The diphtheria toxin produced by lysogenic Corynebacterium diphtheriae is encoded by the tox gene located on a bacteriophage. The enzymatic mechanism of toxin-mediated cell death involves:
- A Cleavage of 28S rRNA, inactivating the 60S ribosomal subunit
- B ADP-ribosylation of elongation factor EF-2 (translocase II), halting protein synthesis ✓
- C Activation of adenylate cyclase, increasing cAMP and causing hypersecretion
- D Formation of pores in the plasma membrane causing cytolysis
Explanation
Diphtheria toxin (encoded by the corynephage beta tox gene) is a single protein with Fragment A (enzymatic) and Fragment B (receptor-binding/translocation). Fragment A catalyses NAD-dependent ADP-ribosylation of a modified histidine residue in EF-2 (eukaryotic elongation factor-2, also called translocase II). This inactivates EF-2, halting translocation during protein synthesis, causing cell death. Because EF-2 is present in all eukaryotic cells, even a single toxin molecule reaching the cytoplasm can kill a cell. This mechanism is shared by Pseudomonas aeruginosa Exotoxin A (which also ADP-ribosylates EF-2) but differs from Shiga toxin (which cleaves 28S rRNA) or cholera toxin (which activates adenylate cyclase).
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.