Figure 2.

A) Effects of the Cys replacements on MotA function. A motAB-deletion strain (RP6894) was transformed with plasmids encoding the indicated motA variants, and wild-type motB, and motility was assayed by rates of colony expansion on soft-agar plates. Downward-pointing arrows indicate double mutants that functioned more poorly than expected on the basis of the single-Cys phenotypes (i.e., synergistic defects). Upward-pointing arrows indicate three instances of mutational suppression in which the motility defect in the L214C mutant (in TM4) was rescued by a second Cys replacement in TM1 or TM2. B) Effects of TM1 and TM2 mutations on MotA function, plotted on helical wheels. Top: Results of Trp-scanning mutagenesis (28). The magnitude of the motility impairment at each position is indicated in grayscale. Arrows are vector sums of the impairments (pointing toward the side most sensitive to mutation). Middle: Effects of Cys replacements made for the present study. Bottom: Relative frequency of synergistic defects in TM1. Grayscale indicates the fraction of double mutants (involving that position) that gave a synergistic defect. (The analogous plot for TM2 is not shown because sampling in that case was sparse, owing to immotility of the G43C mutant.)