Transposon mutagenesis of Erwinia using phage λ vectors

Bacteriophage host range evolution through engineered enrichment bias, exploiting heterologous surface receptor expression

Research on the initial phage-host interaction has been conducted on a limited repertoire of phages and their cognate receptors, such as phage λ and the Escherichia coli LamB (EcLamB) protein. Apart from phage λ, little is known about other phages that target EcLamB. Here, we developed a simple method for isolating novel environmental phages in a predictable way, i.e. isolating phages that target a particular receptor(s) of a bacterium, in this case, the EcLamB protein. A plasmid (pMUT13) encoding the EcLamB porin was transferred into three different enterobacterial genera. By enrichment with these engineered bacteria, a number of phages (ZZ phages) that targeted EcLamB were easily isolated from the environment. Interestingly, although EcLamB-dependent in their recombinant heterologous hosts, these newly isolated ZZ phages also targeted OmpC as an alternative receptor when infecting E. coli. Moreover, the phage host range was readily extended within three different bacterial genera with heterologously expressed EcLamB. Unlike phage λ, which is a member of the Siphoviridae family, these newly isolated EcLamB-dependent phages were more commonly members of the Myoviridae family, based on transmission electron microscopy and genomic sequences. Modifications of this convenient and efficient phage enrichment method could be useful for the discovery of novel phages.

Quorum sensing-controlled Evr regulates a conserved cryptic pigment biosynthetic cluster and a novel phenomycin-like locus in the plant pathogen, Pectobacterium carotovorum

Pectobacterium carotovorum SCRI193 is a phytopathogenic Gram-negative bacterium. In this study, we have identified a novel cryptic pigment biosynthetic locus in P. carotovorum SCRI193 which we have called the Pectobacterium orange pigment (pop) cluster. The pop cluster is flanked by two tRNA genes and contains genes that encode non-ribosomal peptide synthases and polyketide synthase and produces a negatively charged polar orange pigment. Orange pigment production is activated when an adjacent transcriptional activator sharing sequence similarity with the Erwinia virulence regulator (Evr) is overexpressed. Evr was shown to positively activate its own transcription and that of the pigment biosynthetic genes and an unlinked locus encoding a phenomycin homologue. In addition, the expression of Evr and orange pigment production was shown to be regulated by N-(3-oxohexanoyl)-HSL (OHHL) quorum sensing and have a virulence phenotype in potato. Finally, by comparative genomics and Southern blotting we demonstrate that this pigment biosynthetic cluster is present in multiple P. carotovorum spp., Pectobacterium brasiliensis 1692 and a truncated version of the cluster is present in Pectobacterium atrosepticum. The conserved nature of this cluster in P. carotovorum and P. brasiliensis suggests that the pop cluster has an important function in these broad-host-range soft rotting bacteria, which is no longer required in the narrow-host-range P. atrosepticum SCRI1043.

Erwinia carotovora subspecies produce duplicate variants of ExpR, LuxR homologs that activate rsmA transcription but differ in their interactions with N-acylhomoserine lactone signals

The N-acylhomoserine lactone (AHL) signaling system comprises a producing system that includes acylhomoserine synthase (AhlI, a LuxI homolog) and a receptor, generally a LuxR homolog. AHL controls exoprotein production in Erwinia carotovora and consequently the virulence for plants. In previous studies we showed that ExpR, a LuxR homolog, is an AHL receptor and that it activates transcription of rsmA, the gene encoding an RNA binding protein which is a global negative regulator of exoproteins and secondary metabolites. An unusual finding was that the transcriptional activity of ExpR was neutralized by AHL. We subsequently determined that the genomes of most strains of E. carotovora subspecies tested possess two copies of the expR gene: expR1, which was previously studied, and expR2, which was the focus of this study. Comparative analysis of the two ExpR variants of E. carotovora subsp. carotovora showed that while both variants activated rsmA transcription, there were significant differences in the patterns of their AHL interactions, the rsmA sequences to which they bound, and their relative efficiencies of activation of rsmA transcription. An ExpR2- mutant produced high levels of exoproteins and reduced levels of RsmA in the absence of AHL. This contrasts with the almost complete inhibition of exoprotein production and the high levels of RsmA production in an AhlI- mutant that was ExpR1-. Our results suggest that ExpR2 activity is responsible for regulating exoprotein production primarily by modulating the levels of an RNA binding protein.

