Anaerobic nutrition of Saccharomyces cerevisiae. II. Unsaturated fatty acid requirement for growth in a defined medium

Regulation of alcoholic fermentation in batch and chemostat cultures of Kluyveromyces lactis CBS 2359

Kluyveromyces lactis is an important industrial yeast, as well as a popular laboratory model. There is currently no consensus in the literature on the physiology of this yeast, in particular with respect to aerobic alcoholic fermentation ('Crabtree effect'). This study deals with regulation of alcoholic fermentation in K. lactis CBS 2359, a proposed reference strain for molecular studies. In aerobic, glucose-limited chemostate cultures (D = 0.05-0.40 h-1) growth was entirely respiratory, without significant accumulation of ethanol or other metabolities. Alcoholic fermentation occurred in glucose-grown shake-flask cultures, but was absent during batch cultivation on glucose in fermenters under strictly aerobic conditions. This indicated that ethanol formation in the shake-flask cultures resulted from oxygen limitation. Indeed, when the oxygen feed to steady-state chemostat cultures (D = 0.10 h-1) was lowered, a mixed respirofermentative metabolism only occurred at very low dissolved oxygen concentrations (less than 1% of air saturation). The onset of respirofermentative metabolism as a result of oxygen limitation was accompanied by an increase of the levels of pyruvate decarboxylase and alcohol dehydrogenase. When aerobic, glucose-limited chemostat cultures (D = 0.10 h-1) were pulsed with excess glucose, ethanol production did not occur during the first 40 min after the pulse. However, a slow aerobic ethanol formation was invariably observed after this period. Since alcoholic fermentation did not occur in aerobic batch cultures this is probably a transient response, caused by an imbalanced adjustment of enzyme levels during the transition from steady-state growth at mu = 0.10 h to growth at mu max. It is concluded that in K. lactis, as in other Crabtree-negative yeasts, the primary environmental trigger for occurrence of alcoholic fermentation is oxygen limitation.

The transcriptional regulator Hap1p (Cyp1p) is essential for anaerobic or heme-deficient growth of Saccharomyces cerevisiae: Genetic and molecular characterization of an extragenic suppressor that encodes a WD repeat protein

We report here that Hap1p (originally named Cyp1p) has an essential function in anaerobic or heme-deficient growth. Analysis of intragenic revertants shows that this function depends on the amino acid preceding the first cysteine residue of the DNA-binding domain of Hap1p. Selection of recessive extragenic suppressors of a hap1-hem1- strain allowed the identification, cloning, and molecular analysis of ASC1 (Cyp1 Absence of growth Supressor). The sequence of ASC1 reveals that its ORF is interrupted by an intron that shelters the U24 snoRNA. Deletion of the intron, inactivation of the ORF, and molecular localization of the mutations show unambiguously that it is the protein and not the snoRNA that is involved in the suppressor phenotype. ASC1, which is constitutively transcribed, encodes an abundant, cytoplasmically localized 35-kD protein that belongs to the WD repeat family, which is found in a large variety of eucaryotic organisms. Polysome profile analysis supports the involvement of this protein in translation. We propose that the absence of functional Asc1p allows the growth of hap1-hem1- cells by reducing the efficiency of translation. Based on sequence comparisons, we discuss the possibility that the protein intervenes in a kinase-dependent signal transduction pathway involved in this last function.

The two isoenzymes for yeast NAD+-dependent glycerol 3-phosphate dehydrogenase encoded by GPD1 and GPD2 have distinct roles in osmoadaptation and redox regulation

