Enhancing l-glutamine production in Corynebacterium glutamicum by rational metabolic engineering combined with a two-stage pH control strategy

Systems metabolic engineering of Corynebacterium glutamicum for efficient l-tryptophan production

Corynebacterium glutamicum is a versatile industrial microorganism for producing various amino acids. However, there have been no reports of well-defined C. glutamicum strains capable of hyperproducing l-tryptophan. This study presents a comprehensive metabolic engineering approach to establish robust C. glutamicum strains for l-tryptophan biosynthesis, including: (1) identification of potential targets by enzyme-constrained genome-scale modeling; (2) enhancement of the l-tryptophan biosynthetic pathway; (3) reconfiguration of central metabolic pathways; (4) identification of metabolic bottlenecks through comparative metabolome analysis; (5) engineering of the transport system, shikimate pathway, and precursor supply; and (6) repression of competing pathways and iterative optimization of key targets. The resulting C. glutamicum strain achieved a remarkable l-tryptophan titer of 50.5 g/L in 48h with a yield of 0.17 g/g glucose in fed-batch fermentation. This study highlights the efficacy of integrating computational modeling with systems metabolic engineering for significantly enhancing the production capabilities of industrial microorganisms.

De Novo Synthesis of Tyramine in Engineered Escherichia coli Using Two-Stage Dissolved Oxygen-Controlled Fermentation

Metabolic regulation and fermentation strategies limit the synthesis of tyramine. Here, a de novo synthetic tyramine-producing strain, LAN 25, was constructed. First, tyramine-producing strain was obtained by modifying the key metabolic nodes of tyrosine synthesis and overexpressing the tyrosine decarboxylase gene from Lactobacillus brevis. Then, a two-stage dissolved oxygen (DO)-controlled fermentation process was established in a 5 L fed-batch bioreactor. In the first stage, sufficient DO was supplied to accumulate tyrosine precursors, while in the second stage, limited DO was provided to promote tyramine synthesis. Next, ldhA and adhE were deleted to improve the strain's robustness. To enhance the redox flux (ATP/ADP ratio) under limited DO conditions, the anaerobic promoter, Pvgb, was used to control the expression of a ppk-based ATP regeneration system in response to DO changes. Additionally, the tyrosine internal transport system was modified. Finally, a titer of 21.33 g/L of tyramine with a yield of 0.092 g/g glucose was obtained.

Quantitative analysis of amino acid excretion by Methanothermobacter marburgensis under N2-fixing conditions

Methanogenic archaea (methanogens) possess fascinating metabolic characteristics, such as the ability to fix molecular nitrogen (N2). Methanogens are of biotechnological importance due to the ability to produce methane (CH4) from molecular hydrogen (H2) and carbon dioxide (CO2) and to excrete proteinogenic amino acids. This study focuses on analyzing the link between biological methanogenesis and amino acid excretion under N2-fixing conditions. Among five hydrogenotrophic, autotrophic methanogens, Methanothermobacter marburgensis was prioritized and further cultivated in closed batch cultivation mode under N2-fixing conditions. M. marburgensis was grown on chemically defined minimal medium with different concentrations of ammonium in a H2/CO2/N2 atmosphere. This enabled the quantification of ammonia uptake, N2-fixation, amino acid excretion and the conversion of H2/CO2 to CH4. To quantify N2-fixation rates in a mass balance setting a novel method has been established. The method utilizes the pressure drop below a certain threshold pressure in closed batch cultivation mode - the threshold pressure for N2-fixation (THpN2fix). Using the THpN2fix method, volumetric N2-fixation rates of M. marburgensis as high as 0.91 mmol L-1 h-1 were determined. Excretion of amino acids was found with highest detected values of glutamic acid, alanine, glycine and asparagine. The highest total amino acid excretion of 7.5 µmol L-1 h-1 was detected with H2/CO2/N2 at an ammonium concentration of 40 mmol L-1. This study sheds light on the link between methanogenesis, biological N2-fixation, and proteinogenic amino acid excretion. The concomitant production of amino acids and CH4 could become of biotechnological relevance in an integrated approach coupling biomethanation and N2-fixation in a biorefinery concept.

