Brownian ratchet mechanism of translocation in T7 RNA polymerase facilitated by a post-translocation energy bias arising from the conformational change of the enzyme

T7 RNA polymerase can transcribe DNA to RNA by translocating along the DNA. Structural studies suggest that the pivoting rotation of the O helix in the fingers domain may drive the movement of the O helix C-terminal Tyr639 from pre- to post-translocation positions. In a series of all-atom molecular dynamics simulations, we show that the movement of Tyr639 is not tightly coupled to the rotation of the O helix, and that the two processes are only weakly dependent on each other. We also show that the internal potential of the enzyme itself generates a small difference in free energy (ΔE) between the post- and pre-translocation positions of Tyr639. The calculated value of ΔE is consistent with that obtained from single-molecule experimental data. These findings lend support to a model in which the translocation takes place via a Brownian ratchet mechanism, with the small free energy bias ΔE arising from the conformational change of the enzyme itself.