Regulation of tabtoxin production by the lemA gene in Pseudomonas syringae

Innate functions of natural products: A promising path for the identification of novel therapeutics

In the course of evolution, living organisms have become well equipped with diverse natural products that serve important functions, including defence from biotic and abiotic stress, growth regulation, reproduction, metabolism, and epigenetic regulation. It seems to be the organism's ecological niche that influences the natural product's structural and functional diversity. Indeed, natural products constitute the nuts and bolts of molecular co-evolution and ecological relationships among different life forms. Since natural products in the form of specialized secondary metabolites exhibit biological functions via interactions with specific target proteins, they can provide a simultaneous glimpse of both new therapeutics and therapeutic targets in humans as well. In this review, we have discussed the innate role of natural products in the ecosystem and how this intrinsic role provides a futuristic opportunity to identify new drugs and therapeutic targets rapidly.

The Pbo Cluster from Pseudomonas syringae pv. Phaseolicola NPS3121 Is Thermoregulated and Required for Phaseolotoxin Biosynthesis

The bean (Phaseolus vulgaris) pathogen Pseudomonas syringae pv. phaseolicola NPS3121 synthesizes phaseolotoxin in a thermoregulated way, with optimum production at 18 °C. Gene PSPPH_4550 was previously shown to be thermoregulated and required for phaseolotoxin biosynthesis. Here, we established that PSPPH_4550 is part of a cluster of 16 genes, the Pbo cluster, included in a genomic island with a limited distribution in P. syringae and unrelated to the possession of the phaseolotoxin biosynthesis cluster. We identified typical non-ribosomal peptide synthetase, and polyketide synthetase domains in several of the pbo deduced products. RT-PCR and the analysis of polar mutants showed that the Pbo cluster is organized in four transcriptional units, including one monocistronic and three polycistronic. Operons pboA and pboO are both essential for phaseolotoxin biosynthesis, while pboK and pboJ only influence the amount of toxin produced. The three polycistronic units were transcribed at high levels at 18 °C but not at 28 °C, whereas gene pboJ was constitutively expressed. Together, our data suggest that the Pbo cluster synthesizes secondary metabolite(s), which could participate in the regulation of phaseolotoxin biosynthesis.

Two Homologues of the Global Regulator Csr/Rsm Redundantly Control Phaseolotoxin Biosynthesis and Virulence in the Plant Pathogen Pseudomonas amygdali pv. phaseolicola 1448A

The widely conserved Csr/Rsm (carbon storage regulator/repressor of stationary-phase metabolites) post-transcriptional regulatory system controls diverse phenotypes involved in bacterial pathogenicity and virulence. Here we show that Pseudomonas amygdali pv. phaseolicola 1448A contains seven rsm genes, four of which are chromosomal. In RNAseq analyses, only rsmE was thermoregulated, with increased expression at 18 °C, whereas the antagonistic sRNAs rsmX1, rsmX4, rsmX5 and rsmZ showed increased levels at 28 °C. Only double rsmA-rsmE mutants showed significantly altered phenotypes in functional analyses, being impaired for symptom elicitation in bean, including in planta growth, and for induction of the hypersensitive response in tobacco. Double mutants were also non-motile and were compromised for the utilization of different carbon sources. These phenotypes were accompanied by reduced mRNA levels of the type III secretion system regulatory genes hrpL and hrpA, and the flagellin gene, fliC. Biosynthesis of the phytotoxin phaseolotoxin by mutants in rsmA and rsmE was delayed, occurring only in older cultures, indicating that these rsm homologues act as inductors of toxin synthesis. Therefore, genes rsmA and rsmE act redundantly, although with a degree of specialization, to positively regulate diverse phenotypes involved in niche colonization. Additionally, our results suggest the existence of a regulatory molecule different from the Rsm proteins and dependent on the GacS/GacA (global activator of antibiotic and cyanide production) system, which causes the repression of phaseolotoxin biosynthesis at high temperatures.

Inhibitory effect of Thymus vulgaris and Origanum vulgare essential oils on virulence factors of phytopathogenic Pseudomonas syringae strains

Pseudomonas syringae is a phytopathogenic bacterium that causes lesions in leaves during the colonisation process. The damage is associated with production of many virulence factors, such as biofilm and phytotoxins. The essential oils of Thymus vulgaris (thyme) and Origanum vulgare (oregano) have been demonstrated to inhibit P. syringae. The aim of this study was to investigate the effects of T. vulgaris and O. vulgare essential oils on production of virulence factors of phytopathogenic P. syringae strains, including anti-biofilm and anti-toxins activities. The broth microdilution method was used for determination of MIC and biofilm inhibition assays. Coronatine, syringomycin and tabtoxin were pheno- and genotypically evaluated. Both oils showed good inhibitory activity against P. syringae, with MIC values from 1.43 to 11.5 mg·ml^-1 for thyme and 5.8 to 11.6 mg·ml^-1 for oregano. Biofilm formation, production of coronatine, syringomycin and tabtoxin were inhibited by thyme and oregano essential oil in most strains. The results presented here are promising, demonstrating the bactericidal activity and reduction of virulence factor production after treatment with thyme and oregano oil, providing insight into how they exert their antibacterial activity. These natural products could be considered in the future for the control of diseases caused by P. syringae.

Regulation of bacterial virulence by Csr (Rsm) systems

Most bacterial pathogens have the remarkable ability to flourish in the external environment and in specialized host niches. This ability requires their metabolism, physiology, and virulence factors to be responsive to changes in their surroundings. It is no surprise that the underlying genetic circuitry that supports this adaptability is multilayered and exceedingly complex. Studies over the past 2 decades have established that the CsrA/RsmA proteins, global regulators of posttranscriptional gene expression, play important roles in the expression of virulence factors of numerous proteobacterial pathogens. To accomplish these tasks, CsrA binds to the 5' untranslated and/or early coding regions of mRNAs and alters translation, mRNA turnover, and/or transcript elongation. CsrA activity is regulated by noncoding small RNAs (sRNAs) that contain multiple CsrA binding sites, which permit them to sequester multiple CsrA homodimers away from mRNA targets. Environmental cues sensed by two-component signal transduction systems and other regulatory factors govern the expression of the CsrA-binding sRNAs and, ultimately, the effects of CsrA on secretion systems, surface molecules and biofilm formation, quorum sensing, motility, pigmentation, siderophore production, and phagocytic avoidance. This review presents the workings of the Csr system, the paradigm shift that it generated for understanding posttranscriptional regulation, and its roles in virulence networks of animal and plant pathogens.

Mangotoxin production of Pseudomonas syringae pv. syringae is regulated by MgoA

BACKGROUND

The antimetabolite mangotoxin is a key factor in virulence of Pseudomonas syringae pv. syringae strains which cause apical necrosis of mango trees. Previous studies showed that mangotoxin biosynthesis is governed by the mbo operon. Random mutagenesis led to the identification of two other gene clusters that affect mangotoxin biosynthesis. These are the gacS/gacA genes and mgo operon which harbors the four genes mgoBCAD.

