What triggers RNA polymerase to stop transcription in the trp operon?

The mechanism that triggers RNA polymerase to stop transcription in the trp operon is indeed the binding of tryptophan to the repressor. In the trp operon, when tryptophan levels are high, tryptophan molecules bind to a repressor protein, causing a conformational change that activates the repressor. This active repressor then binds to the operator region of the operon, preventing RNA polymerase from transcribing the genes responsible for tryptophan synthesis.

This regulation is a prime example of feedback inhibition, where the presence of the end product (tryptophan) leads to the inhibition of its own synthesis, ensuring that cells do not waste resources producing amino acids when they are abundant. The operon is effectively turned off, allowing the cell to conserve energy and materials, which is crucial for maintaining homeostasis within the cellular environment.