Characterization of the phospholipase C gene of Pseudomonas aeruginosa cloned in Escherichia coli

A simple and reliable method for rapid production and purification of Pseudomonas aeruginosa haemolytic phospholipase C

AIMS

To design a simple method to produce active recombinant Pseudomonas aeruginosa haemolytic phospholipase C (PLC).

METHOD AND RESULTS

Pseudomonas aeruginosa PLC is a virulence factor mainly involved in inflammatory and cytotoxic responses. While ammonium sulphate purification requires large amounts of bacterial suspensions and leads to low yields, production of recombinant protein in Escherichia coli is no more successful because of frequent inclusion bodies and accumulation of inactive PLC in the periplasmic space. Using an inducible system based on the glucose-repressed inv1 promoter in the yeast Schizosaccharomyces pombe, we were able to produce up to 10 IU ml(-1) of pure toxin within 24 h.

CONCLUSIONS

This work describes the first method to easily get recombinant haemolytic PLC.

SIGNIFICANCE AND IMPACT OF THE STUDY

This method provides a powerful tool to study the mechanisms leading to its cellular toxicity.

Type II protein secretion by Pseudomonas aeruginosa: genetic suppression of a conditional mutation in the pilin-like component XcpT by the cytoplasmic component XcpR

Pseudomonas aeruginosa exports a number of hydrolytic enzymes and toxins using the type II or general secretion pathway, found in a variety of Gram-negative bacteria and requiring the functions of at least 12 gene products (XcpP-Z and PilD/XcpA in P. aeruginosa). A number of these gene products are homologues of components of the type IV pilus biogenesis system, including four proteins, XcpT-W, which are highly similar to the pilin subunit in their size, localization and post-translational modifications. These proteins, in addition to the pilin subunit, are cleaved and methylated by the PilD/XcpA prepilin peptidase, but their interactions with other components of the export apparatus are unclear. Using a medium developed for the selection of export-proficient P. aeruginosa strains, we have isolated temperature-sensitive mutations in the xcpT gene and extragenic suppressors for one of the mutants. These suppressors fall into two classes, one that maps outside of the xcpP-Z gene cluster and may define additional cellular functions that are required for export, and a second that maps to the xcpR gene product and indicates a potential protein-protein interaction connecting two different cellular compartments and required for the assembly or function of the export apparatus.

Membrane insertion of the F-pilin subunit is Sec independent but requires leader peptidase B and the proton motive force

F pilin is the subunit required for the assembly of conjugative pili on the cell surface of Escherichia coli carrying the F plasmid. Maturation of the F-pilin precursor, propilin, involves three F plasmid transfer products: TraA, the propilin precursor; TraQ, which promotes efficient propilin processing; and TraX, which is required for acetylation of the amino terminus of the 7-kDa pilin polypeptide. The mature pilin begins at amino acid 52 of the TraA propilin sequence. We performed experiments to determine the involvement of host cell factors in propilin maturation. At the nonpermissive temperature in a LepBts (leader peptidase B) host, propilin processing was inhibited. Furthermore, under these conditions, only full-length precursor was observed, suggesting that LepB is responsible for the removal of the entire propilin leader peptide. Using propilin processing as a measure of propilin insertion into the plasma membrane, we found that inhibition or depletion of SecA and SecY does not affect propilin maturation. Addition of a general membrane perturbant such as ethanol also had no effect. However, dissipation of the proton motive force did cause a marked inhibition of propilin processing, indicating that membrane insertion requires this energy source. We propose that propilin insertion in the plasma membrane proceeds independently of the SecA-SecY secretion machinery but requires the proton motive force. These results present a model whereby propilin insertion leads to processing by leader peptidase B to generate the 7-kDa peptide, which is then acetylated in the presence of TraX.

Common virulence factors for bacterial pathogenicity in plants and animals

A Pseudomonas aeruginosa strain (UCBPP-PA14) is infectious both in an Arabidopsis thaliana leaf infiltration model and in a mouse full-thickness skin burn model. UCBPP-PA14 exhibits ecotype specificity for Arabidopsis, causing a range of symptoms from none to severe in four different ecotypes. In the mouse model, UCBPP-PA14 is as lethal as other well-studied P. aeruginosa strains. Mutations in the UCBPP-PA14 toxA, plcS, and gacA genes resulted in a significant reduction in pathogenicity in both hosts, indicating that these genes encode virulence factors required for the full expression of pathogenicity in both plants and animals.

