Structure of an RNA Polymerase II–TFIIB Complex and the Transcription Initiation Mechanism

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Abstract
Dissecting TFIIB Mechanics: Eukaryotic RNA polymerase II (Pol II) requires five protein cofactors for promoter recognition and initiation of transcription. The factor TFIIB is implicated in start site selection and stabilization of the initial transcript. The co-crystal structure of Pol II and TFIIB showed an N-terminal “finger” region located in the RNA exit channel, but the core C-terminal region of TFIIB was disordered. Now Liu et al. (p. 206 ; published online 12 November) present a structure of the same complex determined under different conditions in which the C-terminal structure is well localized but the finger is disordered. Docking DNA into the structure suggests that the C-terminal region stabilizes initial promoter melting. After transcription of a few bases, TFIIB probably switches to the alternate conformation where the C-terminal region is released and the finger region stabilizes the initial transcript.