Specificity and mechanism of ferrioxamine-mediated iron transport in Streptomyces pilosus

Comparative and functional genomic analyses of iron transport and regulation in Leptospira spp

The spirochetes of the Leptospira genus contain saprophytic and pathogenic members, the latter being responsible for leptospirosis. Despite the recent sequencing of the genome of the pathogen L. interrogans, the slow growth of these bacteria, their virulence in humans, and a lack of genetic tools make it difficult to work with these pathogens. In contrast, the development of numerous genetic tools for the saprophyte L. biflexa enables its use as a model bacterium. Leptospira spp. require iron for growth. In this work, we show that Leptospira spp. can acquire iron from different sources, including siderophores. A comparative genome analysis of iron uptake systems and their regulation in the saprophyte L. biflexa and the pathogen L. interrogans is presented in this study. Our data indicated that, for instance, L. biflexa and L. interrogans contain 8 and 12 genes, respectively, whose products share homology with proteins that have been shown to be TonB-dependent receptors. We show that some genes involved in iron uptake were differentially expressed in response to iron. In addition, we were able to disrupt several putative genes involved in iron acquisition systems or iron regulation in L. biflexa. Comparative genomics, in combination with gene inactivation, gives us significant functional information on iron homeostasis in Leptospira spp.

Ternary Complex Formation Facilitates a Redox Mechanism for Iron Release from a Siderophore

While the naturally occurring reducing agents glutathione (GSH) and ascorbate (H2A) alone are ineffective at reducing iron(III) sequestered by the siderophore ferrioxamine B, the addition of an iron(II) chelator, sulfonated bathophenanthroline (BPDS), facilitates reduction by either reducing agent. A mechanism is described in which a ternary complex is formed between ferrioxamine B and BPDS in a rapidly established pre-equilibrium step, which is followed by rate limiting reduction of the ternary complex by glutathione or ascorbate. Spectral, thermodynamic, and kinetic evidence are given for ternary complex formation. Ascorbate was found to be slightly more efficient at reducing the ternary complex than glutathione (k4=2.1 x 10(-3) M(-1) s(-1) and k4=6.3 x 10(-4) M(-1) s(-1), respectively) at pH 7. Reduction is followed by a rapid ligand exchange step where iron is released from ferrioxamine B to form tris-(BPDS)iron(II). The implications of these results for siderophore mediated iron transport and release are discussed.

Desferrioxamine E produced by Streptomyces griseus stimulates growth and development of Streptomyces tanashiensis

The authors previously reported that interspecific stimulatory events between Streptomyces species for antibiotic production and/or morphological differentiation mediated by putative diffusible metabolites take place at a high frequency. This paper reports the isolation and characterization of a substance produced by Streptomyces griseus that stimulates the growth and development of Streptomyces tanashiensis. The substance was purified from the culture supernatant of S. griseus by using anion-exchange chromatography, gel filtration chromatography and reverse-phase HPLC. FAB-MS and NMR analyses of the purified preparation indicated the substance to be desferrioxamine E (synonym: nocardamine), a siderophore that is widely produced by Streptomyces species and related organisms. Similar stimulatory effects on the growth and development of S. tanashiensis were exerted by desferrioxamine E produced by another actinomycete strain, but not by other siderophores tested, including ferrichrome and nocobactin and free ferric ion. An exogenous supply of desferrioxamine E stimulated secondary metabolite formation and/or morphological differentiation in various actinomycete strains. Disruption of the desferrioxamine biosynthesis gene cluster in Streptomyces coelicolor A3(2) abolished the production of desferrioxamine E and the activity to stimulate the growth and differentiation of S. tanashiensis. The S. coelicolor mutant showed impaired growth and development on Bennett's/glucose agar medium, but it was rescued by the exogenous supply of desferrioxamine E. These results indicate that desferrioxamines play an important role in streptomycete physiology. Similar to several pathogenic bacteria and fungi, S. tanashiensis may be defective in the production of siderophores; however, it can utilize the siderophores excreted by other organisms.

