Overexpression ofOLE1enhances stress tolerance and constitutively activates the MAPK HOG pathway inSaccharomyces cerevisiae

Mechanism and improvement of yeast tolerance to biomass-derived inhibitors: A review

Lignocellulosic biomass is regarded as a potentially valuable second-generation biorefinery feedstock. Yeast has the ability to metabolize this substrate and convert it into fuel ethanol and an array of other chemical products. Nevertheless, during the pretreatment of lignocellulosic biomass, inhibitors (furanaldehydes, carboxylic acids, phenolic compounds, etc.) are generated, which impede the growth and metabolic activities of yeast cells. Consequently, developing yeast strains with enhanced tolerance to these inhibitors is a crucial technological objective, as it can significantly enhance the efficiency of lignocellulosic biorefineries. This review provides a concise overview of the process of inhibitor generation and the detrimental effects of these inhibitors on yeast. It also summarizes the current state of research on the mechanisms of yeast tolerance to these inhibitors, focusing specifically on recent advances in enhancing yeast tolerance to these inhibitors by rational and non-rational strategies. Finally, it discusses the current challenges and future research directions.

An engineered Yarrowia lipolytica with rapid growth and efficient lipid utilization

Yarrowia lipolytica, a safe yeast, efficiently metabolizes lipids for the production of food additives and agricultural products. Boosting its growth and lipid utilization capabilities is crucial to enhancing the overall efficiency Y. lipolytica. Herein, an integrated strategy was implemented to enhance lipid uptake, accumulation and metabolism and systematically promote the growth and lipid utilization of the commonly used Y. lipolytica Po1f strain. The engineered strain had a specific growth rate of 0.32 h-1 and a lipid content of 67.66 % (g/g DCW), which were 54 % and 26 % greater than those of the original strain. β-Carotene was used to verify the production of lipophilic natural compounds, and the highest yield was obtained 48 h earlier using the engineered strain compared to the original strain when consuming same carbon source. These findings show promise in using engineered Y. lipolytica for rapid growth and improved lipid utilization to boost efficiency of lipophilic product production.

Membrane transport engineering for efficient yeast biomanufacturing

Yeast strains have been widely recognized as useful cell factories for biomanufacturing. To improve production efficiency, their biosynthetic pathways and regulatory strategies have been continuously optimized. However, commercial production using yeasts is still limited by low product yield and high production cost. Accumulating evidences have demonstrated the importance of metabolite transport processes in addressing these challenges. Engineering yeast membrane transporters for transporting precursors, substrates, intermediates, products and toxic inhibitors has been successful. In addition, membrane properties are also important for metabolite production. Here we propose membrane transport engineering (MTE) to integrate manipulation of both membrane transporters and membrane properties. We emphasize that systematic optimization of both transporters and membrane lipid bilayers benefits production efficiency. We also envision the potential of artificial intelligence and automation process in MTE for economic and sustainable bioproduction using yeast cell factories.

Alterations in Lipid Saturation Trigger Remodeling of the Outer Mitochondrial Membrane

Lipid saturation is a key determinant of membrane function and organelle health, with changes in saturation triggering adaptive quality control mechanisms to maintain membrane integrity. Among cellular membranes, the mitochondrial outer membrane (OMM) is an important interface for many cellular functions, but how lipid saturation impacts OMM function remains unclear. Here, we show that increased intracellular unsaturated fatty acids (UFAs) remodel the OMM by promoting the formation of multilamellar mitochondrial-derived compartments (MDCs), which sequester proteins and lipids from the OMM. These effects depend on the incorporation of UFAs into membrane phospholipids, suggesting that changes in membrane bilayer composition mediate this process. Furthermore, elevated UFAs impair the assembly of the OMM protein translocase (TOM) complex, with unassembled TOM components captured into MDCs. Collectively, these findings suggest that alterations in phospholipid saturation may destabilize OMM protein complexes and trigger an adaptive response to sequester excess membrane proteins through MDC formation.

Advances in stress-tolerance elements for microbial cell factories

Microorganisms, particularly extremophiles, have evolved multiple adaptation mechanisms to address diverse stress conditions during survival in unique environments. Their responses to environmental coercion decide not only survival in severe conditions but are also an essential factor determining bioproduction performance. The design of robust cell factories should take the balance of their growing and bioproduction into account. Thus, mining and redesigning stress-tolerance elements to optimize the performance of cell factories under various extreme conditions is necessary. Here, we reviewed several stress-tolerance elements, including acid-tolerant elements, saline-alkali-resistant elements, thermotolerant elements, antioxidant elements, and so on, providing potential materials for the construction of cell factories and the development of synthetic biology. Strategies for mining and redesigning stress-tolerance elements were also discussed. Moreover, several applications of stress-tolerance elements were provided, and perspectives and discussions for potential strategies for screening stress-tolerance elements were made.

Engineering industrial yeast for improved tolerance and robustness

As an important cell factory, industrial yeast has been widely used for the production of compounds ranging from bulk chemicals to complex natural products. However, various adverse conditions including toxic products, extreme pH, and hyperosmosis etc., severely restrict microbial growth and metabolic performance, limiting the fermentation efficiency and diminishing its competitiveness. Therefore, enhancing the tolerance and robustness of yeasts is critical to ensure reliable and sustainable production of metabolites in complex industrial production processes. In this review, we provide a comprehensive review of various strategies for improving the tolerance of yeast cells, including random mutagenesis, system metabolic engineering, and material-mediated immobilization cell technology. It is expected that this review will provide a new perspective to realize the response and intelligent regulation of yeast cells to environmental stresses.

Engineering transcriptional regulatory networks for improving second-generation fuel ethanol production in Saccharomyces cerevisiae

Presently, Saccharomyces cerevisiae demonstrates proficient co-fermentation of glucose and xylose, marking a significant advancement in second-generation fuel ethanol production. However, the presence of high concentrations of inhibitors in industrial lignocellulose hydrolysates and post-glucose effect caused by glucose consumption hinders severely impedes yeast robustness and xylose utilization for ethanol fermentation. Even worse, the antagonism between xylose utilization ability and strain robustness was observed, which proposes a difficult challenge in the production of second-generation fuel ethanol by S. cerevisiae. This review introduces the effect of engineering transcriptional regulatory networks on enhancing xylose utilization, improving strain robustness, alleviating antagonism between xylose utilization and strain robustness, and reducing post-glucose effect. Additionally, we provide an outlook on the developmental trends in this field, offering insights into future directions for increasing the production of second-generation fuel ethanol in S. cerevisiae.

Gene Editing of Candida glycerinogenes by Designed Toxin-Antitoxin Cassette

Candida glycerinogenes is an industrial yeast with excellent multistress resistance. However, due to the diploid genome and the lack of meiosis and screening markers, its molecular genetic operation is limited. Here, a gene editing system using the toxin-antitoxin pair relBE from the type II toxin-antitoxin system in Escherichia coli as a screening marker was constructed. The RelBE complex can specifically and effectively regulate cell growth and arrest through a conditionally controlled toxin RelE switch, thereby achieving the selection of positive recombinants. The constructed editing system achieved precise gene deletion, replacement, insertion, and gene episomal expression in C. glycerinogenes. Compared with the traditional amino acid deficiency complementation editing system, this editing system produced higher biomass and the gene deletion efficiency was increased by 3.5 times. Using this system, the production of 2-phenylethanol by C. glycerinogenes was increased by 11.5-13.5% through metabolic engineering and tolerance engineering strategies. These results suggest that the stable gene editing system based on toxin-antitoxin pairs can be used for gene editing of C. glycerinogenes to modify metabolic pathways and promote industrial applications. Therefore, the constructed gene editing system is expected to provide a promising strategy for polyploid industrial microorganisms lacking gene manipulation methods.

