(L)-Valine production with minimization of by-products' synthesis in Corynebacterium glutamicum and Brevibacterium flavum

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.

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.

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.

Blocking the 2,3-butanediol synthesis pathway of Klebsiella pneumoniae resulted in L-valine production

Klebsiella pneumoniae is a 2,3-butanediol producing bacterium. Nevertheless, a design and construction of L-valine production strain was studied in this paper. The first step of 2,3-butanediol synthesis and branched-chain amino acid synthesis pathways share the same step of α-acetolactate synthesis from pyruvate. However, the two pathways are existing in parallel and do not interfere with each other in the wild-type strain. A knockout of budA blocked the 2,3-butanediol synthesis pathway and resulted in the L-valine production. The budA coded an α-acetolactate decarboxylase and catalyzed the acetoin formation from α-acetolactate. Furthermore, blocking the lactic acid synthesis by knocking out of ldhA, which is encoding a lactate dehydrogenase, improved the L-valine synthesis. 2-Ketoisovalerate is the precursor of L-valine, it is also an intermediate of the isobutanol synthesis pathway, while indole-3-pyruvate decarboxylase (ipdC) is responsible for isobutyraldehyde formation from 2-ketoisovalerate. Production of L-valine has been improved by knocking out of ipdC. On the other side, the ilvE, encoding a transaminase B, reversibly transfers one amino group from glutamate to α-ketoisovalerate. Overexpression of ilvE exhibited a distinct improvement of L-valine production. The brnQ encodes a branched-chain amino acid transporter, and L-valine production was further improved by disrupting brnQ. It is also revealed that weak acidic and aerobic conditions favor L-valine production. Based on these findings, L-valine production by metabolically engineered K. pneumonia was examined. In fed-batch fermentation, 22.4 g/L of L-valine was produced by the engineered K. pneumoniae ΔbudA-ΔldhA-ΔipdC-ΔbrnQ-ilvE after 55 h of cultivation, with a substrate conversion ratio of 0.27 mol/mol glucose.

Enhancing β-alanine production from glucose in genetically modified Corynebacterium glutamicum by metabolic pathway engineering

To directly produce β-alanine from glucose by microbial fermentation, a recombinant Corynebacterium glutamicum strain with high efficiency of β-alanine production was constructed in this study. To do this, the biosynthetic pathway of β-alanine in an L-lysine-producing strain XQ-5 was modified by enhancing carbon flux in biosynthetic pathway and limiting carbon flux in competitive pathway. This study showed that replacement of L-aspartate kinase (AK) with wild-type AK and disruption of lactate dehydrogenase and alanine/valine aminotransferases increase β-alanine production because of decreasing the by-products accumulation. Moreover, L-aspartate-α-decarboxylase (ADC) from Bacillus subtilis was designed as the best enzyme for increasing β-alanine production, and its variant (BsADC^E56S/I88M) showed the highest activity for catalyzing L-aspartate to generate β-alanine. To further increase β-alanine production, expression level of BsADC^E56S/I88M was controlled by optimizing promoter and RBS, indicating that P_gro plus ThirRBS is the best combination for BsADC^E56S/I88M expression and β-alanine production. The resultant strain XQ-5.5 produced 30.7 ± 2.3 g/L of β-alanine with a low accumulation of lactate (from 5.2 ± 0.14 to 0.2 ± 0.09 g/L) and L-alanine (from 7.6 ± 0.22 to 3.8 ± 0. 32 g/L) in shake-flask fermentation and produced 56.5 ± 3.2 g/L of β-alanine with a productivity of 0.79 g/(L·h) and the glucose conversion efficiency (α) of 39.5% in feed-batch fermentation. This is the first report of genetically modifying the biosynthetic pathway of β-alanine that improves the efficiency of β-alanine production in an L-lysine-producing strain, and these results give us a new insight for constructing the other valuable biochemical. KEY POINTS: • Optimization and overexpression of the key enzyme BsADC increased the accumulation of β-alanine. • The AK was replaced with wild-type AK to increase the conversion of aspartic acid to β-alanine. • A 56.5-g/L β-alanine production in fed-batch fermentation was achieved.

