[Genetic effects of the breakdown of tritium incorporated into Saccharomyces cerevisiae yeast cells. IV. The lethal and mutagenic effects and the nature of the mutations induced by tritium breakdown in the 5th position of cytosine].

We have studied in lethal and mutagenic effects and the nature of mutations induced by 3H decays in the 5-th position of cytosine (5-3H-C). The lethal efficiency was determined as alpha 1 = (10.3 +/- 6.7) x 10(-3) decay-1 or alpha 1 = (12.9 +/- 9.4) x 10(-5) rad-1 and the mutagen efficiency for ade1, ade2 genes -- as alpha m = (4.3 +/- 2.3) x 10(-7) decay-1 or alpha m = (5.4 +/- 2.9) x 10(-9) rad-1. For ade2 gene the spectrum of mutations induced by 5-3H-C was as follows: 1% of frameshifts and 99% of base pair substitutions -- 9% of transversions, 3% of AT leads to GC transitions and 87% of GC leads to AT transitions. Our results establish the 5-3H-C as one of the most effective and specific mutagens reported so far for yeast. According to the scheme of Krasin with coworkers, the final product of 3H decay in the 5-th position of cytosine is uracil. Our calculations show that more than 90% of uracil residues are removed from the yeast genome by cell repair systems.