The role of the attenuator

The weakening (attenuation) of the expression
of the tryptophan operator is controlled by a
sequence of about 100–140 base pairs (in the 3!
direction) from the starting point of transcription
(tryptophan mRNA leader). In the presence
of tryptophan, the trp mRNA leader is interrupted
in the region of an attenuator sequence
(1), and translation does not take place. In the
absence of tryptophan, translation is continued.
The trp leader peptide contains two tryptophan
residues (2). When tryptophan is deficient,
translation is delayed and a stop signal
weakened.