PHOTOPRODUCT FORMATION IN DNA AT LOW TEMPERATURes.

Abstract— The temperature dependence of thy mine photoproduct formation in Escherichia culi DNA dissolved either in water or in a 50 per cent ethylene glycol solution was studied at temperatures between + 25 and — 196°C. At low temperatures, the formation of thymine dimer was strongly inhibited. A dose of 1 × 10⁴ ergs/mm² at 280 nm converted 2 per cent of the thymine to dimer at 25°C as compared with 0.2 per cent at — 196°C. In addition, a new thymine photo-product which was both nonphotoreversible and nonphotoreactivable was found at low temperatures. On the basis of its chromatographic mobility, this new photoproduct was assumed to be the same as that isolated from irradiated spores of Bacillus megaterium . Extensive irradiation at 254 nrn of DNA at — 120°C resulted in a yield of > 23 per cent for the 'spore-type' photoproduct as compared with 6 per cent for the thymine dimer. In poly d(AT), irradiated at low temperature, no spore-type photoproduct was found; this suggests that adjacent thymine residues are necessary for the formation of the spore-type photoproduct.