Impact of osmotic stress and ethanol inhibition in yeast cells on process oscillation associated with continuous very-high-gravity ethanol fermentation

Assess different fermentation characteristics of 54 lager yeasts based on group classification

Saccharomyces pastorianus, hybrids of Saccharomyces cerevisiae and Saccharomyces eubayanus, were generally regarded as authentic lager beer yeasts. In recent years, with more new findings of other Saccharomyces genus hybrids, yeasts used in lager beer brewing have been proved much more complicated than previous cognition. In this study, we analyzed the different fermentation characteristics of 54 yeast strains used for lager brewing in normal and very high gravity brewing based on group classification. The difference between Group Ⅰ and Group Ⅱ lager yeasts were more striking in very high gravity brewing. However, during our research progress, we realized that some yeasts used in this study were actually hybrids of S. cerevisiae and Saccharomyces kudriavzevii. Features of these hybrids could be beneficial to very high gravity brewing. We further discussed about the mechanism behind their outstanding characteristics and the reason why group classification methods of lager beer yeasts had limitations. Hybridization in yeasts is constantly getting richer. Lager yeasts could have more possibilities based on better understandings of their genetic background and roles of other Saccharomyces genus hybrids. Their heterosis shed light on innovation in brewing and other diverse fermentation industries.

Isolation, screening and identification of ethanol producing yeasts from Ethiopian fermented beverages

The growing demand for renewable energy sources such as bioethanol is facing a lack of efficient ethanologenic microbes. This study aimed to isolate and screen ethanologenic yeasts from Ethiopian fermented beverages. A progressive screening and selection approach was employed. Selected isolates were evaluated for bioethanol production using banana peel waste as substrate. A total of 102 isolates were obtained. Sixteen isolates were selected based on their tolerance to stress conditions and carbohydrate fermentation and assimilation capacity. Most found moderately tolerant to 10 %, but slightly tolerant at 15 and 20 % (v/v) ethanol concentration. They yield 15.3 to 20.1 g/L and 9.1 ± 0.6 to 12.9 ± 1.3 g/L ethanol from 2 % (w/v) glucose and 80 g/L banana peel, respectively. Molecular characterization identified them as Saccharomyces cerevisiae strains. Results demonstrate insight about their potential role in the ethanol industry. Optimization of the fermentation conditions is recommended.

Deletion of NGG1 in a recombinant Saccharomyces cerevisiae improved xylose utilization and affected transcription of genes related to amino acid metabolism

Production of biofuels and biochemicals from xylose using yeast cell factory is of great interest for lignocellulosic biorefinery. Our previous studies revealed that a natural yeast isolate Saccharomyces cerevisiae YB-2625 has superior xylose-fermenting ability. Through integrative omics analysis, NGG1, which encodes a transcription regulator as well as a subunit of chromatin modifying histone acetyltransferase complexes was revealed to regulate xylose metabolism. Deletion of NGG1 in S. cerevisiae YRH396h, which is the haploid version of the recombinant yeast using S. cerevisiae YB-2625 as the host strain, improved xylose consumption by 28.6%. Comparative transcriptome analysis revealed that NGG1 deletion down-regulated genes related to mitochondrial function, TCA cycle, ATP biosynthesis, respiration, as well as NADH generation. In addition, the NGG1 deletion mutant also showed transcriptional changes in amino acid biosynthesis genes. Further analysis of intracellular amino acid content confirmed the effect of NGG1 on amino acid accumulation during xylose utilization. Our results indicated that NGG1 is one of the core nodes for coordinated regulation of carbon and nitrogen metabolism in the recombinant S. cerevisiae. This work reveals novel function of Ngg1p in yeast metabolism and provides basis for developing robust yeast strains to produce ethanol and biochemicals using lignocellulosic biomass.

