The value of electrophoretic fingerprinting and karyotyping in wine yeast breeding programmes

Killer yeasts: expanding frontiers in the age of synthetic biology

Killer yeasts secrete protein toxins that are selectively lethal to other yeast and filamentous fungi. These exhibit exceptional genetic and functional diversity, and have several biotechnological applications. However, despite decades of research, several limitations hinder their widespread adoption. In this perspective we contend that technical advances in synthetic biology present an unprecedented opportunity to unlock the full potential of yeast killer systems across a spectrum of applications. By leveraging these new technologies, engineered killer toxins may emerge as a pivotal new tool to address antifungal resistance and food security. Finally, we speculate on the biotechnological potential of re-engineering host double-stranded (ds) RNA mycoviruses, from which many toxins derive, as a safe and noninfectious system to produce designer RNA.

Yeast's balancing act between ethanol and glycerol production in low-alcohol wines

Alcohol is fundamental to the character of wine, yet too much can put a wine off‐balance. A wine is regarded to be well balanced if its alcoholic strength, acidity, sweetness, fruitiness and tannin structure complement each other so that no single component dominates on the palate. Balancing a wine's positive fruit flavours with the optimal absolute and relative concentration of alcohol can be surprisingly difficult. Over the past three decades, consumers have increasingly demanded wine with richer and riper fruit flavour profiles. In response, grape and wine producers have extended harvest times to increase grape maturity and enhance the degree of fruit flavours and colour intensity. However, a higher degree of grape maturity results in increased grape sugar concentration, which in turn results in wines with elevated alcohol concentration. On average, the alcohol strength of red wines from many warm wine‐producing regions globally rose by about 2% (v/v) during this period. Notwithstanding that many of these ‘full‐bodied, fruit‐forward’ wines are well balanced and sought after, there is also a significant consumer market segment that seeks lighter styles with less ethanol‐derived ‘hotness’ on the palate. Consumer‐focussed wine producers are developing and implementing several strategies in the vineyard and winery to reduce the alcohol concentration in wines produced from well‐ripened grapes. In this context, Saccharomyces cerevisiae wine yeasts have proven to be a pivotal strategy to reduce ethanol formation during the fermentation of grape musts with high sugar content (> 240 g l⁻¹). One of the approaches has been to develop ‘low‐alcohol’ yeast strains which work by redirecting their carbon metabolism away from ethanol production to other metabolites, such as glycerol. This article reviews the current challenges of producing glycerol at the expense of ethanol. It also casts new light on yeast strain development programmes which, bolstered by synthetic genomics, could potentially overcome these challenges.

Sustaining fermentation in high-gravity ethanol production by feeding yeast to a temperature-profiled multifeed simultaneous saccharification and co-fermentation of wheat straw

BACKGROUND

Considerable progress is being made in ethanol production from lignocellulosic feedstocks by fermentation, but negative effects of inhibitors on fermenting microorganisms are still challenging. Feeding preadapted cells has shown positive effects by sustaining fermentation in high-gravity simultaneous saccharification and co-fermentation (SSCF). Loss of cell viability has been reported in several SSCF studies on different substrates and seems to be the main reason for the declining ethanol production toward the end of the process. Here, we investigate how the combination of yeast preadaptation and feeding, cell flocculation, and temperature reduction improves the cell viability in SSCF of steam pretreated wheat straw.

RESULTS

More than 50% cell viability was lost during the first 24 h of high-gravity SSCF. No beneficial effects of adding selected nutrients were observed in shake flask SSCF. Ethanol concentrations greater than 50 g L^-1 led to significant loss of viability and prevented further fermentation in SSCF. The benefits of feeding preadapted yeast cells were marginal at later stages of SSCF. Yeast flocculation did not improve the viability but simplified cell harvest and improved the feasibility of the cell feeding strategy in demo scale. Cultivation at 30 °C instead of 35 °C increased cell survival significantly on solid media containing ethanol and inhibitors. Similarly, in multifeed SSCF, cells maintained the viability and fermentation capacity when the temperature was reduced from 35 to 30 °C during the process, but hydrolysis yields were compromised. By combining the yeast feeding and temperature change, an ethanol concentration of 65 g L^-1, equivalent to 70% of the theoretical yield, was obtained in multifeed SSCF on pretreated wheat straw. In demo scale, the process with flocculating yeast and temperature profile resulted in 5% (w/w) ethanol, equivalent to 53% of the theoretical yield.

