Disruption of the Kluyveromyces lactis GGS1 gene causes inability to grow on glucose and fructose and is suppressed by mutations that reduce sugar uptake

A secondary function of trehalose-6-phosphate synthase is required for resistance to oxidative and desiccation stress in Fusarium verticillioides

The disaccharide trehalose has long been recognized for its role as a stress solute, but in recent years some of the protective effects previously ascribed to trehalose have been suggested to arise from a function of the trehalose biosynthesis enzyme trehalose-6-phosphate (T6P) synthase that is distinct from its catalytic activity. In this study, we use the maize pathogenic fungus Fusarium verticillioides as a model to explore the relative contributions of trehalose itself and a putative secondary function of T6P synthase in protection against stress as well as to understand why, as shown in a previous study, deletion of the TPS1 gene coding for T6P synthase reduces pathogenicity against maize. We report that a TPS1-deletion mutant of F. verticillioides is compromised in its ability to withstand exposure to oxidative stress meant to simulate the oxidative burst phase of maize defense and experiences more ROS-induced lipid damage than the wild-type strain. Eliminating T6P synthase expression also reduces resistance to desiccation, but not resistance to phenolic acids. Expression of catalytically-inactive T6P synthase in the TPS1-deletion mutant leads to a partial rescue of the oxidative and desiccation stress-sensitive phenotypes, suggesting the importance of a T6P synthase function that is independent of its role in trehalose synthesis.

Selection for rapid uptake of scarce or fluctuating resource explains vulnerability of glycolysis to imbalance

Glycolysis is a conserved central pathway in energy metabolism that converts glucose to pyruvate with net production of two ATP molecules. Because ATP is produced only in the lower part of glycolysis (LG), preceded by an initial investment of ATP in the upper glycolysis (UG), achieving robust start-up of the pathway upon activation presents a challenge: a sudden increase in glucose concentration can throw a cell into a self-sustaining imbalanced state in which UG outpaces LG, glycolytic intermediates accumulate and the cell is unable to maintain high ATP concentration needed to support cellular functions. Such metabolic imbalance can result in "substrate-accelerated death", a phenomenon observed in prokaryotes and eukaryotes when cells are exposed to an excess of substrate that previously limited growth. Here, we address why evolution has apparently not eliminated such a costly vulnerability and propose that it is a manifestation of an evolutionary trade-off, whereby the glycolysis pathway is adapted to quickly secure scarce or fluctuating resource at the expense of vulnerability in an environment with ample resource. To corroborate this idea, we perform individual-based eco-evolutionary simulations of a simplified yeast glycolysis pathway consisting of UG, LG, phosphate transport between a vacuole and a cytosol, and a general ATP demand reaction. The pathway is evolved in constant or fluctuating resource environments by allowing mutations that affect the (maximum) reaction rate constants, reflecting changing expression levels of different glycolytic enzymes. We demonstrate that under limited constant resource, populations evolve to a genotype that exhibits balanced dynamics in the environment it evolved in, but strongly imbalanced dynamics under ample resource conditions. Furthermore, when resource availability is fluctuating, imbalanced dynamics confers a fitness advantage over balanced dynamics: when glucose is abundant, imbalanced pathways can quickly accumulate the glycolytic intermediate FBP as intracellular storage that is used during periods of starvation to maintain high ATP concentration needed for growth. Our model further predicts that in fluctuating environments, competition for glucose can result in stable coexistence of balanced and imbalanced cells, as well as repeated cycles of population crashes and recoveries that depend on such polymorphism. Overall, we demonstrate the importance of ecological and evolutionary arguments for understanding seemingly maladaptive aspects of cellular metabolism.

Deletion of the trehalose tps1 gene in Kluyveromyces lactis does not impair growth in glucose

Trehalose is a non-reducing disaccharide composed of two α-glucose molecules and synthesized by an enzyme complex containing four subunits TPS1 (EC 2.4.1.15), TPS2 (EC 3.1.3.12), TPS3 and TSL1. First reports about trehalose classified this sugar as an energy reserve compound like glycogen. However, lately, trehalose is known to assist yeast cells during heat, osmotic and starvation stresses. In Saccharomyces cerevisiae, the deletion of the tps1 encoding gene eliminated the yeast ability to grow on glucose as the sole carbon source. Kluyveromyces lactis is a yeast present in various dairy products and is currently utilized for the synthesis of more than 40 industrial heterologous products. In this study, the deletion of the tps1 gene in K. lactis showed that unlike S. cerevisiae, tps1 gene disruption does not cause growth failure in glucose, galactose, or fructose. The µMAX rate values of K. lactis tps1Δ strains were equal than the non-disrupted strains, showing that the gene deletion does not affect the yeast growth. After gene disruption, the absence of trehalose into the metabolism of K. lactis was also confirmed.

