Genetic mapping of Vibrio cholerae enterotoxin structural genes

Sialoglycan binding patterns of bacterial AB5 toxin B subunits correlate with host range and toxicity, indicating evolution independent of A subunits

Many pathogenic bacteria secrete AB₅ toxins that can be virulence factors. Cytotoxic A subunits are delivered to the cytosol following B subunit binding to specific host cell surface glycans. Some B subunits are not associated with A subunits, for example, YpeB of Yersinia pestis, the etiologic agent of plague. Plague cannot be eradicated because of Y. pestis' adaptability to numerous hosts. We previously showed selective binding of other B₅ pentamers to a sialoglycan microarray, with sialic acid (Sia) preferences corresponding to those prominently expressed by various hosts, for example, N-acetylneuraminic acid (Neu5Ac; prominent in humans) or N-glycolylneuraminic acid (Neu5Gc; prominent in ruminant mammals and rodents). Here, we report that A subunit phylogeny evolved independently of B subunits and suggest a future B subunit nomenclature based on bacterial species names. We also found via phylogenetic analysis of B subunits, which bind Sias, that homologous molecules show poor correlation with species phylogeny. These data indicate ongoing lateral gene transfers between species, including mixing of A and B subunits. Consistent with much broader host range of Y. pestis, we show that YpeB recognizes all mammalian Sia types, except for 4-O-acetylated ones. Notably, YpeB alone causes dose-dependent cytotoxicity, which is abolished by a mutation (Y77F) eliminating Sia recognition, suggesting that cell proliferation and death are promoted via lectin-like crosslinking of cell surface sialoglycoconjugates. These findings help explain the host range of Y. pestis and could be important for pathogenesis. Overall, our data indicate ongoing rapid evolution of both host Sias and pathogen toxin-binding properties.

The Vibrio cholerae genome contains two unique circular chromosomes

Vibrio cholerae, the etiologic agent of the diarrheal disease cholera, is a Gram-negative bacterium that belongs to the gamma subdivision of the family Proteobacteriaceae. The physical map of the genome has been reported, and the genome has been described as a single 3.2-Mb chromosome [Majumder, R., et al. (1996) J. Bacteriol. 178, 1105-1112]. By using pulsed-field gel electrophoresis of genomic DNA immobilized in agarose plugs and digested with the restriction enzymes I-CeuI, SfiI, and NotI, we have also constructed the physical map of V. cholerae. Our analysis estimates the size of the genome at 4.0 Mb, 25% larger than the physical map reported by others. Our most notable finding is, however, that the V. cholerae chromosome appears to be not the single chromosome reported but two unique and separate circular megareplicons.

Genetic analysis in vibrio

Bacteria of the genus Vibrio are remarkably diverse, and until recently the methodology for genetic analysis consisted of a patchwork of different approaches, many of which were narrowly applicable to a single species. The invention of the recombinant DNA technology and the subsequent innovations in transposon mutagenesis and in transductive and conjugative gene transfer techniques have led to the development of very powerful and general strategies for genetic analysis of species of Vibrio. The striking synergy of combining recombinant DNA, transposon, and gene transfer methods is particularly evident in the construction of transposons which generate gene fusions and of broad host range plasmids which deliver transposons and mutated genes and which mobilize chromosomes. With such tools it should be possible to perform advanced genetic analysis on the many undomesticated species of Vibrio still to be explored.

High-frequency spontaneous mutation of classical Vibrio cholerae to a nonmotile phenotype

The species Vibrio cholerae contains within it two biotypes, classical and El Tor, both of which are motile. Phenotypic expression of motility was unaffected by type of growth medium, salt concentration, pH, or temperature of incubation. However, seven strains of classical V. cholerae produced spontaneous nonmotile mutants at an unusually high frequency (ca. 10(-4)), while no mutants were detected for all three El Tor strains examined. No revertants of these nonmotile mutants were detected. Four independent mutants of classical strain 395 were isolated to characterize this phenomenon. By transmission electron microscopy, one of the nonmotile mutants was found to be flagellated, while the other three were found to be aflagellate. Chromosomal DNA from the mutants and parental wild-type strain 395 was examined by Southern blot analysis with, as probes, V. cholerae mutagenic prophages VcA-1 and VcA-2 and six cloned motility gene regions isolated from transposon insertion motility mutants of strains 395 and N16961 (El Tor, Inaba). The parental wild-type strain and all of the mutants exhibited the same pattern of bands when probed with VcA-1 and VcA-2 DNAs. Four of the cloned motility gene regions hybridized to the same fragments of DNA in both the wild-type and mutant isolates. However, two other probes detected a new fragment for a single aflagellate mutant. The observations that spontaneous nonmotile mutants occurred at a high frequency and that these mutants did not revert at a detectable frequency suggested that a genetic event is involved. The phenomenon appears to be limited to classical V. cholerae and may explain why classical V. cholerae is only sporadically associated with disease in the current pandemic.

