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

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.

HyCas9-12aGEP: an efficient genome editing platform for Corynebacterium glutamicum

Corynebacterium glutamicum plays a crucial role as a significant industrial producer of metabolites. Despite the successful development of CRISPR-Cas9 and CRISPR-Cas12a-assisted genome editing technologies in C. glutamicum, their editing resolution and efficiency are hampered by the diverse on-target activities of guide RNAs (gRNAs). To address this problem, a hybrid CRISPR-Cas9-Cas12a genome editing platform (HyCas9-12aGEP) was developed in C. glutamicum in this study to co-express sgRNA (corresponding to SpCas9 guide RNA), crRNA (corresponding to FnCas12a guide RNA), or hfgRNA (formed by the fusion of sgRNA and crRNA). HyCas9-12aGEP improves the efficiency of mapping active gRNAs and outperforms both CRISPR-Cas9 and CRISPR-Cas12a in genome editing resolution and efficiency. In the experiment involving the deletion of the cg0697-0740 gene segment, an unexpected phenotype was observed, and HyCas9-12aGEP efficiently identified the responsible genotype from more than 40 genes. Here, HyCas9-12aGEP greatly improve our capability in terms of genome reprogramming in C. glutamicum.

Construction of a plasmid-free L-leucine overproducing Escherichia coli strain through reprogramming of the metabolic flux

BACKGROUND

L-Leucine is a high-value amino acid with promising applications in the medicine and feed industries. However, the complex metabolic network and intracellular redox imbalance in fermentative microbes limit their efficient biosynthesis of L-leucine.

RESULTS

In this study, we applied rational metabolic engineering and a dynamic regulation strategy to construct a plasmid-free, non-auxotrophic Escherichia coli strain that overproduces L-leucine. First, the L-leucine biosynthesis pathway was strengthened through multi-step rational metabolic engineering. Then, a cooperative cofactor utilization strategy was designed to ensure redox balance for L-leucine production. Finally, to further improve the L-leucine yield, a toggle switch for dynamically controlling sucAB expression was applied to accurately regulate the tricarboxylic acid cycle and the carbon flux toward L-leucine biosynthesis. Strain LEU27 produced up to 55 g/L of L-leucine, with a yield of 0.23 g/g glucose.

CONCLUSIONS

The combination of strategies can be applied to the development of microbial platforms that produce L-leucine and its derivatives.

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.

Improvement of acetyl-CoA supply and glucose utilization increases l-leucine production in Corynebacterium glutamicum

BACKGROUND

l-Leucine is one of important essential amino acids with multiple industrial applications, whose market requirements cannot be met because of the lower productivity.

MAIN METHODS AND MAJOR RESULTS

In this study, a strain of Corynebacterium glutamicum with high l-leucine yield was constructed to enhance its acetyl-CoA supply and glucose utilization. One copy of leuA under the control of a strong promoter was incorporated into the C. glutamicum genome. Then, acetyl-CoA supply was increased by the integration of a terminator in front of gltA and by the heterogeneous overexpression of acetyl-CoA synthetase (Acs) and deacetylase (CobB) derived from Escherichia coli. Next, the transcriptional regulator SugR was deleted to enhance glucose uptake via a phosphotransferase-mediated route. In fed-batch fermentation performed in a 5-L reactor, l-leucine production of 40.11±0.73 g/L was achieved under the optimized conditions, with the l-leucine yield and productivity of 0.25 g/g glucose and 0.59 g/L/h, respectively.

CONCLUSIONS AND IMPLICATIONS

These results represent a significant improvement in the l-leucine titer of C. glutamicum, indicating that the process possesses highly potential for industrial application. These strategies can be also expanded to enable the production of other value-added biochemicals derived from the intermediates of central carbon metabolism. This article is protected by copyright. All rights reserved.

Effect of low-level ultrasound treatment on the production of L-leucine by Corynebacterium glutamicum in fed-batch culture

Various process intensification methods were proposed to improve the yield, quality, and safety of fermented products. Here, we report the enhancement of L-leucine production by Corynebacterium glutamicum CP using ultrasound-assisted fed-batch fermentation. Response surface methodology was employed to optimize the sonication conditions. At an ultrasonic power density of 94 W/L, frequency of 25 kHz, interval of 31 min, and duration of 37 s, C. glutamicum CP produced 52.89 g/L of L-leucine in 44 h, representing a 21.6% increase compared with the control. The production performance of L-leucine was also improved under ultrasonic treatment. Moreover, the effects of ultrasound treatment on the fermentation performance of L-leucine were studied in terms of cell morphology, cell membrane permeability, and enzyme activity. The results indicate that ultrasonication is an efficient method for the intensification of L-leucine production by C. glutamicum CP.

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.

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.

Optimization of CRISPR-Cas9 through promoter replacement and efficient production of L-homoserine in Corynebacterium glutamicum

BACKGROUND

Corynebacterium glutamicum is an important chassis for industrial applications. The low efficiency of commonly used genome editing methods for C. glutamicum limits the rapid multiple engineering of the bacterium.

MAIN METHODS AND MAJOR RESULTS

In this study, chromosome-borne expression of cas9 and recET from Escherichia coli K12-MG1655 was achieved to avoid toxicity to the strain, increase the probability of homologous recombination, and reduce loss of viability caused by double-strand breaks. Constitutive strong promoters, such as P₄₅ , P_trc , and P_H36 , were used to replace P_glyA and to expand the application of the CRISPR-Cas9 system. By using this system, a C. glutamicum strain producing L-homoserine to 22.1 g per L in a 5-L bioreactor after 96 h was obtained.

CONCLUSIONS AND IMPLICATIONS

Through the application of visualized fluorescent protein, the process of plasmid curing was optimized, obtain a continuous and rapid CRISPR-Cas9 genome editing system. The method described here could be useful to construct C. glutamicum mutant rapidly.

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.