Genetics of trehalose biosynthesis in desert-derived Aureobasidium melanogenum and role of trehalose in the adaptation of the yeast to extreme environments

Melanin plays an important role in the stress adaptation of Aureobasidium melanogenum XJ5-1 isolated from the Taklimakan desert. A trehalose-6-phosphate synthase gene (TPS1 gene) was cloned from K5, characterized, and then deleted to determine the role of trehalose in the stress adaptation of the albino mutant K5. No stress response element and heat shock element were found in the promoter of the TPS1 gene. Deletion of the TPS1 gene in the albino mutant rendered a strain DT43 unable to synthesize any trehalose, but DT43 still could grow in glucose, suggesting that its hexokinase was insensitive to inhibition by trehalose-6-phosphate. Overexpression of the TPS1 gene enhanced trehalose biosynthesis in strain ET6. DT43 could not grow at 33 °C, whereas K5, ET6, and XJ5-1 could grow well at this temperature. Compared with K5 and ET6, DT43 was highly sensitive to heat shock treatment, high oxidation, and high desiccation, but all the three strains demonstrated the same sensitivity to UV light and high NaCl concentration. Therefore, trehalose played an important role in the adaptation of K5 to heat shock treatment, high oxidation, and high desiccation.

Yeast metabolic and signaling genes are required for heat-shock survival and have little overlap with the heat-induced genes

Genome-wide gene-expression studies have shown that hundreds of yeast genes are induced or repressed transiently by changes in temperature; many are annotated to stress response on this basis. To obtain a genome-scale assessment of which genes are functionally important for innate and/or acquired thermotolerance, we combined the use of a barcoded pool of ~4,800 nonessential, prototrophic Saccharomyces cerevisiae deletion strains with Illumina-based deep-sequencing technology. As reported in other recent studies that have used deletion mutants to study stress responses, we observed that gene deletions resulting in the highest thermosensitivity generally are not the same as those transcriptionally induced in response to heat stress. Functional analysis of identified genes revealed that metabolism, cellular signaling, and chromatin regulation play roles in regulating thermotolerance and in acquired thermotolerance. However, for most of the genes identified, the molecular mechanism behind this action remains unclear. In fact, a large fraction of identified genes are annotated as having unknown functions, further underscoring our incomplete understanding of the response to heat shock. We suggest that survival after heat shock depends on a small number of genes that function in assessing the metabolic health of the cell and/or regulate its growth in a changing environment.

Impact of temperature stress and validamycin A on compatible solutes and fumonisin production in F. verticillioides: role of trehalose-6-phosphate synthase

Fusarium verticillioides is a pathogen of maize that causes root, stalk and ear rot and produces fumonisins, toxic secondary metabolites associated with disease in livestock and humans. Environmental stresses such as heat and drought influence disease severity and toxin production, but the effects of abiotic stress on compatible solute production by F. verticillioides have not been fully characterized. We found that decreasing the growth temperature leads to a long-term reduction in polyol levels, whereas increasing the temperature leads to a transient increase in polyols. The effects of temperature shifts on trehalose levels are opposite the effects on polyols and more dramatic. Treatment with validamycin A, a trehalose analog with antifungal activity, leads to a rapid reduction in trehalose levels, despite its known role as a trehalase inhibitor. Mutant strains lacking TPS1, which encodes a putative trehalose-6-phosphate synthase, have altered growth characteristics, do not produce detectable amounts of trehalose under any condition tested, and accumulate glycogen at levels significantly higher than wild-type F. verticillioides. TPS1 mutants also produce significantly less fumonisin than wild type and are also less pathogenic than wild type on maize. These data link trehalose biosynthesis, secondary metabolism, and disease, and suggest that trehalose metabolic pathways may be a viable target for the control of Fusarium diseases and fumonisin contamination of maize.

Disruption of Yarrowia lipolytica TPS1 gene encoding trehalose-6-P synthase does not affect growth in glucose but impairs growth at high temperature

We have cloned the Yarrowia lipolytica TPS1 gene encoding trehalose-6-P synthase by complementation of the lack of growth in glucose of a Saccharomyces cerevisiae tps1 mutant. Disruption of YlTPS1 could only be achieved with a cassette placed in the 3' half of its coding region due to the overlap of its sequence with the promoter of the essential gene YlTFC1. The Yltps1 mutant grew in glucose although the Y. lipolytica hexokinase is extremely sensitive to inhibition by trehalose-6-P. The presence of a glucokinase, insensitive to trehalose-6-P, that constitutes about 80% of the glucose phosphorylating capacity during growth in glucose may account for the growth phenotype. Trehalose content was below 1 nmol/mg dry weight in Y. lipolytica, but it increased in strains expressing YlTPS1 under the control of the YlTEF1 promoter or with a disruption of YALI0D15598 encoding a putative trehalase. mRNA levels of YlTPS1 were low and did not respond to thermal stresses, but that of YlTPS2 (YALI0D14476) and YlTPS3 (YALI0E31086) increased 4 and 6 times, repectively, by heat treatment. Disruption of YlTPS1 drastically slowed growth at 35°C. Homozygous Yltps1 diploids showed a decreased sporulation frequency that was ascribed to the low level of YALI0D20966 mRNA an homolog of the S. cerevisiae MCK1 which encodes a protein kinase that activates early meiotic gene expression.

