Outer membrane protein mediating iron uptake via pyoverdinpss, the fluorescent siderophore produced by Pseudomonas syringae pv. syringae

The Role of Iron in Phytopathogenic Microbe-Plant Interactions: Insights into Virulence and Host Immune Response

Iron is an essential element required for the growth and survival of nearly all forms of life. It serves as a catalytic component in multiple enzymatic reactions, such as photosynthesis, respiration, and DNA replication. However, the excessive accumulation of iron can result in cellular toxicity due to the production of reactive oxygen species (ROS) through the Fenton reaction. Therefore, to maintain iron homeostasis, organisms have developed a complex regulatory network at the molecular level. Besides catalyzing cellular redox reactions, iron also regulates virulence-associated functions in several microbial pathogens. Hosts and pathogens have evolved sophisticated strategies to compete against each other over iron resources. Although the role of iron in microbial pathogenesis in animals has been extensively studied, mechanistic insights into phytopathogenic microbe-plant associations remain poorly understood. Recent intensive research has provided intriguing insights into the role of iron in several plant-pathogen interactions. This review aims to describe the recent advances in understanding the role of iron in the lifestyle and virulence of phytopathogenic microbes, focusing on bacteria and host immune responses.

Negatively regulated aerobactin and desferrioxamine E by Fur in Pantoea ananatis are required for full siderophore production and antibacterial activity, but not for virulence

Pantoea ananatis is an emerging plant pathogen that causes disease in economically important crops such as rice, corn, onion, melon, and pineapple, and it also infects humans and insects. In this study, we identified biosynthetic gene clusters of aerobactin and desferrioxamine E (DFO-E) siderophores using the complete genome of P. ananatis PA13 isolated from rice sheath rot. P. ananatis PA13 exhibited the strongest antibacterial activity against Erwinia amylovora and Yersinia enterocolitica (Enterobacterales). Mutants of aerobactin or DFO-E maintained antibacterial activity against E. amylovora and Y. enterocolitica, as well as in a siderophore activity assay. However, double aerobactin- and DFO-E-gene-deletion mutants completely lost siderophore and antibacterial activity. These results reveal that both siderophore biosynthetic gene clusters are essential for siderophore production and antibacterial activity in P. ananatis PA13. A ferric uptake regulator protein (Fur) mutant exhibited a significant increase in siderophore production, and a Fur-overexpressing strain completely lost antibacterial activity. Expression of the iucA, dfoJ, and foxA genes was significantly increased in the Δfur mutant background, and expression of these genes returned to wild type levels after fur compensation. These results indicate that Fur negatively regulates aerobactin and DFO-E siderophores. However, siderophore production was not required for P. ananatis virulence in plants, but it appears to be involved in the microbial ecology surrounding the plant environment. This study is the first to report the regulation and functional characteristics of siderophore biosynthetic genes in P. ananatis. IMPORTANCE Pantoea ananatis is a bacterium that causes diseases in several economically important crops, as well as in insects and humans. This bacterium has been studied extensively as a potentially dangerous pathogen due to its saprophytic ability. Recently, the types, biosynthetic gene clusters, and origin of the siderophores in the Pantoea genus were determined using genome comparative analyses. However, few genetic studies have investigated the characteristics and functions of siderophores in P. ananatis. The results of this study revealed that the production of aerobactin and desferrioxamine E in the rice pathogen P. ananatis PA13 is negatively regulated by Fur, and that these siderophores are essential for antibacterial activity against Erwinia amylovora and Yersinia enterocolitica (Enterobacterales). However, siderophore production was not required for P. ananatis virulence in plants, but it appears to be involved in the microbial ecology surrounding the plant environment.

The Knockout of Enterobactin-Related Gene in Pectobacterium atrosepticum Results in Reduced Stress Resistance and Virulence towards the Primed Plants

Siderophores produced by microorganisms to scavenge iron from the environment have been shown to contribute to virulence and/or stress resistance of some plant pathogenic bacteria. Phytopathogenic bacteria of Pectobacterium genus possess genes for the synthesis of siderophore enterobactin, which role in plant-pathogen interactions has not been elucidated. In the present study we characterized the phenotype of the mutant strain of Pba deficient for the enterobactin-biosynthetic gene entA. We showed that enterobactin may be considered as a conditionally beneficial virulence factor of Pba. The entA knockout did not reduce Pba virulence on non-primed plants; however, salicylic acid-primed plants were more resistant to ΔentA mutant than to the wild type Pba. The reduced virulence of ΔentA mutant towards the primed plants is likely explained by its compromised resistance to oxidative stress.

