Mutants in Disulfide Bond Formation that Disrupt Flagellar Assembly in Escherichia coli

We report the isolation and characterization of Escherichia coli mutants (dsbB) that fail to assemble functional flagella unless cystine is present. Flagellar basal bodies obtained from these mutants are missing the L and P rings. This defect in assembly appears to result from an inability to form a disulfide bond in the P-ring protein (FlgI). Cystine suppresses this defect in dsbB strains. We also show that dsbA strains [Bardwell, J. C. A., McGovern, K. \& Beckwith, J. (1991) Cell 67, 581-589] fail to assemble P rings, apparently from a similar failure in disulfide bond formation. However, cystine does not completely suppress this defect in dsbA strains. Thus, disulfide bond formation in FlgI is essential for assembly. DsbA likely puts in that bond directly, whereas the DsbB product(s) play a role in oxidizing DsbA, so that it can be active.