Expression of the ompF and ompC genes coding for major outer membrane proteins OmpF and OmpC is regulated in opposite directions by medium osmolarity. Chimera genes were constructed by a reciprocal exchange of the promoter-signal sequence region between the two genes. The chimera gene construction was designed so that the proteins synthesized by these genes were essentially the same as the OmpC and OmpF proteins. Studies with the chimera genes demonstrated that the osmoregulation of the OmpF-OmpC synthesis was promoter dependent. They also showed that cells grew normally even when the osmoregulation took place in opposite directions. The effects of the ompR2 and envZ mutations, which suppress ompC and ompF expression, respectively, also became reversed. The reduced expression was still subject to the promoter-controlled osmoregulation. Based on these observations, the mechanism of regulation of the ompF-ompC gene expression and its physiological importance are discussed.