Food-Grade Expression of d-Psicose 3-Epimerase with Tandem Repeat Genes in Bacillus subtilis

Characterization of Runella zeae D-mannose 2-epimerase and its expression in Bacillus subtilis for D-mannose production from D-glucose

D-Mannose 2-epimerase (MEase) catalyzes the bioconversion between D-glucose and D-mannose. It is an important potential biocatalyst for large-scale production of D-mannose, a functional monosaccharide used in pharmaceutical and food industries. In this study, a new microbial MEase was characterized from Runella zeae DSM 19591. The enzyme was purified by one-step nickel-affinity chromatography and determined to be a dimeric protein with two identical subunits of approximately 86.1 kDa by gel filtration. The enzyme showed the highest activity at pH 8.0 and 40 °C, with a specific activity of 2.99 U/mg on D-glucose and 3.71 U/mg on D-mannose. The melting temperature (Tm) was 49.4 °C and the half-life was 115.14 and 3.23 h at 35 and 40 °C, respectively. The purified enzyme (1 U/mL) produced 115.7 g/L of D-mannose from 500 g/L of D-glucose for 48 h, with a conversion ratio of 23.14 %. It was successfully expressed in Bacillus subtilis WB600 via pP43NMK as the vector. The highest fermentation activity was 10.58 U/mL after fed-batch cultivation for 28 h, and the whole cells of recombinant B. subtilis produced 114.0 g/L of D-mannose from 500 g/L of D-glucose, with a conversion ratio of 22.8 %.

Classification, application, multifarious activities and production improvement of lipopeptides produced by Bacillus

Lipopeptides, a class of compounds consisting of a peptide ring and a fatty acid chain, are secondary metabolites produced by Bacillus spp. As their hydrophilic and oleophilic properties, lipopeptides are widely used in food, medicine, environment and other industrial or agricultural fields. Compared with artificial synthetic surfactants, microbial lipopeptides have the advantages of low toxicity, high efficiency and versatility, resulting in urgent market demand and broad development prospect of lipopeptides. However, due to the complex metabolic network and precursor requirements of synthesis, the specific and strict synthesis pathway, and the coexistence of multiple homologous substances, the production of lipopeptides by microorganisms has the problems of high cost and low production efficiency, limiting the mass production of lipopeptides and large-scale application in industry. This review summarizes the types of Bacillus-produced lipopeptides and their biosynthetic pathways, introduces the versatility of lipopeptides, and describes the methods to improve the production of lipopeptides, including genetic engineering and optimization of fermentation conditions.

D-allose, a typical rare sugar: properties, applications, and biosynthetic advances and challenges

D-allose, a C-3 epimer of D-glucose and an aldose-ketose isomer of D-allulose, exhibits 80% of sucrose's sweetness while being remarkably low in calories and nontoxic, making it an appealing sucrose substitute. Its diverse physiological functions, particularly potent anticancer and antitumor effects, render it a promising candidate for clinical treatment, garnering sustained attention. However, its limited availability in natural sources and the challenges associated with chemical synthesis necessitate exploring biosynthetic strategies to enhance production. This overview encapsulates recent advancements in D-allose's physicochemical properties, physiological functions, applications, and biosynthesis. It also briefly discusses the crucial role of understanding aldoketose isomerase structure and optimizing its performance in D-allose synthesis. Furthermore, it delves into the challenges and future perspectives in D-allose bioproduction. Early efforts focused on identifying and characterizing enzymes responsible for D-allose production, followed by detailed crystal structure analysis to improve performance through molecular modification. Strategies such as enzyme immobilization and implementing multi-enzyme cascade reactions, utilizing more cost-effective feedstocks, were explored. Despite progress, challenges remain, including the lack of efficient high-throughput screening methods for enzyme modification, the need for food-grade expression systems, the establishment of ordered substrate channels in multi-enzyme cascade reactions, and the development of downstream separation and purification processes.

