The presence of diverse IS elements and an avrPphD homologue that acts as a virulence factor on the pathogenicity plasmid of Erwinia herbicola pv. gypsophilae

Insights into the Potential Role of Plasmids in the Versatility of the Genus Pantoea

In the past two decades, 25 different species of the genus Pantoea within the Enterobacteriaceae family, have been isolated from different environmental niches. These species have a wide range of biological roles. Versatility in functions and hosts indicate that this genus has undergone extensive genetic diversification, which can be attributed to the different extra-chromosomal genetic elements or plasmids found across this genus. We have analyzed the functions of these plasmids and categorized them into four major groups for a better understanding of their future applications. The first and second group includes plasmids that contribute to genetic diversification and pathogenicity, respectively. The third group comprises cryptic plasmids of Pantoea. The last group includes plasmids that play a role in the metabolic versatility of the genus Pantoea. We have analyzed the data available up to May 2023 from two databases (viz; NCBI and PLSDB). In our analysis we have found a vast gap in knowledge. Complete gene annotations are available for only a few of the plasmids. This review highlights these challenges as an avenue for future research.

Comparative sequence analysis of pPATH pathogenicity plasmids in Pantoea agglomerans gall-forming bacteria

Acquisition of the pathogenicity plasmid pPATH that encodes a type III secretion system (T3SS) and effectors (T3Es) has likely led to the transition of a non-pathogenic bacterium into the tumorigenic pathogen Pantoea agglomerans. P. agglomerans pv. gypsophilae (Pag) forms galls on gypsophila (Gypsophila paniculata) and triggers immunity on sugar beet (Beta vulgaris), while P. agglomerans pv. betae (Pab) causes galls on both gypsophila and sugar beet. Draft sequences of the Pag and Pab genomes were previously generated using the MiSeq Illumina technology and used to determine partial T3E inventories of Pab and Pag. Here, we fully assembled the Pab and Pag genomes following sequencing with PacBio technology and carried out a comparative sequence analysis of the Pab and Pag pathogenicity plasmids pPATHpag and pPATHpab. Assembly of Pab and Pag genomes revealed a ~4 Mbp chromosome with a 55% GC content, and three and four plasmids in Pab and Pag, respectively. pPATHpag and pPATHpab share 97% identity within a 74% coverage, and a similar GC content (51%); they are ~156 kb and ~131 kb in size and consist of 198 and 155 coding sequences (CDSs), respectively. In both plasmids, we confirmed the presence of highly similar gene clusters encoding a T3SS, as well as auxin and cytokinins biosynthetic enzymes. Three putative novel T3Es were identified in Pab and one in Pag. Among T3SS-associated proteins encoded by Pag and Pab, we identified two novel chaperons of the ShcV and CesT families that are present in both pathovars with high similarity. We also identified insertion sequences (ISs) and transposons (Tns) that may have contributed to the evolution of the two pathovars. These include seven shared IS elements, and three ISs and two transposons unique to Pab. Finally, comparative sequence analysis revealed plasmid regions and CDSs that are present only in pPATHpab or in pPATHpag. The high similarity and common features of the pPATH plasmids support the hypothesis that the two strains recently evolved into host-specific pathogens.

An antibacterial T6SS in Pantoea agglomerans pv. betae delivers a lysozyme-like effector to antagonize competitors

The type VI secretion system (T6SS) is deployed by numerous Gram-negative bacteria to deliver toxic effectors into neighbouring cells. The genome of Pantoea agglomerans pv. betae (Pab) phytopathogenic bacteria contains a gene cluster (T6SS1) predicted to encode a complete T6SS. Using secretion and competition assays, we found that T6SS1 in Pab is a functional antibacterial system that allows this pathogen to outcompete rival plant-associated bacteria found in its natural environment. Computational analysis of the T6SS1 gene cluster revealed that antibacterial effector and immunity proteins are encoded within three genomic islands that also harbour arrays of orphan immunity genes or toxin and immunity cassettes. Functional analyses indicated that VgrG, a specialized antibacterial effector, contains a C-terminal catalytically active glucosaminidase domain that is used to degrade prey peptidoglycan. Moreover, we confirmed that a bicistronic unit at the end of the T6SS1 cluster encodes a novel antibacterial T6SS effector and immunity pair. Together, these results demonstrate that Pab T6SS1 is an antibacterial system delivering a lysozyme-like effector to eliminate competitors, and indicate that this bacterium contains additional novel T6SS effectors.

