Differential accumulation of two apo-iso-cytochromes c in processing mutants of yeast

Biogenesis of the bc1 Complex of the Mitochondrial Respiratory Chain

The oxidative phosphorylation system contains four respiratory chain complexes that connect the transport of electrons to oxygen with the establishment of an electrochemical gradient over the inner membrane for ATP synthesis. Due to the dual genetic source of the respiratory chain subunits, its assembly requires a tight coordination between nuclear and mitochondrial gene expression machineries. In addition, dedicated assembly factors support the step-by-step addition of catalytic and accessory subunits as well as the acquisition of redox cofactors. Studies in yeast have revealed the basic principles underlying the assembly pathways. In this review, we summarize work on the biogenesis of the bc₁ complex or complex III, a central component of the mitochondrial energy conversion system.

The flavoprotein Cyc2p, a mitochondrial cytochrome c assembly factor, is a NAD(P)H-dependent haem reductase

Cytochrome c assembly requires sulphydryls at the CXXCH haem binding site on the apoprotein and also chemical reduction of the haem co-factor. In yeast mitochondria, the cytochrome haem lyases (CCHL, CC(1) HL) and Cyc2p catalyse covalent haem attachment to apocytochromes c and c(1) . An in vivo indication that Cyc2p controls a reductive step in the haem attachment reaction is the finding that the requirement for its function can be bypassed by exogenous reductants. Although redox titrations of Cyc2p flavin (E(m) = -290 mV) indicate that reduction of a disulphide at the CXXCH site of apocytochrome c (E(m) = -265 mV) is a thermodynamically favourable reaction, Cyc2p does not act as an apocytochrome c or c(1) CXXCH disulphide reductase in vitro. In contrast, Cyc2p is able to catalyse the NAD(P)H-dependent reduction of hemin, an indication that the protein's role may be to control the redox state of the iron in the haem attachment reaction to apocytochromes c. Using two-hybrid analysis, we show that Cyc2p interacts with CCHL and also with apocytochromes c and c(1) . We postulate that Cyc2p, possibly in a complex with CCHL, reduces the haem iron prior to haem attachment to the apoforms of cytochrome c and c(1) .

Cytochrome c biogenesis in mitochondria--Systems III and V

In c-type cytochromes, heme becomes covalently attached to the polypeptide chain by a reaction between the vinyl groups of the heme and cysteine thiols from the protein. There are two such cytochromes in mitochondria: cytochrome c and cytochrome c(1). The heme attachment is a post-translational modification that is catalysed by different biogenesis proteins in different organisms. Three types of biogenesis system are found or predicted in mitochondria: System I (the cytochrome c maturation system); System III (termed holocytochrome c synthase (HCCS) or heme lyase); and System V. This review focuses primarily on cytochrome c maturation in mitochondria containing HCCS (System III). It describes what is known about the enzymology and substrate specificity of HCCS; the role of HCCS in human disease; import of HCCS into mitochondria; import of apocytochromes c and c(1) into mitochondria and the close relationships with HCCS-dependent heme attachment; and the role of the fungal cytochrome c biogenesis accessory protein Cyc2. System V is also discussed; this is the postulated mitochondrial cytochrome c biogenesis system of trypanosomes and related organisms. No cytochrome c biogenesis proteins have been identified in the genomes of these organisms whose c-type cytochromes also have a unique mode of heme attachment.

c-type cytochrome assembly in Saccharomyces cerevisiae: a key residue for apocytochrome c1/lyase interaction

