The mrp homolog gene cluster mrpCD1D2EFGAB (Ap-mrp) was found in a halotolerant cyanobacterium, Aphanothece halophytica, amplified, and expressed in Escherichia coli mutant TO114. Ap-mrp complemented the salt-sensitive phenotype of TO114 and exhibited Na(+)/H(+) and Li(+)/H(+) exchange activities, indicating that Ap-Mrp functions as a Na(+)/H(+) antiporter.

Hitherto, the roles of specific amino acid residues of ChaA, one of three Na(+)/H(+) antiporters in Escherichia coli, in exchange activity have not been reported. Here we examined the role of acidic amino acid residues, Glu-85 and Glu-325, on the hydrophobic transmembrane domains. It was found that ChaA is involved in salt tolerance at alkaline pH. Mutagenesis analyses revealed the importance of Glu-85, but not Glu-325, in the exchange activity.