Photoreaction of 4,5',8-trimethylpsoralen with superhelical ColE1 and ColE1amp DNA was studied. Changes in mobilities in agarose gels, formation of interstrand cross-links, and DNA strand breaks were determined. Psoralen and light treatment removed negative superhelical turns, and extensive treatments failed to produce positive superhelical turns in covalently closed plasmid DNA. The rate of relaxation of superhelical turns by psoralen Photobinding appeared to be directly proportional to the number of superhelical turns remaining. A unique reaction mechanism is presented to explain these results. By this interpretation the initial rate of psoralen photobinding to superhelical DNA was estimated to be 3 times that for linear DNA, and the ratio of cross-linking to monofuctional adducts appears to be dependent on the superhelical conformation of the DNA. The estimated ratio of psoralen molecules bound to DNA strand breaks was 1.7 . 10(4):1, and 70% of this breakage is caused by the light alone.
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