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 domai n of HAP2 is shown to be a 21 residue region containing three critical histidines and three critical arginines. Mutation of an arginine at p osition 199 to leucine alters the DNA-binding specificity of the compl ex to favor CCAAC over CCAAT. Residues in HAP3 that are critical for D NA-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 mut ations in a different region of HAP2 which promotes subunit assembly o f the complex. These findings suggest that short regions of HAP2 and H AP3 comprise a hybrid DNA-binding domain and that this domain can help hold the two subunits together in the CCAAT-binding complex.