Engineering of Corynebacterium glutamicum for high-yield L-valine production under oxygen deprivation conditions

Omics studies reveal the response mechanisms of Corynebacterium glutamicum to l-homoserine osmotic stress

To investigate the mechanism of osmotic stress produced by Corynebacterium glutamicum (C. glutamicum) in the production of l-homoserine. The present study employed genomic and transcriptomic analyses of both evolved strains and the parental strain grown under l-homoserine osmotic stress to investigate the response mechanisms and identify specific tolerance targets. The results indicated that the evolved strain enhanced its tolerance to l-homoserine stress by inactivating aspartokinase, thereby interrupting the intracellular synthesis pathway of l-homoserine. Early in stress exposure, C. glutamicum suppressed the synthesis of l -homoserine and instead enhanced its catabolic activity. In response to osmotic stress, C. glutamicum also relied on a variety of energy metabolism and ion transport pathways, including ABC transporters and ATP metabolism, which are essential for high-osmolarity tolerance. Given the gradual accumulation of l-homoserine within the cell, this study focused on the transcriptional expression patterns during the adaptation phase, excluding cellular responses during the high-concentration stress phase. These findings provide valuable insights for improving C. glutamicum's tolerance to l-homoserine stress during amino-acid fermentation and highlight potential targets for metabolic engineering strategies.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-025-04304-7.

Construction of a redox-coupled pathway co-metabolizing glucose and acetate for high-yield production of butyl butyrate in Escherichia coli

The carbon and energy efficiency of a biomanufacturing process is of crucial importance in determining its economic viability. Formate dehydrogenase has been demonstrated to be beneficial in regenerating NADH from formate produced during sugar metabolism, thereby creating energy-efficient systems. Nevertheless, introducing enzyme(s) for butyryl butyrate (BB) biosynthesis based on this system, only 1.64 g/L BB with 14.3 % carbon yield was obtained due to an imbalance in NADH-NAD+ turnover. To address the issue of NADH accumulation, a joint redox-balanced pathway for BB biosynthesis was developed in this study by coupling acetate and glucose metabolism. Following overexpression of acetyl-CoA synthetase in the BB-producing strain, acetate and glucose were co-utilized stoichiometrically and intracellular redox homeostasis was achieved. The engineered strain produced 29.02 g/L BB with carbon yield of 43.3 %, representing the highest yield ever reported for fermentative production of BB. It indicated the potential for developing a carbon- and energy-effective route for biomanufacturing.

Metabolic engineering of Escherichia coli for enhanced production of D-pantothenic acid

D-pantothenic acid (D-PA) is an essential vitamin that has been widely used in various industries. However, the low productivity caused by slow D-PA production in fermentation hinders its potential applications. In this study, strategies of engineering the synthetic pathway combined with regulating methyl recycle were employed in E. coli to enhance D-PA production. First, a self-induced promoter-mediated dynamic regulation of D-PA degradation pathway was carried out to improve D-PA accumulation. Then, to drive more carbon flux into D-PA synthesis, the key nodes of the R-pantoate pathway which encoded the essential enzyme were integrated into the genome. Subsequently, the further increase in D-PA production was achieved by promoting the regeneration of methyl donor. The strain L11T produced 86.03 g/L D-PA with a productivity of 0.797 g/L/h, which presented the highest D-PA titer and productivity to date. The strategies could be applied to constructing cell factories for producing other bio-based products.

Microbial Cell Factories in the Bioeconomy Era: From Discovery to Creation

Microbial cell factories (MCFs) are extensively used to produce a wide array of bioproducts, such as bioenergy, biochemical, food, nutrients, and pharmaceuticals, and have been regarded as the "chips" of biomanufacturing that will fuel the emerging bioeconomy era. Biotechnology advances have led to the screening, investigation, and engineering of an increasing number of microorganisms as diverse MCFs, which are the workhorses of biomanufacturing and help develop the bioeconomy. This review briefly summarizes the progress and strategies in the development of robust and efficient MCFs for sustainable and economic biomanufacturing. First, a comprehensive understanding of microbial chassis cells, including accurate genome sequences and corresponding annotations; metabolic and regulatory networks governing substances, energy, physiology, and information; and their similarity and uniqueness compared with those of other microorganisms, is needed. Moreover, the development and application of effective and efficient tools is crucial for engineering both model and nonmodel microbial chassis cells into efficient MCFs, including the identification and characterization of biological parts, as well as the design, synthesis, assembly, editing, and regulation of genes, circuits, and pathways. This review also highlights the necessity of integrating automation and artificial intelligence (AI) with biotechnology to facilitate the development of future customized artificial synthetic MCFs to expedite the industrialization process of biomanufacturing and the bioeconomy.

Metabolic Engineering of Corynebacterium glutamicum for the High-Level Production of l-Valine under Aerobic Conditions

l-Valine, an essential amino acid, serves as a valuable compound in various industries. However, engineering strains with both high yield and purity are yet to be delivered for microbial l-valine production. We engineered a Corynebacterium glutamicum strain capable of highly efficient production of l-valine. We initially introduced an acetohydroxy acid synthase mutant from an industrial l-valine producer and optimized a cofactor-balanced pathway, followed by the activation of the nonphosphoenolpyruvate-dependent carbohydrate phosphotransferase system and the introduction of an exogenous Entner-Doudoroff pathway. Subsequently, we weakened anaplerotic pathways, and attenuated the tricarboxylic acid cycle via start codon substitution in icd, encoding isocitrate dehydrogenase. Finally, to balance bacterial growth and l-valine production, an l-valine biosensor-dependent genetic circuit was established to dynamically repress citrate synthase expression. The engineered strain Val19 produced 103 g/L of l-valine with a high yield of 0.35 g/g glucose and a productivity of 2.67 g/L/h. This represents the highest reported l-valine production in C. glutamicum via direct fermentation and exhibits potential for its industrial-scale production, leveraging the advantages of C. glutamicum over other microbes.

