Patent Publication Number: US-2021189449-A1

Title: Preparation method for novel fusion protein and use of fusion protein for improving protein synthesis

Description:
TECHNICAL FIELD 
     The present invention relates to genetic engineering, and particularly to the preparation of novel fusion proteins and their application in improving protein synthesis. 
     BACKGROUND 
     Proteins are important molecules in cells, which are involved in almost all functions of cells. Different functions of proteins depend on different sequences and structures. In cells, proteins can act as enzymes to catalyze various biochemical reactions, and can act as signal molecules to coordinate various activities of organisms. Proteins can also support biologic forms, store energy, transport molecules and enable organisms to move. In the field of biomedicine, antibodies of protein types, as targeted protein drugs, are an important means to treat diseases such as cancer. 
     In cells, the regulation of protein translation plays an important role in various processes, such as coping with external pressure (e.g., nutritional deficiency), cell development, cell differentiation, and so on. Four processes of protein translation include translation initiation, translation elongation (or translated as translation extension), translation termination and ribosome recycling, among which translation initiation is the most regulated process. In the initiation stage of translation, the small ribosomal subunit (40S) binds (tRNA) i   Met , and recognizes the 5′ terminal of mRNA with the help of various translation initiation factors. The small subunit moves downstream, combines the large ribosomal subunit (60S) at the initiation codon (ATG) to form a complete ribosome, and then enters the stage of translation elongation. 
     In the rapidly dividing yeast cells, the rate of protein synthesis is approximately 13,000 molecules per second. In vivo, the rate of protein synthesis is limited by the number of ribosomes. The average number of ribosomes in one cell is about 200,000, and the number of mRNA molecules is about 15,000 to 60,000. 
     At present, commercial products of in-vitro protein expression systems often used in experiments, include  E. coli  systems ( E. coli  extract, ECE), rabbit reticulocyte systems (rabbit reticulocyte lysate, RRL), wheat germ systems (wheat germ extract, WGE), insect systems (insect cell extract, ICE) and humanized systems. 
     Among the present commercial products of in-vitro protein synthesis systems, the protein yield of a prokaryotic system can reach approximately 0.5 mg/mL, with a cost of about 10 RMB/μg. For eukaryotic systems, the protein yield of a CHO system can reach approximately 0.7 mg/mL, with a cost of about 20 RMB/μg. Therefore, both intracellular protein synthesis systems in nature and artificial extracellular protein synthesis systems have the characteristic of low efficiency and low rate, which greatly limits the application of protein synthesis. 
     Therefore, there is an urgent need in the field to develop an in-vitro protein synthesis system that can effectively enhance the efficiency of in-vitro protein synthesis. 
     SUMMARY 
     The present invention is aimed to provide an in-vitro protein synthesis system that can effectively enhance the efficiency of in-vitro protein synthesis. 
     A first aspect of the present invention provides a fusion protein, wherein, the fusion protein has a structure represented by the general Formula Ia or Formula Ib: 
       S-A-B-C   (Ia)
 
       S-C-B-A   (Ib)
 
     wherein,
     A is a PabI element;   B is none or a linker peptide;   C is an eIF4G element;   S is a signal peptide; and   each “-” is respectively a peptide bond.   

     In another preferred embodiment, the Formula Ia or Ib corresponds to a structure from the N-terminus to the C-terminus. 
     In another preferred embodiment, the element A includes a Pab 1 sequence of wild-type and a Pab 1 sequence of mutant type. 
     In another preferred embodiment, the PabI element is a PabI derived from yeast. 
     In another preferred embodiment, the element A has a sequence as shown in SEQ ID NO.: 1 or an active fragment thereof, or contains a peptide which has a sequence homology of ≥85% (preferably ≥90%; more preferably ≥95%; most preferably ≥97%, such as ≥98%, ≥99%, etc) to the amino acid sequence as shown in SEQ ID NO.: 1 and has the same activity as the sequence of SEQ ID NO.: 1. 
     In another preferred embodiment, the element C includes an eIF4G sequence of wild-type and an eIF4G sequence of mutant type. 
     In another preferred embodiment, the eIF4G element is an eIF4G derived from yeast. 
     In another preferred embodiment, the element C has a sequence as shown in SEQ ID NO.: 2 or an active fragment thereof, or contains a peptide which has a sequence homology of ≥85% (preferably ≥90%; more preferably ≥95%; most preferably ≥97%, such as ≥98%, ≥99%, etc) to the amino acid sequence as shown in SEQ ID NO.: 2 and has the same activity as the sequence of SEQ ID NO.: 2. 
     In another preferred embodiment, the fusion protein is a recombinant protein, and preferably a recombinant protein expressed by yeast. 
     In another preferred embodiment, the yeast is selected from the group consisting of:  Kluyveromyces, Saccharomyces cerevisiae,  and the combination thereof. 
     In another preferred embodiment, the yeast is selected from the group consisting of:  Kluyveromyces lactis, Kluyveromyces marxianus, Kluyveromyces dobzhanskii,  and the combination thereof. 
     In another preferred embodiment, the element A is a Pab1 protein derived from yeast. 
     In another preferred embodiment, the element C is an eIF4G protein derived from yeast. 
     In another preferred embodiment, the peptide linker has a length of 0-50 amino acids, preferably of 10-40 amino acids, and more preferably of 15-25 amino acids. 
     In another preferred embodiment, the fusion protein is selected from the group consisting of: 
     (A) a peptide having an amino acid sequence as shown in SEQ ID NO.: 3; 
     (B) a peptide which has a sequence homology of ≥80% (preferably ≥90%; more preferably ≥95%; most preferably ≥97%, such as ≥98%, ≥99%, etc) to the amino acid sequence as shown in SEQ ID NO.: 3, and the peptide has a function or activity of increasing the efficiency of exogenous protein expression; and 
     (C) a peptide which is formed after any one of the amino acid sequences as shown in SEQ ID NO.: 3 is substituted, deleted or added by 1-15 amino acid residues (preferably, 2-10 amino acid residues, more preferably, 3-8 amino acid residues): and the derived peptide has a function or activity of increasing the efficiency of exogenous protein expression. 
     In another preferred embodiment, the amino acid sequence of the fusion protein is shown in SEQ ID NO.: 3. 
     In another preferred embodiment, the fusion protein has one or more characteristics selected from the group consisting of: 
     (a) improving the efficiency of exogenous protein expression; and (b) improving the efficiency of in-vitro translation. 
     In another preferred embodiment, the exogenous protein is selected from the group consisting of: luciferin or luciferases (such as firefly luciferase), green fluorescent protein, yellow fluorescent protein, aminoacyl tRNA synthetase, glyceraldehyde-3-phosphate dehydrogenase, catalase, actin, the variable region of antibodies, luciferase mutants or variants, α-amylase, enterocin A, hepatitis C virus (HCV) E2 glycoprotein, insulin precursors, interferon alpha A (IFN-αA), interleukin-1β (IL-1β), lysozyme, serum albumin, single-chain fragment variable (scFv) of antibodies, tranthyretin, tyrosinase, xylanase, and the combination thereof. 
     A second aspect of the present invention provides an isolated polynucleotide which encodes the fusion protein described in the first aspect of the present invention. 
     In another preferred embodiment, the polynucleotide is selected from the group consisting of: a DNA sequence and an RNA sequence. 
     In another preferred embodiment, the DNA sequence is selected from the group consisting of: a genomic sequence and a cDNA sequence. 
     In another preferred embodiment, the polynucleotide is mRNA or cDNA, and the polynucleotide has a structured represented by Formula II: 
       A1-C1   (Formula II)
 
     wherein,
     A1 is a nucleotide sequence encoding the above-said A element;   C1 is a nucleotide sequence encoding the above-said C element; and   “-” is a linking bond which connects the A1 element and the C1 element.   

     In another preferred embodiment, the element A1 has a sequence as shown in SEQ ID NO.: 4. 
     In another preferred embodiment, the element C1 has a sequence as shown in SEQ ID NO.: 5. 
     A third aspect of the present invention provides a vector which comprises the polynucleotide described in the second aspect of the present invention. 
     A fourth aspect of the present invention provides a host cell, wherein, the host cell comprises the vector described in the third aspect of the present invention, or the genome of the host cell is integrated with the polynucleotide of the second aspect of the present invention. 
     In another preferred embodiment, the host cell is formed by transforming the expression vector of the third aspect of the present invention or the polynucleotide of the second aspect of the present invention, followed by homologous recombination, so as that the coding sequence of the fusion protein of the first aspect of the present invention is integrated into the genome or chromosome of the host cell. 
     In another preferred embodiment, the host cell is selected from the group consisting of:  Kluyveromyces, Saccharomyces cerevisiae,  and the combination thereof. 
     In another preferred embodiment, the host cell is selected from the group consisting of:  Kluyveromyces lactis, Kluyveromyces marxianus, Kluyveromyces dobzhanskii,  and the combination thereof. 
     In another preferred embodiment, the host cell is  Kluyveromyces lactis.    
     A fifth aspect of the present invention provides an in-vitro protein synthesis system for expressing an exogenous protein, wherein, the reaction system includes:
     (i) a yeast-based in-vitro protein synthesis system comprising (a) yeast cell extract; (b) optional polyethylene glycol; (c) optional exogenous sucrose; and (d) an optional solvent, wherein the solvent is water or an aqueous solvent; and   (ii) the fusion protein according to the first aspect of the present invention.   

     In another preferred embodiment, the reaction system further includes (iii) additionally added eIF4G protein. 
     In another preferred embodiment, the expression of the eIF4G protein is induced by a constitutive or inducible promoter. 
     In another preferred embodiment, the constitutive or inducible promoter is derived from yeast. 
     In another preferred embodiment, the yeast is selected from the group consisting of:  Kluyveromyces, Saccharomyces cerevisiae,  and the combination thereof. 
     In another preferred embodiment, the constitutive or inducible promoter is selected from the group consisting of: pScTEF1, pScPGK1, pKlTEF1, pKlPGK1, pScADH1, pScTPI1, pScTDH3, pKlADH1, pKlTPI1, pKlTDH3, and the combination thereof. 
     A sixth aspect of the present invention provides a method for producing the fusion protein described in the first aspect of the present invention, including:
     (i) culturing host cells according to the fourth aspect of the present invention under a condition suitable for expression to express the fusion protein according to the first aspect of the present invention; and   (ii) isolating the fusion protein.   

     A seventh aspect of the present invention provides use of the fusion protein described in the first aspect of the present invention, wherein, the fusion protein is used for preparing an in-vitro protein synthesis system for expressing an exogenous protein. The in-vitro protein synthesis system is used for increasing the expression efficiency of the exogenous protein. 
     In another preferred embodiment, the reaction system further includes additionally added eIF4G protein. 
     An eighth aspect of the present invention provides use of the fusion protein described in the first aspect of the present invention, wherein, the fusion protein is used for preparing a preparation which can increase the in-vitro protein synthesis ability of an in-vitro protein synthesis system. 
     A ninth aspect of the present invention provides a method for expressing an exogenous protein, including:
     (i) providing a yeast-based in-vitro protein synthesis system which comprises the fusion protein according to the first aspect of the present invention; and   (ii) incubating the yeast-based in-vitro protein synthesis system in the presence of the template of an exogenous protein to express the exogenous protein under a condition suitable for protein expression.   

