Synthetic promoter libraries for Corynebacterium glutamicum

Diffusion-Based Generative Network for de Novo Synthetic Promoter Design

Computer-aided promoter design is a major development trend in synthetic promoter engineering. Various deep learning models have been used to evaluate or screen synthetic promoters, but there have been few works on de novo promoter design. To explore the potential ability of generative models in promoter design, we established a diffusion-based generative model for promoter design in Escherichia coli. The model was completely driven by sequence data and could study the essential characteristics of natural promoters, thus generating synthetic promoters similar to natural promoters in structure and component. We also improved the calculation method of FID indicator, using a convolution layer to extract the feature matrix of the promoter sequence instead. As a result, we got an FID equal to 1.37, which meant synthetic promoters have a distribution similar to that of natural ones. Our work provides a fresh approach to de novo promoter design, indicating that a completely data-driven generative model is feasible for promoter design.

Development of Corynebacterium glutamicum as a monoterpene production platform

Monoterpenes are commonly known for their role in the flavors and fragrances industry and are also gaining attention for other uses like insect repellant and as potential renewable fuels for aviation. Corynebacterium glutamicum, a Generally Recognized as Safe microbe, has been a choice organism in industry for the annual million ton-scale bioproduction of amino acids for more than 50 years; however, efforts to produce monoterpenes in C. glutamicum have remained relatively limited. In this study, we report a further expansion of the C. glutamicum biosynthetic repertoire through the development and optimization of a mevalonate-based monoterpene platform. In the course of our plasmid design iterations, we increased flux through the mevalonate-based bypass pathway, measuring isoprenol production as a proxy for monoterpene precursor abundance and demonstrating the highest reported titers in C. glutamicum to date at 1504.6 mg/L. Our designs also evaluated the effects of backbone, promoter, and GPP synthase homolog origin on monoterpene product titers. Monoterpene production was further improved by disrupting competing pathways for isoprenoid precursor supply and by implementing a biphasic production system to prevent volatilization. With this platform, we achieved 321.1 mg/L of geranoids, 723.6 mg/L of 1,8-cineole, and 227.8 mg/L of linalool. Furthermore, we determined that C. glutamicum first oxidizes geraniol through an aldehyde intermediate before it is asymmetrically reduced to citronellol. Additionally, we demonstrate that the aldehyde reductase, AdhC, possesses additional substrate promiscuity for acyclic monoterpene aldehydes.

Establishment of CRISPR-Cpf1-assisted gene editing tool and engineering of 4-hydroxyisoleucine biosynthesis in Corynebacterium glutamicum

Corynebacterium glutamicum, an important industrial producer, is a model microorganism. However, the limited gene editing methods and their defects limit the efficient genome editing of C. glutamicum. To improve the screening efficiency of second-cross-over strains of traditional SacB editing system, a universal pCS plasmid which harbors CRISPR-Cpf1 system targeting kan gene of SacB system was designed and established to kill the false positive single-cross-over strains remained abundantly after the second-cross-over events. The lethality of pCS plasmid to C. glutamicum carrying kan gene on its genome was as high as 98.6%. In the example of PodhA::PilvBNC replacement, pCS plasmid improved the screening efficiency of second-cross-over bacteria from 5% to over 95%. Then this pCS-assisted gene editing system was applied to improve the supply of precursors and reduce the generation of by-products in the production of 4-hydroxyisoleucine (4-HIL). The 4-HIL titer of one edited strain SC01-TD5IM reached 137.0 ± 33.9 mM, while the weakening of lysE by promoter engineering reduced Lys content by 19.0-47.7% and 4-HIL titer by 16.4-64.5%. These editing demonstrates again the efficiency of this novel CRISPR-Cpf1-assisted gene editing tool, suggesting it as a useful tool for improving the genome editing and metabolic engineering in C. glutamicum.

Systems metabolic engineering upgrades Corynebacterium glutamicum for selective high-level production of the chiral drug precursor and cell-protective extremolyte L-pipecolic acid

The nonproteinogenic cyclic metabolite l-pipecolic acid is a chiral precursor for the synthesis of various commercial drugs and functions as a cell-protective extremolyte and mediator of defense in plants, enabling high-value applications in the pharmaceutical, medical, cosmetic, and agrochemical markets. To date, the production of the compound is unfavorably fossil-based. Here, we upgraded the strain Corynebacterium glutamicum for l-pipecolic acid production using systems metabolic engineering. Heterologous expression of the l-lysine 6-dehydrogenase pathway, apparently the best route to be used in the microbe, yielded a family of strains that enabled successful de novo synthesis from glucose but approached a limit of performance at a yield of 0.18 mol mol^-1. Detailed analysis of the producers at the transcriptome, proteome, and metabolome levels revealed that the requirements of the introduced route were largely incompatible with the cellular environment, which could not be overcome after several further rounds of metabolic engineering. Based on the gained knowledge, we based the strain design on l-l-lysine 6-aminotransferase instead, which enabled a substantially higher in vivo flux toward l-pipecolic acid. The tailormade producer C. glutamicum PIA-7 formed l-pipecolic acid up to a yield of 562 mmol mol^-1, representing 75% of the theoretical maximum. Ultimately, the advanced mutant PIA-10B achieved a titer of 93 g L^-1 in a fed-batch process on glucose, outperforming all previous efforts to synthesize this valuable molecule de novo and even approaching the level of biotransformation from l-lysine. Notably, the use of C. glutamicum allows the safe production of GRAS-designated l-pipecolic acid, providing extra benefit toward addressing the high-value pharmaceutical, medical, and cosmetic markets. In summary, our development sets a milestone toward the commercialization of biobased l-pipecolic acid.

Rapid combinatorial rewiring of metabolic networks for enhanced poly(3-hydroxybutyrate) production in Corynebacterium glutamicum

BACKGROUND

The disposal of plastic waste is a major environmental challenge. With recent advances in microbial genetic and metabolic engineering technologies, microbial polyhydroxyalkanoates (PHAs) are being used as next-generation biomaterials to replace petroleum-based synthetic plastics in a sustainable future. However, the relatively high production cost of bioprocesses hinders the production and application of microbial PHAs on an industrial scale.

RESULTS

Here, we describe a rapid strategy to rewire metabolic networks in an industrial microorganism, Corynebacterium glutamicum, for the enhanced production of poly(3-hydroxybutyrate) (PHB). A three-gene PHB biosynthetic pathway in Rasltonia eutropha was refactored for high-level gene expression. A fluorescence-based quantification assay for cellular PHB content using BODIPY was devised for the rapid fluorescence-activated cell sorting (FACS)-based screening of a large combinatorial metabolic network library constructed in C. glutamicum. Rewiring metabolic networks across the central carbon metabolism enabled highly efficient production of PHB up to 29% of dry cell weight with the highest cellular PHB productivity ever reported in C. glutamicum using a sole carbon source.

