The Pseudomonas syringae pv. syringae 61 hrpH product, an envelope protein required for elicitation of the hypersensitive response in plants

Discovery of the Hrp Type III Secretion System in Phytopathogenic Bacteria: How Investigation of Hypersensitive Cell Death in Plants Led to a Novel Protein Injector System and a World of Inter-Organismal Molecular Interactions Within Plant Cells

In the early 1960s, Pseudomonas syringae and other host-specific phytopathogenic proteobacteria were discovered to elicit a rapid, resistance-associated death when infiltrated at high inoculum levels into nonhost tobacco leaves. This hypersensitive reaction (or response; HR) was a useful indicator of basic pathogenic ability. Research over the next 20 years failed to identify an elicitor of the HR but revealed that its elicitation required contact between metabolically active bacterial and plant cells. Beginning in the early 1980s, molecular genetic tools were applied to the HR puzzle, revealing the presence in P. syringae of clusters of hrp genes, so named because they are required for the HR and pathogenicity, and of avr genes, so named because their presence confers HR-associated avirulence in resistant cultivars of a host plant species. A series of breakthroughs over the next two decades revealed that (i) hrp gene clusters encode a type III secretion system (T3SS), which injects Avr (now "effector") proteins into plant cells, where their recognition triggers the HR; (ii) T3SSs, which are typically present in pathogenicity islands acquired by horizontal gene transfers, are found in many bacterial pathogens of plants and animals and inject many effector proteins, which are collectively essential for pathogenicity; and (iii) a primary function of phytopathogen effectors is to subvert non-HR defenses resulting from recognition of conserved microbial features presented outside of plant cells. In the 2000s, Hrp system research shifted to extracellular components enabling effector delivery across plant cell walls and plasma membranes, regulation, and tools for studying effectors. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

A novel formamidase is required for riboflavin biosynthesis in invasive bacteria

Biosynthesis of riboflavin, the precursor of the redox cofactors FMN and FAD, was thought to be well understood in bacteria, with all the pathway enzymes presumed to be known and essential. Our previous research has challenged this view by showing that, in the bacterium Sinorhizobium meliloti, deletion of the ribBA gene encoding the enzyme that catalyzes the initial steps on the riboflavin biosynthesis pathway only causes a reduction in flavin secretion rather than riboflavin auxotrophy. This finding led us to hypothesize that RibBA participates in the biosynthesis of flavins destined for secretion, while S. meliloti has another enzyme that performs this function for internal cellular metabolism. Here, we identify and biochemically characterize a novel formamidase (SMc02977) involved in the production of riboflavin for intracellular functions in S. meliloti. This catalyst, which we named Sm-BrbF, releases formate from the early riboflavin precursor 2-amino-5-formylamino-6-ribosylamino-4(3H)-pyrimidinone 5'-phosphate (AFRPP) to yield 2,5-diamino-6-ribosylamino-4(3H)-pyrimidinone 5'-phosphate (DARoPP). We show that homologs of this enzyme are present in many bacteria, are highly abundant in the Rhizobiales order, and that sequence homologs from Brucella abortus and Liberobacter solanacearum complement the riboflavin auxotrophy of the Sm1021ΔSMc02977 mutant. Furthermore, we show that the B. abortus enzyme (Bab2_0247, Ba-BrbF) is also an AFRPP formamidase, and that the bab2_0247 mutant is a riboflavin auxotroph exhibiting a lower level of intracellular infection than the wild-type strain. Finally, we show that Sm-BrbF and Ba-BrbF directly interact with other riboflavin biosynthesis pathway enzymes. Together, our results provide novel insight into the intricacies of riboflavin biosynthesis in bacteria.

Sinorhizobium medicae WSM419 Genes That Improve Symbiosis between Sinorhizobium meliloti Rm1021 and Medicago truncatula Jemalong A17 and in Other Symbiosis Systems

Some soil bacteria, called rhizobia, can interact symbiotically with legumes, in which they form nodules on the plant roots, where they can reduce atmospheric dinitrogen to ammonia, a form of nitrogen that can be used by growing plants. Rhizobium-plant combinations can differ in how successful this symbiosis is: for example, Sinorhizobium meliloti Rm1021 forms a relatively ineffective symbiosis with Medicago truncatula Jemalong A17, but Sinorhizobium medicae WSM419 is able to support more vigorous plant growth. Using proteomic data from free-living and symbiotic S. medicae WSM419, we previously identified a subset of proteins that were not closely related to any S. meliloti Rm1021 proteins and speculated that adding one or more of these proteins to S. meliloti Rm1021 would increase its effectiveness on M. truncatula A17. Three genes, Smed_3503, Smed_5985, and Smed_6456, were cloned into S. meliloti Rm1021 downstream of the E. coli lacZ promoter. Strains with these genes increased nodulation and improved plant growth, individually and in combination with one another. Smed_3503, renamed iseA (increased symbiotic effectiveness), had the largest impact, increasing M. truncatula biomass by 61%. iseA homologs were present in all currently sequenced S. medicae strains but were infrequent in other Sinorhizobium isolates. Rhizobium leguminosarum bv. viciae 3841 containing iseA led to more nodules on pea and lentil. Split-root experiments with M. truncatula A17 indicated that S. meliloti Rm1021 carrying the S. medicae iseA is less sensitive to plant-induced resistance to rhizobial infection, suggesting an interaction with the plant's regulation of nodule formation. IMPORTANCE Legume symbiosis with rhizobia is highly specific. Rhizobia that can nodulate and fix nitrogen on one legume species are often unable to associate with a different species. The interaction can be more subtle. Symbiotically enhanced growth of the host plant can differ substantially when nodules are formed by different rhizobial isolates of a species, much like disease severity can differ when conspecific isolates of pathogenic bacteria infect different cultivars. Much is known about bacterial genes essential for a productive symbiosis, but less is understood about genes that marginally improve performance. We used a proteomic strategy to identify Sinorhizobium genes that contribute to plant growth differences that are seen when two different strains nodulate M. truncatula A17. These genes could also alter the symbiosis between R. leguminosarum bv. viciae 3841 and pea or lentil, suggesting that this approach identifies new genes that may more generally contribute to symbiotic productivity.

Arabidopsis bZIP11 Is a Susceptibility Factor During Pseudomonas syringae Infection

The induction of plant nutrient secretion systems is critical for successful pathogen infection. Some bacterial pathogens (e.g., Xanthomonas spp.) use transcription activator-like (TAL) effectors to induce transcription of SWEET sucrose efflux transporters. Pseudomonas syringae pv. tomato strain DC3000 lacks TAL effectors yet is able to induce multiple SWEETs in Arabidopsis thaliana by unknown mechanisms. Because bacteria require other nutrients in addition to sugars for efficient reproduction, we hypothesized that Pseudomonas spp. may depend on host transcription factors involved in secretory programs to increase access to essential nutrients. Bioinformatic analyses identified the Arabidopsis basic-leucine zipper transcription factor bZIP11 as a potential regulator of nutrient transporters, including SWEETs and UmamiT amino acid transporters. Inducible downregulation of bZIP11 expression in Arabidopsis resulted in reduced growth of P. syringae pv. tomato strain DC3000, whereas inducible overexpression of bZIP11 resulted in increased bacterial growth, supporting the hypothesis that bZIP11-regulated transcription programs are essential for maximal pathogen titer in leaves. Our data are consistent with a model in which a pathogen alters host transcription factor expression upstream of secretory transcription networks to promote nutrient efflux from host cells.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

James Robert Alfano, A Giant in Phytopathogenic Bacteria Effector Biology

The worldwide molecular plant-microbe interactions research community was significantly diminished in November 2019 by the death of James "Jim" Robert Alfano at age 56. Jim was a giant in our field, who gained key insights into plant pathogenesis using the model bacterial pathogen Pseudomonas syringae. As a mentor, collaborator, and, above all, a friend, I know Jim's many dimensions and accomplishments and, sadly, the depth of loss being felt by the many people around the world who were touched by him. In tracing the path of Jim's career, I will emphasize the historical context and impact of his advances and, finally, the essence of the person we will so miss.

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.

