Zhan, Y.; Abril-Garrido, J.; Grabbe, F.; Seweryn, P.; Neef, U.; Dienemann, C.; Cramer, P.

Transcription initiation by RNA polymerase II (Pol II) occurs next to a +1 nucleosome that is positioned downstream of the transcription start site (TSS). The +1 nucleosome has been shown to influence the pre-initiation complex (PIC) assembly and Pol II pausing, but it is unclear whether and how it functions in transcription initiation and the transition to elongation. Here, we investigate the transcription initiation-elongation transition in vitro using DNA templates containing a +1 nucleosome, and we present cryo-EM structures of five intermediate states. First, after PIC assembly, the +1 nucleosome evicts TFIID from the PIC upon binding of ATP to TFIIH. Second, after DNA opening, the +1 nucleosome stimulates TFIIH translocase activity and initial RNA synthesis. Finally, after DNA bubble rewinding, the +1 nucleosome removes TFIIH from the early elongation complex for promoter escape. Our findings show that the +1 nucleosome not only acts as a passive border for PIC assembly and a passive barrier for Pol II pausing, but rather has active functions during the initiation-elongation transition of transcription.