In the bacterium *E. coli*, the *trp* operon is repressed when tryptophan is abundant in the environment. Which of the following describes the mechanism by which tryptophan regulates this operon?

Core Concept

The *trp* operon is a repressible operon. When tryptophan (the corepressor) is abundant, it binds to the inactive repressor protein. This alters the repressor's shape, allowing it to bind to the operator and physically block RNA polymerase from transcribing the structural genes.