Genome and Transcriptome Sequences of Pseudomonas syringae pv. syringae B301D-R

Strains of the plant pathogen Pseudomonas syringae are commonly found in the phylosphere and are able to infect a number of agriculturally important crops. Here, we report a high-quality draft genome sequence of Pseudomonas syringae pv. syringae B301D-R, isolated from pears, which is a model strain for phytotoxin research in P. syringae.

Role of iron homeostasis in the virulence of phytopathogenic bacteria: an 'à la carte' menu

The interaction between pathogenic microbes and their hosts is determined by survival strategies on both sides. As a result of its redox properties, iron is vital for the growth and proliferation of nearly all organisms, including pathogenic bacteria. In bacteria-vertebrate interactions, competition for this essential metal is critical for the outcome of the infection. The role of iron in the virulence of plant pathogenic bacteria has only been explored in a few pathosystems in the past. However, in the last 5 years, intensive research has provided new insights into the mechanisms of iron homeostasis in phytopathogenic bacteria that are involved in virulence. This review, which includes important plant pathosystems, discusses the recent advances in the understanding of iron transport and homeostasis during plant pathogenesis. By summarizing the recent progress, we wish to provide an updated view clarifying the various roles played by this metal in the virulence of bacterial phytopathogens as a nutritional and regulatory element. The complex intertwining of iron metabolism and oxidative stress during infection is emphasized.

Characterization of pyoverdine and achromobactin in Pseudomonas syringae pv. phaseolicola 1448a

BACKGROUND

Pseudomonas syringae pv. phaseolicola 1448a (P. syringae 1448a), the causative agent of bean halo blight, is a bacterium capable of occupying diverse biological niches. Under conditions of iron starvation P. syringae 1448a secretes siderophores for active uptake of iron. The primary siderophore of P. syringae 1448a is pyoverdine, a fluorescent molecule that is assembled from amino acid precursors by non-ribosomal peptide synthetase (NRPS) enzymes. Whereas other species of Pseudomonas often exhibit structural variations in the pyoverdine produced by different strains, all P. syringae pathovars previously tested have been found to make an identical pyoverdine molecule. P. syringae 1448a also appears to have the genetic potential to make two secondary siderophores, achromobactin and yersiniabactin, each of which has previously been detected in different P. syringae pathovars.

RESULTS

Five putative pyoverdine NRPS genes in P. syringae 1448a were characterized in-silico and their role in pyoverdine biosynthesis was confirmed by gene knockout. Pyoverdine was purified from P. syringae 1448a and analyzed by MALDI-TOF and MS/MS spectroscopy. Peaks were detected corresponding to the expected sizes for the pyoverdine structure previously found in other P. syringae pathovars, but surprisingly P. syringae 1448a appears to also produce a variant pyoverdine species that has an additional 71 Da monomer incorporated into the peptide side chain. Creation of pyoverdine null mutants of P. syringae 1448a revealed that this strain also produces achromobactin as a temperature-regulated secondary siderophore, but does not appear to make yersiniabactin. Pyoverdine and achromobactin null mutants were characterized in regard to siderophore production, iron uptake, virulence and growth in iron limited conditions.

CONCLUSIONS

This study provides the first evidence of a P. syringae pathovar producing a side chain variant form of pyoverdine. We also describe novel IC₅₀ and liquid CAS assays to quantify the contribution of different siderophores across a range of iron starvation conditions, and show that although achromobactin has potential to contribute to fitness its contribution is masked by the presence of pyoverdine, which is a significantly more effective siderophore. Neither pyoverdine nor achromobactin appear to be required for P. syringae 1448a to cause bean halo blight, indicating that these siderophores are not promising targets for crop protection strategies.

