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Title: Association of improved oxidative stress tolerance and alleviation of glucose repression with superior xylose-utilization capability by a natural isolate of Saccharomyces cerevisiae

Author
item CHENG, CHENG - Dalian University Of Technology
item TANG, RUIQI - School Of Life Sciences And Bioengineering
item XIONG, LIANG - Dalian University Of Technology
item Hector, Ronald - Ron
item BAI, FENGWU - School Of Life Sciences And Bioengineering
item ZHAO, XINQING - School Of Life Sciences And Bioengineering

Submitted to: Biotechnology for Biofuels and Bioproducts
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2018
Publication Date: 2/5/2018
Citation: Cheng, C., Tang, R., Xiong, L., Hector, R.E., Bai, F., Zhao, X. 2018. Association of improved oxidative stress tolerance and alleviation of glucose repression with superior xylose-utilization capability by a natural isolate of Saccharomyces cerevisiae. Biotechnology for Biofuels. 11:28. https://doi.org/10.1186/s13068-018-1018-y.
DOI: https://doi.org/10.1186/s13068-018-1018-y

Interpretive Summary: Brewer’s yeast stains (Saccharomyces cerevisiae) generally have a poor ability to use xylose, the second most abundant sugar in nature. Previous studies of wild brewer’s yeast strains engineered to improve xylose use have shown a dependence on the genetic background of the parent strain. That is, certain strains showed an innate ability which was beneficial to xylose use. This study evaluated one of these strains (YB-2625) to determine which genes are required for the increased use of xylose. Notably, genes for protection against oxidative damage were shown to be elevated. Consistent with the increased expression of these genes, strain YB-2625 was found to be more resistant to oxidative stress. Overexpression of two of these genes in YB-2625 increased cell growth when xylose was used as the sole carbon source, providing up to 18.1% more xylose consumption. Furthermore, it was shown that exposure of cells to xylitol, one of the main metabolic products of xylose, induces oxidative stress. The sum of these results indicates that the increased oxidative stress tolerance of YB-2625 is beneficial to its xylose consumption. The present study provides insights in the innate regulatory mechanisms underlying xylose utilization in brewer’s yeast, which benefits rapid development of robust yeast strains for lignocellulosic biorefineries.

Technical Abstract: Background: Saccharomyces cerevisiae wild strains generally have poor xylose utilization capability, which is the major barrier for efficient bioconversion of lignocellulosic biomass. Laboratory adaption is commonly used to enhance xylose utilization of recombinant S. cerevisiae, whereby yeast cells remodel the metabolic network for improved xylose metabolism. However, it still remains unclear why wild S. cerevisiae uses xylose poorly. Here we analyzed a unique S. cerevisiae natural isolate YB-2625 which has superior xylose metabolism capability to explore the innate regulatory network of xylose utilization. Comparative transcriptomics analyses were performed using YB-2625 grown in the mixture of glucose and xylose, and the model yeast strain S288c served as a control. Global gene transcription was compared at both the early mixed-sugar utilization stage and the latter xylose-utilization stage. Results: Genes involved in endogenous xylose-assimilation (XYL2 and XKS1), gluconeogenesis, and TCA cycle showed higher transcription levels in S. cerevisiae YB-2625 at the xylose-utilization stage, when compared to the reference strain. On the other hand, transcription factors involved in regulation of glucose repression (MIG1, MIG2, and MIG3) and HXK2 displayed decreased transcriptional levels in YB-2625, suggesting alleviation of glucose repression of YB-2625. Notably, genes encoding antioxidant enzymes (CTT1, CTA1, SOD2 and PRX1) showed higher transcription levels in YB-2625 in the xylose-utilization stage than that of the reference strain. Consistently, catalase activity of YB-2625 was 1.9-fold higher than that of S288c during xylose utilization stage. As a result, intracellular reactive oxygen species (ROS) levels of YB-2625 were 43.3% and 58.6% lower than that of S288c at both stages. Overexpression of CTT1 and PRX1 in the recombinant strain S. cerevisiae YRH396 increased cell growth when xylose was used as the sole carbon source, leading to 13.5% and 18.1%, respectively, more xylose consumption, which indicated that enhanced oxidative stress tolerance benefits xylose utilization in S. cerevisiae. Conclusions: Enhanced oxidative stress tolerance and relief of glucose repression are two major mechanisms for superior xylose utilization by S. cerevisiae YB-2625. The present study provides insights in the innate regulatory mechanisms underlying xylose utilization in S. cerevisiae, which benefits rapid development of robust yeast strains for lignocellulosic biorefinery.