The electron transport chains in the membranes of bacteria and organelles generate proton-motive force essential for ATP production. The c-type cytochromes, defined by the covalent attachment of heme to a CXXCH motif, are key electron carriers in these energy-transducing membranes. In mitochondria, cytochromes c and c(1) are assembled by the cytochrome c heme lyases (CCHL and CC(1)HL) and by Cyc2p, a putative redox protein. A cytochrome c(1) mutant with a CAPCH heme-binding site instead of the wild-type CAACH is strictly dependent upon Cyc2p for assembly. In this context, we found that overexpression of CC(1)HL, as well as mutations of the proline in the CAPCH site to H, L, S, or T residues, can bypass the absence of Cyc2p. The P mutation was postulated to shift the CXXCH motif to an oxidized form, which must be reduced in a Cyc2p-dependent reaction before heme ligation. However, measurement of the redox midpoint potential of apocytochrome c(1) indicates that neither the P nor the T residues impact the thermodynamic propensity of the CXXCH motif to occur in a disulfide vs. dithiol form. We show instead that the identity of the second intervening residue in the CXXCH motif is key in determining the CCHL-dependent vs. CC(1)HL-dependent assembly of holocytochrome c(1). We also provide evidence that Cyc2p is dedicated to the CCHL pathway and is not required for the CC(1)HL-dependent assembly of cytochrome c(1).

Cyc2p, a Membrane-bound Flavoprotein Involved in the Maturation of Mitochondrial c-Type Cytochromes

Mitochondrial apocytochrome c and c1 are converted to their holoforms in the intermembrane space by attachment of heme to the cysteines of the CXXCH motif through the activity of assembly factors cytochrome c heme lyase and cytochrome c1 heme lyase (CCHL and CC1HL). The maintenance of apocytochrome sulfhydryls and heme substrates in a reduced state is critical for the ligation of heme. Factors that control the redox chemistry of the heme attachment reaction to apocytochrome c are known in bacteria and plastids but not in mitochondria. We have explored the function of Cyc2p, a candidate redox cytochrome c assembly component in yeast mitochondria. We show that Cyc2p is required for the activity of CCHL toward apocytochrome c and c1 and becomes essential for the heme attachment to apocytochrome c1 carrying a CAPCH instead of CAACH heme binding site. A redox function for Cyc2p in the heme lyase reaction is suggested from 1) the presence of a noncovalently bound FAD molecule in the C-terminal domain of Cyc2p, 2) the localization of Cyc2p in the inner membrane with the FAD binding domain exposed to the intermembrane space, and 3) the ability of recombinant Cyc2p to carry the NADPH-dependent reduction of ferricyanide. We postulate that, in vivo, Cyc2p interacts with CCHL and is involved in the reduction of heme prior to its ligation to apocytochrome c by CCHL.

Enhanced mitochondrial degradation of yeast cytochrome c with amphipathic structures

The dispensable N-terminus of iso-1-cytochrome c (iso-1) in the yeast Saccharomyces cerevisiae was replaced by 11 different amphipathic structures. Rapid degradation of the corresponding iso-1 occurred, with the degree of degradation increasing with the amphipathic moments; and this amphipathic-dependent degradation was designated ADD. ADD occurred with the holo-forms in the mitochondria but not as the apo-forms in the cytosol. The extreme mutant type degraded with a half-life of approximately 12 min, whereas the normal iso-1 was stable over hours. ADD was influenced by the rho+/rho- state and by numerous chromosomal genes. Most importantly, ADD appeared to be specifically suppressed to various extents by deletions of any of the YME1, AFG3, or RCA1 genes encoding membrane-associated mitochondrial proteases, probably because the amphipathic structures caused a stronger association with the mitochondrial inner membrane and its associated proteases. The use of ADD assisted in the differentiation of substrates of different mitochondrial degradation pathways.

