Growth and ethanol fermentation ability on hexose and pentose sugars and glucose effect under various conditions in thermotolerant yeast Kluyveromyces marxianus

Molecular basis and functional development of membrane-based microbial metabolism

My research interest has so far been focused on metabolisms related to the "membrane" of microorganisms, such as the respiratory chain, membrane proteins, sugar uptake, membrane stress and cell lysis, and fermentation. These basic metabolisms are important for the growth and survival of cell, and their knowledge can be used for efficient production of useful materials. Notable achievements in research on metabolisms are elucidation of the structure and function of membrane-bound glucose dehydrogenase as a primary enzyme in the respiratory chain, elucidation of ingenious expression regulation of several operons or by divergent promoters, elucidation of stress-induced programed-cell lysis and its requirement for survival during a long-term stationary phase, elucidation of molecular mechanism of survival at a critical high temperature, elucidation of thermal adaptation and its limit, isolation of thermotolerant fermenting yeast strains, and development of high-temperature fermentation and green energy production technologies. These achievements are described together in this review.

Evolution and functional diversification of yeast sugar transporters

While simple sugars such as monosaccharides and disaccharide are the typical carbon source for most yeasts, whether a species can grow on a particular sugar is generally a consequence of presence or absence of a suitable transporter to enable its uptake. The most common transporters that mediate sugar import in yeasts belong to the major facilitator superfamily (MFS). Some of these, for example the Saccharomyces cerevisiae Hxt proteins have been extensively studied, but detailed information on many others is sparce. In part, this is because there are many lineages of MFS transporters that are either absent from, or poorly represented in, the model S. cerevisiae, which actually has quite a restricted substrate range. It is important to address this knowledge gap to gain better understanding of the evolution of yeasts and to take advantage of sugar transporters to exploit or engineer yeasts for biotechnological applications. This article examines the full repertoire of MFS proteins in representative budding yeasts (Saccharomycotina). A comprehensive analysis of 139 putative sugar transporters retrieved from 10 complete genomes sheds new light on the diversity and evolution of this family. Using the phylogenetic lens, it is apparent that proteins have often been misassigned putative functions and this can now be corrected. It is also often seen that patterns of expansion of particular genes reflects the differential importance of transport of specific sugars (and related molecules) in different yeasts, and this knowledge also provides an improved resource for the selection or design of tailored transporters.

Mechanistic Modelling of Biomass Growth, Glucose Consumption and Ethanol Production by Kluyveromyces marxianus in Batch Fermentation

This paper presents results concerning mechanistic modeling to describe the dynamics and interactions between biomass growth, glucose consumption and ethanol production in batch culture fermentation by Kluyveromyces marxianus (K. marxianus). The mathematical model was formulated based on the biological assumptions underlying each variable and is given by a set of three coupled nonlinear first-order Ordinary Differential Equations. The model has ten parameters, and their values were fitted from the experimental data of 17 K. marxianus strains by means of a computational algorithm design in Matlab. The latter allowed us to determine that seven of these parameters share the same value among all the strains, while three parameters concerning biomass maximum growth rate, and ethanol production due to biomass and glucose had specific values for each strain. These values are presented with their corresponding standard error and 95% confidence interval. The goodness of fit of our system was evaluated both qualitatively by in silico experimentation and quantitative by means of the coefficient of determination and the Akaike Information Criterion. Results regarding the fitting capabilities were compared with the classic model given by the logistic, Pirt, and Luedeking-Piret Equations. Further, nonlinear theories were applied to investigate local and global dynamics of the system, the Localization of Compact Invariant Sets Method was applied to determine the so-called localizing domain, i.e., lower and upper bounds for each variable; whilst Lyapunov's stability theories allowed to establish sufficient conditions to ensure asymptotic stability in the nonnegative octant, i.e., R+,03. Finally, the predictive ability of our mechanistic model was explored through several numerical simulations with expected results according to microbiology literature on batch fermentation.

Non-conventional yeast strains: Unexploited resources for effective commercialization of second generation bioethanol

The conventional yeast (Saccharomyces cerevisiae) is the most studied yeast and has been used in many important industrial productions, especially in bioethanol production from first generation feedstock (sugar and starchy biomass). However, for reduced cost and to avoid competition with food, second generation bioethanol, which is produced from lignocellulosic feedstock, is now being investigated. Production of second generation bioethanol involves pre-treatment and hydrolysis of lignocellulosic biomass to sugar monomers containing, amongst others, d-glucose and D-xylose. Intrinsically, S. cerevisiae strains lack the ability to ferment pentose sugars and genetic engineering of S. cerevisiae to inculcate the ability to ferment pentose sugars is ongoing to develop recombinant strains with the required stability and robustness for commercial second generation bioethanol production. Furthermore, pre-treatment of these lignocellulosic wastes leads to the release of inhibitory compounds which adversely affect the growth and fermentation by S. cerevisae. S. cerevisiae also lacks the ability to grow at high temperatures which favour Simultaneous Saccharification and Fermentation of substrates to bioethanol. There is, therefore, a need for robust yeast species which can co-ferment hexose and pentose sugars and can tolerate high temperatures and the inhibitory substances produced during pre-treatment and hydrolysis of lignocellulosic materials. Non-conventional yeast strains are potential solutions to these problems due to their abilities to ferment both hexose and pentose sugars, and tolerate high temperature and stress conditions encountered during ethanol production from lignocellulosic hydrolysate. This review highlights the limitations of the conventional yeast species and the potentials of non-conventional yeast strains in commercialization of second generation bioethanol.

Exploring Natural Fermented Foods as a Source for New Efficient Thermotolerant Yeasts for the Production of Second-Generation Bioethanol

Considering the cost-effectiveness of bioethanol production at high temperatures, there is an enduring need to find new thermotolerant ethanologenic yeasts. In this study, a total of eighteen thermotolerant yeasts were isolated from various natural fermented products in Morocco. Ethanol production using 50 g/L glucose or 50 g/L xylose as the sole carbon source revealed potential yeasts with high productivities and volumetric ethanol productivities at high temperatures. Based on molecular identification, the selected thermotolerant fermentative isolates were affiliated with Pichia kudriavzevii, Kluyveromyces marxianus, and Kluyveromyces sp. During the simultaneous saccharification and fermentation of lignocellulosic biomass at a high temperature (42 °C), the designated yeast P. kudriavzevii YSR7 produced an ethanol concentration of 22.36 g/L, 18.2 g/L and 6.34 g/L from 100 g/L barley straw (BS), chickpea straw (CS), and olive tree pruning (OTP), respectively. It also exhibited multi-stress tolerance, such as ethanol, acetic acid, and osmotic tolerance. Therefore, the yeast P. kudriavzevii YSR7 showed promising attributes for biorefinery-scale ethanol production in the future.

