Repair of UV-induced DNA damage and survival in yeast. I. Dimer excision

The amount of pyrimidine dimer UV photoproduct lost from the DNA of irradiated yeast cells during dark incubation has been measured in various conditions. It was found that no dimers were lost when cells were incubated in saline. When the cells were incubated, with aeration, in a full growth medium, dimers were lost, most excision being complete within 4 h. Not all dimers were lost and the number lost was a function of UV dose. Maximum loss, amounting to 50 000 dimers per genome was observed after 4000 or 6000 erg/mm2 of UV. At higher doses, the number excised declined. Making the assumptions that dimers are the principal lethal product of UV, that a single dimer remaining in its genome is enough to prevent a cell from multiplying and that excision is the principal dark-repair process in yeast, these data were incorporated into the repair term of an expression relating survival to repair8 and it was found that the survival of yeast at doses up to 2000 erg/mm2 of UV could be quite accurately predicted. This is the first time it has been possible to account for survival in terms of measured repair. It is suggested that the divergence of the predicted and observed curves at higher doses is due to other processes known to exist in yeast.