Discovery of a new peptide natural product by Streptomyces coelicolor genome mining

Analyses of microbial genome sequences reveal numerous examples of gene clusters encoding proteins typically involved in complex natural product biosynthesis but not associated with the production of known natural products. In Streptomyces coelicolor M145 there are several gene clusters encoding new nonribosomal peptide synthetase (NRPS) systems not associated with known metabolites. Application of structure-based models for substrate recognition by NRPS adenylation domains predicts the amino acids incorporated into the putative peptide products of these systems, but the accuracy of these predictions is untested. Here we report the isolation and structure determination of the new tris-hydroxamate tetrapeptide iron chelator coelichelin from S. coelicolor using a genome mining approach guided by substrate predictions for the trimodular NRPS CchH, and we show that this enzyme, which lacks a C-terminal thioesterase domain, together with a homolog of enterobactin esterase (CchJ), are required for coelichelin biosynthesis. These results demonstrate that accurate prediction of adenylation domain substrate selectivity is possible and raise intriguing mechanistic questions regarding the assembly of a tetrapeptide by a trimodular NRPS.

Comparison of the effects of deferiprone versus deferoxamine on growth and virulence of Yersinia enterocolitica

Deferoxamine, a drug used to treat patients with iron overload, has the capacity to promote systemic Y. enterocolitica infections in humans. The aim of this study was to determine whether deferiprone, the only orally active alternative treatment, has the same potential. When Y. enterocolitica IP864 was grown in an iron-poor chemically defined medium, addition of deferoxamine promoted its growth, while various concentrations of deferiprone did not display this activity. Similarly, on iron-poor agar plates, various Y. enterocolitica strains were able to grow around paper disks impregnated with deferoxamine in a dose-dependent manner, while no growth was observed around the deferiprone disks. In a mouse experimental model of infection, the 50% lethal dose (LD(50)) of strain IP864 was decreased by more than 5 log units in mice pretreated with deferoxamine, while a deferiprone pretreatment did not affect it. Therefore, in contrast to deferoxamine, deferiprone does not enhance growth of pathogenic Y. enterocolitica in vitro and does not have the potential to promote Y. enterocolitica septicemia in a mouse model of infection. Deferiprone may thus represent a useful alternative iron-chelation therapy during invasive Y. enterocolitica infections.

Novel plant-microbe rhizosphere interaction involving Streptomyces lydicus WYEC108 and the pea plant (Pisum sativum)

A previously undescribed plant-microbe interaction between a root-colonizing Streptomyces species, S. lydicus WYEC108, and the legume Pisum sativum is described. The interaction is potentially of great importance to the health and growth in nature of this nodulating legume. The root-colonizing soil actinomycete S. lydicus WYEC108 influences pea root nodulation by increasing root nodulation frequency, possibly at the level of infection by Rhizobium spp. S. lydicus also colonizes and then sporulates within the surface cell layers of the nodules. Colonization leads to an increase in the average size of the nodules that form and improves the vigor of bacteroids within the nodules by enhancing nodular assimilation of iron and possibly other soil nutrients. Bacteroid accumulation of the carbon storage polymer, poly-beta-hydroxybutyrate, is reduced in colonized nodules. Root nodules of peas taken from agricultural fields in the Palouse hills of northern Idaho were also found to be colonized by actinomycete hyphae. We hypothesize that root and nodule colonization is one of several mechanisms by which Streptomyces acts as a naturally occurring plant growth-promoting bacterium in pea and possibly other leguminous plants.