Efficient biosynthesis of transglutaminase in Streptomyces mobaraensis via systematic engineering strategies

Transglutaminases (TGases) have been widely used in food, pharmaceutical, biotechnology, and other industries because of their ability to catalyze deamidation, acyl transfer, and crosslinking reactions between Ƴ-carboxamide groups of peptides or protein-bound glutamine and the Ɛ-amino group of lysine. In this study, we demonstrated an efficient systematic engineering strategy to enhance the synthesis of TGase in a recombinant Streptomyces mobaraensis smL2020 strain in a 1000-L fermentor. Briefly, the enzymatic properties of the TGase TGL2020 from S. mobaraensis smL2020 and TGase TGLD from S. mobaraensis smLD were compared to obtain the TGase TGLD with perfected characteristics for heterologous expression in a recombinant S. mobaraensis smL2020ΔTG without the gene tgL 2020. Through multiple engineering strategies, including promoter engineering, optimizing the signal peptides and recombination sites, and increasing copies of the expression cassettes, the final TGLD activity in the recombinant S. mobaraensis smL2020ΔTG: (PL2020-spL2020-protgLD-tgLD)2 (tgL2020and BT1) reached 56.43 U/mL and 63.18 U/mL in shake flask and 1000-L fermentor, respectively, which was the highest reported to date. With the improvement of expression level, the application scope of TGLD in the food industry will continue to expand. Moreover, the genetic stability of the recombinant strain maintained at more than 20 generations. These findings proved the feasibility of multiple systematic engineering strategies in synthetic biology and provided an emerging solution to improve biosynthesis of industrial enzymes.

Enhancement of the d-Allulose 3-Epimerase Expression in Bacillus subtilis through Both Transcriptional and Translational Regulations

d-Allulose, a functional bulk sweetener, has recently attracted increasing attention because of its low-caloric-ness properties and diverse health effects. d-Allulose is industrially produced by the enzymatic epimerization of d-fructose, which is catalyzed by ketose 3-epimerase (KEase). In this study, the food-grade expression of KEase was studied using Bacillus subtills as the host. Clostridium sp. d-allulose 3-epimerase (Clsp-DAEase) was screened from nine d-allulose-producing KEases, showing better potential for expression in B. subtills WB600. Promoter-based transcriptional regulation and N-terminal coding sequence (NCS)-based translational regulation were studied to enhance the DAEase expression level. In addition, the synergistic effect of promoter and NCS on the Clsp-DAEase expression was studied. Finally, the strain with the combination of a PHapII promoter and gln A-Up NCS was selected as the best Clsp-DAEase-producing strain. It efficiently produced Clsp-DAEase with a total activity of 333.2 and 1860.6 U/mL by shake-flask and fed-batch cultivations, respectively.

Establishing Heterologous Production of Microcins J25 and Y in Bacillus subtilis

Microcin J25 (MccJ25) and microcin Y (MccY) are lasso peptides and considered potential alternatives to antibiotics and harmful preservatives. The combination of these two microcins can provide a wide antimicrobial spectrum against food-borne Salmonella. Currently, MccJ25 and MccY are produced using Escherichia coli expression systems; however, the entire production process is accompanied by negative effects from endotoxins. In this study, we identified Bacillus subtilis as a suitable host for MccJ25 and MccY production. High-level production of microcins was achieved by promoter optimization, host strain selection, and recombinant expression. The engineered strains produced maximum yields of 2.827 μM MccJ25 and 1.481 μM MccY. This is the first study to demonstrate the expression of MccJ25 and MccY in B. subtilis, and it offers a few engineered strains that are without antibiotic resistance markers, inducer-free, sporulation-deficient, and free of the negative effects of endotoxins for antibacterial therapy and food preservation.

The Characterization of a Novel D-allulose 3-Epimerase from Blautia produca and Its Application in D-allulose Production

D-allulose is a natural rare sugar with important physiological properties that is used in food, health care items, and even the pharmaceutical industry. In the current study, a novel D-allulose 3-epimerase gene (Bp-DAE) from the probiotic strain Blautia produca was discovered for the production and characterization of an enzyme known as Bp-DAE that can epimerize D-fructose into D-allulose. Bp-DAE was strictly dependent on metals (Mn²⁺ and Co²⁺), and the addition of 1 mM of Mn²⁺ could enhance the half-life of Bp-DAE at 55 °C from 60 to 180 min. It exhibited optimal activity in a pH of 8 and 55 °C, and the K_m values of Bp-DAE for the different substrates D-fructose and D-allulose were 235.7 and 150.7 mM, respectively. Bp-DAE was used for the transformation from 500 g/L D-fructose to 150 g/L D-allulose and exhibited a 30% of conversion yield during biotransformation. Furthermore, it was possible to employ the food-grade microbial species Bacillus subtilis for the production of D-allulose using a technique of whole-cell catalysis to circumvent the laborious process of enzyme purification and to obtain a more stable biocatalyst. This method also yields a 30% conversion yield.

