Pseudomonas aeruginosa cytotoxin: the nucleotide sequence of the gene and the mechanism of activation of the protoxin

Pseudomonas aeruginosa Cytotoxins: Mechanisms of Cytotoxicity and Impact on Inflammatory Responses

Pseudomonas aeruginosa is one of the most virulent opportunistic Gram-negative bacterial pathogens in humans. It causes many acute and chronic infections with morbidity and mortality rates as high as 40%. P. aeruginosa owes its pathogenic versatility to a large arsenal of cell-associated and secreted virulence factors which enable this pathogen to colonize various niches within hosts and protect it from host innate immune defenses. Induction of cytotoxicity in target host cells is a major virulence strategy for P. aeruginosa during the course of infection. P. aeruginosa has invested heavily in this strategy, as manifested by a plethora of cytotoxins that can induce various forms of cell death in target host cells. In this review, we provide an in-depth review of P. aeruginosa cytotoxins based on their mechanisms of cytotoxicity and the possible consequences of their cytotoxicity on host immune responses.

[Genome analysis-based studies on bacterial genetic diversity]

There are a huge number of bacterial species on earth, and a huge intra-species genomic diversity are also observed in many bacteria. The high ability of bacteria to acquire foreign DNA and the presence of various mobile genetic elements contribute the generation of such genomic diversity. During the biochemical and genetic analysis of a Pseudomonas aeruginosa toxin, called cytotoxin, and its converting phage, which I first engaged in my research carrier, I became very interested in the genetic diversity of bacteria and mobile genetic elements such as bacteriophages, and realized the usefulness and power of genome analysis. Since then, I have been involved in genome analyses of various pathogenic bacteria such as enterohemorrhagic Escherichia coli (EHEC), commensal bacteria of human and other animals, and bacteria or bacterial communities in natural environments. I was so lucky that I jumped in this research field at the very begging of genome analyses and experienced a very exciting time of surprisingly rapid advance in genome sequencing technologies which revolutionized a wide range of biology. In this article, I first review the main findings which our group obtained from the genome analyses on the P. aeruginosa cytotoxin converting phage and those on the evolution and genomic diversity of EHEC and related bacteria. The results of our analyses of Rickettsiaceae family genomes, which show surprisingly very low genomic diversity, and genome sequence-based analyses of an intrahospital bacterial outbreak and within-host genomic diversity are also summarized.

Characterization of serracin P, a phage-tail-like bacteriocin, and its activity against Erwinia amylovora, the fire blight pathogen

Serratia plymithicum J7 culture supernatant displayed activity against many pathogenic strains of Erwinia amylovora, the causal agent of the most serious bacterial disease of apple and pear trees, fire blight, and against Klebsiella pneumoniae, Serratia liquefaciens, Serratia marcescens, and Pseudomonas fluorescens. This activity increased significantly upon induction with mitomycin C. A phage-tail-like bacteriocin, named serracin P, was purified from an induced culture supernatant of S. plymithicum J7. It was found to be the only compound involved in the antibacterial activity against sensitive strains. The N-terminal amino acid sequence analysis of the two major subunits (23 and 43 kDa) of serracin P revealed high homology with the Fels-2 prophage of Salmonella enterica, the coliphages P2 and 168, the phiCTX prophage of Pseudomonas aeruginosa, and a prophage of Yersinia pestis. This strongly suggests a common ancestry for serracin P and these bacteriophages.

