Recognition and transport of ferric enterobactin in Escherichia coli.

The specificity of the outer membrane protein receptor for ferric enterobactin transport in Escherichia coli and the mechanism of enterobactin-mediated transport of ferric ions across the outer membrane have been studied. Transport kinetic and inhibition studies with ferric enterobactin and synthetic structural analogs have mapped the parts of the molecule important for receptor binding. The ferric complex of the synthetic structural analog of enterobactin, 1,3,5-N,N',N'-tris-(2,3-dihydroxybenzoyl)triaminomethylbenzene (MECAM), was transported with the same maximum velocity as was ferric enterobactin. A double-label transport assay with [59Fe, 3H]MECAM showed that the ligand and the metal are transported across the outer membrane at an identical rate. Under the growth conditions used, large fractions of the transported complexes were available for exchange across the outer membrane when a large excess of extracellular complex was added to the cell suspension; at least 60% of the internalized [59Fe]enterobactin exchanged with extracellular [55Fe]enterobactin. Internalized [59Fe, 3H]MECAM was released from the cell as the intact complex when either unlabeled Fe-MECAM or Fe-enterobactin was added extracellularly. The results suggest a mechanism of active transport of unmodified coordination complex across the outer membrane with possible accumulation in the periplasm.     

Identification of alcaligin as the siderophore produced by Bordetella pertussis and B. bronchiseptica.

The siderophores produced by iron-starved Bordetella pertussis and B. bronchiseptica were purified and were found to be identical. Using mass spectrometry and proton nuclear magnetic resonance, we determined that the siderophore produced by these organisms was identical to alcaligin, a siderophore produced by Alcaligenes denitrificans.     

The Vibrio cholerae VctPDGC system transports catechol siderophores and a siderophore-free iron ligand

Vibrio cholerae, the causative agent of cholera, has an absolute requirement for iron. It transports the catechol siderophores vibriobactin, which it synthesizes and secretes, and enterobactin. These siderophores are transported across the inner membrane by one of two periplasmic binding protein-dependent ABC transporters, VctPDGC or ViuPDGC. We show here that one of these inner membrane transport systems, VctPDGC, also promotes iron acquisition in the absence of siderophores. Plasmids carrying the vctPDGC genes stimulated growth in both rich and minimal media of a Shigella flexneri mutant that produces no siderophores. vctPDGC also stimulated the growth of an Escherichia coli enterobactin biosynthetic mutant in low iron medium, and this effect did not require feoB, tonB or aroB. A tyrosine to phenylalanine substitution in the periplasmic binding protein VctP did not alter enterobactin transport, but eliminated growth stimulation in the absence of a siderophore. These data suggest that the VctPDGC system has the capacity to transport both catechol siderophores and a siderophore-free iron ligand. We also show that VctPDGC is the previously unidentified siderophore-independent iron transporter in V. cholerae, and this appears to complete the list of iron transport systems in V. cholerae.     

Salmonella typhimurium IroN and FepA Proteins Mediate Uptake of Enterobactin but Differ in Their Specificity for Other Siderophores

Salmonella typhimurium possesses two outer membrane receptor proteins, IroN and FepA, which have been implicated in the uptake of enterobactin. To determine whether both receptors have identical substrate specificities, fepA and iroN mutants and a double mutant were characterized. While both receptors transported enterobactin, the uptake of corynebactin and myxochelin C was selectively mediated by IroN and FepA, respectively.     

Siderophore-mediated iron transport in Bacillus subtilis and Corynebacterium glutamicum

Hexadentate bacillibactin is the siderophore of Bacillus subtilis and is structurally similar to the better known enterobactin of Gram-negative bacteria such as Escherichia coli. Although both are triscatecholamide trilactones, the structural differences of these two siderophores result in opposite metal chiralities, different affinity for ferric ion, and dissimilar iron transport behaviors. Bacillibactin was first reported as isolated from Corynebacterium glutamicum and called corynebactin. However, failure of iron-starved C. glutamicum to transport ⁵⁵Fe bacillibactin and lack of required bacillibactin biosynthetic genes suggest that bacillibactin is not the siderophore produced by this organism. Iron transport mediated by siderophores in B. subtilis occurs through a transport process that is specific for the iron chelating moiety, with parallel pathways for catecholates and hydroxamates. For bacillibactin, enterobactin, and their analogs, neither chirality nor presence of an amino acid spacer affects the uptake and transport process, but alteration of the net charge and size of the molecule impedes the recognition.Paper number 77 in the series Coordination Chemistry of Microbial Iron Transport Compounds. See Abergel et al. [1].     

