[Show abstract][Hide abstract] ABSTRACT: The mechanism of epigenetic inheritance following DNA replication may involve dissociation of chromosomal proteins from parental DNA and reassembly on daughter strands in a specific order. Here we investigated the behaviour of different types of chromosomal proteins using newly developed methods that allow assessment of the assembly of proteins during DNA replication. Unexpectedly, most chromatin-modifying proteins tested, including methylases, demethylases, acetyltransferases and a deacetylase, are found in close proximity to PCNA or associate with short nascent DNA. Histone modifications occur in a temporal order following DNA replication, mediated by complex activities of different enzymes. In contrast, components of several major nucleosome-remodelling complexes are dissociated from parental DNA, and are later recruited to nascent DNA following replication. Epigenetic inheritance of gene expression patterns may require many aspects of chromatin structure to remain in close proximity to the replication complex followed by reassembly on nascent DNA shortly after replication.
[Show abstract][Hide abstract] ABSTRACT: Propagation of gene-expression patterns through the cell cycle requires the existence of an epigenetic mark that re-establishes the chromatin architecture of the parental cell in the daughter cells. We devised assays to determine which potential epigenetic marks associate with epigenetic maintenance elements during DNA replication in Drosophila embryos. Histone H3 trimethylated at lysines 4 or 27 is present during transcription but, surprisingly, is replaced by nonmethylated H3 following DNA replication. Methylated H3 is detected on DNA only in nuclei not in S phase. In contrast, the TrxG and PcG proteins Trithorax and Enhancer-of-Zeste, which are H3K4 and H3K27 methylases, and Polycomb continuously associate with their response elements on the newly replicated DNA. We suggest that histone modification enzymes may re-establish the histone code on newly assembled unmethylated histones and thus may act as epigenetic marks.
[Show abstract][Hide abstract] ABSTRACT: The Drosophila ecdysone receptor (EcR/Usp) is thought to activate or repress gene transcription depending on the presence or absence, respectively, of the hormone ecdysone. Unexpectedly, we found an alternative mechanism at work in salivary glands during the ecdysone-dependent transition from larvae to pupae. In the absense of ecdysone, both ecdysone receptor subunits localize to the cytoplasm, and the heme-binding nuclear receptor E75A replaces EcR/Usp at common target sequences in several genes. During the larval-pupal transition, a switch from gene activation by EcR/Usp to gene repression by E75A is triggered by a decrease in ecdysone concentration and by direct repression of the EcR gene by E75A. Additional control is provided by developmentally timed modulation of E75A activity by NO, which inhibits recruitment of the corepressor SMRTER. These results suggest a mechanism for sequential modulation of gene expression during development by competing nuclear receptors and their effector molecules, ecdysone and NO.
[Show abstract][Hide abstract] ABSTRACT: There is growing awareness of the importance of noncoding (nc)RNAs in the regulation of gene expression during pattern formation in development. Spatial regulation of Hox gene expression in development controls positional identity along the antero-posterior axis. In this review, we will focus on the role of short ncRNAs that repress Hox genes in Drosophila and mammals by RNA interference (RNAi), on long ncRNAs that may repress a Hox in cis in Drosophila by transcriptional interference, and on a novel long ncRNA that functions in trans to regulate Hox genes mammals.
[Show abstract][Hide abstract] ABSTRACT: Polycomb group (PcG) and trithorax group (trxG) proteins act in an epigenetic fashion to maintain active and repressive states of expression of the Hox and other target genes by altering their chromatin structure. Genetically, mutations in trxG and PcG genes can antagonize each other's function, whereas mutations of genes within each group have synergistic effects. Here, we show in Drosophila that multiple trxG and PcG proteins act through the same or juxtaposed sequences in the maintenance element (ME) of the homeotic gene Ultrabithorax. Surprisingly, trxG or PcG proteins, but not both, associate in vivo in any one cell in a salivary gland with the ME of an activated or repressed Ultrabithorax transgene, respectively. Among several trxG and PcG proteins, only Ash1 and Asx require Trithorax in order to bind to their target genes. Together, our data argue that at the single-cell level, association of repressors and activators correlates with gene silencing and activation, respectively. There is, however, no overall synergism or antagonism between and within the trxG and PcG proteins and, instead, only subsets of trxG proteins act synergistically.
