| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
1 Center of Biology, Department of Biology, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal
2 Centro de Recursos Microbiológicos (CREM), Biotechnology Unit, Faculty of Sciences and Technology, New University of Lisbon, 2829-516 Caparica, Portugal
3 Life and Health Sciences Research Institute (ICVS), School of Health Sciences, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal
Correspondence
M. J. Sousa
mjsousa{at}bio.uminho.pt
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
|---|
These authors contributed equally to this work. |
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
|---|
; Almeida & Pais, 1996a). The biotechnological interest in T. delbrueckii has increased in recent years due to its particularly high freezing and osmotic tolerance (Almeida & Pais, 1996b; Ok & Hashinaga, 1997; Hernandez-Lopez et al., 2003; Alves-Araújo et al., 2004a). Moreover, strains of T. delbrueckii have been shown to display dough-raising capacities similar to those of commercial baker's yeasts (Almeida & Pais, 1996b), thus reinforcing their potential application in the baking industry. However, few reports exist on the genetics, biochemistry and physiology of T. delbrueckii, in contrast to the vast knowledge on the traditional baker's yeast S. cerevisiae.
Two main aspects must be considered when selecting a yeast strain for the baking industry (Benitez et al., 1996): effective biomass production in molasses, and dough-leavening ability. Sucrose is the primary carbon and energy source for growth in molasses, the industrial substrate used for large-scale baker's yeast production. Expression of invertase, the hydrolytic enzyme required to convert sucrose into glucose and fructose, is repressed by high glucose concentrations (Mormeneo & Sentandreu, 1982). Despite the high levels of invertase activity required for growth in molasses, there is evidence that the capacity of S. cerevisiae to ferment high sucrose concentrations, like those present in sweet bread doughs, is inversely related to the activity of this enzyme (Attfield & Kletsas, 2000). This is usually ascribed to the reduction in water activity resulting from sucrose hydrolysis and the consequent negative effect on yeast performance. Although there is a small amount of free sugars in the flour (0.3–0.5 % – essentially glucose, fructose, sucrose and maltose), the maltose gradually released from starch as a result of amylolytic activity represents the major fermentable sugar in the dough (Ponte & Reed, 1982). Maltose metabolism requires the presence of both a maltose transporter and a maltase. In S. cerevisiae, the constitutive internal maltase is considered sufficient to hydrolyse maltose, and sugar utilization is limited by maltose uptake (Goldenthal et al., 1987). The efficiency in gas production is determined by high maltase and maltose transport activities (Higgins et al., 1999).
The specific growth rate is a key control parameter in the industrial production of baker's yeast (van Hoek et al., 1998). The biomass productivity of S. cerevisiae is limited by the aerobic fermentation occurring in high-sugar media (Crabtree effect), demanding a high-oxygen fed-batch cultivation method to keep the sugar concentration low and avoid fermentative metabolism. Redirection of the respiro-fermentative flux at high sugar concentrations, and consequent improvements in biomass yields, have been successfully achieved by alleviating glucose repression, either by overexpressing a protein involved in the repressing pathway (Blom et al., 2000) or by engineering glucose uptake rates (Otterstedt et al., 2004). Notably, glucose transport has been shown by different authors to play a fundamental role in the fate of glycolytic flux in S. cerevisiae (Diderich et al., 1999; Ye et al., 1999).
In glucose-limited oxygen-sufficient chemostat cultures, T. delbrueckii shows biomass yields similar to those obtained for S. cerevisiae and consistent with fully respiratory growth. As the oxygen feed rate decreases, S. cerevisiae is the first to switch to a respiro-fermentative metabolism, already showing a decrease in biomass yield at oxygen tensions still able to sustain full respiration in T. delbrueckii. However, T. delbrueckii shows considerably poorer growth than S. cerevisiae under strict anaerobic conditions (Visser et al., 1990; Hanl et al., 2005).
