Vaccinia virus RNA helicase: an essential enzyme related to the DE-H family of RNA-dependent NTPases.

Three distinct nucleic acid-dependent ATPases are packaged within infectious vaccinia virus particles; one of these enzymes (nucleoside triphosphate phosphohydrolase II or NPH-II) is activated by single-stranded RNA. Purified NPH-II is now shown to be an NTP-dependent RNA helicase. RNA unwinding requires a divalent cation and any one of the eight common ribo- or deoxyribonucleoside triphosphates. The enzyme acts catalytically to displace an estimated 10-fold molar excess of duplex RNA under in vitro reaction conditions. NPH-II binds to single-stranded RNA. Turnover of the bound enzyme is stimulated by and coupled to hydrolysis of NTP. Photocrosslinking of radiolabeled RNA to NPH-II results in label transfer to a single 73-kDa polypeptide. The sedimentation properties of the helicase are consistent with NPH-II being a monomer of this protein. Immunoblotting experiments identify NPH-II as the product of the vaccinia virus I8 gene. The I8-encoded protein displays extensive sequence similarity to members of the DE-H family of RNA-dependent NTPases. Mutations in the NPH-II gene [Fathi, Z. & Condit, R.C. (1991) Virology 181, 258-272] define the vaccinia helicase as essential for virus replication in vivo. Encapsidation of NPH-II in the virus particle suggests a role for the enzyme in synthesis of early messenger RNAs by the virion-associated transcription machinery.