Relaxation of Supercoiled Dna by Aminomethyl Trimethylpsoralen and Uv Photons: Action Spectrum

An action spectrum for the relaxation of supercoiled plasmid DNA (induction of the first single-strand break) by photoactivated 4'-aminomethyl-4,5',8-trimethylpsoralen (AMT) has been determined using monochromatic UV photons from 254 to 405 nm. The spectrum of AMT-induced plasmid DNA relaxation fits closely with the absorbance spectrum of AMT in the spectral region between 313 nm and 405 nm but deviates at wavelengths shorter than 313 nm. This assay also reveals that the psoralen photosensitization reaction with DNA also produces piperidine-labile sites. Addition of mannitol and azide partially quenches the supercoil relaxation reaction, evidence for a role of Type II photosensitization pathway.