Thermoresistance in Saccharomyces cerevisiae yeasts

Under natural conditions, yeast Saccharomyces cerevisiae reproduce, as a rule, on the surface of solid or liquid medium. Thus, life cycle of yeast populations is substantially influenced by diurnal changes in ambient temperature. The pattern in the response of unrestricted yeast S. cerevisiae culture to changes in the temperature of cultivation is revealed experimentally. Yeast population, in the absence of environmental constraints on the functioning of cell chemosmotic bioenergetic system, demonstrates the ability of thermoresistance when the temperature of cultivation switches from the range of 12-36 degrees C to 37.5-40 degrees C. During the transient period that is associated with the temperature switching and lasts from 1 to 4 turnover cycles, yeast reproduction rate remains 1.5-2 times higher than under stationary conditions. This is due to evolutionary acquired adaptive activity of cell chemosmotic system. After the adaptive resources exhausting, yeast thermoresistance fully recovers at the temperature range of 12-36 degrees C within one generation time under conditions of both restricted and unrestricted nourishment. Adaptive significance of such thermoresistance seems obvious enough--it allows maintaining high reproduction rate in yeast when ambient temperature is reaching a brief maximum shortly after noon.     

Killer systems of Saccharomyces cerevisiae yeasts

The killer systems of Saccharomyces cerevisiae are a peculiar group of cytoplasmic symbionts of primitive eukaryotes. The genetic material of these symbionts is double-stranded RNA. Their basic properties are linearity of genome, its fragmentation, resulting in two separately replicating major and minor segments, and the ability to control the synthesis of secretory proteins--mycocins which can kill the taxonomically related strains. Secretion of mycocins also confers immunity to their action. The strains containing killer symbionts are toxigenic and resistant to their own toxins, while those with no killer double-stranded RNA are sensitive to mycocins. The killer systems of Saccharomyces cerevisiae possess some properties relevant to viruses and evidently are evolved during the evolution of infectious viruses. Occurrence of such systems in monocellular eucaryotic organisms is an example of genome complication in the course of putting together the virus-like components. The peculiarities of replication and expression of killer systems and their utilization for the construction of vector molecules are discussed.     

Genetic regulation of differentiation towards meiosis in the yeast Saccharomyces cerevisiae

Normally, meiosis and sporulation in Saccharomyces cerevisiae occur only in diploid strains and only when the cells are exposed to starvation conditions. Diploidy is determined by the mating-type system (the genes MAT, RME1, IME1), whereas the starvation signal is transmitted through the adenylate cyclase - protein kinase pathway (the genes CDC25, RAS2, CDC35 (CYR1), BCY1, TPK1, TPK2, TPK3). The two regulatory pathways converge at the gene IME1, which is a positive regulator of meiosis and whose early expression in sporulating cells correlates with the initiation of meiosis. Sites upstream (5') of IME1 appear to mediate in the repression of the gene by repressors originating from both the mating-type and the cyclase--kinase pathways.     

Genetic polymorphism of sherry <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> yeasts

The review deals with natural diversity of sherry (“flor”) Saccharomyces cerevisiae yeasts. Various properties of these yeasts are analyzed: life cycles, fermentation of sugars, sensitivity to methyl violet and killer toxins, and resistance to ethanol and sulfite. Special attention is paid to molecular identification and differentiation of sherry yeasts. The history of their nomenclature is considered, including the names of possible subpopulations: “aceti,” “beticus” (“cheresienses”), “cordubensis,” “gaditensis,” “hispanica” (“prostoserdovii”), and “oxidans.”     

Acetaldehyde production in Saccharomyces cerevisiae wine yeasts

Abstract Eighty-six strains of Saccharomyces cerevisiae were investigated for their ability to produce acetaldehyde in synthetic medium and in grape must. Acetaldehyde production did not differ significantly between the two media, ranging from a few mg/l to about 60 mg/l, and was found to be a strain characteristic. The fermentation temperature of 30°C considerably increased the acetaldehyde produced. This study allowed us to assign the strains to different phenotypes: low, medium and high acetaldehyde producers. The low and high phenotypes differed considerably also in the production of acetic acid, acetoin and higher alcohols and can be useful for studying acetaldehyde production in S. cerevisiae , both from the technological and genetic point of view.     