     In another preferred embodiment, the fusion protein is additionally added. 
     In another preferred embodiment, the fusion protein and other proteins in the yeast-based in-vitro protein synthesis system are extracts derived from the same yeast. 
     In another preferred embodiment, the method is non-diagnostic and non-therapeutic. 
     In another preferred embodiment, the step (ii) further comprises a step (iii): detecting the expression activity of the exogenous protein, (Q1); while incubating a wild-type yeast strain under the same condition as that in the step (ii), detecting the activity of the exogenous protein, (Q2); wherein, if Q1 is significantly higher than Q2, it indicates that expression efficiency of the exogenous protein is significantly improved. 
     In another preferred embodiment, the phrase of “significantly higher than” refers to Q1/Q2≥2, preferably ≥3, and more preferably ≥4. 
     In another preferred embodiment, the yeast-based in-vitro protein synthesis system is an in-vitro protein synthesis system based on a genetically modified  Kluyveromyces  (preferably an in-vitro protein synthesis system based on  Kluyveromyces lactis ). 
     In another preferred embodiment, the coding sequence of the exogenous protein is derived from a prokaryotic organism or a eukaryotic organism. 
     In another preferred embodiment, the coding sequence of the exogenous protein is derived from animal, plant, or pathogen. 
     In another preferred embodiment, the coding sequence of the exogenous protein is derived from mammal, preferably primate or rodent, including human, mouse, rat, etc. 
     In another preferred embodiment, the coding sequence of the exogenous protein is selected from the group consisting of: exogenous DNA sequences encoding luciferin or luciferases (such as firefly luciferase), green fluorescent protein, yellow fluorescent protein, aminoacyl tRNA synthetase, glyceraldehyde-3 -phosphate dehydrogenase, catalase, actin, the variable region of an antibody, DNA of a luciferase mutant (or translated as luciferase variant), and the combination thereof. 
     In another preferred embodiment, the exogenous protein is selected from the group consisting of: luciferin or luciferases (such as firefly luciferase), green fluorescent protein, yellow fluorescent protein, aminoacyl tRNA synthetase, glyceraldehyde-3 -phosphate dehydrogenase, catalase, actin, the variable region of antibodies, luciferase mutants (or translated as luciferase variants), α-amylase, enterocin A, hepatitis C virus (HCV) E2 glycoprotein, insulin precursors, interferon αA, interleukin-1β, lysozyme, serum albumin, single-chain fragment variable (scFv) of antibodies, tranthyretin, tyrosinase, xylanase, and the combination thereof. 
     It should be understood that, within the scope of the present invention, the abovementioned technical features of the present invention and technical features specifically described below (e.g., embodiments) can be combined with each other to form a new or preferred technical solution. These combination details will not be described here, due to specification length and words limit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows the plasmid map of pKM-CAS1.0-KleIF4G. 
         FIG. 2  shows the plasmid map of pKM-pScTEF1-KleIF4G-DD. 
         FIG. 3  shows the plasmid map of pKM-pScPGK1-KleIF4G-DD. 
         FIG. 4  shows the plasmid map of pKM-pKlTEF1-KleIF4G-DD. 
         FIG. 5  shows the plasmid map of pKM-pKlPGK1-KleIF4G-DD. 
         FIG. 6  shows the plasmid map of pKM-CAS1.0-KlTDH3-1. 
         FIG. 7  shows the plasmid map of pKM-CAS1.0-KlTDH3-2. 
         FIG. 8  shows the plasmid map of pKM-KlTDH3-1-F-KleIF4G-DD. 
         FIG. 9  shows the plasmid map of pKM-KlTDH3-2-F-KleIF4G-DD. 
         FIG. 10  shows the plasmid map of pKM-CAS1.0-KlPab1. 
         FIG. 11  shows the plasmid map of pKM-K1Pab1-KleIF4G-DD. 
         FIG. 12  shows the testing results of the in-vitro translation activity based on genetic modified strains, wherein, the fluorescence intensity of the fluorescent protein is used to indicate the protein expression ability of the in-vitro translation system. 
     
    
    
     DETAILED DESCRIPTION 
     After extensive and in-depth research and through a large number of screenings and explorations, a fusion protein having a structure represented by Formula Ia or Formula Ib was unexpectedly discovered for the first time. The fusion protein of the present invention can greatly improve the in-vitro translation efficiency. In addition, in the present invention, it was also found that inserting a constitutive or inducible promoter (e.g., pScTEF1, pScPGK1, pKlTEF1, pKlPGK1, pScADH1, pScTPI1, pScTDH3, pKlADH1, pKlTPI1, pKlTDH3, etc) in front of the eIF4G element can greatly improve the in-vitro protein synthesis ability. 
     What&#39;s more, inventors of the present invention also found that, while the fusion protein of the present invention improved the in-vitro translation efficiency, the expression level of the element eIF4G of the fusion protein of the present invention did not increase. 
     Specifically, in a yeast-based in-vitro protein synthesis system containing the fusion protein of the present invention, the value of relative light units referring to the activity of the synthesized luciferase reaches as high as 1.50×10 9 , and the value of relative light units referring to the activity of the synthesized luciferase with a constitutive or inducible promoter being inserted in front of the eIF4G element reaches as high as 1.57×10 9 , which is far higher than the value of relative light units (4.11×10 8 ) referring to the activity of the synthesized luciferase corresponding to a wild-type yeast strain (e.g., Y1140). On this basis, the present inventors completed the present invention. 
     eIF4G Element 
     In eukaryotic organisms, various translation initiation factors are involved in the process of protein translation initiation (Table 1). Among them, the eIF4F is responsible for the recognition of “cap structure” and the recruitment of downstream translation initiation factors and ribosomes. The eIF4F consists of three protein subunits: eIF4E, eIF4G and eIF4A. The eIF4E specifically binds to the “cap structure” and anchors the eIF4F to the 5′ untranslated region (or 5′ UTR; or 5′ nontranslated region) of mRNA. The eIF4A is an RNA helicase. The eIF4G, which serves as a scaffold protein during almost the entire translation initiation process and can interact with many translation initiation factors, plays an important role in the recruitment of downstream factors. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Translation initiation factors in yeast 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Translation 
                   
                   
                   
               
               
                   
                 Initiation 
                   
                   
                 Protein Length 
               
               
                   
                 Factor 
                 Subunit 
                 Gene 
                 (AA) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 eIF1 
                   
                 SUI1 
                 108 
               
               
                   
                 eIF1A 
                   
                 TIF11 
                 153 
               
               
                   
                 eIF2 
                 α 
                 SUI2 
                 304 
               
               
                   
                   
                 β 
                 SUI3 
                 285 
               
               
                   
                   
                 γ 
                 GCD11 
                 527 
               
               
                   
                 eIF2B 
                 α 
                 GCN3 
                 305 
               
               
                   
                   
                 β 
                 GCD7 
                 381 
               
               
                   
                   
                 γ 
                 GCD1 
                 578 
               
               
                   
                   
                 δ 
                 GCD2 
                 651 
               
               
                   
                   
                 ε 
                 GCD6 
                 712 
               
               
                   
                 eIF3 
                 a 
                 RPG1/TIF32 
                 964 
               
               
                   
                   
                 b 
                 PRT1 
                 763 
               
               
                   
                   
                 c 
                 NIP1 
                 812 
               
               
                   
                   
                 g 
                 TIF35 
                 274 
               
               
                   
                   
                 i 
                 TIF34 
                 347 
               
               
                   
                   
                 j 
                 HCR1 
                 265 
               
               
                   
                 eIF4A 
                   
                 TIF1 
                 395 
               
               
                   
                   
                   
                 TIF2 
                 395 
               
               
                   
                 eIF4B 
                   
                 TIF3/STM1 
                 436 
               
               
                   
                 eIF4E 
                   
                 CDC33 
                 213 
               
               
                   
                 eIF4G 
                   
                 TIF4631 
                 952 
               
               
                   
                   
                   
                 TIF4632 
                 914 
               
               
                   
                 eIF5 
                   
                 TIF5 
                 405 
               
               
                   
                 eIF5B 
                   
                 FUN12 
                 1002 
               
               
                   
                   
               
            
           
         
       
     
     In the present invention, by inserting a constitutive or inducible promoter (e.g., pScTEF1, pScPGK1, pKlTEF1, pKlPGK1, pScADH1, pScTPI1, pScTDH3, pKlADH1, pKlTPI1, pKlTDH3 or the like) derived from yeast such as  Saccharomyces cerevisiae, Kluyveromyces  and the like in front of the eIF4G element, the in-vitro protein synthesis ability was improved. 
     In a preferred embodiment, the eIF4G has a nucleotide sequence as shown in SEQ ID NO.: 5, and an amino acid sequence as shown in SEQ ID NO.: 2. 
     
       
         
           
               
            
               
                 (SEQ ID NO.: 5) 
               
               
                 ATGGGCGAACCTACATCCGATCAGCAACCAGCTGTTGAAGCTCCAGT 
               
               
                   
               
               
                 TGTGCAGGAGGAGACAACCAGTTCTCCGCAAAAAAACAGTGGATATGTC 
               
               
                   
               
               
                 AAGAATACTGCTGGAAGCGGTGCTCCTAGAAATGGGAAATATGATGGTA 
               
               
                   
               
               
                 ACAGGAAGAACTCTAGGCCTTATAACCAAAGAGGTAACAACAACAATAA 
               
               
                   
               
               
                 TAATGGTTCTTCCTCGAATAAGCACTATCAAAAGTATAACCAACCAGCG 
               
               
                   
               
               
                 TACGGTGTTTCTGCGGGATACATTCCGAACTACGGCGTATCGGCAGAGT 
               
               
                   
               
               
                 ACAACCCTCTGTACTATAACCAGTACCAACAGCAGCAACAGCTGTACGC 
               
               
                   
               
               
                 TGCTGCTTACCAGACTCCAATGAGCGGACAAGGTTATGTCCCCCCAGTA 
               
               
                   
               
               
                 GTGTCTCCAGCTGCTGTTTCAGCTAAACCAGCGAAGGTTGAGATTACTA 
               
               
                   
               
               
                 ACAAGTCTGGTGAACACATAGATATTGCTTCCATTGCTCATCCACATAC 
               
               
                   
               
               
                 TCATTCTCATTCTCAATCTCATTCGCGTGCAGTTCCAGTAGTGTCGCCT 
               
               
                   
               
               
                 CCAGCTAACGTTACCGTCGCTGCTGCTGTATCATCCTCTGTGTCTCCAT 
               
               
                   
               
               
                 CAGCTTCTCCAGCTGTCAAAGTACAGAGCCCTGCTGCTAATGGTAAGGA 
               
               
                   
               
               
                 ACAATCTCCAGCTAAGCCTGAAGAACCAAAGAAGGACACTTTAATTGTG 
               
               
                   
               
               
                 AACGATTTCTTGGAACAAGTTAAAAGACGCAAGGCTGCTTTAGCTGCTA 
               
               
                   
               
               
                 AGAAGGCTGTCGAAGAGAAGGGTCCTGAGGAACCGAAGGAATCTGTCGT 
               
               
                   
               
               
                 TGGAACTGACACTGATGCAAGCGTTGATACTAAGACAGGGCCTACAGCC 
               
               
                   
               
               
                 ACTGAATCTGCCAAGTCTGAAGAAGCTCAATCAGAATCACAAGAAAAGA 
               
               
                   
               
               
                 CTAAGGAAGAGGCTCCAGCTGAGCCAAAACCATTGACTTTGGCCGAAAA 
               
               
                   
               
               
                 ATTGAGACTTAAGAGGATGGAAGCTGCAAAGCAAGCTTCTGCTAAGACC 
               
               
                   
               
               
                 GAGGAACTAAAGACTGAAGAATCTAAGCCTGAAGAAACAAAGACCGAGG 
               
               
                   
               
               
                 AGCTAAAGACTGAAGAATCTAAGCCTGAAGAAACAAAGACCGAGGAGCT 
               
               
                   
               
               
                 AAAGACTGAAGAAACAAAGTCCGAGGAACTAAAGACTGAAGAACCTAAG 
               
               
                   
               
               
                 GCGGAAGAATCAAAGGCGGAAGAACCAAAGCCTGAAGAACCAAAGACCG 
               
               
                   
               
               
                 AGGAACCGACGACTGAACAACCAAAGTCAGATGAACCAAAGTCGGAAGA 
               
               
                   
               
               
                 ATCAAAAACTGAAGAGCCAAAAACCGAGGTATTAAAGACTGAAGAACCA 
               
               
                   
               
               
                 AAATCGGAAGAATCAAAGCCTGCAGAACCAAAGACTGAAGAAACAGCAA 
               
               
                   
               
               
                 CTGAAGAAACAGCAACTGAAGCAAACGCCGAAGAAGGTGAACCGGCTCC 
               
               
                   
               
               
                 TGCTGGTCCCGTTGAAACTCCTGCTGATGTTGAAACAAAACCTCGAGAA 
               
               
                   
               
               
                 GAGGCTGAAGTTGAAGACGATGGAAAGATTACCATGACCGATTTCCTAC 
               
               
                   
               
               
                 AGAAGTTGAAAGAGGTTTCTCCAGTTGATGATATTTATTCCTTCCAATA 
               
               
                   
