Cloning of the gene coding for the outer membrane receptor protein for ferric pseudobactin, a siderophore from a plant growth-promoting Pseudomonas strain

The plant pathogen Pseudomonas fuscovaginae contains two conserved quorum sensing systems involved in virulence and negatively regulated by RsaL and the novel regulator RsaM

Pseudomonas fuscovaginae is a Gram-negative fluorescent pseudomonad pathogenic towards several plant species. Despite its importance as a plant pathogen, no molecular studies of virulence have thus far been reported. In this study we show that P. fuscovaginae possesses two conserved N-acyl homoserine lactone (AHL) quorum sensing (QS) systems which we designated PfsI/R and PfvI/R. The PfsI/R system is homologous to the BviI/R system of Burkholderia vietnamiensis and produces and responds to C10-HSL and C12-HSL whereas PfvI/R is homologous to the LasI/R system of Pseudomonas aeruginosa and produces several long-chain 3-oxo-HSLs and responds to 3-oxo-C10-HSL and 3-oxo-C12-HSL and at high AHL concentrations can also respond to structurally different long-chain AHLs. Both systems were found to be negatively regulated by a repressor protein which was encoded by a gene located intergenically between the AHL synthase and LuxR-family response regulator. The pfsI/R system was regulated by a novel repressor designated RsaM while the pfvI/R system was regulated by both the RsaL repressor and by RsaM. The two systems are not transcriptionally hierarchically organized but share a common AHL response and both are required for plant virulence. Pseudomonas fuscovaginae has therefore a unique complex regulatory network composed of at least two different repressors which directly regulate the AHL QS systems and pathogenicity.

LasI/R and RhlI/R quorum sensing in a strain of Pseudomonas aeruginosa beneficial to plants

Pseudomonas aeruginosa possesses three quorum-sensing (QS) systems which are key in the expression of a large number of genes, including many virulence factors. Most studies of QS in P. aeruginosa have been performed in clinical isolates and have therefore focused on its role in pathogenicity. P. aeruginosa, however, is regarded as a ubiquitous organism capable of colonizing many different environments and also of establishing beneficial associations with plants. In this study we examined the role of the two N-acyl homoserine lactone systems known as RhlI/R and LasI/R in the environmental rice rhizosphere isolate P. aeruginosa PUPa3. Both the Rhl and Las systems are involved in the regulation of plant growth-promoting traits. The environmental P. aeruginosa PUPa3 is pathogenic in two nonmammalian infection models, and only the double las rhl mutants are attenuated for virulence. In fact it was established that the two QS systems are not hierarchically organized and that they are both important for the colonization of the rice rhizosphere. This is an in-depth genetic and molecular study of QS in an environmental P. aeruginosa strain and highlights several differences with QS regulation in the clinical isolate PAO1.

Pseudomonas corrugata contains a conserved N-acyl homoserine lactone quorum sensing system; its role in tomato pathogenicity and tobacco hypersensitivity response

Pseudomonas corrugata is a phytopathogenic bacterium, causal agent of tomato pith necrosis, yet it is an ubiquitous bacterium that is part of the microbial community in the soil and in the rhizosphere of different plant species. Although it is a very heterogeneous species, all the strains tested were able to produce short chain acyl homoserine lactone (AHL) quorum sensing signal molecules. The main AHL produced was N-hexanoyl-L-homoserine lactone (C(6)-AHL). An AHL quorum sensing system, designated PcoI/PcoR, was identified and characterized. The role of the quorum sensing system in the expression of a variety of traits was evaluated. Inactivation of pcoI abolished the production of AHLs. The pcoR mutant, but not the pcoI mutant, was impaired in swarming, unable to cause a hypersensitivity response on tobacco and resulted in a reduced tomato pith necrosis phenotype. The pcoI mutant showed a reduced antimicrobial activity against various fungi and bacteria when assayed on King's B medium. These results demonstrate that the AHL quorum sensing in Ps. corrugata regulates traits that contribute to virulence, antimicrobial activity and fitness. This is the first report of genes of Ps. corrugata involved in the disease development and biological control activity.

Regulation of the N-acyl homoserine lactone-dependent quorum-sensing system in rhizosphere Pseudomonas putida WCS358 and cross-talk with the stationary-phase RpoS sigma factor and the global regulator GacA

Quorum sensing is a cell population-density dependent regulatory system which in gram-negative bacteria often involves the production and detection of N-acyl homoserine lactones (AHLs). Some Pseudomonas putida strains have been reported to produce AHLs, and one quorum-sensing locus has been identified. However, it appears that the majority of strains do not produce AHLs. In this study we report the identification and regulation of the AHL-dependent system of rhizosphere P. putida WCS358. This system is identical to the recently identified system of P. putida strain IsoF and very similar to the las system of Pseudomonas aeruginosa. It is composed of three genes, the luxI family member ppuI, the putative repressor rsaL, and the luxR family member ppuR. A genomic ppuR::Tn5 mutant of strain WCS358 was identified by its inability to produce AHLs when it was cross-streaked in close proximity to an AHL biosensor, whereas an rsaL::Tn5 genomic mutant was identified by its ability to overproduce AHL molecules. Using transcriptional promoter fusions, we studied expression profiles of the rsaL, ppuI, and ppuR promoters in various genetic backgrounds. At the onset of the stationary phase, the autoinducer synthase ppuI gene expression is under positive regulation by PpuR-AHL and under negative regulation by RsaL, indicating that the molecules could be in competition for binding at the ppuI promoter. In genomic rsaL::Tn5 mutants ppuI expression and production of AHL levels increased dramatically; however, both processes were still under growth phase regulation, indicating that RsaL is not involved in repressing AHL production at low cell densities. The roles of the global response regulator GacA and the stationary-phase sigma factor RpoS in the regulation of the AHL system at the onset of the stationary phase were also investigated. The P. putida WCS358 gacA gene was cloned and inactivated in the genome. It was determined that the three global regulatory systems are closely linked, with quorum sensing and RpoS regulating each other and GacA positively regulating ppuI expression. Studies of the regulation of AHL quorum-sensing systems have lagged behind other studies and are important for understanding how these systems are integrated into the overall growth phase and metabolic status of the cells.

