Q.10 Prokaryotic mRNAs contain a conserved polypurine tract, consensus AGGAGG,
located about seven nucleotides upstream from the AUG initiation codon. This
conserved hexamer is called
(1) Kozak’s sequences
(2) Shine-Dalgano sequences
(3) Consensus sequences
(4) TATA box sequences
The conserved hexamer AGGAGG in prokaryotic mRNA is called the Shine-Dalgarno sequence. This polypurine tract, located 4-9 nucleotides upstream of the AUG start codon, base-pairs with the 3′ end of 16S rRNA in the 30S ribosomal subunit to position the ribosome accurately for translation initiation.
Option Analysis
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(1) Kozak’s sequences: Eukaryotic consensus (e.g., GCCAUGG) around the start codon that aids ribosome scanning and initiation; absent in prokaryotes.
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(2) Shine-Dalgarno sequences: Correct—Prokaryotic ribosome binding site (RBS) with core AGGAGG; complements anti-SD sequence (CCUCCU) in 16S rRNA for precise AUG alignment.
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(3) Consensus sequences: Generic term for conserved motifs (e.g., -10/-35 in promoters); not specific to this translation initiation element.
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(4) TATA box sequences: Eukaryotic promoter element (-25 bp from TSS) for RNA polymerase II binding; prokaryotes use -10/-35 boxes instead.
The answer is (2) Shine-Dalgarno sequences.
Introduction to Prokaryotic Translation Initiation
Prokaryotic mRNAs contain a conserved polypurine tract, consensus AGGAGG, located about seven nucleotides upstream from the AUG initiation codon. This Shine-Dalgarno sequence ensures accurate ribosome positioning, critical for protein synthesis in bacteria.
Shine-Dalgarno Mechanism
The AGGAGG tract hybridizes with 16S rRNA’s anti-SD (3′-UCCUCC-5′), anchoring the 30S subunit 4-9 nt before AUG. Optimal spacing maximizes translation efficiency.
Distinguishing Other Sequences
Kozak aids eukaryotic scanning; TATA box recruits Pol II; consensus is nonspecific. Only Shine-Dalgarno matches prokaryotic AGGAGG RBS description.
GATE Exam Strategy
Focus: Prokaryotes use Shine-Dalgarno (AGGAGG), eukaryotes Kozak. Test translation vs. transcription elements for molecular biology scoring.