Environmental Impacts of Cultured Meat: A Cradle-to-Gate Life Cycle Assessment

Interest in animal cell-based meat (ACBM) as an environmentally conscious replacement for livestock production has been increasing; however, a life cycle assessment (LCA) for the existing production methods of ACBM has not been conducted. Currently, ACBM products are being produced at a small scale, but ACBM companies are intending to scale-up production. Updated findings from recent technoeconomic assessments (TEAs) of ACBM were utilized to perform an LCA of near-term ACBM production. A scenario analysis was conducted utilizing the metabolic requirements examined in the TEAs of ACBM, and a purification factor was utilized to account for growth medium component processing. The results indicate that the environmental impact of near-term ACBM production has the potential to be significantly higher than beef if a highly refined growth medium is utilized for ACBM production. This study highlights the need to develop a sustainable animal cell growth medium that is optimized for high-density animal cell proliferation for ACBM to generate positive economic and environmental benefits.

Glutamine-derived peptides: Current progress and future directions

Glutamine, the most abundant amino acid in the body, plays a critical role in preserving immune function, nitrogen balance, intestinal integrity, and resistance to infection. However, its limited solubility and instability present challenges for its use a functional nutrient. Consequently, there is a preference for utilizing glutamine-derived peptides as an alternative to achieve enhanced functionality. This article aims to review the applications of glutamine monomers in clinical, sports, and enteral nutrition. It compares the functional effectiveness of monomers and glutamine-derived peptides and provides a comprehensive assessment of glutamine-derived peptides in terms of their classification, preparation, mechanism of absorption, and biological activity. Furthermore, this study explores the potential integration of artificial intelligence (AI)-based peptidomics and synthetic biology in the de novo design and large-scale production of these peptides. The findings reveal that glutamine-derived peptides possess significant structure-related bioactivities, with the smaller molecular weight fraction serving as the primary active ingredient. These peptides possess the ability to promote intestinal homeostasis, exert hypotensive and hypoglycemic effects, and display antioxidant properties. However, our understanding of the structure-function relationships of glutamine-derived peptides remains largely exploratory at current stage. The combination of AI based peptidomics and synthetic biology presents an opportunity to explore the untapped resources of glutamine-derived peptides as functional food ingredients. Additionally, the utilization and bioavailability of these peptides can be enhanced through the use of delivery systems in vivo. This review serves as a valuable reference for future investigations of and developments in the discovery, functional validation, and biomanufacturing of glutamine-derived peptides in food science.

Enhanced ε-Poly-L-Lysine Production in Streptomyces albulus through Multi-Omics-Guided Metabolic Engineering

Safe and eco-friendly preservatives are crucial to preventing food spoilage and illnesses, as foodborne diseases caused by pathogens result in approximately 600 million cases of illness and 420,000 deaths annually. ε-Poly-L-lysine (ε-PL) is a novel food preservative widely used in many countries. However, its commercial application has been hindered by high costs and low production. In this study, ε-PL's biosynthetic capacity was enhanced in Streptomyces albulus WG608 through metabolic engineering guided by multi-omics techniques. Based on transcriptome and metabolome data, differentially expressed genes (fold change >2 or <0.5; p < 0.05) and differentially expressed metabolites (fold change >1.2 or <0.8) were separately subjected to gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis. The integrative analysis of transcriptome, metabolome, and overexpression revealed the essential roles of isocitrate lyase, succinate dehydrogenase, flavoprotein subunit, diaminopimelate dehydrogenase, polyphosphate kinase, and polyP:AMP phosphotransferase in ε-PL biosynthesis. Subsequently, a strain with enhanced ATP supply, L-lysine supply, and ε-PL synthetase expression was constructed to improve its production. Finally, the resulting strain, S. albulus WME10, achieved an ε-PL production rate of 77.16 g/L in a 5 L bioreactor, which is the highest reported ε-PL production to date. These results suggest that the integrative analysis of the transcriptome and metabolome can facilitate the identification of key pathways and genetic elements affecting ε-PL synthesis, guiding further metabolic engineering and thus significantly enhancing ε-PL production. The method presented in this study could be applicable to other valuable natural antibacterial agents.

