the invention is further illustrated in combination with embodiments as follows . embodiment 1 a dna sequence of xylose isomerase from a bovine rumen fluid metagenomic library was cloned the clone of partial dna sequence encoding xylose isomerase between conserved domains of xylose isomerase : by taking a bovine rumen fluid metagenomic library as a template annealed with the primers xicf and xicr ( table 1 ), the product of pcr , which is ˜ 400 bp , was purified and connected into a t vector . 22 recombinant vectors with different pcr fragments were selected randomly and sequenced . sequencing results show that the sequences of the 22 cloned dna inserting fragments all belong to specific sequences of xylose isomerase , wherein 21 dna fragments are part of xylose isomerase gene of e . coli , and only one cloned dna inserting fragment belongs to the xylose isomerase gene conserved sequence from a new source . the fragments were named as 2pxi - 3 and 374 bp . according to the dna sequence information of vector pbk - cmv and 2pxi - 3 , seven specific primers were designed : pbk - f , pbk - r , pbk - r1 , 2pxi - f , 2pxi - r , 2pxi - f2 , and 2pxi - r2 ( table 1 ). by taking the bovine rumen fluid metagenomic library as a template , the upstream and downstream of the dna fragment 2pxi - 3 were amplified by pcr , respectively . using primers pbk - r1 and 2pxi - f , the dna fragment ft1 containing the 3 ′ part of ruxi was obtained ; and using primers pbk - f and 2pxi - r , the dna fragment pr containing the 5 ′ part of ruxi was obtained . the 5 ′ sequence information ( sequence of pr ) and 3 ′ sequence information ( sequence of ft1 ) were spliced , and the open reading frames ( orf ) of xyla gene can be found in the spliced sequence . using bamhi - gxi - f and sbfi - gxi - r ( table 1 ) as specific primers and bovine rumen fluid metagenomic library as the template , a 1320 bp xyla gene was amplified by pcr , and named as ruxi , the sequence was shown in seq id no . 1 . the pfu dna polymerase was used in pcr reaction . the 5 ′ and 3 ′ primers for cloning ruxi were designed base on the sequences of xylose isomerase gene , and sbfi and bamhi restriction sites were contained in the primers , respectively . the pcr product was cloned into pjfe3 vector to obtain recombinant plasmid pjfe3 - ruxi . construction of recombinant s . cerevisiae containing the xylose isomerase gene : the plasmid pjfe3 - ruxi with the xylose isomerase gene was transformed into the s . cerevisiae . in the host s . cerevisiae , the genes in ppp , which are rpe1 , rki1 , tal1 , and tkl1 , together with xks1 were over - expressed , and the gene gre3 encoding aldose reductase was knockouted ; and adaptive evolution was performed in the medium using xylose as the sole carbon source , and the respiratory pathway was blocked . the transformants were screened on a sc - ura plate , and cells which were not transformed cannot grow on the plate . recover the plasmids from recombinant s . cerevisiae transformants and verify that the xylose isomerase expressed in the recombinant cell has the right amino acid sequence shown in seq id no . 2 : plasmids in the recombinant s . cerevisiae were recovered by yeast plasmid kit . the fact that the xylose isomerase expressed in the recombinant cell is the amino acid sequence shown in seq id no . 2 can be verified from two aspects : ( 1 ) by taking the recovered plasmids as the template , ruxi can be amplified by the specific primers bamhi - gxi - f and sbfi - gxi - r ( table 1 ) in pcr ; and ( 2 ) the recovered plasmids were transformed into e . coli dh5α and then extracted from the e . coli cultivation , then the extracted plasmids were double - digested by bamhi and sbfi . meanwhile , the extracted plasmids were sequenced to determine if the sequence is as same as the cloned gene ruxi . sequencing result of the plasmids extracted from the recombinant s . cerevisiae show that the target gene sequence was consistent with the nucleotide sequence shown in seq id no . 1 , and the length of fragments obtained by pcr ( fig1 ) and enzyme digestion ( fig2 ) also demonstrate that the nucleotide sequence shown in seq id no . 1 had been transferred into the s . cerevisiae , namely , the xylose isomerase expressed in the recombinant cell is the amino acid sequence shown in seq id no . 2 . the transformed yeast cell was cultured in glucose medium for 12 h at 30 deg c , the initial od was about 0 . 2 . the cell was recovered and dissolved by using glass beads ( 0 . 10 - 0 . 11 mm ) and lysis buffer ( 100 mm tris - hcl contains protease inhibitor ), and disrupted by using a mechanical vibration cell disruption apparatus , and the mixture was centrifuged after cell disruption ( 11 , 000 g , 4 deg c , 10 min ). the xylose isomerase activity in clarified supernatant , which is the crude enzyme solution , was determined by xi - sdh one - step method ( 100 mm , tris - hcl , ph7 . 