Degradation of Structurally Modified DNAs by Bleomycin Group Antibiotics

Bleomycin-mediated DNA strand scission has been shown to be diminished at certain sequences in proximity to 5-methylcytidines. We have investigated the molecular basis of this observed diminution using selective bleomycin (BLM) modifications at the C-terminus. of the four different bleomycin congeners investigated, only bleomycin A₂and bleomycin BAPP were substantially affected by cytidine methylation. We have also examined the effect of other DNA modifications on bleomycin-mediated strand scission. Methylation at the N⁶position of adenosine resulted in diminution of DNA cleavage by all four bleomycin congeners. The presence of bulky 5-(glucosyloxy)methyl groups in the major groove of T4 DNA had little effect on the efficiency of DNA strand scission mediated by bleomycin A₂or B₂, suggesting the absence of important steric interactions between Fe(II).BLM and DNA in the major groove. In contrast, DNA cleavage mediated by bleomycin congeners was very sensitive to a major DNA conformational change, the B → Z transition. Salt and MgCl₂titrations of the DNA copolymers poly(dG-dC).poly(dG-dC) and poly(dG-^MedC)«poly(dG-^McdC) demonstrated that bleomycin A₂and B₂did not cleave Z-DNA efficiently. In addition, circular dichroism titrations of these copolymers revealed that both bleomycin congeners increased the cation concentration necessary to induce the B→Z transition, implying that bleomycin preferentially binds to and stabilizes B-form DNA. These results are consistent with a model in which cytidine methylation at appropriate sequences of DNA is sufficient to induce subtle conformational changes that render the helix unreceptive to cleavage by some bleomycin congeners.