced heat shock genes are poised for rapid activation, with RNA polymerase II (Pol II) transcriptionally engaged, but paused or stalled, within the promoter-proximal region. Upon heat shock, this Pol II is promptly released from the promoter region and additional Pol II and transcription factors are robustly recruited to the gene. Regulation of the heat shock response relies upon factors that modify the efficiency of elongation through the initially transcribed sequence. This study reports that Pol II is susceptible to transcription arrest within the promoter-proximal region of Drosophila hsp70 and that transcript cleavage factor TFIIS is essential for rapid induction of hsp70 RNA. Moreover, using a tandem RNAi-ChIP assay, it was discovered that TFIIS is not required to establish the stalled Pol II, but that TFIIS is critical for efficient release of Pol II from the hsp70 promoter region and the subsequent recruitment of additional Pol II upon heat induction (Adelman, 2005; full text of article).

In a search for elongation factors that directly affect the heat shock response, a role for the transcript cleavage factor TFIIS was investigated. Like the bacterial Gre factors, TFIIS rescues RNA polymerase that has undergone reverse translocation, or 'backtracking' along the DNA template. Backward movement misaligns the 3' end of the nascent RNA with the RNA polymerase active site, thereby prohibiting continued RNA synthesis. Transcript cleavage factors restart the arrested RNA polymerase by inducing internal cleavage of the RNA by the polymerase active site, creating a new 3' end that is properly aligned for catalysis. The activity of transcript cleavage factors has been reported to stimulate promoter escape and transcription elongation and to decrease pausing. Recently published structural and functional analyses of transcript cleavage factors GreB and TFIIS complexed with their respective RNA polymerases elucidate the mechanism of this activity: TFIIS inserts a long coiled-coil domain into the RNA polymerase secondary channel, helping to coordinate a Mg+2 ion required for the reverse-catalytic reaction. However, although the detailed mechanism of TFIIS activity is known, the in vivo roles for this activity remain poorly defined (Adelman, 2005 and references therein).

To test whether Pol II complexes stalled within the promoter-proximal region were inactive due to transcription arrest, whether they could be rescued by transcript cleavage factor TFIIS was investigated. Stalled early elongation complexes (EEC) formed in a partially fractionated embryo extract lacking TFIIS were isolated and washed before restarting transcription in the presence or absence of purified TFIIS. The data show that the addition of NTPs leads to little or no transcription elongation in the absence of TFIIS. The presence of purified TFIIS alone induced efficient cleavage of RNA products associated with stalled Pol II. The sensitivity of specific RNAs to TFIIS-dependent cleavage signifies that these RNA species are associated with backtracked, arrested Pol II complexes. Cleavage of these RNAs in the presence of TFIIS reactivates the stalled complexes, allowing the labeled RNA species to be elongated upon addition of NTPs. It is concluded that TFIIS-induced cleavage rescues the promoter-proximal stalled, arrested Pol II (Adelman, 2005).

Taken together, these data suggest that intrinsic pause sites within the promoter-proximal region of hsp70 are recognized in vitro, perhaps with the aid of regulatory elongation factors, and that Pol II at these locations rapidly become inactive. However, the experiments demonstrating transcription arrest involve EEC that were artificially stalled and stringently washed prior to analysis, which does not accurately reflect the dynamics of hsp70 transcription. Thus, to investigate whether Pol II actively transcribing through the promoter-proximal region is susceptible to arrest and to determine the role of TFIIS in this process, a transcription assay was performed in a fractionated Drosophila embryo extract that lacked TFIIS (Adelman, 2005).

EEC were radiolabeled during elongation to position +16 nt and washed thoroughly with transcription buffer plus heparin to remove unincorporated NTPs and unbound extract proteins and to prevent reinitiation. The resulting EEC were split into two equivalent reactions, one of which was supplemented with purified TFIIS. Unlabeled NTPs were added to restart transcription, and aliquots were removed various time points. In the absence of TFIIS, Pol II accumulated in the promoter-proximal region and was not able to escape from sites of stalling during the time course. In contrast, inactive Pol II complexes were barely detectable in the presence of TFIIS. Instead, TFIIS stimulated rapid and efficient elongation of the labeled +16 nt RNA through the promoter-proximal region, leading to the formation of increased levels of full-length transcript. TFIIS activity also generated cleavage products that were released from Pol II. These data indicate that the initially transcribed sequence of hsp70 contains intrinsic sites at which Pol II pauses or stalls during active transcription, and that TFIIS is critical for efficient elongation through this region (Adelman, 2005).

To verify the functional relevance of TFIIS in the heat shock response in vivo, TFIIS levels were depleted in Drosophila S2 cells using RNAi. S2 cells that were untreated or treated with dsRNA targeting TFIIS were heat shocked to induce production of hsp70 RNA before harvesting cells and isolating total RNA. The depletion of TFIIS was not complete, perhaps due to the abundance or low turnover of the TFIIS protein; nonetheless, TFIIS-depleted cells were estimated to contain only ~10% of normal levels of TFIIS (Adelman, 2005).

Analysis of hsp70 RNA levels by quantitative RT-PCR reveals that TFIIS-depleted cells are indeed deficient in the heat shock response. In particular, there is a dramatic delay in hsp70 production in TFIIS-depleted cells, with hsp70 levels barely increasing above background after 2.5 min of heat shock. The significant kinetic block in hsp70 RNA production in TFIIS-depleted cells observed after a short heat shock, begins to be overcome at later time points, leading to an overall heat shock response of approximately 50%-60% normal hsp70 levels. These data demonstrate that TFIIS is required in vivo for maximal expression of hsp70 and suggest that TFIIS may serve to regulate the kinetics of the heat shock response by maintaining Pol II in a readily inducible conformation (Adelman, 2005).

These results suggest that TFIIS is involved in mediating the magnitude and efficiency of the heat shock response; additionally, TFIIS has been proposed to function broadly in transcription elongation by Pol II. To view the distribution of TFIIS both over the entire genome and at heat shock loci, Drosophila polytene chromosomes were stained with an antibody that is highly specific for TFIIS. Over 150 specific loci are stained by anti-TFIIS, including several interbands and chromosomal puffs, which contain the Pol II-transcribed developmental genes, the native and transgenic heat shock genes, and the nucleolus organizer, which contains the Pol I-transcribed rRNA genes. The consistent, prominent labeling of the nucleolus organizer suggests that TFIIS plays a role in Pol I elongation. A functional interaction between TFIIS and Pol I has been reported previously; however, conflicting reports have indicated that Pol I transcription is stimulated by a distinct transcript cleavage factor (Adelman, 2005).

Perhaps most surprising is the strong staining of many condensed chromosomal bands. These are sites that are not actively transcribed by RNA Pol I, II, or III. These transcriptionally inactive regions of TFIIS accumulation may be indicative of an as yet uncharacterized function of TFIIS, or may represent storage or proposed transcriptosome assembly loci akin to the TFIIS-containing Cajal Bodies in Xenopus oocytes (Adelman, 2005).

Upon stimulation of the heat shock response, TFIIS accumulates at heat shock loci. However, in contrast to many other transcription factors, TFIIS can still be observed at many other loci on the chromosomes, and in particular, the strong colocalization with condensed DNA bands persists. This result is consistent with recent data on the localization of TFIIS in yeast, where it was noted that TFIIS was not generally required for Pol II transcription but appeared to be specifically recruited to actively transcribed genes during times of cellular stress and when transcription was compromised (i.e., 6-AU treatment or temperature shift). In agreement with these results, Drosophila TFIIS is recruited to heat shock loci rapidly after heat induction and TFIIS appears to travel into the body of the gene along with Pol II, since it can be seen to colocalize throughout the puff with active Pol II (Adelman, 2005).

To analyze the localization of TFIIS at hsp70 at higher resolution, chromatin immunoprecipitation (ChIP) assays were performed followed by real-time PCR. This method allows for quantitative analysis of both the spatial and temporal distribution of TFIIS on the hsp70 gene. Pol II (detected using an antibody that recognizes the Pol II Rpb3 subunit) is associated specifically with the promoter region of hsp70 prior to heat shock (Boehm, 2003). Upon heat induction, Pol II is rapidly detected in the body of the gene and a robust recruitment of additional Pol II is observed (Adelman, 2005).

Strikingly, TFIIS is also present at the uninduced hsp70 promoter. This result is consistent with the idea that TFIIS associates with the promoter-proximal stalled Pol II to rescue it from arrest, thereby maintaining the Pol II in a rapidly responsive, active state. During heat shock, TFIIS is further recruited to the promoter region of hsp70 and TFIIS is seen to track along with the elongating Pol II into the body of the gene, in agreement with its role as an accessory factor for transcription elongation (Adelman, 2005).

All of the above data are consistent with the hypothesis that the promoter-proximal stalled Pol II has a tendency to fall into transcription arrest and that TFIIS serves to rescue the arrested Pol II so that it can be induced to elongate upon heat shock. Thus, one would predict that, in the absence of TFIIS, Pol II that becomes inactive in the promoter-proximal region would remain inactive, thereby presenting a steric obstacle to the rapid recruitment of additional Pol II molecules upon heat shock. The Pol II density at the hsp70 promoter before heat shock would thus remain unchanged (i.e., one Pol II present within each hsp70 promoter region), but the movement of Pol II into the body of the gene and the recruitment of additional Pol II upon heat shock should be diminished or delayed (Adelman, 2005).

To test this idea, a protocol was developed to perform ChIP on S2 cells that had been depleted of TFIIS by RNAi. TFIIS and LacZ RNAi-treated cells were crosslinked directly, or after a short, 2.5 min, heat shock. Depletion of TFIIS has no effect on the level of Pol II detected in the hsp70 promoter region before heat shock. This result indicates that TFIIS is not required for Pol II to stall within the promoter-proximal region. However, depletion of TFIIS leads to a significant reduction in the heat shock-induced recruitment of Pol II to the promoter. In fact, the Pol II density remains equivalent to that observed before heat induction. Moreover, the reduction in recruitment of Pol II is accompanied by a decrease in the Pol II signal throughout the body of the gene. This result indicates that the stalled Pol II is not efficiently released into the gene in the TFIIS-depleted cells, and that this “stuck” Pol II blocks recruitment of additional Pol II (Adelman, 2005).

As a control for the level of depletion, the presence of TFIIS at hsp70 was assayed in LacZ and TFIIS-treated cells. The 10-fold depletion of TFIIS observed by Western analysis leads to a similar reduction in TFIIS detectable on the hsp70 gene under both NHS and HS conditions. Importantly, depletion of TFIIS has no effect on the levels of HSF recruited to hsp70 upon heat shock, indicating that TFIIS-depleted cells did not display a general, nonspecific loss of factor recruitment. These results demonstrate that, while TFIIS is not required to establish the stalled Pol II at hsp70, depletion of TFIIS interferes with efficient release of Pol II from the promoter region and the rapid recruitment of additional Pol II (Adelman, 2005).

Pol II and/or general transcription factors have been found to occupy a growing number of promoters of preactivated genes. These varied promoters may utilize similar mechanisms for selectively recruiting certain components of the transcription machinery and for regulating transcription initiation and elongation through the promoter-proximal region. The efficiency of synthesis through the initially transcribed sequence is particularly sensitive to perturbation and is thus a prime target for gene regulation. Factors that impede the progress of the RNA polymerase within the first 10–40 nt, which often include both protein components and the nucleic acid sequence, have been shown to influence transcriptional pausing, arrest, and termination efficiency. Identification and characterization of the factors that modulate the regulatory pausing and/or stalling of Pol II within the promoter-proximal region is essential to understanding the regulation of genes like hsp70, wherein this step is rate limiting for gene expression (Adelman, 2005).

Transcription of hsp70 in vitro revealed positions of pausing that corresponded faithfully with locations that had been identified in vivo as harboring Pol II complexes that were not efficiently elongated. Likewise, Pol II artificially halted at these positions in vitro rapidly lost the capacity to resume transcription, even after removal of negatively acting elongation factors through stringent washing with sarkosyl. These results demonstrate that Pol II can become inactive within the promoter-proximal region. This work expands upon these observations by establishing that the inactive Pol II can be rescued by transcript cleavage factor TFIIS and thus represent arrested species. It is interesting to note that the predominant sites at which Pol II is found on the uninduced hsp70 gene are positions to which Pol II stably backtracks in vitro (Adelman, 2005).

These data suggest the following model for the role of TFIIS in hsp70 gene expression. Under uninduced conditions, Pol II is recruited to the hsp70 promoter and begins to transcribe through the promoter-proximal region. Intrinsic pause sites within the initially transcribed sequence induce transient stops in elongation, giving the regulatory negative-elongation factors time to bind and impede further movement into the gene. However, Pol II stalled for an extended time at the pausing sites have a tendency to backtrack along the template, displacing the 3′ end of the RNA from the catalytic site and prohibiting further elongation. In the absence of TFIIS, the arrested, inactive Pol II are unable to resume transcription rapidly upon heat induction, even after the negatively acting factors have been removed. However, in the presence of TFIIS, TFIIS-dependent cleavage returns inactive Pol II to a transcriptionally active conformation so that, upon heat shock and the removal of negatively acting factors, Pol II can be rapidly released from the promoter region. The movement of the first Pol II away from the promoter region allows for the recruitment of subsequent Pol II molecules. It is noted that this model is supported by a recent study of factors that interact genetically with Dst1 (the yeast gene encoding TFIIS), which suggested a general role for TFIIS in the transition from initiation to elongation (Adelman, 2005 and references therein).

These results are reminiscent of the role of bacterial Gre factors in mediating transcription efficiency through a regulatory pause in the late gene operon of λ bacteriophage. In the λ system, interactions between the RNA polymerase σ subunit and the promoter-proximal DNA sequence induce a transient pause in transcription, during which the λ Q protein binds and modifies the RNA polymerase, rendering it termination resistant. The Gre proteins modulate the kinetics of transcription through the pause site and are required for efficient function of the λ Q protein. Similarly, the activity of transcript cleavage factor TFIIS is necessary for efficient induction of hsp70 through its activation of promoter-proximally stalled Pol II. Thus, the current results indicate that, in addition to structural and mechanistic similarity between the Gre and TFIIS proteins, these factors may perform similar roles in vivo, serving to mediate the expression of genes that undergo pausing within the initially transcribed sequence (Adelman, 2005).

NELF potentiates gene transcription in the Drosophila embryo

A hallmark of genes that are subject to developmental regulation of transcriptional elongation is association of the negative elongation factor NELF with the paused RNA polymerase complex. This study used a combination of biochemical and genetic experiments to investigate the in vivo function of NELF in the Drosophila embryo. NELF associates with different gene promoter regions in correlation with the association of RNA polymerase II (Pol II) and the initial activation of gene expression during the early stages of embryogenesis. Genetic experiments reveal that maternally provided NELF is required for the activation, rather than the repression of reporter genes that emulate the expression of key developmental control genes. Furthermore, the relative requirement for NELF is dictated by attributes of the flanking cis-regulatory information. It is proposed that NELF-associated paused Pol II complexes provide a platform for high fidelity integration of the combinatorial spatial and temporal information that is central to the regulation of gene expression during animal development (Wang, 2010).

A principle conclusion that emerges from this study is that NELF can play a positive role in supporting transcription in the Drosophila embryo. This finding is somewhat surprising based on NELF's well-characterized properties as a transcriptional inhibitor in vitro and the current view of its role in regulating the hsp70 gene in vivo. So how does a factor that antagonizes transcription elongation play a positive role in promoting gene expression? Depletion of NELF in Drosophila S2 cells leads to reduced expression of a number of genes, and this drop in expression levels correlates with the re-positioning of nucleosomes around the promoter. The idea that NELF stabilizes the local architecture at the promoter that supports transcription is attractive, but the current results indicate these presumptive architectural contributions are not essential for transcription of several endogenous loci in the early embryo (Wang, 2010).

Central to understanding the requirement for NELF in promoting transcription is defining the key differences between the endogenous eve, ftz, rho and slp1 loci and the NELF-dependent reporter genes containing different specific cis-regulatory enhancers from these genes. The results strongly suggest that it is not the basal promoter per se that dictates the requirement for NELF. The basal promoter region contained in the composite slp1[DESE+PESE]-lacZ reporter extends from 260 bp upstream to 121 bp downstream of the transcription start site and includes the entire 5′ untranslated region of the slp1 mRNA. Even more telling is the observation that expression of the ftz-lacC reporter is lost in NELF-A GLC embryos. This reporter contains 6.5 kb of contiguous upstream cis-regulatory DNA extending to 120 bp downstream of the transcription start site. This stands in contrast to the NELF-independent expression of the P{PZ}slp1[05965] enhancer trap inserted 44 bp upstream of the slp1 transcription start site. Transcription of lacZ mRNA from this enhancer trap presumably initiates at the P-element promoter located at the 5′ terminus of this transgene insertion (Wang, 2010).