Quorum-sensing system and stationary-phase sigma factor (rpoS) of the onion pathogen Burkholderia cepacia genomovar I type strain, ATCC 25416

Bacterial strains belonging to Burkholderia cepacia can be human opportunistic pathogens, plant pathogens, and plant growth promoting and have remarkable catabolic activity. B. cepacia consists of several genomovars comprising what is now known as the B. cepacia complex. Here we report the quorum-sensing system of a genomovar I onion rot type strain ATCC 25416. Quorum sensing is a cell-density-dependent regulatory response which involves the production of N-acyl homoserine lactone (HSL) signal molecules. The cep locus has been inactivated in the chromosome, and it has been shown that CepI is responsible for the biosynthesis of an N-hexanoyl HSL (C(6)-HSL) and an N-octanoyl HSL (C(8)-HSL) and that the cep locus regulates protease production as well as onion pathogenicity via the expression of a secreted polygalacturonase. A cep-lacZ-based sensor plasmid has been constructed and used to demonstrate that CepR responded to C(6)-HSL with only 15% of the molar efficiency of C(8)-HSL, that a cepR knockout mutant synthesized 70% less HSLs, and that CepR responded best towards long-chain HSLs. In addition, we also report the cloning and characterization of the stationary-phase sigma factor gene rpoS of B. cepacia ATCC 25416. It was established that quorum sensing in B. cepacia has a negative effect on rpoS expression as determined by using an rpoS-lacZ transcriptional fusion; on the other hand, rpoS-null mutants displayed no difference in the accumulation of HSL signal molecules.

Different responses of pyoverdine genes to autoinduction in Pseudomonas aeruginosa and the group Pseudomonas fluorescens-Pseudomonas putida

We investigated the regulation of the psbA and pvdA pyoverdine biosynthesis genes, which encode the L-ornithine N(5)-oxygenase homologues in Pseudomonas strain B10 and Pseudomonas aeruginosa PAO1, respectively. We demonstrate that pyoverdine(B10), as the end product of its biosynthetic pathway, is a key participant of the control circuit regulating its own production in Pseudomonas strain B10. In P. aeruginosa PAO1, however, pyoverdine(PAO1) has no apparent role in the positive regulation of the pvdA gene.

A siderophore peptide synthetase gene from plant-growth-promoting Pseudomonas putida WCS358

Under iron limiting conditions, Pseudomonas putida WCS358 produces and secretes a fluorescent siderophore called pseudobactin 358 which consists of a nonapeptide linked to a fluorescent dihydroxy quinoline moiety. Previous studies have identified a major gene cluster involved in pseudobactin 358 biosynthesis and several regulators responsible for the activation of biosynthetic genes under iron starving conditions. In this study, we identified the promoter transcribing the pseudobactin 358 synthetase gene. Promoter deletion experiments have demonstrated that the DNA region downstream of the initiation of transcription site is necessary for proper promoter functioning. This promoter controls the expression of a gene designated ppsD which encodes a 2,247-residue protein, PpsD, which has a predicted molecular weight of 247,610 Da and contains two highly homologous domains of approximately 1000 amino acids each. ppsD::Tn5 mutants of strain WCS358 are unable to synthesise pseudobactin 358 and can be complemented when ppsD is provided in trans. It is concluded that ppsD is a peptide synthetase involved in the biosynthesis of the peptide moiety of pseudobactin 358. PpsD displays a very high degree of similarity (52% aa identity) with PvdD from P. aeruginosa, a non-ribosomal peptide synthetase involved in the biosynthesis of pyoverdine, the fluorescent siderophore produced by P. aeruginosa. It also displayed homology with other peptide synthetases from other micro-organisms involved in the biosynthesis of siderophores and peptide antibiotics.

Regulation of rpoS gene expression in Pseudomonas: involvement of a TetR family regulator

The rpoS gene encodes the sigma factor which was identified in several gram-negative bacteria as a central regulator during stationary phase. rpoS gene regulation is known to respond to cell density, showing higher expression in stationary phase. For Pseudomonas aeruginosa, it has been demonstrated that the cell-density-dependent regulation response known as quorum sensing interacts with this regulatory response. Using the rpoS promoter of P. putida, we identified a genomic Tn5 insertion mutant of P. putida which showed a 90% decrease in rpoS promoter activity, resulting in less RpoS being present in a cell at stationary phase. Molecular analysis revealed that this mutant carried a Tn5 insertion in a gene, designated psrA (Pseudomonas sigma regulator), which codes for a protein (PsrA) of 26.3 kDa. PsrA contains a helix-turn-helix motif typical of DNA binding proteins and belongs to the TetR family of bacterial regulators. The homolog of the psrA gene was identified in P. aeruginosa; the protein showed 90% identity to PsrA of P. putida. A psrA::Tn5 insertion mutant of P. aeruginosa was constructed. In both Pseudomonas species, psrA was genetically linked to the SOS lexA repressor gene. Similar to what was observed for P. putida, a psrA null mutant of P. aeruginosa also showed a 90% reduction in rpoS promoter activity; both mutants could be complemented for rpoS promoter activity when the psrA gene was provided in trans. psrA mutants of both Pseudomonas species lost the ability to induce rpoS expression at stationary phase, but they retained the ability to produce quorum-sensing autoinducer molecules. PsrA was demonstrated to negatively regulate psrA gene expression in Pseudomonas and in Escherichia coli as well as to be capable of activating the rpoS promoter in E. coli. Our data suggest that PsrA is an important regulatory protein of Pseudomonas spp. involved in the regulatory cascade controlling rpoS gene regulation in response to cell density.

