SUMMARY: Amongst some 70 recessive suppressors of a mutation in one unstable suppressor was identified. The unstable suppressor, designated , could be maintained on minimal medium, but was lost within 24 h on minimal medium containing more than 1.7 p.p.m. DL-methionine or 0.75 p.p.m. L-methionine. Isolation of hyphal tips from the monokaryotic strain carrying yielded three types of colony: the unstable parental type, the stable auxo-troph and a stable prototroph which was slow-growing and inhibited by methionine in the growth medium. This stable type was recovered with difficulty by resolving dikaryons formed between the unstable strain and strains carrying the wild-type allele of the suppressor gene. From sexual crosses, neither the unstable nor stable type segregated, only the auxotrophic revertant. The unstable strain is thought to have an extra chromosome carrying the mutation. For vigorous growth the wild-type allele, , is indispensable and would be carried on the homologous chromosome. The selective pressures on different media account for loss of the duplicated chromosomes. The results are interpreted as missense suppression by a mutant of an indispensable tRNA.


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