Ty1 copy number oscillation in Saccharomyces.
David J. Garfinkel (1), Sharon P. Moore (1), Gianni Liti (2), Karen M. Stefanisco (1), Katherine M. Nyswaner (1), Caroline Chang (1), Edward J. Louis (2)
(1) National Cancer Institute, National Institutes of Health, PO Box B, Frederick, MD, 21702-1201, USA;
(2) Department of Genetics, University of Leicester, University Road, Leicester LE1 7RH, UK
Ty element gain through retrotransposition is modulated by host defense and cosuppression mechanisms, as well as by element loss to minimize genome expansion. Here we examine the fate of Ty sequences in Saccharomyces strain #337, a strain that lacks Ty elements, contains degenerate solo LTRs, and is more closely related to S. paradoxus. Solo LTRs share the same insertion sites as full-length Ty elements in S. cerevisiae, suggesting that #337 lost Ty elements by LTR-LTR recombination. To understand Ty1 copy-number dynamics, we have estimated the rates of element gain versus loss under genetic and environmental conditions known to affect Ty1 retrotransposition in strains with Ty1 elements at their preferred and selectively neutral genome locations. The results show that Ty1 retrotransposition varies dramatically with copy number, temperature and MATa/alpha cell-type, whereas Ty1 loss is relatively constant. Ty1 cDNA recombination does not appear to be utilized as extensively as gene conversion between chromosomal elements to recycle Ty1 sequences without element gain. Loss of Ty1 in heterozygotes mostly occurs by pathways other than LTR-LTR recombination. Our work suggests that Ty1 copy number can oscillate dramatically over time by altering the rate of retrotransposition, resulting in various copy numbers in different Saccharomyces strains or species, including complete loss in strains like #337.
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