Ferrous Iron Transport

Abstract

Transport systems for both ferric and ferrous iron have been identified in bacteria. Many bacteria produce siderophores to mobilize insoluble ferric iron from their surroundings. One of the first ferrous iron transport systems identified in bacteria, Feo, was found in Escherichia coli. The major transport protein FeoB contains a typical nucleotide-binding motif in the first 160 N-terminal amino acid residues, and there are at least 7 carboxy-terminal α-helices that embed the protein in the cytoplasmic membrane. Although there is evidence that the transport of ferrous iron is energized by ATP hydrolysis, the nucleotide-binding motif in FeoB has more similarities to the GTP-binding sites found in Ras proteins and elongation factors than to the ATP-binding sites in bacterial ABC transporters. Although it is clear that FeoB and the associated GTPase activity are required for ferrous iron transport, the mechanism of transport and precise function of FeoB are unknown. FeoB homologs are found in genomes of archaea and gram-positive and gram-negative bacteria (COGs, or phylogenetic classification of proteins encoded in complete genomes). It has been shown that FeoB-dependent ferrous iron transport in Synechocystis sp. strain PCC6803 is induced under iron-limiting growth conditions. Ferrous iron uptake is also important in the presence of oxygen since many bacteria can reduce extracellular ferric iron. The striking similarity of Feo to G proteins and the wide distribution of this protein make future research on the function of this protein in ferrous iron transport an exciting task.