Bleomycins constitute a group of antitumor antibiotics that are used clinically to treat squamous cell carcinomas, lymphomas and testicular carcinomas. Their generally accepted mechanism of action is to induce single- and double-strand DNA cleavage mainly through abstraction of the C4'-H of pyrimidine nucleotides at 5'-GC and 5'-GT sites. The bleomycins are interesting molecules containing several domains that contribute to their function: a bithiazole domain that binds DNA, a metal-binding domain that is required for DNA sequence selectivity and oxygen activation (via Fe²⁺), and a disaccharide region that may play a role in metal-binding as well as cellular uptake. Although bleomycin ranks as one of the best-studied DNA binding/cleavage reagents, until recently the bleomycin-DNA complex has proven refractory to crystallographic analysis. Previously, 2D NMR data have been reported supporting either minor groove or intercalation modes of interaction for the bithiazole moiety. As an alternative approach to these studies, we are using a host-guest system that employs Moloney murine leukemia virus reverse transcriptase (RT) to crystallize and solve the structure of the bleomycin-DNA complex by X-ray crystallography. We have determined crystal structures of both a DNA-Co(III)⋅bleomycin-B₂ ?green? complex (a structural analogue of Fe(II)⋅bleomycin + O₂) and a structure of the DNA alone. Details of the three-dimensional structure of metallobleomycin-bound DNA will be presented including an analysis of the structural changes induced by the binding of metallobleomycin to a specific DNA sequence. Our goal is to gain detailed insight into the mode of DNA-binding by bleomycin as well as the structural changes that occur during binding and cleavage events. A better understanding of the mechanism of DNA binding and cleavage by bleomycin will provide a model for general DNA-drug interactions and may suggest improvements for drug design to develop better therapeutic treatments for cancer.