Altered protein formation as a result of suppression in Neurospora crassa.

The mutant td201 of Neurospora crassa is mutated in the trp-3 locus and forms an altered tryptophan synthetase. A suppressor mutation, su2-6, in this mutant, unlinked to the trp-3 locus, results in the production of wild-type tryptophan synthetase activity, which accounts for the alleviation of the tryptophan or indole requirement. This enzyme activity is associated with a protein physically dissimilar to the wild-type enzyme. A second altered protein, a serologically cross-reacting material is also formed in the suppressed mutant, in addition to the altered enzyme normally formed by the td201 mutant. Normal growth, equivalent to that of wild type, is not restored in the suppressed mutant even with tryptophan supplementation. The relationship of the data to possible mechanisms of suppression is discussed.