Compare the action of repressor proteins in the lac operon and the trp operon.

In both cases, the active repressor binds to the operator and prevents transcription of the operon. In the lac operon, the repressor is inactivated by allolactose, an isomer of lactose that functions as an inducer. Allolactose is only made when lactose is present; therefore, the operon makes the gene products for lactose catabolism only when lactose is present. On the other hand, the trp repressor is active only when bound to tryptophan. Thus, the gene product for the biosynthesis of tryptophan is not made when sufficient quantities of the amino acid are already present.