p300/CBP sustains Polycomb silencing by non-enzymatic functions (ChIP-seq on inhibitor-treated mouse ES cells). p300/CBP sustains Polycomb silencing by non-enzymatic functions (ChIP-seq on inhibitor-treated mouse ES 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 SUZ12 (PRC2) by ChIP-seq in mouse embryonic stem (ES) cells treated with the p300/CBP inhibitor C646 or DMSO for 1 hour.

维持适当的细胞状态涉及表观遗传调控机制，其中包括多梳蛋白家族（Polycomb group, PcG）介导的转录抑制。尽管已知PcG蛋白可诱导染色质压缩，但PcG蛋白如何在致密染色质中结合DNA以实现长期基因沉默，目前仍知之甚少。本研究显示，在果蝇与小鼠细胞中，p300/CBP共激活因子（p300/CBP co-activator）与三分之二的PcG调控区域存在关联，且对多数此类区域的PcG蛋白占据是必需的。CBP可在PcG结合的抑制位点稳定RNA聚合酶II（RNA polymerase II, Pol II），并独立于其组蛋白乙酰转移酶活性促进Pol II暂停。CBP与Pol II暂停对多梳应答元件（Polycomb Response Elements, PREs）处的RNA-DNA杂交体（RNA-DNA hybrid, R-loop）形成以及核小体耗竭是必需的，而暂停区域之外的转录则无此必要性。上述结果表明，CBP共激活因子的非酶催化活性被重新利用以支持PcG介导的基因沉默，揭示了染色质调控因子之间的相互作用如何维持转录状态。总体实验设计：通过染色质免疫共沉淀测序（ChIP-seq）分析经p300/CBP抑制剂C646或二甲基亚砜（DMSO）处理1小时的小鼠胚胎干细胞（mouse embryonic stem cells, ES细胞）中SUZ12（PRC2）的基因组分布。