Abstract
Reaction of covalently closed circular PM2 bacteriophage DNA with the anticancer drug bleomycin produces nicked circular (form II) and linear duplex (form III) DNA [Lloyd, R.S., Haidle, C.W. & Robberson, D.L. (1978) Biochemistry 17, 1890-1896]. As the reaction proceeds, the frequencies of both form II and form III DNA increase and, concomitantly, an increasing fraction of the DNA mass is found to be in crosslinked structures. Approximately 16% of the PM2 DNA mass is found to be crosslinked after 30 min of reaction with bleomycin at 0.5 microgram/ml. The proportion of each form found in any given crosslinked structure is directly related to the concentration of uncrosslinked (monomeric) forms. Multiple sites of crosslinking occur, and these frequently extend over a region of approximately 500 nucleotide pairs. The intermolecular crosslinked bonds are dissociated by extensive dialysis or by the addition of salt at high concentration (0.8 M NaCl), as would be expected if the bonds were noncovalent. Because intramolecular covalent crosslinks between complementary strands are not detected, it is suggested that intermolecular crosslinks are formed by noncovalent association of bleomycin molecules bound to each of the forms of DNA.