Efficient transformation of Erwinia carotovora subsp. carotovora and E. carotovora subsp. atroseptica

Injectisome T3SS subunits as potential chaperones in the extracellular export of Pectobacterium carotovorum subsp. carotovorum bacteriocins Carocin S1 and Carocin S3 secreted via flagellar T3SS

Pectobacterium carotovorum subsp. carotovorum (Pcc) causes soft-rot disease in a wide variety of plants resulting in economic losses worldwide. It produces various types of bacteriocin to compete against related plant pathogens. Studies on how bacteriocins are extracellularly secreted are conducted to understand the mechanism of interbacterial competition. In this study, the secretion of the low-molecular-weight bacteriocins (LMWB) Carocin S1 and Carocin S3 produced by a multiple-bacteriocin producing strain of Pcc, 89-H-4, was investigated. Tn5 insertional mutagenesis was used to generate a mutant, TH22-6, incapable of LMWBs secretion. Sequence and homology analyses of the gene disrupted by transposon Tn5 insertion revealed that the gene sctT, an essential component of the injectisome type III secretion machinery (T3aSS), is required for the secretion of the bacteriocins. This result raised a question regarding the nature of the secretion mechanism of Pcc bacteriocins which was previously discovered to be secreted via T3bSS, a system that utilizes the bacterial flagellum for extracellular secretions. Our previous report has shown that bacteriocin Carocin S1 cannot be secreted by mutants that are defective of T3bSS-related genes such as flhA, flhC, flhD and fliC. We knocked out several genes making up the significant structural components of both T3aSS and T3bSS. The findings led us to hypothesize the potential roles of the T3aSS-related proteins, SctT, SctU and SctV, as flagellar T3SS chaperones in the secretion of Pcc bacteriocins. This current discovery and the findings of our previous study helped us to conceptualize a unique Type III secretion system for bacteriocin extracellular export which is a hybrid of the injectisome and flagellar secretion systems.

Microbial population dynamics in response to Pectobacterium atrosepticum infection in potato tubers

Endophytes are microbes and fungi that live inside plant tissues without damaging the host. Herein we examine the dynamic changes in the endophytic bacterial community in potato (Solanum tuberosum) tuber in response to pathogenic infection by Pectobacterium atrosepticum, which causes soft rot in numerous economically important crops. We quantified community changes using both cultivation and next-generation sequencing of the 16S rRNA gene and found that, despite observing significant variability in both the mass of macerated tissue and structure of the endophytic community between individual potato tubers, P. atrosepticum is always taken over by the endophytes during maceration. 16S rDNA sequencing revealed bacteria from the phyla Proteobacteria, Actinobacteria, Firmicutes, Bacteroidetes, Fusobacteria, Verrucomicrobia, Acidobacteria, TM7, and Deinococcus-Thermus. Prior to infection, Propionibacterium acnes is frequently among the dominant taxa, yet is out competed by relatively few dominant taxa as the infection proceeds. Two days post-infection, the most abundant sequences in macerated potato tissue are Gammaproteobacteria. The most dominant genera are Enterobacter and Pseudomonas. Eight days post-infection, the number of anaerobic pectolytic Clostridia increases, probably due to oxygen depletion. These results demonstrate that the pathogenesis is strictly initiated by the pathogen (sensu stricto) and proceeds with a major contribution from the endophytic community.

