Abstract
Lignocellulosic hydrolysates used for bioethanol production contain a mixture of sugars, with xylose being the second most abundant after glucose. Since xylose is not a natural substrate for Saccharomyces cerevisiae, recombinant S. cerevisiae strongly prefers glucose over xylose, and the fermentation rate and ethanol yield with xylose are both lower than those with glucose. To determine the molecular basis for glucose and xylose fermentation, we used microarrays to investigate the transcriptional difference of a xylose-utilizing industrial strain cultured in both single sugar media and a mixed sugar medium of glucose and xylose. The transcriptomes were nearly identical between glucose metabolizing cells in the glucose alone medium and those in the glucose fermentation phase in the mixed-sugar medium. Whereas the transcriptomes highly differed between the xylose metabolizing cells in the xylose alone medium and those in the xylose fermentation phase in the mixed sugar medium, and the differences mainly involved sulfur metabolism. When the transcriptional profiles were compared between glucose fermentation state and xylose fermentation state, we found the expression patterns of hexose transporters and glucose signaling pathway differed in response to different sugar sources, and the expression levels of the genes involved in gluconeogenesis, the glyoxylate and tricarboxylic acid cycles and respiration increased with xylose, indicating that the xylose-metabolizing cells had high requirements for maintenance energy and lacked the carbon catabolite repression capability. The effect of carbon catabolite repression by glucose lasted after glucose depletion for specific genes to different extents.Electronic supplementary materialThe online version of this article (doi:10.1186/s13568-016-0223-y) contains supplementary material, which is available to authorized users.
Highlights
Lignocellulosic biomass has been recognized as a sustainable source for fuel ethanol production without affecting the food and feed markets
The presence of glucose repressed the utilization of xylose, and it is found that the transcriptome of the glucose fermentation phase in the mixed-sugar medium was very similar to that in the glucose alone medium
A recently published gene expression study using mixed sugar medium found that respiratory genes were not fully repressed when xylose was present with abundant glucose (Alff-Tuomala et al 2016), which was not observed in our study
Summary
Lignocellulosic biomass has been recognized as a sustainable source for fuel ethanol production without affecting the food and feed markets. It is of economic interest to convert all lignocellulosic sugar fractions, predominantly glucose and xylose, into ethanol at sufficiently high rates and yields. Saccharomyces cerevisiae, which is widely used in bioethanol plants due to its high fermentation efficiency and process robustness, cannot ferment xylose (Batt et al 1986). In the past two decades, fermentation of xylose to ethanol has been achieved in S. cerevisiae by genetic engineering. The xylose-utilizing capacity of the recombinant strains can be further optimized by enhancing the downstream metabolic pathway rationally or through evolutionary engineering (Peng et al 2012). Recombinant strains strongly prefer glucose over xylose, and the co-consumption remains a challenge. What’s more, the specific ethanol productivity from xylose was an order of
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