CONCLUSIONS

Multifeed SSCF was further developed by means of a flocculating yeast and a temperature-reduction profile. Ethanol toxicity is intensified in the presence of lignocellulosic inhibitors at temperatures that are beneficial to hydrolysis in high-gravity SSCF. The counteracting effects of temperature on cell viability and hydrolysis call for more tolerant microorganisms, enzyme systems with lower temperature optimum, or full optimization of the multifeed strategy with temperature profile.

Metabolic engineering of Saccharomyces cerevisiae ethanol strains PE-2 and CAT-1 for efficient lignocellulosic fermentation

In this work, Saccharomyces cerevisiae strains PE-2 and CAT-1, commonly used in the Brazilian fuel ethanol industry, were engineered for xylose fermentation, where the first fermented xylose faster than the latter, but also produced considerable amounts of xylitol. An engineered PE-2 strain (MEC1121) efficiently consumed xylose in presence of inhibitors both in synthetic and corn-cob hydrolysates. Interestingly, the S. cerevisiae MEC1121 consumed xylose and glucose simultaneously, while a CEN.PK based strain consumed glucose and xylose sequentially. Deletion of the aldose reductase GRE3 lowered xylitol production to undetectable levels and increased xylose consumption rate which led to higher final ethanol concentrations. Fermentation of corn-cob hydrolysate using this strain, MEC1133, resulted in an ethanol yield of 0.47 g/g of total sugars which is 92% of the theoretical yield.

Improved xylose and arabinose utilization by an industrial recombinant Saccharomyces cerevisiae strain using evolutionary engineering

BACKGROUND

Cost-effective fermentation of lignocellulosic hydrolysate to ethanol by Saccharomyces cerevisiae requires efficient mixed sugar utilization. Notably, the rate and yield of xylose and arabinose co-fermentation to ethanol must be enhanced.

RESULTS

Evolutionary engineering was used to improve the simultaneous conversion of xylose and arabinose to ethanol in a recombinant industrial Saccharomyces cerevisiae strain carrying the heterologous genes for xylose and arabinose utilization pathways integrated in the genome. The evolved strain TMB3130 displayed an increased consumption rate of xylose and arabinose under aerobic and anaerobic conditions. Improved anaerobic ethanol production was achieved at the expense of xylitol and glycerol but arabinose was almost stoichiometrically converted to arabitol. Further characterization of the strain indicated that the selection pressure during prolonged continuous culture in xylose and arabinose medium resulted in the improved transport of xylose and arabinose as well as increased levels of the enzymes from the introduced fungal xylose pathway. No mutation was found in any of the genes from the pentose converting pathways.

CONCLUSION

To the best of our knowledge, this is the first report that characterizes the molecular mechanisms for improved mixed-pentose utilization obtained by evolutionary engineering of a recombinant S. cerevisiae strain. Increased transport of pentoses and increased activities of xylose converting enzymes contributed to the improved phenotype.

Identification of furfural as a key toxin in lignocellulosic hydrolysates and evolution of a tolerant yeast strain

The production of fuel ethanol from low‐cost lignocellulosic biomass currently suffers from several limitations. One of them is the presence of inhibitors in lignocellulosic hydrolysates that are released during pre‐treatment. These compounds inhibit growth and hamper the production of ethanol, thereby affecting process economics. To delineate the effects of such complex mixtures, we conducted a chemical analysis of four different real‐world lignocellulosic hydrolysates and determined their toxicological effect on yeast. By correlating the potential inhibitor abundance to the growth‐inhibiting properties of the corresponding hydrolysates, we identified furfural as an important contributor to hydrolysate toxicity for yeast. Subsequently, we conducted a targeted evolution experiment to improve growth behaviour of the half industrial Saccharomyces cerevisiae strain TMB3400 in the hydrolysates. After about 300 generations, representative clones from these evolved populations exhibited significantly reduced lag phases in medium containing the single inhibitor furfural, but also in hydrolysate‐supplemented medium. Furthermore, these strains were able to grow at concentrations of hydrolysates that effectively killed the parental strain and exhibited significantly improved bioconversion characteristics under industrially relevant conditions. The improved resistance of our evolved strains was based on their capacity to remain viable in a toxic environment during the prolonged, furfural induced lag phase.

