p300/CBP sustains Polycomb silencing by non-enzymatic functions (ChIP-seq on RNAi-treated Drosophila S2 cells)

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 two-thirds of PcG-regions and required for PcG occupancy at many of these in Drosophila and mouse cells. CBP stabilizes RNA polymerase II (Pol II) at PcG-bound repressive 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. Overall design: Analysis of the genomic distribution of CBP and the PcGs E(z) (PRC2) and Pho (PhoRC) by ChIP-seq in Drosophila late embryonic Schneider 2 cells treated with double-stranded RNA (dsRNA) targeting CBP or GFP for consecutive 72 h treatments. Anti-E(z) was a kind gift of Richard Jones, Southern Methodist University, USA and anti-Pho was a kind gift of Judith Kassis, National Institute of Health, USA.