Symbiotic bacteria of the gall-inducing mite Fragariocoptes setiger (Eriophyoidea) and phylogenomic resolution of the eriophyoid position among Acari

Eriophyoid mites represent a hyperdiverse, phytophagous lineage with an unclear phylogenetic position. These mites have succeeded in colonizing nearly every seed plant species, and this evolutionary success was in part due to the mites' ability to induce galls in plants. A gall is a unique niche that provides the inducer of this modification with vital resources. The exact mechanism of gall formation is still not understood, even as to whether it is endogenic (mites directly cause galls) or exogenic (symbiotic microorganisms are involved). Here we (i) investigate the phylogenetic affinities of eriophyoids and (ii) use comparative metagenomics to test the hypothesis that the endosymbionts of eriophyoid mites are involved in gall formation. Our phylogenomic analysis robustly inferred eriophyoids as closely related to Nematalycidae, a group of deep-soil mites belonging to Endeostigmata. Our comparative metagenomics, fluorescence in situ hybridization, and electron microscopy experiments identified two candidate endosymbiotic bacteria shared across samples, however, it is unlikely that they are gall inducers (morphotype1: novel Wolbachia, morphotype2: possibly Agrobacterium tumefaciens). We also detected an array of plant pathogens associated with galls that may be vectored by the mites, and we determined a mite pathogenic virus (Betabaculovirus) that could be tested for using in biocontrol of agricultural pest mites.

Two Pantoea agglomerans type III effectors can transform nonpathogenic and phytopathogenic bacteria into host-specific gall-forming pathogens

Pantoea agglomerans (Pa), a widespread commensal bacterium, has evolved into a host-specific gall-forming pathogen on gypsophila and beet by acquiring a plasmid harbouring a type III secretion system (T3SS) and effectors (T3Es). Pantoea agglomerans pv. gypsophilae (Pag) elicits galls on gypsophila and a hypersensitive response on beet, whereas P. agglomerans pv. betae (Pab) elicits galls on beet and gypsophila. HsvG and HsvB are two paralogous T3Es present in both pathovars and act as host-specific transcription activators on gypsophila and beet, respectively. PthG and PseB are major T3Es that contribute to gall development of Pag and Pab, respectively. To establish the minimal combinations of T3Es that are sufficient to elicit gall symptoms, strains of the nonpathogenic bacteria Pseudomonas fluorescens 55, Pa 3-1, Pa 98 and Escherichia coli, transformed with pHIR11 harbouring a T3SS, and the phytopathogenic bacteria Erwinia amylovora, Dickeya solani and Xanthomonas campestris pv. campestris were transformed with the T3Es hsvG, hsvB, pthG and pseB, either individually or in pairs, and used to infect gypsophila and beet. Strikingly, all the tested nonpathogenic and phytopathogenic bacterial strains harbouring hsvG and pthG incited galls on gypsophila, whereas strains harbouring hsvB and pseB, with the exception of E. coli, incited galls on beet.

Revealing the inventory of type III effectors in Pantoea agglomerans gall-forming pathovars using draft genome sequences and a machine-learning approach