The two homologous genes GPD1 and GPD2 encode the isoenzymes of NAD-dependent glycerol 3-phosphate dehydrogenase in the yeast Saccharomyces cerevisiae. Previous studies showed that GPD1 plays a role in osmoadaptation since its expression is induced by osmotic stress and gpd1 delta mutants are osmosensitive. Here we report that GPD2 has an entirely different physiological role. Expression of GPD2 is not affected by changes in external osmolarity, but is stimulated by anoxic conditions. Mutants lacking GPD2 show poor growth under anaerobic conditions. Mutants deleted for both GPD1 and GPD2 do not produce detectable glycerol, are highly osmosensitive and fail to grow under anoxic conditions. This growth inhibition, which is accompanied by a strong intracellular accumulation of NADH, is relieved by external addition of acetaldehyde, an effective oxidizer of NADH. Thus, glycerol formation is strictly required as a redox sink for excess cytosolic NADH during anaerobic metabolism. The anaerobic induction of GPD2 is independent of the HOG pathway which controls the osmotic induction of GPD1. Expression of GPD2 is also unaffected by ROX1 and ROX3, encoding putative regulators of hypoxic and stress-controlled gene expression. In addition, GPD2 is induced under aerobic conditions by the addition of bisulfite which causes NADH accumulation by inhibiting the final, reductive step in ethanol fermentation and this induction is reversed by addition of acetaldehyde. We conclude that expression of GPD2 is controlled by a novel, oxygen-independent, signalling pathway which is required to regulate metabolism under anoxic conditions.

Approaches to the study of Rox1 repression of the hypoxic genes in the yeast Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is a facultative aerobe that responds to changes in oxygen tension by changing patterns of gene expression. One set of genes that responds to this environmental cue is the hypoxic genes. Oxygen levels are sensed by changes in heme biosynthesis, which controls the transcription of the ROX1 gene, encoding a protein that binds to the regulatory region of each hypoxic gene to repress transcription. Several experimental molecular and genetic approaches are described here to study Rox1 repression. Derepression of the hypoxic genes is rapid, and one model for such a response requires that Rox1 have a short half-life. This was demonstrated to be the case by immunoblotting using a c-myc epitope-tagged protein. Rox1 repression is mediated through the general repressors Ssn6 and Tup1. To explore possible interactions among these proteins, all three were expressed and partially purified using a baculovirus expression system and histidine-tagged proteins. The effect of Ssn6 and Tup1 on the formation of Rox1-DNA complexes was explored using these purified proteins by both electrophoretic mobility shift and DNase I protection assays. We found that Rox1 DNA-binding activity decayed rapidly and that Ssn6 could stabilize and restore lost activity. Finally, genetic selections are described for the isolation of loss-of-function mutations in Rox1. Also, schemes are proposed for the reversion of such mutations. These selections have been extended to genetic analyses of the TUP1 and SSN6 genes.

Regulation of hypoxic gene expression in yeast

Baker's yeast, Saccharomyces cerevisiae, can adapt to growth under severe oxygen limitation. Two regulatory systems are described here that control this adaptation. The first involves a heme-dependent repression mechanism. Cells sense hypoxia through the inability to maintain oxygen-dependent heme biosynthesis. Under aerobic conditions, heme accumulates and serves as an effector for the transcriptional activator Hap1. The heme-Hap1 complex activates transcription of the ROX1 gene that encodes a repressor of one set of hypoxic genes. Under hypoxic conditions, heme levels fall, and a heme-deficient Hap1 complex represses ROX1 expression. As a consequence, the hypoxic genes are derepressed. The second regulatory system activates gene expression in response to a variety of stress conditions, including oxygen limitation. Oxygen sensing in this system is heme-independent. The same DNA sequence mediates transcriptional activation of each stress signal.

Xylulose and glucose fermentation by Saccharomyces cerevisiae in chemostat culture

