The path to next generation biofuels: successes and challenges in the era of synthetic biology

Meta-analysis of transcriptomic profiles in Dunaliella tertiolecta reveals molecular pathway responses to different abiotic stresses

Microalgae are photosynthetic organisms and a potential source of sustainable metabolite production. However, different stress conditions might affect the production of various metabolites. In this study, a meta-analysis of RNA-seq experiments in Dunaliella tertiolecta was evaluated to compare metabolite biosynthesis pathways in response to abiotic stress conditions such as high light, nitrogen deficiency and high salinity. Results showed downregulation of light reaction, photorespiration, tetrapyrrole and lipid-related pathways occurred under salt stress. Nitrogen deficiency mostly induced the microalgal responses of light reaction and photorespiration metabolism. Phosphoenol pyruvate carboxylase, phosphoglucose isomerase, bisphosphoglycerate mutase and glucose-6-phosphate-1-dehydrogenase (involved in central carbon metabolism) were commonly upregulated under salt, light and nitrogen stresses. Interestingly, the results indicated that the meta-genes (modules of genes strongly correlated) were located in a hub of stress-specific protein-protein interaction (PPI) network. Module enrichment of meta-genes PPI networks highlighted the cross-talk between photosynthesis, fatty acids, starch and sucrose metabolism under multiple stress conditions. Moreover, it was observed that the coordinated expression of the tetrapyrrole intermediated with meta-genes was involved in starch biosynthesis. Our results also showed that the pathways of vitamin B6 metabolism, methane metabolism, ribosome biogenesis and folate biosynthesis responded specifically to different stress factors. Since the results of this study revealed the main pathways underlying the abiotic stress, they might be applied in optimised metabolite production by the microalga Dunaliella in future studies. PRISMA check list was also included in the study.

Fighting the battle against evolution: designing genetically modified organisms for evolutionary stability

Synthetic biology has made significant progress in many areas, but a major challenge that has received limited attention is the evolutionary stability of synthetic constructs made of heterologous genes. The expression of these constructs in microorganisms, that is, production of proteins that are not necessary for the organism, is a metabolic burden, leading to a decrease in relative fitness and make the synthetic constructs unstable over time. This is a significant concern for the synthetic biology community, particularly when it comes to bringing this technology out of the laboratory. In this review, we discuss the issue of evolutionary stability in synthetic biology and review the available tools to address this challenge.

Mass-Activated Droplet Sorting for the Selection of Lysine-Producing Escherichia coli

Synthetic biology relies on engineering cells to have desirable properties, such as the production of select chemicals. A bottleneck in engineering methods is often the need to screen and sort variant libraries for potential activity. Droplet microfluidics is a method for high-throughput sample preparation and analysis which has the potential to improve the engineering of cells, but a limitation has been the reliance on fluorescent analysis. Here, we show the ability to select cell variants grown in 20 nL droplets at 0.5 samples/s using mass-activated droplet sorting (MADS), a method for selecting droplets based on the signal intensity measured by electrospray ionization mass spectrometry (ESI-MS). Escherichia coli variants producing lysine were used to evaluate the applicability of MADS for synthetic biology. E. coli were shown to be effectively grown in droplets, and the lysine produced by these cells was detectable using ESI-MS. Sorting of lysine-producing cells based on the MS signal was shown, yielding 96-98% purity for high-producing variants in the selected pool. Using this technique, cells were recovered after screening, enabling downstream validation via phenotyping. The presented method is translatable to whole-cell engineering for biocatalyst production.

Adding glucose delays the conversion of ethanol and acetic acid to caproic acid in Lacrimispora celerecrescens JSJ-1

Caproic acid is an important fatty acid with diverse applications. In this study, the biomass growth and metabolites of Lacrimispora celerecrescens JSJ-1 were investigated under different carbon sources (ethanol, starch, sucrose, and glucose), with a focus on the effect of the coexistence of glucose and ethanol on the synthesis of caproic acid. The results showed that starch, glucose, and sucrose all contributed to the biomass of L. celerecrescens JSJ-1. Under the three carbon sources, L. celerecrescens JSJ-1 produced acetic acid, butyric acid, lactic acid, ethanol, and butanol, but caproic acid was not produced. Ethanol was the optimal substrate for the production of caproic acid. When glucose and ethanol coexisted, the generation time of caproic acid was delayed compared with that in ethanol sodium acetate medium (ES medium). This was because glucose was preferentially consumed over ethanol. Lactic acid was generated as a result of glucose consumption, which led to a significant decrease in pH from 6.45 to 4.68. The low pH (< 5) inhibited the synthesis of caproic acid. Then, the strain's usage of lactic acid and the reaction between CaCO₃ and lactic acid caused the pH to increase. L. celerecrescens JSJ-1 did not start producing caproic acid using ethanol and acetic acid until the pH increased to 5.8. This research enriches the knowledge regarding the metabolism of L. celerecrescens JSJ-1 and provides guidelines for the industrial production of caproic acid by using L. celerecrescences JSJ-1. KEY POINTS: • Ethanol is the optimal substrate for the synthesis of caproic acid by Lacrimispora celerecrescens JSJ-1. • Lacrimispora celerecrescens JSJ-1 produced lactic acid rapidly when it used glucose, causing a sharp drop in pH. • pH is a crucial factor affecting the synthesis of caproic acid from ethanol by Lacrimispora celerecrescens JSJ-1.

Management of microbial enzymes for biofuels and biogas production by using metagenomic and genome editing approaches

Non-renewable fossil fuels such as bitumen, coal, natural gas, oil shale, and petroleum are depleting over the world owing to unrestricted consumption. Biofuels such as biodiesel, biobutanol, bioethanol, and biogas are considered an eco-friendly and cost-effective alternatives of fossil fuels. For energy sustainability, the production of advanced biofuels is required. The advancement of genetic and metabolic engineering in microbial cells played a significant contribution to biofuels overproduction. Essential approaches such as next-generation sequencing technologies and CRISPR/Cas9-mediated genome editing of microbial cells are required for the mass manufacture of biofuels globally. Advanced "omics" approaches are used to construct effective microorganisms for biofuels manufacturing. A new investigation is required to augment the production of lignocellulosic-based biofuels with minimal use of energy. Advanced areas of metabolic engineering are introduced in the manufacture of biofuels by the use of engineered microbial strains. Genetically modified microorganisms are used for the production of biofuels in large quantities at a low-cost.

