Vaccinia virus is a cytoplasmic, double-stranded DNA orthopoxvirus. Unlike mammalian cells, vaccinia virus produces double-stranded RNA (dsRNA) during its viral life cycle. The protein kinase R, PKR, is one of the principal host defense mechanisms against orthopoxvirus infection. PKR can bind double-stranded RNA and phosphorylate eukaryotic translation initiation factor, eIF2α, shutting down protein synthesis and halting the viral life cycle. To combat host defenses, vaccinia virus encodes E3, a potent inhibitor of the cellular anti-viral eIF2α kinase, PKR.
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