Cloning and expression of the phospholipase D gene from Corynebacterium pseudotuberculosis in Escherichia coli

Protein salvage and repurposing in evolution: Phospholipase D toxins are stabilized by a remodeled scrap of a membrane association domain

The glycerophosphodiester phosphodiesterase (GDPD)-like SMaseD/PLD domain family, which includes phospholipase D (PLD) toxins in recluse spiders and actinobacteria, evolved anciently in bacteria from the GDPD. The PLD enzymes retained the core (β/α)8 barrel fold of GDPD, while gaining a signature C-terminal expansion motif and losing a small insertion domain. Using sequence alignments and phylogenetic analysis, we infer that the C-terminal motif derives from a segment of an ancient bacterial PLAT domain. Formally, part of a protein containing a PLAT domain repeat underwent fusion to the C terminus of a GDPD barrel, leading to attachment of a segment of a PLAT domain, followed by a second complete PLAT domain. The complete domain was retained only in some basal homologs, but the PLAT segment was conserved and repurposed as the expansion motif. The PLAT segment corresponds to strands β7-β8 of a β-sandwich, while the expansion motif as represented in spider PLD toxins has been remodeled as an α-helix, a β-strand, and an ordered loop. The GDPD-PLAT fusion led to two acquisitions in founding the GDPD-like SMaseD/PLD family: (1) a PLAT domain that presumably supported early lipase activity by mediating membrane association, and (2) an expansion motif that putatively stabilized the catalytic domain, possibly compensating for, or permitting, loss of the insertion domain. Of wider significance, messy domain shuffling events can leave behind scraps of domains that can be salvaged, remodeled, and repurposed.

Evolutionary dynamics of origin and loss in the deep history of phospholipase D toxin genes

Background

Venom-expressed sphingomyelinase D/phospholipase D (SMase D/PLD) enzymes evolved from the ubiquitous glycerophosphoryl diester phosphodiesterases (GDPD). Expression of GDPD-like SMaseD/PLD toxins in both arachnids and bacteria has inspired consideration of the relative contributions of lateral gene transfer and convergent recruitment in the evolutionary history of this lineage. Previous work recognized two distinct lineages, a SicTox-like (ST-like) clade including the arachnid toxins, and an Actinobacterial-toxin like (AT-like) clade including the bacterial toxins and numerous fungal homologs.

Results

Here we expand taxon sampling by homology detection to discover new GDPD-like SMase D/PLD homologs. The ST-like clade now includes homologs in a wider variety of arthropods along with a sister group in Cnidaria; the AT-like clade now includes additional fungal phyla and proteobacterial homologs; and we report a third clade expressed in diverse aquatic metazoan taxa, a few single-celled eukaryotes, and a few aquatic proteobacteria. GDPD-like SMaseD/PLDs have an ancient presence in chelicerates within the ST-like family and ctenophores within the Aquatic family. A rooted phylogenetic tree shows that the three clades derived from a basal paraphyletic group of proteobacterial GDPD-like SMase D/PLDs, some of which are on mobile genetic elements. GDPD-like SMase D/PLDs share a signature C-terminal motif and a shortened βα1 loop, features that distinguish them from GDPDs. The three major clades also have active site loop signatures that distinguish them from GDPDs and from each other. Analysis of molecular phylogenies with respect to organismal relationships reveals a dynamic evolutionary history including both lateral gene transfer and gene duplication/loss.

Conclusions

The GDPD-like SMaseD/PLD enzymes derive from a single ancient ancestor, likely proteobacterial, and radiated into diverse organismal lineages at least in part through lateral gene transfer.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1302-2) contains supplementary material, which is available to authorized users.

In vivo and in vitro expression of five genes involved in Corynebacterium pseudotuberculosis virulence

