Nonspecific cleavage of phi X174 RFI deoxyribonucleic acid by bleomycin

The covalently closed circular duplex deoxyribonucleic acid (DNA) of phi X174 underwent progressive conversion to nicked and linear DNA with increasing bleomycin/phi X174 RFI DNA molecule ratios. The formation of linear DNA (a double-strand break) occurred under limited reaction conditions as low as an average of 0.2 single-strand break/phi X174 RFI DNA molecule. As bleomycin-produced linear DNA was further fragmented by bleomycin, a broad distribution of DNA fragments without notable concentrations of unique size was formed. Restriction enzymes PstI and SstII did not generate discrete fragments from bleomycin-produced full-length linear phi X174 DNA, nor did bleomycin cleavage generate discrete fragments from HpaII or PstI digests of phi X174 RFI. These findings suggest that bleomycin does not act at a few specific sites on phi X174 RFI DNA. The single-strand nick appeared to be the preferred site for bleomycin action for a second cleavage in a phi X174 molecule.