Although the requirement for NELF appears not to be dictated by the basal promoter, the observations that NELF is specifically localized to promoter regions and does not travel with elongating Pol II complexes strongly suggest the requirement involves NELF-associated Pol II complexes paused downstream of the promoter. The differential effect of NELF-E depletion on expression of the different slp1-lacZ reporters further indicates that the relative requirement for NELF is a function of the extent of flanking cis-regulatory information. Taken together these observations suggest that the relative requirement for NELF depends on interactions involving these flanking cis-regulatory DNA regions and NELF-associated paused Pol II complexes. It is proposed that NELF interacts with Pol II complexes that have initiated transcription but that are not fully competent to enter productive elongation and helps to stabilize these complexes in a form that is competent for responding to activating (or repressing) cues from enhancer-bound transcription factors. In this model the relative requirement for NELF in allowing for active transcription would depend on the strength of the interaction between a promoter and an enhancer and the relative efficiency of generating productive elongation complexes. Although the enhancers contained in the different lacZ reporters used in these experiments are all clearly capable of communicating with the promoter it would certainly be expected that this communication would be less efficient than for endogenous loci that contain the full complement of flanking DNA that has evolved to optimize the regulation of gene expression at this stage. Importantly, the NELF-dependent expression of these reporters strongly suggests that the generation of a productive Pol II elongation complex is the key step in the transcription cycle that is targeted for integrating the regulatory cues that drive the patterned expression of these genes in the early embryo (Wang, 2010).

NELF clearly has a pleiotropic role during Drosophila development. Perturbations in maternally provided NELF lead to two distinct embryonic lethal phenotypes. The observation that both phenotypes, albeit with different penetrance are produced either by elimination of maternal NELF-A or by reduction of maternal NELF-E strongly suggests both phenotypes are due to decreased activity of the NELF complex. The early arrest phenotype occurs prior to the onset of transcription in the embryo and thus is most likely due to defects that occur during oogenesis. The maternally provided histone mRNAs are one likely candidate as a prospective target of NELF activity during oogenesis. NELF is required for the proper processing of the 3′ termini of replication-dependent histone mRNAs in HeLa cells, and Drosophila embryos with defects in the processing of maternally provided histone mRNAs arrest during the nuclear division cycles that precede the blastoderm stage. Further studies should reveal whether the early arrest of NELF-A and NELF-E GLC embryos reflects a conserved role for NELF in the 3′-end processing of histone mRNAs (Wang, 2010).

The finding that many genes have paused Pol II complexes at their 5′ end strongly suggests that the regulation of transcription elongation is a widespread phenomenon in higher eukaryotes. Recent studies indicate that more than one third of all genes in Drosophila S2 cells generate short, 5′-capped RNAs similar to those produced by stalling of Pol II. The results of Pol II chromatin immunoprecipitation whole genome microarray assays suggest that paused Pol II complexes are formed on approximately 10% of genes in the blastoderm stage Drosophila embryo. This is almost certainly an underestimate as five of the seven genes for which NELF association has been demonstrated were not identified as having paused Pol II complexes at this stage. Indeed, the stringent cut-off used in this study led to the assignment of slp1 as a member of the 27% of genes that have uniform Pol II association in the blastoderm embryo (Wang, 2010).

It is furthermore clear that NELF association is developmentally regulated as neither srya nor any of the five segmentation genes for which NELF association in the early embryo was demonstrated in this study are also associated with NELF in S2 cells. Amongst these six genes with early embryonic association of NELF there are differences in the level of association at different developmental stages. The two genes with the most rapid loss of NELF, ftz and srya show little to no expression after four hours of development. Thus NELF is not involved in the stable maintenance of repression at these later stages, which involves instead other mechanisms such as epigenetic maintenance by the Polycomb group proteins and specific histone methylation marks. The observation that NELF association is also reduced on genes such as en and wg that have increased expression levels at later stages may suggest that NELF is not involved in the ongoing expression of these genes at later stages. However, as the embryo is comprised of a mixture of expressing and non-expressing cells it will be important to examine NELF association specifically in cells expressing these genes before coming to this conclusion (Wang, 2010).

The high levels of NELF association with the promoter regions of a number of genes involved in segmentation and other early developmental processes serves to emphasize the unique and pivotal aspects of this critical stage of Drosophila embryogenesis. Pre-blastoderm nuclei are totipotent and come to be specified in response to maternally-provided positional information and the action of the genetic systems that respond to this information. The regulation of gene transcription is central to the initial specification of cell fates along both the anterior-posterior and dorsal-ventral axes of the early embryo, and it is clear that regulation of transcription elongation is central to this process. Similar to Drosophila blastoderm nuclei, the pluripotent properties of human embryonic stem cells are reflected by the presence of paused Pol II complexes on a wide number of genes, including many key developmental regulators. Further studies on the mechanisms of developmentally regulated transcription elongation are clearly of great importance for understanding the initial programming of cell fates expression during animal embryogenesis (Wang, 2010).

Missra, A. and Gilmour, D. S. (2010). Interactions between DSIF (DRB sensitivity inducing factor), NELF (negative elongation factor), and the Drosophila RNA polymerase II transcription elongation complex. Proc. Natl. Acad. Sci. 107(25): 11301-6. PubMed Citation: 20534440

Interactions between DSIF (DRB sensitivity inducing factor), NELF (negative elongation factor), and the Drosophila RNA polymerase II transcription elongation complex

Negative elongation factor (NELF) and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole sensitivity-inducing factor (DSIF) are involved in pausing RNA Polymerase II (Pol II) in the promoter-proximal region of the hsp70 gene in Drosophila, before heat shock induction. Such blocks in elongation are widespread in the Drosophila genome. However, the mechanism by which DSIF and NELF participate in setting up the paused Pol II remains unclear. The interactions were analyzed among DSIF, NELF, and a reconstituted Drosophila Pol II elongation complex to gain insight into the mechanism of pausing. The results show that DSIF and NELF require a nascent transcript longer than 18 nt to stably associate with the Pol II elongation complex. Protein-RNA cross-linking reveals that Spt5, the largest subunit of DSIF, contacts the nascent RNA as the RNA emerges from the elongation complex. Taken together, these results provide a possible model by which DSIF binds the elongation complex via association with the nascent transcript and subsequently recruits NELF. Although DSIF and NELF were both required for inhibition of transcription, no NELF-RNA contact is detected when the nascent transcript was between 22 and 31 nt long, which encompasses the region where promoter-proximal pausing occurs on many genes in Drosophila. This raises the possibility that RNA binding by NELF is not necessary in promoter-proximal pausing (Missra, 2010).

DSIF and NELF are key factors in pausing Pol II in the promoter-proximal region of genes in Drosophila and human cells. To gain insight into the mechanism by which DSIF and NELF contribute to promoter-proximal pausing, a system was developed in which the physical interaction of DSIF and NELF with a Pol II elongation complex could be monitored using a native gel electrophoresis assay. Previously, it was demonstrated that DSIF alone could associate with the Pol II elongation complex. In this study a method was developed to purify Drosophila NELF, thus allowing exploration of the interplay of DSIF and NELF with the elongation complex (Missra, 2010).

The results show that the association of NELF with the elongation complex is dependent on the presence of DSIF. Previous work provided evidence that NELF associated with preformed complexes of DSIF and Pol II in nuclear extracts but the interaction of DSIF and Pol II was not dependent on NELF. These interactions were likely occurring outside the context of an elongation complex and were relatively weak because the bulk of DSIF, NELF, and Pol II exist independent of each other in nuclear extracts. In contrast, the current results show that NELF can significantly influence the binding of DSIF to Pol II within the context of an elongation complex when limiting amounts of DSIF are present. Since Pol II, DSIF, and NELF have been shown to interact individually with each other, it is likely that this network of interactions contributes to stable association of these proteins in the context of the elongation complex (Missra, 2010).

The binding assays show that the length of the nascent transcript affects the association of DSIF and NELF with the elongation complex. While binding of DSIF alone or in combination with NELF to the elongation complex was evident for an elongation complex with a nascent transcript of 22 nt, no binding was detected when the nascent transcript was 18 nt long. These results are consistent with the finding that human DSIF and NELF require transcripts ≥18 nt long to inhibit transcription, and also a recent study showed human DSIF preferentially bound elongation complexes containing transcripts that were at least 25 nt long. The 5′ end of an 18-nt-long nascent transcript just begins to emerge from the surface of Pol II. Exposure of four additional nucleotides appears to be sufficient for binding of DSIF alone or with NELF. Notably, the association of DSIF with the elongation complex is not simply due to nonspecific interaction with the RNA or DNA because previous experiments show that binding of DSIF to the elongation complex requires specific contacts with Pol II (Missra, 2010).

One way in which nascent transcript length could affect the association of DSIF and NELF is by providing an additional binding site in the elongation complex. Previous results have directed attention at an RRM in NELF-E. Mutations in this RRM impair the capacity of NELF to inhibit elongation in the presence of DSIF. However, these experiments focused on elongation over distances greater than 100 nt. The finding that DSIF associates with elongation complex containing a 22-nt-long radioactive transcript (EC22) but not EC18 suggests that DSIF rather than NELF might be interacting with the nascent transcript, and RNA-protein cross-linking data support this hypothesis. The Spt5 subunit of Drosophila and human DSIF contains five Kyprides, Ouzounis, Woese (KOW) domains. An isolated KOW domain from Aquifex aeolicus NusG has been shown to associate with RNA, so it is possible that one of these domains in Spt5 is contacting the nascent transcript as it emerges from the elongation complex (Missra, 2010).

Cross-linking analysis detected contact between NELF-E and the nascent transcript in EC70 but not in EC31. The 5′ end of the nascent transcript contacts Rpb7 when its length is between 26 to 32 nt. Therefore it is possible that a longer nascent transcript is required to allow contact with NELF. Given that promoter-proximal pausing can occur before Pol II transcribes 30 nt, it is proposed that the RRM of NELF-E is not involved in promoter-proximal pausing. Its role could be limited to processes involving longer nascent transcripts such as regulation that appears to involve the transactivation response element of HIV or 3′ end formation of histone mRNAs (Missra, 2010).

The finding that DSIF and NELF associate with EC22 but not with EC18 is very relevant to the process of promoter-proximal pausing. Permanganate genomic footprinting of over 60 different promoters reveals that Pol II pauses in the promoter-proximal region 20 to 50 nt downstream from the transcription start site. Those cases where the Pol II appeared to be pausing closer to a transcription start site were found to have the start sites inaccurately mapped. Thus, the promoter-proximal limit for the range where Pol II pauses is likely to be dictated by the minimum length of RNA required for DSIF to associate with the elongation complex (Missra, 2010).

From the results presented in this study, it is proposed that the first step in promoter-proximal pausing involves binding of DSIF to the nascent transcript. NELF subsequently associates to form a stable complex. Importantly, this complex alone is not sufficient to stably pause the Pol II as the results show that elongation is slowed but not halted in reactions involving only these three proteins. Hence other factors that remain to be identified are likely to act in concert with this core complex of DSIF, NELF, and Pol II to stably pause Pol II in the promoter-proximal region of genes. Since transcription in vivo occurs on chromatin, nucleosomes may cooperate with DSIF and NELF in setting up the paused polymerase. The experimental approach described in this study could serve as a way to identify additional factors involved in pausing (Missra, 2010).

Negative elongation factor accelerates the rate at which heat shock genes are shut off by facilitating dissociation of heat shock factor

Promoter-proximal pausing of RNA polymerase II (Pol II) occurs on thousands of genes in animal cells. This pausing often correlates with the rapid induction of genes, but direct tests of the relationship between pausing and induction rates are lacking. hsp70 and hsp26 in Drosophila are rapidly induced by heat shock. Contrary to current expectations, depletion of negative elongation factor (NELF), a key factor in setting up paused Pol II, reduced pausing but did not interfere with rapid induction. Instead, depletion of NELF delayed the time taken for these genes to shut off during recovery from heat shock. NELF depletion also delayed the dissociation of HSF from hsp70 and hsp26, and a similar delay was observed when cells were depleted of the histone acetyltransferase CBP. CBP has been reported to associate with Pol II, and acetylation of HSF by CBP has been implicated in inhibiting the DNA-binding activity of HSF. It is proposed that NELF-mediated pausing allows Pol II to direct CBP-mediated acetylation of HSF, thus causing HSF to dissociate from the gene. Activators are typically viewed as controlling Pol II. These results reveal a possible reciprocal relationship in which paused Pol II influences the activator (Ghosh, 2011).

Reduction of promoter proximal pausing on hsp70 does not alter the rate of heat shock induction. Correlations between the presence of paused Pol II and rapid induction of gene expression have led to the hypothesis that promoter proximal pausing provides the basis for rapid induction. However, none of these studies have actually tested if disruption of promoter proximal pausing delays expression of genes. The current analysis reveals that reduction of promoter proximal pausing at hsp70 does not detectably alter the rate of heat shock induction. This was evident by monitoring the association of Pol II at both the 5’ and 3’ ends of hsp70 using permanganate genomic footprinting and also by measuring synthesis of hsp70 RNA. The two types of assays are in good agreement with each other in that a substantial increase in the level of transcript and in the presence of Pol II at the 3’ end of the gene occurs between 2 and 4 minutes after heat shock. Also, the elongation rate of approximately 1.25 kb/min for transcription of hsp70 that can be deduced from permanganate data is in excellent agreement with other types of measurements, thus validating the use of permanganate footprinting as a means for analyzing the kinetics of transcription induction and elongation. It has been proposed that the establishment of paused Pol II at a promoter allows for rapid induction because steps that precede initiation such as chromatin remodeling and Pol II recruitment are bypassed. In the case of the heat shock genes, these steps are still likely to occur prior to heat shock induction even when promoter proximal pausing is impaired. TFIID, which provides the foundation for assembling a preinitiation complex, makes multiple sequence-specific contacts with hsp70 including the TATA box, the initiator, and downstream sequences. In addition, another DNA binding protein called GAGA factor maintains the promoter region in an accessible state even when TFIID binding is impaired. Since HSF associates with a transgenic hsp70 promoter deleted of its TATA box, the accessible state of the promoter in lieu of TFIID appears to be sufficient for HSF binding. Thus access of DNA by the general transcriptional machinery and HSF may not be limiting for hsp70 even when promoter proximal pausing is disrupted. The extent to which promoters retain an accessible state upon loss of paused Pol II is likely to vary. The Drosophila heat shock genes could represent one end of a spectrum where accessibility is retained without sustaining full occupancy of the promoter with paused Pol II because of the binding of GAGA factor and TFIID. At the other end of the spectrum may be genes that require the paused Pol II to maintain a potentially inducible state. Cases have been identified where the paused Pol II appears to contribute to induction by preventing a nucleosome from assembling over the promoter (Ghosh, 2011).

Depletion of NELF impacts the ability of hsp70 to shut-off during recovery from heat shock. Permanganate genomic footprinting provides a high-resolution view of the behavior of Pol II. Permanganate reactivity on hsp70 at positions +7 and +8 are indicative of newly initiated Pol II whereas reactivity downstream from +34 corresponds to Pol II that has read through the pause. Similarly for hsp26, permanganate reactivity at +9 and +10 are indicative of newly initiated Pol II whereas reactivity downstream from +45 corresponds to Pol II that has read through the pause. The patterns of permanganate reactivity that were observed indicate that both initiation and read-through persist for a longer time in NELF-depleted glands than in control glands during recovery from heat shock for both hsp70 and hsp26. Corroborating this conclusion is the demonstration that a brief heat shock causes these two genes to be more highly expressed in NELF-depleted glands than in control glands. Limiting the production of hsp70 protein to a level appropriate to the degree of stress could be critical for proper cell development, since ectopic expression of hsp70 in salivary glands under non-heat shock conditions inhibits growth of the cells in this tissue (Ghosh, 2011).

These results now expand the types of scenarios in which NELF and promoter proximal pausing serve to attenuate gene expression. In the case of the estrogen-mediated activation of the pS2 gene, direct interaction between the NELF-B subunit and the estrogen receptor causes estrogen-mediated association of NELF with the target gene. Depletion of NELF results in hyper-activation of the pS2 gene, indicating that the estrogen receptor is mediating both activation and repression to achieve a particular level of expression in the presence of stimuli. In the case of junB, depletion of NELF increased the expression of junB both before and after serum-mediated activation. The current results now reveal a role for NELF in shutting off gene expression upon withdrawal of the stimulus - this case being heat shock (Ghosh, 2011).

