Abstract
BackgroundTwo heterologous pathways have been used to construct recombinant xylose-fermenting Saccharomyces cerevisiae strains: i) the xylose reductase (XR) and xylitol dehydrogenase (XDH) pathway and ii) the xylose isomerase (XI) pathway. In the present study, the Pichia stipitis XR-XDH pathway and the Piromyces XI pathway were compared in an isogenic strain background, using a laboratory host strain with genetic modifications known to improve xylose fermentation (overexpressed xylulokinase, overexpressed non-oxidative pentose phosphate pathway and deletion of the aldose reductase gene GRE3). The two isogenic strains and the industrial xylose-fermenting strain TMB 3400 were studied regarding their xylose fermentation capacity in defined mineral medium and in undetoxified lignocellulosic hydrolysate.ResultsIn defined mineral medium, the xylose consumption rate, the specific ethanol productivity, and the final ethanol concentration were significantly higher in the XR- and XDH-carrying strain, whereas the highest ethanol yield was achieved with the strain carrying XI. While the laboratory strains only fermented a minor fraction of glucose in the undetoxified lignocellulose hydrolysate, the industrial strain TMB 3400 fermented nearly all the sugar available. Xylitol was formed by the XR-XDH-carrying strains only in mineral medium, whereas in lignocellulose hydrolysate no xylitol formation was detected.ConclusionDespite by-product formation, the XR-XDH xylose utilization pathway resulted in faster ethanol production than using the best presently reported XI pathway in the strain background investigated. The need for robust industrial yeast strains for fermentation of undetoxified spruce hydrolysates was also confirmed.
Highlights
Two heterologous pathways have been used to construct recombinant xylosefermenting Saccharomyces cerevisiae strains: i) the xylose reductase (XR) and xylitol dehydrogenase (XDH) pathway and ii) the xylose isomerase (XI) pathway
Strain construction To generate an isogenic pair of strains, a strain with highlevel expression of xylose isomerases (XI) was constructed similar to the previously generated strain TMB 3057 with high-level expression of XR and XDH [29] (Table 1)
Isogenic strains were used to compare the efficiency of xylose fermentation via the P. stipitis XR and XDH route and the Piromyces XI route
Summary
Two heterologous pathways have been used to construct recombinant xylosefermenting Saccharomyces cerevisiae strains: i) the xylose reductase (XR) and xylitol dehydrogenase (XDH) pathway and ii) the xylose isomerase (XI) pathway. The Pichia stipitis XR-XDH pathway and the Piromyces XI pathway were compared in an isogenic strain background, using a laboratory host strain with genetic modifications known to improve xylose fermentation (overexpressed xylulokinase, overexpressed non-oxidative pentose phosphate pathway and deletion of the aldose reductase gene GRE3). Microbial Cell Factories 2007, 6:5 http://www.microbialcellfactories.com/content/6/1/5 fermentation by S. cerevisiae was first demonstrated by heterologous expression of xylose reductase (XR) and xylitol dehydrogenase (XDH) [4] from Pichia stipitis together with overexpression of the endogenous xylulokinase (XK) [5,6,7,8]. When adaptation [27] or extensive genetic engineering [28] was applied, were growth and fermentation on xylose achieved
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