[Relation between the frequency of various types of reverse mutations in yeasts auxotrophic for adenine and the adenine content of the medium].

As shown in the haploid yeast Saccharomyces cerevisiae, the strain 769-p192-15B-n4 (a ade2-192 lys5-3), the rates of reversion to adenine prototrophy are 0.36 X 10(-8), 1.7 X 10(-8) and 2.7 X 10(-8), when the medium contains 100, 10 and 1 mg/l adenine, respectively. Two types of revertants were taken into account: those prototrophic both for adenine and lysine, i. e. suppressors, and those prototrophic for adenine only, most of them being locus revertants. The proportion of locus revertants at 100, 10 and 1 mg/l adenine does not exceed 2, 25 and 41%, respectively. It is assumed that excess adenine (100 mg/l) suppresses the activity of the genes controlling its synthesis, including the mutant ade2 gene. A hypothesis is forwarded, according to which the genes being in the "active" state mutate significantly more frequently than "not working" genes.