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

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Abstract
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 1.8-2.0 in terms of the RBE [relative biological effectiveness] 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 .fwdarw. G:C transitions. In comparison, the other treatments efficiently induced transitions and transversions. The mutational spectrum resulting from the decay of tritiated water was also determined in the lacI forward-mutagenesis system of E. coli. Transitions predominated at the low dose (2.5 krad), while 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.

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