Effect of citric acid on cell membrane structure and function of Issatchenkia terricola WJL-G4

AIMS

This study aimed to investigate the changes of cell membrane structure and function of Issatchenkia terricola under citric acid by performing physiological analysis.

METHODS AND RESULTS

The membrane integrity, surface hydrophobicity, structure, fluidity, apoptosis, and fatty acid methyl esters composition of I. terricola WJL-G4 cells were determined by propidium iodide staining, microbial adhesion to hydrocarbon test, transmission electron microscopy analysis, fluorescence anisotropy, flow cytometry, and gas chromatography-mass, respectively. The results showed that with the increasing of citric acid concentrations, the cell vitality, membrane integrity, and fluidity of I. terricola reduced; meanwhile, apoptosis rate, membrane permeable, hydrophobicity, and ergosterol contents augmented significantly. Compared to control, the activities of Na+, K+-ATPase, and Ca2+, Mg2+-ATPase increased by 3.73-fold and 6.70-fold, respectively, when citric acid concentration increased to 20 g l-1. The cells cracked and their cytoplasm effused when the citric acid concentration reached 80 g l-1.

CONCLUSIONS

I. terricola could successfully adjust its membrane structure and function below 60 g l-1 of citric acid. However, for citric acid concentrations above 80 g l-1, its structure and function were dramatically changed, which might result in reduced functionality.

Strategies to increase the robustness of microbial cell factories

Engineering microbial cell factories have achieved much progress in producing fuels, natural products and bulk chemicals. However, in industrial fermentation, microbial cells often face various predictable and stochastic disturbances resulting from intermediate metabolites or end product toxicity, metabolic burden and harsh environment. These perturbances can potentially decrease productivity and titer. Therefore, strain robustness is essential to ensure reliable and sustainable production efficiency. In this review, the current strategies to improve host robustness were summarized, including knowledge-based engineering approaches, such as transcription factors, membrane/transporters and stress proteins, and the traditional adaptive laboratory evolution based on natural selection. Computation-assisted (e.g. GEMs, deep learning and machine learning) design of robust industrial hosts was also introduced. Furthermore, the challenges and future perspectives on engineering microbial host robustness are proposed to promote the development of green, efficient and sustainable biomanufacturers.

Regulatory mechanisms and cell membrane properties of Candida glycerinogenes differ under 2-phenylethanol addition or fermentation conditions

The biosynthesis of 2-phenylethanol (2-PE) at high yields and titers is often limited by its toxicity. In this study, we describe the molecular mechanisms of 2-PE tolerance in the multi-stress tolerant industrial yeast, Candida glycerinogenes. They were different under 2-PE addition or fermentation conditions. After extracellular addition of 2-PE, C. glycerinogenes cells became rounder and bigger, which reduced specific surface area. However, during 2-PE fermentation C. glycerinogenes cells were smaller, which increased specific surface area. Other differences in the tolerance mechanisms were studied by analyzing the composition and molecular parameters of the cell membrane. Extracellular 2-PE stress resulted in down-regulation of transcriptional expression of unsaturated fatty acid synthesis genes. This raised the proportion of saturated fatty acids in the cell membrane, which increased rigidity of the cell membrane and reduced 2-PE entry to the cell. However, intracellular 2-PE stress resulted in up-regulation of transcriptional expression of unsaturated fatty acid synthesis genes, and increased the proportion of unsaturated fatty acids in the cell membrane; this in turn enhanced flexibility of the cell membrane which accelerated efflux of 2-PE. These contrasting mechanisms are mediated by transcriptional factors Hog1 and Swi5. Under 2-PE addition, C. glycerinogenes activated Hog1 and repressed Swi5 to upregulate erg5 and erg4 expression, which increased cell membrane rigidity and resisted 2-PE import. During 2-PE fermentation, C. glycerinogenes activated Hog1 and repressed Swi5 to upregulate 2-PE transporter proteins cdr1 and Acyl-CoA desaturase 1 ole1 to increase 2-PE export, thus reducing 2-PE intracellular toxicity. The results provide new insights into 2-PE tolerance mechanisms at the cell membrane level and suggest a novel strategy to improve 2-PE production by engineering anti-stress genes.

Advance of tolerance engineering on microbes for industrial production

Industrial microbes have become the core of biological manufacturing, which utilized as the cell factory for production of plenty of chemicals, fuels and medicine. However, the challenge that the extreme stress conditions exist in production is unavoidable for cell factory. Consequently, to enhance robustness of the chassis cell lays the foundation for development of bio-manufacturing. Currently, the researches on cell tolerance covered various aspects, involving reshaping regulatory network, cell membrane modification and other stress response. In fact, the strategies employed to improve cell robustness could be summarized into two directions, irrational engineering and rational engineering. In this review, the metabolic engineering technologies on enhancement of microbe tolerance to industrial conditions are summarized. Meanwhile, the novel thoughts emerged with the development of biological instruments and synthetic biology are discussed.

Combining Metabolic Engineering and Lipid Droplet Storage Engineering for Improved α-Bisabolene Production in Yarrowia Lipolytica

Bisabolene is a bioactive sesquiterpene with a wide range of applications in food, cosmetics, medicine, and aviation fuels. Microbial production offers a green, efficient, and sustainable alternative. In this study, we focused on improving the titers of α-bisabolene in Yarrowia lipolytica by applying two strategies, (i) optimizing the metabolic flux of α-bisabolene biosynthetic pathway and (ii) sequestering α-bisabolene in lipid droplet, thus alleviating its inherent toxicity to host cells. We showed that overexpression of DGA1 and OLE1 to increase lipid content and unsaturated fatty acid levels was essential for boosting the α-bisabolene synthesis when supplemented with auxiliary carbon sources. The final engineered strain Po1gαB10 produced 1954.3 mg/L α-bisabolene from the waste cooking oil under shake flask fermentation, which was 96-fold higher than the control strain Po1gαB0. At the time of writing, our study represents the highest reported α-bisabolene titer in the engineered Y. lipolytica cell factory. This work describes novel strategies to improve the bioproduction of α-bisabolene that potentially may be applicable for other high-value terpene products.