Siderophore mediated plutonium accumulation by Microbacterium flavescens (JG-9)

Uptake of plutonium and uranium mediated by the siderophore desferrioxamine-B (DFOB) has been studied for the common soil aerobe Microbacterium flavescens(JG-9). M. flavescens does not bind or take up nitrilotriacetic acid (NTA) complexes of U(VI), Fe(III), or Pu(IV) or U(VI)-DFOB but does take up Fe(III)-DFOB and Pu(IV)-DFOB. Pu(IV)-DFOB and Fe(III)-DFOB accumulations are similar: only living and metabolically active bacteria take up these metal-siderophore complexes. The Fe(III)-DFOB and Pu(IV)-DFOB complexes mutually inhibit uptake of the other, indicating that they compete for shared binding sites or uptake proteins. However, Pu uptake is much slower than Fe uptake, and cumulative Pu uptake is less than Fe, 1.0 nmol of Fe vs 0.25 nmol of Pu per mg of dry weight bacteria. The Pu(IV)-DFOB interactions with M. flavescens suggest that Pu-siderophore complexes could generally be recognized by Fe-siderophore uptake systems of many bacteria, fungi, or plants, thereby affecting Pu environmental mobility and distribution. The results also suggest that the siderophore complexes of tetravalent metals can be recognized by Fe-siderophore uptake proteins.

Microbial iron transport via a siderophore shuttle: a membrane ion transport paradigm

A mechanism of ion transport across membranes is reported. Microbial transport of Fe(3+) generally delivers iron, a growth-limiting nutrient, to cells via highly specific siderophore-mediated transport systems. In contrast, iron transport in the fresh water bacterium Aeromonas hydrophila is found to occur by means of an indiscriminant siderophore transport system composed of a single multifunctional receptor. It is shown that (i) the siderophore and Fe(3+) enter the bacterium together, (ii) a ligand exchange step occurs in the course of the transport, and (iii) a redox process is not involved in iron exchange. To the best of our knowledge, there have been no other reports of a ligand exchange mechanism in bacterial iron transport. The ligand exchange step occurs at the cell surface and involves the exchange of iron from a ferric siderophore to an iron-free siderophore already bound to the receptor. This ligand exchange mechanism is also found in Escherichia coli and seems likely to be widely distributed among microorganisms.

Desferrioxamine-dependent iron transport in Erwinia amylovora CFBP1430: cloning of the gene encoding the ferrioxamine receptor FoxR

Iron deprivation of Erwinia amylovora CFBP1430, a species causing fire blight on Pomoïdeae, was shown to induce the production of siderophores of the desferrioxamine (dfo) family and two outer membrane polypeptides with apparent molecular weight of about 70 and 80 kDa, respectively. Cyclic dfo E was characterized as the major metabolite. Phage MudIIpR13 insertional mutagenesis and screening on CAS-agar medium yielded three dfo non-producing and one overproducing clones. These clones failed to grow in the presence of the Fe(III) chelator EDDHA and were determined further as dfo and ferrioxamine transport negative mutants, respectively. The transport mutant which appeared to lack the 70 kDa polypeptide in the outer membrane allowed the purification of dfo E. Growth under iron limitation of dfo negative mutants was stimulated with ferrioxamine E and B but not with other ferrisiderophores tested. The host DNA sequence flanking the left terminal part of the MudIIpR13 prophage responsible for the transport mutation was cloned and used to probe a parental gene library by DNA-DNA hybridization. Two recombinant cosmids restoring the transport mutation to normal were identified. Both cosmids also conferred the ability to utilize ferrioxamine B and E as iron sources on a FhuE- mutant of Escherichia coli. This correlated with the production of an additional polypeptide of 70 kDa in the outer membrane of E. coli transconjugants, thus confirming that this protein serves the ferrioxamine receptor function (FoxR) in E. amylovora.