Comparative analysis of two classes of quorum-sensing signaling systems that control production of extracellular proteins and secondary metabolites in Erwinia carotovora subspecies

In Erwinia carotovora subspecies, N-acyl homoserine lactone (AHL) controls the expression of various traits, including extracellular enzyme/protein production and pathogenicity. We report here that E. carotovora subspecies possess two classes of quorum-sensing signaling systems defined by the nature of the major AHL analog produced as well as structural and functional characteristics of AHL synthase (AhlI) and AHL receptor (ExpR). Class I strains represented by E. carotovora subsp. atroseptica strain Eca12 and E. carotovora subsp. carotovora strains EC153 and SCC3193 produce 3-oxo-C8-HL (N-3-oxooctanoyl-l-homoserine lactone) as the major AHL analog as well as low but detectable levels of 3-oxo-C6-HL (N-3-oxohexanoyl-l-homoserine lactone). In contrast, the members of class II (i.e., E. carotovora subsp. betavasculorum strain Ecb168 and E. carotovora subsp. carotovora strains Ecc71 and SCRI193) produce 3-oxo-C6-HL as the major analog. ExpR species of both classes activate rsmA (Rsm, repressor of secondary metabolites) transcription and bind rsmA DNA. Gel mobility shift assays with maltose-binding protein (MBP)-ExpR(71) and MBP-ExpR(153) fusion proteins show that both bind a 20-mer sequence present in rsmA. The two ExpR functions (i.e., expR-mediated activation of rsmA expression and ExpR binding with rsmA DNA) are inhibited by AHL. The AHL effects are remarkably specific in that expR effect of EC153, a strain belonging to class I, is counteracted by 3-oxo-C8-HL but not by 3-oxo-C6-HL. Conversely, the expR effect of Ecc71, a strain belonging to class II, is neutralized by 3-oxo-C6-HL but not by 3-oxo-C8-HL. The AHL responses correlated with expR-mediated inhibition of exoprotein and secondary metabolite production.

Effects of the two-component system comprising GacA and GacS of Erwinia carotovora subsp. carotovora on the production of global regulatory rsmB RNA, extracellular enzymes, and harpinEcc

Posttranscriptional regulation mediated by the regulator of secondary metabolites (RSM) RsmA-rsmB pair is the most important factor in the expression of genes for extracellular enzymes and HarpinEcc in Erwinia carotovora subsp. carotovora. RsmA is a small RNA-binding protein, which acts by lowering the half-life of a mRNA species. rsmB specifies an untranslated regulatory RNA and neutralizes the RsmA effect. It has been speculated that GacA-GacS, members of a two-component system, may affect gene expression via RsmA. Because expA, a gacA homolog, and expS (or rpfA), a gacS homolog, have been identified in E. carotovora subsp. carotovora, we examined the effects of these gacA and gacS homologs on the expression of rsmA, rsmB, and an assortment of exoprotein genes. The gacA gene of E. carotovora subsp. carotovora strain 71 stimulated transcription of genes for several extracellular enzymes (pel-1, a pectate lyase gene; peh-1, a polygalacturonase gene; and celV, a cellulase gene), hrpNEcc (an E. carotovora subsp. carotovora gene specifying the elicitor of hypersensitive reaction), and rsmB in GacA+ and GacS+ E. carotovora subsp. carotovora strains. Similarly, the E. carotovora subsp. carotovora gacA gene stimulated csrB (rsmB) transcription in Escherichia coli. A GacS- mutant of E. carotovora subsp. carotovora strain AH2 and a GacA- mutant of E. carotovora subsp. carotovora strain Ecc71 compared with their parent strains produced very low levels of rsmB, pel-1, peh-1, celV, and hrpNEcc transcripts but produced similar levels of rsmA RNA and RsmA protein as well as transcripts of hyperproduction of extracellular enzymes (Hex) hexA, kdgR (repressor of genes for uronate and pectate catabolism), rsmC, and rpoS (gene for Sigma-S, an alternate Sigma factor). The levels of rsmB, pel-1, peh-1, celV, and hrpNEcc transcripts as well as production of pectate lyase, polygalacturonase, cellulase, protease, and HarpinEcc proteins were stimulated in GacS- and GacA- mutants by GacS+ or GacA+ plasmids, respectively. The GacA effect on exoenzyme genes and hrpNEcc was abrogated in E. carotovora subsp. carotovora mutants deficient in RsmA and RsmC or RsmA, RsmC, and rsmB RNA. The expression of lacZ transcriptional fusions of rsmB of Erwinia amylovora and Erwinia herbicola pv. gypsophilae was markedly reduced in a GacA- and a GacS- mutant of Pseudomonas syringae pv. syringae. Southern blot hybridization revealed the presence of gacA and gacS homologs in all tested strains of soft-rotting Erwinia spp. and several nonsoft-rotting Erwinia species such as E. amylovora, E. rhapontici, E. herbicola, E. stewartii, and E. herbicola pv. gypsophilae. These findings establish that the GacA-GacS system controls transcription of rsmB of E. carotovora subsp. carotovora, E. amylovora, and E. herbicola pv. gypsophilae and support the hypothesis that the effects of this two-component system on extracellular protein production in E. carotovora subsp. carotovora is mediated, at least in part, via the levels of rsmB transcripts.

