MAT alpha 1 protein, a yeast transcription activator, binds synergistically with a second protein to a set of cell-type-specific genes.

We show by electrophoresis mobility shift and by DNAase I footprinting assays that the alpha 1 product of the yeast alpha mating-type locus binds to homologous sequences within the control regions of the three known alpha-specific genes. Binding requires both alpha 1 and a second yeast protein(s) (called PRTF) that is present in all three cell types (a, alpha, and a/alpha); neither protein binds alone. Binding and competition experiments using synthetic oligonucleotides indicate that PRTF binds to only part of the homology found at alpha-specific genes and imply that alpha 1 binds to the remainder. Our results suggest that alpha 1 renders gene expression alpha-specific by creating a binding site for PRTF. Similar experiments lead to the idea that PRTF also plays a role in transcription of a-specific genes. Perhaps a-specificity is achieved through the occlusion of the PRTF binding site by alpha 2, the negative regulator encoded by the alpha mating-type locus.