Herbaspirillum seropedicae Differentially Expressed Genes in Response to Iron Availability

Herbaspirillum seropedicae Z67 is a nitrogen-fixing endophyte that colonizes many important crops. Like in almost all organisms, vital cellular processes of this endophyte are iron dependent. In order to efficiently acquire iron to fulfill its requirements, this bacterium produces the siderophores serobactins. However, the presence in its genome of many others iron acquisition genes suggests that serobactins are not the only strategy used by H. seropedicae to overcome metal deficiency. The aim of this work was to identify genes and proteins differentially expressed by cells growing in low iron conditions in order to describe H. seropedicae response to iron limitation stress. For this purpose, and by using a transcriptomic approach, we searched and identified a set of genes up-regulated when iron was scarce. One of them, Hsero_2337, codes for a TonB-dependent transporter/transducer present in the serobactins biosynthesis genomic locus, with an unknown function. Another TonB-dependent receptor, the one encoded by Hsero_1277, and an inner membrane ferrous iron permease, coded by Hsero_2720, were also detected. By using a proteomic approach focused in membrane proteins, we identified the specific receptor for iron-serobactin internalization SbtR and two non-characterized TonB-dependent receptors (coded by genes Hsero_1277 and Hsero_3255). We constructed mutants on some of the identified genes and characterized them by in vitro growth, biofilm formation, and interaction with rice plants. Characterization of mutants in gene Hsero_2337 showed that the TonB-dependent receptor coded by this gene has a regulatory role in the biosynthesis of serobactins, probably by interacting with the alternative sigma factor PfrI, coded by gene Hsero_2338. Plant colonization of the mutant strains was not affected, since the mutant strain normally colonize the root and aerial part of rice plants. These results suggest that the strategies used by H. seropedicae to acquire iron inside plants are far more diverse than the ones characterized in this work. In vivo expression studies or colonization competition experiments between the different mutant strains could help us in future works to determine the relative importance of the different iron acquisition systems in the interaction of H. seropedicae with rice plants.

Involvement of type VI secretion system in secretion of iron chelator pyoverdine in Pseudomonas taiwanensis

Rice bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) is one of the most destructive rice diseases worldwide. Therefore, in addition to breeding disease-resistant rice cultivars, it is desirable to develop effective biocontrol agents against Xoo. Here, we report that a soil bacterium Pseudomonas taiwanensis displayed strong antagonistic activity against Xoo. Using matrix-assisted laser desorption/ionization imaging mass spectrometry, we identified an iron chelator, pyoverdine, secreted by P. taiwanensis that could inhibit the growth of Xoo. Through Tn5 mutagenesis of P. taiwanensis, we showed that mutations in genes that encode components of the type VI secretion system (T6SS) as well as biosynthesis and maturation of pyoverdine resulted in reduced toxicity against Xoo. Our results indicated that T6SS is involved in the secretion of endogenous pyoverdine. Mutations in T6SS component genes affected the secretion of mature pyoverdine from the periplasmic space into the extracellular medium after pyoverdine precursor is transferred to the periplasm by the inner membrane transporter PvdE. In addition, we also showed that other export systems, i.e., the PvdRT-OpmQ and MexAB-OprM efflux systems (for which there have been previous suggestions of involvement) and the type II secretion system (T2SS), are not involved in pyoverdine secretion.

Global Regulator IscR Positively Contributes to Antimonite Resistance and Oxidation in Comamonas testosteroni S44

Antimonial compounds can be found as a toxic contaminant in the environment. Knowledge on mechanisms of microbial Sb oxidation and its role in microbial tolerance are limited. Previously, we found that Comamonas testosteroni S44 was resistant to multiple heavy metals and was able to oxidize the toxic antimonite [Sb(III)] to the much less toxic antimonate [Sb(V)]. In this study, transposon mutagenesis was performed in C. testosteroni S44 to isolate genes responsible for Sb(III) resistance and oxidation. An insertion mutation into iscR, which regulates genes involved in the biosynthesis of Fe-S clusters, generated a strain called iscR-280. This mutant strain was complemented with a plasmid carrying iscR to generate strain iscR-280C. Compared to the wild type S44 and iscR-280C, strain iscR-280 showed lower resistance to Sb(III) and a lower Sb(III) oxidation rate. Strain iscR-280 also showed lower resistance to As(III), Cd(II), Cu(II), and H₂O₂. In addition, intracellular γ-glutamylcysteine ligase (γ-GCL) activity and glutathione (GSH) content were decreased in the mutated strain iscR-280. Real-time RT-PCR and lacZ fusion expression assay indicated that transcription of iscR and iscS was induced by Sb(III). Results of electrophoretic mobility shift assay (EMSA) and bacterial one-hybrid (B1H) system demonstrated a positive interaction between IscR and its promoter region. The diverse defective phenotypes and various expression patterns suggest a role for IscR in contributing to multi-metal(loid)s resistance and Sb(III) oxidation via Fe-S cluster biogenesis and oxidative stress protection. Bacterial Sb(III) oxidation is a detoxification reaction.

Sinorhizobium meliloti flavin secretion and bacteria-host interaction: role of the bifunctional RibBA protein

Sinorhizobium meliloti, the nitrogen-fixing bacterial symbiont of Medicago spp. and other legumes, secretes a considerable amount of riboflavin. This precursor of the cofactors flavin mononucleotide and flavin adenine dinucleotide is a bioactive molecule that has a beneficial effect on plant growth. The ribBA gene of S. meliloti codes for a putative bifunctional enzyme with dihydroxybutanone phosphate synthase and guanosine triphosphate (GTP) cyclohydrolase II activities, catalyzing the initial steps of the riboflavin biosynthesis pathway. We show here that an in-frame deletion of ribBA does not cause riboflavin auxotrophy or affect the ability of S. meliloti to establish an effective symbiosis with the host plant but does affect the ability of the bacteria to secrete flavins, colonize host-plant roots, and compete for nodulation. A strain missing the RibBA protein retains considerable GTP cyclohydrolase II activity. Based on these results, we hypothesize that S. meliloti has two partly interchangeable modules for biosynthesis of riboflavin, one fulfilling the internal need for flavins in bacterial metabolism and the other producing riboflavin for secretion. Our data also indicate that bacteria-derived flavins play a role in communication between rhizobia and the legume host and that the RibBA protein is important in this communication process even though it is not essential for riboflavin biosynthesis and symbiosis.

The non-flagellar type III secretion system evolved from the bacterial flagellum and diversified into host-cell adapted systems

Type 3 secretion systems (T3SSs) are essential components of two complex bacterial machineries: the flagellum, which drives cell motility, and the non-flagellar T3SS (NF-T3SS), which delivers effectors into eukaryotic cells. Yet the origin, specialization, and diversification of these machineries remained unclear. We developed computational tools to identify homologous components of the two systems and to discriminate between them. Our analysis of >1,000 genomes identified 921 T3SSs, including 222 NF-T3SSs. Phylogenomic and comparative analyses of these systems argue that the NF-T3SS arose from an exaptation of the flagellum, i.e. the recruitment of part of the flagellum structure for the evolution of the new protein delivery function. This reconstructed chronology of the exaptation process proceeded in at least two steps. An intermediate ancestral form of NF-T3SS, whose descendants still exist in Myxococcales, lacked elements that are essential for motility and included a subset of NF-T3SS features. We argue that this ancestral version was involved in protein translocation. A second major step in the evolution of NF-T3SSs occurred via recruitment of secretins to the NF-T3SS, an event that occurred at least three times from different systems. In rhizobiales, a partial homologous gene replacement of the secretin resulted in two genes of complementary function. Acquisition of a secretin was followed by the rapid adaptation of the resulting NF-T3SSs to multiple, distinct eukaryotic cell envelopes where they became key in parasitic and mutualistic associations between prokaryotes and eukaryotes. Our work elucidates major steps of the evolutionary scenario leading to extant NF-T3SSs. It demonstrates how molecular evolution can convert one complex molecular machine into a second, equally complex machine by successive deletions, innovations, and recruitment from other molecular systems.

