Patent Publication Number: US-2021163958-A1

Title: Recombinant escherichia coli expressing fusion protein of formamidase and phosphite dehydrogenase and construction method and use thereof

Description:
BACKGROUND 
     Technical Field 
     The present invention relates to the field of genetic engineering, and more particularly, to a recombinant  Escherichia coli  expressing a fusion protein of formamidase and phosphite dehydrogenase, a construction method and use thereof. 
     Description of Related Art 
       Escherichia coli  ( E. coli ) is one of the most widely used hosts at present, because the genome of  E. coli  is thoroughly researched, and  E. coli  has a fast multiplication speed and a short fermentation period. Therefore, the  E. coli  receives close concern and attention from entrepreneurs in the fermentation industry. However, in a fermentation process of  E. coli , the problem of microbial contamination is still the most concerned problem for enterprises. In the case of microbial contamination, not only economic losses, and wastes of raw material and time are caused, but also a difficulty is added to waste disposal. Therefore, finding a way to solve the problem that the  E. coli  is contaminated by miscellaneous bacteria in a fermentation process is of great significance to promote the profit growth of enterprises and to the development of society. 
     At present, the research on microbial contamination mostly focuses on aspects such as sources (pathways) and related bacterium shapes of miscellaneous bacteria in  E. coli  fermentation, the influence of microbial contamination on fermentation at different fermentation stages and control measures, the analysis and control from the perspective of equipment, etc. In order to reduce the microbial contamination, enterprises generally focus on improving equipment requirements and technical level of operators. However, microbial contamination also occurs frequently in the fermentation process due to a complex surrounding environment and a hidden part of fermentation equipment. 
     In view of the above phenomenon of microbial contamination in a fermentation process of  E. coli , researches are carried out focusing on the intake of a substrate spectrum of a nutrient substance, to modify a nutrient metabolic pathway of  E. coli , so that the modified  E. coli  strain can express a fusion protein of formamidase and phosphite dehydrogenase. The fusion protein can decompose formamide to form ammonia as a nitrogen source, and oxidize phosphite to phosphate as a phosphorus source. By changing pathways of the nitrogen source and the phosphorus source, the modified  E. coli  strain can normally grow in a specific MOPS medium, while miscellaneous microorganisms not possessing the two metabolic pathways simultaneously will be “starved to death” due to the lack of the nitrogen source or the phosphorus source as sources of nutrient substances. On one hand, the recombinant  E. coli  can express a fusion protein using formamide and phosphite; and on the other hand, the recombinant  E. coli  can also express exogenous genes, including antibodies and other valuable enzyme preparations. Meanwhile, in order to verify an ability of the constructed engineered  E. coli  to synthesize the exogenous genes, a green fluorescent protein gene is also transformed into the engineered  E. coli  for co-expression. 
     SUMMARY 
     An objective of the present invention is, against the deficiency of the technology, to provide a recombinant  E. coli  expressing a fusion protein of formamidase and phosphite dehydrogenase, which is specifically capable of efficiently expressing a fusion protein of formamidase and oxidized phosphite dehydrogenase. The recombinant  E. coli  expresses a fusion protein gene for-Linker-ptx of formamidase gene and phosphite dehydrogenase gene to generate the fusion protein of formamidase and phosphite dehydrogenase, the fusion protein can simultaneously decompose formamide to form NH 4   +  and oxidize phosphite phosphate to phosphate, thereby providing a nitrogen source and a phosphorus source necessary for the growth and multiplication of the recombinant  E. coli , while other microorganisms not possessing the two metabolic pathways simultaneously will be “starved to death” due to the lack of the metabolic pathways of the nitrogen source and the phosphorus source. 
     An objective of the present invention is further to provide a construction method of the recombinant  E. coli  expressing the fusion protein of formamidase and phosphite dehydrogenase. The construction method includes adopting  E. coli  DH5α as a host, amplifying a formamidase gene for-Linker from  Paenibacillus pasadenensis . CS0611 which contains a linker sequence and a phosphite dehydrogenase gene ptx from  Klebsiella pneumonia . OU7, assembling the two genes as a fusion gene for-Linker-ptx by an overlapping PCR technology, ligating the fusion gene to a multiple cloning site of a vector pGEX-2T, and transforming the obtained recombinant plasmid pGEX-for-Linker-ptx into the  E. coli  DH5α; and after successful verification, extracting the plasmid pGEX-for-Linker-ptx and transforming into an expression strain of  E. coli  BL21(DE3) to obtain a recombinant expression strain of  E. coli  BL21 (DE3)(pGEX-for-Linker-ptx), and continuing induction culture in a MOPS medium containing formamide and phosphite. 
     The formamidase gene for and the phosphite dehydrogenase gene ptx that are obtained by amplification are ligated by linker sequence, so that the two enzyme genes are fused and expressed to exert an enzyme catalytic function, thereby realizing normal growth and multiplication of the recombinant  E. coli  in the MOPS medium containing formamide and phosphite, and making the recombinant  E. coli  express its function. 
     An objective of the present invention is further to provide use of the recombinant  E. coli  expressing a fusion protein of formamidase and phosphite dehydrogenase. 
     The objectives of the present invention are achieved by the following technical solutions. 
     A construction method of an  E. coli  expressing a fusion protein of formamidase and phosphite dehydrogenase includes steps as follows: 
     (1) designing primers and amplifying a formamidase gene containing a linker sequence by PCR (the linker sequence at the 3′-end): 
     wherein a forward primer is A1 (5′-CGCGGATCCGATGAACGGACTGGGCGGCTTGAAC-3′), in which an underlined and italic part  GGATCC  is a restriction enzyme cutting site of BamH I; and a reverse primer is A2 
                    (5′- CGACCCACCACCGCCCGAGCCACCGCCACC TCGCGCCGCGCCTCCCT               TCGC-3′),            
in which an underlined part
 
