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

GacA reduces virulence and increases competitiveness in planta in the tumorigenic olive pathogen Pseudomonas savastanoi pv. savastanoi

GacS/GacA is a widely distributed two-component system playing an essential role as a key global regulator, although its characterization in phytopathogenic bacteria has been deeply biased, being intensively studied in pathogens of herbaceous plants but barely investigated in pathogens of woody hosts. P. savastanoi pv. savastanoi (Psv) is characterized by inducing tumours in the stem and branches of olive trees. In this work, the model strain Psv NCPPB 3335 and a mutant derivative with a complete deletion of gene gacA were subjected to RNA-Seq analyses in a minimum medium and a medium mimicking in planta conditions, accompanied by RT-qPCR analyses of selected genes and phenotypic assays. These experiments indicated that GacA participates in the regulation of at least 2152 genes in strain NCPPB 3335, representing 37.9 % of the annotated CDSs. GacA also controls the expression of diverse rsm genes, and modulates diverse phenotypes, including motility and resistance to oxidative stresses. As occurs with other P. syringae pathovars of herbaceous plants, GacA regulates the expression of the type III secretion system and cognate effectors. In addition, GacA also regulates the expression of WHOP genes, specifically encoded in P. syringe strains isolated from woody hosts, and genes for the biosynthesis of phytohormones. A gacA mutant of NCPPB 3335 showed increased virulence, producing large immature tumours with high bacterial populations, but showed a significantly reduced competitiveness in planta. Our results further extend the role of the global regulator GacA in the virulence and fitness of a P. syringae pathogen of woody hosts.

Regulation of the Pseudomonas syringae Type III Secretion System by Host Environment Signals

Pseudomonas syringae are Gram-negative, plant pathogenic bacteria that use a type III secretion system (T3SS) to disarm host immune responses and promote bacterial growth within plant tissues. Despite the critical role for type III secretion in promoting virulence, T3SS-encoding genes are not constitutively expressed by P. syringae and must instead be induced during infection. While it has been known for many years that culturing P. syringae in synthetic minimal media can induce the T3SS, relatively little is known about host signals that regulate the deployment of the T3SS during infection. The recent identification of specific plant-derived amino acids and organic acids that induce T3SS-inducing genes in P. syringae has provided new insights into host sensing mechanisms. This review summarizes current knowledge of the regulatory machinery governing T3SS deployment in P. syringae, including master regulators HrpRS and HrpL encoded within the T3SS pathogenicity island, and the environmental factors that modulate the abundance and/or activity of these key regulators. We highlight putative receptors and regulatory networks involved in linking the perception of host signals to the regulation of the core HrpRS–HrpL pathway. Positive and negative regulation of T3SS deployment is also discussed within the context of P. syringae infection, where contributions from distinct host signals and regulatory networks likely enable the fine-tuning of T3SS deployment within host tissues. Last, we propose future research directions necessary to construct a comprehensive model that (a) links the perception of host metabolite signals to T3SS deployment and (b) places these host–pathogen signaling events in the overall context of P. syringae infection.

Distinctive features of the Gac-Rsm pathway in plant-associated Pseudomonas

Productive plant-bacteria interactions, either beneficial or pathogenic, require that bacteria successfully sense, integrate and respond to continuously changing environmental and plant stimuli. They use complex signal transduction systems that control a vast array of genes and functions. The Gac-Rsm global regulatory pathway plays a key role in controlling fundamental aspects of the apparently different lifestyles of plant beneficial and phytopathogenic Pseudomonas as it coordinates adaptation and survival while either promoting plant health (biocontrol strains) or causing disease (pathogenic strains). Plant-interacting Pseudomonas stand out for possessing multiple Rsm proteins and Rsm RNAs, but the physiological significance of this redundancy is not yet clear. Strikingly, the components of the Gac-Rsm pathway and the controlled genes/pathways are similar, but the outcome of its regulation may be opposite. Therefore, identifying the target mRNAs bound by the Rsm proteins and their mode of action (repression or activation) is essential to explain the resulting phenotype. Some technical considerations to approach the study of this system are also given. Overall, several important features of the Gac-Rsm cascade are now understood in molecular detail, particularly in Pseudomonas protegens CHA0, but further questions remain to be solved in other plant-interacting Pseudomonas.