Construction and characterization of chromosomal insertional mutations of the Pseudomonas aeruginosa exoenzyme S trans-regulatory locus

Exoenzyme S is an ADP-ribosyltransferase produced by Pseudomonas aeruginosa. Synthesis of exoenzyme S depends on an intact trans-regulatory locus encoding three protein products, ExsC, ExsB, and ExsA. To identify the phenotype of ExsC, -B, and -A mutants in exoenzyme S production, specific insertional mutations with the streptomycin resistance-encoding omega interposon were introduced into cloned DNA and returned to the chromosomes of P. aeruginosa PA103, PAO1, and PAK. Southern blot analysis was used to confirm insertion of omega and resolution of vector sequences. Exoenzyme S expression was measured in parental and mutant derivatives by Western blot (immunoblot) analysis and ADP-ribosyltransferase activity measurement. A complete set of mutations were obtained in strains PAK and PAO1, but in strain PA103, only an insertion in the exsA coding region was identified. Southern blot analysis demonstrated that extensive duplication and rearrangement of the PA103 chromosomal trans-regulatory locus occurred when exsC::omega or exsB::omega recombination events were attempted. Exoenzyme S antigen was not detectable in the supernatant or lysate fractions of mutant strains by Western blot analysis. ADP-ribosyltransferase activity was detected in the lysate but not in the supernatant fractions of mutant derivatives. The general secretion pathway appeared to function normally in mutant strains, as elastase, exotoxin A, and phospholipase C were measured in the supernatants of parental and mutant strains. Several differences were noted when the extracellular protein profiles of parental strains were compared with similar samples from the insertional mutant strains. Some of these differences appeared to be unrelated to exoenzyme S. These data suggest that insertional inactivation of the exoenzyme S trans-regulatory locus may affect a subset of other extracellular proteins.

Bacterial phospholipases C

A variety of pathogenic bacteria produce phospholipases C, and since the discovery in 1944 that a bacterial toxin (Clostridium perfringens alpha-toxin) possessed an enzymatic activity, there has been considerable interest in this class of proteins. Initial speculation that all phospholipases C would have lethal properties has not been substantiated. Most of the characterized enzymes fall into one of four groups of structurally related proteins: the zinc-metallophospholipases C, the sphingomyelinases, the phosphatidylinositol-hydrolyzing enzymes, and the pseudomonad phospholipases C. The zinc-metallophospholipases C have been most intensively studied, and lethal toxins within this group possess an additional domain. The toxic phospholipases C can interact with eukaryotic cell membranes and hydrolyze phosphatidylcholine and sphingomyelin, leading to cell lysis. However, measurement of the cytolytic potential or lethality of phospholipases C may not accurately indicate their roles in the pathogenesis of disease. Subcytolytic concentrations of phospholipase C can perturb host cells by activating the arachidonic acid cascade or protein kinase C. Nonlethal phospholipases C, such as the Listeria monocytogenes PLC-A, appear to enhance the release of the organism from the host cell phagosome. Since some phospholipases C play important roles in the pathogenesis of disease, they could form components of vaccines. A greater understanding of the modes of action and structure-function relationships of phospholipases C will facilitate the interpretation of studies in which these enzymes are used as membrane probes and will enhance the use of these proteins as models for eukaryotic phospholipases C.

Components of the protein-excretion apparatus of Pseudomonas aeruginosa are processed by the type IV prepilin peptidase

In the Gram-negative pathogen Pseudomonas aeruginosa, mutants in the gene for the prepilin peptidase (pilD) are pleiotropic, as they not only fail to process pilin but also accumulate in the periplasm, in their mature form, several toxins and hydrolytic enzymes that are normally exported to the external medium (excreted). We have suggested that this excretion defect is due to the lack of PilD-dependent processing of proteins that share sequences in common with the prepilin subunit and that are components of a protein-excretion machinery. In this paper we report the isolation and characterization of transposon-induced excretion mutants with phenotypes similar to that of a pilD gene mutant. Using oligonucleotide probes designed to recognize sequences encoding the cleavage site of the type IV prepilins, we have isolated four linked genes with the predicted putative PilD-dependent cleavage site. Site-specific mutations within these genes have shown that they are required for protein excretion, and PilD-dependent processing of at least one of the four encoded proteins was demonstrated. Evidence suggests that similar components play a role in protein excretion in a wide variety of Gram-negative bacteria.

