Ayca Ozcan, Nicholas Simmons, Larry Lerno, Vladimir Jiranek, Dario Cantu, Anita Oberholster, and David E. Block*  
*Department of Viticulture and Enology, University of California Davis, CA 95616 (deblock@ucdavis.edu)

Different wine yeasts strains (Saccharomyces cerevisiae) vary in cell biomass formation under identical fermentation conditions in a juice or medium. Yeast strains that yield higher biomass complete sugar depletion more efficiently than lower-biomass yielding strains. To understand this variation in nutrient use efficiency (NUE), we used a multifaceted approach to assess metabolic and regulatory differences between yeast strains. Initially, we focused on four commercial wine yeast strains with varying NUE, Montrachet, Cote des Blanc, T306, and Uvaferm 43, using small-scale (400 mL) fermentations in synthetic MMM medium. The maximum cell concentration reached by these strains ranged from OD600 nm 7.4 to 10.2. We analyzed intracellular and extracellular metabolites during fermentation using GC-MS and HPLC-RI, respectively. Complete analysis of the lipid profile of each strain was performed using QqQ LC-MS. In addition, we used a transcriptomic approach (RNA-Seq) to understand relevant transcriptional control mechanisms. Using partial least squares regression with metabolomic and lipidomic data, we observed that certain metabolic pathways, including the pentosephosphate pathway, TCA cycle, and fatty acid synthesis, are most relevant in determining NUE. Lipid profile analysis showed that while higher concentrations of phosphotidylcholines and phosphotidylethanolamine in the yeast cell membrane correlated positively with higher biomass yields, higher concentrations of phos-photidylinositols have the opposite relation with biomass yield. To further confirm our results, we used series of mutants that overexpress key pathway enzymes. As a result of overexpression of FAS1 (fatty acid synthase), statistically significant difference is observed in NUE, biomass yield, and lipid profile. 

Funding Support: American Vineyard Foundation, Ernest Gallo Endowed Chair in Viticulture and Enology