Abstract:
This invention provides use of a series of recombinant  Thermoanaerobacterium saccharolyticum  glucose isomerases with improved catalytic activity obtained by using recombinant techniques. These mutants comprise at least one amino acid variation at position 87, position 139, position 182, position 187, position 217, position 260, position 276, or position 299, and can be used in the conversion of hemicellulose to ethanol.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to molecular biology, and specifically relates to recombinant glucose isomerases with improved activity or both of improved activity and thermostability, the method of preparing the same using recombinant techniques, and use of the same. 
       BACKGROUND OF THE INVENTION 
       [0002]    Glucose isomerase (E.C.5.3.1.5 or xylose isomerase) is a key enzyme in the pentose phosphate pathway. It is one of the most important industrial enzymes (Kaneko et al.,  Bioscience, Biotechnology, and Biochemistry  2000, 64:940-947). In bio-energy industry, people are trying to use the enzyme, together with other key enzymes in the degradation pathway of cellulose and hemicelluloses, to produce bioethanol commercially. For example, the pathway of the enzymatic degradation of xylan to xylulose-5-phosphate is as follows: xylan is converted into xylo-oligosaccharides in the presence of β-1,4-xylanase, xylo-oligosaccharides is further turned into xylose by β-xylosidase. Then, xylose is converted to xylulose by glucose isomerase (xylose isomerase) and xylulose is converted to xylulose-5-phosphate by β-xylulokinase. 
         [0003]    Under the pressure of environmental pollution and the shortage of oil, many countries in the world, such as Brazil, the United States and Canada have been using sugar cane or corn as starting materials to produce fuel ethanol. China has also been using corn or other crops to produce bioethanol. As biomass is very rich in the world, scientists in different countries are exploring ways to use cellulose- and hemicellulose-rich agricultural residues to produce bioethanol industrially. Glucose isomerase is a key enzyme in the biodegradation of hemicellulose. With high activity, or with both of high activity and thermostability glucose isomerase can efficiently reduce the cost of bioethanol production to facilitate industrial production of bioethanol. 
         [0004]    Scientists around the world have been working on the identification of thermostable and highly active glucose isomerases from thermophilic bacteria, and production of the same via protein engineering. J. G. Zeikus and his collaborators isolated and studied thermostable glucose isomerases from thermophilic bacteria, such as  Thermoanaerobacterium saccharolyticum  and  Thermotoga neapolitana  (Lee et al.,  Journal of General Microbiology,  139:1227-1234, 1993; Vieille et al.,  Methods in Enzymology,  330:215-24, 2001; Lee et al.,  Journal of General Microbiology,  139:1241-1243, 1993; Scriprapundh et al.,  Protein Engineering,  13:259-265, 2000; Scriprapundh et al.,  Protein Engineering,  16:683-690, 2003; Zeikus et al., U.S. Pat. No. 5,656,497). Nevertheless, the thermostabilities of the thermostable glucose isomerases from these and other bio-resources are still much to be desired as the activities thereof are low, and thus are not applicable to industrial applications. Therefore, glucose isomerase with high activity, or high activity and thermostability remains desirable. 
       SUMMARY OF THE INVENTION 
       [0005]    The inventors of the present invention have studied the activity of a series of mutants obtained by genetic and protein engineering of  Thermoanaerobacterium saccharolyticum  glucose isomerase. Thus, the present invention provides the use of glucose isomerase mutants with improved catalytic activity in the conversion of D-xylose to D-xylulose under high temperature. 
         [0006]    The inventors of the present invention have introduced mutations, by site-directed mutagenesis, into the  T. saccharolyticum  glucose isomerase gene and obtained a series of highly active or highly active and thermostable glucose isomerase mutants after screening on MacConkey agar. More specifically, the molecular biotechniques being used to generate glucose isomerase mutants include: construction of plasmid carrying the wild-type glucose isomerase gene; design of mutation sites and the mutated amino acids; design of appropriate primers; PCR amplification of DNA fragments using the wild-type glucose isomerase gene or its derivative as template; assembly of the DNA fragments; PCR amplification of the full-length glucose isomerase genes containing the mutation(s); cloning of the mutant genes into appropriate vectors; transformation of the vectors containing the genes into appropriate host cells; screening of the transformants for clones carrying desired glucose isomerase mutants; isolation of the plasmid DNA from the positive clones; and carrying out DNA sequencing to verify the mutations. Finally, the activity of the mutated isomerase is assessed using D-xylose as substrate, and mutated isomerase with higher catalytic activity than that of the wild-type is selected. The selected isomerase is used in conversion of D-xylose to D-xylulose, degradation of hemicellulose and the production of bioethanol. 
         [0007]    For the preparation of the novel glucose isomerases in this invention, suitable vectors include but are not limited to prokaryotic expression vectors pGEMT-Easy, pRSET and pET21; include but are not limited to eukaryotic expression vectors pYD1 and pYES2/GS; include but are not limited to cloning vectors pUC18/19 and pBluescript-SK. 
         [0008]    For the preparation of the glucose isomerase mutants in this invention, the mutated glucose isomerase gene can be expressed intra-cellularly in prokaryotic or eukaryotic cells, or can be expressed extra-cellularly in prokaryotic or eukaryotic cells by using any other techniques known in the art. 
         [0009]    For the preparation of the novel glucose isomerases in this invention, the host cells can be prokaryotic or eukaryotic cells. The prokaryotic cells include but are not limited to  E. coli, Bacillus subtilis, Bacillus brevis, Bacillus megaterium  (e.g.  B. megaterium  BP931),  T. saccharolyticum  and  Streptomyces  (erg.  S. diastaticus  M1033). The eukaryotic cells include but are not limited to  Saccharomyces cerevisiae  and  Pichia pastoris  (e.g.  P. pastoris  GS115/9891). 
         [0010]    The glucose isomerase mutants according to this invention, using Sequence 2 in the Sequence Listing as the reference sequence, comprise at least one mutation, or at least two mutations, or at least three mutations, or at least four mutations, or at least five mutations, or at least six mutations or at least seven mutations at position(s) selected from 87, 139, 182, 187, 217, 260, 276 and 299. And the glucose isomerase mutants have a catalytic activity at least 50% higher than the wild-type in the reaction of xylulose generation using D-xylose as substrate. The glucose isomerase mutants can be used in the degradation of hemicellulose and the production of bioethanol. 
         [0011]    Using Sequence 2 in the Sequence Listing as reference, preferably, the amino acid phenylalanine (Phe) at position 87 is mutated to methionine (Met), or leucine (Leu); tryptophan (Trp) at position 139 is mutated to serine (Ser), or lysine (Lys), or cysteine (Cys), or isoleucine (Ile), or threonine (Thr), or asparagine (Asn), or aspartic acid (Asp); arginine (Arg) at position 182 is mutated to alanine (Ala), or proline (Pro), or serine (Ser), or isoleucine (Ile), or threonine (Thr), or valine (Val); valine (Val) at position 187 is mutated to glycine (Gly), or alanine (Ala), or proline (Pro); valine (Val) at position 217 is mutated to arginine (Arg), or glycine (Gly); aspartic acid (Asp) at position 260 is mutated to glutamic acid (Glu), or alanine (Ala); phenylalanine (Phe) at position 276 is mutated to glycine (Gly), or threonine (Thr); and/or threonine (Thr) at position 299 is mutated to isoleucine (Ile), or tyrosine (Tyr), or cysteine (Cys), or methionine (Met), or glutamic acid (Glu). 
         [0012]    More preferably, the amino acid sequences of the mutants according to this invention comprise any one of the Sequence 5 to Sequence 40 (corresponding to SEQ ID NO.:5 to SEQ ID NO.: 40) as listed in the Sequence Listing. 
         [0013]    Most preferably, the glucose isomerase mutants according to this invention comprise the mutated amino acid sequences in Table 2. The glucose isomerase mutants obtained in this invention possess both of high catalytic activity and thermostability. For example, a seven-mutation mutant MGI4-34 possesses an activity 305.9% higher than that of wild-type and still has 50% or more of the original catalytic activity after heat treatment at 80° C. for 19.2 hours. Another seven-mutation mutant MGI4-35 possesses an activity 296.3% higher than that of wild-type and still has 50% or more of the original catalytic activity after heat treatment at 80° C. for 24 hours. 
