Chemical and structural characterization of the interaction of bleomycin A2 with d(CGCGAATTCGCG)2. efficient, double-strand DNA cleavage accessible without structural reorganization

A detailed description of the interaction between Fe(II).bleomycin A2 and the Dickerson-Drew dodecamer d(CGCGAATTCGCG)2 is presented. The reaction between bleomycin and this substrate leads to DNA cleavage at two major sites, adenosine5 and cytidine11, and two minor sites, cytidine3 and thymidine8. The pattern and relative intensities of cleavage at these sites was not entirely consistent with what would be predicted based on the preference of the drug for cleavage at the pyrimidines of 5'-GC-3' and 5'-GT-3' sites. Insight into the origins of the apparent alteration of selectivity was provided by examination of the structure of the duplex which had been determined by X-ray crystallography. This indicated that the C4' hydrogens of the two nucleotides located at the strongest cleavage sites, C11 on one strand and A5 on the other, were oriented toward each other in the minor groove. Two-dimensional NMR measurements and molecular dynamics modeling indicated that a metalloBLM could bind to the duplex in an orientation that positioned the metal center roughly equally close to each of these hydrogen atoms. On the basis of this observation, it was proposed that these two residues represented a double-stranded BLM cleavage site. This hypothesis was tested through the study of the BLM-mediated cleavage of the related decamer duplex, d(CGCGAATTCG).d(CGAATTCGCG), as well as the hairpin sequence d(CGCGAATTCGIIIITTTTCCCCCGAATTCGCG). By the use of the hairpin oligonucleotide 32P-labeled alternately at the 5' and 3'-ends, unequivocal evidence was obtained for BLM-mediated double-strand cleavage. Quantitative analysis of the proportion of damage involving double-strand cleavage was effected by the use of the hairpin substrate; for damage initiated at the predominant cleavage site (cytidine31, analogous to cytidine11 in the dodecanucleotide), it is estimated that 43% of all damage leads to double-stranded lesions. The exceptional efficiency of double-strand cleavage observed in this system must reflect the spatial proximity and orientation of the two sugar H's whose abstraction is required to produce double-stranded lesions.

Sequence-specific hydrolysis of yeast tRNA(Phe) mediated by metal-free bleomycin

Bleomycin A2 (BLM) was found to mediate sequence specific hydrolysis of tRNA(Phe) in the absence of added metal ions. BLM A2 promoted phosphodiester bond hydrolysis 3' to the pyrimidine residue at all resolved Py-Pu sites not involving modified bases, as demonstrated by high-resolution electrophoretic analysis of 5'- and 3'-32P-end-labeled substrates. The reaction proceeded with surprising facility, approaching efficiency that of oxidative strand scission mediated by the FeII.BLM A2 complex. By the use of a number of BLM congeners, as well as a study of the time, temperature, and salt dependence of the hydrolysis, it was shown that in many respects the hydrolytic reaction parallels the oxidative degradation of RNA and DNA mediated by metallobleomycins. Thus, in contrast to the well-characterized oxidative degradation of DNA and RNA by bleomycin studied for two decades, the present report documents the ability of certain metal-free bleomycins to mediate RNA hydrolysis.