FIG 5.

Lon deficiency expands the potential for trimethoprim-resistant mutations at 3 hot spots of resistance in DHFR. (A and B) MIC₉₉ values of wild-type (A) and Δlon (B) E. coli strains harboring plasmids expressing wild-type DHFR or the indicated mutants at 3 resistance hot spots. Mutants that were at least 2-fold more resistant than the wild type are shown as black bars, while those that showed a wild type-like or hypersensitive phenotype are shown as gray bars. Mutants that conferred resistance only in Δlon E. coli are pointed out with arrows in panel B. Means ± SD of results from at least 3 measurements are plotted. (C) Mean fold differences in expression levels of plasmid-encoded wild-type or mutant DHFR (Δlon/wild type) are plotted along with SEM. Wild-type DHFR showed similar levels of expression on lon⁺ and Δlon backgrounds. Mutants that conferred resistance only in E. coli Δlon are shown as black bars. Statistical significance was tested using a Student's t test. *, P < 0.05.