To investigate why the shut off of the heat shock genes was delayed by depletion of NELF, the association of HSF with hsp70 and hsp26 in salivary glands was measured using chromatin immunoprecipitation. Significantly more HSF was detected on the heat shock genes after 45 minutes of recovery in NELF-depleted than control glands indicating that the delay in shutting off the heat shock genes could be due to a delay in the dissociation of HSF and that NELF is somehow involved in this dissociation. It is unlikely that the dissociation of HSF is due to stress induced by depletion of NELF, since there is no evidence of this depletion inducing a stress response in salivary glands. Prior to heat shock, both control and NELF-depleted glands exhibit low levels of heat shock gene expression, low levels of HSF at heat shock gene promoters, and undetectable levels of HSF DNA-binding activity in amount of HSF detected in glands nor did it alter a heat shock-dependent shift in the mobility of HSF on SDS-PAGE. Approximately 2-fold higher HSF binding activity was detected in extracts from NELF-depleted glands than control glands after recovery from heat shock, but the standard deviations in the measurements indicate that this difference is not significant. Since the results showed that depletion of NELF resulted in a delay in the dissociation of HSF from heat shock genes during the recovery from heat shock, recent indirect evidence in human cells that acetylation of HSF by CBP regulates HSF binding activity was intriguing. The finding that depletion of CBP delays the dissociation of HSF from the heat shock genes provides the first direct evidence that CBP is involved in regulating HSF’s interaction with heat shock genes in vivo. Since depletion of CBP and NELF each delays the dissociation of HSF from the heat shock genes during recovery, it is proposed that their actions are linked. It is posited that NELF-mediated pausing of Pol II could position CBP to acetylate HSF, thus causing HSF to dissociate from the heat shock gene. Several observations provide support for this model. CBP associates with Pol IIa and not Pol IIo and the Pol II paused in the promoter proximal region is in the IIa state. Also, CBP associates with the heat shock genes in Drosophila during heat shock induction. Further investigation of this proposal will be aided by biochemical analyses of interactions between CBP and reconstituted paused elongation complexes. Multiple mechanisms can attenuate the activity of HSF during heat shock, so the possibility cannot be ruled out that the actions of NELF and CBP are independent of each other or that these proteins are acting indirectly on HSF by influencing other regulators of HSF. The heat shock proteins such as hsp70 produced during heat shock repress the activation domain of HSF thus providing a negative feedback loop that limits heat shock gene transcription. These chaperons have also been implicated in attenuating the DNA binding activity of HSF in Drosophila during extended periods of a moderate heat shock. Changes in phosphorylation of HSF have also been implicated in regulating its activity, but such changes do not appear to effect the DNA binding activity of HSF in Drosophila. The mechanisms by which heat shock genes are returned to their uninduced state during recovery from heat shock remain to be fully elucidated. The use of permanganate footprinting to directly monitor the behavior of Pol II at heat shock genes during recovery from heat shock in cells depleted of specific proteins provides a way to identify candidates involved in shutting off transcription of the heat shock genes. A significant advantage of this approach is that it allows one to directly monitor the reestablishment of the paused state (Ghosh, 2011).

Paused Pol II could function as a regulator of gene regulatory factors. Many stimulus-responsive and developmental genes are found to have paused Pol II, and attenuation of transcription of these genes is likely to be critical for establishing appropriate physiological or developmental programs of gene expression. The finding that paused Pol II influences dissociation of an activator from the promoter provides a mechanism for rapidly shutting off gene expression. The pausing of Pol II could function more broadly by allowing the Pol II to remain stationary at a promoter while it serves to recruit modulators of chromatin structure and gene expression (Ghosh, 2011).

Pausing of RNA polymerase II disrupts DNA-specified nucleosome organization to enable precise gene regulation

Metazoan transcription is controlled through either coordinated recruitment of transcription machinery to the gene promoter or regulated pausing of RNA polymerase II (Pol II) in early elongation. This study reports that a striking difference between genes that use these distinct regulatory strategies lies in the 'default' chromatin architecture specified by their DNA sequences. Pol II pausing is prominent at highly regulated genes whose sequences inherently disfavor nucleosome formation within the gene but favor occlusion of the promoter by nucleosomes. In contrast, housekeeping genes that lack pronounced Pol II pausing show higher nucleosome occupancy downstream, but their promoters are deprived of nucleosomes regardless of polymerase binding. These results indicate that a key role of paused Pol II is to compete with nucleosomes for occupancy of highly regulated promoters, thereby preventing the formation of repressive chromatin architecture to facilitate further or future gene activation (Gilchrist, 2011).

The data support a general model for gene regulation wherein the underlying DNA sequence around promoters directly influences both chromatin architecture and the step in the transcription cycle that is rate-limiting for gene expression. Genes with high levels of Pol II pausing inherently favor the formation of nucleosomes over the promoter, establishing an active competition between Pol II and nucleosomes for promoter occupancy. It is proposed that this intrinsically repressive chromatin structure prevents aberrant expression of paused genes, which are often components of highly-regulated pathways. Nucleosome remodeling, likely initiated by proteins such as GAGA factor, would be required to disassemble nucleosomes at these promoters and allow for gene activity. Nucleosome removal would uncover strong promoter motifs that facilitate efficient, stable recruitment of the transcription machinery. Extended NELF-mediated pausing of polymerase at these promoters makes the transition to productive elongation slow. However, upon pause release, low levels of downstream nucleosomes would minimize barriers to transcription elongation and additional Pol II molecules would be rapidly recruited to maintain high Pol II occupancy and prevent nucleosome formation (Gilchrist, 2011).

In contrast, genes that lack extended pausing appear to disfavor promoter nucleosome assembly and instead harbor nucleosomes flanking the nucleosome-deprived promoter region. Localized DNA accessibility near TSSs could both help target the transcription machinery to the promoter region and diminish the requirement for nucleosome remodeling to allow gene activity. The dearth of core promoter elements could make these genes more reliant on activator binding for recruitment of the transcription machinery, and Pol II recruitment would be the rate-limiting step for expression of these genes. Pausing would be short-lived at these genes, and despite higher downstream nucleosome occupancy, polymerase escapes efficiently into productive synthesis (Gilchrist, 2011).

Importantly, these two strategies present different opportunities for gene regulation. Highly paused genes present two distinct steps at which they can be regulated: promoter accessibility and release of Pol II from pausing. It is proposed that this two-step mechanism facilitates precise control of gene expression. It is envisioned that the first step, nucleosome remodeling, functions as a molecular switch that relieves repression by chromatin to permit expression. This step can be temporally uncoupled from gene activation and could potentiate genes for future activation rather than prompting their immediate expression. The second step, release of paused Pol II, might be analogous to a volume dial, which permits fine-tuning of expression levels in response to changing conditions. Transcription levels could be rapidly regulated solely by manipulating the efficiency of P-TEFb recruitment through its interactions with DNA-binding transcription activators and histone modifications. This idea is supported by observations that activation of highly paused genes is both fast and synchronous. In contrast, genes that lack promoter-proximal pausing and nucleosome occupancy rely chiefly on a single-step mechanism to alter gene expression: regulated, step-wise recruitment of the transcription machinery. This mode of regulation has been suggested to be inherently more stochastic and prone to transcriptional noise, which may explain why many genes regulated by recruitment are constitutively active housekeeping genes (Gilchrist, 2011).

This study proviedes evidence that NELF-mediated pausing during early elongation is a general feature of the transcription cycle that is exploited at some genes to regulate transcription output. It is proposed that each round of transcription entails pausing, perhaps serving as an early 'checkpoint' to ensure proper maturation of the elongation complex before release into productive elongation. At some genes, this halt in elongation may be transient, whereas at others it may involve a long-lived paused complex that becomes rate-limiting for gene expression. Importantly, these results imply that the release from pausing through P-TEFb recruitment is an important, regulated step that broadly impacts gene expression, in agreement with recent work. It is noted that general recruitment of NELF during early elongation likely explains the seemingly paradoxical observation made in several systems that NELF levels increase at activated genes that experience robust recruitment of additional Pol II (Gilchrist, 2011).

The data also reveal that the inherent preference towards repression of highly-regulated promoters by nucleosome occlusion is an evolutionarily conserved phenomenon. Moreover, the results are in agreement with recent work in yeast which reveals that Pol II plays a role in displacing nucleosomes from promoter regions. However, in yeast, nucleosome disassembly is coupled directly to gene activation, whereas in Drosophila nucleosome disassembly is coupled to Pol II pausing. Perhaps Drosophila and other metazoans have evolved promoter-proximal pausing as an additional layer of regulation to accommodate increased demands for precise and rapid gene regulation during development and organismal responses to stress. In addition, it might be beneficial to maintain highly-regulated promoters poised in an open chromatin state, to prevent their incorporation into the more inaccessible, condensed heterochromatin that exists in metazoans (Gilchrist, 2011).

In summary, this paper reports that a primary function of paused Pol II is to prevent promoter-proximal nucleosome formation. This represents a fundamental shift in thinking about the role of Pol II pausing, which has long been thought to simply repress gene expression. Instead, it is argued that pausing should be viewed as a mechanism to fine-tune gene expression, and to potentiate genes for further or future activation. In addition, it was shown that sequence-specified 'default' nucleosome architecture instructs the regulatory properties of Drosophila promoters. It is proposed that metazoans have evolved a gene regulatory strategy in which nucleosomes and paused Pol II compete for promoter occupancy, affording multiple opportunities for regulation of gene expression (Gilchrist, 2011).

Regulation of Hox gene activity by transcriptional elongation in Drosophila

Hox genes control the anterior-posterior patterning of most metazoan embryos. Their sequential expression is initially established by the segmentation gene cascade in the early Drosophila embryo. The maintenance of these patterns depends on the Polycomb group (PcG) and trithorax group (trxG) complexes during the remainder of the life cycle. This study provides both genetic and molecular evidence that the Hox genes are subject to an additional tier of regulation, i.e., at the level of transcription elongation. Both Ultrabithorax (Ubx) and Abdominal-B (Abd-B) genes contain stalled or paused RNA polymerase II (Pol II) even when silent. The Pol II elongation factors Elongin-A and Cdk9 are essential for optimal Ubx and Abd-B expression. Mitotic recombination assays suggest that these elongation factors are also important for the regulation of Notch-, EGF-, and Dpp-signaling genes. Stalled Pol II persists in tissues where Ubx and Abd-B are silenced by the PcG complex. It is proposed that stalling fosters both the rapid induction and precise silencing of Hox gene expression during development (Chopra, 2009).

Recent studies suggest that the regulation of polymerase II (Pol II) elongation might be a common feature of developmental gene control in the Drosophila embryo. Chromatin immunoprecipitation (ChIP)-chip assays in cultured cell lines suggest that a significant fraction of all protein-coding genes contain stalled Pol II. As many as 10% of all protein-coding genes in the early Drosophila embryo contain Pol II prior to their expression. Many of these genes are developmental control genes, such as those encoding components of cell-signaling pathways, including Wnt, FGF, and Dpp (TGFβ). Moreover, four of the eight Hox genes in Drosophila appear to contain stalled Pol II (lab, Antp, Ubx, and Abd-B) in the early embryo. This study investigated the role of Pol II elongation factors in Hox gene expression (Chopra, 2009).

To confirm the preliminary evidence for stalled Pol II at the Ubx and Abd-B loci, conventional ChIP assays were performed with different antibodies against Pol II -- namely, 8WG16, which recognizes the CTD of Pol II, and H14, which recognizes the initiating form (Ser-5 phosphorylation) of Pol II. Both of these antibodies have been used in earlier ChIP as well as in ChIP-chip assays to elucidate and map distinct functions of the Pol II complex. Chromatin crosslinking was performed on 0-2 hr wild-type embryos prior to the onset of Hox gene expression. The chromatin was sonicated and precipitated with anti-Pol II antibodies, and then the extracted DNA was used as a template for PCR amplification. Hsp70 was used as a control because it represents the prototypic example of paused Pol II. As expected, the hsp70 promoter region contains strong Pol II signals with both the 8WG16 and H14 antibodies, indicating that an initiated Pol II is bound to the hsp70 promoter prior to heat shock induction. The Ubx and Abd-B promoter regions also exhibit strong signals, whereas PCR amplification performed with exonic probes failed to detect Pol II binding within the main body of the transcription unit. The presence of the H14 signal at these promoters suggests that Ser5 of the Pol II CTD is phosphorylated (initiated Pol II) prior to the activation of Ubx and Abd-B expression. As predicted from the previous ChIP-chip assays, the abd-A promoter region lacks Pol II (Chopra, 2009).

The preceding studies suggest that Ubx and Abd-B contain a stalled form of Pol II in early embryos. Additional assays were done to investigate Pol II binding in wing and haltere imaginal discs. The hsp70 promoter region contains strong Pol II signals in both wing and haltere discs, consistent with previous studies suggesting that the gene is stably paused in most or all tissues prior to induction by heat shoc. The ChIP assays also identify strong Pol II signals in the Ubx promoter region of wing discs, where the gene is silenced by the PcG complex. In contrast, a probe directed against exon 1 failed to detect significant levels of Pol II within the main body of the transcription unit (Chopra, 2009).

Very different results were obtained with haltere discs, in which Ubx is strongly expressed and the resulting Ubx repressor inhibits wing development. In this case, strong Pol II signals are detected in both the promoter region and exon, as would be expected for an actively expressed gene. These findings were strengthened by the use of qPCR assays. For these experiments, ChIP assays were done with a cocktail of Pol II antibodies (both 8WG16 and H14). Pol II signals are detected in both the promoter region and exon of the Ubx locus in haltere discs, where the gene is active. In contrast, there are substantially higher levels of Pol II in the promoter region than exon in wing discs where Ubx is silent. Permanganate protection assays are consistent with the occurrence of paused Pol II located between +18 and +35 bp downstream of the Ubx transcription start site (Chopra, 2009).

Abd-B also exhibits higher levels of Pol II binding in the promoter region as compared with exon 1. However, unlike Ubx, Abd-B is silent in both the wing and haltere discs, so it is not surprising that Pol II is not significantly detected in exon 1 in either tissue. As seen in early embryos, the promoter region of abd-A lacks significant binding of Pol II in wing discs (Chopra, 2009).

Pol II stalling raises the possibility that Ubx might be regulated at the level of transcriptional elongation. A number of elongation factors have been identified in cell culture assays, including negative elongation factors (NELF) A-E, ELONGIN-A (Elo-A), suppressor of termination (SPT) 4 and 5, and cyclin-dependent kinase 9 (CDK9). Reduced levels of Ubx⁺ activity cause a slight transformation of halteres into wings because Ubx functions as a repressor of wing development in the halteres. It was reasoned that, if Ubx is regulated at the level of Pol II elongation, then reduced levels of critical elongation factors should enhance the patterning defects observed in weak Ubx mutants (Chopra, 2009).

mutations in four different elongation factors were specifically examined: Elo-A, Cdk9, Spt4, and Spt5. Cdk9 has been shown to be a critical activator of paused Pol II at the hsp70 promoter. Heterozygotes for each mutation were examined in a Ubx¹/+ background, which displays a weak expansion of the halteres. Elo-A/+; Ubx¹/+ double heterozygotes display an enhanced transformation of halteres into wings. In particular, several wing-like bristles appear at the leading margin of the halteres. A similar phenotype was observed for Cdk9/+; Ubx¹/+ double heterozygotes. Spt4 mutations cause a slight suppression of the Ubx¹/+ phenotype, consistent with their dual activities in both attenuating and augmenting Pol II elongation (Chopra, 2009).

Cdk9 and Elo-A are thought to regulate distinct aspects of Pol II elongation. The Cdk9 kinase phosphorylates Ser-2 of the Pol II CTD, which is critical for the release of Pol II from the pause site in the hsp70 promoter. Inhibition of Cdk9 activity causes a global reduction in Ser-2 phosphorylation. In contrast, Elo-A appears to act at a later point of Pol II elongation after release from the pause site. Mutations in Cdk9 and Elo-A cause an additive enhancement in the Ubx¹/+ phenotype. Triple heterozygotes display an expansion in the overall size of the haltere, and the anterior margin contains a series of bristles like those seen in wings. This phenotype suggests that diminished levels of Cdk9 and Elo-A cause significant reductions in Ubx⁺ activity (Chopra, 2009).

ChIP-chip and conventional ChIP assays suggest that the Abd-B promoter region might also contain a stalled form of Pol II. As seen for Ubx, reduced levels of Cdk9 and Elo-A cause significant enhancements in the Abd-B^M1/+ mutant phenotype. In particular, Abd-B^M1/+ heterozygotes display a weak transformation of posterior abdominal segments into anterior segments, particularly the seventh abdominal segment (A7) into A6 (ectopic partial pigmentation) and A6 to A5 (ectopic bristles in A6 sternite). These phenotypes are augmented by reductions in either Cdk9 or Elo-A activity. Double heterozygotes display a more complete A7-to-A6 transformation, as well as an increase in the number of bristles in A6, suggesting a more severe A6-to-A5 transformation. These segmental transformations are weakly enhanced (not suppressed) by lower levels of Spt4 and Spt5. In contrast, mutations in the negative elongation factor Nelf-E strongly suppress the Abd-B^M1 phenotype, which is consistent with enhanced transcription of Abd-B. Triple heterozygotes, Abd-B^M1/+; Cdk9/+; Elo-A/+, display an even more dramatic transformation of A7 to A6 and A6 to A5. Thus, as seen for Ubx, reduced levels of Cdk9 and Elo-A cause a significant diminishment in Abd-B+ gene activity (Chopra, 2009).