Sue1p is required for degradation of labile forms of altered cytochromes C in yeast mitochondria

Previous studies on certain altered holo-isocytochromes c revealed a rho(-)-dependent degradation (RDD) phenotype, in which certain altered holo-iso-1-cytochromes c are at normal or nearly normal levels in rho+ strains, but are at low levels or absent in rho- strains, although wild-type holo-iso-1-cytochrome c is present at normal levels in both rho+ and related rho- strains. The diminished levels of altered holo-iso-1-cytochrome c are due to the rapid degradation that is carried out by a novel proteolytic pathway in the IMS of mitochondria. SUE1, a nuclear gene that encodes a mitochondrial protein, was identified with a genetic screen for mutants that diminish RDD. The levels of RDD and certain other types of altered holo-iso-1-cytochrome c were elevated in rho- sue1 strains. Also, rho+ sue1 strains containing certain altered holo-iso-1-cytochromes c grew better on non-fermentable carbon sources than the corresponding rho+ SUE1 strains. These results indicate that Sue1p may play an important role in the degradation of abnormal holo-iso-1-cytochrome c in the mitochondria.

Overlapping specificities of the mitochondrial cytochrome c and c1 heme lyases

Heme attachment to the apoforms of fungal mitochondrial cytochrome c and c1 requires the activity of cytochrome c and c1 heme lyases (CCHL and CC1HL), which are enzymes with distinct substrate specificity. However, the presence of a single heme lyase in higher eukaryotes is suggestive of broader substrate specificity. Here, we demonstrate that yeast CCHL is active toward the non-cognate substrate apocytochrome c1, i.e. CCHL promotes low levels of apocytochrome c1 conversion to its holoform in the absence of CC1HL. Moreover, that the single human heme lyase also displays a broader cytochrome specificity is evident from its ability to substitute for both yeast CCHL and CC1HL. Multicopy and genetic suppressors of the absence of CC1HL were isolated and their analysis revealed that the activity of CCHL toward cytochrome c1 can be enhanced by: 1) reducing the abundance of the cognate substrate apocytochrome c, 2) increasing the accumulation of CCHL, 3) modifying the substrate-enzyme interaction through point mutations in CCHL or cytochrome c1, or 4) overexpressing Cyc2p, a protein known previously only as a mitochondrial biogenesis factor. Based on the functional interaction of Cyc2p with CCHL and the presence of a putative FAD-binding site in the protein, we hypothesize that Cyc2p controls the redox chemistry of the heme lyase reaction.

Genetic analysis of chloroplast c-type cytochrome assembly in Chlamydomonas reinhardtii: One chloroplast locus and at least four nuclear loci are required for heme attachment

Chloroplasts contain up to two c-type cytochromes, membrane-anchored cytochrome f and soluble cytochrome c6. To elucidate the post-translational events required for their assembly, acetate-requiring mutants of Chlamydomonas reinhardtii that have combined deficiencies in both plastid-encoded cytochrome f and nucleus-encoded cytochrome c6 have been identified and analyzed. For strains ct34 and ct59, where the phenotype displays uniparental inheritance, the mutations were localized to the chloroplast ccsA gene, which was shown previously to be required for heme attachment to chloroplast apocytochromes. The mutations in another eight strains were localized to the nuclear genome. Complementation tests of these strains plus three previously identified strains of the same phenotype (ac206, F18, and F2D8) indicate that the 11 ccs strains define four nuclear loci, CCS1-CCS4. We conclude that the products of the CCS1-CCS4 loci are not required for translocation or processing of the preproteins but, like CcsA, they are required for the heme attachment step during assembly of both holocytochrome f and holocytochrome c6. The ccsA gene is transcribed in each of the nuclear mutants, but its protein product is absent in ccs1 mutants, and it appears to be degradation susceptible in ccs3 and ccs4 strains. We suggest that Ccsl may be associated with CcsA in a multisubunit "holocytochrome c assembly complex," and we hypothesize that the products of the other CCS loci may correspond to other subunits.