Regulation of the p-hydroxybenzoic acid hydroxylase gene (pobA) in plant-growth-promoting Pseudomonas putida WCS358

The regulation of the p-hydroxybenzoate hydroxylase gene (pobA) of Pseudomonas putida WCS358 involved in the catabolism of p-hydroxybenzoic acid (PHB) to the central intermediate protocatechuate was studied. Protocatechuic acid (PCA) is then degraded via the beta-ketoadipate pathway to form tricarboxylic acid intermediates. In several Gram-negative bacteria pobA has been found genetically linked to a regulator called pobR which activates pobA expression in response to PHB. In this study the identification and characterization of the pobC-pobA locus of P. putida WCS358 is presented. The p-hydroxybenzoate hydroxylase (PobA) is highly identical to other identified PobA proteins, whereas the regulatory protein PobC did not display very high identity to other PobR proteins studied and belonged to the AraC family of regulatory proteins, hence it has been designated POBC: Using the pobA promoter transcriptionally fused to a promoterless lacZ gene it was observed that induction via PobC occurred very efficiently when PHB was present and to a lesser but still significant level also in the presence of PCA. This PobC-PCA response was genetically demonstrated by making use of pobC::Tn5 and pcaH::Tn5 mutants of strain WCS358 constructed in this study. In pobC mutants both the p-hydroxybenzoic and PCA response were not observed, whereas in the pcaH mutant, which lacks a functional protocatechuate 3,4-dioxygenase, the protocatechuic-acid-dependent pobA activation was still observed. Finally, the activation of pobA by PHB varied according to the concentration and it was observed that in the pcaR::Tn5 regulatory mutant of strain WCS358 the pobA promoter activity was reduced. PcaR is a regulator involved in the regulation of several loci of the beta-ketoadipate pathway, one of which is pcaK. It was postulated that the reduction of pobA activation in pcaR::Tn5 mutants was because there was no expression of the pcaK gene encoding the PHB transport protein resulting in lower levels of PHB present inside the cell.

The ferripyoverdine receptor FpvA of Pseudomonas aeruginosa PAO1 recognizes the ferripyoverdines of P. aeruginosa PAO1 and P. fluorescens ATCC 13525

FpvA, the ferripyoverdine outer membrane receptor of Pseudomonas aeruginosa ATCC 15692 (PAO1 strain), is not specific to the pyoverdine produced by PAO1, but is also able to recognize the structurally different (ferri)pyoverdine of P. fluorescens ATCC 13525. The specificity of FpvA was assessed by iron uptake competitions using the wild-type strains P. aeruginosa ATCC 15692 and P. fluorescens ATCC 13525 and their respective ferripyoverdines, and by fpvA gene complementation of a FpvA-deficient mutant of P. aeruginosa ATCC 15692. The receptor mutant was able to utilize none of the two pyoverdines, while the same but fpvA-complemented mutant recovered simultaneously the ability to incorporate iron thanks to each of the two siderophores. The broad specificity of recognition of FpvA is viewed as an advantage for the strain in iron competition. Moreover, it allows an interesting approach for the understanding of the recognition mechanism between a (ferri)pyoverdine and its cognate outer membrane receptor.

Acquisition of iron by the non-siderophore-producing Pseudomonas fragi

The iron requirement, siderophore production and iron uptake mechanisms of the type strain Pseudomonas fragi ATCC 4973 and five P. fragi isolates from meat were analysed. The strains exhibited a high sensitivity to iron starvation: their growth was strongly inhibited in medium supplemented with the iron chelator ethylenediamine di(hydroxyphenylacetic acid) or in medium treated with 8-hydroxyquinoline to remove contaminating iron. No siderophores were detectable in the growth supernatants of iron-starved cells. Cross-feeding experiments in iron-depleted medium showed, however, that the bacterial growth could be strongly stimulated by siderophores of foreign origin including desferriferrioxamine B, enterobactin and some pyoverdines. Moreover, all the strains were capable of efficiently using the iron sources present in their natural environment, i.e., transferrin, lactoferrin and haemoglobin. Iron starvation led to the specific production of supplementary outer-membrane proteins of apparent molecular mass ranging from 80 to 88 kDa. Furthermore, growth in the presence of exogenous siderophores resulted, in some strains, in the induction of siderophore-mediated iron uptake systems. For one strain the concomitant synthesis of an iron-regulated, siderophore-inducible outer-membrane protein was observed.

