A homeo domain protein lacking specific side chains of helix 3 can still bind DNA and direct transcriptional repression.

A series of mutations in the homeo domain of the yeast alpha 2 protein were constructed to test, both in vivo and in vitro, predictions based on the alpha 2-DNA cocrystal structure described by Wolberger et al. (1991). The effects of the mutations were observed in three different contexts using authentic target DNA sequences: alpha 2 binding alone to specific DNA, alpha 2 binding cooperatively with MCM1 to specific DNA, and alpha 2 binding cooperatively with a1 to specific DNA. As expected, changes in the amino acid residues that contact DNA in the X-ray structure severely compromised the ability of alpha 2 to bind DNA alone and to bind DNA cooperatively with MCM1. In contrast, many of these same mutations, including a triple change that altered all the "recognition" residues of helix 3, had little or no effect on the cooperative binding of alpha 2 and a1 to specific DNA, as determined both in vivo and in vitro. These results show that the ability of a homeo domain protein to correctly select and repress target genes does not necessarily depend on the residues commonly implicated in sequence-specific DNA binding.