Genetical analysis of methionine suppressors in Coprinus

Abstract

Seven different genes, me-1 to me-7, controlling the steps in the synthesis of methionine in the Basidiomycete Coprinus lagopus have been tested for the production of prototrophs on minimal medium. Cultures carrying me-1 and me-7 produced prototrophs spontaneously at a rate of 2·6 × 10−5. These prototrophs were the result of mutations of suppressor genes and not due to back-mutation of the me-gene.An intensive study of sixty-four mutants of an independent origin suppressing me-1 has revealed five different suppressor loci.Tests for complementation between suppressor mutants and for their recessiveness to the wild allele were made in dikaryons.All suppressor mutants were recessive to the wild allele. The five suppressor loci were all separated from one another by recombination, twenty-eight map units being the smallest distance between any two pairs. Mutants of the same locus did not complement one another, with few exceptions. Mutants of different loci, as tested in the trans-position in a dikaryon, complemented one another with the exception of pairs between sup-3, sup-4, and sup-5. Sup-3 and sup-4 are 28 units apart and are independent of sup-5 and yet they did not complement. This unique example of long-distance non-complementation is discussed in terms of gene action.