Systematic pathway engineering of Corynebacterium glutamicum S9114 for L-ornithine production

Fermentation improves antioxidant capacity and γ-aminobutyric acid content of Ganmai Dazao Decoction by lactic acid bacteria

Introduction

Ganmai Dazao Decoction is a traditional Chinese recipe, and is composed of licorice, floating wheat, and jujube.

Methods

Effects of lactic acid bacteria fermentation on the physicochemical properties, antioxidant activity, and γ-aminobutyric acid of Ganmai Dazao Decoction were studied. The changes of small and medium molecules in Ganmai Dazao Decoction before and after fermentation were determined by LC-MS non-targeted metabolomics.

Results

The results showed that the contents of lactic acid, citric acid, acetic acid, and total phenol content increased significantly, DPPH free radical clearance and hydroxyl free radical clearance were significantly increased. γ-aminobutyric acid content was 12.06% higher after fermentation than before fermentation. A total of 553 differential metabolites were detected and identified from the Ganmai Dazao Decoction before and after fermentation by partial least squares discrimination and VIP analysis.

Discussion

Among the top 30 differential metabolites with VIP values, the content of five functional substances increased significantly. Our results showed that lactic acid bacteria fermentation of Ganmai Dazao Decoction improves its antioxidant effects and that fermentation of Ganmai Dazao Decoction with lactic acid bacteria is an innovative approach that improves the health-promoting ingredients of Ganmai Dazao Decoction.

Remodeling metabolism of Corynebacterium glutamicum for high-level dencichine production

Dencichine, a sought-after compound in the medical industry, requires a more efficient and sustainable production method than the current plant extraction process. This study successfully remodeled the metabolic pathway of Corynebacterium glutamicum to produce dencichine from the precursors of L-2,3-diaminopropionate (L-DAP) and oxalyl-coenzyme A. Firstly, a synthetic pathway for L-DAP was established by introducing exogenous enzymes ZmaU/ZmaV. This resulted in a production of 628 mg/L by overexpressing key genes and reducing the endogenous competitive pathway. Secondly, an oxalyl-CoA synthetic pathway was created through the enzymatic conversion of glyoxylate by introducing heterologous enzymes. Finally, with the integration of the exogenous enzyme BAHD, de novo synthesis of dencichine in C. glutamicum was achieved, and production reached 31.75 mg/L within 48-hour fermentation. This achievement represents the first successful biosynthesis of dencichine in C. glutamicum, offering a promising approach for natural product through microbial fermentation.

Improvement in L-ornithine production from mannitol via transcriptome-guided genetic engineering in Corynebacterium glutamicum

BACKGROUND

L-Ornithine is an important medicinal intermediate that is mainly produced by microbial fermentation using glucose as the substrate. To avoid competition with human food resources, there is an urgent need to explore alternative carbon sources for L-ornithine production. In a previous study, we constructed an engineered strain, Corynebacterium glutamicum MTL13, which produces 54.56 g/L of L-ornithine from mannitol. However, compared with the titers produced using glucose as a substrate, the results are insufficient, and further improvement is required.

RESULTS

In this study, comparative transcriptome profiling of MTL01 cultivated with glucose or mannitol was performed to identify novel targets for engineering L-ornithine-producing strains. Guided by the transcriptome profiling results, we modulated the expression of qsuR (encoding a LysR-type regulator QsuR), prpC (encoding 2-methylcitrate synthase PrpC), pdxR (encoding a MocR-type regulator PdxR), acnR (encoding a TetR-type transcriptional regulator AcnR), CGS9114_RS08985 (encoding a hypothetical protein), and CGS9114_RS09730 (encoding a TetR/AcrR family transcriptional regulator), thereby generating the engineered strain MTL25 that can produce L-ornithine at a titer of 93.6 g/L, representing a 71.6% increase as compared with the parent strain MTL13 and the highest L-ornithine titer reported so far for C. glutamicum.

