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

Drug resistance in diploid yeast is acquired through dominant alleles, haploinsufficiency, gene duplication and aneuploidy

Previous studies of adaptation to the glucose analog, 2-deoxyglucose, by Saccharomyces cerevisiae have utilized haploid cells. In this study, diploid cells were used in the hope of identifying the distinct genetic mechanisms used by diploid cells to acquire drug resistance. While haploid cells acquire resistance to 2-deoxyglucose primarily through recessive alleles in specific genes, diploid cells acquire resistance through dominant alleles, haploinsufficiency, gene duplication and aneuploidy. Dominant-acting, missense alleles in all three subunits of yeast AMP-activated protein kinase confer resistance to 2-deoxyglucose. Dominant-acting, nonsense alleles in the REG1 gene, which encodes a negative regulator of AMP-activated protein kinase, confer 2-deoxyglucose resistance through haploinsufficiency. Most of the resistant strains isolated in this study achieved resistance through aneuploidy. Cells with a monosomy of chromosome 4 are resistant to 2-deoxyglucose. While this genetic strategy comes with a severe fitness cost, it has the advantage of being readily reversible when 2-deoxyglucose selection is lifted. Increased expression of the two DOG phosphatase genes on chromosome 8 confers resistance and was achieved through trisomies and tetrasomies of that chromosome. Finally, resistance was also mediated by increased expression of hexose transporters, achieved by duplication of a 117 kb region of chromosome 4 that included the HXT3, HXT6 and HXT7 genes. The frequent use of aneuploidy as a genetic strategy for drug resistance in diploid yeast and human tumors may be in part due to its potential for reversibility when selection pressure shifts.

Thermo-adaptive evolution to generate improved Saccharomyces cerevisiae strains for cocoa pulp fermentations

Cocoa pulp fermentation is a consequence of the succession of indigenous yeasts, lactic acid bacteria and acetic acid bacteria that not only produce a diversity of metabolites, but also cause the production of flavour precursors. However, as such spontaneous fermentations are less reproducible and contribute to produce variability, interest in a microbial starter culture is growing that could be used to inoculate cocoa pulp fermentations. This study aimed to generate robust S. cerevisiae strains by thermo-adaptive evolution that could be used in cocoa fermentation. We evolved a cocoa strain in a sugary defined medium at high temperature to improve both fermentation and growth capacity. Moreover, adaptive evolution at high temperature (40 °C) also enabled us to unveil the molecular basis underlying the improved phenotype by analysing the whole genome sequence of the evolved strain. Adaptation to high-temperature conditions occurred at different genomic levels, and promoted aneuploidies, segmental duplication, and SNVs in the evolved strain. The lipid profile analysis of the evolved strain also evidenced changes in the membrane composition that contribute to maintain an appropriate cell membrane state at high temperature. Our work demonstrates that experimental evolution is an effective approach to generate better-adapted yeast strains at high temperature for industrial processes.

'Sugarcoating' 2-deoxyglucose: mechanisms that suppress its toxic effects

Yeast and cancer cells are metabolically similar as they use fermentation of glucose as a primary means of generating energy. Reliance on glucose fermentation makes both of these cell types highly sensitive to the toxic glucose analog, 2-deoxyglucose. Here we review the cellular and metabolic pathways that play a role in 2-deoxyglucose sensitivity and discuss how the modifications to these pathways result in acquisition of 2-deoxyglucose resistance. Insights gained from genetic and proteomic studies in yeast provide new ideas for the design of combinatorial therapies for cancer treatment.

Cellular toxicity of the metabolic inhibitor 2-deoxyglucose and associated resistance mechanisms

Most malignant cells display increased glucose absorption and metabolism compared to surrounding tissues. This well-described phenomenon results from a metabolic reprogramming occurring during transformation, that provides the building blocks and supports the high energetic cost of proliferation by increasing glycolysis. These features led to the idea that drugs targeting glycolysis might prove efficient in the context of cancer treatment. One of these drugs, 2-deoxyglucose (2-DG), is a synthetic glucose analog that can be imported into cells and interfere with glycolysis and ATP generation. Its preferential targeting to sites of cell proliferation is supported by the observation that a derived molecule, 2-fluoro-2-deoxyglucose (FDG) accumulates in tumors and is used for cancer imaging. Here, we review the toxicity mechanisms of this drug, from the early-described effects on glycolysis to its other cellular consequences, including inhibition of protein glycosylation and endoplasmic reticulum stress, and its interference with signaling pathways. Then, we summarize the current data on the use of 2-DG as an anti-cancer agent, especially in the context of combination therapies, as novel 2-DG-derived drugs are being developed. We also show how the use of 2-DG helped to decipher glucose-signaling pathways in yeast and favored their engineering for biotechnologies. Finally, we discuss the resistance strategies to this inhibitor that have been identified in the course of these studies and which may have important implications regarding a medical use of this drug.

Spontaneous mutations that confer resistance to 2-deoxyglucose act through Hxk2 and Snf1 pathways to regulate gene expression and HXT endocytosis

Yeast and fast-growing human tumor cells share metabolic similarities in that both cells use fermentation of glucose for energy and both are highly sensitive to the glucose analog 2-deoxyglucose. Spontaneous mutations in S. cerevisiae that conferred resistance to 2-deoxyglucose were identified by whole genome sequencing. Missense alleles of the HXK2, REG1, GLC7 and SNF1 genes were shown to confer significant resistance to 2-deoxyglucose and all had the potential to alter the activity and or target selection of the Snf1 kinase signaling pathway. All three missense alleles in HXK2 resulted in significantly reduced catalytic activity. Addition of 2DG promotes endocytosis of the glucose transporter Hxt3. All but one of the 2DG-resistant strains reduced the 2DG-mediated hexose transporter endocytosis by increasing plasma membrane occupancy of the Hxt3 protein. Increased expression of the DOG (deoxyglucose) phosphatases has been associated with resistance to 2-deoxyglucose. Expression of both the DOG1 and DOG2 mRNA was elevated after treatment with 2-deoxyglucose but induction of these genes is not associated with 2DG-resistance. RNAseq analysis of the transcriptional response to 2DG showed large scale, genome-wide changes in mRNA abundance that were greatly reduced in the 2DG resistant strains. These findings suggest the common adaptive response to 2DG is to limit the magnitude of the response. Genetic studies of 2DG resistance using the dominant SNF1-G53R allele in cells that are genetically compromised in both the endocytosis and DOG pathways suggest that at least one more mechanism for conferring resistance to this glucose analog remains to be discovered.

