Secretion of cellulases inErwinia chrysanthemiandE. carotovorain species-specific

Population Structure and Biodiversity of Pectobacterium parmentieri Isolated from Potato Fields in Temperate Climate

Pectobacterium parmentieri (formerly Pectobacterium wasabiae) is a newly established species of pectinolytic plant-pathogenic bacteria responsible for the symptoms of soft rot and blackleg on potato. In this work, we describe biodiversity and the population structure of P. parmentieri strains isolated during two consecutive growing seasons from the seed potato fields in Poland. About 450 samples of diseased potato tubers, potato plants, or accompanying weeds were collected throughout the country and tested for the presence of P. parmentieri by molecular identification methods. We found that P. parmentieri strains commonly occur in almost all regions of Poland. Furthermore, these isolates constituted significant fraction of pectinolytic bacteria from seed potato fields because 16% (2013) and 13% (2014) of the analyzed plant samples were infected with P. parmentieri. Subsequently, a detailed characterization of the obtained strains was conducted basing on repetitive sequences profiling, recA-gene-based phylogeny, and phenotypic features. By applying repetitive extragenic palindromic sequence-based polymerase chain reaction (REP-PCR), we revealed the presence of five distinct genomic profiles among P. parmentieri strains, with profile I being the most abundant (approximately 44%). The performed recA gene-based phylogenetic analysis divided P. parmentieri isolates into two distinct clades, although the strains originating from different years did not group separately. Evaluation of the phenotypic traits playing crucial roles for the virulence of pectinolytic bacteria (namely, pectinase, cellulase and protease activities, and siderophore production, in addition to potato tissue maceration, swimming, and swarming motility) indicated some differences among the characterized strains. To the best of our knowledge, this is the first study that describes biodiversity and the population structure of P. parmentieri isolated in two growing seasons under temperate climate conditions and, hence, illustrates high heterogeneity within this species.

Identification of an Extracellular Endoglucanase That Is Required for Full Virulence in Xanthomonas citri subsp. citri

Xanthomonas citri subsp. citri causes citrus canker disease, which is characterized by the formation of water-soaked lesions, white or yellow spongy pustules and brown corky canker. In this work, we report the contribution of extracellular endoglucanase to canker development during infection. The ectopic expression of nine putative cellulases in Escherichia coli indicated that two endoglucanases, BglC3 and EngXCA, show carboxymethyl cellulase activity. Both bglC3 and engXCA genes were transcribed in X. citri subsp. citri, however, only BglC3 protein was detected outside the cell in western blot analysis. The deletion of bglC3 gene resulted in complete loss of extracellular carboxymethyl cellulase activity and delayed the onset of canker symptoms in both infiltration- and wound-inoculation assays. When growing in plant tissue, the cell density of bglC3 mutant was lower than that of the wild type. Our data demonstrated that BglC3 is an extracellular endoglucanase required for the full virulence of X. citri subsp. citri.

Erwinia chrysanthemi iron metabolism: the unexpected implication of the inner membrane platform within the type II secretion system

The type II secretion (T2S) system is an essential device for Erwinia chrysanthemi virulence. Previously, we reported the key role of the OutF protein in forming, along with OutELM, an inner membrane platform in the Out T2S system. Here, we report that OutF copurified with five proteins identified by matrix-assisted laser desorption ionization-time of flight analysis as AcsD, TogA, SecA, Tsp, and DegP. The AcsD protein was known to be involved in the biosynthesis of achromobactin, which is a siderophore important for E. chrysanthemi virulence. The yeast two-hybrid system allowed us to gain further evidence for the OutF-AcsD interaction. Moreover, we showed that lack of OutF produced a pleiotropic phenotype: (i) altered production of the two siderophores of E. chrysanthemi, achromobactin and chrysobactin; (ii) hypersensitivity to streptonigrin, an iron-activated antibiotic; (iii) increased sensitivity to oxidative stress; and (iv) absence of the FbpA-like iron-binding protein in the periplasmic fraction. Interestingly, outE and outL mutants also exhibited similar phenotypes, but, outD and outJ mutants did not. Moreover, using the yeast two-hybrid system, several interactions were shown to occur between components of the T2S system inner membrane platform (OutEFL) and proteins involved in achromobactin production (AcsABCDE). The OutL-AcsD interaction was also demonstrated by Ni(2+) affinity chromatography. These results fully confirm our previous view that the T2S machinery is made up of three discrete blocks. The OutEFLM-forming platform is proposed to be instrumental in two different processes essential for virulence, protein secretion and iron homeostasis.