                              CGACCCACCACCGCCCGAGCCACCGCCACC              
is the linker sequence;
 
     using a genome of  Paenibacillus pasadenensis . CS0611 as a template to clone a formamidase gene sequence for-Linker of 1041 bp containing the linker sequence; ligating the cloned gene sequence for-Linker into a vector pMD-19T Simple, and then transforming the recombinant vector into  E. coli  DH5α to obtain a recombinant  E. coli  DH5α(pMD-19T Simple-for-Linker) containing the recombinant vector pMD-19T Simple-for-Linker; and then amplifying a for-Linker fragment with A1 and B1 
     
       
         
           
               
            
               
                 (5′-TATAACGAGTTTCGGCAGCAT CGACCCACCACCGCCCGAGCCA - 
               
               
                   
               
               
                 3′); 
               
            
           
         
       
     
     (2) designing primers and amplifying a phosphite dehydrogenase gene by PCR: 
     wherein, a forward primer is B2 
                    (5′- TGGCTCGGGCGGTGGTGGGTCG ATGCTGCCGAAACTCGTTATA-               3′),            
in which an underlined part
 
                              TGGCTCGGGCGGTGGTGGGTCG              
is partial DNA of the linker; and a reverse primer is B3 (5′-CCG GAATTC CGACATGCGGCAGGCTCGGCCTTGGGC-3′), in which an underlined and italic part  GAATTC  is a restriction enzyme cutting site of EcoR I;
 
     using a genome of  Klebsiella pneumonia . OU7 as a template to clone a phosphite dehydrogenase gene sequence ptx of1008 bp to obtain aptx DNA fragment; 
     (3) overlapping PCR amplification to obtain a fusion gene for-Linker-ptx: 
     using the for-Linker fragment and the ptx fragment that are obtained by amplification in the step (1) and the step (2) respectively as templates, and using the primer A1 and the primer B3 as the forward primer and the reverse primer respectively, amplifying to obtain the fusion gene for-Linker-ptx of the formamidase gene and the phosphite dehydrogenase gene; and performing double digestion on the fusion gene for-Linker-ptx with BamH I and EcoR I, ligating the fusion gene to a pGEX-2T expression plasmid digested with the same enzyme and transforming the recombinant plasmid into  E. coli  DH5α to obtain a positive clone  E. coli  DH5α(pGEX-for-Linker-ptx); 
     (4) transforming the recombinant plasmid into a recombinant expression strain: 
     extracting a recombinant plasmid pGEX-for-Linker-ptx from the recombinant  E. coli  DH5α(pGEX-for-Linker-ptx), and transforming the recombinant plasmid pGEX-for-Linker-ptx into an expression strain  E. coli  BL21(DE3) to obtain a recombinant expression strain  E. coli  BL21(DE3)(pGEX-for-Linker-ptx); 
     (5) performing induction culture on the recombinant  E. coli:    
     inoculating the obtained recombinant expression strain  E. coli  BL21(DE3)(pGEX-for-Linker-ptx) into a LB medium to perform induction culture, then collecting bacteria and adding the collected strain to a MOPS medium containing formamide and phosphite and continuing to culture to obtain the recombinant  E. coli.    
     Further, in the step (1), the formamidase gene sequence containing the linker sequence from the strain of  Paenibacillus pasadenensis . CS0611 is shown in SEQ1, which has a fragment length of 1041 bp, contains the linker sequence 
                            ( GGTGGCGGTGGCTCGGGCGGTGGTGGGTCG ),            
and encodes 347 amino acids.
 