Yeast and fast-growing human tumor cells share metabolic similarities in that both cells use fermentation of glucose for energy and both are highly sensitive to the glucose analog 2-deoxyglucose. Another similarity between yeast cells and human tumor cells is that both cells can acquire resistance to 2-deoxyglucose, an outcome that can limit the usefulness of some cancer therapeutics. In this study, we used bakers’ yeast as a model organism to better understand the mechanism of toxicity and acquisition of resistance to 2-deoxyglucose. Spontaneous mutations in S. cerevisiae that conferred resistance to 2-deoxyglucose were isolated and identified by whole genome sequencing, a technology that was not available until recently. Our studies indicate that 2-deoxyglucose becomes toxic after it is phosphorylated by an enzyme called hexokinase. One important route to resistance is to reduce hexokinase activity. Other parallel pathways to resistance include increased expression of a hydrolase that degrades the toxic metabolite, altered localization of glucose transporters and altered glucose signal transduction pathways.

Identification of nuclear genes affecting 2-Deoxyglucose resistance in Schizosaccharomyces pombe

2-Deoxyglucose (2-DG) is a toxic glucose analog. To identify genes involved in 2-DG toxicity in Schizosaccharomyces pombe, we screened a wild-type overexpression library for genes which render cells 2-DG resistant. A gene we termed odr1, encoding an uncharacterized hydrolase, led to strong resistance and altered invertase expression when overexpressed. We speculate that Odr1 neutralizes the toxic form of 2-DG, similar to the Saccharomyces cerevisiae Dog1 and Dog2 phosphatases which dephosphorylate 2-DG-6-phosphate synthesized by hexokinase. In a complementary approach, we screened a haploid deletion library to identify 2-DG-resistant mutants. This screen identified the genes snf5, ypa1, pas1 and pho7. In liquid medium, deletions of these genes conferred 2-DG resistance preferentially under glucose-repressed conditions. The deletion mutants expressed invertase activity more constitutively than the control strain, indicating defects in the control of glucose repression. No S. cerevisiae orthologs of the pho7 gene is known, and no 2-DG resistance has been reported for any of the deletion mutants of the other genes identified here. Moreover, 2-DG leads to derepressed invertase activity in S. pombe, while in S. cerevisiae it becomes repressed. Taken together, these findings suggest that mechanisms involved in 2-DG resistance differ between budding and fission yeasts.

Genetic analysis of resistance and sensitivity to 2-deoxyglucose in Saccharomyces cerevisiae

Aerobic glycolysis is a metabolic pathway utilized by human cancer cells and also by yeast cells when they ferment glucose to ethanol. Both cancer cells and yeast cells are inhibited by the presence of low concentrations of 2-deoxyglucose (2DG). Genetic screens in yeast used resistance to 2-deoxyglucose to identify a small set of genes that function in regulating glucose metabolism. A recent high throughput screen for 2-deoxyglucose resistance identified a much larger set of seemingly unrelated genes. Here, we demonstrate that these newly identified genes do not in fact confer significant resistance to 2-deoxyglucose. Further, we show that the relative toxicity of 2-deoxyglucose is carbon source dependent, as is the resistance conferred by gene deletions. Snf1 kinase, the AMP-activated protein kinase of yeast, is required for 2-deoxyglucose resistance in cells growing on glucose. Mutations in the SNF1 gene that reduce kinase activity render cells hypersensitive to 2-deoxyglucose, while an activating mutation in SNF1 confers 2-deoxyglucose resistance. Snf1 kinase activated by 2-deoxyglucose does not phosphorylate the Mig1 protein, a known Snf1 substrate during glucose limitation. Thus, different stimuli elicit distinct responses from the Snf1 kinase.

Role of glucose signaling in yeast metabolism

In this article, knowledge concerning the relation between uptake of and signaling by glucose in the yeast Saccharomyces cerevisiae is reviewed and compared to the analogous process in prokaryotes. It is concluded that (much) more fundamental knowledge concerning these processes is required before rational redesign of metabolic fluxes from glucose in yeast can be achieved. (c) 1996 John Wiley & Sons, Inc.

Reg1 protein regulates phosphorylation of all three Snf1 isoforms but preferentially associates with the Gal83 isoform

The phosphorylation status of the Snf1 activation loop threonine is determined by changes in the rate of its dephosphorylation, catalyzed by the yeast PP1 phosphatase Glc7 in complex with the Reg1 protein. Previous studies have shown that Reg1 can associate with both Snf1 and Glc7, suggesting substrate binding as a mechanism for Reg1-mediated targeting of Glc7. In this study, the association of Reg1 with the three Snf1 isoforms was measured by two-hybrid analysis and coimmunoprecipitation. We found that Reg1 association with Snf1 occurred almost exclusively with the Gal83 isoform of the Snf1 complex. Nonetheless, Reg1 plays an important role in determining the phosphorylation status of all three Snf1 isoforms. We found that the rate of dephosphorylation for isoforms of Snf1 did not correlate with the amount of associated Reg1 protein. Functional chimeric β subunits containing residues from Gal83 and Sip2 were used to map the residues needed to promote Reg1 association with the N-terminal 150 residues of Gal83. The Gal83 isoform of Snf1 is the only isoform capable of nuclear localization. A Gal83-Sip2 chimera containing the first 150 residues of Gal83 was able to associate with the Reg1 protein but did not localize to the nucleus. Therefore, nuclear localization is not required for Reg1 association. Taken together, these data indicate that the ability of Reg1 to promote the dephosphorylation of Snf1 is not directly related to the strength of its association with the Snf1 complex.

