Spectra of Base Substitution Mutations Induced in Escherichia coli by Tritiated Water and the Decay of Incorporated Tritiated Thymidine

The biological effects of tritiated water and of [6-3H]thymidine or [methyl-3H]thymidine incorporated into DNA were compared with those induced by 60Co gamma rays. The killing efficiencies of tritiated water and the tritium-labeled bases were very similar, between 1.8 and 2.0 in terms of the RBE of 60Co gamma rays when compared with the absorbed dose to the bacterial nucleus. The frequency of His+ revertants induced by the decay of [6-3H]thymidine was 3.5 times higher than that induced by [methyl-3H]thymidine or tritiated water; these revertants were most often the result of A:T leads to G:C transitions. In comparison, the other treatments efficiently induced both transitions and transversions. The mutational spectrum resulting from the decay of tritiated water was also determined in the lacI forward-mutagenesis system of Escherichia coli. Transitions predominated at the low dose (2.5 krad), while both transitions and transversions were recovered after a high dose (18 krad). These results are very similar to those observed with 60Co gamma rays and are consistent with the hypothesis that mutagenesis resulting from the decay of [6-3H]thymidine is the result of a position effect, while mutagenesis resulting from the decay of [methyl-3H]thymidine and tritiated water is due to beta-particle ionization.