Epidemic population structure of Pseudomonas aeruginosa: evidence for a clone that is pathogenic to the eye and that has a distinct combination of virulence factors

The genetic structure of a population of Pseudomonas aeruginosa, isolated from patients with keratitis, endophthalmitis, and contact lens-associated red eye, contact lens storage cases, urine, ear, blood, lungs, wounds, feces, and the environment was determined by multilocus enzyme electrophoresis. The presence and characteristics of virulence factors were determined by restriction fragment length polymorphism analysis with DNA probes for lasA, lasB, aprA, exoS, exoT, exoU, and ctx and by zymography of staphylolysin, elastase, and alkaline protease. These analyses revealed an epidemic population structure of P. aeruginosa, characterized by frequent recombination in which a particular successful clone may increase, predominate for a time, and then disappear as a result of recombination. Epidemic clones were found among isolates from patients with keratitis. They were characterized by high activity of a hitherto-unrecognized size variant of elastase, high alkaline protease activity, and possession of the exoU gene encoding the cytotoxic exoenzyme U. These virulence determinants are not exclusive traits in strains causing keratitis, as strains with other properties may cause keratitis in the presence of predisposing conditions. There were no uniform patterns of characteristics of isolates from other types of infection; however, all strains from urinary tract infections possessed the exoS gene, all strains from environment and feces and the major part of keratitis and wound isolates exhibited high elastase and alkaline protease activity, and all strains from feces showed high staphylolysin activity, indicating that these virulence factors may be important in the pathogenesis of these infectious diseases.

The R-type pyocin of Pseudomonas aeruginosa is related to P2 phage, and the F-type is related to lambda phage

Pseudomonas aeruginosa produces three types of bacteriocins: R-, F- and S-type pyocins. The S-type pyocin is a colicin-like protein, whereas the R-type pyocin resembles a contractile but non-flexible tail structure of bacteriophage, and the F-type a flexible but non-contractile one. As genetically related phages exist for each type, these pyocins have been thought to be variations of defective phage. In the present study, the nucleotide sequence of R2 pyocin genes, along with those for F2 pyocin, which are located downstream of the R2 gene cluster on the chromosome of P. aeruginosa PAO1, was analysed in order to elucidate the relationship between the pyocins and bacteriophages. The results clearly demonstrated that the R-type pyocin is derived from a common ancestral origin with P2 phage and the F-type from lambda phage. This notion was supported by identification of a lysis gene cassette similar to those for bacteriophages. The gene organization of the R2 and F2 pyocin gene cluster, however, suggested that both pyocins are not simple defective phages, but are phage tails that have been evolutionarily specialized as bacteriocins. A systematic polymerase chain reaction (PCR) analysis of P. aeruginosa strains that produce various subtypes of R and F pyocins revealed that the genes for every subtype are located between trpE and trpG in the same or very similar gene organization as for R2 and F2 pyocins, but with alterations in genes that determine the receptor specificity.

The complete nucleotide sequence of phi CTX, a cytotoxin-converting phage of Pseudomonas aeruginosa: implications for phage evolution and horizontal gene transfer via bacteriophages

phi CTX is a cytotoxin-converting phage isolated from Pseudomonas aeruginosa. In this study, we determined the complete nucleotide sequence of the phi CTX phage genome. The precise genome size was 35,538 bp with 21 base 5'-extruding cohesive ends. Forty-seven open reading frames (ORFs) were identified on the phi CTX genome, including two previously identified genes, ctx and int. Among them, 15 gene products were identified in the phage particle by protein microsequencing. The most striking feature of the phi CTX genome was an extensive homology with the coliphage P2 and P2-related phages; more than half of the ORFs (25 ORFs) had marked homology to P2 genes with 28.9-65.8% identity. The gene arrangement on the genome was also highly conserved for the two phages, although the G + C content and codon usage of most phi CTX genes were similar to those of the host P. aeruginosa chromosome. In addition, phi CTX was found to share several common features with P2, including the morphology, non-inducibility, use of lipopolysaccharide core oligosaccharide as receptor and Ca(2+)-dependent receptor binding. These findings indicate that phi CTX is a P2-like phage well adapted to P. aeruginosa, and provide clear evidence of the intergeneric spread and evolution of bacteriophages. Furthermore, comparative analysis of genome structures of phi CTX, P2 and other P2 relatives revealed the presence of several hot-spots where foreign DNAs, including the cytotoxin gene, were inserted. They appear to be deeply concerned in the acquisition of various genes that are horizontally transferred by bacteriophage infection.