Microbial production of branched chain amino acids: Advances and perspectives

Branched-chain amino acids (BCAAs) such as L-valine, L-leucine, and L-isoleucine are widely used in food and feed. To comply with sustainable development goals, commercial production of BCAAs has been completely replaced with microbial fermentation. However, the efficient production of BCAAs by microorganisms remains a serious challenge due to their staggered metabolic networks and cell growth. To overcome these difficulties, systemic metabolic engineering has emerged as an effective and feasible strategy for the biosynthesis of BCAA. This review firstly summarizes the research advances in the microbial synthesis of BCAAs and representative engineering strategies. Second, systematic methods, such as high-throughput screening, adaptive laboratory evolution, and omics analysis, can be used to analyses the synthesis of BCAAs at the whole-cell level and further improve the titer of target chemicals. Finally, new tools and engineering strategies that may increase the production output and development direction of the microbial production of BCAAs are discussed.

Combining protein and metabolic engineering strategies for high-level production of L-theanine in Corynebacterium glutamicum

L-theanine is a natural non-protein amino acid with wide applications. Thus, a high yield of L-theanine production is required on an industrial scale. Herein, an efficient L-theanine-producing strain of Corynebacterium glutamicum was constructed by combining protein and metabolic engineering. Firstly, a γ-glutamylmethylamide synthetase from Paracoccus aminovorans (PaGMAS) was isolated and engineered by computer-aided design, the resulting mutant E179K/N105R improved L-theanine yield by 36.61 %. Subsequently, to increase carbon flux towards L-theanine production, the gene ggt which degrades L-theanine, the gene alaT which participated in L-alanine synthesis, and the gene NCgl1221 which encodes glutamate-exporting protein were deleted. Finally, ppk gene was overexpressed to enhance intracellular ATP production. The reprogramed strain produced 44.12 g/L L-theanine with a yield of 57.11 % and productivity of 1.16 g/L/h, which is the highest L-theanine titer reported by Corynebacterium glutamicum. This study provides an efficient and economical biosynthetic pathway for the industrial production of L-theanine.

High-efficient production of L-homoserine in Escherichia coli through engineering synthetic pathway combined with regulating cell division

L-Homoserine is an important amino acid as a precursor in synthesizing many valuable products. However, the low productivity caused by slow L-homoserine production during active cell growth in fermentation hinders its potential applications. In this study, strategies of engineering the synthetic pathway combined with regulating cell division were employed in an L-homoserine-producing Escherichia coli strain for efficiently biomanufacturing L-homoserine. First, the flux-control genes in the L-homoserine degradation pathway were omitted to redistribute carbon flux. To drive more carbon flux into L-homoserine production, the phosphoenolpyruvate-pyruvate-oxaloacetate loop was redrawn. Subsequently, the cell division was engineered by using the self-regulated promoters to coordinate cell growth and L-homoserine production. The ultimate strain HOM23 produced 101.31 g/L L-homoserine with a productivity of 1.91 g/L/h, which presented the highest L-homoserine titer and productivity to date from plasmid-free strains. The strategies used in this study could be applied to constructing cell factories for producing other L-aspartate derivatives.

Application of Atmospheric and Room-Temperature Plasma (ARTP) to Microbial Breeding

Atmospheric and room-temperature plasma (ARTP) is an efficient microbial mutagenesis method with broad application prospects. Compared to traditional methods, ARTP technology can more effectively induce DNA damage and generate stable mutant strains. It is characterized by its simplicity, cost-effectiveness, and avoidance of hazardous chemicals, presenting a vast potential for application. The ARTP technology is widely used in bacterial, fungal, and microalgal mutagenesis for increasing productivity and improving characteristics. In conclusion, ARTP technology holds significant promise in the field of microbial breeding. Through ARTP technology, we can create mutant strains with specific genetic traits and improved performance, thereby increasing yield, improving quality, and meeting market demands. The field of microbial breeding will witness further innovation and progress with continuous refinement and optimization of ARTP technology.