We have undertaken physiological and biochemical studies of T. delbrueckii in batch cultures with sugars present in molasses and in bread dough, using them alone and in mixtures. A strain isolated from traditional corn and rye bread dough in northern Portugal and showing particularly promising characteristics, T. delbrueckii PYCC 5321, was used. The resulting information on sugar utilization patterns, maltase and invertase activities, sugar uptake rates and respiration/fermentation rates contributes to a better evaluation of the potential offered by this yeast to the baking industry.
|
METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
|---|
Since sucrose is the primary carbon and energy substrate present in beet or cane molasses used for industrial baker's yeast production, the inoculum for all experiments was prepared in YPS medium, containing, per litre, 20 g sucrose, 40 g peptone, 20 g yeast extract, 2 g KH2PO4 and 1 g MgSO4.7H2O. Peptone (0118-17) and yeast extract (0127-17) were from Difco and sucrose from Merck. Cells were harvested from a 24 h culture, washed and used as inoculum.
Cultures were carried out in the same YP medium as used to prepare the inoculum but containing 20 g l–1 of sucrose, glucose, maltose or pairwise mixtures of these sugars. When indicated, yeasts were grown in a mineral medium (van Uden, 1967) supplemented with 20 g l–1 glucose, sucrose or maltose, with agitation in an orbital shaker (160 r.p.m.) at 30 °C. Growth was followed by measuring the OD640 of the culture. At specified times during exponential growth, biomass dry weight was also determined.
Analytical procedures.
To determine sugar and ethanol concentrations in the growth medium, the cultures were sampled and immediately centrifuged at 16 000 g for 3 min. The supernatant was frozen and kept at –20 °C until analysed. Quantitative analysis of sugar and ethanol was based on HPLC, using a Gilson chromatograph equipped with a 132-RI detector and a Hypersil-SS-100, H+ column at 30 °C with a 5 mM H2SO4 solution as the mobile phase at a flow rate of 0.45 ml min–1. Due to poor resolution of sucrose and maltose in this system, these two sugars when used in a mixture were determined enzymically using the sucrose/D-glucose UV colorimetric method (Roche, 139041; Boehringer Mannheim) and the enzyme 
-glucosidase (Roche, 124036).
Biomass yields were determined from the slopes of plots of biomass dry weight versus consumed sugar during exponential growth. The ethanol yield was determined by dividing the maximum ethanol concentration obtained by the consumed sugar and expressed as g ethanol per g substrate carbon. Each specific sugar consumption rate (qsugar) was determined by dividing the specific growth rate (µ) by the biomass yield (Yx) during exponential growth on the respective sugar.
Enzyme assays.
To obtain cell-free extracts for the determination of enzymic activities, 25–30 mg of cell mass (wet weight) was collected at different times during growth, sedimented by centrifugation, and washed twice with cold homogenization buffer (0.1 M potassium phosphate buffer, pH 6.5). The pellet was resuspended in 0.45 ml of the same buffer and transferred into a tube containing 0.5 ml acid-treated glass beads (0.5 mm diameter). The mixture was vortexed for four periods of 0.5 min, separated by 1 min intervals on ice, centrifuged for 5 min at 1000 g (4 °C) and the supernatant used immediately for analysis. The total protein content in the cell-free extract was determined with the Bio-Rad protein assay kit based on the Bradford method (Bradford, 1976), and using bovine serum albumin as standard. Maltase activity was determined in the crude extract as described by Okada & Halvorson (1964) using p-nitrophenyl -D-glucopyranoside (pNPG) as substrate. One unit (U) is defined as the amount of enzyme that produces 1 µmol p-nitrophenol in 1 min under the assay conditions. Invertase activity was assayed as described by Niederacher & Entian (1987) and is expressed as µmol glucose released from sucrose in 1 min per mg protein (U mg–1).
Maltose and glucose transport.
