Second cytotoxic pathway of diphtheria toxin suggested by nuclease activity.

Diphtheria toxin (DTx) provokes extensive internucleosomal degradation of DNA before cell lysis. The possibility that DNA cleavage stems from direct chromosomal attack by intracellular toxin molecules was tested by in vitro assays for a DTx-associated nuclease activity. DTx incubated with DNA in solution or in a DNA-gel assay showed Ca2+- and Mg2+-stimulated nuclease activity. This activity proved susceptible to inhibition by specific antitoxin and migrated with fragment A of the toxin. Assays in which supercoiled double-stranded DNA was used revealed rapid endonucleolytic attack. Discovery of a DTx-associated nuclease activity lends support to the model that DTx-induced cell lysis is not a simple consequence of protein synthesis inhibition.