Abstract:
The present invention provides a genetically engineered  Torulopsis glabrata  with enhanced extracellular secretion of pyruvic acid.  T. glabrata  strain was obtained from China Center for Type Culture Collection with CCTCC No: M202019 and over-expressed the optimized CutA (SEQ ID NO:2) encoding stress protein. Both of the temperature tolerance of  T. glabrata  and extracellular concentration of pyruvate were improved by overexpressing the optimized CutA. The optimum growth temperature of genetically engineered  T. glabrata  was increased too. The present invention can be widely used to increase extracellular levels of pyruvate during the fermentation process.

Description:
CROSS-REFERENCES AND RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority to Chinese Application No. 201310722490.1, entitled “A Genetically Engineered  Torulopsis glabrata  with Enhanced Extracellular Secretion of Pyruvic Acid and Its Application”, filed Dec. 24, 2013, which is herein incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to the field of metabolic engineering, and more particularly relates to a genetically engineered strain of  Torulopsis glabrata  with enhanced extracellular secretion of pyruvic acid. 
         [0004]    2. Description of the Related Art 
         [0005]    As one of the important oxo carboxylic acid, pyruvic acid is the precursor of many helpful chemical compounds and plays a key role in bioenergy metabolism. It is also widely used in chemical, pharmaceutical and agrochemical industries and scientific research. Industrial production of pyruvic acid has been achieved through both chemical methods and biological fermentation. With regard to large scale application of the chemical method, high raw material costs and low productivity are limitations. The bio-fermentation process is complex and is easily affected by different growth parameters such as temperature and the presence of metal ions, which can inhibit cell growth and metabolism. Therefore, enhancing microbial tolerance to those unfavorable growth parameters may help to improve pyruvate yield. 
       DETAILED DESCRIPTION 
       [0006]    The goal of the present invention is to provide a genetically engineered  Torulopsis glabrata  (CCTCC No: M202019) with high levels of extracellular pyruvate production, which over-expresses CutA encoding stress protein. 
         [0007]    The nucleotide sequence of CutA is set forth in SEQ ID NO: 2 as follows: 
         [0000]    
       
         
               
               
               
             
           
               
                 ATGATCATCG TTTACACTAC TTTCCCAGAC TGGGAATCTG CTGAAAAGGT TGTTAAGACT 
                 60 
                   
               
               
                   
               
               
                 TTGTTGAAGG AAAGATTGAT CGCTTGTGCT AACTTGAGAG AACACAGAGC TTTCTACTGG 
                 120 
               
               
                   
               
               
                 TGGGAAGGTA AGATCGAAGA AGACAAGGAA GTTGGTGCTA TCTTGAAGAC TAGAGAAGAC 
                 180 
               
               
                   
               
               
                 TTGTGGGAAG AATTGAAGGA AAGAATCAAG GAATTGCACC CATACGACGT TCCAGCTATC 
                 240 
               
               
                   
               
               
                 ATCAGAATCG ACGTTGACGA CGTTAACGAA GACTACTTGA AGTGGTTGAT CGAAGAAACT 
                 300 
               
               
                   
               
               
                 AAGAAGTAAG 
                 310 
               
             
          
         
       
     
         [0008]    There are two methods to construct the genetically engineered strain. 
         [0009]    One of the methods for constructing the genetically engineered strain comprises the following steps: 
         [0000]    (1) Optimizing the CutA derived from  Pyrococcus horikoshii  (SEQ ID NO:1) to obtain SEQ ID NO:2 with codons adapted for usage in  T. glabrata . A codon adaptation tool, Jcat (http://www.jcat.de), was used to substitute synonymous codons of the CutA sequence of  Pyrococcus horikoshii  to optimize the gene for heterologous expression in  T. glabrata.  
 
