Deletion of one of two Escherichia coli genes encoding putative Na+/H+ exchangers (ycgO) perturbs cytoplasmic alkali cation balance at low osmolarity

Two genes in the Escherichia coli genome, b4065 (yjcE) and b1191 (ycgO), are similar to genes encoding eukaryotic Na+/H+ exchangers. Mutants were constructed in which yjcE (GRN11), ycgO (GRF55) or both (GRD22) were inactivated. There was no change in respiration-driven Na+ efflux in any of the mutants when grown in media containing 50-500 mM Na+. The only striking finding was that growth of GRF55 was impaired at low osmolarity. In complex low-salt medium, GRF55 grew at a wild-type rate for three to four generations but then stopped; the growth was partially recovered after a pause, the length of which was dependent on salt concentration. Measurement of cytoplasmic alkali cations showed that an abrupt loss of about one-half of the intracellular K+ preceded the pause. When grown in low-salt medium with only 20 mM added Na+, GRF55 also lost the ability to maintain a sodium concentration gradient. However, this phenomenon appears to be a secondary effect of the ycgO deletion. The double mutant GRD22 has the same properties as GRF55; no additional effect was found. The data indicate that neither ycgO nor yjeE participates in respiration-driven Na+ extrusion. Instead, ycgO is required for growth at low osmolarity. Hence it is concluded that ycgO participates in cell volume regulation, and accordingly it is suggested that ycgO be renamed cvrA.