For sugar transport assays, the cultures were sampled at the indicated times and cells were harvested by centrifugation, washed twice with ice-cold water, suspended in water to a density of 35–45 mg dry weight of cells ml–1 and kept on ice. Zero-trans influx of labelled maltose or glucose (Amersham) was determined at 30 °C. Ten microlitres of cell suspension was mixed with 30 µl 0.1 M potassium phosphate buffer (pH 5.0). The cell suspension was allowed to reach the temperature of the assay and the reaction started by adding 10 µl of an aqueous solution of [U-14C]maltose (specific activity 610 mCi mmol–1; 22.6 GBq mmol–1) or [U-14C]glucose (specific activity 310 mCi mmol–1; 11.5 GBq mmol–1) at the desired concentrations. After incubation for 5 s, 4.5 ml chilled water was added and the mixture immediately filtered through glass fibre filters (GF/C filters, Whatman). The cells on the filter were washed with 15 ml chilled water, the filter immersed in 5 ml scintillation liquid OptiPhase HiSafe II (LKB Scintillation Products) and the radioactivity measured using a Packard Tri-Carb 2200 CA liquid scintillation counter (Packard Instrument Co.), with correction for disintegrations per minute. Non-specific binding of radiolabelled sugar to the yeast cells and filter was determined in parallel by pouring ice-cold water immediately before the addition of the labelled sugar. For each sugar concentration, the reaction was performed in triplicate.
Fermentation and respiration rates.
Fermentation and respiration rates were determined using the standard Warburg method (Umbreit et al., 1964). Yeast strains were grown on YP medium supplemented with 20 g l–1 glucose, sucrose or maltose. Cells were harvested at the exponential growth phase (OD640 0.8–0.9), washed twice with water and suspended in cold water to a cell density 10-fold higher than the original culture. This suspension was diluted in 0.1 M KH2PO4 buffer, pH 5.0, to a cell concentration allowing measurements of CO2 production and O2 consumption in the manometer of the Warburg apparatus during a period of approximately 60 min. The experiments were started by the addition of the sugar solution (final concentration 20 g l–1) to the cell suspension and performed at 30 °C, in duplicate. Fermentation rates are expressed in mmol CO2 produced per g dry weight of cells per hour, and respiration rates expressed in mmol O2 consumed per g dry weight of cells per hour. The respiratory quotient (RQ) was calculated as the ratio between total CO2 produced and the O2 consumed.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
|---|
show the results obtained in sugar mixtures. In glucose-maltose (G-M) medium, maltose consumption became detectable only after glucose was no longer present (Fig. 1a). No diauxic growth curve was observed, i.e. there was no lag period preceding the utilization of the second sugar. In glucose-sucrose (G-S) medium, the utilization of both sugars was simultaneous (Fig. 1b). However, in S-M medium sucrose was apparently preferred to maltose (Fig. 1c). Following an initial slow consumption of maltose, concomitant with sucrose utilization, a lag period was observed which lasted until sucrose had almost disappeared. Only then was maltose consumption resumed.
|
). In single-sugar media, the growth rate values were similar, although slightly higher in glucose and sucrose media than in maltose medium for both yeasts. In mixed-sugar media, the values were similar to those obtained in single-sugar media (Fig. 1
, Table 1). As for biomass yields, typical values for fermentative metabolism were obtained in all cases. However, the biomass yields were slightly lower for S. cerevisiae than for T. delbrueckii (Table 1) in either glucose or maltose medium. Ethanol yields in sugar mixtures were higher when using S. cerevisiae, except in S-M medium, where similar values were observed (Table 1). For comparison, the pattern of maltose utilization when in the presence of glucose was also investigated for S. cerevisiae PYCC 5325. As expected, sequential sugar consumption was observed. However, maltose started to be consumed when glucose was still detectable in the medium (not shown), suggesting a higher glucose control over maltose metabolism in T. delbrueckii than in the commercial baker's yeast.
|
). For all tested media, maltase and invertase activities remained at low levels while glucose (in G-M and G-S media) or sucrose (in S-M medium) were present and increased concomitantly as these sugars approached depletion. Interestingly, maximal maltase activity of T. delbrueckii in G-M medium was comparable to the value obtained with S. cerevisiae, although for the latter species maltase activity was detected before glucose was completely consumed (results not shown), a result in accordance with the observed maltose utilization when glucose was still being consumed. Furthermore, while maltase activity in T. delbrueckii reached higher values whenever maltose was present in the medium, maximal invertase activity in the absence of sucrose (G-M medium) was similar to that found in glucose-sucrose medium (Fig. 1d–f). However, both enzymes were subject to glucose repression. Similar results were observed upon growth in synthetic medium (data not shown), hence excluding the possible interference of residual amounts of sugars contained in YP-based media.
