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

Helsinki Bioenergetics Group, Institute of Biotechnology, PO Box 56 (Viikinkaari 5), FIN-00014 University of Helsinki, Finland¹
Department of Biochemistry, University of Illinois at Urbana-Champaign, 600 South Mathews Street, Urbana, IL 61801, USA²

Author for correspondence: Marina L. Verkhovskaya. Tel: +358 9 191 58002. Fax: +358 9 191 58003. e-mail: Marina.Verkhovskaya{at}Helsinki.Fi

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.

Keywords: NHE-like proteins, Escherichia coli, osmosensitivity

Abbreviations: EIPA, N-ethyl-N-isopropylamiloride

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