Visualising the next frontiers in wine yeast research

A range of game-changing biodigital and biodesign technologies are coming of age all around us, transforming our world in complex ways that are hard to predict. Not a day goes by without news of how data-centric engineering, algorithm-driven modelling, and biocyber technologies—including the convergence of artificial intelligence, machine learning, automated robotics, quantum computing, and genome editing—will change our world. If we are to be better at expecting the unexpected in the world of wine, we need to gain deeper insights into the potential and limitations of these technological developments and advances along with their promise and perils. This article anticipates how these fast-expanding bioinformational and biodesign toolkits might lead to the creation of synthetic organisms and model systems, and ultimately new understandings of biological complexities could be achieved. A total of four future frontiers in wine yeast research are discussed in this article: the construction of fully synthetic yeast genomes, including minimal genomes; supernumerary pan-genome neochromosomes; synthetic metagenomes; and synthetic yeast communities. These four concepts are at varying stages of development with plenty of technological pitfalls to overcome before such model chromosomes, genomes, strains, and yeast communities could illuminate some of the ill-understood aspects of yeast resilience, fermentation performance, flavour biosynthesis, and ecological interactions in vineyard and winery settings. From a winemaker's perspective, some of these ideas might be considered as far-fetched and, as such, tempting to ignore. However, synthetic biologists know that by exploring these futuristic concepts in the laboratory could well forge new research frontiers to deepen our understanding of the complexities of consistently producing fine wines with different fermentation processes from distinctive viticultural terroirs. As the saying goes in the disruptive technology industry, it take years to create an overnight success. The purpose of this article is neither to glorify any of these concepts as a panacea to all ills nor to crucify them as a danger to winemaking traditions. Rather, this article suggests that these proposed research endeavours deserve due consideration because they are likely to cast new light on the genetic blind spots of wine yeasts, and how they interact as communities in vineyards and wineries. Future-focussed research is, of course, designed to be subject to revision as new data and technologies become available. Successful dislodging of old paradigms with transformative innovations will require open-mindedness and pragmatism, not dogmatism—and this can make for a catch-22 situation in an archetypal traditional industry, such as the wine industry, with its rich territorial and socio-cultural connotations.

Tasting the terroir of wine yeast innovation

Wine is an archetypal traditional fermented beverage with strong territorial and socio-cultural connotations. Its 7000 year history is patterned by a tradition of innovation. Every value-adding innovation − whether in the vineyard, winery, supply chain or marketplace − that led to the invention of a new tradition spurred progress and created a brighter future from past developments. In a way, wine traditions can be defined as remembered innovations from the distant past − inherited knowledge and wisdom that withstood the test of time. Therefore, it should not be assumed a priori that tradition and innovation are polar opposites. The relations between the forces driven by the anchors of tradition and the wings of innovation do not necessarily involve displacement, conflict or exclusiveness. Innovation can strengthen wine tradition, and the reinvention of a tradition-bound practice, approach or concept can foster innovation. In cases where a paradigm-shifting innovation disrupts a tradition, the process of such an innovation transitioning into a radically new tradition can become protracted while proponents of divergent opinions duke it out. Sometimes these conflicting opinions are based on fact, and sometimes not. The imperfections of such a debate between the ‘ancients’ and the ‘moderns’ can, from time to time, obscure the line between myth and reality. Therefore, finding the right balance between traditions worth keeping and innovations worth implementing can be complex. The intent here is to harness the creative tension between science fiction and science fact when innovation's first-principles challenge the status quo by re-examining the foundational principles about a core traditional concept, such as terroir. Poignant questions are raised about the importance of the terroir (biogeography) of yeasts and the value of the microbiome of grapes to wine quality. This article imagines a metaphorical terroir free from cognitive biases where diverse perspectives can converge to uncork the effervescent power of territorial yeast populations as well as ‘nomadic’ yeast starter cultures. At the same time, this paper also engages in mental time-travel. A future scenario is imagined, explored, tested and debated where terroir-less yeast avatars are equipped with designer genomes to safely and consistently produce, individually or in combination with region-specific wild yeasts and or other starter cultures, high-quality wine according to the preferences of consumers in a range of markets. The purpose of this review is to look beyond the horizon and to synthesize a link between what we know now and what could be. This article informs readers where to look without suggesting what they must see as a way forward. In the context of one of the world's oldest fermentation industries − steeped in a rich history of tradition and innovation − the mantra here is: respect the past, lead the present and secure the future of wine.

Yeast 2.0-connecting the dots in the construction of the world's first functional synthetic eukaryotic genome

Historians of the future may well describe 2018 as the year that the world's first functional synthetic eukaryotic genome became a reality. Without the benefit of hindsight, it might be hard to completely grasp the long-term significance of a breakthrough moment in the history of science like this. The role of synthetic biology in the imminent birth of a budding Saccharomyces cerevisiae yeast cell carrying 16 man-made chromosomes causes the world of science to teeter on the threshold of a future-defining scientific frontier. The genome-engineering tools and technologies currently being developed to produce the ultimate yeast genome will irreversibly connect the dots between our improved understanding of the fundamentals of a complex cell containing its DNA in a specialised nucleus and the application of bioengineered eukaryotes designed for advanced biomanufacturing of beneficial products. By joining up the dots between the findings and learnings from the international Synthetic Yeast Genome project (known as the Yeast 2.0 or Sc2.0 project) and concurrent advancements in biodesign tools and smart data-intensive technologies, a future world powered by a thriving bioeconomy seems realistic. This global project demonstrates how a collaborative network of dot connectors-driven by a tinkerer's indomitable curiosity to understand how things work inside a eukaryotic cell-are using cutting-edge biodesign concepts and synthetic biology tools to advance science and to positively frame human futures (i.e. improved quality of life) in a planetary context (i.e. a sustainable environment). Explorations such as this have a rich history of resulting in unexpected discoveries and unanticipated applications for the benefit of people and planet. However, we must learn from past explorations into controversial futuristic sciences and ensure that researchers at the forefront of an emerging science such as synthetic biology remain connected to all stakeholders' concerns about the biosafety, bioethics and regulatory aspects of their pioneering work. This article presents a shared vision of constructing a synthetic eukaryotic genome in a safe model organism by using novel concepts and advanced technologies. This multidisciplinary and collaborative project is conducted under a sound governance structure that does not only respect the scientific achievements and lessons from the past, but that is also focussed on leading the present and helping to secure a brighter future for all.

The heterologous expression of polysaccharidase-encoding genes with oenological relevance in Saccharomyces cerevisiae

AIMS

The main objective of this study was to develop polysaccharide-degrading wine strains of Saccharomyces cerevisiae, which are able to improve aspects of wine processing and clarification, as well as colour extraction and stabilization during winemaking.

METHODS AND RESULTS

Two yeast expression/secretion gene cassettes were constructed, namely (i) a pectinase gene cassette (pPPK) consisting of the endo-polygalacturonase gene (pelE) from Erwinia chrysanthemi and the pectate lyase gene (peh1) from Erwinia carotovora and (ii) a glucanase/xylanase gene cassette (pEXS) containing the endo-beta-1,4-glucanase gene (end1) from Butyrivibrio fibrisolvens and the endo-beta-1,4-xylanase gene (xynC) from Aspergillus niger. The commercial wine yeast strain, VIN13, was transformed separately with these two gene cassettes and checked for the production of pectinase, glucanase and xylanase activities. Pinot Noir, Cinsaut and Muscat d'Alexandria grape juices were fermented using the VIN13[pPPK] pectinase- and the VIN13[pEXS] glucanase/xylanase-producing transformants. Chemical analyses of the resultant wines indicated that (i) the pectinase-producing strain caused a decrease in the concentration of phenolic compounds in Pinot Noir whereas the glucanase/xylanase-producing strain caused an increase in phenolic compounds presumably because of the degradation of the grape skins; (ii) the glucanase/xylanase-producing strain caused a decrease in wine turbidity, especially in Pinot Noir wine, as well as a clear increase in colour intensity and (iii) in the Muscat d'Alexandria and Cinsaut wines, the differences between the control wines (fermented with the untransformed VIN3 strain) and the wines produced by the two transformed strains were less prominent showing that the effect of these polysaccharide-degrading enzymes is cultivar-dependent.

CONCLUSIONS

The recombinant wine yeasts producing pectinase, glucanase and xylanase activities during the fermentation of Pinot Noir, Cinsaut and Muscat d'Alexandria grape juice altered the chemical composition of the resultant wines in a way that such yeasts could potentially be used to improve the clarity, colour intensity and stability and aroma of wine.

SIGNIFICANCE AND IMPACT OF THE STUDY

Aspects of commercial-scale wine processing and clarification, colour extraction and stabilization, and aroma enhancement could potentially be improved by the use of polysaccharide-degrading wine yeasts without the addition of expensive commercial enzyme preparations. This offers the potential to further improve the price:quality ratio of wine according to consumer expectations.