A Single-Component Blue Light-Induced System Based on EL222 in Yarrowia lipolytica

Optogenetics has the advantages of a fast response time, reversibility, and high spatial and temporal resolution, which make it desirable in the metabolic engineering of chassis cells. In this study, a light-induced expression system of Yarrowia lipolytica was constructed, which successfully achieved the synthesis and functional verification of Bleomycin resistance protein (BleoR). The core of the blue light-induced system, the light-responsive element (TF), is constructed based on the blue photosensitive protein EL222 and the transcription activator VP16. The results show that the light-induced sensor based on TF, upstream activation sequence (C120)₅, and minimal promoter CYC₁₀₂ can respond to blue light and initiate the expression of GFPMut3 report gene. With four copies of the responsive promoter and reporter gene assembled, they can produce a 128.5-fold higher fluorescent signal than that under dark conditions after 8 h of induction. The effects of light dose and periodicity on this system were investigated, which proved that the system has good spatial and temporal controllability. On this basis, the light-controlled system was used for the synthesis of BleoR to realize the expression and verification of functional protein. These results demonstrated that this system has the potential for the transcriptional regulation of target genes, construction of large-scale synthetic networks, and overproduction of the desired product.

D-allulose, a versatile rare sugar: recent biotechnological advances and challenges

D-Allulose is the C-3 epimer of D-fructose, and widely regarded as a promising substitute for sucrose. It's an excellent low-calorie sweetener, with 70% sweetness of sucrose, 0.4 kcal/g dietary energy, and special physiological functions. It has been approved as GRAS by the U.S. Food and Drug Administration, and is allowed to be excluded from total and added sugar counts on the food labels. Therefore, D-allulose gradually attracts more public attention. Owing to scarcity in nature, the bioproduction of D-allulose by using ketose 3-epimerase (KEase) has become the research hotspot. Herein, we give a summary of the physicochemical properties, physiological function, applications, and the chemical and biochemical synthesis methods of D-allulose. In addition, the recent progress in the D-allulose bioproduction using KEases, and the possible solutions for existing challenges in the D-allulose industrial production are comprehensively discussed, focusing on the molecular modification, immobilization, food-grade expression, utilizing low-cost biomass as feedstock, overcoming thermodynamic limitation, as well as the downstream separation and purification. Finally, Prospects for further development are also proposed.

Bioproduction of D-allulose: Properties, applications, purification, and future perspectives

D-allulose is the C-3 epimer of D-fructose, which rarely exists in nature, and can be biosynthesized from D-fructose by the catalysis of D-psicose 3-epimerase. D-allulose is safe for human consumption and was recently approved by the United States Food and Drug Administration for food applications. It is not only able be used in food and dietary supplements as a low-calorie sweetener, but also modulates a variety of physiological functions. D-allulose has gained increasing attention owing to its excellent properties. This article presents a review of recent progress on the properties, applications, and bioproduction progress of D-allulose.

Research Advances of d-allulose: An Overview of Physiological Functions, Enzymatic Biotransformation Technologies, and Production Processes

d-allulose has a significant application value as a sugar substitute, not only as a food ingredient and dietary supplement, but also with various physiological functions, such as improving insulin resistance, anti-obesity, and regulating glucolipid metabolism. Over the decades, the physiological functions of d-allulose and the corresponding mechanisms have been studied deeply, and this product has been applied to various foods to enhance food quality and prolong shelf life. In recent years, biotransformation technologies for the production of d-allulose using enzymatic approaches have gained more attention. However, there are few comprehensive reviews on this topic. This review focuses on the recent research advances of d-allulose, including (1) the physiological functions of d-allulose; (2) the major enzyme families used for the biotransformation of d-allulose and their microbial origins; (3) phylogenetic and structural characterization of d-allulose 3-epimerases, and the directed evolution methods for the enzymes; (4) heterologous expression of d-allulose ketose 3-epimerases and biotransformation techniques for d-allulose; and (5) production processes for biotransformation of d-allulose based on the characterized enzymes. Furthermore, the future trends on biosynthesis and applications of d-allulose in food and health industries are discussed and evaluated in this review.

