Phage T7 RNA polymerase is a single-subunit transcription enzyme,transcribing template DNA to RNA.Nucleoside triphosphate (NTP) selection and translocation are two critical steps of the transcription elongation.Here,using all-atom molecular dynamics simulations,we found that between pre-and post-translocation states of T7 RNA polymerase an intermediate state exists,where the O helix C-terminal residue tyrosine 639,which plays important roles in translocation,locates between its pre-and post-translocation positions and the side chain of the next template DNA nucleotide has moved into the active site.NTP selection in this intermediate state was studied,revealing that the selection in the intermediate state can be achieved relying on the effect of Watson-Crick interaction between NTP and template DNA nucleotide,effect of stability of the components near the active site such as the nascent DNA-RNA hybrid and role of tyrosine 639.This indicates that another NTP-selection pathway can also exist besides the main pathway where NTP selection begins at the post-translocation state upon the entry of NTP.