Novel pyoverdine biosynthesis gene(s) of Pseudomonas aeruginosa PAO

Conjugational mobilization of a Pseudomonas aeruginosa PAO1 cosmid bank (in pMMB33) into a pyoverdine-deficient (pvd) mutant harbouring a mutation in the 47 min region of the chromosome yielded one clone which restored yellow-green pigmentation and fluorescence when grown on iron-deficient medium. The relevant pMMB33-derivative cosmid, pPYP17, contained a 15.1 kb insert which was subcloned into pKT240 as a 10.8 Sacl-CIal fragment conferring the same phenotype. This derivative, pPYP180, like pPYP17, also conferred an apparent wild-type phenotype on pvd mutants previously shown to map genetically in the 23 min region of the P. aeruginosa PAO chromosomes. Physical mapping indicated that the cloned DNA fragment is located at the 66-70 min region of the PAO chromosome, demonstrating that the restored apparent wild-type phenotype observed for the transconjugants was not the result of a true gene complementation. A gene interruption was obtained by replacing a 0.6 kb BgIll-BgIll region of pPYP180 necessary for the expression of the pigmentation/fluorescence phenotype, by a Hgr interposon (omega Hg). After conjugational transfer and introduction of the mutagenized fragment into the PAO1 chromosome by gene replacement, pyoverdine-deficient mutants were recovered, indicating that the fragment indeed contained at least one gene involved in pyoverdine synthesis. The yellow-green fluorescent compound produced by such cells harbouring plasmids pPYP17 or pPYP180 differed from pyoverdine in several aspects and was consequently named pseudoverdine. Although pseudoverdine was able to complex iron, it was unable to restore growth to pvd mutants in the presence of the iron chelator ethylenediamine di(o-hydroxyphenylacetic acid), or to mediate iron uptake into PAO1. Pseudoverdine lacked a peptide chain but possessed spectral properties similar to pyoverdine, suggesting that it was structurally related to the chromophore of the pyoverdine molecule. The recent structural determination of pseudoverdine as a coumarin derivative confirmed this view and sheds some light on the biosynthetic pathway of the pyoverdine chromophore.

Multiple outer membrane receptors for uptake of ferric pseudobactins in Pseudomonas putida WCS358

Under iron limitation Pseudomonas putida WCS358 produces a fluorescent siderophore, pseudobactin 358, which, after complexing iron, is transported back into the cell via the specific outer membrane receptor PupA. In addition, this strain has the capacity to take up iron via a large variety of siderophores produced by other fluorescent pseudomonads. Putative receptor genes for such siderophores were identified in the chromosome of strain WCS358 by PCR using primers matching two domains conserved in four ferric pseudobactin receptors, including PupA. Eleven amplification products within the expected size range were obtained. Sequence analysis confirmed that the products were derived from genes encoding outer membrane receptors. Two complete receptor genes were isolated from a genomic library of P. putida WCS358. Both protein products are involved in the transport of a limited number of specific ferric pseudobactins. These results indicate that the ability of P. putida WCS358 to exploit many different heterologous pseudobactins is related to the presence of multiple outer membrane receptor proteins.

Iron regulation of siderophore biosynthesis and transport in Pseudomonas putida WCS358: involvement of a transcriptional activator and of the Fur protein

Pseudobactin 358 is the yellow-green fluorescent siderophore produced by Pseudomonas putida WCS358 in conditions of iron limitation. The genes encoding for siderophore biosynthesis are iron-regulated at the transcriptional level. Previous work has shown that a positive regulator, PfrA, is absolutely required for the activation under iron-limiting conditions of pseudobactin 358 biosynthesis. In this study we identified a set of Tn5 insertion mutants of strain WCS358 which lost the ability to activate an iron-regulated siderophore promoter. These mutants no longer produced pseudobactin 358. Molecular analysis revealed that they carried a Tn5 insertion in a gene, designated pfrl (Pseudomonas ferric regulator), which codes for a protein (Pfrl) of 19.5 kDa. Pfrl contains a putative helix-turn-helix motif typical of DNA-binding proteins and has homology to two DNA-binding transcriptional activators, Fecl from Escherichia coli and Pupl from P. putida. The proposed role of Pfrl in strain WCS358 is an activator protein regulating pseudobactin 358 biosynthesis under iron limitation. The pfrl promoter region contains a sequence which displays high identity to the Fur-box consensus. This 19 bp consensus sequence is recognized by Fur, an iron-binding repressor protein found in many different bacteria. The E. coli Fur protein can bind to the pfrl promoter region, indicating that this activator gene is likely to be iron-regulated by Fur. We also report the identification and characterization of the P. putida WCS358 fur gene. The Fur protein of strain WCS358 is structurally and functionally similar to other cloned Fur proteins from other bacterial species.

Utilization of heterologous siderophores and rhizosphere competence of fluorescent Pseudomonas spp

In this study, the potential of different Pseudomonas strains to utilize heterologous siderophores was compared with their competitiveness in the rhizosphere of radish. This issue was investigated in interactions between Pseudomonas putida WCS358 and Pseudomonas fluoresceins WCS374 and in interactions between strain WCS358 and eight indigenous Pseudomonas strains capable of utilizing pseudobactin 358. During four successive plant growth cycles of radish, strain WCS358 significantly reduced rhizosphere population densities of the wild-type strain WCS374 by up to 30 times, whereas derivative strain WCS374(pMR), harboring the siderophore receptor PupA for ferric pseudobactin 358, maintained its population density. Studies involving interactions between strain WCS358 and eight different indigenous Pseudomonas strains demonstrated that despite the ability of these indigenous isolates to utilize pseudobactin 358, their rhizosphere population densities were significantly reduced by strain WCS358 by up to 20 times. Moreover, rhizosphere colonization by WCS358 was not affected by any of these indigenous strains, even though siderophore-mediated growth inhibition of WCS358 by a majority of these strains was demonstrated in a plate bioassay. In conclusion, it can be stated that siderophore-mediated competition for iron is a major determinant in interactions between WCS358 and WCS374 in the rhizosphere. Moreover, our findings support the common assumption that cloning of siderophore receptor genes from one Pseudomonas strain into another can confer a competitive advantage in interactions in the rhizosphere. Interactions between WCS358 and the selected indigenous rhizosphere isolates, however, indicate that other traits also contribute to the rhizosphere competence of fluorescent Pseudomonas spp.Key words: siderophore, siderophore receptors, root colonization, fluorescent Pseudomonas.

