Ribosome–mRNA interaction: On the ribosome, to which component does the mRNA physically bind during translation?

Introduction:
Correct placement of mRNA on the ribosome dictates reading-frame selection and start-codon recognition. While the functional ribosome is a two-subunit assembly, the small subunit is the primary site for mRNA binding and decoding interactions, assisted by initiation factors and rRNA elements.

Given Data / Assumptions:

• Bacterial 30S and eukaryotic 40S subunits engage mRNA early in initiation.
• 16S/18S rRNA participates in mRNA alignment (e.g., Shine–Dalgarno pairing in bacteria).
• Large subunit catalyzes peptide bond formation rather than codon decoding.

Concept / Approach:
mRNA binding determinants—such as the 3′ end of 16S rRNA in bacteria or the eukaryotic cap-binding complex associated with the small subunit—position the start codon in the P site of the small subunit. The large subunit's peptidyl transferase center acts after correct decoding, making it less central to initial mRNA binding.

Step-by-Step Solution:

Verification / Alternative check:
Structural studies show the mRNA channel in the small subunit; mutating 16S/18S rRNA elements alters initiation, confirming primary mRNA binding at the small subunit.

Why Other Options Are Wrong:

• Between subunits/none/exclusive EF binding: contradict structural data.
• Large subunit: main role is catalysis, not initial mRNA binding/decoding.

Common Pitfalls:
Equating the entire ribosome with the decoding function; decoding specificity resides in the small subunit.

Final Answer:
Primarily to the small subunit