Transcriptome analysis of L-leucine-producing Corynebacterium glutamicum under the addition of trimethylglycine

It has been widely reported that the addition of trimethylglycine (betaine) decreases osmotic pressure inhibition for cell growth, leading to increased production of amino acids. However, the underlying mechanism is unclear. To determine the global metabolic differences that occur under the addition of trimethylglycine, transcriptome analysis was performed. Transcriptome analysis of Corynebacterium glutamicum JL1211 revealed that 272 genes exhibited significant changes under trimethylglycine addition. We performed Gene Ontology (GO) and KEGG enrichment pathway analyses on these differentially expressed genes (DEGs). Significantly upregulated genes were mainly involved in the regulation of ABC transporters, especially phosphate transporters and sulfur metabolism. The three phosphate transporter genes pstC, pstA and pstB were upregulated by 13.06-fold, 29.80-fold and 30.49-fold, respectively. Notably, the transcriptional levels of the cysD, cysN, cysH and sir genes were upregulated by 81.5-fold, 57.3-fold, 77.6-fold and 125.4-fold, respectively, consistent with assimilatory sulfate reduction under the addition of trimethylglycine. The upregulation of ilvBN and leuD genes might result in increased L-leucine formation. The data indicated changes in the transcriptome of C. glutamicum with trimethylglycine treatment, thus providing a mechanism supporting the application of trimethylglycine in the production of L-leucine and other amino acids by C. glutamicum strains.

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.

Effect of fed-batch and chemostat cultivation processes of C. glutamicum CP for L-leucine production

Most of the current industrial processes for L-leucine production are based on fermentation, usually in fed-batch operation mode. Although the culture technology has advanced in recent decades, the process still has significant drawbacks. To solve these problems, we investigated the effects of chemostat culture conditions on the production of L-leucine by Corynebacterium glutamicum CP. The dilution rate, the nitrogen source, and the carbon–nitrogen ratio of the medium were optimized. With the addition of ammonium acetate to the chemostat medium, the initial C/N ratio was adjusted to 57.6, and the L-leucine titer reached the highest level at the optimal dilution rate of 0.04 h⁻¹. Compared with fed-batch culture, the L-leucine titer was reduced (from 53.0 to 24.8 g L⁻¹), but the yield from glucose was increased by 10.0% (from 0.30 to 0.33 mol mol⁻¹) and productivity was increased by 58.3% (from 1.2 to 1.9 g L⁻¹ h⁻¹). Moreover, the titer of the by-product L-alanine was significantly reduced (from 8.9 to 0.8 g L⁻¹). In addition, gene expression levels and activity of key enzymes in the synthesis of L-leucine and L-alanine were analyzed to explain the difference of production performance between chemostat culture and fed-batch culture. The results indicate that chemostat culture has great potential to increase the industrial production of L-leucine compared to current fed-batch approaches.

Increasing L-lysine production in Corynebacterium glutamicum by engineering amino acid transporters

Corynebacterium glutamicum has a long and successful history in the biotechnological production of L-lysine. Besides the adjustment of metabolic pathways, intracellular and extracellular transport systems are critical for the cellular metabolism of L-lysine or its by-products. Here, three amino acid transmembrane transporters, namely, GluE, BrnE/BrnF, and LysP, which are widely present in C. glutamicum strains, were each investigated by gene knockout. In comparison with that in the wild-type strain, the yield of L-lysine increased by 9.0%, 12.3%, and 10.0% after the deletion of the gluE, brnE/brnF, and lysP genes, respectively, in C. glutamicum 23,604. Moreover, the amount of by-product amino acids decreased significantly when the gluE and brnE/brnF genes were deleted. It was also demonstrated that there was no effect on the growth of the strain when the gluE or lysP gene was deleted, whereas the biomass of C. glutamicum WL1702 (ΔbrnE/ΔbrnF) in the fermentation medium was significantly reduced in comparison with that of the wild type. These results also provide useful information for enhancing the production of L-lysine or other amino acids by C. glutamicum.

