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STE3 mRNA is present only in Saccharomyces cerevisiae alpha cells, not in a or a/alpha cells, and the transcript level increases about fivefold when cells are treated with a-factor mating pheromone. Deletions in the 5' noncoding region of STE3 defined a 43-base-pair (bp) upstream activation sequence (UAS) that can impart both modes of regulation to a CYC1-lacZ fusion when substituted for the native CYC1 UAS. UAS activity required the alpha 1 product of MAT alpha, which is known to be required for transcription of alpha-specific genes. A chromosomal deletion that removed only 14 bp of the STE3 UAS reduced STE3 transcript levels 50- to 100-fold, indicating that the UAS is essential for expression. The STE3 UAS shares a 26-bp homology with the 5' noncoding sequences of the only other known alpha-specific genes, MF alpha 1 and MF alpha 2. We view the homology as having two components--a nearly palindromic 16-bp "P box" and an adjacent 10-bp "Q box." A synthetic STE3 P box was inactive as a UAS; a perfect palindrome P box was active in all three cell types. We propose that the P box is the binding site for a transcription activator, but that alpha 1 acting via the Q box is required for this activator to bind to the imperfect P boxes of alpha-specific genes. Versions of the P box are also found upstream of a-specific genes, within the binding sites of the repressor alpha 2 encoded by MAT alpha. Thus, the products of MAT alpha may render gene expression alpha or a-specific by controlling access of the same transcription activator to its binding site, the P box.