Chrysobactin-dependent iron acquisition in Erwinia chrysanthemi. Functional study of a homolog of the Escherichia coli ferric enterobactin esterase

A novel phytopathogen Erwinia sorbitola sp. nov., isolated from the feces of ruddy shelducks

The species in the genus Erwinia are Gram-stain-negative, facultatively anaerobic, motile, and rod-shaped. Most species in the genus Erwinia are phytopathogens. Also, Erwinia persicina was involved in several human infections. Based on the reverse microbial etiology principles, it is worth analyzing the pathogenicity of species in this genus. In this study, we isolated and sequenced two species of Erwinia. Phylogenetic, phenotypic, biochemical, and chemotaxonomic analyses were performed to identify its taxonomy position. The virulence tests on plant leaves and pear fruits were used to identify the plant pathogenicity of two species of Erwinia. Bioinformatic methods predicted the possible pathogenic determinants based on the genome sequence. Meanwhile, adhesion, invasion, and cytotoxicity assays on RAW 264.7 cells were applied to identify animal pathogenicity. We isolated two Gram-stain-negative, facultatively anaerobic, motile, and rod-shaped strains from the feces of ruddy shelducks in the Tibet Plateau of China, designated J780T and J316. Distinct phylogenetic, genomic, phenotypic, biochemical, and chemotaxonomic characters of J780T and J316 identified they were novel species and belonged to the genus Erwinia, for which the name Erwinia sorbitola sp. nov. was proposed, the type strain was J780T (= CGMCC 1.17334T = GDMCC 1.1666T = JCM 33839T). Virulence tests showed blight and rot on the leaves and pear fruits confirmed Erwinia sorbitola sp. nov. was a phytopathogen. Predicted gene clusters of motility, biofilm formation, exopolysaccharides, stress survival, siderophores, and Type VI secretion system might be the causes of pathogenicity. In addition, predicted polysaccharide biosynthesis gene clusters on the genome sequence, and the high capacity for adhesion, invasion, and cytotoxicity to animal cells confirmed it has pathogenicity on animals. In conclusion, we isolated and identified a novel phytopathogen Erwinia sorbitola sp. nov. in ruddy shelducks. A predefined pathogen is beneficial for preventing from suffering potential economic losses caused by this new pathogen.

Genomics-driven discovery of chiral triscatechol siderophores with enantiomeric Fe(iii) coordination

Ferric complexes of triscatechol siderophores may assume one of two enantiomeric configurations at the iron site. Chirality is known to be important in the iron uptake process, however an understanding of the molecular features directing stereospecific coordination remains ambiguous. Synthesis of the full suite of (DHB^L/DLys^L/DSer)₃ macrolactone diastereomers, which includes the siderophore cyclic trichrysobactin (CTC), enables the effects that the chirality of Lys and Ser residues exert on the configuration of the Fe(iii) complex to be defined. Computationally optimized geometries indicate that the Λ/Δ configurational preferences are set by steric interactions between the Lys sidechains and the peptide backbone. The ability of each (DHB^L/DLys^L/DSer)₃ diastereomer to form a stable Fe(iii) complex prompted a genomic search for biosynthetic gene clusters (BGCs) encoding the synthesis of these diastereomers in microbes. The genome of the plant pathogen Dickeya chrysanthemi EC16 was sequenced and the genes responsible for the biosynthesis of CTC were identified. A related but distinct BGC was identified in the genome of the opportunistic pathogen Yersinia frederiksenii ATCC 33641; isolation of the siderophore from Y. frederiksenii ATCC 33641, named frederiksenibactin (FSB), revealed the triscatechol oligoester, linear-(DHB^LLys^LSer)₃. Circular dichroism (CD) spectroscopy establishes that Fe(iii)-CTC and Fe(iii)-FSB are formed in opposite enantiomeric configuration, consistent with the results of the ferric complexes of the cyclic (DHB^L/DLys^L/DSer)₃ diastereomers.