Pichia stipitis xylose reductase helps detoxifying lignocellulosic hydrolysate by reducing 5-hydroxymethyl-furfural (HMF)

BACKGROUND

Pichia stipitis xylose reductase (Ps-XR) has been used to design Saccharomyces cerevisiae strains that are able to ferment xylose. One example is the industrial S. cerevisiae xylose-consuming strain TMB3400, which was constructed by expression of P. stipitis xylose reductase and xylitol dehydrogenase and overexpression of endogenous xylulose kinase in the industrial S. cerevisiae strain USM21.

RESULTS

In this study, we demonstrate that strain TMB3400 not only converts xylose, but also displays higher tolerance to lignocellulosic hydrolysate during anaerobic batch fermentation as well as 3 times higher in vitro HMF and furfural reduction activity than the control strain USM21. Using laboratory strains producing various levels of Ps-XR, we confirm that Ps-XR is able to reduce HMF both in vitro and in vivo. Ps-XR overexpression increases the in vivo HMF conversion rate by approximately 20%, thereby improving yeast tolerance towards HMF. Further purification of Ps-XR shows that HMF is a substrate inhibitor of the enzyme.

CONCLUSION

We demonstrate for the first time that xylose reductase is also able to reduce the furaldehyde compounds that are present in undetoxified lignocellulosic hydrolysates. Possible implications of this newly characterized activity of Ps-XR on lignocellulosic hydrolysate fermentation are discussed.

Variability of the response of Saccharomyces cerevisiae strains to lignocellulose hydrolysate

The development of tolerant microorganisms is needed for the efficient fermentation of inhibitory lignocellulose hydrolysates. In the current work, the fermentation performance of six selected strains of Saccharomyces cerevisiae in dilute-acid spruce hydrolysate was compared using two different modes of fermentation; either single pulse addition of hydrolysate to exponentially growing cells or continuous feeding of the same amount of hydrolysate in a controlled fed-batch fermentation was made. All strains performed better in fed-batch mode than when all hydrolysate was added at once. However, the difference between strain performances varied significantly in the two fermentation modes. Large differences were observed between strains during the fed-batch experiments in the in vitro ability to reduce the furan compounds furfural and 5-hydroxymethyl furfural (HMF). A common feature among the strains was the induction of NADPH-coupled reduction of furfural and HMF, with the exception of strain CBS 8066. This strain also performed relatively poorly in both batch and fed-batch fermentations. Strain TMB3000--previously isolated from spent sulphite liquor fermentation--was by far the most efficient strain with respect to specific fermentation rate in both pulse addition and fed-batch mode. This strain was the only strain showing a significant constitutive NADH-coupled in vitro reduction of HMF. The ability to induce NADPH-coupled reduction together with the level of the apparently constitutive NADH-coupled reduction appeared to be key factors for selecting a suitable strain for fed-batch conversion of lignocellulose hydrolysate.

Towards industrial pentose-fermenting yeast strains

Production of bioethanol from forest and agricultural products requires a fermenting organism that converts all types of sugars in the raw material to ethanol in high yield and with a high rate. This review summarizes recent research aiming at developing industrial strains of Saccharomyces cerevisiae with the ability to ferment all lignocellulose-derived sugars. The properties required from the industrial yeast strains are discussed in relation to four benchmarks: (1) process water economy, (2) inhibitor tolerance, (3) ethanol yield, and (4) specific ethanol productivity. Of particular importance is the tolerance of the fermenting organism to fermentation inhibitors formed during fractionation/pretreatment and hydrolysis of the raw material, which necessitates the use of robust industrial strain background. While numerous metabolic engineering strategies have been developed in laboratory yeast strains, only a few approaches have been realized in industrial strains. The fermentation performance of the existing industrial pentose-fermenting S. cerevisiae strains in lignocellulose hydrolysate is reviewed. Ethanol yields of more than 0.4 g ethanol/g sugar have been achieved with several xylose-fermenting industrial strains such as TMB 3400, TMB 3006, and 424A(LNF-ST), carrying the heterologous xylose utilization pathway consisting of xylose reductase and xylitol dehydrogenase, which demonstrates the potential of pentose fermentation in improving lignocellulosic ethanol production.

Simultaneous saccharification and co-fermentation of glucose and xylose in steam-pretreated corn stover at high fiber content with Saccharomyces cerevisiae TMB3400

The two main sugars in the agricultural by-product corn stover are glucose and xylose. Co-fermentation of glucose and xylose at high content of water-insoluble solids (WIS) without detoxification is a prerequisite to obtain high ethanol concentration and to reduce production costs. A recombinant strain of Saccharomyces cerevisiae, TMB3400, was used in simultaneous saccharification and fermentation (SSF) of whole pretreated slurry of corn stover at high WIS. TMB3400 co-fermented glucose and xylose with relatively high ethanol yields giving high final ethanol concentration. The ethanol productivity increased with increasing concentration of pretreatment hydrolysate in the yeast production medium and when SSF was performed in a fed-batch mode.