CONCLUSIONS

This study provides novel indirect genetic targets for enhancing L-ornithine accumulation on mannitol and lays a solid foundation for the biosynthesis of L-ornithine from marine macroalgae, which is farmed globally as a promising alternative feedstock.

Enhanced L-ornithine production from glucose and sucrose via manipulation of the fructose metabolic pathway in Corynebacterium glutamicum

L-Ornithine, an important non-essential amino acid, has considerable medicinal value in the treatment of complex liver diseases. Microbial fermentation strategies using robust engineered strains have remarkable potential for producing L-ornithine. We showed that glucose and sucrose co-utilization accumulate more L-ornithine in Corynebacterium glutamicum than glucose alone. Further manipulating the expression of intracellular fructose-1-phosphate kinase through the deletion of pfkB1resulted in the engineered strain C. glutamicum SO30 that produced 47.6 g/L of L-ornithine, which represents a 32.8% increase than the original strain C. glutamicum SO26 using glucose as substrate (35.88 g/L). Moreover, fed-batch cultivation of C. glutamicum SO30 in 5-L fermenters produced 78.0 g/L of L-ornithine, which was a 78.9% increase in yield compared with that produced by C. glutamicum SO26. These results showed that manipulating the fructose metabolic pathway increases L-ornithine accumulation and provides a reference for developing C. glutamicum to produce valuable metabolites.

Feedback regulation and coordination of the main metabolism for bacterial growth and metabolic engineering for amino acid fermentation

Living organisms such as bacteria are often exposed to continuous changes in the nutrient availability in nature. Therefore, bacteria must constantly monitor the environmental condition, and adjust the metabolism quickly adapting to the change in the growth condition. For this, bacteria must orchestrate (coordinate and integrate) the complex and dynamically changing information on the environmental condition. In particular, the central carbon metabolism (CCM), monomer synthesis, and macromolecular synthesis must be coordinately regulated for the efficient growth. It is a grand challenge in bioscience, biotechnology, and synthetic biology to understand how living organisms coordinate the metabolic regulation systems. Here, we consider the integrated sensing of carbon sources by the phosphotransferase system (PTS), and the feed-forward/feedback regulation systems incorporated in the CCM in relation to the pool sizes of flux-sensing metabolites and αketoacids. We also consider the metabolic regulation of amino acid biosynthesis (as well as purine and pyrimidine biosyntheses) paying attention to the feedback control systems consisting of (fast) enzyme level regulation with (slow) transcriptional regulation. The metabolic engineering for the efficient amino acid production by bacteria such as Escherichia coli and Corynebacterium glutamicum is also discussed (in relation to the regulation mechanisms). The amino acid synthesis is important for determining the rate of ribosome biosynthesis. Thus, the growth rate control (growth law) is further discussed on the relationship between (p)ppGpp level and the ribosomal protein synthesis.

Reconstructing a recycling and nonauxotroph biosynthetic pathway in Escherichia coli toward highly efficient production of L-citrulline

L-citrulline is a high-value amino acid with promising application in medicinal and food industries. Construction of highly efficient microbial cell factories for L-citrulline production is still an open issue due to complex metabolic flux distribution and L-arginine auxotrophy. In this study, we constructed a nonauxotrophic cell factory in Escherichia coli for high-titer L-citrulline production by coupling modular engineering strategies with dynamic pathway regulation. First, the biosynthetic pathway of L-citrulline was enhanced after blockage of the degradation pathway and introduction of heterologous biosynthetic genes from Corynebacterium glutamicum. Specifically, a superior recycling biosynthetic pathway was designed to replace the native linear pathway by deleting native acetylornithine deacetylase. Next, the carbamoyl phosphate and L-glutamate biosynthetic modules, the NADPH generation module, and the efflux module were modified to increase L-citrulline titer further. Finally, a toggle switch that responded to cell density was designed to dynamically control the expression of the argG gene and reconstruct a nonauxotrophic pathway. Without extra supplement of L-arginine during fermentation, the final CIT24 strain produced 82.1 g/L L-citrulline in a 5-L bioreactor with a yield of 0.34 g/g glucose and a productivity of 1.71 g/(L ⋅ h), which were the highest values reported by microbial fermentation. Our study not only demonstrated the successful design of cell factory for high-level L-citrulline production but also provided references of coupling the rational module engineering strategies and dynamic regulation strategies to produce high-value intermediate metabolites.