Acetoin production in Saccharomyces cerevisiae wine yeasts

Abstract One hundred strains of Saccharomyces cerevisiae were examined for the capacity to produce acetoin in synthetic medium and in grape must. The low production of acetoin was found to be the more common pattern in this species. Most strains exhibited a similar distribution in both media, production ranging from non-detectable amounts to 12 mg 1⁻¹. Only four strains produced high quantities of acetoin, up to 29.5 mg l⁻¹ in synthetic medium and up to 194.6 mg l⁻¹ in grape must. This biometric study showed the existence of two phenotypes, "low and high acetoin production", that could be selected for conferring a desirable flavour of the final product.     

Genomic stability of Saccharomyces cerevisiae baker's yeasts.

The objective of this study has been to gather data on genomic stability of baker's yeast strains during long-term mitotic growth under restrictive conditions so that comparisons could be made to other studies indicating genomic instability during meiosis. The work describes the analysis of mitotic stability of the nuclear and mitochondrial genomes in the baker's yeast strain V1 during incubation in continuous culture for 190 generations (300 days). The cells were cultured in complete medium containing 2% glucose and 8 to 12% ethanol, as a mutagenic agent specific for mtDNA. The high concentration of ethanol severely limited the growth rate of the cells. DNA samples were monitored for chromosomal pattern, polymorphisms in selected nuclear genes (SUC2, MALIT, ADH1) and mobile genetic elements (Ty1 and Y'), and for RFLPs in mtDNA. The results show that both the nuclear and mitochondrial genomes of grande cells were very stable. However, the frequency of petite mutants in the population varied dramatically during the course of the experiment, reaching as high as 87% petite during the first 27 days of the experiment and declining to 5.8% petite at the end. This decline can be attributed to selection against petite mutants in media containing high concentrations of ethanol. Moreover, when samples and the parental strain were compared at the end of the experiment, no change could be observed in parameters such as their growth rate in different media, capacity to leave doughs, viability in ethanol or frequency of petite mutants. Results therefore indicated that the majority of the cells in the population were very similar to the parental throughout the experiments, with no apparent molecular or phenotypical changes.     

Fluid-phase endocytosis in yeasts other than Saccharomyces cerevisiae

A FITC-dextran internalization assay with Saccharomyces cerevisiae as positive control was used to determine whether fluid-phase endocytosis is a general characteristic of yeasts. Schizosaccharomyces pombe, Pichia polymorpha, Kluyveromyces phaseolosporus, Yarrowia lipolytica and Candida albicans were clearly positive, whereas results obtained with Debaryomyces marama were inconclusive. In all cases internalized FITC-dextran was found to be localized in the vacuoles and the process was always time- and temperature-dependent. Lower eucaryotes, particularly yeasts, appear to have the ability to incorporate substances from the extracellular medium through fluid-phase endocytosis.     

Metabolic-flux profiling of the yeasts Saccharomyces cerevisiae and Pichia stipitis

The so far largely uncharacterized central carbon metabolism of the yeast Pichia stipitis was explored in batch and glucose-limited chemostat cultures using metabolic-flux ratio analysis by nuclear magnetic resonance. The concomitantly characterized network of active metabolic pathways was compared to those identified in Saccharomyces cerevisiae, which led to the following conclusions. (i) There is a remarkably low use of the non-oxidative pentose phosphate (PP) pathway for glucose catabolism in S. cerevisiae when compared to P. stipitis batch cultures. (ii) Metabolism of P. stipitis batch cultures is fully respirative, which contrasts with the predominantly respiro-fermentative metabolic state of S. cerevisiae. (iii) Glucose catabolism in chemostat cultures of both yeasts is primarily oxidative. (iv) In both yeasts there is significant in vivo malic enzyme activity during growth on glucose. (v) The amino acid biosynthesis pathways are identical in both yeasts. The present investigation thus demonstrates the power of metabolic-flux ratio analysis for comparative profiling of central carbon metabolism in lower eukaryotes. Although not used for glucose catabolism in batch culture, we demonstrate that the PP pathway in S. cerevisiae has a generally high catabolic capacity by overexpressing the Escherichia coli transhydrogenase UdhA in phosphoglucose isomerase-deficient S. cerevisiae.     

