Abstract
Mating type in the yeast Saccharomyces cerevisiae can be one of three types: a, α, and aα. These mating types reflect information present at MAT,which is normally MATa or MATα in haploid cells, and MATa/MATα in diploid cells. MATα and MAT a each have two open reading frames, but functions have only been identified for three gene products, Mata1p, Matα1p, and Matα2p. The MAT gene products are key regulators of the different stages of the yeast life cycle (reviewed in refs. 26,28,30). The Mata1p/Matα2p complex in diploid cells represses transcription of haploid-specific genes, including the repressor of meiosis, RME1,and HO. In MATα haploid cells, Matα1p complexes with Mcm1p to activate α-specific genes. Matα2p complexes with Mcm1p (as well as Tup1p and Ssn6p) to repress a-specific genes. MAT controls mating: a cells mate with α cells but not with a cells, and vice versa, and aα cells do not mate with any of the three cell types. The mating behavior of a and α cells reflects their expression of the haploid-specific mating pheromones, a -and α-factor and the cognate transmembrane receptors. MAT also controls meiosis and sporulation; MATa/MATα diploids can carry out meiosis whereas diploids expressing only one of the two alleles do not. The ability of aα diploids to enter meiosis depends on repression of the regulatory gene RME11,which is turned off by the action of the Mata1p-Matα2p repressor complex. This leads to the expression of many meiosis-specific genes. Although natural aa or αα diploids are rare, they can be easily produced by using a regulated source of HO endonuclease (29) or by deleting one MAT allele (i.e. αΔ or aΔ) (44).