Genetic analysis of the export of an extracellular DNase of Vibrio cholerae using DNase-beta-lactamase fusions

A series of C-terminal deletions of the dns-encoded extracellular deoxyribonuclease (DNS) of Vibrio cholerae, fused to the mature form TEM beta-lactamase (Bla) has been used to analyse the export of the DNase in both V. cholerae and Escherichia coli. All hybrid proteins were localized to the periplasmic space in E. coli and V. cholerae, with specific cleavage of the DNS-Bla fusion occurring in V. cholerae. Periplasmic accumulation of wt DNS was also seen in V. cholerae when present on a multicopy plasmid. DNS fusions retaining all six Cys residues of DNS displayed both DNase and Bla enzymatic activity. While hybrid proteins were unable to be secreted across the outer membrane in V. cholerae, the cleaved (active) DNS portion of these proteins was exported. Taken together, these data suggest that the periplasmic form seen in E. coli is a normal intermediate also seen in V. cholerae, and that the lack of secretion machinery in E. coli prevents further export across the outer membrane. Although the DNS portion of the protein fusions must be able to interact with secretion genes, the whole fusion proteins are not exported.

Mutagenesis and isolation of Aeromonas hydrophila genes which are required for extracellular secretion

Transposon mutagenesis was used to isolate mutants of Aeromonas hydrophila which were deficient in the production of extracellular proteins. The culture supernatants of two of the mutants were essentially devoid of the proteins normally secreted by the parent strain, despite their continued synthesis. Western immunoblot analysis of one of these proteins indicated that normal signal sequence processing occurred but that normal zymogen activation did not, and cell fractionation experiments indicated that both mutants accumulated the three different extracellular proteins assayed in a position external to the cytoplasmic membrane, presumably in the periplasm. The two mutants differed, however, in that one was lysed during the osmotic shock procedures and also contained severely reduced amounts of two of the major protein components of the outer membrane. The wild-type chromosomal regions into which the transposon had been inserted in the two mutants were cloned. In each case, transconjugants of the mutants containing the corresponding cloned fragment were complemented for the defects in secretion, and one of the mutants was complemented by the heterologous clone as well, suggesting the possibility of an interaction between these two genes or gene products. These results indicate that two separate functions which are required for extracellular secretion were interrupted in the insertion mutants and that one of these is also critically important in the biogenesis of the outer membrane.

Nucleotide sequence and expression of the isoamylase gene from an isoamylase-hyperproducing mutant, Pseudomonas amyloderamosa JD210

The isoamylase gene (ISO) of Pseudomonas amyloderamosa JD210, an isoamylase-hyperproducing mutant, was cloned in an isoamylase-deficient and transformable mutant strain K31. By deletion analysis, the ISO gene was found to be located within a 3.3 kilobases BamHI fragment. Its nucleotide sequence contained an open reading frame of 2328 nucleotides (776 amino acids) encoding a secreted isoamylase precursor. The ISO gene fragment was inserted into plasmids pKT230 and pBR 322 in opposite orientations. The expression of the ISO gene in the constructed plasmids was compared in P. amyloderamosa K31, Pseudomonas aeruginosa PAO1-161, Pseudomonas putida mt-2 and Escherichia coli HB101. In all transformed cells, the majority of the isoamylase produced was secreted and higher isoamylase activities were obtained in transformats with the transcriptional direction of the ISO gene similar to the nearby drug-determinant gene of the vector.

Molecular comparison of a nonhemolytic and a hemolytic phospholipase C from Pseudomonas aeruginosa

Pseudomonas aeruginosa produces two secreted phospholipase C (PLC) enzymes. The expression of both PLCs is regulated by Pi. One of the PLCs is hemolytic, and one is nonhemolytic. Low-stringency hybridization studies suggested that the genes encoding these two PLCs shared DNA homology. This information was used to clone plcN, the gene encoding the 77-kilodalton nonhemolytic PLC, PLC-N. A fragment of plcN was used to mutate the chromosomal copy of plcN by the generation of a gene interruption mutation. This mutant produces 55% less total PLC activity than the wild type, confirming the successful cloning of plcN. plcN was sequenced and encodes a protein which is 40% identical to the hemolytic PLC (PLC-H). The majority of the homology lies within the NH2 two-thirds of the proteins, while the remaining third of the amino acid sequence of the two proteins shows very little homology. Both PLCs hydrolyze phosphatidylcholine; however, each enzyme has a distinct substrate specificity. PLC-H hydrolyzes sphingomyelin in addition to phosphatidylcholine, whereas PLC-N is active on phosphatidylserine as well as phosphatidylcholine. These studies suggest structure-function relationships between PLC activity and hemolysis.