Quorum Sensing Regulation in Phytopathogenic Bacteria

Quorum sensing is a type of chemical communication by which bacterial populations control expression of their genes in a coordinated manner. This regulatory mechanism is commonly used by pathogens to control the expression of genes encoding virulence factors and that of genes involved in the bacterial adaptation to variations in environmental conditions. In phytopathogenic bacteria, several mechanisms of quorum sensing have been characterized. In this review, we describe the different quorum sensing systems present in phytopathogenic bacteria, such as those using the signal molecules named N-acyl-homoserine lactone (AHL), diffusible signal factor (DSF), and the unknown signal molecule of the virulence factor modulating (VFM) system. We focus on studies performed on phytopathogenic bacteria of major importance, including Pseudomonas, Ralstonia, Agrobacterium, Xanthomonas, Erwinia, Xylella,Dickeya, and Pectobacterium spp. For each system, we present the mechanism of regulation, the functions targeted by the quorum sensing system, and the mechanisms by which quorum sensing is regulated.

Transposon Insertion Sequencing in a Clinical Isolate of Legionella pneumophila Identifies Essential Genes and Determinants of Natural Transformation

Legionella pneumophila is the etiologic agent of a severe form of nosocomial and community-acquired pneumonia in humans. The environmental life traits of L. pneumophila are essential to its ability to accidentally infect humans.

Legionella pneumophila is a Gram-negative bacterium ubiquitous in freshwater environments which, if inhaled, can cause a severe pneumonia in humans. The emergence of L. pneumophila is linked to several traits selected in the environment, the acquisition of some of which involved intra- and interkingdom horizontal gene transfer events. Transposon insertion sequencing (TIS) is a powerful method to identify the genetic basis of selectable traits as well as to identify fitness determinants and essential genes, which are possible antibiotic targets. TIS has not yet been used to its full power in L. pneumophila, possibly because of the difficulty of obtaining a high-saturation transposon insertion library. Indeed, we found that isolates of sequence type 1 (ST1), which includes the commonly used laboratory strains, are poorly permissive to saturating mutagenesis by conjugation-mediated transposon delivery. In contrast, we obtained high-saturation libraries in non-ST1 clinical isolates, offering the prospect of using TIS on unaltered L. pneumophila strains. Focusing on one of them, we then used TIS to identify essential genes in L. pneumophila. We also revealed that TIS could be used to identify genes controlling vertical transmission of mobile genetic elements. We then applied TIS to identify all the genes required for L. pneumophila to develop competence and undergo natural transformation, defining the set of major and minor type IV pilins that are engaged in DNA uptake. This work paves the way for the functional exploration of the L. pneumophila genome by TIS and the identification of the genetic basis of other life traits of this species.

IMPORTANCELegionella pneumophila is the etiologic agent of a severe form of nosocomial and community-acquired pneumonia in humans. The environmental life traits of L. pneumophila are essential to its ability to accidentally infect humans. A comprehensive identification of their genetic basis could be obtained through the use of transposon insertion sequencing. However, this powerful approach had not been fully implemented in L. pneumophila. Here, we describe the successful implementation of the transposon-sequencing approach in a clinical isolate of L. pneumophila. We identify essential genes, potential drug targets, and genes required for horizontal gene transfer by natural transformation. This work represents an important step toward identifying the genetic basis of the many life traits of this environmental and pathogenic species.

The Evanescent GacS Signal

The GacS histidine kinase is the membrane sensor of the major upstream two-component system of the regulatory Gac/Rsm signal transduction pathway. This pathway governs the expression of a wide range of genes in pseudomonads and controls bacterial fitness and motility, tolerance to stress, biofilm formation, and virulence or plant protection. Despite the importance of these roles, the ligands binding to the sensor domain of GacS remain unknown, and their identification is an exciting challenge in this domain. At high population densities, the GacS signal triggers a switch from primary to secondary metabolism and a change in bacterial lifestyle. It has been suggested, based on these observations, that the GacS signal is a marker of the emergence of nutritional stress and competition. Biochemical investigations have yet to characterize the GacS signal fully. However, they portray this cue as a low-molecular weight, relatively simple and moderately apolar metabolite possibly resembling, but nevertheless different, from the aliphatic organic acids acting as quorum-sensing signaling molecules in other Proteobacteria. Significant progress in the development of metabolomic tools and new databases dedicated to Pseudomonas metabolism should help to unlock some of the last remaining secrets of GacS induction, making it possible to control the Gac/Rsm pathway.

