absent, small, or homeotic discs 1: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References
Gene name - absent, small, or homeotic discs 1

Synonyms -

Cytological map position - 76B8--9

Function - enzyme

Keywords - trithorax group, histone methyl-transferase, chromatin

Symbol - ash1

FlyBase ID: FBgn0005386

Genetic map position - 3-46.6

Classification - histone methyl transferase, BAH (bromo-adjacent homology) domain, PHD-finger, SET domain

Cellular location - nuclear

NCBI link:  Entrez Gene
ash1 orthologs: Biolitmine

Recent literature
McCracken, A. and Locke, J. (2016). Mutations in ash1 and trx enhance P-element-dependent silencing in Drosophila melanogaster. Genome [Epub ahead of print]. PubMed ID: 27373142
In Drosophila melanogaster, the mini-w+ transgene in Pci (a transgene inserted proximally on chromosome 4 between Ribosomal protein S3A (RpS3A) and cubitus interruptus (ci), is normally expressed throughout the adult eye; however, when other P or KP elements are present, a variegated-eye phenotype results, indicating random w+ silencing during development called P-element-dependent silencing (PDS). Mutant Su(var)205 and Su(var)3-7 alleles act as haplo-suppressors/triplo-enhancers of this variegated phenotype, indicating that these heterochromatic modifiers act dose dependently in PDS. Previously, a spontaneous mutation of P{lacW}ciDplac called P{lacW}ciDplacE1 (E1) was recovered that variegated in the absence of P elements, presumably due to the insertion of an adjacent gypsy element. From a screen for genetic modifiers of E1 variegation, this study describes the isolation of five mutations in ash1 and three in trx that enhance the E1 variegated phenotype in a dose-dependent and cumulative manner. These mutant alleles enhance PDS at E1, and in E1/P{lacW}ciDplac, but suppress position effect variegation (PEV) at In(1)wm4. This opposite action is consistent with a model where ASH1 and TRX mark transcriptionally active chromatin domains. If ASH1 or TRX function is lost or reduced, heterochromatin can spread into these domains creating a sink that diverts heterochromatic proteins from other variegating locations, which then may express a suppressed phenotype.
Huang, C., Yang, F., Zhang, Z., Zhang, J., Cai, G., Li, L., Zheng, Y., Chen, S., Xi, R. and Zhu, B. (2017). Mrg15 stimulates Ash1 H3K36 methyltransferase activity and facilitates Ash1 Trithorax group protein function in Drosophila. Nat Commun 8(1): 1649. PubMed ID: 29158494
Ash1 is a Trithorax group protein that possesses H3K36-specific histone methyltransferase activity, which antagonizes Polycomb silencing. This study reports the identification of two Ash1 complex subunits, Mrg15 and Nurf55. In vitro, Mrg15 stimulates the enzymatic activity of Ash1. In vivo, Mrg15 is recruited by Ash1 to their common targets, and Mrg15 reinforces Ash1 chromatin association and facilitates the proper deposition of H3K36me2. To dissect the functional role of Mrg15 in the context of the Ash1 complex, this study identified an Ash1 point mutation (Ash1-R1288A) that displays a greatly attenuated interaction with Mrg15. Knock-in flies bearing this mutation display multiple homeotic transformation phenotypes, and these phenotypes are partially rescued by overexpressing the Mrg15-Nurf55 fusion protein, which stabilizes the association of Mrg15 with Ash1. In summary, Mrg15 is a subunit of the Ash1 complex, a stimulator of Ash1 enzymatic activity and a critical regulator of the TrxG protein function of Ash1 in Drosophila.
Dorafshan, E., Kahn, T. G., Glotov, A., Savitsky, M., Walther, M., Reuter, G. and Schwartz, Y. B. (2019). Ash1 counteracts Polycomb repression independent of histone H3 lysine 36 methylation. EMBO Rep. PubMed ID: 30833342
Polycomb repression is critical for metazoan development. Equally important but less studied is the Trithorax system, which safeguards Polycomb target genes from the repression in cells where they have to remain active. It was proposed that the Trithorax system acts via methylation of histone H3 at lysine 4 and lysine 36 (H3K36), thereby inhibiting histone methyltransferase activity of the Polycomb complexes. This hypothesis was tested by asking whether the Trithorax group protein Ash1 requires H3K36 methylation to counteract Polycomb repression. This study shows that Ash1 is the only Drosophila H3K36-specific methyltransferase necessary to prevent excessive Polycomb repression of homeotic genes. Unexpectedly, the experiments reveal no correlation between the extent of H3K36 methylation and the resistance to Polycomb repression. Furthermore, it was found that complete substitution of the zygotic histone H3 with a variant in which lysine 36 is replaced by arginine does not cause excessive repression of homeotic genes. These results suggest that the model, where the Trithorax group proteins methylate histone H3 to inhibit the histone methyltransferase activity of the Polycomb complexes, needs revision.