The nucleotide sequence of SEQ ID NO:1 is as follows:
 
         [0000]                            ATGATAATAG TTTACACGAC TTTTCCGGAC TGGGAGAGTG CTGAGAAAGT TGTGAAAACT   60                   CTTTTAAAAG AGAGGTTGAT TGCATGCGCA AATTTAAGGG AGCACAGGGC CTTTTACTGG   120               TGGGAAGGTA AGATCGAGGA AGATAAAGAA GTTGGAGCTA TCCTTAAAAC TAGGGAAGAT   180               CTGTGGGAAG AACTTAAGGA AAGGATAAAG GAGCTTCATC CTTACGATGT TCCGGCCATA   240               ATCAGGATTG ACGTTGATGA TGTTAACGAG GATTACCTCA AATGGTTAAT TGAAGAGACG   300               AAAAAATGAG   310            
(2) Constructing a recombinant expression plasmid: synthesize SEQ ID NO:2 by total chemical synthesis; digest the CutA and the plasmid pRS306TEF1 (EUROSARF, Frankfurt, Germany) at the same time using restriction enzyme BamHI and EcoRI, and ligate the digested fragments to obtain a recombinant expression plasmid pRS306TEF1-CutA;
 
(3) Transforming the recombinant expression plasmid pRS306TEF1-CutA into  T. glabrata  (CCTCC No: M202019) ΔURA3 (a  T. glabrata  (CCTCC No: M202019) strain with URA3 gene being disrupted (Zhou, J., et al. (2009). “A reusable method for construction of non-marker large fragment deletion yeast auxotroph strains: A practice in  Torulopsis glabrata.” Journal of Microbiological Methods  76(1): 70-74) by electroporation method, and screening for positive transformants  T. glabrata  Q1 with YNB medium. Since  T. glabrata  ΔURA3 is inable to synthesize uracil itself, it can not grow on YNB medium without exogenous uracil. Only positive transformant cells with plasmid pRS306TEF1 can synthesize uracil, and therefore is selected to grow on YNB medium without added uracil.
 
         [0010]    The other method for constructing the genetically engineered strain comprises the following steps: 
         [0000]    (1) Optimizing the CutA derived from  Pyrococcus horikoshii  (SEQ ID NO:1) to obtain SEQ ID NO:2;
 
(2) Constructing a recombinant expression plasmid: synthesize SEQ ID NO:2 by total chemical synthesis; digest the CutA and the plasmid pRS306TEF1 at the same time using restriction enzyme BamHI and EcoRI, and ligate the digested fragments to obtain a recombinant expression plasmid pRS306TEF1-CutA; synthesize promoter sequence (SEQ ID NO:5) of the gene encoding heat shock protein HSP150 derived from  Saccharomyces cerevisae  by total chemical synthesis; digest the promoter sequence and the plasmid pRS306TEF1-CutA at the same time using restriction enzyme SacI and XhoI and connect the digested fragments to obtain a recombinant expression plasmid pRS306-HSP150-CutA;
 
(3) Transforming the recombinant expression plasmid pRS306HSP150-CutA into  T. glabrata  (CCTCC No: M202019) ΔURA3 by electroporation method, and screening positive transformants  T. glabrata  Q2 with YNB medium.
 
         [0011]    The genetically engineered strain containing the recombinant expression plasmid is inoculated into a 250 mL flask containing 25 mL seed culture medium, and cultured at 28° C., 200 rpm for 24 hours. The cultured cells were inoculated into 3 L fermentor containing 1.5 L medium with an inoculum size of 10% (v/v), and cultured at 30° C., 400 rpm with an aeration rate of 4 vvm. pH and DO were maintained by feeding 8 mol·L −1  NaOH and 2 mol·L −1  HCl with automatic pump. 
         [0012]    Compared with a control group without expressing CutA, the extracellular concentration of pyruvate of the recombinant strain  T. glabrata  Q1 expressing CutA at 33° C. or 36° C.; the biomass increased 12.4% and 20.7% respectively; the extracellular concentration of pyruvate of the recombinant strain  T. glabrata  Q1 increased from 56.8 g/L to 74.2 g/L. 
         [0013]    The present invention provides a method for enhancing temperature tolerance of  T. glabrata  and extracellular concentration of pyruvate by overexpressing the optimized CutA gene. The optimum growth temperature of  T. glabrata  was also increased. 
         [0014]    The nucleotide sequence of SEQ ID NO:5 is as follows: 
         [0000]    
       
