title: Xylose chemostat isolates of Saccharomyces cerevisiae show altered metabolite and enzyme levels compared with xylose, glucose, and ethanol metabolism of the original strain.
abstract: The efficient conversion of xylose-containing biomass hydrolysate by the ethanologenic yeast Saccharomyces cerevisiae to useful chemicals such as ethanol still remains elusive, despite significant efforts in both strain and process development. This study focused on the recovery and characterization of xylose chemostat isolates of a S. cerevisiae strain that overexpresses xylose reductase- and xylitol dehydrogenase-encoding genes from Pichia stipitis and the gene encoding the endogenous xylulokinase. The isolates were recovered from aerobic chemostat cultivations on xylose as the sole or main carbon source. Under aerobic conditions, on minimal medium with 30 g l(-1) xylose, the growth rate of the chemostat isolates was 3-fold higher than that of the original strain (0.15 h(-1) vs 0.05 h(-1)). In a detailed characterization comparing the metabolism of the isolates with the metabolism of xylose, glucose, and ethanol in the original strain, the isolates showed improved properties in the assumed bottlenecks of xylose metabolism. The xylose uptake rate was increased almost 2-fold. Activities of the key enzymes in the pentose phosphate pathway (transketolase, transaldolase) increased 2-fold while the concentrations of their substrates (pentose 5-phosphates, sedoheptulose 7-phosphate) decreased correspondingly. Under anaerobic conditions, on minimal medium with 45 g l(-1) xylose, the ethanol productivity (in terms of cell dry weight; CDW) of one of the isolates increased from 0.012 g g(-1) CDW h(-1) to 0.017 g g(-1) CDW h(-1) and the yield from 0.09 g g(-1) xylose to 0.14 g g(-1) xylose, respectively.
authors.affiliation: VTT Biotechnology, P.O. Box 1500, 02044 VTT, Finland.
authors.forename: Juha-Pekka
authors.initials: JP
authors.lastname: Pitkänen
authors.forename: Eija
authors.initials: E
authors.lastname: Rintala
authors.forename: Aristos
authors.initials: A
authors.lastname: Aristidou
authors.forename: Laura
authors.initials: L
authors.lastname: Ruohonen
authors.forename: Merja
authors.initials: M
authors.lastname: Penttilä
mesh_terms: D000332:Aerobiosis; D000449:Aldehyde Reductase / Q000096:biosynthesis / Q000235:genetics; D001709:Biotechnology; D003470:Culture Media; D050542:D-Xylulose Reductase; D000431:Ethanol / Q000378:metabolism; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000096:biosynthesis; D010843:Pichia / Q000235:genetics; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D013401:Sugar Alcohol Dehydrogenases / Q000096:biosynthesis / Q000235:genetics; D014170:Transformation, Genetic; D014994:Xylose / Q000378:metabolism
chemical_list: D003470:Culture Media; D000431:Ethanol; D014994:Xylose; D013401:Sugar Alcohol Dehydrogenases; D000449:Aldehyde Reductase; D050542:D-Xylulose Reductase; D017853:Phosphotransferases (Alcohol Group Acceptor); C032700:xylulokinase