Histone 3 Lys 27 trimethylation (H3K27me3)-mediated epigenetic silencing plays a critical role in multiple biological processes. However, the H3K27me3 recognition and transcriptional repression mechanisms are only partially understood. Here, we report a new mechanism for H3K27me3 recognition and transcriptional repression. Our structural and biochemical data showed that the BAH domain protein AIPP3 and the PHD proteins AIPP2 and PAIPP2 cooperate to read H3K27me3 and unmodified H3K4 histone marks, respectively, in Arabidopsis. The BAH-PHD bivalent histone reader complex silences a substantial subset of H3K27me3-enriched loci, including a number of development and stress response-related genes such as the RNA silencing effector gene ARGONAUTE 5 (AGO5) and We found that the BAH-PHD module associates with CPL2, a plant-specific Pol II carboxyl terminal domain (CTD) phosphatase, to form the BAH-PHD-CPL2 complex (BPC) for transcriptional repression. The BPC complex represses transcription through CPL2-mediated CTD dephosphorylation, thereby causing inhibition of Pol II release from the transcriptional start site. Our work reveals a mechanism coupling H3K27me3 recognition with transcriptional repression through the alteration of Pol II phosphorylation states, thereby contributing to our understanding of the mechanism of H3K27me3-dependent silencing.

中文翻译：

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H3K27me3识别与拟南芥中通过BAH-PHD-CPL2复合体的转录抑制的耦合

组蛋白3赖氨酸27三甲基化（H3K27me3）介导的表观遗传沉默在多个生物学过程中起着至关重要的作用。但是，H3K27me3识别和转录抑制机制只是部分了解。在这里，我们报告H3K27me3识别和转录抑制的新机制。我们的结构和生化数据表明，BAH域蛋白AIPP3和PHD蛋白AIPP2和PAIPP2协同阅读拟南芥中的H3K27me3和未修饰的H3K4组蛋白标记。BAH-PHD二价组蛋白阅读器复合物沉默了富含H3K27me3的基因座的子集，包括许多与发育和应激反应相关的基因，例如RNA沉默效应基因ARGONAUTE 5（AGO5），我们发现BAH-PHD模块与CPL2相关联，植物特异的Pol II羧基末端结构域（CTD）磷酸酶，形成BAH-PHD-CPL2复合物（BPC）用于转录抑制。BPC复合体通过CPL2介导的CTD去磷酸化抑制转录，从而抑制Pol II从转录起始位点释放。我们的工作揭示了通过改变Pol II磷酸化状态使H3K27me3识别与转录抑制偶联的机制，从而有助于我们对H3K27me3依赖性沉默机制的理解。