Nucleotide sequence of pvdD, a pyoverdine biosynthetic gene from Pseudomonas aeruginosa: PvdD has similarity to peptide synthetases

Pseudomonas aeruginosa secretes a fluorescent siderophore, pyoverdine, when grown under iron-deficient conditions. Pyoverdine consists of a chromophoric group bound to a partly cyclic octapeptide. As a step toward understanding the molecular events involved in pyoverdine synthesis, we have sequenced a gene, pvdD, required for this process. The gene encodes a 2,448-residue protein, PvdD, which has a predicted molecular mass of 273,061 Da and contains two highly similar domains of about 1,000 amino acids each. The protein is similar to peptide synthetases from a range of bacterial and fungal species, indicating that synthesis of the peptide moiety of pyoverdine proceeds by a nonribosomal mechanism. The pvdD gene is adjacent to a gene, fpvA, which encodes an outer membrane receptor protein required for uptake of ferripyoverdine.

Localization of functional domains in the Escherichia coli coprogen receptor FhuE and the Pseudomonas putida ferric-pseudobactin 358 receptor PupA

Transport of ferric-siderophores across the outer membrane of gram-negative bacteria is mediated by specific outer membrane receptors. To localize the substrate-binding domain of the ferric-pseudobactin 358 receptor, PupA, of Pseudomonas putida WCS358, we constructed chimeric receptors in which different domains of PupA were replaced by the corresponding domains of the related ferric-pseudobactin receptors PupB and PupX, or the coprogen receptor FhuE of Escherichia coli. None of the chimeric proteins composed of pseudobactin receptor domains facilitated growth on any of the original substrates, or they showed only an extremely low efficiency. However, these receptors enabled cells of Pseudomonas BN8 to grow on media supplemented with uncharacterized siderophore preparations. These siderophore preparations were isolated from the culture supernatant of WCS358 cells carrying plasmids that contain genes of Pseudomonas B10 required for the biosynthesis of pseudobactin B10. Hybrid proteins that contained at least the amino-terminal 516 amino acids of mature FhuE were active as a receptor for coprogen and interacted with the E. coli TonB protein. A chimeric PupA-FhuE protein, containing the amino-terminal 94 amino acids of mature PupA, was also active as a coprogen receptor, but only in the presence of Pseudomonas TonB. It is concluded that the carboxy-terminal domain of ferric-pseudobactin receptors is important, but not sufficient, for ligand interaction, whereas binding of coprogen by the FhuE receptor is not dependent on this domain. Apparently, the ligand-binding sites of different receptors are located in different regions of the proteins. Furthermore, species-specific TonB binding by the PupA receptor is dependent on the amino-terminal domain of the receptor.

Amplification of the groESL operon in Pseudomonas putida increases siderophore gene promoter activity

Pseudobactin 358 is the yellow-green fluorescent siderophore [microbial iron(III) transport agent] produced by Pseudomonas putida WCS358 under iron-limiting conditions. The genes encoding pseudobactin 358 biosynthesis are iron-regulated at the level of transcription. In this study, the molecular characterization is reported of a cosmid clone of WCS358 DNA that can stimulate, in an iron-dependent manner, the activity of a WCS358 siderophore gene promoter in the heterologous Pseudomonas strain A225. The functional region in the clone was identified by subcloning, transposon mutagenesis and DNA sequencing as the groESL operon of strain WCS358. This increase in promoter activity was not observed when the groESL genes of strain WCS358 were integrated via a transposon vector into the genome of Pseudomonas A225, indicating that multiple copies of the operon are necessary for the increase in siderophore gene promoter activity. Amplification of the Escherichia coli and WCS358 groESL genes also increased iron-regulated promoter activity in the parent strain WCS358. The groESL operon codes for the chaperone proteins GroES and GroEL, which are responsible for mediating the folding and assembly of many proteins.

Isolation of a gene (pbsC) required for siderophore biosynthesis in fluorescent Pseudomonas sp. strain M114

An iron-regulated gene, pbsC, required for siderophore production in fluorescent Pseudomonas sp. strain M114 has been identified. A kanamycin-resistance cassette was inserted at specific restriction sites within a 7 kb genomic fragment of M114 DNA and by marker exchange two siderophore-negative mutants, designated M1 and M2, were isolated. The nucleotide sequence of approximately 4 kb of the region flanking the insertion sites was determined and a large open reading frame (ORF) extending for 2409 bp was identified. This gene was designated pbsC (pseudobactin synthesis C) and its putative protein product termed PbsC. PbsC was found to be homologous to a family of enzymes involved in the biosynthesis of secondary metabolites, including EntF of Escherichia coli. These enzymes are believed to act via ATP-dependent binding of AMP to their substrate. Several areas of high sequence homology between these proteins and PbsC were observed, including a conserved AMP-binding domain. The expression of pbsC is iron-regulated as revealed when a DNA fragment containing the upstream region was cloned in a promoter probe vector and conjugated into the wild-type strain, M114. The nucleotide sequence upstream of the putative translational start site contains a region homologous to previously defined -16 to -25 sequences of iron-regulated genes but did not contain an iron-box consensus sequence. It was noted that inactivation of the pbsC gene also affected other iron-regulated phenotypes of Pseudomonas M114.