The Serratia marcescens Siderophore Serratiochelin Is Necessary for Full Virulence during Bloodstream Infection

Serratia marcescens is a bacterium frequently found in the environment, but over the last several decades it has evolved into a concerning clinical pathogen, causing fatal bacteremia. To establish such infections, pathogens require specific nutrients; one very limited but essential nutrient is iron. We sought to characterize the iron acquisition systems in S. marcescens isolate UMH9, which was recovered from a clinical bloodstream infection. Using RNA sequencing (RNA-seq), we identified two predicted siderophore gene clusters (cbs and sch) that were regulated by iron. Mutants were constructed to delete each iron acquisition locus individually and in conjunction, generating both single and double mutants for the putative siderophore systems. Mutants lacking the sch gene cluster lost their iron-chelating ability as quantified by the chrome azurol S (CAS) assay, whereas the cbs mutant retained wild-type activity. Mass spectrometry-based analysis identified the chelating siderophore to be serratiochelin, a siderophore previously identified in Serratia plymuthica Serratiochelin-producing mutants also displayed a decreased growth rate under iron-limited conditions created by dipyridyl added to LB medium. Additionally, mutants lacking serratiochelin were significantly outcompeted during cochallenge with wild-type UMH9 in the kidneys and spleen after inoculation via the tail vein in a bacteremia mouse model. This result was further confirmed by an independent challenge, suggesting that serratiochelin is required for full S. marcescens pathogenesis in the bloodstream. Nine other clinical isolates have at least 90% protein identity to the UMH9 serratiochelin system; therefore, our results are broadly applicable to emerging clinical isolates of S. marcescens causing bacteremia.

Discovery of a diverse set of esterases from hot spring microbial mat and sea sediment metagenomes

Esterases are an important group of biocatalysts for synthetic organic chemistry. Functional metagenomics allows discovery of novel biocatalysts by providing access to the gene pool of the microbial community of a habitat. Two metagenomic libraries representing the gene pool of sea sediment and hot spring microbial mat were constructed. Functional screening of these libraries resulted in the isolation of total 8 clones with tributyrin hydrolytic activity. Sequence analysis revealed 10 putative lipolytic proteins with 42-99% homology to the protein sequences in the databases, nine of which represented six known esterase families. Four of the encoded proteins represented Family V and amongst others, one each represented the Family VIII, pectin acetylesterase, enterobactin esterase, G-D-S-L family and OsmC domain containing esterase. One unusual lipolytic protein possessed poly-(3-hydroxybutyrate) depolymerase domain fused to lipase/esterase domain. Two phylogenetically related esterases (MLC3 and SLC5) belonging to family V were expressed and purified to homogeneity. The enzymes exhibited environment-adapted temperature optimum and thermostability. MLC3 was able to stereoselectively hydrolyze R-methyl mandelate to produce R-mandelic acid, an important chiral building block, which suggests MLC3 has potential commercial application.

Biosynthetic considerations of triscatechol siderophores framed on serine and threonine macrolactone scaffolds

Bacteria often produce siderophores to facilitate iron uptake. One of the most studied siderophores is enterobactin, the macrolactone trimer of 2,3-dihydroxybenzoyl-l-serine, produced by E. coli and many other enteric bacteria. Other siderophores are variants of enterobactin, with structural modifications including expansion of the tri-serine core to a tetra-serine macrolactone, substitution of l-serine with l-threonine, insertion of amino acids (i.e., Gly, l-Ala, d-Lys, d- and l-Arg, l-Orn), catechol glucosylation, and linearization of the tri-serine macrolactone core. In this review we summarize the current understanding of the biosyntheses of these enterobactin variants, placing them in contrast with the well-established biosynthesis of enterobactin.

Deficiency of α-1-antitrypsin influences systemic iron homeostasis

There is evidence that proteases and antiproteases participate in the iron homeostasis of cells and living systems. We tested the postulate that α-1 antitrypsin (A1AT) polymorphism and the consequent deficiency of this antiprotease in humans are associated with a systemic disruption in iron homeostasis. Archived plasma samples from Alpha-1 Foundation (30 MM, 30 MZ, and 30 ZZ individuals) were analyzed for A1AT, ferritin, transferrin, and C-reactive protein (CRP). Plasma samples were also assayed for metals using inductively coupled plasma atomic emission spectroscopy (ICPAES). Plasma levels of A1AT in MZ and ZZ individuals were approximately 60% and 20% of those for MM individuals respectively. Plasma ferritin concentrations in those with the ZZ genotype were greater relative to those individuals with either MM or MZ genotype. Plasma transferrin for MM, MZ, and ZZ genotypes showed no significant differences. Linear regression analysis revealed a significant (negative) relationship between plasma concentrations of A1AT and ferritin while that between A1AT and transferrin levels was not significant. Plasma CRP concentrations were not significantly different between MM, MZ, and ZZ individuals. ICPAES measurement of metals confirmed elevated plasma concentrations of nonheme iron among ZZ individuals. Nonheme iron concentrations correlated (negatively) with levels of A1AT. A1AT deficiency is associated with evidence of a disruption in iron homeostasis with plasma ferritin and nonheme iron concentrations being elevated among those with the ZZ genotype.