         
               
               
             
           
               
                    1 acggtgaaag gtataacagc gagaccgaat gaggtccggt actctgttgt gccaccatcc 
                   
               
               
                   
               
               
                   61 attttagagc ttggagttaa agttgcccag ggttcaggtg gagcgtacgc agttagcgca 
               
               
                   
               
               
                  121 atactactta atagagatgc tagaaatgct ttcttgtaat gcatattggg gcggtgttca 
               
               
                   
               
               
                  181 cttcttcagt aagttgtata gaattaaaaa atcgagacta aacagaaatt tcgaaacaag 
               
               
                   
               
               
                  241 aagataagta tatgtataca agttatttta ctcttttaac tgttttcgtt tccccactct 
               
               
                   
               
               
                  301 ttcgcaaaaa gaataatttt gtgagtcaaa acagtcgaat tttcttgcac aaaaagtcga 
               
               
                   
               
               
                  361 tattcttttg tgactctgtg taacttactt tctacagctt ttgttactcc acttctctta 
               
               
                   
               
               
                  421 tatataccca atgattcact cgtaatatat cagaggaacg ttagaatttc tgtattttct 
               
               
                   
               
               
                  481 caggaatcct acaaaattag aggtgaaatt gatcaggtta aggagcagaa gcccactata 
               
               
                   
               
               
                  541 taagtaagat gatatcaagc ctagttggat gccttcagga acggcaaata ggatatgtga 
               
               
                   
               
               
                  601 gcttggcaaa ggggttttga tagagtaaca atagatcctt gctagccaat gcgacgttcc 
               
               
                   
               
               
                  661 tctgtagttt ttaccacagt tttacttaat ttcgtgcttt gtcccttttt ggcaaaaaag 
               
               
                   
               
               
                  721 tcggataatg tcttcactat atttgtgttt tcgtctttta ggaaaagcgt actttagtaa 
               
               
                   
               
               
                  781 actacaaatt tgagctgagc taatttcgga ttagtaaaag atgcaaggaa cagtaaaaat 
               
               
                   
               
               
                  841 taaaatagcg attgaccgaa atcagtccaa aaattactac aacaagcaaa tccaattaaa 
               
               
                   
               
               
                  901 aacagtaaaa tactttaagt gcatttaccg ttacgaacaa catatgtcct agttagtata 
               
               
                   
               
               
                  961 gtagtctact aagtagcaaa aaagtagaaa ctctgtcact ataccagcca tcagttcata 
               
               
                   
               
               
                 1021 gtaaaacagt aaaaattgta aaaacaagca aaaaaaaaac aagaaggaac aaatgcacca 
               
               
                   
               
               
                 1081 aactgttgat ctattctgca aaaaaaagta tggtaaattt tttccattat cctggccgct 
               
               
                   
               
               
                 1141 aatccatatg gaggtgaact tagaacttgc acaaggatgc gatgaatgat aggctttgtg 
               
               
                   
               
               
                 1201 ctataattaa tgcaggcagg tccgccatgt ccaacacgtt gctggccgca aaacgagtca 
               
               
                   
               
               
                 1261 atctcactgc tttgccacgc tcatttctcc ccccttctgc ccaattaggc gaccctcaca 
               
               
                   
               
               
                 1321 atgcacatac acatttccca cctctattgg aaggggccgt aaatggtaat tcttgggagt 
               
               
                   
               
               
                 1381 tattcatatt aagtgatctt actatttcct atttcggaaa ttattaaaga caaaaaagct 
               
               
                   