The function of MoGlk1 in integration of glucose and ammonium utilization in Magnaporthe oryzae

Hexokinases are conserved proteins functioning in glucose sensing and signaling. The rice blast fungus Magnaporthe oryzae contains several hexokinases, including MoHxk1 (hexokinase) and MoGlk1 (glucokinase) encoded respectively by MoHXK1 and MoGLK1 genes. The heterologous expression of MoGlk1 and MoHxk1 in Saccharomyces cerevisiae confirmed their conserved functions. Disruption of MoHXK1 resulted in growth reduction in medium containing fructose as the sole carbon source, whereas disruption of MoGLK1 did not cause the similar defect. However, the ΔMoglk1 mutant displayed decreased proton extrusion and a lower biomass in the presence of ammonium, suggesting a decline in the utilization of ammonium. Additionally, the MoGLK1 allele lacking catalytic activity restored growth to the ΔMoglk1 mutant. Moreover, the expression of MoPMA1 encoding a plasma membrane H⁺-ATPase decreased in the ΔMoglk1 mutant that can be suppressed by glucose and G-6-P. Thus, MoGlk1, but not MoHxk1, regulates ammonium utilization through a mechanism that is independent from its catalytic activity.

Life in the midst of scarcity: adaptations to nutrient availability in Saccharomyces cerevisiae

Cells of all living organisms contain complex signal transduction networks to ensure that a wide range of physiological properties are properly adapted to the environmental conditions. The fundamental concepts and individual building blocks of these signalling networks are generally well-conserved from yeast to man; yet, the central role that growth factors and hormones play in the regulation of signalling cascades in higher eukaryotes is executed by nutrients in yeast. Several nutrient-controlled pathways, which regulate cell growth and proliferation, metabolism and stress resistance, have been defined in yeast. These pathways are integrated into a signalling network, which ensures that yeast cells enter a quiescent, resting phase (G0) to survive periods of nutrient scarceness and that they rapidly resume growth and cell proliferation when nutrient conditions become favourable again. A series of well-conserved nutrient-sensory protein kinases perform key roles in this signalling network: i.e. Snf1, PKA, Tor1 and Tor2, Sch9 and Pho85-Pho80. In this review, we provide a comprehensive overview on the current understanding of the signalling processes mediated via these kinases with a particular focus on how these individual pathways converge to signalling networks that ultimately ensure the dynamic translation of extracellular nutrient signals into appropriate physiological responses.

Full dynamic range proteome analysis of S. cerevisiae by targeted proteomics

The rise of systems biology implied a growing demand for highly sensitive techniques for the fast and consistent detection and quantification of target sets of proteins across multiple samples. This is only partly achieved by classical mass spectrometry or affinity-based methods. We applied a targeted proteomics approach based on selected reaction monitoring (SRM) to detect and quantify proteins expressed to a concentration below 50 copies/cell in total S. cerevisiae digests. The detection range can be extended to single-digit copies/cell and to proteins undetected by classical methods. We illustrate the power of the technique by the consistent and fast measurement of a network of proteins spanning the entire abundance range over a growth time course of S. cerevisiae transiting through a series of metabolic phases. We therefore demonstrate the potential of SRM-based proteomics to provide assays for the measurement of any set of proteins of interest in yeast at high-throughput and quantitative accuracy.

The early steps of glucose signalling in yeast

In the presence of glucose, yeast undergoes an important remodelling of its metabolism. There are changes in the concentration of intracellular metabolites and in the stability of proteins and mRNAs; modifications occur in the activity of enzymes as well as in the rate of transcription of a large number of genes, some of the genes being induced while others are repressed. Diverse combinations of input signals are required for glucose regulation of gene expression and of other cellular processes. This review focuses on the early elements in glucose signalling and discusses their relevance for the regulation of specific processes. Glucose sensing involves the plasma membrane proteins Snf3, Rgt2 and Gpr1 and the glucose-phosphorylating enzyme Hxk2, as well as other regulatory elements whose functions are still incompletely understood. The similarities and differences in the way in which yeasts and mammalian cells respond to glucose are also examined. It is shown that in Saccharomyces cerevisiae, sensing systems for other nutrients share some of the characteristics of the glucose-sensing pathways.

Different signalling pathways mediate glucose induction of SUC2, HXT1 and pyruvate decarboxylase in yeast

The glucose sensors Gpr1, Snf3 and Rgt2 generate the earliest signals produced by glucose in yeast. We showed that a lack of Gpr1 or Snf3/Rgt2 decreased by twofold the glucose induction of SUC2, but had no effect on the induction of pyruvate decarboxylase (Pdc). The induction of HXT1 was not affected by the absence of Gpr1. In an hxk1 hxk2 glk1 strain, high glucose fully induced SUC2, caused partial induction of HXT1 and had no effect on Pdc. In this strain, SUC2 induction was dependent on Gpr1, but HXT1 induction was not. Hxk2, required for the high expression of HXT1, was dispensable for the full induction of SUC2 or Pdc. These results indicate that glucose does not induce transcription through a single signalling pathway, but that several pathways may, in different combinations, regulate the transcription of different genes.