Evolution of the metabolic and regulatory networks associated with oxygen availability in two phytopathogenic enterobacteria

Background

Dickeya dadantii and Pectobacterium atrosepticum are phytopathogenic enterobacteria capable of facultative anaerobic growth in a wide range of O₂ concentrations found in plant and natural environments. The transcriptional response to O₂ remains under-explored for these and other phytopathogenic enterobacteria although it has been well characterized for animal-associated genera including Escherichia coli and Salmonella enterica. Knowledge of the extent of conservation of the transcriptional response across orthologous genes in more distantly related species is useful to identify rates and patterns of regulon evolution. Evolutionary events such as loss and acquisition of genes by lateral transfer events along each evolutionary branch results in lineage-specific genes, some of which may have been subsequently incorporated into the O₂-responsive stimulon. Here we present a comparison of transcriptional profiles measured using densely tiled oligonucleotide arrays for two phytopathogens, Dickeya dadantii 3937 and Pectobacterium atrosepticum SCRI1043, grown to mid-log phase in MOPS minimal medium (0.1% glucose) with and without O₂.

Results

More than 7% of the genes of each phytopathogen are differentially expressed with greater than 3-fold changes under anaerobic conditions. In addition to anaerobic metabolism genes, the O₂ responsive stimulon includes a variety of virulence and pathogenicity-genes. Few of these genes overlap with orthologous genes in the anaerobic stimulon of E. coli. We define these as the conserved core, in which the transcriptional pattern as well as genetic architecture are well preserved. This conserved core includes previously described anaerobic metabolic pathways such as fermentation. Other components of the anaerobic stimulon show variation in genetic content, genome architecture and regulation. Notably formate metabolism, nitrate/nitrite metabolism, and fermentative butanediol production, differ between E. coli and the phytopathogens. Surprisingly, the overlap of the anaerobic stimulon between the phytopathogens is also relatively small considering that they are closely related, occupy similar niches and employ similar strategies to cause disease. There are cases of interesting divergences in the pattern of transcription of genes between Dickeya and Pectobacterium for virulence-associated subsystems including the type VI secretion system (T6SS), suggesting that fine-tuning of the stimulon impacts interaction with plants or competing microbes.

Conclusions

The small number of genes (an even smaller number if we consider operons) comprising the conserved core transcriptional response to O₂ limitation demonstrates the extent of regulatory divergence prevalent in the Enterobacteriaceae. Our orthology-driven comparative transcriptomics approach indicates that the adaptive response in the eneterobacteria is a result of interaction of core (regulators) and lineage-specific (structural and regulatory) genes. Our subsystems based approach reveals that similar phenotypic outcomes are sometimes achieved by each organism using different genes and regulatory strategies.

Chrysobactin siderophores produced by Dickeya chrysanthemi EC16

The plant pathogen Dickeya chrysanthemi EC16 (formerly known as Petrobacterium chrysanthemi EC16 and Erwinia chrysanthemi EC16) was found to produce a new triscatecholamide siderophore, cyclic trichrysobactin, the related catecholamide compounds, linear trichrysobactin and dichrysobactin, and the previously reported monomeric siderophore unit, chrysobactin. Chrysobactin is comprised of L-serine, D-lysine, and 2,3-dihydroxybenzoic acid (DHBA). Trichrysobactin is a cyclic trimer of chrysobactin joined by a triserine lactone backbone. The chirality of the ferric complex of cyclic trichrysobactin is found to be in the Λ configuration, similar to Fe(III)-bacillibactin, which contains a glycine spacer between the DHBA and L-threonine components and is opposite that of Fe(III)-enterobactin, which contains DHBA ligated directly to L-serine.

