Na⁺-induced transcription of nhaA, which encodes an Na⁺/H⁺ antiporter in Escherichia coli, is positively regulated by nhaR and affected by hns

nhaA encodes an Na⁺/H⁺ antiporter in Escherichia coli which is essential for adaptation to high salinity and alkaline pH in the presence of Na⁺. We used Northern (RNA) analysis to measure directly the cellular levels of nhaA mRNA. NhaR belongs to the LysR family of regulatory proteins. Consistent with our previous data with tin nhaA'-'lacZ fusion, NhaR was found to be a positive regulator and Na⁺ was found to be a specific inducer of nhaA transcription. In the nhaA'-'lacZ fusion, maximal induction was observed at alkaline pH. In contrast, in the nhaA⁺ strain both the level of nhaA expression and the induction ratio were lower at alkaline pH. This difference may be due to the activity of NhaA in the wild-type strain as NhaA efficiently excreted Na⁺ at alkaline pH and reduced the intracellular concentration of Na⁺, the signal for induction. We also showed that although the global regulator rpoS was not involved in nhaA regulation, the global regulator hns played a role. Thus, the expression of nhaA'-'lacZ was derepressed in strains bearing hns mutations and transformation with a low- copy-number plasmid carrying hns repressed expression and restored Na⁺ induction. The derepression in hns strains was nhaR independent. Most interestingly, multicopy nhaR, which in an hns⁺ background acted only as an Na⁺-dependent positive regulator, acted as a repressor in an hns strain in the absence of Na⁺ but was activated in the presence of the ion. Hence, an interplay between nhaR and hns in the regulation of nhaA was suggested.