Constitutive activation of the fission yeast pheromone-responsive pathway induces ectopic meiosis and reveals Ste11 as a mitogen-activated protein kinase target

In the fission yeast Schizosaccharomyces pombe, meiosis normally takes place in diploid zygotes resulting from conjugation of haploid cells. In the present study, we report that the expression of a constitutively activated version of the pheromone-responsive mitogen-activated protein kinase kinase kinase (MAP3K) Byr2 can induce ectopic meiosis directly in haploid cells. We find that the Ste11 transcription factor becomes constitutively expressed in these cells and that the expression of pheromone-responsive genes no longer depends on nitrogen starvation. Epistasis analysis revealed that these conditions bypassed the requirement for the meiotic activator Mei3. Since Mei3 is normally needed for inactivation of the meiosis-repressing protein kinase Pat1, this finding suggests that the strong Byr2 signal causes inactivation of Patl by an alternative mechanism. Consistent with this possibility, we found that haploid meiosis was dramatically reduced when Ste11 was mutated to mimic phosphorylation by Patl. The mutation of two putative MAPK sites in Ste11 also dramatically reduced the level of haploid meiosis, suggesting that Ste11 is a direct target of Spk1. Supporting this, we show that Split can interact physically with Ste11 and also phosphorylate the transcription factor in vitro. Finally, we demonstrate that ste11 is required for pheromone-induced G, arrest. Interestingly, when we mutated Ste11 in the sites for Patl and Split phosphorylation simultaneously, the cells could still arrest in G, in response to pheromone, suggesting the existence of yet a third bifurcation of the signaling pathway.