Principles and practice of designing microbial biocatalysts for fuel and chemical production

The finite nature of fossil fuels and the environmental impact of its use have raised interest in alternate renewable energy sources. Specifically, non-food carbohydrates, such as lignocellulosic biomass, can be used to produce next generation biofuels, including cellulosic ethanol and other non-ethanol fuels like butanol. However, currently there is no native microorganism that can ferment all lignocellulosic sugars to fuel molecules. Thus, research is focused on engineering improved microbial biocatalysts for production of liquid fuels at high productivity, titer and yield. A clear understanding and application of the basic principles of microbial physiology and biochemistry are crucial to achieve this goal. In this review, we present and discuss the construction of microbial biocatalysts that integrate these principles with ethanol-producing Escherichia coli as an example of metabolic engineering. These principles also apply to fermentation of lignocellulosic sugars to other chemicals that are currently produced from petroleum.

Energy-Saving and Sustainable Separation of Bioalcohols by Adsorption on Bone Char

The separation of ethanol, propanol, and butanol from aqueous solutions was studied using adsorption on bone char. Adsorption kinetics and thermodynamic parameters of this separation method were studied at different conditions of pH and temperature. Results showed that the maximum adsorption capacities of these bioalcohols were obtained at pH 6 and 20°C. An exothermic separation was identified, which can be mainly associated to hydrophobic interactions between bone char surface and bioalcohols. Binary adsorption studies were also performed using mixtures of these bioalcohols. An antagonistic adsorption was observed for all bioalcohols where the ethanol and propanol separation was significantly affected by butanol. A model based on an artificial neural network was proposed to correlate both single and binary adsorption isotherms of these bioalcohols with bone char. It was concluded that the bone char could be an interesting adsorbent for the sustainable separation and recovery of bioalcohols from fermentation broths, which are actually considered emerging liquid biofuels and relevant industrial chemicals.

Modulating redox metabolism to improve isobutanol production in Shimwellia blattae

BACKGROUND

Isobutanol is a candidate to replace gasoline from fossil resources. This higher alcohol can be produced from sugars using genetically modified microorganisms. Shimwellia blattae (p424IbPSO) is a robust strain resistant to high concentration of isobutanol that can achieve a high production rate of this alcohol. Nevertheless, this strain, like most strains developed for isobutanol production, has some limitations in its metabolic pathway. Isobutanol production under anaerobic conditions leads to a depletion of NADPH, which is necessary for two enzymes in the metabolic pathway. In this work, two independent approaches have been studied to mitigate the co-substrates imbalance: (i) using a NADH-dependent alcohol dehydrogenase to reduce the NADPH dependence of the pathway and (ii) using a transhydrogenase to increase NADPH level.

RESULTS

The addition of the NADH-dependent alcohol dehydrogenase from Lactococcus lactis (AdhA) to S. blattae (p424IbPSO) resulted in a 19.3% higher isobutanol production. The recombinant strain S. blattae (p424IbPSO, pIZpntAB) harboring the PntAB transhydrogenase produced 39.0% more isobutanol than the original strain, reaching 5.98 g L^-1 of isobutanol. In both strains, we observed a significant decrease in the yields of by-products such as lactic acid or ethanol.

CONCLUSIONS

The isobutanol biosynthesis pathway in S. blattae (p424IbPSO) uses the endogenous NADPH-dependent alcohol dehydrogenase YqhD to complete the pathway. The addition of NADH-dependent AdhA leads to a reduction in the consumption of NADPH that is a bottleneck of the pathway. The higher consumption of NADH by AdhA reduces the availability of NADH required for the transformation of pyruvate into lactic acid and ethanol. On the other hand, the expression of PntAB from E. coli increases the availability of NADPH for IlvC and YqhD and at the same time reduces the availability of NADH and thus, the production of lactic acid and ethanol. In this work it is shown how the expression of AdhA and PntAB enzymes in Shimwellia blattae increases yield from 11.9% to 14.4% and 16.4%, respectively.

Proteomic Examination for Gluconeogenesis Pathway-Shift during Polyhydroxyalkanoate Formation in Cupriavidus necator Grown on Glycerol

Because of availability and inexpensive, glycerol can be considered as a suitable raw material for polyhydroxyalkanoate (PHA) production with bacterial fermentation. Nevertheless, compared to the production of glucose as a raw precursor, PHA produced from glycerol by Cupriavidus necator was found to produce lower PHA with low bacterial growth rates. According to our study, C. necator was able to synthesize glucose-like intermediates from glycerol via gluconeogenesis. This resulted in a decrease of the cell dry weight and the yield of PHA polymers, especially in the active cell growth phase. It was indicated that glycerol used as a carbon source of the PHA synthesis pathway has glucogenesis-shift, which causes a decrease of the PHA content and productivity. In this research, we investigated the proteins that were closely expressed with the increase of the intracellular PHA and glucose content. For solving the above problem, the proteins inside the bacterial cells were analyzed and compared to the database proteins via mass spectrometry. The proteins were isolated by 1-D SDS-polyacrylamide gel electrophoresis (PAGE) technique and identified by the liquid chromatography mass spectrometry (LC-MS) technique. By using bioinformatics validation, a total number of 1361 proteins were examined and found in the culture bacterial cells. Selective protein expression was correlated with the amount of PHA at each cultivation time and generating glucose by studying the 1361 proteins was elucidated in proteomic information. The results of the cluster of proteins were found to contain 93 proteins using the multiple array viewer (MEV) program with the KMS data analysis model. Protein species with the same expression pattern for PHA and six proteins with similar expression patterns were found to be correlated with generating glucose content. The associations of the two protein groups were then determined through a Stitch program. The protein and chemical associations were analyzed both directly and indirectly through different databases. The proteins of interest were found with research data linked between glycerol and glucose. Five protein types are connecting to glucose and glycerol shift pathway, two of which are glycosyl hydrolase (H16_B1563) and short-chain dehydrogenase (H16_B0687), both of which are enzymes used to break the bonds of complex sugars, possibly related to the partial conversion of glycerol to glucose. The two proteins found in the strains used in the Cupriavidus necator H16 experiment give rise to the break down the bonds of α,α-1,1-glucoside of malto-oligosyltrehalose and short-chain sugar molecules such as mannitol (C₆H₁₄O₆), respectively. In this research, finding the associated expression proteins which is involved in changing the pathway of gluconeogenesis shift to PHA synthesis will be useful information on genetically modifying microorganisms to produce PHA more efficiently, leading to reduction of the production costs.