Branched-chain amino acids: physico-chemical properties, industrial synthesis and role in signaling, metabolism and energy production

Branched-chain amino acids (BCAAs)-leucine (Leu), isoleucine (Ile), and valine (Val)-are essential nutrients with significant roles in protein synthesis, metabolic regulation, and energy production. This review paper offers a detailed examination of the physico-chemical properties of BCAAs, their industrial synthesis, and their critical functions in various biological processes. The unique isomerism of BCAAs is presented, focusing on analytical challenges in their separation and quantification as well as their solubility characteristics, which are crucial for formulation and purification applications. The industrial synthesis of BCAAs, particularly using bacterial strains like Corynebacterium glutamicum, is explored, alongside methods such as genetic engineering aimed at enhancing production, detailing the enzymatic processes and specific precursors. The dietary uptake, distribution, and catabolism of BCAAs are reviewed as fundamental components of their physiological functions. Ultimately, their multifaceted impact on signaling pathways, immune function, and disease progression is discussed, providing insights into their profound influence on muscle protein synthesis and metabolic health. This comprehensive analysis serves as a resource for understanding both the basic and complex roles of BCAAs in biological systems and their industrial application.

Recent Advances in Metabolic Engineering for the Biosynthesis of Phosphoenol Pyruvate-Oxaloacetate-Pyruvate-Derived Amino Acids

The phosphoenol pyruvate-oxaloacetate-pyruvate-derived amino acids (POP-AAs) comprise native intermediates in cellular metabolism, within which the phosphoenol pyruvate-oxaloacetate-pyruvate (POP) node is the switch point among the major metabolic pathways existing in most living organisms. POP-AAs have widespread applications in the nutrition, food, and pharmaceutical industries. These amino acids have been predominantly produced in Escherichia coli and Corynebacterium glutamicum through microbial fermentation. With the rapid increase in market requirements, along with the global food shortage situation, the industrial production capacity of these two bacteria has encountered two bottlenecks: low product conversion efficiency and high cost of raw materials. Aiming to push forward the update and upgrade of engineered strains with higher yield and productivity, this paper presents a comprehensive summarization of the fundamental strategy of metabolic engineering techniques around phosphoenol pyruvate-oxaloacetate-pyruvate node for POP-AA production, including L-tryptophan, L-tyrosine, L-phenylalanine, L-valine, L-lysine, L-threonine, and L-isoleucine. Novel heterologous routes and regulation methods regarding the carbon flux redistribution in the POP node and the formation of amino acids should be taken into consideration to improve POP-AA production to approach maximum theoretical values. Furthermore, an outlook for future strategies of low-cost feedstock and energy utilization for developing amino acid overproducers is proposed.

Efficient Fermentative Production of d-Alanine and Other d-Amino Acids by Metabolically Engineered Corynebacterium glutamicum

d-Amino acids (d-AAs) have wide applications in industries such as pharmaceutical, food, and cosmetics due to their unique properties. Currently, the production of d-AAs has relied on chemical synthesis or enzyme catalysts, and it is challenging to produce d-AAs via direct fermentation from glucose. We observed that Corynebacterium glutamicum exhibits a remarkable tolerance to high concentrations of d-Ala, a crucial characteristic for establishing a successful fermentation process. By optimizing meso-diaminopilmelate dehydrogenases in different C. glutamicum strains and successively deleting l-Ala biosynthetic pathways, we developed an efficient d-Ala fermentation system. The d-Ala titer was enhanced through systems metabolic engineering, which involved strengthening glucose assimilation and pyruvate supply, reducing the formation of organic acid byproducts, and attenuating the TCA cycle. During fermentation in a 5-L bioreactor, a significant accumulation of l-Ala was observed in the broth, which was subsequently diminished by introducing an l-amino acid deaminase. Ultimately, the engineered strain DA-11 produced 85 g/L d-Ala with a yield of 0.30 g/g glucose, accompanied by an optical purity exceeding 99%. The fermentation platform has the potential to be extended for the synthesis of other d-AAs, as demonstrated by the production of d-Val and d-Glu.

Microbial production of branched chain amino acids: Advances and perspectives

Branched-chain amino acids (BCAAs) such as L-valine, L-leucine, and L-isoleucine are widely used in food and feed. To comply with sustainable development goals, commercial production of BCAAs has been completely replaced with microbial fermentation. However, the efficient production of BCAAs by microorganisms remains a serious challenge due to their staggered metabolic networks and cell growth. To overcome these difficulties, systemic metabolic engineering has emerged as an effective and feasible strategy for the biosynthesis of BCAA. This review firstly summarizes the research advances in the microbial synthesis of BCAAs and representative engineering strategies. Second, systematic methods, such as high-throughput screening, adaptive laboratory evolution, and omics analysis, can be used to analyses the synthesis of BCAAs at the whole-cell level and further improve the titer of target chemicals. Finally, new tools and engineering strategies that may increase the production output and development direction of the microbial production of BCAAs are discussed.

System metabolic engineering of Escherichia coli W for the production of 2-ketoisovalerate using unconventional feedstock

Replacing traditional substrates in industrial bioprocesses to advance the sustainable production of chemicals is an urgent need in the context of the circular economy. However, since the limited degradability of non-conventional carbon sources often returns lower yields, effective exploitation of such substrates requires a multi-layer optimization which includes not only the provision of a suitable feedstock but the use of highly robust and metabolically versatile microbial biocatalysts. We tackled this challenge by means of systems metabolic engineering and validated Escherichia coli W as a promising cell factory for the production of the key building block chemical 2-ketoisovalerate (2-KIV) using whey as carbon source, a widely available and low-cost agro-industrial waste. First, we assessed the growth performance of Escherichia coli W on mono and disaccharides and demonstrated that using whey as carbon source enhances it significantly. Second, we searched the available literature and used metabolic modeling approaches to scrutinize the metabolic space of E. coli and explore its potential for overproduction of 2-KIV identifying as basic strategies the block of pyruvate depletion and the modulation of NAD/NADP ratio. We then used our model predictions to construct a suitable microbial chassis capable of overproducing 2-KIV with minimal genetic perturbations, i.e., deleting the pyruvate dehydrogenase and malate dehydrogenase. Finally, we used modular cloning to construct a synthetic 2-KIV pathway that was not sensitive to negative feedback, which effectively resulted in a rerouting of pyruvate towards 2-KIV. The resulting strain shows titers of up to 3.22 ± 0.07 g/L of 2-KIV and 1.40 ± 0.04 g/L of L-valine in 24 h using whey in batch cultures. Additionally, we obtained yields of up to 0.81 g 2-KIV/g substrate. The optimal microbial chassis we present here has minimal genetic modifications and is free of nutritional autotrophies to deliver high 2-KIV production rates using whey as a non-conventional substrate.

