MCQ 692 of 779 | ASRB NET (ICAR-NET) Animal Biochemistry

Standard mechanism under high tryptophan levels: region 2 of the trpL leader sequence is blocked from pairing with region 3(due to rapid ribosome movement), allowing regions 3 and 4 to form a rho-independent terminator hairpin, which halts transcription before the structural genes and then trpL region՚s structure when protein synthesis (translation) has been inhibited in E. coli cells.

Step 1: Recap of Trp Operon Attenuation

The trp operon encodes enzymes for tryptophan biosynthesis and is regulated by both repression and attenuation. Attenuation is a transcription termination mechanism in the trpL leader region (upstream of structural genes), involving four complementary RNA segments that form alternative stem-loop (hairpin) structures:

• Region 1: Contains the leader peptide coding sequence with two tryptophan codons.
• Region 2: Can pair with region 1 or 3.
• Region 3: Can pair with region 2(antiterminator, allows read-through) or 4(terminator, stops transcription).
• Region 4: Forms the terminator with region 3, followed by poly-U for rho-independent termination.

Attenuation depends on transcription-translation coupling and tryptophan-charged tRNA levels:

• High tryptophan: Ribosomes translate the leader peptide quickly, covering region 2 when regions 3 – 4 are transcribed, preventing 2 – 3 pairing. This allows 3 – 4 terminator formation, attenuating (terminating) transcription.
• Low tryptophan: Ribosomes stall at tryptophan codons in region 1(due to uncharged tRNA), leaving region 2 free to pair with 3, forming the 2 – 3 antiterminator. This prevents 3 – 4, allowing full transcription.

Step 2: Effect of Inhibiting Protein Synthesis

Protein synthesis inhibition (e. g. , via antibiotics like chloramphenicol) prevents ribosomes from binding and translating the leader mRNA. Without ribosomes:

• The nascent mRNA folds based on intrinsic stability.
• Region 1 pairs with region 2 to form a stable “preemptor” or anti-antiterminator hairpin, sequestering region 2.
• This prevents region 2 from pairing with region 3, freeing region 3 to pair with region 4, forming the terminator hairpin.
• Transcription terminates prematurely (attenuation occurs), blocking structural gene expression.

This is the default state without ribosomal interference, as confirmed in experimental studies of ribosome-free systems. It mimics high-tryptophan conditions but arises from absent translation rather than rapid translation

Tryptophan is present in the medium aminoacylated tRNAs carrying tryptophan will also available.

• So the ribosomes will translate past the tandem codons in regions I and enter region II.
• This prevent the formation of stem loop 2 - 3 and allows the formation of stem loop 3 - 4 i.e.. , the transcription termination signal.
• This lead to stoppage of transcription and release of RNA polymerase. When tryptophan is absent in the medium the ribosomes will temporarily stop at the tandem codons because of the absence of aminoacylated tRNA carrying tryptophan.
• This allows region 2 to base pair with region 3. This prevent the formation of stem loop 3 - 4 or termination signal. So the RNA polymerase will go carry on transcription.

Alternatively

• Attenuation is possible because of the simultaneous occurrences of transcription and translation. The translation process would begin even before the completion of transcription. When tryptophan is absent in the medium the ribosomes will stop at the tandem codons because of the absence of amino acylated tRNA carrying tryptophan.
• This allows region 2 to base pair with region 3. This is turn prevents the formation of stem loop 3, 4 or the transcription termination signal. So the RNA polymerase will go on carrying out transcription. When tryptophan is available in the medium, aminoactylated tRNAs carrying tryptophan will also be available. So the ribosome will translate past the tandem codon in the region I and enter region II.
• This prevents the loop of stem 2 - 3 and allows the formation 3 - 4 the transcription termination signal. This leads to stoppage of transcription and release of RNA polymerase.