Differential ubiquitin-dependent degradation of the yeast apo-cytochrome c isozymes

The yeast Saccharomyces cerevisiae contains two forms of cytochrome c, iso-1- and iso-2-cytochrome c, which are encoded by the nuclear genes CYC1 and CYC7, respectively. The cytochromes c are synthesized in the cytosol, imported into mitochondria, and subsequently modified by the covalent attachment of heme through the action of cytochrome c heme lyase, which is encoded by CYC3. Apo-iso-2-cytochrome c but not apo-iso-1-cytochrome c was observed in cyc3(-) mutants. Furthermore, pulse-chase experiments previously demonstrated that the lack of apo-iso-1-cytochrome c was due to its rapid degradation. We report herein that this degradation of apo-iso-1-cytochrome c is dependent on ubiquitination and on the action of the proteasome. Diminished degradation of apo-iso-1-cytochrome c was observed in pre2-2 and pre1-1 mutants having altered proteasome subunits; in ubc1, ubc4, and ubc5 strains lacking one or more of the ubiquitin-conjugating enzymes; and in strains blocked in multi-ubiquitination by overproduction of the abnormal ubiquitin-K48R ubiquitin. In addition, we have used epitope-tagged ubiquitin to demonstrate that apo-iso-1-cytochrome c but not apo-iso-2-cytochrome c is ubiquitinated. Furthermore, the degradation of apo-iso-1-cytochrome c was diminished when the N-terminal region was replaced with the N-terminal region of apo-iso-2-cytochrome c, indicating that this region may be the target for degradation. We suggest that ubiquitin-dependent degradation of apo-iso-1-cytochrome c is part of the regulatory process controlling the preferential expression of the iso-cytochromes c.

Enhanced stability in vivo of a thermodynamically stable mutant form of yeast iso-1-cytochrome c

Previous work has established that the N57I amino acid replacement in iso-1-cytochrome c from the yeast Saccharomyces cerevisiae causes an unprecedented increase in thermodynamic stability of the protein in vitro, whereas the N57G replacement diminishes stability. Spectrophotometric measurements of intact cells revealed that the N57I iso-1-cytochrome c is present at higher than normal levels in vivo. Although iso-1-cytochrome c turnover is negligible during aerobic growth, transfer of fully derepressed, aerobically grown cells to anaerobic growth conditions leads to reduction in the levels of all of the cytochromes. Pulse-chase experiments carried out under these anaerobic conditions demonstrated that the N57I iso-1-cytochrome c has a longer half-life than the normal protein. This is the first report of enhanced stability in vivo of a mutant form of a protein that has an enhanced thermodynamic stability in vitro. Although the N57I protein concentration is higher than the normal level, reduced growth in lactate medium indicated that the specific activity of this iso-1-cytochrome c in vivo is diminished relative to wild-type. On the other hand, the level of the thermodynamically labile N57G iso-1-cytochrome c was below normal. The in vivo levels of the N57I and N57G iso-1-cytochrome c suggest that proteins in the mitochondrial intermembrane space can be subjected to degradation, and that this degradation may play a role in controlling their normal levels.

Mutations in the structural gene for cytochrome c result in deficiency of both cytochromes aa3 and c in Neurospora crassa

The cyt-12-12 mutant of Neurospora crassa is characterized by slow growth and a deficiency of spectrophotometrically-detectable cytochromes aa3 and c. Using a sib-selection procedure we have isolated the cyt-12+ allele from a cosmid library of N. crassa genomic DNA. Characterization of the cyt-12+ allele reveals that it encodes the structural gene for cytochrome c. DNA sequence analysis of the cyt-12-12 allele revealed a mutation in the cytochrome c coding sequence that results in replacement of a glycine residue, which is invariant in the cytochrome c of other species, with an aspartic acid. Genetic analysis confirms that cyt-12-12 is allelic with the previously-characterized cyc-1-1 mutant, which was also shown to affect the single locus encoding cytochrome c in N. crassa. We suggest that the amount of functional cytochrome c present in mitochondria influences the level of cytochrome aa3.