Transcriptomic Changes of Piscirickettsia salmonis During Intracellular Growth in a Salmon Macrophage-Like Cell Line

Piscirickettsia salmonis is the causative agent of Piscirickettsiosis, a systemic infection of salmonid fish species. P. salmonis infects and survives in its host cell, a process that correlates with the expression of virulence factors including components of the type IVB secretion system. To gain further insights into the cellular and molecular mechanism behind the adaptive response of P. salmonis during host infection, we established an in vitro model of infection using the SHK-1 cell line from Atlantic salmon head kidney. The results indicated that in comparison to uninfected SHK-1 cells, infection significantly decreased cell viability after 10 days along with a significant increment of P. salmonis genome equivalents. At that time, the intracellular bacteria were localized within a spacious cytoplasmic vacuole. By using a whole-genome microarray of P. salmonis LF-89, the transcriptome of this bacterium was examined during intracellular growth in the SHK-1 cell line and exponential growth in broth. Transcriptome analysis revealed a global shutdown of translation during P. salmonis intracellular growth and suggested an induction of the stringent response. Accordingly, key genes of the stringent response pathway were up-regulated during intracellular growth as well as at stationary phase bacteria, suggesting a role of the stringent response on bacterial virulence. Our results also reinforce the participation of the Dot/Icm type IVB secretion system during P. salmonis infection and reveals many unexplored genes with potential roles in the adaptation to intracellular growth. Finally, we proposed that intracellular P. salmonis alternates between a replicative phase and a stationary phase in which the stringent response is activated.

A revised model for the role of GacS/GacA in regulating type III secretion by Pseudomonas syringae pv. tomato DC3000

GacS/GacA is a conserved two-component system that functions as a master regulator of virulence-associated traits in many bacterial pathogens, including Pseudomonas spp., that collectively infect both plant and animal hosts. Among many GacS/GacA-regulated traits, type III secretion of effector proteins into host cells plays a critical role in bacterial virulence. In the opportunistic plant and animal pathogen Pseudomonas aeruginosa, GacS/GacA negatively regulates the expression of type III secretion system (T3SS)-encoding genes. However, in the plant pathogenic bacterium Pseudomonas syringae, strain-to-strain variation exists in the requirement of GacS/GacA for T3SS deployment, and this variability has limited the development of predictive models of how GacS/GacA functions in this species. In this work we re-evaluated the function of GacA in P. syringae pv. tomato DC3000. Contrary to previous reports, we discovered that GacA negatively regulates the expression of T3SS genes in DC3000, and that GacA is not required for DC3000 virulence inside Arabidopsis leaf tissue. However, our results show that GacA is required for full virulence of leaf surface-inoculated bacteria. These data significantly revise current understanding of GacS/GacA in regulating P. syringae virulence.

The LetA/S two-component system regulates transcriptomic changes that are essential for the culturability of Legionella pneumophila in water

Surviving the nutrient-poor aquatic environment for extended periods of time is important for the transmission of various water-borne pathogens, including Legionella pneumophila (Lp). Previous work concluded that the stringent response and the sigma factor RpoS are essential for the survival of Lp in water. In the present study, we investigated the role of the LetA/S two-component signal transduction system in the successful survival of Lp in water. In addition to cell size reduction in the post-exponential phase, LetS also contributes to cell size reduction when Lp is exposed to water. Importantly, absence of the sensor kinase results in a significantly lower survival as measured by CFUs in water at various temperatures and an increased sensitivity to heat shock. According to the transcriptomic analysis, LetA/S orchestrates a general transcriptomic downshift of major metabolic pathways upon exposure to water leading to better culturability, and likely survival, suggesting a potential link with the stringent response. However, the expression of the LetA/S regulated small regulatory RNAs, RsmY and RsmZ, is not changed in a relAspoT mutant, which indicates that the stringent response and the LetA/S response are two distinct regulatory systems contributing to the survival of Lp in water.