The Model System Saccharomyces cerevisiae Versus Emerging Non-Model Yeasts for the Production of Biofuels

Microorganisms are effective platforms for the production of a variety of chemicals including biofuels, commodity chemicals, polymers and other natural products. However, deep cellular understanding is required for improvement of current biofuel cell factories to truly transform the Bioeconomy. Modifications in microbial metabolic pathways and increased resistance to various types of stress caused by the production of these chemicals are crucial in the generation of robust and efficient production hosts. Recent advances in systems and synthetic biology provide new tools for metabolic engineering to design strategies and construct optimal biocatalysts for the sustainable production of desired chemicals, especially in the case of ethanol and fatty acid production. Yeast is an efficient producer of bioethanol and most of the available synthetic biology tools have been developed for the industrial yeast Saccharomyces cerevisiae. Non-conventional yeast systems have several advantageous characteristics that are not easily engineered such as ethanol tolerance, low pH tolerance, thermotolerance, inhibitor tolerance, genetic diversity and so forth. Currently, synthetic biology is still in its initial steps for studies in non-conventional yeasts such as Yarrowia lipolytica, Kluyveromyces marxianus, Issatchenkia orientalis and Pichia pastoris. Therefore, the development and application of advanced synthetic engineering tools must also focus on these underexploited, non-conventional yeast species. Herein, we review the basic synthetic biology tools that can be applied to the standard S. cerevisiae model strain, as well as those that have been developed for non-conventional yeasts. In addition, we will discuss the recent advances employed to develop non-conventional yeast strains that are efficient for the production of a variety of chemicals through the use of metabolic engineering and synthetic biology.

Modeling a co-culture of Clostridium autoethanogenum and Clostridium kluyveri to increase syngas conversion to medium-chain fatty-acids

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We model a co-culture of C. autoethanogenum and C. kluyveri for syngas fermentation.

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Biomass species ratio affects ethanol and acetate profiles in the co-culture.

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The model predicts that addition of succinate increases caproate production.

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Genetic interventions  in C. autoethanogenum could increase caproate production.

Microbial fermentation of synthesis gas (syngas) is becoming more attractive for sustainable production of commodity chemicals. To date, syngas fermentation focuses mainly on the use of Clostridium species for the production of small organic molecules such as ethanol and acetate. The co-cultivation of syngas-fermenting microorganisms with chain-elongating bacteria can expand the range of possible products, allowing, for instance, the production of medium-chain fatty acids (MCFA) and alcohols from syngas. To explore these possibilities, we report herein a genome-scale, constraint-based metabolic model to describe growth of a co-culture of Clostridium autoethanogenum and Clostridium kluyveri on syngas for the production of valuable compounds. Community flux balance analysis was used to gain insight into the metabolism of the two strains and their interactions, and to reveal potential strategies enabling production of butyrate and hexanoate.

The model suggests that one strategy to optimize the production of medium-chain fatty-acids from syngas would be the addition of succinate. According to the prediction, addition of succinate would increase the pool of crotonyl-CoA and the ethanol/acetate uptake ratio in C. kluyveri, resulting in a flux of up to 60% of electrons into hexanoate. Another potential way to further optimize butyrate and hexanoate production would be an increase of C. autoethanogenum ethanol production. Blocking either acetaldehyde dehydrogenase or formate dehydrogenase (ferredoxin) activity or formate transport, in the C. autoethanogenum metabolic model could potentially lead to an up to 150% increase in ethanol production.

Expression of Metazoan Annexins in Yeast Provides Protection Against Deleterious Effects of the Biofuel Isobutanol

The ability of microorganisms to produce biofuels by fermentation is adversely affected by the perturbing effects of the hydrophobic biofuel on plasma membrane structure. It is demonstrated here that heterologous expression of metazoan, calcium-dependent, membrane-binding proteins of the annexin class can reduce deleterious effects of isobutanol on Saccharomyces cerevisiae viability and complex membrane functions. Therefore, expression of annexins in industrial strains of yeast or bacteria may prove beneficial in biofuel production.

Metabolic traits specific for lipid-overproducing strain of Mucor circinelloides WJ11 identified by genome-scale modeling approach

The genome-scale metabolic model of a lipid-overproducing strain of Mucor circinelloides WJ11 was developed. The model (iNI1159) contained 1,159 genes, 648 EC numbers, 1,537 metabolites, and 1,355 metabolic reactions, which were localized in different compartments of the cell. Using flux balance analysis (FBA), the iNI1159 model was validated by predicting the specific growth rate. The metabolic traits investigated by phenotypic phase plane analysis (PhPP) showed a relationship between the nutrient uptake rate, cell growth, and the triacylglycerol production rate, demonstrating the strength of the model. A putative set of metabolic reactions affecting the lipid-accumulation process was identified when the metabolic flux distributions under nitrogen-limited conditions were altered by performing fast flux variability analysis (fastFVA) and relative flux change. Comparative analysis of the metabolic models of the lipid-overproducing strain WJ11 (iNI1159) and the reference strain CBS277.49 (iWV1213) using both fastFVA and coordinate hit-and-run with rounding (CHRR) showed that the flux distributions between these two models were significantly different. Notably, a higher flux distribution through lipid metabolisms such as lanosterol, zymosterol, glycerolipid and fatty acids biosynthesis in iNI1159 was observed, leading to an increased lipid production when compared to iWV1213. In contrast, iWV1213 exhibited a higher flux distribution across carbohydrate and amino acid metabolisms and thus generated a high flux for biomass production. This study demonstrated that iNI1159 is an effective predictive tool for the pathway engineering of oleaginous strains for the production of diversified oleochemicals with industrial relevance.

An alkaline active feruloyl-CoA synthetase from soil metagenome as a potential key enzyme for lignin valorization strategies

Ferulic acid (FA), a low-molecular weight aromatic compound derived from lignin, represents a high-value molecule, used for applications in the cosmetic and pharmaceutical industries. FA can be further enzymatically converted in other commercially interesting molecules, such as vanillin and bioplastics. In several organisms, these transformations often start with a common step of FA activation via CoA-thioesterification, catalyzed by feruloyl-CoA synthetases (Fcs). In this context, these enzymes are of biotechnological interest for conversion of lignin-derived FA into high value chemicals. In this study, we describe the first structural characterization of a prokaryotic Fcs, named FCS1, isolated from a lignin-degrading microbial consortium. The FCS1 optimum pH and temperature were 9 and 37°C, respectively, with Km of 0.12 mM and Vmax of 36.82 U/mg. The circular dichroism spectra indicated a notable secondary structure stability at alkaline pH values and high temperatures. This secondary structure stability corroborates the activity data, which remains high until pH 9. The Small Angle X-Ray Scattering analyses resulted on the tertiary/quaternary structure and the low-resolution envelope in solution of FCS1, which was modeled as a homodimer using the hyperthermophilic nucleoside diphosphate-forming acetyl-CoA synthetase from Candidatus Korachaeum cryptofilum. This study contributes to the field of research by establishing the first biophysical and structural characterization for Fcs, and our data may be used for comparison against novel enzymes of this class that to be studied in the future.

