CYP1 (HAP1) is a determinant effector of alternative expression of heme-dependent transcribed genes in yeast [corrected].

The CYP1 (HAP1) gene of Saccharomyces cerevisiae is known to activate a number of target genes in response to the presence of heme. Several features of the protein, deduced from the sequence of the gene, suggest that CYP1 is a general sensor of the redox state of the cell. To investigate further the function of CYP1, we analysed its effects on the transcription of two genes, HEM13 and 14DM, which are preferentially expressed in anaerobiosis. HEM13 encodes coproporphyrinogen oxidase which catalyses the sixth enzymatic step in the heme biosynthetic pathway and 14DM encodes lanosterol-14-demethylase which is involved in sterol biosynthesis and is a member of the cytochrome P450 family. Isogenic CYP1+ and cyp1 degree deleted strains, either heme-sufficient or heme-deficient (HEM1 disrupted), were grown in aerobic or anaerobic conditions, and transcripts of HEM13 and 14DM were analysed on Northern blots. The results show that in anaerobic and in heme-deficient cells, CYP1 activates the transcription of HEM13 and inhibits that of 14DM. Opposite effects of CYP1 are observed in aerobic, heme-sufficient cells. We concluded that: (i) CYP1 is an efficient activator especially in heme-depleted cells; (ii) CYP1 exerts both positive and negative regulatory effects; (iii) the nature of the regulatory function of CYP1 depends on the target gene; and (iv) for a given gene, the presence or absence of heme or oxygen reverses the sense of CYP1-dependent regulation.