In Escherichia coli, expression of the major outer membrane proteins, OmpC and OmpF, is regulated through the functions of OmpR and EnvZ at the transcriptional level in response to the medium osmolarity. OmpR is the crucial activator that helps RNA polymerase to efficiently trigger ompC and ompF transcription. This OmpR function is modulated by phosphorylation mediated by the cognate sensory kinase, EnvZ. Phosphorylation at the N-terminal domain of OmpR results in substantial enhancement of the DNA-binding ability of the C-terminal domain, thereby allowing the activation of ompC and ompF transcription by OmpR. Here we isolated an OmpR mutant which lacks the N-terminal half, but can enhance transcription in vivo. This novel type of OmpR mutant was revealed to have a single amino acid replacement of Gly227 to Cys. The newly-introduced-Cys residue allows OmpR molecules to form a stable dimer in vitro without the help of the N-terminal half. This altered C-terminal half is able to bind efficiently and specifically to the cognate DNA in vitro. It can function as an activator for ompC transcription in vitro in a phosphorylation-independent manner. These results suggest that the putative activator domain of OmpR, together with the DNA-binding domain, is most likely located in the C-terminal half. They also suggested that the phosphorylation of OmpR may not be essential for gene activation per se.