Resting cells isobutanol production by Shimwellia blattae (p424IbPSO): Influence of growth culture conditions

Isobutanol is a promising gasoline additive and could even be a potential substitute used directly as combustible. In this work, the production of isobutanol from glucose by Shimwellia blattae (p424IbPSO) in resting cell cultures is studied. This production has two stages, involving a resting cell phase that has not been studied before. The cell growth was carried out under different operating conditions: temperature and medium composition (YE, ammonium, and IPTG concentrations), looking for the highest isobutanol production. Moreover, the cells were collected at three different growth times checking their isobutanol production capacity. The best operating conditions have been determined as: 30°C of temperature, a medium containing 1.5 g L^-1 YE and 1.4 g L^-1 of ammonium as nitrogen sources, adding 0.5 mM IPTG as inducer. The cells collected at early growth times are significantly more active. The use of S. blattae (p424IbPSO) in resting cells is a good strategy for the production of isobutanol from glucose yielding better results than in batch growth cultures, a yield of 60% attainment of theoretical maximum yield is obtained under optimal conditions. In addition, it has been demonstrated that if the cells are cultured at higher temperatures and with high IPTG concentrations, inclusion bodies are formed in the cytoplasm inhibiting the isobutanol production in the resting cell stage.

Lessons in Membrane Engineering for Octanoic Acid Production from Environmental Escherichia coli Isolates

Fermentative production of many attractive biorenewable fuels and chemicals is limited by product toxicity in the form of damage to the microbial cell membrane. Metabolic engineering of the production organism can help mitigate this problem, but there is a need for identification and prioritization of the most effective engineering targets. Here, we use a set of previously characterized environmental Escherichia coli isolates with high tolerance and production of octanoic acid, a model membrane-damaging biorenewable product, as a case study for identifying and prioritizing membrane engineering strategies. This characterization identified differences in the membrane lipid composition, fluidity, integrity, and cell surface hydrophobicity from those of the lab strain MG1655. Consistent with previous publications, decreased membrane fluidity was associated with increased fatty acid production ability. Maintenance of high membrane integrity or longer membrane lipids seemed to be of less importance than fluidity. Cell surface hydrophobicity was also directly associated with fatty acid production titers, with the strength of this association demonstrated by plasmid-based expression of the multiple stress resistance outer membrane protein BhsA. This expression of bhsA was effective in altering hydrophobicity, but the direction and magnitude of the change differed between strains. Thus, additional strategies are needed to reliably engineer cell surface hydrophobicity. This work demonstrates the ability of environmental microbiological studies to impact the metabolic engineering design-build-test-learn cycle and possibly increase the economic viability of fermentative bioprocesses.IMPORTANCE The production of bulk fuels and chemicals in a bio-based fermentation process requires high product titers. This is often difficult to achieve, because many of the target molecules damage the membrane of the microbial cell factory. Engineering the composition of the membrane in order to decrease its vulnerability to this damage has proven to be an effective strategy for improving bioproduction, but additional strategies and engineering targets are needed. Here, we studied a small set of environmental Escherichia coli isolates that have higher production titers of octanoic acid, a model biorenewable chemical, than those of the lab strain MG1655. We found that membrane fluidity and cell surface hydrophobicity are strongly associated with improved octanoic acid production. Fewer genetic modification strategies have been demonstrated for tuning hydrophobicity relative to fluidity, leading to the conclusion that there is a need for expanding hydrophobicity engineering strategies in E. coli.

Modulation of Intracellular O2 Concentration in Escherichia coli Strains Using Oxygen Consuming Devices

The use of cell factories for the production of bulk and value-added compounds is nowadays an advantageous alternative to the traditional petrochemical methods. Nevertheless, the efficiency and productivity of several of these processes can improve with the implementation of micro-oxic or anoxic conditions. In the industrial setting, laccases are appealing catalysts that can oxidize a wide range of substrates and reduce O₂ to H₂O. In this work, several laccase-based devices were designed and constructed to modulate the intracellular oxygen concentration in bacterial chassis. These oxygen consuming devices (OCDs) included Escherichia coli's native laccase (CueO) and three variants of this protein obtained by directed evolution. The OCDs were initially characterized in vitro using E. coli DH5α protein extracts and subsequently using extracts obtained from other E. coli strains and in vivo. Upon induction of the OCDs, no major effect on growth was observed in four of the strains tested, and analysis of the cell extract protein profiles revealed increased levels of laccase. Moreover, oxygen consumption associated with the OCDs occurred under all of the conditions tested, but the performance of the devices was shown to be strain-dependent, highlighting the importance of the genetic background even in closely related strains. One of the laccase variants showed 13- and 5-fold increases in oxidase activity and O₂ consumption rate, respectively. Furthermore, it was also possible to demonstrate O₂ consumption in vivo using l-DOPA as the substrate, which represents a proof of concept that these OCDs generate an intracellular oxygen sink, thereby manipulating the redox status of the cells. In addition, the modularity and orthogonality principles used for the development of these devices allow easy reassembly and fine-tuning, foreseeing their introduction into other chassis/systems.

Identification and manipulation of a novel locus to improve cell tolerance to short-chain alcohols in Escherichia coli

Escherichia coli KO11 is a popular ethanologenic strain, but is more sensitive to ethanol than other producers. Here, an ethanol-tolerant mutant EM was isolated from ultraviolet mutagenesis library of KO11. Comparative genomic analysis added by piecewise knockout strategy and complementation assay revealed EKO11_3023 (espA) within the 36.6-kb deletion from KO11 was the only locus responsible for ethanol sensitivity. Interestingly, when espA was deleted in strain W (the parent strain of KO11), ethanol tolerance was dramatically elevated to the level of espA-free hosts [e.g., MG1655 and BL21(DE3)]. And overexpression of espA in strains MG1655 and BL21(DE3) led to significantly enhanced ethanol sensitivity. In addition to ethanol, deletion of espA also improved cell tolerance to other short-chain (C2–C4) alcohols, including methanol, isopropanol, n-butanol, isobutanol and 2-butanol. Therefore, espA was responsible for short-chain alcohol sensitivity of W-strains compared to other cells, which provides a potential engineering target for alcohols production.

Improved ethanol productivity from lignocellulosic hydrolysates by Escherichia coli with regulated glucose utilization

Background

Lignocellulosic ethanol could offer a sustainable source to meet the increasing worldwide demand for fuel. However, efficient and simultaneous metabolism of all types of sugars in lignocellulosic hydrolysates by ethanol-producing strains is still a challenge.

