Metabolic suppressors of a regulatory mutant in Neurospora

The synthesis of a number of enzymes of sulfur assimilation in Neurospora crassa is controlled by the sulfur source. Mutants in one regulatory gene, cys-3, are unable to make any of the enzymes. This locus is thought to specify a macromolecule that is required for the expression of the structural genes. A mutant, scon ^c, of another regulatory gene is nonrepressible for the synthesis of the enzymes. We report here the isolation of suppressed scon ^c strains which are actually the double mutant, scon ^c,cys-3. These strains are phenotypically indistinguishable from single mutants at the cys-3 locus. Thus cys-3 is epistatic to scon ^c. Evidence that the expression of the cys-3 gene is itself controlled is also presented.This work is supported by a Public Health Service grant, GM-08995, from the National Institute of General Medical Sciences. One of us (R.L.M.) is supported by a Public Health Service Career Development Award.