Sugar transport
To investigate a possible relation between specific sugar consumption rates inferred from the values shown in Table 1 (qsugar=µ/Yx) and the first step of maltose and glucose metabolism, the transport of these two sugars was evaluated during the fermentations in mixed-sugar media (Fig. 1d, e). Just like S. cerevisiae, T. delbrueckii is known to transport maltose through a maltose-H+ symport mechanism, inducible and subject to glucose repression (Alves-Araújo et al., 2004b). Accordingly, in G-M medium we observed that maltose transport capacity increased only after glucose exhaustion (Fig. 1d). The maximum maltose transport capacity obtained [0.41 nmol s–1 (mg dry weight)–1: Fig. 1d)] was about one and a half times lower than that obtained for cells grown in YP maltose medium [Vmax=0.66 nmol s–1 (mg dry weight)–1], which in turn is lower than the estimated specific maltose consumption rate [qmaltose=1.8±0.3 nmol s–1 (mg dry weight)–1]. This suggests that maltose uptake may be limiting maltose metabolism. A similar analysis was conducted for glucose. Glucose transport in T. delbrueckii follows a biphasic Michaelis–Menten kinetics with low- and high-affinity components (Alves-Araújo et al., 2005). During exponential growth in G-M and G-S media, glucose uptake rates were very similar (Fig. 1d, e). The estimated specific glucose consumption rate [qglucose=4.1±0.4 nmol s–1 (mg dry weight)–1] in YP glucose medium was comparable to the total capacity of glucose transport [Vmax=3.96±0.56 nmol s–1 (mg dry weight)–1 in G-M and 3.60±0.32 nmol s–1 (mg dry weight)–1 in G-S], indicating that glucose metabolism may well be limited by glucose transport.
Sugar metabolism
Respiratory and fermentative capacities of T. delbrueckii PYCC 5321 and S. cerevisiae PYCC 5325 cells grown in YP with glucose, maltose or sucrose as the only carbon and energy sources were determined using the Warburg method. The results, expressed as specific CO2 production (qCO2) and oxygen consumption (qO2) rates, are presented in Table 2. The data obtained with T. delbrueckii show that all sugars tested are essentially fermented (77–88 % of the total sugar supplied) and that the fermentation rates were higher for sucrose and glucose than for maltose (Table 2), which is in accordance with the lower values obtained for qmaltose and µmaltose (Table 1). It is noteworthy that the qCO2 values for sucrose and glucose were always similar, irrespective of the sugar used for growth (Table 2). However, fermentation rates obtained with maltose were significantly higher (P<0.001) in maltose-grown cells than in either sucrose- or glucose-grown cells (Table 2).
|
). No significant differences between the fermentation rates of maltose were observed in cells grown with any of the three sugars (Table 2), in contrast to the values obtained for glucose and sucrose fermentation. In both cases, the qCO2 values were found to be higher in glucose- or sucrose-grown cells and lower in maltose-grown cells (P<0.001 and P<0.01, respectively).
A comparative analysis between T. delbrueckii and S. cerevisiae regarding the specific oxygen consumption rates (qO2) estimated with the different sugars showed similar values for sucrose, whereas the values almost doubled for T. delbrueckii in the case of glucose and maltose (Table 2). As stressed above, the relative contribution of respiration to sugar catabolism is always higher in T. delbrueckii, the RQs varying between 3.43 (sucrose) and 2.12 (maltose). For S. cerevisiae, RQ values were in the range 5.41–9.70, which reflects the higher fermentative capacity of this yeast.