A high-throughput dual system to screen polyphosphate kinase mutants for efficient ATP regeneration in L-theanine biocatalysis

ATP, an important cofactor, is involved in many biocatalytic reactions that require energy. Polyphosphate kinases (PPK) can provide energy for ATP-consuming reactions due to their cheap and readily available substrate polyphosphate. We determined the catalytic properties of PPK from different sources and found that PPK from Cytophaga hutchinsonii (ChPPK) had the best catalytic activity for the substrates ADP and polyP6. An extracellular-intracellular dual system was constructed to high-throughput screen for better catalytic activity of ChPPK mutants. Finally, the specific activity of ChPPKD82N-K103E mutant was increased by 4.3 times. Therefore, we focused on the production of L-theanine catalyzed by GMAS as a model of ATP regeneration. Supplying 150 mM ATP, GMAS enzyme could produce 16.8 ± 1.3 g/L L-theanine from 100 mM glutamate. When 5 mM ATP and 5 U/mL ChPPKD82N-K103E were added, the yield of L-theanine was 16.6 ± 0.79 g/L with the conversion rate of 95.6 ± 4.5% at 4 h. Subsequently, this system was scaled up to 200 mM and 400 mM glutamate, resulting in the yields of L-theanine for 32.3 ± 1.6 g/L and 62.7 ± 1.1 g/L, with the conversion rate of 92.8 ± 4.6% and 90.1 ± 1.6%, respectively. In addition, we also constructed an efficient ATP regeneration system from glutamate to glutamine, and 13.8 ± 0.2 g/L glutamine was obtained with the conversion rate of 94.4 ± 1.4% in 4 h after adding 6 U/ mL GS enzyme and 5 U/ mL ChPPKD82N-K103E, which further laid the foundation from glutamine to L-theanine catalyzed by GGT enzyme. This proved that giving the reaction an efficient ATP supply driven by the mutant enzyme enhanced the conversion rate of substrate to product and maximized the substrate value. This is a positively combination of high yield, high conversion rate and high economic value of enzyme catalysis. The mutant enzyme will further power the ATP-consuming biocatalytic reaction platform sustainably.

Engineering of Shikimate Pathway and Terminal Branch for Efficient Production of L-Tryptophan in Escherichia coli

L-tryptophan (L-trp), produced through bio-manufacturing, is widely used in the pharmaceutical and food industries. Based on the previously developed L-trp-producing strain, this study significantly improved the titer and yield of L-trp, through metabolic engineering of the shikimate pathway and the L-tryptophan branch. First, the rate-limiting steps in the shikimate pathway were investigated and deciphered, revealing that the combined overexpression of the genes aroE and aroD increased L-trp production. Then, L-trp synthesis was further enhanced at the shaking flask level by improving the intracellular availability of L-glutamine (L-gln) and L-serine (L-ser). In addition, the transport system and the competing pathway of L-trp were also modified, indicating that elimination of the gene TnaB contributed to the extracellular accumulation of L-trp. Through optimizing formulas, the robustness and production efficiency of engineered strains were enhanced at the level of the 30 L fermenter. After 42 h of fed-batch fermentation, the resultant strain produced 53.65 g/L of L-trp, with a yield of 0.238 g/g glucose. In this study, the high-efficiency L-trp-producing strains were created in order to establish a basis for further development of more strains for the production of other highly valuable aromatic compounds or their derivatives.

