SUMMARY: Spontaneous mutations, and those induced by nitrous acid or ultraviolet light, were studied in the haploid fission yeast 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 mutants in the diauxotrophic strains compared with the 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 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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