Optimization of ribosomal binding site sequences for gene expression and 4-hydroxyisoleucine biosynthesis in recombinant corynebacterium glutamicum

4-Hydroxyisoleucine (4-HIL) has potential value for treating diabetes. α-Ketoglutarate (α-KG)-dependent l-isoleucine dioxygenase (IDO) can convert l-isoleucine (Ile) into 4-HIL. In our previous study, 4-HIL was de novo synthesized from glucose by expressing the ido gene in Corynebacterium glutamicum strain SN01, an Ile producer, and neither Ile nor α-KG was added. In this study, ribosomal binding site (RBS) engineering was applied for gene expression and 4-HIL biosynthesis in C. glutamicum. The 18 tested RBS sequences showed greatly differing strengths for expressing ido, and 8.10-104.22 mM 4-HIL was produced. To supply the cosubstrate α-KG at different levels, the odhI gene was then expressed using the RBS sequences of high, medium, and low strength in the above mentioned optimal strain SF01 carrying R8-ido. However, 4-HIL production decreased to varying amounts, and in some strains, the α-KG was redirected into l-glutamate synthesis. Next, the O₂ supply was further enhanced in three ido-odhI coexpressing strains by overexpressing the vgb gene, and 4-HIL production changed dramatically. 4-HIL (up to 119.27 ± 5.03 mM) was produced in the best strain, SF08, suggesting that the synchronic supply of cosubstrates α-KG and O₂ is critical for the high-yield production of 4-HIL. Finally, the avtA gene and the ldhA-pyk2 cluster were deleted separately in SF08 to reduce pyruvate-derived byproducts, and 4-HIL production increased to 122.16 ± 5.18 and 139.82 ± 1.56 mM, respectively, indicating that both strains were promising candidates for producing 4-HIL. Therefore, fine-tuning ido expression and the cosubstrates supply through RBS engineering is a useful strategy for improving 4-HIL biosynthesis in C. glutamicum.

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.

Metabolic engineering of Corynebacterium glutamicum WM001 to improve l-isoleucine production

In this study, l-isoleucine production in Corynebacterium glutamicum WM001 was improved by deleting three genes in the genome, replacing the native promoter of ilvA in the genome, and overexpression of five genes in an alr-based auxotrophic complementation expression system. The three genes deleted in the genome are alaT, brnQ, and alr. Deletion of alaT improved l-isoleucine production by increasing the supply of pyruvate, whereas deletion of brnQ improved l-isoleucine production by blocking the uptake of extracellular l-isoleucine. Exchange of the native promoter of ilvA with promoter tac or tacM could contribute to l-isoleucine production by increasing 2-ketobutyric acid; tac is better than tacM for improving l-isoleucine yield. Different combinations of the genes ilvBN, ppnK, lrp, and brnFE were overexpressed in an alr-based auxotrophic complementation expression system to further improve l-isoleucine production, and the best yield after 72-H flask fermentation was obtained from the strain WM005/pYCW-1-ilvBN2-ppnK1. Without addition of any antibiotics, WM005/pYCW-1-ilvBN2-ppnK1 could produce 32.1 g/L l-isoleucine after 72-H fed-batch fermentation, which is 34.3% increase compared with the original strain WM001.

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.

Optimizing a CRISPR-Cpf1-based genome engineering system for Corynebacterium glutamicum

Background

Corynebacterium glutamicum is an important industrial strain for the production of a diverse range of chemicals. Cpf1 nucleases are highly specific and programmable, with efficiencies comparable to those of Cas9. Although the Francisella novicida (Fn) CRISPR-Cpf1 system has been adapted for genome editing in C. glutamicum, the editing efficiency is currently less than 15%, due to false positives caused by the poor targeting efficiency of the crRNA.