Glucose signaling in Saccharomyces cerevisiae

Eukaryotic cells possess an exquisitely interwoven and fine-tuned series of signal transduction mechanisms with which to sense and respond to the ubiquitous fermentable carbon source glucose. The budding yeast Saccharomyces cerevisiae has proven to be a fertile model system with which to identify glucose signaling factors, determine the relevant functional and physical interrelationships, and characterize the corresponding metabolic, transcriptomic, and proteomic readouts. The early events in glucose signaling appear to require both extracellular sensing by transmembrane proteins and intracellular sensing by G proteins. Intermediate steps involve cAMP-dependent stimulation of protein kinase A (PKA) as well as one or more redundant PKA-independent pathways. The final steps are mediated by a relatively small collection of transcriptional regulators that collaborate closely to maximize the cellular rates of energy generation and growth. Understanding the nuclear events in this process may necessitate the further elaboration of a new model for eukaryotic gene regulation, called "reverse recruitment." An essential feature of this idea is that fine-structure mapping of nuclear architecture will be required to understand the reception of regulatory signals that emanate from the plasma membrane and cytoplasm. Completion of this task should result in a much improved understanding of eukaryotic growth, differentiation, and carcinogenesis.

The type 1 phosphatase Reg1p-Glc7p is required for the glucose-induced degradation of fructose-1,6-bisphosphatase in the vacuole

Protein phosphatases play an important role in vesicular trafficking and membrane fusion processes. The type 1 phosphatase Glc7p and its regulatory subunit Reg1p were identified as required components in the glucose-induced targeting of the key gluconeogenic enzyme fructose-1,6-bisphosphatase (FBPase) to the vacuole for degradation. The interaction of Reg1p with Glc7p was important for the transport of FBPase from intermediate vacuole import and degradation (Vid) vesicles to vacuoles. The glc7-T152K mutant strain exhibited a reduced Reg1p binding along with defects in FBPase degradation and Vid vesicle trafficking to the vacuole. In this mutant, Vid vesicles were the most defective components, whereas the vacuole was also defective. Shp1p and Glc8p regulate Glc7p phosphatase activity and are required for FBPase degradation. In the Deltashp1 and Deltaglc8 strains, Reg1p-Glc7p interaction was not affected, suggesting that phosphatase activity is also necessary for FBPase degradation. Similar to those seen in the glc7-T152K mutant, the Deltashp1 and Deltaglc8 mutants exhibited severely defective Vid vesicles, but partially defective vacuoles. Taken together, our results suggest that Reg1p-Glc7p interaction and Glc7p phosphatase activity play a required role in the Vid vesicle to vacuole-trafficking step along the FBPase degradation pathway.

Biochemical pathways and mechanisms nitrogen, amino acid, and carbon metabolism

For both nitrogen and carbon metabolism there exist specific regulatory mechanisms to enable cells to assimilate a wide variety of nitrogen and carbon sources. Superimposed are regulatory circuits, the so called nitrogen and carbon catabolite regulation, to allow for selective use of "rich" sources first and "poor" sources later. Evidence points to the importance of specific regulatory mechanisms for short term adaptations, while generalized control circuits are used for long term modulation of nitrogen and carbon metabolism. Similarly a variety of regulatory mechanisms operate in amino acid metabolism. Modulation of enzyme activity and modulation of enzyme levels are the outstanding regulatory mechanisms. In prokaryotes, attenuation and repressor/operator control are predominant, besides a so called "metabolic control" which integrates amino acid metabolism into the overall nutritional status of the cells. In eukaryotic cells compartmentation of amino acid metabolites as well as of part of the pathways becomes an additional regulatory factor; pathway specific controls seem to be rare, but a complex regulatory network, the "general control of amino acid biosynthesis", coordinates the synthesis of enzymes of a number of amino acid biosynthetic pathways.

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.

Affinity partitioning of glucose-6-phosphate dehydrogenase and hexokinase in aqueous two-phase systems with free triazine dye ligands

The partitioning of glucose-6-phosphate dehydrogenase (G6PDH) (E.C. 1.1.1.49) and hexokinase (E.C. 2.7.1.1) in polyethylene glycol (PEG)-hydroxypropyl starch (PES) and PEG-phosphate aqueous two-phase systems was investigated with free triazine dyes, Cibacron Blue F3GA and Procion Red HE3B, as their affinity ligands. It was found that the free reactive triazine dyes, not bound to phase-forming polymers, preferentially partitioned in the top-PEG phase in the PEG-salt and PEG-PES systems. The effect of various parameters such as type and concentration of affinity ligands, pH of the system, molecular mass of PEG and phase composition on partitioning of the enzymes was estimated. Phosphate is a key factor affecting the enzyme partitioning in the PEG-PES system. Cibacron F3GA changed the partition coefficient of G6PDH from 0.73 to 1.59.

Systematic identification of the genes affecting glycogen storage in the yeast Saccharomyces cerevisiae: implication of the vacuole as a determinant of glycogen level

At the onset of nutrient limitation, the yeast Saccharomyces cerevisiae synthesizes glycogen to serve as a carbon and energy reserve. We undertook a systematic survey for the genes that affect glycogen accumulation by taking advantage of the strain deletion set generated by the Saccharomyces Genome Deletion Project. The strain collection analyzed contained some 4600 diploid homozygous null deletants, representing approximately 88% of all viable haploid disruptants. We identified 324 strains with low and 242 with elevated glycogen stores, accounting for 12.4% of the genes analyzed. The screen was validated by the identification of many of the genes known already to influence glycogen accumulation. Many of the mutants could be placed into coherent families. For example, 195 or 60% of the hypoaccumulators carry mutations linked to respiratory function, a class of mutants well known to be defective in glycogen storage. The second largest group consists of approximately 60 genes involved in vesicular trafficking and vacuolar function, including genes encoding 13 of 17 proteins involved in the structure or assembly of the vacuolar ATPase. These data are consistent with our recent findings that the process of autophagy has a significant impact on glycogen storage (Wang, Z., Wilson, W. A., Fujino, M. A., and Roach, P. J. (2001) Antagonistic controls of autophagy and glycogen accumulation by Snf1p, the yeast homolog of AMP-activated protein kinase, and the cyclin-dependent kinase Pho85p. Mol. Cell. Biol. 21, 5742-5752). Autophagy delivers glycogen to the vacuole, and we propose that the impaired vacuolar function associated with ATPase mutants (vma10 or vma22) results in reduced degradation and subsequent hyperaccumulation of glycogen.