Efficient 2-O-α-D-glucopyranosyl-sn-glycerol production by single whole-cell biotransformation through combined engineering and expression regulation with novel sucrose phosphorylase from Leuconostoc mesenteroides ATCC 8293

2-O-α-D-glucopyranosyl-sn-glycerol (2-αGG) is a high value product with wide applications. Here, an efficient, safe and sustainable bioprocesses for 2-αGG production was designed. A novel sucrose phosphorylase (SPase) was firstly identified from Leuconostoc mesenteroides ATCC 8293. Subsequently, SPase mutations were processed with computer-aided engineering, of which the activity of SPaseK138C was 160% higher than that of the wild-type. Structural analysis revealed that K138C was a key functional residue moderating substrate binding pocket and thus influences catalytic activity. Furthermore, Corynebacterium glutamicum was employed to construct microbial cell factories along with ribosome binding site (RBS) fine-tuning and a two-stage substrate feeding control strategy. The maximum production of 2-αGG by these combined strategies reached 351.8 g·L^-1 with 98% conversion rate from 1.4 M sucrose and 3.5 M glycerol in a 5-L bioreactor. This was one of the best performance reported in single-cell biosynthesis of 2-αGG, which paved effective ways for industrial-scale preparation of 2-αGG.

Engineering Streptomyces albulus to enhance ε-poly-L-lysine production by introducing a polyphosphate kinase-mediated ATP regeneration system

BACKGROUND

ε-Poly-L-lysine (ε-PL) is a natural and safe food preservative that is mainly produced by filamentous and aerobic bacteria Streptomyces albulus. During ε-PL biosynthesis, a large amount of ATP is used for the polymerization of L-lysine. A shortage of intracellular ATP is one of the major factors limiting the increase in ε-PL production. In previous studies, researchers have mainly tried to increase the oxygen supply to enhance intracellular ATP levels to improve ε-PL production, which can be achieved through the use of two-stage dissolved oxygen control, oxygen carriers, heterologous expression of hemoglobin, and supplementation with auxiliary energy substrates. However, the enhancement of the intracellular ATP supply by constructing an ATP regeneration system has not yet been considered.

RESULTS

In this study, a polyphosphate kinase (PPK)-mediated ATP regeneration system was developed and introduced into S. albulus to successfully improve ε-PL production. First, polyP:AMP phosphotransferase (PAP) from Acinetobacter johnsonii was selected for catalyzing the conversion of AMP into ADP through an in vivo test. Moreover, three PPKs from different microbes were compared by in vitro and in vivo studies with respect to catalytic activity and polyphosphate (polyP) preference, and PPK2B^cg from Corynebacterium glutamicum was used for catalyzing the conversion of ADP into ATP. As a result, a recombinant strain PL05 carrying coexpressed pap and ppk2B^cg for catalyzing the conversion of AMP into ATP was constructed. ε-PL production of 2.34 g/L was achieved in shake-flask fermentation, which was an increase of 21.24% compared with S. albulus WG608; intracellular ATP was also increased by 71.56%. In addition, we attempted to develop a dynamic ATP regulation route, but the result was not as expected. Finally, the conditions of polyP₆ addition were optimized in batch and fed-batch fermentations, and the maximum ε-PL production of strain PL05 in a 5-L fermenter was 59.25 g/L by fed-batch fermentation, which is the highest ε-PL production reported in genetically engineered strains.

CONCLUSIONS

In this study, we proposed and developed a PPK-mediated ATP regeneration system in S. albulus for the first time and significantly enhanced ε-PL production. The study provides an efficient approach to improve the production of not only ε-PL but also other ATP-driven metabolites.

Adaptive evolutionary strategy coupled with an optimized biosynthesis process for the efficient production of pyrroloquinoline quinone from methanol

BACKGROUND

Pyrroloquinoline quinone (PQQ), a cofactor for bacterial dehydrogenases, is associated with biological processes such as mitochondriogenesis, reproduction, growth, and aging. Due to the extremely high cost of chemical synthesis and low yield of microbial synthesis, the election of effective strains and the development of dynamic fermentation strategies for enhancing PQQ production are meaningful movements to meet the large-scale industrial requirements.

