VuuB and IutB reduce ferric-vulnibactin in Vibrio vulnificus M2799.

Vibrio vulnificus, a pathogenic bacterium that causes serious infections in humans, requires iron for growth. Clinical isolate, V. vulnificus M2799, secretes a catecholate siderophore, namely, vulnibactin, to capture iron (III) from the environment. Growth experiments using a deletion mutant indicated that VuuB, a member of the FAD-containing siderophore-interacting protein family, plays a crucial role in Fe3+-vulnibactin reduction. IutB, a member of the ferric-siderophore reductase family, stands a substitute for VuuB in its absence. It remained unclear why V. vulnificus M2799 has two proteins with relevant functions. Here we biochemically characterized VuuB and IutB using purified recombinant proteins. Purified VuuB, a flavoprotein, catalyzed the reduction of Fe3+-nitrilotriacetic acid as its electron acceptor, in the presence of NADH as its electron donor and FAD as its cofactor. IutB catalyzed the reduction of Fe3+-nitrilotriacetic acid, in the presence of NADH, NADPH, or reduced glutathione as its electron donor. The optimal pH values and temperatures of VuuB and IutB were 7.0 and 37 °C, and 8.5 and 45 °C, respectively. On analyzing their ferric-chelate reductase activities, both VuuB and IutB were found to catalyze the reduction of Fe3+-aerobactin, Fe3+-vibriobactin, and Fe3+-vulnibactin. When the biologically relevant substrate, Fe3+-vulnibactin, was used, the levels of ferric-chelate reductase activities were similar between VuuB and IutB. Finally, the mRNA levels were quantified by qRT-PCR in M2799 cells cultivated under low-iron conditions. The number of vuuB mRNA was 8.5 times greater than that of iutB. The expression ratio correlated with the growth of their mutants in the presence of vulnibactin.