Glutamate dehydrogenase (RocG) in Bacillus licheniformis WX-02: Enzymatic properties and specific functions in glutamic acid synthesis for poly-γ-glutamic acid production

Solar-powered multi-organism symbiont mimic system for beyond natural synthesis of polypeptides from CO2 and N2

Developing artificial symbionts beyond natural synthesis limitations would bring revolutionary contributions to agriculture, medicine, environment, etc. Here, we initiated a solar-driven multi-organism symbiont, which was assembled by the CO₂ fixation module of Synechocystis sp., N₂ fixation module of Rhodopseudomonas palustris, biofunctional polypeptides synthesis module of Bacillus licheniformis, and the electron transfer module of conductive cationic poly(fluorene-co-phenylene) derivative. The modular design broke the pathway to synthesize γ-polyglutamic acid (γ-PGA) using CO₂ and N₂, attributing to the artificially constructed direct interspecific substance and electron transfer. So, the intracellular ATP and NADPH were enhanced by 69 and 30%, respectively, and the produced γ-PGA was enhanced by 104%. The strategy was further extended to produce a commercial antibiotic of bacitracin A. These achievements improve the selectivity and yield of functional polypeptides with one click by CO₂ and N₂, and also provide an innovative strategy for creating photosynthetic systems on demand.

Promoting cell growth for bio-chemicals production via boosting the synthesis of L/D-alanine and D-alanyl-D-alanine in Bacillus licheniformis

Metabolic engineering is a substantial approach for escalating the production of biochemical products. Cell biomass is lowered by system constraints and toxication carried on by the aggregation of metabolites that serve as inhibitors of product synthesis. In order to increase the production of biochemical products, it is important to trace the relationship between alanine metabolism and biomass. According to our investigation, the appropriate concentration of additional L/D-alanine (0.1 g/L) raised the cell biomass (OD₆₀₀) in Bacillus licheniformis in contrast to the control strain. Remarkably, it was also determined that high levels of intracellular L/D-alanine and D-alanyl-D-alanine were induced by the overexpression of the ald, dal, and ddl genes to accelerate cell proliferation. Our findings clearly revealed that 0.2 g/L of L-alanine and D-alanine substantially elevated the titer of poly-γ-glutamic acid (γ-PGA) by 14.89% and 6.19%, correspondingly. And the levels of γ-PGA titer were hastened by the overexpression of the ald, dal, and ddl genes by 19.72%, 15.91%, and 16.64%, respectively. Furthermore, overexpression of ald, dal, and ddl genes decreased the by-products (acetoin, 2,3-butanediol, acetic acid and lactic acid) formation by about 14.10%, 8.77%, and 8.84% for augmenting the γ-PGA production. Our results also demonstrated that overexpression of ald gene amplified the production of lichenysin, pulcherrimin and nattokinase by about 18.71%, 19.82% and 21.49%, respectively. This work delineated the importance of the L/D-alanine and D-alanyl-D-alanine synthesis to the cell growth and the high production of bio-products, and provided an effective strategy for producing bio-products.

A strategy for nitrogen conversion in aquaculture water based on poly-γ-glutamic acid synthesis

Ammonia and nitrite are nitrogenous pollutants in aquaculture effluents, which pose a major threat to the health of aquatic animals. In this study, we developed a nitrogen conversion strategy based on synthesis of poly-γ-glutamic acid (γ-PGA) by Bacillus subtilis NX-2. The nitrogen removal efficiency of NX-2 was closely related to synthesizing γ-PGA, and was positively correlated with the inoculum level. The degradation rates of ammonia nitrogen and nitrite at 10⁴ CFU/mL were 84.42 % and 62.56 %, respectively. Through adaptive laboratory evolution (ALE) experiment, we obtained a strain named ALE 5 M with ammonia degradation rate of 98.03 % and nitrite of 93.62 % at the inoculum level of 10⁴ CFU/mL. Transcriptome analysis showed that the strain was more likely to produce γ-PGA after ALE. By enzyme activity and qPCR analysis, we confirmed that ALE 5 M degraded ammonia nitrogen through γ-PGA synthesis, which provided a new way for nitrogen removal in aquaculture water.

