MgrB and periplasmic redox balance contribute to negative feedback in the PhoQ/PhoP two-component system of Escherichia coli

The PhoQ/PhoP signaling system responds to low magnesium, low pH, and the presence of certain cationic antimicrobial peptides. It regulates genes important for growth under these conditions, as well as additional genes important for virulence in many gram-negative pathogens. PhoQ is a sensor kinase that phosphorylates and activates the transcription factor PhoP. Since feedback inhibition is a common theme in stress-response circuits, we hypothesized that some members of the PhoP-regulon might play such a role in the PhoQ/PhoP pathway. We therefore screened for PhoP-regulated genes that mediate feedback in this system. We found that deletion of mgrB, ( yobG), which encodes a 47 amino acid peptide, results in a potent increase in PhoP-regulated transcription. In addition, over-expression of mgrB decreased transcription at both high and low concentrations of magnesium. Localization and bacterial two-hybrid studies suggest that MgrB resides in the inner-membrane and interacts directly with PhoQ. We further show that MgrB homologs from Salmonella typhimurium and Yersinia pestis also repress PhoP-regulated transcription in these organisms.;In a global transposon mutagenesis screen to identify other factors that regulate the PhoQ/PhoP system, we identified the periplasmic protein disulfide oxidoreductase, DsbA. Deletion of dsbA or its recycling oxidoreductase, dsbB, leads to upregulation of PhoP-regulated genes. Addition of the oxidizing agent CuSO4 to dsbA- cells rescued the transcriptional phenotype, while addition of the reducing agent dithiothreitol to dsbA+ cells phenocopied deletion of dsbA. Interestingly, substitutions in MgrB of C28, C39, or both cysteines with alanines rendered the inhibitor ineffective. Furthermore, deletion of dsbA had no effect on a strain expressing an MgrB variant in which all three cysteines were replaced with alanines.;Our results indicate MgrB is a broadly conserved membrane peptide that is a key mediator of negative feedback in the PhoQ/PhoP circuit. We further demonstrate that the redox state of the periplasm is critical for the action of MgrB and for the proper regulation of PhoP-regulated transcription. We hypothesize that MgrB may function as a point of control that integrates additional input signals to modulate the activity of this important signaling system.