Modification of Phosphorylation Sites in the Yeast Lysine Methyltransferase Set5 Exerts Influences on the Mitogen-Activated Protein Kinase Hog1 under Prolonged Acetic Acid Stress

Responses to acetic acid toxicity in the budding yeast Saccharomyces cerevisiae have widespread implications in the biorefinery of lignocellulosic biomass and food preservation. Our previous studies revealed that Set5, the yeast lysine methyltransferase and histone H4 methyltransferase, was involved in acetic acid stress tolerance. However, it is still mysterious how Set5 functions and interacts with the known stress signaling network. Here, we revealed that elevated phosphorylation of Set5 during acetic acid stress is accompanied by enhanced expression of the mitogen-activated protein kinase (MAPK) Hog1. Further experiments uncovered that the phosphomimetic mutation of Set5 endowed yeast cells with improved growth and fermentation performance and altered transcription of specific stress-responsive genes. Intriguingly, Set5 was found to bind the coding region of HOG1 and regulate its transcription, along with increased expression and phosphorylation of Hog1. A protein-protein interaction between Set5 and Hog1 was also revealed. In addition, modification of Set5 phosphosites was shown to regulate reactive oxygen species (ROS) accumulation, which is known to affect yeast acetic acid stress tolerance. The findings in this study imply that Set5 may function together with the central kinase Hog1 to coordinate cell growth and metabolism in response to stress. IMPORTANCE Hog1 is the yeast homolog of p38 MAPK in mammals that is conserved across eukaryotes, and it plays crucial roles in stress tolerance, fungal pathogenesis, and disease treatments. Here, we provide evidence that modification of Set5 phosphorylation sites regulates the expression and phosphorylation of Hog1, which expands current knowledge on upstream regulation of the Hog1 stress signaling network. Set5 and its homologous proteins are present in humans and various eukaryotes. The newly identified effects of Set5 phosphorylation site modifications in this study benefit an in-depth understanding of eukaryotic stress signaling, as well as the treatment of human diseases.

Proteolysis modification targeting protein corona affects ultrasound-induced membrane homeostasis of saccharomyces cerevisiae: Analysis of lipid relative contributions on membrane properties

Introduction

Protein corona (PCN) adsorbed on the surface of nanoparticles has brought new research perspectives for the interaction between nanoparticles and microorganisms. In this study, the responses of saccharomyces cerevisiae' membrane lipid composition, the average length of the fatty acyl chains and the average number of unsaturation of fatty acids to ultrasound combined with nano-Fe₃O₄@PCN with time-limited proteolysis (nano-Fe₃O₄@TLP-PCN) was investigated.

Methods

Lipidomic data was obtained using Ultra-high performance liquid chromatography coupled with a Q-Exactive plus mass spectrometer. The membrane potential, proton motive force assay and the membrane lipid oxidation were measured using Di-BAC₄(3), DISC₃(5) and C11-BODIPY^581/591 as the probes. Combined with the approach of feasible virtual samples generation, the back propagation artificial neural network (BP-ANN) model was adopted to establish the mapping relationship between lipids and membrane properties.

Results

The time-limited proteolysis targeting wheat PCN-coated Fe₃O₄ nanoparticles resulted in regular changes of hydrodynamic diameters, ζ-potentials, and surface hydrophobicity. In addition, with the prolongation of PCN proteolysis time, disturbances of 3 S.cerevisiae membrane characteristics, and membrane lipidomic remodeling in response to ultrasound+ nano-Fe₃O₄@PCN were observed. The analysis of relative importance which followed revealed that ergosterol, phosphatidylserine, and phosphatidylinositol phosphate had the greatest influence on membrane potential. For membrane lipid oxidation, ceramide, phosphatidylethanolamine, and sitosterol ester contribute 16.2, 14.9, and 13.1%, respectively. The relative contributions of six lysolecithins to the dissipation of proton motive force remained limited.

Discussion

An adaptation mechanism of cell membrane to proteolyzed PCN, wherein lipidome remodeling could preserved functional membrane phenotypes was revealed. Furthermore, it is highlighted that the relative importances of SiE, Cer, PE and PIP in determining membrane potential, PMF dissipation and membrane lipid oxidation by establishing FVSG-BP-ANN model.

High-temperature ethanol fermentation from pineapple waste hydrolysate and gene expression analysis of thermotolerant yeast Saccharomyces cerevisiae

High-temperature ethanol fermentation by thermotolerant yeast is considered a promising technology for ethanol production, especially in tropical and subtropical regions. In this study, optimization conditions for high-temperature ethanol fermentation of pineapple waste hydrolysate (PWH) using a newly isolated thermotolerant yeast, Saccharomyces cerevisiae HG1.1, and the expression of genes during ethanol fermentation at 40 °C were carried out. Three independent variables, including cell concentration, pH, and yeast extract, positively affected ethanol production from PWH at 40 °C. The optimum levels of these significant factors evaluated using response surface methodology (RSM) based on central composite design (CCD) were a cell concentration of 8.0 × 10⁷ cells/mL, a pH of 5.5, and a yeast extract concentration of 4.95 g/L, yielding a maximum ethanol concentration of 36.85 g/L and productivity of 3.07 g/L. Gene expression analysis during high-temperature ethanol fermentation using RT-qPCR revealed that the acquisition of thermotolerance ability and ethanol fermentation efficiency of S. cerevisiae HG1.1 are associated with genes responsible for growth and ethanol stress, oxidative stress, acetic acid stress, DNA repair, the pyruvate-to-tricarboxylic acid (TCA) pathway, and the pyruvate-to-ethanol pathway.

Response and regulatory mechanisms of heat resistance in pathogenic fungi

Abstract

Both the increasing environmental temperature in nature and the defensive body temperature response to pathogenic fungi during mammalian infection cause heat stress during the fungal existence, reproduction, and pathogenic infection. To adapt and respond to the changing environment, fungi initiate a series of actions through a perfect thermal response system, conservative signaling pathways, corresponding transcriptional regulatory system, corresponding physiological and biochemical processes, and phenotypic changes. However, until now, accurate response and regulatory mechanisms have remained a challenge. Additionally, at present, the latest research progress on the heat resistance mechanism of pathogenic fungi has not been summarized. In this review, recent research investigating temperature sensing, transcriptional regulation, and physiological, biochemical, and morphological responses of fungi in response to heat stress is discussed. Moreover, the specificity thermal adaptation mechanism of pathogenic fungi in vivo is highlighted. These data will provide valuable knowledge to further understand the fungal heat adaptation and response mechanism, especially in pathogenic heat-resistant fungi.

Key points

• Mechanisms of fungal perception of heat pressure are reviewed.

• The regulatory mechanism of fungal resistance to heat stress is discussed.

• The thermal adaptation mechanism of pathogenic fungi in the human body is highlighted.

Optimization of microbial cell factories for astaxanthin production: Biosynthesis and regulations, engineering strategies and fermentation optimization strategies

The global market demand for natural astaxanthin is rapidly increasing owing to its safety, the potential health benefits, and the diverse applications in food and pharmaceutical industries. The major native producers of natural astaxanthin on industrial scale are the alga Haematococcus pluvialis and the yeast Xanthopyllomyces dendrorhous. However, the natural production via these native producers is facing challenges of limited yield and high cost of cultivation and extraction. Alternatively, astaxanthin production via metabolically engineered non-native microbial cell factories such as Escherichia coli, Saccharomyces cerevisiae and Yarrowia lipolytica is another promising strategy to overcome these limitations. In this review we summarize the recent scientific and biotechnological progresses on astaxanthin biosynthetic pathways, transcriptional regulations, the interrelation with lipid metabolism, engineering strategies as well as fermentation process control in major native and non-native astaxanthin producers. These progresses illuminate the prospects of producing astaxanthin by microbial cell factories on industrial scale.