Soft rot erwiniae: from genes to genomes

SUMMARY The soft rot erwiniae, Erwinia carotovora ssp. atroseptica (Eca), E. carotovora ssp. carotovora (Ecc) and E. chrysanthemi (Ech) are major bacterial pathogens of potato and other crops world-wide. We currently understand much about how these bacteria attack plants and protect themselves against plant defences. However, the processes underlying the establishment of infection, differences in host range and their ability to survive when not causing disease, largely remain a mystery. This review will focus on our current knowledge of pathogenesis in these organisms and discuss how modern genomic approaches, including complete genome sequencing of Eca and Ech, may open the door to a new understanding of the potential subtlety and complexity of soft rot erwiniae and their interactions with plants.

TAXONOMY

The soft rot erwiniae are members of the Enterobacteriaceae, along with other plant pathogens such as Erwinia amylovora and human pathogens such as Escherichia coli, Salmonella spp. and Yersinia spp. Although the genus name Erwinia is most often used to describe the group, an alternative genus name Pectobacterium was recently proposed for the soft rot species.

HOST RANGE

Ech mainly affects crops and other plants in tropical and subtropical regions and has a wide host range that includes potato and the important model host African violet (Saintpaulia ionantha). Ecc affects crops and other plants in subtropical and temperate regions and has probably the widest host range, which also includes potato. Eca, on the other hand, has a host range limited almost exclusively to potato in temperate regions only. Disease symptoms: Soft rot erwiniae cause general tissue maceration, termed soft rot disease, through the production of plant cell wall degrading enzymes. Environmental factors such as temperature, low oxygen concentration and free water play an essential role in disease development. On potato, and possibly other plants, disease symptoms may differ, e.g. blackleg disease is associated more with Eca and Ech than with Ecc.

USEFUL WEBSITES

http://www.scri.sari.ac.uk/TiPP/Erwinia.htm, http://www.ahabs.wisc.edu:16080/ approximately pernalab/erwinia/index.htm, http://www.tigr.org/tdb/mdb/mdbinprogress.html, http://www.sanger.ac.uk/Projects/E_carotovora/.

Type II protein secretion in gram-negative pathogenic bacteria: the study of the structure/secretion relationships of the cellulase Cel5 (formerly EGZ) from Erwinia chrysanthemi

Erwinia chrysanthemi, a Gram-negative plant pathogen, secretes the cellulase Cel5 (formerly EGZ) via the type II secretion pathway (referred to as Out). Cel5 is composed of two domains, a large N-terminal catalytic domain (390 amino acid residues) and a small C-terminal cellulose-binding domain (62 amino acid residues) separated by a linker region. A combination of mutagenesis and structural analysis permitted us to investigate the structure/secretion relationships with respect to the catalytic domain of Cel5. The 3D structure of the catalytic domain was solved by molecular replacement at 2.3 A resolution. Cel5 exhibits the (beta/alpha)8 structural fold and two extra-barrel features. Our previous genetic study based upon tRNA-mediated suppression allowed us to predict positions of importance in the molecule in relation to structure and catalysis. Remarkably, all of the predictions proved to be correct when compared with the present structural information. Mutations of Arg57, which is located at the heart of the catalytic domain, allowed us to test the consequences of structural modifications on the secretion efficiency. The results revealed that secretability imposes remarkably strong constraints upon folding. In particular, an Arg-to-His mutation yielded a species that folded to a stable conformation close to, but distinct from the wild-type, which however was not secretable. We discuss the relationships between folding of a protein in the periplasm, en route to the cell exterior, and presentation of secretion information. We propose that different solutions have been selected for type II secreted exoproteins in order to meet the constraints imposed by their interaction with their respective secretion machineries. We propose that evolutionary pressure has led to the adaptation of different secretion motifs for different type II exoproteins.

