pH dependence of the function of sodium ion extrusion systems in Escherichia coli

Escherichia coli has three systems for sodium ion extrusion, NhaA, NhaB and ChaA. In this study, we examined the effect of pH on the function of these transporters using mutants having one of them, and found that (1) a mutant having NhaB excreted sodium ions at pH 7.5 but not at pH 8.5, (2) the efflux of sodium ions from mutant cells having ChaA was observed at both pH 7.5 and 8.5, but the activity was lower at pH 7.5, and (3) sodium ions were excreted from mutant cells having NhaA at pH 6.5 to 8.5. The extrusion activity of cells having NhaA was higher than that of cells having NhaB or ChaA. These results indicate that NhaB functions at a pH below 8, and ChaA extrudes sodium ions mainly at an alkaline pH above 8. It was also suggested that the activity of NhaB and ChaA is not enough to maintain a low level of internal sodium ions when the external concentration of sodium ions is high, and NhaA is induced within a wide range of medium pH under such conditions.