Microbial siderophores exert a subtle role in Arabidopsis during infection by manipulating the immune response and the iron status

Siderophores (ferric ion chelators) are secreted by organisms in response to iron deficiency. The pathogenic enterobacterium Erwinia chrysanthemi produces two siderophores, achromobactin and chrysobactin (CB), which are required for systemic dissemination in host plants. Previous studies have shown that CB is produced in planta and can trigger the up-regulation of the plant ferritin gene AtFER1. To further investigate the function of CB during pathogenesis, we analyzed its effect in Arabidopsis (Arabidopsis thaliana) plants following leaf infiltration. CB activates the salicylic acid (SA)-mediated signaling pathway, while the CB ferric complex is ineffective, suggesting that the elicitor activity of this siderophore is due to its iron-binding property. We confirmed this hypothesis by testing the effect of siderophores structurally unrelated to CB, including deferrioxamine. There was no activation of SA-dependent defense in plants grown under iron deficiency before CB treatment. Transcriptional analysis of the genes encoding the root ferrous ion transporter and ferric chelate reductase, and determination of the activity of this enzyme in response to CB or deferrioxamine, showed that these compounds induce a leaf-to-root iron deficiency signal. This root response as well as ferritin gene up-regulation in the leaf were not compromised in a SA-deficient mutant line. Using the Arabidopsis-E. chrysanthemi pathosystem, we have shown that CB promotes bacterial growth in planta and can modulate plant defenses through an antagonistic mechanism between SA and jasmonic acid signaling cascades. Collectively, these data reveal a new link between two processes mediated by SA and iron in response to microbial siderophores.

Siderophore-controlled iron assimilation in the enterobacterium Erwinia chrysanthemi: evidence for the involvement of bacterioferritin and the Suf iron-sulfur cluster assembly machinery

The intracellular fate of iron acquired by bacteria during siderophore-mediated assimilation is poorly understood. We investigated this question in the pathogenic enterobacterium Erwinia chrysanthemi. This bacterium produces two siderophores, chrysobactin and achromobactin, during plant infection. We analyzed the distribution of iron into cytosolic proteins in bacterial cells supplied with 59Fe-chrysobactin using native gel electrophoresis. A parental strain and mutants deficient in bacterioferritin (bfr), miniferritin (dps), ferritin (ftnA), bacterioferredoxin (bfd), or iron-sulfur cluster assembly machinery (sufABCDSE) were studied. In the parental strain, we observed two rapidly 59Fe-labeled protein signals identified as bacterioferritin and an iron pool associated to the protein chain-elongation process. In the presence of increased 59Fe-chrysobactin concentrations, we detected mini-ferritin-bound iron. Iron incorporation into bacterioferritin was severely reduced in nonpolar sufA, sufB, sufD, sufS, and sufE mutants but not in a sufC background. Iron recycling from bacterioferritin did not occur in bfd and sufC mutants. Iron depletion caused a loss of aconitase activity, whereas ferric chrysobactin supplementation stimulated the production of active aconitase in parental cells and in bfr and bfd mutants. Aconitase activity in sufA, sufB, sufD, sufS, and sufE mutant strains was 10 times lower than that in parental cells. In the sufC mutant, it was twice as low as that in the parental strain. Defects observed in the mutants were not caused by altered ferric chrysobactin transport. Our data demonstrate a functional link between bacterioferritin, bacterioferredoxin, and the Suf protein machinery resulting in optimal bacterial growth and a balanced distribution of iron between essential metalloproteins.

Iron(III) uptake and release by chrysobactin, a siderophore of the phytophatogenic bacterium Erwinia chrysanthemi

The plant pathogenic enterobacterium Erwinia chrysanthemi causes important soft-rot disease on a wide range of plants including vegetables and ornamentals of economic importance. It produces a major mono(catecholate) siderophore, chrysobactin (alpha-N-(2,3-dihydroxybenzoyl)-D-lysyl-L-serine). To unravel the role of chrysobactin in the virulence of E. chrysanthemi, its iron(III) coordination properties were thus investigated in aqueous solutions using electrospray ionization mass spectrometric, potentiometric, and spectrophotometric methods. Moreover, kinetic experiments allowed us to determine the uptake and release mechanisms. The formation mechanism of the 1:1 complex reveals a key role of the terminal carboxylic group of chrysobactin in the binding of either FeOH(2+) or Fe2(OH)2(4+). The proton-driven dissociation of the ferric tris-, bis-, and mono(chrysobactin) complexes was also studied. For these three ferric complexes, a single protonation triggers the release of the bound chrysobactin molecule. Interestingly, the dissociation of the last ligand proceeded via the formation of an intermediate for which a salicylate-type mode of bonding was proposed.