RESULTS

The current study shows that disruption of the nonribosomal peptide synthetase (NRPS) gene mgoA resulted in loss of mangotoxin production and reduced virulence on tomato leaves. Transcriptional analyses by qPCR and promoter reporter fusions revealed that mbo expression is regulated by both gacS/gacA and mgo genes. Also, expression of the mgo operon was shown to be regulated by gacS/gacA. Heterologous expression under the native promoter of the mbo operon resulted in mangotoxin production in non-producing P. syringae strains, but not in other Pseudomonas species. Also introduction of the mbo and mgo operons in nonproducing P. protegens Pf-5 did not confer mangotoxin production but did enhance transcription of the mbo promoter.

CONCLUSIONS

From the data obtained in this study, we conclude that both mbo and mgo operons are under the control of the gacS/gacA two-component system and that the MgoA product acts as a positive regulator of mangotoxin biosynthesis.

Multiple classes of immune-related proteases associated with the cell death response in pepper plants

Proteases regulate a large number of biological processes in plants, such as metabolism, physiology, growth, and defense. In this study, we carried out virus-induced gene silencing assays with pepper cDNA clones to elucidate the biological roles of protease superfamilies. A total of 153 representative protease genes from pepper cDNA were selected and cloned into a Tobacco rattle virus-ligation independent cloning vector in a loss-of-function study. Silencing of 61 proteases resulted in altered phenotypes, such as the inhibition of shoot growth, abnormal leaf shape, leaf color change, and lethality. Furthermore, the silencing experiments revealed that multiple proteases play a role in cell death and immune response against avirulent and virulent pathogens. Among these 153 proteases, 34 modulated the hypersensitive cell death response caused by infection with an avirulent pathogen, and 16 proteases affected disease symptom development caused by a virulent pathogen. Specifically, we provide experimental evidence for the roles of multiple protease genes in plant development and immune defense following pathogen infection. With these results, we created a broad sketch of each protease function. This information will provide basic information for further understanding the roles of the protease superfamily in plant growth, development, and defense.

Effect of overexpressing rsmA from Pseudomonas aeruginosa on virulence of select phytotoxin-producing strains of P. syringae

The GacS/GacA two-component system functions mechanistically in conjunction with global post-transcriptional regulators of the RsmA family to allow pseudomonads and other bacteria to adapt to changing environmental stimuli. Analysis of this Gac/Rsm signal transduction pathway in phytotoxin-producing pathovars of Pseudmonas syringae is incomplete, particularly with regard to rsmA. Our approach in studying it was to overexpress rsmA in P. syringae strains through introduction of pSK61, a plasmid constitutively expressing this gene. Disease and colonization of plant leaf tissue were consistently diminished in all P. syringae strains tested (pv. phaseolicola NPS3121, pv. syringae B728a, and BR2R) when harboring pSK61 relative to these isolates harboring the empty vector pME6031. Phaseolotoxin, syringomycin, and tabtoxin were not produced in any of these strains when transformed with pSK61. Production of protease and pyoverdin as well as swarming were also diminished in all of these strains when harboring pSK61. In contrast, alginate production, biofilm formation, and the hypersensitive response were diminished in some but not all of these isolates under the same growth conditions. These results indicate that rsmA is consistently important in the overarching phenotypes disease and endophtyic colonization but that its role varies with pathovar in certain underpinning phenotypes in the phytotoxin-producing strains of P. syringae.

The mbo operon is specific and essential for biosynthesis of mangotoxin in Pseudomonas syringae

Mangotoxin is an antimetabolite toxin produced by certain Pseudomonas syringae pv. syringae strains. This toxin is an oligopeptide that inhibits ornithine N-acetyl transferase, a key enzyme in the biosynthesis of ornithine and arginine. Previous studies have reported the involvement of the putative nonribosomal peptide synthetase MgoA in virulence and mangotoxin production. In this study, we analyse a new chromosomal region of P. syringae pv. syringae UMAF0158, which contains six coding sequences arranged as an operon (mbo operon). The mbo operon was detected in only mangotoxin-producing strains, and it was shown to be essential for the biosynthesis of this toxin. Mutants in each of the six ORFs of the mbo operon were partially or completely impaired in the production of the toxin. In addition, Pseudomonas spp. mangotoxin non-producer strains transformed with the mbo operon gained the ability to produce mangotoxin, indicating that this operon contains all the genetic information necessary for mangotoxin biosynthesis. The generation of a single transcript for the mbo operon was confirmed and supported by the allocation of a unique promoter and Rho-independent terminator. The phylogenetic analysis of the P. syringae strains harbouring the mbo operon revealed that these strains clustered together.

Genes Involved in the Production of Antimetabolite Toxins by Pseudomonas syringae Pathovars

Pseudomonas syringae is pathogenic in a wide variety of plants, causing diseases with economic impacts. Pseudomonas syringae pathovars produce several toxins that can function as virulence factors and contribute to disease symptoms. These virulence factors include antimetabolite toxins, such as tabtoxin, phaseolotoxin and mangotoxin, which target enzymes in the pathways of amino acid metabolism. The antimetabolite toxins are generally located in gene clusters present in the flexible genomes of specific strains. These gene clusters are typically present in blocks of genes that appear to be integrated into specific sites in the P. syringae core genome. A general overview of the genetic organization and biosynthetic and regulatory functions of these genetic traits of the antimetabolite toxins will be given in the present work.

Gene expression of Pht cluster genes and a putative non-ribosomal peptide synthetase required for phaseolotoxin production is regulated by GacS/GacA in Pseudomonas syringae pv. phaseolicola

Pseudomonas syringae pv. phaseolicola is the causal agent of halo blight disease of beans (Phaseolus vulgaris L.), which is characterized by watersoaked lesions surrounded by a chlorotic halo resulting from the action of a non-host specific toxin known as phaseolotoxin. This toxin inhibits the enzyme ornithine carbamoyltransferase involved in the arginine biosynthesis pathway. It was previously reported that genes within the Pht cluster were involved in the regulation and synthesis of phaseolotoxin. The GacS/GacA two-component signal transduction system controls important pathogenicity and virulence mechanisms in several Gram-negative bacteria. Tox(-) phenotype gacA(-) and gacS(-) mutants were obtained and gacA(-) transcriptome analysis revealed that this response activator controls expression of genes within the Pht cluster as well as another gene located in a different region in the bacterial chromosome and that has been unambiguously shown to be directly involved in phaseolotoxin biosynthesis. Results presented in this work suggest that phaseolotoxin biosynthesis involve elements within and outside the Pht Cluster, and that the GacS/GacA two-component system exerts control over them.

Gac two-component system in Pseudomonas syringae pv. tabaci is required for virulence but not for hypersensitive reaction

Pseudomonas syringae pv. tabaci 6605 causes wildfire disease on host tobacco plants. To investigate the regulatory mechanism of the expression of virulence, Gac two-component system-defective mutants, DeltagacA and DeltagacS, and a double mutant, DeltagacADeltagacS, were generated. These mutants produced smaller amounts of N-acyl homoserine lactones required for quorum sensing, had lost swarming motility, and had reduced expression of virulence-related hrp genes and the algT gene required for exopolysaccharide production. The ability of the mutants to cause disease symptoms in their host tobacco plant was remarkably reduced, while they retained the ability to induce hypersensitive reaction (HR) in the nonhost plants. These results indicated that the Gac two-component system of P. syringae pv. tabaci 6605 is indispensable for virulence on the host plant, but not for HR induction in the nonhost plants.