Siderophore receptor PupA as a marker to monitor wild-type Pseudomonas putida WCS358 in natural environments

For application of genetically engineered fluorescent Pseudomonas spp., specific markers are required for monitoring of wild-type Pseudomonas strains and their genetically modified derivatives in natural environments. In this study, the specific siderophore receptor PupA of plant growth-promoting Pseudomonas putida WCS358 was used as a marker to monitor wild-type strain WCS358. After introduction into natural soil and rhizosphere environments, strain WCS358 could be recovered efficiently on a medium amended with 300 microM pseudobactin 358. Although low population densisties of indigenous pseudomonads (less than or equal to 10(3)/g of soil or root) were recovered on the pseudobactin 358-amended medium, subsequent agglutination assays with a WCS358-specific polyclonal antiserum enabled accurate monitoring of populations of wild-type strain WCS358 over a range of approximately 10(3) to 10(7) CFU/g of soil or root. Genetic analysis of the background population by PCR and Southern hybridization revealed that natural occurrence of the pupA gene was limited to a very small number of indigenous Pseudomonas spp. which are very closely related to P. putida WCS358. The PupA marker system enabled the study of differences in rhizosphere colonization among wild-type strain WCS358, rifampin-resistant derivative WCS358rr, and Tn5 mutant WCS358::xylE. Chromosomally mediated rifampin resistance did not affect the colonizing ability of P. putida WCS358. However, Tn5 mutant WCS358::xylE colonized the radish rhizosphere significantly less than did its parental strain.

Cloning and nucleotide sequence of the pvdA gene encoding the pyoverdin biosynthetic enzyme L-ornithine N5-oxygenase in Pseudomonas aeruginosa

The enzyme L-ornithine N5-oxygenase catalyzes the hydroxylation of L-ornithine (L-Orn), which represents an early step in the biosynthesis of the peptidic moiety of the fluorescent siderophore pyoverdin in Pseudomonas aeruginosa. A gene bank of DNA from P. aeruginosa PAO1 (ATCC 15692) was constructed in the broad-host-range cosmid pLAFR3 and mobilized into the L-Orn N5-oxygenase-defective (pvdA) P. aeruginosa mutant PALS124. Screening for fluorescent transconjugants made it possible to identify the trans-complementing cosmid pPV4, which was able to restore pyoverdin synthesis and L-Orn N5-oxygenase activity in the pvdA mutant PALS124. The 17-kb PAO1 DNA insert of pPV4 contained at least two genetic determinants involved in pyoverdin synthesis, i.e., pvdA and pvdC4, as shown by complementation analysis of a set of mutants blocked in different steps of the pyoverdin biosynthetic pathway. Deletion analysis, subcloning, and transposon mutagenesis enabled us to locate the pvdA gene in a minimum DNA fragment of 1.7 kb flanked by two SphI restriction sites. Complementation of the pvdA mutation was under stringent iron control; both pyoverdin synthesis and L-Orn N5-oxygenase activity were undetectable in cells of the trans-complemented mutant which had been grown in the presence of 100 microM FeCl3. The entire nucleotide sequence of the pvdA gene, from which the primary structure of the encoded polypeptide was deduced, was determined. The pvdA structural gene is 1,278 bp; the cloned DNA fragment contains at the 5' end of the gene a putative ribosome-binding site but apparently lacks known promoterlike sequences. The P. aeruginosa L-Orn N5-oxygenase gene codes for a 426-amino-acid peptide with a predicted molecular mass of 47.7 kDa and an isoelectric point of 8.1. The enzyme shows approximately 50% homology with functional analogs, i.e., L-lysine N6-hydroxylase of aerobactin-producing Escherichia coli and L-Orn N5-oxygenase of ferrichrome-producing Ustilago maydis. The pvdA gene was expressed in P. aeruginosa under the control of the T7 promoter. Induction of the T7 RNA polymerase system resulted in parallel increases of the L-Orn N5-oxygenase activity and of the amount of a 47.7-kDa polypeptide. We also constructed a site-specific pvdA mutant by insertion of a tetracycline-resistance cassette in the chromosomal pvdA gene of P. aeruginosa PAO1. Similarly to strain PALS124, the pvdA mutant obtained by gene disruption also disclosed no pyoverdin synthesis, lacked L-Orn N5-oxygenase activity, was complemented by the cloned pvdA gene, and produced pyoverdin at wild-type levels when fed with the biosynthetic precursor L-N5-OH-Orn. Southern blot analysis indicated that genes homologous to pvdA could be located within a 1.7-kb DNA fragment from SphI-digested genomic DNA of different hydroxamate-producing Pseudomonas spp. Our results suggest that omega-amino acid oxygenases have been conserved over a wide evolutionary range and probably evolved from a common ancestor.

Identification and characterization of a siderophore regulatory gene (pfrA) of Pseudomonas putida WCS358: homology to the alginate regulatory gene algQ of Pseudomonas aeruginosa

Genes encoding biosynthesis of pseudobactin 358 (a microbial iron transport agent) and its cognate outer membrane receptor protein, PupA, are transcribed only under iron limitation in plant growth-promoting Pseudomonas putida WCS358. Two cosmid clones were identified from a gene bank of WCS358 DNA which could independently and in an iron-dependent manner activate transcription from a WCS358 siderophore gene promoter in heterologous Pseudomonas strain A225. The functional region of one of the clones was localized by subcloning, transposon Tn3Gus mutagenesis, and DNA sequencing. Genomic transposon insertion mutants in the functional region lost the capacity to activate a siderophore gene promoter fusion transcriptionally; furthermore, these mutants no longer produced pseudobactin 358. The activating region consisted of a single gene designated pfrA (Pseudomonas ferric regulator). The pfrA gene codes for a single polypeptide, PfrA, of approximately 18 kDa, which has 58% identity to AlgQ (also known as AlgR2), a positive regulator involved in transcriptionally regulating alginate biosynthesis in Pseudomonas aeruginosa. Cross-complementation studies between the pfrA gene of P. putida and the algQ gene of P. aeruginosa revealed that pfrA can restore mucoidy (alginate production) in an algQ mutant and that algQ could poorly complement a pfrA genomic mutant. It is concluded that PfrA is involved in the positive regulation of siderophore biosynthetic genes in response to iron limitation; furthermore, pfrA and algQ appeared to be interchangeable between P. putida and P. aeruginosa.