The PDZ domain of OutC and the N-terminal region of OutD determine the secretion specificity of the type II out pathway of Erwinia chrysanthemi

The plant pathogens Erwinia chrysanthemi and Erwinia carotovora secrete multiple exoproteins by a type II pathway, the Out system. Secretion in Erwinia is species-specific: exoproteins of one species cannot be secreted by the other. We analysed the role of two components of the Out system, the bitopic inner membrane protein OutC and the secretin OutD, in the specific recognition of secreted proteins. We demonstrated that the PDZ domain of OutC determines its secretion specificity towards certain exoproteins. The secretin is the major determinant of specificity of the Out system: OutD of E. carotovora changes the secretion specificity of E. chrysanthemi and enables it to secrete heterologous exoproteins. Construction of chimeric OutD showed that the N-terminal region is the specificity domain of the secretin. Thus, both the PDZ domain of OutC and the N-terminal region of OutD are required for specific recognition of secreted proteins. Systematic analysis of the secretion of several exoproteins demonstrated that different exoproteins secreted by the Out machinery have different requirement for their presumed targeting signals on OutC and OutD. This strongly indicates that diverse exoproteins possess a variable number of targeting signals which are recognised by different regions of OutC and OutD.

Exchange of Xcp (Gsp) secretion machineries between Pseudomonas aeruginosa and Pseudomonas alcaligenes: species specificity unrelated to substrate recognition

Pseudomonas aeruginosa and Pseudomonas alcaligenes are gram-negative bacteria that secrete proteins using the type II or general secretory pathway, which requires at least 12 xcp gene products (XcpA and XcpP to -Z). Despite strong conservation of this secretion pathway, gram-negative bacteria usually cannot secrete exoproteins from other species. Based on results obtained with Erwinia, it has been proposed that the XcpP and/or XcpQ homologs determine this secretion specificity (M. Linderberg, G. P. Salmond, and A. Collmer, Mol. Microbiol. 20:175-190, 1996). In the present study, we report that XcpP and XcpQ of P. alcaligenes could not substitute for their respective P. aeruginosa counterparts. However, these complementation failures could not be correlated to species-specific recognition of exoproteins, since these bacteria could secrete exoproteins of each other. Moreover, when P. alcaligenes xcpP and xcpQ were expressed simultaneously in a P. aeruginosa xcpPQ deletion mutant, complementation was observed, albeit only on agar plates and not in liquid cultures. After growth in liquid culture the heat-stable P. alcaligenes XcpQ multimers were not detected, whereas monomers were clearly visible. Together, our results indicate that the assembly of a functional Xcp machinery requires species-specific interactions between XcpP and XcpQ and between XcpP or XcpQ and another, as yet uncharacterized component(s).

Influence of deletions within domain II of exotoxin A on its extracellular secretion from Pseudomonas aeruginosa

Pseudomonas aeruginosa is a gram-negative bacterium that secretes many proteins into the extracellular medium via the Xcp machinery. This pathway, conserved in gram-negative bacteria, is called the type II pathway. The exoproteins contain information in their amino acid sequence to allow targeting to their secretion machinery. This information may be present within a conformational motif. The nature of this signal has been examined for P. aeruginosa exotoxin A (PE). Previous studies failed to identify a common minimal motif required for Xcp-dependent recognition and secretion of PE. One study identified a motif at the N terminus of the protein, whereas another one found additional information at the C terminus. In this study, we assess the role of the central PE domain II composed of six alpha-helices (A to F). The secretion behavior of PE derivatives, individually deleted for each helix, was analyzed. Helix E deletion has a drastic effect on secretion of PE, which accumulates within the periplasm. The conformational rearrangement induced in this variant is predicted from the three-dimensional PE structure, and the molecular modification is confirmed by gel filtration experiments. Helix E is in the core of the molecule and creates close contact with other domains (I and III). Deletion of the surface-exposed helix F has no effect on secretion, indicating that no secretion information is contained in this helix. Finally, we concluded that disruption of a structured domain II yields an extended form of the molecule and prevents formation of the conformational secretion motif.

