Restoration of enzyme activity by recessive missense suppressors in the fungus Coprinus

Summary

The acu-1 locus in Coprinus is the structural gene for acetyl-CoA synthetase. Five suppressor gene mutations, which suppress the acu-1,34 missense allele, were induced by mutagen treatment. All five suppressors were shown to have properties expected for tRNA structural gene mutations: they are recessive, they show a gene dosage effect in any doubly heterozygous combination of two sup⁺ mutations and they are allele specific in action.

Crosses between suppressed mutants established that at least four suppressor loci were represented. Doubly suppressed mutants derived from these crosses were used to show that the gene dosage effect is maintained when two sup⁺ mutations are in cis as well as trans combinations in the two nuclei of the basidiomycete dikaryon.

Extracts of the unsuppressed acu-1.34 mutant contained less than 2% of wild type acetyl-CoA synthetase activity whereas extracts of four of the five suppressor strains showed activities ranging from 28 to 37% of wild type. Only a slight increase in activity was detected in the fifth suppressor strain but this was associated with a temperature sensitive sup⁺ phenotype. All five sup⁺ mutations restored the ability of the acu-1.34 mutant to induce isocitrate lyase, an enzyme which, under the conditions of growth used, can only be induced when acetyl-CoA synthetase activity is present. Thus all five suppressors act to restore normal acu-1 protein function.