The phytopathogen Pseudomonas syringae pv. tomato DC3000 has three high-affinity iron-scavenging systems functional under iron limitation conditions but dispensable for pathogenesis

High-affinity iron scavenging through the use of siderophores is a well-established virulence determinant in mammalian pathogenesis. However, few examples have been reported for plant pathogens. Here, we use a genetic approach to investigate the role of siderophores in Pseudomonas syringae pv. tomato DC3000 (DC3000) virulence in tomato. DC3000, an agronomically important pathogen, has two known siderophores for high-affinity iron scavenging, yersiniabactin and pyoverdin, and we uncover a third siderophore, citrate, required for growth when iron is limiting. Though growth of a DC3000 triple mutant unable to either synthesize or import these siderophores is severely restricted in iron-limited culture, it is fully pathogenic. One explanation for this phenotype is that the DC3000 triple mutant is able to directly pirate plant iron compounds such as heme/hemin or iron-nicotianamine, and our data indicate that DC3000 can import iron-nicotianamine with high affinity. However, an alternative explanation, supported by data from others, is that the pathogenic environment of DC3000 (i.e., leaf apoplast) is not iron limited but is iron replete, with available iron of >1 μM. Growth of the triple mutant in culture is restored to wild-type levels by supplementation with a variety of iron chelates at >1 μM, including iron(III) dicitrate, a dominant chelate of the leaf apoplast. This suggests that lower-affinity iron import would be sufficient for DC3000 iron nutrition in planta and is in sharp contrast to the high-affinity iron-scavenging mechanisms required in mammalian pathogenesis.

Role of the FeoB protein and siderophore in promoting virulence of Xanthomonas oryzae pv. oryzae on rice

Xanthomonas oryzae pv. oryzae causes bacterial blight, a serious disease of rice. Our analysis revealed that the X. oryzae pv. oryzae genome encodes genes responsible for iron uptake through FeoB (homolog of the major bacterial ferrous iron transporter) and a siderophore. A mutation in the X. oryzae pv. oryzae feoB gene causes severe virulence deficiency, growth deficiency in iron-limiting medium, and constitutive production of a siderophore. We identified an iron regulated xss gene cluster, in which xssABCDE (Xanthomonas siderophore synthesis) and xsuA (Xanthomonas siderophore utilization) genes encode proteins involved in biosynthesis and utilization of X. oryzae pv. oryzae siderophore. Mutations in the xssA, xssB, and xssE genes cause siderophore deficiency and growth restriction under iron-limiting conditions but are virulence proficient. An xsuA mutant displayed impairment in utilization of native siderophore, suggesting that XsuA acts as a specific receptor for a ferric-siderophore complex. Histochemical and fluorimetric assays with gusA fusions indicate that, during in planta growth, the feoB gene is expressed and that the xss operon is not expressed. This study represents the first report describing a role for feoB in virulence of any plant-pathogenic bacterium and the first functional characterization of a siderophore-biosynthetic gene cluster in any xanthomonad.

Salicylic acid, yersiniabactin, and pyoverdin production by the model phytopathogen Pseudomonas syringae pv. tomato DC3000: synthesis, regulation, and impact on tomato and Arabidopsis host plants

A genetically tractable model plant pathosystem, Pseudomonas syringae pv. tomato DC3000 on tomato and Arabidopsis thaliana hosts, was used to investigate the role of salicylic acid (SA) and iron acquisition via siderophores in bacterial virulence. Pathogen-induced SA accumulation mediates defense in these plants, and DC3000 contains the genes required for the synthesis of SA, the SA-incorporated siderophore yersiniabactin (Ybt), and the fluorescent siderophore pyoverdin (Pvd). We found that DC3000 synthesizes SA, Ybt, and Pvd under iron-limiting conditions in culture. Synthesis of SA and Ybt by DC3000 requires pchA, an isochorismate synthase gene in the Ybt genomic cluster, and exogenous SA can restore Ybt production by the pchA mutant. Ybt was also produced by DC3000 in planta, suggesting that Ybt plays a role in DC3000 pathogenesis. However, the pchA mutant did not exhibit any growth defect or altered virulence in plants. This lack of phenotype was not attributable to plant-produced SA restoring Ybt production, as the pchA mutant grew similarly to DC3000 in an Arabidopsis SA biosynthetic mutant, and in planta Ybt was not detected in pchA-infected wild-type plants. In culture, no growth defect was observed for the pchA mutant versus DC3000 for any condition tested. Instead, enhanced growth of the pchA mutant was observed under stringent iron limitation and additional stresses. This suggests that SA and Ybt production by DC3000 is costly and that Pvd is sufficient for iron acquisition. Further exploration of the comparative synthesis and utility of Ybt versus Pvd production by DC3000 found siderophore-dependent amplification of ybt gene expression to be absent, suggesting that Ybt may play a yet unknown role in DC3000 pathogenesis.