Differential expression of Bordetella pertussis iron transport system genes during infection

Temporal expression patterns of the Bordetella pertussis alcaligin, enterobactin and haem iron acquisition systems were examined using alcA-, bfeA- and bhuR-tnpR recombinase fusion strains in a mouse respiratory infection model. The iron systems were differentially expressed in vivo, showing early induction of the alcaligin and enterobactin siderophore systems, and delayed induction of the haem system in a manner consistent with predicted changes in host iron source availability during infection. Previous mixed infection competition studies established the importance of alcaligin and haem utilization for B. pertussis in vivo growth and survival. In this study, the contribution of the enterobactin system to the fitness of B. pertussis was confirmed using wild-type and enterobactin receptor mutant strains in similar competition infection experiments. As a correlate to the in vivo expression studies of B. pertussis iron systems in mice, sera from uninfected and B. pertussis-infected human donors were screened for antibody reactivity with Bordetella iron-repressible cell envelope proteins. Pertussis patient sera recognized multiple iron-repressible proteins including the known outer membrane receptors for alcaligin, enterobactin and haem, supporting the hypothesis that B. pertussis is iron-starved and responds to the presence of diverse iron sources during natural infection.     

Siderophore protection against colicins M, B, V, and Ia in Escherichia coli.

A variety of natural and synthetic siderophores capable of supporting the growth of Escherichia coli K-12 on iron-limited media also protect strain RW193+ (tonA+ ent-) from the killing action of colicins B, V, and Ia. Protective activity falls into two categories. The first, characteristic of enterobactin protection against colicin B and ferrichrome protection against colicin M, has properties of a specific receptor competition between the siderophore and the colicin. Thus, enterobactin specifically protects against colicin B in fes- mutants (able to accumulate but unable to utilize enterobactin) as predicted by our proposal that the colicin B receptor functions in the specific binding for uptake of enterobactin (Wayne and Neilands, 1975). Similarly ferrichrome specifically protects against colicin M in SidA mutants (defective in hydroxamate siderophore utilization). The second category of protective response, characteristic of the more general siderophore inhibition of colicins B, V, and Ia, requires the availability or metabolism of siderophore iron. Thus, enterobactin protects against colicins V and Ia, but only when the colicin indicator strain is fes+, and hydroxamate siderophores inhibit colicins B, V, and Ia, but only when the colicin indicator strain is SidA+. Moreover, ferrichrome inhibits colicins B, V, and Ia, yet chromium (III) deferriferrichrome is inactive, and ferrichrome itself does not prevent adsorption of colicin Ia receptor material in vitro. Although the nonspecific protection against colicins B, V, and Ia requires iron, the availability of siderophore iron for cell growth is not sufficient to bring about protection. None of the siderophores tested protect cells against the killing action of colicin E1 or K, or against the energy poisons azide, 2, 4-dinitrophenol, and carbonylcyanide m-chlorophenylhydrazone. We suggest that nonspecific siderophore protection against colicins B, V, and Ia may be due either to an induction of membrane alterations in response to siderophore iron metabolism or to a direct interference by siderophore iron with some unknown step in colicin action subsequent to adsorption.     

Purification of catechol siderophores by boronate affinity chromatography: Identification of chrysobactin from Erwinia carotovora subsp. carotovora

Catechols are co-planar cis-diols known to form stable, isolable complexes with borate under weakly basic conditions. We exploited this chemistry and developed a boronate affinity chromatography for isolating catechol siderophores. The method was applied to the isolation of chrysobactin, enterobactin, and an unknown catechol siderophore produce by Erwinia carotovora subsp. carotovora W3C105. Yields of chrysobactin and enterobactin purified by boronate affinity chromatography were at least two-fold greater than those achieved through alternate methods. The unknown catechol produced by E. carotovora subsp. carotovora W3C105 was isolated by boronate affinity chromatography and shown to be identical to chrysobactin. Boronate affinity chromatography enabled separation of catechol from its rust-colored decomposition products, and simultaneous isolation of catechol and hydroxamate siderophores. Boronate affinity chromatography is a rapid and efficient method for purifying catechol siderophores from bacterial culture supernatants     

Iron supply to Escherichia coli by synthetic analogs of enterochelin.