Results

An engineered strain Escherichia coli B0013-2021HPA with regulated glucose utilization, which could use all monosaccharides in lignocellulosic hydrolysates except glucose for cell growth and glucose for ethanol production, was constructed. In E. coli B0013-2021HPA, pta-ackA, ldhA and pflB were deleted to block the formation of acetate, lactate and formate and additional three mutations at glk, ptsG and manZ generated to block the glucose uptake and catabolism, followed by the replacement of the wild-type frdA locus with the ptsG expression cassette under the control of the temperature-inducible λ pR and pL promoters, and the final introduction of pEtac-PA carrying Zymomonas mobilis pdc and adhB for the ethanol pathway. B0013-2021HPA was able to utilize almost all xylose, galactose and arabinose but not glucose for cell propagation at 34 °C and converted all sugars to ethanol at 42 °C under oxygen-limited fermentation conditions.

Conclusions

Engineered E. coli strain with regulated glucose utilization showed efficient metabolism of mixed sugars in lignocellulosic hydrolysates and thus higher productivity of ethanol production.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0915-x) contains supplementary material, which is available to authorized users.

Reaction kinetics of hydrogen atom abstraction from isopentanol by the H atom and HO2˙ radical

Isopentanol is a potential next-generation biofuel for future applications to Homogeneous Charge Compression Ignition (HCCI) engine concepts. To provide insights into the combustion behavior of isopentanol, especially to its auto-ignition behavior which is linked both to efficiency and pollutant formation in real combustion systems, detailed quantum chemical studies for crucial reactions are desired. H-Abstraction reaction rates from fuel molecules are key initiation steps for chain branching required for auto-ignition. In this study, rate constants are determined for the hydrogen atom abstraction reactions from isopentanol by the H atom and HO2˙ radical by implementing the CBS-QB3 composite method. For the treatment of the internal rotors, a Pitzer-Gwinn-like approximation is applied. On comparing the computed reaction energies, the highest exothermicity (ΔE = -46 kJ mol-1) is depicted for Hα abstraction by the H atom whereas the lowest endothermicity (ΔE = 29 kJ mol-1) is shown for the abstraction of Hα by the HO2˙ radical. The formation of hydrogen bonding is found to affect the kinetics of the H atom abstraction reactions by the HO2˙ radical. Further above 750 K, the calculated high pressure limit rate constants indicate that the total contribution from delta carbon sites (Cδ) is predominant for hydrogen atom abstraction by the H atom and HO2˙ radical.

Applications of CRISPR/Cas System to Bacterial Metabolic Engineering

The clustered regularly interspaced short palindromic repeats/CRISPR-associated (CRISPR/Cas) adaptive immune system has been extensively used for gene editing, including gene deletion, insertion, and replacement in bacterial and eukaryotic cells owing to its simple, rapid, and efficient activities in unprecedented resolution. Furthermore, the CRISPR interference (CRISPRi) system including deactivated Cas9 (dCas9) with inactivated endonuclease activity has been further investigated for regulation of the target gene transiently or constitutively, avoiding cell death by disruption of genome. This review discusses the applications of CRISPR/Cas for genome editing in various bacterial systems and their applications. In particular, CRISPR technology has been used for the production of metabolites of high industrial significance, including biochemical, biofuel, and pharmaceutical products/precursors in bacteria. Here, we focus on methods to increase the productivity and yield/titer scan by controlling metabolic flux through individual or combinatorial use of CRISPR/Cas and CRISPRi systems with introduction of synthetic pathway in industrially common bacteria including Escherichia coli. Further, we discuss additional useful applications of the CRISPR/Cas system, including its use in functional genomics.

Yeasts in sustainable bioethanol production: A review

Bioethanol has been identified as the mostly used biofuel worldwide since it significantly contributes to the reduction of crude oil consumption and environmental pollution. It can be produced from various types of feedstocks such as sucrose, starch, lignocellulosic and algal biomass through fermentation process by microorganisms. Compared to other types of microoganisms, yeasts especially Saccharomyces cerevisiae is the common microbes employed in ethanol production due to its high ethanol productivity, high ethanol tolerance and ability of fermenting wide range of sugars. However, there are some challenges in yeast fermentation which inhibit ethanol production such as high temperature, high ethanol concentration and the ability to ferment pentose sugars. Various types of yeast strains have been used in fermentation for ethanol production including hybrid, recombinant and wild-type yeasts. Yeasts can directly ferment simple sugars into ethanol while other type of feedstocks must be converted to fermentable sugars before it can be fermented to ethanol. The common processes involves in ethanol production are pretreatment, hydrolysis and fermentation. Production of bioethanol during fermentation depends on several factors such as temperature, sugar concentration, pH, fermentation time, agitation rate, and inoculum size. The efficiency and productivity of ethanol can be enhanced by immobilizing the yeast cells. This review highlights the different types of yeast strains, fermentation process, factors affecting bioethanol production and immobilization of yeasts for better bioethanol production.

Cellulolytic RoboLector - towards an automated high-throughput screening platform for recombinant cellulase expression

Background

Cellulases are key player in the hydrolyzation of cellulose. Unfortunately, this reaction is slow and a bottleneck in the process chain from biomass to intermediates and biofuels due to low activities of the enzymes. To overcome this draw back, a lot of effort is put into the area of protein engineering, to modify these enzymes by directed evolution or rational design. Huge clone libraries are constructed and have to be screened for improved variants. High-throughput screening is the method of choice to tackle this experimental effort, but up to now only a few process steps are adapted to automated platforms and little attention has been turned to the reproducibility of clone rankings.

Results

In this study, an extended robotic platform is presented to conduct automated high-throughput screenings of clone libraries including preculture synchronization and biomass specific induction. Automated upstream, downstream and analytical process steps are described and evaluated using E. coli and K. lactis as model organisms. Conventional protocols for media preparation, cell lysis, Azo-CMC assay and PAHBAH assay are successfully adapted to automatable high-throughput protocols. Finally, a recombinant E. coli celA2 clone library was screened and a reliable clone ranking could be realized.

Conclusion

The RoboLector device is a suitable platform to perform all process steps of an automated high-throughput clone library screening for improved cellulases. On-line biomass growth measurement controlling liquid handling actions enables fair comparison of clone variants.

Electronic supplementary material

The online version of this article (doi:10.1186/s13036-016-0043-2) contains supplementary material, which is available to authorized users.

