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The yeast alpha 2 protein, the product of the MAT alpha 2 gene, is a regulator of yeast cell type; it turns off transcription of the a-specific genes by binding to an operator located upstream of each gene. In this paper we describe the domain structure, subunit organization, and some unusual features of the way this protein contacts its operator. We show that the protein is folded into two domains. The carboxy-terminal domain binds specifically to the operator; the amino-terminal domain contains dimerization contacts. The alpha 2 dimer differs from those of the phage repressors in that it is flexible and therefore is able to bind tightly to differently spaced operator half-sites. In the natural operator, the centers of the operator half-sites are two and one-half turns of DNA apart, exposing them on opposite sides of the DNA helix. We show that the design of alpha 2 allows a dimer to reach across its operator such that it occupies the two half-sites but leaves the middle of the operator available to other proteins.

Flexibility of the yeast alpha 2 repressor enables it to occupy the ends of its operator, leaving the center free.