Utilization of Fruit Seed-Based Bioactive Compounds for Formulating the Nutraceuticals and Functional Food: A Review

Fruit seeds include a large number of bioactive substances with potential applications in the culinary and pharmaceutical industries, satisfying current demands for natural ingredients, which are generally preferred since they have fewer adverse effects than artificial components. Researchers have long been interested in the functional features, as well as the proximate and mineral compositions, of diverse fruit seeds such as tomato, apple, guava, and dates, among others. Bioactive components such as proteins (bioactive peptides), carotenoids (lycopene), polysaccharides (pectin), phytochemicals (flavonoids), and vitamins (-tocopherol) are abundant in fruit by-products and have significant health benefits, making them a viable alternative for the formulation of a wide range of food products with significant functional and nutraceutical potential. This article discusses the role and activities of bioactive chemicals found in tomato, apple, dates, and guava seeds, which can be used in a variety of food forms to cure a variety of cardiovascular and neurological disorders, as well as act as an antioxidant, anticancer, and antibacterial agent. The extraction of diverse bioactive components from by-products could pave the path for the creation of value-added products from the fruit industry, making it more commercially viable while also reducing environmental pollution caused by by-products from the fruit industry.

Evolutionary Adaptation by Repetitive Long-Term Cultivation with Gradual Increase in Temperature for Acquiring Multi-Stress Tolerance and High Ethanol Productivity in Kluyveromyces marxianus DMKU 3-1042

During ethanol fermentation, yeast cells are exposed to various stresses that have negative effects on cell growth, cell survival, and fermentation ability. This study, therefore, aims to develop Kluyveromyces marxianus-adapted strains that are multi-stress tolerant and to increase ethanol production at high temperatures through a novel evolutionary adaptation procedure. K. marxianus DMKU 3-1042 was subjected to repetitive long-term cultivation with gradual increases in temperature (RLCGT), which exposed cells to various stresses, including high temperatures. In each cultivation step, 1% of the previous culture was inoculated into a medium containing 1% yeast extract, 2% peptone, and 2% glucose, and cultivation was performed under a shaking condition. Four adapted strains showed increased tolerance to ethanol, furfural, hydroxymethylfurfural, and vanillin, and they also showed higher production of ethanol in a medium containing 16% glucose at high temperatures. One showed stronger ethanol tolerance. Others had similar phenotypes, including acetic acid tolerance, though genome analysis revealed that they had different mutations. Based on genome and transcriptome analyses, we discuss possible mechanisms of stress tolerance in adapted strains. All adapted strains gained a useful capacity for ethanol fermentation at high temperatures and improved tolerance to multi-stress. This suggests that RLCGT is a simple and efficient procedure for the development of robust strains.

Sustainable Use of Apple Pomace (AP) in Different Industrial Sectors

In many countries, apple pomace (AP) is one of the most produced types of agri-food waste (globally, it is produced at a rate of ~4 million tons/year). If not managed properly, such bio-organic waste can cause serious pollution of the natural environment and public health hazards, mainly due to the risk of microbial contamination. This review shows that AP can be successfully reused in different industrial sectors—for example, as a source of energy and bio-materials—according to the idea of sustainable development. The recovered active compounds from AP can be applied as preservatives, antioxidants, anti-corrosion agents, wood protectors or biopolymers. Raw or processed forms of AP can also be considered as feedstocks for various bioenergy applications such as the production of intermediate bioenergy carriers (e.g., biogas and pyrolysis oil), and materials (e.g., biochar and activated carbon). In the future, AP and its active ingredients can be of great use due to their non-toxicity, biodegradability and biocompatibility. Given the increasing mass of produced AP, the commercial applications of AP could have a huge economic impact in the future.

Distinct metabolic flow in response to temperature in thermotolerant Kluyveromyces marxianus

The intrinsic mechanism of the thermotolerance of Kluyveromyces marxianus was investigated by comparison of its physiological and metabolic properties at high and low temperatures. After glucose consumption, the conversion of ethanol to acetic acid became gradually prominent only at high temperature (45°C) and eventually caused a decline in viability, which was prevented by exogenous glutathione. Distinct levels of reactive oxygen species (ROS), glutathione, and NADPH suggest greater accumulation of ROS and enhanced ROS-scavenging activity at a high temperature. Fusion and fission forms of mitochondria were dominantly observed at 30°C and 45°C, respectively. Consistent results were obtained by temperature up-shift experiments including transcriptomic and enzymatic analyses, suggesting a change of metabolic flow from glycolysis to the pentose phosphate pathway. Results of this study suggest that K. marxianus survives at a high temperature by scavenging ROS via metabolic change for a period until a critical concentration of acetate is reached. IMPORTANCE Kluyveromyces marxianus, a thermotolerant yeast, can grow well at temperatures over 45°C, unlike Kluyveromyces lactis, which belongs to the same genus, or Saccharomyces cerevisiae, which is a closely related yeast. K. marxianus may thus bear an intrinsic mechanism to survive at high temperatures. This study revealed the thermotolerant mechanism of the yeast, including ROS scavenging with NADPH, which is generated by changes in metabolic flow.

Adaptive Laboratory Evolution for Multistress Tolerance, including Fermentability at High Glucose Concentrations in Thermotolerant Candida tropicalis

Candida tropicalis, a xylose-fermenting yeast, has the potential for converting cellulosic biomass to ethanol. Thermotolerant C. tropicalis X-17, which was isolated in Laos, was subjected to repetitive long-term cultivation with a gradual increase in temperature (RLCGT) in the presence of a high concentration of glucose, which exposed cells to various stresses in addition to the high concentration of glucose and high temperatures. The resultant adapted strain demonstrated increased tolerance to ethanol, furfural and hydroxymethylfurfural at high temperatures and displayed improvement in fermentation ability at high glucose concentrations and xylose-fermenting ability. Transcriptome analysis revealed the up-regulation of a gene for a glucose transporter of the major facilitator superfamily and genes for stress response and cell wall proteins. Additionally, hydropathy analysis revealed that three genes for putative membrane proteins with multiple membrane-spanning segments were also up-regulated. From these findings, it can be inferred that the up-regulation of genes, including the gene for a glucose transporter, is responsible for the phenotype of the adaptive strain. This study revealed part of the mechanisms of fermentability at high glucose concentrations in C. tropicalis and the results of this study suggest that RLCGT is an effective procedure for improving multistress tolerance.

Coordinately express hemicellulolytic enzymes in Kluyveromyces marxianus to improve the saccharification and ethanol production from corncobs

BACKGROUND

Hemicellulose acts as one factor contributing to the recalcitrance of lignocellulose that prevents cellulases to degrade the cellulose efficiently even in low quantities. Supplement of hemicellulases can enhance the performance of commercial cellulases in the enzymatic hydrolyses of lignocellulose. Kluyveromyce marxianus is an attractive yeast for cellulosic ethanol fermentation, as well as a promising host for heterologous protein production, since it has remarkable thermotolerance, high growth rate, and broad substrate spectrum etc. In this study, we attempted to coordinately express multiple hemicellulases in K. marxianus through a 2A-mediated ribosome skipping to self-cleave polyproteins, and investigated their capabilities for saccharification and ethanol production from corncobs.