Bacterial cytochromes c biogenesis

We report the primary sequence analyses of two loci, hel and ccl, whose gene products are required specifically for the biogenesis of c-type cytochromes in the Gram-negative photosynthetic bacterium Rhodobacter capsulatus. Genetic and molecular analyses show that the hel locus contains at least four genes, helA, helB, helC, and orf52, and the ccl locus contains two genes, ccl1 and ccl2, that are essential for cytochromes c biogenesis. HelA is homologous to a class of proteins called ABC transporters and helA, helB, and helC are proposed to encode an export complex. Cytochrome c2-alkaline phosphatase gene fusions were used to show that apocytochrome c2 synthesis and secretion are not affected by the hel and ccl defects. Ccl1 and Ccl2 possess typical signal sequences to direct them to the periplasm. The periplasmic orientation of Ccl1 was confirmed using a Ccl1-alkaline phosphatase gene fusion. The Ccl1-alkaline phosphatase gene fusion analysis also demonstrated that Ccl1 does not require hel genes for its synthesis and secretion. Ccl1 is homologous to proteins encoded by chloroplast and mitochondrial genes, suggesting analogous functions in these organelles. Taken together, these results support the hypothesis that the hel-encoded proteins are required for the export of heme to the periplasm where it is subsequently ligated to the c-type apocytochromes.

Apo forms of cytochrome c550 and cytochrome cd1 are translocated to the periplasm of Paracoccus denitrificans in the absence of haem incorporation caused either mutation or inhibition of haem synthesis

An apo form of cytochrome c550 can be detected by immunoblotting cell-free extracts of a mutant of Paracoccus denitrificans that is deficient in c-type cytochromes. This apoprotein is found predominantly in the periplasm, the location of the holocytochrome in the wild-type organism, indicating that translocation of the polypeptide occurs in the absence of haem attachment. The polypeptide molecular weight, as judged by sodium dodecyl sulphate/polyacrylamide gel electrophoresis, is indistinguishable from that of the holoprotein and the chemically prepared apoprotein; this suggests that the N-terminal signal sequence is removed in the mutant as in the wild-type organism. In the presence of levulinic acid, an inhibitor of haem biosynthesis, apocytochrome c550 and aponitrite reductase (cytochrome cd1) accumulated in the periplasm of wild-type cells. Synthesis of these apoproteins was blocked by chloramphenicol. Thus in P. denitrificans the synthesis of these polypeptides is neither autoregulated nor regulated by the availability of haem. That the apoproteins appear in the periplasm argues against the possibility of polypeptide/haem co-transport from cytoplasm to periplasm. These observations are related to, and contrasted with, the biosynthesis of c-type cytochromes in eukaryotic cells.

Expression and activity of a gene encoding rat cytochrome c in the yeast Saccharomyces cerevisiae

A rat-processed pseudogene, which encodes normal rat cytochrome c, has been expressed in the yeast, Saccharomyces cerevisiae. The translated region of the chromosomal CYC1+ locus, which encodes yeast iso-1-cytochrome c, was replaced by the translated region of the gene encoding rat cytochrome c (CYC1-RAT), thus preserving the proper CYC1 transcription initiation and termination signals. Although the levels of transcription of the normal CYC1+ gene and the CYC1-RAT gene in yeast were equivalent, rat cytochrome c was produced at approx. 40% of the level of iso-1-cytochrome c. In addition, the specific activity in vivo was estimated to be approx. 60% that of the yeast iso-1-cytochrome c. N-terminal processing of indigenous rat cytochrome c, in which the N-terminal methionine residue is cleaved and the penultimate glycine residue is acetylated, also occurred in yeast. Methionine cleavage was complete, while acetylation proceeded to only 70% completion. Lys-72 was trimethylated to 66% completion in the rat cytochrome c produced in yeast. The near normal expression (40%) and specific activity (60%) in vivo indicates that the 40% difference in amino acid sequence is not critical for mitochondrial import, heme attachment and interactions with redox partners.