Biological synthesis of ursodeoxycholic acid

Ursodeoxycholic acid (UDCA) is a fundamental treatment drug for numerous hepatobiliary diseases that also has adjuvant therapeutic effects on certain cancers and neurological diseases. Chemical UDCA synthesis is environmentally unfriendly with low yields. Biological UDCA synthesis by free-enzyme catalysis or whole-cell synthesis using inexpensive and readily available chenodeoxycholic acid (CDCA), cholic acid (CA), or lithocholic acid (LCA) as substrates is being developed. The free enzyme-catalyzed one-pot, one-step/two-step method uses hydroxysteroid dehydrogenase (HSDH); whole-cell synthesis, mainly uses engineered bacteria (mainly Escherichia coli) expressing the relevant HSDHs. To further develop these methods, HSDHs with specific coenzyme dependence, high enzyme activity, good stability, and high substrate loading concentration, P450 monooxygenase with C-7 hydroxylation activity and engineered strain harboring HSDHs must be exploited.

Engineering of a newly isolated Bacillus tequilensis BL01 for poly-γ-glutamic acid production from citric acid

BACKGROUND

Poly γ-glutamic acid (γ-PGA) is a promising biopolymer for various applications. For glutamic acid-independent strains, the titer of γ-PGA is too low to meet the industrial demand. In this study, we isolated a novel γ-PGA-producing strain, Bacillus tequilensis BL01, and multiple genetic engineering strategies were implemented to improve γ-PGA production.

RESULTS

First, the one-factor-at-a-time method was used to investigate the influence of carbon and nitrogen sources and temperature on γ-PGA production. The optimal sources of carbon and nitrogen were sucrose and (NH₄)₂SO₄ at 37 °C, respectively. Second, the sucA, gudB, pgdS, and ggt genes were knocked out simultaneously, which increased the titer of γ-PGA by 1.75 times. Then, the titer of γ-PGA increased to 18.0 ± 0.3 g/L by co-overexpression of the citZ and pyk genes in the mutant strain. Furthermore, the γ-PGA titer reached 25.3 ± 0.8 g/L with a productivity of 0.84 g/L/h and a yield of 1.50 g of γ-PGA/g of citric acid in fed-batch fermentation. It should be noted that this study enables the synthesis of low (1.84 × 10⁵ Da) and high (2.06 × 10⁶ Da) molecular weight of γ-PGA by BL01 and the engineering strain.

CONCLUSION

The application of recently published strategies to successfully improve γ-PGA production for the new strain B. tequilensis BL01 is reported. The titer of γ-PGA increased 2.17-fold and 1.32-fold compared with that of the wild type strain in the flask and 5 L fermenter. The strain shows excellent promise as a γ-PGA producer compared with previous studies. Meanwhile, different molecular weights of γ-PGA were obtained, enhancing the scope of application in industry.

Efficient molasses utilization for low-molecular-weight poly-γ-glutamic acid production using a novel Bacillus subtilis stain

Background

Poly-γ-glutamic acid (γ-PGA) is a biopolymer and has various applications based on its biocompatibility, non-toxicity, and edibility. Low-molecular-weight (Mw)-γ-PGA has promising applications in agriculture and pharmaceuticals. It is traditionally produced by enzymatic hydrolysis. Cost-effective bioproduction of low-Mw-γ-PGA is essential for commercial application of γ-PGA.

Results

Bacillus subtilis 242 is a newly isolated low-Mw-γ-PGA-producing strain. To develop cost-effective production of γ-PGA using this newly isolated strain, cane molasses and corn steep liquor were used to produce γ-PGA. The concentration of cane molasses was optimized and 100 g/L cane molasses resulted in high γ-PGA production. The effects of yeast extract and corn steep liquor on γ-PGA yield were investigated. High concentration of γ-PGA was obtained in the medium with corn steep liquor. A concentration of 32.14 g/L γ-PGA was achieved in fed-batch fermentation, with a productivity of 0.67 g/L/h and a percentage yield (g_γ-PGA/g_glutamate) of 106.39%. The Mw of γ-PGA was 27.99 kDa.

Conclusion

This study demonstrated the potential application of B. subtilis 242 for cost-effective production of low-Mw-γ-PGA from cane molasses.