         [0014]    The glucose isomerase mutants according to this invention can be used in an unpurified crude enzyme form, or in a partially purified enzyme form, or in a completely purified enzyme preparation. If required, the glucose isomerase mutants can be prepared as immobilized enzyme, or immobilized cells using known immobilization methodologies. The vector with the expressed glucose isomerase mutant can be transformed into bacteria, such as  Klebsiella oxytoca  and  Zymomonas mobilis, Saccharomyces , or other hosts, such as  Pichia pastoris , to speed up the metabolism of xylose. Specifically, the expression bacteria of the glucose isomerase mutants included natural or engineered bacteria for bioethanol fermentation, such as  Klebsiella oxytoca  and  Zymomonas mobilis ; natural or engineered  Saccharomyces  for bioethanol fermentation; natural or engineered  Pichia pastoris  for bioethanol fermentation. 
       DEFINITIONS 
       [0015]    Term “wild-type” as used herein refers to the glucose isomerase from  Thermoanaerobacterium saccharolyticum  ATCC 49915, with its DNA sequence as Sequence 1 in the Sequence Listing, with its amino acid sequence as Sequence 2 in the Sequence Listing. The DNA sequence of the wild-type glucose isomerase in this invention is different in two nucleotides from the published DNA sequence of a glucose isomerase from the same species (Lee et al.,  Journal of General Microbiology,  139:1227-1234, 1993; GenBank L09699); namely, the nucleotides of the wild-type glucose isomerase in this invention at position 241-242 are GC, coding alanine (Ala) at position 81; while the corresponding nucleotides in GenBank L09699 are CG, coding arginine (Arg) at position 81. 
         [0016]    Term “reference sequence” as used herein refers to Sequence 1 in the Sequence Listing when it is a DNA sequence; or Sequence 2 in the Sequence Listing when it is an amino acid sequence. The alignment of the reference sequence and the sequences of the glucose isomerase mutants can be done manually or by computer (e.g. using computer softwares CLUSTALW, AMAS, DIALIGN, etc.). 
         [0017]    Term “position” or “position x”, where x is a numeral, as used herein refers to the position of the nucleotide or amino acid of the mutant sequence in the corresponding reference sequence when the alignment between the glucose isomerase mutants of the present invention and the wild-type glucose isomerase reaches maximum in homology. 
         [0018]    Term “glucose isomerase mutant” as used herein refers to an enzyme using Sequence 2 in the Sequence Listing as the reference sequence contains at least one mutation at position 87, position 139, position 182, position 187, position 217, position 260, position 276 or position 299 and has a catalytic activity at least 50% higher than that of the wild-type glucose isomerase in the reaction of xylulose using D-xylose as substrate. Therefore, in the present invention, the glucose isomerase mutants include mutants with an amino acid sequence that is the same as Sequence 4 in the Sequence Listing, or is a conservative substitution of Sequence 4, or is Sequence 4 with addition, or deletion of one or several amino acids, or is Sequence 4 with amino terminal or carboxyl terminal deletion, or comprises partial or complete repetition of Sequence 4. IUPAC nomenclature and symbolism for amino acid abbreviations are used in the present invention ( European Journal of Biochemistry,  138:9-37, 1984). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  shows the thermal stability of the wild-type glucose isomerase and glucose isomerase mutants MGI-4, MGI4-34 and MGI4-35 at 80° C. Details are described in Examples 9 and 15. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    The examples presented below are for illustration of the invention only and are not intended to be regarded as the limitation of the invention. In the following examples, conventional practice or manufacturers&#39; suggestion/protocol was followed in cases where the conditions were not specified. 
       Example 1 
     Amplification of Wild-type Glucose Isomerase and Construction of pGEMT-TS 
       [0021]    Primers T1 and T2 (Table 1) were designed based on the sequence of GenBank L09699 and used to amplify the wild-type glucose isomerase gene from  T. saccharolyticum  ATCC 49915 (ATCC, USA). 
         [0022]    The amplification condition was: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1, 400 nM primer T2, 1.5 U Taq DNA polymerase (Promega, USA), a loopful of  T. saccharolyticum  colony, and the total volume was adjusted to 50 μl with sterile distilled water. 
         [0023]    The PCR amplification program for the reaction was: 95° C., 3 min; then 40 cycles of 95° C., 50 sec, 50° C., 30 sec, 72° C., 1 min; and finally 72° C., 10 min. The amplified PCR product, about 1.5 kb in length, was ligated into vector pGEMT-Easy to generate pGEMT-TS. The pGEMT-TS was sequenced to determine the DNA sequence of the wild-type glucose isomerase as Sequence 1 in the Sequence Listing and the corresponding amino acid sequence as Sequence 2 in the Sequence Listing. The DNA sequence of the wild-type glucose isomerase is different from that of the published DNA sequence of a glucose isomerase from the same species (GenBank L09699) where the nucleotides of the wild-type glucose isomerase in this invention at position 241-242 are GC, coding alanine (Ala) at the amino acid position 81; while the corresponding nucleotides in GenBank L09699 are CG, coding arginine (Arg) at the position 81. 
       Example 2 
     Site-Directed Mutagenesis of Trp139 of Wild-Type Glucose Isomerase 
       [0024]    The site directed mutagenesis was done as described by Ho et al.,  Gene  77:51-59, 1989 and White et al.,  PCR protocol: current methods and applications . Totowa, N.J.: Humana Press 1993. 
         [0025]    With pGEMT-TS (Example 1) as template, the Trp (W) at position 139 of the wild-type glucose isomerase was mutated to Phe (F) to generate glucose isomerase mutant MGI-W139F by PCR amplification using primers 139FF and 139FR (Table 1) and universal primers T1 and T2 (Example 1). 
         [0026]    Fragment T1FR was amplified using primer pair T1 and 139FR. Fragment FFT2 was amplified using primer pair 139FF and T2. The amplification condition was: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer 139FR (for fragment T1FR) or 400 nM primer T2 and 400 nM primer 139FF (for fragment FFT2), 1.5 U Pfu DNA polymerase (Promega, USA), 20 ng pGEMT-TS, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR products, fragment T1FR and fragment FFT2, were separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit (QIAGEN, German). The full-length glucose isomerase gene was then assembled on the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng fragment T1ER and 20 ng fragment FFT2, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The full-length mutant MGI-W139F was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Plasmid pGEMT-MGI-W139F, generated after ligation of MGI-W139F into pGEMT-Easy, was transformed into competent  E. coli  HB101 and the transformants were screened for glucose isomerase activity on 1% MacConkey plates containing 1% D-xylose and 50 mg/L ampicillin. Plasmid pGEMT-MGI-W139F DNA was then isolated from positive clones and sequenced. The amino acid sequence of MGI-W139F is shown as Sequence 5 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
       Example 3 
     Site-Directed Mutagenesis of Arg182 of Glucose Isomerase 
       [0027]    The site directed mutagenesis was done as described by Ho et al.,  Gene  77:51-59, 1989 and White et al.,  PCR protocol: current methods and applications . Totowa, N.J.: Humana Press 1993. 
         [0028]    Using pGEMT-TS (Example 1) as template, the Arg (R) at position 182 of the wild-type glucose isomerase was mutated to Ala (A) to generate glucose isomerase mutant MGI-R182A by PCR amplification with site-directed primers 182AF and 182AR (Table 1) and universal primers T1 and T2 (Example 1). 
         [0029]    Fragment T1AR was amplified using primer pair T1 and 182AR. Fragment AFT2 was amplified using primer pair 182AF and T2. The amplification condition was: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer 182AR or 400 nM primer T2 and 400 nM primer 182AF, 1.5 U Pfu DNA polymerase, 20 ng pGEMT-TS, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 see, 52° C., 30 see, 72° C., 3 min, and finally 72° C., 5 min. The PCR products, fragment T1AR and fragment AFT2, were separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. The full-length glucose isomerase gene was then assembled on the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng fragment T1AR and 20 ng fragment AFT2, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The full-length mutant MGI-R182A was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Plasmid pGEMT-MGI-R182A, generated after ligation of MGI-R182A into vector pGEMT-Easy, was transformed into competent  E. coli  HB101 and the transformants were screened for glucose isomerase activity on 1% MacConkey plates containing 1% D-xylose and 50 mg/L ampicillin. Plasmid pGEMT-MGI-R182A DNA was then isolated from positive clones and sequenced. The amino acid sequence of MGI-R182A is shown as Sequence 6 in the Sequence Listing. 
       Example 4 
     Site-Directed Mutagenesis of Phe187 of Glucose Isomerase 