Chromosomal transfer and in vivo cloning of genes in Vibrio cholerae using RP4::mini-Mu

The RP4::mini-Mu replicon has been used to transfer chromosomal genes by conjugation and to clone in vivo metabolic, toxin and flagellar genes of Vibrio cholerae. RP4::mini-Mu was introduced into several strains of V. cholerae and these strains were mated with V. cholerae or Escherichia coli K-12 recipients. R'-episomes carrying the respective genes were maintained in recA recipients and were detected by complementation of auxotrophic, nontoxinogenic and aflagellate mutations in V. cholerae.

Genetics of Vibrio cholerae and its bacteriophages

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Plasmid-mediated changes in virulence of Vibrio cholerae

The effects of several plasmids, including cloning vectors and R factors, on the virulence of Vibrio cholerae CA401R were determined by measuring the dose-related diarrheal response in orally challenged infant mice. The plasmids were also examined for their effects on the colonization ability of strain CA401R by joint infection experiments with a spectinomycin-resistant CA401 strain as an internal standard. One V. cholerae R factor, pVH2, enhanced the diarrheal response, while R factors Rts1 and pVH1 reduced it; plasmids RP4, pRK290, Sa, pSJ8, pSJ5, and pBR328 had no effect. The ability of the plasmids to affect in vitro toxin production by CA401R was variable. Cells containing large plasmids all showed a modest decrease in colonization ability. These results showed that some plasmids affected V. cholerae virulence, but that the cloning vectors pBR328, RP4, and pRK290 did not.

Cloning of the Vibrio cholerae recA gene and construction of a Vibrio cholerae recA mutant

A recombinant plasmid carrying the recA gene of Vibrio cholerae was isolated from a V. cholerae genomic library, using complementation in Escherichia coli. The plasmid complements a recA mutation in E. coli for both resistance to the DNA-damaging agent methyl methanesulfonate and recombinational activity in bacteriophage P1 transductions. After determining the approximate location of the recA gene on the cloned DNA fragment, we constructed a defined recA mutation by filling in an XbaI site located within the gene. The 4-base pair insertion resulted in a truncated RecA protein as determined by minicell analysis. The mutation was spontaneously recombined onto the chromosome of a derivative of V. cholerae strain P27459 by screening for methyl methanesulfonate-sensitive variants. Southern blot analysis confirmed the presence of the inactivated XbaI site in the chromosome of DNA isolated from one of these methyl methanesulfonate-sensitive colonies. The recA V. cholerae strain was considerably more sensitive to UV light than its parent, was impaired in homologous recombination, and was deficient in induction of a temperate vibriophage upon exposure to UV light. We conclude that the V. cholerae RecA protein has activities which are analogous to those described for the RecA protein of E. coli.

Effect of a recA mutation on cholera toxin gene amplification and deletion events

The cholera toxin operon (ctxAB) is located on a 7-kilobase pair variable genetic element which undergoes genetic duplication and amplification events in Vibrio cholerae. Amplification of the ctx genetic element was investigated by substituting the resident ctx loci of two V. cholerae strains with a DNA fragment encoding resistance to kanamycin. Although these strains were not normally resistant to greater than 150 micrograms of kanamycin per ml, spontaneous derivatives could be obtained that grew well on 3 mg of kanamycin per ml. Southern blot analysis of these highly resistant isolates demonstrated that the ctx element was amplified approximately 20-fold. This amplification process was completely inhibited in the absence of a functional recA gene. The V. cholerae RecA protein, therefore, is essential for cholera toxin gene amplification. Spontaneous deletions of the ctx structural genes were observed in both recA+ and recA- V. cholerae strains, although such deletions occurred at a 21-fold-lower frequency in the latter case. Structural analysis of these ctx amplification and deletion events supports a model for their formation that involves unequal crossing over between repetitive sequences located upstream and downstream of the ctx operon.

