Secretory expression of xylA under high dissolved oxygen to improve the xylose fermentation efficiency of Saccharomyces cerevisiae

The isomerization of xylose to xylulose is considered the most promising method for xylose utilization. The xylose isomerase (XI) gene xylA from Orpinomyces sp. ukk1 was expressed intracellularly and extracellularly in modified Saccharomyces cerevisiae strains INVSc-xylA and INVSc-SS-xylA, respectively, to enhance the synthesis of bioethanol, which is frequently utilized as a substitute for conventional fossil fuels. Moreover, the xylose transporter gene Xltr1p from Trichoderma reesei was co-expressed with xylA expressed intracellularly for INVSc-xylA-Xltr1p to further improve xylose utilization in glucose and xylose co-fermentation. INVSc-SS-xylA in mixed sugars consumed 8.30 g/L xylose, which was approximately 3- and 2-fold higher than these of INVSc-xylA and INVSc-xylA-Xltr1p, respectively. This result indicated converting xylose to xylulose prior to absorption was more effective for xylose consumption of S. cerevisiae. Furthermore, high dissolved oxygen (DO) promoted xylose utilization, regardless xylA was expressed extracellularly or intracellularly. INVSc-SS-xylA had a 1.5-fold higher xylose consumption rate than INVSc-xylA-Xltr1p. Transcriptome analysis of INVSc-SS-xylA under different DO levels indicated 967 differentially expressed genes (DEGs) were upregulated and 796 DEGs were downregulated. Most upregulated DEGs were related to TCA cycle, amino acid metabolism and energy metabolism, etc. INVSc-SS-xylA contributed to sugar-to-ethanol yield reached 0.25 g/g from pretreated bagasse hydrolysates, without detoxifying or washing pretreated biomass. These results demonstrated xylose metabolism can be improved by xylA expressed extracellularly in high DO production of ethanol from mixed sugars and provide useful methodological guidance in industrial ethanol production using undetoxified biomass.