Rational metabolic engineering of Corynebacterium glutamicum for efficient synthesis of L-glutamate / 生物工程学报

L-glutamic acid is the world's largest bulk amino acid product that is widely used in the food, pharmaceutical and chemical industries. Using Corynebacterium glutamicum G01 as the starting strain, the fermentation by-product alanine content was firstly reduced by knocking out the gene encoding alanine aminotransferase (alaT), a major by-product related to alanine synthesis. Secondly, since the α-ketoglutarate node carbon flow plays an important role in glutamate synthesis, the ribosome-binding site (RBS) sequence optimization was used to reduce the activity of α-ketoglutarate dehydrogenase and enhance the glutamate anabolic flow. The endogenous conversion of α-ketoglutarate to glutamate was also enhanced by screening different glutamate dehydrogenase. Subsequently, the glutamate transporter was rationally desgined to improve the glutamate efflux capacity. Finally, the fermentation conditions of the strain constructed using the above strategy were optimized in 5 L fermenters by a gradient temperature increase combined with a batch replenishment strategy. The glutamic acid production reached (135.33±4.68) g/L, which was 41.2% higher than that of the original strain (96.53±2.32) g/L. The yield was 55.8%, which was 11.6% higher than that of the original strain (44.2%). The combined strategy improved the titer and the yield of glutamic acid, which provides a reference for the metabolic modification of glutamic acid producing strains.

Highly efficient production of L-valine by multiplex metabolic engineering of Corynebacterium glutamicum / 生物工程学报

As a branched chain amino acid, L-valine is widely used in the medicine and feed sectors. In this study, a microbial cell factory for efficient production of L-valine was constructed by combining various metabolic engineering strategies. First, precursor supply for L-valine biosynthesis was enhanced by strengthening the glycolysis pathway and weakening the metabolic pathway of by-products. Subsequently, the key enzyme in the L-valine synthesis pathway, acetylhydroxylate synthase, was engineered by site-directed mutation to relieve the feedback inhibition of the engineered strain. Moreover, promoter engineering was used to optimize the gene expression level of key enzymes in L-valine biosynthetic pathway. Furthermore, cofactor engineering was adopted to change the cofactor preference of acetohydroxyacid isomeroreductase and branched-chain amino acid aminotransferase from NADPH to NADH. The engineered strain C. glutamicum K020 showed a significant increase in L-valine titer, yield and productivity in 5 L fed-batch bioreactor, up to 110 g/L, 0.51 g/g and 2.29 g/(L‧h), respectively.

Biomanufacturing driven by engineered microbes / 生物工程学报

This article summarized the reviews and research articles published in Chinese Journal of Biotechnology in the field of biomanufacturing in 2021. The article covered major chassis cells such as Escherichia coli, Bacillus subtilis, Corynebacterium glutamicum, Saccharomyces cerevisiae, filamentous fungi, non-model bacteria and non-conventional yeasts. Moreover, this article summarized the advances in the production of amino acids, organic acids, vitamins, higher alcohols, natural compounds (terpenoids, flavonoids, alkaloids), antibiotics, enzymes and enzyme-catalyzed products, biopolymers, as well as the utilization of biomass and one-carbon materials. The key technologies used in the construction of cell factories, such as regulation, evolution, and high-throughput screening, were also included. This article may help the readers better understand the R & D trend in biomanufacturing driven by engineered microbes.

