Abstract:
The present invention relates to the use of nucleic acid molecules coding for a bacterial xylose isomerase (XI), preferably coming from  Clostridium phytofermentans , for reaction/metabolization, particularly fermentation, of recombinant microorganisms of biomaterial containing xylose, and particularly for the production of bioalcohols, particularly bioethanol, by means of xylose fermenting yeasts. The present invention further relates to cells, particularly eukaryotic cells, which are transformed utilizing a nucleic acid expression construct which codes for a xylose isomerase, wherein the expression of the nucleic acid expression construct imparts to the cells the capability to directly isomerize xylose into xylulose. Said cells are preferably utilized for reaction/metabolization, particularly fermentation, of biomaterial containing xylose, and particularly for the production of bioalcohols, particularly bioethanol. The present invention also relates to methods for the production of bioethanol, and to methods for the production of further metabolization products, comprising the metabolization of media containing xylose.

Description:
CROSS REFERENCE TO A RELATED APPLICATION 
       [0001]    This application is a Continuation Application of co-pending application Ser. No. 13/001,326, filed Mar. 15, 2011; which is a National Stage Application of International Application Number PCT/EP2009/004762, filed Jul. 1, 2009; which claims priority to German Patent Application No. 102008031350.5, filed Jul. 2, 2008; all of which are incorporated herein by reference in their entirety. 
         [0002]    The Sequence Listing for this application is labeled “SeqList-asfiled.txt”, which was created on Dec. 16, 2014, and is 8 KB. The entire content is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The present invention relates to the use of nucleic acid molecules coding for a bacterial xylose isomerase (XI), preferably coming from  Clostridium phytofermentans , for the conversion/metabolization, particularly fermentation, of biomaterial containing xylose with recombinant microorganisms, and particularly for the production of bioalcohols, particularly bioethanol, by means of xylose-fermenting yeasts. The present invention further relates to cells, particularly eukaryotic cells, which are transformed with a nucleic acid expression construct, which codes for a xylose isomerase (XI), wherein the expression of the nucleic acid expression construct imparts to the cells the capability to directly isomerize xylose into xylulose. Said cells are preferably utilized for the conversion/metabolization, particularly fermentation, of biomaterial containing xylose, and particularly for the production of bioalcohols, particularly bioethanol. The present invention also relates to methods for the production of bioethanol, and to methods for the production of further metabolization products, comprising the metabolization of media containing xylose. 
       BACKGROUND OF THE INVENTION 
       [0004]    Thanks to its capacity to ferment sugar into ethanol and carbon dioxide, the brewer&#39;s, wine and baker&#39;s yeast  Saccharomyces cerevisiae  has already been used for centuries for the production of bread, wine and beer. Apart from the production of heterologous proteins,  S. cerevisiae  is used in biotechnology primarily in the production of ethanol for industrial purposes. In numerous industries, ethanol is used as a starting substrate for syntheses. Due to the ever decreasing oil reserves, increasing oil prices and continuously rising global need for petrol, ethanol is increasingly becoming more important as a fuel alternative. 
         [0005]    To allow for an economic and efficient production of bioethanol, the use of lignocellulose-containing biomass, such as e.g. straw, waste material from the timber industry and agriculture and the organic proportion of everyday domestic refuse, is a prime option as a starting substrate. On the one hand, it is very cheap and, on the other hand, available in great quantities. The three major components of lignocellulose are lignin, cellulose and hemicellulose. Hemicellulose, after cellulose the second most occurring polymer, is a highly branched heteropolymer. It consists of pentoses (L-arabinose, D-xylose), uronic acids (4-0-methyl-D-glucuronic acid, D-galacturonic acid) and hexoses (D-mannose, D-galactose, L-rhamnose, D-glucose) (see  FIG. 1 ). Even though hemicellulose can be more easily hydrolysed than cellulose, it features the pentoses L-arabinose and D-xylose, which normally cannot be converted by the yeast  S. cerevisiae.    
