Figure 1.

Mutant phenotypes. (A) Compilation of RNAP mutants affecting Q activity. Substitutions shown in pairs only diminish Q activity as pairs, and are shown twice to position each bold substitution in the correct spatial group. Mutant classes as discussed in the text are grouped and indicated by background color. β-Gal activity is the average of percent wild-type (% WT) activity at each arabinose concentration. Phage spotting efficiency is relative to the same strain carrying wild-type β or β‘. A value of 4 indicates growth identical to wild type; 0 represents no plaque growth. In vitro Q activity is indicated by two values: the ratio of Q concentrations (WT/mutant) required for half saturation of readthrough activity (1/2^sat); and the fraction of wild-type readthrough obtained at the highest Q concentration (RT^max). (B) Maps of RpoB and RpoC with conserved segments in blue. The regions displaying essentially identical tertiary folds between T. aquaticus RNAP and yeast Pol II are shown beneath in green (Ebright 2000). The rifampicin-binding site is shown (Jin and Gross 1988); the active center is defined by hydroxyl radical cleavage mediated by an Fe²⁺ ion chelated in the active site (Mustaev et al. 1997). Substitutions isolated with the Q^λ reporter are at top (black), and those isolated with the Q⁸² reporter are below (gray); the dot color indicates the mutant class. DR, dispensable region.