Signal transduction and osmoregulation in Escherichia coli. A single amino acid change in the protein kinase, EnvZ, results in loss of its phosphorylation and dephosphorylation abilities with respect to the activator protein, OmpR

The EnvZ protein is a bacterial protein kinase, which specifically phosphorylates the activator protein, OmpR, involved in expression of the ompF and ompC genes in Escherichia coli. The phosphotransfer between the EnvZ and OmpR proteins was postulated to be involved in the signal transduction in response to an environmental osmotic stimulus. In this study, we isolated a novel type of envZ mutant, in which a base substitution resulted in a Tyr-to-Ser conversion at amino acid residue 351 of the EnvZ protein. This single amino acid conversion was found to dramatically affect the functions of the EnvZ protein. The mutant EnvZ protein was defective in its ability not only as to OmpR-phosphorylation but also as to OmpR-dephosphorylation. The envZ mutant, termed envZ30, was isolated as a pseudorevertant, which phenotypically suppresses an ompR3-type mutant exhibiting an OmpF- OmpC-constitutive phenotype. These results will be discussed in relation to the structure and function of the protein kinase, EnvZ.