Treatment and high value utilization of glutamic acid wastewater

Glutamate wastewater has a high yield in the world and is difficult to be treated due to its high acidity, high COD (chemical oxygen demand, reflecting the pollution degree of reduced substances in the water) and high ammonia nitrogen characteristics. In this study, Bacillus licheniformis M 2020051 was used to treat organic wastewater to ferment polyglutamic acid, which reduced the pollution of industrial wastewater and produced polyglutamic acid at a low cost. Firstly, a strain with high salt tolerance and high polyglutamic acid production was isolated from saline soil, and the mechanism of salt tolerance and polyglutamic acid production were also analyzed. Then Single-factor experiment and Response surface methodology (RSM) were used to determine the appropriate fermentation conditions to achieve maximum γ-polyglutamic acid production. After optimization, the yield of polyglutamic acid was increased to 6.91 g·L^-1 by shaking fermentation, an increase of 7.13%. Finally, the agronomic experiments were carried out, and the results showed that γ-polyglutamic acid could significantly increase the germination rate of corn seeds and the growth of rapeseed. These studies will lay the foundation for reducing industrial wastewater pollution and exploring the production model of γ-polyglutamic acid.

Rational engineering of cofactor specificity of glutamate dehydrogenase for poly-γ-glutamic acid synthesis in Bacillus licheniformis

Poly-γ-glutamic acid (γ-PGA) is a multifunctional biopolymer mainly produced by Bacillus. The cofactor specificity of enzymes plays a critical role in regulating metabolic process and metabolite production. Here, we present a novel approach for switching cofactor specificity of glutamate dehydrogenase RocG from nicotinamide adenine dinucleotide phosphate (NADPH) to nicotinamide adenine dinucleotide (NADH) to improve γ-PGA production. Firstly, 3D structural modeling and molecular docking were performed to predict the binding modes of NADH and NADPH. Several site-specific mutants based on the conventional and Random Accelerated Molecular Dynamics simulations were obtained to alter cofactor specificity. Then, the effects of RocG variants overexpressions on γ-PGA production were evaluated. Compared to the wild-type, the mutant RocG^D276E showed highest increase in γ-PGA yield, increased by 40.50%. Meanwhile, yields of main by-products acetoin and 2,3-butandieol were decreased by 21.70% and 16.53%, respectively. Finally, the results of enzymatic properties confirmed that glutamate dehydrogenase mutant RocG^D276E exhibited the higher affinity for NADH, caused a shift in coenzyme preference from NADPH to NADH, with a catalytic efficiency comparable with NADPH-dependent RocG. Taken together, this research demonstrated that switching the cofactor preference of glutamate dehydrogenase via rational design was an effective strategy for high-level production of γ-PGA in Bacillus licheniformis.

Metabolic Engineering of Aspartic Acid Supply Modules for Enhanced Production of Bacitracin in Bacillus licheniformis

Bacitracin, a type of cyclic dodecapeptide antibiotic mainly produced by Bacillus, is widely used in fields of veterinary drug and feed additive. Modularization of metabolic pathways based on the concept of synthetic biology has been widely used in the efficient synthesis of target products. Here, we want to improve bacitracin production through strengthening aspartic acid (Asp) supply in B. licheniformis DW2. First, exogenous Asp addition assays implied that strengthening Asp supply benefited bacitracin production. Second, Asp synthetic pathways were strengthened via overexpressing aspartate dehydrogenase AspD and asparaginase AnsB, attaining recombinant strain DW2-ASP2, and bacitracin yield produced by DW2-ASP2 was 862.81 U/mL, increased by 14.05% compared with that of DW2 (756.49 U/mL). Then, to improve precursor oxaloacetate (OAA) accumulation for Asp synthesis, pyruvate carboxylase PycA and carbonic anhydrase EcaA were co-overexpressed in DW2-ASP2, and malic enzyme gene malS was deleted to weak overflow metabolism of tricarboxylic acid, and the attained strain DW2-ASP7 showed further increased bacitracin production from 862.81 to 989.23 U/mL. Subsequently, transporter YveA was identified as an Asp exporter, and bacitracin yield was increased to 1025.26 U/mL via deleting yveA, attaining strain DW2-ASP9. Finally, Asp ammonia-lyase gene aspA was disrupted to weaken Asp degradation, and bacitracin yield of attained strain DW2-ASP10 reached 1059.86 U/mL, increased by 40.10% compared to DW2. Taken together, this research demonstrated that metabolic engineering of Asp metabolic modules is an efficient strategy for enhancing bacitracin production, and these strategies could also be applied in the production of other peptide-related metabolites.

