On nucleic acid photochemistry

Abstract

I. Frozen aqueous solutions of thymine and N, N'-dimethylthymine have been irradiated at 2537 A to yield two dimers from N,N'-dimethylthymine and a single dimer from thymine. In all cases irradiation of the dimer in liquid aqueous solution at 2537 A causes reversion to the monomer. One of the photodimers obtained from N,N'-dimethylthymine is identical to a product obtained by exhaustively N–methylating the single photodimer obtained from thymine. Nuclear magnetic resonance and ultraviolet absorption data support a structure containing a cyclobutane ring. II. Action spectra for formation of thymine dimer in E. coli DNA have been taken. The initial quantum yield is not strongly dependent on wavelength. The ratio of thymine dimer to thymine in the photo-stationary state is much more dependent on wavelength. At the 235 mµ photo steady state 1.7% of the thymine is present as dimer. This shifts to 6.5% at 254 mµ and to 20% at 275 mµ. While the change in the position of the photosteady state with wavelength fails to fit a simple model, the data do indicate that not all thymines are capable of participating in dimer formation. III. Irradiation of ultraviolet irradiated DNA with photoreactivating light (370 mµ) in the presence of an extract from baker's yeast containing photoreactivating enzyme causes the disappearance of thymine photodimer. The agent causing thymine photodimer to disappear is heat labile. These results suggest that the molecular basis of photoreactivation of biological damage to microorganisms is the reconversion of thymine photodimer to thymine.