Compartmentation prevents a lethal turbo-explosion of glycolysis in trypanosomes

ATP generation by both glycolysis and glycerol catabolism is autocatalytic, because the first kinases of these pathways are fuelled by ATP produced downstream. Previous modeling studies predicted that either feedback inhibition or compartmentation of glycolysis can protect cells from accumulation of intermediates. The deadly parasite Trypanosoma brucei lacks feedback regulation of early steps in glycolysis yet sequesters the relevant enzymes within organelles called glycosomes, leading to the proposal that compartmentation prevents toxic accumulation of intermediates. Here, we show that glucose 6-phosphate indeed accumulates upon glucose addition to PEX14 deficient trypanosomes, which are impaired in glycosomal protein import. With glycerol catabolism, both in silico and in vivo, loss of glycosomal compartmentation led to dramatic increases of glycerol 3-phosphate upon addition of glycerol. As predicted by the model, depletion of glycerol kinase rescued PEX14-deficient cells of glycerol toxicity. This provides the first experimental support for our hypothesis that pathway compartmentation is an alternative to allosteric regulation.

Tps1 regulates the pentose phosphate pathway, nitrogen metabolism and fungal virulence

Trehalose fulfils a wide variety of functions in cells, acting as a stress protectant, storage carbohydrate and compatible solute. Recent evidence, however, indicates that trehalose metabolism may exert important regulatory roles in the development of multicellular eukaryotes. Here, we show that in the plant pathogenic fungus Magnaporthe grisea trehalose-6-phosphate (T6P) synthase (Tps1) is responsible for regulating the pentose phosphate pathway, intracellular levels of NADPH and fungal virulence. Tps1 integrates glucose-6-phosphate (G6P) metabolism with nitrogen source utilisation, and thereby regulates the activity of nitrate reductase. Activity of Tps1 requires an associated regulator protein Tps3, which is also necessary for pathogenicity. Tps1 controls expression of the nitrogen metabolite repressor gene, NMR1, and is required for expression of virulence-associated genes. Functional analysis of Tps1 indicates that its regulatory functions are associated with binding of G6P, but independent of Tps1 catalytic activity. Taken together, these results demonstrate that Tps1 is a central regulator for integration of carbon and nitrogen metabolism, and plays a pivotal role in the establishment of plant disease.

Molecular Cloning, Characterization and Expression of a Novel Trehalose-6-phosphate Synthase Homologue from Ginkgo biloba

In many organisms, trehalose acts as protective metabolite against harsh environmental stresses, such as freezing, drought, nutrient starvation, heat and salt. Herein a cDNA (designated as GbTPS, GenBank Accession Number AY884150) encoding a trehalose-6-phosphate synthase homologue was isolated and characterized from the living fossil plant, Ginkgo biloba, which is highly tolerant to drought and cold. GbTPS encoded an 868-amino-acid polypeptide with a predicted isoelectric point of 5.83 and molecular mass of 97.9 kD. Amino acid sequence alignment revealed that GbTPS shared high identity with class II trehalose-6-phosphate synthase homologues (67% identical to AtTPS7), but had only 17% and 23% of identity with OstA from Escherichia coli and ScTPS1 from S. cerevisiae, respectively. DNA gel blot analysis indicated that GbTPS belonged to a small multi-gene family. The expression analysis by RT-PCR showed that GbTPS expressed in a tissue-specific manner in G. biloba and might involve in leaf development. GbTPS was also found to be induced by a variety of stresses including cold, salt, drought and mannitol.

The Arabidopsis trehalose-6-P synthase AtTPS1 gene is a regulator of glucose, abscisic acid, and stress signaling

In Arabidopsis (Arabidopsis thaliana), trehalose is present at almost undetectable levels, excluding its role as an osmoprotectant. Here, we report that overexpression of AtTPS1 in Arabidopsis using the 35S promoter led to a small increase in trehalose and trehalose-6-P levels. In spite of this, transgenic plants displayed a dehydration tolerance phenotype without any visible morphological alterations, except for delayed flowering. Moreover, seedlings overexpressing AtTPS1 exhibited glucose (Glc)- and abscisic acid (ABA)-insensitive phenotypes. Transgenic seedlings germinated on Glc were visibly larger with green well-expanded cotyledonary leaves and fully developed roots, in contrast with wild-type seedlings showing growth retardation and absence of photosynthetic tissue. An ABA dose-response experiment revealed a higher germination rate for transgenic plants overexpressing AtTPS1 showing insensitive germination kinetics at 2.5 mum ABA. Interestingly, germination in the presence of Glc did not trigger an increase in ABA content in plants overexpressing AtTPS1. Expression analysis by quantitative reverse transcription-PCR in transgenic plants showed up-regulation of the ABI4 and CAB1 genes. In the presence of Glc, CAB1 expression remained high, whereas ABI4, HXK1, and ApL3 levels were down-regulated in the AtTPS1-overexpressing lines. Analysis of AtTPS1 expression in HXK1-antisense or HXK1-sense transgenic lines suggests the possible involvement of AtTPS1 in the hexokinase-dependent Glc-signaling pathway. These data strongly suggest that AtTPS1 has a pivotal role in the regulation of Glc and ABA signaling during vegetative development.

