Evidence for siderophore-dependent iron acquisition in group B streptococcus

Mutagenesis of group B streptococcus (GBS) with TnphoZ, a transposon designed to identify secreted protein genes, identified the gene homologues fhuD and fhuG. The encoded proteins participate in siderophore (hydroxamate)-dependent iron(III) transport in other bacterial species. Sequence analysis of the genome determined that fhuD and fhuG are members of a polycistronic operon comprised of four genes, fhuCDBG, that encode a putative ATPase, cell surface receptor and two transmembrane proteins respectively. We hypothesized that FhuD was a siderophore receptor. Western analysis of cell extracts localized FhuD to the bacterial cell membrane. Fluorescence quenching experiments determined that purified FhuD bound hydroxamate-type siderophores. FhuD displayed highest affinity for iron(III)-desferroxamine, with a K-D (mu M) = 0.05, identical to that described for FhuD2 from Staphylococcus aureus. The role of Fhu in siderophore-iron transport was also characterized. A fhu mutant, ACFhu1, was equally sensitive to the iron-dependent antibiotic streptonigrin as the wild-type strain, suggesting that ACFhu1 was not reduced for intracellular iron concentrations in the absence of exogenous siderophore. However, ACFhu1 transported significantly less siderophore-bound iron in Fe-55 accumulation assays. These data provide the first evidence of siderophore-mediated iron acquisition by GBS.