Abstact

Iron is an essential cofactor for fundamental biological processes. However, Fe(III) is poorly soluble under aerobic conditions, limiting its bioavailability. To secure this essential nutrient, bacteria release high-affinity siderophores that capture environmental Fe(III) and are subsequently imported into the cell as ferric siderophore complexes. While biochemical studies have characterized siderophore uptake in Bacillus species, atomic-level mechanisms of recognition and coordination remain unclear. Here, we investigate the siderophore-binding protein FatB from Bacillus cereus and its interactions with its siderophore, petrobactin (PB), as well as with ferric petrobactin (FePB) and its ferric photoproduct (FePB(nu)). Crystal structures of apo- and ferric-ligand-bound FatB, supported by biophysical and mutational analyses, reveal that ferric-siderophore binding induces substantial domain closure of FatB. This conformational transition involves an extensive ~29-A reorganization of a flexible loop, which positions His252 alongside Tyr317 to directly coordinate the Fe(III) center in the FePB-FatB complex. This protein-derived coordination mode is maintained in the FePB(nu)-FatB complex, where a structured water network preserves interfacial complementarity and functional recognition. These findings provide a structural framework for siderophore recognition and iron acquisition and illustrate how active-site coordination and domain reorganization facilitate robust capture of chemically labile ligands, offering insights for antimicrobial development targeting bacterial iron uptake.