Purification and characterization of procytotoxin of Pseudomonas aeruginosa. Dimer to monomer conversion of protoxin by proteolytic activation

Cytotoxin of Pseudomonas aeruginosa is a cytolytic toxin that forms a pore on the target membrane by oligomerizing into a pentamer. This toxin is produced as an inactive precursor (proCTX) and is converted to an active form by proteolytic cleavage at the C terminus. We purified proCTX to apparent homogeneity and characterized it in a comparison with the active toxin. ProCTX bound to the erythrocyte membrane but did not form an oligomer on the membrane, hence the lack of hemolytic activity in proCTX. Circular dichroic experiments showed that active and proCTX have similar beta-sheet dominant structures. Intrinsic fluorescence analysis indicated that a molecule-buried tryptophan residue(s) of proCTX was exposed to the surface of the molecule as a result of conversion to the active form. In analytical gel filtration, chemical cross-linking, and analytical ultracentrifugation experiments, dimer to monomer conversion occurred with proteolytic activation.

Hyperproduction, purification, and mechanism of action of the cytotoxic enterotoxin produced by Aeromonas hydrophila

A gene encoding the cytotoxic enterotoxin (Act) from Aeromonas hydrophila was hyperexpressed with the pET, pTRX, and pGEX vector systems. Maximum toxin yield was obtained with the pTRX vector. Approximately 40 to 60% of Act was in a soluble form with the pTRX and pET vector systems. The toxin protein was purified to homogeneity by a combination of ammonium sulfate precipitation and fast protein liquid chromatography-based column chromatographies, including hydrophobic, anion-exchange, sizing, and hydroxylapatite chromatographies. Purified mature toxin migrated as a 52-kDa polypeptide on a sodium dodecyl sulfate (SDS)polyacrylamide gel that reacted with Act-specific antibodies in immunoblots. The minimal amount of toxin needed to cause fluid secretion in rat ileal loops was 200 ng, and the 50% lethal dose for mice was 27.5 ng when injected intravenously. Binding of the toxin to erythrocytes was temperature dependent, with no binding occurring at 4 degrees C. However, at 37 degrees C the toxin bound to erythrocytes within 1 to 2 min. It was determined that the mechanism of action of the toxin involved the formation of pores in erythrocyte membranes, and the diameter of the pores was estimated to be 1.14 to 2.8 nm, as determined by the use of saccharides of different sizes and by electron microscopy. Calcium chloride prevented lysis of erythrocytes by the toxin; however, it did not affect the binding and pore-forming capabilities of the toxin. A dose-dependent reduction in hemoglobin release from erythrocytes was observed when Act was preincubated with cholesterol, but not with myristylated cholesterol. With 14C-labeled cholesterol and gel filtration, the binding of cholesterol to Act was demonstrated. None of the other phospholipids and glycolipids tested reduced the hemolytic activity of Act. The toxin also appeared to undergo aggregation when preincubated with cholesterol, as determined by SDS-polyacrylamide gel electorphoresis. As a result of this aggregation, Act's capacity to form pores in the erythrocyte membrane was inhibited.

Lambda-toxin of Clostridium perfringens activates the precursor of epsilon-toxin by releasing its N- and C-terminal peptides

The effect of lambda-toxin, a thermolysin-like metalloprotease of Clostridium perfringens, on the inactive epsilon-prototoxin produced by the same organism was examined. When the purified epsilon-prototoxin was incubated with the purified lambda-toxin at 37 C for 2 hr, the 32.5-kDa epsilon-prototoxin was processed into a 30.5-kDa polypeptide, as determined by SDS-polyacrylamide gel electrophoresis. A mouse lethality test showed that the treatment activated the prototoxin: the 50% lethal doses (LD50) of the prototoxin with and without lambda-toxin treatment were 110 and 70,000 ng/kg of body weight, respectively. The lethal activity of the prototoxin activated by lambda-toxin was comparable to that with trypsin plus chymotrypsin and higher than that with trypsin alone: LD50 of the prototoxin treated with trypsin and trypsin plus chymotrypsin were 320 and 65 ng/kg of body weight, respectively. The epsilon-toxin gene was cloned and sequenced. Determination of the N-terminal amino acid sequence of each activated epsilon-prototoxin revealed that lambda-toxin cleaved between the 10th and 11th amino acid residues from the N-terminus of the prototoxin, while trypsin and trypsin plus chymotrypsin cleaved between the 13th and 14th amino acid residues. The molecular weight of each activated epsilon-prototoxin was also determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The C-terminus deduced from the molecular weight is located at the 23rd or 30th amino acid residue from the C-terminus of the prototoxin, suggesting that removal of not only N-terminal but also C-terminal peptide is responsible for activation of the prototoxin.

