Abstract
BackgroundIn Saccharomyces cerevisiae galactose is initially metabolized through the Leloir pathway after which glucose 6-phosphate enters glycolysis. Galactose is controlled both by glucose repression and by galactose induction. The gene PGM2 encodes the last enzyme of the Leloir pathway, phosphoglucomutase 2 (Pgm2p), which catalyses the reversible conversion of glucose 1-phosphate to glucose 6-phosphate. Overexpression of PGM2 has previously been shown to enhance aerobic growth of S. cerevisiae in galactose medium.ResultsIn the present study we show that overexpression of PGM2 under control of the HXT7'promoter from an integrative plasmid increased the PGM activity 5 to 6 times, which significantly reduced the lag phase of glucose-pregrown cells in an anaerobic galactose culture. PGM2 overexpression also increased the anaerobic specific growth rate whereas ethanol production was less influenced. When PGM2 was overexpressed from a multicopy plasmid instead, the PGM activity increased almost 32 times. However, this increase of PGM activity did not further improve aerobic galactose fermentation as compared to the strain carrying PGM2 on the integrative plasmid.ConclusionPGM2 overexpression in S. cerevisiae from an integrative plasmid is sufficient to reduce the lag phase and to enhance the growth rate in anaerobic galactose fermentation, which results in an overall decrease in fermentation duration. This observation is of particular importance for the future development of stable industrial strains with enhanced PGM activity.
Highlights
In Saccharomyces cerevisiae galactose is initially metabolized through the Leloir pathway after which glucose 6-phosphate enters glycolysis
Galactose catabolism to the glycolytic intermediate glucose 6-phosphate is a complex sequence of reactions including phosphorylation and
Effect of PGM2 gene dosage Two strains with different copy numbers of the gene PGM2 were constructed in the same genetic background (Table 1)
Summary
In Saccharomyces cerevisiae galactose is initially metabolized through the Leloir pathway after which glucose 6-phosphate enters glycolysis. Galactose is controlled both by glucose repression and by galactose induction. Sustainable bioconversion of renewable lignocellulose raw material requires that all sugar components are efficiently utilized [1]. These raw materials are composed of a number of different monosaccharides one of which is galactose. Baker's yeast, which is the preferred organism for industrial ethanolic fermentation [7], catabolizes galactose via the Leloir pathway [8]. Galactose catabolism to the glycolytic intermediate glucose 6-phosphate is a complex sequence of reactions including phosphorylation and
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