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
|---|
; Needleman, 1991; Gancedo, 1998), a few significant differences were observed. In G-M medium the pattern was almost identical, the increase in maltose transport and maltase activities clearly coinciding with the outset of maltose consumption. Hence, there is an apparent co-regulation of these proteins, both being subject to glucose repression and induction by maltose. The results obtained are consistent with the previous identification of a bifunctional MAL promoter in T. delbrueckii PYCC 5321, shared by maltase and maltose transporter genes, including Mig1p and UASMAL consensus binding sites (Alves-Araújo et al., 2004b). The glucose control over maltose metabolism was stricter in T. delbrueckii PYCC 5321, since S. cerevisiae PYCC 5325 started to consume maltose when glucose was still detectable in the medium. In glucose-maltose mixtures, under laboratory culture conditions, this differential behaviour of the two species could lead to an undesirable delay in CO2 production from maltose by T. delbrueckii. However, this advantage exhibited by S. cerevisiae is counteracted under the conditions prevailing in bread dough by the higher osmotolerance of T. delbrueckii. Indeed, the gas production capacity of T. delbrueckii PYCC 5321 in lean dough was slightly lower than the leavening capacity of S. cerevisiae PYCC 5325 (Almeida & Pais, 1996b) and slightly higher than the rates obtained with other commercial baker's yeast strains (Hernandez-Lopez et al., 2003). The differences reported by the latter authors were even more pronounced in sweet, sucrose-added, doughs. In sucrose medium, T. delbrueckii PYCC 5321 showed a lower growth rate than S. cerevisiae PYCC 5325, although the biomass yields were equivalent. This is consistent with the higher contribution of respiration to the overall sugar metabolism in T. delbrueckii. Since the biomass productivity, in industrial fed-batch cultures, is limited not only by the substrate concentration but also by the oxygen available, the growth potential of S. cerevisiae is countered by its requirement for a more careful monitoring of the oxygen tension, to prevent ethanol production. The level of invertase activity in rich YP medium is similar for both yeasts and the regulatory mechanisms for this enzyme appear to be the same. A correlation between invertase activity and sucrose consumption is unclear and needs further investigation.
Overall, the experimental evidence points to sugar transfer rates into the cell limiting the efficiency of the fermentation. In the case of sugar mixtures with maltose, the inhibitory effect of maltose on glucose uptake, which is known to occur in S. cerevisiae (Diderich et al., 1999) and was also found in T. delbrueckii (Alves-Araújo et al., 2005), could reinforce this limitation. In particular, in S-M mixtures (Fig. 1c) the maltose concentration surpasses by far the glucose resulting from extracellular sucrose hydrolysis, thus possibly hindering glucose (and fructose) utilization. This would mean that, in the absence of maltose, sucrose could be consumed faster, which in reality was not observed. On the contrary, it seems that the glucose being released from sucrose through the action of the invertase inhibits maltose metabolism through mechanisms of glucose repression. More detailed studies are required to evaluate these aspects.
Despite the clear fermentative metabolism of T. delbrueckii, with the production of high ethanol yields in batch cultures with each of the sugars tested, our data on the specific rates of CO2 production and O2 consumption, estimated with the Warburg manometric technique, showed a higher contribution of respiration in T. delbrueckii compared to S. cerevisiae. It is worth noting that during batch cultivation the available oxygen rapidly reaches limiting concentrations, thereby favouring fermentative metabolism. In fact, when biomass yields were determined in YP medium, with either glucose, sucrose or maltose, using higher aeration rates a very significant increase in biomass yields (from 20 %, in glucose or sucrose medium, to 80 %, in maltose medium) was observed (not shown). As emphasized above, a more efficient modulation of the respiratory metabolism in T. delbrueckii under aerobic conditions represents an asset for the large-scale production of yeast.
