So, tryptophan acts as a corepressor for its own operon. When tryptophan is absent, the repressor protein is unable to bind to the operator, and the proteins required to synthesize tryptophan are ...

In the case of the trp operon, the ligand tryptophan is required for the repressor ... of genes required for lactose metabolism. In the absence of lactose in the environment, the lac operon ...