Figure 1. Ty1his3AI assay for measuring Ty1 retromobility. A strain with a deletion at the endogenous HIS3 locus harbors a chromosomal Ty1 element with the his3AI indicator gene between Ty1 coding sequences and the 3′ long terminal repeat (white arrowheads indicate long terminal repeats). The strain cannot grow in the absence of histidine because the HIS3 sequence within Ty1 is disrupted by an oppositely oriented intron (AI, labeled “Intron” in drawing) that is not spliced when transcription initiates from the HIS3 promoter. Transcription of Ty1his3AI from the Ty1 promoter allows splicing of AI, and reverse transcription produces a Ty1HIS3 cDNA that can be incorporated into the genome. Cells that acquire Ty1HIS3 through a retromobility event express HIS3 and gain a His+ prototroph phenotype.