Octanoic Acid-An Insecticidal Metabolite of Conidiobolus coronatus (Entomopthorales) That Affects Two Majors Antifungal Protection Systems in Galleria mellonella (Lepidoptera): Cuticular Lipids and Hemocytes

The food flavour additive octanoic acid (C8:0) is also a metabolite of the entomopathogenic fungus Conidiobolus coronatus, which efficiently infects and rapidly kills Galleria mellonella. GC-MS analysis confirmed the presence of C8:0 in insecticidal fraction FR3 extracted from C. coronatus filtrate. Topical administration of C8:0 had a dose-dependent effect on survival rates of larvae but not on pupation or adult eclosion times of the survivors. Topically applied C8:0 was more toxic to adults than larvae (LD100 for adults 18.33 ± 2.49 vs. 33.56 ± 2.57 µg/mg of body mass for larvae). The administration of C8:0 on the cuticle of larvae and adults, in amounts corresponding to their LD50 and LD100 doses, had a considerable impact on the two main defense systems engaged in protecting against pathogens, causing serious changes in the developmental-stage-specific profiles of free fatty acids (FFAs) covering the cuticle of larvae and adults and damaging larval hemocytes. In vitro cultures of G. mellonella hemocytes, either directly treated with C8:0 or taken from C8:0 treated larvae, revealed deformation of hemocytes, disordered networking, late apoptosis, and necrosis, as well as caspase 1-9 activation and elevation of 8-OHdG level. C8:0 was also confirmed to have a cytotoxic effect on the SF-9 insect cell line, as determined by WST-1 and LDH tests.

Wheat Gluten Peptides Enhance Ethanol Stress Tolerance by Regulating the Membrane Lipid Composition in Yeast

Wheat gluten peptides (WGPs), identified as Leu-Leu (LL), Leu-Leu-Leu (LLL), and Leu-Met-Leu (LML), were tested for their impacts on cell growth, membrane lipid composition, and membrane homeostasis of yeast under ethanol stress. The results showed that WGP supplementation could strengthen cell growth and viability and enhance the ethanol stress tolerance of yeast. WGP supplementation increased the expressions of OLE1 and ERG1 and enhanced the levels of oleic acid (C18:1) and ergosterol in yeast cell membranes. Moreover, LLL and LML exhibited a better protective effect for yeast under ethanol stress compared to LL. LLL and LML supplementation led to 20.3 ± 1.5% and 18.9 ± 1.7% enhancement in cell membrane fluidity, 21.8 ± 1.6% and 30.5 ± 1.1% increase in membrane integrity, and 26.3 ± 4.8% and 27.6 ± 4.6% decrease in membrane permeability in yeast under ethanol stress, respectively. The results from scanning electron microscopy (SEM) elucidated that WGP supplementation is favorable for the maintenance of yeast cell morphology under ethanol stress. All of these results revealed that WGP is an efficient enhancer for improving the ethanol stress tolerance of yeast by regulating the membrane lipid composition.

Responses of Issatchenkia terricola WJL-G4 upon Citric Acid Stress

This study aimed to elucidate the responses of a novel characterized Issatchenkia terricola WJL-G4 against citric acid stress by performing physiological analysis, morphology observation, and structural and membrane fatty acid composition analysis. The results showed that under citric acid stress, the cell vitality of I. terricola WJL-G4 was reduced. The cell morphology changed with the unclear, uncompleted and thinner cell wall, and degraded the cell structure. When the citric acid concentration was 20 g/L, I. terricola WJL-G4 could tolerate citric acid and maintain the cell structure by increasing the intracellular pH, superoxide dismutase activity, and contents of unsaturated fatty acids. As the citric acid concentration was ≥80 g/L, the stress has exceeded the cellular anti-stress ability, causing substantial cell damage. The cell membrane permeability, the content of membrane lipids, malondialdehyde and superoxide anion increased, but the intracellular pH and superoxide dismutase activities decreased, accompanying the increase of citric acid concentrations. The findings of this work provided a theoretical basis for the responsive mechanism of I. terricola WJL-G4 under high concentrations of citric acid, and can serve as a reference for biological acid reduction in fruit processing.

Manipulating cell flocculation-associated protein kinases in Saccharomyces cerevisiae enables improved stress tolerance and efficient cellulosic ethanol production

Cell self-flocculation endows yeast strains with improved environmental stress tolerance that benefits bioproduction. Exploration of the metabolic and regulatory network differences between the flocculating and non-flocculating cells is conducive to developing strains with satisfactory fermentation efficiency. In this work, integrated analyses of transcriptome, proteome, and phosphoproteome were performed using flocculating yeast Saccharomyces cerevisiae SPSC01 and its non-flocculating mutant grown under acetic acid stress, and the results revealed prominent changes in protein kinases. Overexpressing the mitogen-activated protein kinase Hog1 upregulated by flocculation led to reduced ROS accumulation and increased glutathione peroxidase activity, leading to improved ethanol production under stress. Among the seven genes encoding protein kinases that were tested, AKL1 showed the best performance when overexpressed, achieving higher ethanol productivity in both corncob hydrolysate and simulated corn stover hydrolysate. These results provide alternative strategies for improving cellulosic ethanol production by engineering key protein kinases in S. cerevisiae.

Involvement of the High-Osmolarity Glycerol Pathway of Saccharomyces Cerevisiae in Protection against Copper Toxicity

Although essential for life, copper is also potentially toxic in concentrations that surpass physiological thresholds. The high-osmolarity glycerol pathway of yeast is the main regulator of adaptive responses and is known to play crucial roles in the responses to various stressors. The objective of this research is to determine whether the HOG pathway could be activated and to investigate the possible interplay of the HOG pathway and oxidative stress due to copper exposure. In this research, we demonstrate that copper could induce oxidative stress, including the elevated concentrations of reactive oxygen species (ROS) and malondialdehyde (MDA). Increased combination with GSH, increased intracellular SOD activity, and the up-regulation of relevant genes can help cells defend themselves against oxidative toxicity. The results show that copper treatment triggers marked and prolonged Hog1 phosphorylation. Significantly, oxidative stress generated by copper toxicity is essential for the activation of Hog1. Activated Hog1 is translocated to the nucleus to regulate the expressions of genes such as CTT1, GPD1, and HSP12, among others. Furthermore, copper exposure induced significant G1-phase cell cycle arrest, while Hog1 partially participated in the regulation of cell cycle progression. These novel findings reveal another role for Hog1 in the regulation of copper-induced cellular stress.

Strategies to increase tolerance and robustness of industrial microorganisms

The development of a cost-competitive bioprocess requires that the cell factory converts the feedstock into the product of interest at high rates and yields. However, microbial cell factories are exposed to a variety of different stresses during the fermentation process. These stresses can be derived from feedstocks, metabolism, or industrial production processes, limiting production capacity and diminishing competitiveness. Improving stress tolerance and robustness allows for more efficient production and ultimately makes a process more economically viable. This review summarises general trends and updates the most recent developments in technologies to improve the stress tolerance of microorganisms. We first look at evolutionary, systems biology and computational methods as examples of non-rational approaches. Then we review the (semi-)rational approaches of membrane and transcription factor engineering for improving tolerance phenotypes. We further discuss challenges and perspectives associated with these different approaches.

