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Molecular and Cellular Biology, August 2001, p. 5374-5388, Vol. 21, No. 16
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.16.5374-5388.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

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

Mary Bryk,1,2 Mukti Banerjee,1 Darryl Conte Jr.,1,dagger and M. Joan Curcio1,*

Molecular Genetics Program, Wadsworth Center and School of Public Health, State University of New York at Albany, Albany, New York 12208,1 and Department of Genetics, Harvard Medical School, Boston, Massachusetts 021152

Received 24 April 2001/Returned for modification 9 May 2001/Accepted 17 May 2001

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 sgs1Delta 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 sgs1Delta mutants, 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 sgs1Delta 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.

* Corresponding author. Mailing address: Wadsworth Center, P.O. Box 22002, Albany, NY 12201-2002. Phone: (518) 473-6078. Fax: (518) 474-3181. E-mail: joan.curcio{at}wadsworth.org.

dagger Present address: Program in Molecular Medicine, University of Massachusetts Cancer Center, Worcester, MA 01605.

Molecular and Cellular Biology, August 2001, p. 5374-5388, Vol. 21, No. 16
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.16.5374-5388.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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