Involvement of RpoN in regulating bacterial arsenite oxidation

In this study with the model organism Agrobacterium tumefaciens, we used a combination of lacZ gene fusions, reverse transcriptase PCR (RT-PCR), and deletion and insertional inactivation mutations to show unambiguously that the alternative sigma factor RpoN participates in the regulation of As(III) oxidation. A deletion mutation that removed the RpoN binding site from the aioBA promoter and an aacC3 (gentamicin resistance) cassette insertional inactivation of the rpoN coding region eliminated aioBA expression and As(III) oxidation, although rpoN expression was not related to cell exposure to As(III). Putative RpoN binding sites were identified throughout the genome and, as examples, included promoters for aioB, phoB1, pstS1, dctA, glnA, glnB, and flgB that were examined by using qualitative RT-PCR and lacZ reporter fusions to assess the relative contribution of RpoN to their transcription. The expressions of aioB and dctA in the wild-type strain were considerably enhanced in cells exposed to As(III), and both genes were silent in the rpoN::aacC3 mutant regardless of As(III). The expression level of glnA was not influenced by As(III) but was reduced (but not silent) in the rpoN::aacC3 mutant and further reduced in the mutant under N starvation conditions. The rpoN::aacC3 mutation had no obvious effect on the expression of glnB, pstS1, phoB1, or flgB. These experiments provide definitive evidence to document the requirement of RpoN for As(III) oxidation but also illustrate that the presence of a consensus RpoN binding site does not necessarily link the associated gene with regulation by As(III) or by this sigma factor.

NTRC and chloroplast-generated reactive oxygen species regulate Pseudomonas syringae pv. tomato disease development in tomato and Arabidopsis

Coronatine (COR)-producing pathovars of Pseudomonas syringae, including pvs. tomato, maculicola, and glycinea, cause important diseases on tomato, crucifers, and soybean, respectively, and produce symptoms with necrotic lesions surrounded by chlorosis. The chlorosis is mainly attributed to COR. However, the significance of COR-induced chlorosis in localized lesion development and the molecular basis of disease-associated cell death is largely unknown. To identify host (chloroplast) genes that play a role in COR-mediated chlorosis, we used a forward genetics approach using Nicotiana benthamiana and virus-induced gene silencing and identified a gene which encodes 2-Cys peroxiredoxin (Prxs) that, when silenced, produced a spreading hypersensitive or necrosis-like phenotype instead of chlorosis after COR application in a COI1-dependent manner. Loss-of-function analysis of Prx and NADPH-dependent thioredoxin reductase C (NTRC), the central players of a chloroplast redox detoxification system, resulted in spreading accelerated P. syringae pv. tomato DC3000 disease-associated cell death with enhanced reactive oxygen species (ROS) accumulation in a COR-dependent manner in tomato and Arabidopsis. Consistent with these results, virulent strain DC3000 suppressed the expression of Prx and NTRC in Arabidopsis and tomato during pathogenesis. However, interestingly, authentic COR suppressed the expression of Prx and NTRC in tomato but not in Arabidopsis, suggesting that COR in conjunction with other effectors may modulate ROS and cell death in different host species. Taken together, these results indicated that NTRC or Prx function as a negative regulator of pathogen-induced cell death in the healthy tissues that surround the lesions, and COR-induced chloroplast-localized ROS play a role in enhancing the disease-associated cell death.

Nitrogen metabolism in Sinorhizobium meliloti-alfalfa symbiosis: dissecting the role of GlnD and PII proteins

To contribute nitrogen for plant growth and establish an effective symbiosis with alfalfa, Sinorhizobium meliloti Rm1021 needs normal operation of the GlnD protein, a bifunctional uridylyltransferase/uridylyl-cleavage enzyme that measures cellular nitrogen status and initiates a nitrogen stress response (NSR). However, the only two known targets of GlnD modification in Rm1021, the PII proteins GlnB and GlnK, are not necessary for effectiveness. We introduced a Tyr→Phe variant of GlnB, which cannot be uridylylated, into a glnBglnK background to approximate the expected state in a glnD-sm2 mutant, and this strain was effective. These results suggested that unmodified PII does not inhibit effectiveness. We also generated a glnBglnK-glnD triple mutant and used this and other mutants to dissect the role of these proteins in regulating the free-living NSR and nitrogen metabolism in symbiosis. The glnD-sm2 mutation was dominant to the glnBglnK mutations in symbiosis but recessive in some free-living phenotypes. The data show that the GlnD protein has a role in free-living growth and in symbiotic nitrogen exchange that does not depend on the PII proteins, suggesting that S. meliloti GlnD can communicate with the cell by alternate mechanisms.

Structural and functional analysis of the type III secretion system from Pseudomonas fluorescens Q8r1-96

Pseudomonas fluorescens Q8r1-96 represents a group of rhizosphere strains responsible for the suppressiveness of agricultural soils to take-all disease of wheat. It produces the antibiotic 2,4-diacetylphloroglucinol and aggressively colonizes the roots of cereal crops. In this study, we analyzed the genome of Q8r1-96 and identified a type III protein secretion system (T3SS) gene cluster that has overall organization similar to that of the T3SS gene cluster of the plant pathogen Pseudomonas syringae. We also screened a collection of 30 closely related P. fluorescens strains and detected the T3SS genes in all but one of them. The Q8r1-96 genome contained ropAA and ropM type III effector genes, which are orthologs of the P. syringae effector genes hopAA1-1 and hopM1, as well as a novel type III effector gene designated ropB. These type III effector genes encoded proteins that were secreted in culture and injected into plant cells by both P. syringae and Q8r1-96 T3SSs. The Q8r1-96 T3SS was expressed in the rhizosphere, but mutants lacking a functional T3SS were not altered in their rhizosphere competence. The Q8r1-96 type III effectors RopAA, RopB, and RopM were capable of suppressing the hypersensitive response and production of reactive oxygen species, two plant immune responses.

GlnB/GlnK PII proteins and regulation of the Sinorhizobium meliloti Rm1021 nitrogen stress response and symbiotic function

The Sinorhizobium meliloti Rm1021 Delta glnD-sm2 mutant, which is predicted to make a GlnD nitrogen sensor protein truncated at its amino terminus, fixes nitrogen in symbiosis with alfalfa, but the plants cannot use this nitrogen for growth (S. N. Yurgel and M. L. Kahn, Proc. Natl. Acad. Sci. U. S. A. 105:18958-18963, 2008). The mutant also has a generalized nitrogen stress response (NSR) defect. These results suggest a connection between GlnD, symbiotic metabolism, and the NSR, but the nature of this connection is unknown. In many bacteria, GlnD modifies the PII proteins, GlnB and GlnK, as it transduces a measurement of bacterial nitrogen status to a cellular response. We have now constructed and analyzed Rm1021 mutants missing GlnB, GlnK, or both proteins. Rm1021 Delta glnK Delta glnB was much more defective in its NSR than either single mutant, suggesting that GlnB and GlnK overlap in regulating the NSR in free-living Rm1021. The single mutants and the double mutant all formed an effective symbiosis, indicating that symbiotic nitrogen exchange could occur without the need for either GlnB or GlnK. N-terminal truncation of the GlnD protein interfered with PII protein modification in vitro, suggesting either that unmodified PII proteins were responsible for the glnD mutant's ineffective phenotype or that connecting GlnD and appropriate symbiotic behavior does not require the PII proteins.

Cell-associated hemolysis activity in the clinical strain of Pseudomonas fluorescens MFN1032

BACKGROUND

MFN1032 is a clinical Pseudomonas fluorescens strain able to grow at 37 degrees C. MFN1032 cells induce necrosis and apoptosis in rat glial cells at this temperature. This strain displays secretion-mediated hemolytic activity involving phospholipase C and cyclolipopeptides. Under laboratory conditions, this activity is not expressed at 37 degrees C. This activity is tightly regulated and is subject to phase variation.

RESULTS

We found that MFN1032 displays a cell-associated hemolytic activity distinct from the secreted hemolytic activity. Cell-associated hemolysis was expressed at 37 degrees C and was only detected in vitro in mid log growth phase in the presence of erythrocytes. We studied the regulation of this activity in the wild-type strain and in a mutant defective in the Gac two-component pathway. GacS/GacA is a negative regulator of this activity. In contrast to the Pseudomonas fluorescens strains PfO-1 and Pf5, whose genomes have been sequenced, the MFN1032 strain has the type III secretion-like genes hrcRST belonging to the hrpU operon. We showed that disruption of this operon abolished cell-associated hemolytic activity. This activity was not detected in P.fluorescens strains carrying similar hrc genes, as for the P. fluorescens psychrotrophic strain MF37.