     Further, in the step (1), the strain of  Paenibacillus pasadenensis . CS0611 was preserved in China Center for Type Culture Collection on Oct. 8, 2014 with a preservation number of CCTCC NO: M2014458. 
     Further, in the step (1), conditions for the PCR amplification are as follows: reacting at 94° C. for 5 minutes; reacting at 98° C. for 10 seconds, reacting at 55° C. for 5 seconds and reacting at 72° C. for 70 seconds, and repeating the reactions for 30 times; then reacting at 72° C. for 7 minutes; and finally, cooling to 16° C. 
     Further, in the step (2), the phosphite dehydrogenase gene sequence from the strain of  Klebsiella pneumonia . OU7 is shown in SEQ2, which has a fragment length of 1008 bp and encodes 336 amino acids. 
     Further, in the step (2), the strain if  Klebsiella pneumonia . OU7 was preserved in China Center for Type Culture Collection on Aug. 24, 2017 with a preservation number of CCTCC NO: M 2017449. 
     Further, in the step (2), conditions for the PCR amplification are as follows: reacting at 94° C. for 5 minutes; reacting at 98° C. for 10 seconds, reacting at 55° C. for 5 seconds and reacting at 72° C. for 70 seconds, and repeating the reactions for 30 times; then reacting at 72° C. for 7 minutes; and finally, cooling to 16° C. 
     Further, in the step (3), the gene sequence of the fusion gene for-Linker-ptx is shown in SEQ3, which has a fragment length of 2049 bp, with the formamidase gene located at a 5′-end and the phosphite dehydrogenase gene located at a 3′-end, and a formamidase gene fragment is ligated to a phosphite dehydrogenase gene fragment by the linker sequence 
                            ( GGTGGCGGTGGCTCGGGCGGTGGTGGGTCG )            
with a fragment length of 30 bp.
 