MexEF-OprN multidrug efflux pump transporter negatively controls N-acyl-homoserine lactone accumulation in pseudomonas syringae pv. Tabaci 6605

Our previous studies revealed that flagellar-motility-defective mutants such as ∆fliC of Pseudomonas syringae pv. tabaci 6605 (Pta6605) have remarkably reduced production of N-acyl-homoserine lactones (AHL), quorum-sensing molecules. To investigate the reason of loss of AHL production in ∆fliC mutant, we carried out transposon mutagenesis. Among approximately 14,000 transconjugants, we found 11 AHL production-recovered (APR) strains. In these APR strains, a transposon was inserted into either mexE or mexF, genes encoding for the multidrug efflux pump transporter MexEF-OprN, and mexT, a gene encoding a putative transcriptional activator for mexEF-oprN. These results suggest that MexEF-OprN is a negative regulator of AHL production. To confirm the negative effect of MexEF-OprN on AHL production, loss- and gain-of-function experiments for mexEF-oprN were carried out. The ∆fliC∆mexF and ∆fliC∆mexT double mutant strains recovered AHL production, whereas the mexT overexpressing strain abolished AHL production, although the psyI, a gene encoding AHL synthase, is transcribed as wild type. Introduction of a mexF or mexT mutation into another flagellar-motility- and AHL production-defective mutant strain, ∆motCD, also recovered the ability to produce AHL. Furthermore, introduction of the mexF mutation into other AHL production-defective mutant strains such as ∆gacA and ∆aefR also recovered AHL production but not to the ∆psyI mutant. These results indicate that MexEF-OprN is a decisive negative determinant of AHL production and accumulation.

N-acylhomoserine lactone-regulation of genes mediating motility and pathogenicity in Pseudomonas syringae pathovar tabaci 11528

Pseudomonas syringae pathovar tabaci 11528 (P. syringae 11528) is a phytopathogen that causes wild-fire disease in soybean and tobacco plants. It utilizes a cell density-dependent regulation system known as quorum sensing (QS). In its QS system, the psyI is responsible for the biosynthesis of N-acylhomoserine lactones (AHLs). By comparing the transcripts from P. syringae 11528 wild-type strain with those of the ΔpsyI mutant using RNA sequencing (RNA-seq) technology, 1118 AHL-regulated genes were identified in the transition from exponential to stationary growth phase. Numerous AHL-regulated genes involved in pathogenicity were negatively controlled, including genes linked to flagella, chemotaxis, pilus, extracellular polysaccharides, secretion systems, and two-component system. Moreover, gene ontology and pathway enrichment analysis revealed that the most pronounced regulation was associated with bacterial motility. Finally, phenotypic assays showed that QS-regulated traits were involved in epiphytic growth of pathogens and disease development in plants. These findings imply that the AHL-mediated QS system in P. syringae 11528 plays significant roles in distinct stages of interactions between plants and pathogens, including early plant colonization and late plant infection.

Characterization of Clinically Isolated Antibiotic-Resistant Salmonella Typhimurium Exposed to Subinhibitory Concentrations of Ceftriaxone and Ciprofloxacin

This study was designed mainly to assess the phenotypic properties of clinically isolated Salmonella Typhimurium exposed to ceftriaxone and ciprofloxacin. The antibiotic susceptibility, β-lactamase activity, efflux activity, bacterial motility, biofilm-forming ability, and gene expression were determined in S. Typhimurium ATCC 19585 and S. Typhimurium CCARM 8009 when exposed to subinhibitory concentrations of ceftriaxone and ciprofloxacin. S. Typhimurium CCARM 8009 was highly resistant to ampicillin, kanamycin, penicillin G, and streptomycin, showing minimum inhibitory concentration values of more than 512 μg/ml, while S. Typhimurium ATCC 19585 showed resistance to erythromycin alone (64 μg/ml). The highest β-lactamase activity was observed in S. Typhimurium CCARM 8009 when exposed to ceftriaxone (8.2 μmol/min/ml), while the least β-lactamase activity was observed in S. Typhimurium ATCC 19585. Compared to S. Typhimurium CCARM 8009, the ethidium bromide (EtBr) accumulation was considerably increased in S. Typhimurium ATCC 19585 when treated with efflux pump inhibitors. S. Typhimurium ATCC 19585 and S. Typhimurium CCARM 8009 were highly susceptible to ciprofloxacin, erythromycin, levofloxacin, and sparfloxacin in the presence of phenylalanine-arginine-β-naphthylamide. The swimming motility of S. Typhimurium ATCC 19585 exposed to ceftriaxone was significantly reduced to 54% when compared to S. Typhimurium CCARM 8009 (93%). The numbers of attached S. Typhimurium CCARM 8009 cells were significantly increased by more than 1 log cfu/ml when exposed to ceftriaxone and ciprofloxacin. The relative gene expression was stable in S. Typhimurium CCARM 8009 in the presence of ceftriaxone and ciprofloxacin compared to the absence of antibiotics. These results suggest that the antibiotic susceptibility of S. Typhimurium having different antibiotic resistance profiles varied depending on the presence of ceftriaxone and ciprofloxacin.