Pantoea agglomerans, a widespread epiphytic bacterium, has evolved into a hypersensitive response and pathogenicity (hrp)-dependent and host-specific gall-forming pathogen by the acquisition of a pathogenicity plasmid containing a type III secretion system (T3SS) and its effectors (T3Es). Pantoea agglomerans pv. betae (Pab) elicits galls on beet (Beta vulgaris) and gypsophila (Gypsophila paniculata), whereas P. agglomerans pv. gypsophilae (Pag) incites galls on gypsophila and a hypersensitive response (HR) on beet. Draft genome sequences were generated and employed in combination with a machine-learning approach and a translocation assay into beet roots to identify the pools of T3Es in the two pathovars. The genomes of the sequenced Pab4188 and Pag824-1 strains have a similar size (∼5 MB) and GC content (∼55%). Mutational analysis revealed that, in Pab4188, eight T3Es (HsvB, HsvG, PseB, DspA/E, HopAY1, HopX2, HopAF1 and HrpK) contribute to pathogenicity on beet and gypsophila. In Pag824-1, nine T3Es (HsvG, HsvB, PthG, DspA/E, HopAY1, HopD1, HopX2, HopAF1 and HrpK) contribute to pathogenicity on gypsophila, whereas the PthG effector triggers HR on beet. HsvB, HsvG, PthG and PseB appear to endow pathovar specificities to Pab and Pag, and no homologous T3Es were identified for these proteins in other phytopathogenic bacteria. Conversely, the remaining T3Es contribute to the virulence of both pathovars, and homologous T3Es were found in other phytopathogenic bacteria. Remarkably, HsvG and HsvB, which act as host-specific transcription factors, displayed the largest contribution to disease development.

The Pseudomonas syringae type III effector HopD1 suppresses effector-triggered immunity, localizes to the endoplasmic reticulum, and targets the Arabidopsis transcription factor NTL9

• Pseudomonas syringae type III effectors are known to suppress plant immunity to promote bacterial virulence. However, the activities and targets of these effectors are not well understood. • We used genetic, molecular, and cell biology methods to characterize the activities, localization, and target of the HopD1 type III effector in Arabidopsis. • HopD1 contributes to P. syringae virulence in Arabidopsis and reduces effector-triggered immunity (ETI) responses but not pathogen-associated molecular pattern-triggered immunity (PTI) responses. Plants expressing HopD1 supported increased growth of ETI-inducing P. syringae strains compared with wild-type Arabidopsis. We show that HopD1 interacts with the membrane-tethered Arabidopsis transcription factor NTL9 and demonstrate that this interaction occurs at the endoplasmic reticulum (ER). A P. syringae hopD1 mutant and ETI-inducing P. syringae strains exhibited enhanced growth on Arabidopsis ntl9 mutant plants. Conversely, growth of P. syringae strains was reduced in plants expressing a constitutively active NTL9 derivative, indicating that NTL9 is a positive regulator of plant immunity. Furthermore, HopD1 inhibited the induction of NTL9-regulated genes during ETI but not PTI. • HopD1 contributes to P. syringae virulence in part by targeting NTL9, resulting in the suppression of ETI responses but not PTI responses and the promotion of plant pathogenicity.

How way leads on to way

In this article, I briefly recount the historical events in my native country that led me to become a plant pathologist. I started as a field pathologist specializing in fungal diseases of legumes, moved to biochemical research on virulence factors, and then on to molecular plant-microbe interactions. I describe the impact my graduate studies at the University of California (UC)-Davis had on my career. My life's work and teaching can be said to reflect the development in plant pathology during the past 40 years. I have included a concise review of the development of plant pathology in Israel and the ways it is funded. Dealing with administrative duties while conducting research has contributed to my belief in the importance of multidisciplinary approaches and of preserving the applied approach in the teaching of plant pathology.

Genome sequence of the biocontrol agent Pantoea vagans strain C9-1

Pantoea vagans is a Gram-negative enterobacterial plant epiphyte of a broad range of plants. Here we report the 4.89-Mb genome sequence of P. vagans strain C9-1 (formerly Pantoea agglomerans), which is commercially registered for biological control of fire blight, a disease of pear and apple trees caused by Erwinia amylovora.

Genomic and phenotypic characterization of a nonpigmented variant of Pantoea vagans biocontrol strain C9-1 lacking the 530-kb megaplasmid pPag3

A 530-kb megaplasmid pPag3 contributing 10.8% of the total genome of Pantoea vagans biocontrol strain C9-1 was sequenced. A rare nonpigmented variant C9-1W was obtained and shown to have lost pPag3, but retained all other plasmids (pPag1, pPag2). Phenotypic characterization of the variant confirmed the function of several annotated genes that may influence ecological fitness and efficacy. Metabolic profiling revealed important plasmid-based carbon utilization phenotypes. Plasmid loss resulted in thiamine auxotrophy, absence of carotenoid pigmentation, desferrioxamine diffusible siderophore biosynthesis, inherent ampicillin resistance and expression of AI-1 quorum-sensing signaling. This confirmed the functional expression of the corresponding genes located on pPag3 in P. vagans.

