Regularities of the induction of point mutations in the yeast Saccharomyces cerevisiae after exposure to gamma-radiation

A tester system consisting of six isogenic strains was used to study the re gularities of the induction of point mutations in the yeast Saccharomyces c erevisiae after exposure to gamma-radiation. This system allowed the identi fication of all six possible substitutions in the Cys-22 codon of the CYC1 gene encoding iso-1-cytochrome c. The dose dependence of the frequency of t hese six base-pair substitutions was shown to be linear-quadratic. The patt ern of base substitutions did not depend on the doses of gamma-irradiation used (from 125 to 1000 Gy) and predominantly included GC --> AT transitions and AT --> TA transversions. The possible mechanisms of gamma-ray mutagene sis leading to a nonlinear dose dependence were considered, and the spectra of mutations obtained for different yeast genes and for Escherichia coil w ere compared.