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Ferrochelatase is the terminal enzyme in the heme biosynthetic pathway. It catalyzes the insertion of ferrous iron into protoporphyrin IX to produce protoheme IX. The crystal structures of ferrochelatase from Saccharomyces cerevisiae in free form, in complex with Co(II), a substrate metal ion, and in complex with two inhibitors, Cd(II) and Hg(I), are presented in this work. The enzyme is a homodimer, with clear asymmetry between the monomers with regard to the porphyrin binding cleft and the mode of metal binding. The Co(II) and Cd(II) complexes reveal the metal binding site which consists of the invariant amino acids H235, E314, and S275 and solvent molecules. The shortest distance to the metal reveals that amino acid H235 is the primary metal binding residue. A second site with bound Cd(II) was found close to the surface of the molecule, approximately 14 A from H235, with E97, H317, and E326 participating in metal coordination. It is suggested that this site corresponds to the magnesium binding site in Bacillus subtilis ferrochelatase. The latter site is also located at the surface of the molecule and thought to be involved in initial metal binding and regulation.

We have constructed a series of chimeric yeast/mouse and yeast/Bacillus subtilis ferrochelatase genes in order to investigate domains of the ferrochelatase that are important for activity and/or association with the membrane. These genes were expressed in a Saccharomyces cerevisiae mutant in which the endogenous ferrochelatase gene (HEM15) had been deleted, and the phenotypes of the transformants were characterized. Exchanging the approximately 40-amino-acid C-terminus between the yeast and mouse ferrochelatases caused a total loss of activity and the hybrid proteins were unstable when overproduced in Escherichia coli. The water-soluble ferrochelatase of B. subtilis did not complement the yeast mutant, although a large amount of active protein accumulated in the cytosol. Addition of the N-terminal leader sequence of yeast ferrochelatase to the B. subtilis enzyme targeted the fusion protein to mitochondria, but both the precursor and the mature forms of the enzyme were inactive in vivo and had residual activity when measured in vitro. An internal approximately 45-amino-acid segment located at the N-terminus of yeast ferrochelatase was identified, which, when replaced with the corresponding 30-amino-acid segment of the B. subtilis enzyme, caused the yeast enzyme to be located in the mitochondrial matrix as a soluble protein. The fusion protein was inactive in vivo and had residual activity in vitro. We speculate that this segment, which shows the greatest variability between species, is responsible for the association of the enzyme with the membrane., (Copyright 1999 Academic Press.)

We have examined the effects of heme or vacuole deficiency on the kinetics of induction of cell surface ferrireductase activity and expression of the FRE1 gene encoding a component of ferrireductase, in response to iron or copper deprivation in S. cerevisiae. Heme deficiency caused a small decrease in the basal expression of FRE1, but did not impair its induction by Fe or Cu limitation. Thus, the absence of ferrireductase activity and its non-inducibility in heme-less cells is not due to the absence of FRE1 expression. Vacuole deficiency led to constitutively high ferrireductase activity slightly induced by Cu limitation, and to high levels of FRE1 expression further inducible by Fe or Cu deprivation. Thus, the vacuole might be a component of the iron signalling pathway.

The Saccharomyces cerevisiae HEM13 gene codes for coproporphyrinogen oxidase (CPO), an oxygen-requiring enzyme catalysing the sixth step of heme biosynthesis. Its transcription is increased 40-50-fold in response to oxygen- or heme-deficiency. We have analyzed CPO activity and HEM13 mRNA levels in a set of isogenic strains carrying single or double deletions of the CYP1 (HAP1), ROX1, SSN6, or TUP1 genes. The cells were grown in the presence or absence of oxygen and under heme-deficiency (hem1 delta background). Both Rox1p and Cyp1p partially repressed HEM13 in aerobic heme-sufficient cells, probably in an independent manner. In the absence of heme, Cyp1p activated HEM13 and strongly repressed 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 HEM13 promoter identified at least four regulatory regions that are required for HEM13 regulation. Two regions, containing motifs similar to the Rox1p consensus sequences, act as repression sites under aerobic growth. The two other sites act as activation sequences required for full induction under oxygen- or heme-deficiency. Taken together, these results suggest that induction of HEM13 occurs in part through relief of repression exerted by Rox1p and Cyp1p, and in part by activation mediated partly by Cyp1p under heme-deficiency and by unknown factors under oxygen-deficiency.