Influence of environmental conditions on putisolvins I and II production inPseudomonas putidastrain PCL1445

Pseudomonas putida strain PCL1445 produces the cyclic lipopeptides putisolvin I and II, which are biosurfactants affecting environmentally important functions including swarming motility, solubility of nutrients and biofilm formation and maintenance. In this study, the effect of relevant nutritional and environmental factors on putisolvin production by PCL1445 was analyzed. Growth and levels of putisolvins I and II biosynthesis were significantly influenced by various carbon sources and by the nature of amino acid supplemented to the culture medium. An increase in the salt concentration of BM medium as well as growth at 1% oxygen significantly increased putisolvin production. In addition, we recently demonstrated that growth at a low temperature has a positive effect on putisolvin production. Analysis of the expression of gacA and gacS in Tn5luxAB mutant strains at a low temperature or under a high salt concentration suggests that biosynthesis of putisolvin is modulated through the GacA/GacS two-component regulatory system in PCL1445 under the tested conditions.

The Pseudomonas syringae Genome Encodes a Combined Mannuronan C-5-epimerase and O-Acetylhydrolase, Which Strongly Enhances the Predicted Gel-forming Properties of Alginates

Alginates are industrially important, linear copolymers of beta-d-mannuronic acid (M) and its C-5-epimer alpha-l-guluronic acid (G). The G residues originate from a postpolymerization reaction catalyzed by mannuronan C-5-epimerases (MEs), leading to extensive variability in M/G ratios and distribution patterns. Alginates containing long continuous stretches of G residues (G blocks) can form strong gels, a polymer type not found in alginate-producing bacteria belonging to the genus Pseudomonas. Here we show that the Pseudomonas syringae genome encodes a Ca(2+)-dependent ME (PsmE) that efficiently forms such G blocks in vitro. The deduced PsmE protein consists of 1610 amino acids and is a modular enzyme related to the previously characterized family of Azotobacter vinelandii ME (AlgE1-7). A- and R-like modules with sequence similarity to those in the AlgE enzymes are found in PsmE, and the A module of PsmE (PsmEA) was found to be sufficient for epimerization. Interestingly, an R module from AlgE4 stimulated Ps-mEA activity. PsmE contains two regions designated M and RTX, both presumably involved in the binding of Ca(2+). Bacterial alginates are partly acetylated, and such modified residues cannot be epimerized. Based on a detailed computer-assisted analysis and experimental studies another PsmE region, designated N, was found to encode an acetylhydrolase. By the combined action of N and A PsmE was capable of redesigning an extensively acetylated alginate low in G from a non gel-forming to a gel-forming state. Such a property has to our knowledge not been previously reported for an enzyme acting on a polysaccharide.

Biocontrol traits of Pseudomonas spp. are regulated by phase variation

Of 214 Pseudomonas strains isolated from maize rhizosphere, 46 turned out to be antagonistic, of which 43 displayed clear colony phase variation. The latter strains formed both opaque and translucent colonies, designated as phase I and phase II, respectively. It appeared that important biocontrol traits, such as motility and the production of antifungal metabolites, proteases, lipases, chitinases, and biosurfactants, are correlated with phase I morphology and are absent in bacteria with phase II morphology. From a Tn5luxAB transposon library of Pseudomonas sp. strain PCL1171 phase I cells, two mutants exhibiting stable expression of phase II had insertions in gacS. A third mutant, which showed an increased colony phase variation frequency was mutated in mutS. Inoculation of wheat seeds with PCL1171 bacteria of phase I morphology resulted in efficient suppression of take-all disease, whereas disease suppression was absent with phase II bacteria. Neither the gacS nor the mutS mutant was able to suppress take-all, but biocontrol activity was restored after genetic complementation of these mutants. Furthermore, in a number of cases, complementation by gacS of wild-type phase II sectors to phase I phenotype could be shown. A PCL1171 phase I mutant defective in antagonistic activity appeared to have a mutation in a gene encoding a lipopeptide synthetase homologue and had lost its biocontrol activity, suggesting that biocontrol by strain PCL1171 is dependent on the production of a lipopeptide. Our results show that colony phase variation plays a regulatory role in biocontrol by Pseudomonas bacteria by influencing the expression of major biocontrol traits and that the gacS and mutS genes play a role in the colony phase variation process. Therefore phase variation not only plays a role in escaping animal defense but it also appears to play a much broader and vital role in the ecology of bacteria producing exoenzymes, antibiotics, and other secondary metabolites.

Identification of Pseudomonas syringae pv. tomato genes induced during infection of Arabidopsis thaliana

Phytopathogenic bacteria possess a large number of genes that allow them to grow and cause disease on plants. Many of these genes should be induced when the bacteria come in contact with plant tissue. We used a modified in vivo expression technology (IVET) approach to identify genes from the plant pathogen Pseudomonas syringae pv. tomato that are induced upon infection of Arabidopsis thaliana and isolated over 500 in planta-expressed (ipx) promoter fusions. Sequence analysis of 79 fusions revealed several known and potential virulence genes, including hrp/hrc, avr and coronatine biosynthetic genes. In addition, we identified metabolic genes presumably important for adaptation to growth in plant tissue, as well as several genes with unknown function that may encode novel virulence factors. Many ipx fusions, including several corresponding to novel genes, are dependent on HrpL, an alternative RNA polymerase sigma factor that regulates the expression of virulence genes. Expression analysis indicated that several ipx fusions are strongly induced upon inoculation into plant tissue. Disruption of one ipx gene, conserved effector locus (CEL) orf1, encoding a putative lytic murein transglycosylase, resulted in decreased virulence of P. syringae. Our results demonstrate that this screen can be used successfully to isolate genes that are induced in planta, including many novel genes potentially involved in pathogenesis.

Regulatory roles of the GacS/GacA two-component system in plant-associated and other gram-negative bacteria

The sensor kinase GacS and the response regulator GacA are members of a two-component system that is present in a wide variety of gram-negative bacteria and has been studied mainly in enteric bacteria and fluorescent pseudomonads. The GacS/GacA system controls the production of secondary metabolites and extracellular enzymes involved in pathogenicity to plants and animals, biocontrol of soilborne plant diseases, ecological fitness, or tolerance to stress. A current model proposes that GacS senses a still-unknown signal and activates, via a phosphorelay mechanism, the GacA transcription regulator, which in turn triggers the expression of target genes. The GacS protein belongs to the unorthodox sensor kinases, characterized by an autophosphorylation, a receiver, and an output domain. The periplasmic loop domain of GacS is poorly conserved in diverse bacteria. Thus, a common signal interacting with this domain would be unexpected. Based on a comparison with the transcriptional regulator NarL, a secondary structure can be predicted for the GacA sensor kinases. Certain genes whose expression is regulated by the GacS/GacA system are regulated in parallel by the small RNA binding protein RsmA (CsrA) at a posttranscriptional level. It is suggested that the GacS/GacA system operates a switch between primary and secondary metabolism, with a major involvement of posttranscriptional control mechanisms.

