Diphtheria Toxin and Related Proteins

The kinetics of diphtheria toxin production by a mutant of the C7(β) strain of Corynebacterium diphtheriae has been studied. This strain, C7-262(β), produces unusually low yields of toxin, although its β-prophage carries a normal tox gene. The isolation of five strains of β-phage, each carrying a different mutation of its tox gene, is described. Lysogens carrying these mutants as prophages produce altered extracellular proteins (cross-reacting materials), serologically related to toxin, each of which has been isolated in purified form. Two of them, cross-reacting materials 30 and 45, are of lower molecular weight than toxin. Although Fragment A, which contains the enzymically active NH2-terminal region of the molecule, isolated from these mutant proteins could not be distinguished from Fragment A derived from toxin itself, cross-reacting materials 30 and 45 are nontoxic for susceptible animals. The other three mutant proteins are serologically indistinguishable from wild type toxin. Cross-reactive materials 197 and 228 are nontoxic because of amino acid substitutions in the Fragment A portion of the molecule causing loss of enzymic activity. Cross-reacting material 176 contains an altered Fragment A which retains about 8 to 10% of the enzymic activity of the wild type. In vivo, however, cross-reacting material 176 is only 0.2 to 0.4% as toxic as intact toxin.