Physiological properties of Saccharomyces cerevisiae from which hexokinase II has been deleted

Hexokinase II is an enzyme central to glucose metabolism and glucose repression in the yeast Saccharomyces cerevisiae. Deletion of HXK2, the gene which encodes hexokinase II, dramatically changed the physiology of S. cerevisiae. The hxk2-null mutant strain displayed fully oxidative growth at high glucose concentrations in early exponential batch cultures, resulting in an initial absence of fermentative products such as ethanol, a postponed and shortened diauxic shift, and higher biomass yields. Several intracellular changes were associated with the deletion of hexokinase II. The hxk2 mutant had a higher mitochondrial H(+)-ATPase activity and a lower pyruvate decarboxylase activity, which coincided with an intracellular accumulation of pyruvate in the hxk2 mutant. The concentrations of adenine nucleotides, glucose-6-phosphate, and fructose-6-phosphate are comparable in the wild type and the hxk2 mutant. In contrast, the concentration of fructose-1,6-bisphosphate, an allosteric activator of pyruvate kinase, is clearly lower in the hxk2 mutant than in the wild type. The results suggest a redirection of carbon flux in the hxk2 mutant to the production of biomass as a consequence of reduced glucose repression.

Mth1 receives the signal given by the glucose sensors Snf3 and Rgt2 in Saccharomyces cerevisiae

We have determined that the mutant genes DGT1-1 and BPC1-1, which impair glucose transport and catabolite repression in Saccharomyces cerevisiae, are allelic forms of MTH1. Deletion of MTH1 had only slight effects on the expression of HXT1 or SNF3, but increased expression of HXT2 in the absence of glucose. A two-hybrid screen revealed that the Mth1 protein interacts with the cytoplasmic tails of the glucose sensors Snf3 and Rgt2. This interaction was affected by mutations in Mth1 and by the concentration of glucose in the medium. A double mutant, snf3 rgt2, recovered sensitivity to glucose when MTH1 was deleted, thus showing that glucose signalling may occur independently of Snf3 and Rgt2. A model for the possible mode of action of Snf3 and Rgt2 is presented.

Functional analysis of the yeast Glc7-binding protein Reg1 identifies a protein phosphatase type 1-binding motif as essential for repression of ADH2 expression

In Saccharomyces cerevisiae, the protein phosphatase type 1 (PP1)-binding protein Reg1 is required to maintain complete repression of ADH2 expression during growth on glucose. Surprisingly, however, mutant forms of the yeast PP1 homologue Glc7, which are unable to repress expression of another glucose-regulated gene, SUC2, fully repressed ADH2. Constitutive ADH2 expression in reg1 mutant cells did require Snf1 protein kinase activity like constitutive SUC2 expression and was inhibited by unregulated cyclic AMP-dependent protein kinase activity like ADH2 expression in derepressed cells. To further elucidate the functional role of Reg1 in repressing ADH2 expression, deletions scanning the entire length of the protein were analyzed. Only the central region of the protein containing the putative PP1-binding sequence RHIHF was found to be indispensable for repression. Introduction of the I466M F468A substitutions into this sequence rendered Reg1 almost nonfunctional. Deletion of the central region or the double substitution prevented Reg1 from significantly interacting with Glc7 in two-hybrid analyses. Previous experimental evidence had indicated that Reg1 might target Glc7 to nuclear substrates such as the Snf1 kinase complex. Subcellular localization of a fully functional Reg1-green fluorescent protein fusion, however, indicated that Reg1 is cytoplasmic and excluded from the nucleus independently of the carbon source. When the level of Adr1 was modestly elevated, ADH2 expression was no longer fully repressed in glc7 mutant cells, providing the first direct evidence that Glc7 can repress ADH2 expression. These results suggest that the Reg1-Glc7 phosphatase is a cytoplasmic component of the machinery responsible for returning Snf1 kinase activity to its basal level and reestablishing glucose repression. This implies that the activated form of the Snf1 kinase complex must cycle between the nucleus and the cytoplasm.

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.

Carbon source-dependent phosphorylation of hexokinase PII and its role in the glucose-signaling response in yeast

The HXK2 gene is required for a variety of regulatory effects leading to an adaptation for fermentative metabolism in Saccharomyces cerevisiae. However, the molecular basis of the specific role of Hxk2p in these effects is still unclear. One important feature in order to understand the physiological function of hexokinase PH is that it is a phosphoprotein, since protein phosphorylation is essential in most metabolic signal transductions in eukaryotic cells. Here we show that Hxk2p exists in vivo in a dimeric-monomeric equilibrium which is affected by phosphorylation. Only the monomeric form appears phosphorylated, whereas the dimer does not. The reversible phosphorylation of Hxk2p is carbon source dependent, being more extensive on poor carbon sources such as galactose, raffinose, and ethanol. In vivo dephosphorylation of Hxk2p is promoted after addition of glucose. This effect is absent in glucose repression mutants cat80/grr1, hex2/reg1, and cid1/glc7. Treatment of a glucose crude extract from cid1-226 (glc7-T152K) mutant cells with lambda-phosphatase drastically reduces the presence of phosphoprotein, suggesting that CID1/GLC7 phosphatase together with its regulatory HEX2/REG1 subunit are involved in the dephosphorylation of the Hxk2p monomer. An HXK2 mutation encoding a serine-to-alanine change at position 15 [HXK2 (S15A)] was to clarify the in vivo function of the phosphorylation of hexokinase PII. In this mutant, where the Hxk2 protein is unable to undergo phosphorylation, the cells could not provide glucose repression of invertase. Glucose induction of HXT gene expression is also affected in cells expressing the mutated enzyme. Although we cannot rule out a defect in the metabolic state of the cell as the origin of these phenomena, our results suggest that the phosphorylation of hexokinase is essential in vivo for glucose signal transduction.