               
               
                 CCCAAGTGACATTACGCCTCCAAATGATAGATATAAAAAGACAAGCATT 
               
               
                   
               
               
                 AAATATGCATACGGACCTGATTTCTTGTATCAGTTCAAAGAAAAGGTCG 
               
               
                   
               
               
                 ATGTTAAATACGATCCAGCGTGGATGGCTGAAATGACGAGTAAAATTGT 
               
               
                   
               
               
                 CATCCCTCCTAAGAAGCCTGGTTCAAGCGGAAGAGGCGAAGATAGATTT 
               
               
                   
               
               
                 AGTAAGGGTAAGGTTGGATCTCTAAGAAGTGAAGGCAGATCGGGTTCCA 
               
               
                   
               
               
                 GGTCCAACTCGAAGAAGAAGTCAAAGAGGGATGATAGAAAATCTAATAG 
               
               
                   
               
               
                 ATCATACACTTCCAGAAAGGACCGTGAAAGATTCAGAGAGGAAGAAGTC 
               
               
                   
               
               
                 GAAGAGCCAAAGGTTGAGGTTGCCCCATTGGTCCCAAGTGCTAATAGAT 
               
               
                   
               
               
                 GGGTTCCTAAATCTAAGATGAAGAAAACAGAAGTCAAGTTAGCTCCAGA 
               
               
                   
               
               
                 CGGAACAGAACTTTACGACGCGGAAGAAGCATCAAGAAAGATGAAGTCA 
               
               
                   
               
               
                 TTGCTGAATAAATTGACATTAGAAATGTTCGAACCTATTTCTGATGATA 
               
               
                   
               
               
                 TCATGAAGATCGCTAACCAATCTAGATGGGAAGAAAAGGGTGAGACTTT 
               
               
                   
               
               
                 GAAGATTGTCATCCAACAAATTTTCAATAAGGCCTGCGATGAACCTCAT 
               
               
                   
               
               
                 TGGTCATCAATGTACGCGCAATTATGTGGTAAGGTCGTTAAAGACTTAG 
               
               
                   
               
               
                 ATGATAGCATTAAAGACTCAGAAACCCCAGATAAGACTGGTTCTCACTT 
               
               
                   
               
               
                 GGTTTTGCATTACTTAGTCCAAAGATGTCAAACTGAATTCCAAACAGGA 
               
               
                   
               
               
                 TGGACTGATCAACTACCTACAAACGAAGACGGTACTCCTCTACAACCTG 
               
               
                   
               
               
                 AAATGATGTCCGATGAATACTATAAGATGGCTGCCGCTAAGAGAAGAGG 
               
               
                   
               
               
                 TTTGGGTTTGGTTCGTTTCATTGGTTTCTTGTACCGTTCGAACTTATTG 
               
               
                   
               
               
                 ACTTCCAGAATGGTCTTCTTCTGTTTCAAGAGACTAATGAAGGATATTC 
               
               
                   
               
               
                 AAAACTCTCCTACTGAAGATACTCTAGAGTCTGTATGTGAACTTTTGGA 
               
               
                   
               
               
                 AACAATTGGTGAACAGTTCGAAGGTGCTCGTATTCAAGTTACTGCAGAA 
               
               
                   
               
               
                 GCTGTCATTGAGGGTTCAAGCTTGCTAGACACACTATTCGACCAAATAA 
               
               
                   
               
               
                 AGAACGTGATCGAAAATGGTGACATCTCCAGCAGAATCAAGTTTAAGTT 
               
               
                   
               
               
                 GATCGACATTGTCGAACTAAGAGAAAAGAGGAACTGGAATAGTAAAAAT 
               
               
                   
               
               
                 AAGAACGATGGTCCAAAGACCATTGCTCAAATTCACGAAGAAGAAGCCT 
               
               
                   
               
               
                 TGAAGAGGGCTTTGGAGGAAAGAGAAAGAGAAAGAGATCGCCATGGGTC 
               
               
                   
               
               
                 CAGAGGTGGTTCCAGACGTATGAATAGCGAGAGAAACTCTTCTAGAAGA 
               
               
                   
               
               
                 GATTTCTCCTCTCATTCTCACAGTCACAATCAAAATAGAGACGGTTTCA 
               
               
                   
               
               
                 CTACTACCAGATCGTCATCAGTGAGATATTCTGAGCCAAAGAAGGAAGA 
               
               
                   
               
               
                 ACAAGCTCCAACTCCAACTAAATCTTCTGGTGGCGCTGCCAACATGTTT 
               
               
                   
               
               
                 GATGCATTGATGGATGCCGAAGATGATTAA 
               
               
                   
               
               
                 (SEQ ID NO.: 2) 
               
               
                 MGEPTSDQQPAVEAPVVQEETTSSPQKNSGYVKNTAGSGAPRNGKYD 
               
               
                   
               
               
                 GNRKNSRPYNQRGNNNNNNGSSSNKHYQKYNQPAYGVSAGYIPNYGVSA 
               
               
                   
               
               
                 EYNPLYYNQYQQQQQLYAAAYQTPMSGQGYVPPVVSPAAVSAKPAKVEI 
               
               
                   
               
               
                 TNKSGEHIDIASIAHPHTHSHSQSHSRAVPVVSPPANVTVAAAVSSSVS 
               
               
                   
               
               
                 PSASPAVKVQSPAANGKEQSPAKPEEPKKDTLIVNDFLEQVKRRKAALA 
               
               
                   
               
               
                 AKKAVEEKGPEEPKESVVGTDTDASVDTKTGPTATESAKSEEAQSESQE 
               
               
                   
               
               
                 KTKEEAPAEPKPLTLAEKLRLKRMEAAKQASAKTEELKTEESKPEETKT 
               
               
                   
               
               
                 EELKTEESKPEETKTEELKTEETKSEELKTEEPKAEESKAEEPKPEEPK 
               
               
                   
               
               
                 TEEPTTEQPKSDEPKSEESKTEEPKTEVLKTEEPKSEESKPAEPKTEET 
               
               
                   
               
               
                 ATEETATEANAEEGEPAPAGPVETPADVETKPREEAEVEDDGKITMTDF 
               
               
                   
               
               
                 LQKLKEVSPVDDIYSFQYPSDITPPNDRYKKTSIKYAYGPDFLYQFKEK 
               
               
                   
               
               
                 VDVKYDPAWMAEMTSKIVIPPKKPGSSGRGEDRFSKGKVGSLRSEGRSG 
               
               
                   
               
               
                 SRSNSKKKSKRDDRKSNRSYTSRKDRERFREEEVEEPKVEVAPLVPSAN 
               
               
                   
               
               
                 RWVPKSKMKKTEVKLAPDGTELYDAEEASRKMKSLLNKLTLEMFEPISD 
               
               
                   
               
               
                 DIMKIANQSRWEEKGETLKIVIQQIFNKACDEPHWSSMYAQLCGKVVKD 
               
               
                   
               
               
                 LDDSIKDSETPDKTGSHLVLHYLVQRCQTEFQTGWTDQLPTNEDGTPLQ 
               
               
                   
               
               
                 PEMMSDEYYKMAAAKRRGLGLVRFIGFLYRSNLLTSRMVFFCFKRLMKD 
               
               
                   
               
               
                 IQNSPTEDTLESVCELLETIGEQFEGARIQVTAEAVIEGSSLLDTLFDQ 
               
               
                   
               
               
                 IKNVIENGDISSRIKFKLIDIVELREKRNWNSKNKNDGPKTIAQIHEEE 
               
               
                   
               
               
                 ALKRALEERERERDRHGSRGGSRRMNSERNSSRRDFSSHSHSHNQNRDG 
               
               
                   
               
               
                 FTTTRSSSVRYSEPKKEEQAPTPTKSSGGAANMFDALMDAEDD 
               
            
           
         
       
     
     Pab1 Element (Pab1 protein) 
     Pab 1 is a 71 kDa RNA-binding protein, which consists of four RNA recognition motif (RRM 1-4) domains and one MLLE domain. Each RRM domain comprises two conserved RNP structures (RNP1/2) which are responsible for binding to RNA. 
     In a preferred embodiment, the Pab 1 has a nucleotide sequence as shown in 
     SEQ ID NO.: 4 and a protein sequence as shown in SEQ ID NO.: 1. 
     
       
         
           
               
            
               
                 (SEQ ID NO.: 4) 
               
               
                 ATGTCTGATATTACTGAAAAAACTGCTGAGCAATTGGAAAACTTGCA 
               
               
                   
               
               
                 GATCAACGATGATCAGCAACCAGCTCAATCTGCCAGTGCTCCATCCACT 
               
               
                   
               
               
                 TCTGCTTCTGAAAGCGAAGCTTCTTCTGTTTCTAAGGTTGAAAACAACA 
               
               
                   
               
               
                 ACGCTTCATTGTACGTTGGTGAATTGGATCCAAACATTACTGAAGCATT 
               
               
                   
               
               
                 GTTGTACGATGTGTTTTCACCATTGGGTCCAATTTCCTCGATCCGTGTT 
               
               
                   
               
               
                 TGTCGTGATGCCGTCACCAAGGCTTCGTTAGGTTACGCTTACGTTAACT 
               
               
                   
               
               
                 ATACTGATTACGAAGCTGGTAAGAAAGCTATTCAAGAATTGAACTATGC 
               
               
                   
               
               
                 TGAAATCAACGGTAGACCATGTAGAATTATGTGGTCCGAACGTGACCCA 
               
               
                   
               
               
                 GCTATCAGAAAGAAGGGTTCTGGTAACATTTTCATCAAGAACTTGCACC 
               
               
                   
               
               
                 CAGCCATTGACAACAAGGCTTTGCATGAAACTTTCTCCACTTTCGGTGA 
               
               
                   
               
               
                 AGTCTTGTCTTGTAAAGTTGCTTTAGATGAGAATGGAAACTCTAGAGGC 
               
               
                   
               
               
                 TTCGGTTTCGTTCATTTCAAGGAAGAATCCGATGCTAAGGATGCTATTG 
               
               
                   
               
               
                 AAGCCGTCAACGGTATGTTGATGAACGGTTTGGAAGTTTACGTTGCCAT 
               
               
                   
               
               
                 GCACGTTCCAAAGAAGGACCGTATCTCCAAGTTGGAAGAAGCCAAGGCT 
               
               
                   
               
               
                 AACTTCACCAACATTTACGTCAAGAACATTGACGTTGAAACCACTGACG 
               
               
                   
               
               
                 AAGAGTTCGAACAGTTGTTCTCCCAATACGGTGAAATTGTCTCTGCTGC 
               
               
                   
               
               
                 TTTGGAAAAGGATGCTGAGGGTAAGCCAAAGGGTTTCGGTTTCGTTAAC 
               
               
                   
               
               
                 TTTGTTGACCACAACGCCGCTGCCAAGGCCGTTGAAGAGTTGAACGGTA 
               
               
                   
               
               
                 AGGAATTCAAGTCTCAAGCTTTGTACGTTGGCAGAGCTCAAAAGAAGTA 
               
               
                   
               
               
                 CGAACGTGCTGAAGAATTGAAGAAACAATACGAACAATACCGTTTGGAA 
               
               
                   
               
               
                 AAATTGGCTAAGTTCCAAGGTGTTAACTTGTTCATCAAGAACTTGGACG 
               
               
                   
               
               
                 ATTCCATCGATGACGAAAAATTGAAGGAAGAATTCGCCCCATACGGTAC 
               
               
                   
               
               
                 CATCACCTCTGCTAGAGTCATGAGAGACCAAGAGGGTAACTCTAAGGGT 
               
               
                   
               
               
                 TTCGGTTTCGTTTGTTTCTCTTCTCCAGAAGAAGCTACCAAGGCTATGA 
               
               
                   
               
               
                 CCGAAAAGAACCAACAAATTGTTGCCGGTAAGCCATTGTACGTTGCCAT 
               
               
                   
               
               
                 TGCTCAAAGAAAGGATGTCAGAAGATCCCAATTGGCTCAACAAATTCAA 
               
               
                   
               
               
                 GCCAGAAACCAAATCAGATTCCAACAACAGCAACAACAACAAGCTGCTG 
               
               
                   
               
               