Study on the mechanism of production of γ-PGA and nattokinase in Bacillus subtilis natto based on RNA-seq analysis

Poly-γ-glutamic acid (γ-PGA) and nattokinase (NK) are the main substances produced by Bacillus subtilis natto in solid-state fermentation and have wide application prospects. We found that our strains had higher activity of nattokinase when soybeans were used as substrate to increase the yield of γ-PGA. Commercial production of γ-PGA and nattokinase requires an understanding of the mechanism of co-production. Here, we obtained the maximum γ-PGA yield (358.5 g/kg, w/w) and highest activity of NK during fermentation and analyzed the transcriptome of Bacillus subtilis natto during co-production of γ-PGA and NK. By comparing changes in expression of genes encoding key enzymes and the metabolic pathways associated with the products in genetic engineering, the mechanism of co-production of γ-PGA and nattokinase can be summarized based on RNA-seq analysis. This study firstly provides new insights into the mechanism of co-production of γ-PGA and nattokinase by Bacillus subtilis natto and reveals potential molecular targets to promote the co-production of γ-PGA and nattokinase.

Natural bio-based monomers for biomedical applications: a review

In recent years, synthetic and semi-synthetic polymer materials have been widely used in various applications. Especially concerning biomedical applications, their biocompatibility, biodegradability, and non-toxicity have increased the interest of researchers to discover and develop new products for the well-being of humanity. Among the synthetic and semi-synthetic materials, the use of natural bio-based monomeric materials presents a possible novel avenue for the development of new biocompatible, biodegradable, and non-toxic products. The purpose of this article is to review the information on the role of natural bio-based monomers in biomedical applications. Increased eco-friendliness, biocompatibility, biodegradability, non-toxicity, and intrinsic biological activity are some of the attributes which make itaconic, succinic, citric, hyaluronic, and glutamic acids suitable potential materials for biomedical applications. Herein, we summarize the most recent advances in the field over the past ten years and specifically highlight new and interesting discoveries in biomedical applications. Natural origin acid-based bio-monomers for biomedical applications.

1H NMR-based chemometric metabolomics characterization of soymilk fermented by Bacillus subtilis BSNK-5

Microbial fermentation can endow food with unique flavors, increase its nutritional value and enhance functional characteristics. Our previous research has shown that liquid fermentation of soymilk by Bacillus subtilis BSNK-5 imparted new functional properties of to the fermented product via production of nattokinase. In this study, in order to further investigate the changes in the flavor, nutritional quality and functional characteristics of soymilk during fermentation using proton nuclear magnetic resonance (¹H NMR) metabolomics to monitor the metabolite profile of BSNK-5-fermented soymilk. A total of 44 differential metabolites were identified between BSNK-5-fermented soymilk and uninoculated/unfermented soymilk, among which the levels of flavor-related substances (acetate, isovalerate and 2-methylbutyrate), nutrient-related substances (12 free amino acids), and functional substances (taurine, GABA and genistein) significantly increased after fermentation. These metabolites were closely associated with eight potential metabolic pathways. This work highlighted the significance of BSNK-5 strain in improving the nutritional quality and functional characteristics of fermented soymilk; however, the use of the strain also caused flavor deterioration. This study lays a theoretical foundation for the improvement and development of fermented soy products via liquid fermentation with B. subtilis.

Isolation of a novel poly-γ-glutamic acid-producing Bacillus licheniformis A14 strain and optimization of fermentation conditions for high-level production

In the present study, bacteria producing poly-γ-glutamic acid were isolated from marine sands, and an efficient producer identified. γ-PGA was rapidly screened by thin-layer chromatography and UV spectrophotometer assay. Media optimization was carried out, and for the cost-effective production of γ-PGA, monosodium glutamate was used as the substrate for the synthesis of γ-PGA instead of glutamic acid. Lastly, Plackett-Buman design (PB) and Response surface methodology (RSM) were used to determine significant media components and their interaction effect to achieve maximum γ-PGA production. With this integrated method, a bacterial strain with a high yield of γ-PGA was obtained rapidly, and the production was increased up to 37.8 g/L after optimization.

