Lysogeny and conversion in mitis and mitis-like Corynebacterium diphtheriae

Conversion by corynephages and its role in the natural history of diphtheria

The conversion of non-toxinogenic Corynebacterium diphtheriae to toxinogeny has been reviewed. The biology of converting phage and the relationship of converting phages to nonconverting phages are summarized. The significance of these findings to the natural history and evolution of diphtheria is assessed.

Corynebacterium ulcerans and Corynebacterium pseudotuberculosis responses to DNA probes derived from corynephage beta and Corynebacterium diphtheriae

Strains of Corynebacterium ulcerans and Corynebacterium pseudotuberculosis (Corynebacterium ovis) were examined for the production of diphtheria toxin. A majority of C. ulcerans strains (25 of 37) and 1 C. pseudotuberculosis strain (1 of 14) gave a positive Elek test for diphtheria toxin, and for all strains but 1, production of diphtheria toxin was inhibited at the same level of Fe2+ as was the Corynebacterium diphtheriae control. All Elek-positive cultures as well as two Elek-negative isolates of C. ulcerans gave a positive signal when hybridized with a DNA probe unambiguous for the diphtheria toxin gene (tox) under conditions of high stringency. The majority of probe-positive C. ulcerans strains contained three or more DNA restriction fragments that hybridized with converting corynephage beta, suggesting that in C. ulcerans as in C. diphtheriae there may be a relationship between toxinogeny and carriage of beta-related phage. Selected strains of C. diphtheriae, C. ulcerans, and C. pseudotuberculosis were examined for DNA homology by a semiquantitative technique. There was very little homology between C. diphtheriae and members of the other two species. Strains of C. ulcerans and C. pseudotuberculosis, although more closely related, appeared to belong to distinct species as well.

Restriction endonuclease map of the nontoxigenic corynephage gamma c and its relationship to the toxigenic corynephage beta c

Clear-plaque-forming mutant gamma tox- corynephages were isolated independently from nontoxigenic lysogenic Corynebacterium diphtheriae strains C7s(gamma tox-) and C4(gamma tox-). A physical map was constructed by using restriction endonucleases BamHI, EcoRI, HindIII, and KpnI. A comparison of nontoxigenic gamma c with toxigenic corynephage beta c revealed large areas of homology, including common regions for cohesive ends (cos) and attachment sites (att). Localization of the att sites on the beta prophage and correlation of the physical and genetic maps defined the orientation of the diphtheria tox operon. Diphtheria tox sequence homologies were mapped on gamma c by hybridizing 32P-labeled diphtheria tox mRNA to restriction fragments of gamma c DNA. Two regions of heterogeneity between phages beta c and gamma c were localized and these regions accounted for the 3-kilobase larger molecular size of gamma c compared with beta c. One change occurs near the tox promoter and may explain the nontoxigenic phenotype of corynephage gamma tox-.

Genetic elements novel for Corynebacterium diphtheriae: specialized transducing elements and transposons

It was shown in an accompanying paper (Buck and Groman, J. Bacteriol. 148: 131-142, 1981) that gamma-tsr-1 phage stocks produced by heat induction of lysogens are a mixture of two phages which differ in the content of their deoxyribonucleic acid (DNA). This difference is evidenced by the appearance of "heterogeneous" (HET) fragments in restriction enzyme digests of gamma-tsr-1 phage DNA. It was estimated that 20 to 80% of the phage in these lysates produced HET fragments. The appearance of HET fragments correlated with the appearance of a DNA insertion (DI-1) in the gamma phage genome as revealed in heteroduplexes of DNA from gamma-tsr-1 and beta corynebacteriophages. The HET fragments were seen in DNA from heat-induced lysates, but not in DNA from phage stocks produced by lytic infection. By DNA-DNA hybridization analysis it was shown that a fraction of gamma-tsr-1 phages from heat-induced lysates carried an insertion of bacterial DNA in the vegetative phage attachment site (attP), and that this insertion was responsible for the formation of HET fragments. Since the phage produced by this event carried a complete phage genome plus a small segment of bacterial DNA, they were called transducing elements. On the basis of these facts it was concluded that heat-induced gamma-tsr-1 prophage was excised at an abnormal site at a very high frequency. Abnormal excision was highly specific, and the change in excision specificity occurred simultaneously with the spontaneous mutation of the phage to heat inducibility. From this and other data it was postulated that a mutation in the immune repressor was reponsible for an alteration in the specificity of the normal excision process. This distinguishes the mechanism of formation of gamma-tsr-1 transducing elements from that employed by other phages. A second DNA insertion (DI-2) in the tox (diphtheria toxin) gene of gamma-tsr-1 and gamma-tsr-2 was also identified as an insertion of bacterial DNA. The DI-2 insertion had a stem-and-loop structure similar to that seen in heteroduplexes visualizing transposons or insertion elements. It seems likely that gamma wild-type phage, which is mutant for tox, was originally tox(+), but that transposition of bacterial DNA into the gene inactivated it.

