A Neurospora mutant deficient in the enzymatic synthesis of tryptophan

Until recently there has been no demonstrated case in Neurospora in which the loss of activity of an essential enzyme could be attributed to gene mutation. It has been assumed from indirect evidence1 that gene mutations do cause such losses and thereby result in "blocks" of essential biochemical reactions. The products of the "blocked" reactions thus become essential growth substances for the mutants. The first success in obtaining direct evidence to support the above assumption has been attained by Wagner and Guirard in studies on the enzymatic synthesis of pantothenic acid by cell-free extracts of Neurospora.