Genetic and transcriptional organization of the hrp cluster of Pseudomonas syringae pv. phaseolicola

Identification of a putative alternate sigma factor and characterization of a multicomponent regulatory cascade controlling the expression of Pseudomonas syringae pv. syringae Pss61 hrp and hrmA genes

The Pseudomonas syringae hrp and hrmA genes controlling pathogenicity and elicitation of the hypersensitive response and the avr genes controlling host range have been shown previously to be regulated by carbon, nitrogen, pH, osmolarity, and hypothetical plant factors. In P. syringae pv. syringae Pss61, inactivation of hrp complementation groups II and XIII reduced expression of a plasmid-borne hrmA'-lacZ fusion. The hrp regions II and XIII were cloned on separate plasmids and shown to enhance the activity of the hrmA promoter in Escherichia coli MC4100 transformants at least 100-fold. The nucleotide sequence of region XIII revealed two open reading frames (hrpR and hrpS) whose deduced products share homology with P. syringae pv. phaseolicola NPS3121 HrpS and are both related to the NtrC family of two-component signal transduction systems. HrpR and HrpS differ from most members of the protein family by lacking an amino-terminal domain which modulates the regulatory activity. A single open reading frame, hrpL, whose product shares homology with AlgU, a putative alternate sigma factor of P. aeruginosa, as well as with the related alternate sigma factors was identified within region II. Key domains are partially conserved. Inactivation of hrpS in Pss61 repressed expression of a plasmid-borne hrpL'-lacZ fusion carried by pYXPL1R, and transformation of MC4100(pYXPL1R) with a plasmid carrying hrpRS increased hrpL promoter activity at least 200-fold. Neither hrpS nor hrpR, when cloned on separate plasmids, activated the hrpL promoter activity individually. The expression of hrpL when directed by a lac promoter was sufficient to express a set of plasmid-borne hrmA'-, hrpJ'-, and hrpZ'-lacZ fusions independently of other hrp genes. The results indicate that hrpRS and hrpL are part of a regulatory cascade in which HrpR and HrpS activate expression of hrpL and HrpL, a putative sigma factor, induces expression of HrpL-responsive genes.

Molecular cloning, chromosomal mapping, and sequence analysis of copper resistance genes from Xanthomonas campestris pv. juglandis: homology with small blue copper proteins and multicopper oxidase

Copper-resistant strains of Xanthomonas campestris pv. juglandis occur in walnut orchards throughout northern California. The copper resistance genes from a copper-resistant strain C5 of X. campestris pv. juglandis were cloned and located on a 4.9-kb ClaI fragment, which hybridized only to DNA of copper-resistant strains of X. campestris pv. juglandis, and was part of an approximately 20-kb region which was conserved among such strains of X. campestris pv. juglandis. Hybridization analysis indicated that the copper resistance genes were located on the chromosome. Plasmids conferring copper resistance were not detected in copper-resistant strains, nor did mating with copper-sensitive strains result in copper-resistant transconjugants. Copper resistance genes from X. campestris pv. juglandis shared nucleotide sequence similarity with copper resistance genes from Pseudomonas syringae pv. tomato, P. syringae, and X. campestris pv. vesicatoria. DNA sequence analysis of the 4.9-kb fragment from strain C5 revealed that the sequence had an overall G+C content of 58.7%, and four open reading frames (ORF1 to ORF4), oriented in the same direction. All four ORFs were required for full expression of copper resistance, on the basis of Tn3-spice insertional inactivation and deletion analysis. The predicted amino acid sequences of ORF1 to ORF4 showed 65, 45, 47, and 40% identity with CopA, CopB, CopC, and CopD, respectively, from P. syringae pv. tomato. The most conserved regions are ORF1 and CopA and the C-terminal region (166 amino acids from the C terminus) of ORF2 and CopB. The hydrophobicity profiles of each pair of predicted polypeptides are similar except for the N terminus of ORF2 and CopB. Four histidine-rich polypeptide regions in ORF1 and CopA strongly resembled the copper-binding motifs of small blue copper proteins and multicopper oxidases, such as fungal laccases, plant ascorbate oxidase, and human ceruloplasmin. Putative copper ligands of the ORF1 polypeptide product are proposed, indicating that the polypeptide of ORF1 might bind four copper ions: one type 1, one type 2, and two type 3.