Differential role of ferritins in iron metabolism and virulence of the plant-pathogenic bacterium Erwinia chrysanthemi 3937

During infection, the phytopathogenic enterobacterium Erwinia chrysanthemi has to cope with iron-limiting conditions and the production of reactive oxygen species by plant cells. Previous studies have shown that a tight control of the bacterial intracellular iron content is necessary for full virulence. The E. chrysanthemi genome possesses two loci that could be devoted to iron storage: the bfr gene, encoding a heme-containing bacterioferritin, and the ftnA gene, coding for a paradigmatic ferritin. To assess the role of these proteins in the physiology of this pathogen, we constructed ferritin-deficient mutants by reverse genetics. Unlike the bfr mutant, the ftnA mutant had increased sensitivity to iron deficiency and to redox stress conditions. Interestingly, the bfr ftnA mutant displayed an intermediate phenotype for sensitivity to these stresses. Whole-cell analysis by Mössbauer spectroscopy showed that the main iron storage protein is FtnA and that there is an increase in the ferrous iron/ferric iron ratio in the ftnA and bfr ftnA mutants. We found that ftnA gene expression is positively controlled by iron and the transcriptional repressor Fur via the small antisense RNA RyhB. bfr gene expression is induced at the stationary phase of growth. The sigmaS transcriptional factor is necessary for this control. Pathogenicity tests showed that FtnA and the Bfr contribute differentially to the virulence of E. chrysanthemi depending on the host, indicating the importance of a perfect control of iron homeostasis in this bacterial species during infection.

Siderophore-based iron acquisition and pathogen control

High-affinity iron acquisition is mediated by siderophore-dependent pathways in the majority of pathogenic and nonpathogenic bacteria and fungi. Considerable progress has been made in characterizing and understanding mechanisms of siderophore synthesis, secretion, iron scavenging, and siderophore-delivered iron uptake and its release. The regulation of siderophore pathways reveals multilayer networks at the transcriptional and posttranscriptional levels. Due to the key role of many siderophores during virulence, coevolution led to sophisticated strategies of siderophore neutralization by mammals and (re)utilization by bacterial pathogens. Surprisingly, hosts also developed essential siderophore-based iron delivery and cell conversion pathways, which are of interest for diagnostic and therapeutic studies. In the last decades, natural and synthetic compounds have gained attention as potential therapeutics for iron-dependent treatment of infections and further diseases. Promising results for pathogen inhibition were obtained with various siderophore-antibiotic conjugates acting as "Trojan horse" toxins and siderophore pathway inhibitors. In this article, general aspects of siderophore-mediated iron acquisition, recent findings regarding iron-related pathogen-host interactions, and current strategies for iron-dependent pathogen control will be reviewed. Further concepts including the inhibition of novel siderophore pathway targets are discussed.

EstB-mediated hydrolysis of the siderophore triacetylfusarinine C optimizes iron uptake of Aspergillus fumigatus

Aspergillus fumigatus excretes the fusarinine-type siderophore desferri-triacetylfusarinine C (DF-TafC) to mobilize iron. DF-TafC is a cyclic peptide consisting of three N(5)-cis-anhydromevalonyl-N(5)-hydroxy-N(2)-acetyl-l-ornithine residues linked by ester bonds; these linkages are in contrast to peptide linkages found for ferrichrome-type siderophores. Subsequent to the binding of iron and uptake, triacetylfusarinine C (TafC) is hydrolyzed, the cleavage products are excreted, and the iron is transferred to the metabolism or to the intracellular siderophore desferri-ferricrocin (DF-FC) for iron storage. Here we report the identification and characterization of the TafC esterase EstB, the first eukaryotic siderophore-degrading enzyme to be characterized at the molecular level. The encoding gene, estB, was found to be located in an iron-regulated gene cluster, indicating a role in iron metabolism. Deletion of estB in A. fumigatus eliminated TafC esterase activity of cellular extracts and caused increased intracellular accumulation of TafC and TafC hydrolysis products in vivo. Escherichia coli-expressed EstB displayed specific TafC esterase activity but did not hydrolyze fusarinine C, which has the same core structure as TafC but lacks three N(2)-acetyl residues. Localization of EstB via enhanced green fluorescent protein tagging suggested that TafC hydrolysis takes place in the cytoplasm. EstB abrogation reduced the intracellular transfer rate of iron from TafC to DF-FC and delayed iron sensing. Furthermore, EstB deficiency caused a decreased radial growth rate under iron-depleted but not iron-replete conditions. Taken together, these data suggest that EstB-mediated TafC hydrolysis optimizes but is not essential for TafC-mediated iron uptake in A. fumigatus.

