Activation of the Kss1 invasive-filamentous growth pathway induces Ty1 transcription and retrotransposition in Saccharomyces cerevisiae

Using a set of genomic TY1A-lacZ fusions, we show that Ste12 and Tec1, two transcription factors of the Kss1 mitogen-activated protein kinase (MAPK) c ascade activate Ty1 transcription in Saccharomyces cerevisiae. This result strongly suggests that the invasive-filamentous pathway regulates Ty1 trans cription. Since this pathway is active in diploid cells, we suspected that Ty1 transposition might occur in this cell type, despite the fact that this event has been never reported before (unless activated by heterologous pro moters such as that of GAL1). We demonstrate here that constitutive activat ion of the invasive-filamentous pathway by the STE11-4 allele or by growth in low-nitrogen medium induces Ty1 transcription and retrotransposition in diploid cells. We show that Ty1 retrotransposition can be activated by STE1 1-4 in haploid cells as well. Our findings provide the first evidence that Ty1 retrotransposition can be activated by environmental signals that affec t differentiation. Activation of the Kss1 MAPK cascade by stress is known t o cause filament formation that permits the search for nutrients away from the colonization site. We propose that activation of Ty1 retrotransposition by this cascade could play a role in adaptive mutagenesis in response to s tress.