The unwinding of plasmid DNA by bleomycin A2(BLM A2) was investigated by use of two-dimensional gel electrophoresis. It was found that Cu2+ions greatly facilitated the unwinding of topoisomers of plasmid DNA by BLM A2at concentrations where cupric ions alone had no effect on DNA supercoiling. The concentration of BLM A2required for observable unwinding was reduced at least 100-fold in the presence of equimolar Cu2+. A plot of [Cu2+] vs extent of DNA unwinding in the presence of 10-4M BLM A2gave a curve consistent with the action of cupric ions on BLM in an allosteric fashion, possibly rearranging the drug into a conformation that facilitates DNA unwinding. The participation of the metal center in enhancing DNA unwinding via direct ionic interaction with one or more negatively charged groups on the DNA duplex also seems possible. Further analysis of the structural factors required for BLM-mediated DNA unwinding was carried out with Cu2++ BLM demethyl A2, the latter of which differs from BLM A2only in that it lacks a methyl group, and associated positive charge, at the C-terminus. Cu(II).BLM demethyl A2was found to be much less effective than Cu(II)-BLM A2as a DNA unwinding agent, emphasizing the strong dependence of this process on the presence of positively charged groups within the BLM molecule. These findings constitute the first direct evidence that the metal center of BLM can participate in DNA interaction, as well as in the previously recognized role of oxygen binding and activation.
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