Transcription elongation of T7 RNAP and mechano–chemical coupling features. (a) The product structure of single subunit T7 RNAP elongation complex (in a surface representation: protein, white; NA, orange; the O-helix on the fingers domain, cyan; PPi, red; Lys472, blue, and the linked loop, light blue). The PPi release is found to be a jump-from-cavity process that is assisted by Lys472 side chain swing.^[42]The release does not appear to be tightly coupled to the O-helix or the fingers domain opening, thus, cannot drive the translocation. (b) A kinetic scheme of T7 elongation used in our recent modeling work,^[53]from NTP binding (pre-insertion) and insertion (the fingers domain or O-helix closing) to chemical reaction and product (PPi) release, followed by translocation. The translocation proceeds in Brownian movements, while the NTP binding serves for a pawl in the Brownian ratchet scenario to prevent backward movements. A small post-translocation free energy bias has been suggested to stabilize Y639 for the incoming nucleotide selection.^[53]