BIOLOGICAL OVERVIEW

Initiation of transcription (the synthesis of messenger RNA) is carried out by a large protein complex, containing at least 30 proteins. As the understanding of the existence and functioning of such a large protein complex has developed, so has the realization that transcriptional initiation is highly regulated, and that each of these 30 protein components plays an important role in mRNA synthesis. The discussion of Taf1 (below) presumes some familiarity with the proteins involved in the initiation of transcription. If you are unfamiliar with these proteins, you might want first refer to RNA polymerase and general transcription factors before continuing to read about TAF1.

If one were to short list proteins according to the significance of their contribution to transcriptional initiation, Taf1 would be somewhere near the top of the list. Taf1 is the largest of the eight or nine transcription associated factors (TAFs) in Drosophila. TAFs are components of TFIID, a target of many transcriptional activators whose interactions with TFIID may enhance promoter function. In the most general case, the orderly process of transcription initiation begins with TFIID recognizing and binding tightly (via TATA binding protein or TBP, a component of TFIID) to the TATA box, a conserved sequence in many promoters. Taf1 has been shown to interact directly with TATA binding protein; Taf1 also harbors kinase activity (Dikstein, 1996) that targets itself (autophosphorylation) and TFIIF, another component of the initiation complex. TFIIF stimulates transcriptional elongation, and is a homolog of the bacterial sigma factor. RNA polymerase II cannot stably associate with a TFIID and TFIIB assembly at the promoter and must be escorted to the promoter by TFIIF (Orphanides, 1996). Therefore TFII250 regulates a protein (TFIIF) that chaperones (accompanies) RNA polymerase to the promoter.

What is the role of Taf1 in transcriptional activation of promoters? Central to eukaryotic transcriptional regulation is how TFIID gains access to a chromatin template and how a stable association is maintained with the chromosomal environment. Drosophila Taf1 and human and yeast homologs possess histone acetyltransferase (HAT) activity. Acetylation of histones is implicated in two related processes in eukaryotes: (1)acetylation is involved in the process of histone assembly into nucleosomes, and 2) histone acetylation plays a positive role in promoting access of transcription factors to nucleosomal DNA. These two processes are discussed more fully at the Histone 4 site. Thus Taf1, by means of a histone transacetylase function, has the potential to modify the protein bound environment of eukaryotic DNA (by acetylation), presumably modifying the access of chromatin, at or near core promoters, to other factors involved in transcriptional regulation (Mizzen, 1996).

Studies in unicellular systems indicate that TAF250 is required for progression through G₁/S of the cell cycle and repression of apoptosis. These in vivo studies have been extended by determining the developmental requirements for TAF250 in a multicellular organism, Drosophila. TAF250 mutants were isolated in a genetic screen that also yielded TAF60 and TAF110 mutants, indicating that TAFs function coordinately to regulate transcription. Null alleles of TAF250 are recessive larval lethal. However, combinations of weak loss-of-function TAF250 alleles survive to adulthood and reveal requirements for TAF250 during ovary, eye, ocelli, wing, bristle, and terminalia development as well as overall growth of the fly. These phenotypes suggest roles for TAF250 in regulating the cell cycle, cell differentiation, cell proliferation, and cell survival. Finally, molecular analysis of TAF250 mutants reveals that the observed phenotypes are caused by mutations in a central region of TAF250 that is conserved among metazoan organisms. This region is contained within the TAF250 histone acetyltransferase domain, but the mutations do not alter the histone acetyltransferase activity of TAF250 in vitro, indicating that some other aspect of TAF250 function is affected. Because this region is not conserved in the yeast TAF250 homolog, TAF145, it may define an activity for TAF250 that is unique to higher eukaryotes (Wassarman, 2000).