Yersiniabactin production by Pseudomonas syringae and Escherichia coli, and description of a second yersiniabactin locus evolutionary group

The siderophore and virulence factor yersiniabactin is produced by Pseudomonas syringae. Yersiniabactin was originally detected by high-pressure liquid chromatography (HPLC); commonly used PCR tests proved ineffective. Yersiniabactin production in P. syringae correlated with the possession of irp1 located in a predicted yersiniabactin locus. Three similarly divergent yersiniabactin locus groups were determined: the Yersinia pestis group, the P. syringae group, and the Photorhabdus luminescens group; yersiniabactin locus organization is similar in P. syringae and P. luminescens. In P. syringae pv. tomato DC3000, the locus has a high GC content (63.4% compared with 58.4% for the chromosome and 60.1% and 60.7% for adjacent regions) but it lacks high-pathogenicity-island features, such as the insertion in a tRNA locus, the integrase, and insertion sequence elements. In P. syringae pv. tomato DC3000 and pv. phaseolicola 1448A, the locus lies between homologues of Psyr_2284 and Psyr_2285 of P. syringae pv. syringae B728a, which lacks the locus. Among tested pseudomonads, a PCR test specific to two yersiniabactin locus groups detected a locus in genospecies 3, 7, and 8 of P. syringae, and DNA hybridization within P. syringae also detected a locus in the pathovars phaseolicola and glycinea. The PCR and HPLC methods enabled analysis of nonpathogenic Escherichia coli. HPLC-proven yersiniabactin-producing E. coli lacked modifications found in irp1 and irp2 in the human pathogen CFT073, and it is not clear whether CFT073 produces yersiniabactin. The study provides clues about the evolution and dispersion of yersiniabactin genes. It describes methods to detect and study yersiniabactin producers, even where genes have evolved.

Identification of the syr-syp box in the promoter regions of genes dedicated to syringomycin and syringopeptin production by Pseudomonas syringae pv. syringae B301D

The phytotoxins syringopeptin and syringomycin are synthesized by nonribosomal peptide synthetases which are encoded by the syringomycin (syr) and syringopeptin (syp) genomic island of Pseudomonas syringae pv. syringae. Previous studies demonstrated that expression of the syr-syp genes was controlled by the salA-syrF regulatory pathway, which in turn was induced by plant signal molecules. In this study, the 132-kb syr-syp genomic island was found to be organized into five polycistronic operons along with eight individual genes based on reverse transcriptional PCR and bioinformatic analysis. The transcriptional start sites of the salA gene and operons III and IV were located 63, 75, and 104 bp upstream of the start codons of salA, syrP, and syrB1, respectively, using primer extension analysis. The predicted -10/-35 promoter region of operon IV was confirmed based on deletion and site-directed mutagenesis analyses of the syrB1::uidA reporter with beta-glucuronidase assays. A 20-bp conserved sequence (TGtCccgN(6)cggGaCA, termed the syr-syp box) with dyad symmetry around the -35 region was identified via computer analysis for the syr-syp genes/operons responsible for biosynthesis and secretion of syringomycin and syringopeptin. Expression of the syrB1::uidA fusion was decreased 59% when 6 bp was deleted from the 5' end of the syr-syp box in the promoter region of operon IV. These results demonstrate that the conserved promoter sequences of the syr-syp genes contribute to the coregulation of syringomycin and syringopeptin production.

Characterization of fluorescent and nonfluorescent peptide siderophores produced by Pseudomonas syringae strains and their potential use in strain identification

Nonfluorescent highly virulent strains of Pseudomonas syringae pv. aptata isolated in different European countries and in Uruguay produce a nonfluorescent peptide siderophore, the production of which is iron repressed and specific to these strains. The amino acid composition of this siderophore is identical to that of the dominant fluorescent peptide siderophore produced by fluorescent P. syringae strains, and the molecular masses of the respective Fe(III) chelates are 1,177 and 1,175 atomic mass units. The unchelated nonfluorescent siderophore is converted into the fluorescent siderophore at pH 10, and colors and spectral characteristics of the unchelated siderophores and of the Fe(III)-chelates in acidic conditions are similar to those of dihydropyoverdins and pyoverdins, respectively. The nonfluorescent siderophore is used by fluorescent and nonfluorescent P. syringae strains. These results and additional mass spectrometry data strongly suggest the presence of a pyoverdin chromophore in the fluorescent siderophore and a dihydropyoverdin chromophore in the nonfluorescent siderophore, which are both ligated to a succinamide residue. When chelated, the siderophores behave differently from typical pyoverdins and dihydropyoverdins in neutral and alkaline conditions, apparently because of the ionization occurring around pH 4.5 of carboxylic acids present in beta-hydroxyaspartic acid residues of the peptide chains. These differences can be detected visually by pH-dependent changes of the chelate colors and spectrophotochemically. These characteristics and the electrophoretic behavior of the unchelated and chelated siderophores offer new tools to discriminate between saprophytic fluorescent Pseudomonas species and fluorescent P. syringae and P. viridiflava strains and to distinguish between the two siderovars in P. syringae pv. aptata.