Molecular analysis of avirulence gene avrRpt2 and identification of a putative regulatory sequence common to all known Pseudomonas syringae avirulence genes

The avrRpt2 locus from Pseudomonas syringae pv. tomato causes virulent strains of P. syringae to be avirulent on some, but not all, lines of Arabidopsis thaliana and Glycine max (soybean). We determined the DNA sequence of the avrRpt2 locus and identified the avrRpt2 gene as a 768-bp open reading frame encoding a putative 28.2-kDa protein. Deletion analysis and transcription studies provided further evidence that this open reading frame encodes AvrRpt2. We found that the avrRpt2 gene also has avirulence activity in P. syringae pathogens of Phaseolus vulgaris (common bean), suggesting that disease resistance genes specific to avrRpt2 are functionally conserved among diverse plant species. The predicted AvrRpt2 protein is hydrophilic and contains no obvious membrane-spanning domains or export signal sequences, and there was no significant similarity of AvrRpt2 to sequences in the GenBank, EMBL, or Swiss PIR data bases. A comparison of the avrRpt2 DNA sequence to nine other P. syringae avirulence genes revealed a highly conserved sequence, GGAACCNA-N14-CCACNNA, upstream of the translation initiation codon. This motif is located 6 to 8 nucleotides upstream of the transcription start site in all four P. syringae avirulence genes for which a transcription start site has been determined, suggesting a role as a binding site for a novel form of RNA polymerase. Regulation of avrRpt2 was similar to other P. syringae avirulence genes; expression was high in minimal medium and low in rich medium and depended on the hrpRS locus and an additional locus at the opposite end of the hrp cluster of P. syringae pv. tomato.

Conservation of secretion pathways for pathogenicity determinants of plant and animal bacteria

Extracellular proteins of plant and animal bacteria are important in virulence. Many of these are secreted through the type I sec-independent and the type II sec-dependent pathways. Recently, a third distinct pathway, involved in secretion of Yops, has been discovered in Yersinia. This pathway has homology with pathways in plant pathogenic bacteria that are putatively involved in the secretion of proteins active on plant cells, such as harpin and possibly some avr gene products

Homology between the HrpO protein of Pseudomonas solanacearum and bacterial proteins implicated in a signal peptide-independent secretion mechanism

A region of approximately 22 kb of DNA defines the large hrp gene cluster of strain GMI1000 of Pseudomonas solanacearum. The majority of mutants that map to this region have lost the ability to induce disease symptoms on tomato plants and are no longer able to elicit a hypersensitive reaction (HR) on tobacco, a non-host plant. In this study we present the complementation analysis and nucleotide sequence of a 4772 bp region of this hrp gene cluster. Three complete open reading frames (ORFs) are predicted within this region. The corresponding putative proteins, HrpN, HrpO and HpaP, have predicted sizes of 357, 690 and 197 amino acids, respectively, and predicted molecular weights of 38,607, 73,990 and 21,959 dalton, respectively. HrpN and HrpO are both predicted to be hydrophobic proteins with potential membrane-spanning domains and HpaP is rich in proline residues. A mutation in hpaP (for hrp associated) does not affect the HR on tobacco or the disease on tomato plants. None of the proteins is predicted to have an N-terminal signal sequence, which would have indicated that the proteins are exported. Considerable sequence similarities were found between HrpO and eight known or predicted prokaryotic proteins: LcrD of Yersinia pestis and Y. enterocolitica, FlbF of Caulobacter crescentus, FlhA of Bacillus subtilis, MxiA and VirH of Shigella flexneri, InvA of Salmonella typhimurium and HrpC2 of Xanthomonas campestris pv. vesicatoria. These homologies suggest that certain hrp genes of phytopathogenic bacteria code for components of a secretory system, which is related to the systems for secretion of flagellar proteins, Ipa proteins of Shigella flexneri and the Yersinia Yop proteins. Furthermore, these homologous proteins have the common feature of being implicated in a distinct secretory mechanism, which does not require the cleavage of a signal peptide. The sequence similarity between HrpO and HrpC2 is particularly high (66% identity and 81% similarity) and the amino acid sequence comparison between these two proteins presented here reveals the first such sequence similarity to be shown between Hrp proteins of P. solanacearum and X. campestris. An efflux of plant electrolytes was found to be associated with the interactions between P. solanacearum and both tomato and tobacco leaves. This phenomenon may be part of the mechanism by which hrp gene products control and determine plant-bacterial interactions, since hrpO mutants induced levels of leakage which were significantly lower than those induced by the wild type on each plant.

