by a vaccinia core protein L4R, an ssDNA binding protein

Four DNA-dependent ATPases were purified from vaccinia virions and tested for DNA helicase activity on two dsDNA substrates. ATPases D6R and D11L were inactive on both substrates, A18R unwound the substrate with a short 20 bp duplex region and 18R unwound both substrates. In addition, the 18R protein was stimulated to unwind longer DNA duplexes by a 25 kDa protein purified from vaccinia virions, representing the cleaved product of the L4R gene, an ssDNA binding protein. Purified recombinant 25 kDa L4R protein also stimulated 18R DNA helicase activity and maximum activity was observed only when there were < 13 nucleotides of DNA per molecule of L4R protein. The DNA helicase activity of the A18R protein was not stimulated by either recombinant 25 kDa L4R protein or by an E. coil ssDNA binding protein. Vaccinia virus (VV) encodes four proteins, D6R, DllL, I8R and A18R, with amino acid similarity to the superfamily helicases (Koonin & Senkevich, 1992). These proteins all have ssDNA-stimulated ATPase activity, a characteristic of DNA helicases, and are all essential for VV replication. The D6R protein is a component of the heterodimeric VV early transcription factor (VETF) (Gershon & Moss, i990) and the Dl lL gene product is nucleoside triphosphate phospho-hydrolase (NPH) I (Paoletti et aI., 1974; Rodriguez eta]., 1986; Broyles & Moss, 1987). The ISR protein is NPH-II, which wa