Saccharomyces mutants with invertase formation resistant to repression by hexoses

Production and partial purification of invertase using Cympopogan caecius leaf powder as substrate

The present study investigates the efficiency of Aspergillus niger to produce invertase, an industrially important enzyme by using powdered stem of Cympopogan caecius (Lemon grass) as sole substrate and sole carbon source for the microorganism. The molecular weight of invertase was estimated to be 66-70 kDa by sodium do decyl sulphate poly acrylamide gel electrophoresis (SDS PAGE). The production of the enzyme was studied at different pH scales ranging from pH 4.0 to 7.0 at a constant temperature of 30°C and 2% substrate concentration. The maximum production of invertase (specific activity -0.0516 μk/mg protein) was obtained at pH 5.5 at 30°C temperature, and incubation for 48 h. The activity was found to be stable at pH 5.5 for 30 min. The enzyme was found to be stable in the temperature range of 20-55°C. The effect of divalent metal ions Cu(2+), Fe(2+), Co(2+) on the activity of the enzyme invertase showed that these ions affected the activity by a certain factor. The study can be further industrially exploited in a country-like India where lemon grass is found in plenty and can be used as substrate for enzyme production. Moreover, the preparation of the substrate is also a simple process.

Production of hantavirus Puumala nucleocapsid protein in Saccharomyces cerevisiae for vaccine and diagnostics

The production of hantavirus Puumala nucleocapsid (N) protein for potential applications as a vaccine and for diagnostic purposes was investigated with Saccharomyces cerevisiae as a recombinant host. The N protein gene and the hexahistidine tagged N (h-N) protein gene were expressed intracellular from a 2-microm plasmid vectors under the control of a fused galactose inducible GAL10-PYK promoter. For monitoring the recombinant gene expression, a h-N and a GFP fusion protein was used. Different cultivation strategies and growth media compositions were tested in shake flasks and a 5 l bioreactor. When using defined YNB growth medium, we found the biomass yield to be unsatisfactorily low. Higher concentrated YNB medium, promoted cell growth but showed a pronounced inhibitory effect on heterologous gene expression. This phenomenon could not be attributed to plasmid losses, as we could demonstrate high stability of the vector under the applied cultivation conditions. Supplementation of YNB medium with extracts of plant origin resulted in increased biomass yields with concomitant high expression levels of the recombinant gene. The modified medium was used for fed-batch cultivations where basic metabolic features as well as growth parameters were determined in addition to recombinant gene expression. The maximal volumetric yield of N protein was 316 mg l(-1), the respective yield of h-N protein was 284 mg l(-1). Our study provides a basis for large-scale production of hantavirus vaccines, which satisfies economic efficiency as well as biosafety regulations for human applications.

Yeast-expressed Puumala hantavirus nucleocapsid protein induces protection in a bank vole model

Hantaviruses are rodent-borne agents that cause severe human diseases. The coding sequences for the authentic and a His-tagged Puumala hantavirus (PUUV) nucleocapsid (N) protein were expressed in yeast (Saccharomyces cerevisiae). N-specific monoclonal antibodies demonstrated native antigenicity of the two proteins. All bank voles vaccinated with the His-tagged N protein in Freund's adjuvant (n=12) were defined as completely protected against subsequent virus challenge, based on the absence of viral N protein, RNA and G2-specific antibodies. In the group vaccinated with the yeast-expressed authentic N protein in Freund's adjuvant, 2/6 animals were defined as completely protected and 4/6 as partially protected. Moreover, when animals were vaccinated with the His-tagged N protein in an adjuvant certified for human use (alum), all (n=8) were at least partially protected (six completely, two partially). The general advantages of the yeast expression system make the described recombinant proteins promising candidate vaccines against hantavirus infection.

Review: the dominant flocculation genes of Saccharomyces cerevisiae constitute a new subtelomeric gene family

The quality of brewing strains is, in large part, determined by their flocculation properties. By classical genetics, several dominant, semidominant and recessive flocculation genes have been recognized. Recent results of experiments to localize the flocculation genes FLO5 and FLO8, combined with the in silicio analysis of the available sequence data of the yeast genome, have revealed that the flocculation genes belong to a family which comprises at least four genes and three pseudogenes. All members of this gene family are located near the end of chromosomes, just like the SUC, MEL and MAL genes, which are also important for good quality baking or brewing strains. Transcription of the flocculation genes is repressed by several regulatory genes. In addition, a number of genes have been found which cause cell aggregation upon disruption or overexpression in an as yet unknown manner. In total, 33 genes have been reported that are involved in flocculation or cell aggregation.

