A new model of uptake of multiple sugars by S. cerevisiae

Barford, J. P.; Phillips, Peter J.; Orlowski, Joanna H.

doi:10.1007/bf00705158

Cited by 13 publications

(10 citation statements)

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“…Indeed, we have observed higher sucrose fermentation rates by the wild-type strain pre-grown on maltose, compared to cells pre-grown on glycerol/ethanol ( fi g. 1 ), despite the higher invertase activity present in these glycerol/ethanol-grown cells ( table 1 ). Thus, our data reinforce the results described by Bradford and co-workers on the importance of direct sucrose transport for the fermentation of this sugar by industrial strains [Orlowski and Barford, 1991;Barford et al, 1992;Mwesigye and Barford, 1996a, b].…”

Section: Discussionsupporting

confidence: 82%

“…Several reports have shown that the fermentation and growth rates on sucrose by this yeast can only fi t a model in which its utilization is composed by the contributions from both the direct uptake of sucrose and the uptake of its hydrolysis products into the cell [Orlowski and Barford, 1991;Barford et al, 1992;Mwesigye and Barford, 1996a, b]. The analysis of direct sucrose uptake by S. cerevisiae cells revealed the presence of active sucrose-H + symport [Santos et al, 1982;Barford, 1994, 1996a], which we have recently shown to be mediated by the AGT1 permease .…”

Section: Introductionmentioning

confidence: 99%

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Sucrose Fermentation by <i>Saccharomyces cerevisiae</i> Lacking Hexose Transport

Batista

Miletti

Stambuk³

2004

Microb Physiol

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Sucrose is the major carbon source used by Saccharomyces cerevisiae during production of baker’s yeast, fuel ethanol and several distilled beverages. It is generally accepted that sucrose fermentation proceeds through extracellular hydrolysis of the sugar, mediated by the periplasmic invertase, producing glucose and fructose that are transported into the cells and metabolized. In the present work we analyzed the contribution to sucrose fermentation of a poorly characterized pathway of sucrose utilization by S. cerevisiae cells, the active transport of the sugar through the plasma membrane and its intracellular hydrolysis. A yeast strain that lacks the major hexose transporters (hxt1–hxt7 and gal2) is incapable of growing on or fermenting glucose or fructose. Our results show that this hxt-null strain is still able to ferment sucrose due to direct uptake of the sugar into the cells. Deletion of the AGT1 gene, which encodes a high-affinity sucrose-H⁺ symporter, rendered cells incapable of sucrose fermentation. Since sucrose is not an inducer of the permease, expression of the AGT1 must be constitutive in order to allow growth of the hxt-null strain on sucrose. The molecular characterization of active sucrose transport and fermentation by S. cerevisiae cells opens new opportunities to optimize yeasts for sugarcane-based industrial processes.

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Section: Discussionsupporting

confidence: 82%

Section: Introductionmentioning

confidence: 99%

Sucrose Fermentation by <i>Saccharomyces cerevisiae</i> Lacking Hexose Transport

Batista

Miletti

Stambuk³

2004

Microb Physiol

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“…This theory alone cannot explain some of the results we have reported in this paper. These results agree with the sucrose-modified cell model proposed earlier (4) in which sucrose utilisation is composed of contributions from both transport of sucrose molecules and its hydrolysis products into the cell. Depending on the conditions in the medium, the two mechanisms can take place concurrently.…”

Section: Mechanism Of Sucrose Utilisationsupporting

confidence: 91%

Mechanism of surcrose utilisation by Saccharomyces cerevisiae.

Mwesigye¹,

Barford²

1996

J. Gen. Appl. Microbiol.

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Saccharomyces cerevisiae was grown on different concentrations of sucrose and glucose mixtures after adapting on sucrose. The yeast cells were found to have two different mechanisms by which sucrose was utilised: hydrolysis outside the cell membrane and direct transport into the cells. The mechanism by which sucrose was utilised depended on the initial concentration of glucose in the mixture and the adaptation state of the cells. In both cases, glucose was utilised first and invertase secretion was repressed when the glucose concentration was higher than 2 g l-'. The major important finding in our results is that, for fully sucrose-adapted cells, even in the presence of a repressive glucose concentration, the yeast cells were able to utilise sucrose, and most importantly, without first being hydrolysed to its constituent monosaccharides. Previously, it has been thought that direct transport of sucrose contributes only a small part of the process of utilisation of sucrose, but as our results indicate it can be the major mechanism of sucrose utilisation in S. cerevisiae if the cells are adapted on sucrose for long periods, glucose concentrations in the medium are high and/or invertase activities are limiting.The initial steps in the utilisation of sugars by yeast is either the passage of intact sugar molecules across the cell membrane or the hydrolysis outside the cell followed by entry of all or some of the hydrolytic products into the cell. Sugar transport into microbial cells across the plasmalemma can take place by active transport, facilitated diffusion and group translocation (19).Sucrose utilisation involves its hydrolysis outside the yeast cell by extracellular enzyme invertase ($-D-fructosidase) into glucose and fructose which are then transported into the cells and metabolised. Most Saccharomyces cerevisiae strains have two forms of invertase: an external large invertase, which is a glycoprotein, and an internal small invertase (9,15). Some strains of S. cerevisiae have been reported to transport sucrose directly into the cell (14,17,20).

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“…In particular, note that even though we speak of E A as an enzyme, the same rate equations arise if the main rate limiting step involves transport across the plasma membrane. Hence our minimalist model is not at odds with the more detailed model of Barford et al for sugar uptake by yeast [15], given a sensible choice of parameters for the kinetics.…”

Section: Resultsmentioning

confidence: 93%

Is Transcriptional Regulation of Metabolic Pathways an Optimal Strategy for Fitness?

et al. 2007

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BackgroundTranscriptional regulation of the genes in metabolic pathways is a highly successful strategy, which is virtually universal in microorganisms. The lac operon of E. coli is but one example of how enzyme and transporter production can be made conditional on the presence of a nutrient to catabolize.MethodologyWith a minimalist model of metabolism, cell growth and transcriptional regulation in a microorganism, we explore how the interaction between environmental conditions and gene regulation set the growth rate of cells in the phase of exponential growth. This in silico model, which is based on biochemical rate equations, does not describe a specific organism, but the magnitudes of its parameters are chosen to match realistic values. Optimizing the parameters of the regulatory system allows us to quantify the fitness benefit of regulation. When a second nutrient and its metabolic pathway are introduced, the system must further decide whether and how to activate both pathways.ConclusionsEven the crudest transcriptional network is shown to substantially increase the fitness of the organism, and this effect persists even when the range of nutrient levels is kept very narrow. We show that maximal growth is achieved when pathway activation is a more or less steeply graded function of the nutrient concentration. Furthermore, we predict that bistability of the system is a rare phenomenon in this context, but outline a situation where it may be selected for.

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A new model of uptake of multiple sugars by S. cerevisiae

Cited by 13 publications

References 13 publications

Sucrose Fermentation by <i>Saccharomyces cerevisiae</i> Lacking Hexose Transport

Sucrose Fermentation by <i>Saccharomyces cerevisiae</i> Lacking Hexose Transport

Mechanism of surcrose utilisation by Saccharomyces cerevisiae.

Is Transcriptional Regulation of Metabolic Pathways an Optimal Strategy for Fitness?

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