Mutations associated with rifamycin resistance hinder AM1 growth. A) Spectrum of mutations to the RNA polymerase beta subunit (RpoB) during past and current selection for rifamycin resistance (RifR). Modern AM1 was selected for RifR in 1984  and acquired two mutations to RpoB: Q521R, and Q1081R (denoted by an asterisk). By recapitulating selection for RifR in replicate Archival populations, we identified a number of other RpoB mutations putatively associated with RifR. B) The effect of RifR mutations on growth rate in the absence of antibiotic. Values represent the mean plus SEM of four biological replicates grown in 48-well plates with succinate. Strains that were significantly slower than Archival are marked with asterisks (p < 0.01, one-way ANOVA with Tukey post-hoc test).