A substantial body of data, largely derived from study of cell extracts, indicates that protein synthesis in adenovirus-infected cells requires VA RNA1 at late times of infection to prevent the activation of a protein kinase known as DAI, and the consequent phosphorylation of the a-subunit of initiation factor eIF-2. To verify this conclusion, we have measured the steady-state levels of eIF-2α phosphorylation in cells infected with wild-type virus (Ad2) and a mutant that produces no VA RNA, (Ad5dl331). Consistent with the proposed mechanism, the a-subunit was very highly phosphorylated (∼90%) at late times of infection with Ad5dl331. Surprisingly, eIF-2α phosphorylation also increased (to ∼30%) at late times of infection with Ad2, suggesting that VA RNA and DAI might be involved in the selective translation of viral mRNA and the shut-off of host cell protein synthesis during the late phase. In agreement with this model, host protein synthesis shut-off is defective in cells expressing low levels of DAI.
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