CONCLUSIONS

To our knowledge this the first demonstration of cell-associated hemolytic activity of a clinical strain of Pseudomonas fluorescens. Moreover, this activity seems to be related to a functional hrpU operon and is independent of biosurfactant production. Precise link between a functional hrpU operon and cell-associated hemolytic activity remains to be elucidated.

Pseudomonas syringae lytic transglycosylases coregulated with the type III secretion system contribute to the translocation of effector proteins into plant cells

Pseudomonas syringae translocates virulence effector proteins into plant cells via a type III secretion system (T3SS) encoded by hrp (for hypersensitive response and pathogenicity) genes. Three genes coregulated with the Hrp T3SS system in P. syringae pv. tomato DC3000 have predicted lytic transglycosylase domains: PSPTO1378 (here designated hrpH), PSPTO2678 (hopP1), and PSPTO852 (hopAJ1). hrpH is located between hrpR and avrE1 in the Hrp pathogenicity island and is carried in the functional cluster of P. syringae pv. syringae 61 hrp genes cloned in cosmid pHIR11. Strong expression of DC3000 hrpH in Escherichia coli inhibits bacterial growth unless the predicted catalytic glutamate at position 148 is mutated. Translocation tests involving C-terminal fusions with a Cya (Bordetella pertussis adenylate cyclase) reporter indicate that HrpH and HopP1, but not HopAJ1, are T3SS substrates. Pseudomonas fluorescens carrying a pHIR11 derivative lacking hrpH is poorly able to translocate effector HopA1, and this deficiency can be restored by HopP1 and HopAJ1, but not by HrpH(E148A) or HrpH(1-241). DC3000 mutants lacking hrpH or hrpH, hopP1, and hopAJ1 combined are variously reduced in effector translocation, elicitation of the hypersensitive response, and virulence. However, the mutants are not reduced in secretion of T3SS substrates in culture. When produced in wild-type DC3000, the HrpH(E148A) and HrpH(1-241) variants have a dominant-negative effect on the ability of DC3000 to elicit the hypersensitive response in nonhost tobacco and to grow and cause disease in host tomato. The three Hrp-associated lytic transglycosylases in DC3000 appear to have overlapping functions in contributing to T3SS functions during infection.

Identification of Pseudomonas syringae pv. syringae 61 type III secretion system Hrp proteins that can travel the type III pathway and contribute to the translocation of effector proteins into plant cells

Pseudomonas syringae translocates effector proteins into plant cells via an Hrp1 type III secretion system (T3SS). T3SS components HrpB, HrpD, HrpF, and HrpP were shown to be pathway substrates and to contribute to elicitation of the plant hypersensitive response and to translocation and secretion of the model effector AvrPto1.

Pleiotropic effects of mutations that alter the Sinorhizobium meliloti cytochrome c respiratory system

Using transposon mutagenesis, mutations have been isolated in several genes (ccdA, cycM, ccmC, ccmB and senC) that play a role in Sinorhizobium meliloti cytochrome metabolism. As in other bacteria, mutations in the S. meliloti ccdA, ccmB and ccmC genes resulted in the absence of all c-type cytochromes. However, the S. meliloti ccdA mutant also lacked cytochrome oxidase aa(3), a defect that does not appear to have been reported for other bacteria. The aa(3)-type cytochromes were also missing from a mutant strain with an insertion into the gene encoding the haem-containing subunit (SU)I of aa(3) cytochrome c oxidase, but not in mutants unable to make SUII or SUIII, indicating that CcdA probably plays a role in assembling SUI. The cytochrome-deficient mutants also had other free-living phenotypes, including a significant decrease in growth rate on rich media and increased motility on minimal media. A senC mutant also had significantly decreased motility, but the motility and growth properties of the cycM mutant were unchanged. Unlike similar mutants in Bradyrhizobium japonicum and Rhizobium leguminosarum, an S. meliloti Rm1021 cycM mutant contained cytochrome oxidase aa(3). Cytochrome maturation in strain Rm1021 appeared to be similar to maturation in other rhizobia, but there were some differences in the cytochrome composition of the strain, and respiration chain function and assembly.

Role of a conserved membrane glycine residue in a dicarboxylate transporter from Sinorhizobium meliloti

Nitrogen-fixing rhizobial bacteroids import dicarboxylates by using the DctA transporter. G114 of DctA is highly conserved. A G114D mutant is inactive, but DctA with a small amino acid (G114A) or a helix disrupter (G114P) retains significant activity. G114 probably interacts with other membrane helices in stabilizing a substrate-binding pocket.

A chaperone-like HrpG protein acts as a suppressor of HrpV in regulation of the Pseudomonas syringae pv. syringae type III secretion system

The cloned hrp/hrc cluster of Pseudomonas syringae pv. syringae 61 (Pss61) contains 28 proteins, and many of those are assembled into a type III secretion system (TTSS) that is responsible for eliciting the hypersensitive response (HR) in non-host plants and causing diseases on host plants (Huang et al., 1995). hrpG, the second gene in the hrpC operon, encodes a 15.4 kDa cytoplasmic protein whose predicted structure is similar to SicP (E-value: 0.19), a TTSS chaperone of Salmonella typhimurium. Two non-polar hrpG mutants, Pss61-N826 and Pss61-N674, were produced to investigate the biological function of hrpG gene. Pss61-N826, generated by replacing the coding sequence of hrpG with an nptII gene lacking both the promoter and the terminator, was found to be capable of eliciting the wild-type HR; whereas Pss61-N674 generated by replacement of a terminatorless nptII gene in the hrpG coding sequence showed the delayed HR phenotype. Northern and Western blotting analyses showed that the expression of hrpZ, hrcJ and hrcQb genes residing on two different operons in Pss61-N674 was reduced due to the nptII promoter-driven constitutive expression of hrpV that codes for a negative regulator. Interestingly, a plasmid-borne hrpG can derepress the hrp expression in Pss61-N674 and in Pss61 overexpressing HrpV without decreasing the hrpV transcript. Moreover, results of yeast two-hybrid assay, pull-down assay and far Western analysis show that HrpG and HrpV interact with each other in vivo and in vitro. Additionally, HrpV interacts with a positive regulator HrpS according to analysis of a yeast two-hybrid system. Based on the results presented in this study, we propose that HrpG acts as a suppressor of the negative regulator HrpV mediated via protein-protein interaction, leading to modulation of hrp/hrc expression subsequently freeing HrpS to promote the activation of other downstream hrp/hrc genes.

Probing the Sinorhizobium meliloti-alfalfa symbiosis using temperature-sensitive and impaired-function citrate synthase mutants

To study the role of the decarboxylating leg of the bacterial TCA cycle in symbiotic nitrogen fixation, we used DNA shuffling and localized random polymerase chain reaction mutagenesis to construct a series of temperature-sensitive and impaired-function mutants in the Sinorhizobium meliloti Rm104A14 citrate synthase (gltA) gene. Reducing citrate synthase (CS) activity by mutation led to a corresponding decrease in the free-living growth rate; however, alfalfa plants formed fully effective nodules when infected with mutants having CS activities as low as 7% of the wild-type strain. Mutants with approximately 3% of normal CS activity formed nodules with lower nitrogenase activity and a mutant with less than 0.5% of normal CS activity formed Fix- nodules. Two temperature-sensitive (ts) mutants grew at a permissive temperature (25 degrees C) with 3% of wild-type CS activities but were unable to grow on minimal medium at 30 degrees C. Alfalfa plants that were inoculated with the ts mutants and grown with a root temperature of 20 degrees C formed functional nodules with nitrogenase activities approximately 20% of the wild type. When the roots of plants infected with the ts mutants were transferred to 30 degrees C, the nodules lost the ability to fix nitrogen over several days. Microscopic examination of these nodules revealed the loss of bacteroids and senescence, indicating that CS activity was essential for nodule maintenance.

