Bacteria Confiscate Unstable Iron-Citrate Complex

A unique bacterial protein selectively binds an unstable triferric citrate complex to import iron into Bacillus cereus cells, reports a team from the University of California, Berkeley (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1210131109). Iron is an essential element that bacteria commonly sequester by sending ligands, called siderophores, to chelate insoluble Fe³⁺ in the environment. Selective binding and transport proteins then convey the complexes back into the cells. Common siderophores include citrate ([COH(COO)(CH₂COO)₂]^3–) and citrate-based ligands. Citrate’s carboxyl and hydroxyl groups coordinate Fe³⁺. Previously, however, citrate-binding proteins were observed harboring only Fe(citrate)₂ and Fe₂(citrate)₂. Kenneth N. Raymond and colleagues have now identified a B. cereus protein, christened ferric citrate-binding protein C, or FctC, that selectively binds Fe₃(citrate)₃ even when other iron-citrate species are present in solution. Only a few closely related species have genes for similar proteins, so FctC may give the B. cereus group an advantage by enabling import of iron complexes that other bacteria cannot sequester.