Effect of enzymatic methylation of apocytochrome c on holocytochrome c formation and proteolysis

1. Methylation of the lysine at residue 72 of yeast apocytochrome c increases its import into mitochondria. 2. Using methylated and unmethylated apocytochrome c as substrate and intact yeast mitochondria and a solubilized mitochondrial fraction as a source of cytochrome c heme lyase, the results show that the methylation state of the apoprotein has no significant effect on its conversion to holoprotein. 3. The above result suggests that the import mechanism is separate from the heme-attaching activity. 4. Unmethylated apocytochrome c was less resistant to a yeast homogenate fraction that methylated apocytochrome c, suggesting that methylation of apocytochrome c alters the conformation of the whole protein.

Isolation of mutants and genes involved in cytochromes c biosynthesis in Rhodobacter capsulatus

Mutants of the photosynthetic bacterium Rhodobacter capsulatus that have combined deficiencies in the cytochrome b/c1 complex and other c-type cytochromes have been isolated. These mutants were unable to grow anaerobically in the light or dark but could grow aerobically. Cosmids with R. capsulatus wild-type DNA that complement the mutants have been used to construct genetic and physical maps of the affected genes. Complementation profiles with Tn5 and mini-Mu insertions in these cosmids and subcloned fragments from them indicated that at least three genes (called helA, helB, and helC) are involved in the defects in cytochromes c biosynthesis. The genes are clustered, and helC is transcribed away from helA and helB. Stable insertion mutants in each gene were constructed. It is postulated that helA, helB, and helC are involved in posttranslational processing during cytochromes c synthesis.

Yeast HAP1 activator competes with the factor RC2 for binding to the upstream activation site UAS1 of the CYC1 gene

We show that the yeast HAP1 activator locus encodes a protein that binds in vitro to the upstream activation site, UAS1, of the CYC1 gene (iso-1-cytochrome c). Binding of wild-type HAP1 and truncated HAP1 derivatives to UAS1 is evident in crudely fractionated yeast extracts using the gel electrophoresis DNA binding assay. The binding of HAP1 in vitro, like the activity of UAS1 in vivo, is stimulated by heme. HAP1 binds to region B, one of two portions of UAS1 shown to be important by genetic analysis of the site. Surprisingly, HAP1 binds to the same sequence as a second factor, RC2. Both HAP1 and RC2 bind to the same side of the helix, and make similar but not identical major and minor groove contacts that span two full turns. An additional factor that binds to the second important part of UAS1, the region A factor (RAF), is also identified. A model depicting the interplay of HAP1, RC2, and RAF in the control of UAS1 is presented.

Import of cytochrome c into mitochondria. Cytochrome c heme lyase

The import of cytochrome c into mitochondria can be resolved into a number of discrete steps. Here we report on the covalent attachment of heme to apocytochrome c by the enzyme cytochrome c heme lyase in mitochondria from Neurospora crassa. A new method was developed to measure directly the linkage of heme to apocytochrome c. This method is independent of conformational changes in the protein accompanying heme attachment. Tryptic peptides of [35S]cysteine-labelled apocytochrome c, and of enzymatically formed holocytochrome c, were resolved by reverse-phase HPLC. The cysteine-containing peptide to which heme was attached eluted later than the corresponding peptide from apocytochrome c and could be quantified by counting 35S radioactivity as a measure of holocytochrome c formation. Using this procedure, the covalent attachment of heme to apocytochrome c, which is dependent on the enzyme cytochrome c heme lyase, could be measured. Activity required heme (as hemin) and could be reversibly inhibited by the analogue deuterohemin. Holocytochrome c formation was stimulated 5--10-fold by NADH greater than NADPH greater than glutathione and was independent of a potential across the inner mitochondrial membrane. NADH was not required for the binding of apocytochrome c to mitochondria and was not involved in the reduction of the cysteine thiols prior to heme attachment. Holocytochrome c formation was also dependent on a cytosolic factor that was necessary for the heme attaching step of cytochrome c import. The factor was a heat-stable, protease-insensitive, low-molecular-mass component of unknown function. Cytochrome c heme lyase appeared to be a soluble protein located in the mitochondrial intermembrane space and was distinct from the previously identified apocytochrome c binding protein having a similar location. A model is presented in which the covalent attachment of heme by cytochrome c heme lyase also plays an essential role in the import pathway of cytochrome c.

