Influence of the concentration of glucose and galactose on the physiology of Saccharomyces cerevisiae in continuous culture

Determination of the oenological properties of yeast strains isolated from spontaneously fermented grape musts obtained from cool climate grape varieties

The international competitiveness of the wine sector and consumer demands for the unique wine styles pose challenges in improving the fermentation process. The basis of proper alcoholic fermentation is knowledge about how individual yeast strains interact with the aroma, taste and color of wine, what results in possibility to select species used as starter cultures. To use the value of non-Saccharomyces yeast strains in wine production and to minimize the possibility of wine deterioration, it is necessary to precisely recognize the yeast cultures present on the fruit of the vine and in grape must, as well as their metabolic properties. The aim of the study was to determine the oenological properties of yeasts isolated from spontaneously fermented grape musts obtained from cool climate grapes. For this purpose, Zweigelt grape must was fermented with yeast monocultures. Alcohol, extract, sugars, glycerol, total acidity and free amine nitrogen were analyzed in the obtained wines. Poor fermentation properties of yeast strains results in obtaining wines with relatively large amounts of residual sugars and low alcohol. A decrease in overall acidity was noted in sets with the participation of M. pulcherrima MG971264, while in other tests the opposite trend was observed. Although some microorganisms have the ability to assimilate organic acids found in wine, they are not able to carry out fermentation or they do it inefficiently. Solution to this problem may, therefore, be use of mixed cultures of noble and non-Saccharomyces yeast, what effectively reduce the concentration of organic acids, while not adversely affecting the organoleptic characteristics of the drink.

Production efficiency of the bacterial non-ribosomal peptide indigoidine relies on the respiratory metabolic state in S. cerevisiae

BACKGROUND

Beyond pathway engineering, the metabolic state of the production host is critical in maintaining the efficiency of cellular production. The biotechnologically important yeast Saccharomyces cerevisiae adjusts its energy metabolism based on the availability of oxygen and carbon sources. This transition between respiratory and non-respiratory metabolic state is accompanied by substantial modifications of central carbon metabolism, which impact the efficiency of metabolic pathways and the corresponding final product titers. Non-ribosomal peptide synthetases (NRPS) are an important class of biocatalysts that provide access to a wide array of secondary metabolites. Indigoidine, a blue pigment, is a representative NRP that is valuable by itself as a renewably produced pigment.

RESULTS

Saccharomyces cerevisiae was engineered to express a bacterial NRPS that converts glutamine to indigoidine. We characterize carbon source use and production dynamics, and demonstrate that indigoidine is solely produced during respiratory cell growth. Production of indigoidine is abolished during non-respiratory growth even under aerobic conditions. By promoting respiratory conditions via controlled feeding, we scaled the production to a 2 L bioreactor scale, reaching a maximum titer of 980 mg/L.

CONCLUSIONS

This study represents the first use of the Streptomyces lavendulae NRPS (BpsA) in a fungal host and its scale-up. The final product indigoidine is linked to the activity of the TCA cycle and serves as a reporter for the respiratory state of S. cerevisiae. Our approach can be broadly applied to investigate diversion of flux from central carbon metabolism for NRPS and other heterologous pathway engineering, or to follow a population switch between respiratory and non-respiratory modes.

Lipid Biosynthesis and its Coordination with Cell Cycle Progression

The activation of cell cycle regulators at the G1/S boundary has been linked to the cellular protein synthesis rate. It is conceivable that regulatory mechanisms are required to allow cells to coordinate the synthesis of other macromolecules with cell cycle progression. The availability of highly synchronized cells and flow cytometric methods facilitates investigation of the dynamics of lipid synthesis in the entire cell cycle of the heterotrophic dinoflagellate Crypthecodinium cohnii. Flow cytograms of Nile red-stained cells revealed a stepwise increase in the polar lipid content and a continuous increase in neutral lipid content in the dinoflagellate cell cycle. A cell cycle delay at early G1, but not G2/M, was observed upon inhibition of lipid synthesis. However, lipid synthesis continued during cell cycle arrest at the G1/S transition. A cell cycle delay was not observed when inhibitors of cellulose synthesis and fatty acid synthesis were added after the late G1 phase of the cell cycle. This implicates a commitment point that monitors the synthesis of fatty acids at the late G1 phase of the dinoflagellate cell cycle. Reduction of the glucose concentration in the medium down-regulated the G1 cell size with a concomitant forward shift of the commitment point. Inhibition of lipid synthesis up-regulated cellulose synthesis and resulted in an increase in cellulosic contents, while an inhibition of cellulose synthesis had no effects on lipid synthesis. Fatty acid synthesis and cellulose synthesis are apparently coupled to the cell cycle via independent pathways.

The effect of stress factors on the spontaneous photon emission from microorganisms

The results of recent work on the photon emission from three yeasts and a bacterium is presented. Both visible region and ultraviolet photon emission is observed; however, no luminescence is observed in the absence of oxygen. The visible region emission is attributed to excited carbonyl groups and excited singlet oxygen dimers formed during the decomposition of lipid hydroperoxides. Possible sources of the ultraviolet photon emission are also examined. The use of microorganisms in the study of ultraweak photon emission and its relation to oxidative, temperature and chemical stress is reviewed and the applications and (or) functions of this photon emission are also discussed.

