Abstract
Understanding factors that regulate the metabolism and growth of an organism is of fundamental biologic interest. This study compared the influence of two different carbon substrates, dextrose and galactose, on the metabolic and growth rates of the yeast Saccharomyces cerevisiae. Yeast metabolic and growth rates varied widely depending on the metabolic substrate supplied. The metabolic and growth rates of a yeast strain maintained under long-term laboratory conditions was compared to strain isolated from natural condition when grown on different substrates. Previous studies had determined that there are numerous genetic differences between these two strains. However, the overall metabolic and growth rates of a wild isolate of yeast was very similar to that of a strain that had been maintained under laboratory conditions for many decades. This indicates that, at in least this case, metabolism and growth appear to be well buffered against genetic differences. Metabolic rate and cell number did not co-vary in a simple linear manner. When grown in either dextrose or galactose, both strains showed a growth pattern in which the number of cells continued to increase well after the metabolic rate began a sharp decline. Previous studied have reported that O2 consumption in S. cerevisiae grown in reduced dextrose levels were elevated compared to higher levels. Low dextrose levels have been proposed to induce caloric restriction and increase life span in yeast. However, there was no evidence that reduced levels of dextrose increased metabolic rates, measured by either O2 consumption or CO2 production, in the strains used in this study.
Highlights
Yeasts in general, and the bakers yeast Saccharomyces cerevisiae in particular, have been extensively used for many decades to examine a wide array of biological processes, including studies on cell cycle control, gene expression regulation and control of metabolic function [1,2,3]
As seen in figure 1A, when grown in 2% dextrose both the BY and RM strains show an exponential increase in CO2/O2 fluxes for,5 h followed by a sharp decline in metabolic rate
The respiratory quotient (Figure 1B) increased over the first several hours and declined sharply. This decrease in RQ is consistent with cells switching from aerobic fermentation of dextrose to the use of ethanol, or stored glycogen, as the metabolic substrates for respiration [19,36]
Summary
The bakers yeast Saccharomyces cerevisiae in particular, have been extensively used for many decades to examine a wide array of biological processes, including studies on cell cycle control, gene expression regulation and control of metabolic function [1,2,3]. While yeast metabolism has been intensely studied, there remain areas of controversy on issues such as the metabolic response of yeast to reduced dextrose levels or the relative use of aerobic fermentation in varied metabolic substrates [9,10]. In high concentrations of dextrose, S. cerevisiae actively represses respiratory enzyme synthesis. In these conditions dextrose is metabolized via fermentation rather than respiration, even when oxygen is abundant [11,12,13,14]. The repression of respiratory enzyme synthesis by fermentation activity is referred to as the Crabtree Effect, and such aerobic glycolysis is often characteristic of mammalian cancer cells [15]. The ethanol produced as a by-product of fermentation can subsequently be utilized as a non-fermentable carbon source in respiration, allowing for the near complete use of all available carbon [17,18]
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