STIMULATION OF VACCINIA VIRION DNA HELICASE 18R, BUT NOT A18R, BY A VACCINIA CORE PROTEIN L4R, AN SSDNA BINDING-PROTEIN

Four DNA-dependent ATPases were purified from vaccinia virions and tes ted for DNA helicase activity on two dsDNA substrates. ATPases D6R and D11L were inactive on both substrates, A18R unwound the substrate wit h a short 20 bp duplex region and 18R unwound both substrates. In addi tion, the 18R protein was stimulated to unwind longer DNA duplexes by a 25 kDa protein purified from vaccinia virions, representing the clea ved product of the L4R gene, an ssDNA binding protein. Purified recomb inant 25 kDa L4R protein also stimulated 18R DNA helicase activity and maximum activity was observed only when there were < 13 nucleotides o f DNA per molecule of L4R protein. The DNA helicase activity of the A1 8R protein was not stimulated by either recombinant 25 kDa L4R protein or by an E. coli ssDNA binding protein.