Pactamycin is an antibiotic that interferes with the initiation of translation. Which specific binding step does pactamycin inhibit during ribosome initiation?

Introduction / Context:
Various antibiotics target distinct steps of translation. Pactamycin is notable for disrupting the formation of the initiation complex. Knowing the precise binding step is important for understanding how it halts protein synthesis and how it differs from elongation inhibitors.

Given Data / Assumptions:

• Organismal contexts include both prokaryotes (30S) and eukaryotes (40S).
• Focus is initiation phase vs. elongation.
• We must choose the exact binding interaction affected by pactamycin.

Concept / Approach:

Pactamycin interferes with initiation by preventing proper binding of the initiator tRNA (fMet-tRNA in bacteria; Met-tRNAi in eukaryotes) to the small ribosomal subunit initiation complex. This blocks assembly of a functional initiation complex and prevents translation from starting.

Step-by-Step Solution:

Verification / Alternative check:

Because initiation is prevented, no peptide bond forms, and subsequent elongation steps never occur—consistent with broad suppression of protein synthesis observed experimentally.

Why Other Options Are Wrong:

• Aminoacyl-tRNA to the A site: that is a tetracycline effect during elongation.
• Peptidyl-tRNA binding to 50S: elongation-related, not the initiation target of pactamycin.
• Formation of the peptide bond: catalyzed by peptidyl transferase of the large subunit; not directly blocked by pactamycin’s initiation action.

Common Pitfalls:

• Confusing initiation inhibitors with elongation inhibitors.
• Assuming all 30S-active antibiotics act at the A site; pactamycin specifically prevents initiator-tRNA docking.

Final Answer:

Initiator-tRNA binding to 30S/40S initiation complexes