Genetic analysis of the cholera toxin-positive regulatory gene toxR

Southern blot analysis with a toxR-specific gene probe indicates that Vibrio cholerae 569B has a 1.2-kilobase deletion near the toxR gene. Heterologous conjugative crosses were carried out between the EI Tor strain RV79 and 569B tox mutants. Tox+ recombinants showed the same linkage properties to the his locus as to the previously mapped tox locus of 569B. Southern blot analysis with the toxR probe of the Tox+ recombinants obtained in these heterologous crosses showed that these recombinants had replaced the V. cholerae 569B (recipient) toxR DNA with the V. cholerae RV79 (donor) toxR DNA, indicating that tox and toxR are the same locus. However, the Tox+ recombinants synthesized an amount of toxin intermediate between the level observed for wild-type RV79 and 569B strains, suggesting there is a difference in the ability of toxR genes from different strains to activate ctx. About half of the mutations which suppress the phenotype of hypertoxinogenic locus htx are unlinked to htx and in addition have a hypotoxinogenic phenotype relative to that of the wild type. Most of these hypotoxinogenic, second-site suppressors show a linkage to his similar to the linkage of toxR to his and are therefore probably mutations in toxR. These results indicate that the toxR gene product is required for ctx expression and that a functional toxR gene is required for the effect of an htx mutation to be seen.

Mapping of a gene that regulates hemolysin production in Vibrio cholerae

A gene that regulates the hemolysin structural gene (hly) was found to be tightly linked to the tox-1000 locus of Vibrio cholerae RJ1 and separated from hly by a large section of the V. cholerae genetic map. This hemolysin regulatory gene was designated hlyR.

Molecular cloning of the hemolysin determinant from Vibrio cholerae El Tor

Vibrio cholerae El Tor RV79 is phenotypically nonhemolytic; however, strongly hemolytic convertants are occasionally observed on blood agar plates. We have cloned DNA sequences corresponding to the hemolysin determinant from RV79 (Hly+) in the lambda L47.1 and pBR322 vectors. A 2.3-kilobase fragment of V. cholerae DNA was found to be necessary for hemolytic activity. This cloned DNA sequence was used as a probe in Southern blot hybridization analysis of chromosomal restriction digests of a variety of El Tor and classical biotype V. cholerae strains. In all cases, DNA fragments with the same electrophoretic mobilities hybridized to the Hly probe. The results presented demonstrate that the cloned hemolysin determinant is the hly locus. By using mutator vibriophage VcA-3 insertion to promote high-frequency transfer, the hly locus was mapped between arg and ilv on the V. cholerae RV79 chromosome.

Synthesis of cholera toxin is positively regulated at the transcriptional level by toxR

We have cloned a positive regulatory gene ( toxR ) from Vibrio cholerae that controls cholera toxin transcription. This was done by first constructing a genetic fusion consisting of the lacZ gene fused to the promoter of the cholera toxin operon ctxAB . This operon fusion was used to screen a V. cholerae genomic library for genes that could activate the ctx promoter in Escherichia coli. This method allowed the identification of a gene, toxR , that increases ctx expression by more than 100-fold. Complementation analysis indicated that certain hypotoxinogenic mutants of V. cholerae 569B probably have mutations in the toxR gene. Southern blot analysis suggests that all V. cholerae, including nontoxinogenic strains, have the toxR gene. Moreover, nontoxinogenic strains not only lack the structural genes for cholera toxin but also sequences associated with the larger 7-kilobase ctx genetic element.

Duplication and amplification of toxin genes in Vibrio cholerae

Vibrio cholerae strains of the classical biotype all contain two widely separated copies of the cholera toxin operon ctxAB. In contrast, EI Tor strains containing multiple copies of ctx have their copies arranged on large tandem repeats which are either 7 or 9.7 kb in length. The variation in size among these large tandem duplications was due to a difference in the copy number of a smaller, 2.7 kb, tandemly repeated sequence (RS1) that is located at the novel joint of these duplications, as well as upstream and downstream of ctx. Southern blot hybridization analysis indicated that amplification of a DNA region carrying ctx and flanked by direct repeats of RS1 may be responsible for the hypertoxinogenic phenotype of EI Tor variants selected by intraintestinal growth in rabbits.