Genoproteomics-assisted improvement of Andrographis paniculata: toward a promising molecular and conventional breeding platform for autogamous plants affecting the pharmaceutical industry

Andrographis paniculata (Burm. f.) Wall. ex Nees. (AP) is a hermaphroditic, self-compatible, and habitual inbreeding plant. Its main bioactive component is andrographolide, which is capable of inducing autophagic cell death in some human cancer cells and helps fight HIV/AIDS. Increasing the andrographolide content by investigating the genetic mechanisms controlling its biosynthesis in order to improve and develop high-yielding cultivars are the main breeding targets for AP. However, there might exist some limitations or barriers for crossability within AP accessions. Recently, this problem was addressed in AP by using a combination of crossbreeding and biotechnology-aided genetic methods. This review emphasizes that development of a breeding platform in a hard-to-breed plant, such as AP, requires the involvement of a broad range of methods from classical genetics to molecular breeding. To this end, a phenological stage (for example, flowering and stigma development) can be simplified to a quantitative morphological trait (for example, bud or stigma length) to be used as an index to express the highest level of receptivity in order to manage outcrossing. The outcomes of the basic crossability research can be then employed in diallel mating and crossbreeding. This review explains how genomic data could produce useful information regarding genetic distance and its influence on the crossability of AP accessions. Our review indicates that co-dominant DNA markers, such as microsatellites, are also capable of resolving the evolutionary pathway and cryptic features of plant populations and such information can be used to select the best breeding strategy. This review also highlights the importance of proteomic analysis as a breeding tool. In this regard, protein diversification, as well as the impact of normal and stress-responsive proteins on morphometric and physiological behaviors, could be used in breeding programs. These findings have immense potential for improving plant production and, therefore, can be regarded as prospective breeding platforms for medicinal plants that have an autogamous mode of reproduction. Finally, this review suggests that novel site-directed genome editing approaches such as TALENs (Transcription Activator-Like Effector Nucleases) and CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 (CRISPR-associated protein-9 nuclease) systems together with other new plant breeding technologies (NPBT) should simultaneously be taken into consideration for improvement of pharmaceutical plants.

High production of fatty alcohols in Escherichia coli with fatty acid starvation

Background

Microbial biofuel synthesis attracting increasing attention. Great advances have been made in producing fatty alcohols from fatty acyl-CoAs and fatty acids in Escherichia coli. However, the low titers and limited knowledge regarding the basic characteristics of fatty alcohols, such as location and toxicity, have hampered large-scale industrialization. Further research is still needed.

Results

In this study, we designed a novel and efficient strategy to enhance fatty alcohol production by inducing fatty acid starvation. We report the first use of deletions of acyl-ACP thioesterases to enhance fatty alcohol production. Transcriptional analysis was conducted to investigate the mechanism of the designed strategy. Then, fatty alcohol production was further enhanced by deletion of genes from competing pathways. Fatty alcohols were shown to be extracellular products with low toxicity. The final strain, E. coli MGL2, produced fatty alcohols at the remarkable level of 6.33 g/L under fed-batch fermentation, representing the highest reported titer of fatty alcohols produced by microorganisms.

Conclusions

Deletions of genes responsible for synthesis of fatty acids and competing products are promising strategies for fatty alcohol production. Our investigation of the location and toxicity of fatty alcohols suggest bright future for fatty alcohol production in E. coli.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0524-5) contains supplementary material, which is available to authorized users.

Role of Escherichia coli in Biofuel Production

Increased energy consumption coupled with depleting petroleum reserves and increased greenhouse gas emissions have renewed our interest in generating fuels from renewable energy sources via microbial fermentation. Central to this problem is the choice of microorganism that catalyzes the production of fuels at high volumetric productivity and yield from cheap and abundantly available renewable energy sources. Microorganisms that are metabolically engineered to redirect renewable carbon sources into desired fuel products are contemplated as best choices to obtain high volumetric productivity and yield. Considering the availability of vast knowledge in genomic and metabolic fronts, Escherichia coli is regarded as a primary choice for the production of biofuels. Here, we reviewed the microbial production of liquid biofuels that have the potential to be used either alone or in combination with the present-day fuels. We specifically highlighted the metabolic engineering and synthetic biology approaches used to improve the production of biofuels from E. coli over the past few years. We also discussed the challenges that still exist for the biofuel production from E. coli and their possible solutions.

Gene Amplification on Demand Accelerates Cellobiose Utilization in Engineered Saccharomyces cerevisiae

Efficient microbial utilization of cellulosic sugars is essential for the economic production of biofuels and chemicals. Although the yeast Saccharomyces cerevisiae is a robust microbial platform widely used in ethanol plants using sugar cane and corn starch in large-scale operations, glucose repression is one of the significant barriers to the efficient fermentation of cellulosic sugar mixtures. A recent study demonstrated that intracellular utilization of cellobiose by engineered yeast expressing a cellobiose transporter (encoded by cdt-1) and an intracellular β-glucosidase (encoded by gh1-1) can alleviate glucose repression, resulting in the simultaneous cofermentation of cellobiose and nonglucose sugars. Here we report enhanced cellobiose fermentation by engineered yeast expressing cdt-1 and gh1-1 through laboratory evolution. When cdt-1 and gh1-1 were integrated into the genome of yeast, the single copy integrant showed a low cellobiose consumption rate. However, cellobiose fermentation rates by engineered yeast increased gradually during serial subcultures on cellobiose. Finally, an evolved strain exhibited a 15-fold-higher cellobiose fermentation rate. To identify the responsible mutations in the evolved strain, genome sequencing was performed. Interestingly, no mutations affecting cellobiose fermentation were identified, but the evolved strain contained 9 copies of cdt-1 and 23 copies of gh1-1 We also traced the copy numbers of cdt-1 and gh1-1 of mixed populations during the serial subcultures. The copy numbers of cdt-1 and gh1-1 in the cultures increased gradually with similar ratios as cellobiose fermentation rates of the cultures increased. These results suggest that the cellobiose assimilation pathway (transport and hydrolysis) might be a rate-limiting step in engineered yeast and copies of genes coding for metabolic enzymes might be amplified in yeast if there is a growth advantage. This study indicates that on-demand gene amplification might be an efficient strategy for yeast metabolic engineering.

IMPORTANCE

In order to enable rapid and efficient fermentation of cellulosic hydrolysates by engineered yeast, we delve into the limiting factors of cellobiose fermentation by engineered yeast expressing a cellobiose transporter (encoded by cdt-1) and an intracellular β-glucosidase (encoded by gh1-1). Through laboratory evolution, we isolated mutant strains capable of fermenting cellobiose much faster than a parental strain. Genome sequencing of the fast cellobiose-fermenting mutant reveals that there are massive amplifications of cdt-1 and gh1-1 in the yeast genome. We also found positive and quantitative relationships between the rates of cellobiose consumption and the copy numbers of cdt-1 and gh1-1 in the evolved strains. Our results suggest that the cellobiose assimilation pathway (transport and hydrolysis) might be a rate-limiting step for efficient cellobiose fermentation. We demonstrate the feasibility of optimizing not only heterologous metabolic pathways in yeast through laboratory evolution but also on-demand gene amplification in yeast, which can be broadly applicable for metabolic engineering.