Caseous lymphadenitis (LC) is a chronic contagious disease caused by Corynebacterium pseudotuberculosis, which mainly affects goats and sheep. Vaccination is an effective but not yet well-established method, partly due to a lack of knowledge surrounding the most effective immunoprotective components. The present study aimed to quantify and compare the in vivo expression of genes pld (phospholipase D), cpp (CP40), nanH (neuraminidase H), sodC (superoxide dismutase C) and spaC (adhesin) using qRT-PCR, with the respective expression in vitro. Caseous material of abscesses removed from five animals was cultured, with colonies suggestive of C. pseudotuberculosis identified. RNA extraction was performed on these samples, as well as on the respective pellets derived from liquid cultures brain heart infusion. After evaluating RNA integrity, complementary DNA was synthesized, followed by the relative quantification each of the genes of interest. Mean mRNA expression of the five genes found in abscesses and in cultures differed significantly, with respective values of: nanH 811.50 ± 198.27 and 359.35 ± 75.45 (p = 0.009); cpp 856.31 ± 385.11 and 154.54 ± 94.34 (p = 0.0039); plD 922.70 ± 450.73 and 212.41 ± 153.10 (p = 0.016); sodC 1,293.53 ± 564.75 and 223.63 ± 145.58 (p = 0.016); spaC 1,157.10 ± 525.13 and 214.26 ± 125.70 (p = 0,016). Expression was observed to be 6–8 times higher in abscesses than in cultures, Indicative that is a genetic expression of the in vitro bacterium exists, yet in vivo has a greater magnitude corroborating to one of these virulence factors in the pathogenesis of LC.

The pan-genome of the animal pathogen Corynebacterium pseudotuberculosis reveals differences in genome plasticity between the biovar ovis and equi strains

Corynebacterium pseudotuberculosis is a facultative intracellular pathogen and the causative agent of several infectious and contagious chronic diseases, including caseous lymphadenitis, ulcerative lymphangitis, mastitis, and edematous skin disease, in a broad spectrum of hosts. In addition, Corynebacterium pseudotuberculosis infections pose a rising worldwide economic problem in ruminants. The complete genome sequences of 15 C. pseudotuberculosis strains isolated from different hosts and countries were comparatively analyzed using a pan-genomic strategy. Phylogenomic, pan-genomic, core genomic, and singleton analyses revealed close relationships among pathogenic corynebacteria, the clonal-like behavior of C. pseudotuberculosis and slow increases in the sizes of pan-genomes. According to extrapolations based on the pan-genomes, core genomes and singletons, the C. pseudotuberculosis biovar ovis shows a more clonal-like behavior than the C. pseudotuberculosis biovar equi. Most of the variable genes of the biovar ovis strains were acquired in a block through horizontal gene transfer and are highly conserved, whereas the biovar equi strains contain great variability, both intra- and inter-biovar, in the 16 detected pathogenicity islands (PAIs). With respect to the gene content of the PAIs, the most interesting finding is the high similarity of the pilus genes in the biovar ovis strains compared with the great variability of these genes in the biovar equi strains. Concluding, the polymerization of complete pilus structures in biovar ovis could be responsible for a remarkable ability of these strains to spread throughout host tissues and penetrate cells to live intracellularly, in contrast with the biovar equi, which rarely attacks visceral organs. Intracellularly, the biovar ovis strains are expected to have less contact with other organisms than the biovar equi strains, thereby explaining the significant clonal-like behavior of the biovar ovis strains.

Intranuclear sphingomyelin is associated with transcriptionally active chromatin and plays a role in nuclear integrity

BACKGROUND INFORMATION

Sphingomyelin is one of the major phospholipids in the cell nucleus. However, its intranuclear distribution with regard to different functional nuclear domains as well as its possible involvement in the nuclear functional architecture remains to be elucidated.

RESULTS

We carried out an ultrastructural cytochemical study of the intranuclear distribution of SM (sphingomyelin) using an in situ binding assay of neutral SMase (sphingomyelinase) conjugated to colloidal gold particles. The enzymatic labelling was carried out on ultrathin sections of different mammalian cells prepared by means of various fixation and resin-embedding protocols. Transmission electron microscopic analysis revealed preferential localization of SM within the PR (perichromatin region), a functionally important nucleoplasmic domain containing sites of pre-mRNA synthesis and processing. In the nucleolus, SM is mostly associated with the dense fibrillar component containing transcriptionally active ribosomal genes. Microinjection of enzymatically active SMase into living cells resulted in a rapid degradation of intranuclear structure.

CONCLUSIONS

Our observations, supported by biochemical data, provide evidence for the involvement of SM in important nuclear functions. They bring additional information pointing out the PR as an essential functional nuclear domain. Furthermore, they suggest a role for SM in the internal nuclear architecture.

