Histone Exchange is Associated with Activator Function at Transcribed Promoters and with Repression at Histone Loci. Histone Exchange is Associated with Activator Function at Transcribed Promoters and with Repression at Histone Loci

Transcription in eukaryotes correlates with major chromatin changes, including the replacement of old nucleosomal histones by new histones at the promoters of genes. The role of these histone exchange events in transcription remains unclear. In particular, the causal relationship between histone exchange and activator binding, preinitiation complex (PIC) assembly, and/or subsequent transcription remains unclear. Here, we provide evidence that histone exchange at gene promoters is not simply a consequence of PIC assembly or transcription but instead is mediated by activators. We further show that not all activators up-regulate gene expression by inducing histone turnover. Thus, histone exchange does not simply correlate with transcriptional activity, but instead reflects the mode of action of the activator. Last, we show that histone turnover is not only associated with activator function but also plays a role in transcriptional repression at the histone loci. Overall design: ChIP-seq of newly incorporated HA-tagged H3 and total H3 in G1-phase synchronized cells upon depletion of TBP

真核生物的转录过程与一系列关键染色质改变密切相关，具体包括在基因启动子区域将旧核小体组蛋白替换为新合成组蛋白的过程。这类组蛋白替换事件在转录过程中的具体作用仍不明确。具体而言，组蛋白替换与激活因子结合、预起始复合物（preinitiation complex, PIC）组装以及后续转录之间的因果关系尚未厘清。本研究提供实验证据表明，基因启动子处的组蛋白替换并非仅仅是预起始复合物组装或转录的结果，而是由激活因子介导的。我们进一步证实，并非所有激活因子都通过诱导组蛋白周转来上调基因表达。由此可见，组蛋白替换并非仅与转录活性相关，而是反映了激活因子的作用模式。最后，本研究还发现，组蛋白周转不仅与激活因子的功能相关，其在组蛋白基因位点的转录抑制过程中同样发挥了重要作用。整体实验设计：在TATA盒结合蛋白（TBP）敲低的G1期同步化细胞中，对新掺入的HA标签组蛋白H3以及总组蛋白H3进行染色质免疫共沉淀测序（ChIP-seq）。