Mating pheromone signaling pathway in S. pombe. (A) MAP kinase cascade of the S. pombe mating pheromone signaling pathway consists of Byr2 (MAPKKK), Byr1 (MAPKK), and Spk1 (MAPK). Spk1 phosphorylates and activates downstream transcription factor, Ste11. (B) Induction of meiosis by activation of the pheromone-responsive MAPK cascade. Expression of a mutant version of Byr1 (an analog form of the activated Byr1, in which serine at residue 214 and threonine at residue 218 are substituted to aspartic acids) can induce meiosis ectopically via activation of Spk1. Expression of a truncated version of Byr2 (residue 340–659) lacking the N-terminal regulatory domain can also activate downstream Byr1 and induce meiosis.