Results

To address this limitation, a screening strategy was developed in this study to systematically evaluate crRNA targeting efficiency in C. glutamicum. We quantitatively examined various parameters of the C. glutamicum CRISPR-Cpf1 system, including the protospacer adjacent motif (PAM) sequence, the length of the spacer sequence, and the type of repair template. We found that the most efficient C. glutamicum crRNA contained a 5′-NYTV-3′ PAM and a 21 bp spacer sequence. Moreover, we observed that linear DNA could be used to repair double strand breaks.

Conclusions

Here, we identified optimized PAM-related parameters for the CRISPR-Cpf1 system in C. glutamicum. Our study sheds light on the function of the FnCpf1 endonuclease and Cpf1-based genome editing. This optimized system, with higher editing efficiency, could be used to increase the production of bulk chemicals, such as isobutyrate, in C. glutamicum.

Electronic supplementary material

The online version of this article (10.1186/s12934-019-1109-x) contains supplementary material, which is available to authorized users.

Metabolic engineering of Corynebacterium glutamicum for improved L-arginine synthesis by enhancing NADPH supply

Corynebacterium glutamicum SNK 118 was metabolically engineered with improved L-arginine titer. Considering the crucial role of NADPH level in L-arginine production, pntAB (membrane-bound transhydrogenase) and ppnK (NAD⁺ kinase) were co-expressed to increase the intracellular NADPH pool. Expression of pntAB exhibited significant effects on NADPH supply and L-arginine synthesis. Furthermore, argR and farR, encoding arginine repressor ArgR and transcriptional regulator FarR, respectively, were removed from the genome of C. glutamicum. The competitive branch pathway gene ldh was also deleted. Eventually, an engineered C. glutamicum JML07 was obtained for L-arginine production. Fed-batch fermentation in 5-L bioreactor employing strain JML07 allowed production of 67.01 g L^-1L-arginine with productivity of 0.89 g L^-1 h^-1 and yield of 0.35 g g^-1 glucose. This study provides a productive L-arginine fermentation strain and an effective cofactor manipulating strategy for promoting the biosynthesis of NADPH-dependent metabolites.

Improved l-Leucine Production in Corynebacterium glutamicum by Optimizing the Aminotransferases

The production of branched-chain amino acids (BCAAs) is still challenging, therefore we rationally engineered Corynebacterium glutamicum FA-1 to increase the l-leucine production by optimizing the aminotransferases. Based on this, we investigated the effects of the native aminotransferases, i.e., branched-chain amino acid aminotransferase (BCAT; encoded by ilvE) and aspartate aminotransferase (AspB; encoded by aspB) on l-leucine production in C. glutamicum. The strain FA-1△ilvE still exhibited significant growth without leucine addition, while FA-1△ilvE△aspB couldn't, which indicated that AspB also contributes to L-leucine synthesis in vivo and the yield of leucine reached 20.81 ± 0.02 g/L. It is the first time that AspB has been characterized for l-leucine synthesis activity. Subsequently, the aromatic aminotransferase TyrB and the putative aspartate aminotransferases, the aspC, yhdR, ywfG gene products, were cloned, expressed and characterized for leucine synthesis activity in FA-1△ilvE△aspB. Only TyrB was able to synthesize l-leucine and the l-leucine production was 18.55 ± 0.42 g/L. The two putative branched-chain aminotransferase genes, ybgE and CaIlvE, were also cloned and expressed. Both genes products function efficiently in BCAAs biosynthesis. This is the first report of a rational modification of aminotransferase activity that improves the l-leucine production through optimizing the aminotransferases.

Transcriptome analysis of the two unrelated fungal β-lactam producers Acremonium chrysogenum and Penicillium chrysogenum: Velvet-regulated genes are major targets during conventional strain improvement programs

BACKGROUND

Cephalosporins and penicillins are the most frequently used β-lactam antibiotics for the treatment of human infections worldwide. The main industrial producers of these antibiotics are Acremonium chrysogenum and Penicillium chrysogenum, two taxonomically unrelated fungi. Both were subjects of long-term strain development programs to reach economically relevant antibiotic titers. It is so far unknown, whether equivalent changes in gene expression lead to elevated antibiotic titers in production strains.

