Characterization of dsbD mutants by Southern blots. SspI-digested chromosomal DNAs were resolved by 0.8% agarose gel. Lanes: 1) wild type strain NMB, 2) the incorrect mutant (dsbD*) obtained from the wild type strain, 3) the correct dsbD mutant created in the complemented strain, 4) dsbD* mutant of the dsbA1 strain, 5) dsbD* mutant of the dsbA2 strain, 6) dsbD* mutant of the dsbA2A3 strain, 7) dsbD mutant of the dsbA2A1 strain, 8) dsbD mutant of the dsbA2A1A3 strain. Removal of an 1.3-kb dsbD coding sequence and insertion of the 800-bp aphA3 cassette gave rise to a smaller fragment that is recognized by the probe made of the remaining 3′ dsbD sequence (A), but not by the probe hybridized to the deleted region (B). The dsbD* designation indicates incorrect dsbD clones that also carry a wild type copy of dsbD. An additional band in lane 3 of both panels represents the second copy of dsbD at the aspC-lctP locus.