LOSS OF NUCLEAR PHOTO-REACTIVATING ENZYME FOLLOWING ULTRAVIOLET-IRRADIATION OF CHLAMYDOMONAS

Abstract

UVS 1 is a mutant of Chlamydomonas reinhardi defective in the dark repair of pyrimidine dimers in 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 in nuclear DNA are repaired by photoreactivation if the irradiated cells are incubated in the dark for 24 h in growth medium. Pyrimidine dimers in chloroplast DNA that are unrepaired during the 24 h post-irradiation incubation can be repaired by photoreactivation. Treatment with methyl methanesulfonate to give a similar survival as the fluence of ultraviolet light did not lead to the inactivation of nuclear photoreactivating enzyme after 24 h in the dark. Assay for photoreactivating enzyme in cell-free extracts showed that about 80% of the photoreactivating enzyme activity disappears after incubating ultraviolet-irradiated cells in the dark for 24 h. No significant disappearance of enzyme activity occurs in the case of unirradiated cells. The mechanism of inactivation of nuclear photoreactivating enzyme is unknown.