                 CCGCTGCTGCTGGTATGCCAGGCCAATACATGCCACAAATGTTCTATGG 
               
               
                   
               
               
                 TGTTATGGCCCCAAGAGGTTTCCCAGGTCCAAACCCAGGTATGAACGGC 
               
               
                   
               
               
                 CCAATGGGTGCCGGTATTCCAAAGAACGGTATGGTCCCACCACCACAAC 
               
               
                   
               
               
                 AATTTGCTGGTAGACCAAACGGTCCAATGTACCAAGGTATGCCACCTCA 
               
               
                   
               
               
                 AAACCAATTCCCAAGACACCAACAACAACACTACATCCAACAACAAAAG 
               
               
                   
               
               
                 CAAAGACAAGCCTTGGGTGAACAATTGTACAAGAAGGTCAGTGCCAAGA 
               
               
                   
               
               
                 TTGACGACGAAAACGCCGCTGGTAAGATCACCGGTATGATCTTGGATCT 
               
               
                   
               
               
                 ACCACCACAGCAAGTCATCCAATTGTTGGACAACGACGAACAATTTGAA 
               
               
                   
               
               
                 CAGCAATTCCAAGAAGCCTTAGCTGCTTACGAAAACTTCAAGAAGGAAC 
               
               
                   
               
               
                 AAGAAGCTCAAGCTTAA 
               
               
                   
               
               
                 (SEQ ID NO.: 1)  
               
               
                 MSDITEKTAEQLENLQINDDQQPAQSASAPSTSASESEASSVSKVENNN 
               
               
                   
               
               
                 ASLYVGELDPNITEALLYDVFSPLGPISSIRVCRDAVTKASLGYAYVNY 
               
               
                   
               
               
                 TDYEAGKKAIQELNYAEINGRPCRIMWSERDPAIRKKGSGNIFIKNLHP 
               
               
                   
               
               
                 AIDNKALHETFSTFGEVLSCKVALDENGNSRGFGFVHFKEESDAKDAIE 
               
               
                   
               
               
                 AVNGMLMNGLEVYVAMHVPKKDRISKLEEAKANFTNIYVKNIDVETTDE 
               
               
                   
               
               
                 EFEQLFSQYGEIVSAALEKDAEGKPKGFGFVNFVDHNAAAKAVEELNGK 
               
               
                   
               
               
                 EFKSQALYVGRAQKKYERAEELKKQYEQYRLEKLAKFQGVNLFIKNLDD 
               
               
                   
               
               
                 SIDDEKLKEEFAPYGTITSARVMRDQEGNSKGFGFVCFSSPEEATKAMT 
               
               
                   
               
               
                 EKNQQIVAGKPLYVAIAQRKDVRRSQLAQQIQARNQIRFQQQQQQQAAA 
               
               
                   
               
               
                 AAAGMPGQYMPQMFYGVMAPRGFPGPNPGMNGPMGAGIPKNGMVPPPQQ 
               
               
                   
               
               
                 FAGRPNGPMYQGMPPQNQFPRHQQQHYIQQQKQRQALGEQLYKKVSAKI 
               
               
                   
               
               
                 DDENAAGKITGMILDLPPQQVIQLLDNDEQFEQQFQEALAAYENFKKEQ 
               
               
                   
               
               
                 EAQA. 
               
            
           
         
       
     
     Fusion Protein 
     As used herein, terms of “fusion protein of the present invention”, “PabI-eIF4G fusion protein of the present invention” and “PabI-eIF4G fusion protein” can be used interchangeably, and refer to a fusion protein formed by fusing the Pab 1 element and the eIF4G element. In the fusion protein of the present invention, a linker peptide or a flexible linker can or cannot be contained between the Pab 1 element and the eIF4G element. In addition, the fusion protein can or cannot contain the starting Met; can or cannot contain a signal peptide; and can or cannot contain a tag sequence (e.g., 6His, etc). 
     In a preferred embodiment, the fusion protein described in the present invention has an above-described structure of Formula Ia or Formula Ib. Preferably, the fusion protein of the present invention has an amino acid sequence as shown in SEQ ID NO.: 3. 
     
       
         
           
               
            
               
                 (SEQ ID NO.: 3) 
               
               
                 MSDITEKTAEQLENLQINDDQQPAQSASAPSTSASESEASSVSKVENNN 
               
               
                   
               
               
                 ASLYVGELDPNITEALLYDVFSPLGPISSIRVCRDAVTKASLGYAYVNY 
               
               
                   
               
               
                 TDYEAGKKAIQELNYAEINGRPCRIMWSERDPAIRKKGSGNIFIKNLHP 
               
               
                   
               
               
                 AIDNKALHETFSTFGEVLSCKVALDENGNSRGFGFVHFKEESDAKDAIE 
               
               
                   
               
               
                 AVNGMLMNGLEVYVAMHVPKKDRISKLEEAKANFTNIYVKNIDVETTDE 
               
               
                   
               
               
                 EFEQLFSQYGEIVSAALEKDAEGKPKGFGFVNFVDHNAAAKAVEELNGK 
               
               
                   
               
               
                 EFKSQALYVGRAQKKYERAEELKKQYEQYRLEKLAKFQGVNLFIKNLDD 
               
               
                   
               
               
                 SIDDEKLKEEFAPYGTITSARVMRDQEGNSKGFGFVCFSSPEEATKAMT 
               
               
                   
               
               
                 EKNQQIVAGKPLYVAIAQRKDVRRSQLAQQIQARNQIRFQQQQQQQAAA 
               
               
                   
               
               
                 AAAGMPGQYMPQMFYGVMAPRGFPGPNPGMNGPMGAGIPKNGMVPPPQQ 
               
               
                   
               
               
                 FAGRPNGPMYQGMPPQNQFPRHQQQHYIQQQKQRQALGEQLYKKVSAKI 
               
               
                   
               
               
                 DDENAAGKITGMILDLPPQQVIQLLDNDEQFEQQFQEALAAYENFKKEQ 
               
               
                   
               
               
                 EAQAGGGGSGGGGSTQDEVQGPHAGKSTVGGGGSGEPTSDQQPAVEAPV 
               
               
                   
               
               
                 VQEETTSSPQKNSGYVKNTAGSGAPRNGKYDGNRKNSRPYNQRGNNNNN 
               
               
                   
               
               
                 NGSSSNKHYQKYNQPAYGVSAGYIPNYGVSAEYNPLYYNQYQQQQQLYA 
               
               
                   
               
               
                 AAYQTPMSGQGYVPPVVSPAAVSAKPAKVEITNKSGEHIDIASIAHPHT 
               
               
                   
               
               
                 HSHSQSHSRAVPVVSPPANVTVAAAVSSSVSPSASPAVKVQSPAANGKE 
               
               
                   
               
               
                 QSPAKPEEPKKDTLIVNDFLEQVKRRKAALAAKKAVEEKGPEEPKESVV 
               
               
                   
               
               
                 GTDTDASVDTKTGPTATESAKSEEAQSESQEKTKEEAPAEPKPLTLAEK 
               
               
                   
               
               
                 LRLKRMEAAKQASAKTEELKTEESKPEETKTEELKTEESKPEETKTEEL 
               
               
                   
               
               
                 KTEETKSEELKTEEPKAEESKAEEPKPEEPKTEEPTTEQPKSDEPKSEE 
               
               
                   
               
               
                 SKTEEPKTEVLKTEEPKSEESKPAEPKTEETATEETATEANAEEGEPAP 
               
               
                   
               
               
                 AGPVETPADVETKPREEAEVEDDGKITMTDFLQKLKEVSPVDDIYSFQY 
               
               
                   
               
               
                 PSDITPPNDRYKKTSIKYAYGPDFLYQFKEKVDVKYDPAWMAEMTSKIV 
               
               
                   
               
               
                 IPPKKPGSSGRGEDRFSKGKVGSLRSEGRSGSRSNSKKKSKRDDRKSNR 
               
               
                   
               
               
                 SYTSRKDRERFREEEVEEPKVEVAPLVPSANRWVPKSKMKKTEVKLAPD 
               
               
                   
               
               
                 GTELYDAEEASRKMKSLLNKLTLEMFEPISDDIMKIANQSRWEEKGETL 
               
               
                   
               
               
                 KIVIQQIFNKACDEPHWSSMYAQLCGKVVKDLDDSIKDSETPDKTGSHL 
               
               
                   
               
               
                 VLHYLVQRCQTEFQTGWTDQLPTNEDGTPLQPEMMSDEYYKMAAAKRRG 
               
               
                   
               
               
                 LGLVRFIGFLYRSNLLTSRMVFFCFKRLMKDIQNSPTEDTLESVCELLE 
               
               
                   
               
               
                 TIGEQFEGARIQVTAEAVIEGSSLLDTLFDQIKNVIENGDISSRIKFKL 
               
               
                   
               
               
                 IDIVELREKRNWNSKNKNDGPKTIAQIHEEEALKRALEERERERDRHGS 
               
               
                   
               
               
                 RGGSRRMNSERNSSRRDFSSHSHSHNQNRDGFTTTRSSSVRYSEPKKEE 
               
               
                   
               
               
                 QAPTPTKSSGGAANMFDALMDAEDD. 
               
            
           
         
       
     
     In the present invention, the fusion protein of the present invention can significantly improve the in-vitro protein synthesis ability of a cell-free and in-vitro protein synthesis system (especially a yeast-based in-vitro protein synthesis system). 
     In-Vitro Protein Synthesis System 
     The fusion protein of the present invention is used for improving the protein synthesis ability of an in-vitro protein synthesis system. 
     A typical in-vitro protein synthesis system is a yeast-based in-vitro protein synthesis system. 
     Yeast has advantages of simple culture, highly efficient protein folding and post-translational modification ability. Wherein,  Saccharomyces cerevisiae  and  Pichia pastoris  are model organisms for expressing complex eukaryotic proteins and membrane proteins. Yeast can also be used as materials for preparing in-vitro translation systems. 
       Kluyveromyces  is a kind of ascosporous yeast, among which  Kluyveromyces marxianus  and  Kluyveromyces lactis  are widely used in industry. For example,  Kluyveromyces lactis  is a kind of yeast which can use lactic acid as its sole carbon source and energy source. Compared with other yeasts,  Kluyveromyces lactis  has many advantages, such as superstrong secretion ability, good large-scale fermentation characteristics, food safety level, post-translational modification ability and the like, and also has shown great potential to be used as a host system to express pharmaceutical proteins. 
     In the present invention, the yeast-based in-vitro protein synthesis system is not particularly limited, and a preferred yeast-based in-vitro protein synthesis system is a  Kluyveromyces  expression system (preferably a  Kluyveromyces lactis  expression system). 
     In the present invention,  Kluyveromyces  (such as  Kluyveromyces lactis ) is not particularly limited and includes any  Kluyveromyces  (such as  Kluyveromyces lactis ) strain that is capable of increasing the efficiency of protein synthesis. 
     In a preferred embodiment, the yeast-based in-vitro protein synthesis system of the present invention is an expression system based on a genetically modified  Kluyveromyces lactis  strain. 
     In a preferred embodiment, the present invention provides an in-vitro cell-free protein synthesis system, including:
     (a) yeast cell extract;   (b) polyethylene glycol;   (c) optional exogenous sucrose; and   (d) an optional solvent, wherein the solvent is water or an aqueous solvent.   

