Fermentation of xylose/glucose mixtures by metabolically engineered Saccharomyces cerevisiae strains expressing XYLl and XYL2 from Pichia stipitis with and without overexpression of TALl

Anaerobic xylose conversion by two metabolically engineered Saccharomyces cerevisiae strains in the presence and absence of simultaneous glucose metabolism was investigated. One strain expressed XYLI encoding xylose reductase (XR) and XYL2 encoding xylitol dehydrogenase (XDH) from Pichia stipitis, whereas the other additionally overexpressed TALI encoding transaldolase (TAL). Both strains formed xylitol as the main product of xylose metabolism. The TALI-overexpressing strain gave a higher biomass yield and produced less carbon dioxide and somewhat less xylitol compared with the XYLl +XYL2 strain, indicating that TAL limited xylose metabolism in the latter. The t:thanol yield was similar with both strains. The simultaneous metabolism of glucose enhanced xylose metabolism by causing a hligher rate of xylose consumption and less xylitol and xylulose excretion, compared with xylose metabolism alone. Simultaneous xylose and glucose metabolism affected the growth rate negatively compared with growth on glucose alone. Additionally, comparison of the specific growth rate of the host strain, a reference strain with a plasmid without XYLI, XYL2 or TALl, the XYLl+XʹrL2 strain and the XYLI+XYL2+TALI strain on glucose, showed that the presence of plasmids and expression of genes on the plasmids caused a decrease in specific growth rates related to the number of plasm ids present and the number of structural genes on the plasmids. Both strains exhibited high XR and XDH activities in batch cultivation, but rapidly lost the activities in chemostat cultivation. Limitations in the xylosemetabolising pathway and further improvement of recomhinant xylose-metaholising S. cerevisiae are discussed