Extrusion of metabolites from baker's yeast during glucose-induced acidification

Extrusion of metabolites (glycerol, lactic, malic, and succinic acid) during the medium acidification caused by resting baker's yeast supplied with 200 mM glucose was studied under aerobic and anaerobic conditions and in the absence and presence of 14 mM KCl. The maximum levels of glycerol and of the sum of acids (about 13 and 8 mM, respectively) were attained anaerobically; aerobiosis reduced the levels by 40-50% and the presence of K+ ions by another 10-20%. The time courses of glucose consumption and medium acidification were similar aerobically and anaerobically. The glucose consumption curves exhibited a short plateau about 2 min after glucose addition, caused probably by a rapid osmotic equilibration of glucose across the cell mambrane. Metabolite extrusion indicates that at high glucose concentrations the alcohol dehydrogenase reaction is supplemented by other reactions aiding in the maintenance of a balanced NAD+/NADH ratio in the cells.

Ammonia assimilation in the fission yeast Schizosaccharomyces pombe 972

Glutamine synthetase (GS) activity of Schizosaccharomyces pombe 972 was high in ammonia-limited cultures, low in phosphate- and sulphate-limited cultures and not detected in glucose-limited cultures. When ammonia was 'pulsed' into an ammonia-limited culture then GS activity decreased at a rate faster than that calculated if enzyme synthesis ceased and enzyme was diluted out by growth. Enzyme activity increased in ammonia-starved, phosphate-limited cultures and in the ammonia 'pulse' system when the added ammonia had been utilised. These increases in enzyme activity were prevented by the presence of 100 mug/ml cycloheximide. GS activity was inversely related to the intracellular concentration of glutamate.

The substrate constant for the ammonium ion of growing Saccharomyces cerevisiae

The relation between ammonium concentration and growth rate was studied in steady state continuous cultures of Saccharomyces cerevisiae in nitrogen-limited glucose ammonium medium. This relation could be described by the Monod equation. A maximum specific growth rate of 0.41 h-1 and a substrate constant for ammonium of 5-11 muM were calculated. Ammonium was determined by a modification of the phenol hypochlorite method. A discussion of the results in view of literature data on the substrate constants for other nutrients is given.

The amino acid pool of Hansenula holstil: characterisation, and changes mediated by environment

Amino acid pools extracted from Hansenula holstii grown in continuous culture with either ammonia or nitrate as sole source of nitrogen, under a variety of substrate limitations, were characterised and quantified. Pools from corresponding cultures were shown to be similar in size and composition, regardless of whether ammonia or nitrate was the nitrogen source. Large changes in pools (both quantitative and qualitative) occurred when cultures were grown under different substrate limitations. Such changes were particularly large in glutamate, glutamine, alanine, lysine and arginine; the possible significance of such environment-mediated changes is discussed.

Inorganic nitrogen assimilation in yeasts: alteration in enzyme activities associated with changes in cultural conditions and growth phase

Ammonia assimilation has been investigated in four strains of Saccharomyces cerevisiae by measuring, at intervals throughout the growth cycle, the activities of several enzymes concerned with inorganic ammonia assimilation. Enzyme activities in extracts of cells were compared after growth in complete and defined media. The effect of shift from growth in a complete to growth in a defined medium (and the reverse) was also determined. The absence of aspartase (EC 4.3.1.1, l-aspartate-ammonia lyase) activity, the low specific activities of alanine dehydrogenase, glutamine synthetase [EC 6.3.1.2, l-glutamate-ammonia ligase (ADP)], and the marked increase in activity of the nicotinamide adenine dinucleotide phosphate-linked glutamate dehydrogenase (NADP-GDH) [EC 1.4.1.4, l-glutamate:NADP-oxidoreductase (deaminating)] during the early stages of growth support the conclusion that yeasts assimilate ammonia primarily via glutamate. The NADP-GDH showed a rapid increase in activity just before the initiation of exponential growth, reached a maximum at the mid-exponential stage, and then gradually declined in activity in the stationary phase. The NADP-GDH reached a higher level of activity when the yeasts were grown on the defined medium as compared with complete medium. The nicotinamide adenine dinucleotide-linked glutamate dehydrogenase (NAD-GDH) [EC 1.4.1.2, l-glutamate:NAD-oxidoreductase (deaminating)] showed only slight increases in activity during the exponential phase of growth. There was an inverse relationship in that the NADP-GDH increased in activity as the NAD-GDH decreased. The NAD-GDH activity was higher after growth on the complete medium. The glutamate-oxaloacetate transaminase (EC 2.6.1.1. l-aspartate:2-oxoglutarate aminotransferase) activity rose and fell in parallel with the NADP-GDH, although its specific activity was somewhat lower. Although other ammonia-assimilatory enzymes were demonstrable, it seems unlikely that their combined activities could account for the remainder of the ammonia-assimilatory capacity not accounted for by the NADP-GDH. The ability of aspartate to serve as effectively as glutamate as the sole source of nitrogen for the growth of yeast apparently resides in their ability to utilize aspartate for amino acid biosynthesis via transamination.