J. Am. Chem. Soc., doi:10.1021/ja406609s

Boron is present at relatively high concentrations in the ocean and has been shown to influence expression of iron uptake genes from Marinobacter algicola: the majority of these genes were upregulated, but the gene encoding the periplasmic binding protein Mb-FbpA was substantially downregulated. As Mb-FbpA homologs can transport free Fe3+ when paired with a synergistic anion, Weerasinghe et al. suspected that this differential regulation might be explained if Mb-FbpA was using a borate anion in a similar manner. Structural alignments suggested the Fe3+-binding site in Mb-FbpA would have sufficient room to accommodate B(OH)4, as the protein mapped more closely to the homologous Mannheimia hemolytica protein—which has a C-terminal domain binding site—than the Neisseria gonorrheae protein, in which the binding site is buried. In vitro characterization confirmed that Mb-FbpA does bind a boron-containing compound, with a Kd comparable to environmental concentrations, but not at acidic pH values, consistent with the relevant species being borate. Analysis of Fe3+ binding in the presence and absence of borate or other anions demonstrated that boron increases the iron affinity of Mb-FbpA at comparable levels to citrate and carbonate, known biologically relevant anions. These results shed new light on a ubiquitous marine element, suggesting further functions may await discovery.