Fus3 controls Ty1 transpositional dormancy through the invasive growth MAPK pathway

Fus3, the mitogen-activated protein kinase (MAPK) of the mating pheromone r esponse pathway, inhibits a post-translational step of Ty1 retrotranspositi on. Fus3 also inhibits haploid invasive growth by blocking cross-activation of invasive growth gene expression by the pheromone response signal cascad e. Here, we show that Fus3 kinase activity and dosage co-ordinately regulat e Ty1 transposition and invasive growth. A chromosomal copy of the kinase-d efective fus3-K42R allele fails to inhibit either Ty1 transposition or inva sive growth. When overexpressed, kinase-defective Fus3 weakly inhibits both Ty1 transposition and invasive growth, but is much less inhibitory than wi ld-type Fus3 expressed at the same level. Moreover, increasing the dosage o f wild-type Fus3 intensifies the inhibition of both Ty1 transposition and i nvasive growth. To demonstrate that Fus3 regulates Ty1 transposition via it s negative regulation of the invasive growth pathway, we show by epistatic analysis that the invasive growth pathway transcription factors Ste12. and Tec1 are both required for Fus3-mediated inhibition of Ty1 transposition. W hen haploid invasive growth is stimulated by high-copy expression of TEC1, by expression of the dominant hypermorphic allele STE11-4 or by deletion of HOG1, Ty1 transposition is concomitantly activated. In summary, these resu lts demonstrate that the haploid invasive growth pathway activates Ty1 tran sposition at both transcriptional and post-transcriptional levels and that Fus3 inhibits Ty1 transposition by inhibiting the invasive growth pathway.