In Saccharomyces cerevisiae, the STE12 protein mediates transcriptional induction of cell type-specific genes in response to pheromones. STE12 binds in vitro to the pheromone response elements (PREs) present in the control region of a-specific genes. STE12 is also required for transcription of o~-specific genes, but there is no evidence that it binds directly to these genes. Instead, the MAToL-encoded protein o~l and the MCM1 product bind to the DNA element that is responsible for ,v-specific and a-factor-inducible expression. To explore the role of STE12 in the pheromone induction of ,v-specific genes, we cloned STE12 and MATal homologs from the related yeast Kluyveromyces lactis. The K. lactis STE12 protein did not cooperate with the S. cerevisiae od protein to promote the overall mating process or the induction of transcription of an s-specific gene. However, introduction of both K. lactis STE12 along with K. lactis od did restore mating, suggesting that an interaction between STE12 and ~1 is important for s-specific gene activation. We also show that bacterially expressed STE12 and r are able to form a complex in vitro. Thus, we demonstrate a coupling in ~ cells between a protein functioning in cell identity, ~1, with a protein responsive to the pheromone-induced signal STE12.