Production of l-glutamate family amino acids in Corynebacterium glutamicum: Physiological mechanism, genetic modulation, and prospects

l-glutamate family amino acids (GFAAs), consisting of l-glutamate, l-arginine, l-citrulline, l-ornithine, l-proline, l-hydroxyproline, γ-aminobutyric acid, and 5-aminolevulinic acid, are widely applied in the food, pharmaceutical, cosmetic, and animal feed industries, accounting for billions of dollars of market activity. These GFAAs have many functions, including being protein constituents, maintaining the urea cycle, and providing precursors for the biosynthesis of pharmaceuticals. Currently, the production of GFAAs mainly depends on microbial fermentation using Corynebacterium glutamicum (including its related subspecies Corynebacterium crenatum), which is substantially engineered through multistep metabolic engineering strategies. This review systematically summarizes recent advances in the metabolic pathways, regulatory mechanisms, and metabolic engineering strategies for GFAA accumulation in C. glutamicum and C. crenatum, which provides insights into the recent progress in l-glutamate-derived chemical production.

Highly efficient biosynthesis of l-ornithine from mannitol by using recombinant Corynebacterium glutamicum

Mannitol is a promising six-carbon sugar alcohol that is widely found in macroalgae. The potential of mannitol as a renewable raw material is of interest due to the advantages of ocean farms. Herein, the biobased production of l-ornithine from mannitol was resoundingly demonstrated for the first time in engineered Corynebacterium glutamicum S9114 through the deletion of the mannitol repressor MtlR. By modulating the expression of mtlD and reinforcing the fructose metabolic pathway, we generated the strain MTL13 that produced 54.56 g/L of l-ornithine with a yield of 0.47 g/g on mannitol. These results illustrate the robust conversion from mannitol to l-ornithine using engineered Corynebacterium glutamicum, providing a reference for the biobased production of additional chemicals from mannitol.

l-arginine production in Corynebacterium glutamicum: manipulation and optimization of the metabolic process

As an important semi-essential amino acid, l-arginine is extensively used in the food and pharmaceutical fields. At present, l-arginine production depends on cost-effective, green, and sustainable microbial fermentation by using a renewable carbon source. To enhance its fermentative production, various metabolic engineering strategies have been employed, which provide valid paths for reducing the cost of l-arginine production. This review summarizes recent advances in molecular biology strategies for the optimization of l-arginine-producing strains, including manipulating the principal metabolic pathway, modulating the carbon metabolic pathway, improving the intracellular biosynthesis of cofactors and energy usage, manipulating the assimilation of ammonia, improving the transportation and membrane permeability, and performing biosensor-assisted high throughput screening, providing useful insight into the current state of l-arginine production.

Development of a DNA double-strand break-free base editing tool in Corynebacterium glutamicum for genome editing and metabolic engineering

As a traditional amino acid producing bacterium, Corynebacterium glutamicum is a platform strain for production of various fine chemicals. Based on the CRISPR (Clustered regularly interspaced short palindromic repeats)-Cas9 system, gene editing tools that enable base conversion in the genome of C. glutamicum have been developed. However, some problems such as genomic instability caused by DNA double-strand break (DSB) and off-target effects need to be solved. In this study, a DSB-free single nucleotide genome editing system was developed by construction of a bi-directional base conversion tool TadA-dCas9-AID. This system includes cytosine base editors (CBEs): activation-induced cytidine deaminase (AID) and adenine deaminase (ABEs): tRNA adenosine deaminase (TadA), which can specifically target the gene through a 20-nt single guide RNA (sgRNA) and achieve the base conversion of C-T, C-G and A-G in the 28-bp editing window upstream of protospacer adjacent motif. Finally, as a proof-of-concept demonstration, the system was used to construct a mutant library of zwf gene in C. glutamicum S9114 genome to improve the production of a typical nutraceutical N-acetylglucosamine (GlcNAc). The GlcNAc titer of the mutant strain K293R was increased by 31.9% to 9.1 ​g/L in shake flask. Here, the developed bases conversion tool TadA-dCas9-AID does not need DNA double-strand break and homologous template, and is effective for genome editing and metabolic engineering in C. glutamicum.