Deciphering metabolic responses of biosurfactant lichenysin on biosynthesis of poly-γ-glutamic acid

Poly-γ-glutamic acid (γ-PGA) is an extracellularly produced biodegradable polymer, which has been widely used as agricultural fertilizer, mineral fortifier, cosmetic moisturizer, and drug carrier. This study firstly discovered that lichenysin, as a biosurfactant, showed the capability to enhance γ-PGA production in Bacillus licheniformis. The exogenous addition of lichenysin improved the γ-PGA yield up to 17.9% and 21.9%, respectively, in the native strain B. licheniformis WX-02 and the lichenysin-deficient strain B. licheniformis WX02-ΔlchAC. The capability of intracellular biosynthesis of lichenysin was positively correlated with γ-PGA production. The yield of γ-PGA increased by 25.1% in the lichenysin-enhanced strain B. licheniformis WX02-Psrflch and decreased by 12.2% in the lichenysin-deficient strain WX02-ΔlchAC. Analysis of key enzyme activities and gene expression in the TCA cycle, precursor glutamate synthesis, and γ-PGA synthesis pathway revealed that the existence of lichenysin led to increased γ-PGA via shifting the carbon flux in the TCA cycle towards glutamate and γ-PGA biosynthetic pathways, minimizing by-product formation, and facilitating the uptake of extracellular substrates and the polymerization of glutamate to γ-PGA. Insight into the mechanisms of enhanced production of γ-PGA by lichenysin would define the essential parameters involved in γ-PGA biosynthesis and provide the basis for large-scale production of γ-PGA.

13C-Metabolic Flux Analysis Reveals the Metabolic Flux Redistribution for Enhanced Production of Poly-γ-Glutamic Acid in dlt Over-Expressed Bacillus licheniformis

Poly-γ-glutamic acid (γ-PGA) is an anionic polymer with various applications. Teichoic acid (TA) is a special component of cell wall in gram-positive bacteria, and its D-alanylation modification can change the net negative charge of cell surface, autolysin activity and cationic binding efficiency, and might further affect metabolic production. In this research, four genes (dltA, dltB, dltC, and dltD) of dlt operon were, respectively, deleted and overexpressed in the γ-PGA producing strain Bacillus licheniformis WX-02. Our results implied that overexpression of these genes could all significantly increase γ-PGA synthetic capabilities, among these strains, the dltB overexpression strain WX-02/pHY-dltB owned the highest γ-PGA yield (2.54 g/L), which was 93.42% higher than that of the control strain WX-02/pHY300 (1.31 g/L). While, the gene deletion strains produced lower γ-PGA titers. Furthermore, ¹³C-Metabolic flux analysis was conducted to investigate the influence of dltB overexpression on metabolic flux redistribution during γ-PGA synthesis. The simulation data demonstrated that fluxes of pentose phosphate pathway and tricarboxylic acid cycle in WX-02/pHY-dltB were 36.41 and 19.18 mmol/g DCW/h, increased by 7.82 and 38.38% compared to WX-02/pHY300 (33.77 and 13.86 mmol/g DCW/h), respectively. The synthetic capabilities of ATP and NADPH were also increased slightly. Meanwhile, the fluxes of glycolytic and by-product synthetic pathways were all reduced in WX-02/pHY-dltB. All these above phenomenons were beneficial for γ-PGA synthesis. Collectively, this study clarified that overexpression of dltB strengthened the fluxes of PPP pathway, TCA cycle and energy metabolism for γ-PGA synthesis, and provided an effective strategy for enhanced production of γ-PGA.

Efficient Biosynthesis of Low-Molecular-Weight Poly-γ-glutamic Acid by Stable Overexpression of PgdS Hydrolase in Bacillus amyloliquefaciens NB

Low-molecular-weight poly-γ-glutamic acid (LMW-γ-PGA) has attracted much attention owing to its great potential in food, agriculture, medicine, and cosmetics. Current methods of LMW-γ-PGA production, including enzymatic hydrolysis, are associated with low operational stability. Here, an efficient method for stable biosynthesis of LMW-γ-PGA was conceived by overexpression of γ-PGA hydrolase in Bacillus amyloliquefaciens NB. To establish stable expression of γ-PGA hydrolase (PgdS) during fermentation, a novel plasmid pNX01 was constructed with a native replicon from endogenous plasmid p2Sip, showing a loss rate of 4% after 100 consecutive passages. Subsequently, this plasmid was applied in a screen of high activity PgdS hydrolase, leading to substantial improvements to γ-PGA titer with concomitant decrease in the molecular weight. Finally, a satisfactory yield of 17.62 ± 0.38 g/L LMW-γ-PGA with a weight-average molecular weight of 20-30 kDa was achieved by direct fermentation of Jerusalem artichoke tuber extract. Our study presents a potential method for commercial production of LMW-γ-PGA.