Identification of Kic1p and Cdc42p as Novel Targets to Engineer Yeast Acetic Acid Stress Tolerance

Robust yeast strains that are tolerant to multiple stress environments are desired for an efficient biorefinery. Our previous studies revealed that zinc sulfate serves as an important nutrient for stress tolerance of budding yeast Saccharomyces cerevisiae. Acetic acid is a common inhibitor in cellulosic hydrolysate, and the development of acetic acid-tolerant strains is beneficial for lignocellulosic biorefineries. In this study, comparative proteomic studies were performed using S. cerevisiae cultured under acetic acid stress with or without zinc sulfate addition, and novel zinc-responsive proteins were identified. Among the differentially expressed proteins, the protein kinase Kic1p and the small rho-like GTPase Cdc42p, which is required for cell integrity and regulation of cell polarity, respectively, were selected for further studies. Overexpression of KIC1 and CDC42 endowed S. cerevisiae with faster growth and ethanol fermentation under the stresses of acetic acid and mixed inhibitors, as well as in corncob hydrolysate. Notably, the engineered yeast strains showed a 12 h shorter lag phase under the three tested conditions, leading to up to 52.99% higher ethanol productivity than that of the control strain. Further studies showed that the transcription of genes related to stress response was significantly upregulated in the engineered strains under the stress condition. Our results in this study provide novel insights in exploring zinc-responsive proteins for applications of synthetic biology in developing a robust industrial yeast.

Real-Time Monitoring of the Yeast Intracellular State During Bioprocesses With a Toolbox of Biosensors

Industrial fermentation processes strive for high robustness to ensure optimal and consistent performance. Medium components, fermentation products, and physical perturbations may cause stress and lower performance. Cellular stress elicits a range of responses, whose extracellular manifestations have been extensively studied; whereas intracellular aspects remain poorly known due to lack of tools for real-time monitoring. Genetically encoded biosensors have emerged as promising tools and have been used to improve microbial productivity and tolerance toward industrially relevant stresses. Here, fluorescent biosensors able to sense the yeast intracellular environment (pH, ATP levels, oxidative stress, glycolytic flux, and ribosome production) were implemented into a versatile and easy-to-use toolbox. Marker-free and efficient genome integration at a conserved site on chromosome X of Saccharomyces cerevisiae strains and a commercial Saccharomyces boulardii strain was developed. Moreover, multiple biosensors were used to simultaneously monitor different intracellular parameters in a single cell. Even when combined together, the biosensors did not significantly affect key physiological parameters, such as specific growth rate and product yields. Activation and response of each biosensor and their interconnection were assessed using an advanced micro-cultivation system. Finally, the toolbox was used to screen cell behavior in a synthetic lignocellulosic hydrolysate that mimicked harsh industrial substrates, revealing differences in the oxidative stress response between laboratory (CEN.PK113-7D) and industrial (Ethanol Red) S. cerevisiae strains. In summary, the toolbox will allow both the exploration of yeast diversity and physiological responses in natural and complex industrial conditions, as well as the possibility to monitor production processes.

Omics analysis reveals mechanism underlying metabolic oscillation during continuous very-high-gravity ethanol fermentation by Saccharomyces cerevisiae

During continuous very-high-gravity (VHG) ethanol fermentation with Saccharomyces cerevisiae, the process exhibits sustained oscillation in residual glucose, ethanol, and biomass, raising a question: how do yeast cells respond to this phenomenon? In this study, the oscillatory behavior of yeast cells was characterized through transcriptome and metabolome analysis for one complete oscillatory period. By analyzing the accumulation of 26 intracellular metabolites and the expression of 90 genes related to central carbon metabolism and stress response, we confirmed that the process oscillation was attributed to intracellular metabolic oscillation with phase difference, and the expression of HXK1, HXT1,2,4, and PFK1 was significantly different from other genes in the Embden-Meyerhof-Parnas pathway, indicating that glucose transport and phosphorylation could be key nodes for regulating the intracellular metabolism under oscillatory conditions. Moreover, the expression of stress response genes was triggered and affected predominately by ethanol inhibition in yeast cells. This progress not only contributes to the understanding of mechanisms underlying the process oscillation observed for continuous VHG ethanol fermentation, but also provides insights for understanding unsteady state that might develop in other continuous fermentation processes operated under VHG conditions to increase product titers for robust production.