hexA of Erwinia carotovora ssp. carotovora strain Ecc71 negatively regulates production of RpoS and rsmB RNA, a global regulator of extracellular proteins, plant virulence and the quorum-sensing signal, N-(3-oxohexanoyl)-L-homoserine lactone

The soft-rotting bacterium, Erwinia carotovora ssp. carotovora (E. c. carotovora), produces an array of extracellular enzymes (= exoenzymes), including pectate lyase (Pel), polygalacturonase (Peh), cellulase (Cel) and protease (Prt), as well as HarpinEcc, the elicitor of hypersensitive reaction (HR). The production of these exoenzymes and HarpinEcc responds to plant products and the quorum-sensing signal [N-(3-oxohexanoyl)-L-homoserine lactone; OHL] and is subject to both transcriptional and post-transcriptional regulation. hexA of E. c. carotovora strain Ecc71 (hereafter hexA71), like that of another E. c. carotovora strain, negatively controls the production of exoenzymes, OHL and virulence in E. c. carotovora strain Ecc71. In addition to exoenzymes, HexA71 negatively regulates the expression of hrpNEcc, the structural gene for HarpinEcc. Exoenzyme overproduction is abolished by OHL deficiency in a HexA- and Ohll- double mutant, indicating that HexA and OHL are components of a common regulatory pathway controlling exoenzyme production. HexA71 negatively affects RpoS, as the levels of this alternative sigma factor are higher in the HexA- mutant than in the HexA+ strain. However, a HexA- and RpoS double mutant produces higher levels of exoenzymes and transcripts of pel-1, peh-1 and celVgenes than the HexA- and RpoS+ parent. Thus, the elevated levels of RpoS protein in the HexA- mutant do not account for exoenzyme overproduction. The following evidence associates for the first time the phenotypic changes in the HexA mutant to overproduction of rsmB RNA, a global regulator of exoenzymes, HarpinEcc, OHL and secondary metabolites. Analyses of rsmB transcripts and expression of an rsmB-lacZoperon fusion in E. c. carotovora strain Ecc71 revealed that HexA71 negatively regulates transcription of rsmB. Multiple copies of hexA71+ DNA suppress various phenotypes, including exoenzyme production in E. c. carotovora strain Ecc71, and concomitantly inhibit the production of rsmB, pel-1, peh-1, celV and hrpNEcc transcripts. Multiple copies of rsmB+ DNA, on the other hand, stimulate exoenzyme production by relieving the negative effects of a chromosomal copy of hexA+. The occurrence of hexA homologues and the negative effect of the dosage of hexA71 DNA on rsmB transcripts were also detected in other E. c. carotovora strains as well as Erwinia carotovora atroseptica and Erwinia carotovora betavasculorum. Extrapolating from the findings with LrhA, the Escherichia coli homologue of HexA, and the presence of sprE homologues in E. carotovora subspecies, we propose that HexA71 controls several regulatory pathways in E. carotovora including rsmB transcription and the production of SprEEcc which, in turn, affects RpoS levels. A model is presented that integrates the findings presented here and our current knowledge of the major regulatory network that controls exoprotein production in soft-rotting Erwinia carotovora subspecies.