CONCLUSIONS

We successfully constructed a heterologous PHB biosynthetic pathway and rapidly optimized metabolic networks across central metabolism in C. glutamicum for enhanced production of PHB using glucose or fructose as a sole carbon source in minimal media. We expect that this FACS-based metabolic rewiring framework will accelerate strain engineering processes for the production of diverse biochemicals and biopolymers.

Synthetic biology tools for engineering Corynebacterium glutamicum

Corynebacterium glutamicum is a promising organism for the industrial production of amino acids, fuels, and various value-added chemicals. From the whole genome sequence release, C. glutamicum has been valuable in the field of industrial microbiology and biotechnology. Continuous discovery of genetic manipulations and regulation mechanisms has developed C. glutamicum as a synthetic biology platform chassis. This review summarized diverse genomic manipulation technologies and gene expression tools for static, dynamic, and multiplex control at transcription and translation levels. Moreover, we discussed the current challenges and applicable tools to C. glutamicum for future advancements.

Identification and combinatorial engineering of indole-3-acetic acid synthetic pathways in Paenibacillus polymyxa

Background

Paenibacillus polymyxa is a typical plant growth-promoting rhizobacterium (PGPR), and synthesis of indole-3-acetic acid (IAA) is one of the reasons for its growth-promoting capacity. The synthetic pathways of IAA in P. polymyxa must be identified and modified.

Results

P. polymyxa SC2 and its spontaneous mutant SC2-M1 could promote plant growth by directly secreting IAA. Through metabonomic and genomic analysis, the genes patA, ilvB3, and fusE in the native IPyA pathway of IAA synthesis in strain SC2-M1 were predicted. A novel strong promoter P₀₄₄₂₀ was rationally selected, synthetically analyzed, and then evaluated on its ability to express IAA synthetic genes. Co-expression of three genes, patA, ilvB3, and fusE, increased IAA yield by 60% in strain SC2-M1. Furthermore, the heterogeneous gene iaam of the IAM pathway and two heterogeneous IPyA pathways of IAA synthesis were selected to improve the IAA yield of strain SC2-M1. The genes ELJP6_14505, ipdC, and ELJP6_00725 of the entire IPyA pathway from Enterobacter ludwigii JP6 were expressed well by promoter P₀₄₄₂₀ in strain SC2-M1 and increased IAA yield in the engineered strain SC2-M1 from 13 to 31 μg/mL, which was an increase of 138%.

Conclusions

The results of our study help reveal and enhance the IAA synthesis pathways of P. polymyxa and its future application.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02181-3.

Verifying an entire native IPyA pathway of IAA synthesis in P. polymyxa.

Introducing heterologous IAM and IPyA pathways of IAA synthesis to P. polymyxa.

Selecting and analyzing a novel strong promoter P₀₄₄₂₀ to express IAA synthesis genes.

Model-Guided Metabolic Rewiring for Gamma-Aminobutyric Acid and Butyrolactam Biosynthesis in Corynebacterium glutamicum ATCC13032

Gamma-aminobutyric acid (GABA) can be used as a bioactive component in the pharmaceutical industry and a precursor for the synthesis of butyrolactam, which functions as a monomer for the synthesis of polyamide 4 (nylon 4) with improved thermal stability and high biodegradability. The bio-based fermentation production of chemicals using microbes as a cell factory provides an alternative to replace petrochemical-based processes. Here, we performed model-guided metabolic engineering of Corynebacterium glutamicum for GABA and butyrolactam fermentation. A GABA biosynthetic pathway was constructed using a bi-cistronic expression cassette containing mutant glutamate decarboxylase. An in silico simulation showed that the increase in the flux from acetyl-CoA to α-ketoglutarate and the decrease in the flux from α-ketoglutarate to succinate drove more flux toward GABA biosynthesis. The TCA cycle was reconstructed by increasing the expression of acn and icd genes and deleting the sucCD gene. Blocking GABA catabolism and rewiring the transport system of GABA further improved GABA production. An acetyl-CoA-dependent pathway for in vivo butyrolactam biosynthesis was constructed by overexpressing act-encoding ß-alanine CoA transferase. In fed-batch fermentation, the engineered strains produced 23.07 g/L of GABA with a yield of 0.52 mol/mol from glucose and 4.58 g/L of butyrolactam. The metabolic engineering strategies can be used for genetic modification of industrial strains to produce target chemicals from α-ketoglutarate as a precursor, and the engineered strains will be useful to synthesize the bio-based monomer of polyamide 4 from renewable resources.

Recent Progress on Feasible Strategies for Arbutin Production

Arbutin is a hydroquinone glucoside and a natural product present in various plants. Arbutin potently inhibits melanin formation. This property has been exploited in whitening cosmetics and pharmaceuticals. Arbutin production relies mainly on chemical synthesis. The multi-step and complicated process can compromise product purity. With the increasing awareness of sustainable development, the current research direction prioritizes environment-friendly, biobased arbutin production. In this review, current strategies for arbutin production are critically reviewed, with a focus on plant extraction, chemical synthesis, biotransformation, and microbial fermentation. Furthermore, the bottlenecks and perspectives for future direction on arbutin biosynthesis are discussed.

Hypersecretion of OmlA antigen in Corynebacterium glutamicum through high-throughput based development process

Outer membrane lipoprotein A (OmlA) is a vaccine antigen against porcine contagious pleuropneumonia (PCP), a disease severely affecting the swine industry. Here, we aimed to systematically potentiate the secretory production of OmlA in Corynebacterium glutamicum (C. glutamicum), a widely used microorganism in the food industry, by establishing a holistic development process based on our high-throughput culture platform. The expression patterns, expression element combinations, medium composition, and induction conditions were comprehensively screened or optimized in microwell plates (MWPs), followed by fermentation parameter optimization in a 4 × 1 L parallel fermentation system (CUBER4). An unprecedented yield of 1.01 g/L OmlA was ultimately achieved in a 5-L bioreactor following the scaling-up strategy of fixed oxygen mass transfer coefficient (k_La), and the produced OmlA antigen showed well-protective immunity against Actinobacillus pleuropneumoniae challenge. This result provides a rapid and reliable pipeline to achieve the hyper-production of OmlA, and possibly other recombinant vaccines, in C. glutamicum. KEY POINTS: • Established a holistic development process and applied it to potentiate the secretion of OmlA. • The secretion of OmlA reached an unprecedented yield of 1.01 g/L. • The recombinant OmlA antigen induced efficient protective immunity.

