Transcriptional regulation of the yeast cytochrome c gene

Biochemical and biophysical methods for studying mitochondrial iron metabolism

Iron is a heavily utilized element in organisms and numerous mechanisms accordingly regulate the trafficking, metabolism, and storage of iron. Despite the high regulation of iron homeostasis, several diseases and mutations can lead to the misregulation and often accumulation of iron in the cytosol or mitochondria of tissues. To understand the genesis of iron overload, it is necessary to employ various techniques to quantify iron in organisms and mitochondria. This chapter discusses techniques for determining the total iron content of tissue samples, ranging from colorimetric determination of iron concentrations, atomic absorption spectroscopy, inductively coupled plasma-optical emission spectroscopy, and inductively coupled plasma-mass spectrometry. In addition, we discuss in situ techniques for analyzing iron including electron microscopic nonheme iron histochemistry, electron energy loss spectroscopy, synchrotron X-ray fluorescence imaging, and confocal Raman microscopy. Finally, we discuss biophysical methods for studying iron in isolated mitochondria, including ultraviolet-visible, electron paramagnetic resonance, X-ray absorbance, and Mössbauer spectroscopies. This chapter should aid researchers to select and interpret mitochondrial iron quantifications.

Evidence for transcriptional regulation of dihydroorotic acid dehydrogenase in Saccharomyces cerevisiae

Two yeast DNA fragments carrying the URA 1 gene have been isolated. URA 1 mRNA levels were measured in wild-type yeast and ppr 1 (constitutive for DHOdehase synthesis) strains. Results favour the hypothesis that ppr 1 increases the transcription of URA 1. The corresponding E. coli PYR D gene has also been cloned.

Regulation of transcription of the Saccharomyces cerevisiae CYC1 gene: Identification of a DNA region involved in heme control

A Saccharomyces cerevisiae mutant (hem1 cycl-1) was transformed with plasmids bearing a chromosomal centromer (CEN3) and a 2 μm DNA replication origin. In one of the plasmids a functional CYC1 gene was present, in a second plasmid an XhoI fragment located between bases -245 and -678 upstream from the translation initiation codon had been deleted, in a third plasmid this region had been inverted. Results of hybridization experiments carried out with mRNA isolated from heme-deficient and heme-containing transformants indicated that heme controls transcription of the CYC1 gene and that DNA sequences located within the upstream XhoI fragment are involved in activation of the gene by heme.

DNA insertions which affect the expression of the yeast iso-2-cytochrome c gene

The plasmid YCpCYC7(2) was constructed containing the Saccharomyces cerevisiae CYC7 gene, encoding the iso-2-cytochrome c protein, replicative sequences and selective markers from both E. coli and yeast, and the centromere of yeast chromosome III. The expression of the plasmid-CYC7 gene in yeast was similar to the low level expression characteristic of the chromosomal CYC7 gene. A number of insertions into the sequences 5' to the gene were constructed in vitro. The insertion at 142 by 5' to the coding sequence of a 400 by fragment which lies 5' to the CYC1 gene and is known to be essential for the high rates of CYC1 transcription increased transcription of the CYC7 gene to levels characteristic of CYC1 transcription. On the other hand, the insertion of random DNA fragments at the same position gave mostly decreased CYC7 transcription. In addition to these in vitro constructions, a mutant plasmid was selected which had increased CYC7 transcription. This mutation was caused by the insertion of the bacterial IS1 element 313 by 5' to the CYC7 coding sequence. The significance of these results is discussed in terms of two alternative models for CYC7 gene expression.

TPS1 terminator increases mRNA and protein yield in a Saccharomyces cerevisiae expression system

Both terminators and promoters regulate gene expression. In Saccharomyces cerevisiae, the TPS1 terminator (TPS1t), coupled to a gene encoding a fluorescent protein, produced more transgenic mRNA and protein than did similar constructs containing other terminators, such as CYC1t, TDH3t, and PGK1t. This suggests that TPS1t can be used as a general terminator in the development of metabolically engineered yeast in high-yield systems.

Glucose signalling pathway controls the programmed ribosomal frameshift efficiency in retroviral-like element Ty3 in Saccharomyces cerevisiae

Ty3 elements of S. cerevisiae contain two overlapping coding regions, GAG3 and POL3, which are functional homologues of retroviral gag and pol genes, respectively. Pol3 is translated as a Gag3‐Pol3 fusion protein dependent on a +1 programmed frameshift at a site with the overlap between the two genes. We show that the Ty3 frameshift frequency varies up to 10‐fold in S. cerevisiae cells depending on carbon source. Frameshift efficiency is significantly lower in cells growing on glucose as carbon source than in cells growing on poor alternative carbon sources (glycerol/lactate or galactose). Our results indicate that Ty3 programmed ribosomal frameshift efficiency in response to glucose signalling requires two protein kinases: Snf1p and cAMP‐dependent protein kinase A (PKA). Increased frameshifting on alternative carbon sources also appears to require cytoplasmic localization of Snf1p, mediated by the Sip2p protein. In addition to the two required protein kinases, our results implicate that Stm1p, a ribosome‐associated protein involved in nutrient sensing, is essential for the carbon source‐dependent regulation of Ty3 frameshifting. These data indicate that Ty3 programmed ribosomal frameshift is not a constitutive process but that it is regulated in response to the glucose‐signalling pathway. Copyright © 2011 John Wiley & Sons, Ltd.

Biophysical characterization of iron in mitochondria isolated from respiring and fermenting yeast

The distributions of Fe in mitochondria isolated from respiring, respiro-fermenting, and fermenting yeast cells were determined with an integrative biophysical approach involving Mossbauer and electronic absorption spectroscopies, electron paramagnetic resonance, and inductively coupled plasma emission mass spectrometry. Approximately 40% of the Fe in mitochondria from respiring cells was present in respiration-related proteins. The concentration and distribution of Fe in respiro-fermenting mitochondria, where both respiration and fermentation occur concurrently, were similar to those of respiring mitochondria. The concentration of Fe in fermenting mitochondria was also similar, but the distribution differed dramatically. Here, levels of respiration-related Fe-containing proteins were diminished approximately 3-fold, while non-heme HS Fe(II) species, non-heme mononuclear HS Fe(III), and Fe(III) nanoparticles dominated. These changes were rationalized by a model in which the pool of non-heme HS Fe(II) ions serves as feedstock for Fe-S cluster and heme biosynthesis. The integrative approach enabled us to estimate the concentration of respiration-related proteins.