TAF250 (also designated TAF230 in Drosophila, CCG1 in humans, and TAF145/135 in yeast), is required for progression through the G₁/S boundary of the cell cycle. Mammalian or yeast cells carrying temperature-sensitive alleles of TAF250 arrest in G₁ at the nonpermissive temperature. Furthermore, following growth arrest, mammalian cells undergo apoptosis. In addition to serving as a scaffold on which other TAFs and TBP are assembled, TAF250 possesses enzymatic, promoter recognition, TBP regulatory, and coactivator activities. Transcriptional defects observed in these cells at the nonpermissive temperature presumably result from elimination of one or more TAF250 activities (Wassarman, 2000 and references therein).

TAF250 contains two independent protein kinase domains and a histone acetyltransferase (HAT) domain. The kinase domains can autophosphorylate as well as transphosphorylate, in vitro, RAP74, the largest subunit of TFIIF. The HAT domain can acetylate lysine residues in vitro, in the N-terminal tails of histones H3 and H4. Because acetylation of histones induces changes in chromatin structure, the HAT activity of TAF250 may provide a mechanism for TFIID to access transcriptionally repressed chromatin. The TAF250 bromodomains may contribute to this process because bromodomains have been shown to bind the N terminus of histone H4 and acetylated Lys residues (Wassarman, 2000 and references therein).

Mutations in Drosophila TAF250 have been identified in a genetic screen that is sensitive to transcription levels. The availability of loss-of-function mutations in TAF250 has allowed for a determination of how Drosophila development is affected by the absence of this broadly acting transcriptional regulator; such mutations allow for the definition of a protein domain that is critical for the function of TAF250 in vivo. Changes in the transcription profile of a cell occur in response to signals mediated by the Ras GTPase. Much work has focused on the gene-specific enhancer-binding proteins that are direct targets of the Ras signal, but components of the transcriptional machinery that interact with enhancer-binding proteins presumably play equally important roles in regulating transcription. Consistent with this proposal, mutations in subunits of TFIIA and TFIID interfere with the ability of Ras1 to specify cell fate decisions during Drosophila eye development. Mutations in TAF60 and TAF110 were isolated in a genetic screen as suppressors of the rough eye phenotype caused by misexpression of a constitutively active form of Ras1 (Ras1^V12) under transcriptional control of the sev enhancer and promoter elements (sev-Ras1^V12). Genetic interaction studies were then used to categorize the isolated modifiers into two general groups: Class I, genes that function directly in the Ras1 signaling pathway, and Class II, genes that function to transcriptionally regulate the sev-Ras1^V12 transgene or downstream gene targets of the pathway. Class I groups have subsequently been shown to include protein kinases that function in a general Ras1 signaling cassette and the only Class II groups characterized thus far, SR3-4B and SR3-3, correspond to TAF60 and TAF110. Thus, it appears likely that additional Class II groups (SR3-2, SR3-4A, SR3-5, SR3-7, and SR3-8) will encode transcriptional regulators. This study focuses on SR3-5 (Wassarman, 2000).

The SR3-5 locus encodes Drosophila TAF250. Searches of the Berkeley Drosophila Genome Project (BDGP) database identified a mutant line, EP(3)0421, in which a P-element is inserted within exon 1 of TAF250, 55 bp downstream of the translation start site. To determine if TAF250 expression is affected in EP(3)0421 flies, ovary extracts were prepared from wild-type and EP(3)0421 heterozygous and homozygous flies and probed with a TAF250 antibody. Relative to TAF110, TAF250 protein expression is drastically reduced in EP(3)0421 homozygous flies. EP(3)0421 is homozygous viable and complements the lethality of SR3-5^S-136 and SR3-5^S-625, but it only partially complements SR3-5^XS-2232 (Wassarman, 2000).