               
               
                 1441 cattaatggc tttccgtctg tagtgataag tcgccaactc agcctaattt ttcatttctt 
               
               
                   
               
               
                 1501 taccagatca ggaaaactaa tagtacaaat gagtgttttc tcaagcggaa caccacattt 
               
               
                   
               
               
                 1561 tgagctaaat ttagattttg gtcaaaataa gaaagatcct 
               
             
          
         
       
     
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1 . Effects of expressing CutA on temperature tolerance of  T. glabrata . A, cell growth at different temperatures for  T. glabrata  C. ▪: 30° C., •: 33° C., ▴: 36° C., ♦: 39° C.; B, cell growth at different temperatures for  T. glabrata  Q1. ▪: 30° C., •: 33° C., ▴: 36° C., ♦: 39° C. 
           [0016]      FIG. 2 . Temperature-induced regulation of CutA heterologous expression.  T. •: glabrata  Q1 , ▪: T. glabrata  C. 
           [0017]      FIG. 3 . Effects of high temperature on pyruvate production in  T. glabrata  strains; ▪:  T. glabrata  C; •:  T. glabrata  Q1. 
       
    
    
     EXAMPLES 
     Materials and Methods 
       [0018]    YPD medium: 5 g·L −1  yeast extract, 10 g·L −1  peptone, 20 g·L −1  dextrose. To make solid medium, add 20 g·L −1  Agar. 
         [0019]    YNB medium: 20 g·L −1  dextrose, 1.7 g·L −1  yeast nitrogen base, and 5 g·L −1  (NH 4 ) 2 SO 4 , adjust pH to 5.0 with 2 mol·L −1  NaOH. To make solid medium, add 20 g·L −1  Agar. 
         [0020]    Seed medium: 20 g·L −1  glucose, 10 g·L −1  peptone, 1 g·L −1  KH 2 PO 4 , 0.5 g·L −1  MgSO 4 .7H 2 O, adjust pH to 5.5 with HCl. To make solid medium, add 20 g·L −1  agar. The sterilization was performed at 115° C. for 15 minutes. 
         [0021]    Fermentation medium: 100 g·L −1  glucose, 7 g·L −1  NH 4 Cl, 5 g·L −1  KH 2 PO 4 , 0.8 g·L −1  MgSO 4 .7H 2 O, 6 g·L −1  Sodium Acetate, 4×10 −3  g·L −1  Nicotinic acid, 30×10 −6  g·L −1  Thiamin hydrochloride, 100×10 −6  g·L −1  Niacin Pyridoxine, 10×10 −6  g·L −1  Biotin, 50×10 −6  g·L −1  Riboflavin. The vitamins were filtrated for sterilization. 
         [0022]    The  Torulopsis glabrata  was obtained from China Center for Type Culture Collection (CCTCC) with CCTCC No: M202019, which is located at Wuhan University, Luojia Shan, Wuhan, Hubei, 430072. 
         [0023]    Determination of extracellular keto acid concentration: fermentation samples were centrifuged at 12000 g for 5 minutes. The supernatant was diluted 50 times with ultrapure water, and keto acid concentration of the sample was determined using HPLC. 
         [0024]    Determination of intercellular keto acid concentration: cells were collected by centrifugation, and washed by 0.9% physiological saline. Cell were resuspended in 10 mL buffer solution containing 0.1 mol·L −1  KH 2 PO 4 —K 2 HPO 4 , 1 mmol·L −1  EDTA, 0.01 mmol·L −1  DTT (pH 7.5). After addition of one volume of acid-washed quartz sand, cells were disrupted by a vortex mixer for 5 minutes, and centrifuged at 13,000 g for 10 minutes to remove the precipitation. 5 ml supernatant was filtered through a membrane with a pore size 0.22 nm. The concentration of keto acid in the supernatant was then measured using HPLC. 
