SINGLE-STEP SELECTION FOR TY1 ELEMENT RETROTRANSPOSITION

The yeast retrotransposon Ty1 has been tagged with a reporter gene that allows selection of RNA-mediated transposition events and is applicable to the study of retroelements in other organisms. The reporter gene is a yeast HIS3 gene interrupted by an artificial intron (AI) in the antisense orientation. The HIS3AI sequences were inserted into a Ty1 element such that the intron is on the sense strand of the Ty1 element; therefore, splicing and retrotransposition of marked Ty1 transcripts can give rise to His+ cells. Fusion of the Ty1-H3mHIS3AI element to the inducible GAL1 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 Ty1 transposition rate to be between 3 X 10(-7) and 1 X 10(-5) transposition per Ty1 element per generation. Variations in the transposition rate of individual Ty1 elements are correlated with the relative abundance of their transcripts.