As a final remark, the strain T. delbrueckii PYCC 5321 used in the present work was reported to display a much higher freezing and osmotic tolerance than S. cerevisiae (Almeida & Pais, 1996a, b; Alves-Araújo et al., 2004a), properties of special interest for the baking industry. In addition to these characteristics, our results show that T. delbruecki behaves very similarly to S. cerevisiae with respect to sugar utilization and regulation patterns. This work also indicated that maltose uptake is a good target for metabolic engineering and improvement of T. delbrueckii's performance in bread doughs. The present study further contributes to the characterization of T. delbrueckii PYCC 5321 at the physiological and biochemical levels, bridging a gap for its exploitation by the baking industry and increasing knowledge on the so-called non-conventional yeast species.
|
ACKNOWLEDGEMENTS |
|---|
Edited by: D. Burke
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
|---|
Almeida, M. J. & Pais, C. S. (1996b). Leavening ability and freeze tolerance of yeasts isolated from traditional corn and rye bread doughs. Appl Environ Microbiol 62, 4401–4404.[Abstract]
Alves-Araújo, C., Almeida, M. J., Sousa, M. J. & Leão, C. (2004a). Freeze tolerance of the yeast Torulaspora delbrueckii: cellular and biochemical basis. FEMS Microbiol Lett 240, 7–14.[CrossRef][Medline]
Alves-Araújo, C., Hernandez-Lopez, M. J., Sousa, M. J., Prieto, J. A. & Randez-Gil, F. (2004b). Cloning and characterization of the MAL11 gene encoding a high-affinity maltose transporter from Torulaspora delbrueckii. FEMS Yeast Res 4, 467–476.[CrossRef][Medline]
Alves-Araújo, C., Hernandez-Lopez, M. J., Prieto, J. A., Randez-Gil, F. & Sousa, M. J. (2005). Isolation and characterization of the LGT1 gene encoding a low-affinity glucose transporter from Torulaspora delbrueckii. Yeast 22, 165–175.[CrossRef][Medline]
Attfield, P. V. & Kletsas, S. (2000). Hyperosmotic stress response by strains of baker's yeasts in high sugar concentration medium. Lett Appl Microbiol 31, 323–327.[CrossRef][Medline]
Benitez, B., Gasent-Ramirez, J. M., Castrejon, F. & Codon, A. C. (1996). Development of new strains for the food industry. Biotechnol Prog 12, 149–163.[CrossRef]
Blom, J., Teixeira de Mattos, M. J. & Grivell, L. A. (2000). Redirection of the respiro-fermentative flux distribution in Saccharomyces cerevisiae by overexpression of the transcription factor Hap4p. Appl Environ Microbiol 66, 1970–1973.
Bradford, M. M. (1976). A rapid and sensitive method for the quantification of microgram quantities of protein utilizing the principle of protein-dye binding. Anal Biochem 72, 248–254.[CrossRef][Medline]
Diderich, J. A., Teusink, B., Valkier, J., Anjos, J., Spencer-Martins, I., van Dam, K. & Walsh, M. C. (1999). Strategies to determine the extent of control exerted by glucose transport on glycolytic flux in the yeast Saccharomyces bayanus. Microbiology 145, 3447–3454.
Gancedo, J. M. (1998). Yeast carbon catabolite repression. Microbiol Mol Biol Rev 62, 334–361.
Goldenthal, M. J., Vanoni, M., Buchferer, B. & Marmur, J. (1987). Regulation of MAL gene expression in yeast: gene dosage effects. Mol Gen Genet 209, 508–517.[CrossRef][Medline]
Hahn, Y.-S. & Kawai, H. (1990). Isolation and characterization of freeze-tolerant yeasts from nature available for the frozen-dough method. Agric Biol Chem 54, 829–831.
Hanl, L., Sommer, P. & Arneborg, N. (2005). The effect of decreasing oxygen feed rates on growth and metabolism of Torulaspora delbrueckii. Appl Microbiol Biotechnol 67, 113–118.[CrossRef][Medline]
Hernandez-Lopez, M. J., Prieto, J. A. & Randez-Gil, F. (2003). Osmotolerance and leavening ability in sweet and frozen dough. Comparative analysis between Torulaspora delbrueckii and Saccharomyces cerevisiae baker's yeast strains. Antonie van Leeuwenhoek 84, 125–134.[CrossRef][Medline]
Higgins, V. J., Braidwood, M., Bell, P., Bissinger, P., Dawes, I. W. & Attfield, P. V. (1999). Genetic evidence that high noninduced maltase and maltose permease activities, governed by MALx3-encoded transcriptional regulators, determine efficiency of gas production by baker's yeast in unsugared dough. Appl Environ Microbiol 65, 680–685.
