Yeast Mating Type

Abstract Mating type in baker's yeast,Saccharomyces cerevisiae, has been extensively studied as a model system for cell fate determination, gene regulation, chromosome structure and genetic recombination. This work has shown that mating type is determined by alleles of a single genetic locus that codes for regulatory proteins governing genes encoding mating pheromones, pheromone receptors and downstream effectors controlling cell type. Some yeast strains are capable of switching mating type, a process involving gene replacement by information transferred from silenced copies of the mating‐type locus found elsewhere in the genome. A combination of genetic, molecular and biochemical approaches has elucidated the mechanisms of silencing and mating‐type switching and has shown them to involve processes that are conserved throughout the eukaryotic kingdom. Key Concepts Mating type in baker's yeast is determined by alleles of a single gene. Products of the mating‐type locus are regulatory proteins that interact to control transcription at multiple other loci that determine cell fate. A genetic approach involving isolation of sterile mutants unable to mate provided the first clues to the structure and function of the mating‐type locus. Mating involves production of soluble protein mating factors and membrane‐bound receptors that trigger a signal transduction cascade upon binding of mating factor to receptor. Mating is coordinated with progression through the yeast cell cycle. Yeast can exist as either haploid or diploid cells with one or two sets of chromosomes; haploids can mate to form diploids. Some yeast strains are homothallic and can switch mating type. Mating‐type switching involves silent copies of the mating‐type locus and a site‐specific endonuclease that directs recombination between the active mating‐type locus and one of the silent copies. The silent copies of the mating‐type locus are present in a heterochromatin‐like configuration that prevents their expression. General recombination functions are required for mating‐type switching.