An NADPH-auxotrophic Corynebacterium glutamicum recombinant strain and used it to construct L-leucine high-yielding strain

The NADPH-regeneration enzymes in Corynebacterium glutamicum were inactivated to construct an NADPH-auxotrophic C. glutamicum strain by gene knockout and gene replacement. The resultant NADPH-auxotrophic C. glutamicum XL-1 ΔZMI_Cg::I_Sm (i.e., strain Leu-1) grew well in the basic medium only with gluconate as carbon source. Replacement of the native glyceraldehyde 3-phosphate dehydrogenase (NAD-GapDH_Cg) by NADP-GapDH_Ca from Clostridium acetobutylicum is an effective strategy for producing L-leucine in NADPH-prototrophic strain XL-1 and NADPH-auxotrophic strain Leu-1, whereas the L-leucine yield did not differ significantly between these strains (14.1 ± 1.8 g/L vs 16.2 ± 1.1 g/L). Enhancing the carbon flux in biosynthetic pathway by recombinant expression plasmid pEC-ABNCE promoted L-leucine production, but the shortage NADPH supply limited the L-leucine yield. The mutated promoters of zwf and icd_Cg were introduced into C. glutamicum with NADP-GapDH_Ca and pEC-ABNCE increased L-leucine yield (54.3 ± 2.9 g/L) and improved cell growth (OD₅₆₂ = 83.4 ± 7.5) in fed-batch fermentation because the resultant strain C. glutamicum XL-1 ΔMI_Cg::I_Sm G_Cg::G_Ca P_zwf-D1 P_icd-D2/pEC-ABNCE (i.e., strain Leu-9) exhibited the proper intracellular NADPH and NADH level. This is the first report of constructing an L-leucine high-yielding strain that reasonably supplies NADPH by optimizing the biosynthetic pathway of NADPH from an NADPH-auxotrophic strain.

Rational metabolic engineering of Corynebacterium glutamicum to create a producer of L-valine

L-Valine is one of the nine amino acids that cannot be synthesized de novo by higher organisms and must come from food. This amino acid not only serves as a building block for proteins, but also regulates protein and energy metabolism and participates in neurotransmission. L-Valine is used in the food and pharmaceutical industries, medicine and cosmetics, but primarily as an animal feed additive. Adding L-valine to feed, alone or mixed with other essential amino acids, allows for feeds with lower crude protein content, increases the quality and quantity of pig meat and broiler chicken meat, as well as improves reproductive functions of farm animals. Despite the fact that the market for L-valine is constantly growing, this amino acid is not yet produced in our country. In modern conditions, the creation of strains-producers and organization of L-valine production are especially relevant for Russia. One of the basic microorganisms most commonly used for the creation of amino acid producers, along with Escherichia coli, is the soil bacterium Corynebacterium glutamicum. This review is devoted to the analysis of the main strategies for the development of L- valine producers based on C. glutamicum. Various aspects of L-valine biosynthesis in C. glutamicum are reviewed: process biochemistry, stoichiometry and regulation, enzymes and their corresponding genes, export and import systems, and the relationship of L-valine biosynthesis with central cell metabolism. Key genetic elements for the creation of C. glutamicum-based strains-producers are identified. The use of metabolic engineering to enhance L-valine biosynthesis reactions and to reduce the formation of byproducts is described. The prospects for improving strains in terms of their productivity and technological characteristics are shown. The information presented in the review can be used in the production of producers of other amino acids with a branched side chain, namely L-leucine and L-isoleucine, as well as D-pantothenate.

Stratifications and foliations in phase portraits of gene network models

Periodic processes of gene network functioning are described with good precision by periodic trajectories (limit cycles) of multidimensional systems of kinetic-type differential equations. In the literature, such systems are often called dynamical, they are composed according to schemes of positive and negative feedback between components of these networks. The variables in these equations describe concentrations of these components as functions of time. In the preparation of numerical experiments with such mathematical models, it is useful to start with studies of qualitative behavior of ensembles of trajectories of the corresponding dynamical systems, in particular, to estimate the highest likelihood domain of the initial data, to solve inverse problems of parameter identification, to list the equilibrium points and their characteristics, to localize cycles in the phase portraits, to construct stratification of the phase portraits to subdomains with different qualities of trajectory behavior, etc. Such an à priori geometric analysis of the dynamical systems is quite analogous to the basic section "Investigation of functions and plot of their graphs" of Calculus, where the methods of qualitative studies of shapes of curves determined by equations are exposed. In the present paper, we construct ensembles of trajectories in phase portraits of some dynamical systems. These ensembles are 2-dimensional surfaces invariant with respect to shifts along the trajectories. This is analogous to classical construction in analytic mechanics, i. e. the level surfaces of motion integrals (energy, kinetic moment, etc.). Such surfaces compose foliations in phase portraits of dynamical systems of Hamiltonian mechanics. In contrast with this classical mechanical case, the foliations considered in this paper have singularities: all their leaves have a non-empty intersection, they contain limit cycles on their boundaries. Description of the phase portraits of these systems at the level of their stratifications, and that of ensembles of trajectories allows one to construct more realistic gene network models on the basis of methods of statistical physics and the theory of stochastic differential equations.

The soil bacterium, Corynebacterium glutamicum, from biosynthesis of value-added products to bioremediation: A master of many trades

Ever since its discovery in 1957, Corynebacterium glutamicum has become a well-established industrial strain and is known for its massive capability of producing various amino acids (like L-lysine and L-glutamate) and other value-added chemicals. With the rising demand for these bio-based products, the revelation of the whole genome sequences of the wild type strains, and the astounding advancements made in the fields of metabolic engineering and systems biology, our perspective of C. glutamicum has been revolutionized and has expanded our understanding of its strain development. With these advancements, a new era for C. glutamicum supremacy in the field of industrial biotechnology began. This led to remarkable progress in the enhancement of tailor-made over-producing strains and further development of the substrate spectrum of the bacterium, to easily accessible, economical, and renewable resources. C. glutamicum has also been metabolically engineered and used in the degradation/assimilation of highly toxic and ubiquitous environmental contaminant, arsenic, present in water or soil. Here, we review the history, current knowledge, progress, achievements, and future trends relating to the versatile metabolic factory, C. glutamicum. This review paper is devoted to C. glutamicum which is one of the leading industrial microbes, and one of the most promising and versatile candidates to be developed. It can be used not only as a platform microorganism to produce different value-added chemicals and recombinant proteins, but also as a tool for bioremediation, allowing to enhance specific properties, for example in situ bioremediation.

