Mechanism of NTP Binding to the Active Site of T7 RNA Polymerase Revealed by Free-Energy Simulation.

In genetic transcription, molecular dynamic details and energetics of NTP binding to the active site of RNA polymerase (RNAP) are poorly understood. In this article, we investigated the NTP binding process in T7 RNAP using all-atom MD simulation combined with the umbrella sampling technique. Based on our simulations, a two-step mechanism was proposed to explain NTP binding: first, substrate NTP in aqueous solution, which carries a magnesium ion, diffuses through a secondary channel of RNAP to attain a pore region, where it undergoes conformational changes to give a correct orientation; next, the NTP establishes initial basepairing contacts with the template nucleoside (TN). Our free-energy calculations suggest that both steps are spontaneous. This mechanism can easily explain the problem of NTP binding with different orientations. Moreover, it is found that the nascent NTP:TN basepair is fragile and easily broken by thermal disturbance. Therefore, we speculate that the fingers domain will be triggered to close, so as to create a steady environment for the next chemical step. The observations from the work provide valuable information for comprehensively understanding the mechanism of the basic step in genetic transcription.