         [0006]    To be able to use pentoses for fermentations, they initially have to get into the cell via the plasma membrane. Although  S. cerevisiae  is not able to metabolize D-xylose, it can absorb it into the cell. However,  S. cerevisiae  does not possess any specific transporters. The transport takes place by means of the numerous hexose transporters. However, the affinity of the transporters for D-xylose is markedly lower than that for D-glucose (Kotter and Ciriacy, 1993). In yeasts, which can metabolize D-xylose, such as e.g.  P. stipitis, C. shehatae  or  P. tannophilus  (Du Preez et al., 1986), both unspecific low-affinity transporters, which transport D-glucose and specific high-affinity proton symporters only for D-xylose are present (Hahn-Hägerdahl et al., 2001). 
       Utilization of D-xylose 
       [0007]    Different bacteria, yeasts and fungi are able to metabolize xylose. In prokaryotes and eukaryotes, the metabolization of xylose mainly differs in the type of isomerization of xylose to xylulose. In prokaryotes, the conversion of xylose to xylulose takes place by means of the enzyme xylose isomerase (XI). In eukaryotes, xylose is mostly isomerized in two steps. Initially, xylose is reduced to xylitol by the NAD(P)H-dependent xylose reductase (XR) and further converted to xylulose by the NAD-dependent xylitol dehydrogenase (XDH). The subsequent phosphorylation reaction takes place in prokaryotes and eukaryotes by means of xylulokinase (XK). 
         [0008]    The resulting intermediate xylulose-5-phosphate is an intermediate of the pentose phosphate pathway. The major part of the xylulose-5-phosphate enters the glycolysis in the form of fructose-6-phosphate and glyceraldehyde-3-phosphate and is therein further converted to pyruvate (Schaaff-Gerstenschläger and Miosga, 1997). Under fermentative conditions, the sugar is degraded further to ethanol by the pyruvate decarboxylase and the alcohol dehydrogenase. Under aerobic conditions, pyruvate can be oxidized to carbon dioxide in the citrate cycle by means of a series of reaction steps. 
         [0000]    Utilization of D-xylose in  S. cerevisiae    
         [0009]    In papers from Kotter and Ciriacy (1993), a recombinant  S. cerevisiae  strain, which was able to metabolize D-xylose was constructed for the first time. For this, the genes of the yeast  Pichia stipitis  coding for D-xylose reductase (XYL1) and xylitol dehydrogenase (XYL2) were heterologously expressed in the yeast  S. cerevisiae . In later works, the endogenous xylulokinase (XKS1) was additionally overexpressed, which improved the D-xylose absorption into the cell as well as its conversion to ethanol (Ho et al., 1998; Eliasson et al., 2000). Despite the achieved improvements, the main by-product of the xylose conversion under oxygen-limiting conditions was xylitol. This is attributed to an imbalance in the redox balance, which is caused by the reaction initially taking place in the metabolic pathway preferably using NADPH, however, the second reaction solely producing NADH (Hahn-Hägerdal et al., 2001). Under aerobic conditions, the NADH formed by the xylitol dehydrogenase can be regenerated to NAD via the respiratory chain. Under anaerobic conditions, NAD cannot be regenerated and accumulation of NADH in the cell results. Without the cofactor NAD, the xylitol dehydrogenase xylitol cannot be converted further to xylulose. 
         [0010]    Although the xylose reductase used in the mentioned paper originates from  P. stipitis , which is able to also use NADH as a cofactor, besides NADPH (Metzger and Hollenberg, 1995), the disruption of the xylose fermentation results under strict anaerobic conditions. 
         [0011]    A solution to the problem was to introduce a redox-neutral metabolic pathway into  S. cerevisiae . In prokaryotes, the conversion of xylose to xylulose takes place by means of the enzyme xylose isomerase (XI). For a complete conversion of D-xylose, only the gene XI would have to be expressed additionally as an endogenous xylulokinase is present. Although a xylose isomerase could be detected in some fungi (Tomoyeda and Horitsu, 1964; Vongsuvanglert and Tani, 1988; Banerjee et al., 1994; Rawat et al., 1996), only the xylose degradation via the enzymes xylose reductase and xylitol dehydrogenase has been shown in eukaryotes. Many efforts to heterologously express a xylose isomerase from different organisms failed (Gárdonyi and Hahn-Hägerdal, 1993). In the majority of cases, the enzymes were not functional in yeast or they were not synthesized to proteins (Sarthy et al., 1987; Amore et al., 1989; Moes et al., 1996). With high activity, only the xylose isomerase could be expressed in yeast from the obligatory anaerobic fungus  Piromyces  sp. E2 (Kyper et al., 2003). When heterologously overexpressing this eukaryotic xylose isomerase (Harhangi et al., 2003),  S. cerevisiae  was able to grow on xylose and also metabolize it under anaerobic conditions (Kuyper et al., 2003). However, further tests showed that the enzyme is strongly inhibited by xylitol, a product of the xylose conversion. Xylitol is formed unspecifically in yeast from xylose by means of aldose reductases. 