RESULTS

Using the sequence of PcbC, a key enzyme of β-lactam antibiotic biosynthesis, from eighteen different pro- and eukaryotic microorganisms, we have constructed a phylogenetic tree to demonstrate the distant relationship of both fungal producers. To address the question whether both fungi have undergone similar genetic adaptions, we have performed a comparative gene expression analysis of wild-type and production strains. We found that strain improvement is associated with the remodeling of the transcriptional landscape in both fungi. In P. chrysogenum, 748 genes showed differential expression, while 1572 genes from A. chrysogenum are differentially expressed in the industrial strain. Common in both fungi is the upregulation of genes belonging to primary and secondary metabolism, notably those involved in precursor supply for β-lactam production. Other genes not essential for β-lactam production are downregulated with a preference for those responsible for transport processes or biosynthesis of other secondary metabolites. Transcriptional regulation was shown to be an important parameter during strain improvement in different organisms. We therefore investigated deletion strains of the major transcriptional regulator velvet from both production strains. We identified 567 P. chrysogenum and 412 A. chrysogenum Velvet target genes. In both deletion strains, approximately 50% of all secondary metabolite cluster genes are differentially regulated, including β-lactam biosynthesis genes. Most importantly, 35-57% of Velvet target genes are among those that showed differential expression in both improved industrial strains.

CONCLUSIONS

The major finding of our comparative transcriptome analysis is that strain improvement programs in two unrelated fungal β-lactam antibiotic producers alter the expression of target genes of Velvet, a global regulator of secondary metabolism. From these results, we conclude that regulatory alterations are crucial contributing factors for improved β-lactam antibiotic titers during strain improvement in both fungi.

Increased glucose utilization and cell growth of Corynebacterium glutamicum by modifying the glucose-specific phosphotransferase system (PTSGlc) genes

The phosphoenolpyruvate:glucose phosphotransferase system (PTSGlc) is the major pathway of glucose uptake in Corynebacterium glutamicum. This study investigated glucose consumption rate, cell growth, and metabolite changes resulting from modification of PTSGlc. The classical l-lysine producer C. glutamicum XQ-8 exhibited low glucose consumption, cell growth, and l-lysine production rates, whereas these parameters were significantly increased during cultivating on glucose plus maltose, through inactivation of SugR, or by overexpression of PTSGlc genes. XQ-8sugR::cat/pDXW-8-ptsI exhibited the highest increase in glucose consumption, growth rate, and l-lysine production, followed by XQ-8sugR::cat/pDXW-8-ptsG. However, overexpression of ptsH had little effect on the above-mentioned factors. Although co-overexpression of ptsGHI led to the highest glucose consumption, growth rate, and final l-lysine production; the l-lysine production rate was lower than that of XQ-8sugR::cat/pDXW-8-ptsIH. In fed-batch fermentation, XQ-8sugR::cat/pDXW-8-ptsIH had a higher growth rate of 0.54 h−1 to a dry cell mass of 66 g·L−1 after 16 h, and had a higher l-lysine production rate of 159.2 g·L−1 after 36 h. These results indicate that modification of the sugar transport systems improves amino acid production, especially for mutants obtained by repeated physical and (or) chemical mutagenesis. However, modification of these systems needs to be performed on a case-by-case basis.

Strategies used for genetically modifying bacterial genome: site-directed mutagenesis, gene inactivation, and gene over-expression

With the availability of the whole genome sequence of Escherichia coli or Corynebacterium glutamicum, strategies for directed DNA manipulation have developed rapidly. DNA manipulation plays an important role in understanding the function of genes and in constructing novel engineering bacteria according to requirement. DNA manipulation involves modifying the autologous genes and expressing the heterogenous genes. Two alternative approaches, using electroporation linear DNA or recombinant suicide plasmid, allow a wide variety of DNA manipulation. However, the over-expression of the desired gene is generally executed via plasmid-mediation. The current review summarizes the common strategies used for genetically modifying E. coli and C. glutamicum genomes, and discusses the technical problem of multi-layered DNA manipulation. Strategies for gene over-expression via integrating into genome are proposed. This review is intended to be an accessible introduction to DNA manipulation within the bacterial genome for novices and a source of the latest experimental information for experienced investigators.

