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Spontaneous mutations, and those induced by nitrous acid or ultraviolet light, were studied in the haploid fission yeast Schizosaccharomyces pombe. Reverse mutations conferring ability to grow on a minimal medium lacking adenine were scored in adenine auxotrophs, and in di-auxotrophs having requirements for both adenine and methionine. The apparently lower mutability of the ad-1 mutants in the ad - met - diauxotrophic strains compared with the ad - strains was shown to be due to the influence of methionine in the plating medium. L-methionine added to the minimal medium at a concentration of 40 μg. per ml. will suppress the appearance of spontaneous and induced revertants of adenine-1 mutants. A similar effect has been shown for reversions of an adenine mutant at another locus, and for reversions of a leucine auxotroph. Methionine has the effect of decreasing the extent of residual divisions undergone by adenine-requiring cells when plated on a minimal medium.
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