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

DNA chips for yeast biotechnology. The case of wine yeasts

The yeast Saccharomyces cerevisiae is one of the most popular model organisms. It was the first eukaryote whose genome was sequenced. Since then many functional analysis projects have tried to find the function of many genes and to understand its metabolism in a holistic way. Apart from basic science this microorganism is of great interest in several biotechnology processes, such as winemaking. Only global studies of the cell as a whole can help us to understand many of the technical problems facing winemaking. DNA chip technology is one of the most promising tools for the analysis of cell physiology. Yeast has been the model organism for the development of this technique. Many of the studies can be applied to improve our knowledge of wine strains. Nevertheless wine strains are quite different in some aspects from the laboratory reference strains so a particular study of wine strains and especially during the winemaking process is needed. During the past two years some groups have started this study and the first results have been published. We review here the current state of the knowledge of wine yeast and the capacity of DNA chip technology for its improvement.

Analysis of yeast populations during alcoholic fermentation: a six year follow-up study

Wine yeasts were isolated from fermenting Garnatxa and Xarel.lo musts fermented in a newly built and operated winery between 1995 and 2000. The species of non-Saccharomyces yeasts and the Saccharomyces cerevisiae strains were identified by ribosomal DNA and mitochondrial DNA RFLP analysis respectively. Non-Saccharomyces yeasts, particularly Hanseniaspora uvarum and Candida stellata, dominated the first stages of fermentation. However Saccharomyces cerevisiae was present at the beginning of the fermentation and was the main yeast in the musts in one vintage (1999). In all the cases, S. cerevisiae took over the process in the middle and final stages of fermentation. The analysis of the S. cerevisiae strains showed that indigenous strains competed with commercial strains inoculated in other fermentation tanks of the cellar. The continuous use of commercial yeasts reduced the diversity and importance of the indigenous S. cerevisiae strains.

Osmotic stress signaling and osmoadaptation in yeasts

The ability to adapt to altered availability of free water is a fundamental property of living cells. The principles underlying osmoadaptation are well conserved. The yeast Saccharomyces cerevisiae is an excellent model system with which to study the molecular biology and physiology of osmoadaptation. Upon a shift to high osmolarity, yeast cells rapidly stimulate a mitogen-activated protein (MAP) kinase cascade, the high-osmolarity glycerol (HOG) pathway, which orchestrates part of the transcriptional response. The dynamic operation of the HOG pathway has been well studied, and similar osmosensing pathways exist in other eukaryotes. Protein kinase A, which seems to mediate a response to diverse stress conditions, is also involved in the transcriptional response program. Expression changes after a shift to high osmolarity aim at adjusting metabolism and the production of cellular protectants. Accumulation of the osmolyte glycerol, which is also controlled by altering transmembrane glycerol transport, is of central importance. Upon a shift from high to low osmolarity, yeast cells stimulate a different MAP kinase cascade, the cell integrity pathway. The transcriptional program upon hypo-osmotic shock seems to aim at adjusting cell surface properties. Rapid export of glycerol is an important event in adaptation to low osmolarity. Osmoadaptation, adjustment of cell surface properties, and the control of cell morphogenesis, growth, and proliferation are highly coordinated processes. The Skn7p response regulator may be involved in coordinating these events. An integrated understanding of osmoadaptation requires not only knowledge of the function of many uncharacterized genes but also further insight into the time line of events, their interdependence, their dynamics, and their spatial organization as well as the importance of subtle effects.

Multidrug resistance as a dominant molecular marker in transformation of wine yeast

Pure wine yeast cultures are increasingly used in winemaking to perform controlled fermentations and produce wine of reproducible quality. For the genetic manipulation of natural wine yeast strains dominant selective markers are obviously useful. Here we demonstrate the successful use of the mutated PDR3 gene as a dominant molecular marker for the selection of transformants of prototrophic wine yeast Saccharomyces cerevisiae. The selected transformants displayed a multidrug resistance phenotype that was resistant to strobilurin derivatives and azoles used to control pathogenic fungi in agriculture and medicine, respectively. Random amplification of DNA sequences and electrophoretic karyotyping of the host and transformed strains after microvinification experiments resulted in the same gel electrophoresis patterns. The chemical and sensory analysis of experimental wines proved that the used transformants preserved all their useful winemaking properties indicating that the pdr3-9 allele does not deteriorate the technological properties of the transformed wine yeast strain.

