Identification of gene loci controlling pectate lyase production and soft-rot pathogenicity in Pseudomonas marginalis

Re-evaluation of a Tn5::gacA mutant of Pseudomonas syringae pv. tomato DC3000 uncovers roles for uvrC and anmK in promoting virulence

Pseudomonas syringae is a taxon of plant pathogenic bacteria that can colonize and proliferate within the interior space of leaf tissue. This process requires P. syringae to rapidly upregulate the production of virulence factors including a type III secretion system (T3SS) that suppress host defenses. GacS/A is a two-component system that regulates virulence of many plant and animal pathogenic bacteria including P. syringae. We recently investigated the virulence defect of strain AC811, a Tn5::gacA mutant of P. syringae pv. tomato DC3000 that is less virulent on Arabidopsis. We discovered that decreased virulence of AC811 is not caused by loss of GacA function. Here, we report the molecular basis of the virulence defect of AC811. We show that AC811 possesses a nonsense mutation in anmK, a gene predicted to encode a 1,6-anhydromuramic acid kinase involved in cell wall recycling. Expression of a wild-type allele of anmK partially increased growth of AC811 in Arabidopsis leaves. In addition to the defective anmK allele, we also show that the Tn5 insertion in gacA exerts a polar effect on uvrC, a downstream gene encoding a regulator of DNA damage repair. Expression of the wild-type anmK allele together with increased expression of uvrC fully restored the virulence of AC811 during infection of Arabidopsis. These results demonstrate that defects in anmK and uvrC are together sufficient to account for the decreased virulence of AC811, and suggest caution is warranted in assigning phenotypes to GacA function based on insertional mutagenesis of the gacA-uvrC locus.

Re-evaluation of a Tn5::gacA mutant of Pseudomonas syringae pv. tomato DC3000 uncovers roles for uvrC and anmK in promoting virulence

Pseudomonas syringae is a taxon of plant pathogenic bacteria that can colonize and proliferate within the interior space of leaf tissue. This process requires P. syringae to rapidly upregulate the production of virulence factors including a type III secretion system (T3SS) that suppress host defenses. GacS/A is a two-component system that regulates virulence of many plant and animal pathogenic bacteria including P. syringae. We recently investigated the virulence defect of strain AC811, a Tn5::gacA mutant of P. syringae pv. tomato DC3000 that is less virulent on Arabidopsis. We discovered that decreased virulence of AC811 is not caused by loss of GacA function. Here, we report the molecular basis of the virulence defect of AC811. We show that AC811 possesses a nonsense mutation in anmK, a gene predicted to encode a 1,6-anhydromuramic acid kinase involved in cell wall recycling. Expression of a wild-type allele of anmK partially increased growth of AC811 in Arabidopsis leaves. In addition to the defective anmK allele, we also show that the Tn5 insertion in gacA exerts a polar effect on uvrC, a downstream gene encoding a regulator of DNA damage repair. Expression of the wild-type anmK allele together with increased expression of uvrC fully restored the virulence of AC811 during infection of Arabidopsis. These results demonstrate that defects in anmK and uvrC are together sufficient to account for the decreased virulence of AC811, and suggest caution is warranted in assigning phenotypes to GacA function based on insertional mutagenesis of the gacA-uvrC locus.

The two-component regulators GacS and GacA positively regulate a nonfluorescent siderophore through the Gac/Rsm signaling cascade in high-siderophore-yielding Pseudomonas sp. strain HYS

Siderophores, which are produced to overcome iron deficiency, are believed to be closely related to the adaptability of bacteria. The high-siderophore-yielding Pseudomonas sp. strain HYS simultaneously secretes the fluorescent siderophore pyoverdine and another nonfluorescent siderophore that is a major contributor to the high siderophore yield. Transposon mutagenesis revealed siderophore-related genes, including the two-component regulators GacS/GacA and a special cluster containing four open reading frames (the nfs cluster). Deletion mutations of these genes abolished nonfluorescent-siderophore production, and expression of the nfs cluster depended on gacA, indicating that gacS-gacA may control the nonfluorescent siderophore through regulation of the nfs cluster. Furthermore, regulation of the nonfluorescent siderophore by GacS/GacA involved the Gac/Rsm pathway. In contrast, inactivation of GacS/GacA led to upregulation of the fluorescent pyoverdine. The two siderophores were secreted under different iron conditions, probably because of differential effects of GacS/GacA. The global GacS/GacA regulatory system may control iron uptake by modulating siderophore production and may enable bacteria to adapt to changing iron environments.

