The tryptophan (trp) operon is repressible operon. Repressible operons turn off the biosynthetic (anabolic) pathway when the end-product is readily available in sufficient amount.
The product of repressor gene is known as apo-repressor which is inactive protein. It cannot bind to Operator region alone. It requires tryptophan to make it active molecule.
When Tryptophan levels are high, it binds to the apo-repressor and thus making it active. Here tryptophan acts as a co-repressor. The active repressor (apo-repressor and co-repressor complex) now binds to Operator region and thus blocks the RNA polymerase activity. This in turn stops the transcription and thereby stopping synthesis of tryptophan synthesizing enzymes. This is called as “switching off” of operon.
When tryptophan level drops, the apo-repressor remains in inactive form as there will be no tryptophan molecules (co-repressor) to activate it. This inactive apo-repressor molecule cannot bind to Operator region and thus making it possible for RNA polymerase to start transcription. This causes the tryptophan synthesizing enzymes to be form and in turn synthesis of tryptophan. This is called as “switching on” of operon.