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A DNA double-strand break-free base editing tool was developed in Corynebacterium glutamicum S9114, which can produce diverse single base mutations.

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The base editing tool can be used for base mutations on genome and metabolic engineering of C. glutamicum S9114.

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High efficiency 20N target sequence linking strategy was developed.

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The base editing tool is used to increase the titer of GlcNAc.

CRISPR-Cpf1-Assisted Engineering of Corynebacterium glutamicum SNK118 for Enhanced L-Ornithine Production by NADP-Dependent Glyceraldehyde-3-Phosphate Dehydrogenase and NADH-Dependent Glutamate Dehydrogenase

Here, Corynebacterium glutamicum SNK118 was metabolically engineered for L-ornithine production through CRISPR-Cpf1-based genome manipulation and plasmid-based heterologous overexpression. Genes argF, argR, and ncgl2228 were deleted to block the degradation of L-ornithine, eliminate the global transcriptional repression, and alleviate the competitive branch pathway, respectively. Overexpression of CsgapC (NADP-dependent glyceraldehyde 3-phosphate dehydrogenases gene from Clostridium saccharobutylicum DSM 13864) and BsrocG (NADH-dependent glutamate dehydrogenase gene from Bacillus subtilis HB-1) resulted markedly increased ornithine biosynthesis. Eventually, the engineered strain KBJ11 (SNK118ΔargRΔargFΔncgl2228/pXMJ19-CsgapC-BsrocG) was constructed for L-ornithine overproduction. In fed-batch fermentation, L-ornithine of 88.26 g/L with productivity of 1.23 g/L/h (over 72 h) and yield of 0.414 g/g glucose was achieved by strain KBJ11 in a 10-L bioreactor. Our result represents the highest titer and yield of L-ornithine production by microbial fermentation. This study suggests that heterologous expression of CsgapC and BsrocG could promote L-ornithine production by C. glutamicum strains.

Recent Advances of L-ornithine Biosynthesis in Metabolically Engineered Corynebacterium glutamicum

L-ornithine, a valuable non-protein amino acid, has a wide range of applications in the pharmaceutical and food industries. Currently, microbial fermentation is a promising, sustainable, and environment-friendly method to produce L-ornithine. However, the industrial production capacity of L-ornithine by microbial fermentation is low and rarely meets the market demands. Various strategies have been employed to improve the L-ornithine production titers in the model strain, Corynebacterium glutamicum, which serves as a major indicator for improving the cost-effectiveness of L-ornithine production by microbial fermentation. This review focuses on the development of high L-ornithine-producing strains by metabolic engineering and reviews the recent advances in breeding strategies, such as reducing by-product formation, improving the supplementation of precursor glutamate, releasing negative regulation and negative feedback inhibition, increasing the supply of intracellular cofactors, modulating the central metabolic pathway, enhancing the transport system, and adaptive evolution for improving L-ornithine production.

Proteome analysis guided genetic engineering of Corynebacterium glutamicum S9114 for tween 40-triggered improvement in L-ornithine production

BACKGROUND

L-ornithine is a valuable amino acid with a wide range of applications in the pharmaceutical and food industries. However, the production of L-ornithine by fermentation cannot compete with other methods, because of the low titers produced with this technique. Development of fermentation techniques that result in a high yield of L-ornithine and efficient strategies for improving L-ornithine production are essential.