RESULTS

A high-titer PQQ-producing mutant strain, Hyphomicrobium denitrificans FJNU-A26, was obtained by integrating ARTP (atmospheric and room‑temperature plasma) mutagenesis, adaptive laboratory evolution and high-throughput screening strategies. Afterward, the systematic optimization of the fermentation medium was conducted using a one-factor-at-a-time strategy and response surface methodology to increase the PQQ concentration from 1.02 to 1.37 g/L. The transcriptional analysis using qRT-PCR revealed that the expression of genes involved in PQQ biosynthesis were significantly upregulated when the ARTP-ALE-derived mutant was applied. Furthermore, a novel two-stage pH control strategy was introduced to address the inconsistent effects of the pH value on cell growth and PQQ production. These combined strategies led to a 148% increase in the PQQ concentration compared with that of the initial strain FJNU-6, reaching 1.52 g/L with a yield of 40.3 mg/g DCW after 144 h of fed-batch fermentation in a 5-L fermenter.

CONCLUSION

The characteristics above suggest that FJNU-A26 represents an effective candidate as an industrial PQQ producer, and the integrated strategies can be readily extended to other microorganisms for the large-scale production of PQQ.

Phosphorylation-Driven Production of d-Allulose from d-Glucose by Coupling with an ATP Regeneration System

d-Allulose is a desirable sucrose substitute with potential applications in food and health care. d-Allulose can be synthesized using d-glucose as a substrate through coupling glucose isomerase with d-allulose 3-epimerase (DAEase); however, the product yield is typically less than 20% at reaction equilibrium and thus limits its use in industrial applications. Here, a 3R-ketose phosphorylation pathway coupled with an adenosine triphosphate (ATP) regeneration system was developed for the efficient synthesis of d-allulose in Escherichia coli using d-glucose as a substrate. The l-rhamnulose kinase (RhaB) was used to break the inherent reaction equilibrium due to its substrate specificity, resulting in increases in d-allulose titer by 69.9% to 4.96 ± 0.49 g/L. By optimizing the whole cell transformation conditions and designing an ATP regeneration module, d-allulose production reached 17.62 ± 0.77 g/L from 30 g/L d-glucose with a final yield of 0.73 g/g without the addition of exogenous ATP. To evaluate the potential industrial application of this multienzyme cascade system, d-allulose was produced from cane molasses (124.16 ± 2.69 g/L glucose equivalent) with a final d-allulose titer of 62.60 ± 3.76 g/L. The present study provides a practical enzymatic approach for the economical synthesis of d-allulose.

Genomics and transcriptomics-guided metabolic engineering Corynebacterium glutamicum for l-arginine production

l-arginine is a semi-essential amino acid that is broadly used as food additives and pharmaceutical intermediates. The synthesis of l-arginine is restricted by complex metabolic mechanisms and suboptimal fermentation conditions. Initially, a mutant strain that accumulated 19.4 g/L l-arginine was generated by random mutagenesis. Subsequently, a mutation of the repressor protein (argRG159D) in the l-arginine operon and glutamate synthase (gltD) with 532-fold upregulation were identified to be vital for l-arginine production by multi-omic analysis. Systematic metabolic engineering was used to modify the strain, which included interfering with α-ketoglutarate dehydrogenase complex (ODHC) activity by knocking out serine/threonine-protein kinase (pknG), enhancing the expression of multiple key enzymes in the l-arginine synthesis pathway, and increasing the availability of intracellular cofactor (NADPH) and energy (ATP). Finally, C. glutamicum ARG12 produced 71.3 g/L l-arginine, with a yield of 0.43 g/g glucose by fermentation optimization. This study provides new ideas to boost l-arginine production.

High-yield ectoine production in engineered Corynebacterium glutamicum by fine metabolic regulation via plug-in repressor library

Ectoine is a high-value protective and stabilizing agent with different applications in biopharmaceuticals, biotechnology, and fine chemicals. Here, efficient production of ectoine in Corynebacterium glutamicum was achieved by combination of metabolic engineering and plug-in repressor library strategy. First, the ectBAC cluster from Pseudomonas stutzeri was introduced into strain K02, and the titer of the obtained strain was 2.12 g/L. Metabolic engineering was then performed for further optimization, including removal of competing pathways (pck and ldh knockout), deletion of glycolysis repressor (sugR knockout), and enhancement of precursor supply (overexpression of Ecasd and CglysC^S301Y). Next, two repressor libraries were designed for targeted flux control to improve ectoine production. Finally, strain CB5L6 produced 45.52 g/L ectoine and had the highest yield in C. glutamicum. For the first time, plug-in repressor library was employed to engineer C. glutamicum for metabolites production, which will provide a guideline for the construction of microbial cell factories.

