MATING IN SACCHAROMYCES-CEREVISIAE - THE ROLE OF THE PHEROMONE SIGNAL-TRANSDUCTION PATHWAY IN THE CHEMOTROPIC RESPONSE TO PHEROMONE

The mating process in yeast has two distinct aspects. One is the induc tion and activation of proteins required for cell fusion in response t o a pheromone signal; the other is chemotropism, i.e., detection of a pheromone gradient and construction of a fusion site available to the signaling cell. To determine whether components of the signal transduc tion pathway necessary for transcriptional activation also play a role in chemotropism, we examined strains with null mutations in component s of the signal transduction pathway for diploid formation, prezygote formation and the chemotropic process of mating partner discrimination when transcription was induced downstream of the mutation. Cells muta nt for components of the mitogen-activated protein (MAP) kinase cascad e (ste5, ste20, ste11, ste7 or fus3 kss1) formed diploids at a frequen cy 1% that of the wild-type control, but formed prezygotes as efficien tly as the wild-type control and showed good mating partner discrimina tion, suggesting that the MAP kinase cascade is not essential for chem otropism. In contrast, cells mutant for the receptor (ste2) or the bet a or gamma subunit (ste4 and ste18) of the G protein were extremely de fective in both diploid and prezygote formation and discriminated poor ly between signaling and nonsignaling mating partners, implying that t hese components are important for chemotropism.