Perturbation of the oxidizing environment of the periplasm stimulates the PhoQ/PhoP system in Escherichia coli.

The PhoQ/PhoP two-component system is repressed by divalent cations, such as Mg(2+) and Ca(2+), in the growth medium and stimulated by low pH and certain cationic antimicrobial peptides. In Escherichia coli, it was recently shown that the histidine kinase PhoQ is also modulated by at least two additional factors, the small membrane proteins SafA and MgrB. This raises the possibility that the PhoQ/PhoP circuit has additional regulatory components and integrates additional input signals. We screened E. coli transposon insertion mutants to look for proteins that modulate the activity of the PhoQ/PhoP system, and we uncovered a role for DsbA, a periplasmic oxidant that facilitates the formation of disulfide bonds. Deletion of dsbA or dsbB, which maintains a pool of oxidized DsbA, leads to increased transcription of at least two PhoP-regulated genes. Addition of the reducing agent dithiothreitol to wild-type cells had a similar effect, and treatment of a dsbA null strain with the oxidant Cu(2+) rescued the reporter gene expression phenotype. We also demonstrated that expression of an MgrB mutant that lacked cysteines blocked the effect of a dsbA null mutation on PhoQ/PhoP activity, suggesting that MgrB acts downstream of DsbA in this pathway. Taken together, these results demonstrate that a decrease in the oxidizing activity of the periplasm stimulates PhoQ/PhoP and may reveal a new input stimulus for this important two-component system.