A Novel Endonuclease of Human Cells Specific for Single-Stranded DNA

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Abstract
Three different enzyme fractions degrading single-stranded DNA were fractionated from human aneuploid cell cultures [EUE human embryo epithelial cells]. One of these DNases was purified and characterized; this is an endonuclease working at alkaline pH (around 9.5) and requiring Mg2+ for its activity. The enzyme degrades denatured DNA over 100 times more efficiently than native DNA in optimal conditions. The termini produced by the enzyme have 5''P and 3''OH ends. The enzyme can attack, though at reduced rate, the supertwisted circular molecule of Simian virus 40 DNA, whereas it is inactive on the nicked circular molecule. UV irradiation of DNA, whether native or denatured, does not affect its efficiency as substrate of the DNase. The properties of this endonuclease distinguish it from those of the other DNases described previously in mammalian cells; the denomination DNase VI is therefore proposed. Its properties are similar to those of DNases described in Ustilago maydis and Bacillus subtilis, for which an essential role in recombination seems likely.