       [0030]    The site directed mutagenesis was done as described by Ho et al.,  Gene  77:51-59, 1989 and White et al.,  PCR protocol: current methods and applications . Totowa, N.J.: Humana Press 1993. 
         [0031]    Using pGEMT-TS (Example 1) as template, the Phe (F) at position 187 of the wild-type glucose isomerase was mutated to Ser (S) to generate glucose isomerase mutant MGI-F187S by PCR amplification with site-directed primers 187SF and 187SR (Table 1) and universal primers T1 and T2 (Example 1). 
         [0032]    Fragment T1SR was amplified using primer pair T1 and 187SR, Fragment SFT2 was amplified using primer pair 187SF and T2. The amplification condition was: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM Q) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer 187SR or 400 nM primer T2 and 400 nM primer 187SF, 1.5 U Pfu DNA polymerase, 20 ng pGEMT-TS, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR products, fragment T1SR and fragment SFT2, were separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. The full-length glucose isomerase gene was then assembled on the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100150 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng fragment T1SR and 20 ng fragment SFT2, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 see, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The full-length mutant MGI-F187S was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Plasmid pGEMT-MGI-F187S, generated after ligation of MGI-F187S into vector pGEMT-Easy, was transformed into competent  E. coli HB 101 and the transformants were screened for glucose isomerase activity on 1% MacConkey plates containing 1% D-xylose and 50 mg/L ampicillin. Plasmid pGEMT-MGI-F187S DNA was then isolated from positive clones and sequenced. The amino acid sequence of MGI-F187S is shown as Sequence 7 in the Sequence Listing. 
       Example 5 
     Site-Directed Mutagenesis of Thr299 of Glucose Isomerase 
       [0033]    The site directed mutagenesis was done as described by Ho et al.,  Gene  77:51-59, 1989 and White et al.,  PCR protocol: current methods and applications . Totowa, N.J.: Humana Press 1993. 
         [0034]    Using pGEMT-TS (Example 1) as template, the Thr (T) at position 299 of the wild-type glucose isomerase was mutated to Gln (Q) to generate glucose isomerase mutant MGI-T299Q by PCR amplification with site-directed primers 299QF and 299QR (Table 1) and universal primers T1 and T2 (Example 1). 
         [0035]    Fragment T1QR was amplified using primer pair T1 and 299QR. Fragment QFT2 was amplified using primer pair 299QF and T2. The amplification condition was: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer 299QR or 400 nM primer T2 and 400 nM primer 299QF, 115 U Pfu DNA polymerase, 20 ng pGEMT-TS, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR products, fragment T1QR and fragment QFT2, were separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. The full-length glucose isomerase gene was then assembled on the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng fragment T1 QR and 20 ng fragment QFT2, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 see, 72° C., 3 min; and finally 72° C., 5 min. The full-length mutant MGI-T299Q was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Plasmid pGEMT-MGI-T299Q, generated after ligation of MGI-T299Q into vector pGEMT-Easy, was transformed into competent  E. coli  HB101 and the transformants were screened for glucose isomerase activity on 1% MacConkey plates containing 1% D-xylose and 50 mg/L ampicillin. Plasmid pGEMT-MGI-T299Q DNA was then isolated from the positive clones and sequenced. The amino acid sequence of MGI-T299Q is shown as Sequence 8 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
       Example 6 
     Generation of Glucose Isomerase Mutant MGI-2 Containing Two Mutations 
       [0036]    The site directed mutagenesis was done as described by Ho et al.,  Gene  77:51-59, 1989 and White et al.,  PCR protocol: current methods and applications . Totowa, N.J.: Humana Press 1993. 
         [0037]    Fragments T1FR and AFT2 were amplified and recovered in accordance with Examples 2 and 3 respectively. Fragment FFAR was amplified using primer pair 139FF (Example 2) and 182AR (Example 3) on the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer 139FF and 400 nM primer 182AR, 1.5 U Pfu DNA polymerase, 20 ng pGEMT-TS, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR product, fragment FFAR, was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. The full length glucose isomerase gene was assembled at the following condition: 20 mm Tris-HCl (pH 8.8), 10 mM KCl, 10 mm (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng fragment T1FR, 20 ng fragment FFAR, 20 ng fragment AFT2 and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The full-length mutant MGI-2 was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Plasmid pGEMT-MGI-2, generated after ligation of MGI-2 into vector pGEMT-Easy, was transformed into competent  E. coli  HB101 and the transformants were screened for glucose isomerase activity on 1% MacConkey plates containing 1% D-xylose and 50 mg/L ampicillin. Plasmid pGEMT-MGI-2 DNA was then isolated from the positive clones and sequenced. The sequence of the MGI-2 contains two mutations including W139F and R182A. The amino acid sequence of MGI-2 is shown as Sequence 9 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
       Example 7 
     Generation of Glucose Isomerase Mutant MGI-3 Containing Three Mutations 
       [0038]    The site directed mutagenesis was done as described by Ho et al.,  Gene  77:51-59, 1989 and White et al.,  PCR protocol: current methods and applications . Totowa, N.J.: Humana Press 1993. 
         [0039]    Fragments T1FR, QFT2 and FFAR were amplified and recovered in accordance with Examples 2, 5 and 6 respectively. Fragment AFQR was amplified using primer pair 182AF (Example 3) and 299QR (Example 5) on the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 dGTP, 400 nM primer 182AF and 400 nM primer 299QR, 1.5 U Pfu DNA polymerase, 20 ng pGEMT-TS, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR product, fragment AFQR, was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. The full length glucose isomerase gene was assembled at the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng fragment T1FR, 20 ng fragment FFAR, 20 ng fragment AFQR and 20 ng fragment QFT2 and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The full-length mutant MGI-3 was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Plasmid pGEMT-MGI-3, generated after ligation of MGI-3 into vector pGEMT-Easy, was transformed into competent  E. coli  HB3101 and the transformants were screened for glucose isomerase activity on 1% MacConkey plates containing 1% D-xylose and 50 mg/L ampicillin. Plasmid pGEMT-MGI-3 DNA was then isolated from the positive clones and sequenced. The sequence of the MGI-3 contains three mutations including W139F, R182A and T299Q. The amino acid sequence of MGI-3 is shown as Sequence 10 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
       Example 8 
     Generation of Glucose Isomerase Mutant MGI-4 Containing Four Mutations 
       [0040]    The site directed mutagenesis was done as described by Ho et al.,  Gene  77:51-59, 1989 and White et al.,  PCR protocol: current methods and applications . Totowa, N.J.: Humana Press 1993. 
         [0041]    Fragments T1FR, QFT2 and FEAR were amplified and recovered in accordance with Examples 2, 5 and 6 respectively. Fragment AFSR was amplified using primer pair 182AF (Example 3) and 187SR Example 4) on the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer 182AF and 400 nM primer 187SR, 1.5 U Pfu DNA polymerase, 20 ng pGEMT-TS, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR product, fragment AFSR, was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Fragment SFQR was amplified using primers 187SF (Example 4) and 299QR (Example 5) at the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 mM primer 187SF and 400 nM primer 299QR, 1.5 U Pfu DNA polymerase, 20 ng pGEMT-TS and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The fragment SFQR was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. The full length glucose isomerase gene was assembled at the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 mM primer T1 and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng fragment T1FR, 20 ng fragment FFAR, 20 ng fragment AFSR, 20 ng fragment SFQR and 20 ng fragment QFT2 and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The full-length mutant MGI-4 was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Plasmid pGEMT-MGI-4, generated after ligation of MGI-4 into vector pGEMT-Easy, was transformed into competent  E. coli  HB101 and the transformants were screened for glucose isomerase activity on 1% MacConkey plates containing 1% D-xylose and 50 mg/L ampicillin. Plasmid pGEMT-MGI-4 DNA was then isolated from the positive clones and sequenced. The sequence of the MGI-4 contains four mutations including W139F, R182A, F187S and T299Q. The amino acid sequence of MGI-4 is shown as Sequence 11 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
       Example 9 