Engineering membrane asymmetry to increase medium-chain fatty acid tolerance in Saccharomyces cerevisiae

Saccharomyces cerevisiae is an attractive chassis for the production of medium-chain fatty acids, but the toxic effect of these compounds often prevents further improvements in titer, yield, and productivity. To address this issue, Lem3 and Sfk1 were identified from adaptive laboratory evolution mutant strains as membrane asymmetry regulators. Co-overexpression of Lem3 and Sfk1 [Lem3(M)-Sfk1(H) strain] through promoter engineering remodeled the membrane phospholipid distribution, leading to an increased accumulation of phosphatidylethanolamine in the inner leaflet of the plasma membrane. As a result, membrane potential and integrity were increased by 131.5% and 29.2%, respectively; meanwhile, the final OD₆₀₀ in the presence of hexanoic acid, octanoic acid, and decanoic acid was improved by 79.6%, 73.4%, and 57.7%, respectively. In summary, this study shows that membrane asymmetry engineering offers an efficient strategy to enhance medium-chain fatty acids tolerance in S. cerevisiae, thus generating a robust industrial strain for producing high-value biofuels.

Strain Degeneration in Pleurotus ostreatus: A Genotype Dependent Oxidative Stress Process Which Triggers Oxidative Stress, Cellular Detoxifying and Cell Wall Reshaping Genes

Strain degeneration has been defined as a decrease or loss in the yield of important commercial traits resulting from subsequent culture, which ultimately leads to Reactive Oxygen Species (ROS) production. Pleurotus ostreatus is a lignin-producing nematophagous edible mushroom. Mycelia for mushroom production are usually maintained in subsequent culture in solid media and frequently show symptoms of strain degeneration. The dikaryotic strain P. ostreatus (DkN001) has been used in our lab as a model organism for different purposes. Hence, different tools have been developed to uncover genetic and molecular aspects of this fungus. In this work, strain degeneration was studied in a full-sib monokaryotic progeny of the DkN001 strain with fast (F) and slow (S) growth rates by using different experimental approaches (light microscopy, malondialdehyde levels, whole-genome transcriptome analysis, and chitosan effect on monokaryotic mycelia). The results obtained showed that: (i) strain degeneration in P. ostreatus is linked to oxidative stress, (ii) the oxidative stress response in monokaryons is genotype dependent, (iii) stress and detoxifying genes are highly expressed in S monokaryons with symptoms of strain degeneration, (iv) chitosan addition to F and S monokaryons uncovered the constitutive expression of both oxidative stress and cellular detoxifying genes in S monokaryon strains which suggest their adaptation to oxidative stress, and (v) the overexpression of the cell wall genes, Uap1 and Cda1, in S monokaryons with strain degeneration phenotype indicates cell wall reshaping and the activation of High Osmolarity Glycerol (HOG) and Cell Wall Integrity (CWI) pathways. These results could constitute a hallmark for mushroom producers to distinguish strain degeneration in commercial mushrooms.

Incorporation of Exogenous Fatty Acids Enhances the Salt Tolerance of Food Yeast Zygosaccharomyces rouxii

Fatty acids have great effects on the maintenance of the cell membrane structure, cell viability, and cell metabolisms. In this study, we sought to elucidate the effects of exogenous fatty acids on the salt tolerance of food yeast Zygosaccharomyces rouxii. Results showed that Z. rouxii can grow by using exogenous fatty acids (C12:0, C14:0, C16:0, C16:1, C18:0, C18:1, and C18:2) as the sole carbon source. Four fatty acids (C12:0, C16:0, C16:1, and C18:1) can improve the salt tolerance of cells, enhance the formation of the cell biofilm, regulate the chemical compositions, restore growth in the presence of cerulenin, regulate the contents of membrane fatty acids, and control the expression of key genes in the fatty acid metabolism. Our results reveal that Z. rouxii can synthesize membrane fatty acids from exogenous fatty acids and the supplementation of these fatty acids can override the need for de novo fatty acid biosynthesis.

The Plasma Membrane at the Cornerstone Between Flexibility and Adaptability: Implications for Saccharomyces cerevisiae as a Cell Factory

In the last decade, microbial-based biotechnological processes are paving the way toward sustainability as they implemented the use of renewable feedstocks. Nonetheless, the viability and competitiveness of these processes are often limited due to harsh conditions such as: the presence of feedstock-derived inhibitors including weak acids, non-uniform nature of the substrates, osmotic pressure, high temperature, extreme pH. These factors are detrimental for microbial cell factories as a whole, but more specifically the impact on the cell’s membrane is often overlooked. The plasma membrane is a complex system involved in major biological processes, including establishing and maintaining transmembrane gradients, controlling uptake and secretion, intercellular and intracellular communication, cell to cell recognition and cell’s physical protection. Therefore, when designing strategies for the development of versatile, robust and efficient cell factories ready to tackle the harshness of industrial processes while delivering high values of yield, titer and productivity, the plasma membrane has to be considered. Plasma membrane composition comprises diverse macromolecules and it is not constant, as cells adapt it according to the surrounding environment. Remarkably, membrane-specific traits are emerging properties of the system and therefore it is not trivial to predict which membrane composition is advantageous under certain conditions. This review includes an overview of membrane engineering strategies applied to Saccharomyces cerevisiae to enhance its fitness under industrially relevant conditions as well as strategies to increase microbial production of the metabolites of interest.

A Gas Chromatography-Mass Spectrometry Method for the Determination of Fatty Acids and Sterols in Yeast and Grape Juice

Lipids are essential components of all living cells. In an oenological context, the supply of unsaturated lipids in grape juice allows the yeasts to grow and ferment, despite very low levels of oxygen. The current study proposes a systematic optimization procedure for the analysis of fatty acids and sterols relevant to the grape fermentation process, including both extracellular and intracellular (i.e., yeast cells) lipids. Even though it was extensive, the sample preparation yielded reproducible results for all compounds of interest. The stability of the analyzed compounds was also tested to offer some implementation flexibility for the extensive procedure. The performance parameters (i.e., selectivity, linearity, limit of detection and quantitation, accuracy, and precision) indicated that the method was suitable for future practical implementation. The proof of concept also suggests that the list of compounds of interest can be expanded if additional peaks are identified. Given the large variation in concentrations, the dilution of the matrix needs to be carefully considered in order to ensure that the lipids of interest are still within the dynamic range and not below the limit of detection and/or quantification.

The metabolism of lipids in yeasts and applications in oenology

Lipids are valuable compounds present in all living organisms, which display an array of functions related to compartmentalization, energy storage and enzyme activation. Furthermore, these compounds are an integral part of the plasma membrane which is responsible for maintaining structure, facilitating the transport of solutes in and out of the cell and cellular signalling necessary for cell survival. The lipid composition of the yeast Saccharomyces cerevisiae has been extensively investigated and the impact of lipids on S. cerevisiae cellular functions during wine alcoholic fermentation is well documented. Although other yeast species are currently used in various industries and are receiving increasing attention in winemaking, little is known about their lipid metabolism. This review article provides an extensive and critical evaluation of our knowledge on the biosynthesis, accumulation, metabolism and regulation of fatty acids and sterols in yeasts. The implications of the yeast lipid content on stress resistance as well as performance during alcoholic fermentation are discussed and a particular emphasis is given on non-Saccharomyces yeasts. Understanding lipid requirements and metabolism in non-Saccharomyces yeasts may lead to a better management of these yeast to enhance their contributions to wine properties.