Rewiring glycerol metabolism for enhanced production of poly-γ-glutamic acid in Bacillus licheniformis

BACKGROUND

Poly-γ-glutamic acid (γ-PGA) is a natural polymer with great potential applications in areas of agriculture, industry, and pharmaceutical. The biodiesel-derived glycerol can be used as an attractive feedstock for γ-PGA production due to its availability and low price; however, insufficient production of γ-PGA from glycerol is limitation.

RESULTS

The metabolic pathway of Bacillus licheniformis WX-02 was rewired to improve the efficiency of glycerol assimilation and the supply of NADPH for γ-PGA synthesis. GlpK, GlpX, Zwf, and Tkt1 were found to be the key enzymes for γ-PGA synthesis using glycerol as a feedstock. Through combinational expression of these key enzymes, the γ-PGA titer increased to 19.20 ± 1.57 g/L, which was 1.50-fold of that of the wild-type strain. Then, we studied the flux distributions, gene expression, and intracellular metabolites in WX-02 and the recombinant strain BC4 (over-expression of the above quadruple enzymes). Our results indicated that over-expression of the quadruple enzymes redistributed metabolic flux to γ-PGA synthesis. Furthermore, using crude glycerol as carbon source, the BC4 strain showed a high productivity of 0.38 g/L/h, and produced 18.41 g/L γ-PGA, with a high yield of 0.46 g γ-PGA/g glycerol.

CONCLUSIONS

The approach to rewiring of metabolic pathways enables B. licheniformis to efficiently synthesize γ-PGA from glycerol. The γ-PGA productivity reported in this work is the highest obtained in glutamate-free medium. The present study demonstrates that the recombinant B. licheniformis strain shows significant potential to produce valuable compounds from crude glycerol.

Enhancement of Bacitracin Production by NADPH Generation via Overexpressing Glucose-6-Phosphate Dehydrogenase Zwf in Bacillus licheniformis

Bacitracin, a kind of cyclic peptide antibiotic mainly produced by Bacillus, has wide ranges of applications. NADPH generation plays an important role in amino acid synthesis, which might influence precursor amino acid supply for bacitracin production. In this study, we want to improve bacitracin yield by enhancing intracellular precursor amino acids via strengthening NAPDH generation pathways in the bacitracin industrial production strain Bacillus licheniformis DW2. Based on our results, strengthening of NADPH pathway genes (zwf, gnd, ppnk, pntAB, and udhA) could all improve bacitracin yields in DW2, and the glucose-6-phosphate dehydrogenase Zwf overexpression strain DW2::Zwf displayed the best performance, the yield of which (886.43 U/mL) was increased by 12.43% compared to DW2 (788.40 U/mL). Then, the zwf transcriptional level and Zwf activity of DW2::Zwf were increased by 12.24-fold and 1.57-fold; NADPH and NADPH/NADH were enhanced by 61.24% and 90.63%, compared with those of DW2, respectively. Moreover, the concentrations of intracellular precursor amino acids (isoleucine, leucine, cysteine, ornithine, lysine, glutamic acid) were all enhanced obviously for bacitracin production in DW2::Zwf. Collectively, this research constructed a promising B. licheniformis strain for industrial production of bacitracin, more importantly, which revealed that strengthening of NADPH generation is an efficient strategy to improve precursor amino acid supplies for bacitracin production.

Enhanced synthesis of poly gamma glutamic acid by increasing the intracellular reactive oxygen species in the Bacillus licheniformis Δ1-pyrroline-5-carboxylate dehydrogenase gene ycgN-deficient strain

Poly gamma glutamic acid (γ-PGA) is an anionic polyamide with numerous applications. Previous studies revealed that L-proline metabolism is implicated in a wide range of cellular processes by increasing intercellular reactive oxygen species (ROS) generation. However, the relationship between L-proline metabolism and γ-PGA synthesis has not yet been analyzed. In this study, our results confirmed that deletion of Δ1-pyrroline-5-carboxylate dehydrogenase gene ycgN in Bacillus licheniformis WX-02 increased γ-PGA yield to 13.91 g L^-1, 85.22% higher than that of the wild type (7.51 g L^-1). However, deletion of proline dehydrogenase gene ycgM had no effect on γ-PGA synthesis. Furthermore, a 2.92-fold higher P5C content (19.24 μmol gDCW^-1) was detected in the ycgN deficient strain WXΔycgN, while the P5C levels of WXΔycgM and the double mutant strain WXΔycgMN showed no difference, compared to WX-02. Moreover, the ROS level of WXΔycgN was increased by 1.18-fold, and addition of n-acetylcysteine (antioxidant) decreased its ROS level, which further reduced γ-PGA synthesis capability of WXΔycgN. Collectively, our results demonstrated that proline catabolism played an important role in maintaining ROS homeostasis, and deletion of ycgN-enhanced P5C accumulation, which induced a transient ROS signal to promote γ-PGA synthesis in B. licheniformis.