Branched-Chain Amino Acids

Branched-chain amino acids (BCAAs), viz., L-isoleucine, L-leucine, and L-valine, are essential amino acids that cannot be synthesized in higher organisms and are important nutrition for humans as well as livestock. They are also valued as synthetic intermediates for pharmaceuticals. Therefore, the demand for BCAAs in the feed and pharmaceutical industries is increasing continuously. Traditional industrial fermentative production of BCAAs was performed using microorganisms isolated by random mutagenesis. A collection of these classical strains was also scientifically useful to clarify the details of the BCAA biosynthetic pathways, which are tightly regulated by feedback inhibition and transcriptional attenuation. Based on this understanding of the metabolism of BCAAs, it is now possible for us to pursue strains with higher BCAA productivity using rational design and advanced molecular biology techniques. Additionally, systems biology approaches using augmented omics information help us to optimize carbon flux toward BCAA production. Here, we describe the biosynthetic pathways of BCAAs and their regulation and then overview the microorganisms developed for BCAA production. Other chemicals, including isobutanol, i.e., a second-generation biofuel, can be synthesized by branching the BCAA biosynthetic pathways, which are also outlined.

RNA-Seq transcriptomic analysis with Bag2D software identifies key pathways enhancing lipid yield in a high lipid-producing mutant of the non-model green alga Dunaliella tertiolecta

BACKGROUND

For many years, increasing demands for fossil fuels have met with limited supply. As a potential substitute and renewable source of biofuel feedstock, microalgae have received significant attention. However, few of the current algal species produce high lipid yields to be commercially viable. To discover more high yielding strains, next-generation sequencing technology is used to elucidate lipid synthetic pathways and energy metabolism involved in lipid yield. When subjected to manipulation by genetic and metabolic engineering, enhancement of such pathways may further enhance lipid yield.

RESULTS

In this study, transcriptome profiling of a random insertional mutant with enhanced lipid production generated from a non-model marine microalga Dunaliella tertiolecta is presented. D9 mutant has a lipid yield that is 2- to 4-fold higher than that of wild type. Using novel Bag2D-workflow scripts developed and reported here, the non-redundant transcripts from de novo assembly were annotated based on the best hits in five model microalgae, namely Chlamydomonas reinhardtii, Coccomyxa subellipsoidea C-169, Ostreococcus lucimarinus, Volvox carteri, Chlorella variabilis NC64A and a high plant species Arabidopsis thaliana. The assembled contigs (~181 Mb) includes 481,381 contigs, covering 10,185 genes. Pathway analysis showed that a pathway from inositol phosphate metabolism to fatty acid biosynthesis is the most significantly correlated with higher lipid yield in this mutant.

CONCLUSIONS

Herein, we described a pipeline to analyze RNA-Seq data without pre-existing transcriptomic information. The draft transcriptome of D. tertiolecta was constructed and annotated, which offered useful information for characterizing high lipid-producing mutants. D. tertiolecta mutant was generated with an enhanced photosynthetic efficiency and lipid production. RNA-Seq data of the mutant and wild type were compared, providing biological insights into the expression patterns of contigs associated with energy metabolism and carbon flow pathways. Comparison of D. tertiolecta genes with homologs of five other green algae and a model high plant species can facilitate the annotation of D. tertiolecta and lead to a more complete annotation of its sequence database, thus laying the groundwork for optimization of lipid production pathways based on genetic manipulation.

De-novo assembly and characterization of Chlorella minutissima UTEX2341 transcriptome by paired-end sequencing and the identification of genes related to the biosynthesis of lipids for biodiesel

Chlorella minutissima is considered to be one of the promising feedstocks for biofuels in the future. In this study, the transcriptome from the oil-rich strain UTEX2341 of C. minutissima was generated based on Illumina paired-end sequencing. Through de-novo assembly, a total of 14,905 isogenes were obtained and compacted into 6216 unigenes. A total of 80% of the unigenes were assigned with GO terms and were further subdivided into 55 sub-categories. KEGG analysis demonstrated that 37.2% of the unigenes could be accessed and mapped into 278 pathways. Interestingly, the genes that encoded key enzymes that are involved in the biosynthesis, elongation, and metabolism of fatty acids were identified, including malonyl-CoA-ACP transacylase, 3-ketoacyl-ACP synthase, 3-ketoacyl-ACP reductase, and others. Moreover, the genes that are involved in triacylglycerol (TAG) biosynthesis and metabolism were also observed. Therefore, the transcriptome analysis of C. minutissima UTEX2341 not only supplies comprehensive insight into the molecular pathway that is involved in the biosynthesis of biofuel precursors but also provides substantial valuable genomic resources to accelerate the further development and utilization of biofuels.

Maximizing biomass concentration in baker's yeast process by using a decoupled geometric controller for substrate and dissolved oxygen

Biomass production by baker's yeast in a fed-batch reactor depends on the metabolic regime determined by the concentration of glucose and dissolved oxygen in the reactor. Achieving high biomass concentration in turn is dependent on the dynamic interaction between the glucose and dissolved oxygen concentration. Taking this into account, we present in this paper the implementation of a decoupled input-output linearizing controller (DIOLC) for maximizing biomass in a fed-batch yeast process. The decoupling is based on the inversion of 2×2 input-output matrix resulting from global linearization. The DIOLC was implemented online using a platform created in LabVIEW employing a TCP/IP protocol via the reactor's built-in electronic system. An improvement in biomass yield by 23% was obtained compared to that using a PID controller. The results demonstrate superior capability of the DIOLC and that the cumulative effect of smoother control action contributes to biomass maximization.

White biotechnology: State of the art strategies for the development of biocatalysts for biorefining

White biotechnology is a term that is now often used to describe the implementation of biotechnology in the industrial sphere. Biocatalysts (enzymes and microorganisms) are the key tools of white biotechnology, which is considered to be one of the key technological drivers for the growing bioeconomy. Biocatalysts are already present in sectors such as the chemical and agro-food industries, and are used to manufacture products as diverse as antibiotics, paper pulp, bread or advanced polymers. This review proposes an original and global overview of highly complementary fields of biotechnology at both enzyme and microorganism level. A certain number of state of the art approaches that are now being used to improve the industrial fitness of biocatalysts particularly focused on the biorefinery sector are presented. The first part deals with the technologies that underpin the development of industrial biocatalysts, notably the discovery of new enzymes and enzyme improvement using directed evolution techniques. The second part describes the toolbox available by the cell engineer to shape the metabolism of microorganisms. And finally the last part focuses on the 'omic' technologies that are vital for understanding and guide microbial engineering toward more efficient microbial biocatalysts. Altogether, these techniques and strategies will undoubtedly help to achieve the challenging task of developing consolidated bioprocessing (i.e. CBP) readily available for industrial purpose.