cDNA-AFLP analysis of differential gene expression in the prokaryotic plant pathogen Erwinia carotovora

For studies of differential gene expression in prokaryotes, methods for synthesizing representative cDNA populations are required. Here, a technique is described for the synthesis of cDNA from the potato pathogens Erwinia carotovora subsp. atroseptica (Eca) and Erwinia carotovora subsp. carotovora (Ecc) using a combination of short oligonucleotide (11-mer) primers that were known to anneal to conserved sequences in the 3' regions of enterobacterial genes. Specific PCR amplifications with primers designed to anneal to 14 known genes from either Eca or Ecc revealed the presence of the corresponding transcripts in cDNA, suggesting that the cDNA represented a broad genomic coverage. cDNA-amplified fragment length polymorphism (cDNA-AFLP) was used to identify differentially expressed genes in Eca, including one that shows significant similarity, at the protein level, to an avirulence gene from Xanthomonas campestris pv. raphani. Northern analysis was used to confirm that differentially amplified cDNA fragments were derived from differentially expressed genes. This is the first report of the use of cDNA-AFLP to study differential gene expression in prokaryotes.

rsmC of the soft-rotting bacterium Erwinia carotovora subsp. carotovora negatively controls extracellular enzyme and harpin(Ecc) production and virulence by modulating levels of regulatory RNA (rsmB) and RNA-binding protein (RsmA)

Previous studies have shown that the production of extracellular enzymes (pectate lyase [Pel], polygalacturonase [Peh], cellulase [Cel], and protease [Prt]) and harpin(Ecc) (the elicitor of hypersensitive reaction) in Erwinia carotovora subsp. carotovora is regulated by RsmA, an RNA-binding protein, and rsmB, a regulatory RNA (Rsm stands for regulator of secondary metabolites) (Y. Liu et al., Mol. Microbiol. 29:219-234, 1998). We have cloned and characterized a novel regulatory gene, rsmC, that activates RsmA production and represses extracellular enzyme and harpin(Ecc) production, rsmB transcription, and virulence in E. carotovora subsp. carotovora. In an rsmC knockout mutant of E. carotovora subsp. carotovora Ecc71 carrying the chromosomal copy of the wild-type rsmA(+) allele, the basal levels of Pel, Peh, Cel, Prt, and harpin(Ecc) as well as the amounts of rsmB, pel-1, peh-1, celV, and hrpN(Ecc) transcripts are high, whereas the levels of rsmA transcripts and RsmA protein are low. Furthermore, the expression of an rsmA-lacZ gene fusion is lower in the RsmC(-) mutant than in the RsmC(+) parent. Conversely, the expression of an rsmB-lacZ operon fusion is higher in the RsmC(-) mutant than in the RsmC(+) parent. These observations establish that RsmC negatively regulates rsmB transcription but positively affects RsmA production. Indeed, comparative studies with an RsmC(-) mutant, an RsmA(-) mutant, and an RsmA(-) RsmC(-) double mutant have revealed that the negative effects on exoprotein production and virulence are due to the cumulative regulatory effects of RsmC on rsmA and rsmB. Exoprotein production by the RsmC(-) mutant is partially dependent on the quorum sensing signal, N-(3-oxohexanoyl)-L-homoserine lactone. Southern blot data and analysis of PCR products disclosed the presence of rsmC sequences in E. carotovora subsp. atroseptica, E. carotovora subsp. betavasculorum, and E. carotovora subsp. carotovora. These findings collectively support the idea that rsmA and rsmB expression in these plant pathogenic Erwinia species is controlled by RsmC or a functional homolog of RsmC.

