PhoP-regulated transcription is redox-sensitive, being activated when the periplasm becomes more reducing. MgrB acts between DsbA/DsbB and PhoP/PhoQ in this pathway. Represses PhoP/PhoQ signaling, possibly by binding to the periplasmic domain of PhoQ, altering its activity and that of downstream effector PhoP. May form homooligomers. Probably interacts with the periplasmic domain of PhoQ. Belongs to the MgrB family. Extended N-terminus.