Question

How does an excess of tryptophan cause a ‘switching-off’ of the tryptophan operon?

Answer 1

The tryptophan (trp) operon is switched off through a negative feedback system involving a repressor protein and a process called attenuation. When tryptophan is present in excess, it acts as a co-repressor by binding to an inactive repressor protein (apo-repressor). This binding changes the repressor’s shape, activating it so it can attach firmly to the operator region of the DNA. Once bound, the repressor physically blocks RNA polymerase from transcribing the structural genes, effectively halting the production of more tryptophan.

Furthermore, the operon utilises attenuation to provide an extra layer of control. When tryptophan levels are high, translation of a leader peptide occurs rapidly, which causes the mRNA to fold into a specific “terminator” hairpin structure. This structure signals RNA polymerase to detach from the DNA before it can complete transcription of the entire operon. Together, these mechanisms ensure that the cell conserves energy by stopping synthesis as soon as the amino acid is abundant.