Yeast flocculation: reconciliation of physiological and genetic viewpoints

Yeast flocculation results from surface expression of specific proteins (lectins). Two flocculation phenotypes were suggested by physiological and biochemical tests, whereas genetic data suggested a larger number of mechanisms of flocculation. After reviewing the biochemistry, physiology and genetics of flocculation, a new hypothesis combining the data available from these different sources, is proposed. Flocculation results when lectins present on flocculent cell walls bind to sugar residues of neighbouring cell walls. These sugar receptors are intrinsic to the mannan comprising cell walls of Saccharomyces cerevisiae. Two lectin phenotypes were revealed by sugar inhibition studies. The gluco- and mannospecific NewFlo phenotype is not, as yet, found in genetically defined strains. Mannospecific flocculation (Flo1 phenotype) is found in strains containing the genes FLO1, FLO5 and FLO8. This phenotype is also found following mutation of the TUP1 or CYC8 loci, in previously non-flocculent strains. It is therefore proposed that the structural gene for mannospecific flocculation is common or possibly ubiquitous in non-flocculent strains and in consequence, FLO1, FLO5 and FLO8 are probably regulatory genes, exerting positive control over the structural gene. Flocculation expression requires lectin secretion to the cell surface. Many of the observed 'suppressions' of flocculation may be due to mutations of the secretory process, involved in transporting structural proteins to the cell wall. The possible involvement of killer L double-stranded RNA with flocculation is suggested, given the lectin properties of viral coat proteins and an association between L double-stranded RNA and the Flo1 phenotype.

Regulation of sugar and ethanol metabolism in Saccharomyces cerevisiae

This review briefly surveys the literature on the nature, regulation, genetics, and molecular biology of the major energy-yielding pathways in yeasts, with emphasis on Saccharomyces cerevisiae. While sugar metabolism has received the lion's share of attention from workers in this field because of its bearing on the production of ethanol and other metabolites, more attention is now being paid to ethanol metabolism and the regulation of aerobic metabolism by fermentable and nonfermentable substrates. The utility of yeast as a highly manipulable organism and the discovery that yeast metabolic pathways are subject to the same types of control as those of higher cells open up many opportunities in such diverse areas as molecular evolution and cancer research.

Isolation and characterization of dominant mutations resistant to carbon catabolite repression of galactokinase synthesis in Saccharomyces cerevisiae

Seven dominant mutations showing greatly enhanced resistance to the glucose repression of galactokinase synthesis have been isolated from GAL81 mutants, which have the constitutive phenotype but are still strongly repressible by glucose for the synthesis of the Leloir enzymes. These glucose-resistant mutants were due to semidominant mutations at either of two loci, GAL82 and GAL83. Both loci are unlinked to the GAL81- gal4, gal80, or gal7 X gal10 X gal1 locus or to each other. The GAL83 locus was mapped on chromosome V at a site between arg9 and cho1. The GAL82 and GAL83 mutations produced partial resistance of galactokinase to glucose repression only when one or both of these mutations were combined with a GAL81 or a gal80 mutation. The GAL82 and GAL83 mutations are probably specific for expression of the Leloir pathway and related enzymes, because they do not affect the synthesis of alpha-D-glucosidase, invertase, or isocitrate lyase.

Isolation of Saccharomyces cerevisiae mutants constitutive for invertase synthesis

A new method for detecting invertase activity in Saccharomyces cerevisiae colonies was used to screen for mutants resistant to catabolite repression of invertase. Mutations causing the highest level of derepression were located in two previously identified genes, cyc8 and tup1. Several of the cyc8 mutations, notably cyc8-10 and cyc8-11, were temperature dependent, repressed at 23 degrees C, and derepressed at 37 degrees C. The kinetics of derepression of invertase mRNA in cyc8-10 cells shifted from 23 to 37 degrees C was determined by Northern blots. Invertase mRNA was detectable at 5 min after the shift, with kinetics of accumulation very similar to that of wild-type cells shifted from high-glucose to low-glucose medium. Assays of representative enzymes showed that many but not all glucose-repressible enzymes are derepressed in both cyc8 and tup1 mutants. cyc8 and tup1 appear to be the major negative regulatory genes controlling catabolite repression in yeasts.