RESULTS

Two polycistronic genes IMPX and IMPαX were constructed to test the self-cleavage of P2A sequence from the Foot-and-Mouth Disease virus (FMDV) in K. marxianus. The IMPX gene consisted of a β-mannanase gene M330 (without the stop codon), a P2A sequence and a β-xylanase gene Xyn-CDBFV in turn. In the IMPαX gene, there was an additional α-factor signal sequence in frame with the N-terminus of Xyn-CDBFV. The extracellular β-mannanase activities of the IMPX and IMPαX strains were 21.34 and 15.50 U/mL, respectively, but the extracellular β-xylanase activity of IMPαX strain was much higher than that of the IMPX strain, which was 136.17 and 42.07 U/mL, respectively. Subsequently, two recombinant strains, the IXPαR and IMPαXPαR, were constructed to coordinately and secretorily express two xylantic enzymes, Xyn-CDBFV and β-D-xylosidase RuXyn1, or three hemicellulolytic enzymes including M330, Xyn-CDBFV and RuXyn1. In fed-batch fermentation, extracellular activities of β-xylanase and β-xylosidase in the IXPαR strain were 1664.2 and 0.90 U/mL. Similarly, the IMPαXPαR strain secreted the three enzymes, β-mannanase, β-xylanase, and β-xylosidase, with the activities of 159.8, 2210.5, and 1.25 U/mL, respectively. Hemicellulolases of both strains enhanced the yields of glucose and xylose from diluted acid pretreated (DAP) corncobs when acted synergistically with commercial cellulases. In hybrid saccharification and fermentation (HSF) of DAP corncobs, hemicellulases of the IMPαXPαR strain increased the ethanol yield by 8.7% at 144 h compared with the control. However, both ethanol and xylose yields were increased by 12.7 and 18.2%, respectively, at 120 h in HSF of aqueous ammonia pretreated (AAP) corncobs with this strain. Our results indicated that coordinate expression of hemicellulolytic enzymes in K. marxianus promoted the saccharification and ethanol production from corncobs.

CONCLUSIONS

The FMDV P2A sequence showed high efficiency in self-cleavage of polyproteins in K. marxianus and could be used for secretory expression of multiple enzymes in the presence of their signal sequences. The IMPαXPαR strain coexpressed three hemicellulolytic enzymes improved the saccharification and ethanol production from corncobs, and could be used as a promising strain for ethanol production from lignocelluloses.

Characterization and optimization of the LAC4 upstream region for low-leakage expression in Kluyveromyces marxianus

Kluyveromyces marxianus is a promising host for the production of heterologous proteins, chemicals, and bioethanol. One superior feature of this species is its capacity to assimilate lactose, which is rendered by the LAC12-LAC4 gene pair encoding a lactose permease and a β-galactosidase enzyme. Little is known about the regulation of LAC4 in K. marxianus. In this study, we showed the presence of weak glucose repression in the regulation of LAC4 and that might contribute to the leaky expression of LAC4 in the glucose medium. In a mutagenesis screen of 1000-bp LAC4 upstream region, one mutant region, named H1, drove low-leakage expression of a URA3 reporter gene in glucose medium. Two mutations inside a polyadenosine stretch (poly(A)) of 5' UTR were major contributors to the low-leakage phenotype of H1. H1 directed low-leakage expression of GFP on a plasmid and that of LAC4 in situ in the glucose medium, which was not due to the reduction of mRNA levels. Meanwhile, H1 did not affect the induction of GFP or LAC4 by lactose. Cre recombinase expressed by H1 caused lower toxicity in the repressive condition and achieved higher yield after induction, compared with that expressed by a wild-type LAC4 upstream region or a strong INU1 promoter. Our study suggested that poly(A) inside 5' UTR played a role in regulating the expression of LAC4 in the repressive condition. Meanwhile, H1 provided a base for the development of a strict inducible system for expressing industrial proteins, especially toxic proteins.

Feeding of a Scleractinian Coral, Goniopora columna, on Microalgae, Yeast, and Artificial Feed in Captivity

Simple Summary

Coral aquaculture is an innovative and sustainable aquaculture industry. Coral husbandry can address ecological environment conservation needs and industrial demand for corals. Many previous studies have confirmed that corals also belong to heterotrophic organisms. Heterotrophic feeding is essential for overcoming nutrient deficiency. The preliminary results of this study indicate that Goniopora columna have high levels of proteases, and artificial feeds containing high protein can be used for feeding during aquaculture, which can increase the growth rate. In conclusion, we have initially explored that Goniopora columna will have better growth by feeding artificial PUFA rich in animal protein. In addition, the best feeding time is 6:00–12:00 in the morning, when there is better digestion and absorption. It is hoped that this research will be helpful to the development of coral aquaculture in the future.

Abstract

Nutritional requirements are critical in the process of coral aquaculture. In addition to energy from symbiotic algae, corals obtain sufficient nutrition through heterotrophic feeding. Microalgae and yeast are commonly used as nutritional supplements for many aquaculture organisms. In addition, if artificial feed can match or improve upon the nutritional supplementation provided by microalgae and yeast in the case of G. columna, then feeding this coral would be markedly easier. Hence, this article preliminarily discusses feeds suitable for G. columna. In this study, artificial PUFA rich in animal protein (R), Saccharomyces cerevisiae, Isochrysis galbana tml, and Nannochloropsis oculate were fed to G. columna at quantities of 5% and 10% of body weight. Growth, survival, body composition, and digestive enzymes were assessed. Regarding body composition, the coral’s protein content is higher than that of carbohydrate or fat; thus, evaluating the heterotrophic nutrition of G. columna by using protein absorption is appropriate. The protease content is also high in digestive enzymes. Protein content, protease activity, and specific growth rate were significantly higher in the R group than in other groups. The number of polyps in the groups fed R at 5% and 10% of body weight increased by 40.00 ± 2.43 and 47.33 ± 0.89 number, respectively, significantly greater increases than those achieved in the other groups (p < 0.05). Changes in body composition and digestive enzymes over a 24-h period were compared to determine the optimal feeding time. Protein content and protease activity increased markedly between 6:00 and 12:00. The experimental results suggest that R can improve the activity of G. columna digestive enzymes and their protein and lipid content in body tissue, shorten the cultivation time, and enhance the profitability of coral aquaculture.

Anoxia-induced mitophagy in the yeast Kluyveromyces marxianus

Kluyveromyces marxianus is a thermotolerant, ethanol-producing yeast that requires oxygen for efficient ethanol fermentation. Under anaerobic conditions, glucose consumption and ethanol production are retarded, suggesting that oxygen affects the metabolic state of K. marxianus. Mitochondria require oxygen to function, and their forms and number vary according to environmental conditions. In this study, the effect of anoxia on mitochondrial behavior in K. marxianus was examined. Under aerobic growth conditions, mitochondria-targeted GFP exhibited a tubular and dotted localization, representing a typical mitochondrial morphology, but under anaerobic conditions, GFP localized in vacuoles, suggesting that mitophagy occurs under anaerobic conditions. To confirm mitophagy induction, the ATG32, ATG8, ATG11 and ATG19 genes were disrupted. Vacuolar localization of mitochondria-targeted GFP under anaerobic conditions was interrupted in the Δatg32 and Δatg8 strains but not the Δatg11 and Δatg19 strains. Electron microscopy revealed mitochondria-like membrane components in the vacuoles of wild-type cells grown under anaerobic conditions. Quantitative analyses using mitochondria-targeted Pho8 demonstrated that mitophagy was induced in K. marxianus by anoxia but not nitrogen starvation. To the best of our knowledge, this is the first demonstration of anoxia-induced mitophagy in yeasts.