Development 08/2008; 135(14):2383-90. DOI:10.1242/dev.023275 · 6.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Much of the genome is transcribed into long untranslated RNAs, mostly of unknown function. Growing evidence suggests that transcription of sense and antisense untranslated RNAs in eukaryotes can repress a neighboring gene by a phenomenon termed transcriptional interference. Transcriptional interference by the untranslated RNA may prevent recruitment of the initiation complex or prevent transcriptional elongation. Recent work in yeast, mammals, and Drosophila highlights the diverse roles that untranslated RNAs play in development. Previously, untranslated RNAs of the bithorax complex of Drosophila were proposed to be required for its activation. Recent studies show that these untranslated RNAs in fact silence Ultrabithorax in early embryos, probably by transcriptional interference.
[Show abstract][Hide abstract] ABSTRACT: Much of the genome is transcribed into long noncoding RNAs (ncRNAs). Previous data suggested that bithoraxoid (bxd) ncRNAs of the Drosophila bithorax complex (BX-C) prevent silencing of Ultrabithorax (Ubx) and recruit activating proteins of the trithorax group (trxG) to their maintenance elements (MEs). We found that, surprisingly, Ubx and several bxd ncRNAs are expressed in nonoverlapping patterns in both embryos and imaginal discs, suggesting that transcription of these ncRNAs is associated with repression, not activation, of Ubx. Our data rule out siRNA or miRNA-based mechanisms for repression by bxd ncRNAs. Rather, ncRNA transcription itself, acting in cis, represses Ubx. The Trithorax complex TAC1 binds the Ubx coding region in nuclei expressing Ubx, and the bxd region in nuclei not expressing Ubx. We propose that TAC1 promotes the mosaic pattern of Ubx expression by facilitating transcriptional elongation of bxd ncRNAs, which represses Ubx transcription.
[Show abstract][Hide abstract] ABSTRACT: There is growing appreciation for the role of non-coding (nc) RNA in regulation of HOX genes of Drosophila. Our data suggest that current models for activation by ncRNA at the bithorax complex (BX-C) genes are mistaken. We propose that bxd and iab ncRNAs repress coding HOX genes Ultrabithorax and abdominal A, respectively, by transcriptional interference. It is not clear how regulation by non-coding RNAs is integrated with other regulatory mechanisms at HOX loci. We suggest that non-coding RNAs regulated by the trithorax group of epigenetic regulators have an early transient role in repression of HOX genes at the bithorax complex. Later, we propose that repression by HOX proteins, and members of the Polycomb group take over from repression by ncRNAs. We discuss emerging research questions in light of this model.
[Show abstract][Hide abstract] ABSTRACT: Rapid induction of the Drosophila melanogaster heat shock gene hsp70 is achieved through the binding of heat shock factor (HSF) to heat shock elements (HSEs) located upstream of the transcription start site (reviewed in ref. 3). The subsequent recruitment of several other factors, including Spt5, Spt6 and FACT, is believed to facilitate Pol II elongation through nucleosomes downstream of the start site. Here, we report a novel mechanism of heat shock gene regulation that involves modifications of nucleosomes by the TAC1 histone modification complex. After heat stress, TAC1 is recruited to several heat shock gene loci, where its components are required for high levels of gene expression. Recruitment of TAC1 to the 5'-coding region of hsp70 seems to involve the elongating Pol II complex. TAC1 has both histone H3 Lys 4-specific (H3-K4) methyltransferase (HMTase) activity and histone acetyltransferase activity through Trithorax (Trx) and CREB-binding protein (CBP), respectively. Consistently, TAC1 is required for methylation and acetylation of nucleosomal histones in the 5'-coding region of hsp70 after induction, suggesting an unexpected role for TAC1 during transcriptional elongation.
[Show abstract][Hide abstract] ABSTRACT: Steroid hormones fulfil important functions in animal development. In Drosophila, ecdysone triggers moulting and metamorphosis through its effects on gene expression. Ecdysone works by binding to a nuclear receptor, EcR, which heterodimerizes with the retinoid X receptor homologue Ultraspiracle. Both partners are required for binding to ligand or DNA. Like most DNA-binding transcription factors, nuclear receptors activate or repress gene expression by recruiting co-regulators, some of which function as chromatin-modifying complexes. For example, p160 class coactivators associate with histone acetyltransferases and arginine histone methyltransferases. The Trithorax-related gene of Drosophila encodes the SET domain protein TRR. Here we report that TRR is a histone methyltransferases capable of trimethylating lysine 4 of histone H3 (H3-K4). trr acts upstream of hedgehog (hh) in progression of the morphogenetic furrow, and is required for retinal differentiation. Mutations in trr interact in eye development with EcR, and EcR and TRR can be co-immunoprecipitated on ecdysone treatment. TRR, EcR and trimethylated H3-K4 are detected at the ecdysone-inducible promoters of hh and BR-C in cultured cells, and H3-K4 trimethylation at these promoters is decreased in embryos lacking a functional copy of trr. We propose that TRR functions as a coactivator of EcR by altering the chromatin structure at ecdysone-responsive promoters.
