Wei Shan, Liu Yanan, Wu Meiling, Ma Tiantai, Bai Xiangzheng, Hou Jin, Shen Yu, Bao Xiaoming
1State Key Laboratory of Microbial Technology, Microbiology and Biotechnology Institute, Shandong University, Shan Da Nan Road 27, Jinan, 250100 China.
2School of Life Science, Shandong University, Shan Da Nan Road 27, Jinan, 250100 China.
Biotechnol Biofuels. 2018 Apr 16;11:112. doi: 10.1186/s13068-018-1112-1. eCollection 2018.
The recombinant strains that acquired the ability to utilize xylose through metabolic and evolutionary engineering exhibit good performance when xylose is the sole carbon source in the medium (designated the X stage in the present work). However, the xylose consumption rate of strains is generally low after glucose depletion during glucose-xylose co-fermentation, despite the presence of xylose in the medium (designated the GX stage in the present work). Glucose fermentation appears to reduce the capacity of these strains to "recognize" xylose during the GX stage, a phenomenon termed the post-glucose effect on xylose metabolism.
Two independent xylose-fermenting strains derived from a haploid laboratory strain and a diploid industrial strain were used in the present study. Their common characteristics were investigated to reveal the mechanism underlying the post-glucose effect and to develop methods to alleviate this effect. Both strains showed lower growth and specific xylose consumption rates during the GX stage than during the X stage. Glycolysis, the pentose phosphate pathway, and translation-related gene expression were reduced; meanwhile, genes in the tricarboxylic acid cycle and glyoxylic acid cycle demonstrated higher expression during the GX stage than during the X stage. The effects of 11 transcription factors (TFs) whose expression levels significantly differed between the GX and X stages in both strains were investigated. Knockout of promoted ribosome synthesis, and the growth rate, specific xylose utilization rate, and specific ethanol production rate of the strain increased by 17.4, 26.8, and 32.4%, respectively, in the GX stage. Overexpression of the ribosome-related genes , and also enhanced xylose utilization in a corresponding manner. Furthermore, the overexpression of , which is related to the cell cycle, increased the growth rate by 8.7%, the xylose utilization rate by 30.0%, and the ethanol production rate by 76.6%.
The TFs Thi2p and Nrm1p exerted unexpected effects on the post-glucose effect, enhancing ribosome synthesis and altering the cell cycle, respectively. The results of this study will aid in maintaining highly efficient xylose metabolism during glucose-xylose co-fermentation, which is utilized for lignocellulosic bioethanol production.
通过代谢和进化工程获得利用木糖能力的重组菌株，当木糖是培养基中的唯一碳源时（在本研究中称为X阶段）表现出良好的性能。然而，在葡萄糖-木糖共发酵过程中葡萄糖耗尽后，尽管培养基中存在木糖，但菌株的木糖消耗率通常较低（在本研究中称为GX阶段）。葡萄糖发酵似乎降低了这些菌株在GX阶段“识别”木糖的能力，这种现象称为葡萄糖对木糖代谢的后效应。
本研究使用了源自单倍体实验室菌株和二倍体工业菌株的两种独立的木糖发酵菌株。研究了它们的共同特征，以揭示葡萄糖后效应的潜在机制，并开发减轻这种效应的方法。两种菌株在GX阶段的生长和木糖比消耗率均低于X阶段。糖酵解、磷酸戊糖途径和翻译相关基因的表达降低；同时，三羧酸循环和乙醛酸循环中的基因在GX阶段的表达高于X阶段。研究了两种菌株在GX和X阶段表达水平有显著差异的11种转录因子（TFs）的作用。敲除促进了核糖体合成，该菌株在GX阶段的生长速率、木糖比利用率和乙醇比生产率分别提高了17.4%、26.8%和32.4%。核糖体相关基因、和的过表达也以相应方式增强了木糖利用。此外，与细胞周期相关的的过表达使生长速率提高了8.7%，木糖利用率提高了30.0%，乙醇生产率提高了76.6%。
转录因子Thi2p和Nrm1p对葡萄糖后效应产生了意想不到的影响，分别增强了核糖体合成和改变了细胞周期。本研究结果将有助于在用于木质纤维素生物乙醇生产的葡萄糖-木糖共发酵过程中维持高效的木糖代谢。