     In another preferred embodiment, said polyethylene glycol is selected from the group consisting of: PEG3000, PEG8000, PEG6000, PEG3350, and the combination thereof. 
     In another preferred embodiment, said polyethylene glycol includes polyethylene glycol with a molecular weight (Da in units) of 200-10000, preferably, polyethylene glycol with a molecular weight of 3000-10000. 
     In another preferred embodiment, in the protein synthesis system, the concentration (v/v) of Component (a) is 20-70%, preferably 30-60%, and more preferably 40-50%, as calculated by the total volume of the protein synthesis system. 
     In another preferred embodiment, in the protein synthesis system, the concentration (w/v, such as g/ml) of Component (b) is 0.1-8%, preferably 0.5-4%, and more preferably 1-2%. 
     In another preferred embodiment, in the protein synthesis system, the concentration of Component (c) is 0.03-40wt %, preferably 0.08-10wt %, and more preferably 0.1-5wt %, as calculated by the total weight of the protein synthesis system. 
     In a particular preferred embodiment, the in-vitro protein synthesis system provided by the present invention consists of: yeast cell extract, 4-hydroxyethyl piperazineethanesulfonic acid, potassium acetate, magnesium acetate, adenosine triphosphate (ATP), guanosine triphosphate (GTP), cytidine triphosphate (CTP), thymine triphosphate (TTP), amino acid mixture, creatine phosphate, dithiothreitol (DTT), creatine phosphate kinase, RNase inhibitor, luciferin, DNA of luciferase and RNA polymerase. 
     In the present invention, the RNA polymerase is not particularly limited, and can be one kind of RNA polymerase or the combination of more kinds of RNA polymerases; a typical RNA polymerase is T7 RNA polymerase. 
     In the present invention, the proportion of the yeast cell extract in the in-vitro protein synthesis system is not particularly limited. Generally, the content of the yeast cell extract is 20-70% relative to the in-vitro protein synthesis system, preferably 30-60%, and more preferably 40-50%. 
     In the present invention, the yeast cell extract does not contain intact cells. A typical yeast cell extract contains factors for protein translation including ribosomes, transfer RNAs and aminoacyl tRNA synthetase, as well as factors required for protein synthesis including initiation factors, elongation factors and release factors mediating termination. In addition, the yeast cell extract also contains some other proteins derived from cytoplasm of the yeast cell, especially soluble proteins. 
     In the present invention, the protein content of the yeast cell extract is 20-100 mg/mL, preferably 50-100 mg/mL. The method for measuring protein content is Coomassie brilliant blue staining method. 
     In the present invention, the preparation method of the yeast cell extract is not limited. A preferred preparation method includes the following steps:
     (i) providing yeast cells;   (ii) washing the yeast cells to obtain washed yeast cells;   (iii) breaking up the washed yeast cells (or translated as “treat the washed cells with a cell lysis process”) to obtain crude yeast extract; and   (iv) treating the crude yeast extract via solid-liquid separation to obtain the liquid phase, that is the yeast cell extract.   

     In the present invention, the solid-liquid separation method is not limited. A preferred method is centrifugation. 
     In a preferred embodiment, the centrifugation is carried out in a liquid state. 
     In the present invention, the centrifugation condition is not particularly limited. A preferred centrifugation condition is 5000-100000 g, preferably 8000-30000 g. 
     In the present invention, the centrifugation time is not particularly limited. A preferred centrifugation time is 0.5 min-2 h, preferably 20-50 min. 
     In the present invention, the centrifugation temperature is not particularly limited. Preferably, the centrifugation is carried out at 1-10° C., preferably at 2-6° C. 
     In the present invention, the washing method is not particularly limited. A preferred washing method is to process at pH 7-8 (preferably pH 7.4) with the use of a washing buffer, wherein the washing buffer is not particularly limited. A typical washing buffer is selected from the group consisting of: potassium 4-hydroxyethyl piperazineethanesulfonate, potassium acetate, magnesium acetate, and the combination thereof. 
     In the present invention, the method for cell lysis (or translated as “the manner for breaking up cells”) is not particularly limited. Preferred cell lysis methods include high-pressure lysis, freeze-thaw (e.g., treatment at liquid-nitrogen low temperature) lysis, etc. 
     The nucleoside triphosphate mixture in the in-vitro protein synthesis system includes adenosine triphosphate, guanosine triphosphate, cytidine triphosphate, and uridine triphosphate. In the present invention, the concentration of each kind of nucleotide is not particularly limited. The concentration of each kind of nucleotide is usually 0.5-5 mM, preferably 1.0-2.0 mM. 
     The amino acid mixture in the in-vitro protein synthesis system can include natural or unnatural amino acids, and can include D-type or L-type amino acids. Representative amino acids include, but are not limited to, 20 kinds of natural amino acids: glycine, alanine, valine, leucine, isoleucine, phenylalanine, proline, tryptophan, serine, tyrosine, cysteine, methionine, asparagine, glutamine, threonine, aspartic acid, glutamic acid, lysine, arginine and histidine. The concentration of each kind of amino acid is usually 0.01-0.5 mM, preferably 0.02-0.2 mM, such as 0.05 mM, 0.06 mM, 0.07 mM, 0.08 mM, etc. 
     In a preferred embodiment, the in-vitro protein synthesis system further contains polyethylene glycol or the like. The concentration of polyethylene glycol or the like is not particularly limited. Usually, the concentration (w/v) of polyethylene glycol or the like is 0.1-8%, preferably 0.5-4%, and more preferably 1-2%, as calculated by the total weight of the protein synthesis system. Representative examples of PEG include, but are not limited to, PEG3000, PEG8000, PEG6000 and PEG3350. It should be understood that the system of the present invention can also include polyethylene glycol with other various molecular weights (e.g., PEG200, 400, 1500, 2000, 4000, 6000, 8000, 10000, etc.). 
     In a preferred embodiment, the in-vitro protein synthesis system further contains sucrose. The concentration of sucrose is not particularly limited. Usually, the concentration of sucrose is 0.03-40wt %, preferably 0.08-10wt %, and more preferably 0.1-5%, as calculated by the total weight of the protein synthesis system. 
     A particularly preferred in-vitro protein synthesis system further contains, besides the yeast extract, the following components including: 22 mM pH7.4 4-hydroxyethyl piperazineethanesulfonic acid, 30-150 mM potassium acetate, 1.0-5.0 mM magnesium acetate, 1.5-4 mM nucleoside triphosphate mixture, 0.08-0.24 mM amino acid mixture, 25 mM creatine phosphate, 1.7 mM dithiothreitol, 0.27 mg/mL creatine phosphate kinase, 1%-4% polyethylene glycol, 0.5%-2% sucrose, 8-20 ng/μl DNA of firefly luciferase, and 0.027-0.054 mg/mL T7 RNA polymerase. 
     In a preferred embodiment, the yeast-based in-vitro protein synthesis system of the present invention also contains: (a) the fusion protein according to the present invention, that is, a Pab1-eIF4G fusion protein. 
     In a preferred embodiment, the yeast-based in-vitro protein synthesis system of the present invention also includes eIF4G protein, wherein the expression of the eIF4G protein of the present invention is induced by a constitutive or inducible promoter (e.g., pScTEF1, pScPGK1, pKlTEF1, pKlPGK1, pScADH1, pScTPI1, pScTDH3, pK1ADH1, pKlTPI1, pKlTDH3 or the like) derived from yeast such as  Saccharomyces cerevisiae,    Kluyveromyces  or the like. 
     In the present invention, the in-vitro protein synthesis system containing the fusion protein of the present invention can significantly enhance the in-vitro protein synthesis ability. In addition, the in-vitro protein synthesis system containing the combination of the fusion protein and the eIF4G protein of the present invention has higher in-vitro protein synthesis ability. 
     A kind of preferred yeast-based in-vitro protein synthesis systems was described in a prior application by the present inventors, CN201710125619.9, which is incorporated entirely herein by reference. The yeast-based in-vitro protein synthesis system in this document does not use the fusion protein described in the present invention. 
     Typically, the yeast-based in-vitro protein synthesis system of the present invention contains: (a) yeast cell extract; (b) optional polyethylene glycol; (c) optional exogenous sucrose; and (d) an optional solvent, wherein the solvent is water or an aqueous solvent; and (ii) the fusion protein according to the present invention. 
     In another preferred embodiment, the cell-free protein synthesis system also includes one or more components selected from the group consisting of:
     (e1) substrate for RNA synthesis;   (e2) substrate for proteins synthesis;   (e3) magnesium ions;   (e3) potassium ions;   (e5) a buffering agent (or translated as a buffer in the present invention);   (e6) RNA polymerase; and   (e7) an energy regeneration system.   

