A Double-Bromodomain Protein, FSH-S, Activates the Homeotic Gene Ultrabithorax through a Critical Promoter-Proximal Region

Abstract

More than a dozen trithorax group (trxG) proteins are involved in activation of Drosophila HOX genes. How they act coordinately to integrate signals from distantly located enhancers is not fully understood. The female sterile (1) homeotic (fs(1)h) gene is one of the trxG genes that is most critical for Ultrabithorax (Ubx) activation. We show that one of the two double-bromodomain proteins encoded by fs(1)h acts as an essential factor in the Ubx proximal promoter. First, overexpression of the small isoform FSH-S, but not the larger one, can induce ectopic expression of HOX genes and cause body malformation. Second, FSH-S can stimulate Ubx promoter in cultured cells through a critical proximal region in a bromodomain-dependent manner. Third, purified FSH-S can bind specifically to a motif within this region that was previously known as the ZESTE site. The physiological relevance of FSH-S is ascertained using transgenic embryos containing a modified Ubx proximal promoter and chromatin immunoprecipitation. In addition, we show that FSH-S is involved in phosphorylation of itself and other regulatory factors. We suggest that FSH-S acts as a critical component of a regulatory circuitry mediating long-range effects of distant enhancers.Drosophila HOX genes control development of body segments via highly restricted expression domains (45). These domains are first established by transiently expressed segmentation genes in early embryos and then maintained in an epigenetically heritable manner by the Polycomb group (PcG) of repressors, and the trithorax group (trxG) of activators (34,53). Like mammalian promoters that are regulated by distant elements, transcriptional regulation of HOX genes also requires coordinated long-range interactions between the basal transcription machinery assembled around the initiation sites and factors recruited at distant regulatory elements (48). How the epigenetic inheritance imposed by PcG and trxG is integrated into the general framework of such long-range interactions remains unclear. Its elucidation should provide an important model for understanding the regulatory mechanisms of genes under strict developmental control (53).PcG repressors form at least two types of multimeric complexes that are targeted by sequence-specific binding proteins to a core PcG response element located ϳ25 kb upstream of the homeotic gene Ultrabithorax (Ubx) (20,27). These complexes may block the access of the regulatory elements (58) or modify chromatin by associated histone deacetylase and histone methyltransferase activities (7,8,13,47,64).In contrast to the highly targeted activities of PcG repressors, trxG activators appear to employ diverse mechanisms for chromatin remodeling and long-range interactions. For example, trithorax (trx) and absent, small or homeotic discs 1 (ash1) encode histone methyltransferases (2, 6, 44, 67) that are targeted to PcG response elements, promoters, and transcribed regions (9,50,51,56). In addition to these targeted activities, brahma, moira, and osa encode subunits of an ATP...

Polycomb purification by in vivo biotinylation tagging reveals cohesin and Trithorax group proteins as interaction partners

“…Moreover, we identified a Pc-interacting protein called Fs(1)h that might, as well, contribute to recruitment of PRCs to chromatin. Fs(1)h interacts strongly with Ubx, trx, and ash1 mutations and leads to homeotic phenotypes when overexpressed (20). Fs(1)h is essential for development and conserved in mammals.…”

“…4A). Fs(1)h was previously reported to bind to Zeste motifs upstream of the Ubx gene supporting Ubx expression (20). Another protein with a putative DNA-binding domain is the histone acetyltransferase (HAT) Enoki mushroom (Enok) of the MYST family.…”

Epe1 recruits BET family bromodomain protein Bdf2 to establish heterochromatin boundaries

“…The fact that BET domain proteins Bdf2 in fission yeast and Bdf1 in budding yeast regulate heterochromatin spreading suggests that BET inhibitors may affect the spreading of heterochromatin and reset the epigenetic landscape in cancer cells. In other multicellular organisms such as zebrafish, worms, and flies, BET proteins are essential for the determination of cell fate during development (Huang and Dawid 1990;Chang et al 2007;Dibenedetto et al 2008;Shibata et al 2010). In particular, the Drosophila BET protein FSH (female sterile 1 homeotic) is classified as a Trithorax group gene that counteracts Polycomb group protein-mediated gene silencing (Chang et al 2007;Kockmann et al 2013).…”

“…In other multicellular organisms such as zebrafish, worms, and flies, BET proteins are essential for the determination of cell fate during development (Huang and Dawid 1990;Chang et al 2007;Dibenedetto et al 2008;Shibata et al 2010). In particular, the Drosophila BET protein FSH (female sterile 1 homeotic) is classified as a Trithorax group gene that counteracts Polycomb group protein-mediated gene silencing (Chang et al 2007;Kockmann et al 2013). Thus, it seems that the function of BET proteins in antagonizing gene silencing is highly conserved across species.…”

Combgap contributes to recruitment of Polycomb group proteins in Drosophila

“…Further, Pho-binding sites alone are not sufficient for PcG recruitment; rather, PREs consist of a collection of binding sites for many proteins (5). Other DNA-binding proteins implicated in PRE activity include Spps (Sp1 factor for pairing-sensitive silencing), GAF/psq (GAGA factor/pipsqueak), Zeste/fs(1)h [female sterile (1) homeotic], Grh (Grainyhead), Dsp1 (dorsal switch protein 1), and Adf1 (alcohol dehydrogenase transcription factor 1) (25)(26)(27)(28)(29). Genome-wide studies have shown that many of these proteins are present at only a subset of PREs, suggesting diversity in PcG protein-complex recruitment.…”