Isolation and functional analysis of OsAOS1 promoter for resistance to Nilaparvata lugens Stål infestation in rice

Insect pests have a great impact on the yield and quality of crops. Insecticide applications are an effective method of pest control, however, they also have adverse effects on the environment. Using insect-inducible promoters to drive insect-resistant genes in transgenic crops is a potential sustainable pest management strategy, but insect-inducible promoters have been rarely reported. In this study, we found rice allene oxide synthase gene (AOS, LOC_Os03g12500) can be highly upregulated following brown planthopper (Nilaparvata lugens Stål, BPH) infestation. Then, we amplified the promoter of OsAOS1 and the β- glucuronidase reporter gene was used to analyze the expression pattern of the promoter. Through a series of 5' truncated assays, three positive regulatory regions in response to BPH infestation in the promoter were identified. The transgenic plants, P1R123-min 35S and P1TR1-min 35S promoter-driven snowdrop lectin (Galanthus nivalis agglutinin, GNA) gene, demonstrated the highest expression levels of GNA and lowest BPH survival. Our work identified a BPH-inducible promoter and three positive regions within it. Transgenic rice with GNA driven by OsAOS1 promoter and positive regions exhibited an expected lethal effect on BPH. This study proved the application potential of BPH-inducible promoter and provided a novel path for the selection of insect-resistant tools in the future.

Precise Prediction of Promoter Strength Based on a De Novo Synthetic Promoter Library Coupled with Machine Learning

Promoters are one of the most critical regulatory elements controlling metabolic pathways. However, the fast and accurate prediction of promoter strength remains challenging, leading to time- and labor-consuming promoter construction and characterization processes. This dilemma is caused by the lack of a big promoter library that has gradient strengths, broad dynamic ranges, and clear sequence profiles that can be used to train an artificial intelligence model of promoter strength prediction. To overcome this challenge, we constructed and characterized a mutant library of Trc promoters (P_trc) using 83 rounds of mutation-construction-screening-characterization engineering cycles. After excluding invalid mutation sites, we established a synthetic promoter library that consisted of 3665 different variants, displaying an intensity range of more than two orders of magnitude. The strongest variant was ∼69-fold stronger than the original P_trc and 1.52-fold stronger than a 1 mM isopropyl-β-d-thiogalactoside-driven P_T7 promoter, with an ∼454-fold difference between the strongest and weakest expression levels. Using this synthetic promoter library, different machine learning models were built and optimized to explore the relationships between promoter sequences and transcriptional strength. Finally, our XgBoost model exhibited optimal performance, and we utilized this approach to precisely predict the strength of artificially designed promoter sequences (R² = 0.88, mean absolute error = 0.15, and Pearson correlation coefficient = 0.94). Our work provides a powerful platform that enables the predictable tuning of promoters to achieve optimal transcriptional strength.

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.

Development of a Hyperosmotic Stress Inducible Gene Expression System by Engineering the MtrA/MtrB-Dependent NCgl1418 Promoter in Corynebacterium glutamicum

Corynebacterium glutamicum is an important workhorse for industrial production of diversiform bioproducts. Precise regulation of gene expression is crucial for metabolic balance and enhancing production of target molecules. Auto-inducible promoters, which can be activated without expensive inducers, are ideal regulatory tools for industrial-scale application. However, few auto-inducible promoters have been identified and applied in C. glutamicum. Here, a hyperosmotic stress inducible gene expression system was developed and used for metabolic engineering of C. glutamicum. The promoter of NCgl1418 (P _NCgl1418 ) that was activated by the two-component signal transduction system MtrA/MtrB was found to exhibit a high inducibility under hyperosmotic stress conditions. A synthetic promoter library was then constructed by randomizing the flanking and space regions of P _NCgl1418 , and mutant promoters exhibiting high strength were isolated via fluorescence activated cell sorting (FACS)-based high-throughput screening. The hyperosmotic stress inducible gene expression system was applied to regulate the expression of lysE encoding a lysine exporter and repress four genes involved in lysine biosynthesis (gltA, pck, pgi, and hom) by CRISPR interference, which increased the lysine titer by 64.7% (from 17.0 to 28.0 g/L) in bioreactors. The hyperosmotic stress inducible gene expression system developed here is a simple and effective tool for gene auto-regulation in C. glutamicum and holds promise for metabolic engineering of C. glutamicum to produce valuable chemicals and fuels.

Green chemical and biological synthesis of cadaverine: recent development and challenges

Cadaverine has great potential to be used as an important monomer for the development of a series of high value-added products with market prospects. The most promising strategies for cadaverine synthesis involve using green chemical and bioconversion technologies. Herein, the review focuses on the progress and strategies towards the green chemical synthesis and biosynthesis of cadaverine. Specifically, we address the specific biosynthetic pathways of cadaverine from different substrates as well as extensively discussing the origination, structure and catalytic mechanism of the key lysine decarboxylases. The advanced strategies for process intensification, the separation and purification of cadaverine have been summarized. Furthermore, the challenging issues of the environmental, economic, and applicable impact for cadaverine production are also highlighted. This review concludes with the promising outlooks of state-of-the-art applications of cadaverine along with some insights toward their challenges and potential improvements.

Progress and strategies towards the green chemo/bio-synthesis of cadaverine with special attention to their environmental, economic, and applicable impact are reviewed.

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.

A Set of Active Promoters with Different Activity Profiles for Superexpressing Rhodococcus Strain

Rhodococcus bacteria are a promising platform for biodegradation, biocatalysis, and biosynthesis, but the use of rhodococci is hampered by the insufficient number of both platform strains for expression and promoters that are functional and thoroughly studied in these strains. To expand the list of such strains and promoters, we studied the expression capability of the Rhodococcus rhodochrous M33 strain, and the functioning of a set of recombinant promoters in it. We showed that the strain supports superexpression of the target enzyme (nitrile hydratase) using alternative inexpensive feedings-acetate and urea-without growth factor supplementation, thus being a suitable expression platform. The promoter set included P_tuf (elongation factor Tu) and P_sod (superoxide dismutase) from Corynebacterium glutamicum ATCC13032, P_cpi (isocitrate lyase) from Rhodococcus erythropolis PR4, and P_nh (nitrile hydratase) from R. rhodochrous M8. Activity levels, regulation possibilities, and growth-phase-dependent activity profiles of these promoters were studied in derivatives of the M33 strain. The activities of the promoters were significantly different (P_cpi < P_sod ≪ P_tuf < P_nh), covering 10³-fold range, and the most active P_nh and P_tuf produced up to a 30-50% portion of target protein in soluble intracellular proteins. On the basis of the mRNA quantification and amount of target protein, the production level of P_nh was positioned close to the theoretical upper limit of expression in a bacterial cell. A selection method for the laboratory evolution of such active promoters directly in Rhodococcus was also proposed. Concerning regulation, P_tuf could not be regulated (2-fold change), while others were tunable (6-fold for P_sod, 79-fold for P_nh, and 44-fold for P_cpi). The promoters possessed four different activity profiles, including three with peak of activity at different growth phases and one with constant activity throughout the growth phases. P_tuf and P_cpi did not change their activity profile under different growth conditions, whereas the P_sod and P_nh profiles changed depending on the growth media. The results allow flexible construction of Rhodococcus strains using the studied promoters, and demonstrate a valuable approach for complex characterization of promoters intended for biotechnological strain construction.

