Abstract
BackgroundThe cost-effective production of second-generation bioethanol, which is made from lignocellulosic materials, has to face the following two problems: co-fermenting xylose with glucose and enhancing the strain’s tolerance to lignocellulosic inhibitors. Based on our previous study, the wild-type diploid Saccharomyces cerevisiae strain BSIF with robustness and good xylose metabolism genetic background was used as a chassis for constructing efficient xylose-fermenting industrial strains. The performance of the resulting strains in the fermentation of media with sugars and hydrolysates was investigated.ResultsThe following two novel heterologous genes were integrated into the genome of the chassis cell: the mutant MGT05196 N360F, which encodes a xylose-specific, glucose-insensitive transporter and is derived from the Meyerozyma guilliermondii transporter gene MGT05196, and Ru-xylA (where Ru represents the rumen), which encodes a xylose isomerase (XI) with higher activity in S. cerevisiae. Additionally, endogenous modifications were also performed, including the overproduction of the xylulokinase Xks1p and the non-oxidative PPP (pentose phosphate pathway), and the inactivation of the aldose reductase Gre3p and the alkaline phosphatase Pho13p. These rationally designed genetic modifications, combined with alternating adaptive evolutions in xylose and SECS liquor (the leach liquor of steam-exploding corn stover), resulted in a final strain, LF1, with excellent xylose fermentation and enhanced inhibitor resistance. The specific xylose consumption rate of LF1 reached as high as 1.089 g g−1 h−1 with xylose as the sole carbon source. Moreover, its highly synchronized utilization of xylose and glucose was particularly significant; 77.6% of xylose was consumed along with glucose within 12 h, and the ethanol yield was 0.475 g g−1, which is more than 93% of the theoretical yield. Additionally, LF1 performed well in fermentations with two different lignocellulosic hydrolysates.ConclusionThe strain LF1 co-ferments glucose and xylose efficiently and synchronously. This result highlights the great potential of LF1 for the practical production of second-generation bioethanol.Electronic supplementary materialThe online version of this article (doi:10.1186/s40643-016-0126-4) contains supplementary material, which is available to authorized users.
Highlights
The cost-effective production of second-generation bioethanol, which is made from lignocellulosic materials, has to face the following two problems: co-fermenting xylose with glucose and enhancing the strain’s tolerance to lignocellulosic inhibitors
XHR11 consumed less xylose either in the steam-exploding corn stover (SECS) hydrolysate (Fig. 2c, d; Table 3, Line 7 vs. 8) or in the inhibitors-free medium (Fig. 3a vs. c, b vs. d; Table 3, Line 9 vs. 10, Line 12 vs. 13); even glucose utilization was slightly decreased (Fig. 3b, d). These results indicated that adaptive evolution effectively improved the tolerance of the strain to inhibitors but affected the metabolic capacities for these sugars, probably due to some unknown changes caused by toxic stress
The following two key novel elements were introduced into the chassis cell: MGT05196N360F, which encodes a xylose-specific, glucose-insensitive transporter derived from the M. guilliermondii transporter gene MGT05196 (Wang et al 2015), and Ru-xylA, which encodes a xylose isomerase (XI) with higher activity in
Summary
The cost-effective production of second-generation bioethanol, which is made from lignocellulosic materials, has to face the following two problems: co-fermenting xylose with glucose and enhancing the strain’s tolerance to lignocellulosic inhibitors. The individual and synergistic negative interactions derived from the numerous inhibitory compounds that are formed during the pretreatment process and the hydrolytic release of sugars exert serious negative effects on the fermentation performance of S. cerevisiae (Ko et al 2016; Palmqvist and Hahn-Hägerdal 2000). For the economically viable and sustainable production of lignocellulosic bioethanol, it is necessary to confer the capacity to co-ferment glucose and xylose on an S. cerevisiae strain and to enhance its resistance to harsh production environments (Demeke et al 2013; Li et al 2015; Sharma et al 2016)
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