PDF(Pubmed)
Abstract：
Mammalian RNA polymerase II preinitiation complexes assemble adjacent to a nucleosome whose proximal edge (NPE) is typically 40 to 50 bp downstream of the transcription start site. At active promoters, that +1 nucleosome is universally modified by trimethylation on lysine 4 of histone H3 (H3K4me3). The Pol II preinitiation complex only extends 35 bp beyond the transcription start site, but nucleosomal templates with an NPE at +51 are nearly inactive in vitro with promoters that lack a TATA element and thus depend on TFIID for promoter recognition. Significantly, this inhibition is relieved when the +1 nucleosome contains H3K4me3, which can interact with TFIID subunits. Here, we show that H3K4me3 templates with both TATA and TATA-less promoters are active with +35 NPEs when transcription is driven by TFIID. Templates with +20 NPE are also active but at reduced levels compared to +35 and +51 NPEs, consistent with a general inhibition of promoter function when the proximal nucleosome encroaches on the preinitiation complex. Remarkably, dinucleosome templates support transcription when H3K4me3 is only present in the distal nucleosome, suggesting that TFIID-H3K4me3 interaction does not require modification of the +1 nucleosome. Transcription reactions performed with an alternative protocol retaining most nuclear factors results primarily in early termination, with a minority of complexes successfully traversing the first nucleosome. In such reactions, the +1 nucleosome does not substantially affect the level of termination even with an NPE of +20, indicating that a nucleosome barrier is not a major driver of early termination by Pol II.
摘要：
哺乳动物RNA聚合酶II预起始复合物与核小体相邻组装，核小体的近端边缘（NPE）通常在转录起始位点（TSS）下游40-50bp。在活跃的启动子，组蛋白H3（H3K4me3）的赖氨酸4上的三甲基化普遍修饰了1个核小体。PolII预起始复合物仅延伸超过TSS35bp，但是NPE为+51的核小体模板在体外几乎无活性，启动子缺乏TATA元件，因此依赖于TFIID进行启动子识别。重要的是,当+1核小体含有H3K4me3时，这种抑制作用得到缓解，H3K4me3可以与TFIID亚基相互作用。在这里，我们显示，当转录由TFIID驱动时，具有TATA和TATA-less启动子的H3K4me3模板具有+35NPE的活性。+20NPE的模板也是活跃的，但与+35和+51NPE相比水平降低，与近端核小体侵占前兆复合物时启动子功能的一般抑制一致。值得注意的是,当H3K4me3仅存在于远端核小体中时，二核小体模板支持转录，这表明TFIID-H3K4me3相互作用不需要修饰+1核小体。用保留大多数核因子的替代方案进行的转录反应主要导致提前终止，少数复合物成功穿过第一个核小体。在此类反应中，即使NPE为20，1核小体也不会实质上影响终止水平，这表明核小体屏障不是PolII早期终止的主要驱动因素。