Stalled Pol II appears to be disproportionately associated with developmental control genes as compared with 'housekeeping' genes that control cell metabolism and proliferation. A substantial fraction of stalled genes exhibit localized patterns of expression during embryogenesis, such as Hox genes and genes encoding components of signaling pathways (e.g., Dpp, FGF, Notch, etc.). Therefore, the possibility was explored that elimination of Cdk9 and Elo-A activity via the production of mitotic clones might produce specific developmental defects in adult appendages. In these experiments, there is no perturbation of Ubx or Abd-B activity. Cdk9 and Elo-A activities are disrupted in an otherwise wild-type background (Chopra, 2009).

The localized loss of Cdk9 or Elo-A activity in the haltere discs leads to weak wing transformation phenotypes, similar to those seen for reductions in Ubx. In particular, there is an expansion in the size of the halteres, and wing-like bristles appear at the margins. At least some of these phenotypes appear to arise from the specific loss of Ubx expression. Haltere discs containing clonal patches of Cdk9⁻/Cdk9⁻ tissue (identified by the loss of GFP expression) display localized reductions in Ubx activity, as judged by the use of an anti-Ubx antibody. This observation suggests that Ubx transcription is particularly sensitive to diminished activities of Pol II elongation factors, which is consistent with the evidence that the Ubx promoter region contains stalled Pol II (Chopra, 2009).

Cdk9 and Elo-A mitotic clones produce a variety of patterning defects in the wing and notum. Most notably, there is notching of the wing margins, ectopic wing veins, short crossveins, and both losses and duplications of macrochaete in the notum. These phenotypes might arise from perturbations in Notch, EGF, and Dpp (TGFβ) signaling. Genes encoding components of each of these pathways appear to contain stalled Pol II in early embryos (Chopra, 2009).

This study has presented evidence that the elongation factors Cdk9 and Elo-A are essential for optimal expression of at least a subset of Drosophila Hox genes, particularly Ubx⁺ activity in the developing halteres. Small patches of Elo-A⁻/Elo-A⁻ or Cdk9⁻/Cdk9⁻ mutant tissue also cause specific patterning defects in the wings and notum. Both Pol II elongation factors are probably required for normal expression of a great number of genes in the Drosophila genome. Indeed, both elongation genes are essential, and every attempt to create large mitotic clones resulted in larval lethality. Such lethality presumably reflects the general role of Elo-A and Cdk9 in gene expression. Previous studies have documented the general importance of the elongation factors ELL and Elo-A in Drosophila larval development and metamorphosis. Nonetheless, it would appear that a small number of patterning genes, including Ubx, are particularly sensitive to the loss of Elo-A and Cdk9 activity (Chopra, 2009).

It has been extensively argued that Polycomb might mediate repression by propagating an inactive form of chromatin, for example, by methylation of H3K27 followed by recruitment of HP1 or other proteins that package chromatin in an inactive state. However, the demonstration that TBP and Pol II are present in the Ubx proximal promoter in wing imaginal discs suggests that PcG silencing does not render the chromatin inaccessible for the binding of even large protein complexes. Instead, it is proposed that paused Pol II could contribute to PcG silencing by excluding the binding of additional Pol II complexes. Such occlusion by steric hindrance might help reduce transcriptional noise and thereby maintain Ubx repression. Mutations in the elongation factor, ELL [Su(Tpl)], suppress Scr phenotypes caused by the Pc⁴ Polycomb mutant, raising the possibility that Pol II elongation factors somehow communicate with the PcG-silencing complex. It is proposed that stalling might serve the dual role of fostering both silencing and rapid induction and thereby provide a sharp on/off switch in Hox regulation (Chopra, 2009).

Pcf11 is a termination factor in Drosophila that dismantles the elongation complex by bridging the CTD of RNA polymerase II to the nascent transcript

The mechanism by which Pol II terminates transcription in metazoans is not understood. This study shows that Pcf11 is directly involved in termination in Drosophila. dPcf11 is concentrated at the 3' end of the hsp70 gene in cells, and depletion of dPcf11 with RNAi causes Pol II to readthrough the normal region of termination. dPcf11 also localizes to most transcribed loci on polytene chromosomes. Biochemical analysis reveals that dPcf11 dismantles elongation complexes by a Pol II C-terminal domain (CTD) dependent but nucleotide-independent mechanism and that dPcf11 forms a bridge between the CTD and RNA. This bridge appears to be crucial because an anti-CTD antibody, which also dismantles the elongation complex, is found to bridge the CTD to RNA. dPcf11 was observed to inhibit transcription at low, but not high, nucleotide levels, suggesting that dPcf11 dismantles paused elongation complexes. These results provide a biochemical basis for the dependency of termination on pausing and the CTD in metazoans (Zhang, 2006).

Termination of Pol II transcription is an essential step in gene expression, but the mechanism is poorly understood. Besides its requirement for recycling use of Pol II, the choice of termination site can influence the availability of splice sites and polyadenylation sites in pre-mRNA. Half of the mRNAs in humans utilize alternate polyadenylation sites, and this can affect the location, stability, and coding potential of the transcripts. Pol II molecules that fail to terminate can inhibit function of downstream promoters by displacing proteins from the DNA. This so-called transcription interference can serve to regulate expression of some genes (Zhang, 2006 and references therein).

Pol II termination is coupled to polyadenylation by the polyadenylation signal in the nascent transcript. Two models have been proposed to explain this coupling. According to the torpedo model, cleavage of the nascent transcript, which precedes polyadenylation, generates an uncapped end on the residual transcript engaged with Pol II. This uncapped end is an entry point for a 5' to 3' exonuclease that chases down the Pol II and induces termination. The torpedo model received recent support with the finding that mutation of a 5' to 3' exonuclease, called Rat1, causes Pol II to readthrough terminators in yeast. Depletion of the homologous protein Xrn2 from human cells also impairs termination on a transiently transfected β globin gene (Zhang, 2006 and references therein).

An alternative model, originally called the antiterminator model but now generalized as the allosteric model, posits that the polyadenylation signal in the nascent transcript causes an allosteric change in Pol II that decreases the processivity of the elongation complex (EC). This could be due to the dissociation of an antiterminator from the EC or association of a factor that depresses processivity. Until recently, the strongest support for the allosteric model was provided by circumstances in which termination occurs in the absence of the cleavage reaction. Under these circumstances, the torpedo model for termination cannot apply, as there is no entry point for the 5' to 3' exonuclease (Zhang, 2006 and references therein).

Recently, a yeast protein called Pcf11 dismantles a yeast Pol II EC. This reaction depends on the CTD of Pol II, thus providing a possible reason for why deletion of the CTD impairs termination in human cells. The CTD corresponds to the unusual C-terminal domain of the largest Pol II subunit and is composed of multiple copies of a heptapeptide with the consensus YSPTSPS. yPcf11 appears to dismantle the EC by bridging the CTD to the nascent transcript. In yeast, mutations in yPcf11 impair both termination and polyadenylation. yPcf11 is in a complex called CF1A, which is involved in processing the 3' end of mRNAs. CF1A recognizes part of the tripartite polyadenylation signal in the GAL7 gene, thus providing a possible basis for how the polyadenylation signal might recruit or regulate the activity of yPcf11. Human Pcf11 is in a complex with at least 15 other polypeptides, and the complex is required for 3' end processing in vitro. The hPcf11 complex interacts with CF1m and CPSF, two proteins that recognize the polyadenylation signal in the nascent transcript. Nothing is known about the role of hPcf11 in termination (Zhang, 2006 and references therein).

Given the results that termination can occur prior to nascent transcript cleavage, and the discovery that yPcf11 could be the engine that drives the termination reaction in yeast it was asked whether Pcf11 is involved in termination in Drosophila (Zhang, 2006).

This study provides evidence that dPcf11 is directly involved in Pol II termination. Immunofluorescence microscopy and ChIP indicate that dPcf11 is concentrated at the 3' end of the hsp70 gene, and depletion of dPcf11 from Drosophila cells increases the level of Pol II normally detected downstream from the polyadenylation signal of hsp70. In addition, the N-terminal region of dPcf11 completely dismantles an elongation complex. This last result sets dPcf11 apart from all other proteins that have been implicated in Pol II termination and is strong evidence that dPcf11 is directly involved in termination. The detection of dPcf11 at most highly transcribed loci in polytene chromosomes suggests that dPcf11 is involved in termination at many genes. dPcf11 provides a basis for connecting three key aspects of termination: the CTD, the polyadenylation signal, and pausing (Zhang, 2006).

A crucial step in the termination reaction mediated by dPcf11 appears to be the formation of a bridge between the CTD and the nascent transcript, since this is the only functional aspect common to the CTD antibody and dPcf11, both of which dismantled the EC. Additional support for the importance of the bridge comes from the analysis of yeast Pcf11: mutations impairing RNA binding or CTD binding each inhibit the dismantling reaction. In addition, the dismantling reaction can be inhibited by hybridizing a DNA oligonucleotide to the nascent transcript in the region just outside from where RNA exits Pol II. Presumably, the oligonucleotide blocks formation of the bridge by interfering with the Pcf11-RNA interaction (Zhang, 2006).

Because the CTD antibody and Pcf11 are structurally unrelated, it is unlikely that the dismantling reaction involves Pcf11 directly recognizing part of the body of Pol II. How the formation of the bridge disrupts the elongation complex is a mystery. One possibility is that constraining the CTD or the RNA causes either of these or Pcf11 itself to contact the RNA exit channel in a way that destabilizes the EC. RNA-protein interactions in the RNA exit channel of bacterial RNA polymerase contribute to pausing and termination. The molecular contacts at the RNA exit channel of the Pol II EC may be uniquely suited for allosteric control of the EC, because it was observed that Rho, which normally functions in termination in bacteria, disrupts Pol II ECs, but not Pol I or Pol III ECs. Rho moves along RNA in a 5' to 3' direction, so it probably collides with the region of Pol II at the RNA exit channel (Zhang, 2006).

dPcf11 seems to interact with a relatively small region of the Drosophila CTD. This is in contrast with the yeast and human CTDs where Pcf11 could in principal coat almost all of the yeast CTD and half of the human CTD. The results from Drosophila suggest that the bridge does not have to form close to the body of the Pol II molecule to dismantle the EC. The binding of dPcf11 to the Drosophila CTD may not be dictated by the heptad per se but by a slightly larger motif that appears four times in the region where dPcf11 bound the CTD. This motif, PSYSPTSP, corresponds to the region of a peptide composed of two consensus heptads that was contacted by yPcf11 in a crystallized complex (Zhang, 2006).

Phosphorylation of the CTD could influence the activity of Pcf11. Phosphorylation of serine 2 in the CTD appears to increase the affinity of yPcf11 for the CTD. Importantly though, yPcf11 binds the unphosphorylated CTD, and there is evidence in yeast indicating that the CTD of Pol II is dephosphorylated just prior to termination. ChIP data indicate that the level of serine 2 phosphorylation increases as Pol II moves from the 5′ to the 3′ end of the hsp70 gene, and the same occurs on several yeast genes (Ahn, 2004). This rising level of serine 2 phosphorylation could contribute to the recruitment of Pcf11 near the 3′ end of the gene. However, the phosphates on the CTD might also antagonize the ability of Pcf11 to form a bridge with the nascent transcript due to electrostatic repulsion. The CTD phosphatase Ssu72 has been implicated in termination. Ssu72 might participate in termination by removing phosphates from the CTD so the bridge can form between the CTD and RNA (Zhang, 2006).

These results show that dPcf11 is concentrated near the polyadenylation signal of hsp70, similar to what was observed for several genes in yeast. Though Pcf11 binds RNA, it seems unlikely that Pcf11 alone recognizes the polyadenylation signal in the nascent transcript for two reasons: (1) equivalent crosslinking is observed to two unrelated RNAs, neither of which contained a polyadenylation signal; (2) amino acids 1-149 of Pcf11 lack any known RNA recognition motifs. Nevertheless, Pcf11 appears to have a surface that interacts specifically with RNA, because mutating one amino acid in yeast Pcf11 impaired RNA binding without affecting CTD binding (Zhang, 2006).

Yeast Pcf11 is part of a complex called CF1A, which contains three other subunits. One subunit, Rna15, recognizes part of the polyadenylation signal, thus providing a way to recruit yPcf11 to the end of the gene after the polyadenylation signal has been transcribed. Human Pcf11 is part of a complex called CFIIA, which itself does not appear to recognize the polyadenylation signal. CFIIA, however, interacts with CPSF and CFI_m, two proteins that recognize different parts of the polyadenylation signal in humans and that are involved in pre-mRNA 3' end processing. If CPSF and CFI_m are involved in recruiting Pcf11 to the 3' end of genes in metazoans, regulation is needed to prevent Pcf11 from prematurely terminating transcription. ChIP detects both CPSF and CFI_m well upstream of the polyadenylation site in the human G6PD gene), and earlier studies indicated that CPSF could be recruited to the 5' end of genes through association with TFIID (Zhang, 2006 and references therein).

The location of pause sites will be a key parameter in dictating where Pcf11 dismantles the elongation complex. As long as the EC is moving, it resists the action of Pcf11. It is suspected that this resistance arises because the RNA reeling out of an actively moving EC interferes with physical interactions that might be required for the dismantling reaction. There are ample data to indicate that pause sites are involved in selection of termination sites. Diverse mechanisms could be used by the cell to cause the EC to pause. These include the presence of pause sites that are intrinsic to the DNA sequence. Intrinsic pauses are found scattered throughout almost any stretch of DNA, so this could account for the stochastic selection of termination sites downstream from a polyadenylation signal. Specific proteins bound to the DNA could cause pausing as appears to be the case for the MAZ protein. Finally, nucleosomes cause ECs to pause. This could explain why chromatin remodeling factors appear to act as terminators (Zhang, 2006 and references therein).

It is concluded that dependence of the Pcf11 dismantling reaction on pausing and the CTD provide possible explanations for why these two things are important for termination. The specificity of termination probably arises from the combinatorial actions of factors that control pausing, the association of Pcf11 with the CTD, and the association of Pcf11 with the nascent transcript. These results provide direct support for an allosteric model of termination but certainly do not preclude possible contributions from an RNA exonuclease after cleavage of the nascent transcript. One possibility that has been proposed is that the exonuclease shortens the residual nascent transcript, forcing Pcf11 to bind close to the RNA exit channel (Zhang, 2006).

Paf1 coordinates histone modifications and changes in nucleosome structure with transcription activation and Pol II elongation

The Paf1 complex in yeast has been reported to influence a multitude of steps in gene expression through interactions with RNA polymerase II (Pol II) and chromatin-modifying complexes; however, it is unclear which of these many activities are primary functions of Paf1 and are conserved in metazoans. The Drosophila homologs of three subunits of the yeast Paf1 complex have been identified and characterized and striking differences were found between the yeast and Drosophila complexes. Although Drosophila Paf1, Rtf1, and Cdc73 (Hyrax) colocalize broadly with actively transcribing, phosphorylated Pol II, and all are recruited to activated heat shock genes with similar kinetics; Rtf1 does not appear to be a stable part of the Drosophila Paf1 complex. RNA interference (RNAi)-mediated depletion of Paf1 or Rtf1 leads to defects in induction of Hsp70 RNA, but tandem RNAi-chromatin immunoprecipitation assays show that loss of neither Paf1 nor Rtf1 alters the density or distribution of phosphorylated Pol II on the active Hsp70 gene. However, depletion of Paf1 reduces trimethylation of histone H3 at lysine 4 in the Hsp70 promoter region and significantly decreases the recruitment of chromatin-associated factors Spt6 and FACT, suggesting that Paf1 may manifest its effects on transcription through modulating chromatin structure. Paf1 therefore directs the histone methyltransferase activity and links active transcription and modifications of chromatin structure. The data support a model in which the Drosophila Paf1 complex plays a key role in coordinating histone modifications and changes in nucleosome structure with transcription activation and Pol II elongation, thereby serving as a critical link between gene expression and chromatin structure (Adelman, 2006; full text of article).

Proper control of gene expression is necessary for the development, differentiation, and survival of the cell, and transcription regulation is a cornerstone of this process. The formation of mRNA in eukaryotes involves a complex multistep pathway wherein each step provides an opportunity for regulation. Once RNA polymerase II (Pol II) has been recruited to a promoter and initiates transcription, it must efficiently escape from the promoter-proximal region and transcribe through a gene that is covered with nucleosomes. The nascent RNA must also be capped, spliced, polyadenylated, and exported to the cytoplasm before it can serve as a template for protein translation. Recent evidence from many laboratories indicates that there is a dynamic interplay between the protein complexes that carry out mRNA transcription, processing, and export, such that the efficiency of one step can have significant consequences for other steps in the pathway. For this reason, many factors that are required for the production of functional, mature RNA and were initially thought to directly stimulate Pol II transcription elongation have since been shown to elicit their primary effects on cotranscriptional processing or RNA export. Thus, a major goal towards understanding the mechanisms of transcription regulation requires the identification of both the direct and indirect activities of the numerous factors implicated in RNA production (Adelman, 2006).