Microbial physiological engineering increases the efficiency of microbial cell factories

Microbial cell factories provide vital platforms for the production of chemicals. Advanced biotechnological toolboxes have been developed to enhance their efficiency. However, these tools have limitations in improving physiological functions, and therefore boosting the efficiency (e.g. titer, rate, and yield) of microbial cell factories remains a challenge. In this review, we propose a strategy of microbial physiological engineering (MPE) to improve the efficiency of microbial cell factories. This strategy integrates tools from synthetic and systems biology to characterize and regulate physiological functions during chemical synthesis. MPE strategies mainly focus on the efficiency of substrate utilization, growth performance, stress tolerance, and the product export capacity of cell factories. In short, this review provides a new framework for resolving the bottlenecks that currently exist in low-efficiency cell factories.

Remodeling of the Histoplasma Capsulatum Membrane Induced by Monoclonal Antibodies

Antibodies play a central role in host immunity by directly inactivating or recognizing an invading pathogen to enhance different immune responses to combat the invader. However, the cellular responses of pathogens to the presence of antibodies are not well-characterized. Here, we used different mass spectrometry techniques to study the cellular responses of the pathogenic fungus Histoplasma capsulatum to monoclonal antibodies (mAb) against HSP60, the surface protein involved in infection. A proteomic analysis of H. capsulatum yeast cells revealed that mAb binding regulates a variety of metabolic and signaling pathways, including fatty acid metabolism, sterol metabolism, MAPK signaling and ubiquitin-mediated proteolysis. The regulation of the fatty acid metabolism was accompanied by increases in the level of polyunsaturated fatty acids, which further augmented the degree of unsaturated lipids in H. capsulatum's membranes and energy storage lipids, such as triacylglycerols, phosphatidylcholines, phosphatidylethanolamines and phosphatidylinositols. MAb treatment also regulated sterol metabolism by increasing the levels of cholesterol and ergosterol in the cells. We also showed that global changes in the lipid profiles resulted in an increased susceptibility of H. capsulatum to the ergosterol-targeting drug amphotericin B. Overall, our data showed that mAb induction of global changes in the composition of H. capsulatum membranes can potentially impact antifungal treatment during histoplasmosis.

High production of triterpenoids in Yarrowia lipolytica through manipulation of lipid components

BACKGROUND

Lupeol exhibits novel physiological and pharmacological activities, such as anticancer and immunity-enhancing activities. However, cytotoxicity remains a challenge for triterpenoid overproduction in microbial cell factories. As lipophilic and relatively small molecular compounds, triterpenes are generally secreted into the extracellular space. The effect of increasing triterpene efflux on the synthesis capacity remains unknown.

RESULTS

In this study, we developed a strategy to enhance triterpene efflux through manipulation of lipid components in Y. lipolytica by overexpressing the enzyme Δ9-fatty acid desaturase (OLE1) and disturbing phosphatidic acid phosphatase (PAH1) and diacylglycerol kinase (DGK1). By this strategy combined with two-phase fermentation, the highest lupeol production reported to date was achieved, where the titer in the organic phase reached 381.67 mg/L and the total production was 411.72 mg/L in shake flasks, exhibiting a 33.20-fold improvement over the initial strain. Lipid manipulation led to a twofold increase in the unsaturated fatty acid (UFA) content, up to 61-73%, and an exceptionally elongated cell morphology, which might have been caused by enhanced membrane phospholipid biosynthesis flux. Both phenotypes accelerated the export of toxic products to the extracellular space and ultimately stimulated the capacity for triterpenoid synthesis, which was proven by the 5.11-fold higher ratio of extra/intracellular lupeol concentrations, 2.79-fold higher biomass accumulation and 2.56-fold higher lupeol productivity per unit OD in the modified strains. This strategy was also highly efficient for the biosynthesis of other triterpenes and sesquiterpenes, including α-amyrin, β-amyrin, longifolene, longipinene and longicyclene.

CONCLUSIONS

In conclusion, we successfully created a high-yield lupeol-producing strain via lipid manipulation. We demonstrated that the enhancement of lupeol efflux and synthesis capacity was induced by the increased UFA content and elongated cell morphology. Our study provides a novel strategy to promote the biosynthesis of valuable but toxic products in microbial cell factories.

Engineering endogenous ABC transporter with improving ATP supply and membrane flexibility enhances the secretion of β-carotene in Saccharomyces cerevisiae

BACKGROUND

Product toxicity is one of the bottlenecks for microbial production of biofuels, and transporter-mediated biofuel secretion offers a promising strategy to solve this problem. As a robust microbial host for industrial-scale production of biofuels, Saccharomyces cerevisiae contains a powerful transport system to export a wide range of toxic compounds to sustain survival. The aim of this study is to improve the secretion and production of the hydrophobic product (β-carotene) by harnessing endogenous ABC transporters combined with physiological engineering in S. cerevisiae.

RESULTS

Substrate inducibility is a prominent characteristic of most endogenous transporters. Through comparative proteomic analysis and transcriptional confirmation, we identified five potential ABC transporters (Pdr5p, Pdr10p, Snq2p, Yor1p, and Yol075cp) for β-carotene efflux. The accumulation of β-carotene also affects cell physiology in various aspects, including energy metabolism, mitochondrial translation, lipid metabolism, ergosterol biosynthetic process, and cell wall synthesis. Here, we adopted an inducible GAL promoter to overexpress candidate transporters and enhanced the secretion and intracellular production of β-carotene, in which Snq2p showed the best performance (a 4.04-fold and a 1.33-fold increase compared with its parental strain YBX-01, respectively). To further promote efflux capacity, two strategies of increasing ATP supply and improving membrane fluidity were following adopted. A 5.80-fold increase of β-carotene secretion and a 1.71-fold increase of the intracellular β-carotene production were consequently achieved in the engineered strain YBX-20 compared with the parental strain YBX-01.

CONCLUSIONS

Overall, our results showcase that engineering endogenous plasma membrane ABC transporters is a promising approach for hydrophobic product efflux in S. cerevisiae. We also highlight the importance of improving cell physiology to enhance the efficiency of ABC transporters, especially energy status and cell membrane properties.

