BackgroundXylose is the second most abundant carbohydrate in the lignocellulosic biomass hydrolysate. The fermentation of xylose is essential for the bioconversion of lignocelluloses to fuels and chemicals. However the wild-type strains of Saccharomyces cerevisiae are unable to utilize xylose. Many efforts have been made to construct recombinant yeast strains to enhance xylose fermentation over the past few decades. Xylose fermentation remains challenging due to the complexity of lignocellulosic biomass hydrolysate. In this study, a modified genome shuffling method was developed to improve xylose fermentation by S. cerevisiae. Recombinant yeast strains were constructed by recursive DNA shuffling with the recombination of entire genome of P. stipitis with that of S. cerevisiae.ResultsAfter two rounds of genome shuffling and screening, one potential recombinant yeast strain ScF2 was obtained. It was able to utilize high concentration of xylose (100 g/L to 250 g/L xylose) and produced ethanol. The recombinant yeast ScF2 produced ethanol more rapidly than the naturally occurring xylose-fermenting yeast, P. stipitis, with improved ethanol titre and much more enhanced xylose tolerance.ConclusionThe modified genome shuffling method developed in this study was more effective and easier to operate than the traditional protoplast-fusion-based method. Recombinant yeast strain ScF2 obtained in this study was a promising candidate for industrial cellulosic ethanol production. In order to further enhance its xylose fermentation performance, ScF2 needs to be additionally improved by metabolic engineering and directed evolution.

Horticultural waste in wood chips form collected from a landscape company in Singapore was utilized as the substrate for the production of cellulase and hemicellulase under solid-state fermentation by Trichoderma reesei RUT-C30. The effects of substrate pretreatment methods, substrate particle size, incubation temperature and time, initial medium pH value, and moisture content on cellulase and hemicellulase production were investigated. Enzyme complex was obtained at the optimal conditions. This enzyme mixture contained FPase (15.0 U/g substrate dry matter, SDM), CMCase (90.5 U/g SDM), beta-glucosidase (61.6 U/g SDM), xylanase (52.1 U/g SDM), and beta-xylosidase (10.4 U/g SDM). The soluble protein concentration in the enzyme complex was 26.1 mg/g SDM. The potential of the crude enzyme complex produced was demonstrated by the hydrolysis of wood chips, wood dust, palm oil fiber, and waste newspaper. The performance of the crude enzyme complex was better than the commercial enzyme blend.

BackgroundXylose isomerase (XI) catalyzes the conversion of xylose to xylulose, which is the key step for anaerobic ethanolic fermentation of xylose. Very few bacterial XIs can function actively in Saccharomyces cerevisiae. Here, we illustrate a group of XIs that would function for xylose fermentation in S. cerevisiae through phylogenetic analysis, recombinant yeast strain construction, and xylose fermentation.ResultsPhylogenetic analysis of deposited XI sequences showed that XI evolutionary relationship was highly consistent with the bacterial taxonomic orders and quite a few functional XIs in S. cerevisiae were clustered with XIs from mammal gut Bacteroidetes group. An XI from Bacteroides valgutus in this cluster was actively expressed in S. cerevisiae with an activity comparable to the fungal XI from Piromyces sp. Two XI genes were isolated from the environmental metagenome and they were clustered with XIs from environmental Bacteroidetes group. These two XIs could not be expressed in yeast with activity. With the XI from B. valgutus expressed in S. cerevisiae, background yeast strains were optimized by pentose metabolizing pathway enhancement and adaptive evolution in xylose medium. Afterwards, more XIs from the mammal gut Bacteroidetes group, including those from B. vulgatus, Tannerella sp. 6_1_58FAA_CT1, Paraprevotella xylaniphila and Alistipes sp. HGB5, were individually transformed into S. cerevisiae. The known functional XI from Orpinomyces sp. ukk1, a mammal gut fungus, was used as the control. All the resulting recombinant yeast strains were able to ferment xylose. The respiration-deficient strains harboring B. vulgatus and Alistipes sp. HGB5 XI genes respectively obtained specific xylose consumption rate of 0.662 and 0.704 g xylose gcdw−1 h−1, and ethanol specific productivity of 0.277 and 0.283 g ethanol gcdw−1 h−1, much comparable to those obtained by the control strain carrying Orpinomyces sp. ukk1 XI gene.ConclusionsThis study demonstrated that XIs clustered in the mammal gut Bacteroidetes group were able to be expressed functionally in S. cerevisiae and background strain anaerobic adaptive evolution in xylose medium is essential for the screening of functional XIs. The methods outlined in this paper are instructive for the identification of novel XIs that are functional in S. cerevisiae.Electronic supplementary materialThe online version of this article (doi:10.1186/s12934-015-0253-1) contains supplementary material, which is available to authorized users.

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