Ability of MtbDsbA to recognize EcDsbB. (a) Restoration of E. coli motility. Constructs expressing MtbDsbA or EcDsbA (control) were transformed into E. coli DsbA null (JCB817) and DsbA/DsbB double null (JCB818) mutant cells. FlgI function is impaired in the absence of EcDsbA or EcDsbB owing to the absence of disulfide-bonding activity (Dailey & Berg, 1993). The ability to recognize EcDsbB and EcDsbA substrates in vivo by MtbDsbA was evaluated by restoration of E. coli motility in the agar, as seen in the induced EcDsbA control. Shown is the summary of three replicates of induced (containing arabinose) and uninduced bacterial swarming plates (not containing arabinose, as a negative control). See Supplementary Fig. S2 for bacterial plate images. (b) Ubiquinone reduction of EcDsbB–UQ1 by MtbDsbA. The data presented here are the normalized mean absorbance of UQ1 from three independent measurements. EcDsbB was added to the EcDsbA/UQ1 mixture to initiate the reaction.