Control of cell growth and division in Saccharomyces cerevisiae

Considerable advances have been made in recent years in our understanding of the biochemistry of protein and nucleic acid synthesis and, particularly, the molecular biology of gene expression in eukaryotes. The yeast Saccharomyces cerevisiae, and to a lesser extent Schizosaccharomyces pombe, has had a preeminent role as a focus for these studies, principally because of the facility with which these organisms can be experimentally manipulated biochemically and genetically. This review will be designed to critically examine and integrate recent advances in several vital areas of regulatory control of enzyme synthesis in yeast: structure and organization of DNA, transcriptional regulation, post-transcriptional modification, control of translation, post-translational modification and secretion, and cell-cycle modulation. It will attempt to emphasize and illustrate, where detailed information is available, principal underlying molecular mechanisms, and it will attempt to make relevant comparisons of this material to inferred and demonstrated facets of regulatory control of enzyme and protein synthesis in higher eukaryotes.

Cell cycle-dependent expression of thymidylate synthase in Saccharomyces cerevisiae

Synchronous populations of Saccharomyces cerevisiae cells, generated by two independent methods, have been used to show that thymidylate synthase, in contrast to the vast majority of cellular proteins thus far examined, fluctuates periodically during the S. cerevisiae cell cycle. The enzyme, as assayed by two different methods, accumulated during S period and peaked in mid to late S phase, and then its level dropped. These observations suggest that both periodic synthesis and the instability of the enzyme contribute to the activity profile seen during the cell cycle. Accumulation of thymidylate synthase is determined at the level of its transcript, with synthase-specific mRNA levels increasing at least 10-fold to peak near the beginning of S period and then falling dramatically to basal levels after the onset of DNA synthesis. This mRNA peak coincided with the time during the cell cycle when thymidylate synthase levels were increasing maximally and immediately preceded the peak of DNA synthesis, for which the enzyme provides precursor dTMP.

Neurospora glucamylase and a mutant affected in its regulation

Neurospora glucamylase is a glucose-repressible extracellular enzyme. The enzyme was purified to homogeneity and found to have a molecular weight of 82,000 and to release glucose from either maltose or amylose. The rate of glucamylase synthesis increases more than 100-fold when cells are transferred from a glucose-containing medium to a glucose-free medium. Increased from a glucose-containing medium to a glucose-free medium. Increased production of glucamylase begins within 30 min of the transfer. Glucamylase is rapidly secreted into the medium. A mutant affecting the ability of glucose to repress the synthesis of the glucose-repressible extracellular enzymes glucamylase and invertase has been isolated and studied. The mutant constitutively synthesizes and secretes a glucamylase which is indistinguishable from the wild-type enzyme.

Cell cycle-dependent expression of thymidylate synthase in Saccharomyces cerevisiae

Synchronous populations of Saccharomyces cerevisiae cells, generated by two independent methods, have been used to show that thymidylate synthase, in contrast to the vast majority of cellular proteins thus far examined, fluctuates periodically during the S. cerevisiae cell cycle. The enzyme, as assayed by two different methods, accumulated during S period and peaked in mid to late S phase, and then its level dropped. These observations suggest that both periodic synthesis and the instability of the enzyme contribute to the activity profile seen during the cell cycle. Accumulation of thymidylate synthase is determined at the level of its transcript, with synthase-specific mRNA levels increasing at least 10-fold to peak near the beginning of S period and then falling dramatically to basal levels after the onset of DNA synthesis. This mRNA peak coincided with the time during the cell cycle when thymidylate synthase levels were increasing maximally and immediately preceded the peak of DNA synthesis, for which the enzyme provides precursor dTMP.

Catabolite repression in yeasts is not associated with low levels of cAMP

relationship between levels of cAMP and catabolite repression in yeasts has been investigated. Strains of Saccharomyces cerevisiae, Schizosaccharomyces pombe and Kluyveromyces fragilis were used. The yeasts were grown on different carbon sources to attain various degrees of repression. Galactose repressed as much as glucose, while maltose was less effective. Full derepression was achieved with ethanol. The enzymes tested were fructose-bisphosphatase, malate dehydrogenase, glutamate dehydrogenase (NAD dependent), cytochrome oxidase and isocitrate lyase (this last enzyme was found to be absent in Schizosaccharomyces). The levels of cAMP were 2-3 times higher in the repressed conditions than in the derepressed ones. It is therefore concluded that in yeasts catabolite repression is not mediated by a lowering of the intracellular concentration of cAMP.

