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Transcription of the genes that encode the major outer membrane porin proteins OmpF and OmpC of Escherichia coli is regulated in response to changes in medium osmolarity by EnvZ and OmpR. EnvZ functions to sense environmental conditions and to relay this information to the DNA-binding protein OmpR. We have used a truncated EnvZ protein (EnvZ115), which is defective in sensory function but able to communicate with OmpR, to study the biochemical interactions between these two proteins and their effects on transcription from the ompF promoter. We show that purified EnvZ115 can phosphorylate OmpR in the presence of ATP. In addition, EnvZ115 stimulates the ability of OmpR to activate ompF transcription in vitro. Using antibodies specific for EnvZ, we have purified the wild-type protein and have shown that it is also an OmpR kinase. These results provide a prokaryotic example of a transmembrane …