Three-step mechanism of promoter escape by RNA polymerase II

SUMMARY 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 (Pol) II and initiation factors are broken to enable promoter escape. Here we reconstitute Pol II promoter escape in vitro , determine high-resolution structures of five transition intermediates, and show that promoter escape occurs in three major steps. First, the growing RNA transcript displaces the B-reader element of the initiation factor TFIIB without evicting TFIIB. Second, rewinding of the upstream edge of the growing DNA bubble evicts TFIIA, TFIIB and TBP and repositions parts of TFIIE and TFIIF. Third, binding of DSIF and NELF evicts TFIIE and TFIIH, establishing the paused elongation complex. This three-step model for promoter escape fills a gap in our understanding of the initiation-elongation transition of Pol II transcription.