     The linker sequence is located at the 3′-end of the formamidase gene, so that 10 hydrophobic amino acids act as a chain bridge between the formamidase and phosphite dehydrogenase, so that these two proteins cannot influence each other when forming an active three-dimensional spatial structure. 
     Further, in the step (3), conditions for the PCR amplification are as follows: reacting at 94° C. for 5 minutes; reacting at 98° C. for 10 seconds, reacting at 55° C. for 5 seconds and reacting at 72° C. for 70 seconds, and repeating the reactions for 30 times; then reacting at 72° C. for 7 minutes; and finally, cooling to 16° C. 
     Further, in the step (5), the induction culture is as follows: culturing in the LB medium at 37 ° C. and 180 rpm for 12 hours to 16 hours, then inoculating in a fresh LB medium, and continuing to culture at 37° C. and 180 rpm until a concentration of the recombinant bacteria reaches OD 600 =0.6, and after cooling to 20° C., adding isopropyl-β-D-thiogalactoside (IPTG) with a final concentration of 0.2 mM for induction for additional 16 hours. 
     Further, in the step (5), the step of collecting bacteria is as follows: centrifuging the bacteria after induction culture at 4° C. and 8000 rpm for 5 minutes, suspending the bacteria with physiological saline precooled to 4° C., centrifuging the bacteria again at 4° C. and 8000 rpm for 5 minutes, repeating the step of suspending with physiological saline and centrifuging twice to remove residual LB medium, and then collecting the bacteria. 
     Further, in the step (5), in the MOPS medium containing formamide and phosphite, a final concentration of formamide is 200 mM and a final concentration of phosphite is 1.32 mM. 
     Further, in the step (5), after adding the collected bacteria into the MOPS medium containing formamide and phosphite, a concentration of the bacteria OD 600  is 0.1 to 0.15. 
     Further, in the step (5), continuing to culture is performed after adding the IPTG with a final concentration of 0.2 mM into the MOPS medium containing formamide and phosphite at 30° C. and 180 rpm for 84 hours to 96 hours. 
     The MOPS medium lacks basic nutrient components NH 4   +  and HPO 4   2− , and is added with formamide and phosphite, and meanwhile, miscellaneous microorganisms do not have a function of decomposing formamide and oxidized phosphite to acquire a nutrient component, so that the miscellaneous microorganisms lack a nitrogen source and a phosphorus source, hence the recombinant  E. coli  acquires sufficient nutrient components to become dominant bacteria due to the function of efficiently decomposing formamide and oxidizing phosphite. 
     A recombinant  E. coli  expressing a fusion protein of formamidase and phosphite dehydrogenase constructed by any one of the above mentioned methods can use formamide and phosphite for growth and multiplication in the MOPS medium, and can co-express an exogenous green fluorescent protein gene to synthesize a green fluorescent protein (GFP), thus having a function of expressing an exogenous gene. 
     The recombinant  E. coli  expressing the fusion protein of formamidase and phosphite dehydrogenase can express the fusion protein to efficiently decompose formamide and oxidize phosphite, and can be applied in industrial fermentation of  E. coli  to synthesize an antibody or a valuable enzyme preparation, which has a very profound significance. 
     Compared with the current technologies, the present invention has the following advantages and beneficial effects: 
     (1) the recombinant  E. coli  according to the present invention can express the fusion protein of formamidase and phosphite dehydrogenase, the fusion protein can simultaneously decompose formamide to form NH 4   +  and oxidize phosphite to phosphate, thereby providing a nitrogen source and a phosphorus source for normal growth and multiplication of the recombinant  E. coli , while other microorganisms cannot grow and multiply in the MOPS medium containing formamide and phosphite due to the lack of the two pathways, the problem of microbial contamination in industrial fermentation of  E. coli  is solved, the requirement on fermentation equipment is reduced, and the 1 engineered  E. coli  can perform its instinctive function of expressing the exogenous gene; 
     (2) the present invention solves the problem of microbial contamination of the in the fermentation process of  E. coli  through the biotechnology of genetic engineering, and the characteristic of the engineered bacteria expressing the exogenous protein cannot be lost; 
     (3) the recombinant  E. coli  according to the present invention can be applied to industrial fermentation of  E. coli  to synthesize an antibody or a valuable protein preparation, so that the process of enzymatically synthesizing an antibody or a valuable protein preparation is efficient, the purity of synthesized substance is high, and the problem of invasion and pollution of miscellaneous microorganisms can be prevented in the production and fermentation processes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a construction process of a recombinant fusion expression plasmid pGEX-for-Linker-ptx; 
         FIG. 2  is a growth curve graph of a recombinant  Escherichia coli  BL21 (DE3)(pGEX-for-Linker-ptx) in a MOPS medium; 
         FIG. 3  is a photofluorogram of the recombinant  Escherichia coli  BL21 (DE3)(pGEX-for-Linker-ptx) expressing a green fluorescent protein (GFP) in the MOPS medium. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     In order to better understand the present invention, the technical solution of the present invention is further described hereinafter with reference to the embodiments and the accompanying drawings, but the description is only used for illustrating the present invention, and shall not and does not limit the present invention. 
       Paenibacillus pasadenensis . CS0611 used in the specific embodiments of the present invention was preserved in China Center for Type Culture Collection on Oct. 8, 2014 with a preservation number of CCTCC NO: M2014458, and a whole genome sequencing has been completed. A fragment of a formamidase gene containing a linker sequence obtained by amplification is shown in SEQ1, which has a fragment length of 1041 bp, contains the linker sequence, and encodes 347 amino acids. 
       Klebsiella pneumonia . OU7 used in the specific embodiments of the present invention was preserved in China Center for Type Culture Collection (Wuhan University, Luojia Mountain, Wuchang Road, Wuhan City, Hubei Province, postcode: 430072) on Aug. 24, 2017 with a preservation number of CCTCC NO: M 2017449, and the  Klebsiella pneumonia . OU7 is obtained by culturing and self-screening by the following method. Through the sequence analysis of phosphite dehydrogenases on the NCBI database and PCR amplification, a fragment of a phosphite dehydrogenase gene is shown in SEQ2, which has a fragment length of 1008 bp and encodes 336 amino acids. 
     Embodiment 1 
     Acquisition of a formamidase gene for (containing a linker sequence) 
       Paenibacillus pasadenensis . CS0611 was cultured in a LB medium at 37° C. and 180 rpm for one day; cultured bacteria were centrifuged at 4° C. and 8000 rpm for 5 minutes to collect the bacteria, the bacteria were washed twice with physiological saline to remove residual medium, and then a genome of the  Paenibacillus pasadenensis . CS0611 was extracted according to a specific method of an OMEGA bacterial genome DNA extraction kit. 
     The extracted genome of the  Paenibacillus pasadenensis . CS0611 was used as a template, A1 (5′-CGC GGATCC GATGAACGGACTGGGCGGCTTGAAC-3′) and A2 
                    (5′- CGACCCACCACCGCCCGAGCCACCGCCACC TCGCGCCGCGCCTCCCT               TCGC-3′)            
were respectively used as a forward primer and a reverse primer, and a formamidase gene for-Linker was amplified by PCR.
 