The yeast Paf1 complex is one example of a factor that has been linked to a number of transcription-related activities. Yeast Paf1 is a complex of at least five polypeptides (Paf1, Rtf1, Cdc73, Leo1, and Ctr9) that has been implicated in processes as divergent as transcription initiation and elongation, modification of histone tails, phosphorylation of the Pol II C-terminal domain (CTD), RNA processing, and export. Although yeast Paf1 was originally thought to be an alternate mediator based upon its direct interactions with Pol II, it has since been found to be recruited throughout the body of active genes and to associate with the elongation-competent form of Pol II. Additional roles for the Paf1 complex have been suggested by the association of Paf1 with several RNA processing and export factors, such as Ccr4, the major yeast deadenylase, and Hpr1, a component of the THO complex that is involved in the export of mRNAs (Adelman, 2006).

Components of the Paf1 complex are nonessential in yeast, but mutations in Paf1 subunits confer sensitivity to 6-azauracil and generate Spt^– phenotypes, which are generally thought to signify defects in transcription elongation. In vitro transcription assays with naked DNA templates suggested that Paf1 and Cdc73 might directly stimulate transcription elongation; however, it is not clear what effects Paf1 has on elongation rates in vivo. In Saccharomyces cerevisiae, deletion of Paf1 or Cdc73 did not alter the distribution of Pol II on an active gene but dramatically decreased the chromatin immunoprecipitation (ChIP) signal observed for serine 2-phosphorylated (Ser2-P) Pol II. Consistent with a Ser2 phosphorylation defect, recruitment of 3' cleavage and processing factors was impaired in the paf1Delta strain and poly(A) tail length was modestly shortened (Adelman, 2006).

A link between the Paf1 complex and the chromatin architecture within transcribed regions has been suggested by genetic interactions between Paf1 components and Chd1, subunits of the yeast FACT complex (see Drosophila FACT complex), and histone assembly factors in the Hir/Hpc pathway. The packaging of template DNA into nucleosomes is known to represent a formidable obstacle to Pol II elongation in vitro, an obstacle that is overcome in vivo by a number of proteins that facilitate Pol II elongation by modifying chromatin structure and/or stability. Examples of factors that have been implicated in transcription through nucleosomes are chromatin remodeling enzymes, such as Chd1 and Swi/Snf, and histone-binding proteins like Spt6 and FACT. The ensemble of these complexes appear to help disassemble nucleosomes to promote efficient Pol II transcription through bound DNA and then to reassemble nucleosomes after the passage of Pol II. Both Spt6 and FACT have recently been shown to help maintain the proper balance between assembly and disassembly of nucleosomes during active transcription by Pol II, with the loss of these factors leading to a net failure to reassemble nucleosomes in the wake of transcription (Adelman, 2006).

The yeast Paf1 complex is required for ubiquitination of histone H2B at lysine 123 in the promoter-proximal region of activated genes. This ubiquitination event is a prerequisite for the methylation of histone H3 (at lysine residues 4 and 79) that accompanies active transcription in yeast; thus, the latter processes are defective in cells lacking functional Paf1. In addition, the Paf1 complex has been reported to be critical for the recruitment of the yeast SET2 histone methyltransferase complex to actively transcribed genes, leading to methylation of histone H3 at residue lysine 36 (Adelman, 2006 and references therein).

Although the yeast Paf1 complex has been studied extensively, a number of important questions remain unanswered. Key questions concern the nature of the interactions between the subunits of the Paf1 complex and their associations with Pol II, as well as the importance of Pol II binding in Paf1 function. A pivotal issue concerns the fact that deletion of Rtf1 or Cdc73 has been reported to reduce the association of all Paf1 components with the Pol II and chromatin yet lead to much weaker phenotypes than does deletion of the other Paf1 components. These results have led some to propose that the critical role of Paf1 occurs when the complex is not chromatin associated; however, the other potential activities of Paf1 have yet to be clearly identified. Furthermore, the subunit composition of the Paf1 complex in human cells appears to differ from that in yeast, since the human Rtf1 protein does not appear to stably associate with the other members of the Paf1 complex (Adelman, 2006).

To address these issues and to investigate the activity of Paf1-associated proteins in Drosophila, the Drosophila homologs of the yeast Paf1, Rtf1, and Cdc73 proteins were identified and characterized. In vivo analyses of the Drosophila Paf1 complex uncover both important similarities to and differences from the reported functions of Paf1 in yeast and provide insight into the connections among histone methylation, nucleosome stability, and transcription activation in a metazoan organism. Strikingly, the Drosophila Paf1 homolog is a previously annotated gene that encodes an essential protein, suggesting that the role of Paf1 has evolved and become more critical in metazoans. Rtf1 is not stably associated with the Drosophila Paf1 and Cdc73 proteins in vivo and shows only a weak interaction with Pol II. Moreover, when Paf1-depleted cells are assayed by tandem RNA interference (RNAi)-ChIP, no changes were observed in the level of Ser2-P Pol II on the Hsp70 gene, in contrast to results obtained with yeast. Interestingly, it appears that major effects of Paf1 depletion are the loss of H3-K4 trimethylation near the Hsp70 promoter and a significant decrease in the recruitment of Spt6 and FACT to the body of the Hsp70 gene, suggesting that Drosophila Paf1 may coordinate the activities of elongating Pol II with factors that maintain the proper chromatin architecture during transcription (Adelman, 2006).

This study shows that the most striking similarities between the yeast and Drosophila Paf1 complexes are their association with elongating RNA Pol II and their roles in gene activation, while the nature of the Pol II association and the composition of the Paf1 complex reflect marked differences between the species. The global view provided by Drosophila polytene chromosomes shows that the chromosome-associated Paf1 and Rtf1 proteins colocalize with active Pol II. This result supports the idea that these proteins participate in most, if not all, Pol II transcription. Remarkably, Paf1 and Rtf1 do appear to be separable from actively elongating Pol II under conditions of heat shock. Although Paf1 and Rtf1 are recruited actively to heat shock loci upon heat stress, these factors also remain associated with a number of additional sites on the chromosome, while Pol II is localized almost exclusively at heat shock loci under these conditions. These data suggest that Paf1 and Rtf1 may remain bound to the chromosome at activated genes through interactions with additional proteins (Adelman, 2006).

It has been suggested that, in yeast, while the Paf1 complex is entirely nuclear in its localization, it has cellular functions that are independent of elongating Pol II. The nucleolar association of Paf1 and Rtf1 observed on Drosophila polytene chromosomes could possibly represent such a function. At the nucleolar organizer, Paf1 shows broad labeling while the Rtf1 signal is restricted to the nucleolar periphery in a manner that is largely nonoverlapping. Interestingly, although the yeast Paf1 complex does not show strong nucleolar association normally, in an Rtf1 mutant, the Paf1 complex shows a strong association that is postulated to be a manifestation of its normal role in nuclear processing or export (Adelman, 2006).

By using ChIP experiments, this study obtained a higher-resolution view of the localization of Paf1, Rtf1, and Cdc73 at the Hsp70 gene. The lack of a ChIP signal at Hsp70 under uninduced conditions demonstrates that the presence of engaged Ser-5-P Pol II or the associated elongation factors such as Spt5 and TFIIS is not sufficient to recruit Paf1, Rtf1, or Cdc73. Upon heat induction, recruitment of all three proteins was observe primarily within the coding regions of active Drosophila genes, rather than regions upstream of the promoter, or downstream of the site for cleavage and polyadenylation. The reduction in the Paf1 signal downstream of the polyadenylation site, which accompanies a decrease in the Pol II signal, likely signifies that Paf1 dissociates from chromatin within this region, consistent with recent results obtained with yeast. However, it is noted that the absence of a significant Paf1 signal obtained with a given primer pair may simply indicate that the interactions of Paf1 with a particular region are transient (Adelman, 2006).

The Paf1 complex in S. cerevisiae has been reported to be required for full Ser-2 phosphorylation of the Pol II CTD. This role of Paf1 in CTD phosphorylation regulation also appears consistent with the fact that rtf1Delta mutants show synthetic lethality with CTD kinase and phosphatase mutants in CTK1 and FCP1. The lack of a Ser-2-P Pol II signal detected in yeast Paf1 mutants resulted in reduced recruitment of cleavage and polyadenylation factors, causing a defect in the polyadenylation of nascent transcripts. However, although depletion of Drosophila Paf1 or Rtf1 has a marked effect on induced Hsp70 RNA levels, no change was seen in the levels of Ser2-P Pol II on the Hsp70 gene in Paf1 or Rtf1 RNAi-treated cells, indicating a difference between the functions of Paf1 in yeast and a metazoan system (Adelman, 2006).

Another fundamental difference that observed between Drosophila and yeast Paf1 complexes is the relationship of the Paf1 and Rtf1 subunits in providing anchorage of the complex to Pol II. In yeast, it has been shown that the association of Paf1 with Pol II and active chromatin depends on the presence of Rtf1. In contrast, this study found that the recruitment of Paf1 to activated Drosophila Hsp70 is independent of Rtf1, while Rtf1 recruitment is dependent on Paf1. These results may reflect the evolution of a more important role for the Paf1 protein in metazoans in providing affinity of the complex for Pol II, while Rtf1 became a more loosely bound component of the complex (Adelman, 2006).

The role was investigated of Drosophila Paf1 in the modification of histones within actively transcribed regions. Whereas yeast Paf1 has been implicated in regulating the bulk levels of methylation of histone H3 at lysine residues 4 and 79, an effect was observed of Paf1 depletion on the trimethylation of H3-K4, but not on di- or trimethylation of H3-K79. Similarly, it was observed that trimethylation of H3-K4 occurred within the promoter-proximal region of Hsp70 and Hsp26 upon heat shock and could be seen to increase from 2.5 to 10 min after heat induction, but no significant levels of H3-K79 dimethylation were observed within the active Hsp70 gene. The latter result differs from results from other systems which link H3-K79 dimethylation with active transcription. However, it is consistent with recent data suggesting that both Grappa, the Drosophila H3-K79 methyltransferase, and the signal corresponding to H3-K79 dimethylation are localized to both active and intergenic regions of Drosophila polytene chromosomes. An alternative possibility is that the apparent differences between yeast and Drosophila result from the experimental systems used; RNAi treatments in Drosophila decrease, but do not completely abolish, their target, and thus the small amount of remaining protein may be sufficient to carry out certain functions. Conversely, the deletion mutants used to investigate yeast Paf1 entirely remove an important protein for many generations of cell growth, raising the possibility that some observed effects are indirect or secondary in nature (Adelman, 2006).

It is interesting that although H3-K4 trimethylation depends upon Paf1 and the recruitment of Paf1 is temporally similar to H3-K4 methylation, the distribution of Paf1 appears to be spatially distinct from the promoter region where the strongest trimethylated H3-K4 signals are observed. Thus, the results suggest that the effects of Paf1 mutants on the modification of promoter-proximal nucleosomes (including the ubiquitination of H2B-K123) may occur through indirect mechanisms. These data are consistent with reports on yeast that indicate that the distribution of Paf1 subunits does not strictly correlate with the patterns of ubiquitinated H2B or methylated histone H3. The localization of H3-K4 trimethylation reported in this study is in agreement with the recently described distribution of Trithorax, a Drosophila H3-K4 methyltransferase. Furthermore, recent studies employing a Drosophila Trithorax mutant fly line suggest that a multiprotein complex that contains Trithorax plays a role in Hsp70 gene activation. However, whether the role of Trithorax in Hsp70 activation is direct or indirect remains to be established. It is noted that no effect of Paf1 depletion is observed on the rates of Pol II recruitment, or distribution over the gene, suggesting that H3-K4 trimethylation may serve as a mark of transcription activation rather than a prerequisite for gene activation (Adelman, 2006).

These studies have provided new insights into the increased importance of the Paf1 complex in a metazoan system. It is significant that Paf1 is recruited in a manner that is spatially and temporally identical to that of chromatin-associated factors Spt6 and FACT. In agreement with the strong colocalization of Paf1 with these nucleosome-associated factors, it was shown that depletion of Paf1 significantly reduces the recruitment of both Spt6 and the FACT subunit SSRP1. A relationship among Paf1, Rtf1, and FACT is consistent with findings that an rtf1Delta mutation shows synthetic lethality with POB3, a subunit of the yeast FACT complex. Moreover, the FACT complex has been shown to interact with the Paf1 complex and the chromodomain-containing Chd1 protein at actively transcribed genes. In vitro, FACT has been shown to function optimally to facilitate transcription through nucleosomes when it is present at approximately one molecule of FACT per two nucleosomes; the effectiveness of FACT in promoting elongation is decreased dramatically below this threshold. If these results reflect the situation in vivo, the greater than 50% decrease in FACT levels at the active Hsp70 gene in Paf1-depleted cells would result in a rather pronounced effect on transcription through nucleosomes (Adelman, 2006).

Furthermore, recent evidence obtained with yeast has shown that mutations of Spt6 or the FACT subunit Spt16 lead to aberrant chromatin architecture in the wake of elongating Pol II, presumably due to defects in reassembly of nucleosome structure. The failure to efficiently repackage transcribed DNA results in transcription initiation from cryptic sites and a reduction in levels of properly initiated and processed RNA. If a primary role of Drosophila Paf1 is to help stably recruit factors like Spt6 and FACT, then loss of Paf1 activity could also lead to the accumulation of nonfunctional or improperly processed RNA species. In support of this idea, a paper that was published during the preparation of this report states that mutations in yeast Spt6 alter the recruitment of Paf1 subunit Ctr9 and lead to defects in 3'-end processing of nascent RNA. It is thus tempting to speculate that the vast array of transcription elongation and RNA processing and export defects reported in yeast Paf1 mutant strains could result from perturbation of the nucleosome structure along actively transcribed genes. Moreover, it may be these chromatin and processing defects that account for the decrease in the amount of Hsp70 mRNA that accumulates in response to heat shock in Paf1- or Rtf1-depleted cells (Adelman, 2006).

Finally, the Paf1 gene in yeast is nonessential while the Paf1 gene in Drosophila is essential. This may reflect the more varied and demanding requirements of the transcription machinery in higher eukaryotes, where chromatin frequently plays a greater and more stringent role in regulation. This, in turn, may place a greater demand on the Paf1 complex, which appears to function at the interface between transcription and chromatin, perhaps serving as a platform that stimulates the association of a number of nucleosome-modifying complexes with actively elongating Pol II (Adelman, 2006).

In summary, the gene for Paf1 is a required Drosophila gene that colocalizes with actively elongating Pol II when chromatin associated and plays a critical role in the activation of stress-induced genes. Furthermore, recent data reveal that mutations in parafibromin, the human homolog of the Paf1 complex subunit Cdc73, are associated with an elevated risk of parathyroid carcinomas; thus, the Paf1 complex may be a key regulator of cellular control in metazoans. The connection between Paf1 and trimethylation of histone H3 at lysine 4 near the promoters of active genes is particularly interesting because a human homolog of Trithorax, the histone methyltransferase implicated in this activity, is ALL-1/MLL-1, which is associated with a number of acute leukemias. Future work to define the way in which Paf1 directs the histone methyltransferase activity of this key enzyme should provide insight into the interaction between active transcription and modifications of chromatin structure. The data support a model in which the Drosophila Paf1 complex plays a key role in coordinating histone modifications and changes in nucleosome structure with transcription activation and Pol II elongation, thereby serving as a critical link between gene expression and chromatin structure (Adelman, 2006).

Phosphorylation of histone H3 at Ser10 by JIL-1 facilitates RNA polymerase II release from promoter-proximal pausing in Drosophila

The Drosophila JIL-1 kinase is known to phosphorylate histone H3 at Ser10 (H3S10) during interphase. This modification is associated with transcriptional activation, but its function is not well understood. Evidence is presented suggesting that JIl-1-mediated H3S10 phosphorylation is dependent on chromatin remodeling by the brahma complex and is required during early transcription elongation to release RNA polymerase II (Pol II) from promoter-proximal pausing. JIL-1 localizes to transcriptionally active regions and is required for activation of the E75A ecdysone-responsive and hsp70 heat-shock genes. The heat-shock transcription factor, the promoter-paused form of Pol II (Pol IIo^ser5), and the pausing factor DSIF (DRB sensitivity-inducing factor) are still present at the hsp70 loci in JIL-1-null mutants, whereas levels of the elongating form of Pol II (Pol IIo^ser2) and the P-TEFb kinase are dramatically reduced. These observations suggest that phosphorylation of H3S10 takes place after transcription initiation but prior to recruitment of P-TEFb and productive elongation. Western analyses of global levels of both forms of Pol II further suggest that JIL-1 plays a general role in early elongation of a broad range of genes. Taken together, the results introduce H3S10 phosphorylation by JIL-1 as a hallmark of early transcription elongation in Drosophila (Ivaldi, 2007).