Iron uptake from ferrioxamine and from ferrirhizoferrin by germinating spores of Rhizopus microsporus

Mucormycosis caused by the fungus Rhizopus has been documented in iron overloaded patients and more particularly in dialysis patients, both when treated with desferrioxamine B (DFO). This iron and aluminium chelator is thought to play a role in the pathogenesis of this infection. We therefore investigated in vitro the cellular pharmacology of iron chelated by DFO in the fungus Rhizopus. In a medium, designed for fungal cultivation, Rhizopus microsporus var. rhizopodiformis takes up iron from ferric-DFO complex (55Fe.DFO) and from 55Fe.rhizoferrin, the siderophore synthesized and secreted by Rhizopus [Drechsel et al., Biol. Metals 4: 238-243, 1991]. In both cases, iron accumulation is partially saturable with the duration of exposure and the chelator concentration. Fe.DFO binds to Rhizopus; iron becomes trapped and remains associated with the fungus, whereas the iron-depleted siderophore is released in the extracellular medium. In a medium designed for mammalian cell cultivation and in the absence of human serum, the fungal iron accumulation both from 55Fe.DFO and from 55Fe.rhizoferrin is proportional to the chelator concentration. Human serum at 40% does not influence the iron accumulation from Fe.DFO but it significantly affects that from Fe.rhizoferrin which, in the presence of serum, only occurs at concentration > 5 microM. This difference finds its explanation in the iron transfer observed between Fe.rhizoferrin and seric apotransferrin, the latter making the metal unavailable to Rhizopus. By contrast, no iron transfer takes place between Fe.DFO and apotransferrin, allowing fungal iron utilization from this complex, even at very low concentrations. The iron uptake, being inhibited by NaN3 and KCN, is energy-dependent; being inhibited by bipyridyl, it requires prior reduction of ferric iron; being unaffected by the covalent linkage of Fe.DFO to albumin, it does not require the entry of Fe.DFO within the fungus. These in vitro results strongly suggest that, upon administration of DFO to iron overloaded or dialysis patients, the formed Fe.DFO is efficiently used as an iron source by Rhizopus, even in the presence of serum apotransferrin or rhizoferrin. The consequent promotion of the growth of Rhizopus helps explain the increased risk of mucormycosis in DFO-treated patients.

Mucormycosis during deferoxamine therapy is a siderophore-mediated infection. In vitro and in vivo animal studies

This study investigates the pathophysiology of mucormycosis caused by Rhizopus, which has been reported in 46 dialysis patients, while treated with deferoxamine (DFO). This drug aggravates mucormycosis, which we experimentally induced in guinea pigs and which lead to a shortened animal survival (P < or = 0.01). The drug's effect on Rhizopus is not mediated through the polymorphonuclear cells. Fe.DFO, the iron chelate of DFO, abolishes the fungistatic effect of serum on Rhizopus and increases the in vitro growth of the fungus (P < or = 0.0001). This effect is present at Fe.DFO concentrations > or = 0.01 microM, at which fungal uptake of radioiron from 55Fe.DFO is observed. A 1,000-fold higher concentration of iron citrate is required to achieve a similar rate of radioiron uptake and of in vitro growth stimulation as observed with Fe.DFO. These in vitro effects of Fe.DFO (1 microM) in serum on radioiron uptake and on growth stimulation are more striking for Rhizopus than for Aspergillus fumigatus and are practically absent for Candida albicans. For these three fungal species, the rates of radioiron uptake from 55Fe.DFO and of growth stimulation in the presence of Fe.DFO in serum are directly related (r = 0.886). These results underscore the major role of Fe.DFO in the pathogenesis of DFO-related mucormycosis. Pharmacokinetic changes in uremia lead to a prolonged accumulation of Fe.DFO after DFO administration, which helps explain the increased sensitivity of dialysis patients to DFO-related mucormycosis.

