The evolution of a Na+-sensitive Vibrio cholerae mutant unmasks the moonlighting aminopeptidase PepA as a regulator of nhaB Na+/H+ antiporter gene expression

The pathogenic bacterium Vibrio cholerae is native to seawater and can therefore be cultivated in nutrient media with increased salt concentration. To maintain osmotic balance, V. cholerae uses Na+/H+ antiporters and an Na+-translocating NADH:quinone oxidoreductase (Na+-NQR). The exact contribution of the various Na+/H+ antiporters to maintain Na+ and H+ homeostasis in V. cholerae is unclear. However, genetic studies indicate the major Na+/H+ antiporter NhaA and the Na+-NQR are required by the bacteria for Na+ resistance at alkaline conditions. Here we show that the growth defect of a V. cholerae nhaA nqr mutant at increased Na+ concentrations and alkaline pH is relieved by the rapid acquisition of suppressor mutations. The suppressor mutants could be assigned to two classes. We identified (i) mutations in the promoter of the nhaB gene encoding a Na+/H+ antiporter and (ii) mutations that either affect the expression level of the pepA gene or DNA binding activity of the encoded multifunctional aminopeptidase PepA. The characterization of the PnhaB and PpepA promoters of the suppressor mutants, membrane potential measurements and comparative proteome analyses identified PepA as a novel factor controlling Na+ homeostasis in V. cholerae. IMPORTANCEThe emergence and spread of multi-resistant bacteria are a major problem for humans and animals. Therefore, the identification of novel targets for the development of novel antibiotics to combat pathogenic bacteria is extremely important. Since sodium ion homeostasis is an essential process in many pathogenic bacteria, especially those living in marine environments, it represents an interesting target for antibiotics. The perturbation of sodium ion homeostasis does indeed impair the fitness of the human pathogenic bacterium Vibrio cholerae but can be restored by the rapid evolution of the bacteria. This suggests that inhibiting multiple targets with different antibiotics might be more effective in preventing the development of resistant bacteria.