FIG. 6.

Mutually exclusive binding of RNA polymerase to P_R and P_RM. Increasing concentrations of RNA polymerase were incubated with a DNA fragment containing wild-type P_R and P_RM (lanes 2 to 4), a fragment bearing a single mutation in the −35 region of P_R and P_RM that simultaneously decreases the strength of P_R and increases the strength of P_RM (lanes 5 to 7) (28), or a template bearing both the −35 mutation and a mutation in the −10 region of P_R (lanes 8 to 10; see Fig. 1 for sequences). The concentration of RNA polymerase was increased in threefold steps starting from 10 nM protein (lanes 2 and 8). RNA polymerase was added to the labeled DNA fragment incubated for 10 min at 37°C before addition of heparin and loading on the gel. RNAP, RNA polymerase; WT, wild type.