Regulation of sugar and ethanol metabolism in Saccharomyces cerevisiae

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

"Active" one-carbon generation in Saccharomyces cerevisiae.

A new mutation introducing a one-carbon requirement (e.g., formate) for the glycine-supplemented growth of a serine-glycine auxotroph (ser1) was correlated with a lack of glycine decarboxylase activity. The presence of oxalate decarboxylase activity or glyoxylate decarboxylase activity did not overcome the one-carbon requirement. Another mutation characterized by the absence of oxalate decarboxylase activity did not introduce a one-carbon requirement. The presence and physiological significance of glycine decarboxylase activity in Saccharomyces are thus inferred.     

Regulation of C1 metabolism by l-methionine in Saccharomyces cerevisiae

1. The concentrations of folate derivatives in aerobic cultures of Saccharomyces cerevisiae (A.T.C.C. 9763) were determined by microbiological assay employing Lactobacillus casei (A.T.C.C. 7469) and Pediococcus cerevisiae (A.T.C.C. 8081). Cells cultured in media lacking l-methionine contained higher concentrations of folate derivatives than cells grown in the same media supplemented with 2.5mumol of l-methionine/ml. The concentrations of highly conjugated derivatives were also decreased by supplementing the growth medium with l-methionine. 2. DEAE-cellulose column chromatography of extracts prepared from cells grown under these conditions revealed that the concentrations of methylated tetrahydrofolates were drastically decreased by the methionine supplement. Smaller decreases were also observed in the concentrations of formylated and unsubstituted derivatives. 3. The concentrations of four enzymes of C(1) metabolism were compared after 6h of growth in the presence and in the absence of l-methionine (2.5mumol/ml). The specific activities of formyltetrahydrofolate synthetase, methylenetetrahydrofolate reductase and serine hydroxymethyltransferase were not altered by this treatment but that of 5-methyltetrahydrofolate-homocysteine methyltransferase was decreased by approx. 65% when l-methionine was supplied. The activities of 5-methyltetrahydrofolate-homocysteine methyltransferase, serine hydroxymethyltransferase and formyltetrahydrofolate synthetase were not appreciably altered by l-methionine in vitro. In contrast this amino acid was found to inhibit the activity of methylenetetrahydrofolate reductase. 4. Feeding experiments employing sodium [(14)C]formate indicated that cells grown in the presence of exogenous methionine, although having less ability to convert formate into methionine, readily incorporated (14)C into serine and the adenosyl moiety of S-adenosylmethionine. 5. It is suggested that exogenous l-methionine controls C(1) metabolism in Saccharomyces principally by regulation of methyl-group biogenesis within the folate pool.     

Regulation of trehalose metabolism by Adox and AdoMet in Saccharomyces cerevisiae

Effect of a potent methylation inhibitor oxidized adenosine (Adox), and a universal methyl group donor S-adenosyl-L-methionine (AdoMet) on trehalose metabolism was studied in two haploids of S. cerevisiae of mating types MATalpha, met3 (6460 -8D) and MATa, leu2, ura3, his4 (8534 -10A). Trehalose level decreased in presence of Adox in both strains. Both neutral trehalase (NT) and trehalose-6-phosphate (tre-6-p) synthase activities increased in presence of Adox in -8D strain. Decrease in trehalose level in -8D thus could not be explained in the light of increased tre-6-p synthase activity; however, it could be correlated with increased NT activity. In strain -10A, NT activity was reduced in presence of Adox while tre-6-p synthase activity increased. Enzyme activity profiles in -10A thus do not explain the reduced trehalose level on Adox treatment. Effect of AdoMet was not very prominent in either strain, though in -8D a small increase in trehalose level was seen on treatment. Intracellular AdoMet level of untreated cells of -10A was seen to be almost six times higher than that of -8D. Further, AdoMet treatment caused increase in its level compared to untreated cells, suggesting AdoMet uptake. No effect of either compound was seen on acid trehalase (AT) activity in any strain. The results suggest that there was a possible effect of demethylation on trehalose metabolism (particularly in the synthetic direction) in both strains, though effect of methylation was not very prominent, the reason for which is not very clear.     