Genetic aspects of targeted insertion mutagenesis in yeasts

Targeted insertion mutagenesis is a main molecular tool of yeast science initially applied in Saccharomyces cerevisiae. The method was extended to fission yeast Schizosaccharomyces pombe and to "non-conventional" yeast species, which show specific properties of special interest to both basic and applied research. Consequently, the behaviour of such non-Saccharomyces yeasts is reviewed against the background of the knowledge of targeted insertion mutagenesis in S. cerevisiae. Data of homologous integration efficiencies obtained with circular, ends-in or ends-out vectors in several yeasts are compared. We follow details of targeted insertion mutagenesis in order to recognize possible rate-limiting steps. The route of the vector to the target and possible mechanisms of its integration into chromosomal genes are considered. Specific features of some yeast species are discussed. In addition, similar approaches based on homologous recombination that have been established for the mitochondrial genome of S. cerevisiae are described.

The importance of a functional trehalose biosynthetic pathway for the life of yeasts and fungi

The view of the role of trehalose in yeast has changed in the last few years. For a long time considered a reserve carbohydrate, it gained new importance when its function in the acquisition of thermotolerance was demonstrated. More recently the cellular processes in which the trehalose biosynthetic pathway has been implicated range from the control of glycolysis to sporulation and infectivity by certain fungal pathogens. There is now enough experimental evidence to conclude that trehalose 6-phosphate, an intermediate of trehalose biosynthesis, is an important metabolic regulator in such different organisms as yeasts or plants. Its inhibition of hexokinase plays a key role in the control of the glycolytic flux in Saccharomyces cerevisiae but other, likely important, sites of action are still unknown. We present examples of the phenotypes produced by mutations in the two steps of the trehalose biosynthetic pathway in different yeasts and fungi, and whenever possible examine the molecular explanations advanced to interpret them.

Uncoupling of the glucose growth defect and the deregulation of glycolysis in Saccharomyces cerevisiae Tps1 mutants expressing trehalose-6-phosphate-insensitive hexokinase from Schizosaccharomyces pombe

In the yeast Saccharomyces cerevisiae inactivation of trehalose-6-phosphate (Tre6P) synthase (Tps1) encoded by the TPS1 gene causes a specific growth defect in the presence of glucose in the medium. The growth inhibition is associated with deregulation of the initial part of glycolysis. Sugar phosphates, especially fructose-1,6-bisphosphate (Fru1,6bisP), hyperaccumulate while the levels of ATP, Pi and downstream metabolites are rapidly depleted. This was suggested to be due to the absence of Tre6P inhibition on hexokinase. Here we show that overexpression of Tre6P (as well as glucose-6-phosphate (Glu6P))-insensitive hexokinase from Schizosaccharomyces pombe in a wild-type strain does not affect growth on glucose but still transiently enhances initial sugar phosphate accumulation. We have in addition replaced the three endogenous glucose kinases of S. cerevisiae by the Tre6P-insensitive hexokinase from S. pombe. High hexokinase activity was measured in cell extracts and growth on glucose was somewhat reduced compared to an S. cerevisiae wild-type strain but expression of the Tre6P-insensitive S. pombe hexokinase never caused the typical tps1Delta phenotype. Moreover, deletion of TPS1 in this strain expressing only the Tre6P-insensitive S. pombe hexokinase still resulted in a severe drop in growth capacity on glucose as well as sensitivity to millimolar glucose levels in the presence of excess galactose. In this case, poor growth on glucose was associated with reduced rather than enhanced glucose influx into glycolysis. Initial glucose transport was not affected. Apparently, deletion of TPS1 causes reduced activity of the S. pombe hexokinase in vivo. Our results show that Tre6P inhibition of hexokinase is not the major mechanism by which Tps1 controls the influx of glucose into glycolysis or the capacity to grow on glucose. In addition, they show that a Tre6P-insensitive hexokinase can still be controlled by Tps1 in vivo.

Genome-wide expression analyses: Metabolic adaptation of Saccharomyces cerevisiae to high sugar stress

The transcriptional response of laboratory strains of Saccharomyces cerevisiae to salt or sorbitol stress has been well studied. These studies have yielded valuable data on how the yeast adapts to these stress conditions. However, S. cerevisiae is a saccharophilic fungus and in its natural environment this yeast encounters high concentrations of sugars. For the production of dessert wines, the sugar concentration may be as high as 50% (w/v). The metabolic pathways in S. cerevisiae under these fermentation conditions have not been studied and the transcriptional response of this yeast to sugar stress has not been investigated. High-density DNA microarrays showed that the transcription of 589 genes in an industrial strain of S. cerevisiae were affected more than two-fold in grape juice containing 40% (w/v) sugars (equimolar amounts of glucose and fructose). High sugar stress up-regulated the glycolytic and pentose phosphate pathway genes. The PDC6 gene, previously thought to encode a minor isozyme of pyruvate decarboxylase, was highly induced under these conditions. Gene expression profiles indicate that the oxidative and non-oxidative branches of the pentose phosphate pathway were up-regulated and might be used to shunt more glucose-6-phosphate and fructose-6-phosphate, respectively, from the glycolytic pathway into the pentose phosphate pathway. Structural genes involved in the formation of acetic acid from acetaldehyde, and succinic acid from glutamate, were also up-regulated. Genes involved in de novo biosynthesis of purines, pyrimidines, histidine and lysine were down-regulated by sugar stress.

