Effect of nitrogen addition timing of an enological yeast strain gene expression

The aim of this study was to evaluate the effect of timing of nitrogen added on a Saccharomyces cerevisiae yeast wine strain. We studied several addition timings and evaluated gene expression at different times after nitrogen addition (30min and 2h). We found that there were no major differences between the two samples after addition. 305 induced genes were common to both addition timings and represented amino acid biosynthetic functions. 147 induced genes were only induced after an addition made at the beginning of the stationary phase and 142 exclusively after an addition made at the end of the stationary phase. These genes represented translation and ribosomal RNA synthesis functions respectively. 150 genes were only repressed after an addition made at the beginning of the stationary phase and represented functions of responses to stress and nutritional deficiencies but also oxidoreductase activity. The repressed genes common to both timings (284) represented the same functions. Finally, the 127 genes only repressed after an addition made close to the end of the fermentation represented functions coding for lipid metabolism and cell wall organization. The yeast Saccharomyces cerevisiae EC1118 is a commercial wine yeast from Lallemand SA. Fermentation experiments were carried out in triplicates. The fermentation medium mimicks a standard natural must and has the same composition as the SM140 with previously described (Bely et al 1990). Fermentations were conducted under constant stirring at 24°C in 1.2 L flasks equipped with locks to maintain anaerobiosis. Production of CO2 was monitored by weighing the flasks every 20 min, to determine weight loss. The rate of CO2 production was estimated using a polynomial smoothing as previously described in Sablayrolles et al (1987). The number of cells was determined with a particle counter (Coulter counter, Beckman Coulter). Two timings of addition were carried out on the fermentations. One addition was made at the beginning of the stationary phase of fermentation (20g/L of CO2 released). The second addition timing corresponds to the three quarters of the stationary phase of fermentation (50g/L of CO2 released). Two samples were taken after each addition: 30min and 2h after nitrogen addition. A control without addition was carried out and the samples were taken at the CO2 released corresponding to the fermentations with nitrogen additions.