Efficient biosynthesis of D-allulose in Bacillus subtilis through D-psicose 3-epimerase translation modification

The combined catalysis of glucose isomerase (GI) and D-psicose 3-epimerase (DPEase) provided a convenient route for the direct synthesis of D-allulose from d-glucose, whose cost is lower than d-fructose. In the present research, the weak activity of DPEase was the key rate-limiting step and resulted in the accumulation of d-fructose in engineered Bacillus subtilis. Then, the 5'-untranslated region (5'-UTR) structure of the mRNA translational initiation region was optimized for the precise control of DPEase expression. The manipulation of the 5'-UTR region promoted the accessibility to ribosome binding and the stability of mRNA, resulting in a maximum of 1.73- and 1.98-fold increase in DPEase activity and intracellular mRNA amount, respectively. Under the optimal catalytic conditions of 75 °C, pH 6.5, 110 g/L d-glucose, and 1 mmol/L Co²⁺, the reaction equilibrium time was reduced from 7.6 h to 6.1 h. We hope that our results could provide a facilitated strategy for large-scale production of D-allulose at low-cost.

Construction of a Super-Folder Fluorescent Protein-Guided Secretory Expression System for the Production of Phospholipase D in Bacillus subtilis

Phospholipids (PLs) are one of the main ingredients in food and nutraceutical, cosmetics, agriculture, and pharmaceutical products. Phospholipase D (PLD) is a crucial enzyme for the biocatalytic synthesis or modification of PLs. Here, to prepare PLD more efficiently, we constructed a PLD expression and secretion system in Bacillus subtilis and developed an environmentally friendly reaction system. A nonclassical secretory pathway where a super-folder green fluorescent protein plays as an N-terminal guide protein was introduced. This expression system can not only achieve rapid screening of high-level expression strains but can also achieve the secretion of the target proteins. Under optimal fermentation conditions, the enzyme activity of the culture medium was 0.35 U/mL, which was 2.05-fold that of the Sec secretion pathway strains. Meanwhile, the effects of several organic solvents in the biphasic reaction media were compared. The results showed that when using cyclopentyl methyl ether as the organic phase, the final conversion rate reached 96.9%. It has shown good application potential in the synthesis of phosphatidylserine, laid the foundation for the synthesis and application of other rare and high-value PLs, and provided a reference for the production of other biocatalysts.

Characterization of Feruloyl Esterase from Bacillus pumilus SK52.001 and Its Application in Ferulic Acid Production from De-Starched Wheat Bran

Feruloyl esterase (FAE; EC 3.1.1.73) catalyzes the hydrolysis of the 4-hydroxy-3-methoxycinnamoyl group in an esterified sugar to assist in waste biomass degradation or to release ferulic acid (FA). An FAE-producing strain was isolated from humus soil samples and identified as Bacillus pumilus SK52.001. The BpFAE gene from B. pumilus SK52.001 was speculated and heterogeneously expressed in Bacillus subtilis WB800 for the first time. The enzyme exists as a monomer with 303 amino acids and a molecular mass of 33.6 kDa. Its specific activity was 377.9 ± 10.3 U/ (mg protein), using methyl ferulate as a substrate. It displays an optimal alkaline pH of 9.0, an optimal temperature of 50 °C, and half-lives of 1434, 327, 235, and 68 min at 50, 55, 60, and 65 °C, respectively. Moreover, the purified BpFAE released 4.98% FA of the alkali-acidic extractable FA from de-starched wheat bran (DSWB). When the DSWB was enzymatically degraded by the synergistic effect of the BpFAE and commercial xylanase, the FA amount reached 49.47%. It suggested that the alkaline BpFAE from B. pumilus SK52.001, which was heterologously expressed in B. subtilis WB800, possesses great potential for biomass degradation and achieving high-added value FA production from food by-products.

Enhanced Production of Transglutaminase in Streptomyces mobaraensis through Random Mutagenesis and Site-Directed Genetic Modification

Streptomyces transglutaminase (TGase) is widely used to improve food texture properties. In this study, random mutagenesis and site-directed genetic modification were used to improve the production of TGase in Streptomyces mobaraensis. First, S. mobaraensis DSM40587 (smWT) was subjected to atmospheric and room-temperature plasma mutagenesis, and then a mutant (smY2019) with a 5.5-fold increase in TGase yield was screened from approximately 3000 × 25 (round) mutants. Compared to smWT, smY2019 exhibits a 3.2-fold higher TGase mRNA level and two site mutations within the -10 region of the TGase promoter. The recombinant expression analysis in the TGase-deficient S. mobaraensis suggests that the mutated TGase promoter is more robust than the wild-type one. Finally, we integrated two additional TGase expression cassettes into the smY2019 genome, yielding the recombinant strain smY2019-3C with a 103% increase in TGase production compared to smY2019. The smY2019-3C strain with 40 U/mL of TGase yield could be a suitable candidate for the industrial production of TGase.