5 ; 500 mm xylose , 10 mm mgcl 2 , crude enzyme solution 10 μl , 1 u sdh , 0 . 15 mm nadh , the amount of oxidized nadh was detected at 340 nm , namely time scanning was conducted at 340 nm ) ( kuyper et al ., fems yeast research , 2003 , 4 : 69 - 78 ). as the sdh used in the determination method of xylose isomerase activity requires appropriate reaction temperature , the xi - sdh two - step method was adopted in the determination of the optimal temperature of the xylose isomerase ( lönn et al ., european journal of biochemistry , 2002 , 269 ( 1 ): 157 - 163 ). the first - step reaction : 100 mm , tris - hcl ph7 . 5 , 500 mm xylose , 10 mm mgcl 2 , crude enzyme solution 10 ul . 50 % trichloroacetic acid was used to terminate the reaction , and 2m sodium carbonate was used for neutralizing the reaction . the second - step reaction : 100 ul reaction solution in the previous step was taken , 100 mm tris - hcl ph7 . 5 , 1 u sdh , 0 . 3 mm nadh , the amount of oxidized nadh was detected at 340 nm , namely time scanning was conducted at 340 nm . under the conditions of 100 mm tris - hcl ph7 . 5 , 10 mm mgcl 2 , the enzyme activity of the xylose isomerase at 20 deg c , 25 deg c , 30 deg c , 35 deg c , 40 deg c , 50 deg c , 60 deg c , 70 deg c and 80 deg c were determined ( fig3 ). the determination of protein content refers to comassie blue method of bradford ( 1976 ). conclusions : the xylose isomerase was expressed in s . cerevisiae with a high activity . the invention relates to a xylose isomerase which is a mesophilic xylose isomerase , with the optimum temperature of 60 deg c ( fig3 ). the xylose isomerase activity was determined by adopting the xi - sdh one - step method at 30 deg c ( kuyper et al ., fems yeast research , 2003 , 4 : 69 - 78 ), and the enzyme activity is 1 . 3 u / mg , which is higher than the activity of xylose isomerase of piromyces sp - e2 being 1 . 1 u / mg ( the data come from determination experiment in a same batch , meanwhile the data are consistent with reports of the document kuyper et al ., fems yeast research , 2003 , 4 : 69 - 78 ). embodiment 3 growth and fermentation of transformed yeast strain on xylose the s . cerevisiae grows in sc medium containing the following components : 0 . 67 ( w / v ) % yeast nitrogen base ; and csm - ura , which is the mixture of purines , pyrimidines , and amino acids except for uracil . for the plate , 1 . 8 % of agar was added in the medium , and ph was adjusted to 6 . 5 to 7 . 0 . host s . cerevisiae strain bspx042 ( phenotype : ura3 - 251 , overexpression of xks1 ; overexpression of rpe1 , rki1 , tal1 , and tkl1 , which are genes in ppp ; knockout of aldose reductase gene gre3 ; and damage of electron transport respiratory chain by deleting gene cox4 after adaptive evolution ), was transformed by empty vector pjfe3 and vector pjfe3 - ruxi inserted with xylose isomerase gene from a new source , respectively , to obtain recombinant strains bsgx000 and bsgx001 respectively . the two recombinant strains were cultured in sc liquid medium taking 2 % glucose and 2 % xylose as carbon source , and sc liquid medium taking 2 % xylose as a sole carbon source , respectively . the growth is monitored by determining the increasing of optical density in a spectrophotometer at 600 nm . results of growth experiment are shown in fig4 and fig5 . in the sc liquid medium taking 2 % glucose and 2 % xylose as the carbon source , the recombinant strains bsgx000 and bsgx001 had similar growth trends in glucose consumption stage , with μ max ( maximum specific growth rate ) being 0 . 31 respectively . after glucose depletion , bsgx001 grew better than bsgx000 ( fig4 ). in the sc liquid medium taking 2 % xylose as the sole carbon source , the recombinant strain bsgx000 which does not contain the xylose isomerase gene did not grow ; and the recombinant strain bsgx001 containing the xylose isomerase gene ruxi grew better , with μ max being 0 . 16 ( fig5 ). embodiment 4 oxygen - limited fermentation of transformed yeast strain taking xylose as the sole carbon source sc liquid medium : 0 . 67 ( w / v ) % yeast nitrogen base ; and csm - ura ; and 2 % xylose . the oxygen - limited fermentation of the recombinant strain bsgx001 was performed in the 100 ml oxygen - limited bottle containing sc liquid medium taking 2 % xylose as the sole carbon source . the fermentation was continued for 72 h , and samples were taken and the metabolites were analyzed by hplc . the recombinant strain bsgx001 containing the xylose isomerase was subjected to oxygen - limited fermentation in the sc liquid medium taking 2 % xylose as the sole carbon source , the ethanol yield reached 0 . 24 gram per gram biomass per hour , the conversion rate of sugar alcohol was up to 44 %, and no detectable by - product xylitol was generated ( fig6 ).