         [0012]    U.S. Pat. No. 6,475,768 describes the use of a prokaryotic thermophilic xylose isomerase from 
         [0013]    Thermus thermophilus and variants of this, respectively, for the metabolization of xylose by yeasts. The optimal temperature for this enzyme or the variants is at a temperature (&gt;70° C.), which is markedly higher than the temperature at which yeast grows and metabolizes (28-35° C.); however, yeast is inactive or dies off at temperatures above 40° C. However, at temperatures of about 30° C., the xylose isomerase from  Thermus thermophilus  and also the variants are virtually inactive. Thus, this enzyme and its variants do not permit the yeast to effectively metabolize xylose. 
         [0014]    Therefore, a need exists in the prior art for pentose isomerases, particularly xylose isomerases allowing for an improved and more efficient pentose conversion, particularly xylose conversion. 
       BRIEF SUMMARY 
       [0015]    It is thus an object of the present invention to provide improved pentose isomerases, particularly xylose isomerases, for the use in the xylose conversion, which in particular can be used for industrial yeast strains. 
       Xylose Isomerase (XI) Constructs and Their Use 
       [0016]    The object is achieved according to the invention by providing a nucleic acid molecule comprising a nucleic acid sequence, which codes for a prokaryotic xylose isomerase (XI), for
       the transformation of a cell, preferably for the recombinant expression and production of the xylose isomerase,   the conversion of xylose to xylulose by the cell, and/or   the formation of secondary products from xylose by the cell.       
 
         [0020]    In particular for the following uses:
       the transformation of a cell, preferably for the recombinant expression/production of the prokaryotic xylose isomerase,   the conversion/metabolization, particularly fermentation, of biomaterial containing xylose,   the production of bio-based chemicals,   the production of biobutanol,   the production of bioethanol.       
 
         [0026]    “Secondary products” should be understood to mean those compounds, which the cell further produces from the xylose converted to xylulose, such as, for example, bio-based chemicals and bioalcohols. 
         [0027]    “Bio-based chemicals” should be understood to mean chemical compounds and substances, which are obtained from biological materials and raw materials (biomass), particularly by using microorganisms. 
         [0028]    The bio-based chemicals can be compounds, which are selected from, but not limited to: lactic acid, acetic acid, succinic acid, malic acid, 1-butanol, isobutanol, 2-butanol, other alcohols, amino acids, 1,3-propanediol, ethylene, glycerine, a β-lactam antibiotic or a cephalosporin, alkanes, terpenes, isoprenoids or the precursor molecule amorphadiene of the antimalarial drug artemisinin. 
         [0029]    The terms “conversion” and “metabolization” are used synonymously and mean the metabolism of a substance or the conversion of a substance in the course of the metabolism, here: the conversion of xylose, particularly the conversion of xylose to xylulose, by a cell, which was transformed with a nucleic acid according to the invention. A preferred conversion/metabolization is fermentation. 
         [0030]    The nucleic acid molecules are recombinant nucleic acid molecules. Furthermore, nucleic acid molecules according to the invention comprise dsDNA, ssDNA, PNA, CNA, RNA or mRNA or combinations thereof. 
       DETAILED DESCRIPTION 
       [0031]    The prokaryotic xylose isomerase (XI) according to the invention comes from  Clostridium phytofermentans.    
         [0032]    In this invention, it was achieved with a test system to express a highly functional prokaryotic xylose isomerase from  Clostridium phytofermentans  in the yeast  S. cerevisiae . It could be shown that the xylose isomerase found allows recombinant yeasts to efficiently metabolize xylose. 
         [0033]    The prokaryotic xylose isomerase (XI) according to the invention can be expressed in cells, particularly eukaryotic cells, in an active form. Additionally, the prokaryotic xylose isomerase (XI) according to the invention is less sensitive to an inhibition by xylitol than the eukaryotic xylose isomerase from an anaerobic fungus known from the prior art. 