Recent evolution of bacterial pathogens: the gall-forming Pantoea agglomerans case

Pantoea agglomerans, a widespread epiphyte and commensal bacterium, has evolved into an Hrp-dependent and host-specific tumorigenic pathogen by acquiring a plasmid containing a pathogenicity island (PAI). The PAI was evolved on an iteron plasmid of the IncN family, which is distributed among genetically diverse populations of P. agglomerans. The structure of the PAI supports the premise of a recently evolved pathogen. This review offers insight into a unique model for emergence of new bacterial pathogens. It illustrates how horizontal gene transfer was the major driving force in the creation of the PAI, although a pathoadaptive mechanism might also be involved. It describes the crucial function of plant-produced indole-3-acetic acid (IAA) and cytokinines (CK) in gall initiation as opposed to the significant but secondary role of pathogen-secreted phytohormones. It also unveils the role of type III effectors in determination of host specificity and evolution of the pathogen into pathovars. Finally, it describes how interactions between the quorum sensing system, hrp regulatory genes, and bacterially secreted IAA or CKs affect gall formation and epiphytic fitness.

Quorum-sensing system affects gall development incited by Pantoea agglomerans pv. gypsophilae

The quorum-sensing (QS) regulatory system of the gall-forming Pantoea agglomerans pv. gypsophilae was identified. Mass spectral analysis, together with signal-specific biosensors, demonstrated that P. agglomerans pv. gypsophilae produced N-butanoyl-l-homoserine lactone (C4-HSL) as a major and N-hexanoyl-l-homoserine lactone (C6-HSL) as a minor QS signal. Homologs of luxI and luxR regulatory genes, pagI and pagR, were characterized in strain P. agglomerans pv. gypsophilae Pag824-1 and shown to be convergently transcribed and separated by 14 bp. The deduced PagI (23.8 kDa) and PagR (26.9 kDa) show high similarity with SmaI (41% identity) and SmaR (43% identity), respectively, of Serratia sp. American Type Culture Collection 39006. PagR possesses characteristic autoinducer binding and a helix-turn-helix DNA-binding domain. Gall formation by P. agglomerans pv. gypsophilae depends on a plasmid-borne hrp/hrc gene cluster, type III effectors, and phytohormones. Disruption of pagI, pagR, or both genes simultaneously in Pag824-1 reduced gall size in gypsophila cuttings by 50 to 55% when plants were inoculated with 10(6) CFU/ml. Higher reductions in gall size (70 to 90%) were achieved by overexpression of pagI or addition of exogenous C4-HSL. Expression of the hrp/hrc regulatory gene hrpL and the type III effector pthG in the pagI mutant, as measured with quantitative reverse-transcriptase polymerase chain reaction, was reduced by 5.8 and 6.6, respectively, compared with the wild type, suggesting an effect of the QS system on the Hrp regulon.

Distribution and replication of the pathogenicity plasmid pPATH in diverse populations of the gall-forming bacterium Pantoea agglomerans

Pantoea agglomerans has been transformed from a commensal bacterium into two related gall-forming pathovars by acquisition of pPATH plasmids containing a pathogenicity island (PAI). This PAI harbors an hrp/hrc gene cluster, type III effectors, and phytohormone biosynthetic genes. DNA typing by pulsed-field gel electrophoresis revealed two major groups of P. agglomerans pv. gypsophilae and one group of P. agglomerans pv. betae. The pPATH plasmids of the different groups had nearly identical replicons (98% identity), and the RepA protein showed the highest level of similarity with IncN plasmid proteins. A series of plasmids, designated pRAs, in which the whole replicon region (2,170 bp) or deleted derivatives of it were ligated with nptI were generated for replicon analysis. A basic 929-bp replicon (pRA6) was sufficient for replication in Escherichia coli and in nonpathogenic P. agglomerans. However, the whole replicon region (pRA1) was necessary for expulsion of the pPATH plasmid, which resulted in the loss of pathogenicity. The presence of direct repeats in the replicon region suggests that the pPATH plasmid is an iteron plasmid and that the repeats may regulate its replication. The pPATH plasmids are nonconjugative but exhibit a broad host range, as shown by replication of pRA1 in Erwinia, Pseudomonas, and Xanthomonas. Restriction fragment length polymorphism analyses indicated that the PAIs in the two groups of P. agglomerans pv. gypsophilae are similar but different from those in P. agglomerans pv. betae. The results could indicate that the pPATH plasmids evolved from a common ancestral mobilizable plasmid that was transferred into different strains of P. agglomerans.