Construction and application of a synthetic promoter library for Corynebacterium glutamicum / 生物工程学报

Promoter is an important genetic tool for fine-tuning of gene expression and has been widely used for metabolic engineering. Corynebacterium glutamicum is an important chassis for industrial biotechnology. However, promoter libraries that are applicable to C. glutamicum have been rarely reported, except for a few developed based on synthetic sequences containing random mutations. In this study, we constructed a promoter library based on the native promoter of odhA gene by mutating the -10 region and the bystanders. Using a red fluorescent protein (RFP) as the reporter, 57 promoter mutants were screened by fluorescence imaging technology in a high-throughput manner. These mutants spanned a strength range between 2.4-fold and 19.6-fold improvements of the wild-type promoter. The strongest mutant exhibited a 2.3-fold higher strength than the widely used strong inducible promoter Ptrc. Sequencing of all 57 mutants revealed that 55 mutants share a 1-4 bases shift (4 bases shift for 68% mutants) of the conserved -10 motif "TANNNT" to the 3' end of the promoter, compared to the wild-type promoter. Conserved T or G bases at different positions were observed for strong, moderate, and weak promoter mutants. Finally, five promoter mutants with different strength were employed to fine-tune the expression of γ-glutamyl kinase (ProB) for L-proline biosynthesis. Increased promoter strength led to enhanced L-proline production and the highest L-proline titer of 6.4 g/L was obtained when a promoter mutant with a 9.8-fold higher strength compared to the wild-type promoter was used for ProB expression. The use of stronger promoter variants did not further improve L-proline production. In conclusion, a promoter library was constructed based on a native C. glutamicum promoter PodhA. The new promoter library should be useful for systems metabolic engineering of C. glutamicum. The strategy of mutating native promoter may also guide the construction of promoter libraries for other microorganisms.

Optimization of CRISPR/Cas9-based multiplex base editing in Corynebacterium glutamicum / 生物工程学报

As a new CRISPR/Cas-derived genome engineering technology, base editing combines the target specificity of CRISPR/Cas and the catalytic activity of nucleobase deaminase to install point mutations at target loci without generating DSBs, requiring exogenous template, or depending on homologous recombination. Recently, researchers have developed a variety of base editing tools in the important industrial strain Corynebacterium glutamicum, and achieved simultaneous editing of two and three genes. However, the multiplex base editing based on CRISPR/Cas9 is still limited by the complexity of multiple sgRNAs, interference of repeated sequence and difficulty of target loci replacement. In this study, multiplex base editing in C. glutamicum was optimized by the following strategies. Firstly, the multiple sgRNA expression cassettes based on individual promoters/terminators was optimized. The target loci can be introduced and replaced rapidly by using a template plasmid and Golden Gate method, which also avoids the interference of repeated sequence. Although the multiple sgRNAs structure is still complicated, the editing efficiency of this strategy is the highest. Then, the multiple gRNA expression cassettes based on Type Ⅱ CRISPR crRNA arrays and tRNA processing were developed. The two strategies only require one single promoter and terminator, and greatly simplify the structure of the expression cassette. Although the editing efficiency has decreased, both methods are still applicable. Taken together, this study provides a powerful addition to the genome editing toolbox of C. glutamicum and facilitates genetic modification of this strain.

Advances and prospects in metabolic engineering for the production of amino acids / 生物工程学报

Fermentative production of amino acids is one of the pillars of the fermentation industry in China. Recently, with the fast development of metabolic engineering and synthetic biology technologies, the metabolic engineering for production of amino acids has been flourishing. Conventional forward metabolic engineering, reversed metabolic engineering based on omics data and in silico simulation, and evolutionary metabolic engineering mimicking the natural evolution, have shown increasingly promising applications. A series of highly efficient and robust amino acids-producing strains have been developed and applied in the industrial production of amino acids. The increasingly fierce market competition has put forward new requirements for strain breeding and selection, such as developing high value-added amino acids, dynamic regulation of cellular metabolism, and adapting to the requirements of new process. This review summarizes the advances and prospects in metabolic engineering for the production of amino acids.

