FIG. 1.

Ty1 transposition assay. The most common way for the His⁻ strains containing the promoterless his3-Δ4 allele diagrammed above to revert to His⁺ is by the insertion of an upstream Ty1 element. Such insertions were detected by the appearance of 400- to 500-bp PCR amplicons when the HIS3 (1His3) and delta element (U5-in) primers were used. No insertions were detected when the HIS3 (1His3) and delta element (U3-in) primers were used, indicating that the Ty1 elements had to be in the indicated orientation to cause a His⁺ reversion. Arrows in boxes indicate the direction of transcription.