When tryptophan is present in the cell, two tryptophan molecules bind to the trp repressor, which changes shape to bind to the trp operator. Binding of the tryptophan–repressor complex at the operator physically prevents the RNA polymerase from binding and transcribing the downstream genes.

When tryptophan is present what happens to the operon?

High tryptophan: The tryptophan binds to the trp repressor and causes it to change shape, converting into its active (DNA-binding) form. The trp repressor with the bound tryptophan attaches to the operator, blocking RNA polymerase from binding to the promoter and preventing transcription of the operon.

Why is the trp operon turned on only when tryptophan is absent from the cell?

When tryptophan levels are high enough, the amino acid will begin inhibiting its own synthesis and transcription will halt. ... Tryptophan's absence switches on the trp operon. Tryptophan acts as corepressor, hence in its presence the operon remains switched off.