Genetic Analysis of tox + and tox − Bacteriophages of Corynebacterium diphtheriae

Abstract

A series of mutants derived from the temperate corynebacteriophages β tox+ , γ tox− , and L tox+ was isolated and characterized. In three-factor crosses between mutant β phages the relative map order of the genetic markers determining extended host ranges ( h and h ′) and loss of ability to lysogenize ( c ) was found to be h -- c -- h ′. Recombination between markers was observed in matings between phage β and the heteroimmune corynebacteriophages γ and L. In such matings between heteroimmune phages the c markers of phages β and γ failed to segregate from the imm markers which determine the specificity of lysogenic immunity in these phages. The factor which directs the synthesis of diphtherial toxin during infection of appropriate corynebacterial hosts by toxinogenic corynebacteriophages is designated tox + . It was possible to show that the tox + determinant of phage β behaves as a single genetic element which occupies a position between the loci h and imm on the genetic map of this phage. Genetic recombination between mutants of phage β occurred at very low frequencies in biparental matings performed by mixed infection of Corynebacterium diphtheriae C7 s (−) tox− . Considerably higher recombination frequencies were observed when lysogenic bacterial strains carrying one parental phage as prophage were induced by ultraviolet irradiation and then superinfected by the second parental phage. Maximal stimulation of genetic recombination between mutant β phages was detected when superinfection followed ultraviolet irradiation of the lysogenic cells within a limited period of time. In matings between phages with incomplete genetic homology, the stimulation of recombination by ultraviolet radiation was much less effective.