Enhanced extracellular gamma glutamyl transpeptidase production by overexpressing of PrsA lipoproteins and improving its mRNA stability in Bacillus subtilis and application in biosynthesis of L-theanine

Enhanced theanine production with reduced ATP supply by alginate entrapped Escherichia coli co-expressing γ-glutamylmethylamide synthetase and polyphosphate kinase

L-theanine is an amino acid with a unique flavor and many therapeutic effects. Its enzymatic synthesis has been actively studied and γ-Glutamylmethylamide synthetase (GMAS) is one of the promising enzymes in the biological synthesis of theanine. However, the theanine biosynthetic pathway with GMAS is highly ATP-dependent and the supply of external ATP was needed to achieve high concentration of theanine production. As a result, this study aimed to investigate polyphosphate kinase 2 (PPK2) as ATP regeneration system with hexametaphosphate. Furthermore, the alginate entrapment method was employed to immobilize whole cells containing both gmas and ppk2 together resulting in enhanced reusability of the theanine production system with reduced supply of ATP. After immobilization, theanine production was increased to 239 mM (41.6 g/L) with a conversion rate of 79.7% using 15 mM ATP and the reusability was enhanced, maintaining a 100% conversion rate up to the fifth cycles and 60% of conversion up to eighth cycles. It could increase long-term storage property for future uses up to 35 days with 75% activity of initial activity. Overall, immobilization of both production and cofactor regeneration system could increase the stability and reusability of theanine production system.

Efficient expression of γ-glutamyl transpeptidase in Bacillus subtilis via CRISPR/Cas9n and its immobilization

In this study, we successfully applied the strategy of combining tandem promoters and tandem signal peptides with overexpressing signal peptidase to efficiently express and produce γ-glutamyl peptidase (GGT) enzymes (BsGGT, BaGGT, and BlGGT) from Bacillus subtilis, Bacillus amyloliquefaciens, and Bacillus licheniformis in Bacillus subtilis ATCC6051Δ5. In order to avoid the problem of instability caused by duplicated strong promoters, we assembled tandem promoters of different homologous genes from different species. To achieve resistance marker-free enzyme in the food industry, we first removed the replication origin and corresponding resistance marker of Escherichia coli from the expression vector. The plasmid was then transformed into the B. subtilis host, and the Kan resistance gene in the expression plasmid was directly edited and silenced using the CRISPR/Cas9n-AID base editing system. As a result, a recombinant protein expression carrier without resistance markers was constructed, and the enzyme activity of the BlGGT strain during shake flask fermentation can reach 53.65 U/mL. The recombinant BlGGT was immobilized with epoxy resin and maintained 82.8% enzyme activity after repeated use for 10 times and 87.36% enzyme activity after storage at 4 °C for 2 months. The immobilized BlGGT enzyme was used for the continuous synthesis of theanine with a conversion rate of 65.38%. These results indicated that our approach was a promising solution for improving enzyme production efficiency and achieving safe production of enzyme preparations in the food industry. KEY POINTS: • Efficient expression of recombinant proteins by a combination of dual promoter and dual signal peptide. • Construction of small vectors without resistance markers in B. subtilis using CRISPR/Cas9n-AID editing system. • The process of immobilizing BlGGT with epoxy resin was optimized.

Enhanced extracellular β-mannanase production by overexpressing PrsA lipoprotein in Bacillus subtilis and optimizing culture conditions

In this study, first, β-mannanase gene man derived from Bacillus amyloliquefaciens CGMCC1.857 was cloned and expressed in Bacillus subtilis 168 to generate B. subtilis M1. However, the extracellular β-mannanase activity of B. subtilis M1 was not very high. To further increase extracellular β-mannanase extracytoplasmic molecular chaperone, PrsA lipoprotein was tandem expressed with man gene in B. subtilis 168 to yield B. subtilis M2. The secretion of β-mannanase of B. subtilis M2 was enhanced by 15.4%, compared with the control B. subtilis M1. Subsequently, process optimization strategies were also developed to enhance β-mannanase production by B. subtilis 168 M2. It was noted that the optimal temperature for β-mannanase production (25°C) was different from the optimal growth temperature (37°C) for B. subtilis. Based on these findings, a two-stage temperature control strategy was proposed where the bacterial culture was maintained at 37°C for the first 12 h to obtain a high rate of cell growth, followed by lowering the temperature to 25°C to enhance β-mannanase production. Using this strategy, the extracellular β-mannanase activity reached 5016 ± 167 U/ml at about 36 h, which was 19.1% greater than the best result obtained using a constant temperature (25°C). The result of this study showed that PrsA lipoprotein overexpression and two-stage temperature control strategy were more efficient for β-mannanase fermentation in B. subtilis.