Functional analysis of the aefR mutation and identification of its binding site in Pseudomonas syringae pv. tabaci 11528

The TetR family transcriptional regulator AefR contributes to the regulation of the quorum-sensing system. However, the role of AefR in the regulatory network of the phytopathogen Pseudomonas syringae pathovars is not known. In this study, the phenotype of a P. syringae pv. tabaci 11528 aefR deletion mutant strain was examined. The aefR gene expression and AefR DNA-binding affinity were examined by quantitative real-time polymerase chain reaction and electrophoretic mobility shift assay, respectively. AefR was found to control quorum-sensing genes as well as the efflux genes mexE, mexF, and oprN via an indirect mechanism. AefR binds to its own operator site as well as to the palindromic sequence between positions -28 and -2 corresponding to the transcription start site of aefR, as determined by dye primer sequencing. These results suggest that P. syringae AefR modulates quorum sensing and efflux as well as its own expression, which can be exploited by strategies developed to manage this plant parasite.

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.

Characterization of quorum sensing-controlled transcriptional regulator MarR and Rieske (2Fe-2S) cluster-containing protein (Orf5), which are involved in resistance to environmental stresses in Pseudomonas syringae pv. tabaci 6605

Pseudomonas syringae pv. tabaci 6605 (Pta6605) produces acyl homoserine lactones (AHLs), quorum sensing (QS) molecules that are indispensable for virulence in host tobacco infection. Genome-wide transcriptional profiling of several QS-defective mutants revealed that the expression of the genes encoding the MarR family transcriptional regulator (MarR) and a Rieske 2Fe-2S cluster-containing protein (Orf5) located adjacent to psyI, a gene encoding AHL synthetase, are significantly repressed. Exogenous application of AHL recovered the expression of both marR and orf5 genes in the ΔpsyI mutant, indicating that AHL positively regulates the expression of these genes. To investigate the role of these genes in the virulence of Pta6605, ΔmarR and Δorf5 mutants were generated. Both mutants showed decreased swimming and swarming motilities, decreased survival ability under oxidative and nitrosative stresses and, consequently, reduced virulence on host tobacco plants. Transmission electron micrographs showed that the structure of the cell membranes of ΔmarR and Δorf5 mutants was severely damaged. Furthermore, not only the ratio of dead cells, but also the amount of flagella, extracellular DNA and protein released into the culture supernatant, was significantly increased in both mutants, indicating that the disruption of marR and orf5 genes might induce structural changes in the membrane and cell lysis. Because both mutants showed partly similar expression profiles, both gene products might be involved in the same regulatory cascades that are required for QS-dependent survival under environmentally stressed conditions.

Physicochemical, mechanical, and molecular properties of nonlysogenic and p22-lysogenic Salmonella typhimurium treated with citrus oil

The aim of this study was to evaluate the phenotypic and genotypic properties of nonlysogenic Salmonella Typhimurium (ST(P22-)) and lysogenic Salmonella Typhimurium (ST(P22+)) in the presence of sublethal concentrations (SLC2D) of citrus essential oils (CEOs), which were used to evaluate antimicrobial susceptibility, cell surface hydrophobicity, autoaggregation ability, bacterial motility, lysogenic conversion, gene expression patterns, and antibiofilm formation. The SLC2D values of non-heat-treated (N-CEO) and heat-treated (H-CEO) CEO in an autoclave at 121°C for 20 min were 2.0 to 2.1 mg/ml against ST(P22-) and 1.7 to 1.9 mg/ml against STP(22+). The rates of injured ST(P22-) and ST(P22+) cells treated with SLC2D of N-CEO and H-CEO ranged from 67 to 83%. The hydrophobicity and autoaggregation were decreased to 2.5 and 19.5% for ST(P22-) and 4.7 and 21.7% for ST(P22+), respectively, in the presence of N-CEO. A noticeable reduction in the swarming motility was observed in ST(P22-) with N-CEO (14.5%) and H-CEO (13.3%). The numbers of CEO-induced P22 were 5.40 log PFU/ml for N-CEO and 5.65 log PFU/ml for H-CEO. The relative expression of hilA, hilC, hilD, invA, invC, invE, invF, sirA, and sirB was down-regulated in ST(P22-) and ST(P22+) with N-CEO and H-CEO. The numbers of adherent ST(P22-) and ST(P22+) were effectively reduced by more than 1 log in the presence of CEO. These results suggest that CEO has potential to be used to control bacterial attachment, colonization, and invasion.