Genetic factors that regulate the attenuation of the general stress response of yeast

The general stress response of yeast involves the induction of approximately 200 genes in response to any one of several stresses. These genes are activated by Msn2 and repressed by the Srb10 kinase, a member of the mediator complex. Normally, Msn2 is exported from the nucleus, and Srb10 represses STRE gene expression. Under stress, Msn2 relocalizes to the nucleus and, with the relief of Srb10 repression, activates transcription. The stress response is rapid, but quickly attenuated. We show here that this attenuation is due to a nuclear-dependent degradation of Msn2. Msn2 rapidly disappeared from cells after heat or osmotic shock. This disappearance was not due to a change in MSN2 RNA levels, which remain constant during stress. Pulse-chase experiments confirmed the stress-dependent Msn2 degradation. The levels of Msn2 were significantly reduced in msn5 deletion cells that have been shown to constitutively retain Msn2 in the nucleus. The degradation was Srb10-dependent; Msn2 was not degraded in an srb10 deletion mutant. An Msn2 internal deletion mutant was insensitive to Srb10 repression, but was degraded by the Srb10-dependent mechanism. Thus, this mutation uncoupled Srb10 repression from degradation.

Transcriptional control of nonfermentative metabolism in the yeast Saccharomyces cerevisiae

Although sugars are clearly the preferred carbon sources of the yeast Saccharomyces cerevisiae, nonfermentable substrates such as ethanol, glycerol, lactate, acetate or oleate can also be used for the generation of energy and cellular biomass. Several regulatory networks of glucose repression (carbon catabolite repression) are involved in the coordinate biosynthesis of enzymes required for the utilization of nonfermentable substrates. Positively and negatively acting complexes of pleiotropic regulatory proteins have been characterized. The Snf1 (Cat1) protein kinase complex, together with its regulatory subunit Snf4 (Cat3) and alternative beta-subunits Sip1, Sip2 or Gal83, plays an outstanding role for the derepression of structural genes which are repressed in the presence of a high glucose concentration. One molecular function of the Snf1 complex is deactivation by phosphorylation of the general glucose repressor Mig1. In addition to regulation of alternative sugar fermentation, Mig1 also influences activators of respiration and gluconeogenesis, although to a lesser extent. Snf1 is also required for conversion of specific regulatory factors into transcriptional activators. This review summarizes regulatory cis-acting elements of structural genes of the nonfermentative metabolism, together with the corresponding DNA-binding proteins (Hap2-5, Rtg1-3, Cat8, Sip4, Adr1, Oaf1, Pip2), and describes the molecular interactions among general regulators and pathway-specific factors. In addition to the influence of the carbon source at the transcriptional level, mechanisms of post-transcriptional control such as glucose-regulated stability of mRNA are also discussed briefly.

Glucose-inducible expression of rrg1+ in Schizosaccharomyces pombe: post-transcriptional regulation of mRNA stability mediated by the downstream region of the poly(A) site

rrg1+(rapid response to glucose) has been isolated previously as a UV-inducible gene in Schizosaccharomyces pombe, designated as uvi22+. However, it was revealed that the transcript level of this gene was regulated by glucose, not by DNA-damaging agents. Glucose depletion led to a rapid decrease in the level of rrg1+ mRNA, by approximately 50% within 30 min. This effect was readily reversed upon re-introduction of glucose within 1 h. High concentrations (4 and 8%) of glucose showed similar effects on increasing the rrg1+ mRNA level compared with 2% glucose, while a low concentration (0.1%) was not effective in raising the rrg1+ mRNA level. In addition, sucrose and fructose could increase rrg1+ mRNA level. Interestingly, the rapid decline in mRNA level seen upon glucose deprivation resulted from precipitous reduction of mRNA half-life. Serial and internal deletions within the 3'-flanking region of rrg1+ revealed that a 210-nt region downstream of the distal poly(A) site was critical for glucose-regulated expression. Moreover, this downstream region participated in 3'-end formation of mRNA. Taken together, this is the first report on glucose-inducible expression regulated post-transcriptionally by control of mRNA stability in S.pombe.

Regulation of expression of the amino acid transporter gene BAP3 in Saccharomyces cerevisiae

The BAP3 gene of Saccharomyces cerevisiae encodes a protein with a high similarity to the BAP2 gene product, a high-affinity permease for branched-chain amino acids. In this paper, we show that, like BAP2, the expression of the BAP3 gene in S. cerevisiae is induced by the addition of branched-chain amino acids to the medium. Unexpectedly, most other naturally occurring L-amino acids found in proteins (with the exception of proline, lysine, arginine and histidine) have the same effect on the expression of BAP3. The induction of BAP3 expression appears to be dependent on Stp1p, a nuclear protein, previously shown to be involved in pre-tRNA maturation and also required for the expression of BAP2, as induction is no longer observed in an stp1 - mutant. The transcriptional regulator Leu3p is not involved in the induction of BAP3 expression, but may act as a repressor of BAP3 expression in the absence of leucine, as can be inferred from a transcriptional analysis in a Deltaleu3 mutant. By extensive deletion analysis of the BAP3 promoter fused to a GUS reporter, as well as by fusions of different parts of the BAP3 promoter to a LacZ reporter, we have found that a portion of the BAP3 promoter from - 418 to - 392 relative to the ATG start codon is both necessary and sufficient for the Stp1p-dependent induction of BAP3 expression by (most) amino acids. We have therefore named this sequence UASaa (amino acid-dependent upstream activator sequence). Neither Stp1p nor Leu3p appear to bind to the UASaa, at least in vitro, as judged from gel retardation assays. Sequences similar to the UASaa can be found in the promoters of BAP2, PTR2 and TAT1; genes that, like BAP3, encode permeases inducible by amino acids, suggesting that amino acid induction of all these genes is exerted via a common mechanism.

Yeast carbon catabolite repression

Glucose and related sugars repress the transcription of genes encoding enzymes required for the utilization of alternative carbon sources; some of these genes are also repressed by other sugars such as galactose, and the process is known as catabolite repression. The different sugars produce signals which modify the conformation of certain proteins that, in turn, directly or through a regulatory cascade affect the expression of the genes subject to catabolite repression. These genes are not all controlled by a single set of regulatory proteins, but there are different circuits of repression for different groups of genes. However, the protein kinase Snf1/Cat1 is shared by the various circuits and is therefore a central element in the regulatory process. Snf1 is not operative in the presence of glucose, and preliminary evidence suggests that Snf1 is in a dephosphorylated state under these conditions. However, the enzymes that phosphorylate and dephosphorylate Snf1 have not been identified, and it is not known how the presence of glucose may affect their activity. What has been established is that Snf1 remains active in mutants lacking either the proteins Grr1/Cat80 or Hxk2 or the Glc7 complex, which functions as a protein phosphatase. One of the main roles of Snf1 is to relieve repression by the Mig1 complex, but it is also required for the operation of transcription factors such as Adr1 and possibly other factors that are still unidentified. Although our knowledge of catabolite repression is still very incomplete, it is possible in certain cases to propose a partial model of the way in which the different elements involved in catabolite repression may be integrated.

