CheY and OmpR proteins are the response regulators of Escherichia coli for signal transductions in chemotaxis and osmoregulation. On the basis of their structural homology and the functional similarity, we constructed a chimeric protein in which N-terminal half of OmpR was substituted by CheY. This CheY-OmpR fusion protein, named Chp, inhibited the expression of ompC and ompF while OmpR, when overexpressed, activated the ompC expression. The degree of ompC inhibition by the Chp was modulated by chemotatic signal as tested in some chemotaxis mutants. This result suggests that the modules in response regulator are functionally separable as well as interchangable even though the mode of informational coupling between the two domains could be altered. The Chp construct, obtained here, may provide a new tool for the study of bacterial chemotaxis.