A dsbA mutant of Pseudomonas syringae exhibits reduced virulence and partial impairment of type III secretion

Abstract To identify virulence genes of P. syringae pv. tomato strain DC3000 we screened for mutants with reduced virulence on its plant hosts, Arabidopsis thaliana and tomato. We isolated a Tn5-insertion mutant that exhibited reduced virulence on both hosts. Further characterization showed that this mutant carried a single Tn5 insertion in the dsbA gene, which encodes a periplasmic disulphide bond-forming protein. In addition to reduced virulence, the dsbA mutant exhibits mucoid colony morphology, loss of fluorescence, decreased motility, and a reduced growth rate in culture. The dsbA mutant is able to multiply in A. thaliana and tomato plants, trigger the hypersensitive response on tobacco and elicit Pto-mediated resistance in tomato, indicating that type III secretion occurs in this background. However, type III secretion appears to function with reduced efficiency in the dsbA mutant, as type III-dependent secretion of HrpZ and AvrRpt2 is impaired. These findings indicate that while the dsbA gene is required for multiple cellular functions in P. syringae, type III secretion in P. syringae is only partially dependent on dsbA.

Production and comparison of peptide siderophores from strains of distantly related pathovars of Pseudomonas syringae and Pseudomonas viridiflava LMG 2352

The production of peptide siderophores and the variation in siderophore production among strains of Pseudomonas syringae and Pseudomonas viridiflava were investigated. An antibiose test was used to select a free amino acid-containing agar medium favorable for production of fluorescent siderophores by two P. syringae strains. A culture technique in which both liquid and solid asparagine-containing culture media were used proved to be reproducible and highly effective for inducing production of siderophores in a liquid medium by the fluorescent Pseudomonas strains investigated. Using asparagine as a carbon source appeared to favor siderophore production, and relatively high levels of siderophores were produced when certain amino acids were used as the sole carbon and energy sources. Purified chelated siderophores of strains of P. syringae pv. syringae, P. syringae pv. aptata, P. syringae pv. morsprunorum, P. syringae pv. tomato, and P. viridiflava had the same amino acid composition and spectral characteristics and were indiscriminately used by these strains. In addition, nonfluorescent strains of P. syringae pv. aptata and P. syringae pv. morsprunorum were able to use the siderophores in biological tests. Our results confirmed the proximity of P. syringae and P. viridiflava; siderotyping between pathovars of P. syringae was not possible. We found that the spectral characteristics of the chelated peptide siderophores were different from the spectral characteristics of typical pyoverdins. Our results are discussed in relation to the ecology of the organisms and the conditions encountered on plant surfaces.

Conservation of the gene for outer membrane protein OprF in the family Pseudomonadaceae: sequence of the Pseudomonas syringae oprF gene

The conservation of the oprF gene for the major outer membrane protein OprF was determined by restriction mapping and Southern blot hybridization with the Pseudomonas aeruginosa oprF gene as a probe. The restriction map was highly conserved among 16 of the 17 serotype strains and 42 clinical isolates of P. aeruginosa. Only the serotype 12 isolate and one clinical isolate showed small differences in restriction pattern. Southern probing of PstI chromosomal digests of 14 species from the family Pseudomonadaceae revealed that only the nine members of rRNA homology group I hybridized with the oprF gene. To reveal the actual extent of homology, the oprF gene and its product were characterized in Pseudomonas syringae. Nine strains of P. syringae from seven different pathovars hybridized with the P. aeruginosa gene to produce five different but related restriction maps. All produced an OprF protein in their outer membranes with the same apparent molecular weight as that of P.aeruginosa OprF. In each case the protein reacted with monoclonal antibody MA4-10 and was similarly heat and 2-mercaptoethanol modifiable. The purified OprF protein of the type strain P. syringae pv. syringae ATCC 19310 reconstituted small channels in lipid bilayer membranes. The oprF gene from this latter strain was cloned and sequenced. Despite the low level of DNA hybridization between P. aeruginosa and P. syringae DNA, the OprF gene was highly conserved between the species with 72% DNA sequence identity and 68% amino acid sequence identity overall. The carboxy terminus-encoding region of P. syringae oprF showed 85 and 33% identity, respectively, with the same regions of the P. aeruginosa oprF and Escherichia coli ompA genes.

