Exploring Yeast's Energy Dynamics: The General Stress Response Lowers Maintenance Energy Requirement

Abstract

In many microbial biotechnology processes, biomass itself is not the product of interest, but rather targeted chemicals or proteins. In these processes, growth should be limited to direct more substrate to product and increase process yields. Under growth-limiting conditions, such as nutrient limitation, microorganisms, including the yeast Saccharomyces cerevisiae, activate a general stress response (GSR). Different hypotheses have been formulated for this activation, including a preparatory role for future stresses or a role in cellular protein density. Here we tested a third hypothesis: the GSR reduces the energy needed to maintain cellular homeostasis, also known as the maintenance energy requirement (MER). The impact of GSR on MER was investigated by assessing the effect of the absence of its key regulators, Msn2 and Msn4, on energy-substrate distribution and stress resistance. Chemostat and fed-batch cultures revealed significant increases in MER of up to 85% in the deletion strain compared to the parental strain. In contrast, maximal biomass yields, growth rates and morphology were unaffected. Our insights highlight an additional role of the GSR, namely saving cellular energy. As the MER is a key determinant of product yields and in process design, especially in low growth processes, our findings can help to optimise microbial bioprocesses.

Keywords: Msn2 and Msn4; energetics; fed‐batch cultures; stress resistance; yeast.

FIGURE 1

Stress resistance of aerobic shake flask cultures of S. cerevisiae CEN.PK113‐7D (blue) and ScNtd002 (red). Average values of two biological duplicates are shown, error bars indicate the standard deviation. (a) Viability of exponentially growing cultures exposed to 53°C. (b) Viability of stationary phase cultures exposed to 53°C. (c) Viability of exponentially growing cultures exposed to hydrogen peroxide. (d) Viability of stationary phase cultures exposed to hydrogen peroxide. (e) Specific growth rates (bars, left y‐axis) and final OD600 values (dots, right y‐axis) in shake flask cultures.

FIGURE 2

(a) Specific glucose consumption rates versus specific growth rates in aerobic glucose‐limited chemostat cultures of S. cerevisiae CEN.PK113‐7D (circles) and ScNtd002 (squares). (b) Maximal biomass yields on glucose (Y _xs ^max ) estimated using chemostat cultures of S. cerevisiae CEN.PK113‐7D (blue) and ScNtd002 (red). (c) Maintenance coefficients estimated using chemostat cultures of S. cerevisiae CEN.PK113‐7D (blue) and ScNtd002 (red). Shown are estimated values of linear regression for chemostat data, where error bars indicate the uncertainty in the regression.

FIGURE 3

(a) Specific growth rate of fed‐batch cultures overtime in average of two replicates. (b) Specific glucose consumption rates versus specific growth rates in aerobic glucose‐limited fed‐batch cultures of S. cerevisiae CEN.PK113‐7D (circles) and ScNtd002 (squares). (c) Maintenance coefficients estimated using fed‐batch cultures of S. cerevisiae CEN.PK113‐7D (blue) and ScNtd002 (red). Shown are the average values of duplicate fed‐batch cultures, where error bars represent the SEM.