Metabolic engineering of Corynebacterium glutamicum for producing branched chain amino acids

Background

Branched chain amino acids (BCAAs) are widely applied in the food, pharmaceutical, and animal feed industries. Traditional chemical synthetic and enzymatic BCAAs production in vitro has been hampered by expensive raw materials, harsh reaction conditions, and environmental pollution. Microbial metabolic engineering has attracted considerable attention as an alternative method for BCAAs biosynthesis because it is environmentally friendly and delivers high yield.

Main text

Corynebacterium glutamicum (C. glutamicum) possesses clear genetic background and mature gene manipulation toolbox, and has been utilized as industrial host for producing BCAAs. Acetohydroxy acid synthase (AHAS) is a crucial enzyme in the BCAAs biosynthetic pathway of C. glutamicum, but feedback inhibition is a disadvantage. We therefore reviewed AHAS modifications that relieve feedback inhibition and then investigated the importance of AHAS modifications in regulating production ratios of three BCAAs. We have comprehensively summarized and discussed metabolic engineering strategies to promote BCAAs synthesis in C. glutamicum and offer solutions to the barriers associated with BCAAs biosynthesis. We also considered the future applications of strains that could produce abundant amounts of BCAAs.

Conclusions

Branched chain amino acids have been synthesized by engineering the metabolism of C. glutamicum. Future investigations should focus on the feedback inhibition and/or transcription attenuation mechanisms of crucial enzymes. Enzymes with substrate specificity should be developed and applied to the production of individual BCAAs. The strategies used to construct strains producing BCAAs provide guidance for the biosynthesis of other high value-added compounds.

Engineering of microbial cells for L-valine production: challenges and opportunities

L-valine is an essential amino acid that has wide and expanding applications with a suspected growing market demand. Its applicability ranges from animal feed additive, ingredient in cosmetic and special nutrients in pharmaceutical and agriculture fields. Currently, fermentation with the aid of model organisms, is a major method for the production of L-valine. However, achieving the optimal production has often been limited because of the metabolic imbalance in recombinant strains. In this review, the constrains in L-valine biosynthesis are discussed first. Then, we summarize the current advances in engineering of microbial cell factories that have been developed to address and overcome major challenges in the L-valine production process. Future prospects for enhancing the current L-valine production strategies are also discussed.

L-valine production in Corynebacterium glutamicum based on systematic metabolic engineering: progress and prospects

L-valine is an essential branched-chain amino acid that cannot be synthesized by the human body and has a wide range of applications in food, medicine and feed. Market demand has stimulated people's interest in the industrial production of L-valine. At present, the mutagenized or engineered Corynebacterium glutamicum is an effective microbial cell factory for producing L-valine. Because the biosynthetic pathway and metabolic network of L-valine are intricate and strictly regulated by a variety of key enzymes and genes, highly targeted metabolic engineering can no longer meet the demand for efficient biosynthesis of L-valine. In recent years, the development of omics technology has promoted the upgrading of traditional metabolic engineering to systematic metabolic engineering. This whole-cell-scale transformation strategy has become a productive method for developing L-valine producing strains. This review provides an overview of the biosynthesis and regulation mechanism of L-valine, and summarizes the current metabolic engineering techniques and strategies for constructing L-valine high-producing strains. Finally, the opinion of constructing a cell factory for efficiently biosynthesizing L-valine was proposed.

Physiological characteristics of Corynebacterium glutamicum as a cell factory under anaerobic conditions

Corynebacterium glutamicum, a gram-positive and facultative anaerobic bacterium, is widely used for the industrial production of amino acids, such as L-glutamate and L-lysine. C. glutamicum grows and produces amino acids under aerobic conditions. When restricted under anaerobic conditions, it produces organic acids, such as L-lactate and succinate, through metabolic shift. With the increasing threat of global warming, these organic acids have drawn considerable attention as bio-based plastic monomers. In addition to the organic acids, the anaerobic bioprocess is also used to produce other value-added compounds, including isobutanol, ethanol, 3-methyl-1-butanol, 2,3-butanediol, L-alanine, and L-valine. Therefore, C. glutamicum is now a versatile cell factory for producing a wide variety of useful chemicals under both aerobic and anaerobic conditions. The growth and metabolism of the bacterium depend on the oxygen levels, which modulate the rearrangement of the carbon flux by reprogramming gene expression patterns and intracellular redox states. Anaerobic cell growth and L-lysine production as well as aerobic succinate production have been demonstrated by engineering the metabolic pathways or supplying a terminal electron acceptor instead of oxygen. In this review, we discuss the physiological and metabolic changes in C. glutamicum associated with its application as a cell factory under different oxygen states. Physiological switching in bacteria is initiated with the sensing of oxygen availability. While such a sensor has not been identified in C. glutamicum yet, the molecular mechanism for oxygen sensing in related bacteria is also discussed. KEY POINTS: • C. glutamicum produces a wide variety of useful compounds under anaerobic conditions. • C. glutamicum is a versatile cell factory under both aerobic and anaerobic conditions. • Metabolic fate can be overcome by engineering metabolic pathways.

Exploring functionality of the reverse β-oxidation pathway in Corynebacterium glutamicum for production of adipic acid

BACKGROUND

Adipic acid, a six-carbon platform chemical mainly used in nylon production, can be produced via reverse β-oxidation in microbial systems. The advantages posed by Corynebacterium glutamicum as a model cell factory for implementing the pathway include: (1) availability of genetic tools, (2) excretion of succinate and acetate when the TCA cycle becomes overflown, (3) initiation of biosynthesis with succinyl-CoA and acetyl-CoA, and (4) established succinic acid production. Here, we implemented the reverse β-oxidation pathway in C. glutamicum and assessed its functionality for adipic acid biosynthesis.

RESULTS

To obtain a non-decarboxylative condensation product of acetyl-CoA and succinyl-CoA, and to subsequently remove CoA from the condensation product, we introduced heterologous 3-oxoadipyl-CoA thiolase and acyl-CoA thioesterase into C. glutamicum. No 3-oxoadipic acid could be detected in the cultivation broth, possibly due to its endogenous catabolism. To successfully biosynthesize and secrete 3-hydroxyadipic acid, 3-hydroxyadipyl-CoA dehydrogenase was introduced. Addition of 2,3-dehydroadipyl-CoA hydratase led to biosynthesis and excretion of trans-2-hexenedioic acid. Finally, trans-2-enoyl-CoA reductase was inserted to yield 37 µg/L of adipic acid.