Knot Formation Caused by Pseudomonas syringae subsp. savastanoi on Olive Plants Is hrp-Dependent

ABSTRACT The virulence of Pseudomonas syringae subsp. savastanoi, which causes hyperplastic symptoms (knots) on olive plants, is associated with secreted phytohormones. We identified a Tn5-induced mutant of P. syringae subsp. savastanoi that did not cause disease symptoms on olive plants although it was still able to produce phytohormones. In addition, the mutant failed to elicit a hypersensitive response in a nonhost plant. Molecular characterization of the mutant revealed that a single Tn5 insertion occurred within an open reading frame encoding a protein 92% identical to the HrcC protein of P. syringae pv. syringae. Moreover, sequence analysis revealed that the gene encoding the HrcC protein in P. syringae subsp. savastanoi was part of an operon that included five genes arranged as in other phytopathogenic bacteria. These results imply that hrp/hrc genes are functional in P. syringae subsp. savastanoi and that they play a key role in the pathogenicity of this plant pathogen.

HrpZPsph from the plant pathogen Pseudomonas syringae pv. phaseolicola binds to lipid bilayers and forms an ion-conducting pore in vitro

The hrp gene clusters of plant pathogenic bacteria control pathogenicity on their host plants and ability to elicit the hypersensitive reaction in resistant plants. Some hrp gene products constitute elements of the type III secretion system, by which effector proteins are exported and delivered into plant cells. Here, we show that the hrpZ gene product from the bean halo-blight pathogen, Pseudomonas syringae pv. phaseolicola (HrpZ(Psph)), is secreted in an hrp-dependent manner in P. syringae pv. phaseolicola and exported by the type III secretion system in the mammalian pathogen Yersinia enterocolitica. HrpZ(Psph) was found to associate stably with liposomes and synthetic bilayer membranes. Under symmetric ionic conditions, addition of 2 nM of purified recombinant HrpZ(Psph) to the cis compartment of planar lipid bilayers provoked an ion current with a large unitary conductivity of 207 pS. HrpZ(Psph)-related proteins from P. syringae pv. tomato or syringae triggered ion currents similar to those stimulated by HrpZ(Psph). The HrpZ(Psph)-mediated ion-conducting pore was permeable for cations but did not mediate fluxes of Cl-. Such pore-forming activity may allow nutrient release and/or delivery of virulence factors during bacterial colonization of host plants.

HrpZ(Psph) from the plant pathogen Pseudomonas syringae pv. phaseolicola binds to lipid bilayers and forms an ion-conducting pore in vitro

The hrp gene clusters of plant pathogenic bacteria control pathogenicity on their host plants and ability to elicit the hypersensitive reaction in resistant plants. Some hrp gene products constitute elements of the type III secretion system, by which effector proteins are exported and delivered into plant cells. Here, we show that the hrpZ gene product from the bean halo-blight pathogen, Pseudomonas syringae pv. phaseolicola (HrpZ(Psph)), is secreted in an hrp-dependent manner in P. syringae pv. phaseolicola and exported by the type III secretion system in the mammalian pathogen Yersinia enterocolitica. HrpZ(Psph) was found to associate stably with liposomes and synthetic bilayer membranes. Under symmetric ionic conditions, addition of 2 nM of purified recombinant HrpZ(Psph) to the cis compartment of planar lipid bilayers provoked an ion current with a large unitary conductivity of 207 pS. HrpZ(Psph)-related proteins from P. syringae pv. tomato or syringae triggered ion currents similar to those stimulated by HrpZ(Psph). The HrpZ(Psph)-mediated ion-conducting pore was permeable for cations but did not mediate fluxes of Cl-. Such pore-forming activity may allow nutrient release and/or delivery of virulence factors during bacterial colonization of host plants.

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

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

New substrates for the dicarboxylate transport system of Sinorhizobium meliloti

The dicarboxylate transport (Dct) system of Sinorhizobium meliloti, which is essential for a functional nitrogen-fixing symbiosis, has been thought to transport only dicarboxylic acids. We show here that the permease component of the Dct system, DctA, can transport orotate, a monocarboxylic acid, with an apparent K(m) of 1.7 mM and a V(max) of 163 nmol min(-1) per mg of protein in induced cells. DctA was not induced by the presence of orotate. The transport of orotate was inhibited by several compounds, including succinamic acid and succinamide, which are not dicarboxylic acids. The dicarboxylic acid maleate (cis-butenedioic acid) was not an inhibitor of orotate transport, which suggests that it was not recognized by DctA. However, maleate was an excellent inducer of DctA expression. Our evaluation of 17 compounds as inducers and inhibitors of transport suggests that substrates recognized by S. meliloti DctA must have appropriately spaced carbonyl groups and an extended conformation, while good inducers are more likely to have a curved conformation.

Pseudomonas syringae Hrp type III secretion system and effector proteins

Pseudomonas syringae is a member of an important group of Gram-negative bacterial pathogens of plants and animals that depend on a type III secretion system to inject virulence effector proteins into host cells. In P. syringae, hrp/hrc genes encode the Hrp (type III secretion) system, and avirulence (avr) and Hrp-dependent outer protein (hop) genes encode effector proteins. The hrp/hrc genes of P. syringae pv syringae 61, P. syringae pv syringae B728a, and P. syringae pv tomato DC3000 are flanked by an exchangeable effector locus and a conserved effector locus in a tripartite mosaic Hrp pathogenicity island (Pai) that is linked to a tRNA(Leu) gene found also in Pseudomonas aeruginosa but without linkage to Hrp system genes. Cosmid pHIR11 carries a portion of the strain 61 Hrp pathogenicity island that is sufficient to direct Escherichia coli and Pseudomonas fluorescens to inject HopPsyA into tobacco cells, thereby eliciting a hypersensitive response normally triggered only by plant pathogens. Large deletions in strain DC3000 revealed that the conserved effector locus is essential for pathogenicity but the exchangeable effector locus has only a minor role in growth in tomato. P. syringae secretes HopPsyA and AvrPto in culture in a Hrp-dependent manner at pH and temperature conditions associated with pathogenesis. AvrPto is also secreted by Yersinia enterocolitica. The secretion of AvrPto depends on the first 15 codons, which are also sufficient to direct the secretion of an Npt reporter from Y. enterocolitica, indicating that a universal targeting signal is recognized by the type III secretion systems of both plant and animal pathogens.

A dsbA mutant of Pseudomonas syringae exhibits reduced virulence and partial impairment of type III secretion

Abstract To identify virulence genes of P. syringae pv. tomato strain DC3000 we screened for mutants with reduced virulence on its plant hosts, Arabidopsis thaliana and tomato. We isolated a Tn5-insertion mutant that exhibited reduced virulence on both hosts. Further characterization showed that this mutant carried a single Tn5 insertion in the dsbA gene, which encodes a periplasmic disulphide bond-forming protein. In addition to reduced virulence, the dsbA mutant exhibits mucoid colony morphology, loss of fluorescence, decreased motility, and a reduced growth rate in culture. The dsbA mutant is able to multiply in A. thaliana and tomato plants, trigger the hypersensitive response on tobacco and elicit Pto-mediated resistance in tomato, indicating that type III secretion occurs in this background. However, type III secretion appears to function with reduced efficiency in the dsbA mutant, as type III-dependent secretion of HrpZ and AvrRpt2 is impaired. These findings indicate that while the dsbA gene is required for multiple cellular functions in P. syringae, type III secretion in P. syringae is only partially dependent on dsbA.

The gene coding for the Hrp pilus structural protein is required for type III secretion of Hrp and Avr proteins in Pseudomonas syringae pv. tomato

Bacterial surface appendages called pili often are associated with DNA and/or protein transfer between cells. The exact function of pili in the transfer process is not understood and is a matter of considerable debate. The Hrp pilus is assembled by the Hrp type III protein secretion system of Pseudomonas syringae pv. tomato (Pst) strain DC3000. In this study, we show that the hrpA gene, which encodes the major subunit of the Hrp pilus, is required for secretion of putative virulence proteins, such as HrpW and AvrPto. In addition, the hrpA gene is required for full expression of genes that encode regulatory, secretion, and effector proteins of the type III secretion system. hrpA-mediated gene regulation apparently is through effect on the mRNA level of two previously characterized regulatory genes, hrpR and hrpS. Ectopic expression of the hrpRS gene operon restored gene expression, but not protein secretion, in the hrpA mutant. Three single amino acid mutations at the HrpA carboxyl terminus were identified that affect the secretion or regulatory function of the HrpA protein. These results define an essential role of the Hrp pilus structural gene in protein secretion and coordinate regulation of the type III secretion system in Pst DC3000.