     Saturated Mutagenesis of Position 139 of Four-Mutation Glucose Isomerase Mutant MGI-4 
       [0042]    The site directed mutagenesis was done as described by Ho et al.,  Gene  77:51-59, 1989 and White et al.,  PCR protocol: current methods and applications . Totowa, N.J.: Humana Press 1993. 
         [0043]    Using pGEMT-MGI-4 Example 8) as template, the Phe (P) at position 139 of the glucose isomerase mutant MGI-4 was mutated to Ser (S) to generate glucose isomerase mutant MGI4-F139S by PCR amplification with site-directed primers 139SF and 139SR (Table 1) and universal primers T1 and T2 (Example 1). 
         [0044]    Fragment T1SR was amplified using primer pair T1 and 139SR. Fragment SFT2 was amplified using primer pair 139SF and T2. The amplification condition was: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer 139SR or 400 nM primer T2 and 400 nM primer 1395, 1.5 U Pfu DNA polymerase (Promega, USA), 20 ng pGEMT-MGI-4, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR products, fragment T1SR and fragment SFT2, were separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit (QIAGEN, German). The full-length glucose isomerase gene was then assembled on the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 no primer T2, 1.5 U Pfu DNA polymerase, 20 ng fragment T1SR and 20 ng fragment SFT2, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 0, 5 min. The full-length mutant MGI4-F139S was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Plasmid pGEMT-MGI4-F139S, generated after ligation of MGI4-F139S into vector pGEMT-Easy, was transformed into competent  E. coli  HB101 and the transformants were screened for glucose isomerase activity on 1% MacConkey plates containing 1% D-xylose and 50 mg/L ampicillin. Plasmid pGEMT-MGI4-F139S DNA was then isolated from the positive clones and sequenced. The sequence of the MGI4-F139S contains four mutations including F139S, R182A, F187S and T299Q. The amino acid sequence of MGI4-F139S is shown as Sequence 12 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
         [0045]    Mutants MGI4-F139K, MGI4-F139C, MGI4-F1391, MGI4-F139T, MGI4-F139N and MGI4-F139D were constructed with similar procedures. The primers used are shown in Table 1. The amino acid sequences are shown as Sequences 13-18 in the Sequence Listing and the specific activities thereof are shown in Table 2. 
       Example 10 
     Saturated Mutagenesis of Position 182 of Four-Mutation Glucose Isomerase Mutant MGI-4 
       [0046]    The site directed mutagenesis was done as described by Ho et al.,  Gene  77:51-59, 1989 and White et al.,  PCR protocol: current methods and applications . Totowa, N.J.: Humana Press 1993. 
         [0047]    Using pGEMT-MGI-4 (Example 8) as template, the Ala (A) at position 182 of the glucose isomerase mutant MGI-4 was mutated to Pro (P) to generate glucose isomerase mutant MGI4-A182P by PCR amplification with site-directed primers 182 PF and 182PR (Table 1) and universal primers T1 and T2 (Example 1). 
         [0048]    Fragment T1PR was amplified using primer pair T1 and 182PR. Fragment PFT2 was amplified using primer pair 182PF and T2. The amplification condition was: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer 182PR or 400 nM primer T2 and 400 nM primer 182 PF, 1.5 U Pfu DNA polymerase, 20 ng pGEMT-MGI-4, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR products, fragment T1PR and fragment PFT2, were separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. The full-length glucose isomerase gene was then assembled on the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng fragment T1PR and 20 ng fragment PFT2, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The full-length mutant MGI4-A182P was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Plasmid pGEMT-MGI4-A182P, generated after ligation of MGI4-A182P into vector pGEMT-Easy, was transformed into competent  E. coli  HB101 and the transformants were screened for glucose isomerase activity on 1% MacConkey plates containing 1% D-xylose and 50 mg/L ampicillin. Plasmid pGEMT-MGI4-A182P DNA was then isolated from the positive clones and sequenced. The sequence of the MGI4-A182P contains four mutations including W139F, A182P, F187S and T299Q. The amino acid sequence of MGI4-A182P is shown as Sequence 19 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
         [0049]    Mutants MGI4-A1825, MGI4-A1821, MGI4-A182T and MGI4-A182V were constructed with similar procedures. The primers used are shown in Table 1. The amino acid sequences are shown as Sequences 20-23 in the Sequence Listing and the specific activities thereof are shown in Table 2. 
       Example 11 
     Saturated Mutagenesis of Position 187 of Four-Mutation Glucose Isomerase Mutant MGI-4 
       [0050]    The site directed mutagenesis was done as described by Ho et al.,  Gene  77:51-59, 1989 and White et al.,  PCR protocol: current methods and applications . Totowa, N.J.: Humana Press 1993. 
         [0051]    Using pGEMT-MGI-4 (Example 8) as template, the Ser (S) at position 187 of the glucose isomerase mutant MGI-4 was mutated to Gly (G) to generate glucose isomerase mutant MGI-4-S187G by PCR amplification with site-directed primers 187GF and 187GR (Table 1) and universal primers T1 and T2 (Example 1). 
         [0052]    Fragment T1GR was amplified using primer pair T1 and 187GR. Fragment GFT2 was amplified using primer pair 187GF and T2. The amplification condition was: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer 187GR or 400 nM primer T2 and 400 nM primer 187GF, 1.5 U Pfu DNA polymerase, 20 ng pGEMT-MGI-4, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR products, fragment T1GR and fragment GFT2, were separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. The full-length glucose isomerase gene was then assembled on the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng fragment T1GR and 20 ng fragment GFT2, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The full-length mutant MGI4-S187G was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Plasmid pGEMT-MGI4-S187G, generated after ligation of MGI4-S187G into vector pGEMT-Easy, was transformed into competent  E. coli  HB101 and the transformants were screened for glucose isomerase activity on 1% MacConkey plates containing 1% D-xylose and 50 mg/L ampicillin. Plasmid pGEMT-MGI4-S187G DNA was then isolated from the positive clones and sequenced. The sequence of the MGI4-S187G contains four mutations including W139F, R182A, S187G and T299Q The amino acid sequence of MGI4-S187G is shown as Sequence 24 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
         [0053]    Mutants MGI4-S187A and MGI4-S187P were constructed with similar procedures. The primers used are shown in Table 1. The amino acid sequences are shown as Sequences 25-26 in the Sequence Listing and the specific activities thereof are shown in Table 2. 
       Example 12 
     Saturated Mutagenesis of Position 299 of Four-Mutation Glucose Isomerase Mutant MGI-4 
       [0054]    The site directed mutagenesis was done as described by Ho et al.,  Gene  77:51-59, 1989 and White et al.,  PCR protocol: current methods and applications . Totowa, N.J.: Humana Press 1993. 
         [0055]    Using pGEMT-MGI-4 (Example 8) as template, the Gln (Q) at position 299 of the glucose isomerase mutant MGI-4 was mutated to Ile (I) to generate glucose isomerase mutant MGI4-Q299I by PCR amplification with site-directed primers 299F and 299IR (Table 1) and universal primers T1 and T2 (Example 1). 
         [0056]    Fragment T1IR was amplified using primer pair T1 and 299IR. Fragment IFT2 was amplified using primer pair 299IF and T2. The amplification condition was: 20 M Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer 299IR or 400 nM primer T2 and 400 nM primer 299IF, 1.5 U Pfu DNA polymerase, 20 ng pGEMT-MGI-4, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR products, fragment T1IR and fragment IFT2, were separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. The full-length glucose isomerase gene was then assembled on the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng fragment T1IR and 20 ng fragment IFT2, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec 52° C., 30 sec, 72° C., 3 min. and finally 72° C., 5 min. The full-length mutant MGI4-Q299I was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Plasmid pGEMT-MGI4-Q299I, generated after ligation of MGI4-Q299I into vector pGEMT-Easy, was transformed into competent  E. coli  HB101 and the transformants were screened for glucose isomerase activity on 1% MacConkey plates containing 1% D-xylose and 50 mg/L ampicillin. Plasmid pGEMT-MGI4-Q299I DNA was then isolated from the positive clones and sequenced. The sequence of the MGI4-Q299I contains four mutations including W139F, R182A, F187S and Q299I. The amino acid sequence of MGI4-Q299I is shown as Sequence 27 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
         [0057]    Mutants MGI4-Q299Y, MGI4-Q299C, MGI4-Q299M and MGI4-Q299E were constructed with similar procedures. The primers used are shown in Table 1. The amino acid sequences are shown as Sequences 28-31 in the Sequence Listing and the specific activities thereof are shown in Table 2. 
       Example 13 
