The tryptophan operon or trp operon is an example of repressible system or feed- back repression of operon by the end product of a metabolic pathway. In this operon there is a promoter sequence, an operator sequence and five structural genes, viz. trp E, trp D, trp C, trp B, trp A, all responsible for synthesis of different enzymes involved in the biosythesis of amino acid tryptophan.
The regulator gene called trp R produces an inactive repressor protein called apo-repressor that cannot bind to the operator. As a result, the operon is turned on normally. But when the amino acid tryptophan is available it acts as a co-repressor / effector and forms an active dimmer with inactive apo-repressor. This active repressor binds to the operator and prevents gene expression. It is to be appreciated that when tryptophan is available readily to the bacteria there is no need for synthesizing the amino acid and, therefore the operon needs to be shut down.