Question

Which of the following features of the trp operon is most essential to the process of attenuation independent of the repressor protein?

The order of the structural genes, E, D, C, B, A

Allosteric modifications as a result of tryptophan binding.

The operator sequence (trpO)

The ability of sequences within the leader mRNA to pair with one another and form stem-lopp structures

Answer 1

Trp operon is regulated by another mechanism called attenuation. This attenuation is possible due to the presence of unique leader sequence in mRNA, which codes for leader peptide. This peptide contains two adjacent trp codons. Attenuator RNA has four short sequences 1-4, which can form 3 distinct secondary structures: 1-2, 2-3, or 3-4. Formation of 2-3 stem loop structure works as an antiterminator form, while 3-4 stem loop works as a terminator form.

If ribosome attempts to translate this peptide while try levels are low, it will stall at the either of the two trp codons, due to lack of sufficient trp. So, at this point the ribosome prevents the formation of 1-2 secondary structure by shielding the sequence 1. Now, sequence 2 is free to hybridize with sequence 3 to form 2-3 structure, which then prevents the formation of the 3-4 termination structure. Therefore, trp genes are transcribes and the cell is able to synthesize trp under low levels.

So, the correct answer is the ability of sequences within the leader mRNA to pair with one another and form stem-lop structures.