Characterisation of the mgo operon in Pseudomonas syringae pv. syringae UMAF0158 that is required for mangotoxin production

Insecticidal features displayed by the beneficial rhizobacterium Pseudomonas chlororaphis PCL1606

The biocontrol rhizobacterium Pseudomonas chlororaphis is one of the bacterial species of the P. fluorescens group where insecticide fit genes have been found. Fit toxin, supported with other antimicrobial compounds, gives the bacterial the ability to repel and to fight against eukaryotic organisms, such as nematodes and insect larvae, thus protecting the plant host and itself. Pseudomonas chlororaphis PCL1606 is an antagonistic rhizobacterium isolated from avocado roots and show efficient biocontrol against fungal soil-borne disease. The main antimicrobial compound produced by P. chlororaphis PCL606 is 2-hexyl-5-propyl resorcinol (HPR), which plays a crucial role in effective biocontrol against fungal pathogens. Further analysis of the P. chlororaphis PCL1606 genome showed the presence of hydrogen cyanide (HCN), pyrrolnitrin (PRN), and homologous fit genes. To test the insecticidal activity and to determine the bases for such activity, single and double mutants on the biosynthetic genes of these four compounds were tested in a Galleria mellonella larval model using inoculation by injection. The results revealed that Fit toxin and HPR in combination are involved in the insecticide phenotype of P. chlororaphis PCL1606, and additional compounds such as HCN and PRN could be considered supporting compounds.

Turning a bacterial gaseous virulence trigger off

Recently, Sieber et al. discovered the new diazeniumdiolate volatile signaling molecule, leudiazen. They confirmed that inactivation of leudiazen by KMnO₄ can reduce the production of mangotoxin. This alleviates the pathogenicity of Pseudomonas syringae pv. syringae on mango trees, providing a new strategy for plant protection, compatible with organic farming.

Mitigation of Pseudomonas syringae virulence by signal inactivation

Pseudomonas syringae is an important plant pathogen of many valuable crops worldwide, with more than 60 identified pathovars. The phytotoxins produced by these organisms were related to the severity of the damage caused to the plant. An emerging strategy to treat bacterial infections relies on interference with their signaling systems. In this study, we investigated P. syringae pv. syringae, which produces the virulence factor mangotoxin that causes bacterial apical necrosis on mango leaves. A previously unknown signaling molecule named leudiazen was identified, determined to be unstable and volatile, and responsible for mangotoxin production. A strategy using potassium permanganate, compatible with organic farming, was developed to degrade leudiazen and thus to attenuate the pathogenicity of P. syringae pv. syringae.

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.

Pseudomonas Virulence Factor Pathway Synthesizes Autoinducers That Regulate the Secretome of a Pathogen

Cell-to-cell communication via chemical signals is an essential mechanism that pathogenic bacteria use to coordinate group behaviors and promote virulence. The Pseudomonas virulence factor (pvf) gene cluster is distributed in more than 500 strains of proteobacteria including both plant and human pathogens. The pvf cluster has been implicated in the production of signaling molecules important for virulence; however, the regulatory impact of these signaling molecules on virulence had not been elucidated. Using the insect pathogen Pseudomonas entomophila L48 as a model, we demonstrated that pvf-encoded biosynthetic enzymes produce PVF autoinducers that regulate the expression of pvf genes and a gene encoding the toxin monalysin via quorum sensing. In addition, PVF autoinducers regulate the expression of nearly 200 secreted and membrane proteins, including toxins, motility proteins, and components of the type VI secretion system, which play key roles in bacterial virulence, colonization, and competition with other microbes. Deletion of pvf also altered the secondary metabolome. Six major compounds upregulated by PVF autoinducers were isolated and structurally characterized, including three insecticidal 3-indolyl oxazoles, the labradorins, and three antimicrobial pyrrolizidine alkaloids, the pyreudiones. The signaling properties of PVF autoinducers and their wide-ranging regulatory effects indicate multifaceted roles of PVF in controlling cell physiology and promoting virulence. The broad genome distribution of pvf suggests that PVF-mediated signaling is relevant to many bacteria of agricultural and biomedical significance.

