Role of Q1235 in discrimination against c2'dNTPs and c3'dNTPs and in catalysis. (a) Kinetics of incorporation of saturating cGTP (1 mM) in ECG1, c2'dATP (4 mM) and c3'dATP (1 mM) in ECA by wild-type (WT; red squares) and Q1235A (violet circles) RNAPs. Kinetic discrimination against c2'dATP and c3'dATP was quantified as a ratio of the rate of cGTP incorporation to the rate of incorporation of corresponding erroneous substrate. (b) Q1235 does not participate in catalysis directly. Kinetics of saturating (1 mM) cGTP incorporation in ECG1 by R1239A/H1242A (orange circles) and Q1235A/R1239A/H1242A (cyan triangles) RNA polymerase. Compare to panel A (left plot) and to Figure 3e.