Enhancement of expression and apparent secretion of Erwinia chrysanthemi endoglucanase (encoded by celZ) in Escherichia coli B

Escherichia coli B has been engineered as a biocatalyst for the conversion of lignocellulose into ethanol. Previous research has demonstrated that derivatives of E. coli B can produce high levels of Erwinia chrysanthemi endoglucanase (encoded by celZ) as a periplasmic product and that this enzyme can function with commercial fungal cellulase to increase ethanol production. In this study, we have demonstrated two methods that improve celZ expression in E. coli B. Initially, with a low-copy-number vector, two E. coli glycolytic gene promoters (gap and eno) were tested and found to be less effective than the original celZ promoter. By screening 18,000 random fragments of Zymomonas mobilis DNA, a surrogate promoter was identified which increased celZ expression up to sixfold. With this promoter, large polar inclusion bodies were clearly evident in the periplasmic space. Sequencing revealed that the most active surrogate promoter is derived from five Sau3A1 fragments, one of which was previously sequenced in Z. mobilis. Visual inspection indicated that this DNA fragment contains at least five putative promoter regions, two of which were confirmed by primer extension analysis. Addition of the out genes from E. chrysanthemi EC16 caused a further increase in the production of active enzyme and facilitated secretion or release of over half of the activity into the extracellular environment. With the most active construct, of a total of 13,000 IU of active enzyme per liter of culture, 7,800 IU was in the supernatant. The total active endoglucanase was estimated to represent 4 to 6% of cellular protein.

GSP-dependent protein secretion in gram-negative bacteria: the Xcp system of Pseudomonas aeruginosa

Bacteria have evolved several secretory pathways to release proteins into the extracellular medium. In Gram-negative bacteria, the exoproteins cross a cell envelope composed of two successive hydrophobic barriers, the cytoplasmic and outer membranes. In some cases, the protein is translocated in a single step across the cell envelope, directly from the cytoplasm to the extracellular medium. In other cases, outer membrane translocation involves an extension of the signal peptide-dependent pathway for translocation across the cytoplasmic membrane via the Sec machinery. By analogy with the so-called general export pathway (GEP), this latter route, including two separate steps across the inner and the outer membrane, was designated as the general secretory pathway (GSP) and is widely conserved among Gram-negative bacteria. In their great majority, exoproteins use the main terminal branch (MTB) of the GSP, namely the Xcp machinery in Pseudomonas aeruginosa, to reach the extracellular medium. In this review, we will use the P. aeruginosa Xcp system as a basis to discuss multiple aspects of the GSP mechanism, including machinery assembly, exoprotein recognition, energy requirement and pore formation for driving through the outer membrane.

Macromolecular assembly and secretion across the bacterial cell envelope: type II protein secretion systems

A decade ago, Pugsley and colleagues reported the existence of a large region of Klebsiella DNA, distinct from the Klebsiella gene encoding pullulanase, which was necessary for secretion of this enzyme to the cell surface in Escherichia coli (d'Enfert et al., 1987a,b). The pul genes it contained proved to be the tip of an iceberg. The sequences reported before 1992 (d'Enfert et al., 1987a,b; d'Enfert & Pugsley, 1989; Pugsley & Reyss, 1990; Reyss & Pugsley, 1990) included only one gene (pulD) that matched any sequence in the data base; a 220 amino acid residue segment of PulD was 32% identical with a portion of the filamentous phage-encoded protein, pIV. But by the time the sequence of the 18.8 kb DNA fragment that contained the pul genes had been completed (Possot et al., 1992), reports of sets of homologous genes in several species of Gram-negative plant and animal pathogens had appeared. For the most part, these gene clusters were cloned by their ability to complement mutants that produced, but failed to secrete, proteins normally found in the extracellular milieu; when tested, the mutants showed reduced pathogenicity or were totally avirulent. The secreted proteins included hydrolytic enzymes such as cellulase and pectinase from plant pathogens, and proteases and toxins from animal pathogens. The multi-gene family necessary for secretion of these enzymes is now known as the type II system or the main terminal branch (MTB) of the general secretion pathway (GSP). As summarized by Pugsley et al. (1997), the current tally includes type II systems from Klebsiella oxytoca (pul), Erwinia chrysanthemi and carotovora (out), Xanthomonas campestris (xps), Pseudomonas aeruginosa (xcp), Aeromonas hydrophila (exe), and Vibrio cholerae (eps). A second type II system (sps) necessary for deposition of the S-layer on the cell surface in A. hydrophila is more similar to the X. campestris than A. hydrophila genes (Thomas & Trust, 1995). The biggest surprise has been the discovery of a complete set of type II secretion genes in E. coli K12. The E. coli genes are not expressed under normal growth conditions, and a search is underway to find inducing conditions and secretion substrates (Francetic & Pugsley, 1996). Impressive progress has already been made in defining components of the pathway. What remains to be understood in mechanistic detail is how this protein secretion system functions.

