Which antibiotic does NOT target the 50S ribosome?

• A. Chloramphenicol
• B. Macrolides
• C. Tetracyclines
• D. Clindamycin

Answer: (C)

Understanding how antibiotics interact with the ribosome helps explain their mechanisms. Chloramphenicol, macrolides, and clindamycin all bind to the 50S ribosomal subunit and disrupt steps of protein synthesis that occur after the ribosome has formed the initiation complex, such as peptidyl transfer or translocation, effectively stopping elongation.

Tetracyclines, however, bind to the 30S ribosomal subunit and prevent aminoacyl-tRNA from attaching to the A site, which blocks the beginning of the elongation process. Because their action is on the 30S subunit rather than the 50S, tetracyclines do not target the 50S ribosome.