GSL2 over-expression confers resistance to Pectobacterium atrosepticum in potato

Over-expression of the potato Gibberellin Stimulated-Like 2 ( GSL2 ) gene in transgenic potato confers resistance to blackleg disease incited by Pectobacterium atrosepticum and confirms a role for GSL2 in plant defence. The Gibberellin Stimulated-Like 2 (GSL2) gene (also known as Snakin 2) encodes a cysteine-rich, low-molecular weight antimicrobial peptide produced in potato plants. This protein is thought to play important roles in the innate defence against invading microbes. Over-expression of the GSL2 gene in potato (cultivar Iwa) was achieved using Agrobacterium-mediated gene transfer of a plant expression vector with the potato GSL2 gene under the regulatory control elements of the potato light-inducible Lhca3 gene. The resulting plants were confirmed as being transgenic by PCR, and subsequently analysed for transcriptional expression of the Lhca3-GSL2-Lhca3 chimeric potato gene. Quantitative RT-PCR analysis demonstrated that the majority of the transgenic potato lines over-expressed the GSL2 gene at the mRNA level. Based on qRT-PCR results and evaluation of phenotypic appearance, eight lines were selected for further characterisation and evaluated in bioassays for resistance to Pectobacterium atrosepticum (formerly Erwinia carotovora subsp. atroseptica), the causal agent of blackleg in potato. Three independent pathogenicity bioassays showed that transgenic lines with significantly increased transcriptional expression of the GSL2 gene exhibit resistance to blackleg disease. This establishes a functional role for GSL2 in plant defence against pathogens in potato.

Role and regulation of the Flp/Tad pilus in the virulence of Pectobacterium atrosepticum SCRI1043 and Pectobacterium wasabiae SCC3193

In this study, we characterized a putative Flp/Tad pilus-encoding gene cluster, and we examined its regulation at the transcriptional level and its role in the virulence of potato pathogenic enterobacteria of the genus Pectobacterium. The Flp/Tad pilus-encoding gene clusters in Pectobacterium atrosepticum, Pectobacterium wasabiae and Pectobacterium aroidearum were compared to previously characterized flp/tad gene clusters, including that of the well-studied Flp/Tad pilus model organism Aggregatibacter actinomycetemcomitans, in which this pilus is a major virulence determinant. Comparative analyses revealed substantial protein sequence similarity and open reading frame synteny between the previously characterized flp/tad gene clusters and the cluster in Pectobacterium, suggesting that the predicted flp/tad gene cluster in Pectobacterium encodes a Flp/Tad pilus-like structure. We detected genes for a novel two-component system adjacent to the flp/tad gene cluster in Pectobacterium, and mutant analysis demonstrated that this system has a positive effect on the transcription of selected Flp/Tad pilus biogenesis genes, suggesting that this response regulator regulate the flp/tad gene cluster. Mutagenesis of either the predicted regulator gene or selected Flp/Tad pilus biogenesis genes had a significant impact on the maceration ability of the bacterial strains in potato tubers, indicating that the Flp/Tad pilus-encoding gene cluster represents a novel virulence determinant in Pectobacterium. Soft-rot enterobacteria in the genera Pectobacterium and Dickeya are of great agricultural importance, and an investigation of the virulence of these pathogens could facilitate improvements in agricultural practices, thus benefiting farmers, the potato industry and consumers.

AepA of Pectobacterium is not involved in the regulation of extracellular plant cell wall degrading enzymes production

Plant cell wall degrading enzymes (PCWDE) are the major virulence determinants in phytopathogenic Pectobacterium, and their production is controlled by many regulatory factors. In this study, we focus on the role of the AepA protein, which was previously described to be a global regulator of PCWDE production in Pectobacterium carotovorum (Murata et al. in Mol Plant Microbe Interact 4:239-246, 1991). Our results show that neither inactivation nor overexpression of aepA affects PCWDE production in either Pectobacterium atrosepticum SCRI1043 or Pectobacterium carotovorum subsp. carotovorum SCC3193. The previously published observation based on the overexpression of aepA could be explained by the presence of the adjacent regulatory rsmB gene in the constructs used. Our database searches indicated that AepA belongs to the YtcJ subfamily of amidohydrolases. YtcJ-like amidohydrolases are present in bacteria, archaea, plants and some fungi. Although AepA has 28% identity with the formamide deformylase NfdA in Arthrobacter pascens F164, AepA was unable to catalyze the degradation of NdfA-specific N-substituted formamides. We conclude that AepA is a putative aminohydrolase not involved in regulation of PCWDE production.