         [0034]    When the nucleic acid sequence coding for the prokaryotic xylose isomerase (XI) is expressed in a cell, the cell is imparted the capability to convert xylose to xylulose, which then may be metabolized further. Through this, the cell is able to grow on xylose as a carbon source. 
         [0035]    The prokaryotic xylose isomerase (XI) according to the invention preferably comprises an amino acid sequence, which is at least 70% identical, preferably at least 80% identical, more preferably at least 90% identical, even more preferably at least 95% identical and yet more preferably 99% identical or identical to the amino acid sequence of SEQ ID NO: 1. 
         [0036]    The nucleic acid sequence coding for a prokaryotic xylose isomerase (XI) preferably comprises a nucleic acid sequence, which is at least 70% identical, preferably at least 80% identical, more preferably at least 90% identical, even more preferably at least 95% identical and yet more preferably 99% identical or identical to the amino acid sequence of SEQ ID NO: 2. 
         [0037]    The nucleic acid molecules according to the invention preferably comprise nucleic acid sequences, which are identical with the naturally occurring nucleic acid sequence or are codon-optimized for the use in a host cell. 
         [0038]    Every amino acid is encrypted on a gene level by a codon. However, there are several different codons, which code for a single amino acid. Thus, the genetic code is degenerated. The preferred choice of a codon for a corresponding amino acid differs from organism to organism. Therefore, problems can arise in heterologously expressed genes if the host organism or the host cell has a very different codon usage. The gene can be expressed not at all or only slowly. Even in genes from different metabolic pathways within an organism, a different codon usage can be discovered. It is known that the glycolysis genes from  S. cerevisiae  are expressed strongly. They have a very restrictive codon usage. It can be assumed that by adapting the codon usage of the bacterial xylose isomerase gene to the codon usage of the glycolysis genes from  S. cerevisiae , an improvement of the xylose conversion in yeast is achieved. 
         [0039]    In a preferred embodiment, the nucleic acid sequence coding for a prokaryotic xylose isomerase (XI) comprises a nucleic acid sequence, which is codon-optimized for the use in a host cell. 
         [0040]    The codon-optimization substantially preferably consists in an adaptation of the codon usage to the codon usage of the host organism/host cell, such as yeast. The codon usage of the bacterial xylose isomerase gene is preferably adapted to the codon usage of the glycolysis gene from  S. cerevisiae . For further details, see also example 2 and table 1. 
         [0041]    The nucleic acid sequence coding for a prokaryotic xylose isomerase (XI) preferably comprises a nucleic acid sequence, which is at least 70% identical, preferably at least 80% identical, more preferably at least 90% identical, even more preferably at least 95% identical and yet more preferably 99% identical or identical to the amino acid sequence of SEQ ID NO: 3. 
         [0042]    The nucleic acid molecule used according to the invention is preferably a nucleic acid expression construct. 
         [0043]    Nucleic acid expression constructs according to the invention are expression cassettes comprising a nucleic acid molecule according to the invention, or expression vectors comprising a nucleic acid molecule according to the invention or an expression cassette, for example. 
         [0044]    A nucleic acid expression construct preferably comprises promoter and terminator sequences, the promoter being operatively linked with the nucleic acid sequence coding for a prokaryotic xylose isomerase (XI). 
         [0045]    Preferred promoter sequences are selected from HXT7, truncated HXT7, PFK1, FBA1, PGK1, ADH1 and TDH3. 
         [0046]    Preferred terminator sequences are selected from CYC1, FBA1, PGK1, PFK1, ADH1 and TDH3. 
         [0047]    The nucleic acid expression construct may further comprise 5′ and/or 3′ recognition sequences and/or selection markers. 
         [0048]    The selection marker is preferably selected from a LEU2 marker gene, a URA3 marker gene and a dominant antibiotic-resistance marker gene. A preferred dominant antibiotic-resistance marker gene is selected from genes, which impart resistances to geneticin, hygromycin and nourseothricin. 
         [0049]    An expression vector can be selected from the group of pRS303X, p3RS305X, p3RS306X, pRS41 H, pRS41 K, pRS41 N, pRS42H, pRS42K, pRS42N or p423HXT7-6HIS, p424HXT7-6HIS, p425HXT7-6HIS, p426HXT7-6HIS. 