Rhodamine-pink as a genetic marker for yeast populations in wine fermentation

Winemaking with selected yeasts requires simple techniques to monitor the inoculated yeast. New high-concentration rhodamine-resistant mutants and low-concentration rhodamine-pink mutants, easy to detect by replica-plate assay, were obtained from selected wine yeasts. The rhodamine-pink mutations were dominant and were located at the pdr5 locus that encodes for the Pdr5 ATP-binding cassette multidrug resistance transporter. The mutants were genetically stable but had lost the killer phenotype of the parent yeast strain. They were genetically improved by elimination of recessive growth-retarding alleles followed by crossing with selected killer wine yeasts. Several spore-clones were selected according to their must fermentation kinetics and the organoleptic quality of the wine. Some spore-clones were tested in industrial winemaking, and they were easily monitored during must fermentation using a simple color-plate assay. They accounted for >96% of the total yeasts in the must, and the resulting wine had as good a quality as those made with standard commercial wine yeasts. The rhodamine-pink yeasts may also be detected by direct seeding onto rhodamine agar or by observation under fluorescence microscopy. These possibilities greatly reduce the time of analysis and make the monitoring procedure for rhodamine-pink yeasts faster, easier, and cheaper than for the genetically marked wine yeasts obtained previously.

Assessment of discriminatory power of three different fingerprinting methods based on killer toxin sensitivity for the differentiation of Saccharomyces cerevisiae strains

AIMS

A panel composed of 44 taxonomically certified strains of Saccharomyces cerevisiae of different origin was used to evaluate the discriminatory power of three different fingerprinting methods based on sensitivity towards 24 killer toxins.

METHODS AND RESULTS

Binary data matrix (BDM), triplet data matrix (TDM) and numerical data matrix (NDM) were used as fingerprinting methods. NDM possessed the highest discriminatory power, assessed through the Simpson's, and Hunter and Gaston's indices for the measurement of diversity. The upper limits of fingerprinting ability expressed by the three above methods have been also discussed.

CONCLUSIONS

NDM determined a significant increase of discriminatory power than the use of BDM or TDM, in terms of an effective amplification of their fingerprinting efficacy.

SIGNIFICANCE AND IMPACT OF THE STUDY

The NDM fingerprinting method could find application in control laboratories for the discrimination of yeast strains of industrial importance or covered by patent.

Generation of the improved recombinant xylose-utilizing Saccharomyces cerevisiae TMB 3400 by random mutagenesis and physiological comparison with Pichia stipitis CBS 6054

The recombinant xylose-utilizing Saccharomyces cerevisiae TMB 3399 was constructed by chromosomal integration of the genes encoding D-xylose reductase (XR), xylitol dehydrogenase (XDH), and xylulokinase (XK). S. cerevisiae TMB 3399 was subjected to chemical mutagenesis with ethyl methanesulfonate and, after enrichment, 33 mutants were selected for improved growth on D-xylose and carbon dioxide formation in Durham tubes. The best-performing mutant was called S. cerevisiae TMB 3400. The novel, recombinant S. cerevisiae strains were compared with Pichia stipitis CBS 6054 through cultivation under aerobic, oxygen-limited, and anaerobic conditions in a defined mineral medium using only D-xylose as carbon and energy source. The mutation led to a more than five-fold increase in maximum specific growth rate, from 0.0255 h(-1) for S. cerevisiae TMB 3399 to 0.14 h(-1) for S. cerevisiae TMB 3400, whereas P. stipitis grew at a maximum specific growth rate of 0.44 h(-1). All yeast strains formed ethanol only under oxygen-limited and anaerobic conditions. The ethanol yields and maximum specific ethanol productivities during oxygen limitation were 0.21, 0.25, and 0.30 g ethanol g xylose(-1) and 0.001, 0.10, and 0.16 g ethanol g biomass(-1) h(-1) for S. cerevisiae TMB 3399, TMB 3400, and P. stipitis CBS 6054, respectively. The xylitol yield under oxygen-limited and anaerobic conditions was two-fold higher for S. cerevisiae TMB 3399 than for TMB 3400, but the glycerol yield was higher for TMB 3400. The specific activity, in U mg protein(-1), was higher for XDH than for XR in both S. cerevisiae TMB 3399 and TMB 3400, while P. stipitis CBS 6054 showed the opposite relation. S. cerevisiae TMB 3400 displayed higher specific XR, XDH and XK activities than TMB 3399. Hence, we have demonstrated that a combination of metabolic engineering and random mutagenesis was successful to generate a superior, xylose-utilizing S. cerevisiae, and uncovered distinctive physiological properties of the mutant.

