Mutations in yeast HAP2/HAP3 define a hybrid CCAAT box binding domain.

We describe a detailed genetic analysis of the DNA‐binding regions in the HAP2/HAP3 CCAAT‐binding heteromeric complex. The DNA‐binding domain of HAP2 is shown to be a 21 residue region containing three critical histidines and three critical arginines. Mutation of an arginine at position 199 to leucine alters the DNA‐binding specificity of the complex to favor CCAAC over CCAAT. Residues in HAP3 that are critical for DNA‐binding comprise a short, seven amino acid region. Three different mutations in the HAP2 DNA‐binding domain are suppressed by a mutation in the HAP3 DNA‐binding domain. This HAP3 mutation also suppresses mutations in a different region of HAP2 which promotes subunit assembly of the complex. These findings suggest that short regions of HAP2 and HAP3 comprise a hybrid DNA‐binding domain and that this domain can help hold the two subunits together in the CCAAT‐binding complex.