Dorafshan, E., Kahn, T. G., Glotov, A., Savitsky, M. and Schwartz, Y. B. (2019). Genetic dissection reveals the role of Ash1 domains in counteracting Polycomb repression. G3 (Bethesda). PubMed ID: 31540973
Antagonistic functions of Polycomb and Trithorax proteins are essential for proper development of all metazoans. While the Polycomb proteins maintain the repressed state of many key developmental genes, the Trithorax proteins ensure that these genes stay active in cells where they have to be expressed. Ash1 is the Trithorax protein that was proposed to counteract Polycomb repression by methylating lysine 36 of histone H3. However, it was recently shown that genetic replacement of Drosophila histone H3 with the variant that carried Arginine instead of Lysine at position 36 did not impair the ability of Ash1 to counteract Polycomb repression. This argues that Ash1 counteracts Polycomb repression by methylating yet unknown substrate(s) and that it is time to look beyond Ash1 methyltransferase SET domain, at other evolutionary conserved parts of the protein that received little attention. This study used Drosophila genetics to demonstrate that Ash1 requires each of the BAH, PHD and SET domains to counteract Polycomb repression, while AT hooks are dispensable. These findings argue that, in vivo, Ash1 acts as a multimer. Thereby it can combine the input of the SET domain and PHD-BAH cassette residing in different peptides. Finally, using new loss of function alleles, zygotic Ash1 was shown to be required to prevent erroneous repression of homeotic genes of the bithorax complex.
Steffen, P. A., Altmutter, C., Dworschak, E., Junttila, S., Gyenesei, A., Zhu, X., Kockmann, T. and Ringrose, L. (2021). The Trithorax group protein ASH1 requires a combination of BAH domain and AT hooks, but not the SET domain, for mitotic chromatin binding and survival. Chromosoma. PubMed ID: 34331109
The Drosophila Trithorax group (TrxG) protein ASH1 remains associated with mitotic chromatin through mechanisms that are poorly understood. ASH1 dimethylates histone H3 at lysine 36 via its SET domain. This study has identified domains of the TrxG protein ASH1 that are required for mitotic chromatin attachment in living Drosophila. Quantitative live imaging demonstrates that ASH1 requires AT hooks and the BAH domain but not the SET domain for full chromatin binding in metaphase, and that none of these domains are essential for interphase binding. Genetic experiments show that disruptions of the AT hooks and the BAH domain together, but not deletion of the SET domain alone, are lethal. Transcriptional profiling demonstrates that intact ASH1 AT hooks and the BAH domain are required to maintain expression levels of a specific set of genes, including several involved in cell identity and survival. This study identifies in vivo roles for specific ASH1 domains in mitotic binding, gene regulation, and survival that are distinct from its functions as a histone methyltransferase.
Chaouch, A., Berlandi, J., Chen, C. C. L., Frey, F., Badini, S., Harutyunyan, A. S., Chen, X., Krug, B., Hebert, S., Jeibmann, A., Lu, C., Kleinman, C. L., Hasselblatt, M., Lasko, P., Shirinian, M. and Jabado, N. (2021). Histone H3.3 K27M and K36M mutations de-repress transposable elements through perturbation of antagonistic chromatin marks. Mol Cell. PubMed ID: 34739871
Histone H3.3 lysine-to-methionine substitutions K27M and K36M impair the deposition of opposing chromatin marks, H3K27me3/me2 and H3K36me3/me2. This study shows that these mutations induce hypotrophic and disorganized eyes in Drosophila eye primordia. Restriction of H3K27me3 spread in H3.3K27M and its redistribution in H3.3K36M result in transcriptional deregulation of PRC2-targeted eye development and of piRNA biogenesis genes, including krimp. Notably, both mutants promote redistribution of H3K36me2 away from repetitive regions into active genes, which associate with retrotransposon derepression in eye discs. Aberrant expression of krimp represses LINE retrotransposons but does not contribute to the eye phenotype. Depletion of H3K36me2 methyltransferase ash1 in H3.3K27M, and of PRC2 component E(z) in H3.3K36M, restores the expression of eye developmental genes and normal eye growth, showing that redistribution of antagonistic marks contributes to K-to-M pathogenesis. These results implicate a novel function for H3K36me2 and showcase convergent downstream effects of oncohistones that target opposing epigenetic marks (Chaouch, 2021).