Enhanced extracellular α-amylase production in Brevibacillus choshinensis by optimizing extracellular degradation and folding environment

A strategy for optimizing the extracellular degradation and folding environment of Brevibacillus choshinensis has been used to enhance the extracellular production of recombinant α-amylase. First, a gene (bcp) encoding an extracellular protease and another encoding an extracellular chaperone (prsC) were identified in the genome of B. choshinensis HPD31-SP3. Then, the effect of extracellular protein degradation on recombinant α-amylase production was investigated by establishing a CRISPR/Cas9n system to knock out bcp. The effect of extracellular folding capacity was investigated separately by coexpressing extracellular chaperones genes from different sources (prsA, prsC, prsL, prsQ) in B. choshinensis. The final recombinant strain (BCPPSQ), which coexpressed prsQ in a genetic background lacking bcp, produced an extracellular α-amylase activity of 6940.9 U/mL during shake-flask cultivation. This was 2.1-fold greater than that of the original strain BCWPS (3367.9 U/mL). Cultivation of BCPPSQ in a 3-L fermenter produced an extracellular α-amylase activity of 17 925.6 U/mL at 72 h, which was 7.6-fold greater than that of BCWPS (2358.1 U/mL). This strategy demonstrates its great potential in enhancing extracellular α-amylase production in B. choshinensis. What's more, this study provides a strategic reference for improving the extracellular production of other recombinant proteins in B. choshinensis.

The multifunctionality of expression systems in Bacillus subtilis: Emerging devices for the production of recombinant proteins

Bacillus subtilis is a successful host for producing recombinant proteins. Its GRAS (generally recognized as safe) status and its remarkable innate ability to absorb and incorporate exogenous DNA into its genome make this organism an ideal platform for the heterologous expression of bioactive substances. The factors that corroborate its value can be attributed to the scientific knowledge obtained from decades of study regarding its biology that has fostered the development of several genetic engineering strategies, such as the use of different plasmids, engineering of constitutive or double promoters, chemical inducers, systems of self-inducing expression with or without a secretion system that uses a signal peptide, and so on. Tools that enrich the technological arsenal of this expression platform improve the efficiency and reduce the costs of production of proteins of biotechnological importance. Therefore, this review aims to highlight the major advances involving recombinant expression systems developed in B. subtilis, thus sustaining the generation of knowledge and its application in future research. It was verified that this bacterium is a model in constant demand and studies of the expression of recombinant proteins on a large scale are increasing in number. As such, it represents a powerful bacterial host for academic research and industrial purposes.

Bacterial Gamma-Glutamyl Transpeptidase, an Emerging Biocatalyst: Insights Into Structure-Function Relationship and Its Biotechnological Applications

Gamma-glutamyl transpeptidase (GGT) enzyme is ubiquitously present in all life forms and plays a variety of roles in diverse organisms. Higher eukaryotes mainly utilize GGT for glutathione degradation, and mammalian GGTs have implications in many physiological disorders also. GGTs from unicellular prokaryotes serve different physiological functions in Gram-positive and Gram-negative bacteria. In the present review, the physiological significance of bacterial GGTs has been discussed categorizing GGTs from Gram-negative bacteria like Escherichia coli as glutathione degraders and from pathogenic species like Helicobacter pylori as virulence factors. Gram-positive bacilli, however, are considered separately as poly-γ-glutamic acid (PGA) degraders. The structure-function relationship of the GGT is also discussed mainly focusing on the crystallization of bacterial GGTs along with functional characterization of conserved regions by site-directed mutagenesis that unravels molecular aspects of autoprocessing and catalysis. Only a few crystal structures have been deciphered so far. Further, different reports on heterologous expression of bacterial GGTs in E. coli and Bacillus subtilis as hosts have been presented in a table pointing toward the lack of fermentation studies for large-scale production. Physicochemical properties of bacterial GGTs have also been described, followed by a detailed discussion on various applications of bacterial GGTs in different biotechnological sectors. This review emphasizes the potential of bacterial GGTs as an industrial biocatalyst relevant to the current switch toward green chemistry.

