Microbial Iron Transport Structure and Function of Siderophores

A new iron(III) chelator of coprogen-type siderophore from the deep-sea-derived fungus Mycosphaerella sp. SCSIO z059

Mycosphazine A (1), a new iron(III) chelator of coprogen-type siderophore, and mycosphamide A (2), a new cyclic amide benzoate, together with six known aryl amides (3-8), were isolated from the fermentation broth of the deep-sea-derived fungus Mycosphaerella sp. SCSIO z059. Alkaline hydrolysis of 1 afforded a new epimer of dimerum acid, mycosphazine B (1a), and a new bi-fusarinine-type siderophore, mycosphazine C (1b). The planar structures of the new compounds were elucidated by extensive spectroscopic data analysis. The absolute configurations of amino acid residues in 1a and 1b were determined by acid hydrolysis. And the absolute configuration of 2 was established by quantum chemical calculations of the electronic circular dichroism (ECD) spectra. Compound 1 is the first siderophore-Fe(III) chelator incorporating both L-ornithine and D-ornithine unites. Compounds 3-8 were reported as natural products for the first time, and the ¹H and ¹³C NMR data of 6 and 8 were assigned for the first time. Compounds 1 and 1a could greatly promote the biofilm formation of bacterium Bacillus amyloliquefaciens with the rate of about 249% and 524% at concentration of 100 μg·mL^-1, respectively.

Pseudomonas orientalis F9: A Potent Antagonist against Phytopathogens with Phytotoxic Effect in the Apple Flower

In light of public concerns over the use of pesticides and antibiotics in plant protection and the subsequent selection for spread of resistant bacteria in the environment, it is inevitable to broaden our knowledge about viable alternatives, such as natural antagonists and their mode of action. The genus Pseudomonas is known for its metabolic versatility and genetic plasticity, encompassing pathogens as well as antagonists. We characterized strain Pseudomonas orientalis F9, an isolate from apple flowers in a Swiss orchard, and determined its antagonistic activity against several phytopathogenic bacteria, in particular Erwinia amylovora, the causal agent of fire blight. P. orientalis F9 displayed antagonistic activity against a broad suite of phytopathogenic bacteria in the in vitro tests. The promising results from this analysis led to an ex vivo assay with E. amylovora CFBP1430Rif and P. orientalis F9 infected detached apple flowers. F9 diminished the fire blight pathogen in the flowers but also revealed phytotoxic traits. The experimental results were discussed in light of the complete genome sequence of F9, which revealed the strain to carry phenazine genes. Phenazines are known to contribute to antagonistic activity of bacterial strains against soil pathogens. When tested in the cress assay with Pythium ultimum as pathogen, F9 showed results comparable to the known antagonist P. protegens CHA0.

Conjugation of hydroxyethyl starch to desferrioxamine (DFO) modulates the dual role of DFO in Yersinia enterocolitica infection

The iron chelator desferrioxamine (DFO) B is widely used in the therapy of patients with iron overload. As a side effect, DFO may favor the occurrence of fulminant Yersinia infections. Previous work from our laboratory showed that this might be due to a dual role of DFO: growth promotion of the pathogen and immunosuppression of the host. In this study, we sought to determine whether conjugation of DFO to hydroxyethyl starch (HES-DFO) may prevent exacerbation of Yersinia infection in mice. We found HES-DFO to promote neither growth of Yersinia enterocolitica nor mitogen-induced T-cell proliferation and gamma interferon production by T cells in vitro. Nevertheless, in vivo HES-DFO promoted growth of Y. enterocolitica possibly due to cleavage of HES and release of DFO. The pretreatment of mice with DFO resulted in death of all mice 2 to 5 days after application of a normally sublethal inoculum of Y. enterocolitica, while none of the mice pretreated with HES-DFO died within the first 7 days postinfection. However, some of the HES-DFO-treated mice died 8 to 14 days postinfection. Thus, due to the delayed in vivo effect HES-DFO failed to trigger Yersinia-induced septic shock, which accounts for early mortality in DFO-associated septicemia. Moreover, our data suggest that DFO needs to be taken up by host cells in order to exert its immunosuppressive action. These results strongly suggest that HES-DFO might be a favorable drug with fewer side effects than DFO in terms of DFO-promoted fulminant infections.