Recent advances in developing enabling technologies for Corynebacterium glutamicum metabolic engineering / 生物工程学报

Corynebacterium glutamicum is an important workhorse of industrial biotechnology, especially for amino acid bioindustry. This bacterium is being used to produce various amino acids at a level of over 6 million tons per year. In recent years, enabling technologies for C. glutamicum metabolic engineering have been developed and improved, which accelerated construction and optimization of microbial cell factoriers, expanding spectra of substrates and products, and facilitated basic researches on C. glutamicum. With these technologies, C. glutamicum has become one of the ideal microbial chasses. This review summarizes recent key technological developments of enabling technologies for C. glutamicum metabolic engineering and focuses on establishment and applications of CRISPR-based genome editing, gene expression regulation, adaptive laboratory evolution, and biosensor technologies.

Current status and future perspectives of metabolic network models of industrial microorganisms / 生物工程学报

Genome-scale metabolic network model (GSMM) is an extremely important guiding tool in the targeted modification of industrial microbial strains, which helps researchers to quickly obtain industrial microbes with specific traits and has attracted increasing attention. Here we reviewe the development history of GSMM and summarized the construction method of GSMM. Furthermore, the development and application of GSMM in industrial microorganisms are elaborated by using four typical industrial microorganisms (Bacillus subtilis, Escherichia coli, Corynebacterium glutamicum, and Saccharomyces cerevisiae) as examples. In addition, prospects in the development trend of GSMM are proposed.

Advances in stress tolerance mechanisms and synthetic biology for the industrial robustness of Corynebacterium glutamicum / 生物工程学报

As a model industrial host and microorganism with the generally regarded as safe (GRAS) status, Corynebacterium glutamicum not only produces amino acids on a large scale in the fermentation industry, but also has the potential to produce various new products. C. glutamicum usually encounters various stresses in the process of producing compounds, which severely affect cell viability and production performance. The development of synthetic biology provides new technical means for improving the robustness of C. glutamicum. In this review, we discuss the tolerance mechanisms of C. glutamicum to various stresses in the fermentation process. At the same time, we highlight new synthetic biology strategies for boosting C. glutamicum robustness, including discovering new stress-resistant elements, modifying transcription factors, and using adaptive evolution strategies to mine stress-resistant functional modules. Finally, prospects of improving the robustness of engineered C. glutamicum strains ware provided, with an emphasis on biosensor, screening and design of transcription factors, and utilizing the multiple regulatory elements.

Engineering the C4 pathway of Corynebacterium glutamicum for efficient production of 5-aminolevulinic acid / 生物工程学报

5-aminolevulinic acid (5-ALA) plays an important role in the fields of medicine and agriculture. 5-ALA can be produced by engineered Escherichia coli and Corynebacterium glutamicum. We systematically engineered the C4 metabolic pathway of C. glutamicum to further improve its ability to produce 5-ALA. Firstly, the hemA gene encoding 5-ALA synthase (ALAS) from Rhodobacter capsulatus and Rhodopseudomonas palustris were heterologously expressed in C. glutamicum, respectively. The RphemA gene of R. palustris which showed relatively high enzyme activity was selected. Screening of the optimal ribosome binding site sequence RBS5 significantly increased the activity of RphemA. The ALAS activity of the recombinant strain reached (221.87±3.10) U/mg and 5-ALA production increased by 14.3%. Subsequently, knocking out genes encoding α-ketoglutarate dehydrogenase inhibitor protein (odhI) and succinate dehydrogenase (sdhA) increased the flux of succinyl CoA towards the production of 5-ALA. Moreover, inhibiting the expression of hemB by means of sRNA reduced the degradation of 5-ALA, while overexpressing the cysteine/O-acetylserine transporter eamA increased the output efficiency of intracellular 5-ALA. Shake flask fermentation using the engineered strain C. glutamicum 13032/∆odhI/∆sdhA-sRNAhemB- RBS5RphemA-eamA resulted in a yield of 11.90 g/L, which was 57% higher than that of the original strain. Fed-batch fermentation using the engineered strain in a 5 L fermenter produced 25.05 g/L of 5-ALA within 48 h, which is the highest reported-to-date yield of 5-ALA from glucose.

