MULTIMERIC ARRAYS OF THE YEAST RETROTRANSPOSON TY

We have identified a novel integrated form of the yeast retrotransposon Ty consisting of multiple elements joined into large arrays. These arrays were first identified among Ty-induced α-pheromone-resistant mutants of MATa cells of Saccharomyces cerevisiae which contain Ty insertions at HMLα that result in the expression of that normally silent cassette. These insertions are multimeric arrays of both the induced genetically marked Ty element and unmarked Ty elements. Structural analysis of the mutations indicated that the arrays include tandem direct repeats of Ty elements separated by only a single long terminal repeat. The Ty-HML junction fragments of one mutant were cloned and shown to contain a 5-base-pair duplication of the target sequence that is characteristic of a Ty transpositional insertion. In addition, the arrays include rearranged Ty elements that do not have normal long terminal repeat junctions. We have also identified multimeric Ty insertions at other chromosomal sites and as insertions that allow expression of a promoterless his3 gene on a plasmid. The results suggest that Ty transposition includes an intermediate that can undergo recombination to produce multimers.