Saccharomyces cerevisiae ATCC 24860 was cultivated in chemostat culture under anoxic conditions with 111.1 mmol of glucose liter-1 alone or with a mixture of 66.7 mmol of xylulose liter-1 and 111.1 mmol of glucose liter-1. The substrate consumption rate was 5.4 mmol g of cells-1 h-1 for glucose, whereas for xylulose it was 1.0 mmol g of cells-1 h-1. The ethanol yield decreased from 0.52 carbon mole of ethanol produced per carbon mole of sugar consumed during the utilization of glucose alone to 0.49 carbon mole produced per carbon mole consumed during the simultaneous utilization of xylulose and glucose, while cell biomass was maintained at 2.04 to 2.10 g liter-1. Xylulose coutilization was accompanied by a shift in product formation from ethanol to acetate and arabinitol. Xylulokinase activity was absent during glucose metabolism but detectable during simultaneous utilization of xylulose and glucose. Xylulose cometabolism resulted in increased in vitro activity of pyruvate decarboxylase and an increased concentration of the intracellular metabolite fructose 1,6-diphosphate without significant changes in the concentrations of 6-phosphogluconate and pyruvate. The results are discussed in relation to (i) altered enzyme activities and (ii) the redox flux of the cell.

Dynamics of ammonia uptake in nitrogen limited anaerobic cultures of Saccharomyces cerevisiae

Dynamics of the ammonia uptake by Saccharomyces cerevisiae under anaerobic conditions was studied in ammonia limited continuous cultures. A large number of pulse additions of ammonia (25-100 mg 1(-1)) were made at different dilution rates (0.05-0.20 h-1). The response was followed by on-line monitoring of the carbon dioxide evolution rate (CER), optical density, and by frequent analysis of extra- and intracellular metabolites. The uptake of a pulse of ammonia proceeded in a qualitatively highly reproducible pattern. Initially, a rapid and growth rate dependent uptake of ammonia was observed (lasting for about 10-15 min). Next followed a phase with little uptake (approx. 5 min). Finally, the rest of the ammonia pulse was taken up at a somewhat smaller rate which also depended on the growth rate. The first phase coincided with an increase in CER caused by mobilization of the intracellular carbohydrate trehalose and subsequently of glycogen. Regardless of dilution rate and the amount of ammonia added, the initial high uptake rate of ammonia was maintained until approximately the same amount of ammonia had been taken up. Transition from the first to the second uptake phase was associated with an increased glycerol production, indicating an elevated anabolic activity.

Xylose-metabolizing Saccharomyces cerevisiae strains overexpressing the TKL1 and TAL1 genes encoding the pentose phosphate pathway enzymes transketolase and transaldolase

Saccharomyces cerevisiae was metabolically engineered for xylose utilization. The Pichia stipitis CBS 6054 genes XYL1 and XYL2 encoding xylose reductase and xylitol dehydrogenase were cloned into S. cerevisiae. The gene products catalyze the two initial steps in xylose utilization which S. cerevisiae lacks. In order to increase the flux through the pentose phosphate pathway, the S. cerevisiae TKL1 and TAL1 genes encoding transketolase and transaldolase were overexpressed. A XYL1- and XYL2-containing S. cerevisiae strain overexpressing TAL1 (S104-TAL) showed considerably enhanced growth on xylose compared with a strain containing only XYL1 and XYL2. Overexpression of only TKL1 did not influence growth. The results indicate that the transaldolase level in S. cerevisiae is insufficient for the efficient utilization of pentose phosphate pathway metabolites. Mixtures of xylose and glucose were simultaneously consumed with the recombinant strain S104-TAL. The rate of xylose consumption was higher in the presence of glucose. Xylose was used for growth and xylitol formation, but not for ethanol production. Decreased oxygenation resulted in impaired growth and increased xylitol formation. Fermentation with strain S103-TAL, having a xylose reductase/xylitol dehydrogenase ratio of 0.5:30 compared with 4.2:5.8 for S104-TAL, did not prevent xylitol formation.