Novel Effective Yeast Strains and Their Performance in High Gravity and Very High Gravity Ethanol Fermentations from Sweet Sorghum Juice

Yeasts were isolated from four potential sources, sweet sorghum juice, sugar cane juice, grapes and rambutan. The 27 yeast isolates were tested for their ethanol tolerance (15% v/v of ethanol) and ethanol fermentation performance in a synthetic ethanol production medium (200 g/L of total sugar). Only five isolates, SCJ04KKU, SCJ07KKU, SCJ09KKU, SCJ14KKU and SSJ01KKU could tolerate 15% ethanol and produce ethanol at levels higher than 55 g/L. The ethanol production efficiency from sweet sorghum juice under high gravity (HG, 200 and 240 g/L of total sugar) and very high gravity (VHG, 280 g/L of total sugar) conditions of the five isolates was tested. Saccharomyces cerevisiae NP01 and S. cerevisiae ATCC4132 were used as reference strains. The results showed that the SSJ01KKU isolate gave the highest ethanol production efficiency under all conditions. Ethanol concentration (PE), yield (YP/S) and productivity (QP) values were 98.89 g/L, 0.50 and 1.18 g/L·h, respectively, with sugar consumption (SC) of 98.96% under the HG condition at 200 g/L of total sugar. Under the HG condition at 240 g/L of total sugar, the PE, YP/S and QP values were 118.12 g/L, 0.51 and 1.41 g/L·h, respectively, with the SC of 95.79%. These values were 82.29 g/L, 0.34 and 0.98 g/L·h, respectively, with the SC of 85.59% under the VHG condition. Addition of urea into the sweet sorghum juice under all conditions significantly shortened the fermentation time, resulting in increased QP values. Based on molecular taxonomic analysis of the five isolates using sequence analysis of the D1/D2 domain and the ITS1 and ITS2 regions, SSJ01KKU is S. cerevisiae, whereas SCJ04KKU, SCJ07KKU, SCJ09KKU and SCJ14KKU are Pichia caribbica.

Metabolism, morphology and transcriptome analysis of oscillatory behavior of Clostridium butyricum during long-term continuous fermentation for 1,3-propanediol production

BACKGROUND

Oscillation is a special cell behavior in microorganisms during continuous fermentation, which poses threats to the output stability for industrial productions of biofuels and biochemicals. In previous study, a spontaneous oscillatory behavior was observed in Clostridium butyricum-intensive microbial consortium in continuous fermentation for 1,3-propanediol (1,3-PDO) production from glycerol, which led to the discovery of oscillation in species C. butyricum.

RESULTS

Spontaneous oscillations by C. butyricum tended to occur under glycerol-limited conditions at low dilution rates. At a glycerol feed concentration of 88 g/L and a dilution rate of 0.048 h^-1, the oscillatory behavior of C. butyricum was observed after continuous operation for 146 h and was sustained for over 450 h with an average oscillation period of 51 h. During oscillations, microbial glycerol metabolism exhibited dramatic periodic changes, in which productions of lactate, formate and hydrogen significantly lagged behind that of other products including biomass, 1,3-PDO and butyrate. Analysis of extracellular oxidation-reduction potential and intracellular ratio of NAD⁺/NADH indicated that microbial cells experienced distinct redox changes during oscillations, from oxidized to reduced state with decreasing of growth rate. Meanwhile, C. butyricum S3 exhibited periodic morphological changes during oscillations, with aggregates, elongated shape, spores or cell debris at the trough of biomass production. Transcriptome analysis indicated that expression levels of multiple genes were up-regulated when microbial cells were undergoing stress, including that for pyruvate metabolism, conversion of acetyl-CoA to acetaldehyde as well as stress response.