     An enzyme reagent used for PCR was PrimeSTAR® HS DNA Polymerase with GC buffer from TaKaRa Company; and a PCR reaction system and conditions were as follows: 
     Composition of PCR reaction liquid (25 μL) 
     
       
         
           
               
               
             
               
                   
                   
               
               
                   
                 Volume 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 2 × Prime STAR buffer 
                 12.5 
                 μL 
               
            
           
           
               
               
               
            
               
                   
                 dNTP mixture 
                 2μ 
               
            
           
           
               
               
               
               
            
               
                   
                 A1 primer 
                 0.5 
                 μL 
               
               
                   
                 A2 primer 
                 0.5 
                 μL 
               
               
                   
                 Template 
                 1 
                 μL 
               
               
                   
                 PrimeSTAR HS DNA polymerase (2.5 U/μL) 
                 0.25 
                 μL 
               
               
                   
                 Deionized water 
                 8.25 
                 μL 
               
               
                   
                   
               
            
           
         
       
     
     PCR reaction conditions were as follows: reacting at 94° C. for 5 minutes; reacting at 98° C. for 10 seconds, reacting at 55° C. for 5 seconds and reacting at 72° C. for 70 seconds in sequence, and repeating the reactions for 30 times; then reacting at 72° C. for 7 minutes; and finally, cooling to 16° C. 
     A DNA product obtained by the PCR amplification was subjected to electrophoresis with 1 wt % agarose gel, a gel extraction kit from the OMEGA Company was used to perform gel extraction purification according to steps in the instruction, then the DNA product was sent for sequencing, and the result showed that a formamidase gene sequence containing a linker sequence with a fragment length of 1041 bp was obtained, and was named as for-Linker. 
     Embodiment 2 
     Acquisition of a phosphite dehydrogenase gene ptx 
     A phosphite dehydrogenase gene was derived from self-screened  Klebsiella pneumonia . OU7, and was screened by our laboratory. 
     A genome of the screened  Klebsiella pneumonia . OU7 was extracted according to a specific method of an OMEGA bacterial genome DNA extraction kit, the genome of the screened  Klebsiella pneumonia . OU7 was used as a template, B2 (5′-TGGCTCGGGCGGTGGTGGGTCGATGCTGCCGAAACTCGTTATA-3′) and B3 (5′-CCG GAATTC CGACATGCGGCAGGCTCGGCCTTGGGC-3′) were respectively used as a forward primer and a reverse primer, and a phosphite dehydrogenase gene ptx was amplified by PCR. 
     An enzyme reagent used for PCR was PrimeSTAR® HS DNA Polymerase with GC buffer from TaKaRa Company; and a PCR reaction system and conditions were as follows: 
     Composition of PCR reaction liquid (25 μL) 
     
       
         
           
               
               
             
               
                   
                   
               
               
                   
                 Volume 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 2 × Prime STAR buffer 
                 12.5 
                 μL 
               
               
                   
                 dNTP substrate 
                 2 
                 μL 
               
               
                   
                 B2 primer 
                 0.5 
                 μL 
               
               
                   
                 B3 primer 
                 0.5 
                 μL 
               
               
                   
                 Template 
                 1 
                 μL 
               
               
                   
                 PrimeSTAR HS DNA polymerase (2.5 U/μL) 
                 0.25 
                 μL 
               
               
                   
                 Deionized water 
                 8.25 
                 μL 
               
               
                   
                   
               