         [0050]    The cell to be transformed is preferably a eukaryotic microorganism, preferably a yeast cell or a filamentous fungal cell. 
         [0051]    The yeast cell is preferably a member of a genus selected from the group of  Saccharomyces, Kluyveromyces, Candida, Pichia, Schizosaccharomyces, Hansenula, Kloeckera, Schwanniomyces, Arxula  and  Yarrowia.    
         [0052]    The yeast cell is more preferably a member of a species selected from the group of  S. cerevisiae, S. bulderi, S. barnetti, S. exiguus, S. uvarum, S. diastaticus, K. lactis, K. marxianus  and  K. fragilis.    
         [0053]    The filamentous fungal cell is preferably a member of a genus selected from the group of  Aspergillus, Trichoderma, Humicola, Acremonium, Fusarium  and  Penicillium.    
       Xylose-fermenting Cells 
       [0054]    The object is achieved according to the invention by providing cells, which are transformed with a nucleic acid expression construct coding for a prokaryotic xylose isomerase (XI). 
         [0055]    A cell according to the invention is preferably a eukaryotic cell. 
         [0056]    A cell according to the invention, particularly a eukaryotic cell, is transformed with a nucleic acid expression construct comprising:
   (a) a nucleic acid sequence coding for a prokaryotic xylose isomerase (XI),   (b) a promoter operatively linked with the nucleic acid sequence, allowing for the expression of the prokaryotic xylose isomerase (XI) in the cell.   
 
         [0059]    In this connection, the expression of the nucleic acid expression construct imparts to the cell the capability to directly isomerize xylose into xylulose. 
         [0060]    As discussed above, the prokaryotic xylose isomerase (XI) according to the invention can be expressed in cells, particularly eukaryotic cells, in an active form such that the cells can thus directly isomerize xylose into xylulose (see also  FIG. 2 ). 
         [0061]    Additionally, the prokaryotic xylose isomerases (XI) according to the invention are less sensitive to an inhibition by xylitol than the eukaryotic xylose isomerases from an anaerobic fungus known from the prior art. 
         [0062]    The inventors have introduced a redox-neutral metabolic pathway into  S. cerevisiae  in which the conversion of xylose to xylulose takes place by means of a xylose isomerase (XI) ( FIG. 2 ). 
         [0063]    When the nucleic acid sequence coding for the prokaryotic xylose isomerase (XI) is expressed in a cell, the cell is imparted the capability to convert xylose to xylulose, which then may be metabolized further. Through this, the cell is able to grow on xylose as a carbon source. 
         [0064]    The prokaryotic xylose isomerase (XI) according to the invention preferably comes from  Clostridium phytofermentans . The xylose isomerase (XI) according to the invention preferably comprises an amino acid sequence, which is at least 70% identical, preferably at least 80% identical, more preferably at least 90% identical, even more preferably at least 95% identical and yet more preferably 99% identical or identical to the amino acid sequence of SEQ ID NO: 1. 
         [0065]    The promoter (b) is preferably selected from HXT7, truncated HXT7, PFK1, FBA1, PGK1, ADH1 and TDH3. 
         [0066]    In a preferred embodiment, the nucleic acid expression construct with which a cell according to the invention is transformed is a nucleic acid molecule according to the invention, as defined herein and above. 
         [0067]    The cell according to the invention is preferably a eukaryotic microorganism, preferably a yeast cell or a filamentous fungal cell. 
         [0068]    A yeast cell according to the invention is preferably a member of a genus selected from the group of  Saccharomyces, Kluyveromyces, Candida, Pichia, Schizosaccharomyces, Hansenula, Kloeckera, Schwanniomyces, Arxula  and  Yarrowia.    
         [0069]    A yeast cell according to the invention is more preferably a member of a species selected from the group of  S. cerevisiae, S. bulderi, S. barnetti, S. exiguus, S. uvarum, S. diastaticus, K. lactis, K. marxianus  and  K. fragilis.    
         [0070]    A yeast cell according to the invention is more preferably the strain Ethanol Red™ or Lallemand1. 
         [0071]    A filamentous fungal cell according to the invention is preferably a member of a genus selected from the group of  Aspergillus, Trichoderma, Humicola, Acremonium, Fusarium  and  Penicillium.    