Isolation and characterization of a pleiotropic glucose repression resistant mutant of Saccharomyces cerevisiae

A new mutation has been described which confers resistance to catabolite repression in Saccharomyces cerevisiae. The mutant allele, termed grr-1 for glucose repression-resistant, is characterized by insensitivity to glucose repression for the cytoplasmic enzymes invertase, maltase, and galactokinase, as well as the mitochondrial enzyme cytochrome c oxidase. Hexokinase levels in grr-1 mutants are approximately 3-fold higher than the corresponding activity of the parental strain. Although the grr-1 allele is expressed phenotypically similarly to the hex-1 (hxk-2) and hex-2 mutations described by Entian et al. (1977) and Zimmermann and Scheel (1977) respectively, we have shown genetically and physiologically that grr-1 represents a new class of mutation.

Comparison of the levels of the 21S mitochondrial rRNA in derepressed and glucose-repressed Saccharomyces cerevisiae

A cDNA preparation, synthesized by using Saccharomyces cerevisiae mitochondrial RNA as template and oligodeoxythymidylic acid as primer, was found to specifically hybridize to the mitochondrial 21S rRNA by the following criteria: (i) it hybridizes only to the 21S RNA species in mitochondrial RNA and not to RNA from a [rho0] mutant, and (ii) it hybridizes to fragments in restriction digests of mitochondrial DNA that contain the 21S rRNA gene but not to nuclear DNA. This cDNA was used as a probe to demonstrate that a 2.6-fold decrease in the cellular level of the mitochondrial large rRNA is associated with glucose repression of mitochondrial function in S. cerevisiae. A corresponding decrease in the level of mitochondrial DNA was not observed.

Comparison of the levels of the 21S mitochondrial rRNA in derepressed and glucose-repressed Saccharomyces cerevisiae

A cDNA preparation, synthesized by using Saccharomyces cerevisiae mitochondrial RNA as template and oligodeoxythymidylic acid as primer, was found to specifically hybridize to the mitochondrial 21S rRNA by the following criteria: (i) it hybridizes only to the 21S RNA species in mitochondrial RNA and not to RNA from a [rho0] mutant, and (ii) it hybridizes to fragments in restriction digests of mitochondrial DNA that contain the 21S rRNA gene but not to nuclear DNA. This cDNA was used as a probe to demonstrate that a 2.6-fold decrease in the cellular level of the mitochondrial large rRNA is associated with glucose repression of mitochondrial function in S. cerevisiae. A corresponding decrease in the level of mitochondrial DNA was not observed.

Recessive mutations conferring resistance to carbon catabolite repression of galactokinase synthesis in Saccharomyces cerevisiae

A total of 37 recessive mutations showing enhanced resistance to the glucose repression of galactokinase synthesis have been isolated by a selection procedure with a GAL81 gal7 double mutant. These mutations were grouped into three different complementation classes. One class, reg1, contains mutants arising from mutations at a site close to, but complementing, the gal3 locus. The reg1 mutant also showed resistance to the glucose repression of invertase synthesis but not to that of alpha-D-glucosidase. The two other classes were identified as arising from recessive mutations at the GAL82 locus and the GAL83 locus, respectively, at which various dominant mutations were isolated previously. When in a constitutive background due to the GAL81 or gal80 mutation, the GAL82 and GAL83 mutations did not show a mutually additive effect on the resistance to glucose repression of galactokinase synthesis, while the reg1 and GAL82 (or GAL83) mutations did. Based upon the specific behavior of cells with various genotypes for the above genes in response to the concentration of galactose and glucose in the medium, we propose a model involving three independent circuits for glucose signals in the regulation of the structural genes for the galactose pathway enzymes.