[Show abstract][Hide abstract] ABSTRACT: Trithorax (Trx) is a member of the trithorax group (trxG) of epigenetic regulators, which is required to maintain active states of Hox gene expression during development. We have purified from Drosophila embryos a trithorax acetylation complex (TAC1) that contains Trx, dCBP, and Sbf1. Like CBP, TAC1 acetylates core histones in nucleosomes, suggesting that this activity may be important for epigenetic maintenance of gene activity. dCBP and Sbf1 associate with specific sites on salivary gland polytene chromosomes, colocalizing with many Trx binding sites. One of these is the site of the Hox gene Ultrabithorax (Ubx). Mutations in either trx or the gene encoding dCBP reduce expression of the endogenous Ubx gene as well as of transgenes driven by the bxd regulatory region of Ubx. Thus Trx, dCBP, and Sbf1 are closely linked, physically and functionally, in the maintenance of Hox gene expression.
[Show abstract][Hide abstract] ABSTRACT: Trithorax (TRX) and ASH1 belong to the trithorax group (trxG) of transcriptional activator proteins, which maintains homeotic gene expression during Drosophila development. TRX and ASH1 are localized on chromosomes and share several homologous domains with other chromatin-associated proteins, including a highly conserved SET domain and PHD fingers. Based on genetic interactions between trx and ash1 and our previous observation that association of the TRX protein with polytene chromosomes is ash1 dependent, we investigated the possibility of a physical linkage between the two proteins. We found that the endogenous TRX and ASH1 proteins coimmunoprecipitate from embryonic extracts and colocalize on salivary gland polytene chromosomes. Furthermore, we demonstrated that TRX and ASH1 bind in vivo to a relatively small (4 kb) bxd subregion of the homeotic gene Ultrabithorax (Ubx), which contains several trx response elements. Analysis of the effects of ash1 mutations on the activity of this regulatory region indicates that it also contains ash1 response element(s). This suggests that ASH1 and TRX act on Ubx in relatively close proximity to each other. Finally, TRX and ASH1 appear to interact directly through their conserved SET domains, based on binding assays in vitro and in yeast and on coimmunoprecipitation assays with embryo extracts. Collectively, these results suggest that TRX and ASH1 are components that interact either within trxG protein complexes or between complexes that act in close proximity on regulatory DNA to maintain Ubx transcription.
[Show abstract][Hide abstract] ABSTRACT: In Drosophila, two classes of genes, the trithorax group and the Polycomb group, are required in concert to maintain gene expression by regulating chromatin structure. We have identified Trithorax protein (TRX) binding elements within the bithorax complex and have found that within the bxd/pbx regulatory region these elements are functionally relevant for normal expression patterns in embryos and confer TRX binding in vivo. TRX was localized to three closely situated sites within a 3-kb chromatin maintenance unit with a modular structure. Results of an in vivo analysis showed that these DNA fragments (each approximately 400 bp) contain both TRX- and Polycomb-group response elements (TREs and PREs) and that in the context of the endogenous Ultrabithorax gene, all of these elements are essential for proper maintenance of expression in embryos. Dissection of one of these maintenance modules showed that TRX- and Polycomb-group responsiveness is conferred by neighboring but separable DNA sequences, suggesting that independent protein complexes are formed at their respective response elements. Furthermore, we have found that the activity of this TRE requires a sequence (approximately 90 bp) which maps to within several tens of base pairs from the closest neighboring PRE and that the PRE activity in one of the elements may require a binding site for PHO, the protein product of the Polycomb-group gene pleiohomeotic. Our results show that long-range maintenance of Ultrabithorax expression requires a complex element composed of cooperating modules, each capable of interacting with both positive and negative chromatin regulators.