Biophysical and Biochemical Measurements 8 . 2 - 1 DNA SUPERCOILING , RADIATION SENSITIVITY AND RADIOPROTECTANT MODIFIERS

F o r m I (underwound families irradiated with fission neutrons at 32OC (in T E buffer) followed the expected expotential dependence on dose. The radiation sensitivity was found to be linearly related t o the average DNA linking difference (DL) with larger DL associated with enhanced r a d i a t i o n sensitivity. WR-33278, a radioprotector, is s h o w n t o s t i m u l a t e e u k a r y o t i c topoisomerase I unwinding of supercoiled DNA and SI endonuclease digestion of singlestranded DNA. Our results suggest that some of the radioprotection ability of amino-thiols is mediated by changes in DNA supercoilness, possibly by reducing the single-stranded character in dA-dT rich regions of negative supercoiled DNA. (topoisomers) of plasmid PI I3 I30 DNA