Sphingomyelin functions as a novel receptor for Helicobacter pylori VacA

The vacuolating cytotoxin (VacA) of the gastric pathogen Helicobacter pylori binds and enters epithelial cells, ultimately resulting in cellular vacuolation. Several host factors have been reported to be important for VacA function, but none of these have been demonstrated to be essential for toxin binding to the plasma membrane. Thus, the identity of cell surface receptors critical for both toxin binding and function has remained elusive. Here, we identify VacA as the first bacterial virulence factor that exploits the important plasma membrane sphingolipid, sphingomyelin (SM), as a cellular receptor. Depletion of plasma membrane SM with sphingomyelinase inhibited VacA-mediated vacuolation and significantly reduced the sensitivity of HeLa cells, as well as several other cell lines, to VacA. Further analysis revealed that SM is critical for VacA interactions with the plasma membrane. Restoring plasma membrane SM in cells previously depleted of SM was sufficient to rescue both toxin vacuolation activity and plasma membrane binding. VacA association with detergent-resistant membranes was inhibited in cells pretreated with SMase C, indicating the importance of SM for VacA association with lipid raft microdomains. Finally, VacA bound to SM in an in vitro ELISA assay in a manner competitively inhibited by lysenin, a known SM-binding protein. Our results suggest a model where VacA may exploit the capacity of SM to preferentially partition into lipid rafts in order to access the raft-associated cellular machinery previously shown to be required for toxin entry into host cells.

Sensitivity to toxins produced by pathogenic bacteria is largely dictated by the presence or absence of toxin receptors on the plasma membrane of host cells. VacA is an important toxin produced by the pathogenic bacterium Helicobacter pylori, which infects the human stomach and causes gastric ulcer disease and stomach cancer. VacA binds and enters human cells, and induces several changes resulting ultimately in the death of the intoxicated cells. However, the identity of the VacA receptor responsible for toxin binding and function has remained a topic of debate. In this paper, we demonstrate that sphingomyelin, a lipid on the surface of cells with important membrane structural and signaling properties, functions as a VacA receptor. We demonstrate that VacA binds to sphingomyelin, and that presence or absence of sphingomyelin on the plasma membrane dictates how much VacA binds to the cell surface, and therefore, how sensitive cells are to the toxin. The identification of sphingomyelin also provides a conceptual framework for how VacA may enter cells through specialized functional domains on the surface of cells. This is the first example of a bacterial toxin that exploits sphingomyelin as a receptor, and future work will focus on developing strategies to block VacA interactions with sphingomyelin, thereby protecting cells from the downstream consequences of toxin action.

Identification and characterization of phospholipase D and its association with drought susceptibilities in peanut (Arachis hypogaea)

Preharvest aflatoxin contamination has been identified by the peanut industry as a serious issue in food safety and human health because of the carcinogenic toxicity. Drought stress is the most important environmental factor exacerbating Aspergillus infection and aflatoxin contamination in peanut. The development of drought-tolerant peanut cultivars could reduce aflatoxin contamination and would represent a major advance in the peanut industry. In this study, we identified a novel PLD gene in peanut (Arachis hypogaea), encoding a putative phospholipase D (PLD, EC 3.1.4.4). The completed cDNA sequence was obtained by using the consensus-degenerated hybrid oligonucleotide primer strategy. The deduced amino acid sequence shows high identity with known PLDs, and has similar conserved domains. The PLD gene expression under drought stress has been studied using four peanut lines: Tifton 8 and A13 (both drought tolerant) and Georgia Green (moderate) and PI 196754 (drought sensitive). Northern analysis showed that PLD gene expression was induced faster by drought stress in the drought-sensitive lines than the drought tolerance lines. Southern analysis showed that cultivated peanut has multiple copies (3 to 5 copies) of the PLD gene. These results suggest that peanut PLD may be involved in drought sensitivity and tolerance responses. Peanut PLD gene expression may be useful as a tool in germplasm screening for drought tolerance.

Use of a real-time polymerase chain reaction-based fluorogenic 5' nuclease assay to evaluate insect vectors of Corynebacterium pseudotuberculosis infections in horses

OBJECTIVE

To develop and use a sensitive molecular assay for detecting the phospholipase D (PLD) exotoxin gene of Corynebacterium pseudotuberculosis in an attempt to identify insect vectors that may be important in transmission of clinical disease in horses.

SAMPLE POPULATION

2,621 flies of various species.

PROCEDURE

A real-time polymerase chain reaction (PCR)-based fluorogenic 5' nuclease (TaqMan) system (ie, TaqMan PCR assay) was developed for the detection of the PLD gene in insects. Flies were collected monthly (May to November 2002) from 5 farms in northern California where C. pseudotuberculosis infection in horses is endemic. Three of the 5 farms (which housed a total of 358 horses) had diseased horses during the study period. A total of 2,621 flies of various species were tested for the PLD gene of C. pseudotuberculosis.