Extracellular secretion of Carocin S1 in Pectobacterium carotovorum subsp. carotovorum occurs via the type III secretion system integral to the bacterial flagellum

BACKGROUND

Pectobacterium carotovorum subsp. carotovorum is a phytopathogenic enterobacterium responsible for soft rot, a disease characterized by extensive maceration of the affected plant tissue. This species also produces two or more antibacterial substances called bacteriocins, which enhance its competitiveness against related rival species. However, the secretion mechanism for low-molecular-weight bacteriocin is still unknown.

RESULTS

A mutant (flhC::Tn5) that did not secrete the low-molecular-weight bacteriocin (LMWB), Carocin S1, was generated by Tn5 insertional mutagenesis. Sequence analysis indicated that this insertion disrupted open reading frame 2 (ORF2) and ORF3 of this strain. Deletion and rescue experiments indicated that ORF2 and ORF3 were both required for extracellular LMWB secretion. The ORF2 and ORF3 sequences showed high homology with the flhD and flhC gene sequences of Pectobacterium carotovorum subsp. atroseptica, Serratia marcescens, Yersinia enterocolitica, and Escherichia coli, indicating that they likely encoded key regulatory components of the type III flagella secretion system.

CONCLUSION

Thus, the extracellular export of Carocin S1 by Pectobacterium carotovorum subsp. carotovorum appears to utilize the type III secretion system integral to bacterial flagella.

Microarray profiling of host-extract-induced genes and characterization of the type VI secretion cluster in the potato pathogen Pectobacterium atrosepticum

Pectobacterium atrosepticum is a Gram-negative plant-pathogenic bacterium that rots potato stems and tubers. Microarray analysis was used to identify genes that were differentially expressed when host extracts were added to the growth medium. Potato extracts downregulated the expression of ribosomal genes and genes related to uptake and metabolism of nutrients, and upregulated genes needed for nitrate or phosphonate use. Some of the observed changes in gene expression in host-extract-induced cultures are similar to those during attachment of the bacterium to host tissues. Other responses indicated defence against toxic metabolites in the extract. Tuber extract induced a large gene cluster having homology to type VI secretion genes shown to be virulence determinants in many, but not all, animal and human pathogens. Two of the genes in the type VI cluster were found to be expressed during infection in potato tubers and stems, and mutants with knockouts of the corresponding genes had increased virulence on potato. One of the type VI secretion mutants was further characterized and found to grow to higher cell density in culture in the presence of host extract and to produce slightly more extracellular tissue-macerating enzymes than the wild-type strain. Analysis of secreted proteins showed that this type VI mutant was affected in the production of haemolysin-coregulated proteins (Hcps), which have been suggested to be secreted by the type VI pathway in other bacteria. The results suggest that the type VI secretion system of P. atrosepticum is needed for secretion of Hcps but not for virulence on its host plant, potato.

Distinguishing bacterial pathogens of potato using a genome-wide microarray approach

A set of 9676 probes was designed for the most harmful bacterial pathogens of potato and tested in a microarray format. Gene-specific probes could be designed for all genes of Pectobacterium atrosepticum, c. 50% of the genes of Streptomyces scabies and c. 30% of the genes of Clavibacter michiganensis ssp. sepedonicus utilizing the whole-genome sequence information available. For Streptomyces turgidiscabies, 226 probes were designed according to the sequences of a pathogenicity island containing important virulence genes. In addition, probes were designed for the virulence-associated nip (necrosis-inducing protein) genes of P. atrosepticum, P. carotovorum and Dickeya dadantii and for the intergenic spacer (IGS) sequences of the 16S-23S rRNA gene region. Ralstonia solanacearum was not included in the study, because it is a quarantine organism and is not presently found in Finland, but a few probes were also designed for this species. The probes contained on average 40 target-specific nucleotides and were synthesized on the array in situ, organized as eight sub-arrays with an identical set of probes which could be used for hybridization with different samples. All bacteria were readily distinguished using a single channel system for signal detection. Nearly all of the c. 1000 probes designed for C. michiganensis ssp. sepedonicus, c. 50% and 40% of the c. 4000 probes designed for the genes of S. scabies and P. atrosepticum, respectively, and over 100 probes for S. turgidiscabies showed significant signals only with the respective species. P. atrosepticum, P. carotovorum and Dickeya strains were all detected with 110 common probes. By contrast, the strains of these species were found to differ in their signal profiles. Probes targeting the IGS region and nip genes could be used to place strains of Dickeya to two groups, which correlated with differences in virulence. Taken together, the approach of using a custom-designed, genome-wide microarray provided a robust means for distinguishing the bacterial pathogens of potato.

