An enzyme aggregate in the tryptophan pathway of Neurospora crassa.

reactions specifically involved in tryptophan biosynthesis are under the T?ntrol of four genes in Neurospora crassa (AHMAD and CATCHESIDE 1960). The complexity of the gene-enzyme relationships in this pathway was discovered by AHMAD and CATCHESIDE (1960) and AHMAD, KHALIL, KHAN and MOZMADAR (1964) who demonstrated that lesions at one of these genes, tryp-2, could prevent either the accumulation or the utilization of anthranilate, an intermediate in tryptophan biosynthesis. We have recently shown that this functional complexity has its basis in the fact that three of the reactions of tryptophan biosynthesis are catalyzed by an enzyme aggregate which is controlled genetically by two unlinked genes, tryp-2 and tryp-2 (DEMOSS and WEGMAN 1965). The three activities, the anthranilate synthetase, PRA isomerase, and InGP synthetase reactions, are associated with a single component in extracts of Neurospora. Purification procedures as well as several physical methods have failed to separate the apparent enzyme aggregate into subunits which are either enzymatically active or capable of reforming the active aggregate. To define the role of the two genetic loci in controlling the structure and activity of the enzyme aggregate, we have carried out a detailed biochemical and genetic analysis of a series of tryp-l and tryp-2 mutants. These studies permit a tentative assignment of the genetic control of various functions of the aggregate to specific regions of the tryp-l and tryp-2 genes.