Which of the following BEST describes attenuation of the trp operon?

a. The ribosome will stall at adjacent trp codons if tryptophan levels in the cell are low.

b. A transcription termination sequence forms in the leader mRNA because the ribosome reaches a translation stop codon if tryptophan levels are high.

c. High levels of tryptophan result in continued translation by the ribosome.

d. The structural genes are not expressed if tryptophan levels in the cell are high.

e. Regulation of trp operon expression by transcriptional repression by the trp repressor when tryptophan levels in the cell are high.

d. The structural genes are not expressed if tryptophan levels in the cell are high.

Explanation:

In tryptophan operon, the regulation is controlled by 2 mechanisms:

(1) Repressible negative control

(2) Attenuation

In tryptophan operon, even if the RNA polymerase has initiated transcription of tryptophan mRNA, the synthesis of entire mRNA will be halted if tryptophan is present in abundance in the cell. This is known as attenuation.

There are two ways in which transcription termination occurs:

(1) Rho dependent transcription termination: A protein named as 'rho' facilitates transcription termination.

(2) Rho independent transcription termination: In rho independent transcription termination, during transcription process if any self complimentary sequence arrives in the mRNA then it leads to the formation of a secondary hair - pin like structure. If this hair pin structure is followed by a poly U sequence immediately, it leads to transcription termination.

Here in tryptophan operon, attenuation occurs through rho independent mechanism if the tryptophan is in abundance. This premature transcription termination will stop the expression of tryptophan bio-synthetic genes (structural genes).