Relative contributions of MCM1 and STE12 to transcriptional activation of a- and alpha-specific genes from Saccharomyces cerevisiae.

We have examined the relative contributions of MCM1 and STE12 to the transcription of the a-specific STE2 gene by using a 367 bp fragment from the STE2 5'-noncoding region to drive expression of a reporter lacZ gene. Mutation of the MCM1 binding site destroyed MCM1.alpha 2-mediated repression in alpha cells and dramatically reduced expression in a cells. The residual expression was highly stimulated by exposure of cells to pheromone. Likewise, the loss of STE12 function reduced lacZ expression driven by the wild-type STE2 fragment. In the absence of both MCM1 and STE12 functions, no residual expression was observed. Thus, the STE2 fragment appears to contain two distinct upstream activation sequences (UASs), one that is responsible for the majority of expression in cells not stimulated by pheromone, and one that is responsible for increased expression upon pheromone stimulation. In further support of this idea, a chemically synthesized version of the STE2 MCM1 binding site had UAS activity, but the activity was neither stimulated by pheromone nor reduced in ste12 mutants. Although transcription of alpha-specific genes also requires both MCM1 and STE12, these genes differ from a-specific genes in that they have a single, MCM1-dependent UAS system. The activity of the minimal 26 bp UAS from the alpha-specific STE3 gene was both stimulated by pheromone and reduced in ste12 mutants. These data suggest that at alpha-specific genes STE12 and MCM1 exert their effects through a single UAS.