Influence of glucose on xylose metabolization by Spathaspora passalidarum

The yeast Spathaspora passalidarum is able to produce ethanol from D-xylose and D-glucose. However, it is not clear how xylose metabolism is affected by D-glucose when both sugars are available in the culture medium. The aims of this work were to evaluate the influence of D-glucose on D-xylose consumption, ethanol production, gene expression, and the activity of key xylose-metabolism enzymes under both aerobic and oxygen-limited conditions. Ethanol yields and productivities were increased in culture media containing D-xylose as the sole carbon source or a mixture of D-xylose and D-glucose. S. passalidarum preferentially consumed D-glucose in the co-fermentations, which is consistent with the reduction in expression of genes encoding the key xylose-metabolism enzymes. In the presence of D-glucose, the specific activities of xylose reductase (XR), xylitol dehydrogenase (XDH), and xylulokinase (XK) were lower. Interestingly, in accordance with other studies, the presence of 2-deoxyglucose (2DG) did not inhibit the growth of S. passalidarum in culture medium containing D-xylose as the sole carbon source. This indicates that a non-canonical repression pathway is acting in S. passalidarum. In conclusion, the results suggest that D-glucose inhibits D-xylose consumption and prevents the D-xylose-mediated induction of the genes encoding XR, XDH, and XK.

Process Intensification in Bio-Ethanol Production–Recent Developments in Membrane Separation

Ethanol is considered as a renewable transport fuels and demand is expected to grow. In this work, trends related to bio-ethanol production are described using Thailand as an example. Developments on high-temperature fermentation and membrane technologies are also explained. This study focuses on the application of membranes in ethanol recovery after fermentation. A preliminary simulation was performed to compare different process configurations to concentrate 10 wt% ethanol to 99.5 wt% using membranes. In addition to the significant energy reduction achieved by replacing azeotropic distillation with membrane dehydration, employing ethanol-selective membranes can further reduce energy demand. Silicalite membrane is a type of membrane showing one of the highest ethanol-selective permeation performances reported today. A silicalite membrane was applied to separate a bio-ethanol solution produced via high-temperature fermentation followed by a single distillation. The influence of contaminants in the bio-ethanol on the membrane properties and required further developments are also discussed.

Kluyveromyces marxianus: a potential biocatalyst of renewable chemicals and lignocellulosic ethanol production

Kluyveromyces marxianus is an ascomycetous yeast which has shown promising results in cellulosic ethanol and renewable chemicals production. It can survive on a variety of carbon sources under industrially favorable conditions due to its fast growth rate, thermotolerance, and acid tolerance. K. marxianus, is generally regarded as a safe (GRAS) microorganism, is widely recognized as a powerhouse for the production of heterologous proteins and is accepted by the US Food and Drug Administration (USFDA) for its pharmaceutical and food applications. Since lignocellulosic hydrolysates are comprised of diverse monomeric sugars, oligosaccharides and potential metabolism inhibiting compounds, this microorganism can play a pivotal role as it can grow on lignocellulosic hydrolysates coping with vegetal cell wall derived inhibitors. Furthermore, advancements in synthetic biology, for example CRISPR-Cas9 (clustered regularly interspaced short palindromic repeats with Cas9)-mediated genome editing, will enable development of an engineered yeast for the production of biochemicals and biopharmaceuticals having a myriad of industrial applications. Genetic engineering companies such as Cargill, Ginkgo Bioworks, DuPont, Global Yeast, Genomatica, and several others are actively working to develop designer yeasts. Given the important traits and properties of K. marxianus, these companies may find it to be a suitable biocatalyst for renewable chemicals and fuel production on the large scale. This paper reviews the recent progress made with K. marxianus biotechnology for sustainable production of ethanol, and other products utilizing lignocellulosic sugars.

Comparative Genomic and Transcriptomic Analysis Reveals Specific Features of Gene Regulation in Kluyveromyces marxianus

Kluyveromyces marxianus is a promising host for producing bioethanol and heterologous proteins. It displays many superior traits to a conventional industrial yeast species, Saccharomyces cerevisiae, including fast growth, thermotolerance and the capacity to assimilate a wider variety of sugars. However, little is known about the mechanisms underlying the fast-growing feature of K. marxianus. In this study, we performed a comparative genomic analysis between K. marxianus and other Saccharomycetaceae species. Genes involved in flocculation, iron transport, and biotin biosynthesis have particularly high copies in K. marxianus. In addition, 60 K. marxianus specific genes were identified, 45% of which were upregulated during cultivation in rich medium and these genes may participate in glucose transport and mitochondrion related functions. Furthermore, the transcriptomic analysis revealed that under aerobic condition, normalized levels of genes participating in TCA cycles, respiration chain and ATP biosynthesis in the lag phase were higher in K. marxianus than those in S. cerevisiae. Levels of highly copied genes, genes involved in the respiratory chain and mitochondrion assembly, were upregulated in K. marxianus, but not in S. cerevisiae, in later time points during cultivation compared with those in the lag phase. Notably, during the fast-growing phase, genes involved in the respiratory chain, ATP synthesis and glucose transport were co-upregulated in K. marxianus. A few shared motifs in upstream sequences of relevant genes might result in the co-upregulation. Specific features in the co-regulations of gene expressions might contribute to the fast-growing phenotype of K. marxianus. Our study underscores the importance of genome-wide rewiring of the transcriptional network during evolution.

Construction of engineered RuBisCO Kluyveromyces marxianus for a dual microbial bioethanol production system

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) genes play important roles in CO₂ fixation and redox balancing in photosynthetic bacteria. In the present study, the kefir yeast Kluyveromyces marxianus 4G5 was used as host for the transformation of form I and form II RubisCO genes derived from the nonsulfur purple bacterium Rhodopseudomonas palustris using the Promoter-based Gene Assembly and Simultaneous Overexpression (PGASO) method. Hungateiclostridium thermocellum ATCC 27405, a well-known bacterium for its efficient solubilization of recalcitrant lignocellulosic biomass, was used to degrade Napier grass and rice straw to generate soluble fermentable sugars. The resultant Napier grass and rice straw broths were used as growth media for the engineered K. marxianus. In the dual microbial system, H. thermocellum degraded the biomass feedstock to produce both C₅ and C₆ sugars. As the bacterium only used hexose sugars, the remaining pentose sugars could be metabolized by K. marxianus to produce ethanol. The transformant RubisCO K. marxianus strains grew well in hydrolyzed Napier grass and rice straw broths and produced bioethanol more efficiently than the wild type. Therefore, these engineered K. marxianus strains could be used with H. thermocellum in a bacterium-yeast coculture system for ethanol production directly from biomass feedstocks.