The Thr505 and Ser557 residues of the AGT1-encoded alpha-glucoside transporter are critical for maltotriose transport in Saccharomyces cerevisiae

AIMS

The main objective of this study was to identify amino acid residues in the AGT1-encoded alpha-glucoside transporter (Agt1p) that are critical for efficient transport of maltotriose in the yeast Saccharomyces cerevisiae.

METHODS AND RESULTS

The sequences of two AGT1-encoded alpha-glucoside transporters with different efficiencies of maltotriose transport in two Saccharomyces strains (WH310 and WH314) were compared. The sequence variations and discrepancies between these two proteins (Agt1p(WH310) and Agt1p(WH314)) were investigated for potential effects on the functionality and maltotriose transport efficiency of these two AGT1-encoded alpha-glucoside transporters. A 23-amino-acid C-terminal truncation proved not to be critical for maltotriose affinity. The identification of three amino acid differences, which potentially could have been instrumental in the transportation of maltotriose, were further investigated. Single mutations were created to restore the point mutations I505T, V549A and T557S one by one. The single site mutant V549A showed a decrease in maltotriose transport ability, and the I505T and T557S mutants showed complete reduction in maltotriose transport.

CONCLUSIONS

The amino acids Thr(505) and Ser(557), which are respectively located in the transmembrane (TM) segment TM(11) and on the intracellular segment after TM(12) of the AGT1-encoded alpha-glucoside transporters, are critical for efficient transport of maltotriose in S. cerevisiae.

SIGNIFICANCE AND IMPACT OF THE STUDY

Improved fermentation of starch and its dextrin products, such as maltotriose and maltose, would benefit the brewing and whisky industries. This study could facilitate the development of engineered maltotriose transporters adapted to starch-efficient fermentation systems, and offers prospects for the development of yeast strains with improved maltose and maltotriose uptake capabilities that, in turn, could increase the overall fermentation efficiencies in the beer and whisky industries.

Assimilable nitrogen utilisation and production of volatile and non-volatile compounds in chemically defined medium by Saccharomyces cerevisiae wine yeasts

Surveys conducted worldwide have shown that a significant proportion of grape musts are suboptimal for yeast nutrients, especially assimilable nitrogen. Nitrogen deficiencies are linked to slow and stuck fermentations and sulphidic off-flavour formation. Nitrogen supplementation of grape musts has become common practice; however, almost no information is available on the effects of nitrogen supplementation on wine flavour. In this study, the effect of ammonium supplementation of a synthetic medium over a wide range of nitrogen values on the production of volatile and non-volatile compounds by two high-nitrogen-demand wine fermentation strains of Saccharomyces cerevisiae was determined. To facilitate this investigation, a simplified chemically defined medium that resembles the nutrient composition of grape juice was used. Analysis of variance revealed that ammonium supplementation had significant effects on the concentration of residual sugar, L-malic acid, acetic acid and glycerol but not the ethanol concentration. While choice of yeast strain significantly affected half of the aroma compounds measured, nitrogen concentrations affected 23 compounds, including medium-chain alcohols and fatty acids and their esters. Principal component analysis showed that branched-chain fatty acids and their esters were associated with low nitrogen concentrations, whereas medium-chain fatty esters and acetic acid were associated with high nitrogen concentrations.

Modulation of volatile sulfur compounds by wine yeast

Sulfur compounds in wine can be a 'double-edged sword'. On the one hand, certain sulfur-containing volatile compounds such as hydrogen sulfide, imparting a rotten egg-like aroma, can have a negative impact on the perceived quality of the wine, and on the other hand, some sulfur compounds such as 3-mercaptohexanol, imparting fruitiness, can have a positive impact on wine flavor and aroma. Furthermore, these compounds can become less or more attractive or repulsive depending on their absolute and relative concentrations. This presents an interesting challenge to the winemaker to modulate the concentrations of these quality-determining compounds in wine in accordance with consumer preferences. The wine yeast Saccharomyces cerevisiae plays a central role in the production of volatile sulfur compounds. Through the sulfate reduction sequence pathway, the HS(-) is formed, which can lead to the formation of hydrogen sulfide and various mercaptan compounds. Therefore, limiting the formation of the HS(-) ion is an important target in metabolic engineering of wine yeast. The wine yeast is also responsible for the transformation of non-volatile sulfur precursors, present in the grape, to volatile, flavor-active thiol compounds. In particular, 4-mercapto-4-methylpentan-2-one, 3-mercaptohexanol, and 3-mercaptohexyl acetate are the most important volatile thiols adding fruitiness to wine. This paper briefly reviews the metabolic processes involved in the production of important volatile sulfur compounds and the latest strategies in the pursuit of developing wine yeast strains as tools to adjust wine aroma to market specifications.

The effect of polysaccharide-degrading wine yeast transformants on the efficiency of wine processing and wine flavour

Commercial polysaccharase preparations are applied to winemaking to improve wine processing and quality. Expression of polysaccharase-encoding genes in Saccharomyces cerevisiae allows for the recombinant strains to degrade polysaccharides that traditional commercial yeast strains cannot. In this study, we constructed recombinant wine yeast strains that were able to degrade the problem-causing grape polysaccharides, glucan and xylan, by separately integrating the Trichoderma reesei XYN2 xylanase gene construct and the Butyrivibrio fibrisolvens END1 glucanase gene cassette into the genome of the commercial wine yeast strain S. cerevisiae VIN13. These genes were also combined in S. cerevisiae VIN13 under the control of different promoters. The strains that were constructed were compared under winemaking conditions with each other and with a recombinant wine yeast strain expressing the endo-beta-1,4-glucanase gene cassette (END1) from B. fibrisolvens and the endo-beta-1,4-xylanase gene cassette (XYN4) from Aspergillus niger, a recombinant strain expressing the pectate lyase gene cassette (PEL5) from Erwinia chrysanthemi and the polygalacturonase-encoding gene cassette (PEH1) from Erwinia carotovora. Wine was made with the recombinant strains using different grape cultivars. Fermentations with the recombinant VIN13 strains resulted in significant increases in free-flow wine when Ruby Cabernet must was fermented. After 6 months of bottle ageing significant differences in colour intensity and colour stability could be detected in Pinot Noir and Ruby Cabernet wines fermented with different recombinant strains. After this period the volatile composition of Muscat d'Alexandria, Ruby Cabernet and Pinot Noir wines fermented with different recombinant strains also showed significant differences. The Pinot Noir wines were also sensorial evaluated and the tasting panel preferred the wines fermented with the recombinant strains.

Domain engineering of Saccharomyces cerevisiae exoglucanases

To illustrate the effect of a cellulose-binding domain (CBD) on the enzymatic characteristics of non-cellulolytic exoglucanases, 10 different recombinant enzymes were constructed combining the Saccharomyces cerevisiae exoglucanases, EXG1 and SSG1, with the CBD2 from the Trichoderma reesei cellobiohydrolase, CBH2, and a linker peptide. The enzymatic activity of the recombinant enzymes increased with the CBD copy number. The recombinant enzymes, CBD2-CBD2-L-EXG1 and CBD2-CBD2-SSG1, exhibited the highest cellobiohydrolase activity (17.5 and 16.3 U mg(-1) respectively) on Avicel cellulose, which is approximately 1.5- to 2-fold higher than the native enzymes. The molecular organisation of CBD in these recombinant enzymes enhanced substrate affinity, molecular flexibility and synergistic activity, contributing to their elevated action on the recalcitrant substrates as characterised by adsorption, kinetics, thermostability and scanning electron microscopic analysis.

The effect of sulphur dioxide and oxygen on the viability and culturability of a strain of Acetobacter pasteurianus and a strain of Brettanomyces bruxellensis isolated from wine

AIMS

The objective of this study was to investigate the effects of free molecular and bound forms of sulphur dioxide and oxygen on the viability and culturability of a selected strain of Acetobacter pasteurianus and a selected strain of Brettanomyces bruxellensis in wine.

METHODS AND RESULTS

Acetic acid bacteria and Brettanomyces/Dekkera yeasts associated with wine spoilage were isolated from bottled commercial red wines. One bacterium, A. pasteurianus strain A8, and one yeast, B. bruxellensis strain B3a, were selected for further study. The resistance to sulphur dioxide and the effect of oxygen addition on these two selected strains were determined by using plating and epifluorescence techniques for monitoring cell viability in wine. Acetobacter pasteurianus A8 was more resistant to sulphur dioxide than B. bruxellensis B3a, with the latter being rapidly affected by a short exposure time to free molecular form of sulphur dioxide. As expected, neither of these microbial strains was affected by the bound form of sulphur dioxide. The addition of oxygen negated the difference observed between plate and epifluorescence counts for A. pasteurianus A8 during storage, while it stimulated growth of B. bruxellensis B3a.

CONCLUSIONS

Acetobacter pasteurianus A8 can survive under anaerobic conditions in wine in the presence of sulphur dioxide. Brettanomyces bruxellensis B3a is more sensitive to sulphur dioxide than A. pasteurianus A8, but can grow in the presence of oxygen. Care should be taken to exclude oxygen from contact with wine when it is being transferred or moved.