Membrane Fluidity of Saccharomyces cerevisiae from Huangjiu (Chinese Rice Wine) Is Variably Regulated by OLE1 To Offset the Disruptive Effect of Ethanol

An evolution and resequencing strategy was used to research the genetic basis of Saccharomyces cerevisiae BR20 (with 18 vol% ethanol tolerance) and the evolved strain F23 (with 25 vol% ethanol tolerance). Whole-genome sequencing and RNA sequencing (RNA-seq) indicated that the enhanced ethanol tolerance under 10 vol% ethanol could be attributed to amino acid metabolism, whereas 18 vol% ethanol tolerance was due to fatty acid metabolism. Ultrastructural analysis indicated that F23 exhibited better membrane integrity than did BR20 under ethanol stress. At low concentrations (<5 vol%), the partition of ethanol into the membrane increased the membrane fluidity, which had little effect on cell growth. However, the toxic effects of medium and high ethanol concentrations (5 to 20 vol%) tended to decrease the membrane fluidity. Under high ethanol stress (>10 vol%), the highly tolerant strain was able to maintain a relatively constant fluidity by increasing the content of unsaturated fatty acid (UFA), whereas less-tolerant strains show a continuous decrease in fluidity and UFA content. OLE1, which was identified as the only gene with a differential single-nucleotide polymorphism (SNP) mutation site related to fatty acid metabolism, was significantly changed in response to ethanol. The role of OLE1 in membrane fluidity was positively validated in its overexpressed transformants. Therefore, OLE1 lowered the rate of decline in membrane fluidity and thus enabled the yeast to better fight the deleterious effects of ethanol.IMPORTANCE Yeasts with superior ethanol tolerance are desirable for winemakers and wine industries. In our previous work, strain F23 was evolved with superior ethanol tolerance and fermentation activity to improve the flavor profiles of Chinese rice wine. Therefore, exploring the genomic variations and ethanol tolerance mechanism of strain F23 could contribute to an understanding of its effect on the flavor characteristics in the resulting Chinese rice wine. The cellular membrane plays a vital role in the ethanol tolerance of yeasts; however, how the membrane is regulated to fight the toxic effect of ethanol remains to be elucidated. This study suggests that the membrane fluidity is variably regulated by OLE1 to offset the disruptive effect of ethanol. Current work will help develop more ethanol-tolerant yeast strains for wine industries and contribute to a deep understanding of its high flavor-producing ability.

CRISPR/Cas-based screening of a gene activation library in Saccharomyces cerevisiae identifies a crucial role of OLE1 in thermotolerance

CRISPR/Cas-based (clustered regularly interspaced short palindromic repeats/CRISPR-associated) screening has been proved to be an efficient method to study functional genomics from yeast to human. In this study, we report the development of a focused CRISPR/Cas-based gene activation library in Saccharomyces cerevisiae and its application in gene identification based on functional screening towards improved thermotolerance. The gene activation library was subjected to screening at 42°C, and the same library cultured at 30°C was set as a control group. After five successive subcultures, five clones were randomly picked from the libraries cultured at 30 and 42°C, respectively. The five clones selected at 30°C contain the specificity sequences of five different single guide RNAs, whereas all the five clones selected at 42°C contain the specificity sequence of one sgRNA that targets the promoter region of OLE1. A crucial role of OLE1 in thermotolerance was identified: the overexpression of OLE1 increased fatty acid unsaturation, and thereby helped counter lipid peroxidation caused by heat stress, rendering the yeast thermotolerant. This study described the application of CRISPR/Cas-based gene activation screening with an example of thermotolerant yeast screening, demonstrating that this method can be used to identify functional genes in yeast.

Metabolic adaptability shifts of cell membrane fatty acids of Komagataeibacter hansenii HDM1-3 improve acid stress resistance and survival in acidic environments

Komagataeibacter hansenii HDM1-3 (K. hansenii HDM1-3) has been widely applied for producing bacterial cellulose (BC). The yield of BC has been frequently limited by the acidification during sugar metabolism, due to the generation of organic acids such as acetic acid. In this study, the acid resistance mechanism of K. hansenii HDM1-3 has been investigated from the aspect of metabolic adaptability of cell membrane fatty acids. Firstly, we observed that the survival rate of K. hansenii HDM1-3 was decreased with lowered pH values (adjusted with acetic acids), accompanied by increased leakage rate. Secondly, the cell membrane adaptability in response to acid stress was evaluated, including the variations of cell membrane fluidity and fatty acid composition. The proportion of unsaturated fatty acids was increased (especially, C18-1w9c and C19-Cyc), unsaturation degree and chain length of fatty acids were also increased. Thirdly, the potential molecular regulation mechanism was further elucidated. Under acid stress, the fatty acid synthesis pathway was involved in the structure and composition variations of fatty acids, which was proved by the activation of both fatty acid dehydrogenase (des) and cyclopropane fatty acid synthase (cfa) genes, as well as the addition of exogenous fatty acids. The fatty acid synthesis of K. hansenii HDM1-3 may be mediated by the activation of two-component sensor signaling pathways in response to the acid stress. The acid resistance mechanism of K. hansenii HDM1-3 adds to our knowledge of the acid stress adaptation, which may facilitate the development of new strategies for improving the industrial performance of this species under acid stress.

CgHog1-Mediated CgRds2 Phosphorylation Alters Glycerophospholipid Composition To Coordinate Osmotic Stress in Candida glabrata

Under stress conditions, Hog1 is required for cell survival through transiently phosphorylating downstream targets and reprogramming gene expression. Here, we report that Candida glabrata Hog1 (CgHog1) interacts with and phosphorylates CgRds2, a zinc cluster transcription factor, in response to osmotic stress. Additionally, we found that deletion of CgRDS2 led to decreases in cell growth and cell survival by 23.4% and 39.6%, respectively, at 1.5 M NaCl, compared with levels of the wild-type strain. This is attributed to significant downregulation of the expression levels of glycerophospholipid metabolism genes. As a result, the content of total glycerophospholipid decreased by 30.3%. Membrane integrity also decreased 47.6% in the Cgrds2Δ strain at 1.5 M NaCl. In contrast, overexpression of CgRDS2 increased the cell growth and cell survival by 10.2% and 6.3%, respectively, owing to a significant increase in the total glycerophospholipid content and increased membrane integrity by 27.2% and 12.1%, respectively, at 1.5 M NaCl, compared with levels for the wild-type strain. However, a strain in which the CgRDS2 gene encodes the replacement of Ser64 and Thr97 residues with alanines (Cgrds2^2A ), harboring a CgRds2 protein that was not phosphorylated by CgHog1, failed to promote glycerophospholipid metabolism and membrane integrity at 1.5 M NaCl. Thus, the above results demonstrate that CgHog1-mediated CgRds2 phosphorylation enhanced glycerophospholipid composition and membrane integrity to resist osmotic stress in C. glabrata IMPORTANCE This study explored the role of CgHog1-mediated CgRds2 phosphorylation in response to osmotic stress in Candida glabrata CgHog1 interacts with and phosphorylates CgRds2, a zinc cluster transcription factor, under osmotic stress. Phosphorylated CgRds2 plays an important role in increasing glycerophospholipid composition and membrane integrity, thereby enhancing cell growth and survival.