Effect of bacteriophage on the susceptibility, motility, invasion, and survival of Salmonella Typhimurium exposed to the simulated intestinal conditions

This study was designed to evaluate the effect of bacteriophage P22 on the susceptibility, swimming motility, invasion gene expression, invasive ability, and intracellular survival of Salmonella Typhimurium exposed to the simulated intestinal conditions. S. Typhimurium cells were inoculated at 37 °C for 4 h in the simulated intestinal conditions with or without bacteriophage P22, including control (0 % bile salts, pH 7.2), SN (0 % bile salts, pH 5.0), SL (0.5 % bile salts, pH 5.0), SH (2.0 % bile salts, pH 5.0), SNp (0 % bile salts + P22, pH 5.0), SLp (0.5 % bile salts + P22, pH 5.0), and SHp (2.0 % bile salts + P22, pH 5.0). The numbers of Typhimurium cells were significantly reduced by 3.30, 3.56, and 3.75 log units, respectively, at SNp, SLp, and SHp. Considerable reduction in the swimming motility was observed at SNp (23 %), SLp (22 %), and SHp (20 %). The transcriptional regulator genes, hilA, hilC, hilD, invA, invE, and invF, were significantly down-regulated with SHp, showing 4.07-fold, 2.87-fold, 3.43-fold, 2.07-fold, 1.44-fold, and 4.83-fold, respectively. The decrease in invasive ability was most significant at SHp (45 %), followed by SLp (49 %). These results suggest that bacteriophage P22 can be used as an alternative to control Salmonella invasion of epithelial cells.

Roles of the Gac-Rsm pathway in the regulation of phenazine biosynthesis in Pseudomonas chlororaphis 30-84

The GacS/GacA two-component regulatory system activates the production of secondary metabolites including phenazines crucial for biological control activity in Pseudomonas chlororaphis 30-84. To better understand the role of the Gac system on phenazine regulation, transcriptomic analyses were conducted by comparing the wild-type strain to a gacA mutant. RNA-seq analysis identified 771 genes under GacA control, including many novel genes. Consistent with previous findings, phenazine biosynthetic genes were significantly downregulated in a gacA mutant. The transcript abundances of phenazine regulatory genes such as phzI, phzR, iopA, iopB, rpoS, and pip also were reduced. Moreover, the transcript abundance of three noncoding RNAs (ncRNAs) including rsmX, rsmY, and rsmZ was significantly decreased by gacA mutation consistent with the presence of consensus GacA-binding sites associated with their promoters. Our results also demonstrated that constitutive expression of rsmZ from a non-gac regulated promoter resulted in complete restoration of N-acyl-homoserine lactone (AHL) and phenazine production as well as the expression of other gac-dependent secondary metabolites in gac mutants. The role of RsmA and RsmE in phenazine production also was investigated. Overexpression of rsmE, but not rsmA, resulted in decreased AHL and phenazine production in P. chlororaphis, and only a mutation in rsmE bypassed the requirement for GacA in phenazine gene expression. In contrast, constitutive expression of the phzI/phzR quorum sensing system did not rescue phenazine production in the gacA mutant, indicating the direct posttranscriptional control by Gac on the phenazine biosynthetic genes. On the basis of these results, we propose a model to illustrate the hierarchic role of phenazine regulators modulated by Gac in the control of phenazine production. The transcriptomic analysis also was used to identify additional genes regulated by GacA that may contribute to the biological control capability of strain 30-84.

Virulence factor regulator (Vfr) controls virulence-associated phenotypes in Pseudomonas syringae pv. tabaci 6605 by a quorum sensing-independent mechanism

