Biochemical and genetic characterization of three hamster cell mutants resistant to diphtheria toxin

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Abstract
Hamster cell mutants (3) are described which are resistant to diphtheria toxin and provide models for investigating some functions required by the toxin for cytotoxic activity expression. Diphtheria toxin inactivates elongation factor 2 (EF-2). Cell-free extracts from mutants Dtxr-1 and Dtxr-3 contained EF-2 resistant to inactivation by the toxin. In somatic cell hybrids, Dtxr-1 phenotype was recessive while that of Dtxr-3 was codominant. The codominant phenotype is the result of mutation in an EF-2 gene coding. The recessive phenotype might arise by alteration of an enzyme which modifies EF-2 structure so that it becomes a substrate for reaction with the toxin. Another mutant, Dtxr-2, contained EF-2 sensitive to the toxin; this phenotype was recessive. Pseudomonas aeruginosa exotoxin inactivates EF-2 as does diphtheria toxin and the mutants were tested for Pseudomonas exotoxin cross-resistance. Dtxr-1 and Dtxr-3 were cross-resistant while Dtxr-2 was not. Diphtheria toxin does not penetrate to mouse cell cytoplasm and these cells have a naturally occurring phenotype of diphtheria toxin resistance. Each mutant was fused with mouse 3T3 [embryo fibroblast] cells and measured hybrid cell resistance to diphtheria toxin. Intraspecies hybrids containing the genome of mutants Dtxr-1 and Dtxr-3 had some resistance while those formed with Dtxr-2 were as sensitive as hybrids derived from fusions between wild-type hamster cells and mouse 3T3 cells.