Comparison of wild-type andReg 1 mutant saccharomyces cerevisiae metabolic levels during glucose and galactose metabolism using31P NMR

Elucidation of the cytoplasmic and vacuolar components in the inorganic phosphate region in the 31P NMR spectrum of yeast

Subcellular compartments, such as the vacuole in yeast, play important roles in cell metabolism and in cell response to external conditions. Concentrations of inorganic phosphate and pH values of the vacuole and cytoplasm were determined for anaerobic Saccharomyces cerevisiae cells based upon (31)P NMR spectroscopy. A new approach allows the determination of these values for the vacuole in cases when the resonance for inorganic phosphate in the cytoplasm overlaps with the resonance for inorganic phosphate in the vacuole. The intracellular inorganic phosphate resonance was first decomposed into two components by computer analysis. The assignments of the components were determined from in vivo correlations of P(i) chemical shift and the chemical shifts of the cytoplasmic sugar phosphates, and the pH dependency of the resonance of pyrophosphate and the terminal phosphate of poly-phosphate (PP(1)) which reside in the vacuole. An in vivo correlation relating PP(1) and P(i) (vac) chemical shifts was established from numerous evaluations of intracellular compositions for several strains of S. cerevisiae. This correlation will aid future analysis of (31)P NMR spectra of yeast and will extend NMR studies of compartmentation to cellular suspensions in phosphate-containing medium. Application of this method shows that both vacuolar and extracellular P(i) were phosphate reserves during glycolysis in anaerobic S. cerevisiae. Net transport of inorganic phosphate across the vacuolar membrane was not correlated with the pH gradient across the membrane.

Optimization of an in vivo plant P450 monooxygenase system in Saccharomyces cerevisiae

Cytochrome P450s are heme-thiolate oxygenases involved in a wide number of reactions such as epoxidation, hydroxylation, and demethylation. Heterologously expressed eukaryotic P450s are potentially useful biocatalysts for stereospecific oxygenation reactions under mild conditions. Numerous factors, such as intracellular pH, cytochrome P450, cytochrome P450 reductase, NADPH, and oxygen concentration all influence the in vivo activity. To systematically examine these factors, we selected ferulate 5-hydroxylase (F5H), a plant P450, with the Saccharomyces cerevisiae WAT11 strain as an expression host. Two media compositions and two cultivation procedures were investigated to optimize the in vivo activity of F5H. We modified a previously published selective growth medium (Pompon et al. [1996] Methods Enzymol 272:51-64) that increased the specific growth rate and cell yield of the host strain. A cultivation procedure with separate growth and induction stages that each contained selective media resulted in a 45% increase of whole cell F5H specific activity. In a medium designed for simultaneous growth and induction, we observed a 2.6-fold higher specific F5H activity, but substantially lower cell yield. Surprisingly, in this medium the higher specific F5H activity did not correlate with a higher P450 concentration. The effects of addition of the first committed heme precursor, delta-aminolevulinic acid, and Fe(III) at the beginning of induction period were also studied for our two-stage procedure. A small, but significant (P < 0.05) increase in whole cell F5H activity was observed following ALA addition.

Dynamic in vivo (31)P nuclear magnetic resonance study of Saccharomyces cerevisiae in glucose-limited chemostat culture during the aerobic-anaerobic shift

The purpose of this work was to analyse in vivo the influence of sudden oxygen depletion on Saccharomyces cerevisiae, grown in glucose-limited chemostat culture, using a recently developed cyclone reactor coupled with (31)P NMR spectroscopy. Before, during and after the transition, intracellular and extracellular phosphorylated metabolites as well as the pHs in the different cellular compartments were monitored with a time resolution of 2.5 min. The employed integrated NMR bioreactor system allowed the defined glucose-limited continuous cultivation of yeast at a density of 75 g DW/l and a p(O(2)) of 30% air saturation. A purely oxidative metabolism was maintained at all times. In vivo (31)P NMR spectra obtained were of excellent quality and even allowed the detection of phosphoenolpyruvate (PEP). During the switch from aerobic to anaerobic conditions, a rapid, significant decrease of intracellular ATP and PEP levels was observed and the cytoplasmic pH decreased from 7.5 to 6.8. This change, which was accompanied by a transient influx of extracellular inorganic phosphate (P(i)), appeared to correlate linearly with the decrease of the ATP concentration, suggesting that the cause of the partial collapse of the plasma membrane pH gradient was a reduced availability of ATP. The complete phosphorous balance established from our measurement data showed that polyphosphate was not the source of the increased intracellular P(i). The derived intracellular P(i), ATP and ADP concentration data confirmed that the glycolytic flux at the level of glyceraldehyde-3-phosphate dehydrogenase, 3-phosphoglycerate kinase and enolase enzymes is mainly controlled by thermodynamic constraints.

