Ty RNA levels determine the spectrum of retrotransposition events that activate gene expression in Saccharomyces cerevisiae

To learn more about the variety of Ty elements capable of activating gene expression, we characterized 206 spontaneous Ty transpositions that activate the promoterless gene his3Δ4. Most of the Ty elements appear to be full-length, although a few deleted elements were recovered. Over 95% of the insertions belong to the Ty1 family, and the rest are Ty2 elements. The excessive number of Ty1 transpositions was unexpected because there are only 2-fold more Ty1 than Ty2 elements in the yeast strains used in the selection. However, there is 20-fold more Ty1 than Ty2 RNA present in these yeast strains. This difference in RNA level explains the greater number of Ty1 verses Ty2 transpositions at his3Δ4, because Ty elements transpose through an RNA intermediate. A similar association between the Ty transcript level and transpositional activation of his3Δ4 is obtained in cells expressing GAL1-promoted Ty2-H556 or Ty2-917 elements, but only if the element does not contain a marker. Genetically marked Ty2-H556 NEO and-917 NEO elements transpose into and activate his3Δ4 with the same efficiency as the previously characterized Ty1H3 NEO element, but are underrepresented relative to the levels of Ty NEO transcript. We also found that chromosomal Ty transcripts are even more abundant than previously estimated and comprise about 1% of total cellular RNA.