Abstract
ABSTRACT UVS 1 is a mutant of Chlamydomonas reinhardi defective in the dark-repair (presumably excision repair) of pyrimidine dimers from nuclear DNA. All of the pyrimidine dimers in nuclear DNA can be repaired upon exposure to photoreactivating light immediately after irradiation. However, none of the dimers are photoreactivable if the cells are incubated for 24 hr in the dark in growth medium. Pyrimidine dimers induced in chloroplast DNA can be photoreactivated even after 24 hr incubation in the dark. Assay for photoreactivating enzyme in extracts shows that about 80% of the photoreactivating enzyme activity disappears after incubating UV-irradiated cells in the dark for 24 hr. No significant disappearance of enzyme activity occurs in the case of unirradiated cells. It is hypothesized that the residual photoreactivating enzyme found in incubated, irradiated cells represents chloroplast enzyme and that UV-irradiation triggers the disappearance of nuclear photoreactivating enzyme.
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