Homeo domain of the yeast repressor alpha 2 is a sequence-specific DNA-binding domain but is not sufficient for repression

The alpha 2 protein, the product of the MAT alpha 2 gene, is a regulator of cell type in the yeast Saccharomyces cerevisiae. It represses transcription of a group of cell type-specific genes by binding to an operator located upstream of each target gene. Fifteen in-frame deletions within the coding region of the MAT alpha 2 gene were constructed. The deletion alleles were examined for phenotypes conferred in vivo, and the encoded mutant proteins were assayed for ability to bind specifically to the operator in vitro. This analysis has revealed that the sequence-specific DNA-binding domain of alpha 2 is located within a region of 68 amino acids. This region of alpha 2 has significant homology with the homeo domain, a conserved sequence found in the products of several Drosophila homeotic and segmentation genes. In addition, there is a class of mutant alpha 2 proteins that binds tightly and specifically to the operator in vitro, but fails to repress transcription in vivo.