APPLICATION OF GLUTAMATE DEHYDROGENASE GDHA OF PEPTOSTREPTOCOCCUS ASACCHAROLYTICUS IN INCREASING YIELD OF POLY- r -GLUTAMIC ACID FROM BACILLUS LICHENIFORMIS

The subject application claims priority on Chinese application no. 201911236047.7 filed on Dec. 5, 2019 in China. The contents and subject matters of the Chinese priority application is incorporated herein by reference.

The invention belongs to the technical field of enzyme engineering and gene engineering, and in particular, relates to the application of glutamate dehydrogenase GdhA of Peptostreptococcus asaccharolyticus in increasing the yield of poly-γ-glutamic acid from Bacillus licheniformis.

Poly-γ-glutamic acid is an anionic polypeptide that is linked by an amide bond between an α-amino group and a γ-carboxylic acid group and is composed of D/L-type glutamic acid residues. The poly-γ-glutamic acid has numerous excellent properties due to its biological structural characteristics. As a water-soluble, biocompatible, edible, and non-toxic biodegradable material, the poly-γ-glutamic acid can be widely applied in fields such as food, agriculture, medicine, cosmetics, and environmental protection. Therefore, poly-γ-glutamic acid has a broad application prospect.

At present, the commercial production of poly-γ-glutamic acid mainly depends on microbiological fermentation. However, due to the need of adding precursors for synthesizing the poly-γ-glutamic acid and excessive fermentation by-products, the rate of conversion from glucose to the poly-γ-glutamic acid is low. It is currently reported that strains for commercially producing the poly-γ-glutamic acid are almost completely dependent on Bacillus, such as Bacillus subtilis, Bacillus amyloliquefaciens, and Bacillus licheniformis. Based on nutritional requirements, these strains for producing the poly-γ-glutamic acid can be divided into an L-glutamate-dependent type and an L-glutamate-independent type. The L-glutamate-dependent strains lead to an increase in the production cost of commercial production. Although the L-glutamate-independent strains are potentially low-cost cell factories, their productivity is greatly restricted. As a key enzyme in the pathway of synthesizing the poly-γ-glutamic acid, glutamate dehydrogenase is responsible for catalyzing α-ketoglutaric acid to form the glutamic acid, which then further reacts by a poly-γ-glutamate synthetase to produce a final product poly-γ-glutamic acid. Since the synthesis and accumulation of intracellular glutamic acid is an essential condition for the efficient synthesis of the poly-γ-glutamic acid, the glutamate dehydrogenase is also a key enzyme in the biosynthesis of the poly-γ-glutamic acid. At present, there is no study on the analysis and investigation of the glutamate dehydrogenase that affects the high yield of the poly-γ-glutamic acid. Peptostreptococcus asaccharolyticus is normal flora in the oral cavity, the upper respiratory tract, and the intestinal tract of a human body. There is neither study showing that the Peptostreptococcus asaccharolyticus has the capability of synthesizing the poly-γ-glutamic acid, nor study where the glutamate dehydrogenase has been investigated and compared with the glutamate dehydrogenase of the Bacillus.

In the invention, the glutamate dehydrogenase of the Bacillus licheniformis per se is replaced with the glutamate dehydrogenase derived from the Peptostreptococcus asaccharolyticus, which significantly increases the level of synthesizing the poly-γ-glutamic acid from the Bacillus licheniformis, and achieve the technical effect of improving the yield of poly-γ-glutamic acid. The invention shows that the glutamate dehydrogenase in the Peptostreptococcus asaccharolyticus is of great significance for the efficient synthesis of the poly-γ-glutamic acid from the Bacillus licheniformis, and provides a new strategy for the efficient biosynthesis of the poly-γ-glutamic acid.

An object of the invention is to provide the application of glutamate dehydrogenase GdhA derived from Peptostreptococcus asaccharolyticus in increasing the yield of poly-γ-glutamic acid from Bacillus licheniformis, and an amino acid sequence of said glutamate dehydrogenase GdhA is as shown in SEQ ID NO:1.

To achieve the object described above, a technical measure employed in the invention is as follows.

The application of glutamate dehydrogenase GdhA of Peptostreptococcus asaccharolyticus in increasing the yield of poly-γ-glutamic acid from Bacillus licheniformis includes: replacing a glutamate dehydrogenase gene in Bacillus licheniformis with a gene encoding the glutamate dehydrogenase of the Peptostreptococcus asaccharolyticus to obtain a recombinant strain, which is then used for producing the poly-γ-glutamic acid through fermentation, wherein an amino acid sequence of the glutamate dehydrogenase gdhA is shown in SEQ ID NO: 1; or expressing the glutamate dehydrogenase GdhA of the Peptostreptococcus asaccharolyticus in the Bacillus licheniformis with a conventional solution in the art, thereby facilitating the synthesis of the poly-γ-glutamic acid.

