Construction of a novel expression system for use in Corynebacterium glutamicum

Metabolic engineering Corynebacterium glutamicum for D-chiro-inositol production

D-chiro-inositol (DCI) is a potential drug for the treatment of type II diabetes and polycystic ovary syndrome. In order to effectively synthesize DCI in Corynebacterium glutamicum, the genes related to inositol catabolism in clusters iol1 and iol2 were knocked out in C. glutamicum SN01 to generate the chassis strain DCI-1. DCI-1 did not grow in and catabolize myo-inositol (MI). Subsequently, different exogenous and endogenous inosose isomerases were expressed in DCI-1 and their conversion ability of DCI from MI were compared. After fermentation, the strain DCI-7 co-expressing inosose isomerase IolI2 and inositol dehydrogenase IolG was identified as the optimal strain. Its DCI titer reached 3.21 g/L in the presence of 20 g/L MI. On this basis, the pH, temperature and MI concentration during whole-cell conversion of DCI by strain DCI-7 were optimized. Finally, the optimal condition that achieved the highest DCI titer of 6.96 g/L were obtained at pH 8.0, 37 °C and addition of 40 g/L MI. To our knowledge, it is the highest DCI titer ever reported.

Rewiring Metabolic Flux in Corynebacterium glutamicum Using a CRISPR/dCpf1-Based Bifunctional Regulation System

Corynebacterium glutamicum, a microorganism classified as generally recognized as safe for use in the industrial production of food raw materials and additives, has encountered challenges in achieving widespread adoption and popularization as microbial cell factories. These obstacles arise from the intricate nature of manipulating metabolic flux through conventional methods, such as gene knockout and enzyme overexpression. To address this challenge, we developed a CRISPR/dCpf1-based bifunctional regulation system to bidirectionally regulate the expression of multiple genes in C. glutamicum. Specifically, through fusing various transcription factors to the C-terminus of dCpf1, the resulting dCpf1-SoxS exhibited both CRISPR interference (CRISPRi) and CRISPR activation (CRISPRa) capabilities in C. glutamicum by altering the binding sites of crRNAs. The bifunctional regulation system was used to fine-tune metabolic flux from shikimic acid (SA) and l-serine biosynthesis, resulting in 27-fold and 10-fold increases in SA and l-serine production, respectively, compared to the original strain. These findings highlight the potential of the CRISPR/dCpf1-based bifunctional regulation system in effectively enhancing the yield of target products in C. glutamicum.

Quorum sensing-mediated dynamic regulation of 4-hydroxyisoleucine biosynthesis in Corynebacterium glutamicum

With the development of synthetic biology, some quorum sensing (QS) systems have been studied and applied to coordinate growth and production. Recently, a novel ComQXPA-P_srfA system with different response strengths was constructed in Corynebacterium glutamicum. However, the plasmid-harbored ComQXPA-P_srfA system lacks genetic stability, which restricts the application of this QS system. In this study, the comQXPA expression cassette was integrated into the chromosome of C. glutamicum SN01, resulting in QSc chassis strain. The green fluorescence protein (GFP) was expressed by the natural and mutant P_srfA promoters (P_srfAM) with various strengths in QSc. All the expressions of gfp were activated to the related level in a cell density-dependent manner. Therefore, ComQXPA-P_srfAM circuit was applied for modulating the dynamic biosynthesis of 4-hydroxyisoleucine (4-HIL). First, the expression of ido encoding α-ketoglutarate (α-KG)-dependent isoleucine dioxygenase was dynamically regulated by P_srfAM promoters, resulting in QSc/_NI. The 4-HIL titer (125.18 ± 11.26 mM) increased by 45.1% compared to static ido expression strain. Then, to coordinate the α-KG supply between TCA cycle and 4-HIL synthesis, the activity of α-KG dehydrogenase complex (ODHC) was dynamically inhibited by regulating the expression of ODHC inhibitor gene odhI under QS-responsive P_srfAM promoters. The highest 4-HIL titer of QSc-₁₁O/₂₀I (145.20 ± 7.80 mM) increased by 23.2% compared to QSc/₂₀I. This study modulated two critical genes expression in both cell growth and 4-HIL de novo synthesis pathways by the stable ComQXPA-P_srfAM system, and 4-HIL was produced responsively with the cell density. This strategy enhanced the 4-HIL biosynthesis efficiently without additional genetic regulation.

