There’s no substitute for good timing. It’s a trait that can keep you out of a dangerous situation, or make you look good on the dance floor. But for cells it’s also critical for maintaining epigenetic states during replication.
Italian researchers at the IRCCS Santa Lucia Foundation recently identified a critical time window right before replication, where high levels of Polycomb Group (PcG) binding and histone marks at Polycomb Response Element (PRE) target sites set the stage for proper transmission of epigenetic signatures to the next cellular generation. The scientists knew from previous work that PcG targets are bivalent sites, containing both active and repressive histone marks, but that during the S phase of the cell cycle, those bivalent marks risk being lost at the replication fork passage.
The proper inheritance of bivalent domains during S phase still remained unexplored, so the team began to study the H3K4me3 active mark at PREs through replication. Their data found an early S-phase spike in H3K4me3 just before replication-dependent dilution, which seems to be how the “bivalency” of PREs are protected.
The IRCCS scientists propose that PcG complex’s binding and enrichment of all histone marks that make PREs “bivalent” occurs simultaneously, and precedes PcG target replication, when epigenetic marks get redistributed on daughter strands. So the histone marks are all protected at the same critical time, which allows them to pass safely through the DNA replication process and be inherited in the next epigenome.
It’s the perfect time to read up on all the details at Cell Cycle, April 2012.