This question wants you to match four well known antibiotics to their real molecular targets, and pick the one whose target is specifically the peptidyl transferase center of the ribosome, the site that forms new peptide bonds during elongation.
- Bleomycin: this drug does not touch the ribosome at all. It works by generating free radicals that cut DNA strands, which is why it is used as an anticancer agent rather than a classic antibacterial protein synthesis inhibitor.
- Rifampicin: this antibiotic blocks bacterial RNA polymerase, stopping transcription (mRNA synthesis) before translation even begins. It never reaches the ribosome's peptidyl transferase site.
- Chloramphenicol: this drug binds the 50S ribosomal subunit right at the peptidyl transferase center and directly stops this rRNA catalyzed reaction from joining the growing chain to the next amino acid, exactly the elongation block the question describes.
- Tetracycline: this one also targets the ribosome, but at the 30S subunit, where it blocks aminoacyl-tRNA from docking onto the A site in the first place, so the new amino acid never arrives, a different point in the cycle from peptidyl transferase itself.
Of the four, only chloramphenicol acts directly on peptidyl transferase, so option (C) is the answer.
Let's summarize:
- Chloramphenicol binds the 50S subunit and blocks peptidyl transferase, stopping peptide bond formation.
- Bleomycin damages DNA, rifampicin blocks RNA polymerase, and tetracycline blocks aminoacyl-tRNA entry, three completely different mechanisms.
So the antibiotic that blocks elongation by preventing peptidyl transferase is chloramphenicol, option (C).