CONCLUSIONS

In the present study, we engineered the reverse β-oxidation pathway in C. glutamicum and assessed its potential for producing adipic acid from glucose as starting material. The presence of adipic acid, albeit small amount, in the cultivation broth indicated that the synthetic genes were expressed and functional. Moreover, 2,3-dehydroadipyl-CoA hydratase and β-ketoadipyl-CoA thiolase were determined as potential target for further improvement of the pathway.

Advances in metabolic engineering of Corynebacterium glutamicum to produce high-value active ingredients for food, feed, human health, and well-being

The soil microbe Corynebacterium glutamicum is a leading workhorse in industrial biotechnology and has become famous for its power to synthetise amino acids and a range of bulk chemicals at high titre and yield. The product portfolio of the microbe is continuously expanding. Moreover, metabolically engineered strains of C. glutamicum produce more than 30 high value active ingredients, including signature molecules of raspberry, savoury, and orange flavours, sun blockers, anti-ageing sugars, and polymers for regenerative medicine. Herein, we highlight recent advances in engineering of the microbe into novel cell factories that overproduce these precious molecules from pioneering proofs-of-concept up to industrial productivity.

The ldhA Gene Encoding Fermentative l-Lactate Dehydrogenase in Corynebacterium Glutamicum Is Positively Regulated by the Global Regulator GlxR

Bacterial metabolism shifts from aerobic respiration to fermentation at the transition from exponential to stationary growth phases in response to limited oxygen availability. Corynebacterium glutamicum, a Gram-positive, facultative aerobic bacterium used for industrial amino acid production, excretes l-lactate, acetate, and succinate as fermentation products. The ldhA gene encoding l-lactate dehydrogenase is solely responsible for l-lactate production. Its expression is repressed at the exponential phase and prominently induced at the transition phase. ldhA is transcriptionally repressed by the sugar-phosphate-responsive regulator SugR and l-lactate-responsive regulator LldR. Although ldhA expression is derepressed even at the exponential phase in the sugR and lldR double deletion mutant, a further increase in its expression is still observed at the stationary phase, implicating the action of additional transcription regulators. In this study, involvement of the cAMP receptor protein-type global regulator GlxR in the regulation of ldhA expression was investigated. The GlxR-binding site found in the ldhA promoter was modified to inhibit or enhance binding of GlxR. The ldhA promoter activity and expression of ldhA were altered in proportion to the binding affinity of GlxR. Similarly, l-lactate production was also affected by the binding site modification. Thus, GlxR was demonstrated to act as a transcriptional activator of ldhA.

Coexistence of the Entner-Doudoroff and Embden-Meyerhof-Parnas pathways enhances glucose consumption of ethanol-producing Corynebacterium glutamicum

BACKGROUND

It is interesting to modify sugar metabolic pathways to improve the productivity of biocatalysts that convert sugars to value-added products. However, this attempt often fails due to the tight control of the sugar metabolic pathways. Recently, activation of the Entner-Doudoroff (ED) pathway in Escherichia coli has been shown to enhance glucose consumption, though the mechanism underlying this phenomenon is poorly understood. In the present study, we investigated the effect of a functional ED pathway in metabolically engineered Corynebacterium glutamicum that metabolizes glucose via the Embden-Meyerhof-Parnas (EMP) pathway to produce ethanol under oxygen deprivation. This study aims to provide further information on metabolic engineering strategies that allow the Entner-Doudoroff and Embden-Meyerhof-Parnas pathways to coexist.

RESULTS

Three genes (zwf, edd, and eda) encoding glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydratase, and 2-keto-3-deoxy-6-phosphogluconate aldolase from Zymomonas mobilis were expressed in a genetically modified strain, C. glutamicum CRZ2e, which produces pyruvate decarboxylase and alcohol dehydrogenase from Z. mobilis. A ¹³C-labeling experiment using [1-¹³C] glucose indicated a distinctive ¹³C distribution of ethanol between the parental and the ED-introduced strains, which suggested an alteration of carbon flux as a consequence of ED pathway introduction. The ED-introduced strain, CRZ2e-ED, consumed glucose 1.5-fold faster than the parental strain. A pfkA deletion mutant of CRZ2e-ED (CRZ2e-EDΔpfkA) was also constructed to evaluate the effects of EMP pathway inactivation, which showed an almost identical rate of glucose consumption compared to that of the parental CRZ2e strain. The introduction of the ED pathway did not alter the intracellular NADH/NAD⁺ ratio, whereas it resulted in a slight increase in the ATP/ADP ratio. The recombinant strains with simultaneous overexpression of the genes for the EMP and ED pathways exhibited the highest ethanol productivity among all C. glutamicum strains ever constructed.

CONCLUSIONS

The increased sugar consumption observed in ED-introduced strains was not a consequence of cofactor balance alterations, but rather the crucial coexistence of two active glycolytic pathways for enhanced glucose consumption. Coexistence of the ED and EMP pathways is a good strategy for improving biocatalyst productivity even when NADPH supply is not a limiting factor for fermentation.

Metabolic engineering of an auto-regulated Corynebacterium glutamicum chassis for biosynthesis of 5-aminolevulinic acid

One challenge in metabolic engineering for industrial applications is the construction of highly efficient microbial cell factories. For this purpose, dynamic regulation of metabolic flux may be indispensable. In this study, an auto-regulated Corynebacterium glutamicum chassis for 5-aminolevulinic acid (5-ALA) biosynthesis was constructed. First, the expression of critical genes involved in 5-ALA synthesis and cofactor regeneration was precisely modulated. Furthermore, odhA expression was controlled using the strategies of static metabolic engineering (SME, with a weak promoter), dynamic metabolic engineering (DME, with a temperature-sensitive plasmid), and auto-inducible metabolic engineering (AME, with a growth-related promoter). The AME strategy showed the best effect and dynamically balanced the tradeoff between cell growth and 5-ALA synthesis. Additionally, the expression of exporter-encoding rhtA was regulated using AME strategy by the two-component system HrrSA in response to extracellular heme. The final strain A30 achieved the highest 5-ALA production (3.16 g/L) ever reported in C. glutamicum through C5 pathway.