Molecular characterization and hrp dependence of the avirulence gene avrPto from Pseudomonas syringae pv. tomato [corrected]

The avrPto avirulence gene from Pseudomonas syringae pv. tomato (Pst) race 0 governs race-specific resistance to bacterial speck disease in tomato cultivars containing the Pto resistance gene. The avrPto gene encodes 0.7 and 0.75 kb mRNAs whose predicted translation product is a mostly hydrophilic 164 amino acid protein of 18.3 kD a that reveals no homology to protein sequences in GenBank or EMBL databases. Highest expression of avrPto in cell culture is observed in minimal media containing sugars and sugar alcohols as carbon sources and lowest expression in minimal media containing tricarboxylic acid intermediates and in complex media. Expression of avrPto in planta is induced within 1 h following infection of both resistant and susceptible tomato plants by Pst, and increases over the first 6 h. Transcription of avrPto requires the hrpSR pathogenicity functions, but is independent of other Pst hrp genes. A region of the avrPto promoter shows homology to hrp box sequences upstream of other P. syringae genes that require the hrpSR locus for expression, and both avirulence activity and avrPto mRNA accumulation are abolished by deletions extending into this region. The avrPto transcription start site maps 31 nucleotides downstream of the hrp box motif.

Generalized Induction of Defense Responses in Bean Is Not Correlated with the Induction of the Hypersensitive Reaction

Transcripts for phenylalanine ammonia-lyase, chalcone synthase, chalcone isomerase, and chitinase accumulated in common bean after infiltration with the Pseudomonas syringae pv tabaci Hrp- mutant Pt11528::Hrp1, even though a hypersensitive reaction did not occur. The temporal pattern of this transcript accumulation was similar to that seen after infiltration with wild-type P. s. tabaci Pt11528, which resulted in a hypersensitive reaction. Escherichia coli DH5[alpha], P. fluorescens Pf101, heat-killed Pt11528 cells, and Pt11528 cells treated with protein synthesis inhibitors also induced accumulation of defense transcripts but not a hypersensitive reaction. In contrast, these transcripts were not detected in plants infiltrated with water or P.s. pv phaseolicola NPS3121, a compatible pathogen that causes halo blight. Phytoalexins were produced in bean after infiltration with Pt11528, Pt11528::Hrp1, Pt11528 cells treated with neomycin, or Pf101, but not in plants infiltrated with NPS3121 or water. These results suggest that there are unique biochemical events associated with the expression of a hypersensitive reaction which are distinct from other plant defense responses such as phytoalexin biosynthesis. In addition, our results support the hypothesis that there is a general, nonspecific mechanism for the induction of defense transcripts and phytoalexins by pathogenic and saprophytic bacteria that is distinct from the more specific mechanism associated with the induction of the hypersensitive reaction.

