Codominant translational mutants of Chinese hamster ovary cells selected with diphtheria toxin.

Diphtheria toxin-resistance markers in two translational mutants, CH-RE1.22c, possessing no toxin-sensitive EF-2 (class IIa), and CH-RE1.32, with 50% toxin-sensitive and 50% toxin-resistant EF-2 (class IIb), behaved codominantly in somatic cell hybrids. There was no complementation in hybrids formed between the two resistant mutants. The mutant parents and their hybrids, except those formed by fusion of CH-RE1.32 and wild-type cells, grew in the presence of toxin. To explain these results we suggest that CHO-K1 cells possess two functional copies of the gene for EF-2 and that CH-RE1.22c and CH-RE1.32 represent the homozygous (R/R) and heterozygous (R/S) states of resistance at the EF-2 gene locus. The failure of hybrids formed between CH-RE1.32 and wild-type cells to grow in toxin is a gene dosage effect. Codominant class IIa translational resistance is a selectable marker for the isolation of hybrids. It can be combined with a second, recessive, marker to provide a cell which is a "universal hybridizer" (10).