Photoreactivation and dark repair of ultraviolet light-induced pyrimidine dimers in chloroplast DNA.

A UV-specific endonuclease was used to detect ultraviolet light-induced pyrimidine dimers in chloroplast DNA of Chlamydomonas reinhardi that was specifically labeled with tritiated thymidine. All of the dimers induced by 100 J/m2 of 254 nm light are removed by photoreaction. Wild-type cells exposed to 50 J/m2 of UF light removed over 80% of the dimers from chloroplast DNA after 24 h of incubation in growth medium in the dark. A UV- sensitive mutant, UVS1, defective in the excision of pyrimidine dimers from nuclear DNA is capable of removing pyrimidine dimers from chloroplast DNA nearly as well as wild-type, suggesting that nuclear and chloroplast DNA dark-repair systems are under separate genetic control.