Disruption of the Candida albicans TPS2 gene encoding trehalose-6-phosphate phosphatase decreases infectivity without affecting hypha formation

Deletion of trehalose-6-phosphate phosphatase, encoded by TPS2, in Saccharomyces cerevisiae results in accumulation of trehalose-6-phosphate (Tre6P) instead of trehalose under stress conditions. Since trehalose is an important stress protectant and Tre6P accumulation is toxic, we have investigated whether Tre6P phosphatase could be a useful target for antifungals in Candida albicans. We have cloned the C. albicans TPS2 (CaTPS2) gene and constructed heterozygous and homozygous deletion strains. As in S. cerevisiae, complete inactivation of Tre6P phosphatase in C. albicans results in 50-fold hyperaccumulation of Tre6P, thermosensitivity, and rapid death of the cells after a few hours at 44 degrees C. As opposed to inactivation of Tre6P synthase by deletion of CaTPS1, deletion of CaTPS2 does not affect hypha formation on a solid glucose-containing medium. In spite of this, virulence of the homozygous deletion mutant is strongly reduced in a mouse model of systemic infection. The pathogenicity of the heterozygous deletion mutant is similar to that of the wild-type strain. CaTPS2 is a new example of a gene not required for growth under standard conditions but required for pathogenicity in a host. Our results suggest that Tre6P phosphatase may serve as a potential target for antifungal drugs. Neither Tre6P phosphatase nor its substrate is present in mammals, and assay of the enzymes is simple and easily automated for high-throughput screening.

Trehalose is required for the acquisition of tolerance to a variety of stresses in the filamentous fungus Aspergillus nidulans

Trehalose is a non-reducing disaccharide found at high concentrations in Aspergillus nidulans conidia and rapidly degraded upon induction of conidial germination. Furthermore, trehalose is accumulated in response to a heat shock or to an oxidative shock. The authors have characterized the A. nidulans tpsA gene encoding trehalose-6-phosphate synthase, which catalyses the first step in trehalose biosynthesis. Expression of tpsA in a Saccharomyces cerevisiae tps1 mutant revealed that the tpsA gene product is a functional equivalent of the yeast Tps1 trehalose-6-phosphate synthase. The A. nidulans tpsA-null mutant does not produce trehalose during conidiation or in response to various stress conditions. While germlings of the tpsA mutant show an increased sensitivity to moderate stress conditions (growth at 45 degrees C or in the presence of 2 mM H(2)O(2)), they display a response to severe stress (60 min at 50 degrees C or in the presence of 100 mM H(2)O(2)) similar to that of wild-type germlings. Furthermore, conidia of the tpsA mutant show a rapid loss of viability upon storage. These results are consistent with a role of trehalose in the acquisition of stress tolerance. Inactivation of the tpsA gene also results in increased steady-state levels of sugar phosphates but does not prevent growth on rapidly metabolizable carbon sources (glucose, fructose) as seen in Saccharomyces cerevisiae. This suggests that trehalose 6-phosphate is a physiological inhibitor of hexokinase but that this control is not essential for proper glycolytic flux in A. nidulans. Interestingly, tpsA transcription is not induced in response to heat shock or during conidiation, indicating that trehalose accumulation is probably due to a post-translational activation process of the trehalose 6-phosphate synthase.

Reconstitution of ethanolic fermentation in permeabilized spheroplasts of wild-type and trehalose-6-phosphate synthase mutants of the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, TPS1-encoded trehalose-6-phosphate synthase (TPS) exerts an essential control on the influx of glucose into glycolysis, presumably by restricting hexokinase activity. Deletion of TPS1 results in severe hyperaccumulation of sugar phosphates and near absence of ethanol formation. To investigate whether trehalose 6-phosphate (Tre6P) is the sole mediator of hexokinase inhibition, we have reconstituted ethanolic fermentation from glucose in permeabilized spheroplasts of the wild-type, tps1Delta and tps2Delta (Tre6P phosphatase) strains. For the tps1Delta strain, ethanol production was significantly lower and was associated with hyperaccumulation of Glu6P and Fru6P. A tps2Delta strain shows reduced accumulation of Glu6P and Fru6P both in intact cells and in permeabilized spheroplasts. These results are not consistent with Tre6P being the sole mediator of hexokinase inhibition. Reconstitution of ethanolic fermentation in permeabilized spheroplasts with glycolytic intermediates indicates additional target site(s) for the Tps1 control. Addition of Tre6P partially shifts the ethanol production rate and the metabolite pattern in permeabilized tps1Delta spheroplasts to those of the wild-type strain, but only with glucose as substrate. This is observed at a very high ratio of glucose to Tre6P. Inhibition of hexokinase activity by Tre6P is less efficiently counteracted by glucose in permeabilized spheroplasts compared to cell extracts, and this effect is largely abolished by deletion of TPS2 but not TPS1. In permeabilized spheroplasts, hexokinase activity is significantly lower in a tps2Delta strain compared to a wild-type strain and this difference is strongly reduced by additional deletion of TPS1. These results indicate that Tps1-mediated protein-protein interactions are important for control of glucose influx into yeast glycolysis, that Tre6P inhibition of hexokinase might not be competitive with respect to glucose in vivo and that also Tps2 appears to play a role in the control of hexokinase activity.