Chromosomal editing of Corynebacterium glutamicum ATCC 13032 to produce gamma-aminobutyric acid

Corynebacterium glutamicum has been used as a sustainable microbial producer for various bioproducts using cheap biomass resources. In this study, a high GABA-producing C. glutamicum strain was constructed by chromosomal editing. Lactobacillus brevis-derived gadB2 was introduced into the chromosome of C. glutamicum ATCC 13032 to produce gamma-aminobutyric acid and simultaneously blocked the biosynthesis of lactate and acetate. GABA transport and degradation in C. glutamicum were also blocked to improve GABA production. As precursor of GABA, l-glutamic acid synthesis in C. glutamicum was enhanced by introducing E. coli gdhA encoding glutamic dehydrogenase, and the copy numbers of gdhA and gadB2 were also optimized for higher GABA production. The final C. glutamicum strain CGY705 could produce 33.17 g/L GABA from glucose in a 2.4-L bioreactor after 78 h fed-batch fermentation. Since all deletion and expression of genes were performed using chromosomal editing, fermentation of the GABA-producing strains constructed in this study does not need supplementation of any antibiotics and inducers. The strategy used in this study has potential value in the development of GABA-producing bacteria.

GEDpm-cg: Genome Editing Automated Design Platform for Point Mutation Construction in Corynebacterium glutamicum

Advances in robotic system-assisted genome editing techniques and computer-aided design tools have significantly facilitated the development of microbial cell factories. Although multiple separate software solutions are available for vector DNA assembly, genome editing, and verification, by far there is still a lack of complete tool which can provide a one-stop service for the entire genome modification process. This makes the design of numerous genetic modifications, especially the construction of mutations that require strictly precise genetic manipulation, a laborious, time-consuming and error-prone process. Here, we developed a free online tool called GEDpm-cg for the design of genomic point mutations in C. glutamicum. The suicide plasmid-mediated counter-selection point mutation editing method and the overlap-based DNA assembly method were selected to ensure the editability of any single nucleotide at any locus in the C. glutamicum chromosome. Primers required for both DNA assembly of the vector for genetic modification and sequencing verification were provided as design results to meet all the experimental needs. An in-silico design task of over 10,000 single point mutations can be completed in 5 min. Finally, three independent point mutations were successfully constructed in C. glutamicum guided by GEDpm-cg, which confirms that the in-silico design results could accurately and seamlessly be bridged with in vivo or in vitro experiments. We believe this platform will provide a user-friendly, powerful and flexible tool for large-scale mutation analysis in the industrial workhorse C. glutamicum via robotic/software-assisted systems.

Synergistic improvement of N-acetylglucosamine production by engineering transcription factors and balancing redox cofactors

The regulation of single gene transcription level in the metabolic pathway is often failed to significantly improve the titer of the target product, and even leads to the imbalance of carbon/nitrogen metabolic network and cofactor network. Global transcription machinery engineering (gTME) can activate or inhibit the synergistic expression of multiple genes in specific metabolic pathways, so transcription factors with specific functions can be expressed according to different metabolic regulation requirements, thus effectively increasing the synthesis of target metabolites. In addition, maintaining intracellular redox balance through cofactor engineering can realize the self-balance of cofactors and promote the efficient synthesis of target products. In this study, we rebalanced the central carbon/nitrogen metabolism and redox metabolism of Corynebacterium glutamicum S9114 by gTME and redox cofactors engineering to promote the production of the nutraceutical N-acetylglucosamine (GlcNAc). Firstly, it was found that the overexpression of the transcription factor RamA can promote GlcNAc synthesis, and the titer was further improved to 16 g/L in shake flask by using a mutant RamA (RamAM). Secondly, a CRISPR interference (CRISPRi) system based on dCpf1 was developed and used to inhibit the expression of global negative transcriptional regulators of GlcNAc synthesis, which promoted the GlcNAc titer to 27.5 g/L. Thirdly, the cofactor specificity of the key enzymes in GlcNAc synthesis pathway was changed by rational protein engineering, and the titer of GlcNAc in shake flask was increased to 36.9 g/L. Finally, the production of GlcNAc was scaled up in a 50-L fermentor, and the titer reached 117.1 ± 1.9 g/L, which was 6.62 times that of the control group (17.7 ± 0.4 g/L), and the yield was increased from 0.19 g/g to 0.31 g/g glucose. The results obtained here highlight the importance of engineering the global regulation of central carbon/nitrogen metabolism and redox metabolism to improve the production performance of microbial cell factories.

