The Escherichia coli EnvZ-OmpR regulatory system is a paradigm of intracellular signal transduction mediated by the well-documented phosphotransfer mechanism, by which the expression of the major outer membrane proteins, OmpC and OmpF, is regulated in response to the medium osmolarity. Although it is clear that the EnvZ histidine(His)-kinase is the major player in the phosphorylation of OmpR, it has been assumed for some time that there may be an alternative phospho-donor(s) that can phosphorylate OmpR under certain in vitro and in vivo conditions. In this study, to address this long-standing issue, extensive genetic studies were done with certain mutant alleles, including ΔenvZ, Δ(ackA-pta), and ΔsixA, as well as ΔompR. Here, for the first time, genetic evidence is provided that, in addition to EnvZ, acetyl phosphate and an as yet unidentified sensor His-kinase can serve as alternative in vivo phospho-donors for OmpR, even in the envZ⁺ background. A model for the alternative phosphotransfer signaling pathway involved in the phosphorylation of OmpR is proposed.

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