Compartmentation protects trypanosomes from the dangerous design of glycolysis

Unlike in other organisms, in trypanosomes and other Kinetoplastida the larger part of glycolysis takes place in a specialized organelle, called the glycosome. At present it is impossible to remove the glycosome without changing much of the rest of the cell. It would seem impossible, therefore, to assess the metabolic consequences of this compartmentation. Therefore, we here develop a computer experimentation approach, which we call computational cell biology. A validated molecular kinetic computer replica was built of glycolysis in the parasite Trypanosoma brucei. Removing the glycosome membrane in that replica had little effect on the steady-state flux, which argues against the prevalent speculation that glycosomes serve to increase flux by concentrating the enzymes. Removal of the membrane did cause (i) the sugar phosphates to rise to unphysiologically high levels, which must have pathological effects, and (ii) a failure to recover from glucose deprivation. We explain these effects on the basis of the biochemical organization of the glycosome. We conclude (i) that the glycosome protects trypanosomes from the negative side effects of the "turbo" structure of glycolysis and (ii) that computer experimentation based on solid molecular data is a powerful tool to address questions that are not, or not yet, accessible to experimentation.

Trehalose hydrolysis is not required for human serum-induced dimorphic transition in Candida albicans: evidence from a tps1/tps1 mutant deficient in trehalose synthesis

Exponential yeast-like cells of a Candida albicans wild-type strain exhibited strong capacity for germ tube formation in a glucose-containing medium (YPD) after induction with human serum at 37 degrees C, whereas the isogenic double disruptant tps1/tps1 mutant, which is deficient in trehalose synthesis, failed to produce germ tubes. In a medium without glucose (YP), the morphological transition fraction was roughly equivalent in both strains. Substitution of glucose by galactose or glycerol increased the number of wild-type proliferating cells able to enter the dimorphic program with no noticeable change in their trehalose content, while stationary cells, which accumulate a large amount of trehalose, did not form germ tubes. When fresh medium was added, a high proportion of these resting cells recovered their ability to carry out dimorphic transition. The tps1/tps1 mutant followed the same pattern of hyphae formation, despite the fact that it was unable to accumulate trehalose either during dimorphism induction or after several stress challenges. Furthermore, trehalose-6-phosphate synthase activity was barely detectable in the mutant. These results strongly suggest that serum-induced dimorphic transition does not require trehalose mobilization; they also support the idea that TPS1 is the only activity involved in trehalose biosynthesis in C. albicans.

The thermophilic yeast Hansenula polymorpha does not require trehalose synthesis for growth at high temperatures but does for normal acquisition of thermotolerance

The TPS1 gene from Hansenula polymorpha, which encodes trehalose-6-phosphate (Tre6P) synthase, has been isolated and characterized. The deletion of TPS1 rendered H. polymorpha cells incapable of trehalose synthesis under conditions where wild-type cells normally accumulate high levels of trehalose. Interestingly, the loss of Tre6P synthase did not cause any obvious growth defects on a glucose-containing medium, even at high temperatures, but seriously compromised the cells' ability to acquire thermotolerance.

A Selaginella lepidophylla trehalose-6-phosphate synthase complements growth and stress-tolerance defects in a yeast tps1 mutant

The accumulation of the disaccharide trehalose in anhydrobiotic organisms allows them to survive severe environmental stress. A plant cDNA, SlTPS1, encoding a 109-kD protein, was isolated from the resurrection plant Selaginella lepidophylla, which accumulates high levels of trehalose. Protein-sequence comparison showed that SlTPS1 shares high similarity to trehalose-6-phosphate synthase genes from prokaryotes and eukaryotes. SlTPS1 mRNA was constitutively expressed in S. lepidophylla. DNA gel-blot analysis indicated that SlTPS1 is present as a single-copy gene. Transformation of a Saccharomyces cerevisiae tps1Delta mutant disrupted in the ScTPS1 gene with S. lepidophylla SlTPS1 restored growth on fermentable sugars and the synthesis of trehalose at high levels. Moreover, the SlTPS1 gene introduced into the tps1Delta mutant was able to complement both deficiencies: sensitivity to sublethal heat treatment at 39 degrees C and induced thermotolerance at 50 degrees C. The osmosensitive phenotype of the yeast tps1Delta mutant grown in NaCl and sorbitol was also restored by the SlTPS1 gene. Thus, SlTPS1 protein is a functional plant homolog capable of sustaining trehalose biosynthesis and could play a major role in stress tolerance in S. lepidophylla.

