α3, a transposable element that promotes host sexual reproduction

Theoretical models predict that selfish DNA elements require host sex to persist in a population. Therefore, a transposon that induces sex would strongly favor its own spread. We demonstrate that a protein homologous to transposases, called alpha 3, was essential for mating type switch in Kluyveromyces lactis. Mutational analysis showed that amino acids conserved among transposases were essential for its function. During switching, sequences in the 59 and 39 flanking regions of the alpha 3 gene were joined, forming a DNA circle, showing that a3 mobilized from the genome. The sequences encompassing the alpha 3 gene circle junctions in the mating type alpha (MAT alpha) locus were essential for switching from MAT alpha to MATa, suggesting that alpha 3 mobilization was a coupled event. Switching also required a DNA-binding protein, Mating type switch 1 (Mts1), whose binding sites in MAT alpha were important. Expression of Mts1 was repressed in MATa/MAT alpha diploids and by nutrients, limiting switching to haploids in low-nutrient conditions. A hairpin-capped DNA double-strand break (DSB) was observed in the MATa locus in mre11 mutant strains, indicating that mating type switch was induced by MAT-specific DSBs. This study provides empirical evidence for selfish DNA promoting host sexual reproduction by mediating mating type switch.