Which of the following best describes the role of the Shine-Dalgarno sequence in prokaryotic translation initiation?
- A It forms a cap structure at the 5' end of mRNA that is recognized by eIF4F in eukaryotes
- B It is a purine-rich sequence ~10 nt upstream of the AUG start codon that base-pairs with the 3' end of 16S rRNA in the 30S subunit, positioning the ribosome at the correct AUG ✓
- C It is the poly-A tail analog in prokaryotes that recruits the ribosome 50S subunit
- D It encodes formyl-methionine (fMet) in the E-site during initiation
Correct answer: B. It is a purine-rich sequence ~10 nt upstream of the AUG start codon that base-pairs with the 3' end of 16S rRNA in the 30S subunit, positioning the ribosome at the correct AUG
Explanation
The Shine-Dalgarno (SD) sequence (consensus: AGGAGG) is found in prokaryotic mRNA approximately 5-10 nucleotides upstream of the AUG initiation codon. It is complementary to the 3' anti-SD sequence of 16S rRNA, allowing direct base-pairing that docks the 30S ribosomal subunit at the correct AUG. This mechanism for translation initiation is prokaryote-specific. In eukaryotes, the equivalent (ribosome scanning for the Kozak sequence around AUG) uses cap-dependent initiation via eIF4F and the 5' m7GTP cap — not an anti-SD rRNA interaction.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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