Engineering yeast for efficient cellulose degradation

Direct and efficient production of ethanol from cellulosic material with a yeast strain displaying cellulolytic enzymes

For direct and efficient ethanol production from cellulosic materials, we constructed a novel cellulose-degrading yeast strain by genetically codisplaying two cellulolytic enzymes on the cell surface of Saccharomyces cerevisiae. By using a cell surface engineering system based on alpha-agglutinin, endoglucanase II (EGII) from the filamentous fungus Trichoderma reesei QM9414 was displayed on the cell surface as a fusion protein containing an RGSHis6 (Arg-Gly-Ser-His(6)) peptide tag in the N-terminal region. EGII activity was detected in the cell pellet fraction but not in the culture supernatant. Localization of the RGSHis6-EGII-alpha-agglutinin fusion protein on the cell surface was confirmed by immunofluorescence microscopy. The yeast strain displaying EGII showed significantly elevated hydrolytic activity toward barley beta-glucan, a linear polysaccharide composed of an average of 1,200 glucose residues. In a further step, EGII and beta-glucosidase 1 from Aspergillus aculeatus No. F-50 were codisplayed on the cell surface. The resulting yeast cells could grow in synthetic medium containing beta-glucan as the sole carbon source and could directly ferment 45 g of beta-glucan per liter to produce 16.5 g of ethanol per liter within about 50 h. The yield in terms of grams of ethanol produced per gram of carbohydrate utilized was 0.48 g/g, which corresponds to 93.3% of the theoretical yield. This result indicates that efficient simultaneous saccharification and fermentation of cellulose to ethanol are carried out by a recombinant yeast cells displaying cellulolytic enzymes.

Meeting the consumer challenge through genetically customized wine-yeast strains

Wine producers are facing intensifying competition brought about by a widening gap between wine production and wine consumption, a shift of consumer preferences away from basic commodity wine to top quality wine, and economic globalization. Consequently, they are calling for a total revolution in the 'magical' world of wine. The process of transforming the wine industry from a production- to a market-orientated industry results in an increasing dependence on, amongst others, biotechnological innovation. Market-orientated wine-yeast strains are currently being developed for the cost-competitive production of wine with minimized resource inputs, improved quality and low environmental impact. The emphasis is on the development of Saccharomyces cerevisiae strains with improved fermentation, processing and biopreservation abilities, and capacities for an increase in the wholesomeness and sensory quality of wine.

Microbial cellulose utilization: fundamentals and biotechnology

Fundamental features of microbial cellulose utilization are examined at successively higher levels of aggregation encompassing the structure and composition of cellulosic biomass, taxonomic diversity, cellulase enzyme systems, molecular biology of cellulase enzymes, physiology of cellulolytic microorganisms, ecological aspects of cellulase-degrading communities, and rate-limiting factors in nature. The methodological basis for studying microbial cellulose utilization is considered relative to quantification of cells and enzymes in the presence of solid substrates as well as apparatus and analysis for cellulose-grown continuous cultures. Quantitative description of cellulose hydrolysis is addressed with respect to adsorption of cellulase enzymes, rates of enzymatic hydrolysis, bioenergetics of microbial cellulose utilization, kinetics of microbial cellulose utilization, and contrasting features compared to soluble substrate kinetics. A biological perspective on processing cellulosic biomass is presented, including features of pretreated substrates and alternative process configurations. Organism development is considered for "consolidated bioprocessing" (CBP), in which the production of cellulolytic enzymes, hydrolysis of biomass, and fermentation of resulting sugars to desired products occur in one step. Two organism development strategies for CBP are examined: (i) improve product yield and tolerance in microorganisms able to utilize cellulose, or (ii) express a heterologous system for cellulose hydrolysis and utilization in microorganisms that exhibit high product yield and tolerance. A concluding discussion identifies unresolved issues pertaining to microbial cellulose utilization, suggests approaches by which such issues might be resolved, and contrasts a microbially oriented cellulose hydrolysis paradigm to the more conventional enzymatically oriented paradigm in both fundamental and applied contexts.

Secretion of endoxylanase A from Penicillium purpurogenum by Saccharomyces cerevisiae transformed with genomic fungal DNA

Saccharomyces cerevisiae was transformed with a genomic library from Penicillium purpurogenum, and an endoxylanase-producing yeast clone (named 44A) that grows on xylose or xylan as sole carbon source was isolated. This yeast synthesizes xynA mRNA and secretes endoxylanase A to culture media when grown on xylan or xylose, but not glucose. Analysis by pulse-field gel electrophoresis and sequencing indicates that xynA, including its eight introns, has been inserted into the yeast genome. It was shown by sequencing that clone 44A is able to correctly splice xynA introns. This is the first successful attempt to express a fungal endoxylanase gene in yeast with correct intron splicing.

Metabolic engineering of Saccharomyces cerevisiae for xylose utilization

Metabolic engineering of Saccharomyces cerevisiae for ethanolic fermentation of xylose is summarized with emphasis on progress made during the last decade. Advances in xylose transport, initial xylose metabolism, selection of host strains, transformation and classical breeding techniques applied to industrial polyploid strains as well as modeling of xylose metabolism are discussed. The production and composition of the substrates--lignocellulosic hydrolysates--is briefly summarized. In a future outlook iterative strategies involving the techniques of classical breeding, quantitative physiology, proteomics, DNA micro arrays, and genetic engineering are proposed for the development of efficient xylose-fermenting recombinant strains of S. cerevisiae.

Gene integration and expression and extracellular secretion of Erwinia chrysanthemi endoglucanase CelY (celY) and CelZ (celZ) in ethanologenic Klebsiella oxytoca P2

The development of methods to reduce costs associated with the solubilization of cellulose is essential for the utilization of lignocellulose as a renewable feedstock for fuels and chemicals. One promising approach is the genetic engineering of ethanol-producing microorganisms that also produce cellulase enzymes during fermentation. By starting with an ethanologenic derivative (strain P2) of Klebsiella oxytoca M5A1 with the native ability to metabolize cellobiose, the need for supplemental beta-glucosidase was previously eliminated. In the current study, this approach has been extended by adding genes encoding endoglucanase activities. Genes celY and celZ from Erwinia chrysanthemi have been functionally integrated into the chromosome of P2 using surrogate promoters from Zymomonas mobilis for expression. Both were secreted into the extracellular milieu, producing more than 20,000 endoglucanase units (carboxymethyl cellulase activity) per liter of fermentation broth. During the fermentation of crystalline cellulose with low levels of commercial cellulases of fungal origin, these new strains produced up to 22% more ethanol than unmodified P2. Most of the beneficial contribution was attributed to CelY rather than to CelZ. These results suggest that fungal enzymes with substrate profiles resembling CelY (preference for long-chain polymers and lack of activity on soluble cello-oligosaccharides of two to five glucosyl residues) may be limiting in commercial cellulase preparations.

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.

Engineering baker's yeast: room for improvement

Bread making is one of the oldest food-manufacturing processes. However, it is only in the past few years that recombinant-DNA technology has led to dramatic changes in formulation, ingredients or processing conditions. New strains of baker's yeast that produce CO2 more rapidly, are more resistant to stress or produce proteins or metabolites that can modify bread flavour, dough rheology or shelf-life are now emerging.