Application of RecET-Cre/loxP system in Corynebacterium glutamicum ATCC14067 for L-leucine production

OBJECTIVE

To explore the RecET-Cre/loxP system for chromosomal replacement of promoter and its application on enhancement L-leucine production in Corynebacterium glutamicum (C. glutamicum) ATCC14067.

RESULTS

 The RecET-Cre/loxP system was used to achieve the chromosomal replacement of promoter in C. glutamicum ATCC14067 to adjust the metabolic flux involving the L-leucine synthetic pathway. First, leuA^r_13032 from C. glutamicum ATCC13032 which carried two mutations was overexpressed to release enzyme feedback inhibition. Then, comparing different mutations in ilvBNC gene clusters, the results indicated that ilvBNC_CP was most effective to enhance the metabolic flux of pyruvate towards L-leucine synthesis. The promoters of pck, odx and pyk2 were overexpressed under the strong promoter Peftu or Psod to improve the supply of pyruvate. Besides, the promoter PilvBNC was employed to dynamically control the transcription level of icd due to its attenuation mechanism by responding to the concentration of L-leucine. The final engineered strain produced 14.05 g L-leucine/L in flask cultivation.

CONCLUSION

 The RecET-Cre/loxP system is effective for gene manipulation in C. glutamicum ATCC14067. Besides, the results demonstrate the potential of C. glutamicum ATCC14067 for L-leucine production and provide new targets and strategies for strain development.

Systematic identification and elimination of flux bottlenecks in the aldehyde production pathway of Synechococcus elongatus PCC 7942

Isotopically nonstationary metabolic flux analysis (INST-MFA) provides a versatile platform to quantitatively assess in vivo metabolic activities of autotrophic systems. By applying INST-MFA to recombinant aldehyde-producing cyanobacteria, we identified metabolic alterations that correlated with increased strain performance in order to guide rational metabolic engineering. We identified four reactions adjacent to the pyruvate node that varied significantly with increasing aldehyde production: pyruvate kinase (PK) and acetolactate synthase (ALS) fluxes were directly correlated with product formation, while pyruvate dehydrogenase (PDH) and phosphoenolpyruvate carboxylase (PPC) fluxes were inversely correlated. Overexpression of enzymes for PK or ALS did not result in further improvements to the previous best-performing strain, while downregulation of PDH expression (through antisense RNA expression) or PPC flux (through expression of the reverse reaction, phosphoenolpyruvate carboxykinase) provided significant improvements. These results illustrate the potential of INST-MFA to enable a systematic approach for iterative identification and removal of pathway bottlenecks in autotrophic host cells.

Rational modification of the carbon metabolism of Corynebacterium glutamicum to enhance l-leucine production

l-Leucine is an essential amino acid that has wide and expanding applications in the industry. It is currently fast-growing market demand that provides a powerful impetus to further increase its bioconversion productivity and production stability. In this study, we rationally engineered the metabolic flux from pyruvate to l-leucine synthesis in Corynebacterium glutamicum to enhance both pyruvate availability and l-leucine synthesis. First, the pyc (encoding pyruvate carboxylase) and avtA (encoding alanine-valine aminotransferase) genes were deleted to weaken the metabolic flux of the tricarboxylic acid cycle and reduce the competitive consumption of pyruvate. Next, the transcriptional level of the alaT gene (encoding alanine aminotransferase) was down regulated by inserting a terminator to balance l-leucine production and cell growth. Subsequently, the genes involved in l-leucine biosynthesis were overexpressed by replacing the native promoters PleuA and PilvBNC of the leuA gene and ilvBNC operon, respectively, with the promoter Ptuf of eftu (encoding elongation factor Tu) and using a shuttle expression vector. The resulting strain WL-14 produced 28.47 ± 0.36 g/L l-leucine in shake flask fermentation.

Anaerobic glucose consumption is accelerated at non-proliferating elevated temperatures through upregulation of a glucose transporter gene in Corynebacterium glutamicum

Cell proliferation is achieved through numerous enzyme reactions. Temperature governs the activity of each enzyme, ultimately determining the optimal growth temperature. The synthesis of useful chemicals and fuels utilizes a fraction of available metabolic pathways, primarily central metabolic pathways including glycolysis and the tricarboxylic acid cycle. However, it remains unclear whether the optimal temperature for these pathways is correlated with that for cell proliferation. Here, we found that wild-type Corynebacterium glutamicum displayed increased glycolytic activity under non-growing anaerobic conditions at 42.5 °C, at which cells do not proliferate under aerobic conditions. At this temperature, glucose consumption was not inhibited and increased by 28% compared with that at the optimal growth temperature of 30 °C. Transcriptional analysis revealed that a gene encoding glucose transporter (iolT2) was upregulated by 12.3-fold compared with that at 30 °C, with concomitant upregulation of NCgl2954 encoding the iolT2-regulating transcription factor. Deletion of iolT2 decreased glucose consumption rate at 42.5 °C by 28%. Complementation of iolT2 restored glucose consumption rate, highlighting the involvement of iolT2 in the accelerating glucose consumption at an elevated temperature. This study shows that the optimal temperature for glucose metabolism in C. glutamicum under anaerobic conditions differs greatly from that for cell growth under aerobic conditions, being beyond the upper limit of the growth temperature. This is beneficial for fuel and chemical production not only in terms of increasing productivity but also for saving cooling costs. KEY POINTS: • C. glutamicum accelerated anaerobic glucose consumption at elevated temperature. • The optimal temperature for glucose consumption was above the upper limit for growth. • Gene expression involved in glucose transport was upregulated at elevated temperature. Graphical abstract.

Enhanced production of recombinant proteins in Corynebacterium glutamicum by constructing a bicistronic gene expression system

BACKGROUND

Corynebacterium glutamicum is a traditional food-grade industrial microorganism, in which an efficient endotoxin-free recombinant protein expression factory is under developing in recent years. However, the intrinsic disadvantage of low recombinant protein expression level is still difficult to be solved. Here, according to the bacteria-specific polycistronic feature that multiple proteins can be translated in one mRNA, efforts have been made to insert a leading peptide gene upstream of target genes as an expression enhancer, and it is found that this can remarkably improve the expression level of proteins under the control of inducible tac promoter in C. glutamicum.

