Single-step selection for Tyl element retrotransposition (Saccharomyces cerevisiae/reverse transcription/artificial intron)

The yeast retrotransposon Tyl has been tag- ged with a reporter gene that allows selection of RNA-mediated transposition events and is applicable to the study of retroele- ments in other organisms. The reporter gene is a yeast HIS3 gene interrupted by an artificial intron (Al) in the antisense orientation. The HIS3AI sequences were inserted into a Tyl element such that the intron is on the sense strand of the Tyl element; therefore, splicing and retrotransposition of marked Tyl transcripts can give rise to His' cells. Fusion of the Tyl-H3mHIS3AI element to the inducible GAL) promoter resulted in a high frequency of histidine prototrophs upon galactose induction. Moreover, spontaneous His+ revertants derived from strains containing genomic TymHIS3AI elements are a result of retrotransposition. By using this assay, we estimated the Tyl transposition rate to be between 3 x 1O-7 and 1 X 10-5 transpositions per Tyl element per generation. Variations in the transposition rate of individual Tyl elements are correlated with the relative abundance of their transcripts.