How does translation initiation differ between prokaryotes and eukaryotes, focusing on the ribosome binding site and initiator tRNA?

• A. Prokaryotes use 5' cap and Kozak; initiator tRNA is Met-tRNAi; Eukaryotes use Shine-Dalgarno.
• B. Prokaryotes use Shine-Dalgarno to align ribosome; initiator tRNA is fMet-tRNA; Eukaryotes use 5' cap and Kozak; initiator tRNA is Met-tRNAi with cap-dependent scanning.
• C. Prokaryotes use Shine-Dalgarno; initiator tRNA is fMet-tRNA; Eukaryotes use 5' cap.
• D. Prokaryotes do not require initiator tRNA; Eukaryotes use a special tRNA.

Answer: (B)

Translation initiation hinges on how the ribosome is recruited to the mRNA and where it begins, and the differences here are about the binding signal and the initiator tRNA. In prokaryotes the small ribosomal subunit is guided to the start site by a Shine-Dalgarno sequence in the mRNA, which base-pairs with the 16S rRNA to position the start codon in the P site; the initiator tRNA is formylmethionyl-tRNA (fMet-tRNA^fMet), delivering a formylated methionine to start translation. In eukaryotes there is no Shine-Dalgarno; instead the 5' cap recruits the ribosome, and the ribosome scans along the mRNA to find an appropriate start codon, with initiation guided by the Kozak sequence; the initiator tRNA is Met-tRNAi^Met, which brings in methionine for the start without being formylated. This combination—SD alignment with fMet for prokaryotes versus cap-dependent scanning with Met-tRNAi^Met and Kozak context for eukaryotes—best captures the distinct initiation mechanisms.