Two genes in the genome, b4065 () and b1191 (), are similar to genes encoding eukaryotic Na/H exchangers. Mutants were constructed in which (GRN11), (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 deletion. The double mutant GRD22 has the same properties as GRF55; no additional effect was found. The data indicate that neither nor participates in respiration-driven Na extrusion. Instead, is required for growth at low osmolarity. Hence it is concluded that participates in ell olume egulation, and accordingly it is suggested that be renamed .


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