RESULTS

Evidence of bacterial DNA for the PLD gene was detected in skin biopsy specimens from clinically affected horses and from 3 fly species collected from farms where affected horses were housed. Farms with a high incidence of diseased horses had a high proportion of insects carrying the organism. High percentages of flies with positive results for the PLD gene were observed in October, when most clinically affected horses were observed.

CONCLUSIONS AND CLINICAL RELEVANCE

Our results are consistent with the hypothesis that C. pseudotuberculosis may be vectored to horses by flies. Three potential vectors were identified, including Haematobia irritans, Stomoxys calcitrans, and Musca domestica. The organism can be identified in up to 20% of house flies (Musca domestica) in the vicinity of diseased horses.

Sphingomyelinase D, a novel probe for cellular sphingomyelin: effects on cholesterol homeostasis in human skin fibroblasts

Sphingomyelin (SM) and free cholesterol (FC) are concentrated in the plasma membranes of eukaryotes; however, the physiological significance of their association is unclear. A common tool for studying the role of membrane SM is digestion with bacterial sphingomyelinase (SMase) C, which hydrolyzes SM to ceramide. However, it is not known whether the observed effects of SMase C treatment are due to the loss of SM per se or to the signaling effects of ceramide. In this study, we tested SMase D from Corynebacterium pseudotuberculosis, which hydrolyzes SM to ceramide phosphate, as an alternative probe. This enzyme specifically hydrolyzed SM in fibroblasts without causing accumulation of ceramide. Treatment of fibroblasts with SMase D stimulated translocation of PM FC to intracellular sites by <20% of the rate observed after SMase C digestion. The cells regenerated SM nearly completely within 5 h after SMase C treatment. However, even after 20 h, no regeneration occurred following SMase D digestion. These findings suggest that the translocation of PM FC caused by SMase C digestion is due to the cellular effects of ceramide rather than the loss of SM. Since ceramide phosphate does not appear to have such effects, we suggest that SMase D is a useful probe of membrane SM.

Identification and role in virulence of putative iron acquisition genes from Corynebacterium pseudotuberculosis

Four genes, fagA, B, C and D, encoding products with 32-47% identity to proteins involved in bacterial iron uptake systems, were identified immediately downstream of the Corynebacterium pseudotuberculosis phospholipase D gene. beta-Galactosidase assays on a C. pseudotuberculosis strain carrying a fagA-lacZ fusion indicated that the putative fagABC operon was poorly expressed in iron-rich media. However, similar experiments in iron-limited media resulted in an approximately three-fold increase in beta-galactosidase activity, suggesting that this operon is regulated by iron in vitro. Although no defect in iron utilization could be determined for a C. pseudotuberculosis fagB(C) mutant in vitro, this mutant showed reduced virulence compared to wild-type in a goat model of caseous lymphadenitis. Thus, expression of the fag genes in the host appears to contribute to virulence.

Enzymatic activity and gene expression under water stress of phospholipase D in two cultivars of Vigna unguiculata L. Walp. differing in drought tolerance

Phospholipase D, a major lipid-degrading enzyme in plants, was studied in two cultivars of Vigna unguiculata L.Walp, differing in their tolerance to drought (cv. EPACE-1, drought-tolerant, and cv. 1183, drought-susceptible). Enzymatic activities, measured with 14C-PC as substrate, increased when plants were submitted to water stress, the increase being much higher in the drought-sensitive cultivar. A 2911 bp cDNA encoding a putative phospholipase D (VuPLD1) was isolated from a cDNA library prepared from V. unguiculata leaves. The deduced amino acid sequence (809 residues) shows 85.5% identity and 91.3% similarity to that of PLD from Ricinus communis. The expression of the VuPLD1 gene in the leaves is differently modulated by water deficit, depending on the intensity of stress and the tolerance or sensitivity of the plants. In the drought-susceptible V. unguiculata cv. 1183, it readily increased under water stress, reaching maximum values at mild water deficit (-1.5 MPa). In the drought-tolerant cv. EPACE-1, VuPLD1 mRNA remained low throughout the whole drought treatment. Dehydration of leaves led to a dramatic increase in transcript level in both cultivars. Changes in protein amounts semi-quantified by immunoblotting correlated well with variations in transcript steady-state level. Taken together, these results showed that phospholipase D in cowpea plants is essentially regulated at the transcriptional level, and that gene expression is strongly stimulated even by moderate water deficit in the drought-sensitive plant. On the contrary, the drought-tolerant plant presents a remarkable stability of PLD gene expression in conditions of water stress.