Approaches to genetic tool development for rapid domestication of non-model microorganisms

Non-model microorganisms often possess complex phenotypes that could be important for the future of biofuel and chemical production. They have received significant interest the last several years, but advancement is still slow due to the lack of a robust genetic toolbox in most organisms. Typically, "domestication" of a new non-model microorganism has been done on an ad hoc basis, and historically, it can take years to develop transformation and basic genetic tools. Here, we review the barriers and solutions to rapid development of genetic transformation tools in new hosts, with a major focus on Restriction-Modification systems, which are a well-known and significant barrier to efficient transformation. We further explore the tools and approaches used for efficient gene deletion, DNA insertion, and heterologous gene expression. Finally, more advanced and high-throughput tools are now being developed in diverse non-model microbes, paving the way for rapid and multiplexed genome engineering for biotechnology.

Improved Plasmid-Based Inducible and Constitutive Gene Expression in Corynebacterium glutamicum

Corynebacterium glutamicum has been safely used in white biotechnology for the last 60 years and the portfolio of new pathways and products is increasing rapidly. Hence, expression vectors play a central role in discovering endogenous gene functions and in establishing heterologous gene expression. In this work, new expression vectors were designed based on two strategies: (i) a library screening of constitutive native and synthetic promoters and (ii) an increase of the plasmid copy number. Both strategies were combined and resulted in a very strong expression and overproduction of the fluorescence protein GfpUV. As a second test case, the improved vector for constitutive expression was used to overexpress the endogenous xylulokinase gene xylB in a synthetic operon with xylose isomerase gene xylA from Xanthomonas campestris. The xylose isomerase activity in crude extracts was increased by about three-fold as compared to that of the parental vector. In terms of application, the improved vector for constitutive xylA and xylB expression was used for production of the N-methylated amino acid sarcosine from monomethylamine, acetate, and xylose. As a consequence, the volumetric productivity of sarcosine production was 50% higher as compared to that of the strain carrying the parental vector.

Fine-Tuning Multi-Gene Clusters via Well-Characterized Gene Expression Regulatory Elements: Case Study of the Arginine Synthesis Pathway in C. glutamicum

Promoters and ribosome binding sites (RBSs) are routinely applied in gene expression regulation, but their orthogonality and combinatorial effects have not yet been systematically studied in Corynebacterium glutamicum. Here, 17 core promoters and 29 RBSs in C. glutamicum were characterized, which exhibited 470-fold and 430-fold in transcriptional and translational activity, respectively. By comparing the expression of two reporter genes regulated by multiple RBSs, the RBS efficacy showed significant dependence on the gene context, besides the RBSs' strength, reflecting the poor orthogonality of RBSs. Bicistron-modified RBS (referred as bc-RBS) was adapted to C. glutamicum, which improved RBS reliability. By coupling a series of promoters with RBSs/bc-RBSs, a much broader regulation range that spanned 4 orders of magnitude was observed compared with that of a sole element, and the contribution to gene expression of RBS was more than that of promoter. Finally, promoters and RBSs were applied as built-in elements to fine-tune the gene cluster in the arginine synthesis pathway in C. glutamicum. Compared with the original strain, more arginine (1.61-fold) or citrulline (2.35-fold) was accumulated in a 7 L bioreactor by strains with the gene expression regulation system rationally engineered. We demonstrated that, via combination of well-characterized gene elements, and overall consideration for both transcription and translation, the biosynthesis pathway can be effectively balanced, and the yield of a target metabolite can be further improved.

Identification, repair and characterization of a benzyl alcohol-inducible promoter for recombinant proteins overexpression in Corynebacterium glutamicum

Corynebacterium glutamicum is an important industrial organism for the production of a variety of biological commodities. We discovered a promoter encoded by the gene NCgl2319 in C. glutamicum, which could be induced by benzyl alcohol, could be used as an efficient tunable expression system. In initial attempts, this promoter failed to function in a recombinant expression system. This was remedied by extending the original genetic context of the promoter, generating a new version Pcat-B. The Pcat-B transcription initiation site, its critical active regions, and its effect of inducers were fully characterized resulting in tunable expression. This approach proved to be very efficient in producing a pharmaceutical protein, N-terminal pro-brain natriuretic peptide (NT-proBNP). Production of approximately 440.43 mg/L NT-proBNP was achieved with the Pcat-B expression system demonstrating its application for controllable pharmaceutical protein production in C. glutamicum.

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.

Cloning and functional identification of a Chilo suppressalis-inducible promoter of rice gene, OsHPL2

BACKGROUND

Promoters play a key role in driving insect-resistant genes during breeding of transgenic plants. In current transgenic procedures for breeding rice resistance to striped stem borer (Chilo suppressalis Walker, SSB), the constitutive promoter is used to drive the insect-resistant gene. To reduce the burden of constitutive promoters on plant growth, isolation and identification of insect-inducible promoters are particularly important. However, few promoters are induced specifically by insect feeding.

RESULTS

We found rice hydroperoxide lyase gene (OsHPL2) (LOC_Os02g12680) was upregulated after feeding by SSB. We subsequently cloned the promoter of OsHPL2 and analysed its expression pattern using the β-glucuronidase (GUS) reporter gene. Histochemical assays and quantitative analyses of GUS activity confirmed that P _HPL2 :GUS was activated by SSB, but did not respond to brown planthopper (Nilaparvata lugens Stål, BPH) infestation, mechanical wounding or phytohormone treatments. A series of 5' truncated assays were conducted and three positive regulatory regions (-1452 to -1213, -903 to -624, and -376 to -176) induced by SSB infestation were identified. P2R123-min 35S and P2TR2-min 35S promoters linked with cry1C of transgenic plants showed the highest levels of Cry1C protein expression and SSB larval mortality.

CONCLUSION

We identified an SSB-inducible promoter and three positive internal regions. Transgenic rice plants with the OsHPL2 promoter and its positive regions driving cry1C exhibited the expected larvicidal effect on SSB. Our study is the first report of an SSB-inducible promoter that could be used as a potential resource for breeding insect-resistant transgenic crops. © 2020 Society of Chemical Industry.

Construction of a 3A system from BioBrick parts for expression of recombinant hirudin variants III in Corynebacterium glutamicum

Standardized parts can be efficiently assembled into novel biological systems using the three antibiotic (3A) system, ensuring the reusability of components and repeatability of experiments. In this study, we created the 3A expression system for easy construction of gene expression cassettes in Corynebacterium glutamicum (C. glutamicum), which was applied to screen combinations of promoters and signal peptides for improved secreted rhv3 production. We first obtained three strong promoters P₂₂₅₂, P_odhi, and P_yweA from all of promoters, which drive the highest expression of green fluorescent protein (egfp). The three promoters were then assembled with different signal peptides to generate a series of constructs using the 3A expression system developed in this study, from which the highest activity of rhv3 reached 3187.5 ATU/L of P_yweA-CspA-rhv3. Further increased production of rhv3 achieved large-scale fermentation using 5-L jar bioreactor, with the highest rhv3 accumulation 1.21 g/L obtained after 40 h of cultivation, which is higher than 0.95 g/L reported in E. coli. To the best of our knowledge, this is the first report of rhv3 secretory expression in C. glutamicum, which could be applied for the production of other recombinant proteins with significant applications.Key points• We have exploited a 3A system for the genetic manipulation in C. glutamicum.• We constructed element libraries for assembling standard sequence in C. glutamicum.• The secreted expression of rhv3 was realized by 3A system in C. glutamicum.

