The diphtheria toxin inhibits protein synthesis by ADP-ribosylating which elongation factor?

• A. EF-2
• B. EF-Tu
• C. EF-G
• D. EF-P

Answer: (A)

Proteins are built step by step during translation, and some toxins halt this process by chemically modifying a specific elongation factor. Diphtheria toxin carries out ADP-ribosylation on elongation factor 2, the factor responsible for moving the ribosome along the mRNA during translocation in eukaryotic cells. This modification blocks EF-2’s function, so the ribosome cannot shift and elongation stops, effectively halting protein synthesis and leading to cell death.

EF-2 is the eukaryotic counterpart to bacterial elongation factors, which is why other factors like EF-Tu or EF-G (which operate in bacteria) aren’t targeted by this toxin. EF-P is a bacterial aid in translation and is not affected by diphtheria toxin.