In vivo nuclear magnetic resonance studies of glycolytic kinetics in Lactococcus lactis

The metabolism of glucose by nongrowing cells of L. lactis strain MG5267 was studied under controlled conditions of pH, temperature, and gas atmosphere (anaerobic and aerobic) using a circulating system coupled to nuclear magnetic resonance (NMR) detection that allowed a noninvasive determination of intracellular pools of intermediate metabolites by 13C-NMR with a time resolution of 30 seconds. In addition, intracellular parameters, such as pH, NTP levels, and concentration of inorganic phosphate in the cytoplasm, could be monitored on-line by 31P-NMR with a time resolution of approx. 3 min. The time course for the concentrations of intracellular fructose 1,6-bisphosphate (FBP), 3-phosphoglycerate (3-PGA), and phosphoenolpyruvate (PEP), together with kinetic measurements of substrate consumption and endproducts formation, were used as a basis for the construction of a mechanistic model for glycolysis. In vivo measurements were complemented with determinations of phosphorylated metabolites in perchloric acid extracts. A top-down model was developed by simplifying the metabolism to the resolution allowed by the experimental data collected by in vivo NMR (grouped in seven metabolic steps). This simplified mechanistic model was adjusted to the metabolite concentrations determined by in vivo NMR. The results obtained led to the rationalization of the dynamics of glucose metabolism as being driven largely by ATP surplus. This excess causes accumulation of FBP due to NAD+ limitation, whose regeneration is dependent on downstream pyruvate reduction. The model was capable of predicting qualitative shifts in the metabolism of glucose when changing from anaerobic to aerobic conditions.

Intracellular pH of Candida albicans blastospores as measured by laser microspectrofluorimetry and 31P-NMR

The intracellular pH (pHi) of Candida albicans blastospores harvested from 8 h or 48 h cultures was determined under identical experimental conditions by two different techniques: 31P-NMR and laser microspectrofluorimetry. Time dependence of pHi was monitored by 31P-NMR on the whole cell population. Microspectrofluorimetry, after loading of the cells with SNARF-1, enabled the determination of pHi in isolated cells and its distribution among the cell population. By this method, the vacuolar pH could not be distinguished from the cytoplasmic pH in C. albicans blastospores, but alkalization of pHi was observed at the beginning of germ tubes. The absolute values of pHi determined by 31P-NMR were slightly different from those obtained by laser microspectrofluorimetry. However, the pH distributions in the cell population were converging. For blastospores in exponential phase a gaussian distribution of pHi was observed with both methods, the cells maintained a steady pHi value when the external pH was varied from 5.5 to 8.5. For cells in stationary phase two pools were identified: the combination of the two techniques demonstrated the presence of two different subpopulations. One of these population (with lower pH) was able to commute to the other one with time as shown by 31P-NMR kinetics. This information is reported here for the first time in C. albicans.

Yeast sugar transporters

Transport of sugars is a fundamental property of all eukaryotic cells. Of particular importance is the uptake of glucose, a preferred carbon and energy source. The rate of glucose utilization in yeast is often dictated by the activity and concentration of glucose transporters in the plasma membrane. Given the importance of transport as a site of control of glycolytic flux, the regulation of glucose transporters is necessarily complex. The molecular analysis of these transporters in Saccharomyces has revealed the existence of a multigene family of sugar carriers. Recent data have raised the question of the actual role of all of these proteins in sugar catabolism, as some appear to be lowly expressed, and point mutations of these genes may confer pleiotropic phenotypes, inconsistent with a simple role as catabolic transporters. The transporters themselves appear to be intimately involved in the process of sensing glucose, a model for which there is growing support.

Dynamics of activation of a galactose-inducible promoter in Saccharomyces cerevisiae

We have investigated the dynamics of accumulation of the Escherichia coli beta-galactosidase (beta-gal) under the control of a promoter containing the galactose-inducible upstream activating sequence (UASG) in single Saccharomyces cerevisiae cells. The accumulation of beta-gal in single cells following the addition of the inducer, galactose, was determined using an in situ combined DNA and immunofluorescent stain in conjunction with flow cytometry. Two strains were studied, D603/2i, which has two copies of the galactose-inducible fusion gene integrated into its genome, and D603/pLGSD5, which carries a 2 microns-based plasmid containing the fusion gene. Flow cytometry results indicate that accumulation of beta-gal within the first three hours following the addition of galactose is dependent on cell cycle position. Two proposed mechanisms explaining this observed behavior are (1) the cell-cycle-dependent synthesis of the fusion protein or (2) the unequal partitioning of the protein at cell division between mother and daughter cells.