Fig. 4.

Substrate selection and mismatch recognition. (A) Long pause density (long pauses per 10 kb transcribed; errors as SEM) for the WT (blue) and E1103G mutant enzymes (red) at subsaturating (0.1 mM) NTPS under the conditions indicated. The long pause density increases for the mutant enzyme when a single NTP is present in excess, indicative of a deficiency in substrate selectivity. (B) The inclusion of 200 μM ITP in a saturating NTP buffer (1 mM NTPs) increases the long pause density for the WT (blue), but decreases the density for the E1103G mutant enzyme (red), indicative of a failure in mismatch recognition for ITP incorporation events.