The identification of novel promoters and terminators for protein expression and metabolic engineering applications in Kluyveromyces marxianus

The K. marxianus has emerged as a potential yeast strain for various biotechnological applications. However, the limited number of available genetic tools has hindered the widespread usage of this yeast. In the current study we have expanded the molecular tool box by identifying novel sets of promoters and terminators for increased recombinant protein expression in K. marxianus. The previously available transcriptomic data were analyzed to identify top 10 promoters of highest gene expression activity. We further characterized and compared strength of these identified promoters using eGFP as a reporter protein, at different temperatures and carbon sources. To examine the regulatory region driving protein expression, serially truncated shorter versions of two selected strong promoters were designed, and examined for their ability to drive eGFP protein expression. The activities of these two promoters were further enhanced using different combinations of native transcription terminators of K. marxianus. We further utilized the identified DNA cassette encoding strong promoter in metabolic engineering of K. marxianus for enhanced β-galactosidase activity. The present study thus provides novel sets of promoters and terminators as well as engineered K. marxianus strain for its wider utility in applications requiring lactose degradation such as in cheese whey and milk.

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Novel promoters and terminators for constitutive gene expression in K. marxianus.

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The promoters show constitutive expression at varying temperature and carbon source.

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K. marxianus strain with improved production of β-galactosidase.

Pentose metabolism and conversion to biofuels and high-value chemicals in yeasts

Pentose sugars are widespread in nature and two of them, D-xylose and L-arabinose belong to the most abundant sugars being the second and third by abundance sugars in dry plant biomass (lignocellulose) and in general on planet. Therefore, it is not surprising that metabolism and bioconversion of these pentoses attract much attention. Several different pathways of D-xylose and L-arabinose catabolism in bacteria and yeasts are known. There are even more common and really ubiquitous though not so abundant pentoses, D-ribose and 2-deoxy-D-ribose, the constituents of all living cells. Thus, ribose metabolism is example of endogenous metabolism whereas metabolism of other pentoses, including xylose and L-arabinose, represents examples of the metabolism of foreign exogenous compounds which normally are not constituents of yeast cells. As a rule, pentose degradation by the wild-type strains of microorganisms does not lead to accumulation of high amounts of valuable substances; however, productive strains have been obtained by random selection and metabolic engineering. There are numerous reviews on xylose and (less) L-arabinose metabolism and conversion to high value substances; however, they mostly are devoted to bacteria or the yeast Saccharomyces cerevisiae. This review is devoted to reviewing pentose metabolism and bioconversion mostly in non-conventional yeasts, which naturally metabolize xylose. Pentose metabolism in the recombinant strains of S. cerevisiae is also considered for comparison. The available data on ribose, xylose, L-arabinose transport, metabolism, regulation of these processes, interaction with glucose catabolism and construction of the productive strains of high-value chemicals or pentose (ribose) itself are described. In addition, genome studies of the natural xylose metabolizing yeasts and available tools for their molecular research are reviewed. Metabolism of other pentoses (2-deoxyribose, D-arabinose, lyxose) is briefly reviewed.

Developing a broad-range promoter set for metabolic engineering in the thermotolerant yeast Kluyveromyces marxianus

Kluyveromyces marxianus is an emerging host for metabolic engineering. This thermotolerant yeast is the fastest growing eukaryote, has high flux through the TCA cycle, and can metabolize a broad range of C5, C6, and C12 carbon sources. In comparison to the common host Saccharomyces cerevisiae, this non-conventional yeast suffers from a lack of metabolic engineering tools to control gene expression over a wide transcriptional range. To address this issue, we designed a library of 25 native-derived promoters from K. marxanius CBS6556 that spans 87-fold transcriptional strength under glucose metabolism. Six promoters from the library were further characterized in both glucose and xylose as well as across various temperatures from 30 to 45 ​°C. The temperature study revealed that in most cases EGFP expression decreased with elevating temperature; however, two promoters, P _SSA3 and P _ADH1 , increased expression above 40 ​°C in both xylose and glucose. The six-promoter set was also validated in xylose for triacetic acid lactone (TAL) production. By controlling the expression level of heterologous 2-pyrone synthase (2-PS), the specific TAL titer increased over 8-fold at 37 ​°C. Cultures at 41 ​°C exhibited a similar TAL biosynthesis capability, while at 30 ​°C TAL levels were lower. Taken together, these results advance the metabolic engineering tool set in K. marxianus and further develop this new host for chemical biosynthesis.

Kluyveromyces marxianus: An emerging yeast cell factory for applications in food and biotechnology

Several yeasts, which are eukaryotic microorganisms, have long been used in different industries due to their potential applications, both for fermentation and for the production of specific metabolites. Kluyveromyces marxianus is one of the most auspicious nonconventional yeasts, generally isolated from wide-ranging natural habitats such as fermented traditional dairy products, kefir grain, sewage from sugar industries, sisal leaves, and plants. This is a food-grade yeast with various beneficial traits, such as rapid growth rate and thermotolerance that make it appealing for different industrial food and biotechnological applications. K. marxianus is a respiro-fermentative yeast likely to produce energy by either respiration or fermentation pathways. It generates a wide-ranging specific metabolites and could contribute to a variety of different food and biotechnological industries. Although Saccharomyces cerevisiae is the most widely used dominant representative in all aspects, many applications of K. marxianus in biotechnology, food and environment have only started to emerge nowadays; some of the most promising applications are reviewed here. The general physiology of K. marxianus is outlined, and then the different applications are discussed: first, the applications of K. marxianus in biotechnology, and then the recent advances and possible applications in food, feed and environmental industries. Finally, this review provides a discussion of the main challenges and some perspectives for targeted applications of K. marxianus in the modern food technology and applied biotechnology in order to exploit the full potential of this yeast which can be used as a cell factory with great efficiency.

Valorization of apple pomace using bio-based technology for the production of xylitol and 2G ethanol

Apple pomace was studied as a raw material for the production of xylitol and 2G ethanol, since this agroindustrial residue has a high concentration of carbohydrate macromolecules, but is still poorly studied for the production of fermentation bioproducts, such as polyols. The dry biomass was subjected to dilute-acid hydrolysis with H₂SO₄ to obtain the hemicellulosic hydrolysate, which was concentrated, detoxified and fermented. The hydrolyzate after characterization was submitted to submerged fermentations, which were carried out in Erlenmeyer flasks using, separately, the yeasts Candida guilliermondii and Kluyveromyces marxianus. High cellulose (32.62%) and hemicellulose (23.60%) contents were found in this biomass, and the chemical hydrolysis yielded appreciable quantities of fermentable sugars, especially xylose. Both yeasts were able to metabolize xylose, but Candida guilliermondii produced only xylitol (9.35 g L^-1 in 96 h), while K. marxianus produced ethanol as the main product (10.47 g L^-1 in 24 h) and xylitol as byproduct (9.10 g L^-1 xylitol in 96 h). Maximum activities of xylose reductase and xylitol dehydrogenase were verified after 24 h of fermentation with C. guilliermondii (0.23 and 0.53 U/mg_prot, respectively) and with K. marxianus (0.08 e 0.08 U/mg_prot, respectively). Apple pomace has shown potential as a raw material for the fermentation process, and the development of a biotechnological platform for the integrated use of both the hemicellulosic and cellulosic fraction could add value to this residue and the apple production chain.