Uroporphyrinogen decarboxylase (Uro-d; EC 4.1.1.37), the fifth enzyme in the heme biosynthetic pathway, which catalyzes the sequential decarboxylation of uroporphyrinogen to coproporphyrinogen, is encoded by the HEM12 gene in Saccharomyces cerevisiae. The HEM12 gene is transcribed into a major short mRNA and a minor longer one, approximately 1.35 and 1.55 kb, respectively, in size, and that differ in the 5' untranslated region. "Uroporphyric" mutants, which have no mutations in the HEM12 gene but accumulate uroporphyrinogen, a phenotype characteristic of partial Uro-d deficiency, were investigated. Genetic analysis showed that the mutant phenotype depends on the combined action of two unlinked mutations, udt1 and either ipa1, ipa2, or ipa3. ipa1 is tightly linked to HEM12. The mutation udt1 apparently acts specifically on the HEM12 gene, and causes a six to tenfold decrease in the levels of the short HEM12 mRNA, in the beta-galactosidase activity of a HEM12-lacZ fusion, in immunodetectable protein and enzyme activity. But heme synthesis is normal and porphyrin accumulation was modest. The mutations ipa1, ipa2, and ipa3 had no phenotype on their own, but they caused an increase in porphyrin accumulation in a udt1 background. This multiplicity of genetic factors leading to uroporphyric yeast cells closely resembles the situation in human porphyria cutanea tarda.

We have isolated a genomic DNA fragment that complements the yeast temperature-sensitive cyt mutation, causing respiratory deficiency and accumulation of porphyrins (Sugimura et al., 1966). Partial DNA sequencing of the complementing region and search for similarity in the DNA and protein databases revealed that (1) the gene had been previously isolated by complementation of the mutation ts2326 (Langgut et al., 1986; accession number X04694), and (2) it encodes a protein with 18-23% identity to uroporphyrinogen III synthases from different sources. This enzyme catalyses the fourth step in the heme biosynthetic pathway and we named its gene HEM4. A hem4 delta disruption mutation was constructed which had phenotypes identical to the cyt mutation. Biochemical analysis confirmed the absence of uroporphyrinogen III synthase activity in both hem4 delta and cyt mutant strains.

The regulation of Cu,Zn- and Mn-superoxide dismutases (SOD) was investigated by Northern blotting and gene fusions of SOD1 and SOD2 promoters with the beta-galactosidase reporter gene. Cu,ZnSOD expression was increased 3-fold under glucose derepressing conditions, and decreased 4- to 6-fold by oxygen or heme deficiency. MnSOD expression was increased 5-fold by glucose derepression, and decreased 8- to 10-fold by anaerobiosis and 4- to 5-fold by heme deficiency. Induction by paraquat was modest, about 50% for SOD1 and 100% for SOD2; it was apparently independent of the respiratory chain function.

The HEM12 gene from Saccharomyces cerevisiae encodes uroporphyrinogen decarboxylase which catalyzes the sequential decarboxylation of the four acetyl side chains of uroporphyrinogen to yield coproporphyrinogen, an intermediate in protoheme biosynthesis. The gene was isolated by functional complementation of a hem12 mutant. Sequencing revealed that the HEM12 gene encodes a protein of 362 amino acids with a calculated molecular mass of 41,348 Da. The amino acid sequence shares 50% identity with human and rat uroporphyrinogen decarboxylase and shows 40% identity with the N-terminus of an open reading frame described in Synechococcus sp. We determined the sequence of two hem12 mutations which lead to a totally inactive enzyme. They correspond to the amino acid changes Gly33----Asp and Gly300----Asp, located in two evolutionarily conserved regions. Each of these substitutions impairs binding of substrates without affecting the overall conformation of the protein. These results argue that a single active center exists in uroporphyrinogen decarboxylase.

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.

Heme-deficient mutants of Saccharomyces cerevisiae have been isolated from two isogenic strains with the use of an enrichment method based on photodynamic properties of Zn-protoporphyrin. They defined seven non-overlapping complementation groups. A mutant representative of each group was further analysed. Genetic analysis showed that each mutant carried a single nuclear recessive mutations. Biochemical studies showed that the observed accumulation and/or excretion of the different heme synthesis precursors by the mutant cells correlated well with the enzymatic deficiencies measured in acellular extracts. Six of the seven mutants were blocked in a different enzyme activity: 5-aminolevulinate synthase, porphobilinogen synthase, uroporphyrinogen I synthase, uroporphyrinogen decarboxylase, coproporphyrinogen III oxidase and ferrochelatase. The other mutant had the same phenotype as the mutant deficient in ferrochelatase activity. However, it possessed a normal ferrochelatase activity when measured in vitro, so this mutant was assumed to be deficient in protoporphyrinogen oxidase activity or in the transport and/or reduction of iron. The absence of PBG synthesis led to a total lack of uroporphyrinogen I synthase activity. The absence of heme, the end product, led to an important increase of coproporphyrinogen III oxidase activity, while the activity of 5-aminolevulinate synthase, the first enzyme of the pathway, was not changed. These results are discussed in terms of possible modes of regulation of heme synthesis pathway in yeast.