Citrate synthase mutants of Sinorhizobium meliloti are ineffective and have altered cell surface polysaccharides

The gltA gene, encoding Sinorhizobium meliloti 104A14 citrate synthase, was isolated by complementing an Escherichia coli gltA mutant. The S. meliloti gltA gene was mutated by inserting a kanamycin resistance gene and then using homologous recombination to replace the wild-type gltA with the gltA::kan allele. The resulting strain, CSDX1, was a glutamate auxotroph, and enzyme assays confirmed the absence of a requirement for glutamate. CSDX1 did not grow on succinate, malate, aspartate, pyruvate, or glucose. CSDX1 produced an unusual blue fluorescence on medium containing Calcofluor, which is different from the green fluorescence found with 104A14. High concentrations of arabinose (0.4%) or succinate (0. 2%) restored the green fluorescence to CSDX1. High-performance liquid chromatography analyses showed that CSDX1 produced partially succinylated succinoglycan. CSDX1 was able to form nodules on alfalfa, but these nodules were not able to fix nitrogen. The symbiotic defect of a citrate synthase mutant could thus be due to disruption of the infection process or to the lack of energy generated by the tricarboxylic acid cycle.

Role of the Hrp type III protein secretion system in growth of Pseudomonas syringae pv. syringae B728a on host plants in the field

hrp genes are reportedly required for pathogenicity in Pseudomonas syringae pv. syringae (Pss) and other phytopathogenic bacterial species. A subset of these genes encodes a type III secretion system through which virulence factors are thought to be delivered to plant cells. In this study, we sought to better understand the role that hrp genes play in interactions of Pss with its host as they occur naturally under field conditions. Population sizes of hrp mutants with defects in genes that encode components of the Hrp secretion system (DeltahrcC::nptII and hrpJ:: OmegaSpc) and a protein secreted via the system (DeltahrpZ::nptII) were similar to B728a on germinating seeds. However, phyllosphere (i.e., leaf) population sizes of the hrcC and hrpJ secretion mutants, but not the hrpZ mutant, were significantly reduced relative to B728a. Thus, the Hrp type III secretion system, but not HrpZ, plays an important role in enabling Pss to flourish in the phyllosphere, but not the spermosphere. The hrcC and hrpJ mutants caused brown spot lesions on primary leaves at a low frequency when they were inoculated onto seeds at the time of planting. Pathogenic reactions also were found when the hrp secretion mutants were co-infiltrated into bean leaves with a non-lesion-forming gacS mutant of B728a. In both cases, the occurrence of disease was associated with elevated population sizes of the hrp secretion mutants. The role of the Hrp type III secretion system in pathogenicity appears to be largely mediated by its requirement for growth of Pss in the phyllosphere. Without growth, disease does not occur.

Characterization of the Pseudomonas syringae pv. tomato AvrRpt2 protein: demonstration of secretion and processing during bacterial pathogenesis

Pseudomonas syringae pv. tomato strain DC3000 (Pst DC3000) expressing avrRpt2 is specifically recognized by plant cells expressing RPS2 activity, resulting in localized cell death and plant resistance. Furthermore, transient expression of this bacterial avrRpt2 gene in plant cells results in RPS2-dependent cell death. This indicates that the AvrRpt2 protein is recognized inside RPS2 plant cells and is sufficient for the activation of disease resistance-mediated cell death in planta. We explored the possibility that Pst DC3000 delivers AvrRpt2 protein to plant cells via the hrp (type III) secretion pathway. We now provide direct evidence that mature AvrRpt2 protein is secreted from Pst DC3000 and that secretion is hrp dependent. We also show that AvrRpt2 is N-terminally processed when Arabidopsis thaliana plants are infected with Pst DC3000 expressing avrRpt2. Similar N-terminal processing of AvrRpt2 occurred when avrRpt2 was stably expressed in A. thaliana. No cleavage of AvrRpt2 was detected in bacteria expressing avrRpt2 in culture or in the plant extracellular fluids. The N-terminus of AvrRpt2 was not required for RPS2 recognition in planta. However, this region of AvrRpt2 was essential for Pst DC3000-mediated elicitation of RPS2-dependent cell death in A. thaliana leaves.

Identification and molecular analysis of rough-colony-specific outer membrane proteins of Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans, a gram-negative bacterium isolated from the human mouth, has been implicated in the pathogenesis of early-onset periodontitis. Primary isolates cultured from subgingival plaque exhibit an adherent, rough colony phenotype which spontaneously converts to a nonadherent, smooth phenotype upon in vitro subculture. The rough colony variant produces abundant fimbriae and autoaggregates, while the smooth colony variant is planktonic and produces scant fimbriae. To begin to understand the significance of colony variation in biofilm formation by A. actinomycetemcomitans, outer membrane protein profiles of four isogenic rough and smooth colony variants were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Two proteins with relative molecular masses of 43 and 20 kDa were expressed by the rough colony variants exclusively. Expression of these proteins was not found to be dependent on growth phase, oxygen tension, or type of complex medium. N-terminal amino acid sequences of these proteins obtained by Edman degradation were compared with sequences from the University of Oklahoma A. actinomycetemcomitans genome database. Two contiguous open reading frames (ORFs) encoding proteins having sequence homology with these proteins were identified. The 43-kDa protein (RcpA [rough colony protein A]) was similar to precursor protein D of the general secretion pathway of gram-negative bacilli, while the 20-kDa protein (RcpB [rough colony protein B]) appeared to be unique. The genes encoding these proteins have been cloned from A. actinomycetemcomitans 283 and sequenced. A BLASTX (gapped BLAST) search of the surrounding ORFs revealed homology with other fimbria-related proteins. These data suggest that the genes encoding the 43-kDa (rcpA) and 20-kDa (rcpB) proteins may be functionally related to each other and to genes that may encode fimbria-associated proteins.

Cellular locations of Pseudomonas syringae pv. syringae HrcC and HrcJ proteins, required for harpin secretion via the type III pathway

The complete hrp-hrc-hrmA cluster of Pseudomonas syringae pv. syringae 61 encodes 28 polypeptides. A saprophytic bacterium carrying this cluster is capable of secreting HrpZ-a harpin encoded by hrpZ-in an hrp-dependent manner, which suggests that this cluster contains sufficient components to assemble functional type III secretion machinery. Sequence data show that HrcJ and HrcC are putative outer membrane proteins, and nonpolar mutagenesis demonstrates they are all required for HrpZ secretion. In this study, we investigated the cellular localization of the HrcC and HrcJ proteins by Triton solubilization, sucrose-gradient isopycnic centrifugation, and immunogold labeling of the bacterial cell surface. Our results indicate that HrcC is indeed an outer membrane protein and that HrcJ is located between both membranes. Their membrane localization suggests that they might be involved in the formation of a supramolecular structure for protein secretion.

The virulence plasmid of Yersinia, an antihost genome

The 70-kb virulence plasmid enables Yersinia spp. (Yersinia pestis, Y. pseudotuberculosis, and Y. enterocolitica) to survive and multiply in the lymphoid tissues of their host. It encodes the Yop virulon, an integrated system allowing extracellular bacteria to disarm the cells involved in the immune response, to disrupt their communications, or even to induce their apoptosis by the injection of bacterial effector proteins. This system consists of the Yop proteins and their dedicated type III secretion apparatus, called Ysc. The Ysc apparatus is composed of some 25 proteins including a secretin. Most of the Yops fall into two groups. Some of them are the intracellular effectors (YopE, YopH, YpkA/YopO, YopP/YopJ, YopM, and YopT), while the others (YopB, YopD, and LcrV) form the translocation apparatus that is deployed at the bacterial surface to deliver the effectors into the eukaryotic cells, across their plasma membrane. Yop secretion is triggered by contact with eukaryotic cells and controlled by proteins of the virulon including YopN, TyeA, and LcrG, which are thought to form a plug complex closing the bacterial secretion channel. The proper operation of the system also requires small individual chaperones, called the Syc proteins, in the bacterial cytosol. Transcription of the genes is controlled both by temperature and by the activity of the secretion apparatus. The virulence plasmid of Y. enterocolitica and Y. pseudotuberculosis also encodes the adhesin YadA. The virulence plasmid contains some evolutionary remnants including, in Y. enterocolitica, an operon encoding resistance to arsenic compounds.

