Represses PhoP/PhoQ signaling, possibly by binding to the periplasmic domain of PhoQ, altering its activity and that of downstream effector PhoP. PhoP-regulated transcription is redox-sensitive, being activated when the periplasm becomes more reducing (deletion of dsbA/dsbB, treatment with dithiothreitol). MgrB acts between DsbA/DsbB and PhoP/PhoQ in this pathway; the 2 periplasmic Cys residues of MgrB are required for its action on PhoQ, and thus PhoP. May form homooligomers. Probably interacts with the periplasmic domain of PhoQ. Induced by low extracellular levels of Mg(2+) via the PhoP/PhoQ two-component regulatory system (PubMed:10464230) (Probable). In exponential phase (at protein level) (PubMed:19121005). By dithiothreitol (PubMed:22267510). Induction of genes regulated by PhoP, not suppressed by a dsbA deletion or 50 uM CuSO(4). Belongs to the MgrB family.