Molecular analysis of a Saccharomyces cerevisiae mutant with improved ability to utilize xylose shows enhanced expression of proteins involved in transport, initial xylose metabolism, and the pentose phosphate pathway

Differences between the recombinant xylose-utilizing Saccharomyces cerevisiae strain TMB 3399 and the mutant strain TMB 3400, derived from TMB 3399 and displaying improved ability to utilize xylose, were investigated by using genome-wide expression analysis, physiological characterization, and biochemical assays. Samples for analysis were withdrawn from chemostat cultures. The characteristics of S. cerevisiae TMB 3399 and TMB 3400 grown on glucose and on a mixture of glucose and xylose, as well as of S. cerevisiae TMB 3400 grown on only xylose, were investigated. The strains were cultivated under chemostat conditions at a dilution rate of 0.1 h(-1), with feeds consisting of a defined mineral medium supplemented with 10 g of glucose liter(-1), 10 g of glucose plus 10 g of xylose liter(-1) or, for S. cerevisiae TMB 3400, 20 g of xylose liter(-1). S. cerevisiae TMB 3400 consumed 31% more xylose of a feed containing both glucose and xylose than S. cerevisiae TMB 3399. The biomass yields for S. cerevisiae TMB 3400 were 0.46 g of biomass g of consumed carbohydrate(-1) on glucose and 0.43 g of biomass g of consumed carbohydrate(-1) on xylose. A K(s) value of 33 mM for xylose was obtained for S. cerevisiae TMB 3400. In general, the percentage error was <20% between duplicate microarray experiments originating from independent fermentation experiments. Microarray analysis showed higher expression in S. cerevisiae TMB 3400 than in S. cerevisiae TMB 3399 for (i) HXT5, encoding a hexose transporter; (ii) XKS1, encoding xylulokinase, an enzyme involved in one of the initial steps of xylose utilization; and (iii) SOL3, GND1, TAL1, and TKL1, encoding enzymes in the pentose phosphate pathway. In addition, the transcriptional regulators encoded by YCR020C, YBR083W, and YPR199C were expressed differently in the two strains. Xylose utilization was, however, not affected in strains in which YCR020C was overexpressed or deleted. The higher expression of XKS1 in S. cerevisiae TMB 3400 than in TMB 3399 correlated with higher specific xylulokinase activity in the cell extracts. The specific activity of xylose reductase and xylitol dehydrogenase was also higher for S. cerevisiae TMB 3400 than for TMB 3399, both on glucose and on the mixture of glucose and xylose.

Differentiation of industrial wine yeast strains using microsatellite markers

AIMS

To differentiate nine industrial wine strains of Saccharomyces cerevisiae using microsatellite (simple sequence repeats, SSR) markers.

METHODS AND RESULTS

Six of the strains were indigenous yeasts currently used as high-density starter monocultures by the Uruguayan wine industry. Unequivocal differentiation of these six native strains and three commercial S. cerevisiae wine strains was achieved by PCR amplification and polymorphism analysis of loci containing microsatellite markers.

CONCLUSION

We recommend the use of this reproducible and simple molecular method to routinely discriminate wine yeast strains.

SIGNIFICANCE AND IMPACT OF THE STUDY

Microsatellites are superior to other methods for typing yeasts because the results can be exchanged as quantitative data. Knowledge of the frequencies of the alleles for different SSR markers will eventually lead to an accurate typing method to identify industrial wine yeast strains.