     In another preferred embodiment, in the protein synthesis system, the concentration of Component (e1) is 0.1-5 mM, preferably 0.5-3 mM, and more preferably 1-1.5 mM. 
     In another preferred embodiment, the yeast cell extract is an aqueous extract of yeast cells. 
     In another preferred embodiment, the yeast cell extract does not contain long-chain nucleic acid molecules endogenously from the yeast. 
     In another preferred embodiment, the substrate for RNA synthesis includes: nucleoside monophosphate, nucleoside triphosphate, and the combination thereof. 
     In another preferred embodiment, the substrate for protein synthesis includes: 1-20 kinds of natural amino acids, and unnatural amino acids. 
     In another preferred embodiment, the magnesium ions derive from a magnesium ion source, and the magnesium ion source is selected from the group consisting of: magnesium acetate, magnesium glutamate and the combination thereof. 
     In another preferred embodiment, the potassium ions derive from a potassium ion source, and the potassium ion source is selected from the group consisting of: potassium acetate, potassium glutamate and the combination thereof. 
     In another preferred embodiment, the energy regeneration system is selected from the group consisting of: a creatine phosphate/creatine phosphate kinase system, an energy system through a glycolytic pathway or the glycolytic intermediate metabolites, and the combination thereof. 
     In another preferred embodiment, the cell-free protein synthesis system also includes (f1) synthetic tRNA. 
     In another preferred embodiment, the buffering agent is selected from the group consisting of: 4-hydroxyethyl piperazineethanesulfonic acid, tris(hydroxymethyl)aminomethane (or trimethylolaminomethane), and the combination thereof. 
     In another preferred embodiment, the cell-free protein synthesis system also includes (g1) exogenous DNA molecules for guiding protein synthesis. 
     In another preferred embodiment, the DNA molecule is linear. 
     In another preferred embodiment, the DNA molecule is circular. 
     In another preferred embodiment, the DNA molecule contains a sequence encoding an exogenous protein. 
     In another preferred embodiment, the sequence encoding an exogenous protein can be a genomic sequence or a cDNA sequence. 
     In another preferred embodiment, the sequence encoding an exogenous protein also includes a promoter sequence, a 5′ untranslated sequence, or/and a 3′ untranslated sequence. 
     Main advantages of the present invention include as follows. 
     (a) The present invention disclosed, for the first time, the modification of genes in cells which thereby improved the protein synthesis efficiency of the in vitro translation system, with the help of a highly efficient cell transformation platform by using genetic engineering technique. 
     (b) The present invention disclosed, for the first time, a fusion protein, and the fusion protein of the present invention can significantly enhance the in-vitro protein synthesis ability. 
     (c) The present invention also disclosed for the first time that the insertion of a constitutive or inducible promoter (e.g., pScTEF1, pScPGK1, pKlTEF1, pKlPGK1, pScADH1, pScTPI1, pScTDH3, pKlADH1, pKlTPI1, pKlTDH3 or the like) in front of the eIF4G element can greatly improve the in-vitro protein synthesis ability. 
     (d) The present invention disclosed, for the first time, the modification of eIF4G through CRISPR-Cas9 gene editing technology, thereby greatly enhancing the in-vitro protein synthesis ability. 
     (e) The present invention disclosed for the first time that while the fusion protein of the present invention improved the efficiency of the in-vitro translation system, the expression level of the element eIF4G of the fusion protein of the present invention did not increase. 
     EXAMPLE 1 
     Theoretical Model for Improving Protein Synthesis Via Genetic Modification 
     In the present invention, the expression of translation initiation factors eIF4G and Pab 1 in  K lactis  is optimized by using CRISPR-Cas9 gene editing technology to improve the efficiency of the in-vitro cell-free translation system. 
     EXAMPLE 2 
     Modification of Translation Initiation Factors By Using CRISPR-Cas9 Technology to Improve the Efficiency of the In-Vitro Translation System. 
     2.1 Insertion of a Strong Promoter in Front of Translation Initiation Factor KleIF4G by using CRISPR-Cas9 technology 
     2.1.1 Retrieval of a KleIF4G Sequence and Determination of a CRISPR gRNA Sequence 
     The eIF4G is an important factor in the process of translation initiation. In the current report, there is no case in which the in-vitro translation activity has been improved by optimizing the expression of endogenous eIF4G by using gene editing technology. In the present invention, the expression of the translation initiation factor KleIF4G was modified by using CRISPR-Cas9 gene editing technology according to the theoretical model in Example 1 to improve the efficiency of the in-vitro cell-free translation system. 
     i. Based on the eIF4G gene sequence in  S. cerevisiae.  BLAST alignment analysis was carried out in the NCBI database to identify an eIF4G-homologous gene sequence in  Kluyveromyces lactis  which was termed as KleIF4G (which was located at 421863 . . . 424928 of chromosome A). The insertion of a segment of labeled DNA at the tail of this gene was taken as an example herein, and other target genes, insertion sites as well as insertion sequences can be manipulated in a similar way. 
     ii. The gRNA sequence was identified by searching for the adjacent PAM sequence (NGG) nearby the initiation codon of the KleIF4G gene. The principle for selecting the gRNA is that the GC content should be moderate (the standard in the present invention is that the GC content is 40-60%); and the existence of a poly T structure should be avoided. Finally, the optimized KleIF4G gRNA sequence determined by the present invention was CGGTTTTTCAAAGCAGATAT (SEQ ID NO.: 6) which was located at 424927 . . . 424936 of the chromosome A. 
     2.1.2 Construction of a Plasmid by Inserting a Strong Promoter in Front of KleIF4G Which was Mediated by CRISPR-Cas9 
     In order to achieve the over-expression of KleIF4G, promoters of pScTEF1, pScPGK1, pKlTEF1 and pKlPGK1 were inserted respectively in front of the KleIF4G gene by using CRISPR-Cas technology in the present invention. The methods for construction and transformation of the plasmid were described below. 
     i. Construction of CRISPR plasmid 
     PCR amplification was carried out by using primer PF1: CGGTTTTTCAAAGCAGATATGTTTAGAGCTAGAAATAGCAAGTTAAAATAAG GCTAGTCCG (SEQ ID NO.: 7), primer PR1: GCTCTAAAACATATCTGCTTTGAAAAACCGAAAGTCCCATTCGCCACCCG (SEQ ID NO.: 8) and a pCAS plasmid as template. 17 μL of the amplification product was mixed, 1 μL of DpnI and 2 μL of 10× digestion buffer were added, and then the mixture was incubated in a bath at 37° C. for 3 h. 10 μL of the DpnI-treated product was added into 100 μL of DH5a competent cells. The mixture was placed on ice for 30 min and heat shocked at 42° C. for 45 s, followed by the addition of 1 mL of LB liquid medium, and then incubated with shaking at 37° C. for 1 h. Thereafter, the mixture was coated onto a Kan-resistant LB solid medium, and then inverted culture was carried out at 37° C. until monoclonal colonies (also translated as monoclones in the present) grew out. Five monoclonal colonies were picked out and then cultured with shaking in an LB liquid medium. After being detected PCR-positive and confirmed by sequencing, the plasmid was extracted and stored, named pKM-CAS1.0-KleIF4G (refer to  FIG. 1 ). 
     ii. Construction and amplification of a donor DNA plasmid 
     In order to facilitate the storage and amplification of the linear donor DNA, in the present invention, the donor DNA was inserted into a pMD18 plasmid first, and then amplified via PCR to obtain the linear donor DNA sequence. 
     PCR amplification was carried out by using a  Kluyveromyces lactis  genomic DNA as template and with primer PF2: GAGCTCGGTACCCGGGGATCCTCTAGAGATAATAAAATTTCAACCTTTAAGCC ATTGA ATTTTACCATTACG (SEQ ID NO.: 9) and primer PR2: GCCAAGCTTGCATGCCTGCAGGTCGAC GATCTTGTTAGTAATCTCAACCTTCGCTGG (SEQ ID NO.: 10); PCR amplification was also carried out by using a pMD18 plasmid as template and with primer pMD18-F: ATCGTCGACCTGCAGGCATG (SEQ ID NO.: 11) and primer pMD18-R: ATCTCTAGAGGATCCCCGGG (SEQ ID NO.: 12). 8.5 μL of each of the two amplification products were mixed, 1 μL of DpnI and 2 μL of 10× digestion buffer were added, and then the mixture was incubated in a bath at 37° C. for 3 h. 10 μL of the DpnI-treated product was added into 100 μL of DH5a competent cells. The mixture was placed on ice for 30 min and heat shocked at 42° C. for 45 s, followed by the addition of 1 mL of LB liquid medium, and then incubated with shaking at 37° C. for 1 h. Thereafter, the mixture was coated onto an Amp-resistant LB solid medium and then inverted culture was carried out at 37° C. until monoclonal colonies grew out. Five monoclonal colonies were picked out and then cultured with shaking in an LB liquid medium. After being detected PCR-positive and confirmed by sequencing, the plasmid was extracted and stored, named pKM-KleIF4G-DD. 
     PCR amplification was carried out by using the pKM-KleIF4G-DD plasmid as template, with primer PF3: ATGGGCGAACCTACATCCGATC (SEQ ID NO.: 13) and primer PR3: ATCTGCTTTGAAAAACCGCTCTTTCTCTC (SEQ ID NO.: 14); PCR amplification (pScTEF1 promoter mediated amplification) was carried out by using an  S. cerevisiae  genomic DNA as template, and with primer PF4: AGAGAGAAAGAGCGGTTTTTCAAAGCAGATCCACACACCATAGCTTCAAAAT GTTTCTAC (SEQ ID NO.: 15) and primer PR4: TGGTTGCTGATCGGATGTAGGTTCGCCCATCTTAGATTAGATTGCTATGCTTTC TTTCTAATGAGC (SEQ ID NO.: 16); PCR amplification (pScPGK1 promoter mediated amplification) was carried out by using an  S. cerevisiae  genomic DNA as template, and with primer PF5: AGAGAGAAAGAGCGGTTTTTCAAAGCAGATAGACGCGAATTTTTCGAAGAA GTACC (SEQ ID NO.: 17) and primer PRS: AGCTTCAACAGCTGGTTGCTGATCGGATGTAGGTTCGCCCATTGTTTTATATTT GTTGTAAAAAGTAGATAATTACTTCCTTGATGATC (SEQ ID NO.: 18); PCR amplification (pK1TEF1 promoter mediated amplification) was carried out by using a  Kluyveromyces lactis  genomic DNA as template, and with primer PF6: AGAGAGAAAGAGCGGTTTTTCAAAGCAGATGAGCCTGTCCAAGCAAATGCC (SEQ ID NO.: 19) and primer PR6: TGGTTGCTGATCGGATGTAGGTTCGCCCATTTTTAATGTTACTTCTCTTGCAGT TAGGGAAC (SEQ ID NO.: 20); PCR amplification (pK1PGK1 promoter mediated amplification) was carried out by using a  Kluyveromyces lactis  genomic DNA as template, and with primer PF7: AGAGAGAAAGAGCGGTTTTTCAAAGCAGATGTTCCTCATCACTAGAAGCCGA ACTG (SEQ ID NO.: 21) and primer PR7: AGCTTCAACAGCTGGTTGCTGATCGGATGTAGGTTCGCCCATTTTTATTAATTC TTGATCGATTTTTTTGTTATTTCTGAAGTAACTCT (SEQ ID NO.: 22). The PF3/PR3 amplification product was mixed with PF4/PR4 amplification product, PF5/PR5 amplification product, PF6/PR6 amplification product and PF7/PR7 amplification product, respectively, to construct pKM-pScTEF1-KleIF4G-DD, pKM-pScPGK1-KleIF4G-DD, pKM-pKlTEF 1-KleIF4G-DD and pKM-pKlPGK1-KleIF4G-DD, respectively (according to  FIGS. 2, 3, 4 and 5 ). The specific steps include as follows: 8.5 μL of each of the two PCR amplification products were mixed, 1 μL of DpnI and 2 μL of 10× digestion buffer were added, and then the mixture was incubated in a bath at 37° C. for 3 h. 10 μL of the DpnI-treated product was added into 100 μL of DH5α competent cells. The mixture was placed on ice for 30 min and heat shocked at 42° C. for 45 s, followed by the addition of 1 mL of LB liquid medium, and then incubated with shaking at 37° C. for 1 h. Thereafter, the culture solution was coated onto an Amp-resistant LB solid medium, and then inverted culture was carried out at 37° C. until single clone colonies grew out. Five monoclonal colonies were picked out and then cultured with shaking in an LB liquid medium. After being detected PCR-positive and confirmed by sequencing, the plasmid was extracted and stored. 
     2.1.3 Transformation of  Kluyveromyces lactis  and Positive Identification 
     i.  A Kluyveromyces lactis  yeast solution was streaked and cultured on YPD solid medium. A single clone was picked out and cultured with shaking in 25 mL 2×YPD liquid medium overnight. 2 mL of the yeast solution was taken and continued to be cultured with shaking in 50 mL 2× YPD liquid medium for 2-8 h. Yeast cells were collected by centrifugation at 3000 g for 5 min at 20° C., resuspended with the addition of 500 μL of sterile water. Cells were collected by centrifugation under the same condition. A competent cell solution (5% v/v glycerol and 10% v/v DMSO) was prepared and the yeast cells were dissolved in 500 L of this solution. 50 μL aliquots were aliquoted into 1.5 mL centrifuge tubes and saved at −80° C. 
     Competent cells were thawed at 37° C. for 15-30 s, centrifuged at 13000 g for 2 min, and the supernatant was removed. Transformation buffer was prepared as follows: 260 μL of PEG3350 (50% (w/v)), 36 μL of LiAc (1.0 M), 20 μL of carrier DNA (5.0 mg/mL), 15 μL of Cas9/gRNA plasmid and 10 μL of donor DNA, with sterile water added until to a final volume of 360 μL. After heat shock, centrifugation was carried out at 13000 g for 30 s to remove the supernatant. 1 mL of YPD liquid medium was added and the mixture was incubated for 2-3 h. 200 μL of the mixture was pipetted and coated onto an YPD (200 μg/mL G418) solid medium and then incubated for 2-3 days until monoclonal colonies appeared. 
     ii. 10-20 monoclonal colonies were picked out from the plate with transformed  Kluyveromyces lactis  cells, and cultured with shaking in 1 mL YPD (200 μg/mL G418) liquid medium overnight. Corresponding samples were detected by PCR by using the yeast solution as template and with CRISPR Insertion Check primers. Strains that were positive for PCR and identified by sequencing, were determined as positive strains. 
     2.2 Fusion of KleIF4G with a High-Expressed Gene Via CRISPR-Cas9 Technology 
     2.2.1 Retrieval of a KlTDH3 Sequence and Determination of a CRISPR gRNA Sequence 
     In  S. cerevisiae,  TDH3 exists in a form of tetramer and is involved in catalytic reactions of the glycolytic pathway. Its promoter pTDH3 is a durable strong promoter widely used in genetic engineering. In order to achieve sufficient expression of KleIF4G in  Kluyveromyces lactis  and to form a locally high concentration when the function of translation initiation is being performed, the KleIF4G gene is linked to the 3′ end of the TDH3 gene ORF (open reading frame) of  Kluyveromyces lactis  in the present invention. 
     i. Based on the TDH3 gene sequence in  S. cerevisiae.  BLAST alignment analysis was carried out in the NCBI database to identify the TDH3-homologous gene sequences in  Kluyveromyces lactis.  After blasting, two TDH3-homologous genes were found in the  Kluyveromyces lactis  genome, which were named KlTDH3-1 (located at 1024297 . . . 1025292 of chromosome A) and KlTDH3-2 (located at 1960417 . . . 1961406 of chromosome F) respectively in the present invention. Inserting a segment of labeled DNA at the tail of this gene was taken as an example herein, and other target genes, insertion sites as well as insertion sequences can be manipulated in a similar way. 
     ii. PAM sequences (NGG) were searched for nearby the termination codon of the KlTDH3 gene, and KLTDH3 gRNA sequences were identified (KlTDH3-1: CTTGTTGCTAAGAACTAAAG (SEQ ID NO.: 23) located at 1024272 . . . 1024291 of chromosome A, and KlTDH3-2: CTCTGAAAGAGTTGTCGATT (SEQ ID NO.: 24) located at 1960378 . . . 1960397 of chromosome F). 
     2.2.2 Construction of a Plasmid by Integrating KleIF4G into the KlTDH3 Site, Which was Mediated by CRISPR-Cas9 
     Construction of CRISPR Plasmid 
     i. For the KlTDH3-1 sequence, PCR amplification was carried out by using a pCAS plasmid as template and with primer PF8: CTTGTTGCTAAGAACTAAAGGTTTTAGAGCTAGAAATAGCAAGTTAAAAT (SEQ ID NO.: 25) and primer PR8: GCTCTAAAACCTTTAGTTCTTAGCAACAAGAAAGTCCCATTCGCCACCCG (SEQ ID NO.: 26). 17 μL of the amplification product was mixed, 1 μL of DpnI and 2 of 10× digestion buffer were added, and the mixture was then incubated in a bath at 37° C. for 3 h. 10 μL of the DpnI-treated product was added into 100 μL of DH5a competent cells. The mixture was placed on ice for 30 min, and heat shocked at 42° C. for 45 s, followed by the addition of 1 mL of LB liquid medium, and then incubated with shaking at 37° C. for 1 h. Thereafter, the mixture was coated onto a Kan-resistant LB solid medium, and then inverted culture was carried out at 37° C. until monoclonal colonies grew out. Five monoclonal colonies were picked out and then cultured with shaking in an LB liquid medium. After being detected PCR-positive and being confirmed by sequencing, the plasmid was extracted and stored, named pKM-CAS1.0-KlTDH3-1 (refer to  FIG. 6 ). 
     ii. For the KlTDH3-2 sequence, PCR amplification was carried out by using a CAS plasmid as template and with primer PF9: CTCTGAAAGAGTTGTCGATTGTTTTAGAGCTAGAAATAGCAAGTTAAAAT (SEQ ID NO.: 27) and primer PR9: GCTCTAAAACAATCGACAACTCTTTCAGAGAAAGTCCCATTCGCCACCCG (SEQ ID NO.: 28). 17 μL of the amplification product was mixed, 1 μL of DpnI and 2 of 10× digestion buffer were added, and then the mixture was incubated in a bath at 37° C. for 3 h. 10 μL of the DpnI-treated product was added into 100 μL of DH5a competent cells. The mixture was placed on ice for 30 min and heat shocked at 42° C. for 45 s, followed by the addition of 1 mL of LB liquid medium, and then incubated with shaking at 37° C. for 1 h. Thereafter, the mixture was coated onto a Kan-resistant LB solid medium, and then inverted culture was carried out at 37° C. until monoclonal colonies grew out. Five monoclonal colonies were picked out and then cultured with shaking in an LB liquid medium. After being detected PCR-positive and being confirmed by sequencing, the plasmid was extracted and stored, namedpKM-CAS1.0-KlTDH3-2 (refer to  FIG. 7 ). 
