AUTHOR=Zhu Yixuan , Zhang Jingtao , Zhu Lang , Jia Zefang , Li Qi , Xiao Wei , Cao Limin 

TITLE=Minimize the Xylitol Production in Saccharomyces cerevisiae by Balancing the Xylose Redox Metabolic Pathway

JOURNAL=Frontiers in Bioengineering and Biotechnology

VOLUME=9

YEAR=2021

URL=https://www.frontiersin.org/journals/bioengineering-and-biotechnology/articles/10.3389/fbioe.2021.639595

DOI=10.3389/fbioe.2021.639595

ISSN=2296-4185

ABSTRACT=<p>Xylose is the second most abundant sugar in lignocellulose, but it cannot be used as carbon source by budding yeast <italic>Saccharomyces cerevisiae</italic>. Rational promoter elements engineering approaches were taken for efficient xylose fermentation in budding yeast. Among promoters surveyed, <italic>HXT7</italic> exhibited the best performance. The <italic>HXT7</italic> promoter is suppressed in the presence of glucose and derepressed by xylose, making it a promising candidate to drive xylose metabolism. However, simple ectopic expression of both key xylose metabolic genes <italic>XYL1</italic> and <italic>XYL2</italic> by the <italic>HXT7</italic> promoter resulted in massive accumulation of the xylose metabolic byproduct xylitol. Through the <italic>HXT7</italic>-driven expression of a reported redox variant, <italic>XYL1-K270R</italic>, along with optimized expression of <italic>XYL2</italic> and the downstream pentose phosphate pathway genes, a balanced xylose metabolism toward ethanol formation was achieved. Fermented in a culture medium containing 50 g/L xylose as the sole carbon source, xylose is nearly consumed, with less than 3 g/L xylitol, and more than 16 g/L ethanol production. Hence, the combination of an inducible promoter and redox balance of the xylose utilization pathway is an attractive approach to optimizing fuel production from lignocellulose.</p>