Required for disulfide bond formation in some periplasmic proteins such as PhoA or OmpA. Acts by oxidizing the DsbA protein. 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. Induction of the PhoP/PhoQ two-component regulatory system. Belongs to the DsbB family. Extended N-terminus.