On the chemistry of RNA degradation by Fe·bleomycin

The chemistry of RNA degradation by Fe·bleomycin was studied using two RNA substrates that are modified efficiently at a small number of sites by the antitumor antibiotic. Cleavage of a tRNA(His) precursor transcript by Fe(II)·BLM A₂ was shown to require O₂; cleavage was also observed when the same substrate was treated with Fe(III)·BLM A₂ + H₂O₂. Consistent with earlier observations made for DNA, the extent of tRNA(His) precursor cleavage was greater for Fe(II)·BLM A₅ than for Fe(II)·BLM A₂; the least cleavage was obtained using Fe(II)·BLM demethyl A₂. By the use of a ³²P end labeled tRNA(His) precursor transcript that was also ³H labeled within the uracil moieties, it was shown that release of uracil was nearly stoichiometric with tRNA strand scission by Fe(II)·BLM A₂. Nonetheless, treatment of the tRNA(His) with hydrazine following BLM-mediated cleavage indicated formation of a new product that- must have derived from a BLM-induced lesion. Also employed for characterization of BLM cleavage of RNA were the octanucleotides CGCTAGCG, C₃-ribo-CGCTAGCG and C₃-ara-CGCTAGCG. Analysis of the products of cleavage indicates that Fe BLM is capable of mediating cleavage by abstraction of a H atom either from C-4' H or C-1' H of the chimeric oligonucleotides.