Abstract
BackgroundSaccharomyces cerevisiae wine strains can develop stuck or sluggish fermentations when nutrients are scarce or suboptimal. Nutrient sensing and signaling pathways, such as PKA, TORC1 and Snf1, work coordinately to adapt growth and metabolism to the amount and balance of the different nutrients in the medium. This has been exhaustively studied in laboratory strains of S. cerevisiae and laboratory media, but much less under industrial conditions.ResultsInhibitors of such pathways, like rapamycin or 2-deoxyglucose, failed to discriminate between commercial wine yeast strains with different nutritional requirements, but evidenced genetic variability among industrial isolates, and between laboratory and commercial strains. Most signaling pathways involve events of protein phosphorylation that can be followed as markers of their activity. The main pathway to promote growth in the presence of nitrogen, the TORC1 pathway, measured by the phosphorylation of Rps6 and Par32, proved active at the very start of fermentation, mainly on day 1, and ceased soon afterward, even before cellular growth stopped. Transcription factor Gln3, which activates genes subject to nitrogen catabolite repression, was also active for the first hours, even when ammonium and amino acids were still present in media. Snf1 kinase was activated only when glucose was exhausted under laboratory conditions, but was active from early fermentation stages. The same results were generally obtained when nitrogen was limiting, which indicates a unique pathway activation pattern in winemaking. As PKA remained active throughout fermentation, it could be the central pathway that controls others, provided sugars are present.ConclusionsWine fermentation is a distinct environmental situation from growth in laboratory media in molecular terms. The mechanisms involved in glucose and nitrogen repression respond differently under winemaking conditions.
Highlights
Saccharomyces cerevisiae wine strains can develop stuck or sluggish fermentations when nutrients are scarce or suboptimal
Chemical screening was performed on 14 commercial strains with different nitrogen requirements by using a series of inhibitors of amino acid biosynthesis and toxic analogs of amino acids and glucose
All industrial strains are more tolerant to rapamycin and 2-deoxyglucose than laboratory strains, and this suggests a common adaptation to certain environmental conditions
Summary
Saccharomyces cerevisiae wine strains can develop stuck or sluggish fermentations when nutrients are scarce or suboptimal. Nutrient sensing and signaling pathways, such as PKA, TORC1 and Snf, work coordinately to adapt growth and metabolism to the amount and balance of the different nutrients in the medium. This has been exhaustively studied in laboratory strains of S. cerevisiae and laboratory media, but much less under industrial conditions. There are two key players in growth and proliferation when the main nutrients (i.e., carbon and nitrogen sources) are present, namely glucose-induced cAMP-dependent protein kinase A (PKA) and the nitrogen-sensing Target Of Rapamycin (TOR) pathway. Snf seems to play a role in the coordination of both types of repression
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