INDUCTION OF THIOGUANINE- AND OUABAIN-RESISTANT MUTANTS AND SINGLE-STRAND BREAKS IN THE DNA OF CHINESE HAMSTER OVARY CELLS BY ³H-THYMIDINE

James E. Cleaver ¹

¹ Laboratory of Radiobiology, University of California, San Francisco, California 94143

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.