     Generation of Five-Mutation Glucose Isomerase Mutants MGI4-F87L, MGI4-F87M, MGI4-V217R, MGI4-D260E and MGI4-F276G 
       [0058]    The site directed mutagenesis was done as described by Ho et al.,  Gene  77:51-59, 1989 and White et al.,  PCR protocol: current methods and applications , Totowa, N.J.: Humana Press 1993. 
         [0059]    Using pGEMT-MGI-4 (Example 8) as template, the Phe (F) at position 87 of the MGI-4 was mutated to Leu (L) to generate glucose isomerase mutant MGI4-F87L by PCR amplification with site-directed primers 87LF and 87LR (Table 1) and universal primers T1 and T2 (Example 1). 
         [0060]    Fragment T1LR was amplified using primer pair T1 and 87LR. Fragment LFT2 was amplified using primer pair 87LF and T2. The amplification condition was: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer 87LR or 400 nM primer T2 and 400 nM primer 87LF, 1.5 U Pfu DNA polymerase (Promega, USA), 20 ng pGEMT-MGI-4, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR products, fragment T1LR and fragment LFT2, were separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit (QIAGEN, German). The full-length glucose isomerase gene was then assembled on the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μA dTTP, 50 μM dCTP, 50 μM dGTP, 400 mM primer T1 and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng fragment T1LR and 20 ng fragment LFT2, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The full-length mutant MGI4-F87L was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Plasmid pGEMT-MGI4-F87L, generated after ligation of MGI4-F87L into vector pGEMT-Easy, was transformed into competent  E. coli  HB101 and the transformants were screened for glucose isomerase activity on 1% MacConkey plates containing 1% D-xylose and 50 mg/L ampicillin. Plasmid pGEMT-MGI4-F87L DNA was then isolated from the positive clones and sequenced. The sequence of the MGI4-F87L contains five mutations including F87L, W139F, R182A, F187S and T299Q. The amino acid sequence of MGI4-F87L is shown as Sequence 32 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
         [0061]    Mutant MGI4-F87M was constructed with similar procedures. The primers used are shown in Table 1. The sequence of the mutant MGI4-F87M contains five mutations including F87M, W139F, R182A, F187S and T299Q. The amino acid sequence is shown as Sequence 33 in the Sequence Listing and the specific activity thereof is shown in Table 2. Mutant MGI4-V217R was constructed with similar procedures. The primers used are shown in Table 1. The sequence of the mutant MGI4-V217R contains five mutations including W139F, R182A, F187S, V217R and T299Q. The amino acid sequence of MGI4-V217R is shown as Sequence 34 in the Sequence Listing and the specific activity thereof is shown in Table 2. Mutant MGI4-D260E was constructed with similar procedures. The primers used are shown in Table 1. The sequence of the mutant contains five mutations including W139F, R192A, F187S, D260E and T299Q. The amino acid sequence of MGI4-D260E is shown as Sequence 35 in the Sequence Listing and the specific activity thereof is shown in Table 2. Mutant MGI4-F276G was constructed with similar procedures. The primers used are shown in Table 1. The sequence of the mutant contains five mutations including W139F, R182A, F117S, F276G and T299Q. The amino acid sequence of MGI4-F276G is shown as Sequence 36 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
       Example 14 
     Generation of Six-Mutation Glucose Isomerase Mutants MGI4-24 and MGI4-25 
       [0062]    The site directed mutagenesis was done as described by Ho et al.,  Gene  77:51-59, 1989 and White et al.,  PCR protocol: current methods and applications . Totowa, N.J.: Humana Press 1993. 
         [0063]    Fragment T1LR was amplified and recovered as in Example 13. Fragment LFAR was amplified using primer pair 87LF and 260AR (Table 1). The amplification condition was: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer 87LF and 400 nM primer 260AR, 1.5 U Pfu DNA polymerase, 20 ng pGEMT-MGI-4, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR product, fragment LFAR, was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Fragment AFT2 was amplified using primer pair 260AF and T2 (Table 1) and recovered. The amplification condition was: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 mM primer 260AF and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng pGEMT-MGI-4, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR product, fragment AFT2 was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. The full-length glucose isomerase gene was then assembled on the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 W dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng fragment T1LR, 20 ng fragment LFAR and 20 ng fragment AFT2, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The full-length mutant MGI4-24 was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Plasmid pGEMT-MGI4-24, generated after ligation of MGI4-24 into vector pGEMT-Easy, was transformed into competent  E. coli  HB101 and the transformants were screened for glucose isomerase activity on 1% MacConkey plates containing 1% D-xylose and 50 mg/L ampicillin. Plasmid pGEMT-MGI4-24 DNA was then isolated from the positive clones and sequenced. The sequence of the MGI4-24 contains six mutations including F87L, W139F, R182A, F187S, T299Q and D260A, The amino acid sequence of MGI4-24 is shown as Sequence 37 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
         [0064]    Mutant MGI4-25 was constructed with similar procedures. The primer pairs used were T1 and 87LR, 87LF and 276TR, 276TF and T2 (Table 1). The sequence of the mutant contains six mutations including F87L, W139F, R182A, F187S, T299Q and F276T. The amino acid sequence is shown as Sequence 38 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
       Example 15 
     Generation of Seven-Mutation Glucose Isomerase Mutants MGI4-34 and MGI4-35 
       [0065]    The site directed mutagenesis was done as described by Ho et al.,  Gene  77:51-59, 1989 and White et al.,  PCR protocol: current methods and applications . Totowa, N.J.: Humana Press 1993. 
         [0066]    Fragment T1LR was amplified and recovered as in Example 13. Fragment LFGR was amplified using primer pair 87LF and 217GR (Table 1). The amplification condition was: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer 87LF and 400 nM primer 217GR, 1.5 U Pfu DNA polymerase, 20 ng pGEMT-MGI-4, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR product, fragment LFGR, was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Fragment GFTR was amplified using primer pair 217GF and 276TR (Table 1) and recovered. The amplification condition was: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer 217GF and 400 nM primer 276TR, 1.5 U Pfu DNA polymerase, 20 ng pGEMT-MGI-4, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 sec, 72° C., 3 min; and finally 72° C., 5 min. The PCR product, fragment GFTR was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Fragment TFT2 was amplified using primer pair 276TF and T2 (Table 1) and recovered. The amplification condition was: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer 276TF and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng pGEMT-MGI-4, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec, 52° C., 30 see, 72° C., 3 min; and finally 72° C., 5 min. The PCR product, fragment TFT2 was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. The full-length glucose isomerase gene was then assembled on the following condition: 20 mM Tris-HCl (pH 8.8), 10 mM KCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 400 nM primer T1 and 400 nM primer T2, 1.5 U Pfu DNA polymerase, 20 ng fragment T1LR, 20 ng fragment LFAR, 20 ng fragment GFTR and 20 ng fragment TFT2, and the total volume was adjusted to 50 μl with sterile distilled water. The PCR amplification program for the reaction was: 95° C., 3 min; then 35 cycles of 95° C., 50 sec. 52° C., 30 sec. 72° C., 3 min; and finally 72° C., 5 min. The fall-length mutant MGI4-34 was separated on 1% agarose gel and recovered using QIAquick Gel Extraction Kit. Plasmid pGEMT-MGI4-34, generated after ligation of MGI4-34 into vector pGEMT-Easy, was transformed into competent  E. coli  HB101 and the transformants were screened for glucose isomerase activity on 1% MacConkey plates containing 1% D-xylose and 50 mg/L ampicillin. Plasmid pGEMT-MGI4-34 DNA was then isolated from the positive clones and sequenced. The sequence of the MGI4-34 contains seven mutations including F87L, W139F, R182A, F187S, T299Q, V217G and F276T. The amino acid sequence of MGI4-34 is shown as Sequence 39 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
         [0067]    Mutant MGI4-35 with seven-mutation was constructed with similar procedures. The primer pairs used were T1 and 87LR, 87LF and 217GR, 217GF and 260AR, 260AF and T2 (Table 1). The sequence of the mutant contains seven mutations including F87L, W139F, R182A, F187S, T299Q, V217G and D260A. The amino acid sequence is shown as Sequence 40 in the Sequence Listing and the specific activity thereof is shown in Table 2. 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 The Primers Used for Amplification of Wild-type 
               