Regulated nuclear translocation of the Mig1 glucose repressor

Glucose represses the transcription of many genes in bakers yeast (Saccharomyces cerevisiae). Mig1 is a Cys2-His2 zinc finger protein that mediates glucose repression of several genes by binding to their promoters and recruiting the general repression complex Ssn6-Tup1. We have found that the subcellular localization of Mig1 is regulated by glucose. Mig1 is imported into the nucleus within minutes after the addition of glucose and is just as rapidly transported back to the cytoplasm when glucose is removed. This regulated nuclear localization requires components of the glucose repression signal transduction pathway. An internal region of the protein separate from the DNA binding and repression domains is necessary and sufficient for glucose-regulated nuclear import and export. Changes in the phosphorylation status of Mig1 are coincident with the changes in its localization, suggesting a possible regulatory role for phosphorylation. Our results suggest that a glucose-regulated nuclear import and/or export mechanism controls the activity of Mig1.

Pho85p, a cyclin-dependent protein kinase, and the Snf1p protein kinase act antagonistically to control glycogen accumulation in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, nutrient levels control multiple cellular processes. Cells lacking the SNF1 gene cannot express glucose-repressible genes and do not accumulate the storage polysaccharide glycogen. The impaired glycogen synthesis is due to maintenance of glycogen synthase in a hyperphosphorylated, inactive state. In a screen for second site suppressors of the glycogen storage defect of snf1 cells, we identified a mutant gene that restored glycogen accumulation and which was allelic with PHO85, which encodes a member of the cyclin-dependent kinase family. In cells with disrupted PHO85 genes, we observed hyperaccumulation of glycogen, activation of glycogen synthase, and impaired glycogen synthase kinase activity. In snf1 cells, glycogen synthase kinase activity was elevated. Partial purification of glycogen synthase kinase activity from yeast extracts resulted in the separation of two fractions by phenyl-Sepharose chromatography, both of which phosphorylated and inactivated glycogen synthase. The activity of one of these, GPK2, was inhibited by olomoucine, which potently inhibits cyclin-dependent protein kinases, and contained an approximately 36-kDa species that reacted with antibodies to Pho85p. Analysis of Ser-to-Ala mutations at the three potential Gsy2p phosphorylation sites in pho85 cells implicated Ser-654 and/or Thr-667 in PHO85 control of glycogen synthase. We propose that Pho85p is a physiological glycogen synthase kinase, possibly acting downstream of Snf1p.

Protein phosphatase type 1 interacts with proteins required for meiosis and other cellular processes in Saccharomyces cerevisiae

Protein phosphatase type I (PP1) is involved in diverse cellular processes, and its activity toward specific substrates is thought to be controlled by different regulatory or targeting subunits. To identify regulatory subunits and substrates of the Saccharomyces cerevisiae PP1, encoded by GLC7, we used the two-hybrid system to detect interacting proteins. Among the many proteins identified were Gac1, a known glycogen regulatory subunit, and a protein with homology to Gac1. We also characterized a new gene designated GIP1, for Glc7-interacting protein. We show that a Gip1 fusion protein coimmunoprecipitates with PP1 from cell extracts. Molecular and genetic analyses indicate that GIP1 is expressed specifically during meiosis, affects transcription of late meiotic genes, and is essential for sporulation. Thus, the Gip1 protein is a candidate for a meiosis-specific substrate or regulator of PP1. Finally, we recovered two genes, RED1 and SCD5, with roles in meiosis and the vesicular secretory pathway, respectively. These results provide strong evidence implicating PP1 function in meiosis. In addition, this study indicates that the two-hybrid system offers a promising approach to understanding the multiple roles and interactions of PP1 in cellular regulation.

The REG2 gene of Saccharomyces cerevisiae encodes a type 1 protein phosphatase-binding protein that functions with Reg1p and the Snf1 protein kinase to regulate growth

The GLC7 gene of Saccharomyces cerevisiae encodes the catalytic subunit of type 1 protein phosphatase (PP1) and is essential for cell growth. We have isolated a previously uncharacterized gene, REG2, on the basis of its ability to interact with Glc7p in the two-hybrid system. Reg2p interacts with Glc7p in vivo, and epitope-tagged derivatives of Reg2p and Glc7p coimmunoprecipitate from cell extracts. The predicted protein product of the REG2 gene is similar to Reg1p, a protein believed to direct PP1 activity in the glucose repression pathway. Mutants with a deletion of reg1 display a mild slow-growth defect, while reg2 mutants exhibit a wild-type phenotype. However, mutants with deletions of both reg1 and reg2 exhibit a severe growth defect. Overexpression of REG2 complements the slow-growth defect of a reg1 mutant but does not complement defects in glycogen accumulation or glucose repression, two traits also associated with a reg1 deletion. These results indicate that REG1 has a unique role in the glucose repression pathway but acts together with REG2 to regulate some as yet uncharacterized function important for growth. The growth defect of a reg1 reg2 double mutant is alleviated by a loss-of-function mutation in the SNF1-encoded protein kinase. The snf1 mutation also suppresses the glucose repression defects of reg1. Together, our data are consistent with a model in which Reg1p and Reg2p control the activity of PP1 toward substrates that are phosphorylated by the Snf1p kinase.

Hexokinase production from S. cerevisiae. Culture conditions

The effects of pH (4.0, 4.5, or 5.0), temperature (T) (30, 35, or 40 degrees C) and dissolved oxygen (DO) (0.2, 2.0, 4.0,or 6.0 mg O2/L) on hexokinase and invertase formation by yeast were studied. The highest enzyme activities were attained at pH 4.0, DO = 4.0 mg O2/L, and T = 35 or 40 degrees C.