The type III effectors HsvG and HsvB of gall-forming Pantoea agglomerans determine host specificity and function as transcriptional activators

Pantoea agglomerans pv. gypsophilae (Pag) elicits galls on gypsophila and a hypersensitive response on beet, whereas P. agglomerans pv. betae (Pab) induces galls on both beet and gypsophila. The pathogenicity of both pathovars is dependent on the presence of a plasmid harbouring type III secretion system (TTSS) components and effectors. The HsvG TTSS effectors of Pag (HsvG-Pag) and Pab (HsvG-Pab) determine the host specificity of both pathovars on gypsophila. Here we describe a novel HsvG homologue, HsvB, which determines the host specificity of Pag and Pab on beet. HsvG requires two direct amino acid repeats for pathogenicity on gypsophila, whereas one repeat in HsvB is sufficient for pathogenicity on beet. Exchanging repeats between HsvG-Pag and HsvB-Pab resulted in a switch of host specificities. Transient expression of GFP-HsvG or GFP-HsvB fusions in gypsophila, beet or melon leaves showed that HsvG and HsvB were localized to the nuclei of host and non-host plants. A yeast one-hybrid assay revealed that a single repeat of HsvG or HsvB was sufficient to activate transcription. By employing random binding-site selection and gel-shift assay HsvG was demonstrated to be a double-stranded DNA-binding protein with an ACACC/aAA consensus binding site. These results suggest that HsvG and HsvB are host-specificity determinants and bear the potential to affect the host transcriptional machinery.

A constitutively expressed, truncated umuDC operon regulates the recA-dependent DNA damage induction of a gene in Acinetobacter baylyi strain ADP1

In response to environmentally caused DNA damage, SOS genes are up-regulated due to RecA-mediated relief of LexA repression. In Escherichia coli, the SOS umuDC operon is required for DNA damage checkpoint functions and for replicating damaged DNA in the error-prone process called SOS mutagenesis. In the model soil bacterium Acinetobacter baylyi strain ADP1, however, the content, regulation, and function of the umuDC operon are unusual. The umuC gene is incomplete, and a remnant of an ISEhe3-like transposase has replaced the middle 57% of the umuC coding region. The umuD open reading frame is intact, but it is 1.5 times the size of other umuD genes and has an extra 5' region that lacks homology to known umuD genes. Analysis of a umuD::lacZ fusion showed that umuD was expressed at very high levels in both the absence and presence of mitomycin C and that this expression was not affected in a recA-deficient background. The umuD mutation did not affect the growth rate or survival after UV-induced DNA damage. However, the UmuD-like protein found in ADP1 (UmuDAb) was required for induction of an adjacent DNA damage-inducible gene, ddrR. The umuD mutation specifically reduced the DNA damage induction of the RecA-dependent DNA damage-inducible ddrR locus by 83% (from 12.9-fold to 2.3-fold induction), but it did not affect the 33.9-fold induction of benA, an unrelated benzoate degradation gene. These data suggest that the response of the ADP1 umuDC operon to DNA damage is unusual and that UmuDAb specifically regulates the expression of at least one DNA damage-inducible gene.