High-level production of L-valine in Escherichia coli using multi-modular engineering

L-valine is a valuable amino acid in mammals that is used as the main component of feed additives. The low efficiency of the fermentation titer limits the industrial application of L-valine. Here, an L-valine-producing strain of Escherichia coli was obtained using a multi-modular strategy. Initially, a chassis strain was generated by mutagenesis and high-throughput screening. The L-valine biosynthetic pathway and transport module were modified to improve the L-valine titer. Subsequently, the transcription factors associated with L-valine biosynthesis were investigated. Overexpression of PdhR and inhibition of the expression of RpoS promoted L-valine synthesis. Finally, the NADPH supply was enhanced after the introduction of the heterologous Entner-Doudoroff (ED) pathway from Zymomonas mobilis. The strain VAL38 produced 92 g/L L-valine in a 5-L bioreactor with a yield of 0.34 g/g glucose. This strategy is provided as a reference for improving the production performance of cell factories for L-valine and its derivatives.

Development of a nonauxotrophic L-homoserine hyperproducer in Escherichia coli by systems metabolic engineering

L-Homoserine is a valuable amino acid as a platform chemical in the synthesis of various important compounds. Development of microbial strains for high-level L-homoserine production is an attractive research direction in recent years. Herein, we converted a wild-type Escherichia coli to a non-auxotrophic and plasmid-free hyperproducer of L-homoserine using systematically metabolic engineer strategies. First, an initial strain was obtained through regulating L-homoserine degradation pathway and enhancing synthetic flow. To facilitate L-homoserine production, flux-control genes were tuned by optimizing the copy numbers in chromosome, and transport system was modified to promote L-homoserine efflux. Subsequently, a strategy of cofactors synergistic utilization was proposed and successfully applied to achieve L-homoserine hyperproduction. The final engineered strain could efficiently produce 85.29 g/L L-homoserine, which was the highest production level ever reported from a plasmid-free, antibiotic-free, inducer-free and nonauxotrophic strain. These strategies used here can be considered for developing microbial cell factory of other L-aspartate derivatives.

Application of Untargeted Metabolomics to Reveal the Taste-Related Metabolite Profiles during Mandarin Fish (Siniperca chuatsi) Fermentation

Spontaneous fermentation is a critical processing step that determines the taste quality of fermented mandarin fish (Siniperca chuatsi). Here, untargeted metabolomics using ultra-high-performance liquid chromatography coupled with Q Exactive tandem mass spectrometry was employed to characterize the taste-related metabolite profiles during the fermentation of mandarin fish. The results demonstrated that the taste profiles of mandarin fish at different stages of fermentation could be distinguished using an electronic tongue technique. Sixty-two metabolites, including amino acids, small peptides, fatty acids, alkaloids, and organic acids, were identified in fermented mandarin fish samples. Additional quantitative analysis of amino acids revealed glutamic acid and aspartic acid as significant contributors to the fresh flavor. Furthermore, the Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis revealed that amino acid metabolism was the dominant pathway throughout the fermentation process. This study provides a scientific and theoretical reference for the targeted regulation of the quality of fermented mandarin fish.