SIGNIFICANCE AND IMPACT OF THE STUDY

Wine spoilage can be avoided by preventing growth of undesirable acetic acid bacteria and Brettanomyces/Dekkera yeasts through the effective use of sulphur dioxide and the management of oxygen throughout the winemaking process.

Discrepancy in glucose and fructose utilisation during fermentation by Saccharomyces cerevisiae wine yeast strains

While unfermented grape must contains approximately equal amounts of the two hexoses glucose and fructose, wine producers worldwide often have to contend with high residual fructose levels (>2 gl(-1)) that may account for undesirable sweetness in finished dry wine. Here, we investigate the fermentation kinetics of glucose and fructose and the influence of certain environmental parameters on hexose utilisation by wine yeast. Seventeen Saccharomyces cerevisiae strains, including commercial wine yeast strains, were evaluated in laboratory-scale wine fermentations using natural Colombard grape must that contained similar amounts of glucose and fructose (approximately 110 gl(-1) each). All strains showed preference for glucose, but to varying degrees. The discrepancy between glucose and fructose utilisation increased during the course of fermentation in a strain-dependent manner. We ranked the S. cerevisiae strains according to their rate of increase in GF discrepancy and we showed that this rate of increase is not correlated with the fermentation capacity of the strains. We also investigated the effect of ethanol and nitrogen addition on hexose utilisation during wine fermentation in both natural and synthetic grape must. Addition of ethanol had a stronger inhibitory effect on fructose than on glucose utilisation. Supplementation of must with assimilable nitrogen stimulated fructose utilisation more than glucose utilisation. These results show that the discrepancy between glucose and fructose utilisation during fermentation is not a fixed parameter but is dependent on the inherent properties of the yeast strain and on the external conditions.

Starch fermentation by recombinant saccharomyces cerevisiae strains expressing the alpha-amylase and glucoamylase genes from lipomyces kononenkoae and saccharomycopsis fibuligera

Lipomyces kononenkoae and Saccharomycopsis fibuligera possess highly efficient alpha-amylase and/or glucoamylase activities that enable both of these yeasts to utilize raw starch as a carbon source. Eight constructs containing the L. kononenkoae alpha-amylase genes (LKA1 and LKA2), and the S. fibuligera alpha-amylase (SFA1) and glucoamylase (SFG1) genes were prepared. The first set of constructs comprised four single gene cassettes each containing one of the individual amylase coding sequences (LKA1, LKA2, SFA1 or SFG1) under the control of the phosphoglycerate kinase gene (PGK1) promoter and terminator, while the second set comprised two single cassettes containing SFA1 and SFG1 linked to their respective native promoters and terminators. The third set of constructs consisted of two double-gene cassettes, one containing LKA1 plus LKA2 under the control of the PGK1 promoter and terminator, and the other SFA1 plus SFG1 controlled by their respective native promoters and terminators. These constructs were transformed into a laboratory strain Saccharomyces cerevisiae (Sigma1278b). Southern-blot analysis confirmed the stable integration of the different gene constructs into the S. cerevisiae genome and plate assays revealed amylolytic activity. The strain expressing LKA1 and LKA2 resulted in the highest levels of alpha-amylase activity in liquid media. This strain was also the most efficient at starch utilization in batch fermentations, utilizing 80% of the available starch and producing 0.61g/100 mL of ethanol after 6 days of fermentation. The strain expressing SFG1 under the control of the PGK1 expression cassette gave the highest levels of glucoamylase activity. It was shown that the co-expression of these heterologous alpha-amylase and glucoamylase genes enhance starch degradation additively in S. cerevisiae. This study has resulted in progress towards laying the foundation for the possible development of efficient starch-degrading S. cerevisiae strains that could eventually be used in consolidated bioprocessing, and in the brewing, whisky, and biofuel industries.

Identification of lactic acid bacteria isolated from South African brandy base wines

In brandy base wines, no sulphur dioxide is used and it therefore is ideal for the proliferation of lactic acid bacteria. As part of an extensive taxonomic survey within the ecological framework of South African vineyards and wineries, and the influence of naturally occurring lactic acid bacteria on the quality of wine and brandy, a total of 54 strains were isolated from grape juice and at different stages of brandy base wine production. The strains were identified using numerical analysis of total soluble cell protein patterns, 16S rRNA sequence analyses and polymerase chain reaction (PCR) using species-specific primers. The predominant species was Oenococcus oeni (22 strains), but Lactobacillus brevis (8 strains), Lactobacillus paracasei (8 strains) and Lactobacillus plantarum (6 strains) were also isolated frequently. Many of the O. oeni strains were isolated from brandy base wines after completion of spontaneous malolactic fermentation (MLF). The Lactobacillus spp. were isolated from all the different stages of brandy base wine production. Lb. plantarum was the dominant species in the juice, but disappeared during the later stages of production. However, Lactobacillus hilgardii, Lb. brevis and Lb. paracasei were also isolated from base wine after spontaneous MLF. Strains identified as Lactobacillus vermiforme were isolated during the alcoholic fermentation and after MLF have been completed. Total soluble cell protein patterns grouped O. oeni strains into two phenotypic groups. Two phenotypic clusters have also been identified for the Lb. brevis isolates. The Lb. paracasei isolates all grouped in one cluster. This is the first report of the presence of Lb. paracasei and Lb. vermiforme in brandy base wines. The presence of the Lactobacillus spp. could be correlated to the decrease in quality of the base wine and distillate, while O. oeni strains were found to have a more favourable influence on the quality of base wine and distillates. These results shed some light on the ecology and oenological influence of lactic acid bacteria (LAB) on the quality of South African brandy.

The Saccharomyces cerevisiae chitinase, encoded by the CTS1-2 gene, confers antifungal activity against Botrytis cinerea to transgenic tobacco

The Saccharomyces cerevisiae chitinase, encoded by the CTS1-2 gene has recently been confirmed by in vitro tests to possess antifungal abilities. In this study, the CTS1-2 gene has been evaluated for its in planta antifungal activity by constitutive overexpression in tobacco plants to assess its potential to increase the plant's defence against fungal pathogens. Transgenic tobacco plants, generated by Agrobacterium-mediated transformation, showed stable integration and inheritance of the transgene. Northern blot analyses conducted on the transgenic tobacco plants confirmed transgene expression. Leaf extracts from the transgenic lines inhibited Botrytis cinerea spore germination and hyphal growth by up to 70% in a quantitative in vitro assay, leading to severe physical damage on the hyphae. Several of the F1 progeny lines were challenged with the fungal pathogen, B. cinerea, in a detached leaf infection assay, showing a decrease in susceptibility ranging from 50 to 70%. The plant lines that showed increased disease tolerance were also shown to have higher chitinase activities.

Expression of the Aspergillus niger glucose oxidase gene in Saccharomyces cerevisiae and its potential applications in wine production

There is a growing consumer demand for wines containing lower levels of alcohol and chemical preservatives. The objectives of this study were to express the Aspergillus niger gene encoding a glucose oxidase (GOX; beta- d-glucose:oxygen oxidoreductase, EC 1.1.3.4) in Saccharomyces cerevisiae and to evaluate the transformants for lower alcohol production and inhibition of wine spoilage organisms, such as acetic acid bacteria and lactic acid bacteria, during fermentation. The A. niger structural glucose oxidase (gox) gene was cloned into an integration vector (YIp5) containing the yeast mating pheromone alpha-factor secretion signal (MFalpha1(S)) and the phosphoglycerate-kinase-1 gene promoter (PGK1(P)) and terminator (PGK1(T)). The PGK1(P)- MFalpha1(S)- gox- PGK1(T) cassette (designated GOX1) was introduced into a laboratory strain (Sigma1278) of S. cerevisiae. Yeast transformants were analysed for the production of biologically active glucose oxidase on selective agar plates and in liquid assays. The results indicated that the recombinant glucose oxidase was active and was produced beginning early in the exponential growth phase, leading to a stable level in the stationary phase. The yeast transformants also displayed antimicrobial activity in a plate assay against lactic acid bacteria and acetic acid bacteria. This might be explained by the fact that a final product of the GOX enzymatic reaction is hydrogen peroxide, a known antimicrobial agent. Microvinification with the laboratory yeast transformants resulted in wines containing 1.8-2.0% less alcohol. This was probably due to the production of d-glucono-delta-lactone and gluconic acid from glucose by GOX. These results pave the way for the development of wine yeast starter culture strains for the production of wine with reduced levels of chemical preservatives and alcohol.

Identification and characterization of Saccharomyces cerevisiae and Saccharomyces paradoxus strains isolated from Croatian vineyards

AIMS

The identification, differentiation and characterization of indigenous Saccharomyces sensu stricto strains isolated from Croatian vineyards and the evaluation of their oenological potential.