New gene from nine Bacillus sphaericus strains encoding highly conserved 35.8-kilodalton mosquitocidal toxins

A new gene encoding a 35.8-kDa mosquitocidal toxin (Mtx3; 326 amino acids) was isolated from Bacillus sphaericus SSII-1 DNA. Mtx3 is a new type of mosquitocidal toxin with homology to the Mtx2 mosquitocidal toxin of B. sphaericus SSII-1, the epsilon-toxin of Clostridium perfringens, and the cytotoxin of Pseudomonas aeruginosa. The mtx3 gene is highly conserved and widely distributed in both high- and low-toxicity mosquito larvicidal strains of B. sphaericus.

A Bacillus sphaericus gene encoding a novel type of mosquitocidal toxin of 31.8 kDa

A size-fractionated genomic library of Bacillus sphaericus strain SSII-1 was constructed and screened for toxicity against larvae of the mosquito Culex quinquefasciatus (Cq). One toxin-producing clone, pS35, was identified and a 2.7-kb subclone was completely sequenced. An open reading frame of 879 bp encoding a 31.8-kDa protein (designated Mtx2) was identified. Purified, recombinant Mtx2 was toxic to Cq larvae. Mtx2 shows no significant homology to known insecticidal toxins, but has homology to two toxins active against mammalian cells, namely the epsilon-toxin of Clostridium perfringens and the cytotoxin of Pseudomonas aeruginosa. Thus, Mtx2 represents a new type of mosquitocidal toxin.

Protein crystallography in Australia

Protein crystallography is the study of the three-dimensional shapes of proteins at near atomic resolution. The field has provided a tremendous insight into the workings of numerous biological processes over the last few decades. The field is presently undergoing a massive worldwide expansion, not only in academic laboratories but also in the pharmaceutical industry. The main driving force for this expansion is the possibility of using the three-dimensional atomic structures of proteins to design lead drugs and to improve the action of existing drugs. This expansion in the field has been mirrored in Australia where the number of protein crystallography groups has more than trebled in the last five years. The work in the Protein Crystallography Unit at St Vincent's Institute of Medical Research has centered on the structural elucidation of membrane proteins and structure-based inhibitor studies of a protein family that attack certain anti-cancer drugs.

Amino-acid residues involved in biological functions of the cytolytic enterotoxin from Aeromonas hydrophila

Some amino acid (aa) residues within the cytolytic enterotoxin (Act) of Aeromonas hydrophila essential for biological activity were identified. Act is a 52-kDa polypeptide, possessing hemolytic, cytotoxic and enterotoxic activities. By deletion analysis, generation of anti-peptide Ab, and site-directed mutagenesis we showed that two regions in Act (aa 245-274 and 361-405) were very important for biological functions. As shown by competitive inhibition assays, peptide 2 (aa 245-274) blocked cytotoxic activity of Act, and aa Tyr256, Trp270 and Gly274 were essential for cytotoxicity. Within peptide 3 (aa 361-405), Trp394 and Trp396 were important for biological activities. Mutations in other regions of the toxin (e.g., Gly169, Asp170, Gly171, Trp172, Asn177,178, Asp179 and His144,209,355) also decreased biological activity. The reactivity of these mutant toxins with Ab in immunoblots was not altered. Data reported in this study suggested the role of some aa residues in biological function(s) of Act.

