Question

Genetic regulation in prokaryotes can involve alterations in RNA secondary structure. What phenomenon occurs in the trp operon that involves such alterations?
A) transcription
B) capping
C) polyadenylation of the 3′ end of the mRNAs
D) intron processing
E) attenuation

Answer 1

Answer:

E) attenuation

Explanation:

Attenuation is a regulatory mechanism in which the transcription is initiated normally but is halted before transcription of operon genes. The trp operon has a leader sequence present at the 5' end of the mRNA. Sequences 3 and 4 of the leader sequence serve as an attenuator by forming a stem-loop structure. Base pairing of sequences 2 and 3 does not allow the formation of the attenuator and the transcription continues.

When the tryptophan levels are high, the sequence 1 is translated and the translation of sequence 2 starts before transcription of sequence 3. The continued transcription allows the formation of attenuator by sequences 3 and 4. When the tryptophan levels are low, the tryptophan code of sequence 1 is not translated by ribosome leading to the base pairing of sequences 2 and 3 to prevent attenuation.