Transcriptional regulation of the Kluyveromyces lactis beta-galactosidase gene

We examined the molecular basis for beta-D-galactosidase (EC 3.2.1.23) induction in the yeast Kluyveromyces lactis. The protein synthesis inhibitor anisomycin effectively blocked both protein synthesis and enzyme induction by lactose. Further, hybridization analysis with the cloned beta-galactosidase gene indicated coordinate increases in the concentration of beta-galactosidase messenger ribonucleic acid and enzyme activity. The half-life of beta-galactosidase messenger ribonucleic acid was the same (4.8 +/- 0.4 min) when measured both before and at succeeding times during enzyme induction. These results strongly support the hypothesis that expression of the yeast beta-galactosidase gene is subject to transcriptional regulation.

The control of mitochondrial respiration in yeast: a possible role of the outer mitochondrial membrane

Mitochondrial respiration in yeast (S. cerevisiae) is regulated by the level of glucose in the medium. Glucose is known to inhibit respiration by repressing key enzymes in the respiratory chain. We present evidence that the early events in this inhibition include the closure of VDAC channels, the primary pathway for metabolite flow across the outer membrane. Aluminum hydroxide is known to inhibit the closure of VDAC. Addition of aluminum acetylacetonate to yeast cells, which should elevate the aluminum hydroxide concentrations in the cytoplasm, caused the inhibition of cell respiration by glucose to be delayed for up to 100 min. No significant effect of aluminum was observed in cells grown on glycerol. Yeast cells lacking the VDAC gene were also unresponsive to the addition of aluminum salt in the presence of glucose. Therefore, the closure of VDAC channels may be an early step in the inhibition of the respiration of yeast by glucose.

Carbon catabolite regulation of transcription of nuclear genes coding for mitochondrial proteins in the yeast Kluyveromyces lactis

Promoter regions of the KlQCR7, KlQCR8 and KlCYC1 genes, coding for subunits of the bc1-complex and cytochrome c respectively, in the short-term Crabtree-negative yeast Kluyveromyces lactis differ markedly in sequence from their Saccharomyces cerevisiae counterparts. They have, however, conserved very similar configurations of binding-site motifs for various transcription factors known to be involved in global and carbon-source regulation in S. cerevisiae. To investigate the carbon source-dependent expression of these genes in K. lactis, we have carried out medium-shift experiments and determined transcript levels during the shifts. In sharp contrast to the situation in S. cerevisiae, the level of expression in K. lactis is not affected when glucose is added to a non-fermentable carbon-source medium. However, the genes are not constitutively expressed, but become significantly induced when the cells are shifted from glucose to a non-fermentable carbon source. Finally, induction of transcriptional activation does not occur in media containing both glucose and non-fermentable carbon sources.

Strain-dependent variation in carbon source regulation of nucleus-encoded mitochondrial proteins of Saccharomyces cerevisiae

Nuclear genes encoding mitochondrial proteins are regulated by carbon source with significant heterogeneity among four Saccharomyces cerevisiae strains. This strain-dependent variation is seen both in respiratory capacity of the cells and in the expression of beta-galactosidase reporter fusions to the promoters of CYB2, CYC1, CYC3, MnSOD, and RPO41.

Initiation of translation can occur only in a restricted region of the CYC1 mRNA of Saccharomyces cerevisiae

The steady-state levels and half-lives of CYC1 mRNAs were estimated in a series of mutant strains of Saccharomyces cerevisiae containing (i) TAA nonsense codons, (ii) ATG initiator codons, or (iii) the sequence ATA ATG ACT TAA (denoted ATG-TAA) at various positions along the CYC1 gene, which encodes iso-1-cytochrome c. These mutational alterations were made in backgrounds lacking all internal in-frame and out-of-frame ATG triplets or containing only one ATG initiator codon at the normal position. The results revealed a "sensitive" region encompassing approximately the first half of the CYC1 mRNA, in which nonsense codons caused Upf1-dependent degradation. This result and the stability of CYC1 mRNAs lacking all ATG triplets, as well as other results, suggested that degradation occurs unless elements associated with this sensitive region are covered with 80S ribosomes, 40S ribosomal subunits, or ribonucleoprotein particle proteins. While elongation by 80S ribosomes could be prematurely terminated by TAA codons, the scanning of 40S ribosomal units could not be terminated solely by TAA codons but could be disrupted by the ATG-TAA sequence, which caused the formation and subsequent prompt release of 80S ribosomes. The ATG-TAA sequence caused degradation of the CYC1 mRNA only when it was in the region spanning nucleotide positions -27 to +37 but not in the remaining 3' distal region, suggesting that translation could initiate only in this restricted initiation region. CYC1 mRNA distribution on polyribosomes confirmed that only ATG codons within the initiation region were translated at high efficiency. This initiation region was not entirely dependent on the distance from the 5' cap site and was not obviously dependent on the short-range secondary structure but may simply reflect an open structural requirement for initiation of translation of the CYC1 mRNA.

Effects of mutating Asn-52 to isoleucine on the haem-linked properties of cytochrome c

Asn-52 of rat cytochrome c and baker's yeast iso-1-cytochrome c was changed to isoleucine by site-directed mutagenesis and the mutated proteins expressed in and purified from cultures of transformed yeast. This mutation affected the affinity of the haem iron for the Met-80 sulphur in the ferric state and the reduction potential of the molecule. The yeast protein, in which the sulphur-iron bond is distinctly weaker than in vertebrate cytochromes c, became very similar to the latter: the pKa of the alkaline ionization rose from 8.3 to 9.4 and that of the acidic ionization decreased from 3.4 to 2.8. The rates of binding and dissociation of cyanide became markedly lower, and the affinity was lowered by half an order of magnitude. In the ferrous state the dissociation of cyanide from the variant yeast cytochrome c was three times slower than in the wild-type. The same mutation had analogous but less pronounced effects on rat cytochrome c: it did not alter the alkaline ionization pKa nor its affinity for cyanide, but it lowered its acidic ionization pKa from 2.8 to 2.2. These results indicate that the mutation of Asn-52 to isoleucine increases the stability of the cytochrome c closed-haem crevice as observed earlier for the mutation of Tyr-67 to phenylalanine [Luntz, Schejter, Garber and Margoliash (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 3524-3528], because of either its effects on the hydrogen-bonding of an interior water molecule or a general increase in the hydrophobicity of the protein in the domain occupied by the mutated residues. The reduction potentials were affected in different ways; the Eo of rat cytochrome c rose by 14 mV whereas that of the yeast iso-1 cychrome c was 30 mV lower as a result of the change of Asn-52 to isoleucine.