Pseudomonas aeruginosa cytotoxin: the Asp197-Gly-Asp-Tyr-His-Tyr-His-Tyr202 containing loop is critical for plasma membrane binding

The cytotoxin from Pseudomonas aeruginosa is a pore-forming toxin that binds specifically to water channel-related molecules of the erythrocyte membrane. Here, we have defined a domain, Asp197-Gly-Asp-Tyr-His-Tyr202 of the cytotoxin, to be essential for receptor binding. Cytotoxin point mutants from the recombinant gene carrying substitutions in the domain were characterized in terms of inhibiting the binding of radioiodinated natural cytotoxin to rat erythrocyte and producing cytotoxic effects in human granulocytes. A synthetic peptide representing residues 191-211 of the cytotoxin acted as a competitive inhibitor at a concentration of 10(-5) M. In contrast, two other cytotoxin-specific peptides were inactive. Structure prediction of the binding sequence shows a loop structure with similarities to the sequence around His332 in Aeromonas aerolysin essential to receptor binding.

Mechanism of the cytolytic action of Pseudomonas aeruginosa cytotoxin: oligomerization of the cytotoxin on target membranes

Pseudomonas aeruginosa cytotoxin (CTX) is thought to be a pore-forming polypeptide of 29 kDa. To study whether CTX assembles into oligomer on target membranes, we solubilized membrane-bound toxin with 1% sodium dodecyl sulfate (SDS) at 25 degrees C and analyzed its molecular size using SDS-polyacrylamide gel electrophoresis and immunoblot analysis. The results indicate that CTX forms a complex of approximately 145 kDa on the surface of erythrocytes and lipid vesicles, and that the complex formation is closely correlated with the toxin-induced permeabilization of target membranes. Thus, CTX may assemble into a pore-forming oligomer on target membranes.

Cytotoxin-converting phages, phi CTX and PS21, are R pyocin-related phages

phi CTX is a temperate phage of Pseudomonas aeruginosa harbouring the ctx gene that encodes cytotoxin (CTX). We identified phi CTX as an R pyocin-related phage, by serological and molecular analysis, based on the findings that the infectivity of the phage was inhibited with the antisera directed R pyocins and R pyocin-related phages and that the phi CTX genome showed DNA homology to the genome of PS17 (a representative of the R pyocin-related phages) as well as to the pyocin R2 genes. Another new CTX-converting, R pyocin-related phage named PS21 was isolated from a CTX-producing strain of P. aeruginosa, suggesting the distribution of the ctx gene by certain members of R pyocin-related phage family.

Identification of the lipopolysaccharide core region as the receptor site for a cytotoxin-converting phage, phi CTX, of Pseudomonas aeruginosa

A temperate phage, phi CTX, is a cytotoxin-converting phage of Pseudomonas aeruginosa. In this study, we characterized the lipopolysaccharide (LPS) structures of phi CTX-resistant mutants derived from phi CTX-sensitive strains. phi CTX infectivity was neutralized by LPS preparations derived from sensitive strains but not by those from resistant strains. phi CTX-resistant mutants had lower-molecular-weight rough (R)-type LPS than the parental strains and lacked the reactivity of some anti-LPS core monoclonal antibodies. Some LPS core components were lacking or significantly decreased in the resistant mutants. These results suggested that a receptor site of the cytotoxin-converting phage phi CTX was the LPS core region and that especially L-rhamnose and D-glucose residues in the outer core were involved in phage binding. The host range of phi CTX was nearly O-serotype dependent, probably because of the diversity of the LPS core structure among P. aeruginosa strains. phi CTX bound to most strains of Homma serotypes A, G, and I but not to strains of serotypes B and E. Furthermore, we found that a genetic locus specifying phi CTX sensitivity (and consequently participating in the biosynthesis of part of the LPS core) existed in or near the locus participating in the determination of O-serotype specificity (somA), which has been mapped between leu-10 and eda-9001. phi CTX, as well as anti-LPS core monoclonal antibodies, will be a good tool for structural characterization of the P. aeruginosa LPS core region.

