Neurospora crassa is widely used as model organism in genetics due to its fast reproducing activity, and can subsist on minimal medium and easy to culture. N. crassa is used in X-ray mutation experiments in turn to find out the mutants that would vary in nutritional requirements and this led to produce one gene- one enzyme hypothesis, in which each enzyme was encoded with its particular gene.
The quaternary structure of a protein is formed by the interaction of subunits. In case of N. crassa, the enzyme activity being examined perhaps develops a protein consisting of two unusual subunits that are necessary for activity and the subunits containing the polypeptides are encoded by distinct and unlinked genes.