Saccharomyces cerevisiae has two haploid cell types, a and alpha, each of which produces a unique set of proteins that participate in the mating process. We sought to determine the minimum set of proteins that must be expressed to allow mating and to confer specificity. We show that the capacity to synthesize alpha-factor pheromone and a-factor receptor is sufficient to allow mating by mat alpha 1 mutants, mutants that normally do not express any alpha- or a-specific products. Likewise, the

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... city to synthesize a-factor receptor and alpha-factor pheromone is sufficient to allow a ste2 ste6 mutants, which do not produce the normal a cell pheromone and receptor, to mate with wild-type a cells. Thus, the a-factor receptor and alpha-factor pheromone constitute the minimum set of alpha-specific proteins that must be produced to allow mating as an alpha cell. Further evidence that the pheromones and pheromone receptors are important determinants of mating specificity comes from studies with mat alpha 2 mutants, cells that simultaneously express both pheromones and both receptors. We created a series of strains that express different combinations of pheromones and receptors in a mat alpha 2 background. These constructions reveal that mat alpha 2 mutants can be made to mate as either a cells or as alpha cells by causing them to express only the pheromone and receptor set appropriate for a particular cell type. Moreover, these studies show that the inability of mat alpha 2 mutants to respond to either pheromone is a consequence of two phenomena: adaptation to an autocrine response to the pheromones they secrete and interference with response to alpha factor by the a-factor receptor.