To determine whether the phenotypes of EP(3)0421-containing flies are caused by insertion of the P-element in the TAF250 gene and to create stronger TAF250 alleles, the EP(3)0421 P-element was excised by introducing a source of transposase. Viable lines in which the P-element was precisely excised no longer display bristle and sterility phenotypes observed in the parental line, indicating that the P-element is the cause of these phenotypes. One of the lethal excision lines, EP(3)0421^EX1, was analyzed in detail. Sequencing of the TAF250 gene from EP(3)0421^EX1 reveals that the parental P-element was internally deleted, leaving behind 52 bp of P-element sequence. The small insertion contains an in-frame stop codon within the first exon of TAF250, and therefore may be a null allele. In fact, like Df(3R)BD5, which deletes the TAF250 gene, EP(3)0421^EX1 fails to complement the lethality of all SR3-5 alleles. EP(3)0421 and SR3-5 mutants have been renamed TAF250 (Wassarman, 2000).

Trans-heterozygous combinations of lethal TAF250 alleles and a viable P-element allele TAF250^EP421 exhibit developmental phenotypes. The severity and nature of these defects depends on the allelic combination. All allelic combinations display defects in oogenesis, ocelli development, and developmental timing; in addition, flies with certain mutant combinations display defects in eye, wing, bristle, and terminalia development, indicating roles for TAF250 in multiple developmental pathways (Wassarman, 2000).

Mutations in TAF250 disrupt development of photosensitive organelles, the eyes and ocelli. Adult compound eyes are composed of hundreds of ommatidia that are arranged in a perfect hexagonal array with single mechanosensory bristles that project from alternate vertices of ommatidia. In TAF250^XS-2232/TAF250^EP421 flies, ommatidia are irregularly shaped and are not aligned in straight rows. In addition, the number of interommatidial bristles is reduced, multiple bristles sometimes originate from a single site, and bristles are randomly distributed (Wassarman, 2000).

The number of photoreceptor cells per ommatidium is altered in TAF250 adult flies. Apical tangential sections through wild-type eyes reveal a trapezoid pattern of photoreceptors, with six large outer photoreceptors (R1-6) forming the perimeter of the trapezoid and one small inner photoreceptor (R7) in the center of the trapezoid. In TAF250^XS-2232/TAF250^EP421 flies, approximately 10% of the ommatidia are missing one outer photoreceptor and 14% have one extra outer photoreceptor. TAF250 mutations also disrupt development of ocelli. Three ocelli are normally arranged in a triangle on the top of the head. Severe TAF250 mutants completely lack individual ocelli whereas less severe mutants have ocelli that are reduced in size (Wassarman, 2000).

Mutations in TAF250 alter bristle development on the thorax and head. The adult cuticle of D. melanogaster has numerous precisely positioned large bristles, macrochaete, and small bristles, microchaete. On the thorax, ten rows of microchaete bristles are positioned between pairs of macrochaete bristles. In TAF250 mutants, several bristles in each row are shifted slightly, disrupting the orderly pattern of bristle rows. Microchaete size and shape do not appear to be affected and all aspects of macrochaete morphology appear normal on the thorax. In contrast, the number of bristles surrounding the ocelli is reduced, including complete elimination of ocellar bristles (Wassarman, 2000).

Mutations in TAF250 result in incomplete rotation of the terminal A9 body segment in males. It has been hypothesized that during pupation, the developing terminalia undergo a 360^o rotation in a clockwise direction to achieve the configuration of ejaculatory and tracheal ducts and the posterior peripheral nerves observed in adult male flies (Wassarman, 2000). Incomplete rotation is observed in approximately 5% of TAF250^XS-2232/TAF250^EP421 males. Different degrees of incomplete rotation occur. In the most extreme phenotype the terminalia have rotated only 30° and the last tergite and analplate protrudes from the body. It should be noted that this phenotype could be interpreted as a 30° overrotation of the terminalia (Wassarman, 2000).