Cloning of the Pseudomonas aeruginosa alkaline protease gene and secretion of the protease into the medium by Escherichia coli

Pseudomonas virulence is thought to depend on multiple characteristics, including the production of an extracellular alkaline protease. We report the isolation, from a PAO1 DNA genomic bank, of a cosmid carrying the structural gene coding for alkaline protease. By in vivo mutagenesis using transposon Tn1735, which functions as a transposable promoter, the expression of an 8.8-kilobase DNA fragment under control the tac promoter was obtained. When expressed in Escherichia coli, active alkaline protease was synthesized and secreted to the extracellular medium in the absence of cell lysis.

Secretion of extracellular proteins by Pseudomonas aeruginosa

Pseudomonas aeruginosa is a bacterial species of commercial value secreting numerous extracellular proteins, involved in pathogenesis. Most strains produce at least a lipase, a phospholipase, an alkaline phosphatase, an exotoxin and 2 proteases (elastase and alkaline protease). Various mechanisms for secretion of exoproteins appear to exist in P aeruginosa. Genetic analysis has led to the identification of 2 secretion pathways: i) a "general" secretion pathway, defined by the xcp mutations, which mediates secretion of most extracellular proteins, and; ii) an independent secretion pathway specific for alkaline protease. Our present knowledge on the pathways and components of the secretion machinery in P aeruginosa is reviewed in this article.

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

A toxic phospholipase D (PLD) is putatively involved in pathogenesis of Corynebacterium pseudotuberculosis infections. We report here the cloning and expression of the PLD gene (pld) in Escherichia coli. A cosmid library of DNA from C. pseudotuberculosis biovar ovis isolate Whetten 1 was constructed and screened for PLD-producing recombinants by plating them on LB agar containing sheep erythrocytes and equi factors. One recombinant, designated pCpO1, yielded a gene product which displayed synergistic hemolytic and sphingomyelinase D activities, both of which are characteristic of PLD. Subcloning into pUC19 yielded a recombinant, pCpO50, which contained a 1.8-kilobase insert. Analysis of supernatant fluids and cell extracts of cultures of E. coli(pCpO50) revealed sphingomyelinase activity and a protein of about 31,000 Mr, neither of which were detected in E. coli(pUC19). The 31-kilodalton protein also reacted with antibodies in serum from a sheep naturally infected with C. pseudotuberculosis, serum which also contained PLD-neutralizing antibodies. When Southern blots of BamHI digests of DNA from biovar ovis and biovar equi isolates of C. pseudotuberculosis were probed with pCpO50, bands of 4.8 and 1.9 kilobases, respectively, were seen, suggesting that the genome organization of pld is different for isolates from the two biovars.

Protein secretion in Pseudomonas aeruginosa: molecular cloning and characterization of the xcp-1 gene

Pleiotropic mutations (xcp) affecting the secretion of proteins in Pseudomonas aeruginosa have been previously characterized and mapped at three different loci on the chromosome. The xcp-1 gene, which is located at the 0-min region, was isolated from a genomic bank containing DNA from P. aeruginosa PAO1. The recombinant cosmid pLX25 complemented the xcp::Tn5-751 insertion mutation previously described. The xcp-1 gene was located on pLX25 by mapping the insertion point of transposon Tn5-751 and by deletion and subcloning analysis. The xcp-1 gene was expressed when transcription was initiated from a tac promoter, and a 26,000-dalton protein was identified in Escherichia coli minicells. The Xcp-1 protein was associated with the membrane fraction of E. coli. A 30-kilodalton outer membrane protein was also affected by the xcp::Tn5-751 mutation in P. aeruginosa. The possible correlation between Xcp-1 and this protein is discussed.