Identification and characterization of the pupB gene encoding an inducible ferric-pseudobactin receptor of Pseudomonas putida WCS358

Pseudomonas putida WCS358 can transport iron complexed to a wide variety of pseudobactins produced by other Pseudomonas strains. The pupB gene encoding an outer membrane ferric-pseudobactin receptor was isolated from a genomic library of P. putida WCS358. The PupB receptor facilitated iron transport via two distinct heterologous siderophores, i.e. pseudobactin BN8 and pseudobactin BN7. The amino acid sequence deduced from the nucleotide sequence consisted of 804 amino acids (molecular weight 88,369) of which the N-terminal part was very similar to a prokaryotic leader peptide. The mature protein shared significant homology with the receptor for ferric-pseudobactin 358 (PupA) and contained three regions common to TonB-dependent receptor proteins of Escherichia coli. Interestingly, PupB expression was only observed in cells cultured in iron-deficient medium containing pseudobactin BN8 or pseudobactin BN7. This expression required a transcriptional unit, pupR, identified upstream of the structural pupB gene. Transposon Tn5 insertion mutants defective in PupB production still exhibited uptake of iron via pseudobactin BN8, although with reduced efficiency. Apparently, an additional transport system for this ferric-siderophore complex operates in this strain. In addition to pseudobactin BN8 also other heterologous siderophores were capable of inducing synthesis of specific high-molecular-weight outer membrane proteins in strain WCS358, which suggests the existence of multiple siderophore-inducible iron transport systems in this strain.

Siderophore-mediated iron transport correlates with the presence of specific iron-regulated proteins in the outer membrane of Rhizobium meliloti

A universal chemical assay used to detect the production of siderophores in a range of Rhizobium strains showed that production is strain specific. Iron nutrition bioassays carried out on Rhizobium meliloti strains to determine cross-utilization of their siderophores showed that R. meliloti 2011, 220-5, and 220-3 could each use the siderophores produced by the other two but not the siderophore produced by R. meliloti DM4 (and vice versa). Mutants of R. meliloti 2011 and 220-5 defective in siderophore production were isolated by Tn5-mob mutagenesis. The Tn5-mob-containing EcoRI fragment of mutant R. meliloti 220-5-1 was cloned into pUC19. By using this fragment as a probe, the presence of a homologous region was observed in R. meliloti 2011 and 220-3 but not in R. meliloti DM4. A complementing cosmid from a gene bank of R. meliloti 2011 was identified by using the same probe. Introduction of this cosmid into R. meliloti 102F34, a strain not producing a siderophore, resulted in the ability of this strain to produce a siderophore and also in the ability to utilize the siderophores produced by R. meliloti 2011, 220-5, and 220-3 but not the siderophore produced by R. meliloti DM4. A comparative analysis of the outer membrane proteins prepared from iron-deficient cultures of R. meliloti 102F34 and 102F34 harboring the cosmid revealed the presence, in the latter, of a low-iron-induced outer membrane protein corresponding to a low-iron-induced protein in R. meliloti 2011, 220-5, and 220-3. This protein is not present in R. meliloti DM4. The results suggest that R. meliloti 2011, 220-5, and 220-3 produce siderophores that are identical or sufficiently similar in structure to be transported by the membrane transport system of each strain while also indicating that utilization of a particular siderophore is correlated with the presence of specific outer membrane proteins.

Regulation of iron assimilation: nucleotide sequence analysis of an iron-regulated promoter from a fluorescent pseudomonad

An iron-regulated promoter was cloned on a 2.1 kb Bg/II fragment from Pseudomonas sp. strain M114 and fused to the lacZ reporter gene. Iron-regulated lacZ expression from the resulting construct (pSP1) in strain M114 was mediated via the Fur-like repressor which also regulates siderophore production in this strain. A 390 bp StuI-PstI internal fragment contained the necessary information for iron-regulated promoter expression. This fragment was sequenced and the initiation point for transcription was determined by primer extension analysis. The region directly upstream of the transcription start point contained no significant homology to known promoter consensus sequences. However the -16 to -25 bp region contained homology to four other iron-regulated pseudomonad promoters. Deletion of bases downstream from the transcriptional start did not affect the iron-regulated expression of the promoter. The -37 and -43 bp regions exhibited some homology to the 19 bp Escherichia coli Fur-binding consensus sequence. When expressed in E. coli (via a cloned transacting factor from strain M114) lacZ expression from pSP1 was found to be regulated by iron. A region of greater than 77 bases but less than 131 upstream from the transcriptional start was found to be necessary for promoter activity, further suggesting that a transcriptional activator may be required for expression.

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.