METHODS AND RESULTS

A total of 47 Saccharomyces sensu stricto strains were isolated from Chardonnay grapes and identified by physiological and molecular genetic methods. By using the standard physiological and biochemical tests, six isolates were identified as Saccharomyces cerevisiae and 41 as Saccharomyces paradoxus. However, PCR-RFLP analyses of the internal transcribed spacer (ITS1) region of the 18S ribosomal DNA identified 12 of the isolates as S.cerevisiae and 35 as S. paradoxus. Fermentation trials in a grape juice medium showed that these isolates ferment vigorously at 18 degrees C and display tolerance to high levels of ethanol. None of these isolates appeared to produce either hydrogen sulphide or killer toxins.

CONCLUSION

Saccharomyces paradoxus, possessing potentially important oenological characteristics, occurs in much higher numbers than S. cerevisiae in the indigenous population of Saccharomyces sensu stricto strains in Croatian vineyards.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study forms an essential step towards the preservation and exploitation of the hidden oenological potential of the untapped wealth of yeast biodiversity in the Croatian grape-growing regions. The results obtained demonstrate the value of using molecular genetic methods, such as PCR-RFLP analyses, in conjunction with the traditional taxonomic methods based on phenotypic characteristics in such ecotaxonomic surveys. The results also shed some light on the ecology and oenological potential of S.paradoxus, which is considered to be the natural parent species of the domesticated species of the Saccharomyces sensu stricto group.

Selection of yeast starter culture strains for the production of marula fruit wines and distillates

Juice of the Sclerocarya birrea subsp. caffra (marula) fruit was fermented by indigenous microflora and different commercial Saccharomyces cerevisiae yeast strains at different temperatures, namely, 15 and 30 degrees C. Volatile acids, esters, and higher alcohols were quantified in the wine and distillates, and the results were interpreted using a multivariate analysis of variance and an average linkage cluster analysis. Significant differences between 15 and 30 degrees C and also among yeasts with respect to volatile compounds were observed. Yeast strains VIN7 and FC consistently produced wines and final distillates significantly different from the other strains. A panel of tasters and marula and brandy producers was asked to select wines and distillates that had an acceptable and typical marula "nose". They were also asked to detect the differences among wines and distillates fermented with the same yeast strain at different temperatures.

Degradation of xylan to D-xylose by recombinant Saccharomyces cerevisiae coexpressing the Aspergillus niger beta-xylosidase (xlnD) and the Trichoderma reesei xylanase II (xyn2) genes

The beta-xylosidase-encoding xlnD gene of Aspergillus niger 90196 was amplified by the PCR technique from first-strand cDNA synthesized on mRNA isolated from the fungus. The nucleotide sequence of the cDNA fragment was verified to contain a 2,412-bp open reading frame that encodes a 804-amino-acid propeptide. The 778-amino-acid mature protein, with a putative molecular mass of 85.1 kDa, was fused in frame with the Saccharomyces cerevisiae mating factor alpha1 signal peptide (MFalpha1(s)) to ensure correct posttranslational processing in yeast. The fusion protein was designated Xlo2. The recombinant beta-xylosidase showed optimum activity at 60 degrees C and pH 3.2 and optimum stability at 50 degrees C. The K(i(app)) value for D-xylose and xylobiose for the recombinant beta-xylosidase was determined to be 8.33 and 6.41 mM, respectively. The XLO2 fusion gene and the XYN2 beta-xylanase gene from Trichoderma reesei, located on URA3-based multicopy shuttle vectors, were successfully expressed and coexpressed in the yeast Saccharomyces cerevisiae under the control of the alcohol dehydrogenase II gene (ADH2) promoter and terminator. These recombinant S. cerevisiae strains produced 1,577 nkat/ml of beta-xylanase activity when expressing only the beta-xylanase and 860 nkat/ml when coexpressing the beta-xylanase with the beta-xylosidase. The maximum beta-xylosidase activity was 5.3 nkat/ml when expressed on its own and 3.5 nkat/ml when coexpressed with the beta-xylanase. Coproduction of the beta-xylanase and beta-xylosidase enabled S. cerevisiae to degrade birchwood xylan to D-xylose.

Carnitine-dependent metabolic activities in Saccharomyces cerevisiae: three carnitine acetyltransferases are essential in a carnitine-dependent strain

L-carnitine is required for the transfer of activated acyl-groups across intracellular membranes in eukaryotic organisms. In Saccharomyces cerevisiae, peroxisomal membranes are impermeable to acetyl-CoA, which is produced in the peroxisome when cells are grown on fatty acids as carbon source. In a reversible reaction catalysed by carnitine acetyltransferases (CATs), activated acetyl groups are transferred to carnitine to form acetylcarnitine which can be shuttled across membranes. Here we describe a mutant selection strategy that specifically selects for mutants affected in carnitine-dependent metabolic activities. Complementation of three of these mutants resulted in the cloning of three CAT encoding genes: CAT2, coding for the carnitine acetyltransferase associated with the peroxisomes and the mitochondria; YAT1, coding for the carnitine acetyltransferase, which is presumably associated with the outer mitochondrial membrane, and YER024w (YAT2), which encodes a third, previously unidentified carnitine acetyltransferase. The data also show that (a) L-carnitine and all three CATs are essential for growth on non-fermentable carbon sources in a strain with a disrupted CIT2 gene; (b) Yat2p contributes significantly to total CAT activity when cells are grown on ethanol; and that (c) the carnitine-dependent transfer of activated acetyl groups plays a more important role in cellular processes than previously realised.

Coexpression of the Bacillus pumilus beta-xylosidase (xynB) gene with the Trichoderma reesei beta xylanase 2 (xyn2) gene in the yeast Saccharomyces cerevisiae

The xynB gene encoding the Bacillus pumilus beta-xylosidase was expressed separately and jointly with the Trichoderma reesei beta-xylanase (xyn2) gene in the yeast Saccharomyces cerevisiae. Both genes were placed under the transcriptional control of the glucose-derepressible alcohol dehydrogenase 2 promoter (ADH2p) and terminator (ADH2T) sequences. The xynB gene was fused in frame to the yeast mating factor alpha1 secretion sequence (MFalpha1s) to effect secretion in S. cerevisiae. The fusion protein was designated Xlo1. Xlo1 produced in S. cerevisiae exhibited low affinity for xylobiose, but eventually hydrolyzed xylobiose and xylotriose to the monomeric constituent, D-xylose. Coproduction of Xyn2 and Xlo1 by S. cerevisiae led to a 25% increase in the amount of reducing sugars released from birchwood xylan compared to S. cerevisiae producing only the Xyn2 beta-xylanase. However, no D-xylose was produced from birchwood xylan, presumably due to very low Xlo1 beta-xylosidase activity and its low affinity for xylobiose.

Electron microscopy of the K2 killer effect of Saccharomyces cerevisiae T206 on a mesophilic wine yeast

A mesophilic wine yeast, Saccharomyces cerevisiae CSIR Y217 K-R- was subjected to the K2 killer effect of Saccharomyces cerevisiae T206 K+R+ in a liquid grape medium. The lethal effect of the K2 mycoviral toxin was confirmed by methylene blue staining. Scanning electron microscopy of cells from challenge experiments revealed rippled cell surfaces, accompanied by cracks and pores, while those unaffected by the toxin, as in the control experiments, showed a smooth surface. Transmission electron microscopy revealed that the toxin damaged the cell wall structure and perturbed cytoplasmic membranes to a limited extent.

Tailoring wine yeast for the new millennium: novel approaches to the ancient art of winemaking

Yeasts are predominant in the ancient and complex process of winemaking. In spontaneous fermentations, there is a progressive growth pattern of indigenous yeasts, with the final stages invariably being dominated by the alcohol-tolerant strains of Saccharomyces cerevisiae. This species is universally known as the 'wine yeast' and is widely preferred for initiating wine fermentations. The primary role of wine yeast is to catalyze the rapid, complete and efficient conversion of grape sugars to ethanol, carbon dioxide and other minor, but important, metabolites without the development of off-flavours. However, due to the demanding nature of modern winemaking practices and sophisticated wine markets, there is an ever-growing quest for specialized wine yeast strains possessing a wide range of optimized, improved or novel oenological properties. This review highlights the wealth of untapped indigenous yeasts with oenological potential, the complexity of wine yeasts' genetic features and the genetic techniques often used in strain development. The current status of genetically improved wine yeasts and potential targets for further strain development are outlined. In light of the limited knowledge of industrial wine yeasts' complex genomes and the daunting challenges to comply with strict statutory regulations and consumer demands regarding the future use of genetically modified strains, this review cautions against unrealistic expectations over the short term. However, the staggering potential advantages of improved wine yeasts to both the winemaker and consumer in the third millennium are pointed out.

