Genome‐wide expression analyses: Metabolic adaptation of Saccharomyces cerevisiae to high sugar stress

The transcriptional response of laboratory strains of Saccharomyces cerevisiae to salt or sorbitol stress has been well studied. These studies have yielded valuable data on how the yeast adapts to these stress conditions. However, S. cerevisiae is a saccharophilic fungus and in its natural environment this yeast encounters high concentrations of sugars. For the production of dessert wines, the sugar concentration may be as high as 50% (w/v). The metabolic pathways in S. cerevisiae under these fermentation conditions have not been studied and the transcriptional response of this yeast to sugar stress has not been investigated. High‐density DNA microarrays showed that the transcription of 589 genes in an industrial strain of S. cerevisiae were affected more than two‐fold in grape juice containing 40% (w/v) sugars (equimolar amounts of glucose and fructose). High sugar stress up‐regulated the glycolytic and pentose phosphate pathway genes. The PDC6 gene, previously thought to encode a minor isozyme of pyruvate decarboxylase, was highly induced under these conditions. Gene expression profiles indicate that the oxidative and non‐oxidative branches of the pentose phosphate pathway were up‐regulated and might be used to shunt more glucose‐6‐phosphate and fructose‐6‐phosphate, respectively, from the glycolytic pathway into the pentose phosphate pathway. Structural genes involved in the formation of acetic acid from acetaldehyde, and succinic acid from glutamate, were also up‐regulated. Genes involved in de novo biosynthesis of purines, pyrimidines, histidine and lysine were down‐regulated by sugar stress.