Synthetic analogs of enterochelin (enterobactin) were tested for their ability to support the growth of Escherichia coli K-12 under iron-limiting conditions. The cyclic compound MECAM [1,3,5-N.N'; N"-tris-(2,3-dihydroxybenzoyl)-triamino-methylbenzene] and its N-methyl derivative Me3MECAM promoted growth, whereas the 2,3-dihydroxy-5-sulfonyl derivatives MECAMS and Me3MECAMS were inactive. The same results were obtained with TRIMCAM [1,3,5-tris(2,3-dihydroxybenzoylcarbamido)-benzene] and TRIMCAMS (the 2,3-dihydroxy-5-sulfonyl derivative of TRIMCAM). However, the sulfonic acid-containing linear compound LICAMS [1,5,10-N,N', N"-tris(5-sulfo-2,3-dihydroxybenzoyl)-triaza-decane] supported growth. In contrast, LIMCAMC, in which the sulfonyl groups at the five position of LICAMS are replaced by carboxyl groups at the four position, was inactive. The uptake of the active analogs required the functions specified by the fepB, fesB, and tonB genes. Surprisingly, growth promotion of mutants lacking the enterochelin receptor protein in the outer membrane was observed. Only MECAM protected cells against colicin B (which kills cells after entering at the enterochelin uptake sites) and transported Fe3+ at about half the enterochelin rate.     

Utilization of exogenous siderophores and natural catechols by Listeria monocytogenes.

Listeria monocytogenes does not produce siderophores for iron acquisition. We demonstrate that a number of microbial siderophores and natural iron-binding compounds are able to promote the growth of iron-starved L. monocytogenes. We suggest that the ability of L. monocytogenes to use a variety of exogenous siderophores and natural catechols accounts for its ubiquitous character.     

Escherichia coli iron enterobactin uptake monitored by Mössbauer spectroscopy.

Iron uptake by Escherichia coli under aerobic conditions of iron deficiency is mediated by a highly stable ferric enterobactin [Fe(ent)3-] siderophore complex. M?ssbauer spectroscopy has been used to monitor the fate of the iron as 57Fe(ent) was taken up by the cells. Osmotic shock experiments were used to distinguish between the iron present in the periplasmic space and that in the cytoplasm of the cell. Iron delivery by a synthetic analog of enterobactin, 1,3,5-N,N',N'- tris-(2,3-dihydroxybenzoyl)triaminomethylbenzene (MECAM), was also studied. Although Fe-MECAM was transported at the same rate as was Fe(ent) across the outer membrane and was apparently accumulated in the periplasmic space, the subsequent behaviors of Fe(ent) and Fe-MECAM were very different. After more than 30 min, a major fraction of the iron originally absorbed as ferric enterobactin appeared as Fe(II), apparently in the cytoplasm of the cell. However, little iron was delivered to the cytoplasm by the MECAM complex. The differences in specificity of these two stages of iron uptake by E. coli are discussed.     

bvg Repression of alcaligin synthesis in Bordetella bronchiseptica is associated with phylogenetic lineage.

Recent studies have shown that Bordetella bronchiseptica utilizes a siderophore-mediated transport system for acquisition of iron from the host iron-binding proteins lactoferrin and transferrin. We recently identified the B. bronchiseptica siderophore as alcaligin, which is also produced by B. pertussis. Alcaligin production by B. bronchiseptica is repressed by exogenous iron, a phenotype of other microbes that produce siderophores. In this study, we report that alcaligin production by B. bronchiseptica RB50 and GP1SN was repressed by the Bordetella global virulence regulator, bvg, in addition to being Fe repressed. Modulation of bvg locus expression with 50 mM MgSO4 or inactivation of bvg by deletion allowed strain RB50 to produce alcaligin. In modulated organisms, siderophore production remained Fe repressed. These observations contrasted with our previous data indicating that alcaligin production by B. bronchiseptica MBORD846 and B. pertussis was repressed by Fe but bvg independent. Despite bvg repression of alcaligin production, strain RB50 was still able to acquire Fe from purified alcaligin, suggesting that expression of the bacterial alcaligin receptor was not repressed by bvg. We tested 114 B. bronchiseptica strains and found that bvg repression of alcaligin production was strongly associated with Bordetella phylogenetic lineage and with host species from which the organisms were isolated.     