The ggpS gene from Synechocystis sp. strain PCC 6803 encoding glucosyl-glycerol-phosphate synthase is involved in osmolyte synthesis

A salt-sensitive mutant of Synechocystis sp. strain PCC 6803 defective in the synthesis of the compatible solute glucosylglycerol (GG) was used to search for the gene encoding GG-phosphate synthase (GGPS), the key enzyme in GG synthesis. Cloning and sequencing of the mutated region and the corresponding wild-type region revealed that a deletion of about 13 kb occurred in the genome of mutant 11. This deletion affected at least 10 open reading frames, among them regions coding for proteins showing similarities to trehalose (otsA homolog)- and glycerol-3-phosphate-synthesizing enzymes. After construction and characterization of mutants defective in these genes, it became obvious that an otsA homolog (sll1566) (T. Kaneko et al., DNA Res. 3:109-136, 1996) encodes GGPS, since only the mutant affected in sll1566 showed salt sensitivity combined with a complete absence of GG accumulation. Furthermore, the overexpression of sll1566 in Escherichia coli led to the appearance of GGPS activity in the heterologous host. The overexpressed protein did not show the salt dependence that is characteristic for the GGPS in crude protein extracts of Synechocystis.

Disruption of the Candida albicans TPS1 gene encoding trehalose-6-phosphate synthase impairs formation of hyphae and decreases infectivity

The TPS1 gene from Candida albicans, which encodes trehalose-6-phosphate synthase, has been cloned by functional complementation of a tps1 mutant from Saccharomyces cerevisiae. In contrast with the wild-type strain, the double tps1/tps1 disruptant did not accumulate trehalose at stationary phase or after heat shock. Growth of the tps1/tps1 disruptant at 30 degreesC was indistinguishable from that of the wild type. However, at 42 degreesC it did not grow on glucose or fructose but grew normally on galactose or glycerol. At 37 degreesC, the yeast-hypha transition in the mutant in glucose-calf serum medium did not occur. During growth at 42 degreesC, the mutant did not form hyphae in galactose or in glycerol. Some of the growth defects observed may be traced to an unbalanced sugar metabolism that reduces the cellular content of ATP. Mice inoculated with 10(6) CFU of the tps1/tps1 mutant did not show visible symptoms of infection 16 days after inoculation, while those similarly inoculated with wild-type cells were dead 12 days after inoculation.

The danger of metabolic pathways with turbo design

Many catabolic pathways begin with an ATP-requiring activation step, after which further metabolism yields a surplus of ATP. Such a 'turbo' principle is useful but also contains an inherent risk. This is illustrated by a detailed kinetic analysis of a paradoxical Saccharomyces cerevisiae mutant; the mutant fails to grow on glucose because of overactive initial enzymes of glycolysis, but is defective only in an enzyme (trehalose 6-phosphate synthase) that appears to have little relevance to glycolysis. The ubiquity of pathways that possess an initial activation step, suggests that there might be many more genes that, when deleted, cause rather paradoxical regulation phenotypes (i.e. growth defects caused by enhanced utilization of growth substrate).

The filamentous fungus Aspergillus niger contains two "differentially regulated" trehalose-6-phosphate synthase-encoding genes, tpsA and tpsB

Two genes encoding trehalose-6-phosphate synthase were cloned from Aspergillus niger. tpsA was cloned using the Saccharomyces cerevisiae GGS1/TPS1 gene as a probe. It encodes a 517-amino acid polypeptide with 64-70% similarity to trehalose-6-phosphate synthase of S. cerevisiae, Kluyveromyces lactis, and Schizosaccharomyces pombe. Its transcription occurs constitutively and is enhanced on carbon-derepressing carbon sources, coinciding with the presence of a CreA-binding nucleotide motif in the 5'-noncoding region of tpsA. Disruption of tpsA only weakly reduces growth on glucose, and neither influences the glucose induction of a low affinity glucose permease nor interferes with the catabolite repression of a pectinase; it causes reduced the heat tolerance of conidia. tpsB was cloned by a polymerase chain reaction-based strategy. Its 480 amino acid sequence showed 76.5% identity to tpsA. Its transcription was hardly detectable at ambient temperatures but was enhanced strongly upon heat shock, which agrees with the presence of several copies of a C4T stress-responsive element in its 5'-upstream sequences. Hence the function of yeast GGS1/TPS1 has been split into two differentially regulated genes in A. niger, of which none appears to be involved in glucose sensing.

Biochemical and molecular-genetic properties of a cytochrome-c-deficient mutant of Kluyveromyces lactis

We have isolated a respiration-deficient nuclear mutant of the yeast Kluyveromyces lactis that exhibited diminished levels of all cytochromes and did not grow on glycerol and other nonfermentable carbon sources. The mutant named cyc1 was transformed with a K. lactis genomic library and the DNA fragment conferring its wild-type properties was isolated and sequenced. The sequence of the isolated gene showed extensive homology with other eukaryotic cytochrome-c genes. The highest level of homology, based on the deduced amino acid sequences, was observed between the gene products of K. lactis and Hansenula anomala.