Regulation of trehalose metabolism in Saccharomyces cerevisiae mutants during temperature shifts

Temperature shifts from 23 degrees C to 36 degrees C resulted in trehalose accumulation in Saccharomyces independently of genetic lesions in the cAMP-protein kinase cascade. In parallel, trehalose 6-phosphate synthase activity increased about 3-fold in all strains; the increase could be inhibited by cycloheximide, suggesting that protein synthesis was required. Heat shock treatment after the temperature shift led to a drastic increase in trehalose activity, and deactivation of the biosynthetic enzyme with a consequent drop in trehalose. Up to now no definite correlation between acquisition of thermotolerance and trehalose accumulation has been made.     

Regulation of enzymes and isoenzymes of carbohydrate metabolism in the yeast Saccharomyces cerevisiae

An electrophoretic method has been devised to investigate the changes in the enzymes and isoenzymes of carbohydrate metabolism, upon adding glucose to derepressed yeast cells. (i) Of the glycolytic enzymes tested, enolase II, pyruvate kinase and pyruvate decarboxylase were markedly increased. This increase was accompanied by an overall increase in glycolytic activity and was prevented by cycloheximide, an inhibitor of protein synthesis. (ii) In contrast, respiratory activity decreased after adding glucose. This decrease was clearly shown to be the result of repression of respiratory enzymes. A rapid decrease within a few minutes of adding glucose, by analogy with the so-called ' Crabtree effect', was not observed in yeast. (iii) The gluconeogenic enzymes, fructose-1,6-bisphosphatase and malate dehydrogenase, which are inactivated after adding glucose, showed no significant changes in electrophoretic mobilities. Hence, there was no evidence of enzyme modifications, which were postulated as initiating degradation. However, it was possible to investigate cytoplasmic and mitochondrial malate dehydrogenase isoenzymes separately. Synthesis of the mitochondrial isoenzyme was repressed, whereas only cytoplasmic malate dehydrogenase was subject to glucose inactivation.     

Regulation of aspartate-derived amino-acid metabolism in Zygosaccharomyces rouxii compared to Saccharomyces cerevisiae

To elucidate the growth inhibitory effect of threonine, the regulation of the aspartate-derived amino-acid metabolism in Zygosaccharomyces rouxii, an important yeast for the flavor development in soy sauce, was investigated. It was shown that threonine inhibited the growth of Z. rouxii by blocking the methionine synthesis. It seemed that threonine blocked this synthesis by inhibiting the conversion of aspartate. In addition, it was shown that the growth of Z. rouxii, unlike that of Saccharomyces cerevisiae, was not inhibited by the herbicide sulfometuron methyl (SMM). From enzyme assays, it was concluded that the acetohydroxy acid synthase in Z. rouxii, unlike that in S. cerevisiae, was not sensitive to SMM. Furthermore, the enzyme assays demonstrated that the activity of threonine deaminase in Z. rouxii, like in S. cerevisiae, was strongly inhibited by isoleucine and stimulated by valine. From this work, it is clear that the aspartate-derived amino-acid metabolism in Z. rouxii only partly resembles that in S. cerevisiae.     

Regulation of macroautophagy in Saccharomyces cerevisiae

Macroautophagy (hereafter autophagy) is a cellular degradation process, which in yeast is induced in response to nutrient deprivation. In this process, a double-membrane vesicle, an autophagosome, surrounds part of the cytoplasm and fuses with the vacuole to allow the breakdown and subsequent recycling of the cargo. In yeast, many autophagy-related (ATG) genes have been identified that are required for selective and/or nonselective autophagy. In all autophagy-related pathways, core Atg proteins are required for the formation of the autophagosome, which is one of the most unique aspects of autophagy and is unlike other vesicle transport events. In contrast to nonselective autophagy, the selective processes are induced in response to various specific physiological conditions such as alterations in the carbon source. In this review, we provide an overview of the common aspects concerning the mechanism of autophagy-related pathways, and highlight recent advances in our understanding of the machinery that controls autophagy induction in response to nutrient starvation conditions.     

Regulation of retrotransposition in Saccharomyces cerevisiae

Retrotransposons are a widely distributed group of eukaryotic mobile genetic elements that transpose through an RNA intermediate. The element Ty (Transposon yeast), found in the yeast Saccharomyces cerevisiae, is a model system for the study of retrotransposons because of the experimental tools that exist to manipulate and detect transposition. Ty transposition can be elevated to levels exceeding one transposition event per cell when an element is expressed from an inducible yeast promoter. In addition, individual genomic Ty elements can be tagged with a retrotransposition indicator gene that allows transposition events occurring at a rate of 10(-5) to 10(-7) per element per cell division to be detected phenotypically. These systems are being used to elucidate the mechanism of Ty transposition and clarify how Ty transposition is controlled.     

