SRA

The established hierarchical model explaining co-occupancy of Polycomb repressor complexes 1 and 2 (PRC 1 and 2) at target loci proposes that the chromodomain of the polycomb protein, a core PRC1 subunit, recognises the H3K27me3 histone modification catalysed by PRC2. We used chromatin immunoprecipitation to analyse PRC1 occupancy at target loci in Eed-/- mouse embryonic stem cells (ESCs) that lack H3K27me3. Occupancy of the core PRC1 proteins Ring1B and Mel18 was strongly reduced, consistent with the hierarchical model. However, levels of H2A ubiquitylation (H2AK119u1), the histone modification catalysed by PRC1, were similar to wild-type cells, suggesting PRC1 recruitment is independent of H3K27me3. ChIP-sequencing analysis of Ring1B occupancy genome wide substantiated this conclusion, demonstrating significant Ring1B levels at polycomb target loci in Eed-/- ESCs. Thus PRC1 and PRC2 are recruited independently to sites that they co-occupy. We conclude that the primary function of H3K27me3 is to increase the residency of PRC1 at target loci and thereby to contribute to the stability of PRC1 mediated silencing. Overall design: Examination of Ring1B binding in WT, Eed ko and Input of ESCs Examination of CBX7 in WT and Eed ko of ESCs