Biochemical and biological characterizations and ribotyping of Actinomyces pyogenes and Actinomyces pyogenes-like organisms from liver abscesses in cattle

Actinomyces pyogenes is the second most frequently encountered pathogen, next only to Fusobacterium necrophorum, in liver abscesses of feedlot cattle. Ninety-one isolates, presumptively identified as A. pyogenes, isolated from liver abscesses of cattle were studied. Biochemical characteristics determined by the API 20 Strep kit were similar to those reported previously for A. pyogenes isolated from other infections, except that 18% of isolates hydrolyzed esculin. Nine isolates that resembled A. pyogenes in morphology and in certain biochemical characteristics, but fermented mannitol and/or raffinose, were called A. pyogenes-like (APL) organisms. The five antimicrobial agents, bacitracin, chlortetracycline, oxytetracycline, tylosin, and virginiamycin were inhibitory to all strains of A. pyogenes and APLs. Generally, APL organisms had higher mean hemolytic and leukotoxic activities than A. pyogenes. All isolates of A. pyogenes and APLs produced proteases and neuraminidases. Ribotyping with endonucleases, including BstEII, ClaI, EcoRI, EcoRV, HaeIII, MboI, PvuII, SalI, and SmaI alone or in combinations, showed considerable genetic heterogeneity in both A. pyogenes and APLs. No specific ribopattern characteristic of each group was observed with any of the endonuclease used. The origin of A. pyogenes and APLs and the relative importance of APLs in causing liver abscesses in feedlot cattle are not known.

The Arcanobacterium (Actinomyces) pyogenes hemolysin, pyolysin, is a novel member of the thiol-activated cytolysin family

Arcanobacterium (Actinomyces) pyogenes, an animal pathogen, produces a hemolytic exotoxin, pyolysin (PLO). The gene encoding PLO was cloned, and sequence analysis revealed an open reading frame of 1,605 bp encoding a protein of 57.9 kDa. PLO has 30 to 40% identity with the thiol-activated cytolysins (TACYs) of a number of gram-positive bacteria. The activity of PLO was found to be very similar to those of other TACYs, except that it was not thiol activated. The highly conserved TACY undecapeptide is divergent in PLO; in particular, the cysteine residue required for thiol activation has been replaced with alanine. However, mutagenesis of the alanine residue to cysteine did not confer thiol activation on PLO, suggesting a conformational difference in the undecapeptide region of this toxin. Specific antibodies against purified, recombinant PLO completely neutralized the hemolytic activity of A. pyogenes, suggesting that this organism produces a single hemolysin. Furthermore, these antibodies could passively protect mice against lethal challenge with A. pyogenes, suggesting that like other TACYs PLO is an important virulence factor in the pathogenesis of this organism.

Characterization of a sphingomyelinase activity in Saccharomyces cerevisiae

Sphingomyelinases (SMase), which hydrolyze sphingolipids to yield ceramide, participate in signal transduction pathways in mammalian cells. Although yeast express many homologs of mammalian signaling proteins, SMase activity had not been previously demonstrated in yeast. In this study, we used an in vitro assay to characterize yeast SMase activity. Activity was detected in yeast membranes at both acid and neutral pH. The enzyme exhibited a requirement for magnesium or manganese, and was sensitive to detergents. The pI of the enzyme was approximately 5.9. SMase was separable from phospholipase D (PLD) activity, and was expressed at normal levels in yeast lacking expression of PLD1. While sphingosine and phytosphingosine inhibited growth, other sphingolipid metabolites had no effect on yeast growth. Intact yeast generate ceramide from exogenous sphingomyelin. These studies demonstrate that yeast express a membrane-localized neutral SMase activity.

Bacterial phospholipases and their role in virulence

Virulence of many bacterial pathogens is based, at least in part, on the action of phospholipases. The consequences may be immediate and direct, as in the action of Clostridium perfringens alpha toxin on red cells or platelets, or subtle, as with phosphatidylinositol-specific phospholipases of Listeria monocytogenes and other bacteria.

