Consequences of phosphoglycerate kinase overproduction for the growth and physiology of Saccharomyces cerevisiae

Summary The physiological consequences of overproduction of the homologous glycolytic enzyme 3-phosphoglycerate kinase (PGK), integrated in 80 PGK1 gene copies in the genome of Saccharomyces cerevisiae are described. This multiple integration and the strong PGK overproduction (maximum 47% of the total soluble cell protein) do not affect the maximal specific growth rate, but cause 40% reduction of the molar growth yield, compared with that of the wild-type host. The extra energy that is needed for protein overproduction is mainly provided by extra fermentation (respirofermentative growth), but respiration is also elevated compared with the reference strains. The increase in the specific oxygen uptake rate indicates that the respiratory capacity of the yeasts is higher than that in the wild-type host, in which the limited capacity of respiration is generally supposed to be at its maximal level at the critical dilution rate, and is thus responsible for the switch to respirofermentative growth. In a medium PGK1 gene copy integrant (about 25 copies), overproduction of 10%–12% PGK has a stimulating effect on the growth yield and energy efficiency. In these cells the growth benefits of overproduction of the glycolytic enzyme are higher than the disadvantages of extra protein synthesis. The overproduction of PGK has also consequences for the glucose affinity of the yeasts: In the more overproducing strain the K_s is increased, compared to its reference strains. Elimination of strong overproducing cells from a glucose-limited chemostat culture is caused by two factors: (a) the excision of the PGK genes from the genome, which is of minor importance for wash-out, but the induction process for this overall decline of overproduction, and (b) the physiological selection process for less overproducing cells, caused by differences in affinity for glucose, most obvious at µ 1/2µ_max. However in batch culture and in a chemostat at low specific growth rates, all the overproducing strains show high genetic stability and constantly provide high PGK quantities.Offprint requests to: P. C. van der Aar