Amino acid exporters and metabolic modification of Corynebacterium glutamicum - a review / 生物工程学报

Amino acids are important compounds with a wide range of applications in the food, medicine and chemical industries. Corynebacterium glutamicum is a powerful workhorse commonly used in industrial amino acid production, with the scale of more than one million tons. In addition to its efficient anabolism, the effective exporters also ensure the high amino acid production by C. glutamicum. In this review, the research progress of amino acid exporter of C. glutamicum is summarized, to provide the foundation for further improving amino acid production by C. glutamicum via metabolic engineering.

Effect of key notes of TCA cycle on L-glutamate production / 生物工程学报

Glutamic acid is an important amino acid with wide range of applications and huge market demand. Therefore, by performing transcriptome sequencing and re-sequencing analysis on Corynebacterium glutamicum E01 and high glutamate-producing strain C. glutamicum G01, we identified and selected genes with significant differences in transcription and gene levels in the central metabolic pathway that may have greatly influenced glutamate synthesis and further increased glutamic acid yield. The oxaloacetate node and α-ketoglutarate node play an important role in glutamate synthesis. The oxaloacetate node and α-ketoglutarate node were studied to explore effect on glutamate production. Based on the integrated strain constructed from the above experimental results, the growth rate in a 5-L fermenter was slightly lower than that of the original strain, but the glutamic acid yield after 48 h reached (136.1±5.53) g/L, higher than the original strain (93.53±4.52) g/L, an increase by 45.5%; sugar-acid conversion rate reached 58.9%, an increase of 13.7% compared to 45.2% of the original strain. The application of the above experimental strategy improved the glutamic acid yield and the sugar-acid conversion rate, and provided a theoretical basis for the metabolic engineering of Corynebacterium glutamicum.

Advances in gene editing of Corynebacterium glutamate / 生物工程学报

Corynebacterium glutamicum, an important microorganism to produce amino acids and organic acids, has been widely applied in food and medicine fields. Therefore, using editing tools to study the function of unknown genes in C. glutamicum has great significance for systematic development of industrial strain with efficient and novel production capability. Recently, gene editing has been greatly developed. Traditional gene editing based on homologous recombination and gene editing mediated by nuclease are successfully applied in C. glutamicum. Among these, the CRISPR system has been developed to be a main tool used for gene knockout of C. glutamicum due to its advantages of efficiency, simplicity and good target specificity. However, more efficient and reliable knockout system is still urgently demanded, to help develop high-performing strains in industrial application.

Anti-Helicobacter pylori, Anti-apoptotic, and Cytoprotective Effects of Threonine Synthesized from Corynebacterium glutamicum in Gastric Epithelial Cells

BACKGROUND/AIMS: Among irritants causing gastric ulcer, Helicobacter pylori (H. pylori) might be pivotal, after which eradication became essential way in either inhibiting ulcerogenesis or preventing ulcer recurrence. Since threonine is essential in either mucus synthesis or cytoprotection, we hypothesized that the dietary threonine from Corynebacterium glutamicum (C. glutamicum) can mitigate the cytotoxicity of H. pylori infection.MATERIALS AND METHODS: RGM-1 cells were challenged with 100 multiplicity of infection H. pylori for 6 hours, during which threonine alone or combination with Corynebacterium sp. was administered and compared for anti-Helicobacter, anti-inflammation, anti-oxidative, and cytoprotective actions.RESULTS: Threonine alone or combination of threonine and C. glutamicum yielded significant bacteriostatic outcomes. The increased expressions of interleukin (IL)-1β, IL-8, Cox-2, and iNOS mRNA after H. pylori infection were significantly decreased with either threonine alone or the combination of threonine and C. glutamicum. The elevated expressions of NF-kB, HIF-1a, and c-jun after H. pylori infection were all significantly decreased with the combination of threonine and broth from C. glutamicum (P < 0.05), leading to significant decreases in 2′,7′-dichlorofluorescein-diacetate (P < 0.01). Tracing further host antioxidative response, the attenuated expression of heme oxygenase-1, Nrf2, and dehydrogenase quinone-1 after H. pylori infection was significantly preserved with combination of threonine and C. glutamicum. H. pylori infection led to significant increases in apoptosis accompanied with Bcl-2 decreases and Bax increases, while the combination of threonine and C. glutamicum significantly attenuated apoptosis, in which attenuated EGF, TGF-β, and VEGF were significantly regulated, while β-catenin did not change.CONCLUSIONS: Threonine synthesized from C. glutamicum significantly alleviated the cytotoxicity of H. pylori in gastric epithelial cells.