In the above-mentioned application, preferably, a nucleotide sequence encoding the amino acid sequence shown in SEQ ID NO:1 is shown in SEQ ID NO:2.

In the above-mentioned application, preferably, the Bacillus licheniformis is Bacillus licheniformis capable of producing the poly-γ-glutamic acid.

In the above-mentioned application, preferably, the Bacillus licheniformis is Bacillus licheniformis WX-02.

In the above-mentioned application, the composition of fermentation media used during the fermentation in the application process is as follows:

30-90 g/L of glucose, 0-30 g/L of sodium glutamate, 0-10 g/L of sodium citrate, 5-10 g/L of NaNO₃, 0-10 g/L of NH₄Cl, 0.5-1 g/L of K₂HPO₄.3H₂O, 0.8-1.2 g/L of MgSO₄.7H₂O, 0.8-1.2 g/L of ZnSO₄.7H₂O, 0.1-0.2 g/L of MnSO₄.H₂O, and 0.8-1.2 g/L of CaCl₂, wherein at most one of the sodium glutamate, the sodium citrate, and the ammonium chloride can be 0 in the content;

or 18-22 g/L of glycerol, 25-35 g/L of sodium glutamate, 8-13 g/L of sodium citrate, 7-12 g/L of NaNO₃, 8-12 g/L of NH₄Cl, 0.8-1.2 g/L of K₂HPO₄.3H₂O, 0.9-1.2 g/L of MgSO₄.7H₂O, 0.8-1.2 g/L of ZnSO₄.7H₂O, 0.1-0.25 g/L of MnSO₄.H₂O, and 0.5-1.5 g/L of CaCl₂.

In the above-mentioned application, the composition of the fermentation media used during the fermentation in the application process is as follows:

30-90 g/L of glucose, 0-30 g/L of sodium glutamate, 9-10 g/L of sodium citrate, 9-10 g/L of NaNO₃, 9-10 g/L of NH₄Cl, 0.8-1 g/L of K₂HPO₄.3H₂O, 0.8-1 g/L of MgSO₄.7H₂O, 0.8-1 g/L of ZnSO₄.7H₂O, 0.1-0.2 g/L of MnSO₄.H₂O, and 0.8-1.2 g/L of CaCl₂, wherein the sodium glutamate and the sodium citrate cannot be 0 in content at the same time.

Compared with the prior art, the invention has the following advantages.

The invention replaced the glutamate dehydrogenase GdhA of the Bacillus licheniformis WX-02 per se with the glutamate dehydrogenase GdhA derived from the Peptostreptococcus asaccharolyticus by means of homologous recombination, which solves the problem of short supply of the glutamic acid in the current synthesis process of the poly-γ-glutamic acid. The modified strains are significantly enhanced in the synthesis capability of the poly-γ-glutamic acid, and is at least increased by 20% in the level of synthesizing the poly-γ-glutamic acid as compared with control strains.

Embodiments below are intended to further explain rather than limiting the invention. The technical solutions of the invention are conventional solutions in the art unless otherwise specified. The reagents or materials described are commercially available unless otherwise specified.

1. Strains: Bacillus licheniformis WX-02, with the Accession No. of CCTCC NO. M208065. The deposit of the biological material was made at China Center for Type Culture Collection (CCTCC), having an address at Wuhan University, Luojiashan, Wuchang, Wuhan, 430072, China, on Apr. 28, 2008; the deposit has the CCTCC Accession No. M208065; the deposited biological material is described as Bacillus licheniformis WX-02; and the deposit has been made under the Budapest Treaty.

The strains E. coli DH5a are commercially available and purchased from Beijing TransGen Biotech Co., Ltd.

2. Enzymes and other biochemical reagents: High-fidelity Taq enzyme was purchased from Wuhan Qingke Biotechnology Co., Ltd. Bacterial genomic DNA extraction kit was purchased from Tiangen; T4 DNA ligases, restriction endonucleases, and other molecular biological reagents were purchased from Nanjing Vazyme Biotech Co., Ltd; and others were domestic reagents (all available from ordinary biochemical reagent companies).

3. Media:

Composition of LB media includes: 10 g/L of tryptone, 5 g/L of yeast powder, 10 g/L of sodium chloride; and the LB media had a pH of 7.0-7.2 and were sterilized at 121° C. for 20 min before use.