Conditional mutations in OutE and OutL block exoenzyme secretion across the Erwinia carotovora outer membrane

The phytopathogen Erwinia carotovora subspecies carotovora secretes pectinases and cellulase via the general secretory pathway, a process requiring at least 13 proteins encoded by the out gene cluster. By exploiting delta::Tn5, a generalised transducing phage (psi KP) and localised mutagenesis of the out gene cluster, we have produced a histidine auxotroph and 19 new secretory mutants, including two (HJN1003 and HJN1004) which were conditional (temperature sensitive) for secretion. All of the mutants accumulated pectinases and cellulase in the periplasm, but in the case of HJN1003 and HJN1004, only at the restrictive temperature. HJN1003 and HJN1004 were complemented by the outE and outL wild-type genes, respectively, and both mutant alleles were cloned and sequenced to reveal single missense substitutions. HJN1003 carries an Arg166 to His alteration in OutE and HJN1004 carries a Pro159 to Leu alteration in OutL. Topology mapping of OutL using a beta-lactamase probe confirmed that OutL is a type II bitopic trans-inner membrane protein and that the mutated Pro159 residue in HJN1004 is located in the cytoplasmic domain of OutL. Hence, the secretion of exoenzymes across the outer membrane is critically dependent on the conformation of secretory components located at the cytoplasmic face of the inner membrane.

Generalized transduction in the potato blackleg pathogen Erwinia carotovora subsp. atroseptica by bacteriophage M1

Summary: Using enrichment methods, a new bacteriophage (M1) was isolated, which is capable of generalized transduction in Erwinia carotovora subsp. atroseptica (Eca) strain SCRI1043. M1 is probably a virulent phage and contains double-stranded DNA of approximately 43 kb. Transduction frequencies for a number of chromosomal markers and plasmid pHCP2 were established, and conditions for transduction optimized. UV irradiation of the lysates prior to transduction enhanced the transduction frequency. M1 infected over 25% of Eca strains tested and so may be useful both for the genetic analysis of a number of Eca isolates and for the transductional transfer of selectable markers between strains.

Global regulation in Erwinia species by Erwinia carotovora rsmA, a homologue of Escherichia coli csrA: repression of secondary metabolites, pathogenicity and hypersensitive reaction

Our previous studies revealed that rsmA of Erwinia carotovora subsp. carotovora strain 71 suppressed the synthesis of the cell density (quorum) sensing signal N-(3-oxohexanoyl)-L-homoserine lactone, the production of extracellular enzymes and tissue macerating ability in soft-rotting Erwinia species and that homologues of this negative regulator gene were present in other Erwinia species. Northern blot data presented here demonstrate that rsmA and rsmA-like genes are also expressed in soft-rotting and non-soft-rotting Erwinia spp. such as E. amylovora, E. carotovora subsp. atroseptica, E. carotovora subsp. betavasculorum, E. carotovora subsp. carotovora, E. chrysanthemi, E. herbicola and E. stewartii. A low-copy plasmid carrying rsmA of E. carotovora subsp. carotovora strain 71 caused suppression of antibiotic production in E. carotovora subsp. betavasculorum, flagellum formation in E. carotovora subsp. carotovora, carotenoid production in E. herbicola and E. stewartii, and indigoidine production in E. chrysanthemi. In E. amylovora, rsmA of E. carotovora subsp. carotovora suppressed the elicitation of the hypersensitive reaction in tobacco leaves and the production of disease symptoms in apple shoots, in addition to repressing motility and extracellular polysaccharide production. We conclude that rsmA homologues function as global regulators of secondary metabolic pathways as well as factors controlling host interaction of Erwinia species.

