Role of Pleiohomeotic in Targeted Gene Silencing by Polycomb Group Proteins

Epigenetic regulation refers to effects on eukaryotic gene expression that are inherited through cell divisions. Research over the last decade has established a critical role for covalent chromatin modifications in the perpetuation of gene expression patterns. However, how specialized DNA sequence elements can bring a linked gene under epigenetic control has remained unclear. Drosophila Polycomb response elements (PREs) are cis-acting epigenetic DNA elements that maintain segment-specific silencing of linked enhancers in a PcG gene-dependent manner and function as chromosomal tethers for Polycomb group proteins (PcG) proteins. The mechanism by which they are targeted to PREs remains largely unclear. One of the major classes of PcG repressor complexes (PRCs) is PRC1. PRC1-type complexes harbor the core subunits Posterior sex combs (PSC), Polyhomeotic (PH), Polycomb (PC) and dRING1, and several other proteins. Because PREs function in transgenes, there is possibly a DNA sequence code that can impose PcG control. However, the nature of this PRE code is still elusive. Recent studies have shed more light in this area. Sequence-specific DNA-binding proteins have been implicated in recruiting PcG complexes to PREs. One of the key players is the Drosophila homologue of the mammalian transcription factor Yin Yang 1 (YY1), which is the DNA-binding protein Pleiohomeotic (PHO). PHO binding elements are essential in PcG silencing in vivo and PHO mutants result in PcG phenotypes in flies. In this thesis, we report on the role of PHO in targeted gene silencing by PcG proteins

dKDM2 couples histone H2A ubiquitylation to histone H3 demethylation during Polycomb group silencing

Transcription regulation involves enzyme-mediated changes in chromatin structure. Here, we describe a novel mode of histone crosstalk during gene silencing, in which histone H2A monoubiquitylation is coupled to the removal of histone H3 Lys 36 dimethylation (H3K36me2). This pathway was uncovered through the identification of dRING-associated factors (dRAF), a novel Polycomb group (PcG) silencing complex harboring the histone H2A ubiquitin ligase dRING, PSC and the F-box protein, and demethylase dKDM2. In vivo, dKDM2 shares many transcriptional targets with Polycomb and counteracts the histone methyltransferases TRX and ASH1. Importantly, cellular depletion and in vitro reconstitution assays revealed that dKDM2 not only mediates H3K36me2 demethylation but is also required for efficient H2A ubiquitylation by dRING/PSC. Thus, dRAF removes an active mark from histone H3 and adds a repressive one to H2A. These findings reveal coordinate trans-histone regulation by a PcG complex to mediate gene repression