Demonstration of a ferric vibrioferrin-binding protein in the outer membrane of Vibrio parahaemolyticus

Under iron-restricted conditions, Vibrio parahaemolyticus produces a siderophore, vibrioferrin, accompanying expression of two major outer membrane proteins of 78 and 83 kDa. Autoradiographic analysis of nondenaturing polyacrylamide gel electrophoregrams of outer membrane preparations previously incubated with [35Fe]ferric vibrioferrin revealed a single radiolabeled band, in which the 78-kDa protein was detected predominantly by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The antiserum against the purified 78-kDa protein partially inhibited Fe-VF binding to isolated OMPs. The 78-kDa protein was cleaved by the treatment of whole cells with proteinase K, indicating that a portion of this protein is exposed on the surface of the outer membrane. The treated cells lost most of their iron uptake activity mediated by vibrioferrin. These results suggest that the ferric vibrioferrin-binding protein of 78 kDa may function as the receptor for ferric vibrioferrin involved in the initial step of vibrioferrin-mediated iron uptake. Immunoblot analysis using the antiserum against the 78-kDa protein demonstrated that the molecular mass and antigenic properties of the protein were highly conserved among V. parahaemolyticus strains examined. The antiserum also recognized an iron-repressible outer membrane protein of 78 kDa from iron-restricted V. alginolyticus strains, some of which appeared to produce vibrioferrin.

Isolation and structure elucidation of acinetobactin, a novel siderophore from Acinetobacter baumannii

A novel siderophore, called acinetobactin, with both catecholate and hydroxamate functional groups was isolated from low-iron cultures of Acinetobacter baumannii ATCC 19606. The structure was elucidated by chemical degradation, fast-atom bombardment mass spectrometry and 1H and 13C NMR spectroscopy. Acinetobactin was composed of omega-N-hydroxyhistamine, threonine and 2,3-dihydroxybenzoic acid, the last two components forming an oxazoline ring. Acinetobactin was structurally related to anguibactin, a plasmid-encoded siderophore of Vibrio anguillarum. The only difference was that acinetobactin possessed an oxazoline ring instead of a thiazoline ring. Four of 12 other clinical A. baumannii strains examined produced acinetobactin, indicative of strain-to-strain variation in the ability to produce acinetobactin. In addition, a relatively small amount of acinetobactin was also detected in A. haemolyticus ATCC 17906.

Siderophore-mediated utilization of iron bound to transferrin by Vibrio parahaemolyticus

Vibrio parahaemolyticus produces a structurally novel type of siderophore, termed vibrioferrin, in response to iron-limitation. This study was performed to examine whether vibrioferrin can assimilate iron from human iron-binding proteins for growth. Comparison of the growth rates between V. parahaemolyticus AQ 3354 and its spontaneously arising, vibrioferrin-deficient mutant revealed that vibrioferrin was able to sequester iron from 30% iron-saturated human transferrin for growth, but not from human lactoferrin even if fully saturated with iron. In both strains, iron limitation induced two high-molecular-weight outer membrane proteins with apparent molecular masses of approximately 78 and 83 kDa. Since only the outer membrane fraction including these proteins showed a binding capacity to ferric vibrioferrin complex, either of them may function as its cell surface receptor. These results suggested that the organism might utilize such a source of host iron through the action of vibrioferrin during in vivo survival and proliferation, although its importance in pathogenesis is unknown.

Reduction and mobilization of iron by a NAD(P)H:flavin oxidoreductase from Escherichia coli

Iron is an essential element in all living cells. Solubilization, uptake and transport of iron by microorganisms is controlled by highly efficient and specific Fe(3+)-chelating agents named siderophores. However, mechanisms of mobilization of iron from ferrisiderophores are still enigmatic. Here, we demonstrate that Escherichia coli contains a powerful enzymatic system for the reduction of ferrisiderophores. Siderophores have a much lower affinity for ferrous iron, which then can be liberated. This system has been previously purified and characterized as a NAD(P)H:flavin oxidoreductase [Fontecave, M., Eliasson, R. and Reichard, P. (1987) J. Biol. Chem. 262, 12,325-12,331)]. It catalyzes the reduction of free flavins, FMN, FAD or riboflavin by NADH or NADPH. Reduced flavins, in turn, transfer their electrons to physiological ferric complexes: ferrisiderophores, ferric citrate and ferritins. The reaction is inhibited by molecular oxygen and greatly stimulated by Fe(2+)-acceptors such as ferrozine or the iron-free form of ribonucleotide reductase subunit R2. We suggest that the reduction and the mobilization of iron from ferrisiderophores in the cell might be regulated by the presence of physiological ferrous traps such as apoproteins.