Inducible Expression Systems Based on Xenogeneic Silencing and Counter-Silencing and Design of a Metabolic Toggle Switch

Inducible expression systems represent key modules in regulatory circuit design and metabolic engineering approaches. However, established systems are often limited in terms of applications due to high background expression levels and inducer toxicity. In bacteria, xenogeneic silencing (XS) proteins are involved in the tight control of horizontally acquired, AT-rich DNA. The action of XS proteins may be opposed by interference with a specific transcription factor, resulting in the phenomenon of counter-silencing, thereby activating gene expression. In this study, we harnessed this principle for the construction of a synthetic promoter library consisting of phage promoters targeted by the Lsr2-like XS protein CgpS of Corynebacterium glutamicum. Counter-silencing was achieved by inserting the operator sequence of the gluconate-responsive transcription factor GntR. The GntR-dependent promoter library is comprised of 28 activated and 16 repressed regulatory elements featuring effector-dependent tunability. For selected candidates, background expression levels were confirmed to be significantly reduced in comparison to established heterologous expression systems. Finally, a GntR-dependent metabolic toggle switch was implemented in a C. glutamicum l-valine production strain allowing the dynamic redirection of carbon flux between biomass and product formation.

Toward fine-tuned metabolic networks in industrial microorganisms

There are numerous microorganisms in nature capable of synthesizing diverse useful compounds; however, these natural microorganisms are generally inefficient in the production of target products on an industrial scale, relative to either chemical synthesis or extraction methods. To achieve industrial production of useful compounds, these natural microorganisms must undergo a certain degree of mutation or effective fine-tuning strategies. This review describes how to achieve an ideal metabolic fine-tuned process, including static control strategies and dynamic control strategies. The static control strategies mainly focus on various matabolic engineering strategies, including protein engineering, upregulation/downregulation, and combinatrorial control of these metabolic engineering strategies, to enhance the flexibility of their application in fine-tuned metabolic metworks. Then, we focus on the dynamic control strategies for fine-tuned metabolic metworks. The design principles derived would guide us to construct microbial cell factories for various useful compounds.

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.

Enhanced production of recombinant proteins in Corynebacterium glutamicum by constructing a bicistronic gene expression system

BACKGROUND

Corynebacterium glutamicum is a traditional food-grade industrial microorganism, in which an efficient endotoxin-free recombinant protein expression factory is under developing in recent years. However, the intrinsic disadvantage of low recombinant protein expression level is still difficult to be solved. Here, according to the bacteria-specific polycistronic feature that multiple proteins can be translated in one mRNA, efforts have been made to insert a leading peptide gene upstream of target genes as an expression enhancer, and it is found that this can remarkably improve the expression level of proteins under the control of inducible tac promoter in C. glutamicum.

RESULTS

In this research, the Escherichia coli (E. coli) tac promoter combined with 24 different fore-cistron sequences were constructed in a bicistronic manner in C. glutamicum. Three strong bicistronic expression vectors were isolated and exhibited high efficiency under different culture conditions. The compatibility of these bicistronic vectors was further validated using six model proteins- aldehyde dehydrogenase (ALDH), alcohol dehydrogenase (ADH), RamA (regulator of acetate metabolism), Bovine interferon-α (BoIFN-α), glycoprotein D protein (gD) of infectious bovine rhinotracheitis virus (IBRV) and procollagen type Ι N-terminal peptide (PΙNP). All examined proteins were highly expressed compared with the original vector with tac promoter. Large-scale production of PΙNP was also performed in fed-batch cultivation, and the highest PΙNP production level was 1.2 g/L.

CONCLUSION

In this study, the strength of the inducible tac promoter for C. glutamicum was improved by screening and inserting fore-cistron sequences in front of the target genes. Those vectors with bicistronic expression patterns have strong compatibility for expressing various heterogeneous proteins in high yield. This new strategy could be used to further improve the performance of inducible promoters, achieving double competence of inducible control and high yield.

Automatic Redirection of Carbon Flux between Glycolysis and Pentose Phosphate Pathway Using an Oxygen-Responsive Metabolic Switch in Corynebacterium glutamicum

Controlling the carbon flux into a desired pathway is important for improving product yield in metabolic engineering. After entering a cell, glucose is channeled into glycolysis and the pentose phosphate pathway (PPP), which decreases the yield of target products whose synthesis relies on NADPH as a cofactor. Here, we demonstrate redirection of carbon flux into PPP under aerobic conditions in Corynebacterium glutamicum, achieved by replacing the promoter of glucose 6-phosphate isomerase gene (pgi) with an anaerobic-specific promoter of the lactate dehydrogenase gene (ldhA). The promoter replacement increased the split ratio of carbon flux into PPP from 39 to 83% under aerobic conditions. The titer, yield, and production rate of 1,5-diaminopentane, whose synthesis requires NADPH as a cofactor, were increased by 4.6-, 4.4-, and 2.6-fold, respectively. This is the largest improvement in the production of 1,5-diaminopentane or its precursor, lysine, reported to date. After aerobic cell growth, pgi expression was automatically induced under anaerobic conditions, altering the carbon flux from PPP to glycolysis, to produce succinate in a single metabolically engineered strain. Such an automatic redirection of metabolic pathway using an oxygen-responsive switch enables two-stage fermentation for efficient production of two different compounds by a single strain, potentially reducing the production costs and time for practical applications.

Comprehensive construction strategy of bidirectional green tissue-specific synthetic promoters

Bidirectional green tissue-specific promoters have important application prospects in genetic engineering and crop genetic improvement. However, there is no report on the application of them, mainly due to undiscovered natural bidirectional green tissue-specific promoters and the lack of a comprehensive approach for the synthesis of these promoters. In order to compensate for this vacancy, the present study reports a novel strategy for the expression regulatory sequence selection and the bidirectional green tissue-specific synthetic promoter construction. Based on this strategy, seven promoters were synthesized and introduced into rice by agrobacterium-mediated transformation. The functional identification of these synthetic promoters was performed by the expression pattern of GFP and GUS reporter genes in two reverse directions in transgenic rice. The results indicated that all the synthetic promoters possessed bidirectional expression activities in transgenic rice, and four synthetic promoters (BiGSSP2, BiGSSP3, BiGSSP6, BiGSSP7) showed highly bidirectional expression efficiencies specifically in green tissues (leaf, sheath, panicle, stem), which could be widely applied to agricultural biotechnology. Our study provided a feasible strategy for the construction of synthetic promoters, and we successfully created four bidirectional green tissue-specific synthetic promoters. It is the first report on bidirectional green tissue-specific promoters that could be efficiently applied in genetic engineering.

