RECOMBINATION OF TY ELEMENTS IN YEAST CAN BE INDUCED BY A DOUBLE-STRAND BREAK

The Ty retrotransposons are the main family of dispersed repeated sequ ences in the yeast Saccharomyces cerevisiae. These elements are flanke d by a pair of long terminal direct repeats (LTRs). Previous experimen ts have shown that Ty elements recombine at low frequencies, despite t he fact that they are present in 30 copies per genome. This frequency is not highly increased by treatments that cause DNA damage, such as U V irradiation. In this study, we show that it is possible to increase the recombination level of a genetically marked Ty by creating a doubl e-strand break in it. This break is repaired by two competing mechanis ms: one of them leaves a single LTR in place of the Ty, and the other is a gene conversion event in which the marked Ty is replaced by an ec topically located one. In a strain in which the marked Ty has only one LTR, the double-strand break is repaired by conversion. We have also measured the efficiency of repair and monitored the progression of the cells through the cell-cycle. We found that in the presence of a doub le-strand break in the marked Ty, a proportion of the cells is unable to resume growth.