The TonB-dependent ferrichrome receptor FcuA of Yersinia enterocolitica: evidence against a strict co-evolution of receptor structure and substrate specificity

A Yersinia enterocolitica receptor mutant was isolated which is impaired in ferrichrome uptake. The receptor-encoding gene fcuA was cloned in Escherichia coli K-12. A fcuA mutant of Y. enterocolitica could be complemented by the cloned DNA fragment. The FcuA-encoding region was sequenced and an open reading frame encoding 758 amino acids including a signal sequence of 36 amino acids was found. FcuA shared 34.6% amino acid sequence homology with FatA, the anguibactin receptor of Vibrio anguillarum, but only 20.6% homology with FhuA, the ferrichrome receptor of E. coli. Since the structure of anguibactin differs strongly from that of ferrichrome there seems to be no co-evolution of receptor structure and substrate specificity. The ferrichrome receptors FcuA from Y. enterocolitica and FhuA from E. coli had slightly different substrate specificities. In contrast to FhuA from E. coli, FcuA from Y. enterocolitica was more stereoselective and failed to transport enantio ferrichrome. Three additional ferrichrome receptors were cloned from Pantoea agglomerans (formerly Erwinia herbicola), Salmonella paratyphi B and Salmonella typhimurium. Their substrate specificity was similar but not identical.

Ferric iron uptake in Erwinia chrysanthemi mediated by chrysobactin and related catechol-type compounds

Erwinia chrysanthemi 3937 possesses a saturable, high-affinity transport system for the ferric complex of its native siderophore chrysobactin, [N-alpha-(2,3-dihydroxybenzoyl)-D-lysyl-L-serine]. Uptake of 55Fe-labeled chrysobactin was completely inhibited by respiratory poison or low temperature and was significantly reduced in rich medium. The kinetics of chrysobactin-mediated iron transport were determined to have apparent Km and Vmax values of about 30 nM and of 90 pmol/mg.min, respectively. Isomers of chrysobactin and analogs with progressively shorter side chains mediated ferric iron transport as efficiently as the native siderophore, which indicates that the chrysobactin receptor primarily recognizes the catechol-iron center. Free ligand in excess only moderately reduced the accumulation of 55Fe. Chrysobactin may therefore be regarded as a true siderophore for E. chrysanthemi.

Cloning of a sequence of Aquaspirillum magnetotacticum that complements the aroD gene of Escherichia coli

A 2 kb DNA fragment isolated from a cosmid library of Aquaspirillum magnetotacticum strain MS-1 complements the aromatic-metabolite requirements and iron-uptake deficiencies of Escherichia coli and Salmonella typhimurium strains that lack a functional aroD (biosynthetic dehydrodquinase) sequence. All recombinant cosmids selected for their aroD complementation property carry this sequence. No DNA sequence homology has, however, been detected by Southern hybridization between the cloned fragment and the aroD gene of E. coli or the qa2 (catabolic dehydroquinase) gene of Neurospora crassa.

Ferrioxamine transport mutants and the identification of the ferrioxamine receptor protein (FoxA) in Erwinia herbicola (Enterobacter agglomerans)

Iron deprivation of Erwinia herbicola (Enterobacter agglomerans) induces the biosynthesis of six high-Mr outer-membrane proteins and large amounts of ferrioxamine E. Mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine and selection with ferrimycin A yielded mutants of E. herbicola K4 (wild type), defective in the expression of a 76-kDa outer-membrane protein, as determined by SDS/polyacrylamide gel electrophoresis. While in bioassays wild-type cells showed growth promotion in the presence of ferrioxamines (B, D1, D2, E, G), enterobactin, citrate, ferrichrome and coprogen, these mutants failed to respond to ferrioxamines. Moreover, experiments with 55Fe-labelled siderophores confirmed that iron transport mediated by ferrioxamine E and B in the mutants was completely inhibited, whereas iron transport by other hydroxamate siderophores, such as ferrichrome and coprogen was unaffected. The results are evidence that the 76-kDa protein in the outer membrane represents the receptor protein (FoxA) for ferrioxamines in E. herbicola.

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.

Kinetics of hemolysin production in bovine group B streptococci

Thirteen strains of group B streptococci etiologically related to bovine infections were investigated in order to observe the effects of non-hemoglobin iron and glucose on growth and hemolysin production; and to determine the necessity to stabilize the hemolysin with a carrier-stabilizer agent. Ferric citrate was diluted to give final concentrations of 1-11 micrograms/ml, added to iron-reduced (by CaCl2 precipitation) brain heart infusion media, inoculated and incubated at 37 degrees C. Parallel experiments were carried out with glucose. A variety of agents was employed to act as hemolysin stabilizers. Hemolysin production was detected by lysis of sheep erythrocytes. Both iron and glucose were crucial in concentrations 5-7 micrograms/ml and 0.5-1% respectively. Bovine serum albumin-starch mixture acted as an excellent stabilizer.

