During growth under
iron limitation, Bacillus cereus and Bacillus anthracis, two human pathogens from the Bacillus cereus group of Gram-positive bacteria, secrete two
siderophores,
bacillibactin (BB) and
petrobactin (PB), for
iron acquisition via membrane-associated substrate-
binding proteins (SBPs) and other
ABC transporter components. Since PB is associated with virulence traits in B. anthracis, the PB-mediated
iron uptake system presents a potential target for antimicrobial
therapies; its characterization in B. cereus is described here. Separate transporters for BB, PB, and several xenosiderophores are suggested by (55)Fe-siderophore uptake studies. The PB precursor,
3,4-dihydroxybenzoic acid (3,4-DHB), and the photoproduct of FePB (FePB(nu)) also mediate
iron delivery into
iron-deprived cells. Putative SBPs were recombinantly expressed, and their
ligand specificity and binding affinity were assessed using fluorescence spectroscopy. The noncovalent complexes of the SBPs with their respective
siderophores were characterized using ESI-MS. The differences between
solution phase behavior and gas phase measurements are indicative of noncovalent interactions between the
siderophores and the binding sites of their respective SBPs. These studies combined with bioinformatics sequence comparison identify SBPs from five putative transporters specific for BB and
enterobactin (FeuA), 3,4-DHB and PB (FatB), PB (FpuA),
schizokinen (YfiY), and
desferrioxamine and
ferrichrome (YxeB). The two PB receptors show different substrate ranges: FatB has the highest affinity for ferric 3,4-DHB,
iron-free PB, FePB, and FePB(nu), whereas FpuA is specific to only apo- and ferric PB. The biochemical characterization of these SBPs provides the first identification of the transporter candidates that most likely play a role in the B. cereus group pathogenicity.