Isolation and properties of phosphoribosyl-aminoimidazole-succinocarboxyamide-synthestase from Saccharomyces cerevisiae yeasts

An isolation procedure for phosphoribosyl succinocarboxamideaminoimidazole synthetase (SAICAR synthetase) (EC 6.3.2.6) has been developed. Pure SAICAR synthetase was found to be a monomeric protein with the apparent molecular weight of 36 kDa. The Michaelis constant for the three substrates of the reaction are 1.6 microM for CAIR, 14 microM for ATP and 960 microM for aspartic acid. The structural analogs of CAIR, 5-aminoimidazole ribotide and 5-aminoimidazole-4-carboxamide ribotide, act as competitive inhibitors of SAICAR synthetase. GTP and 2'-dATP can substitute for ATP in the reaction, while CTP and UTP inhibit the enzyme. No structural analogs of the aspartic acid were found to have affinity for SAICAR synthetase. The optimal reaction conditions for the enzyme were established to be at pH 8.0 and magnesium chloride concentration around 5 mM.     

Comparison of glycolytic NADH oscillations in yeasts Saccharomyces cerevisiae and Saccharomyces carlsbergensis

The possible mechanism of synchronization of NADH oscillations in yeasts were studied. It was shown that the synchronization time depends on cell concentration in suspension. Synchronization of oscillations after acetaldehyde addition was found in Saccharomyces carlsbergensis whereas in S. cerevisiae oscillations were synchronized after adding potassium cyanide. It is possible, that synchronization of oscillations in S. cerevisiae requires low concentration of acetaldehyde and the high acetaldehyde concentration synchronizes oscillations in S. carlsbergensis. In addition, a possible mechanism of synchronization by acetaldehyde in proposed.     

Molecular genetic diversity of the Saccharomyces yeasts in Taiwan: Saccharomyces arboricola, Saccharomyces cerevisiae and Saccharomyces kudriavzevii

Genetic hybridization, sequence and karyotypic analyses of natural Saccharomyces yeasts isolated in different regions of Taiwan revealed three biological species: Saccharomyces arboricola, Saccharomyces cerevisiae and Saccharomyces kudriavzevii. Intraspecies variability of the D1/D2 and ITS1 rDNA sequences was detected among S. cerevisiae and S. kudriavzevii isolates. According to molecular and genetic analyses, the cosmopolitan species S. cerevisiae and S. kudriavzevii contain local divergent populations in Taiwan, Malaysia and Japan. Six of the seven known Saccharomyces species are documented in East Asia: S. arboricola, S. bayanus, S. cerevisiae, S. kudriavzevii, S. mikatae, and S. paradoxus.     

De novo synthesis of monoterpenes by Saccharomyces cerevisiae wine yeasts

This paper reports the production of monoterpenes, which elicit a floral aroma in wine, by strains of the yeast Saccharomyces cerevisiae. Terpenes, which are typical components of the essential oils of flowers and fruits, are also present as free and glycosylated conjugates amongst the secondary metabolites of certain wine grape varieties of Vitis vinifera. Hence, when these compounds are present in wine they are considered to originate from grape and not fermentation. However, the biosynthesis of monoterpenes by S. cerevisiae in the absence of grape derived precursors is shown here to be of de novo origin in wine yeast strains. Higher concentration of assimilable nitrogen increased accumulation of linalool and citronellol. Microaerobic compared with anaerobic conditions favored terpene accumulation in the ferment. The amount of linalool produced by some strains of S. cerevisiae could be of sensory importance in wine production. These unexpected results are discussed in relation to the known sterol biosynthetic pathway and to an alternative pathway for terpene biosynthesis not previously described in yeast.     

Characteristics of the final stages of ergosterol biosynthesis in Saccharomyces cerevisiae yeasts

A scheme of ergosterol cyclic intermediate transformations in S. cerevisiae has been proposed. It is based on the analysis of sterol composition in strains with mutant blocks of one or two reactions of enzymatic synthesis. Biochemical reactions of modification of cyclic part and side branch of sterol molecule are supposed to be carried out independently.