The Sgs1 Helicase of Saccharomyces cerevisiae Inhibits Retrotransposition of Ty1 Multimeric Arrays

Abstract

Ty1 retrotransposons in the yeast Saccharomyces cerevisiae are maintained in a genetically competent but transpositionally dormant state. When located in the ribosomal DNA (rDNA) locus, Ty1 elements are transcriptionally silenced by the specialized heterochromatin that inhibits rDNA repeat recombination. In addition, transposition of all Ty1 elements is repressed at multiple posttranscriptional levels. Here, we demonstrate that Sgs1, a RecQ helicase 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 sgs1Δ mutants. Increased Ty1 mobility was not due to derepression of silent 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 RNA, protein, or cDNA and did not alter the level or specificity of Ty1 integration. Instead, Ty1 cDNA molecules recombined at a high frequency in sgs1Δmutants, resulting in transposition of heterogeneous Ty1 multimers. Formation of Ty1 multimers required the homologous recombination protein Rad52 but did not involve recombination between Ty1 cDNA and genomic Ty1 elements. Therefore, Ty1 multimers that transpose at a high frequency in sgs1Δ mutants are formed by intermolecular recombination between extrachromosomal 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.

ACKNOWLEDGMENTS

We thank Ed Louis, Jeff Smith, and David Garfinkel for supplying plasmids and strains used in this study, the Wadsworth Center Molecular Genetics Core Facility for oligonucleotide synthesis and DNA sequencing, and Abram Gabriel, John Mueller, and Keith Derbyshire for critical reading of the manuscript. Moreover, we are grateful to Fred Winston and Winston lab members for support and encouragement.

M. Bryk is the recipient of a Special Fellowship from the Leukemia & Lymphoma Society. M. Bryk's research in Fred Winston's lab was funded by grant GM32967 from the National Institutes of Health. This work was funded by National Institutes of Health grant GM52072 to M. J. Curcio.