Construction and application of a CRISPR/Cas9-assisted genomic editing system for Corynebacterium glutamicum

Corynebacterium glutamicum is widely used as microbial cell factory for various bioproducts, but its genomic editing efficiency needs to be improved. In this study, a highly efficient CRISPR/Cas9-assisted genomic editing system for C. glutamicum was constructed. This system mainly involves a plasmid and can be used for both gene insertion and deletion in the chromosome of C. glutamicum. The recombinant plasmid for the target gene containing all the editing elements, and first constructed it in E. coli, then purified and transformed it into C. glutamicum. This temperature-sensitive plasmid was cured at high temperature after the genomic editing was completed in C. glutamicum. Using this genetic editing system, the genetic editing efficiency in C. glutamicum ATCC 13032 could reach 95%. The whole work of editing could be done in 8-9 days and showed most time-saving compared to the reported. Using this system, the native promoter of gdhA1 in ATCC 13032 has been replaced with the strong promoter PtacM, and more than 10 genes in ATCC 13032 have been deleted. The results demonstrate that this CRISPR/Cas9-assisted system is highly efficient and very suitable for genome editing in C. glutamicum.

Programming adaptive laboratory evolution of 4-hydroxyisoleucine production driven by a lysine biosensor in Corynebacterium glutamicum

4-Hydroxyisoleucine (4-HIL) is a promising drug for treating diabetes. In our previous study, 4-HIL was synthesized from self-produced L-isoleucine (Ile) in Corynebacterium glutamicum by expressing an Ile dioxygenase gene. Although the 4-HIL production of recombinant strain SZ06 increased significantly, a by-product, L-lysine (Lys) was accumulated because of the share of the first several enzymes in Ile and Lys biosynthetic pathways. In this study, programming adaptive laboratory evolution (ALE) was designed and conducted in SZ06 to promote 4-HIL biosynthesis. At first, a programming evolutionary system pMK was constructed, which contains a Lys biosensor LysG-P_lysE and an evolutionary actuator composed of a mutagenesis gene and a fluorescent protein gene. The evolutionary strain SZ06/pMK was then let to be evolved programmatically and spontaneously by sensing Lys concentration. After successive rounds of evolution, nine mutant strains K1 - K9 with significantly increased 4-HIL production and growth performance were obtained. The maximum 4-HIL titer was 152.19 ± 14.60 mM, 28.4% higher than that in SZ06. This titer was higher than those of all the metabolic engineered C. glutamicum strains ever constructed. The whole genome sequencing of the nine evolved strains revealed approximately 30 genetic mutations in each strain. Only one mutation was directly related to the Lys biosynthetic pathway. Therefore, programming ALE driven by Lys biosensor can be used as an effective strategy to increase 4-HIL production in C. glutamicum.

Advances and Perspectives for Genome Editing Tools of Corynebacterium glutamicum

Corynebacterium glutamicum has been considered a promising synthetic biological platform for biomanufacturing and bioremediation. However, there are still some challenges in genetic manipulation of C. glutamicum. Recently, more and more genetic parts or elements (replicons, promoters, reporter genes, and selectable markers) have been mined, characterized, and applied. In addition, continuous improvement of classic molecular genetic manipulation techniques, such as allelic exchange via single/double-crossover, nuclease-mediated site-specific recombination, RecT-mediated single-chain recombination, actinophages integrase-mediated integration, and transposition mutation, has accelerated the molecular study of C. glutamicum. More importantly, emerging gene editing tools based on the CRISPR/Cas system is revolutionarily rewriting the pattern of genetic manipulation technology development for C. glutamicum, which made gene reprogramming, such as insertion, deletion, replacement, and point mutation, much more efficient and simpler. This review summarized the recent progress in molecular genetic manipulation technology development of C. glutamicum and discussed the bottlenecks and perspectives for future research of C. glutamicum as a distinctive microbial chassis.

