Posttranscriptional heme control of catalase synthesis in the yeast Saccharomyces cerevisiae

Production of glucose oxidase and catalase by Aspergillus niger free and immobilized in alginate-polyvinyl alcohol beads

The production of glucose oxidase (GOX) and catalase (CAT) by submerged fermentation of the free and immobilized xerophytic fungus Aspergillus niger under equal conditions was compared. To immobilize fungal spores, entrapment in PVA/alginate beads treated with NaNO3/CaCl2 was performed. The yield of immobilization in the beads with a diameter less than 1mm was equal to 100%. Fungus growth and substrate consumption were evaluated in both fermentation systems, demonstrating the lag-period presence in the case of the first cycle of immobilized fungus use. The enzyme production by immobilized fungus reuse was carried out. In these cases, greater enzymatic GOX activity was detected, while CAT activity decreased. SEM micrographs for the beads with immobilized fungus applied in the first and second fermentation cycles were obtained, presenting fungus spreading inside the sphere, spore presence and branching hyphae. Immobilization of A. niger on PVA/alginate beads is effective for GOX and CAT production at least on 2-3 repeated fermentative cycles. Thus, immobilization enables repeated use of microbial cells.

Translation and regulation of translation in the yeast Saccharomyces cerevisiae

In recent years the yeast Saccharomyces cerevisiae has become a model system for studies of eukaryotic translation and translation regulation. Analysis of mRNA structure, translation initiation factor sequences and the translation initiation pathway indicate, that translation in S. cerevisiae is very similar to translation in higher eukaryotes. The availability of powerful genetic techniques lead to the dissection in yeast of individual steps in the translation pathway, the detection of biochemical interactions between components involved in translation and the unravelling of complex regulation phenomena.

The 5'-upstream region of the yeast 25S rRNA gene contains a promoter element allowing expression in yeast and E. coli

The 25S rRNA gene of Saccharomyces cerevisiae is preceded by a bona fide TATA sequence which allows the initiation of transcription--presumably by polymerase II--from the same strand as the 25S rRNA gene. When the promoter fragment is cloned in front of a lacZ gene equipped with an initiation codon but lacking a promoter, this element permits formation of beta-galactosidase both in yeast and E. coli. Using RNA from yeast transformed with the fusion plasmid, we mapped by primer elongation a single initiation site 63 bp downstream from the presumed TATA sequence, i.e. about 53 bp 5' of the 25S rRNA gene. A similar signal at about the same position was observed when RNA from untransformed wild-type yeast was used as a template for primer elongation. These results suggest that transcription from this polymerase II promoter-like element occurs in vivo. A regulatory function could not be assigned to this transcript. Its initiation is not significantly influenced by heme or carbon source, although two boxes of high homology with upstream activation sequences (UAS) mediating heme dependent expression of the iso-1-cytochrome c gene (CYC1) precede the promoter at the appropriate distance.

A nuclear yeast gene (GCY) encodes a polypeptide with high homology to a vertebrate eye lens protein

We describe here the nuclear gene for a yeast protein showing unexpectedly high homology with mammalian aldo/keto reductases as well as with p-crystallin, one of the prominent proteins of the frog eye lens. Although it could be proven that the gene occurs as a single copy in the haploid yeast genome, replacement of the intact by a disrupted, nonfunctional allele led to no obvious phenotype, indicating that the gene is dispensable. The gene was assigned to chromosome XV. It is transcribed in vivo into an mRNA of about 1300 bases with a coding capacity for a protein of 312 amino acids (estimated Mr 35,000).

The complete nucleotide sequence of the gene coding for yeast adenylate kinase

The structural gene for yeast adenylate kinase (AKY) has been isolated and analyzed with respect to its nucleotide sequence. Southern and northern analyses imply that the gene is single copy and is transcribed into an mRNA of about 1,100 bases. The flanking regions of the gene contain the canonical elements typical for initiation and termination of transcription of yeast protein coding genes. The amino acid primary structure deduced from the open reading frame is identical with the protein sequence reported for yeast adenylate kinase (Tomasselli et al. 1986) with the exception of an extension of two amino acids (Met-Ser) at the N-terminus and aspartic acid instead of asparagine at the carboxyl end. Yeast adenylate kinase reveals a striking homology with both the mammalian cytosolic and, particularly, with the mitochondrial isozyme. It has an insertion of 31 amino acids in the middle segment of the protein, when compared to the cytosolic version of the mammalian enzyme. A strikingly conserved insert sequence of the same length and at exactly the same position is present in the mammalian mitochondrial isozyme. The question of the subcellular location of the yeast enzyme is discussed.