Selectivity of Ferric Enterobactin Binding and Cooperativity of Transport in Gram-Negative Bacteria

The ligand-gated outer membrane porin FepA serves Escherichia coli as the receptor for the siderophore ferric enterobactin. We characterized the ability of seven analogs of enterobactin to supply iron via FepA by quantitatively measuring the binding and transport of their ⁵⁹Fe complexes. The experiments refuted the idea that chirality of the iron complex affects its recognition by FepA and demonstrated the necessity of an unsubstituted catecholate coordination center for binding to the outer membrane protein. Among the compounds we tested, only ferric enantioenterobactin, the synthetic, left-handed isomer of natural enterobactin, and ferric TRENCAM, which substitutes a tertiary amine for the macrocyclic lactone ring of ferric enterobactin but maintains an unsubstituted catecholate iron complex, were recognized by FepA (K_d ≈ 20 nM). Ferric complexes of other analogs (TRENCAM-3,2-HOPO; TREN-Me-3,2-HOPO; MeMEEtTAM; MeME-Me-3,2-HOPO; K₃MECAMS; agrobactin A) with alterations to the chelating groups and different net charge on the iron center neither adsorbed to nor transported through FepA. We also compared the binding and uptake of ferric enterobactin by homologs of FepA from Bordetella bronchisepticus, Pseudomonas aeruginosa, and Salmonella typhimurium in the native organisms and as plasmid-mediated clones expressed in E. coli. All the transport proteins bound ferric enterobactin with high affinity (K_d ≤ 100 nM) and transported it at comparable rates (≥50 pmol/min/10⁹ cells) in their own particular membrane environments. However, the FepA and IroN proteins of S. typhimurium failed to efficiently function in E. coli. For E. coli, S. typhimurium, and P. aeruginosa, the rate of ferric enterobactin uptake was a sigmoidal function of its concentration, indicating a cooperative transport reaction involving multiple interacting binding sites on FepA.     

Exogenous siderophore-mediated iron uptake in Pseudomonas aeruginosa: possible involvement of porin OprF in iron translocation.

In addition to the two siderophores pyoverdine and pyochelin synthesized by Pseudomonas aeruginosa ATCC 15692 (strain PAO1), several siderophores produced by other bacteria or fungi, namely cepabactin, salicylic acid, desferriferrichrysin, desferriferricrocin, desferriferrioxamine B, desferriferrioxamine E and coprogen, were able to promote iron uptake with variable efficiencies into this bacterium. For most of these siderophores, these results were consistent with the growth stimulation produced by the same compounds in a plate bioassay. Desferriferrichrome A, enterobactin and desferriferrirubin, however, did not promote iron uptake, although enterobactin and desferriferrirubin stimulated bacterial growth. These paradoxical data are discussed in view of siderophore-inducible iron uptake systems, as demonstrated recently for enterobactin. Among the strains tested, including the wild-type PAO1, the pyoverdine-less mutant PAO6606 and the two porin-mutants P. aeruginosa H636 (oprF::omega) and P. aeruginosa H673 (oprD::Tn501), only for the porin-OprF mutant were fewer siderophores able to promote iron uptake compared to the other strains. Such results suggest that beside specific routes for iron uptake P. aeruginosa is also able to take up siderophore-liganded iron through OprF.     

The Pseudomonas aeruginosa pirA gene encodes a second receptor for ferrienterobactin and synthetic catecholate analogues

Actively secreted iron chelating agents termed siderophores play an important role in the virulence and rhizosphere competence of fluorescent pseudomonads, including Pseudomonas aeruginosa which secretes a high affinity siderophore, pyoverdine, and the low affinity siderophore, pyochelin. Uptake of the iron-siderophore complexes is an active process that requires specific outer membrane located receptors, which are dependent of the inner membrane-associated protein TonB and two other inner membrane proteins, ExbB and ExbC. P. aeruginosa is also capable of using a remarkable variety of heterologous siderophores as sources of iron, apparently by expressing their cognate receptors. Illustrative of this feature are the 32 (of which 28 putative) siderophore receptor genes observed in the P. aeruginosa PAO1 genome. However, except for a few (pyoverdine, pyochelin, enterobactin), the vast majority of P. aeruginosa siderophore receptor genes still remain to be characterized. Ten synthetic iron chelators of catecholate type stimulated growth of a pyoverdine/pyochelin deficient P. aeruginosa PAO1 mutant under condition of severe iron limitation. Null mutants of the 32 putative TonB-dependent siderophore receptor encoding genes engineered in the same genetic background were screened for obvious deficiencies in uptake of the synthetic siderophores, but none showed decreased growth stimulation in the presence of the different siderophores. However, a double knock-out mutant of ferrienterobactin receptor encoding gene pfeA (PA 2688) and pirA (PA0931) failed to be stimulated by 4 of the tested synthetic catecholate siderophores whose chemical structures resemble enterobactin. Ferric-enterobactin also failed to stimulate growth of the double pfeA-pirA mutant although, like its synthetic analogues, it stimulated growth of the corresponding single mutants. Hence, we confirmed that pirA represents a second P. aeruginosa ferric-enterobactin receptor. The example of these two enterobactin receptors probably illustrates a more general phenomenon of siderophore receptor redundancy in P. aeruginosa.