Polyketide synthases and nonribosomal peptide synthetases: the emerging view from bacterial genomics

A total of 223 complete bacterial genomes are analyzed, with 281 citations, for the presence of genes encoding modular polyketide synthases (PKS) and nonribosomal peptide synthetases (NRPS). We report on the distribution of these systems in different bacterial taxa and, whenever known, the metabolites they synthesize. We also highlight, in the different bacterial lineages, the PKS and NRPS genes and, whenever known, the corresponding products.

A gene cluster involved in the biosynthesis of vanchrobactin, a chromosome-encoded siderophore produced by Vibrio anguillarum

Vibrio anguillarum serotype O2 strains produce a catechol siderophore named vanchrobactin, which has been identified as N-[N'-(2,3-dihydroxybenzoyl)-arginyl]-serine. This work describes a chromosomal region that harbours the genetic determinants necessary for the biosynthesis of vanchrobactin. The authors have identified the genes involved in 2,3-dihydroxybenzoic acid (DHBA) biosynthesis (vabA, vabB and vabC) and activation (vabE), and a gene (vabF) encoding a non-ribosomal peptide synthetase, which is putatively involved in the assembly of the siderophore components. Also described are the identification and characterization of genes encoding a putative vanchrobactin exporter (vabS) and a siderophore esterase (vabH). In-frame deletion mutants in vabA, vabB, vabC, vabE, vabF and vabH were impaired for growth under conditions of iron limitation, and the analysis of culture supernatants by chrome azurol-S and cross-feeding assays showed almost no production of siderophores in any of the vabABCEF mutants. In addition, deletion mutations of vabA, vabB and vabC abolished production of DHBA, as assessed by chemical and biological analyses. Complementation of each mutant with the corresponding gene provided in trans confirmed the involvement of this gene cluster in the biosynthesis of DHBA and vanchrobactin in V. anguillarum strain RV22. Based on chemical and genetic data, and on published models for other catechol siderophores, a model for vanchrobactin biosynthesis is proposed.

Beyond the Fur paradigm: iron-controlled gene expression in rhizobia

Iron is critical for bacterial growth, but problems arise from the toxicity of excess iron; thus, iron uptake is subject to tight control. The most widely found and best-studied iron-responsive regulator in Gram-negative bacteria is the ferric uptake regulator Fur. In recent years, however, it has become apparent that iron regulation in rhizobia differs from that in many other bacteria. New regulators (RirA, Irr, Mur) were identified which appear to mediate functions that in other bacteria are accomplished by Fur. Even though some of them belong to the Fur family, they exhibit properties that clearly separate them from genuine Fur proteins. This article surveys the principal mechanisms of iron acquisition and uptake in rhizobia, and puts particular emphasis on recent findings on transcriptional regulators and their means to sense the cellular iron status and to regulate gene expression. In this context, we point out differences and similarities with regard to the operators, regulons and structure of the discussed iron regulatory proteins.