Asparagine-linked carbohydrate does not determine the cellular location of yeast vacuolar nonspecific alkaline phosphatase

The nonspecific alkaline phosphatase of Saccharomyces sp. strain 1710 has been shown by phosphatase cytochemistry to be exclusively located in the vacuole, para-Nitrophenyl phosphate-specific alkaline phosphatase is not detected by this procedure because the activity of this enzyme is sensitive to the fixative agent, glutaraldehyde. To determine whether the oligosaccharide of nonspecific alkaline phosphatase is necessary to transport the enzyme into the vacuole, protoplasts were derepressed in the absence or in the presence of tunicamycin, an antibiotic which interferes with the glycosylation of asparagine residues in proteins. The location of the enzyme in the tunicamycin-treated protoplasts, as determined by electron microscopy and subcellular fractionation, was identical to its location in control protoplasts. In addition, carbohydrate-free alkaline phosphatase was found in vacuoles from tunicamycin-treated protoplasts. Our findings indicate that the asparagine-linked carbohydrate moiety does not determine the cellular location of the enzyme.

New genes involved in carbon catabolite repression and derepression in the yeast Saccharomyces cerevisiae

A mutation causing resistance to carbon catabolite repression in gene HEX2, mutant allele hex2-3, causes an extreme sensitivity to maltose when in combination with the genes necessary for maltose metabolism. This provided a convenient system for the selective isolation of mutations in genes specifically required for maltose metabolism and other genes involved in general carbon catabolite repression. In addition to reversion of the hex2-3 allele, mutations in three other genes were detected. These genes were called CAT1, CAT3, and MUR1 and in a mutated form abolished maltose inhibition caused by mutant allele hex2-3. Mutant alleles cat1 and cat3 also restored normal repression in the presence of the hex2-3 allele. Segregants having only mutant alleles cat1 or cat3 were obtained by tetrad analysis. These segregants could not grow on nonfermentable carbon sources. Mutant alleles of gene CAT1 were allelic to a mutant allele cat1-1 previously isolated (Zimmermann et al., Mol. Gen. Genet. 151:95-103). Such mutants prevented derepression not only of the maltose catabolizing system, the selected property, but also of glyoxylate shunt and gluconeogenic enzymes. However, respiratory activities and invertase formation were not affected under derepressing conditions. cat3 mutants had the same phenotypic properties as cat1 mutants. This showed that carbon metabolism in yeast cells is under a very complex and ramified control of repressing and derepressing genes, which are interdependent.

Genetic analysis of the pyruvate decarboxylase reaction in yeast glycolysis

Six different pyruvate decarboxylase mutants of Saccharomyces cerevisiae were isolated. They belong to two unlinked complementation groups. Evidence is presented that one group is affected in a structural gene. The fact that five of the six mutants had residual pyruvate decarboxylase activity provided the opportunity for an intensive physiological characterization. It was shown that the loss of enzyme activity in vitro is reflected in a lower fermentation rate, an increased pyruvate secretion, and slower growth on a 2% glucose medium. The different effects of antimycin A on leaky mutants grown on ethanol versus the same mutants grown on glucose support the view that glucose induces some of the glycolytic enzymes, especially pyruvate decarboxylase.

Regulation of energy metabolism in yeast. Inheritance of a pleiotropic mutation causing defects in metabolism of energy reserves, ethanol utilization and formation of cytochrome a.a3

The recessive, nuclear gene mutation glc1, which causes glycogen deficiency in Saccharomyces cerevisiae, is highly pleiotropic. Studies of the inheritance of glc1 revealed two classes of phenotypic characteristics: I. Traits invariably associated with the mutant gene and II. Traits whose expressions require the presence of glc1 and one or more additional genes. Class I traits include glycogen deficiency and the loss of capacity to accumulate trehalose in nonproliferating conditions. Traits in the second class include a decreased rate of growth on ethanol medium, a deficiency in cytochrome a.a3 and an enhanced accumulation of pigment, probably a metalloporphyrin. Constructed strains containing both glc1 and the constitutive maltose fermentation gene MAL4c can accumulate trehalose but not glycogen during growth on glucose. However, accumulated trehalose is degraded when cells are exposed to nonproliferating conditions. It is proposed that the glc1 mutation affects a regulatory system, probably involving a protein kinase and/or protein phosphatase, which regulates glycogen synthase and trehalase. Independent regulation of trehalose synthesis by a system controlled by MAL4c is indicated.

