The trp operon in E. coli is regulated in such a manner as to allow expression of the genes only when cellular levels of tryptophan are low. When tryptophan levels are adequate, the secondary structure of the mRNA halts translation.

Complete question:

The trp operon in E. coli is regulated in such a manner as to allow expression of the genes only when cellular levels of tryptophan are low. When tryptophan levels are adequate, the secondary structure of the mRNA halts translation. This form of regulation is known as ...

Answer:

Attenuation

Explanation:

Attenuation refers to a mechanism of regulation of transcription wherein the process of transcription is initiated normally. However, transcription is stopped abruptly before the expression of the gene.

The process of attenuation of tryptophan operon depends on the availability of tryptophan. When enough tryptophan is available, the ribosome quickly translates sequence 1 of the leader peptide and blocks sequence 2 before sequence 3 is transcribed. This allows the base pairing of sequences 3 and 4 to form a hairpin-like structure that serves as a terminator and stops the process of transcription.