External loops at the C terminus of Erwinia chrysanthemi pectate lyase C are required for species-specific secretion through the out type II pathway

The type II secretion system (main terminal branch of the general secretion pathway) is used by diverse gram-negative bacteria to secrete extracellular proteins. Proteins secreted by this pathway are synthesized with an N-terminal signal peptide which is removed upon translocation across the inner membrane, but the signals which target the mature proteins for secretion across the outer membrane are unknown. The plant pathogens Erwinia chrysanthemi and Erwinia carotovora secrete several isozymes of pectate lyase (Pel) by the out-encoded type II pathway. However, these two bacteria cannot secrete Pels encoded by heterologously expressed pel genes from the other species, suggesting the existence of species-specific secretion signals within these proteins. The functional cluster of E. chrysanthemi out genes carried on cosmid pCPP2006 enables Escherichia coli to secrete E. chrysanthemi, but not E. carotovora, Pels. We exploited the high sequence similarity between E. chrysanthemi PelC and E. carotovora Pel1 to construct 15 hybrid proteins in which different regions of PelC were replaced with homologous sequences from Pell. The differential secretion of these hybrid proteins by E. coli(pCPP2006) revealed M118 to D175 and V215 to C329 as regions required for species-specific secretion of PelC. We propose that the primary targeting signal is contained within the external loops formed by G274 to C329 but is dependent on residues in M118 to D170 and V215 to G274 for proper positioning.

Secretion of nuclease across the outer membrane of Serratia marcescens and its energy requirements

Extracellular secretion of Serratia marcescens nuclease occurs as a two-step process via a periplasmic intermediate. Unlike other extracellular proteins secreted by gram-negative bacteria by the general secretory pathway, nuclease accumulates in the periplasm in its active form for an unusually long time before its export into the growth medium. The energy requirements for extracellular secretion of nuclease from the periplasm were investigated. Our results suggest that the second step of secretion across the outer membrane is dependent upon the external pH; acidic pH effectively but reversibly blocks extracellular secretion. However, electrochemical proton gradient, and possibly ATP hydrolysis, are not required for this step. We suggest that nuclease uses a novel mechanism for the second step of secretion in S. marcescens.

Identification of two regions of Klebsiella oxytoca pullulanase that together are capable of promoting beta-lactamase secretion by the general secretory pathway

Pullulanase (PulA) is a 116 kDa amylolytic lipoprotein secreted by the Gram-negative bacterium Klebsiella oxytoca via the general secretory pathway. A deletion strategy was used in an attempt to determine the nature and the location of the secretion signal(s) in PulA presumed to be necessary for its specific secretion. The starting material was a gene fusion coding for an efficiently secreted PulA-beta-lactamase hybrid protein. Successive series of exonuclease III-generated deletions were used to remove internal segments of PulA from this hybrid. A simple plate test allowed the identification of truncated hybrids that retained beta-lactamase activity and that were secreted. Two non-adjacent regions, A and B (78 and 80 amino acids, respectively), were together necessary and sufficient to promote beta-lactamase translocation across the outer membrane. Secretion of PulA itself was markedly reduced when either of these regions was deleted, and was completely abolished when both regions were eliminated.

