The pheromone response pathway of the yeast Saccharomyces cerevisiae is necessary for the basal level of transcription of cell-type-specific genes, as well as the induced level observed after pheromone treatment. The STE12 protein binds to the DNA sequence designated the pheromone response element and is a target of the pheromone-induced signal. We generated 6-nucleotide linker insertion mutants, internal-deletion mutants, and carboxy-terminal truncation mutants of STE12 and assayed them for their ability to restore mating and transcriptional activity to a stel2A strain. Two of these mutant proteins retain the capacity to mediate basal transcription but show little or no induced transcription upon pheromone treatment. Cells producing these proteins cannot mate, formally demonstrating that the ability to respond to pheromone by increasing gene expression is essential for the mating process. Since distinct domains of STE12 appear to be required for basal versus induced transcription, we suggest that the pheromone-induced signal is likely to target residues of the protein different from those targeted by the basal signal because of the constitutive activity of the response pathway. Our analysis of mutant STE12 proteins also indicates that only the DNA-binding domain is sensitive to the small changes caused by the linker insertions. In addition, we show that, while the carboxy-terminal sequences necessary for STE12 to form a complex with the transcription factor MCM1 are not essential for mating, these sequences are required for optimal transcriptional activity.Haploid cells of Saccharomyces cerevisiae are of mating type a or a, which differ principally in the kind of pheromone and pheromone receptor produced; a cells synthesize a-factor and the receptor for a-factor, and a cells synthesize a-factor and the receptor for a-factor. Exchange of the pheromones activates a signaling pathway that leads to changes in transcription, cell cycle arrest, and ultimately, mating (for reviews, see references 3, 21, and 38). The ability of a and a cells to display these phenotypic differences is the result of cell-type-specific transcription (for reviews, see references 5 and 16). This transcription is regulated by DNA-binding proteins that are either synthesized in a celltype-specific manner (the AM T products), expressed in both a and a cells with activity regulated by the pheromone signal from the opposite mating type (STE12 product), or ubiquitously expressed and not regulated by signaling (the MCMI product). This combinatorial control, thus, provides a model for understanding cell-type-specific and hormonally induced transcription in more-complex systems.STE12, a protein of 688 amino acids, binds to the pheromone response element found in the upstream region of many genes inducible by a-factor or a-factor (6, 9). The minimal DNA-binding domain of the protein lies between residues 40 and 204 and shows a low level of homology to the homeodomain (42). STE12 becomes rapidly phosphorylated after treatment of cells with pheromone, ...

The pheromone response pathway of the yeast Saccharomyces cerevisiae is necessary for the basal level of transcription of cell-type-specific genes, as well as the induced level observed after pheromone treatment. The STE12 protein binds to the DNA sequence designated the pheromone response element and is a target of the pheromone-induced signal. We generated 6-nucleotide linker insertion mutants, internal-deletion mutants, and carboxy-terminal truncation mutants of STE12 and assayed them for their ability to restore mating and transcriptional activity to a ste12 delta strain. Two of these mutant proteins retain the capacity to mediate basal transcription but show little or no induced transcription upon pheromone treatment. Cells producing these proteins cannot mate, formally demonstrating that the ability to respond to pheromone by increasing gene expression is essential for the mating process. Since distinct domains of STE12 appear to be required for basal versus induced transcription, we suggest that the pheromone-induced signal is likely to target residues of the protein different from those targeted by the basal signal because of the constitutive activity of the response pathway. Our analysis of mutant STE12 proteins also indicates that only the DNA-binding domain is sensitive to the small changes caused by the linker insertions. In addition, we show that, while the carboxy-terminal sequences necessary for STE12 to form a complex with the transcription factor MCM1 are not essential for mating, these sequences are required for optimal transcriptional activity.