Mormeneo, S. & Sentandreu, R. (1982). Regulation of invertase synthesis by glucose in Saccharomyces cerevisiae. J Bacteriol 152, 14–18.
Needleman, R. B. (1991). Control of maltase synthesis in yeast. Mol Microbiol 5, 2079–2084.[Medline]
Niederacher, D. & Entian, K.-D. (1987). Isolation and caracterization of the regulatory HEX2 gene necessary for glucose repression in yeast. Mol Gen Genet 206, 505–509.[CrossRef][Medline]
Ok, T. & Hashinaga, F. (1997). Identification of sugar-tolerant yeasts isolated from high sugar fermented vegetable extracts. J Gen Appl Microbiol 43, 39–47.[Medline]
Okada, H. & Halvorson, H. O. (1964). Uptake of alpha-thioethyl-glucopyranoside by Saccharomyces cerevisiae. 1. The genetic control of facilitated diffusion and active transport. Biochim Biophys Acta 82, 538–542.[Medline]
Otterstedt, K., Larsson, C., Bill, R. M., Stahlberg, A., Boles, E., Hohmann, S. & Gustafsson, L. (2004). Switching the mode of metabolism in the yeast Saccharomyces cerevisiae. EMBO Rep 5, 532–537.[CrossRef][Medline]
Ponte, J. G., Jr & Reed, G. (1982). Bakery foods. In Prescott and Dunn's Industrial Microbiology, pp. 246–292, 4th edn. Edited by G. Reed. Westport, CT: AVI Publishing Co.
Umbreit, W. W., Burris, R. H. & Stauffer, J. F. (1964). Manometric Technics, 4th edn. Minneapolis, MN: Burguess Publishing.
van Hoek, W. P. M., van Dijken, J. P. & Pronk, J. T. (1998). Effect of specific growth rate on fermentative capacity of baker's yeast. Appl Environ Microbiol 64, 4226–4233.
van Uden, N. (1967). Transport-limited fermentation and growth of Saccharomyces cerevisiae and its competitive inhibition. Arch Microbiol 58, 155–168.
Visser, W., Scheffers, W. A., Batenburg-van der Vegte, W. H. & van Dijken, J. P. (1990). Oxygen requirements of yeasts. Appl Environ Microbiol 56, 3785–3792.
Ye, L., Kruckeberg, A. L., Berden, J. A. & van Dam, K. (1999). Growth and glucose repression are controlled by glucose transport in Saccharomyces cerevisiae cells containing only one glucose transporter. J Bacteriol 181, 4673–4675.
Received 16 October 2006;
accepted 22 November 2006.
This article has been cited by other articles:
|
|
 |
|
  P. M. R. Guimaraes, J.-P. Multanen, L. Domingues, J. A. Teixeira, and J. Londesborough Stimulation of Zero-trans Rates of Lactose and Maltose Uptake into Yeasts by Preincubation with Hexose To Increase the Adenylate Energy Charge Appl. Envir. Microbiol., May 15, 2008; 74(10): 3076 - 3084. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. Santos, M. J. Sousa, H. Cardoso, J. Inacio, S. Silva, I. Spencer-Martins, and C. Leao Ethanol tolerance of sugar transport, and the rectification of stuck wine fermentations Microbiology, February 1, 2008; 154(2): 422 - 430. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| INT J SYST EVOL MICROBIOL | MICROBIOLOGY | J GEN VIROL |
| J MED MICROBIOL | ALL SGM JOURNALS | |
, OD640; 
, glucose; 
, maltose; 
, sucrose; 
, ethanol; 
, maltase activity [U (mg protein)–1]; 
, invertase activity [U (mg protein)–1]; 
, 
, rate of glucose (