The Gac/Rsm and cyclic-di-GMP signalling networks coordinately regulate iron uptake in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a versatile bacterial pathogen capable of occupying diverse ecological niches. To cope with iron limitation, P. aeruginosa secretes two siderophores, pyoverdine and pyochelin, whose ability to deliver iron to the cell is crucial for biofilm formation and pathogenicity. In this study, we describe a link between iron uptake and the Gac/Rsm system, a conserved signal transducing pathway of P. aeruginosa that controls the production of extracellular products and virulence factors, as well as the switch from planktonic to biofilm lifestyle. We have observed that pyoverdine and pyochelin production in P. aeruginosa is strongly dependent on the activation state of the Gac/Rsm pathway, which controls siderophore regulatory and biosynthetic genes at the transcriptional level, in a manner that does not involve regulation of ferric uptake regulator (Fur) expression. Gac/Rsm-mediated regulation of iron uptake genes appears to be conserved in different P. aeruginosa strains. Further experiments led to propose that the Gac/Rsm system regulates siderophore production through modulation of the intracellular levels of the second messenger c-di-GMP, indicating that the c-di-GMP and the Gac/Rsm regulatory networks essential for biofilm formation can also coordinately control iron uptake in P. aeruginosa.

Global effect of indole-3-acetic acid biosynthesis on multiple virulence factors of Erwinia chrysanthemi 3937

Production of the plant hormone indole-3-acetic acid (IAA) is widespread among plant-associated microorganisms. The non-gall-forming phytopathogen Erwinia chrysanthemi 3937 (strain Ech3937) possesses iaaM (ASAP16562) and iaaH (ASAP16563) gene homologues. In this work, the null knockout iaaM mutant strain Ech138 was constructed. The IAA production by Ech138 was reduced in M9 minimal medium supplemented with l-tryptophan. Compared with wild-type Ech3937, Ech138 exhibited reduced ability to produce local maceration, but its multiplication in Saintpaulia ionantha was unaffected. The pectate lyase production of Ech138 was diminished. Compared with wild-type Ech3937, the expression levels of an oligogalacturonate lyase gene, ogl, and three endopectate lyase genes, pelD, pelI, and pelL, were reduced in Ech138 as determined by a green fluorescent protein-based fluorescence-activated cell sorting promoter activity assay. In addition, the transcription of type III secretion system (T3SS) genes, dspE (a putative T3SS effector) and hrpN (T3SS harpin), was found to be diminished in the iaaM mutant Ech138. Compared with Ech3937, reduced expression of hrpL (a T3SS alternative sigma factor) and gacA but increased expression of rsmA in Ech138 was also observed, suggesting that the regulation of T3SS and pectate lyase genes by IAA biosynthesis might be partially due to the posttranscriptional regulation of the Gac-Rsm regulatory pathway.

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.

Induction of apoptotic cell death leads to the development of bacterial rot caused by Pseudomonas cichorii

Pseudomonas cichorii is the major causal agent of bacterial rot of lettuce. Collapse and browning symptoms were observed in lettuce leaf tissue from 15 to 24 h after inoculation (HAI) with P. cichorii; superoxide anion generation was detected at 1 to 6 HAI; and cell death was induced at 6 HAI, reaching a maximum at approximately 9 and 12 HAI. Heterochromatin condensation and DNA laddering also were observed within 3 HAI. Pharmacological studies showed that induction of cell death and DNA laddering was closely associated with de novo protein synthesis, protein kinase, intracellular reactive oxygen species, DNase, serine protease, and caspase III-like protease. Moreover, chemicals, which inhibited the induction of cell death and DNA laddering, also suppressed the development of disease symptoms. These results suggest that apoptotic cell death might be closely associated with the development of bacterial rot caused by P. cichorii.