CONCLUSION

This study for the first time systematically investigated the oscillatory behavior of C. butyricum in aspect of occurrence condition, metabolism, morphology and transcriptome. Based on the experimental results, two hypotheses were put forward to explain the oscillatory behavior: disorder of pyruvate metabolism, and excessive accumulation of acetaldehyde.

Improvement of inhibitor tolerance in Saccharomyces cerevisiae by overexpression of the quinone oxidoreductase family gene YCR102C

Budding yeast Saccharomyces cerevisiae is widely used for lignocellulosic biorefinery. However, its fermentation efficiency is challenged by various inhibitors (e.g. weak acids, furfural) in the lignocellulosic hydrolysate, and acetic acid is commonly present as a major inhibitor. The effects of oxidoreductases on the inhibitor tolerance of S. cerevisiae have mainly focused on furfural and vanillin, whereas the influence of quinone oxidoreductase on acetic acid tolerance is still unknown. In this study, we show that overexpression of a quinone oxidoreductase-encoding gene, YCR102C, in S. cerevisiae, significantly enhanced ethanol production under acetic acid stress as well as in the inhibitor mixture, and also improved resistance to simultaneous stress of 40°C and 3.6 g/L acetic acid. Increased catalase activities, NADH/NAD+ ratio and contents of several metals, especially potassium, were observed by YCR102C overexpression under acetic acid stress. To our knowledge, this is the first report that the quinone oxidoreductase family protein is related to acid stress tolerance. Our study provides a novel strategy to increase lignocellulosic biorefinery efficiency using yeast cell factory.

Evaluation of the Fermentative Capacity of Saccharomyces cerevisiae CAT-1 and BB9 Strains and Pichia kudriavzevii BB2 at Simulated Industrial Conditions

The search for promising yeasts that surpass the fermentative capacity of commercial strains, such as Saccharomyces cerevisiae CAT-1, is of great importance for industrial ethanol processes in the world. Two yeasts, Pichia kudriavzevii BB2 and Saccharomyces cerevisiae BB9, were evaluated in comparison to the industrial yeast S. cerevisiae CAT-1. The objective was to evaluate the performance profile of the three studied strains in terms of growth, substrate consumption, and metabolite formation, aiming to determine their behaviour in different media and pH conditions. The results showed that under cultivation conditions simulating the medium used in the industrial process (must at 22° Brix at pH 3.0) the highest ethanol productivity was 0.41 g L^-1 h^-1 for S. cerevisiae CAT-1, compared to 0.11 g L^-1 h^-1 and 0.16 g L^-1 h^-1 for P. kudriavzevii and S. cerevisiae BB2, respectively. S. cerevisiae CAT-1 produced three times more ethanol in must at pH 3.0 (28.30 g L^-1) and in mineral medium at pH 3.0 (29.17 g L^-1) and 5.0 (30.70 g L^-1) when compared to the value obtained in sugarcane must pH 3.0 (9.89 g L^-1). It was concluded that S. cerevisiae CAT-1 was not limited by the variation in pH in the mineral medium due to its nutritional composition, guaranteeing better performance of the yeast even in the presence of stressors. Only S. cerevisiae CAT-1 expressed he constitutive invertase enzyme, which is responsible for hydrolysing the sucrose contained in the must.

Reduction of Fermentation-Associated Stresses by Straw-Based Soluble Saccharides for Enhancing Ethanol Production

In this study, the effect of soluble polysaccharides (SPs) derived from agricultural waste, rice straw, on fermentation-associated stresses (temperature and concentrations of glucose and ethanol) was investigated to achieve high-performance ethanol production. The increase in temperature and concentrations of glucose and ethanol significantly inhibited Saccharomyces cerevisiae growth and lowered ethanol fermentation efficiency. Flow cytometric assays indicated that SPs could alleviate membrane permeability damage caused by fermentation-associated stresses. Atomic force microscopy and transmission electron microscopy revealed that fermentation-associated stresses induced cell surface shrinkage, causing a decrease in the cell size, whereas SPs stimulated the formation of extracellular matrices (EMs), which made the cell surface smooth and the cell morphology regular. Cells with EMs induced by SPs could efficiently produce ethanol under severe stresses. As a result, the titer of ethanol in the fermentation with SPs was 1.40-fold (from 26.40 to 36.98 g/L) higher than that in the fermentation without SPs, suggesting the stress-alleviating effect of SPs on ethanol production.