The interplay between sulphur and selenium metabolism influences the intracellular redox balance in Saccharomyces cerevisiae

Selenium (Se) is an essential element for most eukaryotic organisms, including humans. The balance between Se toxicity and its beneficial effects is very delicate. It has been demonstrated that a diet enriched with Se has cancer prevention potential in humans. The most popular commercial Se supplementation is selenized yeast, which is produced in a fermentation process using an inorganic source of Se. Here, we show that the uptake of Se, Se toxic effects and intracellular Se-metabolite profile are largely influenced by the level of sulphur source supplied during the fermentation. A Yap1-dependent oxidative stress response is active when yeast actively metabolizes Se, and this response is linked to the generation of intracellular redox imbalance. The redox imbalance derives from a disproportionate ratio between the reduced and oxidized forms of glutathione and also from the influence of Se metabolism on the central carbon metabolism. The observed increase in glycerol production rate, concomitant with the inhibition of ethanol formation in the presence of Se, can be ascribed to the occurrence of redox imbalance that triggers glycerol biosynthesis to replenish the pool of NAD(+) .     

Purine metabolism in Saccharomyces cerevisiae.

The synthesis, interconversion, and catabolism of purine bases, ribonucleosides, and ribonucleotides in wild-type Saccharomyces cerevisiae were studied by measuring the conversion of radioactive adenine, hypoxanthine, guanine, and glycine into acid-soluble purine bases, ribonucleosides, and ribonucleotides, and into nucleic acid adenine and guanine. The pathway(s) by which adenine is converted to inosinate is (are) uncertain. Guanine is extensively deaminated to xanthine. In addition, some guanine is converted to inosinate and adenine nucleotides. Inosinate formed either from hypoxanthine or de novo is readily converted to adenine and guanine nucleotides.     

Regulation of phosphatidylcholine biosynthesis in Saccharomyces cerevisiae

Evidence is presented which indicates that the biosynthesis of phosphatidylcholine by the methylation pathway in growing cultures of Saccharomyces cerevisiae is repressed by the presence of choline in the growth medium. This result, obtained previously for glucose-grown cells, was also observed for lactate-grown cells, of which half of the phosphatidylcholine is mitochondrial. A respiration-deficient mutant of the parent wild-type strain has been studied, and its inability to form functional mitochondria cannot be due to an impaired methylation pathway, as it has been shown to incorporate (14)C-CH(3)-methionine into all of the methylated glycerophosphatides. The incorporation rate is depressed by the inclusion of 1 mm choline in the growth medium, suggesting a regulatory effect similar to that demonstrated for the wild-type strain. The effects of choline on the glycerophospholipid composition of lactate and glucose-grown cells is presented. The repressive effects of the two related bases, mono- and dimethylethanolamine, were examined, and reduced levels of (14)C-CH(3)-methionine incorporation were found for cells grown in the presence of these bases. The effect of choline on the methylation rates is reversible and glucosegrown cells regain the nonrepressed level of methylation activity in 60 to 80 min after removal of choline from the growth medium.     

Regulation of Biotin Transport in Saccharomyces cerevisiae

The metabolic control of biotin transport in Saccharomyces cerevisiae was investigated. Nonproliferating cells harvested from cultures grown in excess biotin (25 ng/ml) took up small amounts of biotin, whereas cells grown in biotin-sufficient medium (0.25 ng/ml) accumulated large amounts of the vitamin. Transport was inhibited maximally in cells grown in medium containing 9 ng (or more) of biotin per ml. When avidin was added to biotin-excess cultures, the cells developed the ability to take up large amounts of biotin. Boiled avidin was without effect, as was treatment of cells with avidin in buffer. Avidin did not relieve transport inhibition when added to biotin-excess cultures treated with cycloheximide, suggesting that protein synthesis was required for cells to develop the capacity to take up biotin after removal of extracellular vitamin by avidin. Cycloheximide did not inhibit the activity of the preformed transport system in biotin-sufficient cells. The presence of high intracellular free biotin pools did not inhibit the activity of the transport system. The characteristics of transport in biotin-excess cells (absence of temperature or pH dependence, no stimulation by glucose, absence of iodoacetate inhibition, independence of uptake on cell concentration, and nonsaturation kinetics) indicated that biotin entered these cells by diffusion. The results suggest that the synthesis of the biotin transport system in S. cerevisiae may be repressed during growth in medium containing high concentrations of biotin.