Mutations in TAF250 disrupt wing development. In TAF250 flies, the fourth longitudinal vein (L4) bifurcates along the wing margin forming a delta. The shape and size of the wings appear normal as does the morphology of the other veins. However, the wings are often held out to the sides of the body, rather than crossed over the body. TAF250 mutants are also female sterile. Mutations in TAF250 do not appear to affect the fertility of males. In the most severe case, TAF250^XS-2232/TAF250^EP421, the ovaries are very small and the flies lay very few eggs that do not develop. In a less severe case, TAF250^S-625/TAF250^EP421, the ovaries appear normal in size and the flies lay more eggs but they do not develop. Examination of ovaries dissected from TAF250^S-625/TAF250^EP421 flies reveals that approximately 5% of egg chambers contain 8 cells (7 nurse cells and 1 oocyte) in contrast to wild-type egg chambers that contain 16 cells (15 nurse cells and 1 oocyte). Eight-cell egg chambers may result from a reduction in the number of cell divisions, from four to three, that a single cystoblast normally undergoes to generate a 16-cell egg chamber (Wassarman, 2000).

TAF250 is required for normal developmental timing. Mutations in TAF250 cause a delay in development. This was determined by examining how long transheterozygous TAF250 mutants take to develop from an egg to adult relative to their heterozygous mutant siblings. The strongest affect is observed with TAF250^XS-2232/TAF250^EP421 flies that are delayed approximately two days in development, but all transheterozygous TAF250 combinations are delayed to some extent. Delayed eclosion of mutant flies could reflect slowed developmental processes or an inability to gather enough nutrients.

The SR3-5 mutations identify a conserved region of TAF250 that is critical for its function. The TAF250-coding region of the SR3-5 alleles was sequenced. In each case, single amino acid changes were observed that lie within a 55-amino acid central region of TAF250. The TAF250^S-136 gene product has an Asp-1041 to Asn change; TAF250^XS-2232 has a Val-1072 to Asp change, and TAF250^S-625 has an Arg-1096 to Pro change. These substitutions alter residues that are conserved within metazoan organisms but are not conserved in the homologous yeast protein TAF145 and thus potentially identify a TAF250 function that is specific to metazoan organisms (Wassarman, 2000).

The point mutations in TAF250 fall within a region that has been shown to interact with transcriptional regulatory proteins, such as HIV TAT and retinoblastoma protein, and to possess HAT activity. It should be noted that the TAF250 HAT domain does not contain sequence similarity to the acetyl-CoA binding site defined for acetyltransferases including GCN5. To test whether HAT activity is altered by any of the TAF250 mutations, a recombinant polypeptide encompassing amino acids 612-1,140 (TAF250-M) of Drosophila TAF250 was produced in bacteria, and site-directed mutagenesis was used to generate the identified single amino acid changes within the context of this polypeptide. Purified recombinant proteins were tested in vitro for HAT activity by an activity gel assay. This assay revealed that the TAF250-M region of TAF250 has HAT activity, which narrows down the HAT domain to a small region. Interestingly, the point mutations do not significantly affect this activity. This result indicates that the mutations do not affect the catalytic activity of the TAF250 HAT, but it does not exclude the possibility that they regulate substrate specificity or the kinetics of the acetylation reaction (Wassarman, 2000).

TAFs were originally defined biochemically as components of a complex, TFIID, that is required for activator-responsive transcription in vitro. Subsequent in vitro studies have shown that complexes containing only a subset of TAFs function similarly to holo-TFIID, and specific biochemical functions have been attributed to individual TAFs. These findings raise the question of whether all TAFs are required for the transcription of TAF-dependent genes in vivo. The isolation of mutations in TAF60, TAF110, and TAF250 in the same genetic screen suggests that some genes require the function of multiple TAFs. This is supported by the finding that (1) TAF60 and TAF110 are required for the transcription of twist and snail genes in the Drosophila embryo; (2) TAF60 and TAF250 mutants have similar adult phenotypes; and (3) the transcription of some genes in yeast depends on multiple TAFs. Conversely, because all Drosophila TAFs were not isolated in the sev-Ras1^V12 screen, it suggests that not all TAFs are required for the transcription of TAF-dependent genes or that the dose sensitivities of TAFs are different (Wassarman, 2000).

