We have investigated ascorbate/Cu(II) cleavage of double-stranded DNA in the presence and absence of DNA negative torsion. We found that ascorbate/Cu(II) cleavage shows a site-specificity that is dependent on negative torsion and is influenced by the nature of the salt, ionic strength, and pH. This provides strong evidence for involvement of local DNA conformation in ascorbate/Cu(II) specific cleavage sites, that differs from the previous reports on cleavage of linear double-stranded DNA and secondary structures assumed by single-stranded DNA. The data indicate specific binding of Cu(II) ions to sites in the negatively supercoiled DNA. Fining mapping of the cleavage sites does not reveal any known DNA conformation, nor does it indicate any sequence identity among the sites cleaved. However, identification of a major site of cleavage of supercoiled DNA at physiological ionic strength, pH and temperature, along with fact that ascorbate and Cu(II) are normal cell constituents, suggests the torsion-dependent, site-specific interactions could have biological significance.
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ACKNOWLEDGEMENTS This work was supported by grant 6M 37687 from the National Institute of Health. We wish to thank James Bodley and Walter Sauerbier for helpful discussion.