Cation/proton antiport systems in Escherichia coli. Absence of potassium/proton antiporter activity in a pH-sensitive mutant.

Alkaline-sensitive strains of Escherichia coli were selected by mutagenesis followed by penicillin killing in medium at pH 8.3. One such strain was found to lack K+/H+ exchange. At neutral pH, the doubling time of the mutant was not significantly different from that of the parent, but at pH 8.3 the doubling time of the mutant increased over 5-fold. No energy-dependent uptake of 204Tl+ was observed in the mutant, but addition of the K+/H+ exchanger Nigericin resulted in identical 204Tl+ uptake in the mutant and parent. These results are consistent with the hypothesis that the K+/H+ antiporter is responsible for regulation of cytosolic pH in E. coli. In the absence of antiporter, the organism would be unable to grow at alkaline pH due to alkalinization of the cytosol.