Members of the Polycomb group of repressors and trithorax group of activators maintain heritable states of transcription by modifying nucleosomal histones or remodeling chromatin. Although tremendous progress has been made toward defining the biochemical activities of Polycomb and trithorax group proteins, much remains to be learned about how they interact with each other and the general transcription machinery to maintain on or off states of gene expression. The trithorax group protein Kismet (KIS) is related to the SWI/SNF and CHD families of chromatin remodeling factors. KIS promotes transcription elongation, facilitates the binding of the trithorax group histone methyltransferases ASH1 and TRX to active genes, and counteracts repressive methylation of histone H3 on lysine 27 (H3K27) by Polycomb group proteins. This study sought to clarify the mechanism of action of KIS and how it interacts with ASH1 to antagonize H3K27 methylation in Drosophila. Evidence is presented that KIS promotes transcription elongation and counteracts Polycomb group repression via distinct mechanisms. A chemical inhibitor of transcription elongation, DRB, had no effect on ASH1 recruitment or H3K27 methylation. Conversely, loss of ASH1 function had no effect on transcription elongation. Mutations in kis cause a global reduction in the di- and tri-methylation of histone H3 on lysine 36 (H3K36) - modifications that antagonize H3K27 methylation in vitro. Furthermore, loss of ASH1 significantly decreases H3K36 dimethylation, providing further evidence that ASH1 is an H3K36 dimethylase in vivo. These and other findings suggest that KIS antagonizes Polycomb group repression by facilitating ASH1-dependent H3K36 dimethylation (Dorighi, 2013).

Since KIS promotes transcription elongation, promotes ASH1 binding and counteracts Polycomb repression, it is suspected that these activities might be functionally interdependent. However, the loss of ASH1 function leads to an increase in repressive H3K27 trimethylation without affecting transcription elongation. Furthermore, the treatment of salivary glands with the elongation inhibitor DRB did not affect the level of ASH1 or H3K27me3 associated with polytene chromosomes. It is therefore concluded that KIS promotes transcription elongation and antagonizes Polycomb repression via distinct mechanisms (Dorighi, 2013).

