POSITIVE AND NEGATIVE ELEMENTS INVOLVED IN THE DIFFERENTIAL REGULATION BY HEME AND OXYGEN OF THE HEM13 GENE (COPROPORPHYRINOGEN OXIDASE) INSACCHAROMYCES-CEREVISIAE

The Saccharomyces cerevisiae HEM13 gene codes for coproporphyrinogen o xidase (CPO), an oxygen-requiring enzyme catalysing the sixth step of heme biosynthesis. Its transcription is increased 40-50-fold in respon se to oxygen- or heme-deficiency. We have analyzed CPO activity and HE M13 mRNA levels in a set of isogenic strains carrying single or double deletions of the CYP1 (HAP1), ROX1, SSN6, or TUP1 genes. The cells we re grown in the presence or absence of oxygen and under heme-deficienc y (hem1 Delta background). Both Rox1p and Cyp1p partially repressed HE M13 in aerobic heme-sufficient cells, probably in an independent manne r. In the absence of heme, Cyp1p activated HEM13 and strongly represse d ROX1, allowing de-repression of HEM13. Cyp1p had no effect on HEM13 expression in anaerobic cells. Deletions of SSN6 or TUP1 dramatically de-repressed HEM13 in aerobic cells. A series of deletions in the HEM1 3 promoter identified at least four regulatory regions that are requir ed for HEM13 regulation. Two regions, containing motifs similar to the Rox1p consensus sequences, act as repression sites under aerobic grow th. The two other sites act as activation sequences required for full induction under oxygen- or heme-deficiency. Taken together, these resu lts suggest that induction of HEM13 occurs in part through relief of r epression exerted by Rox1p and Cyp1p, and in part by activation mediat ed partly by Cyp1p under heme-deficiency and by unknown factors under oxygen-deficiency.