Fusion and secretory expression of an exo-inulinase and a d-allulose 3-epimerase to produce d-allulose syrup from inulin

BACKGROUND

This study developed a feasible catalytic method for d-allulose syrup production using a fusion enzyme, either in free or immobilized form, through hydrolysis of inulin extracted from Jerusalem artichoke tubers.

RESULTS

d-Allulose 3-epimerase (DAE) was actively expressed in secretory form by fusing with the extracellular exo-inulinase CSCA in Escherichia coli BL21 (DE3). The best linker ligating the two enzymes was a flexible peptide containing 12 residues (GSAGSAAGSGEF). At 55 °C and pH 8.0, and as with the addition of 1 mmol L^-1 Mn²⁺ , the CSCA-linkerE-DAE fusion enzyme obtained through high cell-density cultivation displayed a maximal exo-inulinase activity of 21.8 U mg^-1 and resulted in a yield of 6.3 g L^-1 d-allulose and 39.2 g L^-1 d-fructose using 60 g L^-1 inulin as the raw material. Catechol-modified alginate with titanium ions (Alg(Ti)PDA) was found to be a promising immobilization material for the fusion enzyme. After conversion for 8 days, the Alg(Ti)PDA-immobilized CSCA-linkerE-DAE (8 U g^-1 ) completed 24 reaction cycles and retained over 80% of its original activity. Each reaction obtained an average of 19.8 g L^-1 d-allulose and 32.7 g L^-1 D-fructose from 60 g L^-1 inulin.

CONCLUSION

This study shed light on a feasible and cost-effective approach for the production of syrup containing d-allulose and D-fructose with inulin as the raw material via the use of a CSCA and DAE fusion enzyme. This syrup is of added value as a functional sweetener. © 2020 Society of Chemical Industry.

Food-Grade Expression and Characterization of a Dextranase from Chaetomium gracile Suitable for Sugarcane Juice Clarification

The microbial production of dextranase using cheap carbon sources is beneficial to solve the economic loss caused by the accumulation of dextran in syrup. A food-grade microbial cell factory was constructed by introducing the dextranase encoding gene DEX from Chaetomium gracile to the chromosome of Bacillus subtilis, and the antibiotic resistance marker gene was subsequently deleted via the Cre/loxP strategy. The dual-promoter system with a sequentially arranged constitutive P43 promoter resulted in an 85 % increase in DEX expression. Under the optimal fermentation conditions of 10 g/L maltose, 15 g/L casein, 1 g/L Na₂ HPO₄ , 1 g/L FeSO₄ and 8 g/L NaCl, DEX activity was increased from 2.625 to 64.34 U/mL. Recombinant DEX was purified 5.98-fold with a recovery ratio of 26.67 % and specific activity of 3935.02 U/mg. Enzyme activity was optimal at 55 °C and pH 5.0 and remained 80.34 % and 71.36 % of the initial activity at 55 °C and pH 4.0 after 60 min, respectively. The enzyme possessed high activity in the presence of Co²⁺ , while Ag⁺ showed the strongest inhibition ability. The optimal substrate was 20 g/L dextran T-2000. The findings could facilitate the low-cost, large-scale production of food-grade DEX for use in the sugar industry.

Expression of d-psicose-3-epimerase from Clostridium bolteae and Dorea sp. and whole-cell production of d-psicose in Bacillus subtilis

Purpose

d-psicose-3-epimerase (DPEase) catalyses the isomerisation of d-fructose to d-psicose, a rare sugar in nature with unique nutritional and biological functions. An effective industrial-scale method is needed for d-psicose production. Herein, the expression of a neutral and a slightly acidic pH DPEase in Bacillus subtilis was evaluated.

Methods

Two DPEase genes from Clostridium bolteae and Dorea sp. were separately expressed in B. subtilis via plasmid pSTOP1622, and an extra P43 promoter was employed to the expression cassette. The fermentation conditions of the engineered B. subtilis strains were also optimised, to facilitate both cell growth and enzyme production.

Result

The introduction of P43 promoter to the two DPEase genes increased enzyme production by about 20%. Optimisation of fermentation conditions increased DPEase production to 21.90 U/g at 55 °C and 24.01 U/g at 70 °C in B. subtilis expressing C. bolteae or Dorea sp. DPEase, equating to a 94.67% and 369.94% increase, respectively, relative to controls.