Siderophore-mediated upregulation of Arabidopsis ferritin expression in response to Erwinia chrysanthemi infection

Ferritins are multimeric iron storage proteins encoded by a four-member gene family in Arabidopsis (AtFer1-4). To investigate whether iron sequestration in ferritins is a part of an iron-withholding defense system induced in response to bacterial invasion, we used Arabidopsis thaliana as a susceptible host for the pathogenic bacterium Erwinia chrysanthemi. In this study, we used a T-DNA insertion mutant line to show that the lack of a functional AtFer1 gene resulted in an enhanced susceptibility of Arabidopsis plants to E. chrysanthemi. We found that the AtFer1 gene is upregulated during infection, with a biphasic accumulation of the transcript at critical time points 0.5 and 24 h post-infection (p.i.). The activation of AtFer1 expression observed at 24 h p.i. was independent of the iron-dependent regulatory sequence (IDRS) known to mediate the transcriptional response of the AtFer1 gene to iron excess and to nitric oxide. Upregulation of AtFer1 gene expression was compromised after inoculation with an E. chrysanthemi siderophore null mutant. Infiltration of the purified siderophores chrysobactin and desferrioxamine strongly increased AtFer1 transcript abundance and it did not occur with the iron-loaded forms of these siderophores. We found that neither oxidative stress nor nitric oxide was involved in the plant response to chrysobactin. Our data show that ferritin accumulation during infection of Arabidopsis by E. chrysanthemi is a basal defense mechanism which is mainly activated by bacterial siderophores. The potential role of siderophores in this process is discussed.

Bacterial iron sources: from siderophores to hemophores

Iron is an essential element for most organisms, including bacteria. The oxidized form is insoluble, and the reduced form is highly toxic for most macromolecules and, in biological systems, is generally sequestrated by iron- and heme-carrier proteins. Thus, despite its abundance on earth, there is practically no free iron available for bacteria whatever biotope they colonize. To fulfill their iron needs, bacteria have multiple iron acquisition systems, reflecting the diversity of their potential biotopes. The iron/heme acquisition systems in bacteria have one of two general mechanisms. The first involves direct contact between the bacterium and the exogenous iron/heme sources. The second mechanism relies on molecules (siderophores and hemophores) synthesized and released by bacteria into the extracellular medium; these molecules scavenge iron or heme from various sources. Recent genetic, biochemical, and crystallographic studies have allowed substantial progress in describing molecular mechanisms of siderophore and hemophore interactions with the outer membrane receptors, transport through the inner membrane, iron storage, and regulation of genes encoding biosynthesis and uptake proteins.

FhuF, Part of a Siderophore−Reductase System†

FhuF is a cytoplasmic 2Fe-2S protein of Escherichia coli loosely associated with the cytoplasmic membrane. E. coli fhuF mutants showed reduced growth on plates with ferrioxamine B as the sole iron source, although siderophore uptake was not defective in transport experiments. Removal of iron from coprogen, ferrichrome, and ferrioxamine B was significantly lower in fhuF mutants compared to the corresponding parental strains, which suggested that FhuF is involved in iron removal from these hydroxamate-type siderophores. A redox potential E(1/2) of -310 +/- 25 mV relative to the normal hydrogen electrode was determined for FhuF by EPR redox titration; this redox potential is sufficient to reduce the siderophores coprogen and ferrichrome. Mössbauer spectra revealed that FhuF in its [Fe(2+)-Fe(3+)] state is also capable of direct reduction of ferrioxamine B-bound ferric iron, thus proving its reductase function. This is the first report on a bacterial siderophore-iron reductase which in vivo seems to be specific for a certain group of hydroxamates.

SufC: an unorthodox cytoplasmic ABC/ATPase required for [Fe-S] biogenesis under oxidative stress

Proteins containing [Fe-S] clusters perform essential functions in all domains of life. Previously, we identified the sufABCDSE operon as being necessary for virulence of the plant pathogen Erwinia chrysanthemi. In addition, we collected preliminary evidence that the sufABCDSE operon might be involved in the assembly of [Fe-S] clusters. Of particular interest are the sufB, sufC and sufD genes, which are conserved among Eubacteria, Archaea, plants and parasites. The present study establishes SufC as an unorthodox ATPase of the ABC superfamily that is located in the cytosol, wherein it interacts with both SufB and SufD. Moreover, under oxidative stress conditions, SufC was found to be necessary for the activity of enzymes containing oxygen-labile [Fe-S] clusters, but dispensable for glutamate synthase, which contains an oxidatively stable [Fe-S] cluster. Lastly, we have shown SufBCD to be essential for iron acquisition via chrysobactin, a siderophore of major importance in virulence. We discuss a model wherein the SufBCD proteins contribute to bacterial pathogenicity via their role in the assembly of [Fe-S] clusters under oxidative stress and iron limitation.