Type III effector proteins from the plant pathogen Xanthomonas and their role in the interaction with the host plant

Pathogenicity of Xanthomonas campestris pathovar (pv.) vesicatoria and most other Gram-negative bacterial plant pathogens largely depends on a type III secretion (TTS) system which is encoded by hypersensitive response and pathogenicity (hrp) genes. These genes are induced in the plant and are essential for the bacterium to be virulent in susceptible hosts and for the induction of the hypersensitive response (HR) in resistant host and non-host plants. The TTS machinery secretes proteins into the extracellular milieu and effector proteins into the plant cell cytosol. In the plant, the effectors presumably interfere with cellular processes to the benefit of the pathogen or have an avirulence activity that betrays the bacterium to the plant surveillance system. Type III effectors were identified by their avirulence activity, co-regulation with the TTS system and homology to known effectors. A number of effector proteins are members of families, e.g., the AvrBs3 family in Xanthomonas. AvrBs3 localizes to the nucleus of the plant cell where it modulates plant gene expression. Another family that is also present in Xanthomonas is the YopJ/AvrRxv family. The latter proteins appear to act as SUMO cysteine proteases in the host. Here, we will present an overview about the regulation of the TTS system and its substrates and discuss the function of the AvrRxv and AvrBs3 family members in more detail.

Analysis of promoters recognized by HrpL, an alternative sigma-factor protein from Pantoea agglomerans pv. gypsophilae

HrpL, an alternative sigma factor, activates the transcription of the Hrp regulon by its binding to a common "hrp box" promoter. Based on computational techniques, the hrp box previously was defined as a consensus bipartite cis element, 5'-GGAACC-N(15-16)-CCACNNA-3'. The present report combines a quantitative in vivo assay for measuring Hrp promoter activity with site-specific mutagenesis to analyze the effect of consensus and nonconsensus nucleotides on promoter activity. The analysis was carried out with Hop effectors of the tumorigenic bacterium Pantoea agglomerans pv. gypsophilae, in which HrpL is indispensable for gall formation. Mutational analysis indicates that the hrp box consensus can be divided into crucial and noncrucial nucleotides. The first 5 nucleotides (nt) of the--35 consensus motif (GGAAC) and the 3 nt of the--10 motif (ACNNA) are crucial, whereas other consensus and adjacent nonconsensus nucleotides exert a significant effect on the promoter's strength. With spacing of 13 or 17 nt between the two motifs, significant activity was still retained. Gel shift assays indicated that deletion of GG from the--35 consensus motif eliminated HrpL binding, whereas mutations in the--10 consensus motif or modification of the spacing, which eliminates promoter activity, did not elicit any effect. The degeneracy in Hrp promoters of four hrp and type III effector genes of P agglomerans pv. gypsophilae indicated significant differences in promoter activity, whereas increasing the promoter strength of the Hop effector, HsvG, resulted in overexpression of gall formation.

Characterization of six flagellin genes in the H3, H53 and H54 standard strains of Escherichia coli

Six flagellin genes in three H standard Escherichia coli strains for H3, H53 and H54 were characterized. Each strain has two flagellin genes, one of which is expressed as its standard H antigen. A pair of flagellin genes flkA3 (encoding for H3 antigen) and fliC16 (H16) was cloned from Bi7327-41, flkA53 (H53) and fliC-53 from E480-68, and flmA54 (H54) and fliC-54 from E223-69. Two fliC genes, fliC-53 and fliC-54, are nonfunctional owing to the insertions of IS1 and IS1222, respectively. The flkA and flmA regions are located in the 3' end of the rnpB gene and near the nlpA gene, respectively. Each of them is followed by a gene homologous to fljA, which is known to repress the expression of fliC(i) in Salmonella enterica serovar Typhimurium. These results suggest that they are derived from the same origin of the fljBA operon. However, these regions contain neither the hin gene nor the invertible H segment. The four flagellin genes, fliC16, flkA3, flkA53 and flmA54, share high homology in nucleotide and amino-acid sequences with one another and with the S. enterica serovar Typhimurium flagellin genes. The promoter sequence of fliC16 is homologous to that of fliC(i), whereas the promoter sequences of flkA and flmA are homologous to that of fljB. The terminator sequences of the fliC16, fliC-53 and fliC-54 genes are conserved among themselves and identical with that of the E. coli fliC48 gene. Three FljA repressors, FljA3, FljA53 and FljA54, are homologous highly with one another and moderately with FljA of Salmonella. These results indicate that six flagellin genes analyzed are markedly similar to the Salmonella flagellin genes, suggesting their lateral transfer from Salmonella.

