Hira-dependent histone H3.3 deposition facilitates PRC2 recruitment at developmental loci in ES cells.

Polycomb repressive complex 2 (PRC2) regulates gene expression during lineage specification through trimethylation of lysine 27 on histone H3 (H3K27me3). In Drosophila, polycomb binding sites are dynamic chromatin regions enriched with the histone variant H3.3. Here, we show that, in mouse embryonic stem cells (ESCs), H3.3 is required for proper establishment of H3K27me3 at the promoters of developmentally regulated genes. Upon H3.3 depletion, these promoters show reduced nucleosome turnover measured by deposition of de novo synthesized histones and reduced PRC2 occupancy. Further, we show H3.3-dependent interaction of PRC2 with the histone chaperone, Hira, and that Hira localization to chromatin requires H3.3. Our data demonstrate the importance of H3.3 in maintaining a chromatin landscape in ESCs that is important for proper gene regulation during differentiation. Moreover, our findings support the emerging notion that H3.3 has multiple functions in distinct genomic locations that are not always correlated with an "active" chromatin state.