Isolation and whole-genome sequencing of Pseudomonas sp. RIT 623, a slow-growing bacterium endowed with antibiotic properties

OBJECTIVE

There is an urgent need for the discovery and/or development of novel antibiotics. We report an exploration of "slow"-growing bacteria, which can be difficult to isolate using rich media as they are usually outcompeted by "fast"-growing bacteria, as potential sources of novel antimicrobials.

RESULTS

Pseudomonas sp. RIT 623 was isolated using pond water agar from a pond located on the campus of the Rochester Institute of Technology (RIT). The genome was sequenced and analyzed for potential secondary metabolite gene clusters. Bioinformatics analysis revealed 14 putative gene clusters predicted to encode pathways for the anabolism of secondary metabolites. Ethyl acetate extracts from spent growth medium of Pseudomonas sp. RIT 623 were tested against two Gram-negative (E. coli ATCC 25922 and P. aeruginosa ATCC 27853) and two Gram-positive (B. subtilis BGSC 168 and S. aureus ATCC 25923) type strains to assess antibiotic activity. The antibiotic assays demonstrated that extracts of Pseudomonas sp. RIT 623 were able to inhibit the growth of the four strains. The active compound was separated using diethyl ether in a multi-solvent extraction and reverse phase chromatography. The bioactive compound/s were subsequently eluted in two consecutive fractions corresponding to approximately 16-22% acetonitrile, indicative of polar compound/s.

Aer Receptors Influence the Pseudomonas chlororaphis PCL1606 Lifestyle

Pseudomonas chlororaphis PCL1606 (PcPCL1606) is a rhizobacterium isolated from avocado roots, which is a favorable niche for its development. This strain extensively interacts with plant roots and surrounding microbes and is considered a biocontrol rhizobacterium. Genome sequencing has shown the presence of thirty-one potential methyl-accepting chemotaxis proteins (MCPs). Among these MCPs, two candidates are putative functional aerotaxis receptors, encoded at locus PCL1606_41090 (aer1-1) and locus PLC1606_20530 (aer1-2), that are homologous to the Aer receptor of Pseudomonas aeruginosa strain PaO1. Single- and double-deletion mutants in one or both genes have led to motility deficiencies in oxygen-rich areas, particularly reduced swimming motility compared with that of wildtype PcPCL1606. No differences in swarming tests were detected, and less adhesion by the aer double mutant was observed. However, the single and double mutants on avocado plant roots showed delayed biocontrol ability. During the first days of the biocontrol experiment, the aer-defective mutants also showed delayed root colonization. The current research characterizes the presence of aer transductors on P. chlororaphis. Thus, the functions of the PCL1606_41090 and PCL1606_20530 loci, corresponding to genes aer1-1 and aer1-2, respectively, are elucidated.

Pseudomonas syringae pv. syringae Associated With Mango Trees, a Particular Pathogen Within the "Hodgepodge" of the Pseudomonas syringae Complex

