Abstract
The structures of two unusual deletions from the yeast Saccharomyces cerevisiae are described. These deletions extend from a single Ty1 retrotransposon to an endpoint near a repetitive tRNA(Gly) gene. The deletions suggest that unique sequences flanked by two nonidentical repetitive sequences, or bordered on only one side by a transposable element, have the potential to be mobilized in the yeast genome. Models for the formation of these two unusual deletions were tested by isolating and analyzing 32 additional unusual deletions of the CYC1 region that extend from a single Ty1 retrotransposon. Unlike the most common class of deletions recovered in this region, these deletions are not attributable solely to homologous recombination among repetitive Ty1 or delta elements. They arose by two distinct mechanisms. In an SPT8 genetic background, most unusual deletions arose by transposition of a Ty1 element to a position adjacent to a tRNA(Gly) gene followed by Ty1-Ty1 recombination. In an spt8 strain, where full-length Ty1 transcription and, therefore, transposition are reduced, most deletions were due to gene conversion of a 7-kb chromosomal interval flanked by a Ty1 element and a tRNA(Gly) gene.