Induction of thioguanine- and ouabain-resistant mutants and single-strand breaks in the DNA of Chinese hamster ovary cells by 3H-thymidine.

Cultured Chinese hamster cells were labeled with 6-3H-thymidine or 5-methyl-3H-thymidine and allowed to accumulate damage from 3H decays for various periods of time while frozen. The frequencies of cells resistant to 6-thioguanine or ouabain and the amount of DNA damage (i.e., number of single-strand breaks) were determined and compared with the mutation frequencies resulting from X and ultraviolet light irradiation. Whereas 3H decays and X rays made only 6-thioguanine-resistant mutants, ultraviolet light made both 6-thioguanine- and ouabain-resistant mutants. 3H decays originating at the 6 position were two to three times as effective as decays at the 5-methyl position in making drug-resistant mutants, but decays at both sites were equally effective in making single-strand breaks. Mutants and strand breaks produced by beta irradiation of the nucleus probably are the same irrespective of the site of the decay in thymine; these results indicate that the local transmutation effects of 3H decay produce more mutations when they occur at the 6 position than at the 5-methyl position.