The Pseudomonas syringae complex comprises different genetic groups that include strains from both agricultural and environmental habitats. This complex group has been used for decades as a "hodgepodge," including many taxonomically related species. More than 60 pathovars of P. syringae have been described based on distinct host ranges and disease symptoms they cause. These pathovars cause disease relying on an array of virulence mechanisms. However, P. syringae pv. syringae (Pss) is the most polyphagous bacterium in the P. syringae complex, based on its wide host range, that primarily affects woody and herbaceous host plants. In early 1990s, bacterial apical necrosis (BAN) of mango trees, a critical disease elicited by Pss in Southern Spain was described for the first time. Pss exhibits important epiphytic traits and virulence factors, which may promote its survival and pathogenicity in mango trees and in other plant hosts. Over more than two decades, Pss strains isolated from mango trees have been comprehensively investigated to elucidate the mechanisms that governs their epiphytic and pathogenic lifestyles. In particular, the vast majority of Pss strains isolated from mango trees produce an antimetabolite toxin, called mangotoxin, whose leading role in virulence has been clearly demonstrated. Moreover, phenotypic, genetic and phylogenetic approaches support that Pss strains producers of BAN symptoms on mango trees all belong to a single phylotype within phylogroup 2, are adapted to the mango host, and produce mangotoxin. Remarkably, a genome sequencing project of the Pss model strain UMAF0158 revealed the presence of other factors that may play major roles in its different lifestyles, such as the presence of two different type III secretion systems, two type VI secretion systems and an operon for cellulose biosynthesis. The role of cellulose in increasing mango leaf colonization and biofilm formation, and impairing virulence of Pss, suggests that cellulose may play a pivotal role with regards to the balance of its different lifestyles. In addition, 62-kb plasmids belonging to the pPT23A-family of plasmids (PFPs) have been strongly associated with Pss strains that inhabit mango trees. Further, complete sequence and comparative genomic analyses revealed major roles of PFPs in detoxification of copper compounds and ultraviolet radiation resistance, both improving the epiphytic lifestyle of Pss on mango surfaces. Hence, in this review we summarize the research that has been conducted on Pss by our research group to elucidate the molecular mechanisms that underpin the epiphytic and pathogenic lifestyle on mango trees. Finally, future directions in this particular plant-pathogen story are discussed.

Comparative genomic analysis of Pseudomonas amygdali pv. lachrymans NM002: Insights into its potential virulence genes and putative invasion determinants

Pseudomonas amygdali pv. lachrymans is currently of important plant pathogenic bacteria that causes cucumber angular leaf spot worldwide. The pathogen has been studied for its roles in pathogenicity and plant inheritance resistance. To further delineate traits critical to virulence, invasion and survival in the phyllosphere, we reported the first complete genome of P. amygdali pv. lachrymans NM002. Analysis of the whole genome in comparison with three closely-related representative pathovars of P. syringae identified the conservation of virulence genes, including flagella and chemotaxis, quorum-sensing systems, two-component systems, and lipopolysaccharide and antiphagocytosis. It also revealed differences of invasion determinants, such as type III effectors, phytotoxin (coronatine, syringomycin and phaseolotoxin) and cell wall-degrading enzyme, which may contribute to infectivity. The aim of this study was to derive genomic information that would reveal the probable molecular mechanisms underlying the virulence, infectivity and provide a better understanding of the pathogenesis of the P. syringae pathovars.

Discovery of (Dihydro)pyrazine N-Oxides via Genome Mining in Pseudomonas

Overexpression of the Pseudomonas virulence factor ( pvf) biosynthetic operon led to the identification of a family of pyrazine N-oxides (PNOs), including a novel dihydropyrazine N,N'-dioxide (dPNO) metabolite. The nonribosomal peptide synthetase responsible for production of (d)PNOs was characterized, and a biosynthetic pathway for (d)PNOs was proposed. This work highlights the unique chemistry catalyzed by pvf-encoded enzymes and sets the stage for bioactivity studies of the metabolites produced by the virulence pathway.

Bioinformatics Analysis of the Complete Genome Sequence of the Mango Tree Pathogen Pseudomonas syringae pv. syringae UMAF0158 Reveals Traits Relevant to Virulence and Epiphytic Lifestyle

