A quantitative study of the relationship between tryptophan and niacin in Neurospora.

Numerous studies elucidating the probable pathway of tryptophan synthesis (1,2) and of niacin synthesis (3-9) in Neurospora have been reported. That tryptophan may serve as a precursor of niacin was first suggested by the results of nutritional studies with mammals (10, 11). This relationship has been confirmed for the rat by tracer experiments (12). A tryptophan-niacin relationship has also been suggested for Neurospora (3), based on the behavior of a mutant strain which utilizes either tryptophan or niacin for growth. Although the existence of mutants requiring either tryptophan or niacin for growth suggests a metabolic relationship between these two compounds, mutants of the type mentioned would not be expected if niacin synthesis in Neurospora proceeds solely by way of tryptophan. Mutant strains should be either tryptophan-requiring or niacinrequiring and should not respond to both compounds. It is difficult, therefore, to account for the unorthodox behavior of strains capable of using either tryptophan or niacin for growth, unless the relationship between tryptophan and niacin is more complex than has been postulated, or our understanding of the genetic control of biochemical reactions is inadequate. It was for the purpose of critically examining these points that the studies reported here were performed. It was felt that quantitative isotope data might indicate whether or not niacin synthesis in Neurospora proceeds solely by way of tryptophan and might also offer an explanation for the behavior of mutant strains capable of using either tryptophan or niacin for growth. A preliminary experiment in this direction was reported (13) in which mutant strain 39401, a strain that responds to either tryptophan or niacin, was grown in the presence of N15-indole. The niacin formed in the mycelium during growth was isolated and it was found that 25 per cent of the niacin synthesized was derived from the indole fed. This experiment demonstrated that niacin is synthesized at least in part from indole in Neurospora, presumably via tryptophan. However, from the limited data obtained it was not possible to account for the niacin which was not de-