The Bradyrhizobium japonicum fsrB gene is essential for utilization of structurally diverse ferric siderophores to fulfill its nutritional iron requirement

In Bradyrhizobium japonicum, iron uptake from ferric siderophores involves selective outer membrane proteins and non-selective periplasmic and cytoplasmic membrane components that accommodate numerous structurally diverse siderophores. Free iron traverses the cytoplasmic membrane through the ferrous (Fe²⁺) transporter system FeoAB, but the other non-selective components have not been described. Here, we identify fsrB as an iron-regulated gene required for growth on iron chelates of catecholate- and hydroxymate-type siderophores, but not on inorganic iron. Utilization of the non-physiological iron chelator EDDHA as an iron source was also dependent on fsrB. Uptake activities of ⁵⁵Fe³⁺ bound to ferrioxamine B, ferrichrome or enterobactin were severely diminished in the fsrB mutant compared with the wild type. Growth of the fsrB or feoB strains on ferrichrome were rescued with plasmid-borne E. coli fhuCDB ferrichrome transport genes, suggesting that FsrB activity occurs in the periplasm rather than the cytoplasm. Whole cells of an fsrB mutant are defective in ferric reductase activity. Both whole cells and spheroplasts catalyzed the demetallation of ferric siderophores that were defective in an fsrB mutant. Collectively, the data support a model whereby FsrB is required for reduction of iron and its dissociation from the siderophore in the periplasm, followed by transport of the ferrous ion into the cytoplasm by FeoAB.