A bridging model for persistence of a polycomb group protein complex through DNA replication in vitro.

Epigenetic regulation may involve heritable chromatin states, but how chromatin features can be inherited through DNA replication is incompletely understood. We address this question using cell-free replication of chromatin. Previously, we showed that a Polycomb group complex, PRC1, remains continuously associated with chromatin through DNA replication. Here we investigate the mechanism of persistence. We find that a single PRC1 subunit, Posterior sex combs (PSC), can reconstitute persistence through DNA replication. PSC binds nucleosomes and self-interacts, bridging nucleosomes into a stable, oligomeric structure. Within these structures, individual PSC-chromatin contacts are dynamic. Stable association of PSC with chromatin, including through DNA replication, depends on PSC-PSC interactions. Our data suggest that labile individual PSC-chromatin contacts allow passage of the DNA replication machinery while PSC-PSC interactions prevent PSC from dissociating, allowing it to rebind to replicated chromatin. This mechanism may allow inheritance of chromatin proteins including PRC1 through DNA replication to maintain chromatin states.