The full-text of this article is not currently available on the LRA. The original published version is available from the publisher's website at http://mcb.asm.org/Expanded trinucleotide repeats underlie a growing number of human diseases. The human FMR1 (CGG)n array can exhibit genetic instability characterized by progressive expansion over several generations leading to gene silencing and the development of the fragile X syndrome. While expansion is dependent upon the length of uninterrupted (CGG)n, instability occurs in a limited germ line and early developmental window, suggesting that lineage-specific expression of other factors determines the cellular environment permissive for expansion. To identify these factors, we have established normal- and premutation-length human FMR1 (CGG)n arrays in the yeast Saccharomyces cerevisiae and assessed the frequency of length changes greater than 5 triplets in cells deficient in various DNA repair and replication functions. In contrast to previous studies with Escherichia coli, we observed a low frequency of orientation-dependent large expansions in arrays carrying long uninterrupted (CGG)n arrays in a wild-type background. This frequency was unaffected by deletion of several DNA mismatch repair genes or deletion of the EXO1 and DIN7 genes and was not enhanced through meiosis in a wild-type background. Array contraction occurred in an orientation-dependent manner in most mutant backgrounds, but loss of the Sgs1p resulted in a generalized increase in array stability in both orientations. In contrast, FMR1 arrays had a 10-fold-elevated frequency of expansion in a rad27 background, providing evidence for a role in lagging-strand Okazaki fragment processing in (CGG)n triplet repeat expansio
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