STUDIES ON THYMINE‐DERIVED UV PHOTO‐PRODUCTS IN DNA—II. A COMPARATIVE ANALYSIS OF DAMAGE CAUSED BY 254 NM IRRADIATION AND TRIPLET‐STATE PHOTOSENSITIZATION

Abstract

Abstract— From the rates of cyclobutyl dipyrimidine (Pyr < > Pyr) formation and the ratio of inactivation of transforming or phage DNA caused by direct (254 nm) or sensitized (1.0 M acetone, 313 nm; 0.02 M acetophenone, 334 nm) irradiation, we conclude that Thy < > Pyr and Cyt < > Pyr are equally lethal, and that they are repaired with equal efficiency by the host cell. Not all the damage formed by photosensitized irradlation can be photoenzymatically repaired, especially when acetone is the sensitizer. We found no compelling evidence for photosensitized interstrand cross‐links or sensitizer‐DNA addition products for the fluence range used in these studies (< 10 6 Jm ‐2 ); moreover, strand breakage can account for only a part of the non‐photorepairable damage. We suggest that a fraction of the damage may be due to Pyr < > Pyr isomers other than the cis, syn type usually formed in native DNA by far‐UV light.