Isolating promoters from Corynebacterium ammoniagenes ATCC 6871 and application in CoA synthesis

BACKGROUND

Corynebacterium ammoniagenes is an important industrial organism that is widely used to produce nucleotides and the potential for industrial production of coenzyme A by C. ammoniagenes ATCC 6871 has been shown. However, the yield of coenzyme A needs to be improved, and the available constitutive promoters are rather limited in this strain.

RESULTS

In this study, 20 putative DNA promoters derived from genes with high transcription levels and 6 promoters from molecular chaperone genes were identified. To evaluate the activity of each promoter, red fluorescence protein (RFP) was used as a reporter. We successfully isolated a range of promoters with different activity levels, and among these a fragment derived from the upstream sequence of the 50S ribosomal protein L21 (Prpl21) exhibited the strongest activity among the 26 identified promoters. Furthermore, type III pantothenate kinase from Pseudomonas putida (PpcoaA) was overexpressed in C. ammoniagenes under the control of Prpl21, CoA yield increased approximately 4.4 times.

CONCLUSIONS

This study provides a paradigm for rational isolation of promoters with different activities and their application in metabolic engineering. These promoters will enrich the available promoter toolkit for C. ammoniagenes and should be valuable in current platforms for metabolic engineering and synthetic biology for the optimization of pathways to extend the product spectrum or improve the productivity in C. ammoniagenes ATCC 6871 for industrial applications.

Construction of Genetic Logic Gates Based on the T7 RNA Polymerase Expression System in Rhodococcus opacus PD630

Rhodococcus opacus PD630 (R. opacus) is a nonmodel, Gram-positive bacterium that holds promise as a biological catalyst for the conversion of lignocellulosic biomass to value-added products. In particular, it demonstrates both a high tolerance for and an ability to consume inhibitory lignin-derived aromatics, generates large quantities of lipids, exhibits a relatively rapid growth rate, and has a growing genetic toolbox for engineering. However, the availability of genetic parts for tunable, high-activity gene expression is still limited in R. opacus. Furthermore, genetic logic circuits for sophisticated gene regulation have never been demonstrated in Rhodococcus spp. To address these shortcomings, two inducible T7 RNA polymerase-based expression systems were implemented for the first time in R. opacus and applied to the construction of AND and NAND genetic logic gates. Additionally, three isopropyl β-d-1-thiogalactopyranoside (IPTG)-inducible promoters were created by inserting LacI binding sites into newly characterized constitutive promoters. Furthermore, four novel aromatic sensors for 4-hydroxybenzoic acid, vanillic acid, sodium benzoate, and guaiacol were developed, expanding the gene expression toolbox. Finally, the T7 RNA polymerase platform was combined with a synthetic IPTG-inducible promoter to create an IMPLY logic gate. Overall, this work represents the first demonstration of a heterologous RNA polymerase system and synthetic genetic logic in R. opacus, enabling complex and tunable gene regulation in this promising nonmodel host for bioproduction.

Diversity, versatility and complexity of bacterial gene regulation mechanisms: opportunities and drawbacks for applications in synthetic biology

Gene expression occurs in two essential steps: transcription and translation. In bacteria, the two processes are tightly coupled in time and space, and highly regulated. Tight regulation of gene expression is crucial. It limits wasteful consumption of resources and energy, prevents accumulation of potentially growth inhibiting reaction intermediates, and sustains the fitness and potential virulence of the organism in a fluctuating, competitive and frequently stressful environment. Since the onset of studies on regulation of enzyme synthesis, numerous distinct regulatory mechanisms modulating transcription and/or translation have been discovered. Mostly, various regulatory mechanisms operating at different levels in the flow of genetic information are used in combination to control and modulate the expression of a single gene or operon. Here, we provide an extensive overview of the very diverse and versatile bacterial gene regulatory mechanisms with major emphasis on their combined occurrence, intricate intertwinement and versatility. Furthermore, we discuss the potential of well-characterized basal expression and regulatory elements in synthetic biology applications, where they may ensure orthogonal, predictable and tunable expression of (heterologous) target genes and pathways, aiming at a minimal burden for the host.

Enhancement of L-arginine production by increasing ammonium uptake in an AmtR-deficient Corynebacterium crenatum mutant

L-Arginine is an important amino acid with extensive application in the food and pharmaceutical industries. The efficiency of nitrogen uptake and assimilation by organisms is extremely important for L-arginine production. In this study, a strain engineering strategy focusing on upregulate intracellular nitrogen metabolism in Corynebacterium crenatum for L-arginine production was conducted. Firstly, the nitrogen metabolism global transcriptional regulator AmtR was deleted, which has demonstrated the beneficial effect on L-arginine production. Subsequently, this strain was engineered by overexpressing the ammonium transporter AmtB to increase the uptake of NH₄⁺ and L-arginine production. To overcome the drawbacks of using a plasmid to express amtB, P_tac, a strong promoter with amtB gene fragment, was integrated into the amtR region on the chromosome in the Corynebacterium crenatum/ΔamtR. The final strain results in L-arginine production at a titer of 60.9 g/L, which was 35.14% higher than that produced by C. crenatum SYPA5-5.

Development of a novel strategy for robust synthetic bacterial promoters based on a stepwise evolution targeting the spacer region of the core promoter in Bacillus subtilis

Background

Promoter evolution by synthetic promoter library (SPL) is a powerful approach to development of functional synthetic promoters to synthetic biology. However, it requires much tedious and time-consuming screenings because of the plethora of different variants in SPL. Actually, a large proportion of mutants in the SPL are significantly lower in strength, which contributes only to fabrication of a promoter library with a continuum of strength. Thus, to effectively obtain the evolved synthetic promoter exhibiting higher strength, it is essential to develop novel strategies to construct mutant library targeting the pivotal region rather than the arbitrary region of the template promoter. In this study, a strategy termed stepwise evolution targeting the spacer of core promoter (SETarSCoP) was established in Bacillus subtilis to effectively evolve the strength of bacterial promoter.