Effect of increased yeast alcohol acetyltransferase activity on flavor profiles of wine and distillates

The distinctive flavor of wine, brandy, and other grape-derived alcoholic beverages is affected by many compounds, including esters produced during alcoholic fermentation. The characteristic fruity odors of the fermentation bouquet are primarily due to a mixture of hexyl acetate, ethyl caproate (apple-like aroma), iso-amyl acetate (banana-like aroma), ethyl caprylate (apple-like aroma), and 2-phenylethyl acetate (fruity, flowery flavor with a honey note). The objective of this study was to investigate the feasibility of improving the aroma of wine and distillates by overexpressing one of the endogenous yeast genes that controls acetate ester production during fermentation. The synthesis of acetate esters by the wine yeast Saccharomyces cerevisiae during fermentation is ascribed to at least three acetyltransferase activities, namely, alcohol acetyltransferase (AAT), ethanol acetyltransferase, and iso-amyl AAT. To investigate the effect of increased AAT activity on the sensory quality of Chenin blanc wines and distillates from Colombar base wines, we have overexpressed the alcohol acetyltransferase gene (ATF1) of S. cerevisiae. The ATF1 gene, located on chromosome XV, was cloned from a widely used commercial wine yeast strain of S. cerevisiae, VIN13, and placed under the control of the constitutive yeast phosphoglycerate kinase gene (PGK1) promoter and terminator. Chromoblot analysis confirmed the integration of the modified copy of ATF1 into the genome of three commercial wine yeast strains (VIN7, VIN13, and WE228). Northern blot analysis indicated constitutive expression of ATF1 at high levels in these yeast transformants. The levels of ethyl acetate, iso-amyl acetate, and 2-phenylethyl acetate increased 3- to 10-fold, 3.8- to 12-fold, and 2- to 10-fold, respectively, depending on the fermentation temperature, cultivar, and yeast strain used. The concentrations of ethyl caprate, ethyl caprylate, and hexyl acetate only showed minor changes, whereas the acetic acid concentration decreased by more than half. These changes in the wine and distillate composition had a pronounced effect on the solvent or chemical aroma (associated with ethyl acetate and iso-amyl acetate) and the herbaceous and heads-associated aromas of the final distillate and the solvent or chemical and fruity or flowery characters of the Chenin blanc wines. This study establishes the concept that the overexpression of acetyltransferase genes such as ATF1 could profoundly affect the flavor profiles of wines and distillates deficient in aroma, thereby paving the way for the production of products maintaining a fruitier character for longer periods after bottling.

Divergent regulation of the evolutionarily closely related promoters of the Saccharomyces cerevisiae STA2 and MUC1 genes

The 5' upstream regions of the Saccharomyces cerevisiae glucoamylase-encoding genes STA1 to -3 and of the MUC1 (or FLO11) gene, which is critical for pseudohyphal development, invasive growth, and flocculation, are almost identical, and the genes are coregulated to a large extent. Besides representing the largest yeast promoters identified to date, these regions are of particular interest from both a functional and an evolutionary point of view. Transcription of the genes indeed seems to be dependent on numerous transcription factors which integrate the information of a complex network of signaling pathways, while the very limited sequence differences between them should allow the study of promoter evolution on a molecular level. To investigate the transcriptional regulation, we compared the transcription levels conferred by the STA2 and MUC1 promoters under various growth conditions. Our data show that transcription of both genes responded similarly to most environmental signals but also indicated significant divergence in some aspects. We identified distinct areas within the promoters that show specific responses to the activating effect of Flo8p, Msn1p (or Mss10p, Fup1p, or Phd2p), and Mss11p as well as to carbon catabolite repression. We also identified the STA10 repressive effect as the absence of Flo8p, a transcriptional activator of flocculation genes in S. cerevisiae.

The development of bactericidal yeast strains by expressing the Pediococcus acidilactici pediocin gene (pedA) in Saccharomyces cerevisiae

The excessive use of sulphur dioxide and other chemical preservatives in wine, beer and other fermented food and beverage products to prevent the growth of unwanted microbes holds various disadvantages for the quality of the end-products and is confronted by mounting consumer resistance. The objective of this study was to investigate the feasibility of controlling spoilage bacteria during yeast-based fermentations by engineering bactericidal strains of Saccharomyces cerevisiae. To test this novel concept, we have successfully expressed a bacteriocin gene in yeast. The pediocin operon of Pediococcus acidilactici PAC1.0 consists of four clustered genes, namely pedA (encoding a 62 amino acid precursor of the PA-1 pediocin), pedB (encoding an immunity factor), pedC (encoding a PA-1 transport protein) and pedD (encoding a protein involved in the transport and processing of PA-1). The pedA gene was inserted into a yeast expression/secretion cassette and introduced as a multicopy episomal plasmid into a laboratory strain (Y294) of S. cerevisiae. Northern blot analysis confirmed that the pedA structural gene in this construct (ADH1P-MFa1S-pedA-ADH1T, designated PED1), was efficiently expressed under the control of the yeast alcohol dehydrogenase I gene promoter (ADH1P) and terminator (ADH1T). Secretion of the PED1-encoded pediocin PA-1 was directed by the yeast mating pheromone alpha-factor's secretion signal (MFa1S). The presence of biologically active antimicrobial peptides produced by the yeast transformants was indicated by agar diffusion assays against sensitive indicator bacteria (e.g. Listeria monocytogenes B73). Protein analysis indicated the secreted heterologous peptide to be approximately 4.6 kDa, which conforms to the expected size. The heterologous peptide was present at relatively low levels in the yeast supernatant but pediocin activity was readily detected when intact yeast colonies were used in sensitive strain overlays. This study could lead to the development of bactericidal yeast strains where S. cerevisiae starter cultures not only conduct the fermentations in the wine, brewing and baking industries but also act as biological control agents to inhibit the growth of spoilage bacteria.

Functional analysis of multiple AUG codons in the transcripts of the STA2 glucoamylase gene from Saccharomyces cerevisiae

A scanning ribosome will usually initiate translation as soon as it encounters the first favourable AUG codon and only a few eukaryotic transcripts have more complex arrangements. These relatively few complex transcripts are normally characterized by structural features such as multiple AUGs and significant secondary structure. However, the functional relevance of these features has rarely been established. We present here a study of the functional significance of the multiple AUGs in the leader of STA2 transcripts of the budding yeast Saccharomyces cerevisiae, and extrapolate, where applicable, these results to a co-regulated gene, MUC1. The STA2 gene (a representative member of the polymorphic STA1-3 gene family), encodes an extracellular glucoamylase, and is evolutionarily linked to, and transcriptionally co-regulated with, the MUC1 gene, which encodes a mucin-like protein essential for pseudohyphal/invasive growth and cell-adhesion in S. cerevisiae. Each of these genes contains a putative upstream ORF, while STA2 has two additional in-frame AUG codons 5' to the major cistron. We show that utilization of the alternative translational start-sites of STA2 results in glucoamylases that differ at their N-termini, which are associated with differences in their localization patterns. Analysis of mutants revealed the presence of a putative secretion-enhancing signal that might prove to be relevant to the alternative targeting mechanism recently uncovered in S. cerevisiae. We show that a short up-stream ORF present in the leaders of STA1-3 and MUC1 is probably bypassed by a process of leaky scanning.

Msn1p/Mss10p, Mss11p and Muc1p/Flo11p are part of a signal transduction pathway downstream of Mep2p regulating invasive growth and pseudohyphal differentiation in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, a network of signal transduction pathways governs the switch from yeast-type growth to pseudohyphal and invasive growth that occurs in response to nutrient limitation. Important elements of this network have been identified, including nutrient signal receptors, GTP-binding proteins, components of the pheromone-dependent MAP kinase cascade and several transcription factors. However, the structural and functional mapping of these pathways is far from complete. Here, we present data regarding three genes, MSN1/MSS10, MSS11 and MUC1/FLO11, which form an essential part of the signal transduction network establishing invasive growth. Both MSN1 and MSS11 are involved in the co-regulation of starch degradation and invasive growth. Msn1p and Mss11p act downstream of Mep2p and Ras2p and regulate the transcription of both STA2 and MUC1. We show that MUC1 mediates the effect of Msn1p and Mss11p on invasive growth. In addition, our results suggest that the activity of Msn1p is independent of the invasive growth MAP kinase cascade, but the Mss11p is required for the activation of pseudohyphal and invasive growth by Ste12p. We also show that starch metabolism in S. cerevisiae is subject to regulation by components of the MAP kinase cascade.

Genetic and fermentation properties of the K2 killer yeast, Saccharomyces cerevisiae T206

Saccharomyces cerevisiae T206 K+R+, a K2 killer yeast, was differentiated from other NCYC killer strains of S. cerevisiae on the basis of CHEF-karyotyping and mycoviral RNA separations. Genomic DNA of strain T206 was resolved into 13 chromosome bands, ranging from approximately 0.2 to 2.2 Mb. The resident virus in strain T206 yielded L and M RNA species of approximately 5.1 kb and 2.0 kb, respectively. In micro-scale vinifications, strain T206 showed a lethal effect on a K-R- mesophilic wine yeast. Metabolite accumulation and toxin activity were measured over a narrow pH range of 3.2 to 3.5. Contrary to known fermentation trends, the challenged fermentations were neither stuck nor protracted although over 70% of the cell population was killed. Toxin-sensitive cells showed cytosolic efflux.