Pseudomonas aeruginosa GacA, a factor in multihost virulence, is also essential for biofilm formation

We have investigated a potential role for GacA, the response regulator of the GacA/GacS two-component regulatory system, in Pseudomonas aeruginosa biofilm formation. When gacA was disrupted in strain PA14, a 10-fold reduction in biofilm formation capacity resulted relative to wild-type PA14. However, no significant difference was observed in the planktonic growth rate of PA14 gacA(-). Providing gacA in trans on the multicopy vector pUCP-gacA abrogated the biofilm formation defect. Scanning electron microscopy of biofilms formed by PA14 gacA(-) revealed diffuse clusters of cells that failed to aggregate into microcolonies, implying a deficit in biofilm development or surface translocation. Motility assays revealed no decrease in PA14 gacA(-) twitching or swimming abilities, indicating that the defect in biofilm formation is independent of flagellar-mediated attachment and solid surface translocation by pili. Autoinducer and alginate bioassays were performed similarly, and no difference in production levels was observed, indicating that this is not merely an upstream effect on either quorum sensing or alginate production. Antibiotic susceptibility profiling demonstrated that PA14 gacA(-) biofilms have moderately decreased resistance to a range of antibiotics relative to PA14 wild type. This study establishes GacA as a new and independent regulatory element in P. aeruginosa biofilm formation.

Effects of the two-component system comprising GacA and GacS of Erwinia carotovora subsp. carotovora on the production of global regulatory rsmB RNA, extracellular enzymes, and harpinEcc

Posttranscriptional regulation mediated by the regulator of secondary metabolites (RSM) RsmA-rsmB pair is the most important factor in the expression of genes for extracellular enzymes and HarpinEcc in Erwinia carotovora subsp. carotovora. RsmA is a small RNA-binding protein, which acts by lowering the half-life of a mRNA species. rsmB specifies an untranslated regulatory RNA and neutralizes the RsmA effect. It has been speculated that GacA-GacS, members of a two-component system, may affect gene expression via RsmA. Because expA, a gacA homolog, and expS (or rpfA), a gacS homolog, have been identified in E. carotovora subsp. carotovora, we examined the effects of these gacA and gacS homologs on the expression of rsmA, rsmB, and an assortment of exoprotein genes. The gacA gene of E. carotovora subsp. carotovora strain 71 stimulated transcription of genes for several extracellular enzymes (pel-1, a pectate lyase gene; peh-1, a polygalacturonase gene; and celV, a cellulase gene), hrpNEcc (an E. carotovora subsp. carotovora gene specifying the elicitor of hypersensitive reaction), and rsmB in GacA+ and GacS+ E. carotovora subsp. carotovora strains. Similarly, the E. carotovora subsp. carotovora gacA gene stimulated csrB (rsmB) transcription in Escherichia coli. A GacS- mutant of E. carotovora subsp. carotovora strain AH2 and a GacA- mutant of E. carotovora subsp. carotovora strain Ecc71 compared with their parent strains produced very low levels of rsmB, pel-1, peh-1, celV, and hrpNEcc transcripts but produced similar levels of rsmA RNA and RsmA protein as well as transcripts of hyperproduction of extracellular enzymes (Hex) hexA, kdgR (repressor of genes for uronate and pectate catabolism), rsmC, and rpoS (gene for Sigma-S, an alternate Sigma factor). The levels of rsmB, pel-1, peh-1, celV, and hrpNEcc transcripts as well as production of pectate lyase, polygalacturonase, cellulase, protease, and HarpinEcc proteins were stimulated in GacS- and GacA- mutants by GacS+ or GacA+ plasmids, respectively. The GacA effect on exoenzyme genes and hrpNEcc was abrogated in E. carotovora subsp. carotovora mutants deficient in RsmA and RsmC or RsmA, RsmC, and rsmB RNA. The expression of lacZ transcriptional fusions of rsmB of Erwinia amylovora and Erwinia herbicola pv. gypsophilae was markedly reduced in a GacA- and a GacS- mutant of Pseudomonas syringae pv. syringae. Southern blot hybridization revealed the presence of gacA and gacS homologs in all tested strains of soft-rotting Erwinia spp. and several nonsoft-rotting Erwinia species such as E. amylovora, E. rhapontici, E. herbicola, E. stewartii, and E. herbicola pv. gypsophilae. These findings establish that the GacA-GacS system controls transcription of rsmB of E. carotovora subsp. carotovora, E. amylovora, and E. herbicola pv. gypsophilae and support the hypothesis that the effects of this two-component system on extracellular protein production in E. carotovora subsp. carotovora is mediated, at least in part, via the levels of rsmB transcripts.

Detection of tabtoxin-producing strains of Pseudomonas syringae by PCR

AIMS

The present study describes a system based on PCR to distinguish tabtoxin-producing strains of Pseudomonas syringae from other Ps. syringae plant pathogens that produce chlorosis-inducing phytotoxins.

METHODS AND RESULTS

Thirty-two strains of Ps. syringae and related species were examined. Two sets of PCR primers were developed to amplify genes (tblA and tabA) required for tabtoxin production. Only a PCR product of 829 bp or 1020 bp was produced in PCR reactions with the tblA or tabA primer sets, respectively, and cells from tabtoxin-producing pathovars of Pseudomonas syringae. All known non-tabtoxin producing bacterial species failed to produce an amplification product with either primer set.

CONCLUSIONS

PCR of genes required for tabtoxin production is a simple, rapid and reliable method for identifying tabtoxin-producing strains of Ps. syringae.

SIGNIFICANCE AND IMPACT OF THE STUDY

The protocol can effectively distinguish tabtoxin-producing strains of Ps. syringae from other Ps. syringae pathovars and Ps. syringae pv. tabaci strains from other tabtoxin-producing Ps. syringae pathovars.

Bacteria in the leaf ecosystem with emphasis on Pseudomonas syringae-a pathogen, ice nucleus, and epiphyte

The extremely large number of leaves produced by terrestrial and aquatic plants provide habitats for colonization by a diversity of microorganisms. This review focuses on the bacterial component of leaf microbial communities, with emphasis on Pseudomonas syringae-a species that participates in leaf ecosystems as a pathogen, ice nucleus, and epiphyte. Among the diversity of bacteria that colonize leaves, none has received wider attention than P. syringae, as it gained notoriety for being the first recombinant organism (Ice(-) P. syringae) to be deliberately introduced into the environment. We focus on P. syringae to illustrate the attractiveness and somewhat unique opportunities provided by leaf ecosystems for addressing fundamental questions of microbial population dynamics and mechanisms of plant-bacterium interactions. Leaf ecosystems are dynamic and ephemeral. The physical environment surrounding phyllosphere microbes changes continuously with daily cycles in temperature, radiation, relative humidity, wind velocity, and leaf wetness. Slightly longer-term changes occur as weather systems pass. Seasonal climatic changes impose still a longer cycle. The physical and physiological characteristics of leaves change as they expand, mature, and senesce and as host phenology changes. Many of these factors influence the development of populations of P. syringae upon populations of leaves. P. syringae was first studied for its ability to cause disease on plants. However, disease causation is but one aspect of its life strategy. The bacterium can be found in association with healthy leaves, growing and surviving for many generations on the surfaces of leaves as an epiphyte. A number of genes and traits have been identified that contribute to the fitness of P. syringae in the phyllosphere. While still in their infancy, such research efforts demonstrate that the P. syringae-leaf ecosystem is a particularly attractive system with which to bridge the gap between what is known about the molecular biology of genes linked to pathogenicity and the ecology and epidemiology of associated diseases as they occur in natural settings, the field.