Evidence that the hrpB gene encodes a positive regulator of pathogenicity genes from Pseudomonas solanacearum

The hrp gene cluster of Pseudomonas solanacearum GMI1000 strain encodes functions that are essential for pathogenicity on tomato and for the elicitation of the hypersensitive response on tobacco. In this study, we present the nucleotide sequence of one of the hrp genes (hrpB) located at the left-hand end of the cluster and we show that hrpB encodes a positive regulator controlling the expression of hrp genes. hrpB has a coding capacity for a 477-amino-acid polypeptide, which shows significant similarity to several prokaryotic transcriptional activators including the AraC protein of Escherichia coli, the XylS protein of Pseudomonas putida and the VirF protein of Yersinia enterocolitica. The predicted hrpB gene product belongs to a family of bacterial regulators different from the previously described HrpS protein of the hrp gene cluster of Pseudomonas syringae pv. phaseolicola. Genetic evidence demonstrates that the hrpB gene product acts as a positive regulator of the expression in minimal medium of all but one of the putative transcription units of the hrp gene cluster and also controls the expression of genes located outside this cluster. We also show in this paper that the transcription of hrpB is induced in minimal medium and is partly autoregulated.

Plant and environmental sensory signals control the expression of hrp genes in Pseudomonas syringae pv. phaseolicola

The hrp genes of Pseudomonas syringae pv. phaseolicola control the development of primary disease symptoms in bean plants and the elicitation of the hypersensitive response in resistant plants. We examined the expression of the seven operons located in the 22-kb hrp cluster (L. G. Rahme, M. N. Mindrinos, and N. J. Panopoulos, J. Bacteriol. 173:575-586, 1991) in planta and in vitro under different physiological and nutritional conditions by using chromosomally located hrp::inaZ reporter fusions. We show that (i) a plant signal(s) is specifically required for the induction of the seven hrp operons, during both compatible and incompatible interactions; (ii) hrpL and hrpRS are regulated by different mechanisms in planta and in vitro; and (iii) expression of individual hrp loci is differentially affected by pH, osmotic strength, and type of carbon source: hrpAB, hrpC, and hrpD were downregulated similarly by osmolarity, pH, and certain carbon sources; hrpE expression was affected strongly by pH and carbon substrate and slightly by osmolarity; and hrpF was not substantially affected by any of these factors. These findings suggest complex signaling mechanisms taking place during plant-pathogen interactions.

Organization and environmental regulation of the Pseudomonas syringae pv. syringae 61 hrp cluster

The ability of Pseudomonas syringae pv. syringae 61 to elicit the hypersensitive response in nonhost plant species has been linked to a cluster of hrp/hrm genes whose expression appears to be environmentally regulated. To understand the genetic organization of this hrp/hrm gene cluster and its expression during the interaction with nonhost plant species better, we constructed a set of chromosomal hrp-uidA fusions in P. syringae pv. syringae 61 by Tn5-gusA1 mutagenesis of the cloned hrp/hrm gene cluster and transferred them into the genome by marker exchange mutagenesis. Complementation analysis employing plasmid-borne Tn5-gusA1 insertions and previously characterized chromosomal TnphoA mutations defined at least eight apparent transcriptional units within the hrp/hrm cluster, several of which were multicistronic. The expression of hrp-uidA fusions in seven of these apparent hrp transcriptional units increased following inoculation into tobacco leaves. Enhanced expression from a representative fusion was detected 1 h after inoculation of tobacco leaves. The induction observed in planta was similar to the levels detected following culture of the bacteria in minimal-salts medium: irrespective of the carbon source. Complex amino acid sources, such as peptone, repressed the expression of P. syringae pv. syringae 61 hrp genes at levels exceeding 0.028%. The results indicate that enhanced expression of hrp genes occurs early in the interaction with nonhost plant species in an apparent response to altered nutritional conditions.

