STUDIES ON MUTATIONS INDUCED BY ULTRAVIOLET RADIATION IN PARAMECIUM AURELIA WITH SPECIAL EMPHASIS ON PHOTOREVERSAL

Studies were made of mutations induced by ultraviolet radiation in Paramecium aurelia. The mutation yield is a maximum when the paramecia are exposed to UV just before or perhaps during the S period and is less the longer the time between irradiation and S. The interpretation is made that UV-induced premutational damage can undergo dark repair until S and that this repair is nearly error-free since the yield when the time to S becomes long enough is nearly zero. About half themutation yield can be eliminated by photoreversal and arguments are presented that this portion of the mutation yield is attributable to pyrimidine dimers. The remainder of the yield could be due to some other lesion or to dimers that are unavailable to the photoreversing enzyme. Photoreversal becomes ineffective in reducing the mutation yield while the cells are still in G1. Data are given to show that this is because the nucleus loses the ability to photoreverse even newly produced lesions. This loss in ability to photoreverse lesions is dependent upon prior irradiation; i.e.. it does not occur in unirradiated cells progressing normally through G1. Thus, loss of photoreversibility cannot be used in this system to test for mutation fixation. © 1966.