Cyclic AMP may not be involved in catabolite repression in Saccharomyes cerevisiae: evidence from mutants capable of utilizing it as an adenine source

Mutants able to utilize 5'-AMP or cyclic AMP as the adenine source were isolated from an ade6 ade10 double mutant by ethyl methane sulfonate mutagenesis. A single amp1 mutation, primarily selected on 5'-AMP medium, confers the phenotype for utilization of exogenous 5'-AMP as the adenine source. From the ade6 ade10 amp1 triple mutant, a mutant able to utilize cyclic AMP was isolated, and the mutant phenotype was proven to be due to the simultaneous occurrence of triple mutations designated as cam1, cam2, and cam3. The cam3 mutation, but not cam1 or cam2, also confers the phenotype for utilizing 5'-AMP, the same phenotype as the amp1 mutation. All of these mutations are recessive to the respective wild-type counterparts. Cells having the ade6 ade10 amp1 cam1 cam2 cam3 genotype showed significant ability to take up exogenous cyclic AMP, whereas no differences were observed in cyclic AMP phosphodiesterase activity in comparison with that of the original strains used in the mutant isolation. Since glucose severely repressed galactokinase synthesis in the constitutive GAL81 mutant having the ade6 ade10 amp1 cam1 cam2 cam3 genotype, irrespective of the presence or absence of cyclic AMP in the medium, it was suggested that cyclic AMP is not involved in the mechanism of catabolite repression in Saccharomyces cerevisiae. It does, however, have a stimulative effect on the galactokinase synthesis in the GAL81 mutant in the absence of glucose.

Distinct repressible mRNAs for cytoplasmic and secreted yeast invertase are encoded by a single gene

We have studied regulation of invertase putative structural genes (SUC) in S. cerevisiae and the synthetic relationship between secreted, glycosylated invertase (E.C.3.2.1.26) and the cytoplasmic, nonglycosylated form of the enzyme. Using immunoprecipitation and gel electrophoresis, we have analyzed invertase polypeptides and glycopeptides synthesized in vitro and in vivo. Analysis of size-fractionated mRNA from a SUC2 strain has shown that three mature, catabolite-repressible mRNA species direct the in vitro synthesis of three invertase polypeptides that have differing molecular weights. Two of these polypeptides, P63 and P62 (63 and 62 kd), are larger than the polypeptides of the secreted enzyme and are cotranslationally processed by microsomal membranes in vitro to yield secreted invertase glycopeptides (GP90 and GP87). The smallest polypeptide, P60 (60 kd), which comigrates electrophoretically with cytoplasmic invertase, is not processed. Posttranslationally, a microsomal-membrane detergent extract removes approximately 20 aminoacids from P62 but not from P60. In vitro translations of mRNAs from a genetically confirmed suc3 mutant strain, from the parental SUC3 strain and from derivative meiotic segregants have shown that the three polypeptides (and therefore three mRNA species) are encoded by one gene. Analysis of in vivo radiolabeled invertase from the same SUC3 and suc3 strains has verified that the SUC3 locus contains the structural gene for secreted and cytoplasmic invertase. Through the derepressed synthesis of multiple primary or processed transcripts, the SUC2 and SUC3 genes are regulated to produce multiple invertase polypeptides. The larger two polypeptides appear to be processed and secreted to yield glycosylated invertase, while the smallest remains in the cytoplasm.

Isolation and characterization of dominant mutations resistant to carbon catabolite repression of galactokinase synthesis in Saccharomyces cerevisiae

Seven dominant mutations showing greatly enhanced resistance to the glucose repression of galactokinase synthesis have been isolated from GAL81 mutants, which have the constitutive phenotype but are still strongly repressible by glucose for the synthesis of the Leloir enzymes. These glucose-resistant mutants were due to semidominant mutations at either of two loci, GAL82 and GAL83. Both loci are unlinked to the GAL81- gal4, gal80, or gal7 X gal10 X gal1 locus or to each other. The GAL83 locus was mapped on chromosome V at a site between arg9 and cho1. The GAL82 and GAL83 mutations produced partial resistance of galactokinase to glucose repression only when one or both of these mutations were combined with a GAL81 or a gal80 mutation. The GAL82 and GAL83 mutations are probably specific for expression of the Leloir pathway and related enzymes, because they do not affect the synthesis of alpha-D-glucosidase, invertase, or isocitrate lyase.