     2.2.3 Construction and Amplification of a Donor DNA Plasmid 
     In order to facilitate the storage and amplification of the linear donor DNA, in the present invention, the donor DNA was inserted into a pMD18 plasmid first, and then amplified via PCR to obtain the linear donor DNA sequence. 
     i. For the KlTDH3-1, PCR amplification was carried out by sing a  Kluyveromyces lactis  genomic DNA as template and with primer PF10: GAGCTCGGTACCCGGGGATCCTCTAGAGATCATCCACTCCATCACCGCTACCC AA (SEQ ID NO.: 29) and primerPR10: GCCAAGCTGCATGCCTGCAGGTCGACGATCAACGTCCCCATCTACAAGAGC (SEQ ID NO.: 30); PCR amplification was carried out by using a pMD18 plasmid as template and with primer pMD18-F: ATCGTCGACCTGCAGGCATG (SEQ ID NO.: 31) and primer pMD18-R: ATCTCTAGAGGATCCCCGGG (SEQ ID NO.: 32). 8.5 μL of each of the two amplification products were mixed, 1 μL of DpnI and 2 μL of 10× digestion buffer were added, and then the mixture was incubated in a bath at 37° C. for 3 h. 10 μL of the DpnI-treated product was added into 100 μL of DH5α competent cells. The mixture was placed on ice for 30 min and heat shocked at 42° C. for 45 s, followed by the addition of 1 mL of LB liquid medium, and then incubated with shaking at 37° C. for 1 h. Thereafter, the mixture was coated onto an Amp-resistant LB solid medium, and then inverted culture was carried out at 37° C. until monoclonal colonies grew out. Five monoclonal colonies were picked out and then cultured with shaking in an LB liquid medium. After being detected PCR-positive and being confirmed by sequencing, the plasmid was extracted and stored, named pKM-K1TDH3-1-DD. 
     Amplification was carried out by using pKM-K1TDH3-1-DD as template and with primers PF 11: GATGCATTGATGGATGCCGAAGATGATTAAAGAGGTTGATGTAATTGATATTTT CCTGATAAAATTACTATTG (SEQ ID NO.: 33) and PR11: AGCTGGTTGCTGATCGGATGTAGGTTCGCCAGATCCACCTCCTTCCACGTTTG TTGGTCTTGATCCACCTCCACCGTTCTTAGCAACAAGTTCGACCAAATCG (SEQ ID NO.: 34); amplification was carried out by using a K lactis genomic DNA as template and with primers PF12: GGCGAACCTACATCCGATCAGC (SEQ ID NO.: 35) and PR12: TTAATCATCTTCGGCATCCATCAATGC (SEQ ID NO.: 36). 8.5 μL of each of the two amplification products, 1 μL of DpnI and 2 μL of 10× digestion buffer were mixed, and then the mixture was incubated in a bath at 37° C. for 3 h. 10 μL of the DpnI-treated product was added into 100 μL of DH5α competent cells. The mixture was placed on ice for 30 min and heat shocked at 42° C. for 45 s, followed by the addition of 1 mL of LB liquid medium, and then incubated with shaking at 37° C. for 1 h. Thereafter, the mixture was coated onto an Amp-resistant LB solid medium, and then inverted culture was carried out at 37° C. until monoclonal colonies grew out. Five monoclonal colonies were picked out and then cultured with shaking in an LB liquid medium. After being detected PCR-positive and being confirmed by sequencing, the plasmids was extracted and stored, named pKM-KlTDH3-1-F-KleIF4G-DD (refer to  FIG. 8 ). 
     Amplification was carried out by using the pKM-KlTDH3-1-F-KleIF4G-DD plasmid as template and with primers M13-F: GTAAAACGACGGCCAGT (SEQ ID NO.: 37) and M13-R: CAGGAAACAGCTATGAC (SEQ ID NO.: 38) to obtain the linear donor DNA. 
     ii. For the KlTDH3-2 sequence, PCR amplification was carried out by using a  Kluyveromyces lactis  genomic DNA as template and with primers PF13: GAGCTCGGTACCCGGGGATCCTCTAGAGATGAAGCTTTGATGACTACCGTTC (SEQ ID NO.: 39) and PR13: GCCAAGCTTGCATGCCTGCAGGTCGACGATGTCTATTGTATCGGAAGAACTGT CA (SEQ ID NO.: 40); PCR amplification was carried out by using a pMD18 plasmid as template and with primers pMD18-F: ATCGTCGACCTGCAGGCATG (SEQ ID NO.: 41) and pMD18-R: ATCTCTAGAGGATCCCCGGG (SEQ ID NO.: 42). 8.5 μL of each of the two amplification products were mixed, 1 μL of DpnI and 2 μL of 10× digestion buffer were added, and then the mixture was incubated in a bath at 37° C. for 3 h. 10 μL of the DpnI-treated product was added into 100 μL of DH5α competent cells. The mixture was placed on ice for 30 min and heat shocked at 42° C. for 45 s, followed by the addition of 1 mL of LB liquid medium, and then incubated with shaking at 37° C. for 1 h. Thereafter, the culture solution was coated onto an Amp-resistant LB solid medium, and then inverted culture was carried out at 37° C. until monoclonal colonies grew out. Five monoclonal colonies were picked out and then incubated with shaking in an LB liquid medium. After being detected PCR-positive and being confirmed by sequencing, the plasmid was extracted and stored, namedpKM-KlTDH3-2-DD. 
     Amplification was carried out by using the pKM-KlTDH3-2-DD as template and with primers PF14: GATGCATTGATGGATGCCGAAGATGATTAAATTACTCTTTTAAGTTAACGAACG CTTTTGATGAG (SEQ ID NO.: 43) and PR14: AGCTGGTTGCTGATCGGATGTAGGTTCGCCAGATCCACCTCCTTCCACGTTTG TTGGTCTTGATCCACCTCCACCAGCAACGTGCTCAACtAAgTCaACgACcCTTT CAGAGTAACCGTATTCGTTATCG (SEQ ID NO.: 44); amplification was carried out by using a  Kluyveromyces lactis  DNA as template and with primers PF15: GGCGAACCTACATCCGATCAGC (SEQ ID NO.: 45) and PR15: TTAATCATCTTCGGCATCCATCAATGC (SEQ ID NO.: 46). 8.5 μL of each of the two amplification products, 1 μL of DpnI and 2 μL of 10× digestion buffer were mixed, and then the mixture was incubated in a bath at 37° C. for 3 h. 10 μL of the DpnI-treated product was added into 100 μL of DH5α competent cells. The mixture was placed on ice for 30 min and heat shocked at 42° C. for 45 s, followed by the addition of 1 mL of LB liquid medium, and then incubated with shaking at 37° C. for 1 h. Thereafter, the culture solution was coated onto an Amp-resistant LB solid medium, and then inverted culture was carried out at 37° C. until monoclonal colonies grew out. Five monoclonal colonies were picked out and then cultured with shaking in an LB liquid medium. After being detected PCR-positive and being confirmed by sequencing, the plasmid was extracted and stored, named pKM-KlTDH3-2-F-KleIF4G-DD (refer to  FIG. 9 ). 
     Amplification was carried out by using the pKM-KlTDH3-2-F-KleIF4G-DD plasmid as template, and with primers M13-F: GTAAAACGACGGCCAGT (SEQ ID NO.: 47) and M13-R: CAGGAAACAGCTATGAC (SEQ ID NO.: 48) to obtain the linear donor DNA. 
     2.2.4 Transformation of  Kluyveromyces lactis  and positive identification 
     i.  A Kluyveromyces lactis  yeast solution was streaked and cultured on YPD solid medium. A single clone was picked out and cultured with shaking in 25 mL 2× YPD liquid medium overnight. 2 mL of the yeast solution was taken and continued to be cultured with shaking in 50 mL 2× YPD liquid medium for 2-8 h. Yeast cells were collected by centrifugation at 3000 g for 5 min under the condition of 20° C., resuspended with the addition of 500 μL of sterile water. Cells were collected by centrifugation under the same condition. A competent cell solution (5% v/v glycerol and 10% v/v DMSO) was prepared and the yeast cells were dissolved in 500 μL of this solution. 50 μL aliquots were aliquoted into 1.5 mL centrifuge tubes and saved at −80° C. 
     Competent cells were thawed at 37° C. for 15-30 s, centrifuged at 13000 g for 2 min, and the supernatant was removed. Transformation buffer was prepared as follows: 260 μL of PEG3350 (50% (w/v)), 36 μL of LiAc (1.0 M), 20 μL of carrier DNA (5.0 mg/mL), 15 μL of Cas9/gRNA plasmid and 10 μL of donor DNA, with sterile water added until the final volume was 360 μL. After heat shock, centrifugation was carried out at 13000 g for 30 s to remove the supernatant. 1 mL of YPD liquid medium was added and the mixture was incubated for 2-3 h. 200 μL of the culture solution was pipetted and coated onto an YPD (200 μg/mL G418) solid medium and then incubated for 2-3 days until monoclonal colonies appeared. 
     ii. 10-20 monoclonal colonies were picked out from the culture plate with transformed  Kluyveromyces lactis  cells, and cultured with shaking in 1 mL YPD (200 μg/mL G418) liquid medium overnight. PCR amplification was carried out by using the yeast solution as template, and by using CRISPR Insertion Check primer KlTDH3-1-CICF1 (the inner primer for the KlTDH3-1 sequence): CTTCTACTGCTCCAATGTTCGTCGTT (SEQ ID NO.: 49) and primer KlTDH3-2-CICF1 (the inner primer for the KlTDH3-2 sequence): TTAACGAAGACAAGTACAACGGTGA (SEQ ID NO.: 50), which are paired with primer KleIF4G-CICR2 (the inner primer for the KleIF4G sequence): TTCTCTTCGACAGCCTTCTTAGCAG (SEQ ID NO.:51), respectively. The insertion of KleIF4G at KlTDH3-1 and KlTDH3-2 sites was characterized. Strains with positive PCR results and identified by sequencing were determined as positive strains. 
     2.3 Fusion of KleIF4G with its Interacting Protein via CRISPR-Cas9 Technology 
     2.3.1 Retrieval of the KlPab1 Sequence and Determination of the CRISPR gRNA Sequence 
     As mentioned above, the Pab 1 protein and the eIF4G protein interact with each other in the translation initiation process. In the present invention, KlPab 1 and KleIF4G were fused by using CRISPR-Cas9 gene editing technology to promote the interaction between them and thus to improve the in-vitro translation efficiency. 
     Based on the Pab 1 sequence, a KlPab1 gene sequence (located at 1553322 . . . 1555100 of chromosome C) in  Kluyveromyces lactis  was obtained. PAM sequences (NGG) were searched for nearby the termination codon of the KlPab1 gene, and a gRNA sequence was identified. The principle for selecting a gRNA sequence is that the GC content should be moderate (the standard in the present invention is that the GC content is 40-60%); and the existence of a poly(T) structure should be avoided. Finally, the KlPab1 gRNA sequence determined by the present invention was TGCTTACGAAAACTTCAAGA (SEQ ID NO.: 52) which was located at 1555058 . . . 1555077 of the chromosome C. 
     2.3.2 Construction of a Plasmid with KleIF4G Integrated at KlPab1 Site Via CRISPR-Cas9 Technology 
     i. Construction of a CRISPR Plasmid 
     PCR amplification was carried out by using a pCAS plasmid as template and with primers PF16: TGCTTACGAAAACTTCAAGAGTTTTAGAGCTAGAAATAGCAAGTTAAAATAA GGCTAGTCCG (SEQ ID NO.: 53) and PR16: GCTCTAAAACTCTTGAAGTTTTCGTAAGCAAAAGTCCCATTCGCCACCCG (SEQ ID NO.: 54). 17 μL, of the amplification product was mixed, 1 μL of DpnI and 2 μL , of 10× digestion buffer were added, and then the mixture was incubated in a bath at 37° C. for 3 h. 10 μL, of the DpnI-treated product was added into 100 μL of DH5α competent cells. The mixture was placed on ice for 30 min and heat shocked at 42° C. for 45 s, followed by the addition of 1 mL of LB liquid medium, and then incubated with shaking for at 37° C. 1 h. Thereafter, the mixture was coated onto a Kan-resistant LB solid medium, and then inverted culture was carried out at 37° C. until monoclonal colonies grew out. Five monoclonal colonies were picked out and then cultured with shaking in an LB liquid medium. After being detected PCR-positive and confirmed by sequencing, the plasmid was extracted and stored, named pKM-CAS1.0-K1Pab1 (refer to  FIG. 10 ). 
     ii. Construction and amplification of a KlPab1-KleIF4G donor DNA plasmid 
     In order to facilitate the storage and amplification of the linear donor DNA, the donor DNA was inserted into a pMD18 plasmid first, and then amplified via PCR to obtain the linear donor DNA sequence. 
     PCR amplification was carried out by using a  Kluyveromyces lactis  genomic DNA as template and with primers PF17: GAGCTCGGTACCCGGGGATCCTCTAGAGATCCGGTAAGCCATTGTACGTTGCC AT (SEQ ID NO.: 55) and PR17: GCCAAGCTTGCATGCCTGCAGGTCGACGATCAGTATACCGTCCATGTTGATGA CT (SEQ ID NO.: 56); PCR amplification was carried out by using a pMD18 plasmid as template and with primers pMD18-F: ATCGTCGACCTGCAGGCATG (SEQ ID NO.: 57) and pMD18-R: ATCTCTAGAGGATCCCCGGG (SEQ ID NO.: 58). 8.5 μL of each of the two amplification products were mixed, 1 μL of DpnI and 2 μL of 10× digestion buffer were added, and then the mixture was incubated in a bath at 37° C. for 3 h. 10 μL of the DpnI-treated product was added into 100 μL of DH5α competent cells. The mixture was placed on ice for 30 min and heat shocked at 42° C. for 45 s, followed by the addition of 1 mL of LB liquid medium, and then incubated with shaking at 37° C. for 1 h. Thereafter, the mixture was coated onto an Amp-resistant LB solid medium, and then inverted culture was carried out at 37° C. until monoclonal colonies grew out. Five monoclonal colonies were picked out and then cultured with shaking in an LB liquid medium. After being detected PCR-positive and confirmed by sequencing, the plasmid was extracted and stored, named pKM-KlPab1-DD. 
     Amplification was carried out by using the pKM-KlPab1-DD as template and with primer PF18: GATGCATTGATGGATGCCGAAGATGATTAAACTTGATTTTTTGACCTTGATCTT CATCTTGTC (SEQ ID NO.: 59) and primer PR18: CTTGAACTTCATCTTGAGTTGAACCTCCACCTCCAGATCCACCTCCACCAGCT TGAGCTTCTTGTTCtTTtTTaAAaTTcTCGTAAGCAGCTAAGGCTTC (SEQ ID NO.: 60); amplification was carried out by using a  Kluyveromyces lactis  DNA as template and with primer PF19: GTGGAGGTTCAACTCAAGATGAAGTTCAAGGTCCACATGCTGGTAAGTCTAC TGTTGGTGGAGGTGGATCTGGCGAACCTACATCCGATCAGC (SEQ ID NO.: 61) and primer PR19: TTAATCATCTTCGGCATCCATCAATGC (SEQ ID NO.: 62). 8.5 of each of the two amplification products, 1 μL of DpnI and 2 μL of 10× digestion buffer were mixed, and then the mixture was incubated in a bath at 37° C. for 3 h. 10 of the DpnI-treated product was added into 100 μL of DH5α competent cells. The mixture was placed on ice for 30 min, treated by heat shock at 42° C. for 45 s, followed by the addition of LB liquid medium 1mL, and then incubated with shaking at 37° C. for 1 h. Thereafter, the mixture was coated onto an Amp-resistant LB solid medium, and then inverted culture was carried out at 37° C. until monoclonal colonies grew out. Five monoclonal colonies were picked out and then cultured with shaking in an LB liquid medium. After being detected PCR-positive and being confirmed by sequencing, the plasmid was extracted and stored, named pKM-KlPab1-KleIF4G-DD (refer to  FIG. 11 ). 
     Amplification was carried out by using the pKM-KlPab1-KleIF4G-DD plasmid as template and with primers M13-F: GTAAAACGACGGCCAGT (SEQ ID NO.: 63) and M13-R: CAGGAAACAGCTATGAC (SEQ ID NO.: 64) to obtain the linear donor DNA. 
     2.3.3 Transformation of  Kluyveromyces lactis  and Positive Identification 
     i.  A Kluyveromyces lactis  yeast solution was streaked and cultured on YPD solid medium. A single clone was picked out and cultured with shaking in 25 mL 2× YPD liquid medium overnight. 2 mL of the yeast solution was taken and continued to be cultured with shaking in 50 mL 2× YPD liquid medium for 2-8 h. Yeast cells were collected by centrifugation at 3000 g for 5 min under the condition of 20° C., resuspended with the addition of 500 μL of sterile water. Cells were collected by centrifugation under the same condition. A competent cell solution (5% v/v glycerol and10% v/v DMSO) was prepared and the yeast cells were dissolved in 500 μL of this solution. 50 μL aliquots were aliquoted into 1.5 mL centrifuge tubes and saved at −80° C. 
     Competent cells were thawed at 37° C. for 15-30 s, centrifuged at 13000 g for 2 min, and the supernatant was removed. Transformation buffer was prepared as follows: 260 μL of PEG3350 (50% (w/v)), 36 μL of LiAc (1.0 M), 20 μL of carrier DNA (5.0 mg/mL), 15 μL of Cas9/gRNA plasmid and 10 μL of donor DNA, with sterile water added to a final volume 360 μL. After heat shock, centrifugation was carried out at 13000 g for 30 s to remove the supernatant. 1 mL of YPD liquid medium was added and the mixture was incubated for 2-3 h. 200 μL of the culture solution was pipetted and coated onto an YPD (200 μg/mL G418) solid medium and then incubated for 2-3 days until monoclonal colonies appeared. 
     ii. 10-20 monoclonal colonies were picked out from the plate with transformed  Kluyveromyces lactis  cells, and cultured with shaking in 1 mL YPD (200 μg/mL G418) liquid medium overnight. PCR amplification was carried out by using the yeast solution as template and with primers: KlPAB1-CICF1 (the inner primer for the KlPAB1 sequence): TCTCCAGAAGAAGCTACCAAGGCTA (SEQ ID NO.: 65) and KleIF4G-CICR2 (the inner primer for the KleIF4G sequence): TTCTCTTCGACAGCCTTCTTAGCAG (SEQ ID NO.: 66). The insertion of KleIF4G at the KlPAB1 site was detected. Strains with positive PCR results and identified by sequencing were determined as positive strains. 
     EXAMPLE 3 
     Determination of the In-Vitro Translation Activity of the Modified Strains 
     The genetically modified  Kluyveromyces lactis  strains were prepared into in-vitro protein synthesis systems, and firefly luciferase (Fluc) gene as DNA template was added to detect the ability of protein translation of the modified strains. The above-said reaction systems were placed at room temperature 25-30° C., let stand and incubated for about 2-6 h. After the reaction was completed, equal volumes of substrate for Fluc (luciferin) were added into the 96-well white plates or 384-well white plates. The plate was immediately placed in an Envision 2120 multi-mode microplate reader (Perkin Elmer). Read values to obtain the Fluc activity, wherein, the unit for activity is relative light unit (RLU). 
     Among the modified structures, both the pKlPGK1:: KleIF4G structure formed by inserting the pKlPGK1 promoter in front of KleIF4G and the KlPab1-KleIF4G structure formed by fusing KleIF4G to the C-terminus of KlPab 1, showed stronger protein synthesis ability in vitro than that based on the wild-type yeast strain Y1140. The values of relative light units released by corresponding encoded Fluc protein reached 1.57×10 9  and 1.50×10 9 , respectively, while the value of relative light units of the Fluc protein synthesized in vitro based on the wild-type yeast strain Y1140 was only 4.11×10 8 . It indicates that the modification of KleIF4G can effectively enhance the protein synthesis efficiency of the yeast-based in vitro protein synthesis system (refer to  FIG. 12 ). 
     Specific effects are shown in Table 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Dilution 
                 Final activity 
               