Localization of the upstream regulatory sites of yeast iso2-cytochrome c gene

In order to study the regulation of expression of the iso2-cytochrome c gene, we have constructed a fused gene between the 5'flanking region of the gene coding for the yeast iso2-cytochrome c and the coding region of the E. coli beta-galactosidase lacZ gene. When introduced in yeast cells this hybrid gene is expressed and regulated like the production of iso2-cytochrome c: it is under the control of the general catabolic repression and of the unlinked trans-acting CYP1 gene whose CYP1-18 allele causes an overproduction of iso2-cytochrome c. The expression of hybrid genes whose upstream region has been progressively shortened or altered by internal deletions was studied either in wild-type CYP1+ cells or in cells carrying the CYP1-18 allele grown either on glucose or on glycerol. It appears that the expression and the regulation of the iso2-cytochrome c gene is controlled by an upstream regulatory site composed of a positive and a negative element. This site is the target of regulation by the CYP1 gene product and, directly or through this gene, of the control by the general catabolic repression.

In situ assay for 5-aminolevulinate dehydratase and application to the study of a catabolite repression-resistant Saccharomyces cerevisiae mutant

To facilitate the study of the effects of carbon catabolite repression and mutations on 5-aminolevulinate dehydratase (EC 4.2.1.24) from Saccharomyces cerevisiae, a sensitive in situ assay was developed, using cells permeabilized by five cycles of freezing and thawing. Enzymatic activity was measured by colorimetric determination of porphobilinogen with a modified Ehrlich reagent. For normal strains, porphobilinogen production was linear for 15 min, and the reaction rate was directly proportional to the permeabilized cell concentration up to 20 mg (dry weight) per ml. The reaction exhibited Michaelis-Menten-type kinetics, and an apparent Km of 2.6 mM was obtained for 5-aminolevulinic acid. This value is only slightly higher than the value of 1.8 mM obtained for the enzyme assayed in cell extracts. The in situ assay was used to assess catabolite repression-dependent changes in 5-aminolevulinate dehydratase during batch culture on glucose medium. In normal S. cerevisiae cells, the enzyme is strongly repressed as long as glucose is present in the medium. In contrast, a strain bearing the hex2-3 mutation exhibits derepressed levels of enzyme activity during growth on glucose. Synthesis of cytochromes by this strain is also resistant to catabolite repression. Similar studies employing a strain containing the glc1 mutation, which enhances porphyrin accumulation, did not reveal any significant phenotypic change in catabolite regulation of 5-aminolevulinate dehydratase.

Heme regulates transcription of the CYC1 gene of S. cerevisiae via an upstream activation site

We show that expression of the iso-1-cytochrome c gene of Saccharomyces cerevisiae, CYC1, is tightly regulated by levels of intracellular heme. Expression is reduced at least 200-fold in cells grown under conditions of heme deficiency. Studies on the regulation of a CYC1-lacZ fused gene and direct determination of mRNA levels indicate that this control is transcriptional. Furthermore, we show that the heme regulatory site in the CYC1 promoter region is an upstream activation site (UASc) centered about 275 bp upstream from the region of transcriptional initiation. The latter region is required for optimal transcription and contains three TATA box sequences and six prominent mRNA initiation sites that span 34 bp. Substitution of the UASc with the UAS of the yeast GAL10 gene results in activation of the normal set of CYC1 transcripts. In this case, however, transcription is independent of regulation by heme, suggesting that in the wildtype, heme controls initiation per se and not translation or mRNA stability.