Mutations in the hemolytic-phospholipase C operon result in decreased virulence of Pseudomonas aeruginosa PAO1 grown under phosphate-limiting conditions

The phospholipase C (PLC) operon of Pseudomonas aeruginosa consists of plcS, which encodes a heat-labile secreted hemolysin, and two in-phase, overlapping genes, plcR1 and plcR2, which may encode Pi-regulatory genes. A 2.8-kilobase-pair deletion mutation in this operon was constructed, and a tetracycline resistance (Tcr) cartridge replaced the deleted sequences. A deletion mutant of strain PAO1 was obtained through recombination between the flanking regions of the mutated cloned PLC operon and the homologous chromosomal regions. The deletion of the chromosomal PLC operon and its replacement by the Tcr cartridge was confirmed by Southern hybridization. The deletion strain, PLC SR, is nonhemolytic. However, it retains PLC activity when measured on a synthetic substrate. A second mutant strain, PLC R, contains a deletion in the plcR genes. This mutant is more hemolytic and produces more enzymatic activity than PAO1. The virulence of both of these mutants was compared with that of PAO1 in the mouse burn model of infection. When mice were infected with cultures grown in a high-Pi medium, there was a 10-fold increase in the 50% lethal dose of the mutants compared with PAO1. In contrast, when the inoculum originated from low-Pi cultures, there was a 200- to 10,000-fold increase in the 50% lethal dose of the mutants over PAO1.

Characterization and expression in Escherichia coli of an endoglucanase gene of Pseudomonas fluorescens subsp. cellulosa

An endoglucanase gene of Pseudomonas fluorescens subsp. cellulosa present on plasmid pRUCL150 and expressed in Escherichia coli was subcloned in plasmid pBR322. Plasmid pRUCL153 contained the smallest DNA insert (2.9 kb) with endoglucanase activity. The plasmids directed the synthesis of a mostly periplasmic enzyme in E. coli and the level of enzyme activity was comparable in several strains. Analysis by non-denaturing polyacrylamide gel electrophoresis of the endoglucanase produced with various recombinant plasmids showed that it was unique. The endoglucanase gene on plasmid pRUCL153 was localized by physical mapping of independent transposon Tn5 insertions. Hence, its size was estimated to be approx. 1.3 kb. In vivo radioactive labelling of plasmid-encoded proteins using minicells, followed by denaturing polyacrylamide gel electrophoresis, allowed us to determine the size of the endoglucanase: Mr 40,000 for the precursor and Mr 38,000 for the mature enzyme. It was demonstrated that no cellulase operon, but a single gene, was cloned. The direction of transcription of the gene was determined by placing it under the control of the promoter of the lactose operon.

Exotoxin A of Pseudomonas aeruginosa: active, cloned toxin is secreted into the periplasmic space of Escherichia coli

We subcloned the structural gene for exotoxin A (ETA) of Pseudomonas aeruginosa in front of the tac promoter in an Escherichia coli expression vector and studied the intracellular location and properties of the protein product. The E. coli K-12 strain that carried this recombinant plasmid produced an immunoreactive protein that was identical to authentic ETA in size and in cytotoxic and ADP-ribosyl transferase activities per unit of immunoreactive material. The protein was predominantly in the periplasmic fraction; and a mutation in the secA gene blocked secretion, processing, and conversion of the protein to a fully toxic conformation. The results indicate that expression of the ETA gene in E. coli yields native ETA, which is localized within the periplasmic space. This organism may therefore serve as a useful host for studying structure and function in ETA.

Identification of a new phospholipase C activity by analysis of an insertional mutation in the hemolytic phospholipase C structural gene of Pseudomonas aeruginosa

The phospholipase C (PLC) gene of Pseudomonas aeruginosa encodes a heat-labile secreted hemolysin which is part of a Pi-regulated operon. The structural gene for PLC, plcS, was mutated in vitro by insertion of a tetracycline resistance gene cartridge. Gene replacement techniques were used to introduce the mutated plcS gene into the P. aeruginosa chromosome in place of the wild-type gene. The precise replacement of wild-type sequences by mutant sequences was confirmed by Southern hybridization. The mutant strain, designated PLC S, is nonhemolytic and lacks a 78-kilodalton protein corresponding to the size of the wild-type PLC. However, there is an additional phospholipase activity present in PLC S capable of hydrolyzing p-nitrophenylphosphorylcholine, a synthetic PLC substrate, and phosphatidylcholine. This enzymatic activity is not a result of a truncated product produced from the mutated plcS gene. The phospholipase activity of PLC S was identified as a nonhemolytic PLC.