Secretory production of xylanase in Corynebacterium glutamicum using its endogenous elements / 生物工程学报

We constructed bicistronic expression system containing AH6 promoter, 5' UTR and its fore 38 bp sequence from Corynebacterium glutamicum, followed by a conserved Shine-Dalgarno (SD) sequence for xylanase expression. The two major secretory pathways signal peptide in C. glutamicum, Tat (CgR0949) and Sec (CspB) dependent signal peptide were added before xylanase for its secretion. Fed-batch cultivation was done in a 5 L jar for high-level xylanase secretion. The enzyme properties of the purified xylanase were then studied, including the effect of temperature and pH on its activity. The xylanase could be secreted into the culture supernatant when the Sec-dependent signal peptide CspB was used, but none was detected when CgR0949 was used. The secretory production level of xylanase in a flask was 486.2 U/mL and become 1 648.7 U/mL when in a 5 L jar, which was 3.4 fold as in the flask. The optimal pH and temperature of xylanase were pH 4.5 and 45 ℃, respectively. Its activity was 80% of initial activity after pretreatment at 4 ℃ for 24 h at pH 4-11, 95% after incubation below 50 ℃ for 15 min, and 20% when the temperature above 60 ℃. The xylanase could be efficiently secreted into the culture medium by C. glutamicum using its own genetic elements, and the secretion level could be improved through large-scale fed-batch cultivation. This bicistronic expression system can provide a useful tool for heterologous proteins secretion in C. glutamicum. In addition, the catalyze activity of xylanase could be further improved by enzyme properties study.

Screening efficient constitutive promoters in Corynebacterium glutamicum based on time-series transcriptome analysis / 生物工程学报

Promoter, an essential regulatory element, is widely used for metabolic engineering of industrial strains. Corynebacterium glutamicum is an important industrial workhorse to produce various amino acids. However, strong constitutive promoters that are applicable to C. glutamicum are rarely reported. In this study, we first performed a time-series transcriptome analysis of a glutamate hyper-producing strain C. glutamicum SL4 by using RNA-Seq. Overall, we picked 10 samples at different time during the fermentation process. By analyzing the time-series transcriptome data, we selected 10 candidate genes with the highest transcriptional level. These genes were all transcribed stably during the fermentation process. We subsequently cloned the promoter sequences and evaluated the promoters' strength in strain SL4 using a red fluorescent protein reporter system. To evaluate the universality of the promoters in different C. glutamicum strains, we further tested the performance of some promoters in wild type C. glutamicum strains, including ATCC 13869 and ATCC 13032. The strongest promoter was further characterized using LacZ as a reporter in all the three C. glutamicum strains. Finally, we successfully obtained three constitutive promoters with universality, PcysK, PgapA and PfumC. PcysK is the most efficient promoter among the three C. glutamicum strains. In strains SL4 and ATCC 13869, the strength of PcysK is 2-fold of the strong inducible promoter Ptac using the red fluorescent protein as a reporter and 4-fold of Ptac using LacZ as a reporter. Moreover, the strength of PcysK reaches 30%-40% of Ptac in strain ATCC 13032. The promoter PcysK is identified as a strong promoter for the first time, which can be used as an efficient biobrick for metabolic engineering of synthesis pathways in C. glutamicum.

