Copper stress causes an in vivo requirement for the Escherichia coli disulfide isomerase DsbC.

In Escherichia coli, the periplasmic disulfide oxidoreductase DsbA is thought to be a powerful but nonspecific oxidant, joining cysteines together the moment they enter the periplasm. DsbC, the primary disulfide isomerase, likely resolves incorrect disulfides. Given the reliance of protein function on correct disulfide bonds, it is surprising that no phenotype has been established for null mutations in dsbC. Here we demonstrate that mutations in the entire DsbC disulfide isomerization pathway cause an increased sensitivity to the redox-active metal copper. We find that copper catalyzes periplasmic disulfide bond formation under aerobic conditions and that copper catalyzes the formation of disulfide-bonded oligomers in vitro, which DsbC can resolve. Our data suggest that the copper sensitivity of dsbC- strains arises from the inability of the cell to rearrange copper-catalyzed non-native disulfides in the absence of functional DsbC. Absence of functional DsbA augments the deleterious effects of copper on a dsbC- strain, even though the dsbA- single mutant is unaffected by copper. This may indicate that DsbA successfully competes with copper and forms disulfide bonds more accurately than copper does. These findings lead us to a model in which DsbA may be significantly more accurate in disulfide oxidation than previously thought, and in which the primary role of DsbC may be to rearrange incorrect disulfide bonds that are formed during certain oxidative stresses.