Malo-ethanolic fermentation in grape must by recombinant strains of Saccharomyces cerevisiae

Recombinant strains of Saccharomyces cerevisiae with the ability to reduce wine acidity could have a significant influence on the future production of quality wines, especially in cool climate regions. L-Malic acid and L-tartaric acid contribute largely to the acid content of grapes and wine. The wine yeast S. cerevisiae is unable to effectively degrade L-malic acid, whereas the fission yeast Schizosaccharomyces pombe efficiently degrades high concentrations of L-malic acid by means of a malo-ethanolic fermentation. However, strains of Sz. pombe are not suitable for vinification due to the production of undesirable off-flavours. Heterologous expression of the Sz. pombe malate permease (mae1) and malic enzyme (mae2) genes on plasmids in S. cerevisiae resulted in a recombinant strain of S. cerevisiae that efficiently degraded up to 8 g/l L-malic acid in synthetic grape must and 6.75 g/l L-malic acid in Chardonnay grape must. Furthermore, a strain of S. cerevisiae containing the mae1 and mae2 genes integrated in the genome efficiently degraded 5 g/l of L-malic acid in synthetic and Chenin Blanc grape musts. Furthermore, the malo-alcoholic strains produced higher levels of ethanol during fermentation, which is important for the production of distilled beverages.

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.

Yeast killer systems

The killer phenomenon in yeasts has been revealed to be a multicentric model for molecular biologists, virologists, phytopathologists, epidemiologists, industrial and medical microbiologists, mycologists, and pharmacologists. The surprisingly widespread occurrence of the killer phenomenon among taxonomically unrelated microorganisms, including prokaryotic and eukaryotic pathogens, has engendered a new interest in its biological significance as well as its theoretical and practical applications. The search for therapeutic opportunities by using yeast killer systems has conceptually opened new avenues for the prevention and control of life-threatening fungal diseases through the idiotypic network that is apparently exploited by the immune system in the course of natural infections. In this review, the biology, ecology, epidemiology, therapeutics, serology, and idiotypy of yeast killer systems are discussed.

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.

Molecular analysis of the malic enzyme gene (mae2) of Schizosaccharomyces pombe

Sequence analysis of a 4.6-kb HindIII fragment containing the malic enzyme gene (mae2) of Schizosaccharomyces pombe, revealed the presence of an open reading frame of 1695 nucleotides, coding for a 565 amino acid polypeptide. The mae2 gene is expressed constitutively and encodes a single mRNA transcript of 2.0 kb. The mae2 gene was mapped on chromosome III by chromoblotting. The coding region and inferred amino acid sequence showed significant homology with 12 malic enzyme genes and proteins from widely different origins. Eight highly homologous regions were found in these malic enzymes, suggesting that they contain functionally conserved amino acid sequences that are indispensable for activity of malic enzymes. Two of these regions have previously been reported to be NAD- and NADP-binding sites.

A note on the primary structure and expression of an Erwinia carotovora polygalacturonase-encoding gene (peh1) in Escherichia coli and Saccharomyces cerevisiae

A 1209-base pair (bp) DNA fragment containing the endopolygalacturonase-encoding gene (peh1) from Erwinia carotovora subsp. carotovora was amplified by the polymerase chain reaction (PCR) technique and expressed in Escherichia coli. The nucleotide sequence of the PCR product was determined and found to be highly homologous to the primary structures of other polygalacturonase-encoding genes. The peh1 DNA fragment encoding the mature polygalacturonase was inserted between two different yeast expression-secretion cassettes and a yeast gene terminator, generating recombinant yeast-integrating shuttle plasmids pAMS10 and pAMS11. These YIp5-derived plasmids were transformed and stably integrated into the genome of a laboratory strain of Saccharomyces cerevisiae. Transcription initiation signals present in these expression-secretion cassettes were derived from the yeast alcohol dehydrogenase (ADC1P) or mating pheromone alpha-factor (MF alpha 1P) gene promoters. The transcription termination signals were derived from the yeast tryptophan synthase gene terminator (TRP5T). Secretion of polygalacturonase was directed by the signal sequence of the yeast mating pheromone alpha-factor (MF alpha 1S). Northern blot analysis revealed the presence of peh1 mRNA in the yeast transformants and a polypectate agarose test was used to monitor polygalacturonase production.

The use of karyotyping in the systematics of yeasts

The use of electrophoretic karyotyping in systematics of yeasts is discussed. New data are provided on the karyotypes of the medically important fungi Hortaea werneckii, Filobasidiella (= Cryptococcus) neoformans, and Malassezia species. Hortaea werneckii has twelve to eighteen bands of chromosomal DNA, ranging in size between 500 and 2300 kb. The karyotypes of Filobasidiella neoformans consist of seven to fourteen bands of chromosomal DNA. The varieties neoformans and bacillispora cannot be separated by their karyotypes, and no obvious correlation was found with serotypes, geography or habitat. All strains of Malassezia pachydermatis studied have similar karyotypes consisting of five bands, whereas in M. furfur, four different karyotypes are prevalent. However, each of these karyotypes is stable.