Effects of growth conditions on mitochondrial morphology in Saccharomyces cerevisiae

Effects of growth conditions on mitochondrial morphology were studied in living Saccharomyces cerevisiae cells by vital staining with the fluorescent dye dimethyl-aminostyryl-methylpyridinium iodine (DASPMI), fluorescence microscopy, and confocal-scanning laser microscopy. Cells from respiratory, ethanol-grown batch cultures contained a large number of small mitochondria. Conversely, cells from glucose-grown batch cultures, in which metabolism was respiro-fermentative, contained small numbers of large, branched mitochondria. These changes did not significantly affect the fraction of the cellular volume occupied by the mitochondria. Similar differences in mitochondrial morphology were observed in glucose-limited chemostat cultures. In aerobic chemostat cultures, glucose metabolism was strictly respiratory and cells contained a large number of small mitochondria. Anaerobic, fermentative chemostat cultivation resulted in the large, branched mitochondrial structures also seen in glucose-grown batch cultures. Upon aeration of a previously anaerobic chemostat culture, the maximum respiratory capacity increased from 10 to 70 mumole.min-1.g dry weight-1 within 10 h. This transition resulted in drastic changes of mitochondrial number, morphology and, consequently, mitochondrial surface area. These changes continued for several hours after the respiratory capacity had reached its maximum. Cyanide-insensitive oxygen consumption contributed ca. 50% of the total respiratory capacity in anaerobic cultures, but was virtually absent in aerobic cultures. The response of aerobic cultures to oxygen deprivation was qualitatively the reverse of the response of anaerobic cultures to aeration. The results indicate that mitochondrial morphology in S. cerevisiae is closely linked to the metabolic activity of this yeast: conditions that result in repression of respiratory enzymes generally lead to the mitochondrial morphology observed in anaerobically grown, fermenting cells.

Xylitol formation and reduction equivalent generation during anaerobic xylose conversion with glucose as cosubstrate in recombinant Saccharomyces cerevisiae expressing the xyl1 gene

Glucose was used as a cosubstrate under anaerobic conditions in the conversion of xylose to xylitol by a recombinant Saccharomyces cerevisiae strain expressing the xyl1 gene. Glucose was metabolized mainly through glycolysis, with carbon dioxide, acetate, and ethanol as end products and with reduction equivalents generated in the glyceraldehyde-3-phosphate dehydrogenase and acetaldehyde dehydrogenase reactions. At a high glucose supply rate, generation of surplus reduction equivalents resulted in simultaneous ethanol formation. On the other hand, at a low glucose supply rate, additional reduction equivalents were generated by simultaneous ethanol consumption. A significantly lower xylitol formation rate was observed.

Chemostat cultivation as a tool for studies on sugar transport in yeasts

Chemostat cultivation enables investigations into the effects of individual environmental parameters on sugar transport in yeasts. Various means are available to manipulate the specific rate of sugar uptake (qs) in sugar-limited chemostat cultures. A straightforward way to manipulate qs is variation of the dilution rate, which, in substrate-limited chemostat cultures, is equal to the specific growth rate. Alternatively, qs can be varied independently of the growth rate by mixed-substrate cultivation or by variation of the biomass yield on sugar. The latter can be achieved, for example, by addition of nonmetabolizable weak acids to the growth medium or by variation of the oxygen supply. Such controlled manipulation of metabolic fluxes cannot be achieved in batch cultures, in which various parameters that are essential for the kinetics of sugar transport cannot be controlled. In sugar-limited chemostat cultures, yeasts adapt their sugar transport systems to cope with the low residual sugar concentrations, which are often in the micromolar range. Under the conditions, yeasts with high-affinity proton symport carriers have a competitive advantage over yeasts that transport sugars via facilitated-diffusion carriers. Chemostat cultivation offers unique possibilities to study the energetic consequences of sugar transport in growing cells. For example, anaerobic, sugar-limited chemostat cultivation has been used to quantify the energy requirement for maltose-proton symport in Saccharomyces cerevisiae. Controlled variation of growth conditions in chemostat cultures can be used to study the differential expression of genes involved in sugar transport and as such can make an important contribution to the ongoing studies on the molecular biology of sugar transport in yeasts.