Pore-forming Pseudomonas aeruginosa cytotoxin

Pseudomonas aeruginosa procytotoxin protein is processed C-terminally during bacterial autolysis to generate the active 29-kDa cytotoxin molecule. Binding to target cell membranes is dependent upon Cys23 and Cys215 and a domain flanked to Cys215. On rabbit erythrocytes, cytotoxin binds to a 28-kDa peptide of a glycoprotein, its N-terminus shows high homology to channel integral membrane protein CHIP28. At concentrations of more than 3 x 10(-9) M, cytotoxin increases plasma membrane permeability of most eucaryotic cells investigated. The role of cytotoxin in the formation of pores with a diameter of 2 nm on mammalian cells is discussed. The cytotoxin effects are coordinated with other pseudomonal products and the resultant concept of pathogenesis is presented.

Pseudomonas aeruginosa cytotoxin as a pathogenicity factor in a systemic infection of leukopenic mice

The effect of Pseudomonas aeruginosa cytotoxin was assessed in leukopenic outbred Swiss male mice (20 g) using a high cytotoxin-producing strain (PA158) and its cytotoxin-deficient isogenic mutant (PA114F5) generated by Tn7::Tn5 transposon mutagenesis of PA158. Leukopenia was induced by intraperitoneal (i.p.) administration of cyclophosphamide (150 micrograms/g). Anesthetized mice were infected via a 4 mm incision on the shaved back with 300 CFU/mouse (9 LD50; expected death rate 85%). Precleared mouse cytotoxin-specific heat inactivated rabbit polyclonal antibody (RPA) was administered i.p. (0.2 ml) 24 hr before challenge. Controls received i.p. normal rabbit serum, RPA, cyclophosphamide alone, or a sham procedure. Challenge with the high cytotoxin-producing strain PA158 caused earlier and a significantly greater mortality than that observed with a cytotoxin-deficient strain PA114F5 (P < 0.01). Cytotoxin-specific polyclonal antibody was protective. Pretreatment with antibody decreased the mortality rate following challenge with PA158 from 88.9% to 27.8% (P < 0.01). Pretreatment with antibody decreased the mortality rate following challenge with PA114F5 from 27.8% to 5.6% (P < 0.05). These results demonstrate that P. aeruginosa cytotoxin contributes to the pathogenicity of the organism and that cytotoxin antibody is protective in a systemic P. aeruginosa infection in leukopenic mice.

The two Staphylococcal bi-component toxins, leukocidin and gamma-hemolysin, share one component in common

Staphylococcal bi-component toxins, leukocidin and gamma-hemolysin, consist of two protein components, i.e. F and S for leukocidin and H gamma I and H gamma II for gamma-hemolysin. In this study we purified H gamma I and H gamma II to homogeneity from the culture medium of Staphylococcus aureus RIMD 310925 and compared their properties with those of F and S purified from the same source. The N-terminal 59- and C-terminal 2-residue amino acid sequences, apparent molecular mass, and isoelectric point of purified H gamma I were the same as those of F. In an Ouchterlony double diffusion test a fused line without spur was formed between F and H gamma I using either anti-F or anti-H gamma I antibodies. A synergistic action of F and H gamma II caused hemolysis of human red blood cells, and H gamma I acted synergistically with S to exhibit leukocidin activity. We conclude that the two toxins share one protein component (F = H gamma I) in common and leukocidin- and gamma-hemolysin-specific activities are determined by S and H gamma II, respectively. It is also reported that the N-terminal 58-residue sequence of H gamma II is 72% similar to the corresponding sequence of S.

