Coexpression of β-xylosidase and xylose isomerase in Saccharomyces cerevisiae improves the efficiency of saccharification and fermentation from xylo-oligosaccharides

The depolymerization of xylo-oligosaccharides to xylose by beta-xylosidase and the subsequent ethanol production from xylose are important for reducing inhibition towards cellulase and hemicellulase and for taking full advantage of all sugars in lignocellulose hydrolysate. The activity of beta-xylosidase in most cellulases produced by filamentous fungi is usually deficient, and thus recombinant xylose-fermenting Saccharomyces cerevisiae capable of secreting highly active beta-xylosidase is increasingly recognized as an excellent candidate for addressing this issue. To this end, a prominent chassis cell, BSPX042, constructed in our previous work with a well-modified downstream metabolic pathway of xylose through multiple chromosome manipulations and beneficial mutations after evolution engineering, was selected as the host. The xylose isomerase coding gene Ru-xylA (where Ru represents the rumen), which was screened from the metagenomics library of bovine rumen contents, was shown to exhibit a high activity level in S. cerevisiae, and the glycoside hydrolase family 3 beta-xylosidase gene xyl3A from Penicillium oxalicum was coexpressed in BSPX042. The recombinant strain BSGIBX with the highest extracellular activity of beta-xylosidase was determined by comparing different signal peptides. The hydrolysis of xylobiose and xylotriose by BSGIBX culture further confirmed its beta-xylosidase activity. BSGIBX can grow and produce ethanol with xylo-oligosaccharides as the sole carbon source. When xylo-oligosaccharides were pretreated by effective xylanase, the majority of the xylo-oligosaccharides were consumed rapidly and produced similar to 19.4 g L-1 ethanol at 48 h with a yield of 0.473 g g(-1) consumed sugars in mixture with glucose. These results indicate the successful coexpression of beta-xylosidase and xylose isomerase in ethanol-producing S. cerevisiae, and provide theoretical support for our future work in robust industrial yeast strains.