Kinetics of growth and sugar consumption in yeasts

An overview is presented of the steady- and transient state kinetics of growth and formation of metabolic byproducts in yeasts. Saccharomyces cerevisiae is strongly inclined to perform alcoholic fermentation. Even under fully aerobic conditions, ethanol is produced by this yeast when sugars are present in excess. This so-called 'Crabtree effect' probably results from a multiplicity of factors, including the mode of sugar transport and the regulation of enzyme activities involved in respiration and alcoholic fermentation. The Crabtree effect in S. cerevisiae is not caused by an intrinsic inability to adjust its respiratory activity to high glycolytic fluxes. Under certain cultivation conditions, for example during growth in the presence of weak organic acids, very high respiration rates can be achieved by this yeast. S. cerevisiae is an exceptional yeast since, in contrast to most other species that are able to perform alcoholic fermentation, it can grow under strictly anaerobic conditions. 'Non-Saccharomyces' yeasts require a growth-limiting supply of oxygen (i.e. oxygen-limited growth conditions) to trigger alcoholic fermentation. However, complete absence of oxygen results in cessation of growth and therefore, ultimately, of alcoholic fermentation. Since it is very difficult to reproducibly achieve the right oxygen dosage in large-scale fermentations, non-Saccharomyces yeasts are therefore not suitable for large-scale alcoholic fermentation of sugar-containing waste streams. In these yeasts, alcoholic fermentation is also dependent on the type of sugar. For example, the facultatively fermentative yeast Candida utilis does not ferment maltose, not even under oxygen-limited growth conditions, although this disaccharide supports rapid oxidative growth.

Optimization of the simultaneous saccharification and fermentation process using thermotolerant yeasts

Different treatments to improve the thermotolerance of fermenting yeasts for simultaneous ethanol saccharification and fermentation process of cellulosic materials have been examined. Yeasts of the genera Saccharomyces and Kluyveromyces were tested for growth and fermentation at progressively higher temperatures in the range of 42-47 degrees C. The best results were obtained with K. marxianus LG, which was then submitted to different treatments in order to achieve thermotolerant clones. A total of 35 new clones were obtained that dramatically improved the SSF of 10% Solka-floc substrate at 45 degrees C when compared to the original strain, some with ethanol concentrations as high as 33 g/L.

Determinants of flocculence of brewer's yeast during fermentation in wort

Ability of Saccharomyces cerevisiae MPY3 cells to flocculate during fermentation in wort was found to be triggered after growth limitation by oxygen shortage and to coincide with a sharp increase in cell surface hydrophobicity of the cells. Presence of oxygen in the pitching wort influenced final cell number, flocculence of the cells and cell surface hydrophobicity. Flocculation ability of cells grown in air-depleted pitching wort was hampered, concomitant with a decrease in final cell number and in final cell surface hydrophobicity. Addition of ergosterol and Tween 80 to air-depleted wort restored normal growth of the cells as well as flocculation ability and the increase in cell surface hydrophobicity. The same parameters increased in value after addition of ergosterol and Tween 80 to a fermentation with air-saturated pitching wort. Hydrophobicity of a non-flocculent mutant of S. cerevisiae strain MPY3, fermenting in air-saturated pitching wort, did not increase at cell division arrest. These results support the hypothesis that cell surface hydrophobicity is a major determinant for yeast cells to become flocculent, and suggest that shortage of sterols and unsaturated fatty acids precedes flocculence under brewing conditions.

Regulation of gene expression by oxygen in Saccharomyces cerevisiae

The oxygen regulation of two broad categories of yeast genes is discussed in this review. The first is made up of genes regulated by heme, and the second is made up of genes whose regulation is heme independent. Heme-regulated genes fall into two classes: heme-activated and heme-repressed genes. Activation is achieved through one of two transcriptional activators, the heme-dependent HAP1 protein or the heme-activated, glucose-repressed HAP2/3/4 complex. Some of the properties and the DNA-binding sites of these activators are discussed. Heme repression is achieved through the action of the ROX1 repressor, the expression of which is transcriptionally activated by heme. Once ROX1 is synthesized, its function is heme independent. Evidence that ROX1 binds to DNA or is part of a DNA-binding complex is described. Factors which modulate the function of these regulatory proteins are discussed, and a schematic of heme activation and repression is presented. The mitochondrial subunits of cytochrome c oxidase are induced by oxygen in a heme-independent fashion. The translation of one, cytochrome c oxidase subunit III, is dependent upon three nucleus-encoded initiation factors. One of these, PET494, is itself translationally regulated by oxygen in a heme-independent fashion. The expression of at least four other mitochondrially encoded cytochrome subunits is dependent upon specific translation factors, raising the potential for translational regulation as a general mechanism. Finally, a number of anaerobic genes that show heme-independent, oxygen-repressed expression have been identified. These fall into two kinetic classes, suggesting that there are at least two different regulatory circuitries.

