Vaccinia virus RNA helicase. Directionality and substrate specificity.

Vaccinia virus RNA helicase (NPH-II) catalyzes unidirectional unwinding of 3'-tailed duplex RNAs in the presence of a divalent cation and any one of the eight common nucleoside triphosphates (NTP). The helicase binds stably to the tailed RNA in the absence of any cofactor; strand displacement by the bound protein is coupled to NTP hydrolysis. Although the helicase is capable of binding to tailed duplex DNA as well as to tailed RNA, the enzyme is unable to unwind duplex DNA. It is suggested that NTP hydrolysis by RNA-bound NPH-II drives processive translocation of the protein in a 3' to 5' direction along the RNA strand, whereas energy utilization by DNA-bound enzyme leads to dissociation without extensive protein movement. A role for the RNA helicase in vaccinia mRNA synthesis is proposed.