Essential Genetics 6th Edition by Daniel Hartl

Exercise 11

A mutant is isolated with a defect in one of the enzymes in the metabolic pathway in Problem 20, but it is not known which step (A, B, or C) is blocked. The final product W of the pathway is essential for growth. When mutant cells are placed in cultures lacking W, they cannot grow; but when W is added to the medium, they can grow. Experiments are carried out to determine whether any of the intermediates can substitute for W in supporting growth. The mutant cells are found to grow in the presence of Z but not in the presence of X or Y. Deduce from these data what step in the pathway is blocked in the mutant.