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

Abstract

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 an 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.

Chloride, a New Environmental Signal Molecule Involved in Gene Regulation in a Moderately Halophilic Bacterium,Halobacillus halophilus

“…Upon salt stress, Na ϩ enters the cell and activates expression of genes catalyzing energy-dependent efflux systems of cytotoxic Na ϩ . Intracellular Na ϩ binds to a transcriptional regulator which then activates expression of Na ϩ /H ϩ antiporter genes (6). Interestingly, optimal gene expression is achieved at internal Na ϩ concentrations of Ϸ20 mM (7, 23), a rather high concentration when compared with that of other ions involved in gene regulation, but this simply reflects the rather high intracellular and extracellular concentrations of Na ϩ required by microorganisms.…”

Transcription ofnhaA, the Main Na+/H+Antiporter ofEscherichia coli, Is Regulated by Na+and Growth Phase

“…The culture was divided in two, and incubation was continued for 2 h in the presence (induced) or absence (control) of 300 mM NaCl. These conditions were found to cause maximal induction of nhaA transcription in the wild-type strain (10). At the end of each experiment total RNA was isolated.…”

“…An interplay between NhaR and H-NS in the regulation of nhaA has been suggested. Deleting hns causes derepression of nhaA, and both repression and Na ϩ inducibility are restored upon transformation with plasmidic nhaR (10).…”

“…This system is specifically induced by intracellular Na ϩ , and neither osmolarity nor ionic strength affect nhaA transcription (10). Only one additional regulatory system that is induced specifically by Na ϩ is known but has not been studied intensively yet (20).…”

“…The pattern of regulation of nhaA during exponential growth, studied in a strain carrying an nhaAЈ-ЈlacZ fusion (16) and in the wild-type strain (10), reflects the importance of nhaA in salt adaptation. This system is specifically induced by intracellular Na ϩ , and neither osmolarity nor ionic strength affect nhaA transcription (10).…”