Yeasts present during wine fermentation: comparative analysis of conventional plating and PCR-TTGE

Yeasts isolated from must before and during fermentation at a wine cellar of La Mancha region in Spain were characterised using Polymerase Chain Reaction / Restriction Fragments Lengths Polymorphism and Polymerase Chain Reaction / Temporal Temperature Gradient Gel Electrophoresis. S. cerevisiae strains were differentiated using mtDNA restriction analysis. Direct PCR-TTGE was also used to study biodiversity during wine fermentation, and revealed the variations in the population. It was observed that isolation by conventional plating may afford a skewed view of the strains taking part in wine fermentation.

Characterization of protein biomarkers desorbed by MALDI from whole fungal cells

In this publication, the use of ultraviolet (UV) matrix-assisted laser desorption/ionization (MALDI) time-of-fight (TOF) mass spectrometry (MS) for rapid identification and characterization of Saccharomyces cerevisiae, a fungus, is reported. S. cerevisiae is a unicellular eukaroyte that can serve as a model to study more complex organisms. We have determined that the best technique for cell wall lyses for MALDI involves the use of high concentrations of formic acid solutions. We also have shown that different fungal species exhibit different mass spectra, which can be used to distinguish them readily. Protein peaks from S. cerevisiae spectra have been tentatively identified using bioinformatics and are mainly assigned to ribosomal and mitochondrion-related proteins.

Glycerol export and glycerol-3-phosphate dehydrogenase, but not glycerol phosphatase, are rate limiting for glycerol production in Saccharomyces cerevisiae

Glycerol, one of the most important by-products of alcoholic fermentation, has positive effects on the sensory properties of fermented beverages. It was recently shown that the most direct approach for increasing glycerol formation is to overexpress GPD1, which encodes the glycerol-3-phosphate dehydrogenase (GPDH) isoform Gpd1p. We aimed to identify other steps in glycerol synthesis or transport that limit glycerol flux during glucose fermentation. We showed that the overexpression of GPD2, encoding the other isoform of glycerol-3-phosphate dehydrogenase (Gpd2p), is equally as effective as the overexpression of GPD1 in increasing glycerol production (3.3-fold increase compared to the wild-type strain) and has similar effects on yeast metabolism. In contrast, overexpression of GPP1, encoding glycerol 3-phosphatase (Gpp1p), did not enhance glycerol production. Strains that simultaneously overexpress GPD1 and GPP1 did not produce higher amounts of glycerol than a GPD1-overexpressing strain. These results demonstrate that GPDH, but not the glycerol 3-phosphatase, is rate-limiting for glycerol production. The channel protein Fps1p mediates glycerol export. It has recently been shown that mutants lacking a region in the N-terminal domain of Fps1p constitutively release glycerol. We showed that cells producing truncated Fps1p constructs during glucose fermentation compensate for glycerol loss by increasing glycerol production. Interestingly, the strain with a deregulated Fps1 glycerol channel had a different phenotype to the strain overexpressing GPD genes and showed poor growth during fermentation. Overexpression of GPD1 in this strain increased the amount of glycerol produced but led to a pronounced growth defect.

The effect of oxygen on the survival of non-Saccharomyces yeasts during mixed culture fermentations of grape juice with Saccharomyces cerevisiae

AIMS

The effect of oxygen on the survival of Torulaspora delbrueckii and Kluyveromyces thermotolerans during mixed culture fermentations in grape juice with Saccharomyces cerevisiae was investigated.

METHODS AND RESULTS

Fermentations were carried out in two simple fermentation systems differing in the availability of oxygen. At low available oxygen conditions, T. delbrueckii and K. thermotolerans began to die off after two days of mixed culture fermentation. In filtrates from 2-day-old mixed cultures, single cultures of T. delbrueckii and K. thermotolerans survived and actively produced ethanol to concentrations of approx. 65 and 70 g l-1, respectively, at low available oxygen conditions. Oxygen clearly increased the survival time and decreased the death rate of T. delbrueckii and K. thermotolerans in mixed cultures, whereas it did not affect the growth and survival of S. cerevisiae.

CONCLUSION

Our results show that the deaths of T. delbrueckii and K. thermotolerans in mixed cultures at low available oxygen conditions are not due to toxic metabolites produced by the yeasts but rather to the lack of oxygen. Furthermore, they indicate that T. delbrueckii and K. thermotolerans are less tolerant to low available oxygen conditions than S. cerevisiae.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our study reveals new knowledge on the mechanisms underlying the succession of yeasts during wine fermentations. This knowledge may be of importance when creating defined, mixed starter cultures for the controlled production of wines with a wide range of flavour compositions.