Evidence for different pyoverdine-mediated iron uptake systems among Pseudomonas aeruginosa strains

Fourteen strains of Pseudomonas aeruginosa (P. aeruginosa ATCC 15692, P. aeruginosa ATCC 27853, and 12 clinical isolates) were checked for the production of pyoverdine and for pyoverdine-mediated iron uptake. Under iron restriction, two isolates produced undetectable amounts of pyoverdine, but all the other strains produced a compound with physicochemical properties identical or close to those of the pyoverdine of P. aeruginosa ATCC 15692 (strain PAO1). The pyoverdines were purified and tested for their growth-promoting activity and for their ability to facilitate 59Fe uptake in homologous experiments involving each pyoverdine and its producing strain, as well as in heterologous systems involving all the other strains. The results of both types of experiments suggested the existence of three specificity groups. This was confirmed by analysis of the amino acid composition of the pyoverdines, which differed for each group. A partially purified polyclonal antiserum raised against a major 80-kilodalton (kDa) iron-regulated outer membrane protein (IROMP) of P. aeruginosa PAO1 recognized the 80-kDa IROMPs from P. aeruginosa PAO1 and the clinical isolates belonging to the same group, whereas the IROMPs from the strains belonging to the two other groups were not detected. A second antiserum raised against the P. aeruginosa ATCC 27853 80-kDa IROMP gave similar results by reacting specifically with the 80-kDa IROMP from the strains belonging to this group. Thus, together with the already known pyoverdine from P. aeruginosa PAO1, two new types of pyoverdines produced by strains belonging to this species were characterized.

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.

Synthesis of a mucin-type O-glycosylated amino acid, beta-Gal-(1----3)-[alpha-Neu5Ac-(2----6)]-alpha-GalNAc-(1----3)- Ser

Total synthesis of O-beta-D-galactopyranosyl-(1----3)-O-[(5-acetamido-3,5-dideoxy- D-glycero-alpha-D-galacto-2-nonulopyranosylonic acid)-(2----6)]-O-(2-acetamido-2-deoxy-alpha-D-galactopyranosyl)-(1----3 )-L- serine was achieved by use of the key glycosyl donor O-(2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl)-(1----3)-O- [methyl (5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-alpha-D-galact o-2- nonulopyranosyl)onate-(2----6)]-4-O-acetyl-2-azido-2-deoxy-a lpha-D- galactopyranosyl trichloroacetimidate and the key glycosyl acceptor N-(benzyloxycarbonyl)-L- serine benzyl ester in a regiocontrolled way.

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.

Genetic organization and transcriptional analysis of a major gene cluster involved in siderophore biosynthesis in Pseudomonas putida WCS358

In iron-limited environments, the plant-growth-stimulating Pseudomonas putida WCS358 produces a yellow-green fluorescent siderophore called pseudobactin 358. The transcriptional organization and the iron-regulated expression of a major gene cluster involved in the biosynthesis and transport of pseudobactin 358 were analyzed in detail. The cluster comprises a region with a minimum length of 33.5 kilobases and contains at least five transcriptional units, of which some are relatively large. The directions of transcription of four transcriptional units were determined by RNA-RNA hybridization and by analysis in Escherichia coli minicells. The latter also demonstrated that large polypeptides were encoded by these transcriptional units. The results allowed us to localize several promoter regions on the DNA. The iron-dependent expression of at least two genes within this cluster appears to be regulated at the transcriptional level.

Characterization of Pyoverdin pss , the Fluorescent Siderophore Produced by Pseudomonas syringae pv. syringae

Pseudomonas syringae pv. syringae B301D produces a yellow-green, fluorescent siderophore, pyoverdin pss , in large quantities under iron-limited growth conditions. Maximum yields of pyoverdin pss of approximately 50 μg/ml occurred after 24 h of incubation in a deferrated synthetic medium. Increasing increments of Fe(III) coordinately repressed siderophore production until repression was complete at concentrations of ≥ 10 μM. Pyoverdin pss was isolated, chemically characterized, and found to resemble previously characterized pyoverdins in spectral traits (absorbance maxima of 365 and 410 nm for pyoverdin pss and its ferric chelate, respectively), size (1,175 molecular weight), and amino acid composition. Nevertheless, pyoverdin pss was structurally unique since amino acid analysis of reductive hydrolysates yielded β-hydroxyaspartic acid, serine, threonine, and lysine in a 2:2:2:1 ratio. Pyoverdin pss exhibited a relatively high affinity constant for Fe(III), with values of 10 25 at pH 7.0 and 10 32 at pH 10.0. Iron uptake assays with [ 55 Fe]pyoverdin pss demonstrated rapid active uptake of 55 Fe(III) by P. syringae pv. syringae B301D, while no uptake was observed for a mutant strain unable to acquire Fe(III) from ferric pyoverdin pss . The chemical and biological properties of pyoverdin pss are discussed in relation to virulence and iron uptake during plant pathogenesis.

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

In an iron-limited environment Pseudomonas syringae pv. syringae B301D produces a yellow-green fluorescent siderophore called pyoverdinpss which functions in high-affinity iron transport. Two-dimensional electrophoretic comparisons of the outer membrane proteins of strain B301D identified nine proteins which were expressed at low (50 nM) but not at high (10 microM) iron concentrations. Except for the minor protein 8e, the iron-regulated proteins exhibited high molecular weights ranging from approximately 74,000 to 80,000. A mutant of strain B301D incapable of iron uptake (Iu-) from ferric pyoverdinpss lacked the 74,000-molecular-weight protein 4a, which was the major iron-regulated outer membrane protein. In contrast, a nonfluorescent mutant (Flu-) unable to synthesize pyoverdinpss showed no quantitative or qualitative difference in its outer membrane profile from that of the wild-type strain. In plant pathogenicity tests the Iu- and Flu- strains caused typical brown necrotic and sunken lesions in immature sweet cherry fruit which were indistinguishable from those of the wild-type strain. Thus, excretion of pyoverdinpss and subsequent Fe(III) uptake do not have a determinative role in the pathogenicity or virulence of P. syringae pv. syringae.