The transcriptional activator NhaR is responsible for the osmotic induction of osmC(p1), a promoter of the stress-inducible gene osmC in Escherichia coli.

Two overlapping promoters, osmC(p1) and osmC(p2), direct the transcription of the osmC gene of Escherichia coli. The proximal promoter, osmC(p2), is induced upon entry into stationary phase under the control of Esigma(s), the RNA polymerase that uses the sigma(s) (RpoS) sigma factor. Transcription from the distal promoter, osmC(p1), is independent of sigma(s). Previous analysis demonstrated that the osmolarity of the growth medium modulates expression of both promoters. The use of an E. coli genomic library showed that the cloned nhaR gene was able to stimulate transcription of an osmC-lac reporter fusion. NhaR is a positive regulator of the LysR family, previously identified as an activator of nhaA, a gene encoding a Na+/H+ antiporter involved in adaptation to Na+ and alkaline pH in E. coli and other enteric bacteria. NhaR was shown to activate only the expression of osmC(p1) and to be necessary for the induction of this promoter by LiCl, NaCl and sucrose. Therefore, activation by NhaR is responsible for the osmotic induction of osmC(p1). In contrast to its action on nhaA, NhaR activation of osmC(p1) is independent of H-NS. Activation of osmC(p1) by NhaR requires a site located just upstream of the atypical -35 region of the promoter.