A method for simultaneous gene overexpression and inactivation in the Corynebacterium glutamicum genome

The gene integration method is an important tool to stably express desirable genes in bacteria. To avoid heavy workload and cost, we constructed a rapid and efficient method for genome modification. This method depended on a mobilizable plasmid, which contains a P tac promoter, an introduced multiple cloning site (iMCS), and rrnBT1T2 terminator. Briefly, the mobilizable plasmid pK18-MBPMT with the P tac-iMCS-rrnBT1T2 cartridge derived from pK18mobsacB was prepared to directly integrate hetero-/homologous DNA into the Corynebacterium glutamicum genome. Like our previous method, this method was based on insertional inactivation and double-crossover homologous recombination, which simultaneously achieved gene overexpression and inactivation in the genome without the use of genetic markers. Compared to the previous method, this protocol omitted the construction of a recombinant expression plasmid and clone of the target gene(s) cassette, which significantly decreased the workload, cost, and operational time. Using this method, the heterologous gene amy and the homologous gene lysC (T311I) were successfully integrated into the C. glutamicum genome at alaT and avtA loci, respectively. Moreover, the operation time of this method was shorter than that of the previous method, especially for repeated integration. This method, which is based on the mobilizable plasmid pK18-MBPMT, thus represents a potentially attractive protocol for the integration of genes in the course of genetic modification of C. glutamicum.

Enhancing pentose phosphate pathway in Corynebacterium glutamicum to improve l-isoleucine production

Three genes, gnd, pgl, and fbp, relevant to the pentose phosphate pathway (PPP) were overexpressed in Corynebacterium glutamicum IWJ001, leading to increase of l-isoleucine production. The transcriptional levels of gnd, pgl, and fbp significantly increased in IWJ001/pDXW-8-gnd-fbp-pgl. Compared with the control strain IWJ001/pDXW-8, intracellular NADPH/NADP⁺ ratios in IWJ001/pDXW-8-gnd and IWJ001/pDXW-8-gnd-fbp cells grown for 36 H increased threefold and fourfold, respectively, indicating that overexpression of gnd and fbp redirected the carbon flux to PPP. Intracellular NADPH/NADP⁺ ratio in IWJ001/pDXW-8-gnd-fbp-pgl grown for 36 H was similar to IWJ001/pDXW-8, suggesting that the NADPH produced by PPP could be quickly consumed for l-isoleucine production. 10.9 and 28.96 g/L of l-isoleucine was produced in IWJ001/pDXW-8-gnd-fbp-pgl in shake flask cultivation and fed-batch fermentation, respectively. In addition, IWJ001/pDXW-8-gnd-fbp-pgl grew fast, its dry cell weight reached 49 g/L after 48 H, whereas the start strain IWJ001/pDXW-8 reached only 40 g/L. After 96 H fermentation, l-isoleucine yield on glucose in IWJ001/pDXW-8-gnd-fbp-pgl reached 0.138 g/g. The results demonstrate that carbon flux redirection to PPP is an efficient approach to enhance l-isoleucine production in C. glutamicum.

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.