(1) a gdhA gene (as shown in SEQ ID NO: 2) is synthesized according to genomic DNA sequence of Peptostreptococcus asaccharolyticus DSM 20463 (GenBank Access No. NZ_FWWR00000000.1), wherein primers for amplifying the gene included T2-F2 of SEQ ID NO: 3 and T2-R2 SEQ ID NO: 4; and an upstream homologous arm (T2-F1 SEQ ID NO: 5 and T2-R1 SEQ ID NO: 6 as primers) and a downstream homologous arm (T2-F3 SEQ ID NO: 7 and T2-R3 SEQ ID NO: 8 as primers) of a glutamate dehydrogenase gene rocG (GenBank Access NO. AKQ74236.1) of the Bacillus licheniformis per se are amplified through PCR with the genomic DNA of the Bacillus licheniformis WX-02 (GenBank Access No. NZ_CP012110.1) as a template;

(2) the upstream homologous arm of the gene rocG (SEQ ID NO: 13), the amplified gdhA gene and the downstream homologous arm of the gene rocG (SEQ ID NO: 14) are linked through overlap-extension PCR to form a target gene fragment, which has an order as follows: the upstream homologous arm of the gene rocG-the amplified gdhA gene-the downstream homologous arm of the gene rocG;

(3) double digestion is performed on the target gene fragment using restriction endonucleases SacI and XbaI to obtain digested gene fragments, and meanwhile, double digestion is performed on a plasmid T2(2)-Ori using the restriction endonucleases SacI and XbaI to obtain linear plasmid fragments;

(4) the digested target fragments obtained in step (3) is linked with the linear plasmid fragments obtained in step (3) via T4-DNA ligases, and the correctness is verified to obtain plasmids T2(2)-gdhA;

(5) the plasmids T2(2)-gdhA are transferred into the Bacillus licheniformis WX-02, and screened by media resistant to kanacillin to obtain transformants, and the plasmids are picked from the transformants for colony PCR verification;

(6) positive transformants obtained in step (4) are transferred to cultured on the media resistant to kanacillin at 45° C. 3 times, each for 12 hours, and colony PCR is performed to detect single-exchange strains with T2-KYF and gdhA-R as primers;

(7) strains obtained in step (5) and the single-exchange strains obtained in step (6) are mixed and inoculated, and then transferred to and cultured in media containing no kanacillin at 37° C. several times; the transformants are picked for colony PCR verification to obtain positive transformants; and DNA sequencing is performed on the positive transformants for further verification, thereby obtaining successfully double-exchanged recombinant strains. Then, the positive transformants are obtained. Subsequently, DNA sequencing is performed on the positive transformants for further verification, and successfully double-exchanged gdhA strains (i.e., Bacillus licheniformis WX-gdhA) is obtained.

The recombinant strains obtained in Example 1 is inoculated into the LB media and cultured at 37° C. for 14 h; 50 mL of poly-γ-glutamic acid fermentation medium (Table 1) is put into a 500 mL Erlenmeyer flask, and then a seed-culturing bacteria liquid is inoculated into the fermentation media at an inoculum amount of 3% (volume percentage). The culture is performed at the speed of 230 r/min and at the temperature of 37° C. for a fermentation period of 36 hours.

In this example, the effect of the Bacillus licheniformis WX-gdhA on the synthesis level of the poly-γ-glutamic acid is investigated with respect to different compositions of the fermentation media (at the same time, these 24 media were inoculated with the same inoculation amount of Bacillus licheniformis WX-02 as controls). The specific compositions of the 24 media are shown in Table 1:

The media compositions above are all in g/L, and the fermentation media have the pH of 6.5-7.2 and are sterilized at 115° C. for 20 min before use.

The yield of the poly-γ-glutamic acid is measured by a dry weight method, with the specific operation steps as follows. A certain volume of fermentation broth sample is taken, adjusted to 3.0 in pH with 6 mol/L HCl and centrifuged at 12000 r/min for 10 min; bacterial precipitates are dried in an oven at 80° C.; the dry weight of the bacteria is measured. A supernatant is taken, adjusted to neutrality in pH with 6 mol/L NaOH, added with ethanol to precipitate the poly-γ-glutamic acid, wherein the volume of the ethanol is 3 times that of the supernatant; a resultant product is centrifuged to collect flocculent precipitates of the poly-γ-glutamic acid, and the precipitates are dried in the oven at 80° C. and measured in dry weight. The yield of the poly-γ-glutamic acid in the fermentation broth is calculated according to the dry weight method (see Table 2).

The invention provides a new strategy for the efficient production of poly-γ-glutamic acid.