In bacteria, what is the role of the Shine-Dalgarno sequence?

• A. It aligns the ribosome with the start codon for translation initiation.
• B. It signals transcription termination.
• C. It recruits tRNA synthetase for charging tRNA.
• D. It stabilizes the mRNA 5' cap.

Answer: (A)

The Shine-Dalgarno sequence helps initiate translation by aligning the ribosome with the start codon. It’s a short, purine-rich stretch located just upstream of the start codon in bacterial mRNAs and pairs with a complementary region at the 3′ end of the 16S rRNA in the small ribosomal subunit. This base-pairing positions the AUG start codon in the ribosome’s P site, ensuring that the initiator tRNA brings in formylmethionine at the correct reading frame and that the large subunit can join to start protein synthesis efficiently.

This function is distinct from transcription termination, which ends RNA synthesis, or from recruiting aminoacyl-tRNA synthetases, which charge tRNAs before they participate in translation. It’s also unrelated to a 5′ mRNA cap, a feature not used in bacteria (they typically don’t rely on a cap to recruit the ribosome).