               
                 NO. 
                 Data1 
                 Data2 
                 Data3 
                 Activity 
                 ratio 
                 (RLU) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 pKlPGK1_KleIF4G 
                 33216610 
                 28584890 
                 32598650 
                 31466717 
                 50 
                 1.57 × 10 9   
               
               
                 pKlTEF1_KleIF4G 
                 6685609 
                 10189150 
                 8594529 
                 8489763 
                 50 
                 4.24 × 10 8   
               
               
                 pScPGK1_KleIF4G 
                 4130719 
                 8605461 
                 4555399 
                 5763860 
                 50 
                 2.88 × 10 8   
               
               
                 pScTEF1_KleIF4G 
                 8230202 
                 6415045 
                 7578242 
                 7407830 
                 50 
                 3.70 × 10 8   
               
               
                 KlTDH3_1_KleIF4G 
                 788821 
                 751243 
                 941381 
                 827148.3 
                 50 
                 4.13 × 10 7   
               
               
                 KlTDH3_2_KleIF4G 
                 8676941 
                 6496592 
                 7904461 
                 7692665 
                 50 
                 3.85 × 10 8   
               
               
                 KlPAB1_KleIF4G 
                 22155330 
                 33507550 
                 34075530 
                 29912803 
                 50 
                 1.50 × 10 9   
               
               
                 Y1140 
                 10925600 
                 6729764 
                 6997436 
                 8217600 
                 50 
                 4.11 × 10 8   
               
               
                 NC 
                 707 
                 965 
               
               
                   
               
               
                 NC represents negative control. 
               
            
           
         
       
     
     The above experimental results show that the fusion protein of the present invention can significantly enhance the protein synthesis efficiency of the yeast-based in-vitro protein synthesis system through the related modification of the KleIF4G gene of  Kluyveromyces lactis.    
     All documents mentioned in the present invention are incorporated by reference in this application, as if each document is individually incorporated by reference. In addition, it should be understood that those skilled in the art, after reading the above-mentioned description, can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the claims attached to the Application.