The structural basis of the transition from initiation to elongation phases of transcription, as well as translocation and strand separation, by T7 RNA polymerase.

The RNA polymerase from phage T7 is a 99kDa single polypeptide that is unrelated to the multisubunit cellular RNA polymerases, but exhibits nearly all of their properties. Six separate crystal structures have enhanced our understanding of promoter DNA recognition, duplex DNA opening, and the transition from the abortive initiation phase to the elongation phase. A major conformational change in the N-terminal domain removes the promoter-binding site, accounting for promoter clearance, and creates a tunnel through which the transcript passes, accounting for the processivity of the elongation phase. Structures of substrate and product complexes show that a rotational conformational change of the fingers domain is associated with translocation and downstream strand separation. The rotation that results in translocation is powered by the release of the pyrophosphate product.