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.
