Question

In the E. coli trp operon, what would be the effect of a deletion in the region 2 sequence, which would prevent it from forming stable RNA hairpin structure with either regions 1 or 3?

A) The Trp repressor could not bind to the operator and trp transcription will be high, even in the presence of high tryptophan concentration.
B) Translation of the leader sequence could not be initiated.
C) No effect on trp operon transcription.
D) Transcription of the trp operon could not be initiated.
E) Hairpin 3:4 would form even when tryptophan is low, and this would constitutively attenuate transcription of the Trp operon.

Answer 1

E) Hairpin 3:4 would form even when tryptophan is low, and this would constitutively attenuate transcription of the Trp operon.

Tryptophan (trp) operon is regulated by the TrpR repressor protein and fine-tuned by the 4 sequences 1, 2, 3, and 4 which has the capability of forming hairpin structures, 1-2, 2-3, and 3-4. Though 1-2 hairpin render an attenuation signal, it is very rare as sequence 1 is usually blocked by the RNA polymerase, but hairpin 2-3 and 3-4 play a significant role in attenuation. If there is significant trp present in the cell, the RNA polymerase will get pass sequence 2 which enable formation of 3-4 hairpin rendering the attenuation. But if there aren't much trp present in the cell the RNA Pol. cannot get pass the sequence 2 which enables formation of 2-3 hairpin which prevents the formation of 1-2 or 3-4 terminator hairpin.

As the sequence 2 is deleted the trp-operon will constitutively form 3-4 terminator hairpin, constitutively attenuating the trp-operon.