A functional screen for the type III (Hrp) secretome of the plant pathogen Pseudomonas syringae

Type III secreted "effector" proteins of bacterial pathogens play central roles in virulence, yet are notoriously difficult to identify. We used an in vivo genetic screen to identify 13 effectors secreted by the type III apparatus (called Hrp, for "hypersensitive response and pathogenicity") of the plant pathogen Pseudomonas syringae. Although sharing little overall homology, the amino-terminal regions of these effectors had strikingly similar amino acid compositions. This feature facilitated the bioinformatic prediction of 38 P. syringae effectors, including 15 previously unknown proteins. The secretion of two of these putative effectors was shown to be type III--dependent. Effectors showed high interstrain variation, supporting a role for some effectors in adaptation to different hosts.

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.

SirA orthologs affect both motility and virulence

The sirA gene of Salmonella enterica serovar Typhimurium encodes a two-component response regulator of the FixJ family that has a positive regulatory influence on the expression of type III secretion genes involved with epithelial cell invasion and the elicitation of bovine gastroenteritis. SirA orthologs in Pseudomonas, Vibrio, and Erwinia control the expression of distinct virulence genes in these genera, but an evolutionarily conserved target of SirA regulation has never been identified. In this study we tested the hypothesis that sirA may be an ancient member of the flagellar regulon. We examined the effect of a sirA mutation on transcriptional fusions to flagellar promoters (flhD, fliE, fliF, flgA, flgB, fliC, fliD, motA, and fliA) while using fusions to the virulence gene sopB as a positive control. SirA had only small regulatory effects on all fusions in liquid medium (less than fivefold). However, in various types of motility agar plates, sirA was able to activate a sopB fusion by up to 63-fold while repressing flagellar fusions by values exceeding 100-fold. Mutations in the sirA orthologs of Escherichia coli, Vibrio cholerae, Pseudomonas fluorescens, and Pseudomonas aeruginosa result in defects in either motility or motility gene regulation, suggesting that control of flagellar regulons may be an evolutionarily conserved function of sirA orthologs. The implications for our understanding of virulence gene regulation in the gamma Proteobacteria are discussed.

Identification of UvrY as the cognate response regulator for the BarA sensor kinase in Escherichia coli

BarA is a membrane-associated protein that belongs to a subclass of tripartite sensors of the two-component signal transduction system family. In this study, we report that UvrY is the cognate response regulator for BarA of Escherichia coli. This conclusion is based upon homologies with analogous two-component systems and demonstrated by both biochemical and genetic means. We show that the purified BarA protein is able to autophosphorylate when incubated with [gamma-(32)P]ATP but not with [alpha-(32)P]ATP or [gamma-(32)P]GTP. Phosphorylated BarA, in turn, acts as an efficient phosphoryl group donor to UvrY but not to the non-cognate response regulators ArcA, PhoB, or CpxR. The specificity of the transphosphorylation reaction is further supported by the fact that UvrY can receive the phosphoryl group from BarA-P but not from the non-cognate tripartite sensor ArcB-P or ATP. In addition, genetic evidence that BarA and UvrY mediate the same signal transduction pathway is provided by the finding that both uvrY and barA mutant strains exhibit the same hydrogen peroxide hypersensitive phenotype. These results provide the first biochemical evidence as well as genetic support for a link between BarA and UvrY, suggesting that the two proteins constitute a new two-component system for gene regulation in Escherichia coli.