Induction and genetics of two alpha-galactosidase activities in Saccharomyces cerevisiae

The induction of alpha-galactosidase of Saccharomyces cerevisiae was investigated. We have demonstrated the existence of inducible internal and external alpha-galactosidase activities and have studied the relationship between the two alpha-galactosidases by examining a mutant strain which lacks both the internal and external activities. The mutant possesses a mutation in a single locus (mel1-1) which does not affect the synthesis of the other galactose pathway enzymes or the ability of the yeast to grow on media containing only galactose as the carbon source. Genetic studies of the mutant indicate that mel 1-1 is recessive and allelic to the wild tye allele for melibiose fermentation Mel 1.

Extracellular protein release and its response to pH level in Saccharomyces cerevisiae

Saccharomyces cerevisiae grown in batch culture at pH 5.5 releases 0.1 to 0.2 pg protein per cell to the external medium over a period of four to five days, final concentration 20-40 micrograms/ml. Cells grown at pH 3.0 release 10-fold this quantity (1-2 pg/cell, final concentration 100-200 micrograms/ml). A kinetic model based on published behavior of periplasmic protein gave a good fit to the observed kinetics of exoprotein yield. The electrophoretic pattern of exoprotein differed from that of cell lysate protein, and exoprotein synthesis was apparently limited to early stages of the life cycle. These results are consistent with the identification of exoprotein as periplasmic protein released to the external medium through the cell wall. Analysis of the observed kinetics of exoprotein yield, utilizing the kinetic model suggests that the greater exoprotein production of cells grown at pH 3.0 was due entirely to greater synthesis of periplasmic proteins while the fraction of periplasmic protein released per unit time was greater for cells grown at pH 5.5. The latter conclusion is supported by thicker cell walls of cells grown at pH 3.0 as observed by electron microscopy. At an applied level the apparent limitation of exoprotein synthesis to the first few hours of cell life, the slow leakage of exoprotein through the cell wall, and the dilute nature of a yeast suspension do not favor the utilization of yeast cells for direct conversion of substrate into protein released to the external medium.

Pleiotropic glucose repression-resistant mutation in Saccharomyces carlesbergensis

We describe the characterization of a mutation of the locus GLR1. This mutation allowed for (i) the glucose repression-insensitive synthesis ot the enzymes maltase, galactokinase, alpha-galactosidase, reduced nicotinamide adenine dinucleotide-cytochrome c reductase, and cytochrome c oxidase and (ii) growth on maltose in the presence of the gratuitous glucose repressor D-glucosamine. The glucosamine resistance cosegregated with the glucose-insensitive synthesis of the enzymes listed above. In addition, crosses between the glucosamine-resistant mutant and isogenic sensitive strains gave only tetrads containing two resistant and two sensitive spores. Thus, a single pleiotropic mutation is responsible for both phenotypes. We call the locus GLR1, for glucose regulation, and the glucose repression-insensitive mutation glr1-1.

Glycolytic enzymes and intermediates in carbon catabolite repression mutants of Saccharomyces cerevisiae

Glycolytic parameters were determined in recessive yeast mutants with partial defects in carbon catabolite repression. Specific activities of pyruvate kinase and pyruvate decarboxylase in glucose grown cells of all mutant and wild type strains were 4--5 times higher than in ethanol grown cells. Mutants of gene HEX1 had a reduced hexose phosphorylating activity on all media whereas those of gene HEX2 had elevated levels but only in glucose grown cells. Mutants of gene CAT80 were normal in this respect. All other glycolytic enzymes were normal in all mutants. This was also true for glycolytic intermediates. Only hex1-mutants showed a reduced fermentation of repressing sugars. The three genes appear to be involved in catabolite repression of several but not of all repressible enzymes. Even though all three types of mutants show a limited overlap in their effects on certain enzymes, they still are distinctly different in their action spectra. Carbon catabolite repression apparently does not depend on the sole accumulation of glycolytic intermediates. The activity of the products of the three genes HEX1, HEX2 and CAT80 are required directly or indirectly for triggering carbon catabolite repression. Even a small segment of carbon catabolite repression is controlled by several genes with regulatory functions indicating that the entire regulatory circuit is highly complex.

