Efficient ammonium uptake and mobilization of vacuolar arginine by

Abstract

Under N-limiting conditions, Saccharomyces cerevisiae strains display a substantial variability in their biomass yield from consumed nitrogen -in particular wine yeasts exhibit high growth abilities- that is correlated with their capacity to complete alcoholic fermentation, a trait of interest for fermented beverages industries. The aim of the present work was to assess the contribution of nitrogen availability to the strain-specific differences in the ability to efficiently use N-resource for growth and to identify the underlying mechanisms. We compared the profiles of assimilation of several nitrogen sources (mostly ammonium, glutamine, and arginine) for high and low biomass-producing strains in various conditions of nitrogen availability. We also analyzed the intracellular fate of nitrogen compounds. Strains clustered into two groups at initial nitrogen concentrations between 85 and 385 mg N.L−1: high biomass producers that included wine strains, were able to complete fermentation of 240 g.L−1 glucose and quickly consume nitrogen, in contrast to low biomass producers. The two classes of strains exhibited distinctive characteristics that contributed to their differential capacity to produce biomass. The contribution of each characteristic varied according to nitrogen availability. In high biomass producers, the high rate of ammonium uptake resulted in an important consumption of this preferred nitrogen source that promoted the growth of these yeasts when nitrogen was provided in excess. Both classes of yeast accumulated poor nitrogen sources, mostly arginine, in vacuoles during the first stages of growth. However, at end of the growth phase when nitrogen had become limiting, high biomass producers more efficiently used this vacuolar nitrogen fraction for protein synthesis and further biomass formation than low biomass producers. Overall, we demonstrate that the efficient management of the nitrogen resource, including efficient ammonium uptake and efficient use of the amino acids stored in vacuoles during the late stages of growth, might lead to high biomass production by wine yeasts.