TAF250 mutants were isolated as modifiers of a Ras1-induced phenotype and display phenotypes similar to those of flies mutant for components of Ras1 signaling pathways. In Drosophila, the epidermal growth factor receptor (EGFR) tyrosine kinase signals through Ras1 to control cell proliferation, survival, and differentiation. Similar to mutations in TAF250, mutations in EGFR or downstream components of EGFR signaling pathways cause female sterility, rough eyes with associated bristle defects, altered photoreceptor number, loss of ocelli and ocellar bristles, and extra wing vein material. In addition, misrotated terminalia are observed in the head involution defective (hid) mutant. hid encodes a pro-apoptotic factor that is transcriptionally regulated by the EGFR/Ras1 pathway. The correlation between TAF250 and EGFR pathway mutant phenotypes suggests that TAF250 transcriptionally regulates genes in the EGFR pathway (possibly EGFR, which is a target for TAF250 in mammalian cells, or downstream genes in the pathway, like hid). Alternatively, some of the TAF250 phenotypes may result from altered transcription of ribosomal protein genes. Minute mutants encode ribosomal proteins and have common phenotypes, including delayed development, female sterility, and short and slender bristles . Furthermore, some Minutes have rough eyes, missing ocelli, or misrotated terminalia. TAF250 mutants display all of these phenotypes, except for changes in bristle morphology, suggesting that, as in yeast, TAF250 regulates ribosomal protein gene transcription. Finally, the oogenesis defect suggests a role for TAF250 in cell cycle progression, as has been reported for yeast and mammalian cells. Mutations in cyclin E, which is necessary for the G₁/S transition, also cause the formation of eight-cell egg chambers, suggesting that TAF250 may regulate transcription of cyclin E (Wassarman, 2000 and references therein).

A sequence-specific core promoter-binding transcription factor recruits TRF2 to coordinately transcribe ribosomal protein genes

Ribosomal protein (RP) genes must be coordinately expressed for proper assembly of the ribosome yet the mechanisms that control expression of RP genes in metazoans are poorly understood. Recently, TATA-binding protein-related factor 2 (TRF2) rather than the TATA-binding protein (TBP) was found to function in transcription of RP genes in Drosophila. Unlike TBP, TRF2 lacks sequence-specific DNA binding activity, so the mechanism by which TRF2 is recruited to promoters is unclear. This study shows that the transcription factor M1BP, which associates with the core promoter region, activates transcription of RP genes. Moreover, M1BP directly interacts with TRF2 to recruit it to the RP gene promoter. High resolution ChIP-exo was used to analyze in vivo the association of M1BP, TRF2 and TFIID subunit, TAF1. Despite recent work suggesting that TFIID does not associate with RP genes in Drosophila, it was found that TAF1 is present at RP gene promoters and that its interaction might also be directed by M1BP. Although M1BP associates with thousands of genes, its colocalization with TRF2 is largely restricted to RP genes, suggesting that this combination is key to coordinately regulating transcription of the majority of RP genes in Drosophila (Baumann, 2017).

This study shows that M1BP activates transcription of RP genes in Drosophila and that it can do so by recruiting TRF2 to RP gene promoters in cells. These conclusions are based on the demonstration that M1BP is detected in the core promoter region of the majority of RP genes in cells and that mutation of Motif 1 diminished the level of expression from RP reporter genes. Additionally, it was demonstrated that M1BP activates transcription of RP gene promoters in nuclear extracts. Also, M1BP was shown to recruit TRF2 to promoter DNA in vitro and that M1BP and TRF2 colocalize on the RP gene promoters in cells. M1BP, therefore, is the first sequence-specific DNA-binding protein that has been directly shown to activate RP gene transcription in metazoans. DREF is possibly the only other protein, but it remains to be determined if it activates RP genes in vitro. Since these transcription factors associate with a broad spectrum of genes, loss of function assays in cells must be viewed with caution as it is difficult to distinguish between direct and indirect effects regardless of whether the protein can be detected at a particular genez (Baumann, 2017).

