Comparison of process configurations for ethanol production from acid- and alkali-pretreated corncob by Saccharomyces cerevisiae strains with and without β-glucosidase expression

Comparison of the Unfolded Protein Response in Cellobiose Utilization of Recombinant Angel- and W303-1A-Derived Yeast Expressing β-Glucosidase

The unfolded protein response (UPR) is one of the most important protein quality control mechanisms in cells. At least, three factors are predicted to activate the UPR in yeast cells during fermentation. Using UPRE-lacZ as a reporter, we constructed two indicator strains, KZ and WZ, based on Angel-derived K-a and W303-1A strains, respectively, and investigated their UPR response to tunicamycin, ethanol, and acetic acid. Then, four strains carrying plasmids BG-cwp2 and BG were obtained to realize the displaying and secretion of β-glucosidase, respectively. The results of cellobiose utilization assays indicated interactions between the UPR and the metabolic burden between the strain source, anchoring moiety, oxygen supply, and cellobiose concentration. Meanwhile, as expected, growth (OD600), β-glucosidase, and β-galactosidase activities were shown to have a positive inter-relationship, in which the values of the KZ-derived strains were far lower than those of the WZ-derived strains. Additionally, extra metabolic burden by displaying over secreting was also much more serious in strain KZ than in strain WZ. The maximum ethanol titer of the four strains (KZ (BG-cwp2), KZ (BG), WZ (BG-cwp2), and WZ (BG)) in oxygen-limited 10% cellobiose fermentation was 3.173, 5.307, 5.495, and 5.486% (v/v), respectively, and the acetic acid titer ranged from 0.038 to 0.060% (v/v). The corresponding maximum values of the ratio of β-galactosidase activity to that of the control were 3.30, 5.29, 6.45, and 8.72, respectively. Under aerobic conditions with 2% cellobiose, those values were 3.79, 4.97, 6.99, and 7.67, respectively. A comparison of the results implied that β-glucosidase expression durably induced the UPR, and the effect of ethanol and acetic acid depended on the titer produced. Further study is necessary to identify ethanol- or acid-specific target gene expression. Taken together, our results indicated that the host strain W303-1A is a better secretory protein producer, and the first step to modify strain K-a for cellulosic ethanol fermentation would be to relieve the bottleneck of UPR capacity. The results of the present study will help to identify candidate host strains and optimize expression and fermentation by quantifying UPR induction.

Repetitive δ-integration of a cellulase-encoding gene into the chromosome of an industrial Angel Yeast-derived strain by URA3 recycling

Genetic modification of industrial yeast strains often faces more difficulties than that of laboratory strains. Thus, new approaches are still required. In this research, the Angel Yeast-derived haploid strain Kα was genetically modified by multiple rounds of δ-integration, which was achieved via URA3 recycling. Three δ-integrative plasmids, pGδRU, pGδRU-BGL, and pGδRU-EG, were first constructed with two 167 bp δ sequences and a repeat-URA3-repeat fragment. Then, the δ-integrative strains containing the bgl1 or egl2 gene were successfully obtained by one-time transformation of the linearized pGδRU-BGL or pGδRU-EG fragment, respectively. Their counterparts in which the URA3 gene was looped out were also easily isolated by selection for growth on 5´-fluoroorotic acid plates, although the ratio of colonies lacking URA3 to the total number of colonies decreased with increasing copy number of the corresponding integrated cellulase-encoding gene. Similar results were observed during the second round of δ-integration, in which the δ-integration strain Kα(δ::bgl1-repeat) obtained from the first round was transformed with a linearized pGδRU-EG fragment. After 10 rounds of cell growth and transfer to fresh medium, the doubling times and enzyme activities of Kα(δ::bgl1-repeat), Kα(δ::egl2-repeat), and Kα(δ::bgl1-repeat)(δ::egl2-repeat) showed no significant change and were stable. Further, their maximum ethanol concentrations during simultaneous saccharification and fermentation of pretreated corncob over a 7-day period were 46.35, 33.13, and 51.77 g/L, respectively, which were all substantially higher than the parent Kα strain. Thus, repetitive δ-integration with URA3 recycling can be a feasible and valuable method for genetic engineering of Angel Yeast. These results also provide clues about some important issues related to δ-integration, such as the structural stability of δ-integrated genes and the effects of individual integration-site locations on gene expression. Further be elucidation of these issues should help to fully realize the potential of δ-integration-based methods in industrial yeast breeding.

Extra metabolic burden by displaying over secreting: Growth, fermentation and enzymatic activity in cellobiose of recombinant yeast expressing β-glucosidase

β-Glucosidase was selected to be a reporter to study metabolic burden imposed by its expression in yeast. Cell growth, fermentation yield and enzymatic activity were used as indicators of the metabolic burden borne by 14 recombinant yeast strains. Various factors were found to affect metabolic burden, including BGLI gene source, gene dose, trafficking of the enzyme (either cell-surface display or secretion), and oxygen supply. While BGLI gene from Aspergillus aculeatus provided better performance for the host cells than that from Saccharomycopsis fibuligera, displaying β-glucosidase on the cell surface generally led to lower μ_m, total activity and ethanol titer, and longer lag period, lower (aerobic condition) or higher (anaerobic condition) biomass yield than that of secreting β-glucosidase. The negative effect on growth increased with gene dose level until a final failure to grow. This growth difference implies that displaying β-glucosidase on the cell surface imposes an extra metabolic burden. The molecular basis and mechanisms for this phenomenon need to further be investigated in order to develop better strategies for utilizing displayed and secreted enzymes in biotechnology and yeast breeding.

