Bleomycin is an antitumor agent whose activity has long been thought to derive from its ability to degrade DNA. Recent findings suggest that cellular RNA may be a therapeutically relevant locus. At micromolar concentrations, Fe(II)-bleomycin readily cleaved a Bacilus subtilis tRNAHis precursor in a highly selective fashion, but Escherichia coli tRNATYr precursor was largely unaffected even under more forcing conditions. Other substrates included an RNA transcript encoding a large segment of the reverse transcriptase from human immunodeficiency virus 1. RNA cleavage was oxidative, %10-fold more selective than DNA cleavage, and largely unaffected by nonsubstrate RNAs. RNA sequence analysis suggested recognition of RNA tertiary structure, rather than recognition of specific sequences; subsets of nucleotides at the junction of single-and double-stranded regions were especially susceptible to cleavage. The ready accessibility of cellular RNAs to xenobiotic agents, the high selectivity of bleomycin action on RNAs, and the paucity ofmechanisms for RNA repair suggest that RNA may be a therapeutically relevant target for bleomycin.The bleomycins (BLMs) are antitumor antibiotics believed to function by DNA degradation (1, 2). DNA degradation is sequence-selective, occurring primarily at a subset of all 5'-GT-3' and 5'-GC-3' sequences (3, 4). A wealth of information exists concerning the mechanism of BLM-mediated DNA degradation in cell-free (1, 2, 5) and cellular (5, 6) systems.In contrast, few studies have dealt with RNA as a substrate for BLM. Further, in spite of the compartmentalization of DNA and RNA in nucleated cells, most studies of RNA cleavage have been carried out with DNA present. For example, BLM degraded only the poly(dT) strand of a poly(rA)-poly(dT) hybrid (7,8). Hori (9) showed that relaxation of simian virus 40 form I DNA was hardly affected by even a large excess of Escherichia coli tRNA. Those experiments that employed RNA alone were carried out in the absence of added metal ions (10, 11). Magliozzo et al. (12) have reported degradation of yeast tRNAPhe by high concentrations of Fe(II)-BLM; product bands corresponding to <5-10% of the starting tRNA were observed by methylene blue staining of a polyacrylamide gel. Thin layer chromatographic analysis indicated products that comigrated with adenine and uracil.Although none ofthe foregoing studies suggested that RNA might constitute an efficient substrate for BLM, ongoing studies in this laboratory of BLM-mediated DNA oligonucleotide degradation have demonstrated that alteration of DNA conformation can significantly change the pattern and extent of cleavage normally obtained with B-form DNA (13,14). On the chance that the limited RNA cleavage observed (vide supra) might actually have resulted from highly efficient cleavage at a few conformationally unique sites, we studied several RNAs as substrates. Presently, we show that Fe(II)-BLM does mediate RNA degradation and that the process is highly site-selective.

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