Overexpression of ribosome elongation factor G and recycling factor increases L-isoleucine production in Corynebacterium glutamicum

Ribosome elongation factor G encoded by fusA promotes the translocation step of protein synthesis in bacteria; ribosome recycling factor encoded by frr, together with the elongation factor G, dissociates ribosomes from messenger RNA after the termination of translation. Both factors play important roles during protein synthesis in bacteria. In this study, we found that overexpression of fusA and/or frr led to the increase of L-isoleucine production in Corynebacterium glutamicum IWJ001, an L-isoleucine production strain generated by random mutagenesis. Reverse transcription polymerase chain reaction analysis showed that transcriptional levels of genes lysC, hom, thrB, ilvA, ilvBN, and ilvE encoding the key enzymes in the biosynthetic pathway of L-isoleucine increased in C. glutamicum IWJ001 when fusA and/or frr were overexpressed. Co-overexpression of fusA and frr, together with genes ilvA, ilvB, ilvN, and ppnk in C. glutamicum IWJ001, led to 76.5 % increase of L-isoleucine production in flask cultivation and produced 28.5 g/L L-isoleucine in 72-h fed-batch fermentation. The results demonstrate that overexpressing ribosome elongation factor G and ribosome recycling factor is an efficient approach to enhance L-isoleucine production in C. glutamicum.

L-Serine overproduction with minimization of by-product synthesis by engineered Corynebacterium glutamicum

The direct fermentative production of L-serine by Corynebacterium glutamicum from sugars is attractive. However, superfluous by-product accumulation and low L-serine productivity limit its industrial production on large scale. This study aimed to investigate metabolic and bioprocess engineering strategies towards eliminating by-products as well as increasing L-serine productivity. Deletion of alaT and avtA encoding the transaminases and introduction of an attenuated mutant of acetohydroxyacid synthase (AHAS) increased both L-serine production level (26.23 g/L) and its productivity (0.27 g/L/h). Compared to the parent strain, the by-products L-alanine and L-valine accumulation in the resulting strain were reduced by 87 % (from 9.80 to 1.23 g/L) and 60 % (from 6.54 to 2.63 g/L), respectively. The modification decreased the metabolic flow towards the branched-chain amino acids (BCAAs) and induced to shift it towards L-serine production. Meanwhile, it was found that corn steep liquor (CSL) could stimulate cell growth and increase sucrose consumption rate as well as L-serine productivity. With addition of 2 g/L CSL, the resulting strain showed a significant improvement in the sucrose consumption rate (72 %) and the L-serine productivity (67 %). In fed-batch fermentation, 42.62 g/L of L-serine accumulation was achieved with a productivity of 0.44 g/L/h and yield of 0.21 g/g sucrose, which was the highest production of L-serine from sugars to date. The results demonstrated that combined metabolic and bioprocess engineering strategies could minimize by-product accumulation and improve L-serine productivity.

Metabolic engineering Corynebacterium glutamicum for the L-lysine production by increasing the flux into L-lysine biosynthetic pathway

The experiments presented here were based on the conclusions of our previous results. In order to avoid introduction of expression plasmid and to balance the NADH/NAD ratio, the NADH biosynthetic enzyme, i.e., NAD-dependent glyceraldehyde-3-phosphate dehydrogenase (GADPH), was replaced by NADP-dependent GADPH, which was used to biosynthesize NADPH rather than NADH. The results indicated that the NADH/NAD ratio significantly decreased, and glucose consumption and L-lysine production drastically improved. Moreover, increasing the flux through L-lysine biosynthetic pathway and disruption of ilvN and hom, which involve in the branched amino acid and L-methionine biosynthesis, further improved L-lysine production by Corynebacterium glutamicum. Compared to the original strain C. glutamicum Lys5, the L-lysine production and glucose conversion efficiency (α) were enhanced to 81.0 ± 6.59 mM and 36.45% by the resulting strain C. glutamicum Lys5-8 in shake flask. In addition, the by-products (i.e., L-threonine, L-methionine and L-valine) were significantly decreased as results of genetic modification in homoserine dehydrogenase (HSD) and acetohydroxyacid synthase (AHAS). In fed-batch fermentation, C. glutamicum Lys5-8 began to produce L-lysine at post-exponential growth phase and continuously increased over 36 h to a final titer of 896 ± 33.41 mM. The L-lysine productivity was 2.73 g l(-1) h(-1) and the α was 47.06% after 48 h. However, the attenuation of MurE was not beneficial to increase the L-lysine production because of decreasing the cell growth. Based on the above-mentioned results, we get the following conclusions: cofactor NADPH, precursor, the flux through L-lysine biosynthetic pathway and DCW are beneficial to improve L-lysine production in C. glutamicum.

