EVIDENCE FOR THE ROLE OF RECOMBINATION IN THE REGULATORY EVOLUTION OFSACCHAROMYCES-CEREVISIAE TY ELEMENTS

The recent completion of the sequencing of the Saccharomyces cerevisia e genome provides a unique opportunity to analyze the evolutionary rel ationships existing among the entire complement of retrotransposons re siding within a single genome. In this article we report the results o f such an analysis of two closely related families of yeast long termi nal repeat (LTR) retrotransposons, Ty1 and Ty2. In our study, we analy zed the molecular variation existing among the 32 Ty1 and 13 Ty2 eleme nts present within the S. cerevisiae genome recently sequenced within the context of the yeast genome project. Our results indicate that whi le the Ty1 family is most likely ancestral to Ty2 elements, both famil ies of elements are relatively recent components of the S. cerevisiae genome. Our results also indicate that both families of elements have been subject to purifying selection within their protein coding region s. Finally, and perhaps most interestingly, our results indicate that a relatively recent recombination event has occurred between Ty2 and a subclass of Ty1 elements involving the LTR regulatory region. We disc uss the possible biological significance of these findings and, in par ticular, how they contribute to a better overall understanding of LTR retrotransposon evolution.