         [0025]    Conditions for HPLC analysis: pyruvate was simultaneously determined by HPLC (Agilent 1200 series, Santa Clara, Calif.) with a Aminex HPX-87H ion exchange column (300 mm×7.8 mm; Bio-Rad Laboratories Inc., Hercules, Calif.). The mobile phase was 5 mmol·L −1  sulfuric acid in distilled, de-ionized water filtered through a 0.22 lam pore size membrane. The mobile phase flow rate was 0.6 mL·min −1  The column temperature was maintained at 35° C., and the injection volume was 10 μL. The pyruvate was detected by UV (wavelength at 210 nm) detector. 
         [0026]    Transformation of  Torulopsis glabrata : A freshly grown single colony of  T. glabrata  ΔURA3 cells were transferred into liquid YPD medium and cultured at 28° C., 200 rpm overnight. The  T. glabrata  ΔURA3 cells were transferred into new liquid YPD medium by an inoculum size of 10% (v/v), cultured at 28° C., 200 rpm until the OD 600 =1.2. The cells were collected by centrifugation, and resuspended at 8×10 8  cells/mL in 8 mL buffer solution (100 mmol·L −1  LiAc, 10 mmol·L −1  DTT, 0.6 mol·L −1  sorbitol 10 mmol·L −1  Tris-HCL, pH=7.5) and incubated at 30° C. for 30 minutes. Collect cells again by centrifugation and wash the cells by ice-chilled 5 mL 1 mol·L −1  sorbitol solution three times, and resuspend cells to the concentration of 10 10  cell mL −1  in the sorbitol solution. 1 μg purified recombinant plasmid pRS306TEF1-CutA was added to the cell suspension, incubated on ice for 5 min, and transferred to a ice-chilled 0.2-cm electric rotor. The electroporation shock was performed at 2.5 KV, 25 μF, 200Ω, and 1 mL ice-chilled 1 M sorbitol solution was immediately added afterwards. The mixture was incubated at room temperature for 1 hour. 0.2 mL cells, which have been electrically shocked, were spread on the selective culture plates containing YNB medium, and cultured at 28° C. for 96-144 hour. The YNB medium is a basic medium containing no uracil and is used as a selective medium. The  T. glabrata  ΔURA3 host cells lacks a functional URA3 gene, therefore unable to grow on YNB media. The plasmid pRS306TEF1 harbors a URA3 gene, enabling positive transformants to grow on YNB media without exogenous uracil. The sequence of pRS306TEF1 can be find in the this references (Sikorski, R. S. and P. Hieter (1989). “A system of shuttle vectors and yeast host strains designed for efficient manipulation of DNA in  Saccharomyces cerevisiae .” Genetics 122(1): 19-27). The heterologous expression of CutA was verified by PCR using primer P1/P2 (Table 1) and genomic DNAs of recombinant  T. glabrata  cells. The recombinant strain expressing CutA was named  T. glabrata  Q1, while the recombinant strain expressing control plasmid pRS306TEF1 was named  T. glabrata  C. 
         [0027]    The promoter sequence (SEQ ID NO:5) of the gene encoding heat shock protein HSP150 (GeneID: 853281) derived from  Saccharomyces cerevisae  was synthesized by total chemical synthesis. Digest the HSP150 promoter sequence and the plasmid pRS306TEF1-CutA at the same time using restriction enzyme SacI and XhoI to replace promoter TEF1 in plasmid pRS306TEF1, and ligate the digested fragments to obtain a recombinant expression plasmid pRS306-HSP150-CutA, where CutA gene is under control of HSP150 promoter. Transform the recombinant expression plasmid pRS306-HSP150-CutA into  T. glabrata  (CCTCC No: M202019) ura3 by electroporation method, and screen positive transformants  T. glabrata  Q2 with YNB medium. The heterologous expression was verified by PCR using primer P1/P2 (Table 1) and genome of original  T. glabrata . The recombinant strain expressing CutA was named  T. glabrata  Q2. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Primer sequence 
               
             
          