Expression of the Xanthomonas campestris pv. vesicatoria hrp gene cluster, which determines pathogenicity and hypersensitivity on pepper and tomato, is plant inducible

The hrp gene cluster from Xanthomonas campestris pv. vesicatoria determines functions necessary not only for pathogenicity on the host plants pepper and tomato but also for the elicitation of the hypersensitive reaction on resistant host and nonhost plants. Transcriptional orientation and expression of the hrp loci were determined with hrp::Tn3-gus fusions. In addition, expression of the hrp loci was studied by RNA hybridization experiments. Expression of the hrp genes was not detectable after growth of the bacteria in complex medium or in minimal medium. However, high levels of induction of hrp gene expression were measured during growth of the bacteria in the plant. To search for a plant molecule responsible for this induction, we examined a variety of materials of plant origin for their ability to induce hrp gene expression. Filtrates from plant suspension cultures induced hrp genes to levels comparable to those induced in the plant. The inducing molecule(s) was found to be heat stable and hydrophilic and to have a molecular mass of less than 1,000 daltons.

Opine catabolism and conjugal transfer of the nopaline Ti plasmid pTiC58 are coordinately regulated by a single repressor

The Ti plasmids of Agrobacterium tumefaciens are conjugal elements whose transfer is strongly repressed. Transfer is induced by the conjugal opines, a group of unique carbon compounds synthesized in crown gall tumors. The opines also induce Ti plasmid-encoded genes required by the bacteria for opine catabolism. We have cloned and sequenced a gene from the Ti plasmid pTiC58, whose product mediates the opine-dependent regulation of conjugal transfer and catabolism of the conjugal opines, agrocinopines A and B. The gene, accR, is closely linked to the agrocinopine catabolic locus. A spontaneous mutant Ti plasmid, pTiC58Trac, which constitutively expresses conjugal transfer and opine catabolism, was complemented in trans by a clone of wild-type accR. Comparative sequence analysis identified a 5-base-pair deletion close to the 5' end of the mutant accR allele from pTiC58Trac. Analysis of lacZ fusions in conjugal transfer and opine catabolic structural genes demonstrated that the accR-encoded function is a transcriptional repressor. accR can encode a 28-kDa protein. This protein is related to a class of repressor proteins that includes LacR, GutR, DeoR, FucR, and GlpR that regulate sugar catabolic systems in several bacterial genera.

Expression of the avirulence gene avrBs3 from Xanthomonas campestris pv. vesicatoria is not under the control of hrp genes and is independent of plant factors

The avirulence gene avrBs3 from Xanthomonas campestris pv. vesicatoria pepper race 1 is responsible for the induction of a race-specific hypersensitive reaction in resistant pepper cultivars. A DNA region of 3.7 kb, containing several open reading frames and an internal repetitive region, was shown previously to be necessary for avirulence activity (U. Bonas, R. E. Stall, and B. Staskawicz, Mol. Gen. Genet. 218:127-136, 1989). The promoter of avrBs3 was identified by using gene fusions to beta-glucuronidase. Also, we mapped the transcription start site and showed that the avrBs3 gene is expressed constitutively in cells grown in minimal or complex medium and in planta. Polyclonal antibodies raised against a fusion protein produced in Escherichia coli allowed the identification of a 122-kDa protein in X. campestris pv. vesicatoria cells expressing the avrBs3 gene. The antibody is specific for AvrBs3 in X. campestris pv. vesicatoria cells but also recognizes homologous proteins in other pathovars of X. campestris. We found that AvrBs3 is localized intracellularly in X. campestris pv. vesicatoria and is mainly in the soluble fraction. The effect of mutations in the hrp gene cluster on the function of AvrBs3 was examined. Expression of AvrBs3 in X. campestris pv. vesicatoria grown in minimal or complex medium is independent of the hrp gene cluster that determines pathogenicity and hypersensitivity to X. campestris pv. vesicatoria. In the plant, however, the hrp genes are required for elicitation of a race-specific resistance response.