Dynamic Control of 4-Hydroxyisoleucine Biosynthesis by Modified l-Isoleucine Biosensor in Recombinant Corynebacterium glutamicum

4-Hydroxyisoleucine (4-HIL), a promising drug for treating diabetes, can be synthesized from the self-produced l-isoleucine (Ile) by expressing the Ile dioxygenase gene ido in Corynebacterium glutamicum. However, the requirement of three substrates, Ile, α-ketoglutarate (α-KG), and O₂, makes such de novo biosynthesis difficult to be fulfilled effectively under static engineering conditions. In this study, dynamic control of 4-HIL biosynthesis by the Ile biosensor Lrp-P_brnFE was researched. The native P_brnFE promoter of natural Ile biosensor was still weak even under Ile induction. Through tetA dual genetic selection, several modified stronger P_brnFEN promoters were obtained from the synthetic library of the Ile biosensor. Dynamic regulation of ido expression by modified Ile biosensors increased the 4-HIL titer from 24.7 mM to 28.9-74.4 mM. The best strain ST04 produced even a little more 4-HIL than the static strain SN02 overexpressing ido by the strong P_tacM promoter (69.7 mM). Further dynamic modulation of α-KG supply in ST04 by expressing different P_brnFEN-controlled odhI decreased the 4-HIL production but increased the l-glutamate or Ile accumulation. However, synergistic modulation of α-KG supply and O₂ supply in ST04 by different combinations of P_brnFEN-odhI and P_brnFEN-vgb improved the 4-HIL production significantly, and the highest titer (135.3 mM) was obtained in ST17 strain regulating all the three genes by P_brnFE7. This titer was higher than those of all the static metabolic engineered C. glutamicum strains ever constructed. Therefore, dynamic regulation by modified Ile biosensor is a predominant strategy for enhancing 4-HIL de novo biosynthesis in 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.

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.

Obtaining a series of native gradient promoter-5'-UTR sequences in Corynebacterium glutamicum ATCC 13032

BACKGROUND

Corynebacterium glutamicum is an important industrial microorganism used for the production of many valuable compounds, especially amino acids and their derivatives. For fine-tuning of metabolic pathways, synthetic biological tools are largely based on the rational application of promoters. However, the limited number of promoters make it difficult.

RESULTS

In this study, according to the analysis of RNA-Seq data, 90 DNA fragments with lengths of 200-500 bp that may contain promoter-5'-UTR (PUTR) sequences were amplified and linked to a fluorescent protein gene. When compared with the common strong PUTR P_sodUTR, 17 strong PUTRs were obtained, which maintained stable expression strengths from the early to post stationary phase. Among them, P_NCgl1676UTR was the strongest and its fluorescent protein expression level was more than five times higher than that of P_sodUTR. Furthermore, nine typical chemicals related to the biosynthesis of sulfur-containing amino acids (such as L-methionine, L-cysteine) were selected as stress substances to preliminarily explore the stress on these PUTRs. The results showed that the expression of P_brnFUTR was activated by L-methionine, while that of P_NCgl1202UTR was severely inhibited by L-lysine.

CONCLUSIONS

These findings demonstrated that the selected PUTRs can stably express different genes, such as the red fluorescence protein gene, and can be useful for fine-tuning regulation of metabolic networks in C. glutamicum or for establishing high-throughput screening strategies through biosensor for the production of useful compounds.

A novel expression vector for Corynebacterium glutamicum with an auxotrophy complementation system

Corynebacterium glutamicum is an important industrial strain used for the production of amino acids and vitamins. Most tools developed for overexpression of genes in C. glutamicum are based on the inducible promoter regulated by the lacI^q gene or contain an antibiotic resistance gene as a selection marker. These vectors are essential for rapid identification of recombinant strains and detailed study of gene functions, but, as a considerable disadvantage, these vectors are not suitable for large-scale industrial production due to the need for the addition of isopropyl-β-D-thiogalactopyranoside (IPTG) and antibiotics. In this study, the novel Escherichia coli-C. glutamicum shuttle expression vector pLY-4, derived from the expression vector pXMJ19, was constructed. The constitutive vector pLY-4 contains a large multiple cloning site, the strong promoter tacM and two selective markers: the original chloramphenicol resistance gene cat is used for molecular cloning operations, and the auxotrophy complementation marker alr, which can be stably replicated in the auxotrophic host strain without antibiotic selection pressure, is used for industrial fermentation. Heterologous expression of the gapC gene using the vector pLY-4 in C. glutamicum for L-methionine production indicated the potential application of pLY-4 in the development of C. glutamicum strain engineering and industrial fermentation.

