Saccharomyces cerevisiae takes up siderophorebound iron through two distinct systems, one that requires siderophore transporters of the ARN family and one that requires the high affinity ferrous iron transporter on the plasma membrane. Uptake through the plasma membrane ferrous iron transporter requires that the iron first must dissociate from the siderophore and undergo reduction to the ferrous form. FRE1 and FRE2 encode cell surface metalloreductases that are required for reduction and uptake of free ferric iron. The yeast genome contains five additional FRE1 and FRE2 homologues, four of which are regulated by iron and the major iron-dependent transcription factor, Aft1p, but whose function remains unknown. Fre3p was required for the reduction and uptake of ferrioxamine B-iron and for growth on ferrioxamine B, ferrichrome, triacetylfusarinine C, and rhodotorulic acid in the absence of Fre1p and Fre2p. By indirect immunofluorescence, Fre3p was expressed on the plasma membrane in a pattern similar to that of Fet3p, a component of the high affinity ferrous transporter. Enterobactin, a catecholate siderophore, was not a substrate for Fre3p, and reductive uptake required either Fre1p or Fre2p. Fre4p could facilitate utilization of rhodotorulic acid-iron when the siderophore was present in higher concentrations. We propose that Fre3p and Fre4p are siderophore-iron reductases and that the apparent redundancy of the FRE genes confers the capacity to utilize iron from a variety of siderophore sources.Virtually every organism on earth requires iron as an essential nutrient. Although iron is the second most abundant metal in the crust of the earth, the bioavailability of iron can be extremely low. This poor bioavailability occurs because iron is rapidly oxidized in an aerobic environment to the ferric form (Fe(III)), 1 which is poorly soluble in water and forms precipitates of oxyhydroxides. Microorganisms have the capacity to scavenge iron from insoluble precipitates by secreting and taking up siderophores, low molecular weight compounds that bind to Fe(III) with very high affinity and specificity. Siderophores are synthesized and secreted in the iron-free form, which then binds and solubilizes Fe(III) in the extracellular environment. The Fe(III)-siderophore complex is then recognized and selectively taken up by specific transport mechanisms. Many microorganisms synthesize one or a few types of siderophores, yet have the capacity to take up iron from a variety of siderophores secreted by other species of bacteria and fungi (1). Budding and fission yeast appear to be an exception; they neither synthesize nor secrete these compounds (2, 3). Saccharomyces cerevisiae can, however, recognize and take up iron from a variety of structurally distinct siderophores (4 -10).S. cerevisiae has two genetically separable systems for the uptake of siderophore-bound iron. One system depends on a family of homologous transporters of the major facilitator superfamily that is expressed as part of the AFT1 regulon and are termed ARN1, ARN2...

The ferric siderophore transporters of the Gram-negative bacterial outer membrane manifest a unique architecture: Their N termini fold into a globular domain that lodges within, and physically obstructs, a transmembrane porin ␤-barrel formed by their C termini. We exchanged and deleted the N termini of two such siderophore receptors, FepA and FhuA, which recognize and transport ferric enterobactin and ferrichrome, respectively. The resultant chimeric proteins and empty ␤-barrels avidly bound appropriate ligands, including iron complexes, protein toxins, and viruses. Thus, the ability to recognize and discriminate these molecules fully originates in the transmembrane ␤-barrel domain. Both the hybrid and the deletion proteins also transported the ferric siderophore that they bound. The FepA constructs showed less transport activity than wild type receptor protein, but the FhuA constructs functioned with turnover numbers that were equivalent to wild type. The mutant proteins displayed the full range of transport functionalities, despite their aberrant or missing N termini, confirming (Braun, M., Killmann, H., and Braun, V. (1999) Mol. Microbiol. 33, 1037-1049) that the globular domain within the pore is dispensable to the siderophore internalization reaction, and when present, acts without specificity during solute uptake. These and other data suggest a transport process in which siderophore receptors undergo multiple conformational states that ultimately expel the N terminus from the channel concomitant with solute internalization.

We created hybrid proteins to study the functions of TonB. We first fused the portion of Escherichia coli tonB that encodes the C-terminal 69 amino acids (amino acids 170 to 239) of TonB downstream from E. coli malE (MalE-TonB69C). Production of MalE-TonB69C in tonB ؉ bacteria inhibited siderophore transport. After overexpression and purification of the fusion protein on an amylose column, we proteolytically released the TonB C terminus and characterized it. Fluorescence spectra positioned its sole tryptophan (W213) in a weakly polar site in the protein interior, shielded from quenchers. Affinity chromatography showed the binding of the TonB C-domain to other proteins: immobilized TonB-dependent (FepA and colicin B) and TonB-independent (FepA⌬3-17, OmpA, and lysozyme) proteins adsorbed MalE-TonB69C, revealing a general affinity of the C terminus for other proteins. Additional constructions fused full-length TonB upstream or downstream of green fluorescent protein (GFP). TonB-GFP constructs had partial functionality but no fluorescence; GFP-TonB fusion proteins were functional and fluorescent. The activity of the latter constructs, which localized GFP in the cytoplasm and TonB in the cell envelope, indicate that the TonB N terminus remains in the inner membrane during its biological function. Finally, sequence analyses revealed homology in the TonB C terminus to E. coli YcfS, a proline-rich protein that contains the lysin (LysM) peptidoglycan-binding motif. LysM structural mimicry occurs in two positions of the dimeric TonB C-domain, and experiments confirmed that it physically binds to the murein sacculus. Together, these findings infer that the TonB N terminus remains associated with the inner membrane, while the downstream region bridges the cell envelope from the affinity of the C terminus for peptidoglycan. This architecture suggests a membrane surveillance model of action, in which TonB finds occupied receptor proteins by surveying the underside of peptidoglycan-associated outer membrane proteins.Iron is one target of gram-negative bacterial cell envelope transport systems, and microbes elaborate high-affinity siderophores that complex extracellular iron (70). However, ferric siderophores, like ferric enterobactin (FeEnt), are too large (716 Da) to pass through general porins in the outer membrane (OM), necessitating a different type of transporter to acquire them. On the basis of their 22-stranded transmembrane ␤-barrels, OM metal transporters like FepA belong to the porin superfamily (89). Ligand binding to such receptors initiates the transport reaction through their transmembrane channels, which led to their designation as ligand-gated porins (LGP) (88), by analogy to the family of eukaryotic ligand-gated ion channels. It is noteworthy that LGP are mechanistically distinct from general, diffusive porins because they bind metal complexes with high affinity and actively transport them against a concentration gradient into the cell. Once in the periplasm, binding proteins adsorb ferric siderophores and del...