               
                 Glucose Isomerase and the Mutants in Examples 1-15. 
               
             
          
           
               
                 Product 
                 Primer Pair 
               
               
                   
               
               
                 Wild-type 
                 T1: 5′ AGCCTAGGTTAATTAACTTTAAGAAGGAGATATACATAT 
               
               
                   
                 GAATAAATATTTTGAGA 3′ 
               
               
                   
                 T2: 5′ ATAAGCTCAGCGGCGCGCCTTATTCTGCAAACAAATAC 3′ 
               
               
                   
               
               
                 Mutant MGI-W139F 
                 139FF: 5′ AAAAGTTTTGTTTGGTACCGCAAATCTTTTCTC 3′ 
               
               
                   
                 139FR: 5′ TTGCGGTACCAAACAAAACTTTTGTCTTGCTGG 3′ 
               
               
                   
               
               
                 Mutant MGI4-F139S 
                 139SF: 5′ AAGTTTTGAGCGGTACCGCAAATCTTTTCT 3′ 
               
               
                   
                 139SR: 5′ TGCGGTACCGCTCAAAACTTTTGTCTTGCT 3′ 
               
               
                   
               
               
                 Mutant MGI4-F139K 
                 139KF: 5′ AAGTTTTGAAAGGTACCGCAAATCTTTTCT 3′ 
               
               
                   
                 139KR: 5′ TGCGGTACCTTTCAAAACTTTTGTCTTGCT 3′ 
               
               
                   
               
               
                 Mutant MGI4-F139C 
                 139CF: 5′ AAGTTTTGTGCGGTACCGCAAATCTTTTCT 3′ 
               
               
                   
                 139CR: 5′ TGCGGTACCGCACAAAACTTTTGTCTTGCT 3′ 
               
               
                   
               
               
                 Mutant MGI4-F139I 
                 139IF: 5′ AAGTTTTGATTGGTACCGCAAATCTTTTCT 3′ 
               
               
                   
                 139IR: 5′ TGCGGTACCAATCAAAACTTTTGTCTTGCT 3′ 
               
               
                   
               
               
                 Mutant MGI4-F139T 
                 139TF: 5′ AAGTTTTGACAGGTACCGCAAATCTTTTCT 3′ 
               
               
                   
                 139TR: 5′ TGCGGTACCTGTCAAAACTTTTGTCTTGCT 3 
               
               
                   
               
               
                 Mutant MGI4-F139N 
                 139NF: 5′ AAGTTTTGAACGGTACCGCAAATCTTTTCT 3′ 
               
               
                   
                 139NR: 5′ TGCGGTACCGTTCAAAACTTTTGTCTTGCT 3′ 
               
               
                   
               
               
                 Mutant MGI4-F139D 
                 139DF: 5′ AAGTTTTGGATGGTACCGCAAATCTTTTCT 3′ 
               
               
                   
                 139DR: 5′ TGCGGTACCATCCAAAACTTTTGTCTTGCT 3′ 
               
               
                   
               
               
                 Mutant MGI-R182A 
                 182AF: 5′ GGAGCTTGGCGCGGAAAACTACGTATTTTGGGG 3′ 
               
               
                   
                 182AR: 5′ CGTAGTTTTCCGCGCCAAGCTCCTTAGTAATCT 3′ 
               
               
                   
               
               