Physiology of yeasts in relation to biomass yields

The stoichiometric limit to the biomass yield (maximal assimilation of the carbon source) is determined by the amount of CO2 lost in anabolism and the amount of carbon source required for generation of NADPH. This stoichiometric limit may be reached when yeasts utilize formate as an additional energy source. Factors affecting the biomass yield on single substrates are discussed under the following headings: Energy requirement for biomass formation (YATP). YATP depends strongly on the nature of the carbon source. Cell composition. The macroscopic composition of the biomass, and in particular the protein content, has a considerable effect on the ATP requirement for biomass formation. Hence, determination of for instance the protein content of biomass is relevant in studies on bioenergetics. Transport of the carbon source. Active (i.e. energy-requiring) transport, which occurs for a number of sugars and polyols, may contribute significantly to the calculated theoretical ATP requirement for biomass formation. P/O-ratio. The efficiency of mitochondrial energy generation has a strong effect on the cell yield. The P/O-ratio is determined to a major extent by the number of proton-translocating sites in the mitochondrial respiratory chain. Maintenance and environmental factors. Factors such as osmotic stress, heavy metals, oxygen and carbon dioxide pressures, temperature and pH affect the yield of yeasts. Various mechanisms may be involved, often affecting the maintenance energy requirement. Metabolites such as ethanol and weak acids. Ethanol increases the permeability of the plasma membrane, whereas weak acids can act as proton conductors. Energy content of the growth substrate. It has often been attempted in the literature to predict the biomass yield by correlating the energy content of the carbon source (represented by the degree of reduction) to the biomass yield or the percentage assimilation of the carbon source. An analysis of biomass yields of Candida utilis on a large number of carbon sources indicates that the biomass yield is mainly determined by the biochemical pathways leading to biomass formation, rather than by the energy content of the substrate.

Involvement of heme components in sterol metabolism of Saccharomyces cerevisiae

There is an intimate association between sterol biosynthesis in yeast and aerobicity. Besides the requirement for molecular oxygen for the epoxidation of squalene, cytochrome hemoproteins are involved in demethylation and desaturation steps. Regulatory effects of hemes on sterol formation have been demonstrated using specifically defective mutants of yeast. Heme competency participates in a mechanism whereby wild-type cells are prevented from taking exogenous sterols from the growth media. The multiple interactions of hemes and sterols appear to be associated with the variously defined functions for sterols in the yeast cells.

Oxygen requirements of yeasts

Type species of 75 yeast genera were examined for their ability to grow anaerobically in complex and mineral media. To define anaerobic conditions, we added a redox indicator, resazurin, to the media to determine low redox potentials. All strains tested were capable of fermenting glucose to ethanol in oxygen-limited shake-flask cultures, even those of species generally regarded as nonfermentative. However, only 23% of the yeast species tested grew under anaerobic conditions. A comparative study with a number of selected strains revealed that Saccharomyces cerevisiae stands out as a yeast capable of rapid growth at low redox potentials. Other yeasts, such as Torulaspora delbrueckii and Candida tropicalis, grew poorly mu max, 0.03 and 0.05 h-1, respectively) under anaerobic conditions in mineral medium supplemented with Tween 80 and ergosterol. The latter organisms grew rapidly under oxygen limitation and then displayed a high rate of alcoholic fermentation. It can be concluded that these yeasts have hitherto-unidentified oxygen requirements for growth.