Results

The native promoter, P_srfA, from B. subtilis, which exhibits higher strength than the strong promoter P43, was set as the parental template. According to the comparison of conservation of the spacer sequences between − 35 box and − 10 box among a set of strong and weak native promoter, it revealed that 7-bp sequence immediately upstream of the − 10 box featured in the regulation of promoter strength. Based on the conservative feature, two rounds of consecutive evolution were performed targeting the hot region of P_srfA. In the first round, a primary promoter mutation library (pPML) was constructed by mutagenesis targeting the 3-bp sequence immediately upstream of the − 10 box of the P_srfA. Subsequently, four evolved mutants from pPML were selected to construction of four secondary promoter mutation libraries (sPMLs) based on mutagenesis of the 4-bp sequence upstream of the first-round target. After the consecutive two-step evolution, the mutant P_BH4 was identified and verified to be a highly evolved synthetic promoter. The strength of P_BH4 was higher than P_srfA by approximately 3 times. Moreover, P_BH4 also exhibited broad suitability for different cargo proteins, such as β-glucuronidase and nattokinase. The proof-of-principle test showed that SETarSCoP successfully evolved both constitutive and inducible promoters.

Conclusion

Comparing with the commonly used SPL strategy, SETarSCoP facilitates the evolution process to obtain strength-evolved synthetic bacterial promoter through fabrication and screening of small-scale mutation libraries. This strategy will be a promising method to evolve diverse bacterial promoters to expand the toolbox for synthetic biology.

Electronic supplementary material

The online version of this article (10.1186/s12934-019-1148-3) contains supplementary material, which is available to authorized users.

Identification of a native promoter PLH-77 for gene expression in Paenibacillus polymyxa

Paenibacillus polymyxa is a rhizobacterium that has attracted substantial attention due to its ability to produce functional metabolites and promote plant growth. Metabolic and genetic improvements in this species will benefit research and other applications of the bacterium. However, a suitable gene expression system has not been established in this species. In this study, a promoter trap system based on a green fluorescent protein and a chloramphenicol-resistance gene was developed to isolate native promoters of P. polymyxa SC2-M1 to regulate gene expression. Through high-throughput screening, the novel promoter P_LH-77 was identified, sequenced, and subsequently characterized. Promoter P_LH-77 is a strong, continuous expression system containing the typical -10 and -35 motifs regions. Its effective sequence was evaluated and then cascaded to improve the promotion efficiency. To further verify the existence of P_LH-77, a heterogenous xylose isomerase was expressed by P_LH-77 in P. polymyxa SC2-M1. In the resulting strain, the amount of xylose consumed was increased by 2.5 g/L during the 78 h fermentation period. Meanwhile, the production levels of lactate and acetate increased. It was confirmed that promoter P_LH-77 could effectively mediate gene expression in P. polymyxa SC2-M1 and will further benefit the quantitative monitoring of gene expression in P. polymyxa.

Development of a novel, robust and cost-efficient process for valorizing dairy waste exemplified by ethanol production

BACKGROUND

Delactosed whey permeate (DWP) is a side stream of whey processing, which often is discarded as waste, despite of its high residual content of lactose, typically 10-20%. Microbial fermentation is one of the most promising approaches for valorizing nutrient rich industrial waste streams, including those generated by the dairies. Here we present a novel microbial platform specifically designed to generate useful compounds from dairy waste. As a starting point we use Corynebacterium glutamicum, an important workhorse used for production of amino acids and other important compounds, which we have rewired and complemented with genes needed for lactose utilization. To demonstrate the potential of this novel platform we produce ethanol from lactose in DWP.

RESULTS

First, we introduced the lacSZ operon from Streptococcus thermophilus, encoding a lactose transporter and a β-galactosidase, and achieved slow growth on lactose. The strain could metabolize the glucose moiety of lactose, and galactose accumulated in the medium. After complementing with the Leloir pathway (galMKTE) from Lactococcus lactis, co-metabolization of galactose and glucose was accomplished. To further improve the growth and increase the sugar utilization rate, the strain underwent adaptive evolution in lactose minimal medium for 100 generations. The outcome was strain JS95 that grew fast in lactose mineral medium. Nevertheless, JS95 still grew poorly in DWP. The growth and final biomass accumulation were greatly stimulated after supplementation with NH₄⁺, Mn²⁺, Fe²⁺ and trace minerals. In only 24 h of cultivation, a high cell density (OD₆₀₀ of 56.8 ± 1.3) was attained. To demonstrate the usefulness of the platform, we introduced a plasmid expressing pyruvate decarboxylase and alcohol dehydrogenase, and managed to channel the metabolic flux towards ethanol. Under oxygen-deprived conditions, non-growing suspended cells could convert 100 g/L lactose into 46.1 ± 1.4 g/L ethanol in DWP, a yield of 88% of the theoretical. The resting cells could be re-used at least three times, and the ethanol productivities obtained were 0.96 g/L/h, 2.2 g/L/h, and 1.6 g/L/h, respectively.

CONCLUSIONS

An efficient process for producing ethanol from DWP, based on C. glutamicum, was demonstrated. The results obtained clearly show a great potential for this newly developed platform for producing value-added chemicals from dairy waste.

Advancement of Metabolic Engineering Assisted by Synthetic Biology

Synthetic biology has undergone dramatic advancements for over a decade, during which it has expanded our understanding on the systems of life and opened new avenues for microbial engineering. Many biotechnological and computational methods have been developed for the construction of synthetic systems. Achievements in synthetic biology have been widely adopted in metabolic engineering, a field aimed at engineering micro-organisms to produce substances of interest. However, the engineering of metabolic systems requires dynamic redistribution of cellular resources, the creation of novel metabolic pathways, and optimal regulation of the pathways to achieve higher production titers. Thus, the design principles and tools developed in synthetic biology have been employed to create novel and flexible metabolic pathways and to optimize metabolic fluxes to increase the cells’ capability to act as production factories. In this review, we introduce synthetic biology tools and their applications to microbial cell factory constructions.

Identification and application of a growth-regulated promoter for improving L-valine production in Corynebacterium glutamicum

Background

Promoters are commonly used to regulate the expression of specific target genes or operons. Although a series of promoters have been developed in Corynebacterium glutamicum, more precise and unique expression patterns are needed that the current selection of promoters cannot produce. RNA-Seq technology is a powerful tool for helping us to screen out promoters with expected transcriptional strengths.

Results

The promoter P_{CP_2836} of an aldehyde dehydrogenase coding gene from Corynebacterium glutamicum CP was identified via RNA-seq and RT-PCR as a growth-regulated promoter. Comparing with the strong constitutive promoter P_tuf, the transcriptional strength of P_{CP_2836} showed a significant decrease that from about 75 to 8% in the stationary phase. By replacing the native promoters of the aceE and gltA genes with P_{CP_2836} in the C. glutamicum ATCC 13032-derived l-valine-producing strain AN02, the relative transcriptional levels of the aceE and gltA genes decreased from 1.2 and 1.1 to 0.35 and 0.3, and the activity of their translation products decreased to 43% and 35%, respectively. After 28 h flask fermentation, the final cell density of the obtained strains, GRaceE and GRgltA, exhibited a 7–10% decrease. However, l-valine production increased by 23.9% and 27.3%, and the yield of substrate to product increased 43.8% and 62.5%, respectively. In addition, in the stationary phase, the intracellular citrate levels in GRaceE and GRgltA decreased to 27.0% and 33.6% of AN02, and their intracellular oxaloacetate levels increased to 2.7 and 3.0 times that of AN02, respectively.