Utilization of microbial siderophores by mycorrhizal and non-mycorrhizal pine roots

In iron-deficient conditions, most bacteria and fungi are known to release siderophores, iron-chelating compounds. Most plants do not produce siderophores, but seem to use microbial siderophores as iron sources. Although ectomycorrhizal fungi have been found to release siderophores in pure culture, little research has addressed production by mycorrhizas and the consequences for plant iron nutrition. The objectives of this study were to determine the effect of an ectomycorrhizal fungus [Pisolithus tinctorius (Pers.) Coker and Couch] on the utilization of siderophore (ferrioxamine B) by slash pine (Pinus elliottii Engelm.) roots grown under iron-deficient or iron-sufficient conditions. Experiments were conducted with excised roots and whole seedlings. Uptake of ⁵⁵ Fe from ferrioxamine B was lower by mycorrhizal roots than non-mycorrhizal roots. Growth under iron-deficient conditions had little effect on iron uptake by non-mycorrhizal roots but increased the uptake by mycorrhizal roots. Uptake of iron from a non-purified siderophore isolated from a pure culture of P. tinctorius was also lower by mycorrhizal roots. The uptake of iron was not dependent on the pH of the uptake solution. Differential responses could be attributed to different mechanisms of iron uptake between fungal cells and root cells. However, the higher iron content of mycorrhizal roots may indicate a negative feedback effect on uptake.

The specificity of bacterial siderophore receptors probed by bioassays

The ability to utilize siderophores of bacterial and fungal origin has been studied in wild-type and mutant strains of the enterobacterial genera Salmonella, Escherichia, Shigella, Moellerella, Klebsiella, Enterobacter, Hafnia, Pantoea, Ewingella, Tatumella, Yersinia, and in the non-enterics Aeromonas, Pseudomonas and Aureobacterium. Although only a few representative strains were tested, the results show characteristic genus-specific differences in the utilization of hydroxamate and catecholate siderophores. Moreover, the different response to structural alterations of certain siderophore classes by some wild-type and mutant strains points to variable interacting receptor domains.

Staphyloferrin A: a structurally new siderophore from staphylococci

Two ferric ion-binding compounds, designated staphyloferrin A and B, were detected in the culture filtrates of staphylococci grown under iron-deficient conditions. Staphyloferrin A was isolated from cultures of Staphylococcus hyicus DSM 20459. The structural elucidation of this highly hydrophilic, acid-labile compound revealed a novel siderophore, N2,N5-di-(1-oxo-3-hydroxy-3,4-dicarboxybutyl)-D-ornithine, which consists of one ornithine and two citric acid residues linked by two amide bonds. The two citric acid components of staphyloferrin A provide two tridentate pendant ligands, comprising of a beta-hydroxy, beta-carboxy-substituted carboxylic acid derivative, for octahedral metal chelation. The CD spectrum of the staphyloferrin A ferric complex indicates a predominant A configuration about the ferric ion center. The uptake of ferric staphyloferrin A by S. hyicus obeys Michaelis-Menten kinetics (Km = 0.246 microM; vmax = 82 pmol.mg-1.min-1), indicating active transport of this siderophore. The staphyloferrin A transport system is different from that of the ferrioxamines as shown by an antagonism test. Production of staphyloferrin A is strongly iron-dependent and is stimulated by supplementation of the medium with either D- or L-ornithine. DL-[5-14C]ornithine was incorporated into staphyloferrin A, demonstrating that ornithine is an intermediate in staphyloferrin A biosynthesis.

Kinetics of iron acquisition from ferric siderophores by Paracoccus denitrificans