Effect of nystatin, amphotericin B and amphotericin B methyl ester on Saccharomyces cerevisiae with different lipid composition

Saccharomyces cerevisiae was cultured under anaerobiosis in semi-complete medium to which either palmitoleic or oleic acid was added. Cells were grown at 20 degrees C or 30 degrees C. The levels of total lipids, total sterols, and phospholipids were higher in cells grown at 20 degrees C than at 30 degrees C. The effects of nystatin (NYS), amphotericin B (AMB), and amphotericin B methyl ester (AME) were evaluated by determining cell viability and liberation of intracellular compounds. The loss of cell viability is higher in the first 30 minutes of incubation with the drugs and is the same regardless of the type of cells obtained. Low molecular weight compounds and ions such as K+ are liberated a few minutes after incubation with the drugs whereas proteins and substances absorbing at 260 nm are liberated later. Phosphate liberation comes after K+ and before compounds of higher molecular weights.

Growth and metabolism of Pneumocystis carinii in axenic culture

Recent analyses of rRNA have suggested a taxonomic relationship of Pneumocystis carinii with fungi. We investigated various liquid and solid media without feeder cells for the ability to support growth of P. carinii. Organisms were obtained from the lungs of immunosuppressed rats and separated from host cells by treatment with hypotonic solutions and a series of microfiltrations. The growth of P. carinii was quantitated by staining nuclei with Diff-Quik. Optimal replication was observed in media with neopeptone and N-acetylglucosamine at low pH. Increases of at least 8- to 10-fold occurred during the first 24 to 72 h. P. carinii replication was influenced by pH, temperature, inoculum size, and anti-P. carinii drugs. The viability of organisms was confirmed by incorporation of radiolabeled uracil, methionine, and N-acetylglucosamine. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of trichloroacetic acid-precipitable compounds showed that [35S]methionine was incorporated into specific P. carinii proteins. Thus, P. carinii can metabolize and replicate in the absence of mammalian cells. This system should have applications for in vitro drug screening and other molecular and metabolic analyses.

Competition for glucose between the yeasts Saccharomyces cerevisiae and Candida utilis

The competition between the yeasts Saccharomyces cerevisiae CBS 8066 and Candida utilis CBS 621 for glucose was studied in sugar-limited chemostat cultures. Under aerobic conditions, C. utilis always successfully completed against S. cerevisiae. Only under anaerobic conditions did S. cerevisiae become the dominant species. The rationale behind these observations probably is that under aerobic glucose-limited conditions, high-affinity glucose/proton symporters are present in C. utilis, whereas in S. cerevisiae, glucose transport occurs via facilitated diffusion with low-affinity carriers. Our results explain the frequent occurrence of infections by Crabtree-negative yeasts during bakers' yeast production.

Intracellular accumulation of AMP as a cause for the decline in rate of ethanol production by Saccharomyces cerevisiae during batch fermentation

A general hypothesis is presented for the decline in the rate of ethanol production (per unit of cell protein) during batch fermentation. Inhibition of ethanol production is proposed to result from the intracellular accumulation of AMP during the transition from growth to the stationary phase. AMP acts as a competitive inhibitor of hexokinase with respect to ATP. When assayed in vitro in the presence of ATP and AMP concentrations equivalent to those within cells at different stages of fermentation, hexokinase activity declined in parallel with the in vivo decline in the rate of ethanol production. The coupling of glycolytic flux and fermentation to cell growth via degradation products of RNA may be of evolutionary advantage for Saccharomyces cerevisiae. Such a coupling would reduce the exposure of nongrowing cells to potentially harmful concentrations of waste products from metabolism and would conserve nutrients for future growth under more favorable conditions.

