Shine-Dalgarno sequence
Summary
The Shine-Dalgarno sequence is a purine-rich ribosomal binding site (consensus AGGAGG) found in prokaryotic mRNA, located 6-10 base pairs upstream of the start codon (AUG). It base-pairs with a complementary sequence on the 16S rRNA of the 30S ribosomal subunit, positioning the ribosome to initiate translation. This is a key difference between prokaryotic and eukaryotic translation initiation, making it a target for antibiotic mechanisms and a common board exam distinguishing feature.
Detail
The Shine-Dalgarno (SD) sequence is a ribosomal binding site found exclusively in prokaryotic (bacterial) mRNA transcripts. It consists of a purine-rich consensus sequence (typically AGGAGG) located approximately 6-10 nucleotides upstream (5') of the start codon (AUG) in the 5' untranslated region (UTR) of mRNA.
Mechanism: The SD sequence is complementary to a pyrimidine-rich sequence at the 3' end of the 16S ribosomal RNA (rRNA), which is a component of the 30S small ribosomal subunit. Through Watson-Crick base pairing between the SD sequence and the 16S rRNA (specifically the anti-Shine-Dalgarno sequence), the ribosome is correctly positioned on the mRNA to identify the start codon and initiate translation. This interaction is crucial for accurate and efficient translation initiation in prokaryotes.
Clinical/Board Relevance: 1. Prokaryotic vs Eukaryotic Translation: This is a classic distinguishing feature tested on USMLE. Eukaryotes lack a Shine-Dalgarno sequence; instead, eukaryotic ribosomes recognize the 5' cap structure and use a scanning mechanism (Kozak sequence) to locate the start codon. 2. This difference in initiation mechanisms is one reason why certain antibiotics (aminoglycosides, macrolides, tetracyclines) can selectively target bacterial ribosomes without significantly affecting human ribosomes, forming the basis for antimicrobial selectivity. 3. Polycistronic mRNA: Because bacterial mRNA is often polycistronic (encoding multiple proteins), each gene within the mRNA can have its own Shine-Dalgarno sequence, allowing independent translation initiation for each cistron. 4. Molecular biology techniques: Understanding SD sequences is important in recombinant DNA technology when expressing genes in bacterial systems (e.g., E. coli) for protein production—proper spacing and sequence of the SD site is critical for efficient translation of the inserted gene.
High-Yield Point: If asked to differentiate prokaryotic translation from eukaryotic translation, remember: Shine-Dalgarno (prokaryotes) vs Kozak sequence with 5' cap (eukaryotes).
Sources
- First Aid for the USMLE Step 1
- Lippincott Illustrated Reviews: Biochemistry
- Molecular Biology of the Cell (Alberts et al.)
- Lewin's Genes XII
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.