Semi-quantitative activity assays for high-throughput screening of higher activity gamma glutamyl transferase and enzyme immobilization to efficiently synthesize L-theanine

The bio-production of theanine is currently of significant interest due to its wide applications in food and healthcare products. Gamma glutamyl transferase (GGT) has been widely applied in L-theanine synthesis, but L-theanine yields remain prohibitively low for commercial production. In this study, a robust high-throughput screening process for isolating GGT mutants was developed through a combination of error-prone PCR techniques and a colorimetric reaction. The co-expression of PrsA lipoprotein enhances the secretion of GGT, thus GGT could be obtained quickly and easily without crushing cells. Random mutations on ggt genes were introduced by using error-prone PCR kits to build a large mutant library. A colorless compound generated by the reaction between NH⁴⁺ (released from L-theanine synthesis) and OPA was measured quantitatively by UV/visible spectroscopy when mixed with TCA and DMSO. Approximately 30 positive clones with improved color formation on the 96-well plates were identified, and mutants T413P and T463S with more than by 30 % higher transpeptidation activity versus the original GGT were isolated. To improve the operational stability and economical use, mutant GGT was immobilized on a prepared oxidized cellulose nanofiber membrane. The remaining activity of immobilized GGT was 88 % versus 72 % of free enzyme over 15 h. A fed-batch conversion was performed with the immobilized GGT, and over 70 g/L L-theanine could be accumulated within 18 h after feeding twice. Versus other studies, this is one of the best L-theanine synthesis systems using immobilized GGT.

γ-Glutamyltranspeptidase from Bacillus amyloliquefaciens: transpeptidation activity enhancement and L-theanine production

L-theanine, a unique amino acid in green tea with health benefits, can be enzymatically synthesized by γ-glutamyltranspeptidase (γ-GT; EC 2.3.2.2). Here, a salt-tolerant γ-glutamyltranspeptidase from a marine bacterium Bacillus amyloliquefaciens was expressed in Escherichia. coli BL21 (DE3) and was shown to be optimally active at 55 °C, pH 8.5 and alkali stable. A mutant, with higher transpeptidation activity, was obtained following two rounds of directed evolution using error-prone PCR and site-saturation mutagenesis. The mutation increased the ratio of transpeptidation to hydrolysis from 1.6 to 35.6. Additionally, Kinetic analysis exhibited 17.5% decrease of K_m, 13.0-fold increase of K_cat, and 16.3-fold increase of K_cat/K_m in mutant V319A/S437 G versus the wild-type. The 3-D modelling analysis revealed a tighter binding pocket in mutant V319A/S437 G. The frequency of hydrogen bond between donor substrate and two residues in the catalytic pocket (Gly437 and Thr375) was enhanced, which stabilized the ligand binding and thus improved the catalytic efficiency. The optimal conditions for the biocatalytic synthesis were determined as pH 10.0, 20 μg mL^-1BaGT, 200 mM L-glutamine, 2 M ethylamine, and a reaction time of 5 h. The V319A/S437 G mutant was shown to increase the percentage yield of L-theanine from 58% to 83%. These results indicate the great potential of V319A/S437 G in L-theanine production after further study.

Complete genome sequencing and comparative genomic analyses of Bacillus sp. S3, a novel hyper Sb(III)-oxidizing bacterium

BACKGROUND

Antimonite [Sb(III)]-oxidizing bacterium has great potential in the environmental bioremediation of Sb-polluted sites. Bacillus sp. S3 that was previously isolated from antimony-contaminated soil displayed high Sb(III) resistance and Sb(III) oxidation efficiency. However, the genomic information and evolutionary feature of Bacillus sp. S3 are very scarce.

RESULTS

Here, we identified a 5,436,472 bp chromosome with 40.30% GC content and a 241,339 bp plasmid with 36.74% GC content in the complete genome of Bacillus sp. S3. Genomic annotation showed that Bacillus sp. S3 contained a key aioB gene potentially encoding As (III)/Sb(III) oxidase, which was not shared with other Bacillus strains. Furthermore, a wide variety of genes associated with Sb(III) and other heavy metal (loid) s were also ascertained in Bacillus sp. S3, reflecting its adaptive advantage for growth in the harsh eco-environment. Based on the analysis of phylogenetic relationship and the average nucleotide identities (ANI), Bacillus sp. S3 was proved to a novel species within the Bacillus genus. The majority of mobile genetic elements (MGEs) mainly distributed on chromosomes within the Bacillus genus. Pan-genome analysis showed that the 45 genomes contained 554 core genes and many unique genes were dissected in analyzed genomes. Whole genomic alignment showed that Bacillus genus underwent frequently large-scale evolutionary events. In addition, the origin and evolution analysis of Sb(III)-resistance genes revealed the evolutionary relationships and horizontal gene transfer (HGT) events among the Bacillus genus. The assessment of functionality of heavy metal (loid) s resistance genes emphasized its indispensable role in the harsh eco-environment of Bacillus genus. Real-time quantitative PCR (RT-qPCR) analysis indicated that Sb(III)-related genes were all induced under the Sb(III) stress, while arsC gene was down-regulated.

CONCLUSIONS

The results in this study shed light on the molecular mechanisms of Bacillus sp. S3 coping with Sb(III), extended our understanding on the evolutionary relationships between Bacillus sp. S3 and other closely related species, and further enriched the Sb(III) resistance genetic data sources.