Expression of the Escherichia coli outer membrane proteins, OmpC and OmpF, is regulated in response to the medium osmolarity. The OmpR and EnvZ proteins are transcriptional factors involved in this osmotic regulation of the ompC and ompF genes. In particular, expression of the ompC gene is activated by the positive regulator, OmpR, in response to high osmolarity of the medium. In this study, we succeeded in defining a functional OmpR-binding sequence by analyzing a set of synthetic oligonucleotides, and propose a consensus motif for OmpR-binding. It was also demonstrated that the asymmetric OmpR-binding sequence, thus identified, can activate the canonical ompC promoter in an orientation independent-manner, providing that this sequence is placed closely and stereo-specifically with respect to the -35 region.