Nucleotide sequences and expression in Escherichia coli of the in-phase overlapping Pseudomonas aeruginosa plcR genes

The translation products of chromosomal DNAs of Pseudomonas aeruginosa encoding phospholipase C (heat-labile hemolysin) have been examined in T7 promoter plasmid vectors and expressed in Escherichia coli cells. A plasmid carrying a 4.7-kilobase (kb) DNA fragment was found to encode the 80-kilodalton (kDa) phospholipase C as well as two more proteins with an apparent molecular mass of 26 and 19 kDa. Expression directed by this DNA fragment with various deletions suggested that the coding region for the two smaller proteins was contained in a 1-kb DNA region. Moreover, the size of both proteins was reduced by the same amount by an internal BglII-BglII DNA deletion, suggesting that they were translated from overlapping genes. Similar results were obtained with another independently cloned 6.1-kb Pseudomonas DNA, which in addition coded for a 31-kDa protein of opposite orientation. The nucleotide sequence of the 1-kb region above revealed an open reading frame with a signal sequence typical of secretory proteins and a potential in-phase internal translation initiation site. Pulse-chase and localization studies in E. coli showed that the 26-kDa protein was a precursor of a secreted periplasmic 23-kDa protein (PlcR1) while the 19-kDa protein (PlcR2) was mostly cytoplasmic. These results indicate the expression of Pseudomonas in-phase overlapping genes in E. coli.

Analysis of transcription from the trfA promoter of broad host range plasmid RK2 in Escherichia coli, Pseudomonas putida, and Pseudomonas aeruginosa

Reverse transcriptase mapping has been used to analyze transcription from the trfA promoter of broad host range plasmid RK2. The results show that trfA operon mRNA has the same 5' end in Pseudomonas aeruginosa, Pseudomonas putida, and Escherichia coli. The strengths of wild-type and mutant trfA promoters, which differ by defined base substitutions, have been compared and the positions of their transcriptional start sites determined. While these base substitutions do not alter the transcriptional start site, they do have marked effects on promoter strength which are broadly similar in each of the host species. A single base pair substitution, which lies in the region corresponding to the E. coli promoter consensus, brings about a large reduction in gene expression while the introduction of a second mutation, at a locus outside this region, has no further effect on promoter strength. The results indicate that these Pseudomonas species possess an RNA polymerase which recognizes the same region of the trfA promoter as that utilized by E. coli RNA polymerase. Within the limits of these observations it is clear that the trfA operon is transcribed from a single promoter which can function efficiently in diverse species, a property which may be important for its broad host range.

Nucleotide sequence and expression of a phosphate-regulated gene encoding a secreted hemolysin of Pseudomonas aeruginosa

A 3.3-kilobase-pair fragment of Pseudomonas aeruginosa DNA containing the phospholipase C (heat-labile hemolysin) gene was sequenced, and the location of the gene was determined. The gene product contains at its NH2 terminus a 38-amino acid sequence which structurally resembles the signal peptides of other secreted proteins but is unusually long and positively charged (6+). The location of the translation start codon was determined by constructing a series of plasmids in which the promoter of a transcription vector was ligated to Pseudomonas DNA containing deletions at the 5' end of the gene. The plasmids were used to transform Escherichia coli, and the resulting clones were assayed for hemolysin activity. In addition, sizes of truncated proteins produced by mutants with translation terminators introduced at specific sites were analyzed in E. coli maxicells. The gene is transcribed, starting just upstream of the hemolysin gene, as an mRNA of approximately 2,800 bases. Analysis of the nucleotide sequence, analysis of mutants in maxicells, and transcriptional studies indicate that the hemolysin is part of an operon composed of two genes. Phosphate regulation of the operon is at the transcriptional level. The location of the 5' end of the transcript was determined by S1 mapping.

Expression of the Pseudomonas aeruginosa PAK pilin gene in Escherichia coli

Pseudomonas aeruginosa is a piliated opportunistic pathogen. We have recently reported the cloning of the structural gene for the pilus protein, pilin, from P. aeruginosa PAK (B. L. Pasloske, B. B. Finlay, and W. Paranchych, FEBS Lett. 183:408-412, 1985), and in this paper we present evidence that this chimera (pBP001) expresses P. aeruginosa PAK pilin in Escherichia coli independent of a vector promoter. The strength of the promoter for the PAK pilin gene was assayed, and the cellular location of the pilin protein within E. coli was examined. This protein was present mainly in the inner membrane fraction both with and without its six-amino-acid leader sequence, but it was not assembled into pili.

