The transition from transcription initiation to elongation is highly regulated in human cells but remains incompletely understood at the structural level. In particular, it is unclear how interactions between RNA polymerase II (RNA Pol II) and initiation factors are broken to enable promoter escape. Here, we reconstitute RNA Pol II promoter escape in vitro and determine high-resolution structures of initially transcribing complexes containing 8-, 10-, and 12-nt ordered RNAs and two elongation complexes containing 14-nt RNAs. We suggest that promoter escape occurs in three major steps. First, the growing RNA displaces the B-reader element of the initiation factor TFIIB without evicting TFIIB. Second, the rewinding of the transcription bubble coincides with the eviction of TFIIA, TFIIB, and TBP. Third, the binding of DSIF and NELF facilitates TFIIE and TFIIH dissociation, establishing the paused elongation complex. This three-step model for promoter escape fills a gap in our understanding of the initiation-elongation transition of RNA Pol II transcription.

从转录起始到延伸的转变在人类细胞中受到高度调控，但在结构水平上仍不完全了解。特别是，尚不清楚 RNA 聚合酶 II (RNA Pol II) 和起始因子之间的相互作用如何被破坏以使启动子逃脱。在这里，我们在体外重建了RNA Pol II启动子逃逸，并确定了包含8、10和12 nt有序RNA的初始转录复合物以及包含14 nt RNA的两个延伸复合物的高分辨率结构。我们认为启动子逃逸分三个主要步骤发生。首先，生长的 RNA 会取代起始因子 TFIIB 的 B 阅读器元件，但不会驱逐 TFIIB。其次，转录泡沫的倒带与 TFIIA、TFIIB 和 TBP 的驱逐同时发生。第三，DSIF 和 NELF 的结合促进 TFIIE 和 TFIIH 解离，建立暂停延伸复合物。这种启动子逃逸的三步模型填补了我们对 RNA Pol II 转录起始-延伸转变的理解空白。