Molecular analysis of a cytotoxin-converting phage, phi CTX, of Pseudomonas aeruginosa: structure of the attP-cos-ctx region and integration into the serine tRNA gene

The Pseudomonas aeruginosa ctx gene encoding cytotoxin is carried by a temperate phage phi CTX. The genome of phi CTX is a 35.5 kb double-stranded DNA with cohesive ends (cos). It is unique in that the ctx gene and attP site of phi CTX exist very close to the respective cohesive ends. In this study, we determined the structure of this attP-cos-ctx region. The termini of phi CTX are 21-base 5' extended-single-stranded DNAs. The ctx gene is located 361 bp downstream of the left end (cosL). The attP core sequence of 30 bp exists only 647 bp apart from the right end (cosR). The attP-cos-ctx region contains six kinds of repeats and integration host factor-binding sequences and showed sequence-directed static bends, suggesting its potential to form a highly ordered structure. In addition, phi CTX was found to integrate into the serine tRNA gene which was mapped to the 43-45 min region on the P. aeruginosa chromosome.

Site-directed mutagenesis of the Pseudomonas aeruginosa cytotoxin for probing toxic activity

The location of amino acids with direct influence on toxic activity in the pore-forming cytotoxin from Pseudomonas aeruginosa was tested by site-directed mutagenesis. Mutant fragments obtained by the polymerase-chain reaction were subcloned into a cytotoxin gene-bearing plasmid to minimize the possibility of amplification error. Our data suggest an important role of cysteines for toxic activity of cytotoxin. Furthermore, domain-facilitating-binding to plasma membranes is located on the C-terminal side of Cys215.

Nucleotide sequence of leukocidin S-component gene (lukS) from methicillin resistant Staphylococcus aureus

The nucleotide sequence of lukS gene encoding S-component of Staphylococcal leukocidin from methicillin resistant Staphylococcus aureus (MRSA) was determined. The structural gene of lukS consisted of 857 base pairs. An open reading frame that could encode a 35,556 dalton polypeptide consisting of 315 amino acids was assigned. The molecular size of the polypeptide predicted from the amino acid composition was close to the value of pre-matured S-component determined in DNA-directed transcription/translation system. Inspection of the amino acid sequence deduced from nucleotide sequence of lukS and that from S-component of leukocidin clarified that pre-matured S-component contains a typical signal sequence at the NH2 terminus. The amino acid sequence of predicted matured S-component correlated exactly with the known N-terminal 50 amino acid sequence of S-component from MRSA and S. aureus V8. The molecular size of the predicted matured protein was also close to the value of S-component determined in both MRSA and S. aureus V8. The nucleotide sequence of the 5'-flanking region showed the presence of the consensus sequence of ribosome binding site, Pribnow box and the RNA polymerase recognition site in Escherichia coli.

Physical mapping of virulence-associated genes in Pseudomonas aeruginosa by transverse alternating-field electrophoresis

The relative chromosomal locations of 20 virulence-associated genes in four clinical isolates of Pseudomonas aeruginosa were investigated by using transverse alternating-field electrophoresis. Each strain had a characteristic restriction pattern when digested with either SpeI or DraI and electrophoresed with 15-s pulses. All four strains had restriction fragments that hybridized with each of the gene probes used, although there were variations in fragment size. An SpeI physical map constructed by Ratnaningsih et al. (E. Ratnaningsih, S. Dharmsthiti, V. Krishnapillai, A. Morgan, M. Sinclair, and B. W. Holloway, J. Gen. Microbiol. 136:2351-2357, 1990) for one of these strains, PAO1, was used to identify the location of 11 previously unmapped genes. The physical locations of the remaining genes were found to be consistent with their genetically mapped loci. Whereas phospholipase C and alginate structural and regulatory genes were associated in three separate clusters in the early, middle, and late regions of the chromosome, no virulence cluster was identified. Our data suggest that the pathogenicity of P. aeruginosa results from the gradual acquisition of genes encoding various virulence determinants.