            
           
         
       
     
     PCR reaction conditions were as follows: reacting at 94° C. for 5 minutes; reacting at 98° C. for 10 seconds, reacting at 55° C. for 5 seconds and reacting at 72° C. for 70 seconds in sequence, and repeating the reactions for 30 times; then reacting at 72° C. for 7 minutes; and finally, cooling to 16° C. 
     A DNA product obtained by the PCR amplification was subjected to electrophoresis with 1 wt % agarose gel, a gel extraction kit from the OMEGA Company was used to perform gel extraction purification according to steps in the instruction, then the DNA product was sent for detection and sequencing, and the result showed that a phosphite dehydrogenase gene sequence with a fragment length of 1008 bp was obtained, and was named as ptx. 
     Embodiment 3 
     Acquisition of a fusion gene for-Linker-ptx by overlapping PCR amplification 
     The for-Linker and ptx fragments obtained by amplification were used as templates, A1 (5′-CGC GGATCC GATGAACGGACTGGGCGGCTTGAAC-3′) and B3 (5′-CCG GAATTC CGACATGCGGCAGGCTCGGCCTTGGGC-3′) were respectively used as a forward primer and a reverse primer, and a fusion gene for-Linker-ptx was obtained by amplification. 
     Amplification conditions were as follows: reacting at 94° C. for 5 minutes; reacting at 98° C. for 10 seconds, reacting at 55° C. for 5 seconds and reacting at 72° C. for 70 seconds in sequence, and repeating the reactions for 30 times; then reacting at 72° C. for 7 minutes; and finally, cooling to 16° C. 
     Embodiment 4 
     Construction of a recombinant  E. coli  BL21(DE3) (pGEX-for-Linker-ptx) 
     Double digestion was performed on the fusion gene for-Linker-ptx obtained in the embodiment 3 and a plasmid pGEX-2T respectively with BamH I and EcoR I, an underlined and italic part of a forward primer was a restriction enzyme cutting site of BamH I, an underlined and italic part of a reverse primer was a restriction enzyme cutting site of EcoR I, and digestion conditions were as follows: digesting at 37° C. for 120 minutes; 
     Digestion system: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Fusion gene (μL) 
                 Plasmid (μL) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 ddH 2 O 
                 5 
                 0 
               
               
                   
                 10 × buffer 
                 3 
                 2 
               
               
                   
                 Fragment/plasmid 
                 20 
                 16 
               
               
                   
                 BamH I + EcoR I 
                 1 + 1 
                 1 + 1 
               
               
                   
                 Total volume 
                 30 
                 20 
               
               
                   
                   
               
            
           
         
       
     
     Gel extraction purification was performed on a digested product respectively, and a extraction method and steps referred to a gel extraction kit from the OMEGA Company; the digested product was subjected to electrophoresis with 1 wt % agarose gel after extraction, and a extraction rate was detected; and then a extracted target fragment was ligated to a plasmid, T4 DNA Ligase from Thermo Fisher SCIENTIFIC Company was used as a ligation kit, and a ligation system was as follows: 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 DNA fragment 
                 6 
                 μL 
               
               
                   
                 Plasmid 
                 9 
                 μL 
               
               
                   
                 10 × T4 buffer 
                 2 
                 μL 
               
            
           
           
               
               
               
            
               
                   
                 T4 DNA ligase 
                 1 Weiss U 
               
            
           
           
               
               
               
               
            
               
                   
                 Deionized water 
                 2 
                 μL 
               
               
                   
                 Total volume 
                 20 
                 μL 
               
               
                   
                   
               
            
           
         
       
