Final answer:
When tryptophan is not present in the cell, the trp operon is active and tryptophan is synthesized. However, when tryptophan accumulates in the cell, the trp operon is repressed and the synthesis of tryptophan is halted.
Step-by-step explanation:
The operon is active and tryptophan is produced when tryptophan is absent from the cell because the repressor does not attach to the operator on its own. But as tryptophan builds up in the cell, two tryptophan molecules attach to the trp repressor molecule, changing its structure so that the trp operator can bind to it. Expression of the operon is halted when the active form of the trp repressor binds to the operator, preventing RNA polymerase from transcription of the structural genes. Therefore, the operon's expression is regulated by the actual byproduct of the metabolic pathway that the operon controls.