Figure 1. Genes active at the mating type locus in a or haploids, or in a/ diploids. The different colours indicate that there is a portion of this locus that is not the same in the two mating types. (a) In mating type a cells, the MATa1 gene is expressed. a-specific genes are expressed by a default pathway; -specific genes are not expressed; haploid-specific genes are expressed by default pathways; and one of the haploid-specific genes is a repressor of meiosis. (b) In mating type cells, the MAT1 and MAT2 genes are both expressed. The former is an activator of -specific genes and the latter a repressor of a-specific genes. Haploid-specific genes are expressed, including the repressor of meiosis. (c) When both alleles are present in an a/ diploid, MAT2 protein continues to repress a-specific genes as in (b), and interacts with the product of MATa1 to repress the haploid-specific genes and allow meiosis and sporulation.

Figure 2. Signal transduction pathway for response to mating pheromone. Mating pheromone is an extracellular peptide that interacts with the extracellular portion of the appropriate transmembrane receptor (Ste2 or Ste3 protein). Binding of pheromone to receptor causes a conformational change in the associated G protein, which consists of three subunits (G, product of the GPA1 gene, G, product of STE4 and G, product of STE18). This conformational change results in dissociation of the G and G subunits, which activates Ste20 protein, a protein kinase. Ste20p phosphorylates the mitogen-activated protein (MAP) kinase kinase kinase (MAPKKK), Ste11p. This in turn phosphorylates the MAP kinase kinase (MAPKK) Ste7p, which phosphorylates the MAP kinase (Fus3p). Fus3p phosphorylates target proteins that regulate the cell cycle or activate transcription of genes required for mating.