Toxic phospholipases D of Corynebacterium pseudotuberculosis, C. ulcerans and Arcanobacterium haemolyticum: cloning and sequence homology

The genes encoding toxic phospholipases D (PLD) from Corynebacterium pseudotuberculosis (Cp)biovar equi and C. ulcerans (Cu) have been cloned and sequenced. The deduced proteins are 307 amino acids (aa) in length and include a putative signal sequences of 26-aa. A molecular mass of 31.2 and 31.0 kDa and pI values of 8.84 and 6.73 are predicted for the secreted (mature) proteins from Cp and Cu, respectively. Comparison of the deduced primary structure of the two proteins to those of the PLD produced by Cp biovar ovis and Arcanobacterium haemolyticum (Ah) revealed that the four enzymes share 64-97% identity. The aa sequences of this group of proteins were unique when compared to the sequences of other phospholipases in GenBank and were found to share only small regions of homology with other proteins, including two conserved domains of glyceraldehyde-3-phosphate dehydrogenase (G3PD). The similarity of PLD from Cp biovar equi, Cu and Ah to the PLD of Cp biovar ovis suggests that these enzymes may act as virulence determinants.

Molecular and biochemical characterization of a protective 40-kilodalton antigen from Corynebacterium pseudotuberculosis

A 40-kDa protein from Corynebacterium pseudotuberculosis has been previously identified as a protective antigen against ovine caseous lymphadenitis. From genomic DNA libraries of C. pseudotuberculosis, we have cloned and sequenced the 40-kDa protein gene, which was found to contain an open reading frame of 1,137 bp encoding a protein of 379 amino acids. No significant homology with previously published DNA or amino acid sequence data was found in databases, suggesting that this is a novel protein. Recombinant 40-kDa protein was overexpressed as a fusion protein to 15% of total cell proteins in Escherichia coli. Biochemical analysis of native and recombinant 40-kDa proteins has revealed associated proteolytic activity, which was shown to be of the serine protease type through the use of specific inhibitors. We suggest that this novel protective antigen be termed corynebacterial protease 40 (CP40).

Protection of sheep against caseous lymphadenitis by use of a single oral dose of live recombinant Corynebacterium pseudotuberculosis

An inactive form of the Corynebacterium pseudotuberculosis phospholipase D (PLD) gene was constructed and expressed in a PLD-negative strain (designated Toxminus) of C. pseudotuberculosis. Antibody responses specific to Toxminus and both Toxminus and PLD proteins were detected in sheep following oral administration of Toxminus or Toxminus expressing the PLD toxoid, respectively. However, only those sheep vaccinated with Toxminus expressing PLD toxoid were protected against wild-type challenge. These results confirm the importance of PLD as a protective antigen and demonstrate both the potential for developing an oral caseous lymphadenitis vaccine and C. pseudotuberculosis Toxminus as a live vaccine vector.

Targeted mutagenesis of the phospholipase D gene results in decreased virulence of Corynebacterium pseudotuberculosis

The chromosomal gene encoding the phospholipase D from Corynebacterium pseudotuberculosis (biovar ovis) isolate Whetten 1 was replaced with an allele containing a nonsense mutation. The virulence of the mutant strain (W1.31r1) and the isogenic parental strain were then compared by inoculation of goats. The wild-type strain caused abscessation at the site of infection, which then spread to the regional lymph node, while W1.31r1 had a reduced ability to establish a primary infection and was incapable of dissemination. Our results confirm that phospholipase D is a virulence determinant of C. pseudotuberculosis that increases the persistence and spread of the bacteria within the host.

A cytolysin encoded by Salmonella is required for survival within macrophages

A Salmonella gene encoding a cytolysin has been identified by screening for hemolysis on blood agar. DNA sequence analyses together with genetic mapping in Salmonella suggest that it is unrelated to other toxins or hemolysins. The gene (slyA) is present in every strain of Salmonella examined, in Shigella, and in enteroinvasive Escherichia coli but not in other Enterobacteriaceae. SlyA (salmolysin) purified from a derivative of the original clone has hemolytic and cytolytic activity and has a molecular weight predicted by the DNA sequence. The median lethal dose and infection kinetics in mice suggest that the toxin is required for virulence and facilitates Salmonella survival within mouse peritoneal macrophages.

