Previously, it was shown that the CYP1(HAP1) gene product mediates the transcription of several oxygen‐regulated genes through a metabolic co‐effector, heme, in the yeast Saccharomyces cerevisiae. This study investigates the overproduction of the CYP1 protein when the CYP1(HAP1) gene is placed under the control of the GAL10‐CYC1 hybrid promoter (either at the locus of the CYP1(HAP1) gene or cloned in a high‐copy‐number plasmid). In these conditions, the CYP1 protein is detected by Western blot analysis and has a molecular mass in agreement with the open reading frame sequence. Band‐shift experiments show that the CYP1(HAP1) protein is able to interact specifically with its target sequences in vitro without addition of hemin, and forms a large complex with one or several unidentified factors denoted as X. Addition of hemin allows the formation of a new complex which has a lower molecular mass. The internal deletion of the seven repeated amino acid sequences containing the KCPVDH motif in the CYP1(HAP1) protein modifies the heme responsiveness phenomenon observed in vitro in the band‐shift experiments and in vivo in the transcription of the CYB2, CYC1, CYP3(CYC7) and ERG11 genes. On the basis of these data, we propose a new model for heme‐induced activation of the CYP1 protein.