NA-INDUCED TRANSCRIPTION OF NHAA, WHICH ENCODES AN NA+()H+ ANTIPORTERIN ESCHERICHIA-COLI, IS POSITIVELY REGULATED BY NHAR AND AFFECTED BY HNS/

nhaA encodes an Na+/H+ antiporter in Escherichia coli which is essenti al for adaptation to high salinity and alkaline pH in the presence of Na+, We used Northern (RNA) analysis to measure directly the cellular le,cls of nhaA mRNA. NhaR belongs to the LysR family of regulatory pro teins. Consistent with our previous data with an nhaA'-'lacZ fusion, N haR was found to be a positive regulator and Na+ was found to be a spe cific inducer of nhaA transcription. In the nhaA'-'lacZ fusion, maxima l induction was observed at alkaline pH, In contrast, in the nhaA(+) s train both the level of nhaA expression and the induction ratio were l ower at alkaline pH. This difference may be due to the activity of Nha A 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 ro le, Thus, the expression of nhaA'-'lacZ was derepressed in strains bea ring hns mutations and transformation with a low-copy-number plasmid c arrying hns repressed expression and restored Na+ induction, The derep ression in hns strains was nhaR independent, Most interestingly, multi copy nhaR, which in an hns(+) background acted only as an Nac-dependen t positive regulator, acted as a repressor in an hns strain in the abs ence of Na+ but was activated in the presence of the ion, Hence, an in terplay between nhaR and hns in the regulation of nhaA was suggested.