Identification of a global repressor gene, rsmA, of Erwinia carotovora subsp. carotovora that controls extracellular enzymes, N-(3-oxohexanoyl)-L-homoserine lactone, and pathogenicity in soft-rotting Erwinia spp

The production of extracellular enzymes such as pectate lyase (Pel), polygalacturonase (Peh), cellulase (Cel), and protease (Prt) is activated by the cell density (quorum)-sensing signal, N-(3-oxohexanoyl)-L-homoserine lactone (HSL); plant signals; and aep genes during postexponential growth of Erwinia carotovora subsp. carotovora 71. Studies with mutants of E. carotovora subsp. carotovora 71 derepressed in exoenzyme production led to the identification of a negative regulator gene, rsmA (rsm, repressor of secondary metabolites). Nucleotide sequencing, transcript assays, and protein analysis established that a 183-bp open reading frame encodes the 6.8-kDa RsmA. rsmA has extensive homology with the csrA gene of Escherichia coli, which specifies a negative regulator of carbon storage. Moreover, the suppression of glycogen synthesis in E. coli by rsmA indicates that the Erwinia gene is functionally similar to csrA. Southern hybridizations revealed the presence of rsmA homologs in soft-rotting and non-soft-rotting Erwinia spp. and in other enterobacteria such as Enterobacter aerogenes, E. coli, Salmonella typhimurium, Shigella flexneri, Serratia marcescens, and Yersinia pseudotuberculosis. rsmA suppresses production of Pel, Peh, Cel, and Prt, plant pathogenicity, and synthesis of HSL in E. carotovora subsp. atroseptica, E. carotovora subsp. betavasculorum, E. carotovora subsp. carotovora, and E. chrysanthemi. In the E. carotovora subsp. carotovora 71, rsmA reduces the levels of transcripts of hslI, a luxI homolog required for HSL biosynthesis. This specific effect and the previous finding that HSL is required for extracellular enzyme production and pathogenicity in soft-rotting Erwinia spp. support the hypothesis that rsmA controls these traits by modulating the levels of the cell density (quorum)-sensing signal.

Regulation of the production of extracellular pectinase, cellulase, and protease in the soft rot bacterium Erwinia carotovora subsp. carotovora: evidence that aepH of E. carotovora subsp. carotovora 71 activates gene expression in E. carotovora subsp. carotovora, E. carotovora subsp. atroseptica, and Escherichia coli

The production of pectolytic enzymes (pectate lyase [Pel] and polygalacturonase [Peh]), cellulase (Cel), and protease (Prt) is activated in the soft rot bacterium Erwinia carotovora subsp. carotovora by aepA (activator of extracellular protein production) and celery extract (Y. Liu, H. Murata, A. Chatterjee, and A. K. Chatterjee, Mol. Plant-Microbe Interact. 6:299-308, 1993). We recently isolated a new class of mutants of strain E. carotovora subsp. carotovora 71 which overproduces Pel, Peh, Cel, and Prt. From the overproducing strain AC5034, we identified an activator locus, designated aepH*, which stimulated Pel, Peh, Cel, and Prt production in E. carotovora subsp. carotovora 71 or its derivatives. The nucleotide sequence of the aepH* DNA segment revealed an open reading frame of 141 bp that could encode a small (5.45-kDa) highly basic (pI 11.7) protein of 47 amino acid residues. Analyses of deletions and MudI insertions indicated that the activator function required the 508-bp DNA segment which contains this open reading frame. The wild-type locus, aepH+, is localized within a DNA segment upstream of aepA. An AepH- strain constructed by exchanging aepH+ with aepH*::MudI was deficient in Pel, Peh, Cel, and Prt production; exoenzyme production was restored upon the introduction of a plasmid carrying aepH+ or aepH*. Plasmids carrying either aepH+ or aepH* activated the production of Pel-1, Peh-1, and Cel in Escherichia coli HB101 carrying the cognate genes. The aepH effect in E. coli was due to the activation of transcription, as indicated by assays of pel-1 and peh-1 mRNAs. The aepH+ and aepH* plasmids also stimulated Pel, Peh, Cel, and Prt production in other wild-type E. carotovora subsp. carotovora strains as well as in E. carotovora subsp. atroseptica. Although the stimulatory effect was generally more pronounced with aepH* than with aepH+, the extent of activation in the wild-type strains depended upon the bacterial strain and the growth medium. Southern blot hybridization revealed the presence of aepH homologs in E. carotovora subsp. carotovora and E. carotovora subsp. atroseptica, and provided physical evidence for linkage between aepA and aepH homologs in genomes of these bacteria. We conclude that aepH-mediated activation of exoprotein gene expression is a feature common to most strains of E. carotovora.