These findings suggest that the major mechanism by which KIS antagonizes Polycomb group repression is by promoting the association of the trithorax group histone methyltransferases ASH1 and TRX with chromatin. Recent biochemical studies have suggested several mechanisms by which ASH1 and TRX counteract Polycomb repression. A histone modification catalyzed by TRX in vitro (H3K4 trimethylation) disrupts interactions between PRC2 and its nucleosome substrate. H3K4me3 directly interferes with the binding of the PRC2 subunit NURF55 (CAF1) to nucleosomes and inhibits the catalytic activity of E(Z) allosterically through interactions with the SU(Z)12 subunit of PRC2. The relevance of this modification to TRX function in vivo is not clear, however, as the bulk of H3K4 trimethylation in Drosophila is catalyzed by the histone methyltransferase SET1. Another mechanism by which TRX counteracts Polycomb repression was suggested by its physical association with the histone acetyltransferase CBP in the TAC1 complex. The acetylation of H3K27 by CBP directly blocks the methylation of this residue by PRC2. It is therefore tempting to speculate that the diminished binding of TAC1 to active genes contributes to the increased methylation of H3K27me3 observed in kis mutants (Dorighi, 2013).

Other histone modifications, including both the di- and tri- methylation of H3K36, also block the catalytic activity of PRC2 in vitro. In Drosophila, H3K36 trimethylation is catalyzed by SET2, which associates with the elongating RNA Pol II via its phosphorylated CTD. In this way, H3K36me3 becomes concentrated at the 3' ends of genes where it plays a role in preventing cryptic initiation. Consistent with its role in transcription elongation, kis mutations decreased the level of H3K36me3 on polytene chromosomes. Interestingly, H3K36me3 blocks the methylation of H3K27 at genes expressed in the C. elegans germline. Thus, H3K36 trimethylation might represent a conserved mechanism for antagonizing PRC2 function to maintain appropriate patterns and steady-state levels of transcription. Transcription- dependent H3K36 trimethylation is unlikely to be the sole mechanism by which KIS counteracts Polycomb repression, however, because blocking transcription elongation with DRB did not increase the level of H3K27me3 on polytene chromosomes. Furthermore, ash1 mutants display elevated levels of H3K27 methylation without a reduction in H3K36 trimethylation or transcription elongation, suggesting that additional mechanisms exist to counteract repressive H3K27 methylation (Dorighi, 2013).

An antagonism between H3K36 dimethylation and H3K27 trimethylation was suggested by the recent discovery that H3K36me2 inhibits PRC2 function in vitro. This finding, together with recent evidence that ASH1 dimethylates H3K36 in vitro, prompted an investigation of whether ASH1 also dimethylates H3K36 in vivo. The chromosomal distributions of ASH1 and H3K36me2 overlap significantly, consistent with their localization at the 5' end of active genes. Furthermore, the levels of H3K36me2 on the polytene chromosomes of both ash1 and kis mutant larvae were significantly reduced, consistent with the role of KIS in promoting ASH1 binding. Taken together, these observations strongly suggest that KIS antagonizes Polycomb repression by promoting the ASH1-dependent dimethylation of H3K36 (Dorighi, 2013).

The differences in the chromosomal distributions of ASH1 and H3K36me2 and the residual H3K36me2 observed in ash1 mutants are probably due to the presence of another H3K36 dimethylase (MES-4) in Drosophila. In addition to dimethylating H3K36, MES-4 is required for SET2-dependent H3K36 trimethylation in vivo, as revealed by RNAi knockdown of MES-4 both in larvae and in cultured cells. By contrast, this study failed to observe a significant reduction in H3K36me3 levels in ash1 mutant larvae. The findings suggest that ASH1 and MES-4 play non-redundant roles in H3K36 methylation in vivo (Dorighi, 2013).

It is becoming increasingly clear that multiple mechanisms (including the trimethylation of H3K4, the di- and tri-methylation of H3K36 and the acetylation of H3K27) antagonize repressive H3K27 methylation catalyzed by Polycomb group proteins. The current findings suggest that KIS plays a central role in coordinating these activities. By facilitating the binding of TRX and ASH1, KIS promotes H3K27 acetylation and H3K36 dimethylation in the vicinity of active promoters. By stimulating elongation, KIS promotes H3K36 trimethylation over the body of transcribed genes. Thus, KIS appears to counteract Polycomb group repression by promoting multiple histone modifications that inhibit H3K27 methylation by the E(Z) subunit of PRC2 (Dorighi, 2013).