Isolation and characterization of tox mutants of corynebacteriophage beta

Seventeen nontoxinogenic (tox) mutants of corynebacteriophage beta have been isolated by using a tissue culture screening technique. The mutants fall into four major classes. Two of the classes, I and II, appear to contain missense and nonsense mutants, respectively. However, classes III and IV have not been previously described. Class III mutants produce two proteins (CRMs) seriologically related to diphtheria toxin, but efforts to demonstrate the presence of more than one tox gene have been successful. Class IV mutants are phenotypically CRM-, failing to produce any detectable protein serologically related to diphtheria toxin. Genetic studies indicate that the mutations in class IV strains are not in a gene distinct form the structural gene for toxin, and that the CRM- strains retain at least a portion of that gene. A natural phage isolate, gamma, behaves in a completely parallel fashion to the class IV mutants. The production of tox+ recombinants through recombination of various pairs of tox phage mutants has been demonstrated. The implications of these findings for the natural history of diphtheria are discussed.

Comparative studies with tox plus and tox minus corynebacteriophages

The characteristics of nine inducible temperate corynebacteriophages designated alpha(tox+), beta(tox+), P(tox+), gamma(tox-), pi(tox+), K(tox-), rho(tox-), L(tox+), and delta(tox+) have been compared. Virion morphology and ability to recombine genetically with the well-studied phage beta(tox+) have been correlated with other properties of the phages, and the distribution of the genetic marker tox+ among related and relatively unrelated corynebacteriophages has been analyzed. The immunity specificity, host range, and plaque morphology of each phage were determined. The phages can be separated into five groups with different immunity specificities. Each type of host range previously recognized in mutants of phage beta(tox+) was present in one or more of the phages included in the present study, and the phages were found to produce plaques of several different morphological types. Representative phages with each of the five types of immunity specificity were further characterized with respect to virion morphology, ability to recombine with phage beta(tox+), latent period, average burst size, and neutralization by homologous and heterologous antiphage sera. All of these phages have polyhedral heads and long slender tails, but two distinct morphological types were distinguished by the sizes and proportions of the components of the virions. Only phages of the same morphological type as beta(tox+) were capable of genetic recombination with beta(tox+), but morphological similarity between phages was not sufficient to insure interfertility. The phages which recombined with beta(tox+) resembled one another in plaque morphology, latent period, and average burst size, whereas phages which failed to recombine with beta(tox+) differed in these characteristics. The phages capable of genetic recombination with beta(tox+) were found to differ from each other in immunity specificity, host range, neutralization by antiphage sera, and toxinogenicity. Thus, these latter characteristics are of limited value in establishing the extent of relatedness between corynebacteriophages. The genetic marker tox+ was not consistently correlated with any other property of the corynebacteriophages analyzed in this study. The most striking finding regarding the distribution of the tox+ marker is its presence both in beta(tox+) and delta(tox+), phages which fail to recombine genetically and which differ in virion morphology. The presence of the tox+ marker in genetically unrelated corynebacteriophages poses many questions concerning the origin(s) of tox+ and the evolution of the phage-host interactions which determine the ability of corynebacteria to synthesize diphtherial toxin.