                 Mutant MGI4-A182P 
                 182PF: 5′ AGCTTGGCCCGGAAAACTACGTATTTTGGG 3′ 
               
               
                   
                 182PR: 5′ GTAGTTTTCCGGGCCAAGCTCCTTAGTAAT 3′ 
               
               
                   
               
               
                 Mutant MGI4-A182S 
                 182SF: 5′ AGCTTGGCTCAGAAAACTACGTATTTTGGG 3′ 
               
               
                   
                 182SR: 5′ GTAGTTTTCTGAGCCAAGCTCCTTAGTAAT 3′ 
               
               
                   
               
               
                 Mutant MGI4-A182I 
                 182IF: 5′ AGCTTGGCATTGAAAACTACGTATTTTGGG 3′ 
               
               
                   
                 182IR: 5′ GTAGTTTTCAATGCCAAGCTCCTTAGTAAT 3′ 
               
               
                   
               
               
                 Mutant MGI4-A182T 
                 182TF: 5′ AGCTTGGCACAGAAAACTACGTATTTTGGG 3′ 
               
               
                   
                 182TR: 5′ GTAGTTTTCTGTGCCAAGCTCCTTAGTAAT 3′ 
               
               
                   
               
               
                 Mutant MGI4-A182V 
                 182VF: 5′ AGCTTGGCGTGGAAAACTACGTATTTTGGG 3′ 
               
               
                   
                 182VR: 5′ GTAGTTTTCCACGCCAAGCTCCTTAGTAAT 3′ 
               
               
                   
               
               
                 Mutant MGI-F187S 
                 187SF: 5′ ACTACGTAAGCTGGGGTGGAAGAGAAGGGT 3′ 
               
               
                   
                 187SR: 5′ CCACCCCAGCTTACGTAGTTTTCGCGGCCA 3′ 
               
               
                   
               
               
                 Mutant MGI4-S187G 
                 187GF: 5′ ACTACGTAGGCTGGGGTGGAAGAGAAGGGT 3′ 
               
               
                   
                 187GR: 5′ CCACCCCAGCCTACGTAGTTTTCGCGGCCA 3′ 
               
               
                   
               
               
                 Mutant MGI4-S187A 
                 187AF: 5′ ACTACGTAGCGTGGGGTGGAAGAGAAGGGT 3′ 
               
               
                   
                 187AR: 5′ CCACCCCACGCTACGTAGTTTTCGCGGCCA 3′ 
               
               
                   
               
               
                 Mutant MGI4-S187P 
                 187PF: 5′ ACTACGTACCGTGGGGTGGAAGAGAAGGGT 3′ 
               
               
                   
                 187PR: 5′ CCACCCCACGGTACGTAGTTTTCGCGGCCA 3′ 
               
               
                   
               
               
                 Mutant MGI-T299Q 
                 299QF: 5′ TGACGCAAATCAAGGCGACATGCTTTTGGGATG 3′ 
               
               
                   
                 299QR: 5′ GCATGTCGCCTTGATTTGCGTCGATTGATCCTA 3′ 
               
               
                   
               
               
                 Mutant MGI4-Q299I 
                 299IF: 5′ GACGCAAATATTGGCGACATGCTTTTAGGAT 3′ 
               
               
                   
                 299IR: 5′ CATGTCGCCAATATTTGCGTCGATTGATCCT 3′ 
               
               
                   
               
               
                 Mutant MGI4-Q299Y 
                 299YF: 5′ GACGCAAATTATGGCGACATGCTTTTAGGAT 3′ 
               
               
                   
                 299YR: 5′ CATGTCGCCATAATTTGCGTCGATTGATCCT 3′ 
               
               
                   
               
               
                 Mutant MGI4-Q299C 
                 299CF: 5′ GACGCAAATTGCGGCGACATGCTTTTAGGAT 3′ 
               
               
                   
                 299CR: 5′ CATGTCGCCGCAATTTGCGTCGATTGATCCT 3′ 
               
               
                   
               
               
                 Mutant MGI4-Q299M 
                 299MF: 5′ GACGCAAATATGGGCGACATGCTTTTAGGAT 3′ 
               
               
                   
                 299MR: 5′ CATGTCGCCCATATTTGCGTCGATTGATCCT 3′ 
               
               
                   
               
               
                 Mutant MGI4-Q299E 
                 299EF: 5′ GACGCAAATGAAGGCGACATGCTTTTAGGAT 3′ 
               
               
                   
                 299ER: 5′ CATGTCGCCTTCATTTGCGTCGATTGATCCT 3′ 
               
               
                   
               
               
                 Mutant MGI4-F87L 
                 87LF: 5′ GAAGCAGCACTGGAGTTTTTTGATAAGATAA 3′ 
               
               
                   
                 87LR: 5′ AAAAACTCCAGTGCTGCTTCTACCCTTGCTTTC 3′ 
               
               
                   
               
               
                 Mutant MGI4-F87M 
                 87MF: 5′ GAAGCAGCAATGGAGTTTTTTGATAAGATAA 3′ 
               
               
                   
                 87MR: 5′ AAAAACTCCATTGCTGCTTCTACCCTTGCTTTC 3′ 
               
               
                   
               
               
                 Mutant MGI4-V217R 
                 217RF: 5′ ACATGGCTCGCGACTATGCAAAGGAAATCG 3′ 
               
               
                   
                 217RR: 5′ GCATAGTCGCGAGCCATGTGCAAAAATCTT 3′ 
               
               
                   
               
               
                 Mutant MGI4-V217G 
                 217GF: 5′ ACATGGCTGGCGACTATGCAAAGGAAATCG 3′ 
               
               
                   
                 217GR: 5′ GCATAGTCGCCAGCCATGTGCAAAAATCTT 3′ 
               
               
                   
               
               
                 Mutant MGI4-D260E 
                 260EF: 5′ ACGACCTTGAAAAATATTTCAAAGTAAATA 3′ 
               
               
                   
                 260ER: 5′ AAATATTTTTCAAGGTCGTATTTTCTCAAG 3′ 
               
               
                   
               
               
                 Mutant MGI4-D260A 
                 260AF: 5′ ACGACCTTGCGAAATATTTCAAAGTAAATA 3′ 
               
               
                   
                 260AR: 5′ AAATATTTCGCAAGGTCGTATTTTCTCAAG 3′ 
               
               
                   
               
               
                 Mutant MGI4-F276G 
                 276GF: 5′ ACATTGGCAGGCCACGACTTCCAACATGAGC 3′ 
               
               
                   
                 276GR: 5′ GAAGTCGTGGCCTGCCAATGTCGCATGGTTT 3′ 
               
               
                   
               
               
                 Mutant MGI4-F276T 
                 276TF: 5′ ACATTGGCAACCCACGACTTCCAACATGAGC 3′ 
               
               
                   
                 276TR: 5′ GAAGTCGTGGGTTGCCAATGTCGCATGGTTT 3′ 
               
               
                   
               
             
          
         
       
     
