Originally published as Genetics Published Articles Ahead of Print on April 16, 2005.

Genetics, Vol. 170, 601-612, June 2005, Copyright © 2005
doi:10.1534/genetics.104.026278

Length and Sequence Heterozygosity Differentially Affect HRAS1 Minisatellite Stability During Meiosis in Yeast

Peter A. Jauert and David T. Kirkpatrick1

Department of Genetics, Cell Biology and Development, University of Minnesota, Minneapolis, Minnesota 55455

1 Corresponding author: Department of Genetics, Cell Biology and Development, University of Minnesota, 6-160 Jackson Hall, 321 Church St. SE, Minneapolis MN 55455.
E-mail: dkirkpat{at}cbs.umn.edu

Minisatellites, one of the major classes of repetitive DNA sequences in eukaryotic genomes, are stable in somatic cells but destabilize during meiosis. We previously established a yeast model system by inserting the human Ha-ras/HRAS1 minisatellite into the HIS4 promoter and demonstrated that our system recapitulates all of the phenotypes associated with the human minisatellite. Here we demonstrate that meiotic minisatellite tract-length changes are half as frequent in diploid cells harboring heterozygous HRAS1 minisatellite tracts in which the two tracts differ by only two bases when compared to a strain with homozygous minisatellite tracts. Further, this decrease in alteration frequency is entirely dependent on DNA mismatch repair. In contrast, in a diploid strain containing heterozygous minisatellite tract alleles differing in length by three complete repeats, length alterations are observed at twice the frequency seen in a strain with homozygous tracts. Alterations consist of previously undetectable gene conversion events, plus nonparental length alteration events seen previously in strains with homozygous tracts. A strain containing tracts with both base and length heterozygosity exhibits the same level of alteration as a strain containing only length heterozygosity, indicating that base heterozygosity-dependent tract stabilization does not affect tract-length alterations occurring by gene conversion.