Haploinsufficiency for CHD7, a KIS homolog in humans, is the major cause of CHARGE syndrome, a serious developmental disorder affecting ~1 in 10,000 live births (Janssen, 2012). Infants born with CHARGE syndrome often have severe health complications due to defects in the development of tissues derived from the neural crest, including coloboma of the eye, cranial nerve abnormalities, ear defects and hearing loss, congenital heart defects, genital abnormalities and narrowing or blockage of the nasal passages. Based on the phenotypes associated with kis mutations in Drosophila, it seems likely that some of these defects may stem from changes in gene expression resulting from loss of transcription elongation and inappropriate gene silencing by Polycomb group proteins. The current findings suggest that changes in histone H3 modifications resulting from the loss of CHD7 function might contribute to the broad spectrum of developmental defects associated with CHARGE syndrome (Dorighi, 2013).

Noncoding RNAs of trithorax response elements recruit Drosophila Ash1 to Ultrabithorax

Alternative epigenetic chromatin states of polycomb target genes

Polycomb (PcG) regulation has been thought to produce stable long-term gene silencing. Genomic analyses in Drosophila and mammals, however, have shown that it targets many genes, which can switch state during development. Genetic evidence indicates that critical for the active state of PcG target genes are the histone methyltransferases Trithorax (Trx) and Ash1. This study has analyzed the repertoire of alternative states in which PcG target genes are found in different Drosophila cell lines and the role of PcG proteins Trx and Ash1 in controlling these states. Using extensive genome-wide chromatin immunoprecipitation analysis, RNAi knockdowns, and quantitative RT-PCR, it was shown that, in addition to the known repressed state, PcG targets can reside in a transcriptionally active state characterized by formation of an extended domain enriched in Ash1, the N-terminal, but not C-terminal moiety of Trx and H3K27ac. Ash1/Trx N-ter domains and transcription are not incompatible with repressive marks, sometimes resulting in a 'balanced' state modulated by both repressors and activators. Often however, loss of PcG repression results instead in a 'void' state, lacking transcription, H3K27ac, or binding of Trx or Ash1. It is concluded that PcG repression is dynamic, not static, and that the propensity of a target gene to switch states depends on relative levels of PcG, Trx, and activators. N-ter Trx plays a remarkable role that antagonizes PcG repression and preempts H3K27 methylation by acetylation. This role is distinct from that usually attributed to Trx/MLL proteins at the promoter. These results have important implications for Polycomb gene regulation, the 'bivalent' chromatin state of embryonic stem cells, and gene expression in development (Schwartz, 2010).

Key to the current understanding of PcG mechanisms is the fact that, while PcG proteins are present in most kinds of cells, the decision whether or not to repress a target gene depends crucially on whether that gene had been repressed in the previous cell cycle. This effect is responsible for the epigenetic maintenance of the repressed state and associated chromatin modifications. Similarly, through the action of Trx and Ash1, a PcG target gene that had not been repressed tends not to become repressed in the subsequent cell cycle and remains susceptible to transcriptional activators. By comparing PcG/TrxG and transcriptional landscapes in three lines of Drosophila cultured cells it was found that the full repertoire of chromatin states that PcG target genes can assume is not limited to the repressed state dominated by PcG mechanisms and the transcriptionally active state governed by TrxG proteins but in addition includes transcriptionally active 'balanced' states subjected to simultaneous or at least rapidly alternating control by both PcG and TrxG proteins, and a transcriptionally inactive 'void' state lacking both PcG and TrxG control. Thus, although PcG mechanisms first achieved fame for producing stable long-term silenced states in Drosophila homeotic genes, it is clear that, in the general case, PcG states are not necessarily stable nor long-term (Schwartz, 2010).