AlgT (sigma22) controls alginate production and tolerance to environmental stress in Pseudomonas syringae

Pseudomonas aeruginosa and the phytopathogen P. syringae produce the exopolysaccharide alginate, which is a copolymer of D-mannuronic and L-guluronic acids. One of the key regulatory genes controlling alginate biosynthesis in P. aeruginosa is algT, which encodes the alternate sigma factor, sigma(22). In the present study, the algT gene product from P. syringae pv. syringae showed 90% amino acid identity with its P. aeruginosa counterpart, and sequence analysis of the region flanking algT in P. syringae revealed the presence of nadB, mucA, and mucB in an arrangement virtually identical to that of P. aeruginosa. An algT mutant of P. syringae was defective in alginate production but could be complemented with wild-type algT from P. syringae or P. aeruginosa when expressed in trans. The algT mutant also displayed increased sensitivity to heat, paraquat, and hydrogen peroxide (H(2)O(2)); the latter two compounds are known to generate reactive oxygen intermediates. Signals for activation of algT gene expression in P. syringae were investigated with an algT::uidA transcriptional fusion. Like that in P. aeruginosa, algT transcription in P. syringae was activated by heat shock. However, algT expression in P. syringae was also stimulated by osmotic stress and by exposure to paraquat, H(2)O(2), and copper sulfate. The latter two compounds are frequently encountered during colonization of plant tissue and may be unique signals for algT activation in P. syringae.

dnaK and the heat stress response of Pseudomonas syringae pv. glycinea

The dnaK gene from Pseudomonas syringae pv. glycinea PG4180 was cloned and sequenced. The dnaK coding region was 1,917 bp and contained a putative sigma 32 heat shock promoter 86 bp upstream of the translational start site. grpE, another heat shock gene, was found immediately upstream of the putative dnaK promoter. The predicted amino acid sequence of dnaK showed relatedness to the ATPase and substrate binding domains commonly found in heat shock proteins, as well as the highly conserved signature sequence motifs belonging to the Hsp70 protein family. Furthermore, the PG4180 dnaK gene complemented an Escherichia coli dnaK mutant for growth at temperatures above 37 degrees C, indicating that a fully functional dnaK homologue had been cloned from P. syringae pv. glycinea. All attempts to eliminate dnaK function by insertion mutagenesis failed, possibly because DnaK performs essential functions in P. syringae pv. glycinea. Expression of dnaK in P. syringae pv. glycinea PG4180 was investigated by constructing dnaK::uidA transcriptional fusions; expression of dnaK increased markedly when cells were preincubated at 18 degrees C and then shifted to 35 degrees C. An anti-DnaK monoclonal antibody was used to detect DnaK; in P. syringae pv. glycinea race 4, DnaK levels followed cell density during a 6-h incubation at 26 degrees C. When cells were shifted from 26 degrees C to either 32 or 38 degrees C, DnaK levels increased transiently, and then decreased rapidly. Although the cells continued to grow when incubated at 32 degrees C, growth was not supported at 38 degrees C. Our results indicate that P. syringae pv. glycinea responds to heat shock by producing DnaK, but DnaK does not aid in acclimation to sustained elevated temperatures.

Regulation of alginate biosynthesis in Pseudomonas syringae pv. syringae

Both Pseudomonas aeruginosa and the phytopathogen P. syringae produce the exopolysaccharide alginate. However, the environmental signals that trigger alginate gene expression in P. syringae are different from those in P. aeruginosa with copper being a major signal in P. syringae. In P. aeruginosa, the alternate sigma factor encoded by algT (sigma22) and the response regulator AlgR1 are required for transcription of algD, a gene which encodes a key enzyme in the alginate biosynthetic pathway. In the present study, we cloned and characterized the gene encoding AlgR1 from P. syringae. The deduced amino acid sequence of AlgR1 from P. syringae showed 86% identity to its P. aeruginosa counterpart. Sequence analysis of the region flanking algR1 in P. syringae revealed the presence of argH, algZ, and hemC in an arrangement virtually identical to that reported in P. aeruginosa. An algR1 mutant, P. syringae FF5.32, was defective in alginate production but could be complemented when algR1 was expressed in trans. The algD promoter region in P. syringae (PsalgD) was also characterized and shown to diverge significantly from the algD promoter in P. aeruginosa. Unlike P. aeruginosa, algR1 was not required for the transcription of algD in P. syringae, and PsalgD lacked the consensus sequence recognized by AlgR1. However, both the algD and algR1 upstream regions in P. syringae contained the consensus sequence recognized by sigma22, suggesting that algT is required for transcription of both genes.

Pseudomonas syringae phytotoxins: mode of action, regulation, and biosynthesis by peptide and polyketide synthetases

Coronatine, syringomycin, syringopeptin, tabtoxin, and phaseolotoxin are the most intensively studied phytotoxins of Pseudomonas syringae, and each contributes significantly to bacterial virulence in plants. Coronatine functions partly as a mimic of methyl jasmonate, a hormone synthesized by plants undergoing biological stress. Syringomycin and syringopeptin form pores in plasma membranes, a process that leads to electrolyte leakage. Tabtoxin and phaseolotoxin are strongly antimicrobial and function by inhibiting glutamine synthetase and ornithine carbamoyltransferase, respectively. Genetic analysis has revealed the mechanisms responsible for toxin biosynthesis. Coronatine biosynthesis requires the cooperation of polyketide and peptide synthetases for the assembly of the coronafacic and coronamic acid moieties, respectively. Tabtoxin is derived from the lysine biosynthetic pathway, whereas syringomycin, syringopeptin, and phaseolotoxin biosynthesis requires peptide synthetases. Activation of phytotoxin synthesis is controlled by diverse environmental factors including plant signal molecules and temperature. Genes involved in the regulation of phytotoxin synthesis have been located within the coronatine and syringomycin gene clusters; however, additional regulatory genes are required for the synthesis of these and other phytotoxins. Global regulatory genes such as gacS modulate phytotoxin production in certain pathovars, indicating the complexity of the regulatory circuits controlling phytotoxin synthesis. The coronatine and syringomycin gene clusters have been intensively characterized and show potential for constructing modified polyketides and peptides. Genetic reprogramming of peptide and polyketide synthetases has been successful, and portions of the coronatine and syringomycin gene clusters could be valuable resources in developing new antimicrobial agents.

