Fig. 7. Modes of the action of RNAP active center. The schemes present the results of molecular modeling and proposed reaction mechanisms when coordination of Mg-II is effected in different modes. (A) The 3′–5′ exonuclease in the absence of additional stabilization of Mg-II. (B) The polymerase with Mg-II stabilized by the incoming NTP (shown in blue). (C) The 3′–5′ exonuclease with Mg-II stabilized by non-complementary NTP (red). (D) The pyrophosphorylase with stabilizing pyrophosphate bound in two alternative modes (red or blue). (E) The 3′–5′ exonuclease with Mg-II stabilized by D814 residue shift resulting from disrupted interaction with R1106.