The results establish clearly that in robust PcG target regions (i.e. Class I PcG target regions) PcG and TrxG regulation are tightly coupled. Considering the role of MLL1 in the regulation of HOX genes and the similarity between PcG complexes in flies and mammals, it is expected that the same holds true for mammalian cells. It is possible that PcG and Trx recruitment to PRE/TREs share some DNA-binding proteins or DNA motifs. It will be important to determine whether PcG and Trx bind simultaneously or alternate over time. The nature of the Trx complex that binds to PRE/TREs remains enigmatic. To date the only Trx complex characterized biochemically is TAC1, purified from Drosophila embryos (Petruk, 2001). It is said to contain uncleaved full length Trx, anti-phosphatase Sbf1 and histone acetyltransferase dCBP. This study could detect no uncleaved Trx in the nuclei of cultured cells indicating that the Trx bound at PRE/TREs of repressed genes does not represent TAC1. Proteolytically cleaved human orthologs of Trx, MLL1 and MLL2 have been purified as part of complexes similar in composition to the yeast COMPASS4. Although the PRE/TRE binds both parts of the cleaved Trx, it lacks some COMPASS components and lacks H3K4 trimethylation, suggesting that it involves a different complex whose composition is yet to be characterized (Schwartz, 2010).

Consistent with genetic evidence, the presence of Ash1 and Trx at PcG target regions is linked to their transcriptional activity. However the two proteins show important differences in their behavior: binding of Ash1 is limited to transcriptionally active (fully derepressed or balanced) PcG targets and is not detected at completely repressed target loci. Trx is more complex. Both N-ter and C-ter parts of the protein associate with PREs regardless of the transcriptional status of their target genes and bind in the vicinity of TSS specifically when a target gene is transcriptionally active. In addition, the N-ter moiety of Trx together with Ash1 forms broad domains that encompass transcriptionally active PcG target genes. The different behavior of N-ter and C-ter parts of Trx may account for the discrepancy between reports of the co-localization of Trx and PcG proteins at many chromosomal sites and reports claiming that Trx binds exclusively to transcriptionally active target genes. The different accounts are due to the use of anti-Trx antibodies specific to different parts of the protein (Schwartz, 2010).

Whether C-ter or N-ter specific antibodies were used, the number of Trx bound regions detected in these experiments is small compared to the number of active genes. This argues against a general role for Trx in transcription and is consistent with the limited number of regions detected on polytene chromosomes by various antibodies directed against C-ter or N-ter Trx. In marked contrast to these observations, Schuettengruber (2009) has recently reported exclusive association of N-ter but not C-ter moiety of Trx with TSS of most active transcription units in the chromatin of embryonic cells. The same report also asserted that in embryonic cells Trx C-ter but not Trx N-ter is bound at PREs. Remarkably the Trx N-ter specific antibody used by Schuettengruber is reportedly the same as one of the two used in experiments from this lab. While the possibility cannot be excluded that the behavior of the N-ter moiety of Trx in embryos is totally different from that in cultured or salivary gland cells, it is suspected that more likely the preparation of the antibody used by Schuettengruber cross-reacted with some general transcription factor particularly abundant in embryonic cells. This emphasizes the importance, even the necessity, of using two or more independent antibodies to verify genome-wide ChIP results (Schwartz, 2010).

The RNAi knockdown experiments show that the broad binding of Ash1 and Trx N-ter within transcriptionally active PcG target regions is interdependent. This is consistent with the reported dissociation of Ash1 from polytene chromosomes of the salivary gland cells subjected to Trx RNAi and the severe reduction of Trx N-ter binding to polytene chromosomes of ash1 mutant larvae. Despite interdependence in binding there is no compelling evidence that Ash1 and Trx N-ter are in the same protein complex. Although an interaction between Trx and Ash1 has been reported, it was said to require the intact SET domain of Trx, which is absent from its N-ter moiety. This study has not found that Trx N-ter and Ash1 co-precipitate from nuclear extracts, strengthening the impression that the two peptides do not interact directly (Schwartz, 2010).

