Polycomb Complexes and the Propagation of the Methylation Mark at the Drosophila Ubx Gene

Abstract

Polycomb group proteins are transcriptional repressors that control many developmental genes. The Polycomb group protein Enhancer of Zeste has been shown in vitro to methylate specifically lysine 27 and lysine 9 of histone H3 but the role of this modification in Polycomb silencing is unknown. We show that H3 trimethylated at lysine 27 is found on the entire Ubx gene silenced by Polycomb. However, Enhancer of Zeste and other Polycomb group proteins stay primarily localized at their response elements, which appear to be the least methylated parts of the silenced gene. Our results suggest that, contrary to the prevailing view, the Polycomb group proteins and methyltransferase complexes are recruited to the Polycomb response elements independently of histone methylation and then loop over to scan the entire region, methylating all accessible nucleosomes. We propose that the Polycomb chromodomain is required for the looping mechanism that spreads methylation over a broad domain, which in turn is required for the stability of the Polycomb group protein complex. Both the spread of methylation from the Polycomb response elements, and the silencing effect can be blocked by the gypsy insulator.

Comparing active and repressed expression states of genes controlled by the Polycomb/Trithorax group proteins

“…Our results for SF4 cells are consistent with the data obtained by Schwartz et al (28) who also used a Schneider cell derivative for ChIP studies (29). PRC1 binds to discrete sequence elements, whereas H3K27me3 covers large genomic domains, including genic and intergenic regions.…”

“…2). Previous studies revealed a central role of trimethylated H3K27 at PcG regulated genes (16,28,29). Supporting this data, we detected H3K27me3 in broad regions encompassing entire inactive transcription units such as the AbdB domain in Kc cells ( Fig.…”

In situ dissection of a Polycomb response element in Drosophila melanogaster

“…2B). The first conversion constructs targeted the DNA segments that showed the strongest activity in a transgene assay (23) and the highest levels of POLYCOMB cross-linking (15). This region contains clusters of binding sites for the pleiohomeotic protein (called PHO sites) and for the GAGA factor or the pipsqueak protein (called GAGA sites) (Fig.…”

“…PcG products act at the PREs in such constructs to repress or restrict expression of certain reporter genes (5)(6)(7)(8), and PRE transgenes can recruit the POLY-COMB protein to new chromosomal locations (6,(9)(10)(11). The binding of PcG proteins to PREs can also be assayed by formaldehyde cross-linking of nuclei from cultured cells, whole embryos, or imaginal discs (12)(13)(14)(15). These assays may define where PREs lie on the DNA sequence, but do not reveal how they function in their original context.…”

Association of cohesin and Nipped-B with transcriptionally active regions of the Drosophila melanogaster genome

“…This region is flanked on both sides by histone H3 lysine 27 trimethylation (H3K27Me3) domains associated with PcG silencing ( Fig. 7; Kahn et al 2006;Schwartz et al 2006). The lack of cohesin binding to the silent Ubx and abd-A genes suggested that cohesin might bind Abd-B only when it is expressed.…”