Common mechanisms for pathogens of plants and animals

The vast evolutionary gulf between plants and animals--in terms of structure, composition, and many environmental factors--would seem to preclude the possibility that these organisms could act as receptive hosts to the same microorganism. However, some pathogens are capable of establishing themselves and thriving in members of both the plant and animal kingdoms. The identification of functionally conserved virulence mechanisms required to infect hosts of divergent evolutionary origins demonstrates the remarkable conservation in some of the underlying virulence mechanisms of pathogenesis and is changing researchers' thinking about the evolution of microbial pathogenesis.

Production of pectate lyases and cellulases by Chryseomonas luteola strain MFCL0 depends on the growth temperature and the nature of the culture medium: evidence for two critical temperatures

Several extracellular enzymes that are responsible for plant tissue maceration were detected in culture supernatant of the psychrotrophic bacterium Chryseomonas luteola MFCL0. Isoelectrofocusing experiments showed that pectate lyase (PL) activity resulted from the cumulative action of three major isoenzymes, designated PLI, PLII, and PLIII. Cellulolytic activity was also detected in culture supernatants. These enzymes exhibited different behaviors with respect to growth temperature. PLII was not regulated by temperature, whereas PLI and PLIII were regulated similarly by growth temperature. Maximal levels of PLI and PLIII were produced at 14 degrees C when cells were grown in polygalacturonate-containing synthetic medium and at around 20 to 24 degrees C in nutrient broth. In contrast, thermoregulation of cellulolytic activity production differed from thermoregulation of PL. The level of cellulolytic activity was low in all media at temperatures up to 20 degrees C, and then it increased dramatically until the temperature was 28 degrees C, which is the optimal temperature for growth of C. luteola. Previously, we defined the critical temperature by using the modified Arrhenius equation to characterize bacterial behavior. This approach consists of monitoring changes in the maximal specific growth rate as a function of temperature. Our most striking result was the finding that the temperature at which maximum levels of PLI and PLIII were produced in two different media was the same as the critical temperature for growth observed in these two media.

Characterization of two novel regulatory genes affecting Salmonella invasion gene expression

A Salmonella typhimurium chromosomal deletion removing approximately 19 kb of DNA at centisome 65 reduces invasion of cultured epithelial cells as well as the expression of lacZY operon fusions to several genes required for the invasive phenotype. As the deleted region contains no genes previously known to affect Salmonella invasion, we investigated the roles of individual genes in the deleted region using a combination of cloning, complementation and directed mutation. We find that the deletion includes two unrelated regulatory genes. One is the Salmonella homologue of Escherichia coli barA (airS ), which encodes a member of the multistep phosphorelay subgroup of two-component sensor kinases. The action of BarA is coupled to that of SirA, a member of the phosphorylated response regulator family of proteins, and includes both HilA-dependent and HilA-independent components. The other regulatory gene removed by the deletion is the Salmonella homologue of E. coli csrB, which specifies a regulatory RNA implicated in controlling specific message turnover in E. coli. These results identify a protein that is likely to play a key role in the environmental control of Salmonella invasion gene expression, and they also suggest that transcriptional control of invasion genes could be subject to refinement at the level of message turnover.

Salmonella SirA is a global regulator of genes mediating enteropathogenesis

SirA of Salmonella typhimurium is known to regulate the hilA and prgH genes within Salmonella pathogenicity island 1 (SPI1). To identify more members of the SirA regulon, we screened 10,000 random lacZY fusions (chromosomal MudJ insertions) for regulation by SirA and identified 10 positively regulated fusions. Three fusions were within the SPI1 genes hilA (an SPI1 transcriptional regulator), spaS (a component of the SPI1 type III export apparatus) and sipB (a substrate of the SPI1 export apparatus). Two fusions were within the sopB gene (also known as sigD). sopB is located within SPI5, but encodes a protein that is exported via the SPI1 export apparatus. In addition, five fusions were within genes of unknown function that are located in SPI4. As spaS and sipB were likely to be hilA dependent, we tested all of the fusions (except hilA) for hilA dependence. Surprisingly, we found that all of the fusions require hilA for expression and that plasmid-encoded SirA cannot bypass this requirement. Therefore, SirA regulates hilA, the product of which regulates genes within SPI1, SPI4 and SPI5. Both sirA and hilA mutants are dramatically attenuated in a bovine model of gastroenteritis, but have little or no effect in the mouse model of typhoid fever. This study establishes the SirA/HilA regulatory cascade as the primary regulon controlling enteropathogenic virulence functions in S. typhimurium. Because S. typhimurium causes gastroenteritis in both cattle and humans, we believe that this information may be directly applicable to the human disease.