Integration of comprehensive data and biotechnological tools for industrial applications of Kluyveromyces marxianus

Among the so-called non-conventional yeasts, Kluyveromyces marxianus has extremely potent traits that are suitable for industrial applications. Indeed, it has been used for the production of various enzymes, chemicals, and macromolecules in addition to utilization of cell biomass as nutritional materials, feed and probiotics. The yeast is expected to be an efficient ethanol producer with advantages over Saccharomyces cerevisiae in terms of high growth rate, thermotolerance and a wide sugar assimilation spectrum. Results of comprehensive analyses of its genome and transcriptome may accelerate studies for applications of the yeast and may further increase its potential by combination with recent biotechnological tools including the CRISPR/Cas9 system. We thus review published studies by merging with information obtained from comprehensive data including genomic and transcriptomic data, which would be useful for future applications of K. marxianus.

MIG1 as a positive regulator for the histidine biosynthesis pathway and as a global regulator in thermotolerant yeast Kluyveromyces marxianus

Kmmig1 as a disrupted mutant of MIG1 encoding a regulator for glucose repression in Kluyveromyces marxianus exhibits a histidine-auxotrophic phenotype. Genome-wide expression analysis revealed that only HIS4 in seven HIS genes for histidine biosynthesis was down-regulated in Kmmig1. Consistently, introduction of HIS4 into Kmmig1 suppressed the requirement of histidine. Considering the fact that His4 catalyzes four of ten steps in histidine biosynthesis, K. marxianus has evolved a novel and effective regulation mechanism via Mig1 for the control of histidine biosynthesis. Moreover, RNA-Seq analysis revealed that there were more than 1,000 differentially expressed genes in Kmmig1, suggesting that Mig1 is directly or indirectly involved in the regulation of their expression as a global regulator.

Removal of aromatic inhibitors produced from lignocellulosic hydrolysates by Acinetobacter baylyi ADP1 with formation of ethanol by Kluyveromyces marxianus

BACKGROUND

Lignocellulosic biomass is an attractive, inexpensive source of potentially fermentable sugars. However, hydrolysis of lignocellulose results in a complex mixture containing microbial inhibitors at variable composition. A single microbial species is unable to detoxify or even tolerate these non-sugar components while converting the sugar mixtures effectively to a product of interest. Often multiple substrates are metabolized sequentially because of microbial regulatory mechanisms. To overcome these problems, we engineered strains of Acinetobacter baylyi ADP1 to comprise a consortium able to degrade benzoate and 4-hydroxybenzoate simultaneously under batch and continuous conditions in the presence of sugars. We furthermore used a thermotolerant yeast, Kluyveromyces marxianus, to convert the glucose remaining after detoxification to ethanol.

RESULTS

The two engineered strains, one unable to metabolize benzoate and another unable to metabolize 4-hydroxybenzoate, when grown together removed these two inhibitors simultaneously under batch conditions. Under continuous conditions, a single strain with a deletion in the gcd gene metabolized both inhibitors in the presence of sugars. After this batch detoxification using ADP1-derived mutants, K. marxianus generated 36.6 g/L ethanol.

CONCLUSIONS

We demonstrated approaches for the simultaneous removal of two aromatic inhibitors from a simulated lignocellulosic hydrolysate. A two-stage batch process converted the residual sugar into a non-growth-associated product, ethanol. Such a two-stage process with bacteria (A. baylyi) and yeast (K. marxianus) is advantageous, because the yeast fermentation occurs at a higher temperature which prevents growth and ethanol consumption of A. baylyi. Conceptually, the process can be extended to other inhibitors or sugars found in real hydrolysates. That is, additional strains which degrade components of lignocellulosic hydrolysates could be made substrate-selective and targeted for use with specific complex mixtures found in a hydrolysate.

Increased flux in acetyl-CoA synthetic pathway and TCA cycle of Kluyveromyces marxianus under respiratory conditions

Yeasts are extremely useful, not only for fermentation but also for a wide spectrum of fuel and chemical productions. We analyzed the overall metabolic turnover and transcript dynamics in glycolysis and the TCA cycle, revealing the difference in adaptive pyruvate metabolic response between a Crabtree-negative species, Kluyveromyces marxianus, and a Crabtree-positive species, Saccharomyces cerevisiae, during aerobic growth. Pyruvate metabolism was inclined toward ethanol production under aerobic conditions in S. cerevisiae, while increased transcript abundances of the genes involved in ethanol metabolism and those encoding pyruvate dehydrogenase were seen in K. marxianus, indicating the augmentation of acetyl-CoA synthesis. Furthermore, different metabolic turnover in the TCA cycle was observed in the two species: malate and fumarate production in S. cerevisiae was higher than in K. marxianus, irrespective of aeration; however, fluxes of both the reductive and oxidative TCA cycles were enhanced in K. marxianus by aeration, implying both the cycles contribute to efficient electron flux without producing ethanol. Additionally, decreased hexokinase activity under aerobic conditions is expected to be important for maintenance of suitable carbon flux. These findings demonstrate differences in the key metabolic trait of yeasts employing respiration or fermentation, and provide important insight into the metabolic engineering of yeasts.

Release of glucose repression on xylose utilization in Kluyveromyces marxianus to enhance glucose-xylose co-utilization and xylitol production from corncob hydrolysate

BACKGROUND

Lignocellulosic biomass is one of the most abundant materials for biochemicals production. However, efficient co-utilization of glucose and xylose from the lignocellulosic biomass is a challenge due to the glucose repression in microorganisms. Kluyveromyces marxianus is a thermotolerant and efficient xylose-utilizing yeast. To realize the glucose-xylose co-utilization, analyzing the glucose repression of xylose utilization in K. marxianus is necessary. In addition, a glucose-xylose co-utilization platform strain will facilitate the construction of lignocellulosic biomass-utilizing strains.

RESULTS

Through gene disruption, hexokinase 1 (KmHXK1) and sucrose non-fermenting 1 (KmSNF1) were proved to be involved in the glucose repression of xylose utilization while disruption of the downstream genes of cyclic AMP-protein kinase A (cAMP-PKA) signaling pathway or sucrose non-fermenting 3 (SNF3) glucose-sensing pathway did not alleviate the repression. Furthermore, disruption of the gene of multicopy inhibitor of GAL gene expression (KmMIG1) alleviated the glucose repression on some nonglucose sugars (galactose, sucrose, and raffinose) but still kept glucose repression of xylose utilization. Real-time PCR analysis of the xylose utilization related genes transcription confirmed these results, and besides, revealed that xylitol dehydrogenase gene (KmXYL2) was the critical gene for xylose utilization and stringently regulated by glucose repression. Many other genes of candidate targets interacting with SNF1 were also evaluated by disruption, but none proved to be the key regulator in the pathway of the glucose repression on xylose utilization. Therefore, there may exist other signaling pathway(s) for glucose repression on xylose consumption. Based on these results, a thermotolerant xylose-glucose co-consumption platform strain of K. marxianus was constructed. Then, exogenous xylose reductase and xylose-specific transporter genes were overexpressed in the platform strain to obtain YHY013. The YHY013 could efficiently co-utilized the glucose and xylose from corncob hydrolysate or xylose mother liquor for xylitol production (> 100 g/L) even with inexpensive organic nitrogen sources.

