Spt5 histone binding activity preserves chromatin during transcription by RNA polymerase II

Nucleosomes are disrupted transiently during eukaryotic transcription, yet the displaced histones must be retained and redeposited onto upstream DNA, to preserve nucleosome density and associated histone modifications. Here we show that the essential Spt5 processivity factor of RNA polymerase II (Pol II) plays a direct role in this process in budding yeast. Functional orthologues of eukaryotic Spt5 are present in archaea and bacteria, reflecting its universal role in RNA polymerase processivity. However, eukaryotic Spt5 is unique in having an acidic amino terminal tail (Spt5N) that is sandwiched between the downstream nucleosome and the upstream DNA that emerges from Pol II. We show that Spt5N contains a histone-binding motif that is required for viability in yeast cells and prevents loss of nucleosomal histones within actively transcribed regions. These findings indicate that eukaryotic Spt5 combines two essential activities, which together couple processive transcription to the efficient capture and re-deposition of nucleosomal histones.核小体（nucleosome）在真核转录过程中会发生暂时性解离，但解离后的组蛋白必须被保留并重新沉积至上游DNA区域，以维持核小体密度及其相关联的组蛋白修饰。本研究表明，RNA聚合酶II（RNA polymerase II，Pol II）的必需持续合成因子Spt5，在酿酒酵母的该过程中发挥直接调控作用。真核生物Spt5的功能同源物广泛存在于古菌与细菌中，这反映出其在RNA聚合酶持续合成能力维持方面具有通用性功能。然而，真核Spt5拥有独特的酸性氨基末端尾部（Spt5N），该结构域恰好夹在下游核小体与从RNA聚合酶II中伸出的上游DNA之间。我们证实，Spt5N包含一个组蛋白结合基序，该基序对酵母细胞的存活性至关重要，且可阻止活跃转录区域内的核小体组蛋白丢失。上述研究结果表明，真核Spt5兼具两项核心功能，二者协同将持续转录过程与核小体组蛋白的高效捕获及重新沉积过程相偶联。