OmpR is the response regulator of a two-component regulatory system that controls the expression of the porin genes ompF and ompC in Escherichia coli. This regulator consists of two domains joined by a flexible linker region. The amino-terminal domain is phosphorylated by the sensor kinase EnvZ, and the carboxyl- terminal domain binds DNA via a winged helix-turn-helix motif. In vitro studies have shown that amino-terminal phosphorylation enhances the DNA binding affinity of OmpR and, conversely, that DNA binding by the carboxyl terminus increases OmpR phosphorylation. In the present work, we demonstrate that the linker region contributes to this communication between the two domains of OmpR. Changing the specific amino acid composition of the linker alters OmpR function, as does increasing or decreasing its length. Three linker mutants give rise to an OmpF+ OmpC- phenotype, but the defects are not due to a shared molecular mechanism. Currently, functional homology between response regulators is predicted based on similarities in the amino and carboxyl-terminal domains. The results presented here indicate that linker length and composition should also be considered. Furthermore, classification of response regulators in the same subfamily does not necessarily imply that they share a common response mechanism.
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