A Mutation of the RNA Polymerase β′ Subunit ( rpoC ) Confers Cephalosporin Resistance in Bacillus subtilis

In bacteria, mutations affecting the major catalytic subunits of RNA polymerase (encoded byrpoB andrpoC ) emerge in response to a variety of selective pressures. Here we isolated aBacillus subtilis strain with high-level resistance to cefuroxime (CEF). Whole-genome resequencing revealed only one missense mutation affecting an invariant residue in close proximity to the C-terminal DNA-binding domain of RpoC (G1122D). Genetic reconstruction experiments demonstrate that this substitution is sufficient to confer CEF resistance. The G1122D mutation leads to elevated expression of stress-responsive regulons, including those of extracytoplasmic function (ECF) σ factors (σM , σW , and σX ) and the general stress σ factor (σB ). The increased CEF resistance of therpoC G1122D strain is lost in thesigM rpoC G1122D double mutant, consistent with a major role for σM in CEF resistance. However, asigM mutant is very sensitive to CEF, and this sensitivity is still reduced by the G1122D mutation, suggesting that other regulatory effects are also important. Indeed, the ability of the G1122D mutation to increase CEF resistance is further reduced in a triple mutant strain lacking three ECF σ factors (σM , σW , and σX ), which are known from prior studies to control overlapping sets of genes. Collectively, our findings highlight the ability of mutations in RNA polymerase to confer antibiotic resistance by affecting the activity of alternative σ factors that control cell envelope stress-responsive regulons.