Signal transduction and osmoregulation in Escherichia coli: a novel mutant of the positive regulator, OmpR, that functions in a phosphorylation-independent manner.

In Escherichia coli, expression of the outer membrane proteins, OmpF and OmpC, is regulated by the regulatory factors, EnvZ and OmpR, at the transcriptional level in response to the medium osmolarity. In this particular osmotic regulation, phosphorylation of OmpR at an aspartate residue (Asp-55) by EnvZ plays an important role. The previously isolated mutant, ompR55Q, with the amino acid replacement of Asp-55 to Gln, exhibits an OmpF- and OmpC- phenotype. In this study, we isolated a novel type of ompR mutant, in which the defect caused by the ompR55Q mutation is suppressed. The intragenic suppressor mutation we isolated results in the amino acid replacement of Tyr-102 to Cys in the N-terminal domain of OmpR, and exhibits an OmpF+ and OmpC+ phenotype in response to the medium osmolarity in an EnvZ-independent manner. It was revealed that this amino acid replacement in OmpR enhances the in vitro DNA-binding ability to the cognate DNAs. These results suggested that OmpR is capable of functioning in a phosphorylation-independent manner under certain in vivo conditions, and further suggested that an EnvZ-independent mechanism may also be involved in the osmotically regulated expression of ompF and ompC.