Mechanisms by which TRF2 associates with promoters are not well understood. DREF was purified in a complex with TRF2 but no direct measurement of TRF2 recruitment to DNA by this complex was provided. An uncharacterized TRF2 complex associates with promoters bearing the DPE and canonical initiator, but RP genes lack both of these DNA elements. This study provides a direct mechanism that involves M1BP associating with its cognate-binding site and interacting directly with TRF2. Since there is little overlap between M1BP and TRF2 outside of RP gene promoters, it follows that additional cis-elements are required for TRF2's association with M1BP. It is suspected that the TCT motif, along with M1BP and DREF, may be an additional key contributor to TRF2's association with gene promoters. Additionally, ChIP-exo data reveals that TAF1 is present at virtually all promoters that are associated with Pol II including most promoters that associate with TRF2. Therefore, since TAF1 recognizes sequences at the initiator and the DPE, TAF1 could be part of the currently uncharacterized TRF2-containing complex that selectively binds the initiator-DPE-containing promoters (Baumann, 2017).

The total number of TRF2 peaks that were observed is considerably lower than that reported previously. There could be a couple reasons for this discrepancy. First, the previous study used 2~4 h embryos, whereas this study used S2R+ cells. It is possible that TRF2 functions at a broader spectrum of developmentally regulated genes in the early embryo than in S2R+ cells. Additionally, the difference could be due to the increased signal to noise ratio afforded by ChIP-exo which results in more reliable peak detection (Baumann, 2017).

Detection of TAF1 on RP gene promoters was unexpected because TAF1 is best known for being a subunit of the general transcription factor, TFIID and biochemical evidence argued against TFIID being involved in RP gene transcription. Moreover, previous analysis of the PCNA promoter showed that immunodepletion of TFIID with TAF1 antibody from a Drosophila transcription reaction did not inhibit transcription of a TRF2-dependent promoter. ChIP-exo data provide evidence for M1BP being in close proximity to and potentially interacting with, TRF2 and TAF1 on RP gene promoters. The ChIP-exo data showed a peak of M1BP contacts downstream from the TSS yet Motif 1 that binds M1BP typically resides upstream from the TSS. Since the ChIP-exo data for M1BP and TAF1 display overlapping peaks in the +30 to +50 region, it is proposed that M1BP is in contact with TAF1 and that the ChIP-exo signal for M1BP in this region is a consequence of M1BP crosslinking to TAF1 and TAF1 in turn crosslinking to the +30 to +50 region. In contrast, the ChIP exo signals for M1BP and TRF2 are shifted relative to each other by ∼10 nts suggesting that M1BP might position TRF2 on the DNA adjacent to M1BP (Baumann, 2017).

A unique feature of the RP gene promoters in Drosophila and humans is the presence of the TCT motif located at the TSS. What recognizes this motif is currently not known. Since TAF1 is known to recognize the canonical initiator element, its presence at RP gene promoters raises the possibility that this TAF also recognizes the TCT motif. DNAse I footprinting analysis of TFIID binding to RP gene promoters indicated that binding was extremely weak. However, close inspection of the DNase I cutting patterns in the absence and presence of TFIID reveals the appearance of weak hypersensitive cut sites near the TCT initiator. One possibility is that M1BP together with TRF2 enhance the affinity of TAF1 for the RP gene initiator (Baumann, 2017).

It is proposed that M1BP functions as a hub to recruit TRF2 and TAF1. Since the only known TAF1-containing complex in metazoans is TFIID, it is proposed that TFIID still binds to RP gene promoters along with TRF2. One possibility is that TRF2 displaces TBP at the RP gene promoter. A recent model of TFIID bound to promoter DNA indicates that TFIIA is involved in connecting TBP to TAF1. Since TRF2 associates with TFIIA , displacement of TBP from TAF1 by TRF2 is tenable (Baumann, 2017).

