p300/CBP Sustains Polycomb Silencing by Non-Enzymatic Functions

Maintenance of appropriate cell states involves epigenetic mechanisms, including Polycomb group (PcG)-mediated transcriptional repression. While PcG proteins are known to induce chromatin compaction, how PcG proteins gain access to DNA in compact chromatin to achieve long-term silencing is poorly understood. Here we show that the p300/CBP co-activator is associated with repressive chromatin regions and required for PcG occupancy in Drosophila and mouse cells. CBP stabilizes RNA polymerase II (Pol II) at PcG bound sites and promotes Pol II pausing independently of its histone acetyltransferase activity. CBP and Pol II pausing are necessary for RNA-DNA hybrid (R-loop) formation and nucleosome depletion at Polycomb Response Elements (PREs), whereas transcription beyond the pause region is not. These results suggest that non-enzymatic activities of the CBP co-activator have been repurposed to support PcG-mediated silencing, revealing how chromatin regulator interplay maintains transcriptional states.