Solvents Production from a Mixture of Glucose and Xylose by Mixed Fermentation of Clostridium acetobutylicum and Saccharomyces cerevisiae

To overcome the xylose utilization defect in ethanol fermentation by wide-type Saccharomyces cerevisiae and alleviate the carbon catabolite repression (CCR) in acetone-butanol-ethanol (ABE) fermentation by Clostridium acetobutylicum, a novel mixed fermentation of S. cerevisiae and C. acetobutylicum was developed. When S. cerevisiae was inoculated 24 h earlier than C. acetobutylicum CH02, a higher solvents yield was achieved with 0.41 g/g, compared to 0.38 g/g in ABE fermentation, and when S. cerevisiae and C. acetobutylicum CH02 were inoculated simultaneously, a higher productivity was achieved with 0.32 g/L/h, compared to 0.15 g/L/h in ABE fermentation. The total solvents yield was improved by the high ethanol yield from glucose. The CCR in mixed fermentation was alleviated when glucose was utilized quickly by S. cerevisiae, and therefore, the productivity was improved. This study suggests that mixed fermentation is an effective solvents production method from a mixture of glucose and xylose.

An endophyte of Picrorhiza kurroa Royle ex. Benth, producing menthol, phenylethyl alcohol and 3-hydroxypropionic acid, and other volatile organic compounds

An endophytic fungus, PR4 was found in nature associated with the rhizome of Picrorhiza kurroa, a high altitude medicinal plant of Kashmir Himalayas. The fungus was found to inhibit the growth of several phyto-pathogens by virtue of its volatile organic compounds (VOCs). Molecular phylogeny, based on its ITS1-5.8S-ITS2 ribosomal gene sequence, revealed the identity of the fungus as Phomopsis/Diaporthe sp. This endophyte was found to produce a unique array of VOCs, particularly, menthol, phenylethyl alcohol, (+)-isomenthol, β-phellandrene, β-bisabolene, limonene, 3-pentanone and 1-pentanol. The purification of compounds from the culture broth of PR4 led to the isolation of 3-hydroxypropionic acid (3-HPA) as a major metabolite. This is the first report of a fungal culture producing a combination of biologically and industrially important metabolites—menthol, phenylethyl alcohol, and 3-HPA. The investigation into the monoterpene biosynthetic pathway of PR4 led to the partial characterization of isopiperitenone reductase (ipr) gene, which seems to be significantly distinct from the plant homologue. The biosynthesis of plant-like-metabolites, such as menthol, is of significant academic and industrial significance. This study indicates that PR4 is a potential candidate for upscaling of menthol, phenylethyl alcohol, and 3-HPA, as well as for understanding the menthol/monoterpene biosynthetic pathway in fungi.

A paper-based microbial fuel cell array for rapid and high-throughput screening of electricity-producing bacteria

In this work, a 48-well, paper-based sensing platform was developed for the high-throughput and rapid characterization of the electricity-producing capability of microbes.

Desorption of 1-butanol from polymeric resin: experimental studies and mathematical modeling

Modeling of desorption kinetics and dynamic desorption process of 1-butanol from a polymeric resin.

Near-real-time analysis of the phenotypic responses of Escherichia coli to 1-butanol exposure using Raman Spectroscopy

Raman spectroscopy was used to study the time course of phenotypic responses of Escherichia coli (DH5α) to 1-butanol exposure (1.2% [vol/vol]). Raman spectroscopy is of interest for bacterial phenotyping because it can be performed (i) in near real time, (ii) with minimal sample preparation (label-free), and (iii) with minimal spectral interference from water. Traditional off-line analytical methodologies were applied to both 1-butanol-treated and control cells to draw correlations with Raman data. Here, distinct sets of Raman bands are presented that characterize phenotypic traits of E. coli with maximized correlation to off-line measurements. In addition, the observed time course phenotypic responses of E. coli to 1.2% (vol/vol) 1-butanol exposure included the following: (i) decreased saturated fatty acids levels, (ii) retention of unsaturated fatty acids and low levels of cyclopropane fatty acids, (iii) increased membrane fluidity following the initial response of increased rigidity, and (iv) no changes in total protein content or protein-derived amino acid composition. For most phenotypic traits, correlation coefficients between Raman spectroscopy and traditional off-line analytical approaches exceeded 0.75, and major trends were captured. The results suggest that near-real-time Raman spectroscopy is suitable for approximating metabolic and physiological phenotyping of bacterial cells subjected to toxic environmental conditions.

Metabolic engineering and classical selection of the methylotrophic thermotolerant yeast Hansenula polymorpha for improvement of high-temperature xylose alcoholic fermentation

Background

The methylotrophic yeast, Hansenula polymorpha is an industrially important microorganism, and belongs to the best studied yeast species with well-developed tools for molecular research. The complete genome sequence of the strain NCYC495 of H. polymorpha is publicly available. Some of the well-studied strains of H. polymorpha are known to ferment glucose, cellobiose and xylose to ethanol at elevated temperature (45 – 50°C) with ethanol yield from xylose significantly lower than that from glucose and cellobiose. Increased yield of ethanol from xylose was demonstrated following directed metabolic changes but, still the final ethanol concentration achieved is well below what is considered feasible for economic recovery by distillation.

Results

In this work, we describe the construction of strains of H. polymorpha with increased ethanol production from xylose using an ethanol-non-utilizing strain (2EthOH⁻) as the host. The transformants derived from 2EthOH⁻ overexpressing modified xylose reductase (XYL1m) and native xylitol dehydrogenase (XYL2) were isolated. These transformants produced 1.5-fold more ethanol from xylose than the original host strain. The additional overexpression of XYL3 gene coding for xylulokinase, resulted in further 2.3-fold improvement in ethanol production with no measurable xylitol formed during xylose fermentation. The best ethanol producing strain obtained by metabolic engineering approaches was subjected to selection for resistance to the known inhibitor of glycolysis, the anticancer drug 3-bromopyruvate. The best mutant selected had an ethanol yield of 0.3 g/g xylose and produced up to 9.8 g of ethanol/l during xylose alcoholic fermentation at 45°C without correction for ethanol evaporation.

Conclusions

Our results indicate that xylose conversion to ethanol at elevated temperature can be significantly improved in H. polymorpha by combining methods of metabolic engineering and classical selection.