A method for gene amplification and simultaneous deletion in Corynebacterium glutamicum genome without any genetic markers

A method for the simultaneous replacement of a given gene by a target gene, leaving no genetic markers, has been developed. The method is based on insertional inactivation and double-crossover homologous recombination. With this method, the lysC(T311I), fbp and ddh genes were inserted into Corynebacterium glutamicum genome, and the pck, alaT and avtA genes were deleted. Mobilizable plasmids with lysC(T311I), fbp and ddh cassettes and two homologous arms on the ends of pck, alaT and avtA were constructed, and then transformed into C. glutamicum. The target-expression cassettes were inserted in the genome via the first homologous recombination, and the genetic markers were removed via the second recombination. The target-transformants were sequentially screened from kanamycin-resistance and sucrose-resistance plates. The enzyme activities of transformants were stably maintained for 30 generations under non-selective culture conditions, suggesting that the integrated cassettes in host were successfully expressed and maintained as stable chromosomal insertions in C. glutamicum. The target-transformants were used to optimize the l-lysine production, showing that the productions were strongly increased because the selected genes were closely linked to l-lysine production. In short, this method can be used to construct amino acid high-producing strains with unmarked gene amplification and simultaneous deletion in genome.

Corynebacterium glutamicum promoters: a practical approach

Transcription initiation is the key step in gene expression in bacteria, and it is therefore studied for both theoretical and practical reasons. Promoters, the traffic lights of transcription initiation, are used as construction elements in biotechnological efforts to coordinate ‘green waves’ in the metabolic pathways leading to the desired metabolites. Detailed analyses of Corynebacterium glutamicum promoters have already provided large amounts of data on their structures, regulatory mechanisms and practical capabilities in metabolic engineering. In this minireview the main aspects of promoter studies, the methods developed for their analysis and their practical use in C. glutamicum are discussed. These include definitions of the consensus sequences of the distinct promoter classes, promoter localization and characterization, activity measurements, the functions of transcriptional regulators and examples of practical uses of constitutive, inducible and modified promoters in biotechnology. The implications of the introduction of novel techniques, such as in vitro transcription and RNA sequencing, to C. glutamicum promoter studies are outlined.

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

We previously demonstrated efficient L-valine production by metabolically engineered Corynebacterium glutamicum under oxygen deprivation. To achieve the high productivity, a NADH/NADPH cofactor imbalance during the synthesis of l-valine was overcome by engineering NAD-preferring mutant acetohydroxy acid isomeroreductase (AHAIR) and using NAD-specific leucine dehydrogenase from Lysinibacillus sphaericus. Lactate as a by-product was largely eliminated by disrupting the lactate dehydrogenase gene ldhA. Nonetheless, a few other by-products, particularly succinate, were still produced and acted to suppress the L-valine yield. Eliminating these by-products therefore was deemed key to improving theL-valine yield. By additionally disrupting the phosphoenolpyruvate carboxylase gene ppc, succinate production was effectively suppressed, but both glucose consumption and L-valine production dropped considerably due to the severely elevated intracellular NADH/NAD(+) ratio. In contrast, this perturbed intracellular redox state was more than compensated for by deletion of three genes associated with NADH-producing acetate synthesis and overexpression of five glycolytic genes, including gapA, encoding NADH-inhibited glyceraldehyde-3-phosphate dehydrogenase. Inserting feedback-resistant mutant acetohydroxy acid synthase and NAD-preferring mutant AHAIR in the chromosome resulted in higher L-valine yield and productivity. Deleting the alanine transaminase gene avtA suppressed alanine production. The resultant strain produced 1,280 mM L-valine at a yield of 88% mol mol of glucose(-1) after 24 h under oxygen deprivation, a vastly improved yield over our previous best.