Combined acid/alkaline-peroxide pretreatment of olive tree biomass for bioethanol production

Olive tree biomass (OTB) can be used for producing second generation bioethanol. In this work, extracted OTB was subjected to fractionation using a sequential acid/alkaline oxidative pretreatment. In the first acid stage, the effects of sulfuric acid concentration and reaction times at 130°C were investigated. Up to 71% solubilization of hemicellulosic sugars was achieved under optimized conditions (2.4% H₂SO₄, 84min). In the second stage, the influence of hydrogen peroxide concentration and process time were evaluated at 80°C. Approximately 80% delignification was achieved under the best operational conditions (7% H₂O₂, 90min) within the experimental range studied. This pretreatment produced a substrate with 72% cellulose that was highly accessible to enzymatic attack, yielding 82g glucose/100g glucose in delignified OTB. Ethanol production from both hemicellulosic sugars solubilized in the acid pretreatment and glucose from enzymatic hydrolysis of delignified OTB yielded 15g ethanol/100g OTB.

Reducing β-glucosidase supplementation during cellulase recovery using engineered strain for successive lignocellulose bioconversion

Enzyme recycling by re-adsorption is one of the primary methods for reducing enzyme usage in lignocellulose conversion. This work proposes the combination of an engineered yeast strain that expresses β-glucosidase with enzyme recycling to reduce the amount of supplemented β-glucosidase in enzyme recycling experiments. Using the engineered strain, a slight increase in ethanol concentration was obtained after a 96-h fermentation of pretreated corncobs. Ethanol concentrations increased by 34.7% and 62.7% in the following two recycle rounds using the engineered strain compared with those using its parental strain without β-glucosidase addition. Furthermore, with the addition of β-glucosidase at 30CBU/g cellulose, the ethanol concentration after two recycle rounds exceeded 90% of that observed in the first SSF round with the engineered strain at a high initial cellulase loading of 45FPU/g cellulose.

Development of a cellulolytic Saccharomyces cerevisiae strain with enhanced cellobiohydrolase activity

Consolidated bioprocessing (CBP) is a promising technology for lignocellulosic ethanol production, and the key is the engineering of a microorganism that can efficiently utilize cellulose. Development of Saccharomyces cerevisiae for CBP requires high level expression of cellulases, particularly cellobiohydrolases (CBH). In this study, to construct a CBP-enabling yeast with enhanced CBH activity, three cassettes containing constitutively expressed CBH-encoding genes (cbh1 from Aspergillus aculeatus, cbh1 and cbh2 from Trichoderma reesei) were constructed. T. reesei eg2, A. aculeatus bgl1, and the three CBH-encoding genes were then sequentially integrated into the S. cerevisiae W303-1A chromosome via δ-sequence-mediated integration. The resultant strains W1, W2, and W3, expressing uni-, bi-, and trifunctional cellulases, respectively, exhibited corresponding cellulase activities. Furthermore, both the activities and glucose producing activity ascended. The growth test on cellulose containing plates indicated that CBH was a necessary component for successful utilization of crystalline cellulose. The three recombinant strains and the control strains W303-1A and AADY were evaluated in acid- and alkali-pretreated corncob containing media with 5 FPU exogenous cellulase/g biomass loading. The highest ethanol titer (g/l) within 7 days was 5.92 ± 0.51, 18.60 ± 0.81, 28.20 ± 0.84, 1.40 ± 0.12, and 2.12 ± 0.35, respectively. Compared with the control strains, W3 efficiently fermented pretreated corncob to ethanol. To our knowledge, this is the first study aimed at creating cellulolytic yeast with enhanced CBH activity by integrating three types of CBH-encoding gene with a strong constitutive promoter Ptpi.

Improving bgl1 gene expression in Saccharomyces cerevisiae through meiosis in an isogenic triploid

Introducing large numbers of target genes into the chromosome of Saccharomyces cerevisiae via δ-sequence-mediated integration is a good strategy for exploring the effects of gene dosage on expression and secretion of heterologous proteins. The expression of exogenous genes might be further improved through meiosis in an isogenic triploid. Here, a stable strain A-8 was screened from 35 sexual spore colonies obtained from an isogenic triploid integratively expressing bgl1 from Aspergillus aculeatus. The corresponding β-glucosidase activity in this strain was increased by ~120 % compared with the parent strain BGL-a. Measurement of doubling time, flow cytometry, and mating experiments further confirmed that A-8 was a spore-forming strain obtained from a triploid parent. Thus, combining δ-integration and meiosis in an isogenic triploid is a promising approach for improving the expression of exogenous proteins in S. cerevisiae.