Coregulation of starch degradation and dimorphism in the yeast Saccharomyces cerevisiae

Saccharomyces cerevisiae, the exemplar unicellular eukaryote, can only survive and proliferate in its natural habitats through constant adaptation within the constraints of a dynamic ecosystem. In every cell cycle of S. cerevisiae, there is a short period in the G1 phase of the cell cycle where "sensing" transpires; if a sufficient amount of fermentable sugars is available, the cells will initiate another round of vegetative cell division. When fermentable sugars become limiting, the yeast can execute the diauxic shift, where it reprograms its metabolism to utilize nonfermentable carbon sources. S. cerevisiae can also initiate the developmental program of pseudohyphal formation and invasive growth response, when essential nutrients become limiting. S. cerevisiae shares this growth form-switching ability with important pathogens such as the human pathogen, Candida albicans, and the corn smut pathogen Ustilago maydis. The pseudohyphal growth response of S. cerevisiae has mainly been implicated as a means for the yeast to search for nutrients. An important observation made was that starch-degrading S. cerevisiae strains have the added ability to form pseudohyphae and grow invasively into a starch-containing medium. More significantly, it was also shown that the STA1-3 genes encoding three glucoamylase isozymes responsible for starch hydrolysis in S. cerevisiae are coregulated with a gene, MUC1, essential for pseudohyphal and invasive growth. At least two putative transcriptional activators, Mss10p and Mss11p, are involved in this regulation. The Muc1p is a putative integral membrane-bound protein similar to mammalian mucin-like proteins that have been implicated in the ability of cancer cells to invade other tissues. This provided us with an excellent example of integrative control between nutrient sensing, signaling, and differential development.

Engineering yeast for efficient cellulose degradation

Saccharomyces cerevisiae produces several beta-1,3-glucanases, but lacks the multicomponent cellulase complexes that hydrolyse the beta-1,4-linked glucose polymers present in cellulose-rich biomass as well as in haze-forming glucans in certain wines and beers. We have introduced into S. cerevisiae a functional cellulase complex for efficient cellulose degradation by cloning the Endomyces fibuliger cellobiase (BGL1) gene and co-expressing it with the Butyrivibrio fibrisolvens endo-beta-1,4-glucanase (END1), the Phanerochaete chrysosporium cellobiohydrolase (CBH1) and the Ruminococcus flavefacies cellodextrinase (CEL1) gene constructs in this yeast. The END1, CBH1 and CEL1 genes were inserted into yeast expression/secretion cassettes. Expression of END1, CBH1 and CEL1 was directed by the promoter sequences derived from the alcohol dehydrogenase II (ADH2), the phosphoglycerate kinase I (PKG1) and the alcohol dehydrogenase I (ADH1) genes, respectively. In contrast, BGL1 was expressed under the control of its native promoter. Secretion of End1p and Cel1p was directed by the signal sequence of the yeast mating pheromone alpha-factor (MF alpha 1), whereas Cbh1p and Bgl1p were secreted using their authentic leader peptides. The construction of a fur1 ura3 S. cerevisiae strain allowed for the autoselection of this multicopy URA3-based plasmid in rich medium. S. cerevisiae transformants secreting biologically active endo-beta-1,4-glucanase, cellobiohydrolase, cellodextrinase and cellobiase were able to degrade various substrates including carboxymethylcellulose, hydroxyethylcellulose, laminarin, barley glucan, cellobiose, polypectate, birchwood xylan and methyl-beta-D-glucopyranoside. This study could lead to the development of industrial strains of S. cerevisiae capable of converting cellulose in a one-step process into commercially important commodities.

Identification of a functional TATA element in the STA2 glucoamylase gene promoter from Saccharomyces cerevisiae

Accurate transcription by RNA polymerase II is usually dependent on the presence of a TATA element, and/or an initiator element, in the promoters of protein-encoding genes. The STA1-3 genes, encoding three glucoamylase isozymes (Sta1p, Sta2p and Sta3p, respectively) responsible for starch hydrolysis in the yeast Saccharomyces cerevisiae, have been shown to contain long and complex promoters with several regulatory regions. These promoters are also virtually identical to the yeast MUC1 gene promoter; this gene encodes a mucin-like protein and is evolutionary linked to, and transcriptionally co-regulated with, STA1-3. The STA1-3 genes contain two putative TATA sequences; one conforming to the typical TATA box sequence, TATAAA, and another with the sequence of TATAAT. Here we present a study into the functional relevance of these putative TATA sequences and their effects on the transcription of the STA2 gene (as a representative model of the STA1-3 multigene family) and, by analogy, the MUC1 gene. We show that the TATAAA motif is the functional TATA box for STA2 and influences transcript levels, transcript initiation sites, and glucoamylase activities.

MSS11, a novel yeast gene involved in the regulation of starch metabolism

Expression of the STA1-3 glucoamylase genes, responsible for starch degradation in Saccharomyces cerevisiae, is down regulated by the presence of STA10. In order to elucidate the role of STA10 in the regulation of the glucoamylase system, a multicopy genomic library was constructed and screened for genes that enhanced growth of a STA2-STA10 S. cerevisiae strain on starch media. This screen allowed us to clone and characterize a novel activator gene of STA2 (and by extrapolation, STA1 and STA3), designated MSS11. A strain transformed with multiple copies of MSS11 exhibits increased levels of STA2 mRNA and, consequently, increased glucoamylase activity. Deletion of MSS11, located on chromosome XIII, results in media-dependent absence of glucoamylase synthesis. MSS11 has not been cloned previously and the encoded protein, Mss11p, is not homologous to any other known protein. An outstanding feature of Mss11p is that the protein contains regions of 33 asparagine residues interrupted by only three serine residues, and 35 glutamine residues interrupted by a single histidine residue. Epistasis studies showed that deletion of MSS11 abolishes the activation of STA2 caused by the over-expression of MSS10, a previously identified gene. In turn, it was found that deletion of MSS10 still allows activation of STA2 by over-expression of MSS11. Mss11p therefore appears to be positioned below Mss10p in a signal transduction pathway.

Over-expression of the Saccharomyces cerevisiae exo-beta-1,3-glucanase gene together with the Bacillus subtilis endo-beta-1,3-1,4-glucanase gene and the Butyrivibrio fibrisolvens endo-beta-1,4-glucanase gene in yeast

The EXG1 gene encoding the main Saccharomyces cerevisiae exo-beta-1,3-glucanase was cloned and over-expressed in yeast. The Bacillus subtilis endo-1,3-1,4-beta-glucanase gene (beg1) and the Butyrivibrio fibrisolvens endo-beta-1,4-glucanase gene (end1) were fused to the secretion signal sequence of the yeast mating pheromone alpha-factor (MF alpha 1S) and inserted between the yeast alcohol dehydrogenase II gene promoter (ADH2P) and terminator (ADH2T). Constructs ADH2P-MF alpha 1S-beg1-ADH2T and ADH2P-MF alpha 1S-end 1-ADH2T designated BEG1 and END1, respectively, were expressed separately and jointly with EXG1 in S. cerevisiae. The construction of fur 1 ura3 S. cerevisiae strains allowed for the autoselection of these multicopy URA3-based plasmids in rich medium. Enzyme assays confirmed that co-expression of EXG1, BEG1 and END1 enhanced glucan degradation by S. cerevisiae.

Cloning of the Bacillus pumilus beta-xylosidase gene (xynB) and its expression in Saccharomyces cerevisiae

A genomic DNA library of the bacterium Bacillus pumilus PLS was constructed and the beta-xylosidase gene (xynB) was amplified from a 3-kb genomic DNA fragment with the aid of the polymerase chain reaction technique. The amplified xynB gene was inserted between the yeast alcohol dehydrogenase II gene promoter (ADH2P) and terminator (ADH2T) sequences on a multicopy episomal plasmid (pDLG11). The xynB gene was also fused in-frame to the secretion signal sequence of the yeast mating pheromone alpha-factor (MF alpha 1S) before insertion between the ADH2P and ADH2T sequences on a similar multicopy episomal plasmid (pDLG12). The resulting construct ADH2P-MF alpha 1S-xynB-ADH2T was designated XLO1. Both plasmids pDLG11 and PDLG12 were introduced into Saccharomyces cerevisiae but only the expression of the XLO1 gene yielded biologically functional beta-xylosidase. The total beta-xylosidase activity remained cell-associated with a maximum activity of 0.09 nkat/ml obtained when the recombinant S. cerevisiae strain was grown for 143 h in synthetic medium. The temperature and pH optima of the recombinant Xlo1 enzyme were 45-50 degrees C and pH 6.6 respectively. The enzyme was thermostable at 45 degrees C; however, at 60 degrees C most of the Xlo1 was inactive after 5 min.

