Microaerobic conversion of xylose to ethanol in recombinant Saccharomyces cerevisiae SX6 MUT expressing cofactor-balanced xylose metabolic enzymes and deficient in ALD6

Construction of an economical xylose-utilizing Saccharomyces cerevisiae and its ethanol fermentation

Traditional industrial Saccharomyces cerevisiae could not metabolize xylose due to the lack of a specific enzyme system for the reaction from xylose to xylulose. This study aims to metabolically remould industrial S. cerevisiae for the purpose of utilizing both glucose and xylose with high efficiency. Heterologous gene xylA from Piromyces and homologous genes related to xylose utilization were selected to construct expression cassettes and integrated into genome. The engineered strain was domesticated with industrial material under optimizing conditions subsequently to further improve xylose utilization rates. The resulting S. cerevisiae strain ABX0928-0630 exhibits a rapid growth rate and possesses near 100% xylose utilization efficiency to produce ethanol with industrial material. Pilot-scale fermentation indicated the predominant feature of ABX0928-0630 for industrial application, with ethanol yield of 0.48 g/g sugars after 48 hours and volumetric xylose consumption rate of 0.87 g/l/h during the first 24 hours. Transcriptome analysis during the modification and domestication process revealed a significant increase in the expression level of pathways associated with sugar metabolism and sugar sensing. Meanwhile, genes related to glycerol lipid metabolism exhibited a pattern of initial increase followed by a subsequent decrease, providing a valuable reference for the construction of efficient xylose-fermenting strains.

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

Xylose is the second most abundant sugar in lignocellulose, but it cannot be used as carbon source by budding yeast Saccharomyces cerevisiae. Rational promoter elements engineering approaches were taken for efficient xylose fermentation in budding yeast. Among promoters surveyed, HXT7 exhibited the best performance. The HXT7 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 XYL1 and XYL2 by the HXT7 promoter resulted in massive accumulation of the xylose metabolic byproduct xylitol. Through the HXT7-driven expression of a reported redox variant, XYL1-K270R, along with optimized expression of XYL2 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.

Multi-omic characterization of laboratory-evolved Saccharomyces cerevisiae HJ7-14 with high ability of algae-based ethanol production

In this study, an evolved Saccharomyces cerevisiae HJ7-14 with high ability of algae-based ethanol production was characterized by multi-omic approaches. Genome sequencing of the HJ7-14 revealed a point mutation in the GAL83 gene (G703A) involved in the catabolite repression as well as the galactose metabolism. Cultural and transcriptional analyses of a S. cerevisiae mutant with chromosomal GAL83(G703A) indicated that the catabolite repression onto the galactose metabolism was considerably relieved in all cell growth stages. Untargeted metabolomic approach revealed that metabolic phenotypes between the control D452-2 and HJ7-14 strains were clearly discriminated in time-dependent manner. Especially in early growth stage at 6 h, the HJ7-14 showed dramatic and coordinated alteration in central carbon and amino acid metabolisms. Through metabolomic re-organization, fold changes in fatty acid metabolism and metabolites related to stress response system were also found upon glucose depletion and active galactose utilization. Multi-omic characterization using genome sequencing, transcription, and metabolome profiling clearly unveiled that the GAL83 gene mutation partially relieved glucose-dependent catabolite repression and allowed the evolved HJ7-14 to efficiently convert algal sugars to ethanol. Our finding could be applicable for engineering of S. cerevisiae able to covert red algal biomass to other biofuels and biochemicals.

Metabolic engineering of the 2-ketobutyrate biosynthetic pathway for 1-propanol production in Saccharomyces cerevisiae

Background

To produce 1-propanol as a potential biofuel, metabolic engineering of microorganisms, such as E. coli, has been studied. However, 1-propanol production using metabolically engineered Saccharomyces cerevisiae, which has an amazing ability to produce ethanol and is thus alcohol-tolerant, has infrequently been reported. Therefore, in this study, we aimed to engineer S. cerevisiae strains capable of producing 1-propanol at high levels.