Host-extract induced changes in the secretome of the plant pathogenic bacterium Pectobacterium atrosepticum

Pectobacterium atrosepticum is a Gram-negative plant pathogenic bacterium that causes rotting in potato stems and tubers. The secreted proteins of this pathogen were analyzed with proteomics from culture supernatant of cells grown in minimal medium supplemented with host extracts. More than 40 proteins were identified, among them known virulence determinants, such as pectic enzymes, metalloprotease, and virulence protein Svx, along with flagella proteins, GroEL and cyclophilin-type chaperones and several hypothetical proteins or proteins with unknown function. Some of the identified proteins may be involved in utilization of nutrients or transport of minerals. Northern and real-time RT-PCR analyses suggested that most of the proteins upregulated by plant extract were transcriptionally regulated. Among the identified proteins were VgrG and four homologs of hemolysin-coregulated proteins (Hcps). A mutant strain lacking one of the hcp genes was not affected in virulence, while a bacterial strain overexpressing the same gene was shown to have increased virulence, which suggests that these proteins may be new virulence determinants of P. atrosepticum. Comparison of the secretomes of wild type cells and hrcC mutant defective in Type III secretion suggested that the production of the identified proteins was independent of functional Type III secretion system.

Cloning and expression of the Erwinia carotovora subsp. carotovora gene encoding the low-molecular-weight bacteriocin carocin S1

The purpose of this study was to clone the carocin S1 gene and express it in a non-carocin-producing strain of Erwinia carotovora. A mutant, TH22-10, which produced a high-molecular-weight bacteriocin but not a low-molecular-weight bacteriocin, was obtained by Tn5 insertional mutagenesis using H-rif-8-2 (a spontaneous rifampin-resistant mutant of Erwinia carotovora subsp. carotovora 89-H-4). Using thermal asymmetric interlaced PCR, the DNA sequence from the Tn5 insertion site and the DNA sequence of the contiguous 2,280-bp region were determined. Two complete open reading frames (ORF), designated ORF2 and ORF3, were identified within the sequence fragment. ORF2 and ORF3 were identified with the carocin S1 genes, caroS1K (ORF2) and caroS1I (ORF3), which, respectively, encode a killing protein (CaroS1K) and an immunity protein (CaroS1I). These genes were homologous to the pyocin S3 gene and the pyocin AP41 gene. Carocin S1 was expressed in E. carotovora subsp. carotovora Ea1068 and replicated in TH22-10 but could not be expressed in Escherichia coli (JM101) because a consensus sequence resembling an SOS box was absent. A putative sequence similar to the consensus sequence for the E. coli cyclic AMP receptor protein binding site (-312 bp) was found upstream of the start codon. Production of this bacteriocin was also induced by glucose and lactose. The homology search results indicated that the carocin S1 gene (between bp 1078 and bp 1704) was homologous to the pyocin S3 and pyocin AP41 genes in Pseudomonas aeruginosa. These genes encode proteins with nuclease activity (domain 4). This study found that carocin S1 also has nuclease activity.

