Regulation of oxygen dependent gene expression. It modulates the expression of Iso-1 (CYP1) and Iso-2 (CYP3) cytochrome c. In response to heme, promotes transcription of genes encoding functions required for respiration, controlling oxidative damage and repression of anaerobic genes. Binds to the sequence 5'-CGGNNNTNNCGG-3' (By similarity). Binds DNA as a homodimer. Interacts with SRO9 and YDJ1. In the absence of heme, binds to at least four cellular proteins, including YDJ1 and SRO9, forming a high-molecular-weight complex (HMC) which results in repression of its activity and dictates its DNA-binding specificity (By similarity). Heme is an effector molecule for CYP1/HAP1. The repeat region (see FT table) mediates heme induction by masking the DNA-binding domain in the absence of inducer (By similarity).