Posttranslational modification of elongation factor 2 in diphtheria-toxin-resistant mutants of CHO-K1 cells.

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Abstract
Two types of mutants of Chinese hamster ovary [CHO] 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), were identified. The first is mutant in the gene for EF-2 and possesses permanently altered, TOXR gene product. The second lacks a component of a post-translational modification system that converts TOXR EF-2 to the toxin-sensitive (TOXS) state. 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. The second type was designated as MOD- mutants. The missing or nonfunctional component in the MOD- mutants can be restored by hybridizing them with either normal TOXS cells or with EF-2 structural gene mutants. The TOXR EF-2 from MOD- mutants is also converted to toxin sensitivity in vitro by incubation with extracts of TOXS or EF-2 gene mutant cells in the presence of an energy-generating system. EF-2 can be synthesized and released from ribosomes in a toxin-resistant form and then converted to toxin sensitivity by post-translational modification.