           
               
                   
                 Primers 
                 Primer sequence (5′-3′) 
               
               
                   
               
               
                   
                 P1 (SEQ ID NO: 3) 
                 CTTTCCCAGACTGGGAATCTG 
               
               
                   
               
               
                   
                 P2 (SEQ ID NO: 4) 
                 TTAGTGGTGGTGGTGGTGG 
               
               
                   
               
             
          
         
       
     
       Example 1 
     Comparison of Cell Growth of Recombinant Strain  T. glabrata  Q1 and  T. glabrata  C Under Different Temperatures 
       [0028]    Freshly grown single colonies of  T. glabrata  C and  T. glabrata  Q1 were transferred into 250 mL flasks containing 25 mL liquid YPD medium and cultured at different temperatures: 30° C., 33° C., 36° C. and 39° C., 200 rpm. Dry cell weight (DCW) was measured every 4 hours ( FIG. 1 ). 
         [0029]    As it was shown in  FIG. 1  that when the strains were cultured at 30° C., there was no obvious difference between cell growth of  T. glabrata  C and  T. glabrata  Q1. When the strains were cultured at 36° C. and 39° C., cell growth of  T. glabrata  C was strongly inhibited and its maximum DCW dropped to 87.4% and 63.6% compared with that obtained at 30° C. When the strains were cultured at 33° C. and 36° C., cell growth of  T. glabrata  Q1 was promoted and its maximum DCW increased to 112.4% and 120.7% compared with that obtained at 30° C. However, cell growth of  T. glabrata  Q1 was also slightly inhibited at 39° C. and the maximum DCW dropped to 96.2% compared with that obtained at 30° C. 
       Example 2 
     Temperature-Induced Regulation of CutA Heterologous Expression 
       [0030]    To verify the effects of CutA over-expression on temperature tolerance of  T. glabrata , freshly grown single colonies of  T. glabrata  C and  T. glabrata  Q2 were transferred into 250 mL flask containing 25 mL liquid YPD medium and cultured at 30° C., 200 rpm for 8 hours. Afterwards, the culture temperature was increased to 37° C. to induce the expression of CutA for 16 hours in  T. glabrata  Q2. Dry cell weight (DCW) was measured every 4 hours ( FIG. 2 ). 
         [0031]    As shown in  FIG. 2 , when the strains were cultured at 30° C., there was no obvious difference between cell growth of  T. glabrata  C and  T. glabrata  Q2. However, when the strains were cultured at 37° C., the cell growth of  T. glabrata  C was strongly inhibited while the cell growth of  T. glabrata  Q2 was increased. 
       Example 3 
     Effects of High Temperature on Pyruvate Synthesis by Recombinant Strains 
       [0032]    Fresh  T. glabrata  Q1 cultures were transferred from solid slant to 500-mL flasks containing 50 mL liquid YPD medium and cultured at 30° C., 200 rpm for 24 hours. The cultured cells were inoculated into fermentation medium with an inoculum size of 10% (v/v), and cultured at 36° C., 200 rpm for 72 hours.  T. glabrata  C cells were inoculated at a rate of 10% (v/v), and was cultured at 30° C., 200 rpm for 72 hours. The comparison of extracellular pyruvate levels of  T. glabrata  C and Q1 was made under their respective optimum culture temperatures (30° C. for  T. glabrata  C and 36° C. for  T. glabrata  Q1). Extracellular concentration of pyruvic acid in the fermentation medium was measured as described above. As shown in  FIG. 3 , compared with  T. glabrata  C, extracellular concentration of pyruvate of  T. glabrata  Q1 increased from 56.8 g·L −1  to 74.2 g·L −1 . 
         [0033]    While the present invention has been described in some detail for purposes of clarity and understanding, one skilled in the art will appreciate that various changes in form and detail can be made without departing from the true scope of the invention. All figures, tables, appendices, patents, patent applications and publications, referred to above, are hereby incorporated by reference.