University of Toronto (UofT) BIO230H1 From Genes to Organisms Midterm Practice Exam

The binding of tryptophan to the Trp repressor results in the inhibition of the expression of biosynthesis genes. This occurs because the Trp repressor is a regulatory protein that, when bound by tryptophan, undergoes a conformational change that enhances its ability to bind to the operator region of the tryptophan operon. Once bound to the operator, the Trp repressor blocks RNA polymerase from transcribing the genes involved in tryptophan biosynthesis.

This mechanism is an example of negative regulation in gene expression, where the presence of a molecule (tryptophan, in this case) leads to a decrease in the expression of genes that are responsible for producing that molecule. As a result, when tryptophan levels are high and the metabolic demand for its synthesis is low, the cell conserves energy and resources by shutting down the synthesis of tryptophan. Thus, the correct answer highlights the critical role of tryptophan in regulating its own biosynthesis through feedback inhibition.