The eukaryotic cell packages its DNA wrapped around histone proteins to form nucleosomes, the basic units of chromatin. These nucleosomes assemble into higher-order chromatin structures through which the transcription machinery must navigate each time it is signaled to transcribe. Mechanisms have consequently evolved to maintain a flexible chromatin state that can readily respond to intrinsic and extrinsic stimuli and accordingly modulate gene expression. Most prominently, histone-modifying enzymes can methylate, acetylate, and phosphorylate various amino acid residues of histone N termini, thereby changing their affinity for different transcriptional regulators. ATP-dependent chromatin remodeling complexes can also be recruited to alter the position and accessibility of the nucleosome. The binding of specific transcription factors triggers a cascade of events during which these diverse chromatin modulators work in concert to allow the RNA polymerase II (Pol II) machinery to bind target genes, initiate transcription, and elongate the messenger RNA (mRNA). These regulators maintain tight control of transcription throughout the elongation process by continuously communicating with the C-terminal domain (CTD) of the largest subunit of Pol II (Ivaldi, 2007 and references therein).

The CTD of Pol II consists of a heptad repeat (Tyr-Ser-Pro-Thr-Ser-Pro-Ser) that is conserved from yeast to humans. It integrates transcriptional events by interacting with distinct regulatory proteins that recognize different patterns of CTD phosphorylation. When Pol II is first recruited to the promoter as part of the preinitiation complex, its CTD is hypophosphorylated. After Pol II disengages from the promoter, the CTD becomes phosphorylated at Ser5 (Pol IIo^ser5) by TFIIH, a general transcription factor that is part of the Pol II machinery. As part of an early elongation complex, Pol II progresses 20-40 base pairs (bp) downstream from the promoter. It then pauses in a process referred to as promoter-proximal pausing to allow for capping of the nascent mRNA. DRB sensitivity-inducing factor (DSIF, Spt5) and negative elongation factor (NELF) cooperate to repress transcription elongation and maintain this pause. Pol II is released once the P-TEFb kinase is recruited to relieve the negative effects of DSIF and NELF and phosphorylate the CTD at Ser2 (Pol IIo^ser2), marking the onset of productive elongation. The various transcriptional steps are associated with distinct histone modifications and chromatin remodeling complexes. Set1, the enzyme responsible for methylating Lys4 of histone H3 (H3K4) in Saccharomyces cerevisiae, is known to physically associate with the CTD of Pol II when it is phosphorylated at Ser5. At the same time, trimethylation of H3K4 has been found concentrated at the 5' end of transcribed genes. Methylation of Lys36 of H3 (H3K36), on the other hand, is associated with a later step in elongation; this mark accumulates further downstream from the promoter and associates with the CTD when phosphorylated at Ser2. Other modifications, such as lysine acetylation, arginine methylation, and serine phosphorylation, have also been associated with activation of gene expression. Of interest, phosphorylation of histone H3 at the Ser10 residue (H3S10) has been shown to be important for activation of transcription in yeast, Drosophila, and mammalian cells, but its precise role in this process is not well understood (Ivaldi, 2007 and references therein).

Several studies have suggested an important role for H3S10 phosphorylation in specific transcriptional responses to signaling stimuli. The yeast Snf1 kinase phosphorylates H3S10 upon activation of the INO1 gene. In mammalian fibroblasts, rapid phosphorylation of histone H3 concomitant with activation of immediate-early (IE) response genes takes place when cells are treated with growth factors and various stress-inducing agents. Further, Coffin-Lowry syndrome is characterized by impaired transcriptional activation of the c-fos gene and a loss of EGF-induced phosphorylation of histone H3S10. Treatment of immature rat ovarian granulosa cells with follicle-stimulating hormone produces rapid H3S10 phosphorylation in a PKA-dependent manner, suggesting a role for histone phosphorylation in cellular differentiation. Additionally, H3S10 phosphorylation follows the stimulation of the suprachiasmatic nucleus of rats with light and activation of hippocampal neurons. It further appears to play a central role during cytokine-induced gene expression mediated by IkappaB kinase α (IKK-α). What remains unclear from these studies is whether H3S10 phosphorylation is limited to mediating signal transduction events or whether it plays a more general role in the activation of gene expression in vertebrates (Ivaldi, 2007).

Studies in Drosophila suggest that this modification may be required for the transcription of most genes in this organism. Using the heat-shock response as a model system, it has been established that H3S10 phosphorylation patterns parallel those of active genes. Drosophila responds to a rise in temperature by rapidly increasing the transcription of heat-shock genes while repressing genes expressed previously. Before heat shock, phosphorylated H3S10 localizes to euchromatic regions of polytene chromosomes and colocalizes with Pol II. After heat shock, this modification redistributes to the active heat-shock loci and disappears from the rest of the chromosome, where genes are now repressed (Nowak, 2000; Ivaldi, 2007).

Despite these observations, the precise role of H3 phosphorylation in gene activation remains elusive. The mammalian MSK1 and MSK2 kinases, among others, have been shown to be responsible for H3S10 phosphorylation associated with transcription. The Drosophila homolog of MSK1/2, the JIL-1 threonine/serine kinase, has been shown to phosphorylate H3S10 in vitro. H3S10 phosphorylation levels in vivo are dramatically reduced in JIL-1^z2-null mutants. The JIL-1 protein localizes to interband regions of polytene chromosomes and is found up-regulated on the male X chromosome. Furthermore, the JIL-1^z2 allele enhances the phenotype of trx-G mutations. These data indirectly suggest that JIL-1-mediated H3S10 phosphorylation plays an important role in transcriptional activation (Ivaldi, 2007).

This study further characterizes the role of JIL-1-mediated H3S10 phosphorylation in transcription. JIL-1 is required for the transcription of the majority of, if not all, Drosophila genes. Mechanistic analyses place the phosphorylation event subsequent to transcription initiation but prior to productive elongation; JIL-1 plays an integral role in the release of Pol II from promoter-proximal pausing. The data therefore highlight H3S10 phosphorylation as a novel hallmark of early productive elongation in Drosophila (Ivaldi, 2007).

These results establish H3S10 phosphorylation by JIL-1 as a key event during early elongation of transcription in Drosophila. JIL-1 appears to interact with the transcription machinery at most or all actively transcribed regions on Drosophila polytene chromosomes, including active ecdysone and heat-shock genes. At the same time, expression levels of the hsp70 and E75A genes are decreased in JIL-1-null mutants. Importantly, when JIL-1 is mutated, a global decrease in the phosphorylation levels of elongating RNA polymerase II is observed, suggesting that JIL-1 is required for transcription of the majority of genes (Ivaldi, 2007).

The results further elucidate the timing of H3S10 phosphorylation within the framework of the cascade of events that lead to activation of transcription in eukaryotes. Phosphorylation of H3S10 is not required for transcription factor recruitment, since loss of JIL-1 does not affect binding of HSF at the hsp70 genes after heat shock. Also, H3S10 phosphorylation is dependent on BRM chromatin remodeling, which is required genome-wide prior to the recruitment of Pol II. Transcription initiation can take place independently of JIL-1, as shown by the normal levels of Pol IIo^ser5 and H3K4 methylation in JIL-1^z2 mutants, indicating that the chromatin environment in the absence of JIL-1 is still suitable for transcription initiation. However, productive elongation is impaired in these mutants, as is evident by the decrease in Pol IIo^ser2 levels. These findings introduce H3S10 phosphorylation as a new component of an increasingly complex chromatin environment that is required at the onset of transcription elongation in Drosophila, suggesting a role for JIL-1 in the release of Pol II from promoter-proximal pausing and facilitation of early elongation. Specifically, in JIL-1 mutants, P-TEFb is not detected at the induced hsp70 genes while levels of DSIF are maintained. In the absence of P-TEFb, neither DSIF nor Pol II can be phosphorylated, which is sufficient to block productive elongation. It is likely that Pol II arrests in a paused state and cannot elongate in these mutants. It is also possible that Pol II continues to elongate but is unable to communicate with the proper mRNA processing machinery, which is normally contingent on Ser2 phosphorylation of its CTD (Orphanides, 2002). In this case, the mRNA would be produced but quickly degraded, leading to the transcription defects observed in the Northern analyses. Further work is needed to distinguish between these two possibilities (Ivaldi, 2007).

Although JIL-1 is required for transcription, its presence is not sufficient to ensure gene activation, since JIL-1 is present at all previously transcribed genes that are silenced after heat shock, whereas phosphorylated H3S10 is found exclusively at the transcriptionally active heat-shock genes (Nowak, 2000). Nevertheless, recruitment of JIL-1 to the hsp70 gene is transcription dependent. One possibility is that JIL-1 can exist in both active and inactive states. Once recruited to activate a gene, it may eventually be repressed by inactivation rather than disassociation. Alternatively, the net levels of phosphorylated H3S10 could result from a delicate balance between kinase and phosphatase activities. It has been proposed previously that phosphatase 2A (PP2A) plays a major role in transcription-dependent H3S10 phosphorylation (Nowak, 2003). Therefore, even if JIL-1 is actively maintained at silent genes, its action may be counterbalanced by PP2A. Further studies are required to shed light on how JIL-1 activity can be regulated to affect transcription (Ivaldi, 2007).

In vertebrates, phosphorylation of H3S10 seems to be limited to transcription activation of specific genes in the context of particular signal transduction pathways. In fact, activation of the hsp70 genes by different stressors in mammalian cells is associated with distinct signaling pathways that are not always linked to H3S10 phosphorylation. Contrary to the Drosophila response, heat shock elicits histone H4 acetylation instead of H3S10 phosphorylation at the hsp70 loci in mouse fibroblasts. In contrast, both H3S10 phosphorylation and H4 acetylation are detected at the hsp70 genes upon arsenite treatment of the same cells (Thomson. 2004). Therefore, mammals appear to have more diverse mechanisms of transcription activation and may partially rely on H3S10 phosphorylation in a context-dependent manner. In yeast, substituting the H3 Ser10 for an Ala prevents the recruitment of the TATA-binding protein to the INO1 and GAL1 gene promoters, suggesting that H3S10 phosphorylation is required for the assembly of the preinitiation complex. It would be interesting to explore the significance of this apparent diversity across species (Ivaldi, 2007 and references therein).

The results presented in this study shed light on the mechanism of transcription regulation by H3S10 phosphorylation. It has been recently shown that H3S10 phosphorylation antagonizes the binding of the heterochromatin protein HP1 to histone H3 methylated in Lys9 (H3K9) during mitosis in mammalian cells. It was consequently proposed that JIL-1 maintains chromosome structure in Drosophila by counteracting heterochromatin formation and preventing its spreading into euchromatin. This model for JIL-1 activity could explain a lack of transcription in JIL-1^z2 mutants, since any ectopic heterochromatin would make the DNA inaccessible to the Pol II machinery. However, contrary to such a prediction, the current results show that heat-shock puffs are still formed in JIL-1^z2mutants, and transcription factors and the Pol II machinery retain the ability to bind despite the disruption of chromatin structure. Furthermore, transcription can be initiated, as is evident by the phosphorylation of Pol II at Ser5. This requires several components of the core transcription machinery and the procession of Pol II a few bases downstream from the promoter. These results suggest that, rather than contribute to global chromosome structure, JIL-1-mediated H3S10 phosphorylation may be required to maintain a local chromatin environment that serves as a platform for the recruitment of P-TEFb and the consequent release of Pol II from promoter-proximal pausing (Ivaldi, 2007).

It has become increasingly evident that transcription elongation is a rate-limiting step of gene expression that requires tight regulation. It was reported recently that the majority of gene promoters in human embryonic stem cells are occupied by a promoter-proximally paused Pol II, poised for productive elongation (Guenther, 2007). This suggests that the expression of these genes is predominantly regulated at the level of Pol II release rather than during preinitiation. The exact mechanism of P-TEFb recruitment, a key step in this process, remains to be determined. Several transcription regulators have been shown to recruit P-TEFb, but this is the first evidence of a histone modification required precisely at the timing of recruitment (Ivaldi, 2007).

The exact contribution of H3S10 phosphorylation to P-TEFb recruitment remains open to further investigation. Recent reports have shown that the ubiquitous protein 14-3-3 binds to H3 only when phosphorylated at Ser10, and this interaction could provide a mechanistic link between H3S10 phosphorylation and P-TEFb (Macdonald, 2005). It is possible that 14-3-3 interacts with P-TEFb directly or indirectly through other transcription regulators that are known to recruit it. Alternatively, 14-3-3 is known to interact with many chromatin-related proteins, thus providing another avenue to manipulate the local chromatin environment to support P-TEFb recruitment and early elongation. Further analyses will be necessary to test these hypotheses and clarify the role and mechanism of regulation of JIL-1 and H3S10 phosphorylation in gene expression (Ivaldi, 2007).

Set2 associates with hyperphosphorylated RNAPII

Recent reports have shown that Set2 from various organisms binds to the hyperphosphorylated CTD of RNAPII, implying that K36 methylation plays an important role in the transcription elongation process. The presence of both the WW and SRI domains suggested that Set2 may associate with RNAPII also in Drosophila. To address this issue, extracts prepared from control or Set2 RNAi embryos were immunoprecipitated with antibodies directed against Ser5-phosphorylated CTD, followed by immunoblotting with antibodies directed against Set2 and Ser5-phosphorylated CTD form of RNAPII. Immunoprecipitation of Ser5-phosphorylated CTD resulted in strong immunoreactivity of both phosphorylated CTD and Set2 in control embryos whereas no Set2 is detected in extracts from RNAi embryos. This result was corroborated by showing co-localization of Set2 and elongating RNAPII on salivary gland chromosomes. While these results demonstrate that Set2 is associated with the elongating form of RNAPII in Drosophila, the precise role of this association is currently unclear. However, the fact that a loss of Set2/K36 methylation results in mutant phenotypes associated with defects in the ecdysone response indicates that Set2/K36 methylation plays an important role in the ecdysone regulatory hierarchy (Stabell, 2007).

Histone H3 K36 methylation is mediated by a trans-histone methylation pathway involving an interaction between Set2 and histone H4

Set2-mediated H3 K36 methylation is an important histone modification on chromatin during transcription elongation. Although Set2 associates with the phosphorylated C-terminal domain (CTD) of RNA polymerase II (RNAPII), the mechanism of Set2 binding to chromatin and subsequent exertion of its methyltransferase activity is relatively uncharacterized. This study identified a critical lysine residue in histone H4 that is needed for interaction with Set2 and proper H3 K36 di- and trimethylation. It was also determined that the N terminus of Set2 contains a histone H4 interaction motif that allows Set2 to bind histone H4 and nucleosomes. A Set2 mutant lacking the histone H4 interaction motif is able to bind to the phosphorylated CTD of RNAPII and associate with gene-specific loci but is defective for H3 K36 di- and trimethylation. In addition, this Set2 mutant shows increased H4 acetylation and resistance to 6-Azauracil. Overall, this study defines a new interaction between Set2 and histone H4 that mediates trans-histone regulation of H3 K36 methylation, which is needed for the preventative maintenance and integrity of the genome (Du, 2008).

Transcribing RNA polymerase II is phosphorylated at CTD residue serine-7

RNA polymerase II is distinguished by its large carboxyl-terminal repeat domain (CTD), composed of repeats of the consensus heptapeptide Tyr¹-Ser²-Pro³-Thr⁴-Ser⁵-Pro⁶-Ser⁷. Differential phosphorylation of serine-2 and serine-5 at the 5' and 3' regions of genes appears to coordinate the localization of transcription and RNA processing factors to the elongating polymerase complex. Using monoclonal antibodies, serine-7 phosphorylation has been revealed on transcribed genes. This position does not appear to be phosphorylated in CTDs of less than 20 consensus repeats. The position of repeats where serine-7 is substituted influenced the appearance of distinct phosphorylated forms, suggesting functional differences between CTD regions. These results indicate that restriction of serine-7 epitopes to the Linker-proximal region limits CTD phosphorylation patterns and is a requirement for optimal gene expression (Chapman, 2007).

Differential phosphorylation of CTD residues of the large subunit of eukaryotic RNA polymerase II (Pol II) occurs during the transcription cycle and appears to orchestrate the recruitment, activation, and displacement of various factors involved in transcription and mRNA processing. A variety of kinases have been identified, with phosphorylation activity directed toward the amino acids tyrosine-1 (Abl1/2), serine-2 (CTDK1, CDK9, and DNA-PK), serine-5 (ERK1/2 and CDK7-9), and serine-7 (DNA-PK). The mammalian CTD is >99% conserved across species and possesses almost double the length of its yeast counterparts. A minimum length of CTD is required to support the growth of yeast or mammalian cells. However, this is dependent on the number and position of consensus and nonconsensus repeats, which suggests that CTD function is composed of both sequence and length. Of the 52 mammalian CTD repeats, 21 obey the consensus sequence and lie largely proximal to the Linker region. The distal C-terminal region deviates from this consensus, predominantly at position 7. These nonconsensus repeats may affect the binding of specific factors or may serve to prevent phosphorylation at the position of deviation. Indeed, studies in vivo suggest that they are equivalent to consensus repeats for functions such as splicing of the fibronectin extra domain I exon but not for maintenance of long-term cell viability (Chapman, 2007).