The kinetics of iron accumulation by iron-starved Paracoccus denitrificans during the first 2 min of exposure to 55Fe-labeled ferric siderophore chelates is described. Iron is acquired from the ferric chelate of the natural siderophore L-parabactin in a process exhibiting biphastic kinetics by Lineweaver-Burk analysis. The kinetic data for 1 microM less than [Fe L-parabactin] less than 10 microM fit a regression line which suggests a low-affinity system (Km = 3.9 +/- 1.2 microM, Vmax = 494 pg-atoms of 55Fe min-1 mg of protein-1), whereas the data for 0.1 microM less than or equal to [Fe L-parabactin] less than or equal to 1 microM fit another line consistent with a high-affinity system (Km = 0.24 +/- 0.06 microM, Vmax = 108 pg-atoms of 55Fe min-1 mg of protein-1). The Km of the high-affinity uptake is comparable to the binding affinity we had previously reported for the purified ferric L-parabactin receptor protein in the outer membrane. In marked contrast, ferric D-parabactin data fit a single regression line corresponding to a simple Michaelis-Menten process with comparatively low affinity (Km = 3.1 +/- 0.9 microM, Vmax = 125 pg-atoms of 55Fe min-1 mg of protein-1). Other catecholamide siderophores with an intact oxazoline ring derived from L-threonine (L-homoparabactin, L-agrobactin, and L-vibriobactin) also exhibit biphasic kinetics with a high-affinity component similar to ferric L-parabactin. Circular dichroism confirmed that these ferric chelates, like ferric L-parabactin, exist as the lambda enantiomers. The A forms ferric parabactin (ferrin D- and L-parabactin A), in which the oxazoline ring is hydrolyzed to the open-chain threonyl structure, exhibit linear kinetics with a comparatively high Km (1.4 +/- 0.3 microM) and high Vmax (324 pg-atoms of 55Fe min-1 of protein-1). Furthermore, the marked stereospecificity seen between ferric D- and L-parabactins is absent; i.e., iron acquisition from ferric parabactin A is non stereospecific. The mechanistic implications of these findings in relation to a stereospecific high-affinity binding followed by a nonstereospecific postreceptor processing is discussed.

Structural and stereochemical aspects of iron transport in fungi

A variety of fungi are known to overproduce and excrete desferri-siderophores under iron limitation. After complexing with ferric iron, octahedral complexes are formed and taken up by siderophore-specific transport systems. These systems represent energy consuming systems as inferred from their sensitivity to respiratory inhibitors, uncouplers and changes of the membrane potential and are able to recognize structure and stereochemical configuration of the various siderophore molecules. Ferrichromes, the most common siderophores in fungi, are generally recognized as Lambda-cis coordination complexes. Triacetylfusarinins, although prevailing as Delta-cis optical isomers in aqueous solution, are assumed to be taken up after isomerization to the corresponding Lambda-cis complexes. However, coprogens which also show a predominant Delta-absolute configuration in solution seem to be transported without prior isomerization. When both, ferrichromes as well as triacetylfusarines or coprogens are taken up, competition during uptake is observed, suggesting the presence of a common transport system during translocation of siderophores across the fungal plasma membrane.

Characterization of siderophore-mediated iron transport in Geotrichum candidum, a non-siderophore producer

Geotrichum candidum is capable of utilizing iron from hydroxamate siderophores of different structural classes. The relative rates of iron transport for ferrichrome, ferrichrysin, ferrioxamine B, fusigen, ferrichrome A, rhodotorulic acid, coprogen B, dimerium acid and ferrirhodin were 100%, 98%, 74%, 59%, 49%, 35%, 24%, 12% and 11% respectively. Ferrichrome, ferrichrysine and ferrichrome A inhibited [59Fe]ferrioxamine-B-mediated iron transport by 71%, 68% and 28% respectively when added at equimolar concentrations to the radioactive complex. The inhibitory mechanism of [59Fe]ferrioxamine B uptake by ferrichrome was noncompetitive (Ki 2.4 microM), suggesting that the two siderophores do not share a common transport system. Uptake of [59Fe]ferrichrome, [59Fe]rhodotorulic acid and [59Fe]fusigen was unaffected by competition with the other two siderophores or with ferrioxamine B. Thus, G. candidum may possess independent transport systems for siderophores of different structural classes. The uptake rates of [14C]ferrioxamine B and 67Ga-desferrioxamine B were 30% and 60% respectively, as compared to [59Fe]ferrioxamine B. The specific ferrous chelates, dipyridyl and ferrozine at 6 mM, caused 65% and 35% inhibition of [59Fe]ferrioxamine uptake. From these results we conclude that, although about 70% of the iron is apparently removed from the complex by reduction prior to being transported across the cellular membrane, a significant portion of the chelated ligand may enter the cell intact. The former and latter mechanisms seem not to be mutually exclusive.

