Repair of pyrimidine dimer damage induced in yeast by ultraviolet light

Crude extracts from ultraviolet (UV)-irradiated yeast cells compete with UV-irradiated transforming deoxyribonucleic acid (DNA) for photoreactivating enzyme. The amount of competition is taken as a measure of the level of cyclobutyl pyrimidine dimers in the yeast DNA. A calibration of the competition using UV-irradiated calf thymus DNA indicates that an incident UV dose (1,500 ergs/mm 2 ) yielding 1% survivors of wild-type cells produces between 2.5 × 10 4 to 5 × 10 4 dimers per cell. Wild-type cells irradiated in the exponential phase of growth remove or alter more than 90% of the dimers within 220 min after irradiation. Pyrimidine dimers induced in stationary-phase wild-type cells appear to remain in the DNA; however, with incubation, they become less photoreactivable in vivo, although remaining photoreactivable in vitro. In contrast, exponentially growing or stationary-phase UV-sensitive cells ( rad2-17 ) show almost no detectable alteration of dimers. We conclude that the UV-sensitive cells lack an early step in the repair of UV-induced pyrimidine dimers.