The Prefoldin Complex Regulates Chromatin Dynamics during Transcription Elongation

Transcriptional elongation requires the concerted action of several factors that allow RNA polymerase II to advance through chromatin in a highly processive manner. In order to identify novel elongation factors, we performed systematic yeast genetic screening based on the GLAM (Gene Length-dependent Accumulation of mRNA) assay, which is used to detect defects in the expression of long transcription units. Apart from well-known transcription elongation factors, we identified mutants in the prefoldin complex subunits, which were among those that caused the most dramatic phenotype. We found that prefoldin, so far involved in the cytoplasmic co-translational assembly of protein complexes, is also present in the nucleus and that a subset of its subunits are recruited to chromatin in a transcription-dependent manner. Prefoldin influences RNA polymerase II the elongation rate in vivo and plays an especially important role in the transcription elongation of long genes and those whose promoter regions contain a canonical TATA box. Finally, we found a specific functional link between prefoldin and histone dynamics after nucleosome remodeling, which is consistent with the extensive network of genetic interactions between this factor and the machinery regulating chromatin function. This study establishes the involvement of prefoldin in transcription elongation, and supports a role for this complex in cotranscriptional histone eviction.

转录延伸（Transcriptional elongation）需要多种因子协同作用，使RNA聚合酶II（RNA polymerase II）以高度持续合成的方式穿过染色质。为鉴定新型转录延伸因子，我们基于GLAM（mRNA长度依赖性积累，Gene Length-dependent Accumulation of mRNA）测定法开展了系统性酵母遗传筛选，该方法用于检测长转录单位的表达缺陷。除已报道的经典转录延伸因子外，我们还鉴定出了前折叠素（prefoldin）复合物亚基的突变体，这类突变体是引发最为显著表型的突变体之一。我们发现，此前仅被认为参与蛋白质复合物胞质共翻译组装的前折叠素，同样定位于细胞核中，且其部分亚基会以转录依赖性的方式被招募至染色质。前折叠素可在体内影响RNA聚合酶II的延伸速率，并在长基因以及启动子区域含有经典TATA框的基因的转录延伸中发挥尤为关键的作用。最终，我们发现前折叠素与核小体重塑后的组蛋白动态变化之间存在特异性功能关联，这与该因子与调控染色质功能的分子机器之间广泛的遗传互作网络相符。本研究证实了前折叠素参与转录延伸过程，并支持该复合物在共转录组蛋白移除中发挥功能。