Final answer:
Srb Horowitz identified the mutant enzyme by supplementing the minimal medium with intermediates in the pathway, leading to the identification of different classes of arginine mutants. Each mutant was defective in a different gene encoding a different enzyme in the arginine biosynthesis pathway.
Step-by-step explanation:
In the known biosynthetic pathway, Srb Horowitz identified the mutant enzyme by supplementing the minimal medium with intermediates in the pathway. They identified three classes of arginine mutants, each differing in their ability to grow in the presence of intermediates in the arginine biosynthesis pathway. From this, they concluded that each mutant was defective in a different gene encoding a different enzyme in the arginine biosynthesis pathway, supporting the one gene-one enzyme hypothesis.