Cloning and expression of the pilin gene of Pseudomonas aeruginosa PAK in Escherichia coli

Many strains of Pseudomonas aeruginosa possess pili which have been implicated in the pathogenesis of the organism. This report presents the cloning and expression in Escherichia coli of the gene encoding the structural subunit of the pili of P. aeruginosa PAK. Total DNA from this strain was partially digested with Sau3A and inserted into the cloning vector pUC18. Recombinant E. coli clones were screened with oligonucleotide probes prepared from the constant region of the previously published amino acid sequence of the mature pilin subunit. Several positive clones were identified, and restriction maps were generated. Each clone contained an identical 1.1-kilobase HindIII fragment which hybridized to the oligonucleotide probes. Western blot analysis showed that all of the clones expressed small amounts of the P. aeruginosa pilin subunit, which has a molecular mass of ca. 18,000. This expression occurred independently of the orientation of the inserted DNA fragments in the cloning vector, indicating that synthesis was directed from an internal promoter. However, subclones containing the 1.1-kilobase HindIII fragment in a specific orientation produced an order of magnitude more of the pilin subunit. While the expressed pilin antigen was located in both the cytoplasmic and outer membrane fractions of E. coli, none appeared to be polymerized into a pilus structure.

[Infections of the respiratory tract with Pseudomonas aeruginosa in cystic fibrosis]

The main cause of death in cystic fibrosis (CF) patients is progressive pulmonary insufficiency frequently associated with chronic infections of the respiratory tract by Pseudomonas aeruginosa. Bacteria of this species synthesize numerous extracellular products contributing to its pathogenicity. An alginate-like exopolysaccharide is characteristic for mucoid mutants predominating among P. aeruginosa isolates from CF patients. It interferes with immune defense mechanisms of the host and probably protects the bacteria against certain antibiotics. Furthermore, it is involved in the formation of bacterial microcolonies that resist mucociliary clearance, opsonisation, and phagocytosis. Exotoxin A and elastase are regarded as the most important among various extracellular enzymes involved in pulmonary injury in CF patients. Exotoxin A inhibits eukaryotic protein synthesis leading to necrosis; elastase, together with other Pseudomonas-proteases, induces hemorrhagic lesions and necrosis and seems to inactivate immunoglobulins and complement factors. Phospholipase C and glycolipid represent two hemolysins of P. aeruginosa that may contribute to cytopathogenic effects in infected lungs. No primary defect in the immunological defense mechanisms of CF patients has been described so far. Antibodies against various P. aeruginosa antigens including those mentioned above have been demonstrated, but a complete elimination of the bacteria from infected lungs has not been observed. Therapy of pulmonary P. aeruginosa infections in CF patients usually includes combinations of antibiotics of the beta-lactam and aminoglycoside type. Difficulties arise from an unusually high intrinsic resistance of P. aeruginosa as well as from poor penetration of many antibiotics into the sputum of CF patients. Therefore, future efforts to manage the Pseudomonas problem in CF will probably concentrate on prophylactic therapy, e.g. childhood vaccination of CF patients in order to prevent bacterial colonization of the respiratory tract.

Orientation and expression of the cloned hemolysin gene of Pseudomonas aeruginosa

The structural gene for Pseudomonas aeruginosa hemolysin, carried on recombinant plasmid pSL2 and cloned in Escherichia coli, was analyzed by insertional and deletional mutagenesis. Expression of the hemolysin was blocked by insertion of transposon Tn5 into different locations. Two of the mutants allowed detectable synthesis of truncated hemolysin polypeptides of two different sizes and thus defined the structural gene. The location of the hemolysin gene in the recombinant plasmid, and the direction of transcription, were further established by nuclease BAL 31 digestion, and by construction of gene fusions between hemolysin and beta-galactosidase. Evidently, the tet promoter contributed to the majority of the expression of cloned hemolysin gene, but the Pseudomonas promoter was present in the cloned DNA and was functional in E. coli since inactivation of the tet promoter either by Tn5 insertion or by deletion decreased synthesis of the 80-kDal hemolysin but did not fully abolish it.