Cloning and expression of the alpha-L-arabinofuranosidase gene (ABF2) of Aspergillus niger in Saccharomyces cerevisiae

First-strand cDNA was prepared from mRNA of Aspergillus niger MRC11624 induced on oat spelts xylan. Using the cDNA as a template, the alpha-L-arabinofuranosidase gene (abfB) was amplified with the polymerase chain reaction technique. The abfB DNA fragment was inserted between the yeast phosphoglycerate kinase I gene promoter (PGK1P) and terminator (PGK1T) sequences on a multicopy episomal plasmid. The resulting construct PGK1P-abfB-PGK1T was designated ABF2. The ABF2 gene was expressed successfully in Saccharomyces cerevisiae and functional alpha-L-arabinofuranosidase was secreted from the yeast cells. The ABF2 nucleotide sequence was determined and verified to encode a 449-amino-acid protein (Abf2) that is 94% identical to the alpha-L-arabinofuranosidase B of A. niger N400. Maximum alpha-L-arabinofuranosidase activities of 0.020 U/ml and 1.40 U/ml were obtained with autoselective recombinant S. cerevisiae strains when grown for 48 h in synthetic and complex medium respectively.

Muc1, a mucin-like protein that is regulated by Mss10, is critical for pseudohyphal differentiation in yeast

Pseudohyphal differentiation in Saccharomyces cerevisiae was first described as a response of diploid cells to nitrogen limitation. Here we report that haploid and diploid starch-degrading S. cerevisiae strains were able to switch from a yeast form to a filamentous pseudohyphal form in response to carbon limitation in the presence of an ample supply of nitrogen. Two genes, MSS10 and MUC1, were cloned and shown to be involved in pseudohyphal differentiation and invasive growth. The deletion of MSS10 resulted in extremely reduced amounts of pseudohyphal differentiation and invasive growth, whereas the deletion of MUC1 abolished pseudohyphal differentiation and invasive growth completely. Mss10 appears to be a transcriptional activator that responds to nutrient limitation and coregulates the expression of MUC1 and the STA1-3 glucoamylase genes, which are involved in starch degradation. MUC1 encodes a 1367-amino acid protein, containing several serine/threonine-rich repeats. Muc1 is a putative integral membrane-bound protein, similar to mammalian mucin-like membrane proteins that have been implicated to play a role in the ability of cancer cells to invade other tissues.

Cloning, mapping and characterization of a genomic copy of the Lipomyces kononenkoae alpha-amylase-encoding gene (LKA1)

The expression in Saccharomyces cerevisiae and Schizosaccharomyces pombe of a cDNA copy of the Lipomyces kononenkoae IGC4052B alpha-amylase gene (LKA1), linked to the phosphoglycerate kinase gene (PGK1) promoter, resulted in the extracellular production of biologically active alpha-amylase (LKA1). However, transformation of S. cerevisiae and Schiz. pombe with a cosmid clone containing the complete genomic copy of LKA1, expressed from its native promoter, did not result in secretion of active alpha-amylase by any of the transformants. When the cDNA copy of LKA1 was expressed in S. cerevisiae under control of the wild-type L, kononenkoae promoter, biologically active alpha-amylase was secreted into the culture medium, indicating the recognition of the LKA1 promoter in S. cerevisiae. Sequence analysis of the GC-rich LKA1 promoter revealed canonical sequences that are homologous to the TATAAA, CAAT and CCAAT boxes and GCN4-binding sites that are present in several promoter sequences of S. cerevisiae. Primer extension analysis of LKA1 transcripts in L. kononenkoae indicated major initiation sites at nucleotides -64 and -65. S. cerevisiae and Schiz. pombe cells transformed with a plasmid containing the open reading frame of the genomic copy of LKA1, linked to the PGK1 promoter, did not produce alpha-amylase. Polymerase chain reaction mapping and sequence analysis revealed the presence of a 61-bp intron in the genomic copy of LKA1 that impaired synthesis of biologically active alpha-amylase in S. cerevisiae and Schiz. pombe. This intron contains donor, acceptor and branch sequences that correlate with the consensus sequences identified in the introns of split genes from Schiz. pombe and mammals. Pulsed-field gradient gel electrophoresis resolved at least eight chromosomal DNAs for L. kononenkoae IGC4052B and chromoblot analysis indicated that LKA1 is located on the second smallest chromosome, designated chromosome II.

Co-expression of a Phanerochaete chrysosporium cellobiohydrolase gene and a Butyrivibrio fibrisolvens endo-beta-1,4-glucanase gene in Saccharomyces cerevisiae

A cDNA fragment encoding the Phanerochaete chrysosporium cellobiohydrolase (cbh1-4) was amplified and cloned with the aid of the polymerase chain reaction (PCR) technique. The cbh1-4 gene and the Butyrivibrio fibrisolvens endo-beta-1,4-glucanase (end1) gene were successfully expressed in Saccharomyces cerevisiae under the control of the phosphoglycerate kinase-I (PGK1) and alcohol dehydrogenase-II (ADH2) gene promoters and terminators, respectively. The native P. chrysosporium signal sequence mediated secretion of cellobiohydrolase in S. cerevisiae, whereas secretion of the endo-beta-1,4-glucanase was directed by the signal sequence of the yeast mating pheromone alpha-factor (MFalpha1S). These constructs, designated CBH1 and END1, respectively, were expressed separately and jointly in S. cerevisiae. The construction of fur1 ura3 S. cerevisiae strains allowed for the autoselection of these multicopy URA3-based plasmids in rich medium. Enzyme assays confirmed that co-expression of CBH1 and END1 synergistically enhanced cellulose degradation by S. cerevisiae.

A multicopy suppressor gene, MSS10, restores STA2 expression in Saccharomyces cerevisiae strains containing the STA10 repressor gene

Transcription of the three unlinked, homologous STA1-3 glucoamylase-encoding genes, involved in starch degradation by Saccharomyces cerevisiae, was previously shown to be down-regulated by the presence of STA10, acting via three upstream repression sequence regions that were identified in the STA2 promoter. Here we report the cloning and characterization of a putative transcriptional activator gene, MSS10 (multicopy suppressor of STA10), which, when present in multiple copies, overcomes STA10 repression. Deletion of MSS10, located on chromosome XV, resulted in media-specific extinction of glucoamylase synthesis. The nucleotide sequence of MSS10 is identical to three other genes from S. cerevisiae identified as: FUP1, a gene that enhances iron-limited growth; PHD2, a gene identified for its ability to induce pseudohyphal growth in diploid cells grown on nitrogen-limited media; and MSN1, a gene encoding a transcriptional activator involved in invertase regulation.

Expression of a Trichoderma reesei beta-xylanase gene (XYN2) in Saccharomyces cerevisiae

The XYN2 gene encoding the main Trichoderma reesei QM 6a endo-beta-1,4-xylanase was amplified by PCR from first-strand cDNA synthesized on mRNA isolated from the fungus. The nucleotide sequence of the cDNA fragment was verified to contain a 699-bp open reading frame that encodes a 223-amino-acid propeptide. The XYN2 gene, located on URA3-based multicopy shuttle vectors, was successfully expressed in the yeast Saccharomyces cerevisiae under the control of the alcohol dehydrogenase II (ADH2) and phosphoglycerate kinase (PGK1) gene promoters and terminators, respectively. The 33-amino-acid leader peptide of the Xyn2 beta-xylanase was recognized and cleaved at the Kex2-like Lys-Arg residues, enabling the efficient secretion and glycosylation of the heterologous beta-xylanase. The molecular mass of the recombinant beta-xylanase was estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to be 27 kDa. The construction of fur1 ura3 S. cerevisiae strains allowed for the autoselection of the URA3-based XYN2 shuttle vectors in nonselective complex medium. These autoselective S. cerevisiae strains produced 1,200 and 160 nkat of beta-xylanase activity per ml under the control of the ADH2 and PGK1 promoters in rich medium, respectively. The recombinant enzyme showed highest activity at pH 6 and 60 degrees C and retained more than 90% of its activity after 60 min at 50 degrees C.

Cloning, sequence analysis and expression in yeasts of a cDNA containing a Lipomyces kononenkoae alpha-amylase-encoding gene

The yeast Lipomyces kononenkoae (Lk) secretes a highly active raw starch-degrading alpha-amylase (alpha Amy) that liberates reducing groups from glucose polymers containing both alpha-1,4 and alpha-1,6 bonds. The LKA1 gene encoding this industrially important alpha Amy was cloned as a 2261-bp cDNA fragment from a glucose-derepressed mutant (IGC4052B) of Lk and characterized. The nucleotide (nt) sequence of the cDNA fragment was determined, revealing an open reading frame of 1872 bp, encoding a 596 amino-acid (aa) mature protein (LKA1) with a calculated M(r) of 65,706. The similarity between the aa sequence of LKA1 and those of other alpha Amy showed four common conserved regions characteristic of the alpha Amy protein family: (A) 264DIVVNH269, (B) 349GLRIDTVKH357, (B') 376GEVFD380 and (C) 439FLENQD444. The deduced aa sequence revealed significant homology to the aa sequences of the Aspergillus oryzae, Schwanniomyces occidentalis and Saccharomycopsis fibuligera alpha Amy, various bacterial cyclomaltodextrin glucanotransferases, a beta-amylase and the 5'-region of a glucoamylase. LKA1 was expressed in Saccharomyces cerevisiae (Sc) under the control of the phosphoglycerate kinase (PGK1) promoter and Northern blot analysis showed the presence of a single 2.3-kb transcript. The 28-aa signal peptide of the LKA1 protein efficiently directed its secretion into the medium when expressed in Sc.

