Question

Which one of the following antibiotics blocks protein chain elongation by preventing the action of peptidyl transferase?

• A. Bleomycin
• B. Rifampicin
• C. Chloramphenicol
• D. Tetracycline

Hint:

Peptidyl transferase is an rRNA catalyzed activity of the 50S subunit; chloramphenicol is the antibiotic that binds there directly and blocks peptide bond formation.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks which antibiotic, among four with very different mechanisms, works specifically by blocking peptidyl transferase, the enzyme activity that forms peptide bonds during protein synthesis.

Step 2: Key Formula or Approach:
Peptidyl transferase is not a separate protein; it is a ribozyme activity carried by the 23S rRNA of the large (50S) ribosomal subunit in bacteria. It catalyzes the peptide bond between the growing polypeptide chain (on the peptidyl-tRNA in the P site) and the incoming amino acid (on the aminoacyl-tRNA in the A site).

Step 3: Detailed Explanation:
Chloramphenicol binds directly to the 50S subunit, close to the peptidyl transferase center, and physically prevents this enzyme activity from forming the new peptide bond, which halts elongation of the polypeptide chain.
Bleomycin (option A) is not a translation inhibitor at all; it is a glycopeptide antibiotic (used as an anticancer agent) that causes single and double strand breaks in DNA by generating free radicals, so it acts on DNA, not on the ribosome.
Rifampicin (option B) inhibits bacterial DNA-dependent RNA polymerase, blocking the initiation of transcription, a completely separate step from protein synthesis on the ribosome.
Tetracycline (option D) does act on the ribosome, but at the 30S subunit, where it blocks the binding of aminoacyl-tRNA to the A site; it prevents a new amino acid from arriving at all, rather than blocking the peptide bond forming enzyme once the amino acid is already there.
Only chloramphenicol matches the specific mechanism described, blocking the peptidyl transferase reaction itself.

Step 4: Final Answer:
Chloramphenicol blocks protein chain elongation by binding the 50S ribosomal subunit and preventing peptidyl transferase from forming new peptide bonds. \[ \boxed{\text{(C) Chloramphenicol}} \]