       Example 16 
     Isolation and Purification of Wild-Type Glucose Isomerase 
       [0068]    The isolation and purification of wild-type glucose isomerase were carried out in accordance with Lee et al.,  Journal of General Microbiology,  139:1227-1234, 1993. 
         [0069]    Plasmid pGEMT-TS transformed  E. coli  HB101 cells were incubated on MacConkey plate containing 1% D-xylose and 50 mg/L ampicillin at 37° C. for 36 hours. A single colony from the plate was inoculated and cultivated in 5 ml LB supplemented with 50 mg/L ampicillin for 16 hours. The bacterial cells were pelleted, resuspended in 1 ml mM sodium phosphate buffer (pH 6.5), added CoCl 2  and MgCl 2  to final concentrations of 250 μM and 5 mM respectively, disrupted using ultrasonication and centrifuged at 17,800 g for 15 min at 10° C. to collect the supernatant as crude protein. The crude protein was heated at 80° C. for 10 min and centrifuged at 17,800 g for 15 min at 10° C. to remove the precipitate. The resultant partially purified glucose isomerase was used in the subsequent assays. 
       Example 17 
     Isolation and Purification of Glucose Isomerase Mutants 
       [0070]    The isolation and purification of glucose isomerase mutant MGI4-35 were carried out as described in Example 16, except the plasmid used was pGEMT-MGI4-35. Other glucose isomerase mutants were also isolated and purified as described in Example 16. 
       Example 18 
     Activity Assay of Wild-Type Glucose Isomerase 
       [0071]    Stock substrate solution A containing 1.0 M D-xylose, 20 mM sodium phosphate buffer (pH6.5), 250 μM CoCl 2  (final concentration) and 5 mM MgCl 2  (final concentration) was adjusted to pH 6.5. Ninety μl of the stock substrate solution A were mixed with 10 μl of the glucose isomerase prepared as described in Example 16, incubated at 80° C. for 10 min and quenched on ice immediately. The D-xylulose formed was measured by the cysteine-carbazole method (Dische et al.,  Journal of Biological Chemistry,  192:583-587, 1951; and Nakamura,  Agricultural and Biological Chemistry,  32:701-706, 1968). Protein concentration was determined using Coomassie® Plus Protein Assay Reagent Kit (Pierce, USA) and SDS-PAGE. One unit of enzyme activity is defined as the amount of enzyme that produces xylulose from 1 μmole of D-xylose per min under the assay condition. Table 2 below shows the specific activity of wild-type glucose isomerase. 
       Example 19 
     Activity Assay of Glucose Isomerase Mutants 
       [0072]    The activities of glucose isomerase mutant MGI4-35 and other mutants were measured as described in Example 18. Table 2 below shows the comparison of the specific activities of wild-type glucose isomerase and the mutants. 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 The Specific Activities of Wild-type 
               
               
                 Glucose Isomerase and the Mutants 
               
             
          
           
               
                   
                   
                 SEQ ID NO. of the 
                 Relative Specific 
               
               
                   
                 Enzyme 
                 Amino Acid Sequence 
                 Activity (%) 
               
               
                   
                   
               
               
                   
                 Wild-type 
                 SEQ ID NO.: 2 
                 100.0 
               
               
                   
                 MGI-W139F 
                 SEQ ID NO.: 5 
                 169.3 
               
               
                   
                 MGI-F187S 
                 SEQ ID NO.: 7 
                 217.6 
               
               
                   
                 MGI-T299Q 
                 SEQ ID NO.: 8 
                 192.4 
               
               
                   
                 MGI-2 
                 SEQ ID NO.: 9 
                 166.8 
               
               
                   
                 MGI-3 
                 SEQ ID NO.: 10 
                 190.9 
               
               
                   
                 MGI-4 
                 SEQ ID NO.: 11 
                 273.4 
               
               
                   
                 MGI4-F139S 
                 SEQ ID NO.: 12 
                 329.6 
               
               
                   
                 MGI4-F139K 
                 SEQ ID NO.: 13 
                 238.2 
               
               
                   
                 MGI4-F139C 
                 SEQ ID NO.: 14 
                 219.7 
               
               
                   
                 MGI4-F139I 
                 SEQ ID NO.: 15 
                 334.9 
               
               
                   
                 MGI4-F139T 
                 SEQ ID NO.: 16 
                 314.0 
               
               
                   
                 MGI4-F139N 
                 SEQ ID NO.: 17 
                 322.7 
               
               
                   
                 MGI4-F139D 
                 SEQ ID NO.: 18 
                 212.4 
               
               
                   
                 MGI4-A182P 
                 SEQ ID NO.: 19 
                 227.4 
               
               
                   
                 MGI4-A182S 
                 SEQ ID NO.: 20 
                 333.1 
               
               
                   
                 MGI4-A182I 
                 SEQ ID NO.: 21 
                 253.6 
               
               
                   
                 MGI4-A182T 
                 SEQ ID NO.: 22 
                 288.8 
               
               
                   
                 MGI4-A182V 
                 SEQ ID NO.: 23 
                 307.4 
               
               
                   
                 MGI4-S187G 
                 SEQ ID NO.: 24 
                 393.9 
               
               
                   
                 MGI4-S187A 
                 SEQ ID NO.: 25 
                 327.2 
               
               
                   
                 MGI4-S187P 
                 SEQ ID NO.: 26 
                 186.9 
               
               
                   
                 MGI4-Q299I 
                 SEQ ID NO.: 27 
                 153.5 
               
               
                   
                 MGI4-Q299Y 
                 SEQ ID NO.: 28 
                 255.5 
               
               
                   
                 MGI4-Q299C 
                 SEQ ID NO.: 29 
                 265.5 
               
               
                   
                 MGI4-Q299M 
                 SEQ ID NO.: 30 
                 246.6 
               
               
                   
                 MGI4-Q299E 
                 SEQ ID NO.: 31 
                 292.9 
               
               
                   
                 MGI4-F87L 
                 SEQ ID NO.: 32 
                 170.7 
               
               
                   
                 MGI4-F87M 
                 SEQ ED NO.: 33 
                 457.2 
               
               
                   
                 MGI4-V217R 
                 SEQ ED NO.: 34 
                 209.4 
               
               
                   
                 MGI4-D260E 
                 SEQ ID NO.: 35 
                 344.2 
               
               
                   
                 MGI4-F276G 
                 SEQ ID NO.: 36 
                 378.3 
               
               
                   
                 MGI4-24 
                 SEQ ID NO.: 37 
                 154.0 
               
               
                   
                 MGI4-25 
                 SEQ ID NO.: 38 
                 255.4 
               
               
                   
                 MGI4-34 
                 SEQ ID NO.: 39 
                 405.9 
               
               
                   
                 MGI4-35 
                 SEQ ID NO.: 40 
                 396.3 
               
               
                   
                   
               
             
          
         
       
     
       Example 20 
     Thermostability of Wild-Type Glucose Isomerase 
       [0073]    Two hundred μl of the partially purified wild-type glucose isomerase obtained as described in Example 16 were added to each of seven 1.5 ml microfuge tubes, and overlaid with 200 μl mineral oil. The tubes were placed in an 80° C. water bath. One of the seven tubes was removed from the water bath at a time interval of 0 h, 4 h, 12 h and 24 h, and centrifuged at 17,800 g for 20 min at 10° C. The residual protein and the residual glucose isomerase activity of the supernatants were determined as described in Example 18.  FIG. 1  shows the thermostability of wild-type glucose isomerase at 80° C. 
       Example 21 
     Thermostability of Glucose Isomerase Mutants 
       [0074]    The thermostability of glucose isomerase mutants MGI-4, MGI4-34 or MGI4-35 was measured as described in Example 20 and was shown in  FIG. 1 . As shown in  FIG. 1 , the half-life of the activity of wild-type glucose isomerase at 80° C. was 13.4 hours, that of MGI-4 was 21.4 hours, that of MGI4-34 was 19.2 hours and that of MGI4-35 was greater than 24 hours. 
         [0075]    This invention is not limited by the detailed description in the Examples above. Various modifications can be made by those skilled in the art without departing from the scope of the invention.