Characterization of a novel alpha-amylase from Lipomyces kononenkoae and expression of its gene (LKA1) in Saccharomyces cerevisiae

A highly active alpha-amylase (76,250 Da) secreted by the raw starch-degrading yeast Lipomyces kononenkoae strain IGC4052B was purified and characterized. Using high performance liquid chromatography (HPLC), end-product analysis indicated that the L. kononenkoae alpha-amylase acted by endo-hydrolysis on glucose polymers containing alpha-1,4 and alpha-1,6 bonds, producing mainly maltose, maltotriose and maltotetraose. The following NH2-terminal amino acids were determined for the purified enzyme: Asp-Cys-Thr-Thr-Val-Thr-Val-Leu-Ser-Ser-Pro- Glu-Ser-Val-Thr-Gly. The L. kononenkoae alpha-amylase-encoding gene (LKA1), previously cloned as a cDNA fragment, was expressed in Saccharomyces cerevisiae under the control of the PGK1 promoter. The native signal sequence efficiently directed the secretion of the glycosylated protein in S. cerevisiae. De-glycosylation of the enzyme indicated that post-translational glycosylation is different in S. cerevisiae from that in L. kononenkoae. Zymogram analysis indicated that glycosylation of the protein in S. cerevisiae had a negative effect on enzyme activity. Southern-blot analysis revealed that there is only a single LKA1 gene present in the genome of L. kononenkoae.

Cloning and expression of an Aspergillus kawachii endo-1,4-beta-xylanase gene in Saccharomyces cerevisiae

First-strand cDNA was prepared from mRNA isolated from Aspergillus kawachii IFO4308 and the beta-xylanase gene (xynC) amplified by using the polymerase chain reaction (PCR) technique. This gene was inserted between the yeast phosphoglycerate kinase (PGK1) gene promoter (PGK1p) and terminator (PGK1T) sequences. The PGK1p-xynC-PGK1T construct (designated XYN3) was cloned into a multicopy episomal plasmid and the XYN3 gene was expressed in Saccharomyces cerevisiae. Functional beta-xylanase (Xyn3) was produced and secreted by the recombinant yeast. Xyn3 was stable between 30 and 50 degrees C, and the optimum temperature and pH were shown to be at 60 degrees C and lower than pH 3, respectively. An autoselective furl::LEU2 XYN3 recombinant strain was developed that allowed beta-xylanase production at a level of 300 nkat/ml in a non-selective complex medium.

The S1, S2 and SGA1 ancestral genes for the STA glucoamylase genes all map to chromosome IX in Saccharomyces cerevisiae

The polymorphic extracellular glucoamylase-encoding genes STA1 (chr. IV), STA2 (chr. II) and STA3 (chr. XIV), from Saccharomyces cerevisiae var. diastaticus probably evolved by genomic rearrangement of DNA regions (S1, S2 and SGA1) present in S. cerevisiae, and subsequent translocation to unlinked regions of chromosomal regions. S1, encoding a homologue to the threonine/serine-rich domain of STA glucoamylases (GAI-III), mapped to the right arm of chromosome IX. S2, encoding the hydrophobic leader peptide of GAI-III), was also mapped on the right arm of chromosome IX, next to S1, close to DAL81. The SGA1 sporulation-specific, intracellular glucoamylase-encoding gene is located on the left arm of chromosome IX, 32 kb proximal of HIS5.

One-step enzymatic hydrolysis of starch using a recombinant strain of Saccharomyces cerevisiae producing alpha-amylase, glucoamylase and pullulanase

A recombinant strain of Saccharomyces cerevisiae was constructed that contained the genes encoding a bacterial alpha-amylase (AMY1), a yeast glucoamylase (STA2) and a bacterial pullulanase (pulA). The Bacillus amyloliquefaciens alpha-amylase and S. cerevisiae var. diastaticus glucoamylase genes were expressed in S. cerevisiae using their native promoters and the encoded enzymes secreted under direction of their native leader sequences. In contrast, the Klebsiella pneumoniae pullulanase gene was placed under the control of the yeast alcohol dehydrogenase gene promoter (ADC1P) and secreted using the yeast mating pheromone alpha-factor secretion signal (MF alpha 1S). Transcription termination of the pullulanase gene was effected by the yeast tryptophan synthase gene terminator (TRP5T), whereas termination of the glucoamylase and alpha-amylase genes was directed by their native terminators. Pullulanase (PUL1) produced by recombinant yeasts containing ADC1P MF alpha 1S pulA TRP5T (designated PUL1) was further characterized and compared to its bacterial counterpart (PulA). The different genes were introduced into S. cerevisiae in different combinations and the various amylolytic Saccharomyces transformants compared to Schwanniomyces occidentalis. Introduction of PUL1 into a S. cerevisiae strain containing both STA2 and AMY1, resulted in 99% assimilation of starch.

Expression of human P450c17 as an export protein in Saccharomyces cerevisiae

Cytochrome P450c17 (P450c17), together with cytochrome P450c21 (P450c21), plays an important role in progesterone metabolism in the mammalian adrenal cortex. Low levels of expression and the presence of other steroidogenic enzymes in adrenal cortex endoplasmic reticulum (ER) impedes purification and characterisation of wild type as well as mutant forms of the hemoprotein. Heterologous gene expression systems have previously been used successfully to express active P450c17. Heterologous expression can also be used for the preparation of anti-P450c17-IgG. For antibody production larger amounts of pure P450c17 peptide, rather than the active protein, is, however, desirable. If the expressed protein can be affinity tagged and secreted into the medium, isolation and purification will be facilitated. Saccharomyces cerevisiae, YPH259, was transformed with a modified YCplac111 yeast expression-secretion vector (pPRL2). The gene coding for a truncated human P450c17 (signal anchor sequence 1-18 was removed) was inserted, in reading frame, downstream from the leader sequence MF alpha. A histidine tag was incorporated at the C-terminus. The modified yeast expression vector was expressed in yeast, the secreted P450c17-peptide purified by affinity chromatography and identified by immunoblot analysis.

Expression of the Butyrivibrio fibrisolvens endo-beta-1,4-glucanase gene together with the Erwinia pectate lyase and polygalacturonase genes in Saccharomyces cerevisiae

Recombinant Saccharomyces cerevisiae strains capable of simultaneous secretion of bacterial glucanase and pectinase enzymes have been developed. The Butyrivibrio fibrrisolvens endo-beta-1,4-glucanase gene (end1), the Erwinia chrysanthemi pectate lyase gene (pelE) and E. carotovora polygalacturonase gene (peh1) were each inserted between a yeast expression-secretion cassette and yeast gene terminator, and cloned into yeast-centromeric shuttle vectors. Transcription initiation signals present in the expression-secretion cassette were derived from the yeast alcohol dehydrogenase gene promoter (ADC1P), whereas the transcription termination signals were derived from the yeast tryptophan synthase gene terminator (TRP5T). Secretion of glucanase and pectinases was directed by the signal sequence of the yeast mating pheromone alpha-factor (MF alpha 1S). These YCplac111-based constructs, designated END1, PEL5, AND PEH1, respectively, were transformed into S. cerevisiae. The END1, PEL5 and PEH1 constructs were co-expressed in laboratory strains of S. cerevisiae as well as in wine and distillers' yeasts. DNA-RNA hybridization analysis showed the presence of END1, PEL5 and PEH1 transcripts. Carboxymethylcellulose and polypectate agarose assays revealed the production of biologically active endo-beta-1,4-glucanase, pectate lyase and polygalacturonase by the S. cerevisiae transformants. Interestingly, although the same expression-secretion cassette was used in all three constructs, time-course assays indicated that the pectinases were secreted before the glucanase. It is tempting to speculate that the bulkiness of the END1-encoded protein and the five alternating repeats of Pro-Asp-Pro-Thr(Gln)-Pro-Val-Asp within the glucanase moiety could be involved in the delayed secretion of the glucanase.

Multiple positive and negative cis-acting elements of the STA2 gene regulate glucoamylase synthesis in Saccharomyces cerevisiae

Expression of the glucoamylase-encoding gene (STA2) in Saccharomyces cerevisiae was previously shown to be regulated transcriptionally by both positive and negative factors. The objective of this work was to identify the cis-acting elements responsible for STA2 transcriptional activation as well as the transcriptional repressor effects of STA10 and MATa/MAT alpha. We identified two upstream activation regions (UAS). Three repressor regions responsive to STA10-mediated repression were identified, as well as two regions for down-regulation of STA2 expression. MATa/MAT alpha repression appears to effect STA2 expression either downstream from the translational start site or, indirectly, since no functional a1/alpha 2-responsive sequence was identified in the promoter region.