Identification and validation of appropriate reference genes for qRT-PCR analysis in Corynebacterium glutamicum

Real-time quantitative PCR (qRT-PCR) is a fast and efficient technology for detecting gene expression levels in the study of the Corynebacterium glutamicum protein expression system, but it requires normalization to ensure the reliability of the results obtained. We selected 13 genes from the commonly used housekeeping genes and from transcriptome data as candidate reference genes. The Ct values of the 13 genes were obtained by qRT-PCR at different fermentation stages and under three stress conditions (temperature, acid and salt). The expression stability of the reference genes was evaluated by geNorm and NormFinder software. For the study of different growth stages, the most appropriate reference genes are Ncgl2772 and leua, which encode acetyl-CoA carboxylase beta subunit and 2-isopropylmalate synthase, separately. For the study of different stress factors, the optimal minimum number of reference genes is 3, with Ncgl2772, gyrb encoding DNA gyrase B and siga encoding RNA polymerase sigma factor A as the most suitable combination. Additionally, clpx and clpc, encoding ClpX and ClpC protease subunits, were used to validate the candidate reference genes. The identification of new reference genes makes qRT-PCR more convenient, and using these genes for normalization can improve the accuracy and reliability of the measurements of target gene expression levels obtained by qRT-PCR for C. glutamicum.

Construction of a novel Escherichia coli expression system: relocation of lpxA from chromosome to a constitutive expression vector

The selective marker in the plasmid-based expression system is usually a gene that encodes an antibiotic-resistant protein; therefore, the antibiotic has to add to maintain the plasmid when growing the bacteria. This antibiotic addition would lead to increase of production cost and the environment contamination. In this study, a novel Escherichia coli expression system, the lpxA deletion mutant harboring an lpxA-carrying vector, was developed. To develop this system, three plasmids pCas9Cre, pTF-A-UD, and pRSFCmlpxA were constructed. The plasmid pCas9Cre produces enzymes Cas9, λ-Red, and Cre and can be cured by growing at 42 °C; pTF-A-UD contains several DNA fragments required for deleting the chromosomal lpxA and can be cured by adding isopropyl-D-thiogalactopyranoside; pRSFCmlpxA contains the lpxA mutant lpxA123 and CamR. When E. coli were transformed with these three plasmids, the chromosomal lpxA and the CamR in pRSFCmlpxA can be efficiently removed, resulting in an E. coli lpxA mutant harboring pRSFlpxA. The lpxA is essential for the growth of E. coli; its relocation from chromosome to a constitutive expression vector is an ideal strategy to maintain the vector without antibiotic addition. The lpxA123 in pRSFlpxA can complement the deletion of the chromosomal lpxA and provide a strong selective pressure to maintain the plasmid pRSFlpxA. This study provides an experimental evidence that this novel expression system is convenient and efficient to use and can be used to improve L-threonine biosynthesis in the wild type E. coli MG1655 and an L-threonine producing E. coli TWF006.

Metabolic engineering of Corynebacterium glutamicum S9114 based on whole-genome sequencing for efficient N-acetylglucosamine synthesis

Glucosamine (GlcN) and its acetylated derivative N-acetylglucosamine (GlcNAc) are widely used in the pharmaceutical industries. Here, we attempted to achieve efficient production of GlcNAc via genomic engineering of Corynebacterium glutamicum. Specifically, we ligated the GNA1 gene, which converts GlcN-6-phosphate to GlcNAc-6-phosphate by transferring the acetyl group in Acetyl-CoA to the amino group of GlcN-6-phosphate, into the plasmid pJYW4 and then transformed this recombinant vector into the C. glutamicum ATCC 13032, ATCC 13869, ATCC 14067, and S9114 strains, and we assessed the GlcNAc titers at 0.5 g/L, 1.2 g/L, 0.8 g/L, and 3.1 g/L from each strain, respectively. This suggested that there were likely to be significant differences among the key genes in the glutamate and GlcNAc synthesis pathways of these C. glutamicum strains. Therefore, we performed whole genome sequencing of the S9114 strain, which has not been previously published, and found that there are many differences among the genes in the glutamate and GlcNAc synthesis pathways among the four strains tested. Next, nagA (encoding GlcNAc-6-phosphate deacetylase) and gamA (encoding GlcN-6-phosphate deaminase) were deleted in C. glutamicum S9114 to block the catabolism of intracellular GlcNAc, leading to a 54.8% increase in GlcNAc production (from 3.1 to 4.8 g/L) when grown in a shaker flask. In addition, lactate synthesis was blocked by knockout of ldh (encoding lactate dehydrogenase); thus, further increasing the GlcNAc titer to 5.4 g/L. Finally, we added a key gene of the GlcN synthetic pathway, glmS, from different sources into the expression vector pJYW-4-ceN, and the resulting recombinant strain CGGN2-GNA1-CgglmS produced the GlcNAc titer of 6.9 g/L. This is the first report concerning the metabolic engineering of C. glutamicum, and the results of this study provide a good starting point for further metabolic engineering to achieve industrial-scale production of GlcNAc.

