In prokaryotic translation initiation, where is the ribosome binding site (Shine–Dalgarno sequence) located relative to the coding sequence?

Introduction / Context:
Translation initiation in bacteria relies on base pairing between a purine-rich ribosome binding site (RBS; Shine–Dalgarno sequence) in the mRNA and a complementary sequence at the 3' end of 16S rRNA. This interaction positions the start codon in the P site of the small ribosomal subunit.

Given Data / Assumptions:

• RBS is an mRNA feature, not a DNA promoter element.
• RBS lies in the 5' untranslated region (5' UTR) just before AUG (or alternative start codon).
• Spacing to AUG is typically ~5–10 nucleotides.

Concept / Approach:
Because the RBS aligns the ribosome, it must be close to the start codon. It is not upstream of the promoter (a DNA region), nor is it within the start codon. While common in polycistronic operons, monocistronic bacterial mRNAs also possess RBS elements to ensure correct initiation.

Step-by-Step Solution:
Identify the biological level: mRNA leader region (not DNA).Choose the positional relationship: immediately upstream of the start codon.Select the option that states this clearly.

Verification / Alternative check:
Mutating the Shine–Dalgarno or its spacing to AUG disrupts translation initiation efficiency, confirming its positional and functional role.

Why Other Options Are Wrong:

• Stem–loop far downstream: unrelated to canonical RBS function.
• Upstream of promoter: confuses DNA transcription control with mRNA translation initiation.
• ‘‘Mostly operons’’: RBS is needed broadly, not just in operons.
• Inside AUG: incorrect; it is adjacent but distinct.

Common Pitfalls:
Mixing transcription (DNA promoter) and translation (mRNA RBS) control elements; they act at different molecular stages.

Final Answer:
Immediately upstream of the start codon within the mRNA leader