Assessment of microbial quality and health risks associated with traditional rice wine starter Xaj-pitha of Assam, India: a step towards defined and controlled fermentation

This study reports the microbial quality of ethnic starter culture Xaj-pitha used for rice wine fermentation in Assam. Here, we collected 60 Xaj-pitha samples belonging to Ahom community of the state and enumerated the microorganisms using spread plate technique. Illumina-based whole genome shotgun sequencing detected the presence of microbial contaminants like Acidovorax, Herbaspirillum, Methylobacterium, Pantoea, Pseudomonas, Stenotrophomonas, Staphylococcus, Micrococcus, Acinetobacter, etc. Presence of major health hazards associated with spontaneous rice wine fermentation necessitated method optimization through the development of a defined mixed starter culture. For this, functionally important α-amylase producers viz., Penicillium sp. ABTSJ23, Rhizopus oryzae ABTSJ63, Mucor guilliermondii ABTSJ72 and Amylomyces rouxii ABTSJ82 and eight yeasts viz., Saccharomyces cerevisiae ABTY1J, ABTY1S, ADJ5 & ADJ1, Wickerhamomyces anomalus ADJ2, Saccharomycopsis malanga ADJ3, Saccharomycopsis fibuligera ADJ4 and Saccharomycopsis malanga ADJ6 were retrieved using appropriate media. All the mould cultures tested negative for aflotoxins production. Among the yeasts, Saccharomyces cerevisiae ABTY1S and ADJ1 decarboxylated lysine HCl and tyramine HCl, respectively, indicating their biogenic amine production ability. For defined mixed starter culture, Amylomyces rouxii ABT82 with α-amylase (5.92 U/ml) and glucoamylase (7.50 U/ml) activities was selected as fungal partner; while Saccharomycopsis fibuligera ADJ4 and Saccharomyces cerevisiae ABT-Y1J with high ethanol production (up to 10.11% and 9.88% v/v, respectively) were selected as yeast partners. The mixed culture was able to produce high amount of glucose, ethanol and liquid (glucose 10.91% w/v; ethanol 7.5% w/v; liquid 51.0% w/v). Therefore, this study demonstrated the efficiency of mixed starter cultures for safe and controlled rice wine production.

Association of improved oxidative stress tolerance and alleviation of glucose repression with superior xylose-utilization capability by a natural isolate of Saccharomyces cerevisiae

BACKGROUND

Saccharomyces cerevisiae wild strains generally have poor xylose-utilization capability, which is a major barrier for efficient bioconversion of lignocellulosic biomass. Laboratory adaption is commonly used to enhance xylose utilization of recombinant S. cerevisiae. Apparently, yeast cells could remodel the metabolic network for xylose metabolism. However, it still remains unclear why natural isolates of S. cerevisiae poorly utilize xylose. Here, we analyzed a unique S. cerevisiae natural isolate YB-2625 which has superior xylose metabolism capability in the presence of mixed-sugar. Comparative transcriptomic analysis was performed using S. cerevisiae YB-2625 grown in a mixture of glucose and xylose, and the model yeast strain S288C served as a control. Global gene transcription was compared at both the early mixed-sugar utilization stage and the latter xylose-utilization stage.