To investigate the role of the CTD repeat structure on its phosphorylation, a system was established that allows the comparison of CTDs of different lengths and repeat compositions in vivo. Recombinant polymerases are engineered with a point mutation conferring resistance to α-amanitin, allowing the endogenous polymerase to be inhibited (and degraded) after addition of α-amanitin but without affecting recombinant polymerase activity. Monoclonal antibodies (mAbs) were produced against the CTD phosphoserine epitopes Ser²-P, Ser⁵-P, and Ser⁷-P. In preparing these antibodies, earlier findings were considered that showed that the functional unit of the CTD is not the heptad repeat itself but is in a sequence lying within heptapeptide pairs. Thus, in the production and testing of these antibodies, a panel of di-heptapeptides with various modifications was used. Analysis of these antibodies and commercially available antibodies revealed that some recognition profiles were limited by modifications on neighboring repeats. For example, the α-Ser⁷-P antibody (4E12) is affected by upstream, but not downstream, Ser⁵-P (Chapman, 2007).

Combining these tools, the phosphorylation of wild-type (WT) CTD was compared with that of different lengths of consensus repeats. If all repeats are equally accessible to CTD kinases, intensities of phosphorylation signals should be expected for WT and mutants 1 to 8 proportional to CTD length. Dual labeling of membranes with α-Rpb1 antibody (mAb Pol3/3 recognizes an epitope outside the CTD) and with α-phospho-CTD antibody reveals forms of different mobility -- the rapidly migrating, unmodified IIa form and the slower, modified IIo form. For mutants containing 16 to 24 consensus repeats, the majority of Pol II is not efficiently phosphorylated and accumulates in the IIa form. Within the IIo form, Ser²-P appears in a sharp, slow migrating band, whereas in longer CTDs, Ser⁵-P appears largely in a band migrating between the Ser²-P band and IIa, which suggests that at least two populations of phosphorylated CTD exist in vivo at any time: Ser²-P alone and Ser⁵-P alone. These data are supported by both the recognition profiles of the antibodies and previous work showing a shift in IIo to a faster migrating form upon treatment with a Ser²-kinase inhibitor. Antibody raised against Ser⁷-P revealed the existence of this epitope in vivo, which is distributed among the major Ser²-P and Ser⁵-P reactive bands. The epitope is lacking from the Ser⁵-P band that appears just above the IIa form. Strong reactivity of α-Ser⁷-P is detectable for a band between IIa and IIo. Furthermore, although Ser²-P and Ser⁵-P appear in all mutants, Ser⁷-P appears only in mutants with more than 24 repeats (Chapman, 2007).

To investigate the effect of nonconsensus repeats on the distribution of phosphorylation, a panel of CTD mutants was analyzed for their reactivity against phospho-CTD antibodies. α-Ser⁷-P does not recognize a mutant lacking Ser⁷ but strongly recognizes mutants containing Ser⁷ substituted with glutamic acid (S7E), indicating either that this antibody recognizes a CTD conformation or that S7E can structurally mimic Ser⁷-P for antibody recognition. Furthermore, replacement of Ser⁷ with alanine prevents recognition of the intermediate band between IIa and IIo by α-Ser⁵-P, suggesting that this form may be Ser⁷-P-dependent (Chapman, 2007).

Because deviations from serine at position 7 in the WT CTD are concentrated in its distal region, chimeras were produced to assess the effect of proximal and distal positioning of nonconsensus repeats. The two chimeras of consensus repeats, and repeats containing S7E substitutions, produce a form that migrates between IIa and IIo. The proximal positioning of nonconsensus repeats (S7A and S7T/K) affects the appearance of a form similar in mobility to the intermediate IIo Ser^5/7-P-reactive band seen in mutants of >35 pure consensus repeats (Chapman, 2007).

To determine whether Ser⁷ phosphorylation is a physiological event during the transcription cycle, chromatin immunoprecipitation (ChIP) experiments were conducted. A detailed example is shown for the T cell receptor beta (TCRβ) gene locus. Ser⁷ was phosphorylated on transcribing Pol II, appearing strongly at the promoter and increasing toward the 3' region of TCRβ. The differences in Ser² phosphorylation that was observe, compared with earlier data, may result from the antibodies used, because the H5 antibody preferentially recognizes repeats with phosphorylated Ser² and Ser⁵ (Chapman, 2007).

Given that Ser⁷ is phosphorylated across TCRβ and all other genes tested (GAPDH, RPLPO, and RPS27), the ability of synthetic polymerases to transcribe and produce mature mRNA from the c-myc and pes1 genes was analyzed. The effect on c-myc and pes1 mRNA levels of Ser⁷ substitution to E or K/T appears dependent on its position, either proximal or distal to the Linker, suggesting again that functional differences exist between these regions. Substitution of Ser⁷ to the non-phosphoacceptor, alanine, did not obviously affect mRNA levels, nor did it affect the long-term growth of cell lines, although viability was compromised. This may be due to the effect of this mutation on small nuclear RNA genes (Chapman, 2007).

ChIP experiments revealed that S7E-containing mutants do not stably associate with any of the genes tested, providing an explanation for the deficit in mRNA observed for mutants containing S7E in the Linker-proximal region. Mutants containing either 48 consensus or S7A repeats appear to be recruited to protein coding genes at similar levels (Chapman, 2007).

It is concluded that the nature of the amino acid at position 7 of the Linker-proximal CTD region is important in steps involved in the stable association of Pol II with class II genes. Accumulation of Ser⁷-P in the 3' region of the TCRβ gene may suggest a role in transcription and/or 3' RNA processing of some protein-coding genes. Previous models can now be expanded for the cycle of CTD modification across genes that are transcribed by RNA polymerase II, not only to show how potential phosphorylation patterns change from 5' to 3' regions across a gene but also to speculate as to the region of the CTD in which they occur. Phosphorylation of Ser⁷ in the proximal part of CTD and replacement of Ser⁷ by other amino acids in the distal part of CTD may constitute an added layer of gene regulation by mammalian RNA polymerases (Chapman, 2007).

Drosophila MCRS2 associates with RNA polymerase II complexes to regulate transcription

Drosophila MCRS2 (dMCRS2; MCRS2/MSP58 and its splice variant MCRS1/p78 in humans) belongs to a family of forkhead-associated (FHA) domain proteins. Whereas human MCRS2 proteins have been associated with a variety of cellular processes, including RNA polymerase I transcription and cell cycle progression, dMCRS2 has been largely uncharacterized. Recent data show that MCRS2 is purified as part of a complex containing the histone acetyltransferase MOF (males absent on first) in both humans and flies. MOF mediates H4K16 acetylation and regulates the expression of a large number of genes, suggesting that MCRS2 could also have a function in transcription regulation. This study shows that dMCRS2 copurifies with RNA polymerase II (RNAP II) complexes and localizes to the 5' ends of genes. Moreover, dMCRS2 is required for optimal recruitment of RNAP II to the promoter regions of cyclin genes. In agreement with this, dMCRS2 is required for normal levels of cyclin gene expression. A model is proposed whereby dMCRS2 promotes gene transcription by facilitating the recruitment of RNAP II preinitiation complexes (PICs) to the promoter regions of target genes (Anderson, 2010).

The initiation of mRNA transcription involves the assembly of a transcription preinitiation complex (PIC), which as a minimum includes RNA polymerase II (RNAP II), Mediator, and six general transcription factors (TFIIA, -B, -D, -E, -F, and -H) at the core promoter DNA region. PIC assembly is initiated by the binding of the TATA box binding protein (TBP) subunit of TFIID to the promoter, which is stabilized in the presence of TFIIA and Mediator. Subsequently, TFIIB binds to and stabilizes the TFIIA-TFIIB-Mediator-DNA complex and functions as an adaptor that recruits the preformed RNAP II-TFIIF complex to the promoter. TFIIE and TFIIH then join to form the complete PIC (Anderson, 2010).

Once the PIC has been assembled on the promoter, transcription initiation occurs in several steps, which involve extensive phosphorylation of the C-terminal domain (CTD) of RNAP II. Early on in the transition from preinitiation to elongation, phosphorylation of Ser5s in the CTD heptapeptide repeats takes place, and this depends on the activity of the TFIIH-associated kinase cyclin-dependent kinase 7 (Cdk7; mammals)/Kin28 (yeast). Subsequently, Ser2s are phosphorylated by the elongation phase kinase Cdk9 (mammals)/CTDK-1 (yeast) to generate elongation-proficient RNAP II complexes. Another Cdk, Cdk8, can negatively regulate RNAP II transcription, partially via its inhibitory effect on Cdk7 activity. More recently, it has been suggested that Cdk11^p110 regulates RNAP II transcription in humans. Thus, Cdk11^p110 binds to hypo- and hyper-phosphorylated RNAP II, and antibody-mediated repression of Cdk11^p110 activity results in inhibition of RNAP II transcription (Anderson, 2010 and references therein).

In addition to the phosphorylation events that control RNAP II activity, modification of the chromatin structure represents an important mechanism for regulating gene expression. When the chromatin is in its repressed state, the DNA is wrapped tightly around the histones, creating a barrier to the assembly of the RNAP II PIC at the promoter region. Activation of gene expression is associated with a number of histone modifications that loosen the chromatin structure, including acetylation, methylation, ubiquitylation, and phosphorylation. Histone H3 and H4 acetylations are particularly frequent toward the 5' ends of actively transcribed genes and presumably facilitate the initial assembly of the PICs at the promoter region. MOF (males absent on first) is a histone H4 lysine 16 (H4K16)-specific histone acetyltransferase (HAT) in both mammals and Drosophila. MOF is part of several complexes, including the Drosophila male-specific lethal (MSL) complex, which is required for X chromosome dosage compensation, the mammalian counterpart of the MSL complex, and the MOF-MSL1v1 complex, which mediates p53 acetylation at K120. In addition, MOF copurifies with a number of other proteins, such as the forkhead-associated (FHA) domain-containing protein MCRS2, NSL1-3 (for nonspecific lethal 1 to 3), and MBD-R2, as part of the NSL complex (Anderson, 2010).

This study focuses on the function of Drosophila MCRS2 (dMCRS2), the Drosophila ortholog of human MCRS2 (also known as MSP58). Whereas human MCRS1 and -2 proteins have been associated with a variety of cellular processes, including RNA polymerase I transcription and cell cycle progression, dMCRS2 is largely uncharacterized. In addition to the recent observation that human and Drosophila MCRS2s form complexes with MOF (Cai, 2010; Prestel, 2010; Mendjan, 2006; Raja, 2010), several other reports suggest that MCRS1 and -2 proteins could function in transcription regulation via interactions with the transcriptional repressor Daxx or the basic region leucine zipper factor Nrf1 (Anderson, 2010).

Drosophila MCRS2 and its human homologue, MCRS2, are 59% identical, with the highest level of homology being in the FHA domain. Whereas dMCRS2 is largely uncharacterized, MCRS1 and -2 have been linked with a variety of cellular processes, RNAP I-dependent transcription, transcriptional repression, and cell cycle control, though these functions remain poorly understood (Anderson, 2010).

This study shows that dMCRS2 is an essential nuclear protein required for cell cycle progression and growth during development. The data show that dMCRS2 physically associates with Cdk11 and RNAP II and colocalizes with RNAP II PICs on polytene chromosomes in vivo. Consistent with this, dMCRS2 is required for optimal binding of RNAP II components to the cyclin promoter regions and for normal levels of cyclin gene expression (Anderson, 2010).

The demonstration of colocalization of dMCRS2 with RNAP II on numerous sites on polytene chromosomes is in agreement with a recent ChIP-seq analysis, which revealed that dMCRS2 is present on the promoters of over 4,000 genes, correlating with 55% of active genes (Raja, 2010). Furthermore, gene expression profiling studies show that dMCRS2 depletion elicits the downregulation of over 5,000 genes. This essential function as a broad-specificity transcriptional regulator is reflected by the extreme growth defect of dMCRS2-depleted cells both in vivo and in cell culture and in the fact that dMCRS2 has been recovered as a hit in RNAi screens for diverse cellular functions such as centrosome maturation and Hedgehog signaling (Anderson, 2010).

In accordance with its pleiotropic function, dMCRS2 can be purified with a number of proteins, from NSL components to members of the RNAP II machinery. Moreover, dMCRS2 colocalizes with RNAP II PICs on polytene chromosomes in vivo, suggesting that it may regulate an early step in the recruitment and/or assembly of RNAP II PICs. This is consistent with the majority of dMCRS2 binding to the promoter regions of autosomal and X-linked genes and the fact that dMCRS2 is required for the loading of RNAP II components to cyclin gene promoters. Thus, dMCRS2 appears to be an important transcriptional regulator, and the data represent the first evidence for a physical connection between dMCRS2 and the core transcriptional machinery. While the results suggest that dMCRS2 associates with RNAP II complexes via protein-protein interactions, future studies will need to establish the exact molecular nature of this connection (Anderson, 2010).

Interestingly, MCRS2 and dMCRS2 copurify with the MOF HAT independently of the dosage compensation MSL complex. Furthermore, it was observed that dMCRS2 coimmunoprecipitates and colocalizes extensively with MOF on polytene chromosomes. MOF, as well as binding to the 3' ends of MSL targets along the male X chromosome, is also found on numerous promoter regions, both on the X chromosome and on autosomes in both sexes. Since MCRS2 also binds to promoters, it is possible that dMCRS2 and MOF could function in concert in transcriptional regulation. However, despite the evidence that MOF regulates a broad range of both X-linked and autosomal genes, no physical connection between the putative dMCRS2-MOF NSL complex and RNAP II complexes has been established so far. This study shows that both dMCRS2 and MOF associate with core RNAP II complexes in cultured cells (Anderson, 2010).

dMCRS2 may promote transcription by different mechanisms. Through its HAT activity, dMCRS2-associated MOF may create a relaxed chromatin state favorable to PIC assembly, either by inducing the physical weakening of DNA/histone or histone/histone interactions or by promoting the recruitment of bromodomain-containing factors. dMCRS2 may also induce PIC formation by recruiting the preformed RNAP II/TFIIF complex and/or promoting transcription elongation through the recruitment of CK2 and the FACT complex, which facilitates transcription elongation by remodeling chromatin. However, whether these different dMCRS2-containing complexes regulate common target genes or whether they represent distinct transcriptional regulators remains to be investigated. In summary, a model is proposed where dMCRS2 binds to multiple sites along the chromosomes and promotes the recruitment of RNAP II PICs to target genes (Anderson, 2010).

Sequential changes at differentiation gene promoters as they become active in a stem cell lineage

Transcriptional silencing of terminal differentiation genes by the Polycomb group (PcG) machinery is emerging as a key feature of precursor cells in stem cell lineages. How, then, is this epigenetic silencing reversed for proper cellular differentiation? This study investigate how the developmental program reverses local PcG action to allow expression of terminal differentiation genes in the Drosophila male germline stem cell (GSC) lineage. It was found that the silenced state, set up in precursor cells, is relieved through developmentally regulated sequential events at promoters once cells commit to spermatocyte differentiation. The programmed events include global downregulation of Polycomb repressive complex 2 (PRC2) components, recruitment of hypophosphorylated RNA polymerase II (Pol II) to promoters, as well as the expression and action of testis-specific homologs of TATA-binding protein-associated factors (tTAFs). In addition, action of the testis-specific meiotic arrest complex (tMAC; Drosophila RB, E2F and Myb), a tissue-specific version of the mammalian MIP/dREAM complex, is required both for recruitment of tTAFs to target differentiation genes and for proper cell type-specific localization of PRC1 components and tTAFs within the spermatocyte nucleolus. Together, the action of the tMAC and tTAF cell type-specific chromatin and transcription machinery leads to loss of Polycomb and release of stalled Pol II from the terminal differentiation gene promoters, allowing robust transcription (Chen, 2011).

The results suggest a stepwise series of developmentally programmed events as terminal differentiation genes convert from a transcriptionally silent state in precursor cells to full expression in differentiating spermatocytes. In precursor cells, differentiation genes are repressed and associated with background levels of hypophosphorylated Pol II and H3K4me3. These genes also display elevated levels of H3K27me3 and Polycomb at the promoter region, suggesting that they are acted upon by the PcG transcriptional silencing machinery. Notably, the differentiation genes studied in precursor cells in this study did not show the hallmark bivalent chromatin domains enriched for both the repressive H3K27me3 mark and the active H3K4me3 mark that have been characterized for a cohort of differentiation genes in mammalian ESCs (Chen, 2011).