Nucleotide sequence of the Saccharomyces cerevisiae CTT1 gene and deduced amino-acid sequence of yeast catalase T

A 2642-base-pair DNA fragment containing the catalase T (CTT1) structural gene of the yeast Saccharomyces cerevisiae and its flanking regions has been sequenced. The gene codes for a protein of 562 amino acids (relative molecular mass 64,449) and appears to contain no intron. The amino acid sequence of catalase T derived from the DNA sequence shows 40.7% homology (52.2% including conservative replacements) to that of bovine liver catalase. All amino acids previously postulated to participate directly in catalysis by liver catalase and most of the amino acids of the immediate environment of hemin, the prosthetic group of catalase, are conserved in catalase T. The data obtained indicate that the folding of polypeptide chains of the two catalases compared has been conserved within a central region consisting mainly of the beta-barrel domain, which bears the prosthetic group, and a major part of the "wrapping domain". N- and C-terminal regions involved in subunit interactions are less well conserved. It is suggested that their structure is more similar to that of the corresponding regions of Penicillium vitale catalase. However, catalase T lacks the C-terminal flavodoxin-like domain present in this protein.

Isolation of the catalase A gene of Saccharomyces cerevisiae by complementation of the cta1 mutation

As a first step in an analysis of the DNA regions involved in the control of the catalase A gene of Saccharomyces cerevisiae by glucose, heme, and oxygen this gene has been cloned. Catalase A-deficient mutants were obtained by UV mutagenesis of a ctt1 mutant strain specifically lacking catalase T. All the catalase A-deficient mutants obtained fall into one complementation group. The single recessive mutation causing specific lack of catalase A was designated cta1. Several overlapping DNA fragments complementing the cta1 mutation were obtained by transforming ctt1 cta1 double mutants with a yeast gene library in vector YEp13. Hybrid selection of RNA with the help of one of the cloned DNAs followed by in vitro translation of this RNA and identification of the protein synthesized with catalase A-specific antibodies showed that the catalase A structural gene has been cloned. A single copy of this gene is present in the yeast genome. Transcription of the catalase A gene cloned into vector YEp13 is repressed by glucose. The DNA isolated hybridizes to a 1.6 kb polyA+-RNA virtually absent from heme-deficient cells, presumably catalase A mRNA.

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.

Regulation of synthesis of catalases and iso-1-cytochrome c in Saccharomyces cerevisiae by glucose, oxygen and heme

The regulation of the hemoproteins catalase T, catalase A and iso-1-cytochrome c was studied in the yeast Saccharomyces cerevisiae. Levels of catalase T and catalase A mRNAs are low or undetectable in anaerobic and heme-deficient cells, and in wild type strains grown on high glucose concentrations. Regulatory mutants (cgr4 and cas1), which have previously been shown to have high catalase T activity when grown in the absence of oxygen or on high glucose concentrations, have high levels of catalase T mRNA when grown under glucose repression conditions. Whereas no catalase T mRNA could be detected in a heme-deficient (ole3) single mutant, double mutants (ole3 cgr4) and (ole3 cas1) contain mature catalase T mRNA. Catalase T and A mRNAs are accumulated rapidly during adaptation of anaerobic cells to oxygen. Anaerobic and heme-deficient cells lack or have extremely low levels of iso-1-cytochrome c mRNA, which, like catalase mRNAs, is accumulated rapidly during oxygen adaptation. The results obtained demonstrate that glucose, oxygen and heme regulate the synthesis of the hemoproteins studied by controlling mRNA levels. In addition, posttranscriptional, probably translational control has to be postulated at least in the case of catalases, to explain the results obtained.