FE-CENTER-DOT-BLEOMYCIN CLEAVES A TRANSFER-RNA PRECURSOR AND ITS TRANSFER DNA ANALOG AT THE SAME MAJOR SITE

Previously, Fe.bleomycin (BLM) has been shown to mediate RNA cleavage in a fashion more highly selective than that of DNA. Because RNAs ofte n assume secondary and tertiary structures not commonly encountered wi th DNAs, it was not clear whether the greater selectivity of RNA cleav age was a consequence of differences in the mononucleotide constituent s of RNA and DNA, or of the three-dimensional structures of the indivi dual substrates. Accordingly, we prepared a ''tDNA'' identical in sequ ence with Bacillus subtilis tRNA(His) precursor, the latter of which i s known to be a good substrate for Fe(II).BLM A2 and which undergoes o xidative cleavage predominantly at U35. Remarkably, the tDNA underwent cleavage predominantly at T35. At higher concentrations of Fe(II).BLM A2, the tDNA was extensively degraded, while the tRNA(His) precursor was not. Competition experiments suggested that this was not due to mo re efficient binding of Fe.BLM to the tDNA; in fact the tRNA precursor appeared to be bound more efficiently. The lesser cleavage of the tRN A(His) may be due to limitations in the facility of chemical transform ation following Fe.BLM binding, or else to the formation of RNA lesion s that do not lead directly to RNA strand scission.