Cloning, nucleotide sequence, and expression in Escherichia coli of levansucrase genes from the plant pathogens Pseudomonas syringae pv. glycinea and P. syringae pv. phaseolicola

Plant-pathogenic bacteria produce various extracellular polysaccharides (EPSs) which may function as virulence factors in diseases caused by these bacteria. The EPS levan is synthesized by the extracellular enzyme levansucrase in Pseudomonas syringae, Erwinia amylovora, and other bacterial species. The lsc genes encoding levansucrase from P. syringae pv. glycinea PG4180 and P. syringae pv. phaseolicola NCPPB 1321 were cloned, and their nucleotide sequences were determined. Heterologous expression of the lsc gene in Escherichia coli was found in four and two genomic library clones of strains PG4180 and NCPPB 1321, respectively. A 3. 0-kb PstI fragment common to all six clones conferred levan synthesis on E. coli when further subcloned. Nucleotide sequence analysis revealed a 1,248-bp open reading frame (ORF) derived from PG4180 and a 1,296-bp ORF derived from NCPPB 1321, which were both designated lsc. Both ORFs showed high homology to the E. amylovora and Zymomonas mobilis lsc genes at the nucleic acid and deduced amino acid sequence levels. Levansucrase was not secreted into the supernatant but was located in the periplasmic fraction of E. coli harboring the lsc gene. Expression of lsc was found to be dependent on the vector-based Plac promoter, indicating that the native promoter of lsc was not functional in E. coli. Insertion of an antibiotic resistance cassette in the lsc gene abolished levan synthesis in E. coli. A PCR screening with primers derived from lsc of P. syringae pv. glycinea PG4180 allowed the detection of this gene in a number of related bacteria.

A newly identified regulator is required for virulence and toxin production in Pseudomonas syringae

The genes lemA (which we here redesignate gacS) and gacA encode members of a widely conserved two-component regulatory system. In Pseudomonas syringae strain B728a, gacS and gacA are required for lesion formation on bean, as well as for the production of protease and the toxin syringomycin. A gene, designated salA, was discovered that restored syringomycin production to a gacS mutant when present on a multiple-copy plasmid. Disruption of chromosomal salA resulted in loss of syringomycin production and lesion formation in laboratory assays. Sequence analysis of salA suggests that it encodes a protein with a DNA-binding motif but without other significant similarity to proteins in current databases. Chromosomal reporter fusions revealed that gacS and gacA positively regulate salA, that salA upregulates its own expression and that salA positively regulates the expression of a syringomycin biosynthetic gene, syrB. Loss of syringomycin production does not account for the salA mutant's attenuated pathogenicity, as a syrB mutant was found to retain full virulence. The salA gene did not similarly suppress the protease deficient phenotype of gacS mutants, nor were salA mutants affected for protease production. A gacS/gacA-dependent homoserine lactone activity as detected by bioassay was also unaffected by the disruption of salA. Thus, salA appears to encode a novel regulator that activates the expression of at least two separate genetic subsets of the gacS/gacA regulon, one pathway leading to syringomycin production and the other resulting in plant disease.

Biosynthesis and regulation of coronatine, a non-host-specific phytotoxin produced by Pseudomonas syringae

Many P. syringae pathovars are known to produce low-molecular-weight, diffusible toxins in infected host plants. These phytotoxins reproduce some of the symptoms of the relevant bacterial disease and are effective at very low concentrations. Phytotoxins generally enhance the virulence of the P. syringae pathovar which produces them, but are not required for pathogenesis. Genes encoding phytotoxin production have been identified and cloned from several P. syringae pathovars. With the exception of coronatine, toxin biosynthetic gene clusters are generally chromosomally encoded. In several pathovars, the toxin biosynthetic gene cluster also contains a resistance gene which functions to protect the producing strain from the biocidal effects of the toxin. In the case of phaseolotoxin, a resistance gene (argK) has been utilized to engineer phaseolotoxin-resistant tobacco plants. Although P. syringae phytotoxins can induce very similar effects in plants (chlorosis and necrosis), their biosynthesis and mode of action can be quite different. Knowledge of the biosynthetic pathways to these toxins and the cloning of the structural genes for their biosynthesis has relevance to the development of new bioactive compounds with altered specificity. For example, polyketides constitute a huge family of structurally diverse natural products including antibiotics, chemotherapeutic compounds, and antiparasitics. Most of the research on polyketide synthesis in bacteria has focused on compounds synthesized by Streptomyces or other actinomycetes. It is also important to note that it is now possible to utilize a genetic rather than synthetic approach to biosynthesize novel polyketides with altered biological properties (Hutchinson and Fujii, 1995; Kao et al., 1994; Donadio et al., 1993; Katz and Donadio, 1993). Most of the reprogramming or engineering of novel polyketides has been done using actinomycete PKSs, but much of this technology could also be applied to polyketides synthesized by Pseudomonas when sufficient sequence information is available. It is important to note that Pseudomonas produces a variety of antimicrobial compounds from the polyketide pathway, including mupirocin (pseudomonic acid) (Feline et al., 1977), pyoluteorin (Cuppels et al., 1986), and 2-4 diacetylphloroglucinol (Phl) (Bangera and Thomashow, 1996). Pseudomonic acid is valued for its pharmaceutical properties as an antibiotic (Aldridge, 1992), whereas pyoluteorin and Phl have antifungal properties (Howell and Stipanovic, 1980; Keel et al., 1992). A thorough understanding of the biosynthetic pathway to polyketide phytotoxins such as coronatine may ultimately lead to the development of novel compounds with altered biological properties. Thus, specific genes in the biosynthetic pathways of P. syringae phytotoxins could be deployed in other systems to develop new compounds with a wide range of activities.

Contribution of the Regulatory Gene lemA to Field Fitness of Pseudomonas syringae pv. syringae

In Pseudomonas syringae pv. syringae, lemA is required for brown spot lesion formation on snap bean and for production of syringomycin and extracellular proteases (E. M. Hrabak and D. K. Willis, J. Bacteriol. 174: 3011-3022, 1992; E. M. Hrabak and D. K. Willis, Mol. Plant-Microbe Interact. 6:368-375, 1993; D. K. Willis, E. M. Hrabak, J. J. Rich, T. M. Barta, S. E. Lindow, and N. J. Panopoulos, Mol. Plant-Microbe Interact. 3:149-156, 1990). The lemA mutant NPS3136 (lemA1::Tn5) was previously found to be indistinguishable from its pathogenic parent B728a in its ability to grow when infiltrated into bean leaves of plants maintained under controlled environmental conditions (Willis et al., Mol. Plant-Microbe Interact. 3:149-156, 1990). We compared population sizes of NPS3136 and B728aN (a Nal(supr) clone of wild-type B728a) in two field experiments to determine the effect of inactivation of lemA on the fitness of P. syringae pv. syringae. In one experiment, the bacterial strains were spray inoculated onto the foliage of 25-day-old bean plants. In the other, seeds were inoculated at the time of planting. In both experiments, the strains were inoculated individually and coinoculated in a 1:1 ratio. NPS3136 and B728aN achieved similar large population sizes on germinating seeds. However, in association with leaves, population sizes of NPS3136 were diminished relative to those of B728aN in both experiments. Thus, lemA contributed significantly to the fitness of P. syringae pv. syringae in association with bean leaves but not on germinating seeds under field conditions. When NPS3136 was coinoculated with B728aN, the mutant behaved as it did when inoculated alone. However, population sizes of B728aN in the coinoculation treatment were much lower than those when it was inoculated alone. Inactivation of the lemA gene appeared to have rendered the mutant suppressive to B728aN.