Characterization of the hrpC and hrpRS operons of Pseudomonas syringae pathovars syringae, tomato, and glycinea and analysis of the ability of hrpF, hrpG, hrcC, hrpT, and hrpV mutants to elicit the hypersensitive response and disease in plants

The species Pseudomonas syringae encompasses plant pathogens with differing host specificities and corresponding pathovar designations. P. syringae requires the Hrp (type III protein secretion) system, encoded by a 25-kb cluster of hrp and hrc genes, in order to elicit the hypersensitive response (HR) in nonhosts or to be pathogenic in hosts. DNA sequence analysis of the hrpC and hrpRS operons of P. syringae pv. syringae 61 (brown spot of beans), P. syringae pv. glycinea U1 (bacterial blight of soybeans), and P. syringae pv. tomato DC3000 (bacterial speck of tomatos) revealed that the 13 genes comprising the right half of the hrp cluster (including those in the previously sequenced hrpZ operon) are conserved and identically arranged. The hrpC operon is comprised of hrpF, hrpG, hrcC, hrpT, and hrpV. hrcC encodes a putative outer membrane protein that is conserved in all type III secretion systems. The other four genes appear to be characteristic of group I Hrp systems, such as those possessed by P. syringae and Erwinia amylovora. The predicted products of these four genes in P. syringae pv. syringae 61 are HrpF (8 kDa), HrpG (15.4 kDa), HrpT (7.5 kDa), and HrpV (13.4 kDa). HrpT is a putative outer membrane lipoprotein. HrpF, HrpG, and HrpV are all hydrophilic proteins lacking N-terminal signal peptides. The HrpG, HrcC, HrpT, and HrpV proteins of P. syringae pathovars syringae and tomato (the two most divergent pathovars) had at least 76% amino acid identity with each other, whereas the HrpF proteins of these two pathovars had only 36% amino acid identity. The HrpF proteins of P. syringae pathovars syringae and glycinea also showed significant similarity to the HrpA pilin protein of P. syringae pathovar tomato. Functionally nonpolar mutations were introduced into each of the genes in the hrpC operon of P. syringae pv. syringae 61 by insertion of an nptII cartridge lacking a transcription terminator. The mutants were assayed for their ability to elicit the HR in nonhost tobacco leaves or to multiply and cause disease in host bean leaves. Mutations in hrpF, hrcC, and hrpT abolished or greatly reduced the ability of P. syringae pv. syringae 61 to elicit the HR in tobacco. The hrpG mutant had only weakly reduced HR activity, and the activity of the hrpV mutant was indistinguishable from that of the wild type. Each of the mutations could be complemented, but surprisingly, the hrpV subclone caused a reduction in the HR elicitation ability of the DeltahrpV::nptII mutant. The hrpF and hrcC mutants caused no disease in beans, whereas the hrpG, hrpT, and hrpV mutants had reduced virulence. Similarly, the hrcC mutant grew little in beans, whereas the other mutants grew to intermediate levels in comparison with the wild type. These results indicate that HrpC and HrpF have essential functions in the Hrp system, that HrpG and HrpT contribute quantitatively but are not essential, and that HrpV is a candidate negative regulator of the Hrp system.

The hrpC and hrpN operons of Erwinia chrysanthemi EC16 are flanked by plcA and homologs of hemolysin/adhesin genes and accompanying activator/transporter genes

The hrpC operon of Erwinia chrysanthemi EC16 encodes five genes conserved in Erwinia amylovora and Pseudomonas syringae. Mutagenesis indicated that hrcC is required for elicitation of the hypersensitive reaction in tobacco leaves. The unexpected presence of plcA and homologs of hemolysin/activator genes in the regions flanking the hrcC and hrpN operons is reported.

Type III protein secretion systems in bacterial pathogens of animals and plants

Various gram-negative animal and plant pathogens use a novel, sec-independent protein secretion system as a basic virulence mechanism. It is becoming increasingly clear that these so-called type III secretion systems inject (translocate) proteins into the cytosol of eukaryotic cells, where the translocated proteins facilitate bacterial pathogenesis by specifically interfering with host cell signal transduction and other cellular processes. Accordingly, some type III secretion systems are activated by bacterial contact with host cell surfaces. Individual type III secretion systems direct the secretion and translocation of a variety of unrelated proteins, which account for species-specific pathogenesis phenotypes. In contrast to the secreted virulence factors, most of the 15 to 20 membrane-associated proteins which constitute the type III secretion apparatus are conserved among different pathogens. Most of the inner membrane components of the type III secretion apparatus show additional homologies to flagellar biosynthetic proteins, while a conserved outer membrane factor is similar to secretins from type II and other secretion pathways. Structurally conserved chaperones which specifically bind to individual secreted proteins play an important role in type III protein secretion, apparently by preventing premature interactions of the secreted factors with other proteins. The genes encoding type III secretion systems are clustered, and various pieces of evidence suggest that these systems have been acquired by horizontal genetic transfer during evolution. Expression of type III secretion systems is coordinately regulated in response to host environmental stimuli by networks of transcription factors. This review comprises a comparison of the structure, function, regulation, and impact on host cells of the type III secretion systems in the animal pathogens Yersinia spp., Pseudomonas aeruginosa, Shigella flexneri, Salmonella typhimurium, enteropathogenic Escherichia coli, and Chlamydia spp. and the plant pathogens Pseudomonas syringae, Erwinia spp., Ralstonia solanacearum, Xanthomonas campestris, and Rhizobium spp.

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

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

Environmental regulation of Salmonella typhi invasion-defective mutants

Salmonella typhi is the etiologic agent of human typhoid. During infection, S. typhi adheres to and invades epithelial and M cells that line the distal ileum. To survive in the human host, S. typhi must overcome numerous complex extracellular and intracellular environments. Since relatively little is known about S. typhi pathogenesis, studies were initiated to identify S. typhi genes involved in the early steps of interaction with the host and to evaluate the environmental regulation of these genes. In the present study, TnphoA mutagenesis was used to study these early steps. We isolated 16 Salmonella typhi TnphoA mutants that were defective for both adherence and invasion of the human small intestinal epithelial cell line Int407. Twelve of sixteen mutations were identified in genes homologous to the S. typhimurium invG and prgH genes, which are known to be involved in the type III secretion pathway of virulence proteins. Two additional insertions were identified in genes sharing homology with the cpxA and damX genes from Escherichia coli K-12, and two uncharacterized invasion-deficient mutants were nonmotile. Gene expression of TnphoA fusions was examined in response to environmental stimuli. We found that the cpxA, invG, and prgH genes were induced when grown under conditions of high osmolarity (0.3 M NaCl). Expression of invG and prgH genes was optimal at pH 6.5 and strongly reduced at low pH (5.0). Transcription of both invG and prgH TnphoA gene fusions was initiated during the late logarithmic growth phase and was induced under anaerobic conditions. Finally, we show that both invG and prgH genes appear to be regulated by DNA supercoiling, a mechanism influenced by environmental factors. These results are the first to demonstrate that in S. typhi, (i) the prgH and cpxA genes are osmoregulated, (ii) the invG gene is induced under low oxygen conditions, (iii) the invG gene is pH regulated and growth phase dependent, and (iv) the prgH gene appears to be regulated by DNA supercoiling. Since our experimental conditions were designed to mimic the in vivo environmental milieu, our results suggest that specific environmental conditions act as signals to induce the expression of S. typhi invasion genes.

Type III protein secretion systems in plant and animal pathogenic bacteria

Among many interesting and sophisticated mechanisms used by bacterial pathogens to subvert eukaryotic hosts is a class of specialized protein secretion systems (known as type III protein secretion systems) that deliver bacterial virulence proteins directly into the host cell. Recent studies have revealed four important features of these secretion systems. First, they are widespread among plant and animal bacterial pathogens, and mutations affecting type III protein secretion often eliminate bacterial virulence completely. Second, at least eight type III secretion components share sequence similarities with those of the flagellar assembly machinery and flagellum-like structures are associated with type III secretion, raising the possibility that these secretion systems are derived from the presumably more ancient flagellar assembly apparatus. Third, type III secretion is activated in vivo upon contact with host cells. Fourth, the type III secretion mechanism is Sec-independent and the effector proteins may possess mRNA-based targeting signals. This review highlights the similarities and differences among type III secretion systems of selected model plant and animal pathogenic bacteria.