Virulence factor regulator (Vfr) is a member of the cyclic 3',5'-adenosine monophosphate (cAMP) receptor proteins that regulate the expression of many important virulence genes in Pseudomonas aeruginosa. The role of Vfr in pathogenicity has not been elucidated fully in phytopathogenic bacteria. To investigate the function of Vfr in Pseudomonas syringae pv. tabaci 6605, the vfr gene was disrupted. The virulence of the vfr mutant towards host tobacco plants was attenuated significantly, and the intracellular cAMP level was decreased. The vfr mutant reduced the expression of flagella-, pili- and type III secretion system-related genes and the defence response in nonhost Arabidopsis leaves. Furthermore, the expression levels of achromobactin-related genes and the iron uptake ability were decreased, suggesting that Vfr regulates positively these virulence-related genes. In contrast, the vfr mutant showed higher tolerance to antimicrobial compounds as a result of the enhanced expression of the resistance-nodulation-division family members, the mexA, mexB and oprM genes. We further demonstrated that the mutant strains of vfr and cyaA, an adenylate cyclase gene responsible for cAMP synthesis, showed a similar phenotype, suggesting that Vfr regulates virulence factors in a cAMP-dependent manner. Because there was no significant difference in the production of acylhomoserine lactone (AHL) quorum sensing molecules in the wild-type, vfr and cyaA mutant strains, Vfr might control important virulence factors by an AHL-independent mechanism in an early stage of infection by this bacterium.

Carbon source and cell density-dependent regulation of type III secretion system gene expression in Pseudomonas syringae pathovar tomato DC3000

Pseudomonas syringae utilizes a type III secretion system (T3SS) encoded by the hrp/hrc genes to translocate virulence proteins called effectors into plant cells. To ensure that the T3SS functions at appropriate times during infection, hrp/hrc and effector gene expression is modulated by environmental conditions and a complex network of transcription factors. The sigma factor HrpL activates hrp/hrc and effector genes, while σ(54) and enhancer binding proteins HrpR and HrpS regulate hrpL. To better understand how environmental conditions control the T3SS regulatory cascade in P. syringae pathovar tomato strain DC3000, we tested the effects of various growth media and carbon sources on expression of the hrpRS operon, hrpL, and the effector avrPto. Fructose optimally induced hrpRS expression, while most other carbon sources had only mild stimulatory effects. In contrast, hrpL and avrPto were highly induced by several sugars and organic acids, yet expression decreased as cultures reached higher cell densities. This cell density-dependent regulation was not due to alteration of the pH of the medium, although involvement of a quorum sensing signal was also not apparent. Our findings may explain conflicting results from previous studies and additionally indicate that culture conditions should be considered carefully when examining T3SS gene expression.

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.

Characterization of each aefR and mexT mutant in Pseudomonas syringae pv. tabaci 6605

To investigate the mechanism of activation of the genes for resistance-nodulation-division (RND) family members MexE, MexF, and OprN for multidrug resistance (MDR), we mutagenized aefR and mexT, the potential regulators of mexEF/oprN transcription in Pseudomonas syringae pv. tabaci 6605 (Pta 6605). AefR is a member of the TetR transcription factors, and is known to be required for production of the quorum-sensing molecules, acyl homoserine lactones (AHL), in P. syringae. Furthermore, we found that AHL-synthesis-defective mutant strains in Pta 6605 showed enhanced expression of mexEF/oprN, and were highly tolerant to antimicrobial compounds such as chloramphenicol. MexT is a LysR-type transcription factor and is known to positively regulate transcription of mexEF/oprN in Pseudomonas aeruginosa. The ∆aefR mutant reduced the amount of growth in in vitro culture, caused the loss of AHL production, reduced the swarming motility, virulence and expression of psyI (AHL synthase) and psyR (AHL transcriptional regulator), and enhanced mexEF/oprN expression and tolerance to chloramphenicol, whereas the ∆mexT mutant retained the ability to produce AHL and did not show remarkable changes in in vitro growth, tolerance to antimicrobial compounds or virulence. Furthermore, unlike P. aeruginosa, the expression of mexEF/oprN is independent of MexT. These results indicate that (1) AefR is a regulator for the quorum-sensing system and MDR, and is required for swarming motility and virulence toward the host tobacco plant, and (2) MexT is not involved in the expression of mexEF/oprN in this bacterium.

The role of a periplasmic gluconolactonase (PpgL)-like protein in Pseudomonas syringae pv. syringae B728a

