Polycomb group proteins: remembering how to catch chromatin during replication.

Polycomb group (PcG) proteins maintain the expression state of PcG-responsive genes during development of multicellular organisms. Recent observations suggest that "the H3K27me3 modification" acts to maintain Polycomb repressive complex (PRC) 2, the enzyme that creates this modification, on replicating chromatin. This could in turn promote propagation of H3K27me3 on newly replicated daughter chromatin, and promote recruitment of PRC1. Other work suggests that PRC1-class complexes can be maintained on replicating chromatin, at least in vitro, independently of H3K27me3. Thus, histone modifications and PcG proteins themselves may both be maintained through replication.