A beta-related corynebacteriophage which lacks a tox allele but can acquire it by recombination with converting phage

Corynebacteriophage 782, a phage highly related to the β family of corynebacteriophages but lacking a tox allele, was isolated from a nontoxinogenic clinical isolate of Corynebacterium diphtheriae. Phage 782 exhibits β immunity but has a wider host range than β, forming plaques on strains of C. ulcerans and in their pseudotuberculosis as well as on C. diphtheriae. Phage 782 and β differed in their DNA mass and in their restriction endonulease digest patterns, but were similar in possessing cos (cohesive) and attP (phage attachment) sites. Moreover, all the BamHI fragments of 782 and β except one hybridized with a DNA probe of the other. The exception in both cases was the attP-containing fragment, which in β also carries the tox gene. Recombinants between phage 782 and π phage, a tox⁺ β-related phage, were isolated which contained ca. 70% of phage 782 DNA but carried the attP-tox-bearing fragment of π and were thus now converting phages. The recombinants had lost the wide-host-range phenotype of 782 and had the narrower host range of π. The significance of the tox-less, β-related phages to the natural history of diphtheria is discussed.