A carbon source-responsive promoter element necessary for activation of the isocitrate lyase gene ICL1 is common to genes of the gluconeogenic pathway in the yeast Saccharomyces cerevisiae

The expression of yeast genes encoding gluconeogenic enzymes depends strictly on the carbon source available in the growth medium. We have characterized the control region of the isocitrate lyase gene ICL1, which is derepressed more than 200-fold after transfer of cells from fermentative to nonfermentative growth conditions. Deletion analysis of the ICL1 promoter led to the identification of an upstream activating sequence element, UASICL1 (5' CATTCATCCG 3'), necessary and sufficient for conferring carbon source-dependent regulation on a heterologous reporter gene. Similar sequence motifs were also found in the upstream regions of coregulated genes involved in gluconeogenesis. This carbon source-responsive element (CSRE) interacts with a protein factor, designated Ang1 (activator of nonfermentative growth), detectable only in extracts derived from derepressed cells. Gene activation mediated by the CSRE requires the positively acting derepression genes CAT1 (= SNF1 and CCR1) and CAT3 (= SNF4). In the respective mutants, Ang1-CSRE interaction was no longer observed under repressing or derepressing conditions. Since binding of Ang1 factor to the CSRE could be competed for by an upstream sequence derived from the fructose-1,6-bisphosphatase gene FBP1, we propose that the CSRE functions as a UAS element common to genes of the gluconeogenic pathway.

Heterologous expression of plant vacuolar pyrophosphatase in yeast demonstrates sufficiency of the substrate-binding subunit for proton transport

The membrane bounding the vacuole of plant cells contains two electrogenic proton pumps. These are the vacuolar H(+)-ATPase (EC 3.6.1.3), an enzyme common to all eukaryotes, and a vacuolar H(+)-translocating pyrophosphatase (EC 3.6.1.1), which is ubiquitous in plants but otherwise known in only a few phototrophic bacteria. Although the substrate-binding subunit of the vacuolar H(+)-pyrophosphatase has been identified and purified and cDNAs encoding it have been isolated and characterized, the minimal unit competent in pyrophosphate (PPi)-energized H+ translocation is not known. Here we address this question and show that heterologous expression of the cDNA (AVP) encoding the substrate-binding subunit of the vacuolar H(+)-pyrophosphatase from the vascular plant Arabidopsis thaliana in the yeast Saccharomyces cerevisiae results in the production of vacuolarly localized functional enzyme active in PPi-dependent H+ translocation. Since the heterologously expressed pump is indistinguishable from the native plant enzyme with respect to PPi hydrolysis, H+ translocation, activation by potassium, and selective inhibition by calcium and 1,1-diphosphonates, it is concluded that all of the known catalytic functions of the enzyme map to the one subunit encoded by AVP.

A carbon source-responsive promoter element necessary for activation of the isocitrate lyase gene ICL1 is common to genes of the gluconeogenic pathway in the yeast Saccharomyces cerevisiae

The expression of yeast genes encoding gluconeogenic enzymes depends strictly on the carbon source available in the growth medium. We have characterized the control region of the isocitrate lyase gene ICL1, which is derepressed more than 200-fold after transfer of cells from fermentative to nonfermentative growth conditions. Deletion analysis of the ICL1 promoter led to the identification of an upstream activating sequence element, UASICL1 (5' CATTCATCCG 3'), necessary and sufficient for conferring carbon source-dependent regulation on a heterologous reporter gene. Similar sequence motifs were also found in the upstream regions of coregulated genes involved in gluconeogenesis. This carbon source-responsive element (CSRE) interacts with a protein factor, designated Ang1 (activator of nonfermentative growth), detectable only in extracts derived from derepressed cells. Gene activation mediated by the CSRE requires the positively acting derepression genes CAT1 (= SNF1 and CCR1) and CAT3 (= SNF4). In the respective mutants, Ang1-CSRE interaction was no longer observed under repressing or derepressing conditions. Since binding of Ang1 factor to the CSRE could be competed for by an upstream sequence derived from the fructose-1,6-bisphosphatase gene FBP1, we propose that the CSRE functions as a UAS element common to genes of the gluconeogenic pathway.

Expression of the Saccharomyces cerevisiae CYT2 gene, encoding cytochrome c1 heme lyase

In this paper we examine the expression of the Saccharomyces cerevisiae CYT2 gene, which encodes cytochrome c1 heme lyase. This enzyme is required for covalent attachment of heme to apocytochrome c1, a subunit of the mitochondrial respiratory chain. Transcription of the 1-kb CYT2 mRNA initiates at four prominent sites at a distance of 52-225 bp in front of the AUG start codon. The level of CYT2 mRNA is not influenced by the presence or absence of oxygen or of heme, but it is subject to carbon-source control. The concentration of the CYT2 mRNA is significantly reduced in glucose-grown cells as compared to cells grown under non-repressing conditions. Neither the HAPp activator proteins nor MIG1p, a repressor protein involved in glucose repression, seem to mediate this effect.

Characterization of the Saccharomyces cerevisiae nuclear gene CYB3 encoding a cytochrome b polypeptide of respiratory complex II

Computer-assisted structural analysis of the predicted product of the previously described open reading frame (ORF) YKL4 located on the left arm of chromosome XI of Saccharomyces cerevisiae revealed a high degree of similarity (> 50%) to bovine cytochrome b560, the sdhC polypeptide of the Escherichia coli succinate dehydrogenase (SDH) complex and the protein specified by ORF137 located on the chloroplast DNA of Marchantia polymorpha. Disruption of the yeast gene severely impaired mitochondrial function, while Northern analysis showed it to be subject to catabolite repression. Deletion analysis of the CYB3 promoter identified a single HAP2/3/4-binding element that is necessary and sufficient for carbon source-dependent transcriptional regulation. These experiments also suggested the presence of additional, as yet unidentified, transcriptional control elements, both negative and positive. Taken together, these data lead us to conclude that the CYB3 gene encodes the yeast homolog of the bovine cytochrome b560 component of complex II of the mitochondrial electron transport chain.