CONCLUSIONS

The analysis of the glucose repression in K. marxianus laid the foundation for construction of the glucose-xylose co-utilizing platform strain. The efficient xylitol production strain further verified the potential of the platform strain in exploitation of lignocellulosic biomass.

Alleviation of catabolite repression in Kluyveromyces marxianus: the thermotolerant SBK1 mutant simultaneously coferments glucose and xylose

BACKGROUND

Simultaneous cofermentation of glucose and xylose mixtures would be a cost-effective solution for the conversion of cellulosic biomass to high-value products. However, most yeasts ferment glucose and xylose sequentially due to glucose catabolite repression. A well known thermotolerant yeast, Kluyveromyces marxianus, was selected for this work because it possesses cost-effective advantages over Saccharomyces cerevisiae for biofuel production from cellulosic biomass.

RESULTS

In the present study, we employed a directed evolutionary approach using 2-deoxyglucose to develop a thermotolerant mutant capable of simultaneous cofermentation of glucose and xylose by alleviating catabolite repression. The selected mutant, K. marxianus SBK1, simultaneously cofermented 40 g/L glucose and 28 g/L xylose to produce 23.82 g/L ethanol at 40 °C. This outcome corresponded to a yield of 0.35 g/g and productivity of 0.33 g/L h, representing an 84% and 129% improvement, respectively, over the parental strain. Interestingly, following mutagenesis the overall transcriptome of the glycolysis pathway was highly downregulated in K. marxianus SBK1, except for glucokinase-1 (GLK1) which was 21-fold upregulated. Amino acid sequence of GLK1 from K. marxianus SBK1 revealed three amino acid mutations which led to more than 22-fold lower enzymatic activity compared to the parental strain.

CONCLUSIONS

We herein successfully demonstrated that the cofermentation of a sugar mixture is a promising strategy for the efficient utilization of cellulosic biomass by K. marxianus SBK1. Through introduction of additional biosynthetic pathways, K. marxianus SBK1 could become a chassis-type strain for the production of fuels and chemicals from cellulosic biomass.

Improved secretory expression of lignocellulolytic enzymes in Kluyveromyces marxianus by promoter and signal sequence engineering

BACKGROUND

Taking into account its thermotolerance, high growth rate, and broad substrate spectrum, Kluyveromyces marxianus can be considered an ideal consolidated bioprocessing (CBP). A major obstacle to ethanol production using K. marxianus is the low production of lignocellulolytic enzymes, which reduces the cellulose hydrolysis and ethanol production. Thus, further improvement of enzyme expression and secretion is essential.

RESULTS

To improve the expression of lignocellulolytic enzymes, the inulinase promoter and signal sequence from K. marxianus was optimized through mutagenesis. A T(-361)A mutation inside the promoter, a deletion of AT-rich region inside 5'UTR (UTR∆A), and a P10L substitution in the signal sequence increased the secretory expression of the feruloyl esterase Est1E by up to sixfold. T(-361)A and UTR∆A increased the mRNA expression, while the P10L substitution extended the hydrophobic core of signal sequence and promoted secretion of mature protein. P10L and T(-361)A mutations increased secretory expressions of other types of lignocellulolytic enzymes by up to threefold, including endo-1,4-β-glucanase RuCelA, endo-1,4-β-endoxylanase Xyn-CDBFV, and endo-1,4-β-mannanase MAN330. During the fed-batch fermentation of strains carrying optimized modules, the peak activities of RuCelA, Xyn-CDBFV, MAN330, and Est1E reached 24 U/mL, 25,600 U/mL, 10,200 U/mL, and 1220 U/mL, respectively. Importantly, higher yield of enzymes by optimized promoter and signal sequence were achieved in all tested carbon sources, including the major end products of lignocellulose saccharification and fermentation, with growth on xylose resulting in the highest production.

CONCLUSIONS

The engineered promoter and signal sequence presented increased secretory expressions of different lignocellulolytic enzymes in K. marxianus by means of various carbon resources. Activities of lignocellulolytic enzymes in fed-batch fermentation were the highest activities reported for K. marxianus so far. Our engineered modules are valuable in producing lignocellulolytic enzymes by K. marxianus and in constructing efficient CBP strains for cellulosic ethanol production.

Highly efficient conversion of xylose to ethanol without glucose repression by newly isolated thermotolerant Spathaspora passalidarum CMUWF1-2

BACKGROUND

Efficient bioconversion of lignocellulosic biomass to bioethanol is one of key challenges in the situation of increasing bioethanol demand. The ethanologenic microbes for such conversion are required to possess abilities of utilization of various sugars including xylose and arabinose in lignocellulosic biomass. As required additional characteristics, there are a weak or no glucose repression that allows cells to simultaneously utilize various sugars together with glucose and thermotolerance for fermentation at high temperatures, which has several advantages including reduction of cooling cost. Spathaspora passalidarum ATCC MYA-4345, a type strains, isolated previously have mainly of these abilities or characteristics but its thermotolerance is not so strong and its glucose repression on xylose utilization is revealed.

RESULTS

Newly isolated S. passalidarum CMUWF1-2 was found to have a high ability to produce ethanol from various sugars included in lignocellulosic biomass at high temperatures. The strain achieved ethanol yields of 0.43 g, 0.40 g and 0.20 g ethanol/g xylose at 30 °C, 37 °C and 40 °C, respectively. Interestingly, no significant glucose repression was observed in experiments with mixed sugars, being consistent with the strong resistance to 2-deoxyglucose, and antimycin A showed no effect on its growth in xylose medium. Moreover, the strain was tolerant to glucose and ethanol at concentrations up to 35.0% (w/v) and 8.0% (v/v), respectively.

CONCLUSIONS

S. passalidarum CMUWF1-2 was shown to achieve efficient production of ethanol from various sugars and a high ethanol yield from xylose with little accumulation of xylitol. The strain also exhibited stress-resistance including thermotolerance and no detectable glucose repression as beneficial characteristics. Therefore, S. passalidarum CMUWF1-2 has remarkable potential for conversion of lignocellulosic biomass to bioethanol.

Uptake of iron by Kluyveromyces marxianus DSM 5422 cultivated in a whey-based medium

The ability of Kluyveromyces marxianus for converting lactose into ethyl acetate offers a chance for the economical reuse of whey. Iron plays a significant role in this process as ester synthesis requires a low intracellular iron content, x_Fe . The iron content in turn is decreased by growth due to cell expansion and increased by iron uptake. Thus, the iron-uptake rate, ψ, is important for the considered process. Iron uptake by K. marxianus DSM 5422 was studied in aerobic cultivation on a whey-borne medium with varied initial iron content, in part combined with a feed of iron under intensive growth conditions. A possible precipitation of iron that would pretend iron uptake was verified not to have occurred. Regularly measured dissolved iron concentrations, C_Fe,L , allowed the x_Fe and ψ parameters to be obtained by model-based iron balancing. The achieved data were used for establishing a ψ(C_Fe,L , x_Fe ) model. Mathematical simulations based on this iron-uptake model reproduced the performed cultivation processes. The created iron-uptake model allows for a future predictive system to be developed for the optimization of biotechnological ester production.