Results

We found that the activity of endogenous 2-keto acid decarboxylase and alcohol/aldehyde dehydrogenase is sufficient to convert 2-ketobutyrate (2 KB) to 500 mg/L 1-propanol in yeast. Production of 1-propanol could be increased by: (i) the construction of an artificial 2 KB biosynthetic pathway from pyruvate via citramalate (cimA); (ii) overexpression of threonine dehydratase (tdcB); (iii) enhancement of threonine biosynthesis from aspartate (thrA, thrB and thrC); and (iv) deletion of the GLY1 gene that regulates a competing pathway converting threonine to glycine. With high-density anaerobic fermentation of the engineered S. cerevisiae strain YG5C4231, we succeeded in producing 180 mg/L 1-propanol from glucose.

Conclusion

These results indicate that the engineering of a citramalate-mediated pathway as a production method for 1-propanol in S. cerevisiae is effective. Although optimization of the carbon flux in S. cerevisiae is necessary to harness this pathway, it is a promising candidate for the large-scale production of 1-propanol.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0883-1) contains supplementary material, which is available to authorized users.

Sugar and ethanol production from woody biomass via supercritical water hydrolysis in a continuous pilot-scale system using acid catalyst

The aim of this study were to efficiently produce fermentable sugars by continuous type supercritical water hydrolysis (SCWH) of Quercus mongolica at the pilot scale with varying acid catalyst loading and to use the obtained sugars for ethanol production. The SCWH of biomass was achieved in under one second (380°C, 230bar) using 0.01-0.1% H₂SO₄. With 0.05% H₂SO₄, 49.8% of sugars, including glucose (16.5% based on biomass) and xylose monomers (10.8%), were liberated from biomass. The hydrolysates were fermented with S. cerevisiae DXSP and D452-2 to estimate ethanol production. To prepare the fermentation medium, the hydrolysates were detoxified using activated charcoal and then concentrated. The ethanol yield of fermentation with S. cerevisiae DXSP was 14.1% (based on biomass). The proposed system has potential for improvement in yield through process optimization. After further development, it is expected to be a competitive alternative to traditional systems for ethanol production from woody biomass.

Construction of efficient xylose-fermenting Saccharomyces cerevisiae through a synthetic isozyme system of xylose reductase from Scheffersomyces stipitis

Engineered Saccharomyces cerevisiae has been used for ethanol production from xylose, the abundant sugar in lignocellulosic hydrolyzates. Development of engineered S. cerevisiae able to utilize xylose effectively is crucial for economical and sustainable production of fuels. To this end, the xylose-metabolic genes (XYL1, XYL2 and XYL3) from Scheffersomyces stipitis have been introduced into S. cerevisiae. The resulting engineered S. cerevisiae strains, however, often exhibit undesirable phenotypes such as slow xylose assimilation and xylitol accumulation. This work was undertaken to construct an improved xylose-fermenting strain by developing a synthetic isozyme system of xylose reductase (XR). The DXS strain having both wild XR and mutant XR showed low xylitol accumulation and fast xylose consumption compared to the engineered strains expressing only one type of XRs, resulting in improved ethanol yield and productivity. These results suggest that the introduction of the XR-based synthetic isozyme system is a promising strategy to develop efficient xylose-fermenting strains.

Expression of a mutated SPT15 gene in Saccharomyces cerevisiae enhances both cell growth and ethanol production in microaerobic batch, fed-batch, and simultaneous saccharification and fermentations

The SPT15 gene encodes a Saccharomyces cerevisiae TATA-binding protein, which is able to globally control the transcription levels of various metabolic and regulatory genes. In this study, a SPT15 gene mutant (S42N, S78R, S163P, and I212N) was expressed in S. cerevisiae BY4741 (BSPT15-M3), of which effects on fermentative yeast properties were evaluated in a series of culture types. By applying different nitrogen sources and air supply conditions in batch culture, organic nitrogen sources and microaerobic condition were decided to be more favorable for both cell growth and ethanol production of the BSPT15-M3 strain than the control S. cerevisiae BY4741 strain expressing the SPT15 gene (BSPT15wt). Microaerobic fed-batch cultures of BSPT15-M3 with glucose shock in the presence of high ethanol content resulted in a 9.5-13.4% higher glucose consumption rate and ethanol productivity than those for the BSPT15wt strain. In addition, BSPT15-M3 showed 4.5 and 3.9% increases in ethanol productivity from cassava hydrolysates and corn starch in simultaneous saccharification and fermentation processes, respectively. It was concluded that overexpression of the mutated SPT15 gene would be a potent strategy to develop robust S. cerevisiae strains with enhanced cell growth and ethanol production abilities.