Expression of sodium/proton antiporter NhaA at various pH values in Escherichia coli.

It was reported that NhaA, one of sodium/proton antiporters in Escherichia coli, was expressed at alkaline pH [J. Biol. Chem. 266 (1991) 21753]. In disagreement with their results, expression of an nhaA-lacZ fusion gene was found to be very low in an E. coli strain derived from MC4100 within the wide pH range from 5 to 9. When nhaB was deleted, the fusion gene was expressed at pH values below 8, while the expression was observed at alkaline pH after chaA was deleted. The internal level of sodium ions was increased by deletion of nhaA in strains deficient in nhaB and chaA at low and high pH values, respectively. These results suggested that nhaA is induced only when a low level of internal sodium ions is not kept by NhaB and ChaA. Strains used in the previous study may have low active ChaA.