The Sgs1 helicase of Saccharomyces cerevisiae inhibits retrotransposition of Ty1 multimeric arrays

Ty1 retrotransposons in the yeast Saccharomyces cerevisiae are maintained i n a genetically competent but transpositionally dormant state. When located in the ribosomal DNA (rDNA) locus, Ty1 elements are transcriptionally sile nced by the specialized heterochromatin that inhibits rDNA repeat recombina tion. In addition, transposition of all Ty1 elements is repressed at multip le posttranscriptional levels. Here, we demonstrate that Sgs1, a RecQ helic ase required for genome stability, inhibits the mobility of Ty1 elements by a posttranslational mechanism. Using an assay for the mobility of Ty1 cDNA via integration or homologous recombination, we found that the mobility of both euchromatic and rDNA-Ty1 elements was increased 32- to 79-fold in sgs 1 Delta mutants. Increased Ty1 mobility was not due to derepression of sile nt rDNA-Ty1 elements, since deletion of SGS1 reduced the mitotic stability of rDNA-Ty1 elements but did not stimulate their transcription. Furthermore , deletion of SGS1 did not significantly increase the levels of total Ty1 R NA, protein, or cDNA and did not alter the level or specificity of Ty1 inte gration. Instead, Ty1 cDNA molecules recombined at a high frequency in sgs1 Delta mutants, resulting in transposition of heterogeneous Ty1 multimers. Formation of Ty1 multimers required the homologous recombination protein Ra d52 but did not involve recombination between Ty1 cDNA and genomic Ty1 elem ents. Therefore, Ty1 multimers that transpose at a high frequency in sgs1 D elta mutants are formed by intermolecular recombination between extrachromo somal Ty1 cDNA molecules before or during integration. Our data provide the first evidence that the host cell promotes retrotransposition of monomeric Ty1 elements by repressing cDNA recombination.