Characterization of the alginate biosynthetic gene cluster in Pseudomonas syringae pv. syringae

Alginate, a copolymer of D-mannuronic acid and L-guluronic acid, is produced by a variety of pseudomonads, including Pseudomonas syringae. Alginate biosynthesis has been most extensively studied in P. aeruginosa, and a number of structural and regulatory genes from this species have been cloned and characterized. In the present study, an alginate-defective (Alg-) mutant of P. syringae pv. syringae FF5 was shown to contain a Tn5 insertion in algL, a gene encoding alginate lyase. A cosmid clone designated pSK2 restored alginate production to the algL mutant and was shown to contain homologs of algD, alg8, alg44, algG, algX (alg60), algL, algF, and algA. The order and arrangement of the structural gene cluster were virtually identical to those previously described for P. aeruginosa. Complementation analyses, however, indicated that the structural gene clusters in P. aeruginosa and P. syringae were not functionally interchangeable when expressed from their native promoters. A region upstream of the algD gene in P. syringae pv. syringae was shown to activate the transcription of a promoterless glucuronidase (uidA) gene and indicated that transcription initiated upstream of algD as described for P. aeruginosa. Transcription of the algD promoter from P. syringae FF5 was significantly higher at 32 degrees C than at 18 or 26 degrees C and was stimulated when copper sulfate or sodium chloride was added to the medium. Alginate gene expression was also stimulated by the addition of the nonionic solute sorbitol, indicating that osmolarity is a signal for algD expression in P. syringae FF5.

Multiple loci of Pseudomonas syringae pv. syringae are involved in pathogenicity on bean: restoration of one lesion-deficient mutant requires two tRNA genes

A mutational analysis of lesion-forming ability was undertaken in Pseudomonas syringae pv. syringae B728a, causal agent of bacterial brown spot disease of bean. Following a screen of 6,401 Tn5-containing derivatives of B728a on bean pods, 26 strains that did not form disease lesions were identified. Nine of the mutant strains were defective in the ability to elicit the hypersensitive reaction (HR) and were shown to contain Tn5 insertions within the P. syringae pv. syringae hrp region. Ten HR+ mutants were defective in the production of the toxin syringomycin, and a region of the chromosome implicated in the biosynthesis of syringomycin was deleted in a subset of these mutants. The remaining seven lesion-defective mutants retained the ability to produce protease and syringomycin. Marker exchange mutagenesis confirmed that the Tn5 insertion was causal to the mutant phenotype in several lesion-defective, HR+ strains. KW239, a lesion- and syringomycin-deficient mutant, was characterized at the molecular level. Sequence analysis of the chromosomal region flanking the Tn5 within KW239 revealed strong similarities to a number of known Escherichia coli gene products and DNA sequences: the nusA operon, including the complete initiator tRNA(Met) gene, metY; a tRNA(Leu) gene; the tpiA gene product; and the MrsA protein. Removal of sequences containing the two potential tRNA genes prevented restoration of mutant KW239 in trans. The Tn5 insertions within the lesion-deficient strains examined, including KW239, were not closely linked to each other or to the lemA or gacA genes previously identified as involved in lesion formation by P. syringae pv. syringae.

Characterization of dapB, a gene required by Pseudomonas syringae pv. tabaci BR2.024 for lysine and tabtoxinine-beta-lactam biosynthesis

The dapB gene, which encodes L-2,3-dihydrodipicolinate reductase, the second enzyme of the lysine branch of the aspartic amino acid family, was cloned and sequenced from a tabtoxin-producing bacterium, Pseudomonas syringae pv. tabaci BR2.024. The deduced amino acid sequence shared 60 to 90% identity to known dapB gene products from gram-negative bacteria and 19 to 21% identity to the dapB products from gram-positive bacteria. The consensus sequence for the NAD(P)H binding site [(V/I)(A/G)(V/I)XGXXGXXG)] and the proposed substrate binding site (HHRHK) were conserved in the polypeptide. A BR2.024 dapB mutant is a diaminopimelate auxotroph and tabtoxin negative. The addition of a mixture of L-,L-, D,D-, and meso-diaminopimelate to defined media restored growth but not tabtoxin production. Cloned DNA fragments containing the parental dapB gene restored the ability to grow in defined media and tabtoxin production to the dapB mutant. These results indicate that the dapB gene is required for both lysine and tabtoxin biosynthesis, thus providing the first genetic evidence that the biosynthesis of tabtoxin proceeds in part along the lysine biosynthetic pathway. These data also suggest that L-2,3,4,5-tetrahydrodipicolinate is a common intermediate for both lysine and tabtoxin biosynthesis.

Suppression of a sensor kinase-dependent phenotype in Pseudomonas syringae by ribosomal proteins L35 and L20

The lemA gene of Pseudomonas syringae pv. syringae encodes the sensor kinase of a bacterial two-component signal transduction system. Phenotypes that are lemA dependent in P. syringae include lesion formation on bean and production of extracellular protease and the antibiotic syringomycin. Recently, the gacA gene has been identified as encoding the response regulator of the lemA regulon. To identify additional components that interact with LemA, suppressors of a lemA mutation were sought. A locus was identified that, when present in multiple copies, restores extracellular protease production to a lemA insertion mutant of P. syringae pv. syringae. This locus was found to encode the P. syringae homologs of translation initiation factor IF3 and ribosomal proteins L20 and L35 of Escherichia coli and other bacteria. Deletion analysis and data from Western immunoblots with anti-IF3 antiserum suggest that protease restoration does not require IF3. Deletion of both the L35 and L20 genes resulted in loss of protease restoration, whereas disruption of either gene alone increased protease restoration. Our results suggest that overexpression of either L20 or L35 is sufficient for protease restoration. It is unclear how alteration of ribosomal protein expression compensates in this instance for loss of a transcriptional activator, but a regulatory role for L20 and L35 apart from their function in the ribosome may be indicated.