Conclusion

Enhanced DPEase production was achieved in B. subtilis expressing C. bolteae or Dorea sp. DPEase genes.

A Novel d-Allulose 3-Epimerase Gene from the Metagenome of a Thermal Aquatic Habitat and d-Allulose Production by Bacillus subtilis Whole-Cell Catalysis

A novel d-allulose 3-epimerase gene (daeM) has been identified from the metagenomic resource of a hot-water reservoir. The enzyme epimerizes d-fructose into d-allulose, a functional sugar of rare abundance in nature. The metagenomic DNA fragment was cloned and expressed in Escherichia coli The purified recombinant protein (DaeM) was found to be metal dependent (Co2+ or Mn2+). It displayed the maximal levels of catalytic activity in a pH range of 6 to 11 and a temperature range of 75°C to 80°C. The enzyme exhibited remarkably high thermal stability at 60°C and 70°C, with half-life values of 9,900 and 3,240 min, respectively. To the best of our knowledge, this is the highest thermal stability demonstrated by a d-allulose 3-epimerase that has been characterized to date. The enzymatic treatment of 700 mg·ml-1 d-fructose yielded about 217 mg·ml-1 d-allulose, under optimal condition. The catalytic product was purified, and its nuclear magnetic resonance (NMR) spectra were found to be indistinguishable from those of standard d-allulose. For biomolecule production, the whole-cell catalysis procedure avoids the tedious process of extraction and purification of enzyme and also offers better biocatalyst stability. Further, it is desirable to employ safe-grade microorganisms for the biosynthesis of a product. The daeM gene was expressed intracellularly in Bacillus subtilis A whole-cell catalysis reaction performed with a reaction volume of 1 liter at 60°C yielded approximately 196 g·liter-1 d-allulose from 700 g·liter-1 d-fructose. Further, the whole recombinant cells were able to biosynthesize d-allulose in apple juice, mixed fruit juice, and honey.IMPORTANCE d-Allulose is a noncaloric sugar substitute with antidiabetes and antiobesity potential. With several characteristics of physiological significance, d-allulose has wide-ranging applications in the food and pharmacology industries. The development of a thermostable biocatalyst is an objective of mainstream research aimed at achieving industrial acceptability of the enzyme. Aquatic habitats of extreme temperatures are considered a potential metagenomic resource of heat-tolerant biocatalysts of industrial importance. The present study explored the thermal-spring metagenome of the Tattapani geothermal region, Chhattisgarh, India, discovering a novel d-allulose 3-epimerase gene, daeM, encoding an enzyme of high-level heat stability. The daeM gene was expressed in the microbial cells of a nonpathogenic and safe-grade species, B. subtilis, which was found to be capable of performing d-fructose to d-allulose interconversion via a whole-cell catalysis reaction. The results indicate that DaeM is a potential biocatalyst for commercial production of the rare sugar d-allulose. The study established that extreme environmental niches represent a genomic resource of functional sugar-related biocatalysts.

Economical production of isomaltulose from agricultural residues in a system with sucrose isomerase displayed on Bacillus subtilis spores

A safe, efficient, environmentally friendly process for producing isomaltulose is needed. Here, the biocatalyst, sucrose isomerase (SIase) from Erwinia rhapontici NX-5, displayed on the surface of Bacillus subtilis 168 spores (food-grade strain) was applied for isomaltulose production. The anchored SIase showed relatively high bioactivity, suggesting that the surface display system using CotX as the anchoring protein was successful. The stability of the anchored SIase was also significantly better. Thermal stability analysis showed that 80% of relative activity was retained after incubation at 40 °C and 45 °C for 60 min. To develop an economical industrial fermentation medium, untreated beet molasses (30 g/L) and cold-pressed soybean powder (50 g/L) were utilised as the main broth components for SIase pilot-scale production. Under the optimal conditions, the productive spores converted 92% of sucrose after 6 h and the conversion rate was 45% after six cycles. Isomaltulose production with this system using the agricultural residues, untreated beet molasses and soybean powder, as substrates is cost-effective and environmentally friendly and can help to overcome issues due to the genetic background.