Efficient production of lycopene in Saccharomyces cerevisiae by enzyme engineering and increasing membrane flexibility and NAPDH production

Lycopene is a red carotenoid pigment with strong antioxidant activity. Saccharomyces cerevisiae is considered a promising host to produce lycopene, but lycopene toxicity is one of the limiting factors for high-level production. In this study, we used heterologous lycopene biosynthesis genes crtE and crtI from Xanthophyllomyces dendrorhous and crtB from Pantoea agglomerans for lycopene production in S. cerevisiae. The crtE, crtB, and crtI genes were integrated into the genome of S. cerevisiae CEN.PK2-1C strain, while deleting DPP1 and LPP1 genes to inhibit a competing pathway producing farnesol. Lycopene production was further improved by inhibiting ergosterol production via downregulation of ERG9 expression and by deleting ROX1 or MOT3 genes encoding transcriptional repressors for mevalonate and sterol biosynthetic pathways. To further increase lycopene production, CrtE and CrtB mutants with improved activities were isolated by directed evolution, and subsequently, the mutated genes were randomly integrated into the engineered lycopene-producing strains via delta-integration. To relieve lycopene toxicity by increasing unsaturated fatty acid content in cell membranes, the OLE1 gene encoding stearoyl-CoA 9-desaturase was overexpressed. In combination with the overexpression of STB5 gene encoding a transcription factor involved in NADPH production, the final strain produced up to 41.8 mg/gDCW of lycopene, which is approximately 74.6-fold higher than that produced in the initial strain.

Oleaginicity of the yeast strain Saccharomyces cerevisiae D5A

BACKGROUND

The model yeast, Saccharomyces cerevisiae, is not known to be oleaginous. However, an industrial wild-type strain, D5A, was shown to accumulate over 20% storage lipids from glucose when growth is nitrogen-limited compared to no more than 7% lipid accumulation without nitrogen stress.

METHODS AND RESULTS

To elucidate the mechanisms of S. cerevisiae D5A oleaginicity, we compared physiological and metabolic changes; as well as the transcriptional profiles of the oleaginous industrial strain, D5A, and a non-oleaginous laboratory strain, BY4741, under normal and nitrogen-limited conditions using analytic techniques and next-generation sequencing-based RNA-Seq transcriptomics. Transcriptional levels for genes associated with fatty acid biosynthesis, nitrogen metabolism, amino acid catabolism, as well as the pentose phosphate pathway and ethanol oxidation in central carbon (C) metabolism, were up-regulated in D5A during nitrogen deprivation. Despite increased carbon flux to lipids, most gene-encoding enzymes involved in triacylglycerol (TAG) assembly were expressed at similar levels regardless of the varying nitrogen concentrations in the growth media and strain backgrounds. Phospholipid turnover also contributed to TAG accumulation through increased precursor production with the down-regulation of subsequent phospholipid synthesis steps. Our results also demonstrated that nitrogen assimilation via the glutamate-glutamine pathway and amino acid metabolism, as well as the fluxes of carbon and reductants from central C metabolism, are integral to the general oleaginicity of D5A, which resulted in the enhanced lipid storage during nitrogen deprivation.

CONCLUSION

This work demonstrated the disequilibrium and rebalance of carbon and nitrogen contribution to the accumulation of lipids in the oleaginous yeast S. cerevisiae D5A. Rather than TAG assembly from acyl groups, the major switches for the enhanced lipid accumulation of D5A (i.e., fatty acid biosynthesis) are the increases of cytosolic pools of acetyl-CoA and NADPH, as well as alternative nitrogen assimilation.

Med15B Regulates Acid Stress Response and Tolerance in Candida glabrata by Altering Membrane Lipid Composition

Candida glabrata is a promising producer of organic acids. To elucidate the physiological function of the Mediator tail subunit Med15B in the response to low-pH stress, we constructed a deletion strain, C. glabratamed15BΔ, and an overexpression strain, C. glabrata HTUΔ/CgMED15B Deletion of MED15B caused biomass production, glucose consumption rate, and cell viability to decrease by 28.3%, 31.7%, and 26.5%, respectively, compared with those of the parent (HTUΔ) strain at pH 2.0. Expression of lipid metabolism-related genes was significantly downregulated in the med15BΔ strain, whereas key genes of ergosterol biosynthesis showed abnormal upregulation. This caused the proportion of C_18:1 fatty acids, the ratio of unsaturated to saturated fatty acids (UFA/SFA), and the total phospholipid content to decrease by 11.6%, 27.4%, and 37.6%, respectively. Cells failed to synthesize fecosterol and ergosterol, leading to the accumulation and a 60.3-fold increase in the concentration of zymosterol. Additionally, cells showed reductions of 69.2%, 11.6%, and 21.8% in membrane integrity, fluidity, and H⁺-ATPase activity, respectively. In contrast, overexpression of Med15B increased the C_18:1 levels, total phospholipids, ergosterol content, and UFA/SFA by 18.6%, 143.5%, 94.5%, and 18.7%, respectively. Membrane integrity, fluidity, and H⁺-ATPase activity also increased by 30.2%, 6.9%, and 51.8%, respectively. Furthermore, in the absence of pH buffering, dry weight of cells and pyruvate concentrations were 29.3% and 61.2% higher, respectively, than those of the parent strain. These results indicated that in C. glabrata, Med15B regulates tolerance toward low pH via transcriptional regulation of acid stress response genes and alteration in lipid composition.IMPORTANCE This study explored the role of the Mediator tail subunit Med15B in the metabolism of Candida glabrata under acidic conditions. Overexpression of MED15B enhanced yeast tolerance to low pH and improved biomass production, cell viability, and pyruvate yield. Membrane lipid composition data indicated that Med15B might play a critical role in membrane integrity, fluidity, and H⁺-ATPase activity homeostasis at low pH. Thus, controlling membrane composition may serve to increase C. glabrata productivity at low pH.

Engineering Saccharomyces cerevisiae fatty acid composition for increased tolerance to octanoic acid

Biorenewable chemicals such as short and medium chain fatty acids enable functional or direct substitution of petroleum-derived building blocks, allowing reduction of anthropogenic greenhouse gases while meeting market needs of high-demand products like aliphatic alcohols and alpha olefins. However, producing these fatty acids in microorganisms can be challenging due to toxicity issues. Octanoic acid (C8) can disrupt the integrity of the cell membrane in yeast, and exogenous supplementation of oleic acid has been shown to help alleviate this. We recently engineered the Saccharomyces cerevisiae enzyme acetyl-CoA carboxylase by replacing serine residue 1157 with alanine to prevent deactivation by phosphorylation. Expression of Acc1^S1157A in S. cerevisiae resulted in an increase in total fatty acid production, with the largest increase for oleic acid. In this study, we evaluated the effect of this modified lipid profile on C8 toxicity to the yeast. Expression of Acc1^S1157A in S. cerevisiae BY4741 increased the percentage of oleic acid 3.1- and 1.6-fold in the absence and presence of octanoic acid challenge, respectively. Following exposure to 0.9 mM of C8 for 24 h, the engineered yeast had a 10-fold higher cell density relative to the baseline strain. Moreover, overexpressing Acc1^S1157A allowed survival at C8 concentrations that were lethal for the baseline strain. This marked reduction of toxicity was shown to be due to higher membrane integrity as an 11-fold decrease in leakage of intracellular magnesium was observed. Due to the increase in oleic acid, this approach has the potential to reduce toxicity of other valuable bioproducts such as shorter chain aliphatic acids and alcohols and other membrane stressors. In an initial screen, increased resistance to n-butanol, 2-propanol, and hexanoic acid was demonstrated with cell densities 3.2-, 1.8-, and 29-fold higher than the baseline strain, respectively. Biotechnol. Bioeng. 2017;114: 1531-1538. © 2017 Wiley Periodicals, Inc.