Characterization of detergent-soluble proteins of Corynebacterium pseudotuberculosis

Whole cells and culture supernatant of isolates of Corynebacterium pseudotuberculosis were studied by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting (immunoblotting). SDS-PAGE analysis of detergent-solubilized whole cells revealed more than 20 bands in silver-stained gels. However, the SDS-soluble proteins that are present in all the isolates of the bacterium can be separated into four groups, as follows, (i) high molecular mass proteins that are greater than 119 kDa, (ii) a set of three proteins with molecular mass of 84, 64 and 58 kDa, (iii) a doublet consisting of proteins of molecular mass 33 to 30 kDa, and (iv) low molecular mass proteins of 25 to 20 kDa. SDS-PAGE analysis of ammonium sulphate concentrated culture supernatant demonstrated more than seven bands in silver-stained gels ranging in molecular mass from 64-14 kDa. Sera from goats with C. pseudotuberculosis-induced disease were used to probe immunoblots of electrophoresed SDS-soluble proteins. Ten or more SDS-soluble proteins from whole cells, ranging in molecular mass from 119-20 kDa were recognized by antibodies in sera of naturally infected goats. These sera also recognized up to five molecules ranging from 64-30 kDa, on immunoblots of ammonium sulfate concentrated culture supernatant.

Arcanobacterium haemolyticum phospholipase D is genetically and functionally similar to Corynebacterium pseudotuberculosis phospholipase D

Arcanobacterium haemolyticum, a pathogen of the human upper respiratory tract and other systems, has been reported to produce soluble toxins, including a phospholipase D (PLD). We confirmed production of PLD by this organism and cloned and sequenced pld. Arcanobacterial PLD (PLD-A) was found to be a protein of approximately 31.5 kDa with a pI of approximately 9.4. Cosmid cloning, followed by subcloning into phagemid pBluescriptIISK+, yielded Escherichia coli(pAh140), a recombinant with a gene product corresponding to PLD-A. Evidence of PLD activity was found by three assays in supernatant fluid of cultures of E. coli(pAh140) and A. haemolyticum, but not in E. coli(pBluescriptIISK+). Experiments to determine if this protein was secreted were not conducted, but previous work with PLD from Corynebacterium pseudotuberculosis suggested that the presence of the enzyme in culture supernatant fluids was due to lysis of E. coli rather than to active transport. Antibodies in polyclonal sera from goats immunized with native or recombinant PLD-A neutralized native and recombinant PLD-A activity, and antibodies against native or recombinant PLD from C. pseudotuberculosis (PLD-P) partially neutralized native and recombinant PLD-A. Antibodies prepared against recombinant PLD-A labelled both recombinant and native PLD-A in Western blots (immunoblots) and dot blots, but antibodies against PLD-P did not. Sequencing of the insert in pAh140 revealed an open reading frame of 930 bp coding for 309 amino acids, including a putative signal sequence of 26 amino acids (3.2 kDa, determined on the basis of homology with the 24-amino-acid signal sequence of pld from C. pseudotuberculosis bv. ovis) and the mature PLD protein (31.5 kDa). Sequence comparisons of coding regions revealed 65% DNA homology with pld genes from C. pseudotuberculosis and Corynebacterium ulcerans. Comparison of amino acid sequences revealed 64% homology of PLD-A both with PLD-P and with PLD produced by C. ulcerans.

Production of an enzymatically inactive analog of phospholipase D from Corynebacterium pseudotuberculosis

The gene pld, encoding the phospholipase D (PLD) of Corynebacterium pseudotuberculosis, was mutagenized using formic acid and then expressed in Escherichia coli. Mutagenesis was targeted at the coding region of pld, so as to produce only one or a limited number of point mutations. Transformants were screened for the enzymatic and immunological properties of their PLD products. One clone was found to produce a protein which was enzymatically inactive, but which was comparable to the wild-type PLD in size and antigenicity. The sequence of the pld mutant revealed a single base change. As a consequence, the codon for His20 was converted to Tyr. These results suggest that His20 forms part of the active site of the PLD molecule. If this protein is immunogenic in sheep, it would form the basis of a genetically inactivated vaccine.