Pore-forming bacterial protein hemolysins (cytolysins)

Protein toxins forming pores in biological membranes occur frequently in Gram-positive and Gram-negative bacteria. They kill either bacteria or eukaryotic cells (at most, a few seem to act on both groups of organisms). Most of the toxins affecting eukaryotes have clearly been shown to be related to the pathogenicity of the producing organisms. Toxin formation frequently involves a number of genes which encode the toxin polypeptide as well as proteins for toxin activation and secretion. Regulation of toxin production is usually coupled with that of the synthesis of a number of other virulence factors. Iron is the only known environmental factor that regulates transcription of a number of toxin genes by a Fur repressor-type mechanism, as has been originally described in Escherichia coli. Interestingly, the thiol-activated hemolysins (cytolysins) of Gram-positive bacteria contain a single cysteine which can be replaced by alanine without affecting the cytolytic activity. The Gram-negative hemolysins (cytolysins) are usually synthesized as precursor proteins, then covalently modified to yield an active hemolysin and secreted via specific export systems, which differ for various types of hemolysins.

Phage-conversion of cytotoxin production in Pseudomonas aeruginosa

We isolated a temperate phage which carried the cytotoxin gene (ctx) from a cytotoxin (CTX)-producing Pseudomonas aeruginosa strain, PA158. The phage, phi CTX, had a head with a hexagonal outline and a contractile tail with tail fibres. The phage genome was a linear double-stranded 35.5 kb DNA with single-stranded cohesive ends (cos). The attP, cos and ctx genes were all located very close to one another within a 2.3 kb segment on the phage genome in the order given (in the circular form). phi CTX converted CTX non-producing P. aeruginosa strains into CTX producers. A single copy of phi CTX DNA was integrated at the same site on the host chromosome (attB) in every lysogen, including PA158. However, the amount of CTX produced in these lysogens varied from strain to strain and was less than that in PA158.

Aerolysin of Aeromonas sobria: evidence for formation of ion-permeable channels and comparison with alpha-toxin of Staphylococcus aureus

Aerolysin from Aeromonas sobria AB3 was isolated and purified. The pure toxin formed sodium dodecyl sulfate-insoluble oligomers in a lipidic environment. The addition of aerolysin to the aqueous phase bathing lipid bilayer membranes resulted in the formation of ion-permeable channels which had a single-channel conductance of about 70 pS in 0.1 M KCl. This defines the toxin as a channel-forming component similar to other toxins but without any indication for an association-dissociation reaction, since the channels had a long lifetime at low voltages. At voltages higher than 50 mV, the aerolysin channel switched into a closed state with a low residual conductance. The single-channel conductance was a linear function of the total aqueous conductance, which suggested that the toxin oligomers formed aqueous channels with an estimated minimal diameter of about 0.7 nm. The aerolysin pores were found to be slightly anion selective. The pore-forming properties of aerolysin were compared with those of alpha-toxin of Staphylococcus aureus. Both aerolysin and alpha-toxin share secondary structure features, must oligomerize to form pores in lipid bilayer membranes, and form channels with similar properties.

The cytotoxin of Pseudomonas aeruginosa: cytotoxicity requires proteolytic activation

The primary structure of a cytotoxin from Pseudomonas aeruginosa was determined by sequencing of the structural gene. The cytotoxin (31,700 Mr) lacks an N-terminal signal sequence for bacterial secretion but contains a pentapeptide consensus sequence commonly found in prokaryotic proteins which function in a TonB-dependent manner. The cytotoxin gene has a [G + C]-content of 53.8% which is considerably lower than generally observed for genes from Pseudomonas aeruginosa. The cytotoxin gene was exclusively detected in strain 158 but not in three other clinical isolates, as determined by Southern and Northern hybridization. The latter technique revealed that the toxin is translated from monocistronic mRNA. The promoter of the cytotoxin is inactive in Escherichia coli. Upon site-directed modification of the 5'-noncoding region by the polymerase chain reaction the gene was expressed under control of the trc-promoter. The gene product obtained in Escherichia coli was nontoxic. Toxicity was induced by subsequent treatment with trypsin. [35S]methionine-labeled cytotoxin with high specific radioactivity was obtained by in vitro transcription/translation. Like [125I] labeled material from Pseudomonas aeruginosa this polypeptide bound to membrane preparations from Ehrlich ascites cells, as evidenced by sedimentation through a sucrose gradient at neutral pH.