Pyoverdin-facilitated iron uptake in Pseudomonas aeruginosa: immunological characterization of the ferripyoverdin receptor

A purified polyclonal antiserum directed against the isolated main 80 kD IROMP (iron-regulated outer-membrane protein) from Pseudomonas aeruginosa PAO1 detected only the 80 kD polypeptide of outer-membrane proteins from PAO1 cells grown in iron deficiency in Western blots. It was also shown to inhibit the uptake of 59Fe pyoverdin by PAO1 cells as well as its binding to purified outer membranes. Immunofluorescence experiments with intact PAO1 cells confirmed that the receptor is present only at the surface of cells grown under conditions of iron deficiency. All these data allow us to conclude that the 80 kD main IROMP of P. aeruginosa is indeed the receptor for the siderophore ferripyoverdin.

Outer membrane proteins of Pseudomonas

In this review, we describe the outer membrane proteins of Pseudomonas aeruginosa and related strains from the Pseudomonas fluorescens rRNA homology group of the Pseudomonadaceae, with emphasis on the physiological function and biochemical characteristics of these proteins. The use of opr (for outer membrane protein) is proposed as the genetic designation for the P. aeruginosa outer membrane proteins and letters are assigned, in conjunction with this designation, to known outer membrane proteins. Proteins whose primary functions involve pore formation, transport of specific substrates, cell structure determination and membrane stabilization are discussed. The conservation of selected proteins in the above Pseudomonas species is also examined.

Structural and chemical characterization of the S layer of a Pseudomonas-like bacterium

Sections and freeze-fractured preparations showed an S layer on the surface of Pseudomonas-like strain EU2. Polyacrylamide gel electrophoresis of cell envelopes extracted with 1% sodium dodecyl sulfate (SDS) at room temperature showed three proteins (45K, 55K, and 110K). The 55K protein was identified as the S-layer protein. Incubation in 1.5 M guanidine hydrochloride removed the S layer from cell envelopes and dissociated the structure into subunits. The soluble 55K protein reassembled into planar sheets upon removal of the guanidine hydrochloride by dialysis. Electron microscopy and image processing indicated that these sheets had p4 symmetry in projection with a lattice constant of 13.2 +/- 0.1 nm (corresponding to 9.3 nm between adjacent fourfold axes). In some instances these reassemblies appeared to form small three-dimensional crystals which gave particularly clear views of the structure in projection because of the superimposition of information from a number of layers. A model is proposed with molecules having rounded lobes connected by a narrower linker region and joining at the lobes to form the fourfold axes of the array. The pattern superficially resembles those of other bacterial S layers, such as those of Aeromonas salmonicida, Aeromonas hydrophila, and Azotobacter vinelandii. Extraction of cell envelopes with 1% SDS at 50 degrees C released the 110K protein from the envelopes and removed an amorphous backing layer from the S layer. The 45K protein displayed heat-modifiable migration in SDS-polyacrylamide gel electrophoresis and was insoluble in SDS at 50 degrees C or in high concentrations of guanidine hydrochloride, suggesting that it was associated with the peptidoglycan.

Iron-regulated envelope proteins of mycobacteria grown in vitro and their occurrence in Mycobacterium avium and Mycobacterium leprae grown in vivo

Several iron-regulated envelope proteins (IREPs), 11-180 kDa, have been detected in preparations of walls and membranes of Mycobacterium smegmatis, in an armadillo-derived mycobacterium (ADM) and in M. avium. The same sized proteins from M. vacae appeared under both iron-deficient and iron-sufficient growth conditions. Two larger proteins, of 240 and 250 kDa, appeared in the membranes of M. smegmatis and M. avium only when grown iron-sufficiently but were constitutively present in both ADM and M. vaccae. The IREPs from M. smegmatis were not induced under zinc-deficient growth conditions. Three of the four IREPs (14, 21 and 29 kDa) recognized in M. avium grown in vitro were also recovered from membrane fractions of the same strain grown in mice. In addition, these membranes contained both the high-molecular-mass proteins associated with iron-sufficient growth conditions. Membranes of M. leprae, recovered from infected armadillos, showed the faint presence of a possible IREP at 29 kDa and wall preparations showed the presence of a 21-kDa protein. Membranes also contained the two larger proteins at 240 and 250 kDa. An explanation for the simultaneous occurrence of both low-iron-regulated and high-iron-regulated proteins is offered.