Construction of second generation protease-deficient hosts of Bacillus subtilis for secretion of foreign proteins

Although one of the major factors limiting the application of Bacillus subtilis as an expression host has been its production of at least eight extracellular proteases, researchers have also noticed that some proteases benefited the secretion of foreign proteins at times. Therefore, to maximize the yield of a foreign protein, the proteases should be selectively inactivated. This raises a new question that how to identify the favorable and unfavorable proteases for a target protein. Here, an evaluation system containing nine mutant strains of B. subtilis 168 was developed to address this question. The mutant strain PD8 has all the eight proteases inactivated whereas each of the other eight mutant strains expresses only one kind of these eight proteases. The target protein is secreted in these nine mutant strains; if the production of target protein in a mutant strain is higher than that in strain PD8, the corresponding protease is regarded as favorable. Accordingly, the optimal protease-deficient host is constructed through inactivating the unfavorable proteases. The effectiveness of this system was confirmed by expressing three foreign proteins. This study provides a strategy for improving the secretion of a foreign protein in B. subtilis through tailoring a personalized protease-deficient host.

Improving the Production of Salt-Tolerant Glutaminase by Integrating Multiple Copies of Mglu into the Protease and 16S rDNA Genes of Bacillus subtilis 168

In this study, the Micrococcus luteus K-3 glutaminase was successfully over-expressed in the GRAS (Generally Recognized as Safe) Bacillus subtilis strain 168 by integration of the Mglu gene in the 16S rDNA locus. This was done in order to screen a strain producing high levels of recombinant glutaminase from the selected candidates. The transcription of the glutaminase genes in the B. subtilis 168 chromosome and the expression of glutaminase protein was further assessed by qPCR, SDS-PAGE analysis and an enzyme activity assay. To further increase the production of glutaminase, the nprB and nprE genes, which encode specific proteases, were disrupted by integration of the Mglu gene. After continuous cell culturing without the addition of antibiotics, the integrated recombinant strains showed excellent genetic stability, demonstrating favorable industrialization potential. After the fermentation temperature was optimized, a 5-L bioreactor was used for fed-batch fermentation of the recombinant glutaminase producing strain at 24 °C, and the highest enzyme activity achieved was approximately 357.6 U/mL.

Food-grade expression of an iron-containing acid urease in Bacillus subtilis

Enzymatic degradation of urea, the precursor of carcinogenic compound ethylcarbamate in rice wine, is always attractive. In the present study, we achieved high efficient production of Bacillus paralicheniformis iron-containing urease (Bp_Urease) in B. subtilis with the food-grade expression system. After reassembly of the urease gene cluster with inserting ribosome binding site (RBS), the production was increased from 38 U/L to 187 U/L. After altering the position of ureC and co-expressing the iron transporter encoding gene ureH, the activity was further increased to 1307 U/L. Eventually, the urease production was improved to 21,964 U/L in 3-L fermentor, which is the highest reported value to date. Food-grade production of the iron-containing urease would be favorable to the practical applications in food industries.

Development of food-grade expression system for d-allulose 3-epimerase preparation with tandem isoenzyme genes in Corynebacterium glutamicum and its application in conversion of cane molasses to D-allulose

D-Allulose 3-epimerase (DAE) has been applied to produce D-allulose, a low-calorie and functional sweetener. In this study, a new DAE from Paenibacillus senegalensis was characterized in Escherichia coli. Furthermore, we presented a tandem isoenzyme gene expression strategy to express multiple DAEs in one cell and construct food-grade expression systems based on Corynebacterium glutamicum. Seventeen expression cassettes based on three DAE genes from different organisms were constructed. Among all recombinant strains, DAE16 harboring three DAE genes in an expression vector exhibited the highest enzyme activity with 22.7 U/mg. Whole-cell transformation of DAE16 produced 225 g/L D-allulose with a volumetric productivity of 353 g·g ^-1 ·hr ^-1 . The catalytic efficiency of strain C-DAE9 integrating total 11 DAE genes in chromosome was 16.4-fold higher than strains carrying one DAE. Fed-batch culture of C-DAE9 gave enzyme activity of 44,700 U/L. We also expressed a thermostable invertase in C. glutamicum and obtained enzyme activity of 29 U/mg. Immobilized cells expressing DAE or invertase exhibited 80% of retained activity after 30 cycles of catalytic reactions. Those immobilized cells were coupled to produce 61.2 g/L D-allulose from cane molasses in a two-step reaction process. This study provided an efficient approach for enzyme preparation and allowed access to produce D-allulose from other abundant and low-cost feedstock enriched with sucrose.