Regulation of nuclear genes encoding mitochondrial proteins in Saccharomyces cerevisiae

Selection for mutants which release glucose repression of the CYB2 gene was used to identify genes which regulate repression of mitochondrial biogenesis. We have identified two of these as the previously described GRR1/CAT80 and ROX3 genes. Mutations in these genes not only release glucose repression of CYB2 but also generally release respiration of the mutants from glucose repression. In addition, both mutants are partially defective in CYB2 expression when grown on nonfermentable carbon sources, indicating a positive regulatory role as well. ROX3 was cloned by complementation of a glucose-inducible flocculating phenotype of an amber mutant and has been mapped as a new leftmost marker on chromosome 2. The ROX3 mutant has only a modest defect in glucose repression of GAL1 but is substantially compromised in galactose induction of GAL1 expression. This mutant also has increased SUC2 expression on nonrepressing carbon sources. We have also characterized the regulation of CYB2 in strains carrying null mutation in two other glucose repression genes, HXK2 and SSN6, and show that HXK2 is a negative regulator of CYB2, whereas SSN6 appears to be a positive effector of CYB2 expression.

The expression of a specific 2-deoxyglucose-6P phosphatase prevents catabolite repression mediated by 2-deoxyglucose in yeast

2-deoxyglucose (2-DOG), a non-metabolize analogue of glucose, is taken up by yeast using the same transporter(s) as glucose and is phosphorylated by hexokinases producing 2-deoxyglucose-6-P. We found that in DOGR yeasts, 2-DOG was not able to trigger glucose repression, even at concentrations of 0.5%. This result suggests that the specific 2-DOG-6P phosphatase, the enzyme responsible for the DOGR phenotype, may be involved in inhibiting the process of catabolite repression mediated by 2-DOG.

Molecular and biochemical characterization of the hexokinase from the starch-utilizing yeast Schwanniomyces occidentalis

Hexose-phosphorylating enzymes from the starch-utilizing yeast Schwanniomyces occidentalis were purified and two isoenzymes separated. The substrate pattern characterized one of these as a hexokinase phosphorylating glucose and fructose and the other as a glucokinase unable to phosphorylate fructose. The purified Schw. occidentalis hexokinase had a KM value of 0.98 mM for glucose and 9.3 mM for fructose. The hexokinase gene was cloned by cross hybridization with a probe from the Saccharomyces cerevisiae HXK2 gene. Deletion of Schw. occidentalis hexokinase by gene replacement yielded a mutant unable to grow on fructose as sole carbon source, but still growing on glucose. Deletion mutants of Schw. occidentalis hexokinase prevented glucose repression of invertase and maltase. Growth deficiencies and the defect of glucose repression of a S. cerevisiae hexokinase null mutant could be restored by heterologous expression of the Schw. occidentalis hexokinase. Moreover, the results clearly showed the existence of a separate glucokinase in Schw. occidentalis.

Glucose repression in fungi

In many organisms, glucose represses genes that are used to metabolize other carbon sources. Work in yeast and filamentous fungi has revealed a mechanism for glucose repression in eukaryotes that is different from that found in bacteria. Zinc finger proteins, such as Mig1 and CREA, that bind GC-boxes play a key role in mediating this response.

The mutation DGT1-1 decreases glucose transport and alleviates carbon catabolite repression in Saccharomyces cerevisiae

Glucose in ethanol-glycerol mixtures inhibits growth of Saccharomyces cerevisiae mutants lacking phosphoglycerate mutase. A suppressor mutation that relieved glucose inhibition was isolated. This mutation, DGT1-1 (decreasing glucose transport), was dominant and produced pleiotropic effects even in an otherwise wild-type background. Growth of the DGT1-1 mutant in glucose was dependent on respiration, and no ethanol was detected in the medium within 7 h of glucose addition. When grown on glucose, the mutant had a reduced glucose uptake and both the low- and high-affinity transport systems were affected. In galactose-grown cells, only the high-affinity glucose transport system was detected. This system had similar kinetic characteristics in the wild type and in the mutant. Catabolite repression of several enzymes was absent in the mutant during growth in glucose but not during growth in galactose. In contrast with the wild type, the mutant grown in glucose had high transcription of the glucose transporter gene SNF3 and no transcription of HXT1 and HXT3. Expression of multicopy plasmids carrying the HXT1, HXT2, or HXT3 gene allowed partial recovery of both fermentative capacity and catabolite repression in the mutant. The results suggest that DGT1 codes for a regulator of the expression of glucose transport genes. They also suggest that glucose flux might determine the levels of molecules implicated as signals in catbolite repression.

Industrial production of heterologous proteins by fed-batch cultures of the yeast Saccharomyces cerevisiae

This review concerns the issues involved in the industrial development of fed-batch culture processes with Saccharomyces cerevisiae strains producing heterologous proteins. Most of process development considerations with fed-batch recombinant cultures are linked to the reliability and reproducibility of the process for manufacturing environments where quality assurance and quality control aspects are paramount. In this respect, the quality, safety and efficacy of complex biologically active molecules produced by recombinant techniques are strongly influenced by the genetic background of the host strain, genetic stability of the transformed strain and production process factors. An overview of the recent literature of these culture-related factors is coupled with our experience in yeast fed-batch process development for producing various therapeutic grade proteins. The discussion is based around three principal topics: genetics, microbial physiology and fed-batch process design. It includes the fundamental aspects of yeast strain physiology, the nature of the recombinant product, quality control aspects of the biological product, features of yeast expression vectors, expression and localization of recombinant products in transformed cells and fed-batch process considerations for the industrial production of Saccharomyces cerevisiae recombinant proteins. It is our purpose that this review will provide a comprehensive understanding of the fed-batch recombinant production processes and challenges commonly encountered during process development.