Cloning and characterization of a gene encoding an outer membrane protein required for siderophore-mediated uptake of Fe3+ in Pseudomonas putida WCS358

In iron-limited environments plant-growth-stimulating Pseudomonas putida WCS358 produces a yellow-green fluorescent siderophore called pseudobactin 358. Ferric pseudobactin 358 is efficiently taken up by cells of WCS358 but not by cells of another rhizophere-colonizing strain, Pseudomonas fluorescens WCS374. A gene bank containing partial Sau3A DNA fragments from WCS358 was constructed in a derivative of the broad-host-range cosmid pLAFR1. By mobilization of this gene bank to strain WCS374 a cosmid clone, pMR, which made WCS374 competent for the utilization of pseudobactin 358 was identified. By subcloning of the 29.4-kilobase (kb) insert of pMR the essential genetic information was localized on a BglII fragment of 5.3 kb. Tn5 mutagenesis limited the responsible gene to a region of approximately 2.5 kb within this fragment. Since the gene encodes an outer membrane protein with a predicted molecular mass of 90,000 daltons, it probably functions as the receptor for ferric pseudobactin 358. The gene is flanked by pseudobactin 358 biosynthesis genes on both sides and is on a separate transcriptional unit. WCS374 cells carrying pMR derivatives with Tn5 insertions in the putative receptor gene did not produce the 90,000-dalton protein anymore and were unable to take up Fe3+ via pseudobactin 358. In WCS358 cells as well as in WCS374 cells the gene is expressed only under iron-limited conditions.

Specificity of pyoverdine-mediated iron uptake among fluorescent Pseudomonas strains

Pyoverdine-mediated iron transport was determined for seven fluorescent Pseudomonas strains belonging to different species. For all strains, cell or cell outer membrane and iron(III)-pyoverdine combinations were compared with their homologous counterparts in uptake, binding, and cross-feeding experiments. For four strains (Pseudomonas putida ATCC 12633, Pseudomonas fluorescens W, P. fluorescens ATCC 17400, and Pseudomonas tolaasii NCPPB 2192), the pyoverdine-mediated iron transport appeared to be strictly strain specific; pyoverdine-facilitated iron uptake by iron-starved cells and binding of ferripyoverdine to the purified outer membranes of such cells were efficient only in the case of the homologous systems. Cross-feeding assays, in liquid or solid cultures, resulted, however, especially for P. fluorescens ATCC 17400, in some discrepancies compared with uptake and binding assays, suggesting that growth experiments are the least likely to yield correct information on specificity of the pyoverdine-mediated iron transport. For the three other strains (P. fluorescens ATCC 13525, P. chlororaphis ATCC 9446, and P. aeruginosa ATCC 15692), cross-reactivity was demonstrated by the uptake, binding, and cross-feeding experiments. In an attempt to determine which parts of the iron transport system were responsible for the specificity, the differences in amino acid composition of the pyoverdines, together with the differences observed at the level of the iron-sensitive outer membrane protein pattern of the seven strains, are discussed.

Systemic virulence of Erwinia chrysanthemi 3937 requires a functional iron assimilation system

In Erwinia chrysanthemi, conditions of iron starvation initiate production of a catechol-type siderophore and enhance production of three outer membrane polypeptides. Twenty-two mutants affected in the different stages of this iron assimilation system were isolated by mini-Mu insertion mutagenesis. All of them failed to induce systemic soft rot on axenically grown Saintpaulia plants. From the siderophore auxotrophs and the iron uptake mutants, clones having recovered the missing function(s) were isolated by using the in vivo cloning vector pULB113 (RP4::mini-Mu). An R-prime plasmid containing a ca. 35.5-kilobase-pair DNA insert was identified. Restoration of the iron functions restored partially, if not completely, the virulence of the parental strain.