A histone mark associated with Ash1/Trx N-ter domains is H3K27ac. Acetylation of H3K27 can antagonize PcG activity by competing with the placement of the H3K27me3 mark, which in our model is needed for effective contact of the PRE complex with the promoter, as well as for stable PcG binding. In fact, targeting a histone H3 acetylase to a PRE is sufficient to prevent the epigenetic maintenance of repression indicates that in Drosophila the HAT responsible for bulk acetylation of H3K27 is CREB-binding protein (dCBP), encoded by the nejire gene. Direct association of both Trx and Ash1 with dCBP has been previously reported. Consistent with this, the Trx knock-down experiment, which also impairs Ash1 binding, shows that either or both proteins promote acetylation of K27 in the chromatin of active PcG targets. It also shows, however, that the level of H3K27ac is not directly related to the amount of Ash1 or Trx N-ter bound. It is possible that a small amount of Trx and/or Ash1 remaining on the chromosomes after RNAi depletion is sufficient to target enough HAT activity to maintain nearly normal levels of H3K27ac. Alternatively H3K27 acetylation may be produced by a process that is not mechanistically linked to Ash1 or Trx but is promoted by the two. A global reduction of immunostaining of polytene chromosomes with anti-H3K27ac antibodies and a global elevation of immunostaining with anti-H3K27me3 antibodies in trx mutant larvae was recently reported. The current study did not detect any global changes in either H3K27ac or H3K27me3 levels. It is possible that the effects are generally weak and could only be detected on polytene chromosomes that consist of thousands of chromatin fibers bundled together. It is noted, however, that the reported changes of H3K27 acetylation and trimethylation levels involved numerous chromosomal sites, most of which, according to current data, do not stably bind PcG or TrxG, suggesting that in these cases the effect of trx mutation may have been indirect (Schwartz, 2010).

The microarray data show that narrower peaks of H3K27ac are also found near numerous active promoters, at genes not known to be PcG targets. A role of H3K27ac at these promoters may be to antagonize dimethylation of H3K27, which is abundantly distributed throughout the genome and may have a general negative effect on transcription (Schwartz, 2010).

In Drosophila cultured cells most PcG target genes are either completely repressed or fully derepressed or entirely devoid of both PcG and TrxG regulation. However in about 5% of cases, exemplified by the Psc-Su(z)2 or inv-en loci, binding of PcG complexes does not result in complete transcriptional silencing and can coexist with binding of Ash1/Trx N-ter. Whether a PcG target gene is capable of and will assume a 'balanced' state may depend on the nature of the PRE, its binding complexes, and the promoter of the target gene. More likely, however, the major determinants are the nature and concentration of activators and repressors that act in concert with PcG/TrxG. Consistent with this idea, in imaginal disc cells, which are controlled by much more complex regulatory networks than cultured cells, simultaneous presence of both PcG and TrxG proteins at the transcriptionally active PcG target genes appears to be more common. Interestingly a common feature of the 'balanced' chromatin state in both cultured and imaginal disc cells is the confinement of Ash1/Trx N-ter binding to the regions immediately around the promoters. This is taken as a hint that the formation of a broad Ash1/Trx N-ter domain starts in the vicinity of the TSS (Schwartz, 2010).

Much has been said about the 'bivalent' state, i.e. containing both PcG repressive marks and transcriptional activity marks, as characteristic of genes in mammalian embryonic stem cells. In Drosophila, in cases such as those of the Psc-Su(z)2 or inv-en loci, the balanced action of PcG and TrxG results in chromatin states similar in appearance to the 'bivalent' state. It is supposed that, like the 'balanced' chromatin state of Drosophila PcG targets, the 'bivalent' domains of embryonic stem cells would be associated with both PcG proteins and mammalian orthologs of Trx and Ash1 (Schwartz, 2010).