RESULTS

In this research, the Escherichia coli (E. coli) tac promoter combined with 24 different fore-cistron sequences were constructed in a bicistronic manner in C. glutamicum. Three strong bicistronic expression vectors were isolated and exhibited high efficiency under different culture conditions. The compatibility of these bicistronic vectors was further validated using six model proteins- aldehyde dehydrogenase (ALDH), alcohol dehydrogenase (ADH), RamA (regulator of acetate metabolism), Bovine interferon-α (BoIFN-α), glycoprotein D protein (gD) of infectious bovine rhinotracheitis virus (IBRV) and procollagen type Ι N-terminal peptide (PΙNP). All examined proteins were highly expressed compared with the original vector with tac promoter. Large-scale production of PΙNP was also performed in fed-batch cultivation, and the highest PΙNP production level was 1.2 g/L.

CONCLUSION

In this study, the strength of the inducible tac promoter for C. glutamicum was improved by screening and inserting fore-cistron sequences in front of the target genes. Those vectors with bicistronic expression patterns have strong compatibility for expressing various heterogeneous proteins in high yield. This new strategy could be used to further improve the performance of inducible promoters, achieving double competence of inducible control and high yield.

Microbial Engineering for Production of N-Functionalized Amino Acids and Amines

N-functionalized amines play important roles in nature and occur, for example, in the antibiotic vancomycin, the immunosuppressant cyclosporine, the cytostatic actinomycin, the siderophore aerobactin, the cyanogenic glucoside linamarin, and the polyamine spermidine. In the pharmaceutical and fine-chemical industries N-functionalized amines are used as building blocks for the preparation of bioactive molecules. Processes based on fermentation and on enzyme catalysis have been developed to provide sustainable manufacturing routes to N-alkylated, N-hydroxylated, N-acylated, or other N-functionalized amines including polyamines. Metabolic engineering for provision of precursor metabolites is combined with heterologous N-functionalizing enzymes such as imine or ketimine reductases, opine or amino acid dehydrogenases, N-hydroxylases, N-acyltransferase, or polyamine synthetases. Recent progress and applications of fermentative processes using metabolically engineered bacteria and yeasts along with the employed enzymes are reviewed and the perspectives on developing new fermentative processes based on insight from enzyme catalysis are discussed.

Requirement of de novo synthesis of pyruvate carboxylase in long-term succinic acid production in Corynebacterium glutamicum

Protein turnover through de novo synthesis is critical for sustainable cellular functions. We previously found that glucose consumption rate in Corynebacterium glutamicum under anaerobic conditions increased at temperature higher than the upper limit of growth temperature. Here, we showed that production of lactic and succinic acids increased at higher temperature for long-term (48 h) anaerobic reaction in metabolically engineered strains. At 42 °C, beyond the upper limit of growth temperature range, biomass-specific lactic acid production rate was 8% higher than that at 30 °C, the optimal growth temperature. In contrast, biomass-specific succinic acid production rate was highest at 36 °C, 28% higher than that at 30 °C, although the production at 42 °C was still 23% higher than that at 30 °C. As enzymes are usually unstable at high temperatures, we investigated whether protein turnover of metabolic enzymes is required for the production of lactic and succinic acids under these conditions. Interestingly, when de novo protein synthesis was inhibited by addition of chloramphenicol, after 6 h, only succinic acid production was inhibited. Because glycolytic enzymes are involved in both lactic and succinic acids synthesis, enzymes in the anaplerotic pathway and the tricarboxylic acid (TCA) cycle leading to succinic acid synthesis were likely to be responsible for its decreased production. Among the five enzymes examined, the specific activity of only pyruvate carboxylase was drastically decreased after 48 h at 42 °C. Thus, the de novo synthesis of pyruvate carboxylase is required for long-term production of succinic acid. Graphical abstract KEY POINTS: • Long-term reaction for organic acids can be improved at temperature beyond ideal growth conditions. • De novo synthesis of pyruvate carboxylase is required for long-term succinic acid production.

Improvement of l-Valine Production by Atmospheric and Room Temperature Plasma Mutagenesis and High-Throughput Screening in Corynebacterium glutamicum

As one of the branched-chain amino acids, l-valine is an essential nutrient for most mammalian species. In this study, the l-valine producer Corynebacterium glutamicum ΔppcΔaceEΔalatΔpqo was first constructed. Additionally, an improved biosensor based on the Lrp-type transcriptional regulator and temperature-sensitive replication was built. Then, the C. glutamicum strain was mutagenized by atmospheric and room temperature plasma. A sequential three-step procedure was carried out to screen l-valine-producing strains, including the fluorescence-activated cell sorting (FACS), 96-well plate screening, and flask fermentation. The final mutant HL2-7 obtained by screening produced 3.20 g/L of l-valine, which was 21.47% higher than the titer produced by the starting strain. This study demonstrates that the l-valine-producing mutants can be successfully isolated based on the Lrp sensor system in combination with FACS screening after random mutagenesis.

Establishment and application of multiplexed CRISPR interference system in Bacillus licheniformis

Bacillus licheniformis has been regarded as an outstanding microbial cell factory for the production of biochemicals and enzymes. Due to lack of genetic tools to repress gene expression, metabolic engineering and gene function elucidation are limited in this microbe. In this study, an integrated CRISPR interference (CRISPRi) system was constructed in B. licheniformis. Several endogenous genes, including yvmC, cypX, alsD, pta, ldh, and essential gene rpsC, were severed as the targets to test this CRISPRi system, and the repression efficiencies were ranged from 45.02 to 94.00%. Moreover, the multiple genes were simultaneously repressed with high efficiency using this CRISPRi system. As a case study, the genes involved in by-product synthetic and L-valine degradation pathways were selected as the silence targets to redivert metabolic flux toward L-valine synthesis. Repression of acetolactate decarboxylase (alsD) and leucine dehydrogenase (bcd) led to 90.48% and 80.09 % increases in L-valine titer, respectively. Compared with the control strain DW9i△leuA (1.47 g/L and 1.79 g/L), the L-valine titers of combinatorial strain DW9i△leuA/pHYi-alsD-bcd were increased by 1.27-fold and 2.89-fold, respectively, in flask and bioreactor. Collectively, this work provides a feasible approach for multiplex metabolic engineering and functional genome studies of B. licheniformis.