The genome sequence of more than 100 Pseudomonas syringae strains has been sequenced to date; however only few of them have been fully assembled, including P. syringae pv. syringae B728a. Different strains of pv. syringae cause different diseases and have different host specificities; so, UMAF0158 is a P. syringae pv. syringae strain related to B728a but instead of being a bean pathogen it causes apical necrosis of mango trees, and the two strains belong to different phylotypes of pv.syringae and clades of P. syringae. In this study we report the complete sequence and annotation of P. syringae pv. syringae UMAF0158 chromosome and plasmid pPSS158. A comparative analysis with the available sequenced genomes of other 25 P. syringae strains, both closed (the reference genomes DC3000, 1448A and B728a) and draft genomes was performed. The 5.8 Mb UMAF0158 chromosome has 59.3% GC content and comprises 5017 predicted protein-coding genes. Bioinformatics analysis revealed the presence of genes potentially implicated in the virulence and epiphytic fitness of this strain. We identified several genetic features, which are absent in B728a, that may explain the ability of UMAF0158 to colonize and infect mango trees: the mangotoxin biosynthetic operon mbo, a gene cluster for cellulose production, two different type III and two type VI secretion systems, and a particular T3SS effector repertoire. A mutant strain defective in the rhizobial-like T3SS Rhc showed no differences compared to wild-type during its interaction with host and non-host plants and worms. Here we report the first complete sequence of the chromosome of a pv. syringae strain pathogenic to a woody plant host. Our data also shed light on the genetic factors that possibly determine the pathogenic and epiphytic lifestyle of UMAF0158. This work provides the basis for further analysis on specific mechanisms that enable this strain to infect woody plants and for the functional analysis of host specificity in the P. syringae complex.

Genome mining and metabolic profiling of the rhizosphere bacterium Pseudomonas sp. SH-C52 for antimicrobial compounds

The plant microbiome represents an enormous untapped resource for discovering novel genes and bioactive compounds. Previously, we isolated Pseudomonas sp. SH-C52 from the rhizosphere of sugar beet plants grown in a soil suppressive to the fungal pathogen Rhizoctonia solani and showed that its antifungal activity is, in part, attributed to the production of the chlorinated 9-amino-acid lipopeptide thanamycin (Mendes et al., 2011). To get more insight into its biosynthetic repertoire, the genome of Pseudomonas sp. SH-C52 was sequenced and subjected to in silico, mutational and functional analyses. The sequencing revealed a genome size of 6.3 Mb and 5579 predicted ORFs. Phylogenetic analysis placed strain SH-C52 within the Pseudomonas corrugata clade. In silico analysis for secondary metabolites revealed a total of six non-ribosomal peptide synthetase (NRPS) gene clusters, including the two previously described NRPS clusters for thanamycin and the 2-amino acid antibacterial lipopeptide brabantamide. Here we show that thanamycin also has activity against an array of other fungi and that brabantamide A exhibits anti-oomycete activity and affects phospholipases of the late blight pathogen Phytophthora infestans. Most notably, mass spectrometry led to the discovery of a third lipopeptide, designated thanapeptin, with a 22-amino-acid peptide moiety. Seven structural variants of thanapeptin were found with varying degrees of activity against P. infestans. Of the remaining four NRPS clusters, one was predicted to encode for yet another and unknown lipopeptide with a predicted peptide moiety of 8-amino acids. Collectively, these results show an enormous metabolic potential for Pseudomonas sp. SH-C52, with at least three structurally diverse lipopeptides, each with a different antimicrobial activity spectrum.

Intrinsic terminators in Mycoplasma hyopneumoniae transcription

Background

Mycoplasma hyopneumoniae, an important pathogen of swine, exhibits a low guanine and cytosine (GC) content genome. M. hyopneumoniae genome is organised in long transcriptional units and promoter sequences have been mapped upstream of all transcription units. These analysis provided insights into the gene organisation and transcription initiation at the genome scale. However, the presence of transcriptional terminator sequences in the M. hyopneumoniae genome is poorly understood.

Results

In silico analyses demonstrated the presence of putative terminators in 82% of the 33 monocistronic units (mCs) and in 74% of the 116 polycistronic units (pCs) considering different classes of terminators. The functional activity of 23 intrinsic terminators was confirmed by RT-PCR and qPCR. Analysis of all terminators found by three software algorithms, combined with experimental results, allowed us to propose a pattern of RNA hairpin formation during the termination process and to predict the location of terminators in the M. hyopneumoniae genome sequence.