Genetic control of invertase formation in Saccharomyces cerevisiae. II. Isolation and characterization of mutants conferring invertase hyperproduction in strain EK-6B carrying the SUC3 gene

Invertase formation in the yeast Saccharomyces cerevisiae is subject to repression by hexoses in the growth medium. Mutagen-induced (ethyl methanesulfonate or N-methyl-N-nitro-nitrosoguanidine) invertase hyperproducer mutants have been derived from the SUC3 MAL3 strain EK-6B by selecting for their ability to grow on media containing the sugar raffinose plus 2-deoxy-D-glucose (2DG). Raffinose like sucrose is a betta-fructoside which can be hydrolyzed by yeast invertase (beta-fructoside which can be hydrolyzed by yeast invertase (beta-fructofuranoside fructohydrolase). These mutants, designated dgr, produce higher levels of invertase (pi-glucosidase levels are also elevated but to a lesser extent) under conditions normally repressing invertase biosynthesis in the parent. Invertases of mutants dgr2 and dgr3 are indistinguishable from that of EK-6B with respect to their Km's for sucrose and thermal labilities. Genetic studies revealed that dgr2 and dgr3 are recessive and unlinked to the SUC3 gene.

Relationship of large and small invertases in Saccharomyces: mutant selectively deficient in small invertase

A mutant strain of Saccharomyces cerevisiae (D10-ER1) has been isolated after a two-step mutagenesis of strain 4059-358D (SUC 1) using ethyl methane sulfonate. Cells of this new strain produced a level of total invertase equaling that of 4059 but contained only trace amounts of the small, internal, aglycan form of the enzyme (less than 0.1% of total in D10-ER1 compared with 6% in 4059). When D10.ER1 was crossed with an invertase-hyperproducing strain dgr3 (SUC3), progeny were isolated (HZ400-5A and HZ400-2C) in which levels of total invertase had at least quadrupled. The percentage of small invertase, however, remained insignificant. Levels of small invertase in strain HZ400-5A were determined by affinity chromatography on conconavalin A-Sepharose, gel permeation chromatography, and isopycnic centrifugation in CsCl. The large invertase of the SUC1 yeasts described here was found to contain a form apparently greater in size than the large invertase of the SUC2 strain FH4C; this probably reflects a higher content of carbohydrate. The overall results of this study do not support a direct structural relationship between large and small invertases. The implications on invertase biosynthesis and structure are discussed.

A yeast mutant with glucose-resistant formation of mitochondrial enzymes

Yeast mutants with glucose-insensitive formation of mitochondrial enzymes were isolated starting with a strain completely lacking alcohol dehydrogenase activity. The mutations could uniquely be attributed to a single nuclear gene, designated CCR80. They were largely dominant. Glucose-resistant enzyme formation was most prominent with regard to mitochondrial enzymes succinate dehydrogenase and NADH: cytochrome c oxidoreductase. The effect of CCR80r mutations was rather small but significant on the gluconeogenetic enzymes isocitrate lyase, malate synthase and fructose-1,6-bisphosphatase and on invertase synthesis. The repressive effect of maltose in CCR80r mutants was also reduced showing that glucose-resistance is not caused by a mere hexose uptake defect. This regulatory disorders were not accompanied by reduced levels of glycolytic enzymes or drastically altered levels of glycolytic intermediates. Aerobic fermentation of glucose was almost completely inhibited in the mutants; anaerobic glucose degradation was reduced but not completely abolished. Therefore, the mutants appear to be altered in the regulation of glycolysis. A largely glucose-resistant synthesis of respiratory enzymes is obviously a corollary of this alteration.

Preparation of mutants of Trichoderma reesei with enhanced cellulase production

The development of an agar plate screening technique has allowed the isolation of a range of mutants of Trichoderma reesei capable of synthesizing cellulase under conditions of high catabolite repression. The properties of one of these mutants (NG-14) is described to illustrate the use of this technique. NG-14 produced five times the filter paper-degrading activity per ml of culture medium and twice the specific activity per mg of excreted protein in submerged culture when compared with the best existing mutant, QM9414. NG-14 also showed enhanced endo-beta-glucanase and beta-glucosidase production. Although these mutants were isolated as cellulase producers in the presence of 5% glycerol on agar plates, in similar liquid medium, NG-14 exhibits only partial derepression of the cellulase complex. Since the proportions of filter paper activity, endo-beta-glucanase, and cellobiase were not the same in mutants NG-14 and QM9414, and the yields of each enzyme under conditions repressive for cellulase synthesis were different, differential control of each enzyme of the cellulase complex is implied. These initial results suggest that the selective technique for isolating hyper-cellulase-producing mutants of Trichoderma will be of considerable use in the development of commercially useful cellulolytic strains.