PcG target genes may also assume a state lacking both PcG and TrxG proteins. The fact that in several instances the same locus resides in this 'void' chromatin state in cultured cells of completely different origin argues against it being a product of genomic aberrations. Several experimental observations also indicate that the 'void' state is not a peculiarity of cultured cells. Thus, in salivary glands the hh gene lacks PcG binding and H3K27me3 but remains transcriptionally inactive, as in D23 cells. More recently a comparison of embryonic and imaginal disc cells showed that in many cases lack of PC binding was not accompanied by transcriptional activity. The void chromatin state might be simply interpreted as a derepressed region that is transcriptionally inactive because the needed activator is absent. However, this does not explain why in this state Trx is also absent from the PRE, implying that neither PcG nor Trx binding is the default state of the PRE or that some specific condition prevents the recruitment of both. No other known repressive marks such as H3K9 methylation has been detected at these sites (Schwartz, 2010).

In line with the finding that the lack of PcG repression in the 'void' chromatin state does not automatically lead to the activation of a target gene is the observation that PC knock-down elicits a very specific genomic response. Remarkably Sex combs reduced (Scr), Antennapedia (Antp) and Abd-B, the HOX genes whose derepression in heterozygous +/Pc- flies gives the famous Polycomb phenotype, are also among the genes most sensitive to PC knockdown in BG3 cells. The sensitivity of these genes cannot be explained by intrinsically poor recruitment of PcG proteins as Abd-B and Scr are controlled by multiple strong PREs capable of robust repression when placed next to a reporter gene. It is supposed that the reason for the differential sensitivity to PC levels lies in the availability of the corresponding transcriptional activators. It is propose that in the BG3 cells the transcriptional activators of sensitive PcG target genes are present but at levels insufficient to override repression under normal conditions. The knockdown of PC lowers the threshold required for derepression. It is suggested that a general role of PcG and TrxG mechanisms is to modulate the constraints on the levels of transcriptional activators required to switch the expression of PcG target genes. This concept helps to explain why, despite the implication of the PcG system in the control of all morphogenetic pathways, the reduction of PcG levels during differentiation of mammalian cell lineages or tissue regeneration in flies results in the execution of very specific genomic programs (Schwartz, 2010).

Please note that the Sanchez-Elsner paper has been retracted (see retraction report)

Homeotic genes contain cis-regulatory trithorax response elements (TREs) that are targeted by epigenetic activators and transcribed in a tissue-specific manner. The transcripts of three TREs located in the Drosophila homeotic gene Ultrabithorax mediate transcription activation by recruiting the epigenetic regulator Ash1 to the template TREs. TRE transcription coincides with Ubx transcription and recruitment of Ash1 to TREs in Drosophila. The SET domain of Ash1 binds all three TRE transcripts, with each TRE transcript hybridizing with and recruiting Ash1 only to the corresponding TRE in chromatin. Transgenic transcription of TRE transcripts restores recruitment of Ash1 to Ubx TREs and restores Ubx expression in Drosophila cells and tissues that lack endogenous TRE transcripts. Small interfering RNA-induced degradation of TRE transcripts attenuates Ash1 recruitment to TREs and Ubx expression, which suggests that noncoding TRE transcripts play an important role in epigenetic activation of gene expression (Sanchez-Elsner, 2006).


cDNA clone length - 7053

Bases in 5' UTR - 548

Exons - 6

Bases in 3' UTR - 525


Amino Acids - 2144

Structural Domains

The primary translation product of the 7.5-kb ash1 transcript is predicted to be a basic protein of 2144 amino acids. The ASH1 protein contains a SET domain, a PHD finger and a BAH (Bromo adjacent homology) domain (see NCBI Conserved Domain Summary. These motifs are found in the products of some trithorax group and Polycomb group genes (Tripoulas, 1996).

absent, small, or homeotic discs 1: Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References

date revised: 22 March 2022

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