Cloning and characterisation of the cytochrome c gene of Aspergillus nidulans

The cytochrome c gene (cycA) of the filamentous fungus Aspergillus nidulans has been isolated and sequenced. The gene is present in a single copy per haploid genome and encodes a polypeptide of 112 amino acid residues. The nucleotide sequence of the A. nidulans cycA gene shows 87% identity to the DNA sequence of the Neurospora crassa cytochrome c gene, and approximately 72% identity to the sequence of the Saccharomyces cerevisiae iso-1-cytochrome c gene (CYC1). The S. cerevisiae CYC1 gene was used as a heterologous probe to isolate the homologous gene in A. nidulans. The A. nidulans cytochrome c sequence contains two small introns. One of these is highly conserved in terms of position, but the other has not been reported in any of the cytochrome c genes so far sequenced. Expression of the cycA gene is not affected by glucose repression, but has been shown to be induced approximately tenfold in the presence of oxygen and three- to fourfold under heat-shock conditions.

The RHO4a and NUD1 genes on Saccharomyces cerevisiae chromosome XI

Sequence analysis of a 4 kb fragment from the right arm of Saccharomyces cerevisiae chromosome XI, in combination with Northern hybridization experiments revealed the presence of two genes, designated RHO4a and NUD1. The first gene encodes a 32 kDa protein showing significant sequence similarity with members of the ras family. Its 3'-terminal sequence is virtually identical to a sequence published previously as the RHO4 gene [Matsui and Toh-e, Gene 114 (1992), 43-49], which, however, appears to start at an internal ATG codon. The RHO4a sequence also overlaps the 5'-terminal sequence of the RNC1 gene [Chow et al., Nucl. Acids Res. 20 (1992), 5215-5221] proposed to encode the yeast yNucR endo/exonuclease. The remainder of this RNC1 gene overlaps with the 5'-end of the NUD1 gene. However, the RNC1 sequence lacks a portion of 276 bp that in our fragment is part of the intergenic region separating RHO4a and NUD1. From these results we conclude that the proposed RNC1 gene is the result of a cloning artefact and that the yNucR protein is instead encoded by the NUD1 gene.

The glucose-6-phosphate-isomerase reaction is essential for normal glucose repression in Saccharomyces cerevisiae

Wild-type Saccharomyces cerevisiae and a strain carrying a deletion in the glucose-6-phosphate-isomerase gene (pgi1) were grown in carbon-limited continuous cultures on a mixture of fructose and galactose. Pulses of glucose, fructose and galactose were given to these cultures to investigate whether the pgi1 strain was capable of normal glucose repression. Glucose and galactose pulses inhibited fructose consumption and thus glycolysis in the pgi1 strain by a combination of competition between glucose and fructose at the uptake and/or phosphorylation level and inhibition of fructose uptake and/or phosphorylation by glucose 6-phosphate. Fructose pulses administered to the pgi1 strain transiently decreased the glycolytic flux downstream of fructose-1,6-bisphosphate. Transcriptional induction of the PDC1 gene (encoding pyruvate decarboxylase) was observed after glucose or galactose pulses were applied to the pgi1 strain, demonstrating that metabolism of these sugars beyond glucose 6-phosphate is dispensable for PDC1 induction. Fructose also induced PDC1 transcription, indicating that intracellular sugars could act as trigger for PDC1 induction or, alternatively, that two inductors are present. In contrast to the wild-type transcriptional inhibition of the glucose-repressible genes, HXK1 and GAL10 (encoding hexokinase isoenzyme 1 and uridine diphosphoglucose-4-epimerase, respectively) did not occur upon addition of glucose or fructose to the pgi1 mutant. Transcriptional repression was observed after application of the fructose pulse when the yeast had resumed metabolism of fructose. These results demonstrate that the initial signal for catabolite repression is not generated by high sugar concentrations or high concentrations of intermediates; moreover a simple role for the hexokinases can also be excluded. The absence of an increased glycolytic flux in the pgi1 mutant after administration of the sugar pulses while the concentrations of sugar and glycolytic intermediates were high, suggests that the initial signal for glucose repression could be linked to an increased glycolytic flux. The occurrence of PDC1 induction in the pgi1 strain while GAL10/HXKI repression is absent, demonstrates that the initial signals for catabolite induction and catabolite repression are different.

The Saccharomyces cerevisiae SPR1 gene encodes a sporulation-specific exo-1,3-beta-glucanase which contributes to ascospore thermoresistance

A number of genes have been shown to be transcribed specifically during sporulation in Saccharomyces cerevisiae, yet their developmental function is unknown. The SPR1 gene is transcribed during only the late stages of sporulation. We have sequenced the SPR1 gene and found that it has extensive DNA and protein sequence homology to the S. cerevisiae EXG1 gene which encodes an exo-1,3-beta-glucanase expressed during vegetative growth (C. R. Vasquez de Aldana, J. Correa, P. San Segundo, A. Bueno, A. R. Nebrada, E. Mendez, and F. del Ray, Gene 97:173-182, 1991). We show that spr1 mutant cells do not hydrolyze p-nitrophenyl-beta-D-glucoside or laminarin in a whole-cell assay for exo-1,3-beta-glucanases. In addition to the absence of this enzymatic activity, spr1 mutant spores exhibit reduced thermoresistance relative to isogenic wild-type spores. These observations are consistent with the notion that SPR1 encodes a sporulation-specific exo-1,3-beta-glucanase.

Structural and putative regulatory sequences of Kluyveromyces ribosomal protein genes

The transcription of the majority of the ribosomal protein (rp) genes of Saccharomyces cerevisiae is activated by cis-acting elements, designated RPG boxes, which specifically bind the multifunctional protein RAP1 in vitro. To investigate to what extent this global system of transcription regulation has been conserved, we have isolated a number of rp genes of the related yeast species Kluyveromyces lactis and Kluyveromyces marxianus, whose counterparts in Saccharomyces are controlled by RAP1. The coding regions of these genes showed a sequence similarity of about 90% when compared to their Saccharomyces counterparts. In contrast, little or no sequence similarity was found between the upstream regions and the intervening sequences of Kluyveromyces and Saccharomyces homologs. However, the occurrence and the position of the introns is conserved. The sequence data also show that the physical linkage that exists in S. cerevisiae between the rp genes encoding RP59 (CRY1), S24 and L46 is conserved in Kluyveromyces. Northern analysis demonstrated that each of the isolated Kluyveromyces genes is transcriptionally active. By sequence comparison we identified a number of conserved sequences in the upstream region of each of the Kluyveromyces rp genes, which we designated the X, Z and RPGK boxes. The last one is highly similar, though not identical, to the S. cerevisiae RPG box. Functional analysis of the intergenic region between the genes encoding Kluyveromyces ribosomal proteins S24 and L46 showed that the RPGK box (+Z box) functions as a transcriptional activator, while the X box acts as a transcriptional repressor. Band-shift assays confirmed the existence of a RAP1-like protein in Kluyveromyces that binds to the RPGK box but not to the S. cerevisiae RPG box. In contrast, S. cerevisiae RAP1 did recognize the RPGK box.