Regulation of sesquiterpene cyclase gene expression. Characterization of an elicitor- and pathogen-inducible promoter

The promoter for a tobacco (Nicotiana tabacum) sesquiterpene cyclase gene, a key regulatory step in sesquiterpene phytoalexin biosynthesis, has been analyzed. The EAS4 promoter was fused to the beta-glucuronidase (GUS) reporter gene, and the temporal and spatial expression patterns of GUS activity were examined in stably transformed plants and in transient expression assays using electroporated protoplasts of tobacco. No GUS activity was observed in any tissues under normal growth conditions. A low level of GUS activity was detected in wounded leaf, root, and stem tissues, whereas a much higher level was observed when these tissues were challenged with elicitors or microbial pathogens. The GUS expression pattern directed by the EAS4 promoter was identical to the induction patterns observed for the endogenous sesquiterpene cyclase genes. Neither exogenous salicylic acid nor methyl jasmonate induced GUS expression; and H2O2 induced GUS expression to only a limited extent. Although the EAS4 promoter contains cis-sequences resembling previously identified transcriptional control motifs, other cis-sequences important for quantitative and qualitative gene expression were identified by deletion and gain-of-function analyses. The EAS4 promoter differs from previously described pathogen-/elicitor-inducible promoters because it only supports inducible gene expression and directs unique spatial expression patterns.

Altered localization of HrpZ in Pseudomonas syringae pv. syringae hrp mutants suggests that different components of the type III secretion pathway control protein translocation across the inner and outer membranes of gram-negative bacteria

Pseudomonas syringae pv. syringae 61 (Pss61) secretes the HrpZ harpin by a type III protein secretion pathway encoded by a cluster of hrp (hypersensitive response and pathogenicity) and hrc genes. The nine hrc genes represent a subset of hrp genes that are also conserved in the type III virulence protein secretion systems of animal pathogenic Yersinia, Shigella, and Salmonella spp. The hrpJ and hrpU operons contain seven hrc genes (counting hrcQ(A) and hrcQ(B) as one gene), all with additional homologs involved in flagellar biogenesis and secretion, and five of which encode predicted inner membrane proteins. The hrpC and hrpZ operons encode HrcC and HrcJ, respectively, which are associated with the outer membrane. Interposon mutants affected in all of the hrc genes in the hrpJ and hrpU operons and TnphoA-induced hrcC and hrcJ mutants were assayed for altered localization of HrpZ in mid-log-phase cultures by immunoblotting sodium dodecyl sulfate-polyacrylamide gels that were run with various cell fractions. The hrpJ and hrpU operon mutants revealed a novel phenotype of partially reduced accumulation of HrpZ in the total culture (despite wild-type levels of hrpZ operon transcription), all of which was cell bound and equivalent in level to that of cell-bound HrpZ in the wild type. The hrcC and hrcJ mutant cultures accumulated the same total amount of HrpZ as the wild type, but the HrpZ was cell bound. Among all the strains tested, only the hrcC mutant accumulated significant amounts of HrpZ in the periplasm, as indicated by selective release through spheroplasting. Analysis of nonpolar mutations in the hrpU and hrpC operons support the results obtained with polar mutations. These observations indicate that a constant pool of HrpZ is maintained in the cytoplasm of Pss61 despite secretion deficiencies, that the hrpJ and hrpU operons encode an alternative to the Sec (general protein export) pathway for translocation across the inner membrane, that genes in the hrpC operon are necessary for translocation across the outer membrane, and that the Pss61 Hrp system permits study of two genetically distinguishable stages in type III protein secretion.

Hrp pilus: an hrp-dependent bacterial surface appendage produced by Pseudomonas syringae pv. tomato DC3000

Hypersensitive response and pathogenicity (hrp) genes control the ability of major groups of plant pathogenic bacteria to elicit the hypersensitive response (HR) in resistant plants and to cause disease in susceptible plants. A number of Hrp proteins share significant similarities with components of the type III secretion apparatus and flagellar assembly apparatus in animal pathogenic bacteria. Here we report that Pseudomonas syringae pv. tomato strain DC3000 (race 0) produces a filamentous surface appendage (Hrp pilus) of 6-8 nm in diameter in a solid minimal medium that induces hrp genes. Formation of the Hrp pilus is dependent on at least two hrp genes, hrpS and hrpH (recently renamed hrcC), which are involved in gene regulation and protein secretion, respectively. Our finding of the Hrp pilus, together with recent reports of Salmonella typhimurium surface appendages that are involved in bacterial invasion into the animal cell and of the Agrobacterium tumefaciens virB-dependent pilus that is involved in the transfer of T-DNA into plant cells, suggests that surface appendage formation is a common feature of animal and plant pathogenic bacteria in the infection of eukaryotic cells. Furthermore, we have identified HrpA as a major structural protein of the Hrp pilus. Finally, we show that a nonpolar hrpA mutant of P. syringae pv. tomato DC3000 is unable to form the Hrp pilus or to cause either an HR or disease in plants.

The hrpA and hrpC operons of Erwinia amylovora encode components of a type III pathway that secretes harpin

A 6.2-kb region of DNA corresponding to complementation groups II and III of the Erwinia amylovora hrp gene cluster was analyzed. Transposon mutagenesis indicated that the two complementation groups are required for secretion of harpin, an elicitor of the hypersensitive reaction. The sequence of the region revealed 10 open reading frames in two putative transcription units: hrpA, hrpB, hrcJ, hrpD, and hrpE in the hrpA operon (group III) and hrpF, hrpG, hrcC, hrpT, and hrpV in the hrpC operon (group II). From promoter regions of the hrpA, hrpC, and hrpN operons, sequences similar to those of the HrpL-dependent promoters of Pseudomonas syringae pathovars were identified with a consensus sequence of 5'-GGAAC-N17-18-CACTNAA-3'. The protein products of seven genes, hrpA, hrcJ, hrpE, hrpF, hrpG, hrcC, and hrpV, were visualized with a T7 polymerase/promoter expression system. HrcC, HrcJ, and HrpT sequences contained potential signal peptides, and HrcC appeared to be envelope associated based on a TnphoA translational fusion. Comparison of deduced amino acid sequences indicated that many of the proteins are homologous to proteins that function in the type III protein secretion pathway. HrcC is a member of the YscC-containing subgroup in the PulD/pIV superfamily of outer membrane proteins. HrcJ is a member of a lipoprotein family that includes YscJ of Yersinia spp., MxiJ of Shigella flexneri, and NolT of Rhizobim fredii. Additional similarities were detected between HrpB and YscI and between HrpE and YscL. HrcJ and HrpE were similar to flagellar biogenesis proteins FliF and FliH, respectively. In addition, HrpA, HrpB, HrcJ, HrpD, HrpE, HrpF, and HrcC showed various degrees of similarity to corresponding proteins of P. syringae. Comparison of hrp clusters with respect to gene organization and similarity of individual proteins confirms that the hrp systems of E. amylovora and P. syringae are closely related to each other and distinct from those of Ralstonia (Pseudomonas) solanacearum and Xanthomonas campestris. Possible implications of extensive similarities between the E. amylovora and P. syringae hrp systems in pathogenesis mechanisms are discussed.

The role of hrp genes during plant-bacterial interactions

hrp genes control the ability of phytopathogenic bacteria to cause disease and to elicit hypersensitive reactions on resistant plants. Genetic and biochemical studies have demonstrated that Hrp proteins are components of Type III secretion systems, regulatory proteins, proteinaceous elicitors of the hypersensitive reaction, and enzymes needed for synthesis of periplasmic glucans. Significantly, Type III secretion systems are involved with the secretion of pathogenicity proteins in bacterial pathogens of animals. The transcriptional activation of a number of bacterial avirulence (avr) genes is controlled by Hrp regulatory proteins, and recent experimental evidence suggests that Avr proteins may be transported by Hrp secretion systems. It has also been hypothesized that pathogenicity and/or virulence gene products exit bacterial phytopathogens via Hrp pathways. Thus, hrp genes may be one of the most important groups of genes found in phytopathogenic bacteria in relationship to pathogenicity and host range.