The cell fate switch from proliferating spermatogonia to the spermatocyte differentiation program initiates both global and local changes in the transcriptional regulatory landscape, starting a cell type-specific gene expression cascade that eventually leads to robust transcription of the terminal differentiation genes. Globally, soon after the switch from spermatogonia to spermatocytes, core subunits of the PRC2 complex are downregulated, including E(z), the enzyme that generates the H3K27me3 mark. Locally, after male germ cells become spermatocytes, Pol II accumulates at the terminal differentiation gene promoters, although these genes still remain transcriptionally silent, with low H3K4me3 and high Polycomb protein levels near their promoters (Chen, 2011).

The next step awaits the expression of spermatocyte-specific forms of core transcription machinery and chromatin-associated regulators, including homologs of subunits of both the general transcription factor TFIID (tTAFs) and the MIP/dREAM complex (Aly and other testis-specific components of tMAC). The tMAC complex acts either locally or globally, perhaps at the level of chromatin or directly through interaction with tTAFs, to allow recruitment of tTAFs to promoters of target terminal differentiation genes. The action of tTAFs then allows full and robust transcription of the terminal differentiation genes, partly by displacing Polycomb from their promoters (Chen, 2011).

Strikingly, the two major PcG protein complexes appear to be regulated differently by the germ cell developmental program: whereas the PRC2 components E(z) and Su(z)12 are downregulated, the PRC1 components Polycomb, Polyhomeotic and dRing continue to be expressed in spermatocytes. The global downregulation of the epigenetic 'writer' E(z) in spermatocytes might facilitate displacement of the epigenetic 'reader', the PRC1 complex, from the differentiation genes, with the local action of tTAFs at promoters serving to select which genes are relieved of PRC1. In addition, the tTAFs act at a second level to regulate Polycomb by recruiting and accompanying Polycomb and several other PRC1 components to a particular subnucleolar domain in spermatocytes. It is not yet known whether sequestering of PRC1 to the nucleolus by tTAFs plays a role in the activation of terminal differentiation genes, perhaps by lowering the level of PRC1 that is available to exchange back on to differentiation gene promoters. Conversely, recruitment of PRC1 to the nucleolar region might have a separate function, such as in chromatin silencing in the XY body as observed in mammalian spermatocytes (Chen, 2011).

The findings indicate that, upon the switch from spermatogonia to spermatocytes, the terminal differentiation genes go through a poised state, marked by presence of both active Pol II and repressive Polycomb, before the genes are actively transcribed. Stalled Pol II and abortive transcript initiation are emerging as a common feature in stem/progenitor cells. This mechanism may prime genes to rapidly respond to developmental cues or environmental stimuli. Stalled Pol II could represent transcription events that have initiated elongation but then pause and await further signals, as in the regulation of gene expression by the androgen receptor or by heat shock. Alternatively, Pol II might be trapped at a nascent preinitiation complex, without melting open the DNA, as found in some instances of transcriptional repression by Polycomb. Although ChIP analyses did not have the resolution to distinguish whether Pol II was stalled at the promoter or had already initiated a short transcript, the results with antibodies specific for unphosphorylated Pol II suggest that Pol II is trapped in a nascent preinitiation complex. The PRC1 component dRing has been shown to monoubiquitylate histone H2A on Lys119 near or just downstream of the transcription start site. It is proposed that in early spermatocytes, before expression of the tTAFs and tMAC, the local action of PRC1 in causing H2AK119ub at the terminal differentiation gene promoters might block efficient clearing of Pol II from the preinitiation complex and prevent transcription elongation (Chen, 2011).

Removal of PRC1 from the promoter and full expression of the terminal differentiation genes in spermatocytes require the expression and action of tMAC and tTAFs. Cell type-specific homologs of TFIID subunits have been shown to act gene-selectively to control developmentally programmed gene expression. For example, incorporation of one subunit of the mammalian TAF4b variant into TFIID strongly influences transcriptional activation at selected promoters, directing a generally expressed transcriptional activator to turn on tissue-specific gene expression (Chen, 2011).

The local action of the tTAFs to relieve repression by Polycomb at target gene promoters provides a mechanism that is both cell type specific and gene selective, allowing expression of some Polycomb-repressed genes while keeping others silent. Similar developmentally programmed mechanisms may also reverse PcG-mediated epigenetic silencing in other stem cell systems. Indeed, striking parallels between the current findings and recent results from mammalian epidermis suggest that molecular strategies are conserved from flies to mammals. In mouse epidermis, the mammalian E(z) homolog Ezh2 is expressed in stem/precursor cells at the basal layer of the skin. Strikingly, as was observed for E(z) and Su(z)12 in the Drosophila male GSC lineage, the Ezh2 level declines sharply as cells cease DNA replication and the epidermal differentiation program is turned on. Overexpression of Ezh2 in epidermal precursor cells delays the onset of terminal differentiation gene expression, and removal of the Ezh2-generated H3K27me3 mark by the Jmjd3 (Kdm6b) demethylase is required for epidermal differentiation (Chen, 2011 and references therein).

In particular, the results suggest a possible explanation for the conundrum that, although PcG components are bound at many transcriptionally silent differentiation genes in mammalian ESCs, loss of function of PcG components does not cause loss of pluripotency but instead causes defects during early embryonic differentiation. In Drosophila male germ cells, events during the switch from precursor cell proliferation to differentiation are required to recruit Pol II to the promoters of differentiation genes. Without this differentiation-dependent recruitment of Pol II, loss of Polycomb is not sufficient to precociously turn on terminal differentiation genes in precursor cells. Rather, Polycomb that is pre-bound at the differentiation gene promoters might serve to delay the onset of their transcription after the mitosis-to-differentiation switch. Robust transcription must await the expression of cell type- and stage-specific components of the transcription machinery. These might in turn guide gene-selective reversal of Polycomb repression to facilitate appropriate differentiation gene expression in specific cell types (Chen, 2011).

Immediate mediators of the inflammatory response are poised for gene activation through RNA polymerase II stalling

The kinetics and magnitude of cytokine gene expression are tightly regulated to elicit a balanced response to pathogens and result from integrated changes in transcription and mRNA stability. Yet, how a single microbial stimulus induces peak transcription of some genes (TNFalpha) within minutes whereas others (IP-10) require hours remains unclear. This study dissected activation of several lipopolysaccharide (LPS)-inducible genes in macrophages, an essential cell type mediating inflammatory response in mammals. A key difference between the genes is the step of the transcription cycle at which they are regulated. Specifically, at TNFalpha, RNA Polymerase II initiates transcription in resting macrophages, but stalls near the promoter until LPS triggers rapid and transient release of the negative elongation factor (NELF) complex and productive elongation. In contrast, no NELF or polymerase is detectible near the IP-10 promoter before induction, and LPS-dependent polymerase recruitment is rate limiting for transcription. It was further demonstrated that this strategy is shared by other immune mediators and is independent of the inducer and signaling pathway responsible for gene activation. Finally, as a striking example of evolutionary conservation, the Drosophila homolog of the TNFalpha gene, eiger, displayed all of the hallmarks of NELF-dependent polymerase stalling. It is proposed that polymerase stalling ensures the coordinated, timely activation the inflammatory gene expression program from Drosophila to mammals (Adelman, 2010).

Search PubMed for articles about Drosophila Pol II

Adelman, K., et al. (2005). Efficient release from promoter-proximal stall sites requires transcript cleavage factor TFIIS. Mol. Cell 17(1): 103-12. Medline abstract: 15629721

Adelman, K., Wei, W., Ardehali, M. B., Werner, J., Zhu, B. Reinberg, D. and Lis, J. T. (2006). Drosophila Paf1 modulates chromatin structure at actively transcribed genes. Mol. Cell. Biol. 26(1): 250-60. Medline abstract: 16354696

Adelman, K., et al. (2010). Immediate mediators of the inflammatory response are poised for gene activation through RNA polymerase II stalling. Proc. Natl. Acad. Sci. 106(43): 18207-12. PubMed Citation: 19820169

Andersen, D. S., et al. (2010). Drosophila MCRS2 associates with RNA polymerase II complexes to regulate transcription. Mol. Cell. Biol. 30(19): 4744-55. PubMed Citation: 20679484

Bieniasz, P.D., et al. (1999). Recruitment of cyclin T1/P-TEFb to an HIV type 1 long terminal repeat promoter proximal RNA target is both necessary and sufficient for full activation of transcription. Proc. Natl. Acad. Sci. 96: 7791-7796. 10393900

Boehm, A. K., Saunders, A., Werner, J. and Lis, J. T. (2003). Transcription factor and polymerase recruitment, modification, and movement on dhsp70 in vivo in the minutes following heat shock. Mol Cell Biol. 23(21): 7628-37. Medline abstract: 14560008

Cai, Y., et al. (2010). Subunit composition and substrate specificity of a MOF-containing histone acetyltransferase distinct from the male-specific lethal (MSL) complex. J. Biol. Chem. 285: 4268-4272. PubMed Citation: 20018852

Chapman, R. D., et al. (2007). Transcribing RNA polymerase II is phosphorylated at CTD residue serine-7. Science 318(5857): 1780-2. PubMed citation: 18079404

Chen, X., Lu, C., Prado, J. R., Eun, S. H. and Fuller, M. T. (2011). Sequential changes at differentiation gene promoters as they become active in a stem cell lineage. Development 138(12): 2441-50. PubMed Citation: 21610025

Chopra, V. S., Hong, J. W. and Levine, M. (2009). Regulation of Hox gene activity by transcriptional elongation in Drosophila. Curr. Biol. 19(8): 688-93. PubMed Citation: 19345103

Du, H. N., Fingerman, I. M. and Briggs, S. D. (2008). Histone H3 K36 methylation is mediated by a trans-histone methylation pathway involving an interaction between Set2 and histone H4. Genes Dev. 22(20): 2786-98. PubMed Citation: 18923077

Eissenberg, J. C., Shilatifard, A., Dorokhov, N., Michener, D. E.. (2007). Cdk9 is an essential kinase in Drosophila that is required for heat shock gene expression, histone methylation and elongation factor recruitment. Mol. Genet. Genomics. 277(2): 101-14. Medline abstract: 17001490

Flanagan, J. F., Mi, L.-Z., Chruszcz, M., Cymborowski, M., Clines, K. L., Kim, Y., Minor, W., Rastinejad, F., Khorasanizadeh, S. (2005). Double chromodomains cooperate to recognize the methylated histone H3 tail. Nature 438: 1181-1185. Medline abstract: 16372014

Ghosh, S. K., Missra, A., Gilmour, D. S. (2011). Negative elongation factor accelerates the rate at which heat shock genes are shut off by facilitating dissociation of heat shock factor. Mol. Cell Biol. 31(20): 4232-43. PubMed Citation: 21859888

Gilchrist, D. A., et al. (2011). Pausing of RNA polymerase II disrupts DNA-specified nucleosome organization to enable precise gene regulation. Cell 143(4): 540-51. PubMed Citation: 21074046

Guenther, M. G., Levine, S. S., Boyer, L. A., Jaenisch, R., and Young, R. A. (2007). A chromatin landmark and transcription initiation at most promoters in human cells. Cell 130: 77-88. Medline abstract: 17632057

Ivaldi, M. S., Karam, C. S. and Corces, V. G. (2007). Phosphorylation of histone H3 at Ser10 facilitates RNA polymerase II release from promoter-proximal pausing in Drosophila. Genes Dev. 21(21): 2818-31. Medline abstract: 17942706

Lindstrom, D. L. and Hartzog, G. A. (2001). Genetic interactions of Spt4-Spt5 and TFIIS with the RNA polymerase II CTD and CTD modifying enzymes in Saccharomyces cerevisiae. Genetics 159: 487-497. Medline abstract: 11606527

Lindstrom, D. L., et al. (2003). Dual roles for Spt5 in pre-mRNA processing and transcription elongation revealed by identification of Spt5-associated proteins. Mol. Cell Biol. 23: 1368-1378. Medline abstract: 12556496

Lis, J. T., et al. (2000). P-TEFb kinase recruitment and function at heat shock loci. Genes Dev. 14: 792-803. Medline abstract: 10766736

Macdonald, N., et al. (2005). Molecular basis for the recognition of phosphorylated and phosphoacetylated histone h3 by 14-3-3. Mol. Cell 20: 199-211. Medline abstract: 16246723

Mancebo. H. S., et al. (1997). P-TEFb kinase is required for HIV Tat transcriptional activation in vivo and in vitro. Genes Dev. 11(20): 2633-44. Medline abstract: 9334326

Marshall, N. F. and D. H. Price. (1995). Purification of P-TEFb, a transcription factor required for the transition into productive elongation. J. Biol. Chem. 270: 12335-12338. 7759473

Mendjan, S., et al. (2006). Nuclear pore components are involved in the transcriptional regulation of dosage compensation in Drosophila. Mol. Cell 21: 811-823. PubMed Citation: 16543150

Muse, G. W., et al. (2007). RNA polymerase is poised for activation across the genome. Nat. Genet. 39(12): 1507-11. Medline abstract: 17994021

Ni, Z., Schwartz, B. E., Werner, J., Suarez, R.-R. and Lis, J. T. (2004). Coordination of transcription, RNA processing, and surveillance by P-TEFb kinase on heat shock genes. Mol. Cell 13: 55-65. Medline abstract: 14731394

Ni, Z., Saunders, A., Fuda, N. J., Yao, J., Suarez, J. R., Webb, W. W. and Lis, J. T. (2008). P-TEFb is critical for the maturation of RNA polymerase II into productive elongation in vivo. Mol. Cell Biol. 28(3): 1161-70. PubMed Citation: 18070927

Nowak, S. J. and Corces, V. G. (2000). Phosphorylation of histone H3 correlates with transcriptionally active loci. Genes Dev. 14: 3003-3013. Medline abstract: 11114889

Nowak, S. J., Pai, C. Y., and Corces, V. G. (2003). Protein phosphatase 2A activity affects histone H3 phosphorylation and transcription in Drosophila melanogaster. Mol. Cell. Biol. 23: 6129-6138. Medline abstract: 12917335

Orphanides, G. and Reinberg, D. (2002). A unified theory of gene expression. Cell 108: 439-451. Medline abstract: 11909516

Peng, J., Marshall, N. F. and Price, D. H. (1998). Identification of a Cyclin subunit required for the function of Drosophila P-TEFb. J. Biol. Chem. 273: 13855-13860.

Prestel, M., et al. (2010). The activation potential of MOF is constrained for dosage compensation. Mol. Cell 38: 815-826. PubMed Citation: 20620953

Raja, S. J., et al. (2010). The nonspecific lethal complex is a transcriptional regulator in Drosophila. Mol. Cell 38: 827-841. PubMed Citation: 20620954

Smith, E. R., Winter, B., Eissenberg, J. C. and Shilatifard, A. (2008). Regulation of the transcriptional activity of poised RNA polymerase II by the elongation factor ELL. Proc. Natl. Acad. Sci. 105(25): 8575-9. PubMed Citation: 18562276

Stabell, M., Larsson, J., Aalen, R. B. and Lambertsson, A. (2007). Drosophila dSet2 functions in H3-K36 methylation and is required for development. Biochem. Biophys. Res. Commun. 359(3): 784-9. PubMed citation: 17560546

Thomson, S., Hollis, A., Hazzalin, C. A., and Mahadevan, L. C. (2004). Distinct stimulus-specific histone modifications at hsp70 chromatin targeted by the transcription factor heat shock factor-1. Mol. Cell 15: 585-594. Medline abstract: 15327774

Wang, X., Lee, C., Gilmour, D. S. and Gergen, J. P. (2007). Transcription elongation controls cell fate specification in the Drosophila embryo. Genes Dev. 21(9): 1031-6. Medline abstract: 17473169

Wang, X., Hang, S., Prazak, L. and Gergen, J. P. (2010). NELF potentiates gene transcription in the Drosophila embryo. PLoS One 5(7): e11498. PubMed Citation: 20634899

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Wu, C.-H., et al. (2003). NELF and DSIF cause promoter proximal pausing on the hsp70 promoter in Drosophila. Genes Dev. 17: 1402-1414. Medline abstract: 12782658

Yao, J., et al. (2006). Dynamics of heat shock factor association with native gene loci in living cells. Nature 442(7106): 1050-3. Medline abstract: 16929308

Yoh, S, M,, Cho, H., Pickle, L., Evans, R. M. and Jones, K. A. (2007). The Spt6 SH2 domain binds Ser2-P RNAPII to direct Iws1-dependent mRNA splicing and export. Genes Dev. 21(2): 160-74. PubMed Citation: 17234882

Zeitlinger, J., et al. (2007). RNA polymerase stalling at developmental control genes in the Drosophila melanogaster embryo. Nat. Genet. 39(12): 1512-6 . Medline abstract: 17994019

Zhang, Z. and Gilmour, D. S. (2006). Pcf11 is a termination factor in Drosophila that dismantles the elongation complex by bridging the CTD of RNA polymerase II to the nascent transcript. Mol. Cell 21(1): 65-74. Medline abstract: 16387654

date revised: 28 December 2011

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