Structure and expression of the ABF1-regulated ribosomal protein S33 gene in Kluyveromyces

The abundant multifunctional protein ABF1 of Saccharomyces cerevisiae binds to the upstream region of several genes, including some ribosomal protein genes like the one encoding protein S33. Deletion of the ABF1-binding sequence lowers the transcription of these genes three- to more than ten-fold. We have isolated the S33 genes of two related yeast species, Kluyveromyces lactis and Kluyveromyces marxianus. Comparison of the nucleotide sequences of these S33 genes with their counterpart from S. cerevisiae shows a strong sequence similarity covering the whole of the coding regions. In contrast, little or no sequence similarity is found in the 5'-flanking regions of the three genes. Also the trailer regions differ considerably in both length and sequence from one species to another. An ABF1-binding site is present in the upstream region of the S33 gene of K. marxianus. Retardation analyses showed that this sequence is able to bind a protein present in Kluyveromyces cells with a molecular mass somewhat lower than that of S. cerevisiae ABF1. Functional analyses, using a beta-glucuronidase reporter system, showed that the ABF1-binding site is indeed involved in transcription activation of the K. marxianus S33 gene in Kluyveromyces cells. A S. cerevisiae ABF1-gene-specific probe showed only weak hybridization with Kluyveromyces DNA and Northern blots did not show a signal. These results indicate that S. cerevisiae and Kluyveromyces contain functionally related but structurally dissimilar ABF1-type proteins.

Early meiotic transcripts are highly unstable in Saccharomyces cerevisiae

Meiosis in Saccharomyces cerevisiae requires the induction of a large number of genes whose mRNAs accumulate at specific times during meiotic development. This study addresses the role of mRNA stability in the regulation of meiosis-specific gene expression. Evidence is provided below demonstrating that the levels of meiotic mRNAs are exquisitely regulated by both transcriptional control and RNA turnover. The data show that (i) early meiotic transcripts are extremely unstable when expressed during either vegetative growth or sporulation, and (ii) transcriptional induction, rather than RNA turnover, is the predominant mechanism responsible for meiosis-specific transcript accumulation. When genes encoding the early meiotic mRNAs are fused to other promoters and expressed during vegetative growth, their mRNA half-lives, of under 3 min, are among the shortest known in S. cerevisiae. Since these mRNAs are only twofold more stable when expressed during sporulation, we conclude that developmental regulation of mRNA turnover can be eliminated as a major contributor to meiosis-specific mRNA accumulation. The rapid degradation of the early mRNAs at all stages of the yeast life cycle, however, suggests that a specific RNA degradation system operates to maintain very low basal levels of these transcripts during vegetative growth and after their transient transcriptional induction in meiosis. Studies to identify specific cis-acting elements required for the rapid degradation of early meiotic transcripts support this idea. A series of deletion derivatives of one early meiosis-specific gene, SPO13, indicate that its mRNA contains determinants, located within the coding region, which contribute to the high instability of this transcript. Translation is another component of the degradation mechanism since frameshift and nonsense mutations within the SPO13 mRNA stabilize the transcript.

MOL1, a Saccharomyces cerevisiae gene that is highly expressed in early stationary phase during growth on molasses

We have isolated a new Saccharomyces cerevisiae gene, MOL1, that is transiently expressed at high levels in the early stationary phase of batch cultures growing on industrial molasses medium. The DNA sequence of the MOL1 gene (for MOLasses-inducible) with its flanking regions was determined (EMBL accession number X61669). It encodes a polypeptide of M(r) 35 kDa that is closely related to stress-inducible proteins of similar size from two Fusarium species. Unlike ST135 of Fusarium, MOL1 is not induced by ethanol or heat shock. MOL1 expression is absent in rich (YP) medium, and only very low levels of expression are detectable in minimal (YNB) medium. The gene is not essential, and a MOL1 disruption strain showed no apparent phenotype under a variety of growth conditions. The 5' region of MOL1 contains the complete sequence previously determined for the SUF4 locus, encoding a tRNA-gly (UCC) gene, which has been mapped to chromosome VII.

An examination of adaptive reversion in Saccharomyces cerevisiae

Reversion to Lys+ prototrophy in a haploid yeast strain containing a defined lys2 frameshift mutation has been examined. When cells were plated on synthetic complete medium lacking only lysine, the numbers of Lys+ revertant colonies accumulated in a time-dependent manner in the absence of any detectable increase in cell number. An examination of the distribution of the numbers of early appearing Lys+ colonies from independent cultures suggests that the mutations to prototrophy occurred randomly during nonselective growth. In contrast, an examination of the distribution of late appearing Lys+ colonies indicates that the underlying reversion events occurred after selective plating. No accumulation of Lys+ revertants occurred when cells were starved for tryptophan, leucine or both lysine and tryptophan prior to plating selectively for Lys+ revertants. These results indicate that mutations accumulate more frequently when they confer a selective advantage, and are thus consistent with the occurrence of adaptive mutations in yeast.

Early meiotic transcripts are highly unstable in Saccharomyces cerevisiae

Meiosis in Saccharomyces cerevisiae requires the induction of a large number of genes whose mRNAs accumulate at specific times during meiotic development. This study addresses the role of mRNA stability in the regulation of meiosis-specific gene expression. Evidence is provided below demonstrating that the levels of meiotic mRNAs are exquisitely regulated by both transcriptional control and RNA turnover. The data show that (i) early meiotic transcripts are extremely unstable when expressed during either vegetative growth or sporulation, and (ii) transcriptional induction, rather than RNA turnover, is the predominant mechanism responsible for meiosis-specific transcript accumulation. When genes encoding the early meiotic mRNAs are fused to other promoters and expressed during vegetative growth, their mRNA half-lives, of under 3 min, are among the shortest known in S. cerevisiae. Since these mRNAs are only twofold more stable when expressed during sporulation, we conclude that developmental regulation of mRNA turnover can be eliminated as a major contributor to meiosis-specific mRNA accumulation. The rapid degradation of the early mRNAs at all stages of the yeast life cycle, however, suggests that a specific RNA degradation system operates to maintain very low basal levels of these transcripts during vegetative growth and after their transient transcriptional induction in meiosis. Studies to identify specific cis-acting elements required for the rapid degradation of early meiotic transcripts support this idea. A series of deletion derivatives of one early meiosis-specific gene, SPO13, indicate that its mRNA contains determinants, located within the coding region, which contribute to the high instability of this transcript. Translation is another component of the degradation mechanism since frameshift and nonsense mutations within the SPO13 mRNA stabilize the transcript.