Selective inactivation of heterokaryons of Candida albicans by anaerobiosis

Heterokaryons (hets), but not monokaryons of Candida albicans die when grown anaerobically on minimal medium. Their rates of inactivation increase with decreases in growth temperatures from 37 degrees C to 25 degrees C. At 10 degrees C, however, anaerobiosis is not lethal and suppresses the inactivation which normally occurs among hets cultured aerobically at that temperature. Killing of hets by anaerobiosis can be altered significantly by certain exogenously provided amino acids or intermediates of oxidative respiration. Aspartic acid alone promotes inactivation whereas alanine, glutamic acid or lysine individually have no effects. However, glutamate and lysine combined afford slight protection against inactivation while aspartate and glutamate combined, with or without lysine, are highly protective: the activity of the aspartate-glutamate combination is completely negated by the addition of alanine. Other common amino acids have no effects on het responses to anaerobiosis other than the ability, when combined, to relieve the antagonism of alanine for the aspartate-glutamate combination. Anaerobic survivals are also enhanced by oxalacetic acid or alpha-ketoglutaric acid, and even more so by a combination of these two intermediates. The resistances to inactivation elicited by the oxalacetate alpha-ketoglutarate or aspartate-glutamate combinations are not additive. These relationships are interpreted to signify that inactivation of hets by anaerobic growth is largely, if not exclusively, due to depletion of their oxalacetic acid and alpha-ketoglutaric acid contents for amino acid biosyntheses, and the unique inability of het cells to replenish those keto acids upon subsequent return to aerobic conditions. The observations are consistent with previous indications that mitochondria formed by hets are functionally abnormal.

Ethanol production during batch fermentation with Saccharomyces cerevisiae: changes in glycolytic enzymes and internal pH

During batch fermentation, the rate of ethanol production per milligram of cell protein is maximal for a brief period early in this process and declines progressively as ethanol accumulates in the surrounding broth. Our studies demonstrate that the removal of this accumulated ethanol does not immediately restore fermentative activity, and they provide evidence that the decline in metabolic rate is due to physiological changes (including possible ethanol damage) rather than to the presence of ethanol. Several potential causes for the decline in fermentative activity have been investigated. Viability remained at or above 90%, internal pH remained near neutrality, and the specific activities of the glycolytic and alcohologenic enzymes (measured in vitro) remained high throughout batch fermentation. None of these factors appears to be causally related to the fall in fermentative activity during batch fermentation.

Ethanol tolerance in yeasts

It is now certain that the inherent ethanol tolerance of the Saccharomyces strain used is not the prime factor regulating the level of ethanol that can be produced in a high sugar brewing, wine, sake, or distillery fermentation. In fact, in terms of the maximum concentration that these yeasts can produce under batch (16 to 17% [v/v]) or fed-batch conditions, there is clearly no difference in ethanol tolerance. This is not to say, however, that under defined conditions there is no difference in ethanol tolerance among different Saccharomyces yeasts. This property, although a genetic determinant, is clearly influenced by many factors (carbohydrate level, wort nutrition, temperature, osmotic pressure/water activity, and substrate concentration), and each yeast strain reacts to each factor differently. This will indeed lead to differences in measured tolerance. Thus, it is extremely important that each of these be taken into consideration when determining "tolerance" for a particular set of fermentation conditions. The manner in which each alcohol-related industry has evolved is now known to have played a major role in determining traditional thinking on ethanol tolerance in Saccharomyces yeasts. It is interesting to speculate on how different our thinking on ethanol tolerance would be today if sake fermentations had not evolved with successive mashing and simultaneous saccharification and fermentation of rice carbohydrate, if distillers' worts were clarified prior to fermentation but brewers' wort were not, and if grape skins with their associated unsaturated lipids had not been an integral part of red wine musts. The time is now ripe for ethanol-related industries to take advantage of these findings to improve the economies of production. In the authors' opinion, breweries could produce higher alcohol beers if oxygenation (leading to unsaturated lipids) and "usable" nitrogen source levels were increased in high gravity worts. White wine fermentations could also, if desired, match the higher ethanol levels in red wines if oxygenation (to provide the unsaturated lipids deleted in part by the removal of the grape skins) were practiced and if care were given to assimilable nitrogen concentrations. This would hold true even at 10 to 14 degrees C, and the more rapid fermentations would maximize utilization of winery tankage.(ABSTRACT TRUNCATED AT 400 WORDS)