Iron metabolism of Escherichia coli studied by Mössbauer spectroscopy and biochemical methods

To date it has barely been recognized that the nature of about 75% of the Escherichia coli iron pool is unknown. Here we report the isolation of two iron species representing major components of iron metabolism in various growth states of E. coli. In vivo Mössbauer spectroscopy was applied to obtain information on the intracellular distribution pattern of iron in E. coli K12 W3110. Only two types of iron could be detected in the cell spectra: hexacoordinated Fe2+ and Fe3+ high-spin complexes. Other iron-requiring compounds are at least one order of magnitude less abundant in E. coli. The Mössbauer parameters of these complexes fit neither cytochromes nor iron-sulfur proteins nor ferric holo-bacterioferritin. They are sensitive to metabolic changes and inhibitors. The ratio of Fe/subunit, Fe2+/Fe3+ interconversion, chromatographic and electrophoretic data exclude bacterioferritin as the main iron metabolite in E. coli. Bacterioferritin can be observed only at very high ferric ion concentrations in the medium. The 55Fe fluorograms of both cytoplasmic and membrane fractions exhibit two exclusive bands with apparent molecular masses of 17 and 15 kDa, respectively. The two bands comprised 70% of the applied radioactivity. In gel filtration the main iron peak elutes at 155 kDa yielding two bands with apparent molecular masses of 17 and 15 kDa on SDS/PAGE. We therefore conclude that the iron species form a protein with an apparent molecular mass of 155 kDa containing 17-kDa and 15-kDa subunits. The iron content of the protein is 44 micrograms Fe/mg protein which corresponds to approximately 13 iron ions/subunit. No iron protein exhibiting the observed features has been described so far. Additional Mössbauer experiments suggest that these novel iron proteins are not restricted to E. coli but that similar components are detectable in several bacterial and fungal systems, thus pointing to a general occurrence.

Iron limitation and its effect on membrane proteins and siderophore transport in Neurospora crassa

Cells of the fungus Neurospora crassa were grown under iron-deficient and iron-sufficient conditions and their plasma membrane proteins were compared. Three strains were studied: N. crassa 74A (wild type), a siderophore-free mutant N. crassa (arg-5 ota aga) as well as a 'slime' variant of N. crassa which lacks a cell wall. Plasma membranes were purified, solubilized and analyzed by one-dimensional SDS/polyacrylamide gel electrophoresis yielding approximately 50 distinct protein bands with molecular masses in the range 14-160 kDa. Iron-sufficient and iron-deficient growth resulted in nearly identical plasma membrane protein profiles in all strains. Although minor alterations in the proportion of certain proteins could be detected, significant overproduction of certain membrane proteins during iron limitation could not be observed. Transport of 55Fe-labeled siderophores seems to be correlated to the degree of iron limitation. For example, transport rates were enhanced fivefold after 16 h of growth in iron-deficient medium compared to growth in iron-sufficient medium. Extraction and HPLC measurement of siderophores from conidiospores yielded approximately 10(-15) mol/spore, indicating that germination tubes and young cells used for transport measurements are not iron-deficient. It is suggested that the putative transport systems for siderophores in fungal plasma membranes are constitutively expressed and enhanced uptake of siderophores during iron limitation is rather the result of cellular transport regulation mechanisms.

Mechanistically novel iron(III) transport system in Serratia marcescens

A novel iron(III) transport system of Serratia marcescens, named SFU, was cloned and characterized in Escherichia coli. Iron acquisition by this system differed from that by E. coli and related organisms. No siderophore production and no receptor protein related to the SFU system could be detected. In addition, iron uptake was independent of the TonB and ExbB functions. On the cloned 4.8-kilobase sfu fragment, two loci encoding a 36-kilodalton (kDa) protein and three proteins with molecular masses of 40, 38, and 34 kDa were identified; the 40-kDa protein represents a precursor form. Furthermore, chromosomally encoded functions of E. coli were required for the uptake of iron by this system.

Characterization of ferrioxamine E as the principal siderophore of Erwinia herbicola (Enterobacter agglomerans)

Several strains of the enterobacterial group Erwinia herbicola (Enterobacter agglomerans) were screened for siderophore production. After 3 days of growth in a low-iron medium, all strains studied produced hydroxamate siderophores. The retention values of the main siderophore during thin-layer chromatography on silica gel plates and on HPLC reversed-phase columns were identical with those of an authentic sample of ferrioxamine E (norcardamine). Gas-chromatographic analysis of the HI hydrolyzate yielded succinic acid and 1,5-diaminopentane in equimolar amounts; fast-atom-bombardment (FAB) mass spectroscopy showed a molecular mass of 653 Da. Iron from 55Fe-labelled ferrioxamine E was well taken up by iron-starved cells of E. herbicola (Km = 0.1 microM, Vmax = 8 pmol mg-1 min-1). However, besides ferrioxamine E (100%), several exogenous siderophores such as enterobactin (94.5%), ferric citrate (78.5%), coprogen (63.5%) and ferrichrome (17.5%) served as siderophores, suggesting the presence of multiple siderophore receptors in the outer membrane of E. herbicola.