A pleiotropic reduced virulence (Rvi-) mutant of Erwinia carotovora subspecies atroseptica is defective in flagella assembly proteins that are conserved in plant and animal bacterial pathogens

Erwinia carotovora subsp. atroseptica was mutagenized and assayed for virulence in planta. Those mutants which exhibited reduced virulence (Rvi-) were assayed for growth rate, auxotrophy and extracellular enzyme secretion and seven mutants were found to be wild type for all of these phenotypes. When screened for other phenotypes, two were found to be non-motile. One mutant was complemented for motility by a heterologous gene library. A 2.7kb XmaIII-ClaI complementing fragment was sequenced and the gene products were found to have similarity to flagella biosynthesis gene products from several bacteria. Further similarity was found to a pathogenicity protein from the plant pathogen Xanthomonas campestris pv. glycines and to the Spa pathogenicity proteins of the human pathogen Shigella flexneri, which are involved in the surface presentation of antigens. These studies highlight the emergence of common themes in the molecular strategies employed by both plant and animal bacterial pathogens for the targeting of proteins involved in the elaboration of disease.

Molecular cloning and characterization of 13 out genes from Erwinia carotovora subspecies carotovora: genes encoding members of a general secretion pathway (GSP) widespread in gram-negative bacteria

The chemical mutagen ethylmethanesulphonate (EMS) has been used to generate mutants of Erwinia carotovora subspecies carotovora which are defective in the secretion of pectinases (Pel) and cellulases (Cel) but unaltered for protease (Prt) secretion. Such mutants, called Out-, still synthesize Pel and Cel but these enzymes accumulate within the periplasm. Cosmid clones carrying wild-type E. carotovora ssp. carotovora DNA, identified by their ability to restore the Out+ phenotype when transferred to some Out- mutants, were classified into six complementation groups using cosmids and cosmid derivatives. Analysis of the nucleotide sequence of a 12.7 kb DNA fragment, encompassing complementing cosmid inserts, revealed a coding capacity for 13 potential open reading frames (ORFs), and these were designated outC-outO. Some of the out gene products were visualized using a T7 gene 10 expression system. The predicted Out proteins are highly similar to components of extracellular enzyme secretion systems from a diverse range of eubacteria including Erwinia chrysanthemi, Klebsiella oxytoca, Aeromonas hydrophila, Pseudomonas aeruginosa and Xanthomonas campestris. Lower levels of similarity exist between Ecc Out proteins and components of macromolecular trafficking systems from Bacillus subtilis, Haemophilus influenzae, Agrobacterium tumefaciens, Yersinia pestis and a protein involved in the morphogenesis of filamentous bacteriophages such as M13.

TnblaM: a transposon for directly tagging bacterial genes encoding cell envelope and secreted proteins

A transposon, TnblaM, designed for the direct selection of bacterial mutants with insertions in genes encoding cell envelope and secreted proteins, was constructed and subcloned into plasmid and bacteriophage lambda delivery vectors. TnblaM is a spectinomycin-resistant derivative of Tn5 with an unexpressed open reading frame encoding mature beta-lactamase (BlaM) at its left end. Therefore, when it inserts into genes in the correct orientation and reading frame, gene fusions encoding hybrid proteins are generated. By introducing TnblaM into bacterial cells and selecting ampicillin-resistant (ApR) colonies, the subset of isolates producing extracytoplasmic BlaM, and hence containing TnblaM inserted in genes encoding secreted proteins and cell envelope proteins, can be directly selected. TnblaM, like TnphoA, can therefore be used to preferentially mutagenise genes encoding extracytoplasmic proteins, but it has the advantage over TnphoA that the desired mutants can be isolated by direct selection (as ApR colonies) rather than by phenotypic screening. Isolates in which TnblaM occupies sites in the chromosome from which it can transpose at high frequency are readily identifiable, and constitute TnblaM donors, with which to simply and efficiently generate rare types of insertion mutants. Moreover, the ApR selection that is used with TnblaM can be fine-tuned to obtain blaM fusions to poorly or well-expressed genes.