A new and widely applicable technique has been elaborated for enrichment of mutants of Saccharomyces cerevisiae deficient in porphyrin and heme synthesis. The method is based on selective photooxidative killing of the wild-type cells, sensitited by Zn-protoporphyrin synthesized and accumulated in wild-type cells under defined conditions. Using a single cycle of selection, heme-deficient mutants have been obtained with a frequency of 2-2.3%.

The biosynthesis of yeast 5-aminolevulinate (ALA) synthase, a mitochondrial protein encoded by the nuclear HEM1 gene, has been studied in vitro in a cell-free translation system and in vivo in whole cells. In vitro translation of mRNA hybrid-selected by the cloned HEM1 gene, or of total RNA followed by immunoprecipitation with anti-(ALA synthase) antibody yielded a single polypeptide of higher molecular mass than the purified ALA synthase. This larger form, also seen in pulse-labeled cells, can be post-translationally processed by isolated mitochondria. These results show that the cytoplasmically made ALA synthase is synthesized with a cleavable extension which was estimated to be about 3.5 kDa by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The complete nucleotide sequence of the HEM1 gene and its flanking regions was determined. The 5' ends of the HEM1 mRNAs map from -76 to -63 nucleotides upstream of the translation initiation codon. The open reading frame of 1644 base pairs encodes a protein of 548 amino acids with a calculated Mr of 59,275. The predicted amino-terminal sequence of the protein is strongly basic (five basic and no acidic amino acids within the first 35 residues), rich in serine and threonine and must represent the transient presequence that targets this protein to the mitochondria. Comparison of deduced amino acid sequences indicates a clear homology between the mature yeast and chick embryo liver ALA synthases.

Two new haem-deficient mutants of Saccharomyces cerevisiae were isolated on the basis of their catalase deficiency. Mutant H11 accumulated and excreted coproporphyrin III and was completely deficient in haem; the cell-free extract had no coproporphyrinogen oxidase activity. Mutant H12 accumulated uroporphyrin to coproporphyrin III and excreted coproporphyrin III, and contained a small amount of haem; the cell-free extract had a residual coproporphyrinogen oxidase activity. The two mutations were allelic and the mutant phenotypes were under the control of a single, recessive nuclear gene.

We have cloned the structural gene HEM1 for 5-aminolevulinate (ALA) synthase from Saccharomyces cerevisiae by transformation and complementation of a yeast hem1-5 mutant which was previously shown to lack ALA synthase activity (Urban-Grimal and Labbe Bois 1981) and had no immunodetectable ALA synthase protein when tested with yeast ALA synthase antiserum. The gene was selected from a recombinant cosmid pool which contained wild-type yeast genomic DNA fragments of an average size of 40 kb. The cloned gene was identified by the restauration.of growth on a non fermentable carbon source without addition of exogenous ALA. Sub cloning of partial Sau3A digests and functional analysis by transformation allowed us to isolate three independent plasmids, each carrying a 6 kb yeast DNA fragment inserted in either orientation into the single BamHI site of the vector pHCG3 and able to complement hem1-5 mutation. Analysis of the three plasmids by restriction endonucleases showed that HEM1 is contained within a 2.9 kb fragment. The three corresponding yeast trans formants present a 1, 2.5 and 16 fold increase in ALA synthase activity as compared to the wild-type strain. The gene product immunodetected in the transformant yeast cells has identical size as the wild-type yeast ALA synthase and its amount correlates well with the increase in ALA synthase activity.

Mutants of Saccharomyces cerevisiae, described as catalase and cytochromes deficient (Pachecka et al., 1974), have been analyzed for heme biosynthesis ability. Some enzymatic activities involved in protoheme synthesis were measured in acellular extracts, whereas whole cells were analyzed for cytochrome spectra and for possible accumulation of porphyrin synthesis intermediates. A good correlation was found between these in vitro and in vivo studies. Results show that two mutants were impaired in 5-aminolevulinate synthesis, two mutants were devoid of uroporphyrinogen I synthetase activity and one mutant presented defects in coproporphyrinogen III oxidase activity.