Glucose uptake and metabolism in grr1/cat80 mutants of Saccharomyces cerevisiae

Glucose repression in the yeast Saccharomyces cerevisiae designates a global regulatory system controlling the expression of various sets of genes required for the utilization of alternate carbon sources. In a screen, designed for the selection of mutants with reduced glycolytic flux we obtained isolates which were shown by complementation of the cloned wild-type gene to be allelic to the glucose repression mutants grr1/cat80/cot2 previously described. We demonstrate that the grr1 lesion lead to a concentration-dependent decrease in glycolytic flux on glucose. It is very likely that this is caused by a significant decrease in the expression of various genes encoding hexose transporters (HXT1,3) leading to a reduced glucose-uptake rate. In contrast, expression of the maltose permease gene (MAL11) and maltose utilization is normal. There is indirect evidence that grr1 affects the uptake of amino acids, and others have shown that the sugar-induced transport of divalent cations is impaired. These effects are not glucose-specific. We suggest that Grr1, a putative cytoplasmic protein, has a central function in the sensing of nutritional conditions for a variety of unrelated substances, and that relief from glucose repression may be a corollary of this defect in sensing.

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.

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.

Glutamine synthesis is a regulatory signal controlling glucose catabolism in Saccharomyces cerevisiae

The effect of glutamine biosynthesis and degradation on glucose catabolism in Saccharomyces cerevisiae was studied. A wild-type strain and mutants altered in glutamine biosynthesis and degradation were analyzed. Cells having low levels of glutamine synthetase activity showed high ATP/ADP ratios and a diminished rate of glucose metabolism. It is proposed that glutamine biosynthesis plays a role in the regulation of glucose catabolism.

Induction of pyruvate decarboxylase in glycolysis mutants of Saccharomyces cerevisiae correlates with the concentrations of three-carbon glycolytic metabolites

Pyruvate decarboxylase, PDCase, activity in wild-type yeast cells growing on ethanol is quite low but increases up to tenfold upon addition of glucose, less with galactose and only slightly with glycerol. PDCase levels in glycolysis mutant strains growing on ethanol or acetate were higher than in the wild-type strain. These levels correlated with the sum of the concentrations of three-carbon glycolytic metabolites. The highest accumulation was observed in a fructose bisphosphate aldolase deletion mutant concomitant with the highest PDCase activity ever observed under gluconeogenic conditions. Glucose addition induced an increase in PDCase activity in all mutants. However, the enzyme activities never reached wild-type level. On the other hand, the PDCase levels in the different mutants again correlated with the sum of the concentrations of the three-carbon glycolytic metabolites. This was interpreted to mean that full induction of PDCase activity requires the accumulation of hexose- and triosephosphates.

Glucose uptake and catabolite repression in dominant HTR1 mutants of Saccharomyces cerevisiae

Growth and carbon metabolism in triosephosphate isomerase (delta tpi1) mutants of Saccharomyces cerevisiae are severely inhibited by glucose. By using this feature, we selected for secondary site revertants on glucose. We defined five complementation groups, some of which have previously been identified as glucose repression mutants. The predominant mutant type, HTR1 (hexose transport regulation), is dominant and causes various glucose-specific metabolic and regulatory defects in TPI1 wild-type cells. HTR1 mutants are deficient in high-affinity glucose uptake and have reduced low-affinity transport. Transcription of various known glucose transporter genes (HXT1, HXT3, and HXT4) was defective in HTR1 mutants, leading us to suggest that HTR mutations affect a negative factor of HXT gene expression. By contrast, transcript levels for SNF3, which encodes a component of high-affinity glucose uptake, were unaffected. We presume that HTR1 mutations affect a negative factor of HXT gene expression. Multicopy expression of HXT genes or parts of their regulatory sequences suppresses the metabolic defects of HTR1 mutants but not their derepressed phenotype at high glucose concentrations. This suggests that the glucose repression defect is not a direct result of the metabolically relevant defect in glucose transport. Alternatively, some unidentified regulatory components of the glucose transport system may be involved in the generation or transmission of signals for glucose repression.

Alteration of cell population structure due to cell lysis in Saccharomyces cerevisiae cells overexpressing the GAL4 gene

Transformed Saccharomyces cerevisiae cells overexpressing the Escherichia coli LacZ gene and the transcriptional activator GAL4, release in the external medium a fraction (from 2 to 10%) of the total beta-galactosidase activity (Porro et al., 1992b). It is known that this abnormal release of a cytoplasmic protein is related to a partial cell lysis of the yeast population, which is likely to be caused by the overexpression of the transcriptional activator GAL4. In the present paper we have characterized the GAL4-induced cell lysis phenomenon. The expression of the GAL4 gene causes morphological modifications and alteration of the cell size distribution. The cell lysis is independent of the expression of the heterologous LacZ gene and occurs in a specific subpopulation of cells (the parent cells) independently of the genealogical age, growth phase conditions and cell cycle progression. Lysis is preceded by a loss of the plasma membrane integrity as indicated by the uptake of ethidium bromide in unfixed cells. Computer analysis of simulated protein distributions indicates that cell lysis takes place in a sizeable aliquot (about 50%) of the parent cells, therefore profoundly altering the age structure of the population.

Regulation of sugar utilization by Saccharomyces cerevisiae

There are several kinds of regulation that enable microbes to cope with rapidly changing supplies of nutrients. This is exemplified by sugar metabolism in Saccharomyces cerevisiae. Some readily reversible controls affect the activity of enzymes, either by allosteric activation and deactivation, which often occur within seconds, or by covalent modification, within minutes. Other controls regulate the amount of enzyme present in the cells, either by irreversible proteolytic inactivation of the enzyme, or by influencing enzymic synthesis. The nomenclature of these processes is often confused.