Increased flux through the TCA cycle enhances bacitracin production by Bacillus licheniformis DW2

The dodecapeptide antibiotic bacitracin, produced by several strains of Bacillus licheniformis and Bacillus subtilis, is widely used as an antibacterial animal feed additive. Several genetic strategies were explored to enhance its production. The availability of building block amino acids for bacitracin production was found to play an important role in its synthesis. In this study, the TCA cycle in the industrial strain B. licheniformis DW2 was strengthened by overexpression of the key enzymes citrate synthase and isocitrate dehydrogenase (ICDH). As the central metabolic pathway, the TCA cycle is a major source for energy supply and intermediates for anabolism. By enhancing flux through the TCA cycle, more energy and precursors were generated for amino acid biosynthesis and uptake, resulting in enlarged intracellular pool of bacitracin-containing amino acids for bacitracin production. This study unveiled the metabolic responses of the increased TCA cycle flux in B. licheniformis and provided a novel strategy for enhancing bacitracin production.

Enhanced Production of Poly-γ-glutamic acid by Overexpression of the Global Anaerobic Regulator Fnr in Bacillus licheniformis WX-02

Poly-γ-glutamic acid is a multi-functional biopolymer with various applications. ATP supply plays an important role in poly-γ-glutamic acid (γ-PGA) synthesis. Global anaerobic regulator Fnr plays a key role in anaerobic adaptation and nitrate respiration, which might affect ATP generation during γ-PGA synthesis. In this study, we have improved γ-PGA production by overexpression of Fnr in Bacillus licheniformis WX-02. First, the gene fnr was knocked out in WX-02, and the γ-PGA yields have no significant differences between WX-02 and the fnr-deficient strain WXΔfnr in the medium without nitrate (BFC medium). However, the γ-PGA yield of 8.95 g/L, which was produced by WXΔfnr in the medium with nitrate addition (BFCN medium), decreased by 74% compared to WX-02 (34.53 g/L). Then, the fnr complementation strain WXΔfnr/pHY-fnr restored the γ-PGA synthesis capability, and γ-PGA yield was increased by 13% in the Fnr overexpression strain WX/pHY-fnr (39.96 g/L) in BFCN medium, compared to WX/pHY300 (35.41 g/L). Furthermore, the transcriptional levels of narK, narG, and hmp were increased by 5.41-, 4.93-, and 3.93-fold in WX/pHY-fnr, respectively, which led to the increases of nitrate consumption rate and ATP supply for γ-PGA synthesis. Collectively, Fnr affects γ-PGA synthesis mainly through manipulating the expression level of nitrate metabolism, and this study provides a novel strategy to improve γ-PGA production by overexpression of Fnr.

A novel approach to improve poly-γ-glutamic acid production by NADPH Regeneration in Bacillus licheniformis WX-02

Poly-γ-glutamic acid (γ-PGA) is an important biochemical product with a variety of applications. This work reports a novel approach to improve γ-PGA through over expression of key enzymes in cofactor NADPH generating process for NADPH pool. Six genes encoding the key enzymes in NADPH generation were over-expressed in the γ-PGA producing strain B. licheniformis WX-02. Among various recombinants, the strain over-expressing zwf gene (coding for glucose-6-phosphate dehydrogenase), WX-zwf, produced the highest γ-PGA concentration (9.13 g/L), 35% improvement compared to the control strain WX-pHY300. However, the growth rates and glucose uptake rates of the mutant WX-zwf were decreased. The transcriptional levels of the genes pgsB and pgsC responsible for γ-PGA biosynthesis were increased by 8.21- and 5.26-fold, respectively. The Zwf activity of the zwf over expression strain increased by 9.28-fold, which led to the improvement of the NADPH generation, and decrease of accumulation of by-products acetoin and 2,3-butanediol. Collectively, these results demonstrated that NADPH generation via over-expression of Zwf is as an effective strategy to improve the γ-PGA production in B. licheniformis.