Genome sequence of the enterobacterial phytopathogen Erwinia carotovora subsp. atroseptica and characterization of virulence factors

The bacterial family Enterobacteriaceae is notable for its well studied human pathogens, including Salmonella, Yersinia, Shigella, and Escherichia spp. However, it also contains several plant pathogens. We report the genome sequence of a plant pathogenic enterobacterium, Erwinia carotovora subsp. atroseptica (Eca) strain SCRI1043, the causative agent of soft rot and blackleg potato diseases. Approximately 33% of Eca genes are not shared with sequenced enterobacterial human pathogens, including some predicted to facilitate unexpected metabolic traits, such as nitrogen fixation and opine catabolism. This proportion of genes also contains an overrepresentation of pathogenicity determinants, including possible horizontally acquired gene clusters for putative type IV secretion and polyketide phytotoxin synthesis. To investigate whether these gene clusters play a role in the disease process, an arrayed set of insertional mutants was generated, and mutations were identified. Plant bioassays showed that these mutants were significantly reduced in virulence, demonstrating both the presence of novel pathogenicity determinants in Eca, and the impact of functional genomics in expanding our understanding of phytopathogenicity in the Enterobacteriaceae.

Identification and cloning of an Erwinia carotovora subsp. carotovora bacteriocin regulator gene by insertional mutagenesis

Avirulent Erwinia carotovora subsp. carotovora CGE234-M403 produces two types of bacteriocin. For the purpose of cloning the bacteriocin genes of strain CGE234M403, a spontaneous rifampin-resistant mutant of this strain, M-rif-11-2, was isolated. By Tn5 insertional mutagenesis using M-rif-11-2, a mutant, TM01A01, which produces the high-molecular-weight bacteriocin but not the low-molecular-weight bacteriocin was obtained. By thermal asymmetric interlaced PCR, the DNA sequence from the Tn5 insertion site and the DNA sequence of a contiguous 1,280-bp region were determined. One complete open reading frame (ORF), designated ORF2, was identified within the sequenced fragment. The 3' end of another ORF, ORF1, was located upstream of ORF2. A noncoding region and a putative promoter were located between ORF1 and ORF2. Downstream from ORF2, the 5' end of another ORF (ORF3) was found. Deduction from the nucleotide sequence indicated that ORF2 encodes a protein of 99 amino acids, which showed high homology with Yersinia enterocolitica Yrp, a regulator of enterotoxin (Y-ST) production; Escherichia coli host factor 1, required for Qbeta-replicase; and Azorhizobium caulinodans NrfA, required for the expression of nifA. ORF2 was designated brg, bacteriocin regulator gene. A fragment containing ORF2 and its promoter was amplified and cloned into pBR322 and pHSG415r, and the recombinant plasmids, pBYL1 and pHYL1, were transferred into E. coli DH5. Plasmid pBYL1 was reisolated and transferred into the insertion mutant TM01A01. Transformants carrying the plasmid, which was reisolated and designated pBYL1, re-produced the low-molecular-weight bacteriocin.

Two Genomic Regions Involved in Catechol Siderophore Production by Erwinia carotovora

Two regions involved in catechol biosynthesis ( cbs ) of Erwinia carotovora W3C105 were cloned by functional complementation of Escherichia coli mutants that were deficient in the biosynthesis of the catechol siderophore enterobactin ( ent ). A 4.3-kb region of genomic DNA of E. carotovora complemented the entB402 mutation of E. coli. A second genomic region of 12.8 kb complemented entD, entC147, entE405 , and entA403 mutations of E. coli. Although functions encoded by catechol biosynthesis genes ( cbsA, cbsB, cbsC, cbsD , and cbsE ) of E. carotovora were interchangeable with those encoded by corresponding enterobactin biosynthesis genes ( entA, entB, entC, entD , and entE ), only cbsE hybridized to its functional counterpart ( entE ) in E. coli. The cbsEA region of E. carotovora W3C105 hybridized to genomic DNA of 21 diverse strains of E. carotovora but did not hybridize to that of a chrysobactin-producing strain of Erwinia chrysanthemi. Strains of E. carotovora fell into nine groups on the basis of sizes of restriction fragments that hybridized to the cbsEA region, indicating that catechol biosynthesis genes were highly polymorphic among strains of E. carotovora.

