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.
andMolecular 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 sgs1
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 sgs1
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 sgs1
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.
Present address: Program in Molecular Medicine, University of
Massachusetts Cancer Center, Worcester, MA 01605.
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