RESULTS

This study demonstrates that tween 40, a surfactant promoter of the production of glutamate and arginine, improves L-ornithine production titers in engineered C. glutamicum S9114. The intracellular metabolism under tween 40 triggered fermentation conditions was explored using a quantitative proteomic approach, identifying 48 up-regulated and 132 down-regulated proteins when compared with the control. Numerous proteins were identified as membrane proteins or functional proteins involved in the biosynthesis of the cell wall. Modulation of those genes revealed that the overexpression of CgS9114_09558 and the deletion of CgS9114_13845, CgS9114_02593, and CgS9114_02058 improved the production of L-ornithine in the engineered strain of C. glutamicum Orn8. The final strain with all the exploratory metabolic engineering manipulations produced 25.46 g/L of L-ornithine, and a yield of 0.303 g L-ornithine per g glucose, which was 30.6% higher than that produced by the original strain (19.5 g/L).

CONCLUSION

These results clearly demonstrate the positive effect of tween 40 addition on L-ornithine accumulation. Proteome analysis was performed to examine the impact of tween 40 addition on the physiological changes in C. glutamicum Orn8 and the results showed several promising modulation targets for developing L-ornithine-producing strains.

Development of a System of High Ornithine and Citrulline Production by a Plant-Derived Lactic Acid Bacterium, Weissella confusa K-28

As a bacterium used in industry for production of several amino acids, an endotoxin-free Corynebacterium (C.) glutamicum is well known. However, it is also true that the endotoxin-producing other Corynebacterium species is present. An aim of this study is to obtain a lactic acid bacterium (LAB) that produces ornithine and citrulline at high levels. We successfully isolated a strain, designated K-28, and identified it as Weissella (W.) confusa. The production of ornithine and citrulline by K-28 was 18 ± 1 and 10 ± 2 g/L, respectively, with a 100 ± 9% conversion rate when arginine was continuously fed into a jar fermenter. Although the ornithine high production using C. glutamicum is industrially present, the strains have been genetically modified. In that connection, the wild-type of C. glutamicum produces only 0.5 g/L ornithine, indicating that W. confusa K-28 is superior to C. glutamicum to use a probiotic microorganism. We confirmed that W. confusa K-28 harbors an arginine deiminase (ADI) gene cluster, wkaABDCR. The production of ornithine and the expression of these genes significantly decreased under the aerobic condition rather than anaerobic one. The expression level of the five genes did not differ with or without arginine, suggesting that the production of amino acids in the K-28 strain was not induced by exogenous arginine.

Metabolic engineering of Corynebacterium glutamicum S9114 to enhance the production of l-ornithine driven by glucose and xylose

l-ornithine, an important amino acid, is widely used in food and medicine industries. l-ornithine production mainly relies on microbial fermentation, which may not meet the industrial requirement owing to the poor fermentation ability of available strains. Herein, mscCG2 deletion, CgS9114_12202 (gdh2) overexpression and rational modulation in tricarboxylic acid cycle was firstly demonstrated to increase l-ornithine production in engineered Corynebacterium glutamicum S9114. By further modulate glucose utility result in strain SO26 that produced 38.5 g/L or 43.6 g/L of l-ornithine in shake flask and fed-batch fermentation, respectively. This was 25% higher than that of the original strain (30.8 g/L) and exhibits highest titer reported in shake-flask. Moreover, the incorporation of xylose pathway in the engineered strain resulted in the highest l-ornithine production titer (18.9 g/L) and yield (0.40 g/g xylose) with xylose substrate. These results illustrate the tremendous potential of the engineered strain C. glutamicum S9114 in l-ornithine production.

Engineering Corynebacterium glutamicum triggers glutamic acid accumulation in biotin-rich corn stover hydrolysate

BACKGROUND

Lignocellulose biomass contains high amount of biotin and resulted in an excessive biotin condition for cellulosic glutamic acid accumulation by Corynebacterium glutamicum. Penicillin or ethambutol triggers cellulosic glutamic acid accumulation, but they are not suitable for practical use due to the fermentation instability and environmental concerns. Efficient glutamic acid production from lignocellulose feedstocks should be achieved without any chemical inductions.