Molecular analysis of the major cellulase (CelV) of Erwinia carotovora: evidence for an evolutionary "mix-and-match" of enzyme domains

The structural gene for the major cellulase of Erwinia carotovora subspecies carotovora (Ecc) was isolated and expressed in Escherichia coli. Sequencing of the gene (celV) revealed a typical signal sequence and two functional domains in the enzyme; a catalytic domain linked by a short proline/threonine-rich linker to a cellulose-binding domain (CBD). The deduced amino acid sequence of the catalytic domain showed homology with cellulases of Family A, including enzymes from Bacillus spp. and Erwinia chrysanthemi CelZ, whereas the CBD showed homology with cellulases from several diverse families, supporting a "mix-and-match" hypothesis for evolution of this domain. Analysis of the substrate specificity of CelV showed it to be an endoglucanase with some exoglucanase activity. The pH optimum is about 7.0 and the temperature optimum about 42 degrees C. CelV is secreted by Ecc and by the taxonomically related Erwinia carotovora subspecies atroseptica (Eca) but not by E. coli. Overproduction of the enzyme from multicopy plasmids in Ecc appears to overload the secretory mechanism.

Characterization of the nucM gene coding for a nuclease of the phytopathogenic bacteria Erwinia chrysanthemi

The gene nucM encoding a nuclease was cloned from a genomic library of Erwinia chrysanthemi. The nucM gene was subcloned, and mutagenized by insertion of a uidA-KanR cartridge. This mutation was introduced by recombination into the Erwinia chrysanthemi chromosome. The nucM mutant lost NucM activity when tested on a DNA plate after 24 hours, but still possessed secondary weak nuclease activity. The nucleotide sequence of nucM was determined. It presents a 798 bp open reading frame, coding for a 266-amino-acid protein, with a predicted molecular mass of 29,910 Da. The deduced NucM protein shows 59% sequence identity with the DNase I precursor from Vibrio cholerae. It contains a typical leader sequence. Experiments of cell fractionation showed that NucM is periplasmic in E. chrysanthemi. The transcription start has been determined by S1 mapping. The -10 and -35 regions do not show homology with consensus sequence of the promoters recognized by sigma 70. In fact, the promoter seems to be dependent on the sigma 70, but the first transcription nucleotide is unusually far from the -10 region. nucM seems to be expressed constitutively.

High-affinity iron uptake systems present in Erwinia carotovora subsp. carotovora include the hydroxamate siderophore aerobactin

The phytopathogenic bacterium Erwinia carotovora subsp. carotovora W3C105 produced the hydroxamate siderophore aerobactin under iron-limiting conditions. A survey of 22 diverse strains of E. carotovora revealed that strain W3C105 alone produced aerobactin. The ferric-aerobactin receptor of strain W3C105 was an 80-kDa protein, identified by immunoblots of Sarkosyl-soluble proteins obtained from E. carotovora cells grown in iron-depleted medium and probed with antiserum raised against the 74-kDa ferric-aerobactin receptor encoded by the pColV-K30 plasmid of Escherichia coli. Genes determining aerobactin biosynthesis and uptake were localized to an 11.3-kb EcoRI-HindIII chromosomal fragment of strain W3C105. A 10-kb subclone of the fragment conferred on E. coli DH5 alpha both aerobactin biosynthesis and uptake, determined by cloacin DF13 sensitivity, the presence of the 80-kDa receptor protein, and iron-independent growth of E. coli clones. The aerobactin biosynthesis genes of E. carotovora W3C105 hybridized to those of the pColV-K30 plasmid of E. coli, but the restriction patterns of the aerobactin regions of E. coli and E. carotovora differed. Although the aerobactin region of enteric bacteria is commonly flanked by IS1-like sequences, IS1 sequences were not detected in the genomic DNA or the cloned aerobactin region of E. carotovora. E. coli DH5 alpha cells harboring cloned aerobactin biosynthesis genes from E. carotovora W3C105 produced greater quantities of aerobactin and the 80-kDa ferric-aerobactin receptor when grown in iron-limited than in iron-replete medium. Strain W3C105 grew on an iron-limited medium, whereas derivatives that lacked a functional aerobactin iron acquisition system did not grow on the medium. These results provide evidence for the occurrence and heterogeneity of aerobactin as a high-affinity iron uptake system of both clinical and phytopathogenic species of the Enterobacteriaceae. Although future studies may reveal a role for aerobactin in the virulence or ecology of strain W3C105, a functional aerobactin iron acquisition system is not necessary for the pathogenicity of E. carotovora.