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.

pthG from Pantoea agglomerans pv. gypsophilae encodes an avirulence effector that determines incompatibility in multiple beet species

SUMMARY Pantoea agglomerans pv. gypsophilae (Pag) causes root and crown gall disease on gypsophila, whereas P. agglomerans pv. betae (Pab) induces the disease on beet as well as gypsophila. Both pathovars harbour a pathogenicity plasmid (pPATH(Pag) or pPATH(Pab)) that determines disease development. We have previously isolated and partially characterized a pleiotropic gene from the pPATH(Pag), designated as pthG, that encodes a virulence factor in gypsophila and an elicitor of a hypersensitive-like response in beet roots. The present study was undertaken to characterize pthG further as an avr gene. The infiltration of beet leaves with strains expressing PthG (i.e. Pag or Pab containing pthG in trans) caused an hypersensitive reaction (HR) response within 48 h, whereas strains lacking intact pthG (i.e. Pab or Pag mutated in pthG) resulted in gall formation after 5 days. A hypersensitive reaction was elicited by PthG on multiple beet species, whereas a marker exchange mutant of Pag in pthG extended its host range on these beet species. A marker exchange mutant of Pag in hrpJ, encoding a component of the Type III secretion system, prevented HR elicitation. Mutations in each of the hrp regulatory genes (hrpY, hrpS and hrpL) substantially reduced the transcriptional activity of pthG in gypsophila cuttings. PthG could only be detected inside Pag cells during over-expression of hrpS or hrpL. Particle bombardment of GFP-PthG fusion caused cell death in beet, but not in non-host (melon) leaves. Present and previous results have established pthG as a broad-host-range avr gene that functions in multiple host plant species and the first functional avr gene in Pantoea spp.

Pathogenicity and other genomic islands in plant pathogenic bacteria

SUMMARY Pathogenicity islands (PAIs) were first described in uropathogenic E. coli. They are now defined as regions of DNA that contain virulence genes and are present in the genome of pathogenic strains, but absent from or only rarely present in non-pathogenic variants of the same or related strains. Other features include a variable G+C content, distinct boundaries from the rest of the genome and the presence of genes related to mobile elements such as insertion sequences, integrases and transposases. Although PAIs have now been described in a wide range of both plant and animal pathogens it has become evident that the general features of PAIs are displayed by a number of regions of DNA with functions other than pathogenicity, such as symbiosis and antibiotic resistance, and the general term genomic islands has been adopted. This review will describe a range of genomic islands in plant pathogenic bacteria including those that carry effector genes, phytotoxins and the type III protein secretion cluster. The review will also consider some medically important bacteria in order to discuss the range, acquisition and stabilization of genomic islands.

Pantoea agglomerans pvs. gypsophilae and betae, recently evolved pathogens?

SUMMARY Pantoea agglomerans pvs. gypsophilae and betae

TAXONOMY

Bacteria; Proteobacteria; gamma subdivision; order Enterobacteriales; family Enterobacteriaceae; species Pantoea agglomerans. Microbiological properties: Gram-negative, non-capsulated, non-spore-forming, predominately motile rode. Disease symptoms: Gall formation at wound sites, mainly in the crown region of the stem. The host range of P. agglomerans pv. gypsophilae is restricted to Gypsophila paniculata, whereas P. agglomerans pv. betae is pathogenic on Beta vulgaris and gypsophila. Disease control: Pathogenic-free transplants and sanitation. No resistant cultivars are available. Major virulence determinants: Pathogenicity plasmid (pPATH), hrp cluster, type III virulence effectors, phytohormones.