RESULTS

Genes involved in endogenous xylose-assimilation (XYL2 and XKS1), gluconeogenesis, and TCA cycle showed higher transcription levels in S. cerevisiae YB-2625 at the xylose-utilization stage, when compared to the reference strain. On the other hand, transcription factor encoding genes involved in regulation of glucose repression (MIG1, MIG2, and MIG3) as well as HXK2 displayed decreased transcriptional levels in YB-2625, suggesting the alleviation of glucose repression of S. cerevisiae YB-2625. Notably, genes encoding antioxidant enzymes (CTT1, CTA1, SOD2, and PRX1) showed higher transcription levels in S. cerevisiae YB-2625 in the xylose-utilization stage than that of the reference strain. Consistently, catalase activity of YB-2625 was 1.9-fold higher than that of S. cerevisiae S288C during the xylose-utilization stage. As a result, intracellular reactive oxygen species levels of S. cerevisiae YB-2625 were 43.3 and 58.6% lower than that of S288C at both sugar utilization stages. Overexpression of CTT1 and PRX1 in the recombinant strain S. cerevisiae YRH396 deriving from S. cerevisiae YB-2625 increased cell growth when xylose was used as the sole carbon source, leading to 13.5 and 18.1%, respectively, more xylose consumption.

CONCLUSIONS

Enhanced oxidative stress tolerance and relief of glucose repression are proposed to be two major mechanisms for superior xylose utilization by S. cerevisiae YB-2625. The present study provides insights into the innate regulatory mechanisms underlying xylose utilization in wild-type S. cerevisiae, which benefits the rapid development of robust yeast strains for lignocellulosic biorefineries.

Deletion of acetate transporter gene ADY2 improved tolerance of Saccharomyces cerevisiae against multiple stresses and enhanced ethanol production in the presence of acetic acid

The aim of this work was to study the effects of deleting acetate transporter gene ADY2 on growth and fermentation of Saccharomyces cerevisiae in the presence of inhibitors. Comparative transcriptome analysis revealed that three genes encoding plasma membrane carboxylic acid transporters, especially ADY2, were significantly downregulated under the zinc sulfate addition condition in the presence of acetic acid stress, and the deletion of ADY2 improved growth of S. cerevisiae under acetic acid, ethanol and hydrogen peroxide stresses. Consistently, a concomitant increase in ethanol production by 14.7% in the presence of 3.6g/L acetic acid was observed in the ADY2 deletion mutant of S. cerevisiae BY4741. Decreased intracellular acetic acid, ROS accumulation, and plasma membrane permeability were observed in the ADY2 deletion mutant. These findings would be useful for developing robust yeast strains for efficient ethanol production.

Overproduction of cellulase by Trichoderma reesei RUT C30 through batch-feeding of synthesized low-cost sugar mixture

Cellulase is a prerequisite for the bioconversion of lignocellulosic biomass, but its high cost presents the biggest challenge. In this article, low-cost mixture was produced from glucose through the transglycosylation reaction catalyzed by β-glucosidase for cellulase overproduction by Trichodema reesei RUT C30. As a result, cellulase titer of 90.3FPU/mL, which was more than 10 folds of that achieved with lactose as inducer, was achieved at 144h. Meanwhile, cellulase productivity was drastically increased to 627.1FPU/L/h, at least 3-5 folds higher than previously reported by the fungal species. The crude enzyme was further tested by hydrolyzing NaOH-pretreated corn stover with 15% solid loading, and 96.6g/L glucose was released with 92.6% sugar yield at 96h and 44.8g/L ethanol was obtained.

Development of stress tolerant Saccharomyces cerevisiae strains by metabolic engineering: New aspects from cell flocculation and zinc supplementation