Conclusions

The P_{CP_2836} promoter displayed a significant difference on its transcriptional strength in different cell growth phases. With using P_{CP_2836} to replace the native promoters of aceE and gltA genes, both the transcriptional levels of the aceE and gltA genes and the activity of their translation products demonstrated a significant decrease in the stationary phase. Thus, the availability of pyruvate was significantly increased for the synthesis of l-valine without any apparent irreversible negative impacts on cell growth. Use of this promoter can enhance the selectivity and control of gene expression and could serve as a useful research tool for metabolic engineering.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-1031-7) contains supplementary material, which is available to authorized users.

Recombinant Protein Expression System in Corynebacterium glutamicum and Its Application

Corynebacterium glutamicum, a soil-derived gram-positive actinobacterium, has been widely used for the production of biochemical molecules such as amino acids (i.e., L-glutamate and L-lysine), nucleic acids, alcohols, and organic acids. The metabolism of the bacterium has been engineered to increase the production of the target biochemical molecule, which requires a cytosolic enzyme expression. As recent demand for new proteinaceous biologics (such as antibodies, growth factors, and hormones) increase, C. glutamicum is attracting industrial interest as a recombinant protein expression host for therapeutic protein production due to the advantages such as low protease activity without endotoxin activity. In this review, we have summarized the recent studies on the heterologous expression of the recombinant protein in C. glutamicum for metabolic engineering, expansion of substrate availability, and recombinant protein secretion. We have also outlined the advances in genetic components such as promoters, surface anchoring systems, and secretory signal sequences in C. glutamicum for effective recombinant protein expression.

One-step process for production of N-methylated amino acids from sugars and methylamine using recombinant Corynebacterium glutamicum as biocatalyst

N-methylated amino acids are found in Nature in various biological compounds. N-methylation of amino acids has been shown to improve pharmacokinetic properties of peptide drugs due to conformational changes, improved proteolytic stability and/or higher lipophilicity. Due to these characteristics N-methylated amino acids received increasing interest by the pharmaceutical industry. Syntheses of N-methylated amino acids by chemical and biocatalytic approaches are known, but often show incomplete stereoselectivity, low yields or expensive co-factor regeneration. So far a one-step fermentative process from sugars has not yet been described. Here, a one-step conversion of sugars and methylamine to the N-methylated amino acid N-methyl-l-alanine was developed. A whole-cell biocatalyst was derived from a pyruvate overproducing C. glutamicum strain by heterologous expression of the N-methyl-l-amino acid dehydrogenase gene from Pseudomonas putida. As proof-of-concept, N-methyl-l-alanine titers of 31.7 g L⁻¹ with a yield of 0.71 g per g glucose were achieved in fed-batch cultivation. The C. glutamicum strain producing this imine reductase enzyme was engineered further to extend this green chemistry route to production of N-methyl-l-alanine from alternative feed stocks such as starch or the lignocellulosic sugars xylose and arabinose.

Construction, Model-Based Analysis, and Characterization of a Promoter Library for Fine-Tuned Gene Expression in Bacillus subtilis

Promoters are among the most-important and most-basic tools for the control of metabolic pathways. However, previous research mainly focused on the screening and characterization of some native promoters in Bacillus subtilis. To develop a broadly applicable promoter system for this important platform organism, we created a de novo synthetic promoter library (SPL) based on consensus sequences by analyzing the microarray transcriptome data of B. subtilis 168. A total of 214 potential promoters spanning a gradient of strengths was isolated and characterized by a green fluorescence assay. Among these, a detailed intensity analysis was conducted on nine promoters with different strengths by reverse-transcription polymerase chain reaction (RT-PCR) and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Furthermore, reconstructed promoters and promoter cassettes (tandem promoter cluster) were designed via statistical model-based analysis and tandem dual promoters, which showed strength that was increased 1.2- and 2.77-fold compared to that of promoter P43, respectively. Finally, the SPL was employed in the production of inosine and acetoin by repressing and over-expressing the relevant metabolic pathways, yielding a 700% and 44% increase relative to the respective control strains. This is the first report of a de novo synthetic promoter library for B. subtilis, which is independent of any native promoter. The strategy of improving and fine-tuning promoter strengths will contribute to future metabolic engineering and synthetic biology projects in B. subtilis.

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.

Ribosomal binding site sequences and promoters for expressing glutamate decarboxylase and producing γ-aminobutyrate in Corynebacterium glutamicum

Glutamate decarboxylase (GAD) converts l-glutamate (Glu) into γ-aminobutyric acid (GABA). Corynebacterium glutamicum that expresses exogenous GAD gene, gadB2 or gadB1, can synthesize GABA from its own produced Glu. To enhance GABA production in C. glutamicum, ribosomal binding site (RBS) sequence and promoter were searched and optimized for increasing the expression efficiency of gadB2. R4 exhibited the highest strength among RBS sequences tested, with 6 nt the optimal aligned spacing (AS) between RBS and start codon. This combination of RBS sequence and AS contributed to gadB2 expression, increased GAD activity by 156% and GABA production by 82% compared to normal strong RBS and AS combination. Then, a series of native promoters were selected for transcribing gadB2 under optimal RBS and AS combination. P_dnaK, P_dtsR, P_odhI and P_clgR expressed gadB2 and produced GABA as effectively as widely applied P_tuf and P_cspB promoters and more effectively than P_sod promoter. However, each native promoter did not work as well as the synthetic strong promoter P_tacM, which produced 20.2 ± 0.3 g/L GABA. Even with prolonged length and bicistronic architecture, the strength of P_dnaK did not enhance. Finally, gadB2 and mutant gadB1 were co-expressed under the optimal promoter and RBS combination, thus converted Glu into GABA completely and improved GABA production to more than 25 g/L. This study provides useful promoters and RBS sequences for gene expression in C. glutamicum.

Model-based reconstruction of synthetic promoter library in Corynebacterium glutamicum

OBJECTIVE

To develop an efficient synthetic promoter library for fine-tuned expression of target genes in Corynebacterium glutamicum.

RESULTS

A synthetic promoter library for C. glutamicum was developed based on conserved sequences of the - 10 and - 35 regions. The synthetic promoter library covered a wide range of strengths, ranging from 1 to 193% of the tac promoter. 68 promoters were selected and sequenced for correlation analysis between promoter sequence and strength with a statistical model. A new promoter library was further reconstructed with improved promoter strength and coverage based on the results of correlation analysis. Tandem promoter P70 was finally constructed with increased strength by 121% over the tac promoter. The promoter library developed in this study showed a great potential for applications in metabolic engineering and synthetic biology for the optimization of metabolic networks.

CONCLUSIONS

To the best of our knowledge, this is the first reconstruction of synthetic promoter library based on statistical analysis of C. glutamicum.