Rapid genomic response after relief from nitrogen starvation of an enological strain of Saccharomyces cerevisiae. Rapid genomic response after relief from nitrogen starvation of an enological strain of Saccharomyces cerevisiae

The aim of this study was to determine how nitrogen repletion affected the genomic cell response of a Saccharomyces cerevisiae wine yeast strain, in particular within the first hour following relief from nutrient starvation. We found almost 4000 genes induced or repressed sometimes within minutes of nutrient changes. Some of the transcriptional responses to nitrogen depended on the TOR pathway which control positively ribosomal protein genes, amino acid and purine biosynthesis or amino acid permease genes and negatively stress-response genes, RTG specific TCA cycle genes and NCR sensitive genes. Some unexpected transcriptional responses concerned all the glycolytic genes, the starch and glucose metabolism and citrate cycle related genes which were down-regulated, as well as genes from the lipid metabolism. Overall design: 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 SM300 previously described (Bely et al. 1990), except for the total concentration of assimilable nitrogen which was 100 mg/L and the addition of 17 g/L FeCl3, 6H2O. 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). Preliminary experiments have shown that under this condition, cells were starved for nitrogen (i.e. reached stationary phase) upon a release of 17 g of CO2. Some cells were collected at this stage as controls (t = 0), then diammonium phosphate (DAP) was added to the culture medium at a final concentration of 300 mg/L. Before DAP addition, an equivalent volume of medium was removed to keep the total volume unchanged. Samplings were then performed 15, 30, 45 and 60 min after DAP addition and cells were quickly recovered by filtration as previously described in Tesnière et al. (2017).