Two-component regulators involved in the global control of virulence in Erwinia carotovora subsp. carotovora

Production of extracellular, plant cell wall degrading enzymes, the main virulence determinants of the plant pathogen Erwinia carotovora subsp. carotovora, is coordinately controlled by a complex regulatory network. Insertion mutants in the exp (extracellular enzyme production) loci exhibit pleiotropic defects in virulence and the growth-phase-dependent transcriptional activation of genes encoding extracellular enzymes. Two new exp mutations, designated expA and expS, were characterized. Introduction of the corresponding wild-type alleles to the mutants complemented both the lack of virulence and the impaired production of plant cell wall degrading enzymes. The expA gene was shown to encode a 24-kDa polypeptide that is structurally and functionally related to the uvrY gene product of Escherichia coli and the GacA response regulator of Pseudomonas fluorescens. Functional similarity of expA and uvrY was demonstrated by genetic complementation. The expA gene is organized in an operon together with a uvrC-like gene, identical to the organization of uvrY and uvrC in E. coli. The unlinked expS gene encodes a putative sensor kinase that shows 92% identity to the recently described rpfA gene product from another E. carotovora subsp. carotovora strain. Our data suggest that ExpS and ExpA are members of two-component sensor kinase and response regulator families, respectively. These two proteins might interact in controlling virulence gene expression in E. carotovora subsp. carotovora.

Biochemical and genetic characterization of an extracellular protease from Pseudomonas fluorescens CY091

Pseudomonas fluorescens CY091 cultures produce an extracellular protease with an estimated molecular mass of 50 kDa. Production of this enzyme (designated AprX) was observed in media containing CaCl2 or SrCl2 but not in media containing ZnCl2, MgCl2, or MnCl2. The requirement of Ca2+ (or Sr2+) for enzyme production was concentration dependent, and the optimal concentration for production was determined to be 0.35 mM. Following ammonium sulfate precipitation and ion-exchange chromatography, the AprX in the culture supernatant was purified to near electrophoretic homogeneity. Over 20% of the enzyme activity was retained in the AprX sample which had been heated in boiling water for 10 min, indicating that the enzyme is highly resistant to heat inactivation. The enzyme activity was almost completely inhibited in the presence of 1 mM 1,10-phenanthroline, but only 30% of the activity was inhibited in the presence of 1 mM EGTA. The gene encoding AprX was cloned from the genome of P. fluorescens CY091 by isolating cosmid clones capable of restoring the protease production in a nonproteolytic mutant of strain CY091. The genomic region of strain CY091 containing the aprX gene was located within a 7.3-kb DNA fragment. Analysis of the complete nucleotide sequence of this 7.3-kb fragment revealed the presence of a cluster of genes required for the production of extracellular AprX in P. fluorescens and Escherichia coli. The AprX protein showed 50 to 60% identity in amino acid sequence to the related proteases produced by Pseudomonas aeruginosa and Erwinia chrysanthemi. Two conserved sequence domains possibly associated with Ca2+ and Zn2+ binding were identified. Immediately adjacent to the aprX structural gene, a gene (inh) encoding a putative protease inhibitor and three genes (aprD, aprE, and aprF), possibly required for the transport of AprX, were also identified. The organization of the gene cluster involved in the synthesis and secretion of AprX in P. fluorescens CY091 appears to be somewhat different from that previously demonstrated in P. aeruginosa and E. chrysanthemi.