RESULTS

An industrial strain C. glutamicum S9114 was metabolically engineered to achieve efficient glutamic acid accumulation in biotin-excessive corn stover hydrolysate. Among the multiple metabolic engineering efforts, two pathway regulations effectively triggered the glutamic acid accumulation in lignocellulose hydrolysate. The C-terminal truncation of glutamate secretion channel MscCG (ΔC110) led to the successful glutamic acid secretion in corn stover hydrolysate without inductions. Then the α-oxoglutarate dehydrogenase complex (ODHC) activity was attenuated by regulating odhA RBS sequence, and glutamic acid accumulation was further elevated for more than fivefolds. The obtained C. glutamicum XW6 strain reached a record-high titer of 65.2 g/L with the overall yield of 0.63 g/g glucose using corn stover as the starting feedstock without any chemical induction.

CONCLUSIONS

Metabolic engineering method was successfully applied to achieve efficient glutamic acid in biotin-rich lignocellulose hydrolysate for the first time. This study demonstrated the high potential of glutamic acid production from lignocellulose feedstock.

Optimization of ʟ-ornithine production in recombinant Corynebacterium glutamicum S9114 by cg3035 overexpression and manipulating the central metabolic pathway

Background

ʟ-Ornithine is an important amino acid with broad applications in pharmaceutical and food industries. Despite lagging ʟ-ornithine productivity and cost reduction, microbial fermentation is a promising route for sustainable ʟ-ornithine production and thus development of robust microbial strains with high stability and productivity is essential.

Results

Previously, we systematically developed a new strain, SO1 originate from Corynebacterium glutamicum S9114, for ʟ-ornithine production. In this work, overexpression of cg3035 encoding N-acetylglutamate synthase (NAGS) using a plasmid or by inserting a strong P_tac promoter into the chromosome was found to increase ʟ-ornithine production in the engineered C. glutamicum SO1. The genome-based cg3035 modulated strain was further engineered by attenuating the expression of pta and cat, inserting a strong P_eftu promoter in the upstream region of glycolytic enzymes such as pfkA, gap, and pyk, and redirecting carbon flux to the pentose phosphate pathway. The final strain with all the exploratory metabolic engineering manipulations produced 32.3 g/L of ʟ-ornithine, a yield of 0.395 g ornithine per g glucose, which was 35.7% higher than that produced by the original strain (23.8 g/L).

Conclusion

These results clearly demonstrated that enhancing the expression of NAGS promoted ʟ-ornithine production and provide a promising alternative systematic blueprint for developing ʟ-ornithine-producing C. glutamicum strains.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0940-9) contains supplementary material, which is available to authorized users.

Improved l-ornithine production in Corynebacterium crenatum by introducing an artificial linear transacetylation pathway

l-Ornithine is a non-protein amino acid with extensive applications in the food and pharmaceutical industries. In this study, we performed metabolic pathway engineering of an l-arginine hyper-producing strain of Corynebacterium crenatum for L-ornithine production. First, we amplified the L-ornithine biosynthetic pathway flux by blocking the competing branch of the pathway. To enhance L-ornithine synthesis, we performed site-directed mutagenesis of the ornithine-binding sites to solve the problem of l-ornithine feedback inhibition for ornithine acetyltransferase. Alternatively, the genes argA from Escherichia coli and argE from Serratia marcescens, encoding the enzymes N-acetyl glutamate synthase and N-acetyl-l-ornithine deacetylase, respectively, were introduced into Corynebacterium crenatum to mimic the linear pathway of L-ornithine biosynthesis. Fermentation of the resulting strain in a 5-L bioreactor allowed a dramatically increased production of L-ornithine, 40.4 g/L, with an overall productivity of 0.673 g/L/h over 60 h. This demonstrates that an increased level of transacetylation is beneficial for L-ornithine biosynthesis.