Impact of mycolic acid deficiency on cells of Corynebacterium glutamicum ATCC13869

Mycolic acid (MA) plays important role in Corynebacterium glutamicum, but the key enzymes in the biosynthetic pathway of MA in C. glutamicum ATCC13869 have not been characterized. Since the locus BBD29_RS14045 in C. glutamicum ATCC13869 shows high similarity to the gene Cgl2871, which encodes Pks13, the key enzyme for synthesizing MA in C. glutamicum ATCC13032, it was deleted, resulting in the mutant WG001. Compared with the wild-type ATCC13869, MA was not synthesized in WG001, but more phosphatidylglycerol and phosphatidylinositol containing longer unsaturated fatty acids were produced. WG001 cells also show hindered cell growth and defective cell separation when compared with ATCC13869 cells. Transcriptomic analysis shows that many genes relevant to the pathways of fatty acids, inositol, phospholipids, cell wall, and cell division were significantly regulated in WG001 cells when compared with ATCC13869 cells. This study demonstrates that the locus BBD29_RS14045 encodes a key enzyme that plays important role for synthesizing MA in C. glutamicum ATCC13869.

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.

Deletion of odhA or pyc improves production of γ-aminobutyric acid and its precursor L-glutamate in recombinant Corynebacterium glutamicum

OBJECTIVES

To enhance γ-aminobutyric acid (GABA) production in recombinant Corynebacterium glutamicum, metabolic engineering strategies were used to improve the supply of the GABA precursor, L-glutamate.

RESULTS

C. glutamicum ATCC13032 co-expressing two glutamate decarboxylase genes (gadB1 and gadB2) was constructed in a previous study Shi et al. (J Ind Microbiol Biotechnol 40:1285-1296, 2013) to synthesize GABA from endogenous L-glutamate. To improve its L-glutamate supply, new strains were constructed here. First, the odhA and pyc genes were deleted separately. Then, a gadB1-gadB2 co-expression plasmid was transferred into ΔodhA, Δpyc, and ATCC13032, resulting in recombinant strains SNW201, SNW202, and SNW200, respectively. After fermenting for 72 h, GABA production increased to 29.5 ± 1.1 and 24.9 ± 0.7 g/l in SNW201 and SNW202, respectively, which was significantly higher than that in SNW200 (19.4 ± 2.6 g/l). The GABA conversion ratios of SNW201 and SNW202 reached 0.98 and 0.96 mol/mol, respectively.

CONCLUSION

The recombinant strains SNW201 and SNW202 can be used as candidates for GABA production.

Metabolic engineering of Corynebacterium glutamicum strain ATCC13032 to produce L-methionine

L-Methionine-producing strain QW102/pJYW-4-hom(m) -lysC(m) -brnFE was developed from Corynebacterium glutamicum strain ATCC13032, using metabolic engineering strategies. These strategies involved (i) deletion of the gene thrB encoding homoserine kinase to increase the precursor supply, (ii) deletion of the gene mcbR encoding the regulator McbR to release the transcriptional repression to various genes in the l-methionine biosynthetic pathway, (iii) overexpression of the gene lysC(m) encoding feedback-resistant aspartate kinase and the gene hom(m) encoding feedback-resistant homoserine dehydrogenase to further increase the precursor supply, and (iv) overexpression of the gene cluster brnF and brnE encoding the export protein complex BrnFE to increase extracellular l-methionine concentration. QW102/pJYW-4-hom(m) -lysC(m) -brnFE produced 42.2 mM (6.3 g/L) l-methionine after 64-H fed-batch fermentation. These results suggest that l-methionine-producing strains can be developed from wild-type C. glutamicum strains by rationally metabolic engineering.