Repair of UV-irradiated plasmid DNA in Saccharomyces cerevisiae Inability to complement mutational defects in excision repair by in vitro treatment with Micrococcus luteus UV endonuclease

Excision repair defects of Saccharomyces cerevisiae rad1-1, rad4-4, rad7-1 and rad14 mutants were examined. As previously found, transformation of such cells with UV-irradiated plasmid DNA is poor compared to wild-type yeast. Treatment of UV-irradiated YRp12 plasmid DNA with crude preparations of Micrococcus luteus UV endonuclease before introducing it into rad1-1 cells increased transformation efficiency to wild-type levels. This is consistent with earlier reports of rad1-1 mutants being defective in the incision step of excision repair. However, with purified UV endonuclease little or no rescue occured when the UV-irradiated plasmid was incised before transformation into rad1-1 or rad4-4 cells. Furthermore, the purified UV endonuclease reduced transformation of rad7-1 and rad14 mutants to levels seen in rad1-1 and rad4-4 cells. In contrast such treatment caused only a small decrease in the transforming ability of UV-irradiated DNA in wild-type cells. These results show that yeast can normally process pre-incised, UV-irradiated DNA and that this activity is absent in rad1-1, rad4-4, rad7-1 and rad14 mutants. Thus, in addition to their previously reported roles in incision, the RAD1, 4, 7 and 14 gene products are also required for repair to continue after the incision of DNA lesions.