Conclusions

The stem-loop structures of intrinsic terminators of mycoplasma diverge from the pattern of terminators found in other bacteria due the low content of guanine and cytosine. In M. hyopneumoniae, transcription can end after a transcriptional unit and before its terminator sequence and can also continue past the terminator sequence with RNA polymerases gradually releasing the RNA.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1468-6) contains supplementary material, which is available to authorized users.

Transcriptional analysis of the global regulatory networks active in Pseudomonas syringae during leaf colonization

The plant pathogen Pseudomonas syringae pv. syringae B728a grows and survives on leaf surfaces and in the leaf apoplast of its host, bean (Phaseolus vulgaris). To understand the contribution of distinct regulators to B728a fitness and pathogenicity, we performed a transcriptome analysis of strain B728a and nine regulatory mutants recovered from the surfaces and interior of leaves and exposed to environmental stresses in culture. The quorum-sensing regulators AhlR and AefR influenced few genes in planta or in vitro. In contrast, GacS and a downstream regulator, SalA, formed a large regulatory network that included a branch that regulated diverse traits and was independent of plant-specific environmental signals and a plant signal-dependent branch that positively regulated secondary metabolite genes and negatively regulated the type III secretion system. SalA functioned as a central regulator of iron status based on its reciprocal regulation of pyoverdine and achromobactin genes and also sulfur uptake, suggesting a role in the iron-sulfur balance. RetS functioned almost exclusively to repress secondary metabolite genes when the cells were not on leaves. Among the sigma factors examined, AlgU influenced many more genes than RpoS, and most AlgU-regulated genes depended on RpoN. RpoN differentially impacted many AlgU- and GacS-activated genes in cells recovered from apoplastic versus epiphytic sites, suggesting differences in environmental signals or bacterial stress status in these two habitats. Collectively, our findings illustrate a central role for GacS, SalA, RpoN, and AlgU in global regulation in B728a in planta and a high level of plasticity in these regulators’ responses to distinct environmental signals.

Leaves harbor abundant microorganisms, all of which must withstand challenges such as active plant defenses and a highly dynamic environment. Some of these microbes can influence plant health. Despite knowledge of individual regulators that affect the fitness or pathogenicity of foliar pathogens, our understanding of the relative importance of various global regulators to leaf colonization is limited. Pseudomonas syringae strain B728a is a plant pathogen and a good colonist of both the surfaces and interior of leaves. This study used global transcript profiles of strain B728a to investigate the complex regulatory network of putative quorum-sensing regulators, two-component regulators, and sigma factors in cells colonizing the leaf surface and leaf interior under stressful in vitro conditions. The results highlighted the value of evaluating these networks in planta due to the impact of leaf-specific environmental signals and suggested signal differences that may enable cells to differentiate surface versus interior leaf habitats.

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.

Genomics-Based Exploration of Virulence Determinants and Host-Specific Adaptations of Pseudomonas syringae Strains Isolated from Grasses

The Pseudomonas syringae species complex has recently been named the number one plant pathogen, due to its economic and environmental impacts, as well as for its role in scientific research. The bacterium has been repeatedly reported to cause outbreaks on bean, cucumber, stone fruit, kiwi and olive tree, as well as on other crop and non-crop plants. It also serves as a model organism for research on the Type III secretion system (T3SS) and plant-pathogen interactions. While most of the current work on this pathogen is either carried out on one of three model strains found on dicot plants with completely sequenced genomes or on isolates obtained from recent outbreaks, not much is known about strains isolated from grasses (Poaceae). Here, we use comparative genomics in order to identify putative virulence-associated genes and other Poaceae-specific adaptations in several newly available genome sequences of strains isolated from grass species. All strains possess only a small number of known Type III effectors, therefore pointing to the importance of non-Type III secreted virulence factors. The implications of this finding are discussed.