This analysis of STARR-seq data indicates that RP gene promoters can act as enhancers and that they are selective in activating housekeeping gene core promoters and not core promoters of developmental and stress activated genes. The RP gene promoters more strongly activated the candidate RP gene promoter over all the other tested candidates. This selectivity could establish a network in which active RP genes and other housekeeping genes act reciprocally to activate each other. In addition, the selectivity of the enhancer activity of these RP promoters would prevent them from inadvertently activating nearby developmentally regulated genes (Baumann, 2017).

GENE STRUCTURE

Bases in 5' UTR - 77

Bases in 3' UTR - 248

PROTEIN STRUCTURE

Amino Acids - 2068

Structural Domains

DTAFII230 (p230) can be divided into four subdomains. The small amino-terminal domain (I) (amino acids 1-235) is a highly acidic region that is moderately conserved between Drosophila and human (25% identity). This domain is also rich in glycine and serine residues, suggesting that it might form a fairly flexible and/or closely packaged structure as a result of the small side chains of these amino acids. In contrast, the large domain II ( amino acids 236-1706) is well conserved (55% identity), especially in its central region (78% identity for amino acids 610-1178). Domain II contains three intriguing structural motifs: an HMG box (amino acids 1246-1360); a potential nuclear localization signal (amino acids 1445-1452); and 130-amino-acid direct repeats containing bromodomain in each repeat (amino acids 1457-1577 and 1578-1699). Domain III (1707-1993 amino acids) is barely conserved in primary sequence (15% identity) between Drosophila and human, but both species contain a remarkable number of acidic residues (Drosophila 24%). This acidic domain has two consecutive negative charges, both of which are consensus target sites for casein kinase II, which is known to control the activity of several transcription factors. The most C-terminal domain (IV) (amino acids 1994-2068) is a glutamine-rich tail present in the Drosophila sequence, but not in the human sequence (Kokubo, 1993).

Remarkably, only the HMG box of p230 is interrupted by the insertion of a 35-amino acid sequence. Other HMG box proteins of Drosophila include HMG-D and Sox box protein 70D(also known as Dichaete). It is unknown whether the HMG box of p230 is active in DNA binding and bending. The bromodomain is a feature common to several global transcriptional activators, such as SNF2 of yeast and Brahma of Drosophila, as well as Imitation SWI and Male specific lethal 2 (Kokubo, 1993).

A kinase domain resides within the N-terminal portion of Taf1. There are several patches of amino acid sequences with significant but weak homology to the protein kinase family. The best match to multiple kinase homologies is found within the N-terminal region of both Drosophila and human Taf1. A minimal kinase domain is located in human Taf1, mapping to residues 1-434. Deletion of residues 1-161 abrogates phosphorylation of RAP74, also known as TFIIF (see RNA polymerase and general transcription factors) as well as the potential for autophosphorylation. A second distinct kinase domain is located at the C-terminal region of Taf1. An autophosphorylation domain is intrinisic to the C-terminal 468 amino acid residues of hTaf1. The amino acid sequence of this putative C-terminal kinase (CTK) shares no apparent homology to other known protein kinases (Dikstein, 1996).

Drosophila Taf1 possesses a histone transacetylase (HAT) activity that targets histone H3 and histone H4. The putative catalytic domain of Drosophila Taf1 is located between residues 1 and 1140. Furthermore, sequences between residues 885 and 1140 are critical for HAT activity. This region corresponds to a highly conserved domain shared with yeast TAFII130, the homolog of Drosophila Taf1. The central portion of yeast TAFII130 possesses the HAT activity. There is no similarity of Taf1 or yeast TAFII130 to other known acetyltransferase proteins. These results suggest that the HAT catalytic domain of Taf1 may represent a second type of HAT domain, unrelated to previously characterized histone acetyltransferases (Mizzen, 1996).

date revised: 25 April 2019 

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