Single-Stranded Deoxyribonucleic Acid-specific Nuclease from Vaccinia Virus

A deoxyribonuclease purified from vaccinia virus was shown to have a specificity for single-stranded DNA. Undenatured HeLa cell DNA was digested at less than 2 % of the rate of thermally denatured DNA. The low rate of digestion was probably due to single-stranded regions in the isolated HeLa cell DNA since no nicking of native T7 DNA was detected by alkaline sucrose gradient sedimentation. Evidence of endonucleolytic activity was obtained by sedimentation, gel filtration, and DEAE-cellulose chromatography of the digestion products. Exonucleolytic activity was also indicated by the high percentage of mononucleotides formed. The single-strand specificity and the endo- and exonucleolytic activities are similar to the previously described properties of the S1 nuclease from Aspergillus oryzae. The vaccinia deoxyribonuclease was also able to nick closed single-stranded circular DNA as well as closed superhelical double-stranded circles. The susceptibility of the latter probably resulted from the presence of weakly hydrogen-bonded regions in the DNA.