In Pseudomonas syringae pv. syringae B728a, the Psyr_1712 locus ID encodes a putative protein with a signal peptide and a COG2706 domain of the type present in 3-carboxy-cis,cis-muconate lactonizing enzymes. An amino acid sequence alignment of the P. aeruginosa PpgL with other genome sequenced fluorescent pseudomonads such as P. syringae Psyr_1712 showed that they have the same enzymatic active site residue comprising one histidine, one glutamic acid and two arginines. Based on the similarity of the Psyr_1712 locus ID and PpgL of P. aeruginosa, it was designated as PspL (P seudomonas s yringae PpgL- like) protein. Deletion of the pspL gene caused a delay in lag phase growth of bacterium. Mutants lacking pspL were defective in N-acylhomoserine lactones production. The PspL with signal peptide was expressed in a ppgL mutant of P. aeruginosa and restored the defects. The presence of a lux-like box sequence in upstream of pspL along with decreased expression level of the pspL gene in an ahlI negative mutant indicated that the pspL gene is under control of quorum sensing. Furthermore, two acylhomoserinelactone regulated phenotypes, swarming motility and susceptibility to hydrogen peroxide were enhanced in ΔpspL mutant. Together, this work reveals the important role of the new PpgL-like protein PspL in quorum sensing of P. syringae pv. syringae B728a.

The siderophore pyoverdine of Pseudomonas syringae pv. tabaci 6605 is an intrinsic virulence factor in host tobacco infection

To investigate the role of iron uptake mediated by the siderophore pyoverdine in the virulence of the plant pathogen Pseudomonas syringae pv. tabaci 6605, three predicted pyoverdine synthesis-related genes, pvdJ, pvdL, and fpvA, were mutated. The pvdJ, pvdL, and fpvA genes encode the pyoverdine side chain peptide synthetase III L-Thr-L-Ser component, the pyoverdine chromophore synthetase, and the TonB-dependent ferripyoverdine receptor, respectively. The Delta pvdJ and Delta pvdL mutants were unable to produce pyoverdine in mineral salts-glucose medium, which was used for the iron-depleted condition. Furthermore, the Delta pvdJ and Delta pvdL mutants showed lower abilities to produce tabtoxin, extracellular polysaccharide, and acyl homoserine lactones (AHLs), which are quorum-sensing molecules, and consequently had reduced virulence on host tobacco plants. In contrast, all of the mutants had accelerated swarming ability and increased biosurfactant production, suggesting that swarming motility and biosurfactant production might be negatively controlled by pyoverdine. Scanning electron micrographs of the surfaces of tobacco leaves inoculated with the mutant strains revealed only small amounts of extracellular polymeric matrix around these mutants, indicating disruption of the mature biofilm. Tolerance to antibiotics was drastically increased for the Delta pvdL mutant, as for the Delta psyI mutant, which is defective in AHL production. These results demonstrated that pyoverdine synthesis and the quorum-sensing system of Pseudomonas syringae pv. tabaci 6605 are indispensable for virulence in host tobacco infection and that AHL may negatively regulate tolerance to antibiotics.

Systems level analysis of two-component signal transduction systems in Erwinia amylovora: role in virulence, regulation of amylovoran biosynthesis and swarming motility

BACKGROUND

Two-component signal transduction systems (TCSTs), consisting of a histidine kinase (HK) and a response regulator (RR), represent a major paradigm for signal transduction in prokaryotes. TCSTs play critical roles in sensing and responding to environmental conditions, and in bacterial pathogenesis. Most TCSTs in Erwinia amylovora have either not been identified or have not yet been studied.

RESULTS

We used a systems approach to identify TCST and related signal transduction genes in the genome of E. amylovora. Comparative genomic analysis of TCSTs indicated that E. amylovora TCSTs were closely related to those of Erwinia tasmaniensis, a saprophytic enterobacterium isolated from apple flowers, and to other enterobacteria. Forty-six TCST genes in E. amylovora including 17 sensor kinases, three hybrid kinases, 20 DNA- or ligand-binding RRs, four RRs with enzymatic output domain (EAL-GGDEF proteins), and two kinases were characterized in this study. A systematic TCST gene-knockout experiment was conducted, generating a total of 59 single-, double-, and triple-mutants. Virulence assays revealed that five of these mutants were non-pathogenic on immature pear fruits. Results from phenotypic characterization and gene expression experiments indicated that several groups of TCST systems in E. amylovora control amylovoran biosynthesis, one of two major virulence factors in E. amylovora. Both negative and positive regulators of amylovoran biosynthesis were identified, indicating a complex network may control this important feature of pathogenesis. Positive (non-motile, EnvZ/OmpR), negative (hypermotile, GrrS/GrrA), and intermediate regulators for swarming motility in E. amylovora were also identified.

CONCLUSION

Our results demonstrated that TCSTs in E. amylovora played major roles in virulence on immature pear fruit and in regulating amylovoran biosynthesis and swarming motility. This suggested presence of regulatory networks governing expression of critical virulence genes in E. amylovora.