VACCINIA VIRION PROTEIN I8R HAS BOTH DNA AND RNA HELICASE ACTIVITIES - IMPLICATIONS FOR VACCINIA VIRUS TRANSCRIPTION

A nucleic acid-dependent ATPase was purified from vaccinia virions and shown to have both DNA:DNA and RNA:RNA helicase activities, This is o nly the third helicase to be identified that can unwind both DNA and R NA duplexes, The DNA helicase activity copurified with nucleoside trip hosphate phosphohydrolase II (NPHII), an RNA helicase encoded by gene I8R (S. Shuman, Proc, Natl, Acad, Sci, USA 89:10935-10939, 1992), Immu nodepletion with two antisera to NPHII and analysis of recombinant NPH II protein (C. H. Gross and S. Shuman, J. Virol, 69:4727-4736, 1995) c onfirmed that the DNA helicase activity was encoded by the I8R gene. T he I8R DNA helicase unwound DNA in a 3'-to-5' direction only, unwound duplexes of 35 bp but not 45 bp, and could be stimulated to unwind lon ger duplexes by the Escherichia coli single-stranded DNA-binding prote in. DNA helicase activity was not stimulated by salt and was sensitive to 100 mM NaCl or KCI, The I8R protein has amino acid similarity to h uman RNA helicase A and to nuclear DNA helicase II, a bovine DNA and R NA helicase. On the basis of the phenotype of I8R temperature-sensitiv e mutants, it was suggested that the I8R protein is not required for D NA replication but might aid in the extrusion of early mRNA from the v irus core. The DNA helicase activity of the ISR protein allows another interpretation of the mutant phenotype, namely, that the I8R DNA heli case activity is required for initiation of early transcription from w ithin vaccinia virions.