Corynebacterium diphtheriae toxin is only produced by strains lysogenised by a specific bacteriophage. The gene for diphtheria toxin is carried on which phage, and what host protein does the toxin ADP-ribosylate to inhibit protein synthesis?

• A Beta (β) corynephage (phage β); toxin ADP-ribosylates elongation factor 2 (EF-2) at a modified histidine residue called diphthamide ✓
• B Lambda phage (λ); toxin ADP-ribosylates ribosomal protein S6
• C Phage CTXφ; toxin ADP-ribosylates Gsα protein activating adenylate cyclase
• D Phage 80α; toxin ADP-ribosylates actin polymerisation factor

Explanation

The diphtheria toxin structural gene (tox) is carried on the lysogenic corynebacteriophage β (beta phage). The toxin (Fragment A) ADP-ribosylates elongation factor-2 (EF-2) at the unique amino acid diphthamide (a modified histidyl residue found only in eukaryotic EF-2), irreversibly blocking translocation of peptidyl-tRNA on the ribosome and halting protein synthesis — a single molecule can kill a cell. CTXφ carries cholera toxin (ADP-ribosylates Gsα). Phage CTXφ and phage 80α are unrelated to diphtheria.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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