RNA polymerase II transcription: Structure and mechanism

► TFIIB positions promoter DNA above the RNA polymerase II cleft. ► TFIIB guides descent of the template DNA strand into the polymerase cleft. ► TFIIB stabilizes an early transcribing complex. ► The fidelity of transcription is enhanced by the “trigger loop”. ► The translocation step may entail bending of the “bridge helix”.