Highly efficient production of Clostridium cellulolyticum H10 D-psicose 3-epimerase in Bacillus subtilis and use of these cells to produce D-psicose

Background

d-Psicose 3-epimerase (DPEase) catalyzes the isomerization of d-fructose to the rare sugar d-psicose, which may help prevent obesity, reduce blood sugar and blood fat, and inhibit intra-abdominal fat accumulation.

Results

In this study, the DPEase of Clostridium cellulolyticum H10 was expressed in the food-grade host Bacillus subtilis. Optimization of the culture medium during shake-flask experiments yielded a DPEase activity of 314 U/mL. The optimal medium included 20 g/L peptone, 15 g/L corn steep powder, 5 g/L glycerol, and 1 mM Ca²⁺. Controlling the carbon source concentration was important because elevated concentrations can result in catabolite metabolic suppression (CCR). To avoid CCR and increase DPEase expression, we developed a fed-batch strategy in a 3.6-L fermenter. We altered the ratio of carbon source to nitrogen source (C/N) in the feeding medium and employed a constant feeding rate (6 g/L/h). This strategy improved the DPEase activity to 2246 U/mL (7.8 g/L), which is almost 15 times higher than that observed in the original shake-flask cultures. Finally, we used the DPEase-expressing B. subtilis cells to produce d-psicose from d-fructose, and a 28% conversion yield was achieved with these cells, demonstrating their potential use in d-psicose production.

Conclusions

This is the first report to enhance recombinant DPEase production in B. subtilis using efficient and convenient fermentation strategy, and the DPEase yield is three times higher than the highest yield reported to date. The recombinant B. subtilis cells were further used in the efficient synthesis of d-psicose. This study provides a basis for the industrial production of d-psicose.

Cell regeneration and cyclic catalysis of engineered Kluyveromyces marxianus of a D-psicose-3-epimerase gene from Agrobacterium tumefaciens for D-allulose production

D-Allulose as a low-energy and special bioactive monosaccharide sugar is essential for human health. In this study, the D-psicose-3-epimerase gene (DPEase) of Agrobacterium tumefaciens was transferred into thermotolerant Kluyveromyces marxianus to decrease the production cost of D-allulose and reduce the number of manufacturing procedures. The cell regeneration of K. marxianus and cyclic catalysis via whole-cell reaction were investigated to achieve the sustainable application of K. marxianus and the consumption of residual D-fructose. Results showed that DPEase, encoding a 33 kDa protein, could be effectively expressed in thermotolerant K. marxianus. The engineered K. marxianus produced 190 g L^-1 D-allulose with 750 g L^-1 D-fructose as a substrate at 55 °C within 12 h. Approximately 100 g of residual D-fructose was converted into 34 g of ethanol, and 15 g of the engineered K. marxianus cells was regenerated after fermentation at 37 °C for 21 h. The purity of D-allulose of more than 90% could be obtained without isolating it from D-allulose and D-fructose mixture through residual D-fructose consumption. This study provided a valuable pathway to regenerate engineered K. marxianus cells and achieve cyclic catalysis for D-allulose production.

An integrated bio-process for production of functional biomolecules utilizing raw and by-products from dairy and sugarcane industries

The study investigated an integrated bioprocessing of raw and by-products from sugarcane and dairy industries for production of non-digestible prebiotic and functional ingredients. The low-priced feedstock, whey, molasses, table sugar, jaggery, etc., were subjected to transglucosylation reactions catalyzed by dextransucrase from Leuconostoc mesenteroides MTCC 10508. HPLC analysis approximated production of about 11-14 g L^-1 trisaccharide i.e. 2-α-D-glucopyranosyl-lactose (4-galactosyl-kojibiose) from the feedstock prepared from table sugar, jaggery, cane molasses and liquid whey, containing about 30 g L^-1 sucrose and lactose each. The trisaccharide was hydrolysed into the prebiotic disaccharide, kojibiose, by employing recombinant β-galactosidase from Escherichia coli. The enzyme β-galactosidase achieved about 90% conversion of 2-α-D-glucopyranosyl-lactose into kojibiose. The D-fructose generated by catalytic reactions of dextransucrase was targeted for catalytic transformation into rare sugar, D-allulose (or D-psicose), by treating the samples with Smt3-D-psicose 3-epimerase. The catalytic reactions resulted in the conversion of ~ 25% D-fructose to D-allulose. These bioactive compounds are known to exert a plethora of benefits to human health, and therefore, are preferred ingredients for making functional foods.