Analysis of the immunodominant antigens of Corynebacterium pseudotuberculosis

Antibodies to seven antigens in a whole cell lysate of Corynebacterium pseudotuberculosis ranging in molecular mass from 22 to 120 kilodaltons (kDa) were present in sera of 40 sheep and goats infected with C. pseudotuberculosis. Three antigens of about 120, 68, and 31.5 kDa in size were consistently detected with sera from all animals and twenty-two sera had antibodies to 64, 43, 40, and 22 kDa antigens. None of these antigens were detected by sera from 160 sheep in a C. pseudotuberculosis-free research flock. An NaCl extract of C. pseudotuberculosis cells contained one major protein of about 31.5 kDa and four minor proteins of 68, 64, 43, and 22 kDa in molecular mass as shown by Coomassie Blue staining. Immunoblot analysis demonstrated that the three immunodominant antigens identified in the whole cell extract were contained in the NaCl extract. The 31.5-kDa protein was purified from the NaCl extract by fast-protein liquid chromatography gel filtration to near homogeneity. The purified 31.5-kDa protein showed phospholipase D activity as indicated by synergistic hemolysis with Rhodococcus equi factors and sphingomyelinase activity. The 31.5-kDa protein reacted with antibodies in serum from a sheep naturally infected with C. pseudotuberculosis. This serum also had phospholipase D neutralizing activity. On the basis of its molecular mass, biological activity, N-terminal amino acid sequence analysis, and immunoreactivity, the 31.5-kDa protein was identified as the phospholipase D exotoxin of C. pseudotuberculosis.

Phospholipase activity of Mycobacterium leprae harvested from experimentally infected armadillo tissue

Three types of phospholipase activity--phospholipase A1, A2, and lysophospholipase--were detected in Mycobacterium leprae harvested from armadillo tissue at about 25% of the specific activity found in a slowly growing mycobacterium, Mycobacterium microti, which was grown in medium to optimize its phospholipase activity. The highest activity found was lysophospholipase, which released fatty acid from 2-lyso-phosphatidylcholine. Phospholipase activity was detected by using phosphatidylcholine and phosphatidylethanolamine. Differences in relative activities with these three types of substrate distinguished phospholipase activity in M. leprae extracts from armadillo liver extracts. Furthermore, retention of activity in M. leprae after NaOH treatment showed that the activity associated with M. leprae was not host derived. The specific activity of phospholipase was 20 times higher in extracts of M. leprae than in intact M. leprae organisms. Diazotization, a treatment which abolishes activities of surface enzymes exposed to the environment by the formation of covalent azide bonds with exposed amino groups, did not affect M. leprae's phospholipase activity, with one exception: release of arachidonic acid from phosphatidylcholine, which was partially inhibited. Phenolic glycolipid I, the major excreted amphipathic lipid of M. leprae, inhibited phospholipase activity, including release of arachidonic acid, for both M. leprae- and armadillo-derived activity.

Phospholipase D in cell signalling and its relationship to phospholipase C

Phospholipases C and D are phosphodiesterases which act on phospholipid head groups. Although the presence of these enzymes in living organisms has long been known, it is only recently that their role in cell signal transduction has been appreciated. The new developments on phospholipases D (PLD) are especially noteworthy, since these enzymes catalyze a novel pathway for second messenger generation. In a variety of mammalian cell systems, several biological or chemical agents have recently been shown to stimulate PLD activity. Depending on the system, activation of PLD has been suggested to be either dependent on, or independent of, Ca2+ and protein kinase C. PLD primarily hydrolyses phosphatidylcholine (PC) but phosphatidylinositol and phosphatidylethanolamine have also been reported as substrates. Different forms of endogenous PLD may also exist in cells. Exogenous addition of PLD causes alterations in cellular functions. In many instances, Ca2+ mobilizing agonists may stimulate both PLC and PLD pathways. Interestingly, several metabolites of these two enzymes are second messengers and are common to both pathways (e.g. phosphatidic acid, diglyceride). This has raised the issue of the interrelationship between these pathways. The regulation of either PLC or PLD by cellular components, e.g. guanine nucleotide binding proteins or protein kinases, is under intense investigation. These recent advances are providing novel information on the significance of phospholipase C and D mediated phospholipid turnover in cellular signalling. This review highlights some of these new discoveries and emerging issues, as well as challenges for future research on phospholipases.

Coryneform bacteria in infectious diseases: clinical and laboratory aspects

Coryneform isolates from clinical specimens frequently cannot be identified by either reference laboratories or research laboratories. Many of these organisms are skin flora that belong to a large number of taxonomic groups, only 40% of which are in the genus Corynebacterium. This review provides an update on clinical presentations, microbiological features, and pathogenic mechanisms of infections with nondiphtheria Corynebacterium species and other pleomorphic gram-positive rods. The early literature is also reviewed for a few coryneforms, especially those whose roles as pathogens are controversial. Recognition of newly emerging opportunistic coryneforms is dependent on sound identification schemes which cannot be developed until cell wall analyses and nucleic acid studies have defined the taxonomic groups and all of the reference strains within each taxon have been shown by molecular methods to be authentic members. Only then can reliable batteries of biochemical tests be selected for distinguishing each taxon.