Posttranslational Modification of Elongation Factor 2 in Diphtheriatoxin-Resistant Mutants of CHO-K1 Cells

We have identified two types of mutants of Chinese hamster ovary cells in which the unique ADP-ribose attachment site in elongation factor 2 (EF-2) is altered, thereby rendering them resistant to diphtheria and Pseudomonas toxins (TOXR). The first is mutant in the gene for EF-2 and possesses a permanently altered, TOXRgene product. The second lacks a component of a posttranslational modification system that converts TOXREF-2 to the toxin-sensitive (TOXS) state. We postulate that this modification system is involved in the conversion of a single histidine residue in EF-2 to the specific target of toxin-catalyzed ADP-ribosylation, the novel amino acid X. We have designated the second type MOD-mutants. The missing or nonfunctional component in the MOD-mutants can be restored by hybridizing them with either normal TOXScells or with EF-2 structural gene mutants. The TOXREF-2 from MOD-mutants is also converted to toxin sensitivity in vitro by incubation with extracts of TOXSor EF-2 gene mutant cells in the presence of an energy-generating system. Our results demonstrate that EF-2 can be synthesized and released from ribosomes in a toxin-resistant form and then converted to toxin sensitivity by posttranslational modification.