Binds to the upstream activation site (UAS) of the CYC1 gene and other genes involved in mitochondrial electron transport and activates their expression. Recognizes the sequence CCAAT. Two complexes bind CCAAT; complex I, that consists of HAP2/3/5 and complex II, that consists of HAP2/3/5/4. Belongs to the NFYB/HAP3 subunit family.