Characterization of transposon insertion out- mutants of Erwinia carotovora subsp. carotovora defective in enzyme export and of a DNA segment that complements out mutations in E. carotovora subsp. carotovora, E. carotovora subsp. atroseptica, and Erwinia chrysanthemi

Soft-rotting Erwinia spp. export degradative enzymes to the cell exterior (Out+), a process contributing to their ability to macerate plant tissues. Transposon (Tn5, Tn10, Tn10-lacZ) insertion Out- mutants were obtained in Erwinia carotovora subsp. carotovora 71 by using plasmid and bacteriophage lambda delivery systems. In these mutants, pectate lyases, polygalacturonase, and cellulase, which are normally excreted into the growth medium, accumulated in the periplasm. However, localization of the extracellular protease was not affected. The Out- mutants were impaired in their ability to macerate potato tuber tissue. Out+ clones were identified in a cosmid library of E. carotovora subsp. carotovora 71 by their ability to complement mutants. Localization of cyclic phosphodiesterase in the periplasm indicated that the Out+ plasmids did not cause lysis or a nonspecific protein release. The Out+ derivatives of the E. carotovora subsp. carotovora 71 mutants regained the ability to macerate potato tuber tissue. Our data indicate that a cluster of several genes is required for the Out+ phenotype. While one plasmid, pAKC260, restored the Out+ phenotype in each of the 31 mutants of E. carotovora subsp. carotovora, E. carotovora subsp. atroseptica, and Erwinia chrysanthemi, it failed to render Escherichia coli export proficient. Homologs of E. carotovora subsp. carotovora 71 out DNA were detected by Southern hybridizations in subspecies of E. carotovora under high-stringency conditions. In contrast, E. chrysanthemi sequences bearing homology to the E. carotovora subsp. carotovora 71 out DNA were detectable only under low-stringency hybridization. Thus, although the out genes are functional in these two soft-rotting bacterial groups, the genes appear to have diverged.

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.

Isolation and characterisation of transposon-induced mutants of Erwinia carotovora subsp. atroseptica exhibiting reduced virulence

The blackleg pathogen Erwinia carotovora subsp. atroseptica (Eca) causes an economically important disease of potatoes. We selected a genetically amenable Eca strain for the genetic analysis of virulence. Tn5 mutagenesis was used to generate nine mutants which exhibited reduced virulence (Rvi-) of strain SCRI1043. Following physiological characterisation, mutants were divided into three classes: (1) auxotrophs; (2) extracellular enzyme mutants; and (3) a growth rate mutant. The isolation of these Rvi- mutants has allowed us to consider some factors that affect Eca virulence.

Use of TnphoA to enrich for extracellular enzyme mutants of Erwinia carotovora subspecies carotovora

Soft rot Erwinia species secrete a range of enzymes into the extracellular environment. Therefore, the genetically amenable Erwinia system is a useful model for the study of protein secretion by Gram-negative bacteria. We have used a lambda-sensitive derivative of Erwinia carotovora subspecies carotovora (Ecc) and the transposon TnphoA, to isolate a range of extracellular enzyme mutants. The use of TnphoA provides an enrichment for extracellular enzyme mutants over other transposon-based systems. In these mutants, the alkaline phosphatase activity of the hybrid protein is found in the periplasm, and is under the control of the Ecc promoters. Three TnphoA-induced extracellular enzyme mutants were studied in detail. One proved to be an enzyme structural gene mutant, whilst the other two appeared to be secretory mutants.

Use of lambda vehicles to isolate ompC-lacZ gene fusions in Salmonella typhimurium LT2

A novel plasmid vector, pAMH70 carrying both the lamB and nusA genes of Escherichia coli K12 was constructed. Introduction of this plasmid into Salmonella typhimurium LT2 renders this bacterium both sensitive to lambda adsorption and able to sustain growth and lysogenization by lambda. Using this strain as a recipient, stable gene fusions to the gene encoding a major outer membrane porin protein OmpC, were constructed with a lambda vehicle lambda placMu. To confirm the actual site of fusions they were genetically mapped and transducing phages carrying the ompC-lacZ fusion were isolated and relysogenized. The fusions were also shown to be to ompC by their regulatory properties.