Control of mRNA turnover as a mechanism of glucose repression in Saccharomyces cerevisiae

We have examined the expression of the gene encoding the iron-protein subunit (Ip) of succinate dehydrogenase in Saccharomyces cerevisiae. The gene had been cloned by us and shown to be subject to glucose regulation (A. Lombardo, K. Carine, and I. E. Scheffler, J. Biol. Chem. 265:10419-10423, 1990). We discovered that a significant part of the regulation of the Ip mRNA levels by glucose involves the regulation of the turnover rate of this mRNA. In the presence of glucose, the half-life appears to be less than 5 min, while in glycerol medium, the half-life is greater than 60 min. The gene is also regulated transcriptionally by glucose. The upstream promoter sequence appeared to have four regulatory elements with consensus sequences shown to be responsible for the interaction with the HAP2/3/4 regulatory complex. A deletion analysis has shown that the two distal elements are redundant. These measurements were carried out by Northern (RNA) analyses of Ip mRNA transcripts as well as by assays of beta-galactosidase activity in cells carrying constructs of the Ip promoter linked to the lacZ coding sequence. These observations on the regulation of mRNA stability were also extended to the mRNA of the flavoprotein subunit of succinate dehydrogenase and in some experiments of iso-1-cytochrome c.

The rate-limiting step in yeast PGK1 mRNA degradation is an endonucleolytic cleavage in the 3'-terminal part of the coding region

Insertion of an 18-nucleotide-long poly(G) tract into the 3'-terminal untranslated region of yeast phosphoglycerate kinase (PGK1) mRNA increases its chemical half-life by about a factor of 2 (P. Vreken, R. Van der Veen, V. C. H. F. de Regt, A. L. de Maat, R. J. Planta, and H. A. Raué, Biochimie 73:729-737, 1991). In this report, we show that this insertion also causes the accumulation of a degradation intermediate extending from the poly(G) sequence down to the transcription termination site. Reverse transcription and S1 nuclease mapping experiments demonstrated that this intermediate is the product of shorter-lived primary fragments resulting from endonucleolytic cleavage immediately downstream from the U residue of either of two 5'-GGUG-3' sequences present between positions 1100 and 1200 close to the 3' terminus (position 1251) of the coding sequence. Similar endonucleolytic cleavages appear to initiate degradation of wild-type PGK1 mRNA. Insertion of a poly(G) tract just upstream from the AUG start codon resulted in the accumulation of a 5'-terminal degradation intermediate extending from the insertion to the 1100-1200 region. RNase H degradation in the presence of oligo(dT) demonstrated that the wild-type and mutant PGK1 mRNAs are deadenylated prior to endonucleolytic cleavage and that the half-life of the poly(A) tail is three- to sixfold lower than that of the remainder of the mRNA. Thus, the endonucleolytic cleavage constitutes the rate-limiting step in degradation of both wild-type and mutant PGK1 transcripts, and the resulting fragments are degraded by a 5'----3' exonuclease, which appears to be severely retarded by a poly(G) sequence.

Osmostress-induced changes in yeast gene expression

When Saccharomyces cerevisiae cells are exposed to high concentration of NaCl, they show reduced viability, methionine uptake and protein biosynthesis. Cells can acquire tolerance against a severe salt shock (up to 1.4 M NaCl) by a previous treatment with 0.7 M NaCl, but not by a previous heat shock. Two-dimensional analysis of [3H]-leucine-labelled proteins from salt-shocked cells (0.7 M NaCl) revealed the elevated rate of synthesis of nine proteins, among which were the heat-shock proteins hsp12 and hsp26. Northern analysis using gene-specific probes confirmed the identity of the latter proteins and, in addition, demonstrated the induction of glycerol-3-phosphate dehydrogenase gene expression. The synthesis of the same set of proteins is induced or enhanced upon exposure of cells to 0.8 M sucrose, although not as dramatically as in an iso-osmolar NaCl concentration (0.7 M).

Control of mRNA turnover as a mechanism of glucose repression in Saccharomyces cerevisiae

We have examined the expression of the gene encoding the iron-protein subunit (Ip) of succinate dehydrogenase in Saccharomyces cerevisiae. The gene had been cloned by us and shown to be subject to glucose regulation (A. Lombardo, K. Carine, and I. E. Scheffler, J. Biol. Chem. 265:10419-10423, 1990). We discovered that a significant part of the regulation of the Ip mRNA levels by glucose involves the regulation of the turnover rate of this mRNA. In the presence of glucose, the half-life appears to be less than 5 min, while in glycerol medium, the half-life is greater than 60 min. The gene is also regulated transcriptionally by glucose. The upstream promoter sequence appeared to have four regulatory elements with consensus sequences shown to be responsible for the interaction with the HAP2/3/4 regulatory complex. A deletion analysis has shown that the two distal elements are redundant. These measurements were carried out by Northern (RNA) analyses of Ip mRNA transcripts as well as by assays of beta-galactosidase activity in cells carrying constructs of the Ip promoter linked to the lacZ coding sequence. These observations on the regulation of mRNA stability were also extended to the mRNA of the flavoprotein subunit of succinate dehydrogenase and in some experiments of iso-1-cytochrome c.

The rate-limiting step in yeast PGK1 mRNA degradation is an endonucleolytic cleavage in the 3'-terminal part of the coding region

Insertion of an 18-nucleotide-long poly(G) tract into the 3'-terminal untranslated region of yeast phosphoglycerate kinase (PGK1) mRNA increases its chemical half-life by about a factor of 2 (P. Vreken, R. Van der Veen, V. C. H. F. de Regt, A. L. de Maat, R. J. Planta, and H. A. Raué, Biochimie 73:729-737, 1991). In this report, we show that this insertion also causes the accumulation of a degradation intermediate extending from the poly(G) sequence down to the transcription termination site. Reverse transcription and S1 nuclease mapping experiments demonstrated that this intermediate is the product of shorter-lived primary fragments resulting from endonucleolytic cleavage immediately downstream from the U residue of either of two 5'-GGUG-3' sequences present between positions 1100 and 1200 close to the 3' terminus (position 1251) of the coding sequence. Similar endonucleolytic cleavages appear to initiate degradation of wild-type PGK1 mRNA. Insertion of a poly(G) tract just upstream from the AUG start codon resulted in the accumulation of a 5'-terminal degradation intermediate extending from the insertion to the 1100-1200 region. RNase H degradation in the presence of oligo(dT) demonstrated that the wild-type and mutant PGK1 mRNAs are deadenylated prior to endonucleolytic cleavage and that the half-life of the poly(A) tail is three- to sixfold lower than that of the remainder of the mRNA. Thus, the endonucleolytic cleavage constitutes the rate-limiting step in degradation of both wild-type and mutant PGK1 transcripts, and the resulting fragments are degraded by a 5'----3' exonuclease, which appears to be severely retarded by a poly(G) sequence.

