Final answer:
Tryptophan acts as a co-repressor in the trp operon, allowing transcription to continue when tryptophan levels are low.
Step-by-step explanation:
The trp operon continues transcription when tryptophan levels are low because tryptophan serves as a co-repressor, which binds to the repressor protein and changes its conformation. This allows the repressor to bind to the operator and prevent the transcription of the trp operon. When tryptophan levels are low, there is not enough tryptophan to bind to the repressor, so the repressor remains inactive and transcription of the operon continues.