PHOTOREACTIVATION and EXCISION REPAIR OF UV INDUCED PYRIMIDINE DIMERS IN THE UNICELLULAR CYANOBACTERIUM GLOEOCAPSA ALPICOLA (SYNECHOCYSTIS PCC 6308)

Abstract

Abstract— The survival curve obtained after UV irradiation of the unicellular cyanobacterium Synecho-cystis is typical of a DNA repair competent organism. Inhibition of DNA replication, by incubating cells in the dark, increased resistance to the lethal effects of UV at higher fluences. Exposure of irradiated cells to near ultraviolet light(350–500 nm) restored viability to pre-irradiation levels. In order to measure DNA repair activity, techniques have been developed for the chromatographic analysis of pyrimidine dimers in Synechocystis. The specificity of this method was established using a haploid strain of Sacchar-omyces cerevisiae. In accordance with the physiological responses of irradiated cells to photoreactivating light, pyrimidine dimers were not detected after photoreactivation treatment. Incubation of irradiated cells under non-photoreactivating growth conditions for 15 h resulted in complete removal of pyrimidine dimers. It is concluded that Synechocystis contains photoreactivation and excision repair systems for the removal of pyrimidine dimers.