Genetic analysis of the yeast NUD1 endo-exonuclease: a role in the repair of DNA double-strand breaks

The deoxyribonucleases (DNases) have been shown genetically to be important in the vital processes of DNA repair and recombination, The NUD1 gent, whi ch codes for an endo-exonuclease of Saccharomyces cerevisiae. was analyzed for its role in the DNA double-strand break (DSB) repair processes, While t he nud1 strain is only slightly sensitive to ionizing radiation, expression of the HO-endonuclease to introduce a DSB at the MAT locus in that strain results in cell death. Cell survival is inversely proportional to the durat ion of HO-endonuclease expression. Analysis of the surviving colonies from the nud1 strain indicated that many of the survivors an sterile and that th e proportion Of these sterile survivors increases with the time of HO-endon uclease expression. On the other hand, the surviving, colonies from the iso genic NUD1 strain are mating-proficient:. interestingly, double mutants of nud1 rad52 are more resistant to ionizing irradiation than the rad52 strain and have a cell-survival fraction of 32% for rad52-1 nud1 and 9% for rad52 ::URA3 nud1 following prolonged HO-endonuclease expression, indicating that nud1 has a suppressor effect on the DSB-induced lethality in rad52. Polyme rase chain reaction analysis show;ed that many of the nud1 survivors contai ned small,alternations within the,the Mat locus. suggesting that the surviv ors arose through the process of non-homologous end-joining. These results suggest that the endo-exonuclease acts at a DSB to promote DNA repair via t he homologous recombination pathway.