Treatment of livestock carcasses in soil using Corynebacterium glutamicum and lysosomal application to livestock burial

A method of rapidly decaying livestock carcasses is sought through Corine glutamicum, and furthermore, lysosomes are used to remove toxic microorganisms from livestock carcasses. The landfill was constructed on a laboratory scale. Optimized growth conditions of C. glutamicum that could quickly decay livestock carcasses were determined. Lysosomes were extracted from egg whites and used to treat contaminated soil to confirm their antimicrobial activities. Condition of C. glutamicum was activated, regardless both anaerobic and aerobic conditions, soil exists and, to be close to the optimum conditions as possible temperatures, moisture content was about 1/10 of the culture. Lysosomes were found to be effective in clearing soil contamination. C. glutamicum can accelerate the decay of livestock carcasses. A combination of C. glutamicum and lysomes could be used to treat soil contamination caused by decomposition of livestock.

Treatment of livestock carcasses in soil using Corynebacterium glutamicum and lysosomal application to livestock burial

A method of rapidly decaying livestock carcasses is sought through Corine glutamicum, and furthermore, lysosomes are used to remove toxic microorganisms from livestock carcasses. The landfill was constructed on a laboratory scale. Optimized growth conditions of C. glutamicum that could quickly decay livestock carcasses were determined. Lysosomes were extracted from egg whites and used to treat contaminated soil to confirm their antimicrobial activities. Condition of C. glutamicum was activated, regardless both anaerobic and aerobic conditions, soil exists and, to be close to the optimum conditions as possible temperatures, moisture content was about 1/10 of the culture. Lysosomes were found to be effective in clearing soil contamination. C. glutamicum can accelerate the decay of livestock carcasses. A combination of C. glutamicum and lysomes could be used to treat soil contamination caused by decomposition of livestock.

Characterization of L-aspartate-α-decarboxylase from Bacillus subtilis / 生物工程学报

As an important material in pharmaceutical and chemical industry, β-alanine was mainly produced by chemical methods. L-aspartate-α-decarboxylase could catalyze the α-decarboxylation from L-aspartate to β-alanine. Determinations for specific activities of PanDs from Escherichia coli, Corynebacterium glutamicum and Bacillus subtilis were performed in this study (0.98 U/mg, 7.52 U/mg and 8.4 U/mg respectively). The optimal temperature and pH of PanDs from C. glutamicum and B. subtilis were 65 degrees C, pH 6.5 and 60 degrees C, pH 6.5 respectively. According to our research, PanD from B. subtilis could be more appropriate for industrial application because of the higher activity and thermostability when compared to PanDs from E. coli and C. glutamicum which had been the most studied. We also analyzed and discussed the special post-translation self-cleavage phenomenon and the mechanism based inactivation.

Reducing lactate secretion by ldhA Deletion in L-glutamate- producing strain Corynebacterium glutamicum GDK-9

L-lactate is one of main byproducts excreted in to the fermentation medium. To improve L-glutamate production and reduce L-lactate accumulation, L-lactate dehydrogenase-encoding gene ldhA was knocked out from L-glutamate producing strain Corynebacterium glutamicum GDK-9, designated GDK-9ΔldhA. GDK-9ΔldhA produced approximately 10.1% more L-glutamate than the GDK-9, and yielded lower levels of such by-products as α-ketoglutarate, L-lactate and L-alanine. Since dissolved oxygen (DO) is one of main factors affecting L-lactate formation during L-glutamate fermentation, we investigated the effect of ldhA deletion from GDK-9 under different DO conditions. Under both oxygen-deficient and high oxygen conditions, L-glutamate production by GDK-9ΔldhA was not higher than that of the GDK-9. However, under micro-aerobic conditions, GDK-9ΔldhA exhibited 11.61% higher L-glutamate and 58.50% lower L-alanine production than GDK-9. Taken together, it is demonstrated that deletion of ldhA can enhance L-glutamate production and lower the unwanted by-products concentration, especially under micro-aerobic conditions.