Periplasmic production of correctly processed human growth hormone in Escherichia coli: natural and bacterial signal sequences are interchangeable

We have studied the synthesis, secretion, and processing of human growth hormone (hGH) in Escherichia coli transformed with plasmids engineered for the expression of hGH as a secreted product. In one plasmid, pPreHGH207-2, the coding sequence of the natural hGH precursor (pre-hGH) is placed under the control of the E. coli trp promoter. In a second plasmid, pAPH-1, a DNA fragment containing the E. coli alkaline phosphatase promoter and signal sequence codons is fused to the mature hGH coding sequence (pho-hGH). Most of the hGH was present in the osmotic shock fluids of E. coli cells containing either plasmid, indicating transport to the periplasmic space. Amino acid sequencing of the N termini of the pre-hGH and pho-hGH gene products revealed that both were processed correctly. Electrophoretic analysis of these polypeptides on reducing and nonreducing sodium dodecyl sulfate (SDS)-polyacrylamide (PA) gels indicates that periplasmic hGH is monomeric and contains the same two disulfide bonds as authentic hGH.

Legionella pneumophila surface antigens cloned and expressed in Escherichia coli are translocated to the host cell surface and interact with specific anti-Legionella antibodies

Escherichia coli clones that express Legionella pneumophila antigens were isolated from a plasmid genomic library, and their antigens were characterized by immunoblotting with rabbit anti-L. pneumophila sera. Because previous studies of L. pneumophila antigens by whole-cell radioimmunoprecipitation suggested that comigrating native antigens were surface localized, we conducted experiments to determine if the cloned antigens were surface expressed in E. coli. Aliquots of antisera were absorbed by intact cells of three representative antigen-producing E. coli clones, and surface-bound antibodies were acid eluted from the intact cells. Immunoblots made with selectively absorbed antisera and eluted antibodies confirmed that reactivity to the homologous cloned antigens could be specifically absorbed from the antisera and then eluted from the cells, demonstrating a surface (antibody-accessible) localization in the cloned state. Antibodies eluted from the surface of an E. coli clone that expressed a 19-kilodalton antigen reacted with the surface of L. pneumophila in a liquid-phase, whole-cell enzyme-linked immunosorbent assay. In addition, intact cells of this clone were used in an enzyme-linked immunosorbent assay to detect serum antibody. E. coli cells that express foreign antigens on their surfaces can be used to develop antigen-specific immunoassays and to affinity purify monospecific antibodies.

Cloning, nucleotide sequence, and expression in Escherichia coli of the exotoxin A structural gene of Pseudomonas aeruginosa

A 2760-base pair DNA segment of the Pseudomonas aeruginosa strain PA103 chromosome encoding the exotoxin A (ETA) structural gene has been cloned in Escherichia coli and the nucleotide sequence has been determined. Analysis of the 5'- and 3'-flanking regions indicate that ETA is translated from a monocistronic message. Comparison of the deduced NH2-terminal amino acid sequence with that determined by sequence analysis of the secreted protein indicates that ETA is made as a 638 amino acid precursor from which a highly hydrophobic leader peptide of 25 amino acids is removed during the secretion process. Data are presented that indicate a COOH-terminal location of the ADP-ribosylation activity of the molecule. Expression of the ETA coding sequence in E. coli under control of the E. coli trp promoter, but not the ETA promoter, results in the production of enzymatically and immunologically active protein. However, most of this material appears to be neither processed nor secreted. Comparison of the ETA amino acid and nucleotide sequences to those of diphtheria toxin revealed no significant homologies, indicating that these functionally similar toxins evolved from different genes.

Mechanism of protein excretion by gram-negative bacteria: Pseudomonas aeruginosa exotoxin A

Excretion of proteins by a cell with a double membrane may involve mechanisms different from secretion across a single membrane. We studied this problem with Pseudomonas aeruginosa exotoxin A. This 68,000-dalton protein was released as rapidly as it was completed; even after short pulse-labeling the cells contained neither the toxin nor a larger precursor. Excretion is evidently cotranslational, since in fractionated lysates the toxin was formed (almost entirely in the mature form) by the membrane-polysome complexes but not by the free polysomes. When the membrane was perturbed by 10% ethanol, the cells stopped excreting the toxin and they accumulated an immunoprecipitable, enzymatically active precursor of 71,000 daltons. The precursor was located entirely in the outer membrane on its outer surface. On removal of the ethanol, the cells again excreted mature toxin, but they did not process or release the previously accumulated precursor. Based on these data, a model for the excretion of exotoxin A is presented.