Transcriptomic analysis of thermotolerant yeastKluyveromyces marxianusin multiple inhibitors tolerance

Global transcriptional response ofK. marxianusto multiple inhibitors including acetic acid, phenols, furfural and HMF at 42 °C.

Physiological growth and galactose utilization by dairy yeast Kluyveromyces marxianus in mixed sugars and whey during fermentation

The dairy yeast Kluyveromyces marxianus represents a promising industrial strain useful for the production of bioethanol from cheese whey. Physiology of the five K. marxianus strains on galactose was examined during batch cultivation under controlled aerobic conditions on minimal media and one of the strains designated K. marxianus strain 6C17 which presented the highest specific galactose consumption rate. A maximum specific growth rate of 0.34 and 0.37 h^-1, respectively, was achieved using batch cultivation in a minimal medium and a complex medium amended with galactose (50 g/L) at 37 °C. The sugar was metabolized for the production of ethanol as the chief metabolite with a maximum ethanol yield of 0.39 g/g of galactose. Different growth behaviors were observed when galactose was used with other sugar such as glucose, lactose and fructose. The growth rates on hydrolyzed cheese whey were also measured, and a maximum specific growth rate of 0.39 and 0.32 h^-1 was observed with glucose and galactose, respectively, with the maximum flux diverted toward ethanol production. This approach of studying the physiology of thermotolerant K. marxianus on hydrolyzed whey during fermentation would be helpful in achieving higher yields of ethanol.

Thermotolerant genes essential for survival at a critical high temperature in thermotolerant ethanologenic Zymomonas mobilis TISTR 548

BACKGROUND

High-temperature fermentation (HTF) technology is expected to reduce the cost of bioconversion of biomass to fuels or chemicals. For stable HTF, the development of a thermotolerant microbe is indispensable. Elucidation of the molecular mechanism of thermotolerance would enable the thermal stability of microbes to be improved.

RESULTS

Thermotolerant genes that are essential for survival at a critical high temperature (CHT) were identified via transposon mutagenesis in ethanologenic, thermotolerant Zymomonas mobilis TISTR 548. Surprisingly, no genes for general heat shock proteins except for degP were included. Cells with transposon insertion in these genes showed a defect in growth at around 39 °C but grew normally at 30 °C. Of those, more than 60% were found to be sensitive to ethanol at 30 °C, indicating that the mechanism of thermotolerance partially overlaps with that of ethanol tolerance in the organism. Products of these genes were classified into nine categories of metabolism, membrane stabilization, transporter, DNA repair, tRNA modification, protein quality control, translation control, cell division, and transcriptional regulation.

CONCLUSIONS

The thermotolerant genes of Escherichia coli and Acetobacter tropicalis that had been identified can be functionally classified into 9 categories according to the classification of those of Z. mobilis, and the ratio of thermotolerant genes to total genomic genes in Z. mobilis is nearly the same as that in E. coli, though the ratio in A. tropicalis is relatively low. There are 7 conserved thermotolerant genes that are shared by these three or two microbes. These findings suggest that Z. mobilis possesses molecular mechanisms for its survival at a CHT that are similar to those in E. coli and A. tropicalis. The mechanisms may mainly contribute to membrane stabilization, protection and repair of damage of macromolecules and maintenance of cellular metabolism at a CHT. Notably, the contribution of heat shock proteins to such survival seems to be very low.

Cellulases and beyond: the first 70 years of the enzyme producer Trichoderma reesei

More than 70 years ago, the filamentous ascomycete Trichoderma reesei was isolated on the Solomon Islands due to its ability to degrade and thrive on cellulose containing fabrics. This trait that relies on its secreted cellulases is nowadays exploited by several industries. Most prominently in biorefineries which use T. reesei enzymes to saccharify lignocellulose from renewable plant biomass in order to produce biobased fuels and chemicals. In this review we summarize important milestones of the development of T. reesei as the leading production host for biorefinery enzymes, and discuss emerging trends in strain engineering. Trichoderma reesei has very recently also been proposed as a consolidated bioprocessing organism capable of direct conversion of biopolymeric substrates to desired products. We therefore cover this topic by reviewing novel approaches in metabolic engineering of T. reesei.

Efficient conversion of xylose to ethanol by stress-tolerant Kluyveromyces marxianus BUNL-21

The fermentation ability of thermotolerant Kluyveromyces marxianus BUNL-21 isolated in Laos was investigated. Comparison with thermotolerant K. marxianus DMKU3-1042 as one of the most thermotolerant yeasts isolated previously revealed that the strain possesses stronger ability for conversion of xylose to ethanol, resistance to 2-deoxyglucose in the case of pentose, and tolerance to various stresses including high temperature and hydrogen peroxide. K. marxianus BUNL-21 was found to have ethanol fermentation activity from xylose that is slightly lower and much higher than that of Scheffersomyces stipitis (Pichia stipitis) at 30 °C and at higher temperatures, respectively. The lower ethanol production seems to be due to large accumulation of acetic acid. The possible mechanism of acetic acid accumulation is discussed. In addition, it was found that both K. marxianus strains produced ethanol in the presence of 10 mM hydroxymethylfurfural or furfural, at a level almost equivalent to that in their absence. Therefore, K. marxianus BUNL-21 is a highly competent yeast for high-temperature ethanol fermentation with lignocellulosic biomass.

Data for rapid ethanol production at elevated temperatures by engineered thermotolerant Kluyveromyces marxianus via the NADP(H)-preferring xylose reductase-xylitol dehydrogenase pathway

A thermo-tolerant NADP(H)-preferring xylose pathway was constructed in Kluyveromyces marxianus for ethanol production with xylose at elevated temperatures (Zhang et al., 2015 [25]). Ethanol production yield and efficiency was enhanced by pathway engineering in the engineered strains. The constructed strain, YZJ088, has the ability to co-ferment glucose and xylose for ethanol and xylitol production, which is a critical step toward enabling economic biofuel production from lignocellulosic biomass. This study contains the fermentation results of strains using the metabolic pathway engineering procedure. The ethanol-producing abilities of various yeast strains under various conditions were compared, and strain YZJ088 showed the highest production and fastest productivity at elevated temperatures. The YZJ088 xylose fermentation results indicate that it fermented well with xylose at either low or high inoculum size. When fermented with an initial cell concentration of OD₆₀₀=15 at 37 °C, YZJ088 consumed 200 g/L xylose and produced 60.07 g/L ethanol; when the initial cell concentration was OD₆₀₀=1 at 37 °C, YZJ088 consumed 98.96 g/L xylose and produced 33.55 g/L ethanol with a productivity of 0.47 g/L/h. When fermented with 100 g/L xylose at 42 °C, YZJ088 produced 30.99 g/L ethanol with a productivity of 0.65 g/L/h, which was higher than that produced at 37 °C.