Bacteriophage lambda-mediated transposon mutagenesis of phytopathogenic and epiphytic Erwinia species is strain dependent

Using transformation and conjugal mobilization, plasmids carrying the lamB gene of Escherichia coli were transferred to a range of Erwinia strains. The resultant strains were infected with lambda 467, and kanamycin resistant transductants were screened for various mutant phenotypes including auxotrophy and altered extracellular enzyme activities. Reversion analysis suggested that most mutant phenotypes were due to Tn5 insertion. The applicability of the techniques was highly strain dependent. However a rapid and simple route to mutant isolation was obtained, which could allow the use of other lambda-related genetic techniques in several important species which, to date, have not been genetically manipulated.

Expanded linkage map of Erwinia chrysanthemi strain 3937

In this paper we describe the chromosomal location of various loci in Erwinia chrysanthemi strain 3937. Auxotrophic markers were obtained by chemical mutagenesis, antibiotic resistances were isolated spontaneously and mutations in sugar utilization were obtained by means of Mu insertions. These markers were located on the genetic linkage map of strain 3937 by using a conjugative system mediated by RP4::mini-Mu plasmids which permitted transfer of genetic material from any point of origin. The location of these markers was compared to that of previously located mutations. Many genes involved in pectinolysis were also located on the E. chrysanthemi 3937 map. These results permitted us to present a new genetic map containing 61 markers distributed over 34 widely scattered loci on the chromosome. Some pairs of markers giving high cotransfer frequencies were tested for cotransduction mediated by the generalized transducing phage phi-EC2; nine cotransducing pairs were found. It appears that the chromosomal locations of many of these loci are quite different to those of the well-known enterobacterium Escherichia coli but seem similar to those described for other E. chrysanthemi strains.

Marker-exchange mutagenesis of a pectate lyase isozyme gene in Erwinia chrysanthemi

The phytopathogenic enterobacterium Erwinia chrysanthemi contains pel genes encoding several different isozymes of the plant-tissue-disintegrating enzyme pectate lyase (PL). The pelC gene, encoding an isozyme with an approximate isoelectric point of 8.0, was mutagenized by a three-step procedure involving (i) insertional inactivation of the cloned gene by ligation of a kan-containing BamHI fragment from pUC4K with a partial Sau3A digest of E. chrysanthemi pelC DNA in pBR322; (ii) mobilization of the pBR322 derivative from Escherichia coli to E. chrysanthemi by the helper plasmids R64drd11 and pLVC9; and (iii) exchange recombination of the pelC::kan mutation into the E. chrysanthemi chromosome by selection for kanamycin resistance in transconjugants cultured in phosphate-limited medium (which renders pBR322 unstable). The resulting E. chrysanthemi mutant was Kanr Amps, lacked pBR322 sequences, and was deficient in only one of the four major PL isozymes, PLc, as determined by activity-stained isoelectric-focusing polyacrylamide gels. The rates of PL induction and cell growth in a medium containing polygalacturonic acid as the sole carbon source were not significantly reduced in the mutant. No difference was detected in the ability of the mutant to macerate potato tuber tissue. The evidence suggests that this isozyme is not necessary for soft-rot pathogenesis.