Correlation between acetaldehyde and ethanol resistance and expression of HSP genes in yeast strains isolated during the biological aging of sherry wines.

In the production of sherry wines, the process of biological aging is essential for the development of their organoleptic properties. This process involves velum formation by "flor" yeasts. Several of these yeast strains have been isolated and characterized with regard to their genetic, physiological and metabolic properties. In this work, we studied their resistance to cold-, osmotic-, oxidative-, ethanol- and acetaldehyde-stress, and found, in most cases, a correlation between resistance to acetaldehyde stress and ethanol stress and isolation from "soleras." Moreover, gene expression analysis revealed induction of the heat shock protein (HSP) genes HSP12, HSP82, and especially HSP26 and HSP104, under acetaldehyde stress in most of the strains. In strain C, there was a clear correlation between resistance to ethanol and acetaldehyde, the high induction of HSP genes by these compounds and its presence as the predominant strain in most levels of several soleras.

Aranda A ，Querol A ，del Olmo Ml 《ARCHIVES OF MICROBIOLOGY》

Response to acetaldehyde stress in the yeast Saccharomyces cerevisiae involves a strain-dependent regulation of several ALD genes and is mediated by the general stress response pathway.

One of the stress conditions that yeast may encounter is the presence of acetaldehyde. In a previous study we identified that, in response to this stress, several HSP genes are induced that are also involved in the response to other forms of stress. Aldehyde dehydrogenases (ALDH) play an important role in yeast acetaldehyde metabolism (e.g. when cells are growing in ethanol). In this work we analyse the expression of the genes encoding these enzymes (ALD) and also the corresponding enzymatic activities under several growth conditions. We investigate three kinds of yeast strains: laboratory strains, strains involved in the alcoholic fermentation stage of wine production and flor yeasts (responsible for the biological ageing of sherry wines). The latter are very important to consider because they grow in media containing high ethanol concentrations, and produce important amounts of acetaldehyde. Under several growth conditions, further addition of acetaldehyde or ethanol in flor yeasts induced the expression of some ALD genes and led to an increase in ALDH activity. This result is consistent with their need to obtain energy from ethanol during biological ageing processes. Our data also suggest that post-transcriptional and/or post-translational mechanisms are involved in regulating the activity of these enzymes. Finally, analyses indicate that the Msn2/4p and Hsf1p transcription factors are necessary for HSP26, ALD2/3 and ALD4 gene expression under acetaldehyde stress, while PKA represses the expression of these genes.

Aranda A ，del Olmo Ml Ml 《YEAST》

Btn2p is involved in ethanol tolerance and biofilm formation in flor yeast.

Flor yeasts are a particular kind of Saccharomyces cerevisiae strains involved in Sherry wine biological ageing. During this process, yeasts form a film on the wine surface and use ethanol as a carbon source, producing acetaldehyde as a by-product. Acetaldehyde induces BTN2 transcription in laboratory strains. Btn2p is involved in the control of the subcellular localization of different proteins. The BTN2 gene shows a complex expression pattern in wine yeast, increasing its expression by acetaldehyde, but repressing it by ethanol. A flor yeast strain transcribes more BTN2 than a first fermentation yeast during growth, but less under different stress conditions. BTN2 deletion decreases flor yeast resistance to high ethanol concentrations. Surprisingly, this effect is suppressed by the addition of high amounts of amino acids to the growth medium, indicating that the role of Btn2p protein in amino acid transport is important for ethanol resistance. Btn2p deletion increases the fermentative capacity of flor yeast and its overexpression prevents its growth on nonfermentable carbon sources. BTN2 deletion also affects the biofilm formation ability of flor yeast, and it increases its sliding motility, resulting in increased mat formation. This correlates with an increased transcription of the FLO11 gene, a gene essential for biofilm formation.

Espinazo-Romeu M ，Cantoral JM ，Matallana E ，Aranda A ... -  《FEMS YEAST RESEARCH》

Overexpression of stress-related genes enhances cell viability and velum formation in Sherry wine yeasts.

Fierro-Risco J ，Rincón AM ，Benítez T ，Codón AC ... -  《-》

Acetaldehyde and ethanol are responsible for mitochondrial DNA (mtDNA) restriction fragment length polymorphism (RFLP) in flor yeasts.

Flor yeasts grow and survive in fino sherry wine where the frequency of respiratory-deficient (petite) mutants is very low. Mitochondria from flor yeasts are highly acetaldehyde- and ethanol-tolerant, and resistant to oxidative stress. However, restriction fragment length polymorphism (RFLP) of mtDNA from flor yeast populations is very high and reflects variability induced by the high concentrations of acetaldehyde and ethanol of sherry wine on mtDNA. mtDNA RFLP increases as the concentration of these compounds also increases, but is followed by a total loss of mtDNA in petite cells. Yeasts with functional mitochondria (grande) are target of continuous variability, so that flor yeast mtDNA can evolve extremely rapidly and may serve as a reservoir of genetic diversity, whereas petite mutants are eventually eliminated because metabolism in sherry wine is oxidative.

Castrejón F ，Codón AC ，Cubero B ，Benítez T ... -  《SYSTEMATIC AND APPLIED MICROBIOLOGY》