Copper as a signal for alginate synthesis in Pseudomonas syringae pv. syringae

Plant-associated pseudomonads are commonly exposed to copper bactericides, which are applied to reduce the disease incidence caused by these bacteria. Consequently, many of these bacteria have acquired resistance or tolerance to copper salts. We recently conducted a survey of 37 copper-resistant (Cur) Pseudomonas spp., including P. cepacia, P. fluorescens, P. syringae, and P. viridiflava, and found that a subset of the P. syringae strains showed a dramatic increase in exopolysaccharide (EPS) production on mannitol-glutamate medium containing CuSO4 at 250 micrograms/ml. A modified carbazole assay indicated that the EPS produced on copper-amended media contained high levels of uronic acids, suggesting that the EPS was primarily alginic acid. Uronic acids extracted from selected strains were further confirmed to be alginate by demonstrating their sensitivity to alginate lyase and by descending paper chromatography following acid hydrolysis. Subinhibitory levels of arsenate, cobalt, lithium, rubidium, molybdenum, and mercury did not induce EPS production, indicating that alginate biosynthesis is not induced in P. syringae cells exposed to these heavy metals. A 200-kb plasmid designated pPSR12 conferred a stably mucoid phenotype to several P. syringae recipients and also increased their resistance to cobalt and arsenate. A cosmid clone constructed from pPSR12 which conferred a stably mucoid phenotype to several P. syringae strains but not to Pseudomonas aeruginosa was obtained. Results obtained in this study indicate that some of the signals and regulatory genes for alginate production in P. syringae differ from those described for alginate production in P. aeruginosa.

Genetic evidence that the gacA gene encodes the cognate response regulator for the lemA sensor in Pseudomonas syringae

Mutational analysis of the bean-pathogenic Pseudomonas syringae pv. syringae strain B728a has led to the genetic identification of the gacA gene as encoding the response regulator for the unlinked lemA sensor kinase. The analysis of a collection of spontaneous mutants of P. syringae pv. syringae suggested that the gacA gene was involved in lesion formation and the production of protease and syringomycin. The gacA gene originally was identified as a regulator of extracellular antibiotic production by Pseudomonas fluorescens, and the predicted GacA protein is a member of the FixJ family of bacterial response regulators. The sequence of the putative B728a GacA protein revealed 92% identity with the P. fluorescens GacA protein. An insertional mutation within the P. syringae pv. syringae gacA gene abrogated lesion formation on beans, production of extracellular protease, and production of the toxin syringomycin, the same phenotypes affected by a lemA mutation. DNA sequence analysis identified the P. syringae pv. syringae uvrC gene immediately downstream of the gacA gene, an arrangement conserved in P. fluorescens and Escherichia coli. The gacA insertional mutant was sensitive to UV, presumably because of polarity on transcription of the downstream uvrC gene. Southwestern (DNA-protein) analysis revealed that the lemA and gacA genes were required for the full expression of a DNA binding activity.

Restriction fragment length polymorphism evidence for genetic homology within a pathovar of Pseudomonas syringae

Pseudomonas syringae pv. phaseolicola NPS3121 hrp sequences were used as hybridization probes in a restriction fragment length polymorphism (RFLP) analysis of 24 P. syringae pv. tabaci strains as a means to evaluate the genetic and taxonomic relationship of pathovars of P. syringae. Southern blot analyses of genomic restriction digests, with hrpA-S sequences as hybridization probes, and restriction analyses of PCR-amplified DNA of regions within hrpD were conducted. The resulting RFLP patterns were uniform for 23 of the 24 isolates tested, with strain BR2R having a unique pattern. BR2R is a pathogen of bean which was classified as pathovar tabaci because of its ability to produce tabtoxin, but unlike the other 23 tabaci strains in this study, it does not incite disease symptoms on tobacco. When a DNA fragment containing hrpM sequences was used as a hybridization probe, the tabaci isolates could be divided into three groups on the basis of the RFLP patterns : BR2R, Pt11528R and Pt113R, and the remaining strains. For all of the above analyses, BR2R shared identical RFLP patterns with P. syringae pv. phaseolicola NPS3121, also a bean pathogen which does not cause disease on tobacco. However, BR2R AND NPS3121 could be differentiated from each other on the basis of the RFLP patterns from restriction analysis of PCR-amplified DNA of argF, while the remaining tabaci strains had a third pattern. These studies indicate that hrp genes and argF are conserved in strains of P. syringae pathogenic to tobacco, suggesting that P. syringae strains pathogenic to specific hosts may have a high level of genetic similarity. We believe that these analyses have shown that distinct identifiable genetic differences may be correlated with host range and suggest that such information may be useful for assigning pathovar designations.

DNA sequence and transcriptional analysis of the tblA gene required for tabtoxin biosynthesis by Pseudomonas syringae

The tblA gene of Pseudomonas syringae is required for tabtoxin biosynthesis and is under the control of a regulatory gene, lemA. We have determined the nucleotide sequence of the tblA gene and identified the 5' end of the tblA gene transcript. The sequence of the tblA gene was identified to that of the recently reported open reading frame 1 gene of the tabA region of the BR2 chromosome. The open reading frame of the tblA gene potentially encodes a protein of 231 amino acids. mRNA from the tblA gene was detected at all phases of cells grown in minimal medium. This result is correlated with the constitutive production of tabtoxinine-beta-lactam (the biologically active part of the toxin) by P. syringae BR2R in minimal medium, as quantitated by a phenylisothiocyanate derivatization method.

Regulation of tabtoxin production by the lemA gene in Pseudomonas syringae

Pseudomonas syringae pv. coronafaciens, a pathogen of oats, was mutagenized with Tn5 to generate mutants defective in tabtoxin production. From a screen of 3,400 kanamycin-resistant transconjugants, seven independent mutants that do not produce tabtoxin (Tox-) were isolated. Although the Tn5 insertions within these seven mutants were linked, they were not located in the previously described tabtoxin biosynthetic region of P. syringae. Instead, all of the insertions were within the P. syringae pv. coronafaciens lemA gene. The lemA gene is required by strains of P. syringae pv. syringae for pathogenicity on bean plants (Phaseolus vulgaris). In contrast to the phenotype of a P. syringae pv. syringae lemA mutant, the Tox- mutants of P. syringae pv. coronafaciens were still able to produce necrotic lesions on oat plants (Avena sativa), although without the chlorosis associated with tabtoxin production. Northern (RNA) hybridization experiments indicated that a functional lemA gene was required for the detection of a transcript produced from the tblA locus located in the tabtoxin biosynthetic region. Marker exchange mutagenesis of the tblA locus resulted in loss of tabtoxin production. Therefore, both the tblA and lemA genes are required for tabtoxin biosynthesis, and the regulation of tabtoxin production by lemA probably occurs at the transcriptional level.

The lemA gene required for pathogenicity of Pseudomonas syringae pv. syringae on bean is a member of a family of two-component regulators

The lemA gene of the plant pathogen Pseudomonas syringae pv. syringae is required for disease lesion formation on bean plants. Cosmid clones that complemented a lemA mutant in trans were isolated previously. The lemA gene was localized by subcloning and transposon mutagenesis. The lemA region and flanking DNA were sequenced, and an open reading frame of 2.7 kb was identified. The nucleotide and predicted amino acid sequences of the lemA gene showed sequence similarity to a family of prokaryotic two-component regulatory proteins. Unlike most of the previously described two-component systems, the lemA gene product contained homology to both components in one protein. Mutations introduced upstream and downstream of the lemA gene failed to locate a gene for a second protein component but identified the putative cysM gene of P. syringae pv. syringae. The cysM gene was located upstream of the lemA gene and was divergently transcribed. The lemA gene product was expressed at low levels in P. syringae pv. syringae and appeared to be positively auto-regulated.