The Pseudomonas syringae type III-secreted protein HopPtoD2 possesses protein tyrosine phosphatase activity and suppresses programmed cell death in plants

The bacterial plant pathogen Pseudomonas syringae possesses a type III protein secretion system that delivers many virulence proteins into plant cells. A subset of these proteins (called Avr proteins) is recognized by the plant's innate immune system and triggers defences. One defence-associated response is the hypersensitive response (HR), a programmed cell death (PCD) of plant tissue. We have previously identified HopPtoD2 as a type III secreted protein from P. s. pv. tomato DC3000. Sequence analysis revealed that an N-terminal domain shared homology with AvrPphD and a C-terminal domain was similar to protein tyrosine phosphatases (PTPs). We demonstrated that purified HopPtoD2 possessed PTP activity and this activity required a conserved catalytic Cys residue (Cys(378)). Interestingly, HopPtoD2 was capable of suppressing the HR elicited by an avirulent P. syringae strain on Nicotiana benthamiana. HopPtoD2 derivatives that lacked Cys(378) no longer suppressed the HR indicating that HR suppression required PTP activity. A constitutively active MAPK kinase, called NtMEK2DD, is capable of eliciting an HR-like cell death when transiently expressed in tobacco. When NtMEK2DD and HopPtoD2 were co-delivered into plant cells, the HR was suppressed indicating that HopPtoD2 acts downstream of NtMEK2DD. DC3000 hopPtoD2 mutants were slightly reduced in their ability to multiply in planta and displayed an enhanced ability to elicit an HR. The identification of HopPtoD2 as a PTP and a PCD suppressor suggests that the inactivation of MAPK pathways is a virulence strategy utilized by bacterial plant pathogens.

The regulatory cascade that activates the Hrp regulon in Erwinia herbicola pv. gypsophilae

The pathogenicity of Erwinia herbicola pv. gypsophilae (Ehg) is dependent on a plasmid (pPATH(Ehg)) that harbors the hrp gene cluster and additional virulence genes. The hrp regulatory cascade of Ehg comprises an hrpXY operon encoding a two-component system; hrpS encoding a transcriptional factor of the NtrC family and hrpL encoding an alternative sigma factor. Results obtained suggest the following signal transduction model for activating the Hrp regulon: phosphorylated HrpY activates hrpS, HrpS activates hrpL, and HrpL activates genes containing "hrp box" promoter. This model was supported by studies on the effects of mutations in the regulatory genes on pathogenicity and complementation analysis. Nonpolar mutations in hrpX did not affect virulence or transcription of downstream genes. Site-directed mutagenesis of the conserved aspartate 57 in HrpY suggested that its phosphorylation is crucial for activating the hrp regulatory cascade. Studies on the effects of mutations in the hrp regulatory genes on transcriptional activity of downstream genes or of their isolated promoters in planta showed dependency of hrpS expression on active HrpY, of hrpL expression on active HrpS, and of hrpN or hrpJ expression on active HrpL. These results were also partially supported by overexpression of regulatory genes under in vitro conditions. The hrpXY is constitutively expressed with high basal levels under repressive conditions, in contrast to hrpS and hrpL, which exhibit low basal expression levels and are environmentally regulated.

Presence of a characteristic D-D-E motif in IS1 transposase

Transposases encoded by various transposable DNA elements and retroviral integrases belong to a family of proteins with three conserved acidic amino acids, D, D, and E, constituting the D-D-E motif that represents the active center of the proteins. IS1, one of the smallest transposable elements in bacteria, encodes a transposase which has been thought not to belong to the family of proteins with the D-D-E motif. In this study, we found several IS1 family elements that were widely distributed not only in eubacteria but also in archaebacteria. The alignment of the transposase amino acid sequences from these IS1 family elements showed that out of 14 acidic amino acids present in IS1 transposase, three (D, D, and E) were conserved in corresponding positions in the transposases encoded by all the elements. Comparison of the IS1 transposase with other proteins with the D-D-E motif revealed that the polypeptide segments surrounding each of the three acidic amino acids were similar. Furthermore, the deduced secondary structures of the transposases encoded by IS1 family elements were similar to one another and to those of proteins with the D-D-E motif. These results strongly suggest that IS1 transposase has the D-D-E motif and thus belongs to the family of proteins with the D-D-E motif. In fact, mutant IS1 transposases with an amino acid substitution for each of the three acidic amino acids possibly constituting the D-D-E motif were not able to promote transposition of IS1, supporting this hypothesis. The D-D-E motif identified in IS1 transposase differs from those in the other proteins in that the polypeptide segment between the second D and third E in IS1 transposase is the shortest, 24 amino acids in length. Because of this difference, the presence of the D-D-E motif in IS1 transposase has not been discovered for some time.