C. diphtheriae Produces a potent exotoxin encoded by a lysogenic beta-prophage. The toxin has a mol. Wt of 61,150 daltons 2 pas; A: 21,150 & B:40,000; A: active; B: binding Following proteolytic cleavage, the A segment is released into the host cell where it inhibits protein synthesis ADP ribosylation of elongation factor/ EF-2 (which is a t RNA translocase). Inhibition of protein synthesis disrupts normal cellular physiologic functions that are believed to be responsible for the necrotizing and neurotoxic effects of diphtheria toxin.