Complementation of deletion mutations in a cloned functional cluster of Erwinia chrysanthemi out genes with Erwinia carotovora out homologues reveals OutC and OutD as candidate gatekeepers of species-specific secretion of proteins via the type II pathway

The type II or Sec-dependent secretion system is used by diverse Gram-negative bacteria for secretion of extracellular proteins. Of the 12-15 proteins involved in secretion, the requirement for many has not been demonstrated and little is known about their functions in the secretion process. The plant pathogens Erwinia chrysanthemi and Erwinia carotovora secrete extra-cellular pectate lyases (Pels) using the type II or Out pathway. However, these two bacteria cannot secrete Pels encoded by heterologously expressed genes from the other species, suggesting the presence of species-specific recognition factors in the Out systems of the two Erwinia species. We previously reported the isolation of a cosmid clone, pCPP2OO6, from E. chrysanthemi EC16, which enables Escherichia coil to secrete heterologously expressed E. chrysanthemi Pels. Sequencing in a region required for secretion revealed the presence of 12 genes, outC-M and outO. We report here the construction of functionally non-polar mutations in each gene in the outC-M operon and outS and outB using a polA(ts) strain of E. coli to facilitate homologous recombination between out genes carrying deletions and their wild-type copies on pCPP2006. By testing for complementation of each deletion with wild-type out genes from E. chrysanthemi EC16 and E. carotovora SCRI193 we have demonstrated that: (i) each out gene is required for secretion of E. chrysanthemi PelE from E. coli with the exception of outH; (ii) each mutation can be complemented by its homologue from E. carotovora, except for outC and outD; (iii) outC and outD from E. carotovora do not confer secretion of Pel1 on the E. chrysanthemi Out system; and (iv) Pel1 secretion can be conferred on the E. chrysanthemi Out system by the presence of outC-M, S and B from E. carotovora. The data suggest that OutC and OutD are gatekeepers of the Out system involved in recognition of Pels targeted for secretion but that OutC and OutD from E. carotovora cannot be successfully assembled into the E. chrysanthemi Out system.

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.

Membrane traffic wardens and protein secretion in gram-negative bacteria

Recent progress in the genetic analysis of protein secretion in diverse Gram-negative bacteria has revealed three major, highly conserved but functionally independent pathways that involve accessory apparatus proteins. Protein secretion via the Type I pathway is signal sequence-independent with no free periplasmic intermediate. Secretion by the Type II pathway is signal sequence-dependent and via the periplasm. Recent results also suggest that a third (Type III) secretory pathway exists in which protein secretion is signal sequence-independent.

Analysis of eight out genes in a cluster required for pectic enzyme secretion by Erwinia chrysanthemi: sequence comparison with secretion genes from other gram-negative bacteria

Many extracellular proteins produced by Erwinia chrysanthemi require the out gene products for transport across the outer membrane. In a previous report (S. Y. He, M. Lindeberg, A. K. Chatterjee, and A. Collmer, Proc. Natl. Acad. Sci. USA 88:1079-1083, 1991) cosmid pCPP2006, sufficient for secretion of Erwinia chrysanthemi extracellular proteins by Escherichia coli, was partially sequenced, revealing four out genes sharing high homology with pulH through pulK from Klebsiella oxytoca. The nucleotide sequence of eight additional out genes reveals homology with pulC through pulG, pulL, pulM, pulO, and other genes involved in secretion by various gram-negative bacteria. Although signal sequences and hydrophobic regions are generally conserved between Pul and Out proteins, four out genes contain unique inserts, a pulN homolog is not present, and outO appears to be transcribed separately from outC through outM. The sequenced region was subcloned, and an additional 7.6-kb region upstream was identified as being required for secretion in E. coli. out gene homologs were found on Erwinia carotovora cosmid clone pAKC651 but were not detected in E. coli. The outC-through-outM operon is weakly induced by polygalacturonic acid and strongly expressed in the early stationary phase. The out and pul genes are highly similar in sequence, hydropathic properties, and overall arrangement but differ in both transcriptional organization and the nature of their induction.