The effect of synthetic iron chelators on bacterial growth in human serum

The effect of synthetic iron chelators of the 1-alkyl-3-hydroxy-2-methylpyrid-4-one class (the L1 series) and 1-hydroxypyrid-2-one (L4) on bacterial growth in human serum was compared with those of the plant iron chelators mimosine and maltol and of the microbial siderophore desferrioxamine. None of the synthetic chelators enhanced growth of 3 Gram-negative organisms (Yersinia enterocolitica, Escherichia coli and Pseudomonas aeruginosa); in some cases they were even inhibitory. L4 strongly stimulated growth of Staphylococcus epidermidis, but the L1 series had only a marginal effect. Maltol was mildly inhibitory to all 4 bacterial species, while mimosine enhanced the growth of S. epidermidis and Y. enterocolitica but had little effect on E. coli or P. aeruginosa. Desferrioxamine enhanced the growth of all except E. coli. These results suggest that the chelators of synthetic or plant origin may carry less risk of increasing susceptibility to bacterial infection in patients undergoing chelation therapy for iron overload than does desferrioxamine, the drug currently in clinical use.

Inhibitory effect of the partially resolved coordination isomers of chromic desferricoprogen on coprogen uptake in Neurospora crassa

Two partially resolved chromatographic fractions of geometrical and optical isomers of the chromic complexes of desferricoprogen, a siderophore from Neurospora crassa, were obtained from high-pressure liquid chromatography on a reverse-phase matrix. The first fraction was identified as a cis complex with a 20% diastereomeric excess of the lambda isomer. The second fraction was identified as a mixture of several of the possible trans isomers with a net 20% diastereomeric excess of the delta isomers. These fractions were used to evaluate the stereospecificity of the coprogen-mediated iron uptake system with respect to the metal coordination center. Fraction II competitively inhibited coprogen uptake, whereas fraction I showed only slight inhibition. N. crassa accumulated chromium from fraction II faster than the rate of chromium uptake from fraction I. Neither fraction had a significant effect on the uptake of ferricrocin, suggesting that coprogen and ferricrocin are taken up by different receptor systems.

Iron transport in Streptomyces pilosus mediated by ferrichrome siderophores, rhodotorulic acid, and enantio-rhodotorulic acid

Streptomyces pilosus is one of several microbes which produce ferrioxamine siderophores. In the accompanying paper (G. Müller and K. Raymond, J. Bacteriol. 160:304-312), the mechanism of iron uptake mediated by the endogenous ferrioxamines B, D1, D2, and E was examined. Here we report iron transport behavior in S. pilosus as mediated by the exogenous siderophores ferrichrome, ferrichrysin, rhodotorulic acid (RA), and synthetic enantio-RA. In each case iron acquisition depended on metabolic energy and had uptake rates comparable to that of [55Fe]ferrioxamine B. However, the synthetic ferric enantio-RA (which has the same preferred chirality at the metal center as ferrichrome) was twice as effective in supplying iron as was the natural ferric RA complex, suggesting that stereospecific recognition at the metal center is involved in the transport process. Iron uptake mediated by ferrichrome and ferric enantio-RA was strongly inhibited by kinetically inert chromic complexes of desferrioxamine B. These inhibition experiments indicate that iron from these exogenous siderophores is transported by the same uptake system as ferrioxamine B. Since the ligands have no structural similarity to ferrioxamine B except for the presence of three hydoxamate groups, we conclude that only the hydroxamate iron center and its direct surroundings are important for recognition and uptake. This hypothesis is supported by the fact that ferrichrome A and ferrirubin, which are both substituted at the hydroxamate carbonyl groups, were not (or were poorly) effective in supplying iron to S. pilosus.