Evidence for trehalose-6-phosphate-dependent and -independent mechanisms in the control of sugar influx into yeast glycolysis

In the yeast Saccharomyces cerevisiae, trehalose-6-phosphate (tre-6-P) synthase encoded by GGS1/TPS1, is not only involved in the production of trehalose but also in restriction of sugar influx into glycolysis in an unknown fashion; it is therefore essential for growth on glucose or fructose. In this work, we have deleted the TPS2 gene encoding tre-6-P phosphatase in a strain which displays very low levels of Ggs1/TPS1, as a result of the presence of the byp 1-3 allele of GGS1/TPS1. The byp 1-3 tps2 delta double mutant showed elevated tre-6-P levels along with improved growth and ethanol production, although the estimated concentrations of glycolytic metabolites indicated excessive sugar influx. In the wild-type strain, the addition of glucose caused a rapid transient increase of tre-6-P. In tps 2 delta mutant cells, which showed a high tre-6-P level before glucose addition, sugar influx into glycolysis appeared to be diminished. Furthermore, we have confirmed that tre-6-P inhibits the hexokinases in vitro. These data are consistent with restriction of sugar influx into glycolysis through inhibition of the hexokinases by tre-6-P during the switch to fermentative metabolism. During logarithmic growth on glucose the tre-6-P level in wild-type cells was lower than that of the byp 1-3 tps2 delta mutant. However, the latter strain arrested growth and ethanol production on glucose after about four generations. Hence, other mechanisms, which also depend on Ggs1/Tps1, appear to control sugar influx during growth on glucose. In addition, we provide evidence that the requirement for Ggs1/Tps1 for sporulation may be unrelated to its involvement in trehalose metabolism or in the system controlling glycolysis.

Trehalose synthase: guard to the gate of glycolysis in yeast?

The addition of glucose to cells of the yeast Saccharomyces cerevisiae triggers a variety of regulatory phenomena. Initial glucose metabolism is required for the induction of most of them. Mutants deficient in both glucose-induced signalling and the control of initial glucose metabolism have a defect in the trehalose-6-phosphate synthase catalytic subunit of the trehalose synthase complex. This finding has raised novel questions about the control of glucose influx into glycolysis in yeast and its connection to the glucose-sensing mechanism. This dual function of the trehalose-6-phosphate synthase subunit has been found in several yeast species, suggesting that this control system might be widespread in fungi and possibly also in other organisms.

The byp1-3 allele of the Saccharomyces cerevisiae GGS1/TPS1 gene and its multi-copy suppressor tRNA(GLN) (CAG): Ggs1/Tps1 protein levels restraining growth on fermentable sugars and trehalose accumulation

Byp1-3 is an amber nonsense allele of the Saccharomyces cerevisiae GGS1/TPS1 gene which encodes the small subunit of the trehalose synthase complex. Mutations in this gene confer an inability to grow on glucose or fructose but the phenotype of byp1-3 mutants is leaky in a strain-dependent manner. Overexpression of the isolated byp1-3 allele suppressed the growth defect of a ggs1/tps1 delta mutant. Expression of an in-vitro-generated mutant allele of GGS1/TPS1 that lacks all the coding sequences downstream from the byp1-3 mutation led to the production of a shortened protein that did not complement the ggs1/tps1 delta mutant. We have isolated, as an allele-specific multi-copy suppressor of the growth defect of the byp1-3 mutant on fructose, the gene for tRNA(GLN) (CAG). Thus the leaky phenotype of byp1-3 mutants is due to a low level of read through of the internal nonsense codon by tRNA(GLN) (CAG). Using overexpression of the isolated byp1-3 allele, as well as of the tRNA(GLN) (CAG) gene, we were able to demonstrate that as little as about 10% of the normal Ggs1/Tps1 protein level is sufficient for slow growth on fructose. We also show a correlation between the level of Ggs1/Tps1, the ability to accumulate trehalose in stationary phase and the ability to grow on fermentable sugars. Sequence analysis of the cloned tRNA(GLN) (CAG) gene showed that it is located 700 bp upstream of URA10. However, we found considerable differences to the reported sequence of URA10, in particular in the non-coding region.

Trehalose-6-P synthase is dispensable for growth on glucose but not for spore germination in Schizosaccharomyces pombe

Trehalose-6-P inhibits hexokinases in Saccharomyces cerevisiae (M. A. Blázquez, R. Lagunas, C. Gancedo, and J. M. Gancedo, FEBS Lett. 329:51-54, 1993), and disruption of the TPS1 gene (formerly named CIF1 or FDP1) encoding trehalose-6-P synthase prevents growth in glucose. We have found that the hexokinase from Schizosaccharomyces pombe is not inhibited by trehalose-6-P even at a concentration of 3 mM. The highest internal concentration of trehalose-6-P that we measured in S. pombe was 0.75 mM after heat shock. We have isolated from S. pombe the tps1+ gene, which is homologous to the Saccharomyces cerevisiae TPS1 gene. The DNA sequence from tps1+ predicts a protein of 479 amino acids with 65% identity with the protein of S. cerevisiae. The tps1+ gene expressed from its own promoter could complement the lack of trehalose-6-P synthase in S. cerevisiae tps1 mutants. The TPS1 gene from S. cerevisiae could also restore trehalose synthesis in S. pombe tps1 mutants. A chromosomal disruption of the tps1+ gene in S. pombe did not have a noticeable effect on growth in glucose, in contrast with the disruption of TPS1 in S. cerevisiae. However, the disruption prevented germination of spores carrying it. The level of an RNA hybridizing with an internal probe of the tps1+ gene reached a maximum after 20 min of heat shock treatment. The results presented support the idea that trehalose-6-P plays a role in the control of glycolysis in S. cerevisiae but not in S. pombe and show that the trehalose pathway has different roles in the two yeast species.