Metabolic engineering of l-leucine production in Escherichia coli and Corynebacterium glutamicum: a review

l-Leucine, as an essential branched-chain amino acid for humans and animals, has recently been attracting much attention because of its potential for a fast-growing market demand. The applicability ranges from flavor enhancers, animal feed additives and ingredients in cosmetic to specialty nutrients in pharmaceutical and medical fields. Microbial fermentation is the major method for producing l-leucine by using Escherichia coli and Corynebacterium glutamicum as host bacteria. This review gives an overview of the metabolic pathway of l-leucine (i.e. production, import and export systems) and highlights the main regulatory mechanisms of operons in E. coli and C. glutamicum l-leucine biosynthesis. We summarize here the current trends in metabolic engineering techniques and strategies for manipulating l-leucine producing strains. Finally, future perspectives to construct industrially advantageous strains are considered with respect to recent advances in biology.

Improvement of l-Leucine Production in Corynebacterium glutamicum by Altering the Redox Flux

The production of l-leucine was improved by the disruption of ltbR encoding transcriptional regulator and overexpression of the key genes (leuAilvBNCE) of the l-leucine biosynthesis pathway in Corynebacterium glutamicum XQ-9. In order to improve l-leucine production, we rationally engineered C. glutamicum to enhance l-leucine production, by improving the redox flux. On the basis of this, we manipulated the redox state of the cells by mutating the coenzyme-binding domains of acetohydroxyacid isomeroreductase encoded by ilvC, inserting NAD-specific leucine dehydrogenase, encoded by leuDH from Lysinibacillus sphaericus, and glutamate dehydrogenase encoded by rocG from Bacillus subtilis, instead of endogenous branched-chain amino acid transaminase and glutamate dehydrogenase, respectively. The yield of l-leucine reached 22.62 ± 0.17 g·L^-1 by strain ΔLtbR-acetohydroxyacid isomeroreductase (AHAIR)^M/ABNC^ME, and the concentrations of the by-products (l-valine and l-alanine) increased, compared to the strain ΔLtbR/ABNCE. Strain ΔLtbR-AHAIR^MLeuDH/ABNC^MLDH accumulated 22.87±0.31 g·L^-1 l-leucine, but showed a drastically low l-valine accumulation (from 8.06 ± 0.35 g·L^-1 to 2.72 ± 0.11 g·L^-1), in comparison to strain ΔLtbR-AHAIR^M/ABNC^ME, which indicated that LeuDH has much specificity for l-leucine synthesis but not for l-valine synthesis. Subsequently, the resultant strain ΔLtbR-AHAIR^MLeuDHRocG/ABNC^MLDH accumulated 23.31 ± 0.24 g·L^-1 l-leucine with a glucose conversion efficiency of 0.191 g·g^-1.

Application of leucine dehydrogenase Bcd from Bacillus subtilis for l-valine synthesis in Escherichia coli under microaerobic conditions

Microaerobic cultivation conditions have been shown experimentally and theoretically to improve the performance of a number of bioproduction systems. However, under these conditions, the production of l-valine by Escherichia coli is decreased mainly because of a redox cofactor imbalance and a decreased l-glutamate supply. The synthesis of one mole of l-valine from one mole of glucose generates two moles of NADH via glycolysis but consumes a total of two moles of NADPH, one in the ketol-acid reductoisomerase (KARI) reaction and the other in the regeneration of l-glutamate as an amino group donor for the branched-chain amino acid aminotransferase (BCAT) reaction. The improvement of l-valine synthesis under oxygen deprivation may be due to solving these problems. Increased l-valine synthesis under oxygen deprivation conditions was previously shown in Corynebacterium glutamicum (Hasegawa et al., 2012). In this study, we have proposed the use of NADH-dependent leucine dehydrogenase (LeuDH; EC 1.4.1.9) Bcd from B. subtilis instead of the native NADPH-dependent pathway including aminotransferase encoded by ilvE to improve l-valine production in E. coli under microaerobic conditions. We have created l-valine-producing strains on the base of the aminotransferase B-deficient strain V1 (B-7 ΔilvBN ΔilvIH ΔilvGME::P_L-ilvBN^N17KDA) by introducing one chromosomal copy of the bcd gene or the ilvE gene. Evaluation of the l-valine production by the obtained strains under microaerobic and aerobic conditions revealed that leucine dehydrogenase Bcd had a higher potential for l-valine production under microaerobic conditions. The Bcd-possessing strain exhibited 2.2-fold higher l-valine accumulation (up to 9.1 g/L) and 2.0-fold higher yield (up to 35.3%) under microaerobic conditions than the IlvE-possessing strain. The obtained results could be interpreted as follows: an altering of redox cofactor balance in the l-valine biosynthesis pathway increased the production and yield by E. coli cells under microaerobic conditions. Thus, the effective synthesis of l-valine by means of “valine fermentation” was shown in E. coli. This methodology has the advantages of being an economical and environmentally friendly process.

Genome shuffling and high-throughput screening of Brevibacterium flavum MDV1 for enhanced L-valine production

L-valine is an essential branched-amino acid that is widely used in multiple areas such as pharmaceuticals and special dietary products and its use is increasing. As the world market for L-valine grows rapidly, there is an increasing interest to develop an efficient L-valine-producing strain. In this study, a simple, sensitive, efficient, and consistent screening procedure termed 96 well plate-PC-HPLC (96-PH) was developed for the rapid identification of high-yield L-valine strains to replace the traditional L-valine assay. L-valine production by Brevibacterium flavum MDV1 was increased by genome shuffling. The starting strains were obtained using ultraviolet (UV) irradiation and binary ethylenimine treatment followed by preparation of protoplasts, UV irradiation inactivation, multi-cell fusion, and fusion of the inactivated protoplasts to produce positive colonies. After two rounds of genome shuffling and the 96-PH method, six L-valine high-yielding mutants were selected. One genetically stable mutant (MDVR2-21) showed an L-valine yield of 30.1 g/L during shake flask fermentation, 6.8-fold higher than that of MDV1. Under fed-batch conditions in a 30 L automated fermentor, MDVR2-21 accumulated 70.1 g/L of L-valine (0.598 mol L-valine per mole of glucose; 38.9% glucose conversion rate). During large-scale fermentation using a 120 m3 fermentor, this strain produced > 66.8 g/L L-valine (36.5% glucose conversion rate), reflecting a very productive and stable industrial enrichment fermentation effect. Genome shuffling is an efficient technique to improve production of L-valine by B. flavum MDV1. Screening using 96-PH is very economical, rapid, efficient, and well-suited for high-throughput screening.