Iron(II) Bleomycin-Mediated Degradation of a DNA-RNA Heteroduplex

The effect of iron(II) bleomycin on a DNA-RNA heteroduplex was investigated using a substrate formed by reverse transcription of Escherichia coli 5S ribosomal RNA. Both strands of the heteroduplex were cleaved by Fe^II-BLM A₂ at comparable concentrations; complete digestion of both strands was observed using 5 µM Fe^II·BLM A₂. The DNA strand of the heteroduplex was cleaved predominantly at S′-G-pyr-B′ sites; the sites of cleavage of the DNA strand were a subset of those observed for the corresponding DNA strand of a DNA duplex of identical sequence. The sites of cleavage of the RNA strand of the heteroduplex involved both purines and pyrimidines and were found to be different than the sites of cleavage of the 5S rRNA alone, demonstrating that cleavage of the former must actually have involved heteroduplex recognition by Fe^II·BLM A₂. Both the DNA and RNA strands of the heteroduplex were cleaved by Fe^II-BLM A₂ in the presence of physiological concentrations of Mg²⁺, consistent with the possibility that DNA-RNA heteroduplexes may be therapeutically relevant targets for bleomycin.