A cell-free extract from yeast cells for studying mRNA turnover

We have isolated a cell-free extract from yeast cells that reproduces the differences observed in vivo in the rate of turnover of individual yeast mRNAs. Detailed analysis of the degradation of yeast phosphoglycerate kinase (PGK) mRNA in this system demonstrated that both natural and synthetically prepared PGK transcripts are degraded by the same pathway previously established by us in vivo, consisting of endonucleolytic cleavage at a number of 5'-GGUG-3' sequence motifs within a short target region located close to the 3'-end of the coding sequence followed by 5'-3' exonucleolytic removal of the resulting fragments. The extract, therefore, is suitable for studying the mechanistic details of mRNA turnover in yeast. As a first application of this system we have performed a limited mutational analysis of two of the GGUG motifs within the endonucleolytic target region of the PGK transcript. The results show that sequence changes in either motif abolish cleavage at the mutated site only, indicating the involvement of the residues in question in selection of the cleavage positions.

Molecular cloning and physical analysis of an 8.2 kb segment of chromosome XI of Saccharomyces cerevisiae reveals five tightly linked genes

The nucleotide sequence of 6472 base pairs of an 8.2 kb segment of Saccharomyces cerevisiae chromosome XI has been determined. The sequence contains a cluster of four long open reading frames (ORF) designated YKL2, YKL3, YKL4 and TGL1 in the same orientation, flanked at the 5'-end by a divergent incomplete ORF (YKL1). Transcription and Southern analysis of the four complete ORFs showed that all are expressed and are present in single copy on the haploid genome. The average codon adaptation index of the coding regions is approximately 0.2, suggesting that these genes are lowly expressed. The upstream regions of all four genes as well as the YKL1 ORF contain putative promoter elements previously found to be characteristic of nuclear genes encoding mitochondrial proteins. Significant sequence similarities were found between the YKL3 protein and Escherichia coli ribosomal protein S2 as well as between the TGL1 protein and triglyceride lipases from rat salivary gland and human gastric tissue. The 3'-end of the 6472 bp nucleotide sequence overlaps with the upstream region of the previously identified CTK1 gene, encoding the largest subunit of CTD kinase (Lee, J.M. and Greenleaf, A.L., 1991, Gene Expression 2, 149-167), thereby increasing the number of genes on the 8.2 kb fragment to at least five. The transcripts of these genes represent approximately 83% of the DNA fragment, making it one of the most highly transcribed regions of the yeast chromosome analysed to date.

Regulation of the yeast CYT1 gene encoding cytochrome c1 by HAP1 and HAP2/3/4

Mitochondrial biogenesis requires the coordinate induction of hundreds of genes that reside in the nucleus. We describe here a study of the regulation of the nuclear-encoded cytochrome c1 of the b-c1 complex. Unlike cytochrome c, which is encoded by two genes, CYC1 and CYC7, c1 is encoded by a single gene, CYT1. The regulatory region of the CYT1 promoter contains binding sites for the HAP1 and HAP2/3/4 transactivators that regulate CYC1. The binding of HAP1 to the CYT1 element was studied in detail and found to differ in two important respects from binding to the CYC1 element. First, while CYC1 contains two sites that bind HAP1 cooperatively, CYT1 has a single high-affinity site. Second, while the CYT1 site and the stronger HAP1-binding site of CYC1 share a large block of homology, the HAP1 footprints at these sites are offset by several nucleotides. We discuss how these differences in HAP1 binding might relate to the difference in the biology of cytochrome c and cytochrome c1.

Regulation of the yeast CYT1 gene encoding cytochrome c1 by HAP1 and HAP2/3/4

Mitochondrial biogenesis requires the coordinate induction of hundreds of genes that reside in the nucleus. We describe here a study of the regulation of the nuclear-encoded cytochrome c1 of the b-c1 complex. Unlike cytochrome c, which is encoded by two genes, CYC1 and CYC7, c1 is encoded by a single gene, CYT1. The regulatory region of the CYT1 promoter contains binding sites for the HAP1 and HAP2/3/4 transactivators that regulate CYC1. The binding of HAP1 to the CYT1 element was studied in detail and found to differ in two important respects from binding to the CYC1 element. First, while CYC1 contains two sites that bind HAP1 cooperatively, CYT1 has a single high-affinity site. Second, while the CYT1 site and the stronger HAP1-binding site of CYC1 share a large block of homology, the HAP1 footprints at these sites are offset by several nucleotides. We discuss how these differences in HAP1 binding might relate to the difference in the biology of cytochrome c and cytochrome c1.

Expression of the alpha-galactosidase from Cyamopsis tetragonoloba (guar) by Hansenula polymorpha

The methylotrophic yeast Hansenula polymorpha, a host organism for the production of heterologous proteins, has been applied to produce the alpha-galactosidase from the plant Cyamopsis tetragonoloba (guar). The yeast/Escherichia coli shuttle expression vector used is based on the origin of replication of the endogenous 2 microns plasmid of Saccharomyces cerevisiae and the LEU2 gene of S. cerevisiae for selection in H. polymorpha. In the expression vector, the alpha-galactosidase is controlled by the methanol-regulated promoter from the methanol oxidase gene, MOX, of H. polymorpha. The signal sequence of SUC2 (invertase) from the yeast S. cerevisiae, was used to ensure secretion of the alpha-galactosidase enzyme. After transformation and stabilization, the expression vector was stably integrated in the genome. The active alpha-galactosidase enzyme was efficiently secreted (greater than 85%) and after methanol induction, the expression level was 42 mg/l. Amino-terminal sequencing of the purified alpha-galactosidase enzyme synthesized by H. polymorpha showed that the S. cerevisiae invertase signal sequence was correctly processed by H. polymorpha. The secreted alpha-galactosidase was glycosylated and had a sugar content of 9.5%. The specific activity of the alpha-galactosidase produced by H. polymorpha was 38 U mg-1 compared to 100 U mg-1 for the guar alpha-galactosidase. Deglycosylation of the H. polymorpha alpha-galactosidase restored the specific activity completely.