     
     A molar ratio of the fusion gene to the pGEX-2T plasmid was 5:1. 
     A ligation product was transformed into an  E. coli  DH5α, and transformation steps were as follows: 10 μL of the ligation product was mixed with 100 μL of competent cells of the  E. coli  DH5α, the mixture was placed into ice bath for 30 minutes, heat shock at 42° C. for 90 seconds, and then ice bath for 2 minutes, then 890 μL of LB medium was added, and after shaking culture at 37° C. and 180 rpm for 1 hour, the mixture was centrifuged at 4000 rpm for 5 minutes to collect bacteria; 890 μL of supernatant medium was taken, the bacteria at a bottom of a tube were resuspended, evenly coated on a LB solid plate containing ampicillin (containing 100 μg/mL ampicillin sodium), and cultured at 37° C. for 16 hours; and after transformants grew on the plate, the transformants were selected for PCR verification and sent for detection and sequencing, and ORF search was performed on the sequencing result using DNAssist software. 
     The result shows that the obtained fusion gene sequence (for-Linker-ptx) has been correctly inserted into a multiple cloning site of pGEX-2T, and the pGEX-for-Linker-ptx plasmid has been successfully obtained and transformed into  E. coli  BL21(DE3) competent cell to obtain a recombinant  E. coli  BL21(DE3)(pGEX-for-Linker-ptx). 
     A construction process of the recombinant  E. coli  BL21(DE3)(pGEX-for-Linker-ptx) assimilating and metabolizing formamide and phosphite to become dominant engineered bacteria in a medium is shown in  FIG. 1 . Due to insertion of a fusion gene (a formamidase gene+Linker+a phosphite dehydrogenase gene) into the  E. coli  BL21(DE3)(pGEX-for-Linker-ptx), the modified  E. coli  BL21(DE3)(pGEX-for-Linker-ptx) not only can decompose formamide into NH 4   +  but also can oxidize phosphite to phosphate, thus providing a nitrogen source and a phosphorus source for the growth of the  E. coli  BL21(DE3)(pGEX-for-Linker-ptx), while other miscellaneous microorganisms cannot grow due to the lack of the two pathways. 
     Embodiment 5 
     Growth of an  E. coli  BL21(DE3)(pGEX-for-Linker-ptx) in a specific MOPS medium 
     Induction culture was performed on the obtained recombinant expression strain  E. coli  BL21(DE3)(pGEX-for-Linker-ptx), and a specific process was as follows: 
       E. coli  BL21(DE3)(pGEX-for-Linker-ptx) was inoculated to 30 mL of LB medium according to a volume ratio of 1:100, and cultured at 37° C. and 180 rpm overnight for 16 hours; induction culture was performed, 1 mL of recombinant  E. coli  cultured overnight was inoculated to 100 mL of fresh LB medium, and cultured at 37° C. and 180 rpm until a concentration of recombinant bacteria reached OD 600 =0.6, and after cooling to 20° C., IPTG with a final concentration of 0.2 mM was added for induction for 16 hours, and bacteria were collected by centrifuging at 4° C. and 8000 rpm for 5 minutes. 
     The collected bacteria were suspended with physiological saline (precooled at 4° C.), and centrifuged at 4° C. and 8000 rpm for 5 minutes to collect bacteria, and this step was repeated twice to remove residual LB medium; the bacteria were added to a basic MOPS medium containing formamide (200 mM) and phosphite (1.32 mM) to make OD 600  of the bacteria be 0.1, IPTG with a final concentration of 0.2 mM was added, and the recombinant  E. coli  was continued to be cultured at 30° C. and 180 rpm. 
     The growth of the recombinant  E. coli  in the MOPS medium is observed, a growth curve graph of the recombinant  E. coli  BL21(DE3)(pGEX-for-Linker-ptx) in the MOPS medium containing formamide (200 mM) and phosphite (1.32 mM) is shown in  FIG. 2 . It can be seen from  FIG. 2  that the recombinant  E. coli  BL21(DE3)(pGEX-for-Linker-ptx) can grow in this medium, a concentration of bacteria reaches a maximum value after the third day, and A600 is 2.223, while a control strain  E. coli  BL21(DE3) basically does not grow in the MOPS medium. 
     In order to verify an express ability of the recombinant  E. coli  BL21(DE3)(pGEX-for-Linker-ptx) to synthesize exogenous gene in the MOPS medium, a green fluorescent protein (GFP) gene was used to verify an ability of the recombinant  E. coli  to express an exogenous gene. The pET-28a-GFP plasmid was transformed into  E. coli  BL21(DE3)(pGEX-for-Linker-ptx) to form the recombinant  E. coli , which was named as  E. coli BL 21(DE3)(pGEX-for-Linker-ptx+pET-28a-GFP), and the obtained recombinant  E. coli  was induced to express GFP in the MOPS medium containing formamide and phosphite. Then, the cultured and fermented recombinant  E. coli  was observed with a fluorescence inversion microscope with an excitation wavelength of 488 nm and an emission wavelength of 507 nm. It can be seen from  FIG. 3  that the recombinant  E. coli  can grow normally in the MOPS medium and can express an exogenous green fluorescent protein gene.