Pathway engineering in Corynebacterium glutamicum S9114 for 5-aminolevulinic acid production

5-Aminolevulinic acid (ALA) is a non-protein amino acid with a significant potential for cancer treatment and plant stress resistance. Microbial fermentation has gradually replaced the traditional chemical-based method for ALA production, thus increasing the need for high-ALA-producing strains. In this study, we engineered the glutamate producing strain, Corynebacterium glutamicum S9114, for ALA production. To efficiently convert l-glutamate to ALA, hemA and hemL from Salmonella typhimurium and Escherichia coli were tandemly overexpressed. In addition, ncgl1221 encoding a glutamate transporter was deleted to block glutamate secretion and thus improve ALA production. Furthermore, the intrinsic ribosome-binding site (RBS) of hemB was replaced by a relatively weak RBS to reduce the conversion of ALA to porphyrin. Transcriptional and fermentation data confirmed that inactivation of lysE and putP reduced the conversion of glutamate to arginine and proline, which also contribute to ALA production. The final SA14 strain produced 895 mg/L concentration of ALA after 72 h incubation in a shake flask. This amount was 58-fold higher than that obtained by the parent strain C. glutamicum S9114. The results demonstrate the potential of C. glutamicum S9114 for efficient ALA production and provide new targets for the development of ALA-producing strains.

Improvement of L-ornithine production by attenuation of argF in engineered Corynebacterium glutamicum S9114

l-Ornithine, a non-essential amino acid, has enormous industrial applications in food, pharmaceutical, and chemical industries. Currently, l-ornithine production is focused on microorganism fermentation using Escherichia coli or Corynebacterium glutamicum. In C. glutamicum, development of high l-ornithine producing C. glutamicum was achieved by deletion of argF, but was accompanied by growth deficiency and arginine auxotrophy. l-Arginine has been routinely added to solve this problem; however, this increases production cost and causes feedback inhibition of N-acetyl-l-glutamate kinase activity. To avoid the drawbacks of growth disturbance due to disruption of ArgF, strategies were adopted to attenuate its expression. Firstly, ribosome binding site substitution and start codon replacement were introduced to construct recombinant C. glutamiucm strains, which resulted in an undesirable l-ornithine production titer. Then, we inserted a terminator (rrnB) between argD and argF, which significantly improved l-ornithine production and relieved growth disturbance. Transcription analysis confirmed that a terminator can be used to downregulate expression of argF and simultaneously improve the transcriptional level of genes in front of argF. Using disparate terminators to attenuate expression of argF, an optimal strain (CO-9) with a T4 terminator produced 6.1 g/L of l-ornithine, which is 42.8% higher than that produced by strain CO-1, and is 11.2-fold higher than that of the parent CO strain. Insertion of terminators with gradient termination intensity can be a stable and powerful method to exert precise control of the expression level of argF in the development of l-ornithine producing strains, with potential applications in metabolic engineering and synthetic biology.

Enhanced l-ornithine production by systematic manipulation of l-ornithine metabolism in engineered Corynebacterium glutamicum S9114

l-Ornithine is a non-protein amino acid with extensive applications in medicine and the food industry. Currently, l-ornithine is produced by microbial fermentation; however, this process needs to be further improved in terms of l-ornithine productivity and cost reduction. In this study, overexpression of LysE was observed to increase l-ornithine production in engineered Corynebacterium glutamicum S9114. To overcome the drawbacks of using a plasmid to express LysE, P_tac, a strong promoter, was inserted in the upstream region of lysE on the chromosome. This strain was further engineered by attenuating the expression of ncgl2228 and proB, and enhancing the expression of gdh and argCJBD. Combination of those targets resulted in l-ornithine production at a titer of 25 g/L, which was 63.4% higher than that produced by the original strain (15.3 g/L). These results demonstrated the positive effects of overexpressing LysE on l-ornithine production and provided novel targets for developing l-ornithine-producing C. glutamicum strains.