[Escherichia coli K-12 mutants with increased resistance to ionizing radiation. V. The effect of mutations on spontaneous and radiation mutagenesis].

The frequencies of spontaneous mutations (reversions his-4----His+ and forward mutations to rifampicin-, nalidixic acid- or valine-resistance) in radiation-resistant mutants Gamr444 and Gamr445 are much lower than in the wild-type strain AB1157. His+ revertants and rifampicin-resistant mutants Rifr are induced by low doses of gamma-rays more efficiently than in the wild-type. Low doses of UV light only enhanced mutagenic activity in Gamr strains for induction of His+ reversions but not for Rifr mutations. For the wild-type strain the frequencies of His+ and Rifr mutations increase proportionally to the square of dose both of UV light and gamma-rays. For the most radioresistant Gamr444 mutant the frequencies of UV- and gamma-rays-induced Rifr mutations and of gamma-rays-induced reversions increase linearly with the dose. Possible reasons for these anomalies of radiation-induced mutagenesis in Gamr mutants are discussed.