Abstract:
The adapted metabolic response of commercial wine yeast under prolonged
exposure to concentrated solutes present in Icewine juice is not fully understood.
Presently, there is no information regarding the transcriptomic changes in gene
expression associated with the adaptive stress response ofwine yeast during Icewine
fermentation compared to table wine fermentation. To understand how and why wine
yeast respond differently at the genomic level and ultimately at the metabolic level during
Icewine fermentation, the focus ofthis project was to identify and compare these
differences in the wine yeast Saccharomyces cerevisiae KI-Vll16 using cDNA
microarray technology during the first five days of fermentation. Significant differences
in yeast gene expression patterns between fermentation conditions were correlated to
differences in nutrient utilization and metabolite production. Sugar consumption, nitrogen
usage and metabolite levels were measured using enzyme assays and HPLC. Also, a
small subset of differentially expressed genes was verified using Northern analysis. The
high osmotic stress experienced by wine yeast throughout Icewine fermentation elicited
changes in cell growth and metabolism correlating to several fermentation difficulties,
including reduced biomass accumulation and fermentation rate. Genes associated with
carbohydrate and nitrogen transport and metabolism were expressed at lower levels in
Icewine juice fermenting cells compared to dilute juice fermenting cells. Osmotic stress,
not nutrient availability during Icewine fermentation appears to impede sugar and
nitrogen utilization. Previous studies have established that glycerol and acetic acid
production are increased in yeast during Icewine fermentation. A gene encoding for a
glycerollW symporter (STL1) was found to be highly expressed up to 25-fold in the
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Icewine juice condition using microarray and Northern analysis. Active glycerol transport
by yeast under hyperosmotic conditions to increase cytosolic glycerol concentration may
contribute to reduced cell growth observed in the Icewine juice condition. Additionally,
genes encoding for two acetyl CoA synthetase isoforms (ACSl and ACS2) were found to
be highly expressed, 19- and II-fold respectively, in dilute juice fermenting cells relative
to the Icewine juice condition. Therefore, decreased conversion of acetate to acetyl-CoA
may contribute to increased acetic acid production during Icewine fermentation. These
results further help to explain the response of wine yeast as they adapt to Icewine juice
fermentation.
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