The mangotoxin biosynthetic operon (mbo) is specifically distributed within Pseudomonas syringae genomospecies 1 and was acquired only once during evolution

Mangotoxin production was first described in Pseudomonas syringae pv. syringae strains. A phenotypic characterization of 94 P. syringae strains was carried out to determine the genetic evolution of the mangotoxin biosynthetic operon (mbo). We designed a PCR primer pair specific for the mbo operon to examine its distribution within the P. syringae complex. These primers amplified a 692-bp DNA fragment from 52 mangotoxin-producing strains and from 7 non-mangotoxin-producing strains that harbor the mbo operon, whereas 35 non-mangotoxin-producing strains did not yield any amplification. This, together with the analysis of draft genomes, allowed the identification of the mbo operon in five pathovars (pathovars aptata, avellanae, japonica, pisi, and syringae), all of which belong to genomospecies 1, suggesting a limited distribution of the mbo genes in the P. syringae complex. Phylogenetic analyses using partial sequences from housekeeping genes differentiated three groups within genomospecies 1. All of the strains containing the mbo operon clustered in groups I and II, whereas those lacking the operon clustered in group III; however, the relative branching order of these three groups is dependent on the genes used to construct the phylogeny. The mbo operon maintains synteny and is inserted in the same genomic location, with high sequence conservation around the insertion point, for all the strains in groups I and II. These data support the idea that the mbo operon was acquired horizontally and only once by the ancestor of groups I and II from genomospecies 1 within the P. syringae complex.

Comparative genomics of plant-associated Pseudomonas spp.: insights into diversity and inheritance of traits involved in multitrophic interactions

We provide here a comparative genome analysis of ten strains within the Pseudomonas fluorescens group including seven new genomic sequences. These strains exhibit a diverse spectrum of traits involved in biological control and other multitrophic interactions with plants, microbes, and insects. Multilocus sequence analysis placed the strains in three sub-clades, which was reinforced by high levels of synteny, size of core genomes, and relatedness of orthologous genes between strains within a sub-clade. The heterogeneity of the P. fluorescens group was reflected in the large size of its pan-genome, which makes up approximately 54% of the pan-genome of the genus as a whole, and a core genome representing only 45–52% of the genome of any individual strain. We discovered genes for traits that were not known previously in the strains, including genes for the biosynthesis of the siderophores achromobactin and pseudomonine and the antibiotic 2-hexyl-5-propyl-alkylresorcinol; novel bacteriocins; type II, III, and VI secretion systems; and insect toxins. Certain gene clusters, such as those for two type III secretion systems, are present only in specific sub-clades, suggesting vertical inheritance. Almost all of the genes associated with multitrophic interactions map to genomic regions present in only a subset of the strains or unique to a specific strain. To explore the evolutionary origin of these genes, we mapped their distributions relative to the locations of mobile genetic elements and repetitive extragenic palindromic (REP) elements in each genome. The mobile genetic elements and many strain-specific genes fall into regions devoid of REP elements (i.e., REP deserts) and regions displaying atypical tri-nucleotide composition, possibly indicating relatively recent acquisition of these loci. Collectively, the results of this study highlight the enormous heterogeneity of the P. fluorescens group and the importance of the variable genome in tailoring individual strains to their specific lifestyles and functional repertoire.

We sequenced the genomes of seven strains of the Pseudomonas fluorescens group that colonize plant surfaces and function as biological control agents, protecting plants from disease. In this study, we demonstrated the genomic diversity of the group by comparing these strains to each other and to three other strains that were sequenced previously. Only about half of the genes in each strain are present in all of the other strains, and each strain has hundreds of unique genes that are not present in the other genomes. We mapped the genes that contribute to biological control in each genome and found that most of the biological control genes are in the variable regions of the genome, which are not shared by all of the other strains. This finding is consistent with our knowledge of the distinctive biology of each strain. Finally, we looked for new genes that are likely to confer antimicrobial traits needed to suppress plant pathogens, but have not been identified previously. In each genome, we discovered many of these new genes, which provide avenues for future discovery of new traits with the potential to manage plant diseases in agriculture or natural ecosystems.

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.