Budding yeast Saccharomyces cerevisiae is widely studied for the production of biofuels from lignocellulosic biomass. However, economic production is currently challenged by the repression of cell growth and compromised fermentation performance of S. cerevisiae strains in the presence of various environmental stresses, including toxic level of final products, inhibitory compounds released from the pretreatment of cellulosic feedstocks, high temperature, and so on. Therefore, it is important to improve stress tolerance of S. cerevisiae to these stressful conditions to achieve efficient and economic production. In this review, the latest advances on development of stress tolerant S. cerevisiae strains are summarized, with the emphasis on the impact of cell flocculation and zinc addition. It was found that cell flocculation affected ethanol tolerance and acetic acid tolerance of S. cerevisiae, and addition of zinc to a suitable level improved stress tolerance of yeast cells to ethanol, high temperature and acetic acid. Further studies on the underlying mechanisms by which cell flocculation and zinc status affect stress tolerance will not only enrich our knowledge on stress response and tolerance mechanisms of S. cerevisiae, but also provide novel metabolic engineering strategies to develop robust yeast strains for biofuels production.

Lactobacillus casei as a biocatalyst for biofuel production

Microbial fermentation of sugars from plant biomass to alcohols represents an alternative to petroleum-based fuels. The optimal biocatalyst for such fermentations needs to overcome hurdles such as high concentrations of alcohols and toxic compounds. Lactic acid bacteria, especially lactobacilli, have high innate alcohol tolerance and are remarkably adaptive to harsh environments. This study assessed the potential of five Lactobacillus casei strains as biocatalysts for alcohol production. L. casei 12A was selected based upon its innate alcohol tolerance, high transformation efficiency and ability to utilize plant-derived carbohydrates. A 12A derivative engineered to produce ethanol (L. casei E1) was compared to two other bacterial biocatalysts. Maximal growth rate, maximal optical density and ethanol production were determined under conditions similar to those present during alcohol production from lignocellulosic feedstocks. L. casei E1 exhibited higher innate alcohol tolerance, better growth in the presence of corn stover hydrolysate stressors, and resulted in higher ethanol yields.

Improved growth and ethanol fermentation of Saccharomyces cerevisiae in the presence of acetic acid by overexpression of SET5 and PPR1

To better understand the contribution of zinc-finger proteins to environmental stress tolerance, particularly inhibition from acetic acid, which is a potent inhibitor for cellulosic ethanol production by microbial fermentations, SET5 and PPR1 were overexpressed in Saccharomyces cerevisiae BY4741. With 5 g/L acetic acid addition, engineered strains BY4741/SET5 and BY4741/PPR1 showed improved growth and enhanced ethanol fermentation performance compared to that with the control strain. Similar results were also observed in ethanol production using corn stover hydrolysate. Further studies indicated that SET5 and PPR1 overexpression in S. cerevisiae significantly improved activities of antioxidant enzymes and ATP generation in the presence of acetic acid, and consequently decreased intracellular accumulation of reactive oxygen species (50.9 and 45.7%, respectively). These results revealed the novel functions of SET5 and PPR1 for the improvement of yeast acetic acid tolerance, and also implicated the involvement of these proteins in oxidative stress defense and energy metabolism in S. cerevisiae. This work also demonstrated that overexpression of SET5 and PPR1 would be a feasible strategy to increase cellulosic ethanol production efficiency.

Metabolomic analysis of acid stress response in Saccharomyces cerevisiae

Acid stress has been reported to inhibit cell growth and decrease productivity during bio-production processes. In this study, a metabolomics approach was conducted to understand the effect of lactic acid induced stress on metabolite pools in Saccharomyces cerevisiae. Cells were cultured with lactic acid as the acidulant, with or without initial pH control, i.e., at pH 6 or pH 2.5, respectively. Under conditions of low pH, lactic acid led to a decrease in the intracellular pH and specific growth rate; however, these parameters remained unaltered in the cultures with pH control. Capillary electrophoresis-mass spectrometry followed by a statistical principal component analysis was used to identify the metabolites and measure the increased concentrations of ATP, glutathione and proline during severe acid stress. Addition of proline to the acidified cultures improved the specific growth rates. We hypothesized that addition of proline protected the cells from acid stress by combating acid-induced oxidative stress. Lactic acid diffusion into the cell resulted in intracellular acidification, which elicited an oxidative stress response and resulted in increased glutathione levels.