Transcriptional Regulation of the Creatine Utilization Genes of Corynebacterium glutamicum ATCC 14067 by AmtR, a Central Nitrogen Regulator

In the genus Corynebacterium, AmtR is a key component of the nitrogen regulatory system, and it belongs to the TetR family of transcription regulators. There has been much research on AmtR structure, functions, and regulons in the type strain C. glutamicum ATCC 13032, but little research in other C. glutamicum strains. In this study, chromatin immunoprecipitation and massively parallel DNA sequencing (ChIP-seq) was performed to identify the AmtR regulon in C. glutamicum ATCC 14067. Ten peaks were obtained in the C. glutamicum ATCC 14067 genome including two new peaks related to three operons (RS_01910-RS_01915, RS_15995, and RS_16000). The interactions between AmtR and the promoter regions of the three operons were confirmed by electrophoretic mobility shift assays (EMSAs). The RS_01910, RS_01915, RS_15995, and RS_16000 are not present in the type strain C. glutamicum ATCC 13032. Sequence analysis indicates that RS_01910, RS_01915, RS_15995, and RS_16000, are related to the degradation of creatine and creatinine; RS_01910 may encode a protein related to creatine transport. The genes RS_01910, RS_01915, RS_15995, and RS_16000 were given the names crnA, creT, cshA, and hyuB, respectively. Real-time quantitative PCR (RT-qPCR) analysis and sfGFP (superfolder green fluorescent protein) analysis reveal that AmtR directly and negatively regulates the transcription and expression of crnA, creT, cshA, and hyuB. A growth test shows that C. glutamicum ATCC 14067 can use creatine or creatinine as a sole nitrogen source. In comparison, a creT deletion mutant strain is able to grow on creatinine but loses the ability to grow on creatine. This study provides the first genome-wide captures of the dynamics of in vivo AmtR binding events and the regulatory network they define. These elements provide more options for synthetic biology by extending the scope of the AmtR regulon.

A Novel Approach to Develop Lager Yeast with Higher NADH Availability to Improve the Flavor Stability of Industrial Beer

Flavor stability is important for beer quality and extensive efforts have been undertaken to improve this. In our previous work, we proved a concept whereby metabolic engineering lager yeast with increased cellular nicotinamide adenine dinucleotide hydride (NADH) availability could enhance the flavor stability of beer. However, the method for breeding non-genetically modified strains with higher NADH levels remains unsolved. In the current study, we reported a novel approach to develop such strains based on atmospheric and room temperature plasma (ARTP) mutagenesis coupled with 2,4-dinitrophenol (DNP) selection. As a result, we obtained a serial of strains with higher NADH levels as well as improved flavor stability. For screening an optimal strain with industrial application potential, we examined the other fermentation characteristics of the mutants and ultimately obtained the optimal strain, YDR-63. The overall fermentation performance of the strain YDR-63 in pilot-scale fermentation was similar to that of the parental strain YJ-002, but the acetaldehyde production was decreased by 53.7% and the resistance staling value of beer was improved by 99.8%. The forced beer aging assay further demonstrated that the favor stability was indeed improved as the contents of 5-hydroxymethylfurfural in YDR-63 was less than that in YJ-002 and the sensory notes of staling was weaker in YDR-63. We also employed this novel approach to another industrial strain, M14, and succeeded in improving its flavor stability. All the findings demonstrated the efficiency and versatility of this new approach in developing strains with improved flavor stability for the beer industry.

MarR-type transcription factor RosR regulates glutamate metabolism network and promotes accumulation of L-glutamate in Corynebacterium glutamicum G01

Transcription factors (TFs) perform a crucial function in the regulation of amino acids biosynthesis. Here, TFs involved in L-glutamate biosynthesis in Corynebacterium glutamicum were investigated. Compared to transcriptomic results of C. glutamicum 13032, 7 TFs regulated to glutamate biosynthesis were indentifed in G01 and E01. Among them, RosR was demonstrated to regulate L-glutamate metabolic network by binding to the promoters of glnA, pqo, ilvB, ilvN, ilvC, ldhA, odhA, dstr1, fas, argJ, ak and pta. Overexpression of RosR in G01 resulted in significantly decreased by-products yield and improved L-glutamate titer (130.6 g/L) and yield (0.541 g/g from glucose) in fed-batch fermentation. This study demonstrated the L-glutamate production improved by the expression of TFs in C. glutamicum, which provided a good reference for the transcriptional regulation engineering of strains for amino acid biosynthesis and suggested further metabolic engineering of C. glutamicum for L-glutamate production.