         [0072]    The cell according to the invention is preferably a cell maintained in a cell culture or a cultured cell. 
         [0073]    The cells according to the invention are transiently or stably transformed with the nucleic acid expression construct or the nucleic acid molecule, as defined herein. 
         [0074]    In one embodiment, a cell according to the invention furthermore expresses one or more enzymes, which impart to the cell the capability to produce one or more further metabolization products. 
         [0075]    In this connection, such a further metabolization product is preferably selected from, but not limited to, the group of bio-based chemicals, such as lactic acid, acetic acid, succinic acid, malic acid, 1-butanol, isobutanol, 2-butanol, other alcohols, amino acids, 1,3-propanediol, ethylene, glycerol, a β-lactam antibiotic or a cephalosporin, alkanes, terpenes, isoprenoids or the precursor molecule amorphadiene of the antimalarial drug artemisinin. 
         [0076]    The object is achieved according to the invention by using the cells according to the invention for the conversion/metabolization, particularly fermentation, of biomaterial containing xylose and/or for the production of bioethanol. 
         [0077]    The object is achieved according to the invention by using the corresponding cells according to the invention for the conversion/metabolization, particularly fermentation, of biomaterial containing xylose and/or for the production of a metabolization product. 
         [0078]    In this connection, the metabolization product is preferably selected from the group of bio-based chemicals (but not limited to this group of bio-based chemicals), such as lactic acid, acetic acid, succinic acid, malic acid, 1-butanol, isobutanol, 2-butanol, other alcohols, amino acids, 1,3-propanediol, ethylene, glycerol, a β-lactam antibiotic or a cephalosporin, alkanes, terpenes, isoprenoids or the precursor molecule amorphadiene of the antimalarial drug artemisinin. 
         [0079]    The object is achieved according to the invention by providing a method for the production of bioethanol. 
         [0080]    The method according to the invention comprises the following steps:
   (a) converting a medium containing a xylose source with a cell according to the invention, which converts xylose to ethanol,   (b) optionally obtaining the bioethanol.   
 
         [0083]    The bioethanol is obtained by isolation, for example. 
         [0084]    The medium may also contain another additional carbon source, particularly glucose. 
         [0085]    The production of bioethanol preferably takes place at a rate of at least 0.03 g of ethanol per g of yeast dry weight and hour. 
         [0086]    The ethanol yield is preferably at least 0.3 g of ethanol per g of xylose. 
         [0087]    The object is achieved according to the invention by providing a method for the production of a metabolization product. 
         [0088]    In this connection, such a further metabolization product is preferably selected from, but not limited to, the group of bio-based chemicals, such as lactic acid, acetic acid, succinic acid, malic acid, 1-butanol, isobutanol, 2-butanol, other alcohols, amino acids, 1,3-propanediol, ethylene, glycerol, a β-lactam antibiotic or a cephalosporin, alkanes, terpenes, isoprenoids or the precursor molecule amorphadiene of the antimalarial drug artemisinin. 
         [0089]    The method according to the invention comprises the following steps:
   (a) converting/metabolizing, particularly fermenting, a medium containing a xylose source with a corresponding cell according to the invention, which converts xylose to produce the metabolization product,   (b) optionally obtaining the metabolization product.   
 
         [0092]    The metabolization product is obtained by isolation, for example. 
         [0093]    The medium may also contain another additional carbon source, particularly glucose. 
         [0094]    The inventors have succeeded to introduce a redox-neutral metabolic pathway into  S. cerevisiae  in which the conversion of xylose to xylulose takes place by means of a xylose isomerase (XI) ( FIG. 2 ). 
         [0095]    In this invention, it was achieved with a test system to express a highly functional prokaryotic xylose isomerase from  Clostridium phytofermentans  in the yeast  S. cerevisiae . It could be shown that the xylose isomerase found allows recombinant yeasts to efficiently metabolize xylose. 
         [0096]    Furthermore, a plurality of experimental obstacles and difficulties had to be overcome in finding a functional xylose isomerase:
       5 genes had to be overexpressed for the construction of the test strain MKY09.   The choice of the xylose isomerases to be tested was not trivial.   All the bacterial xylose isomerases hitherto tested showed no to very low activity in yeast.   High expenditure in the cultivation of the organisms to be tested, which were needed for the screen.   The xylose isomerase according to the invention is the first described highly active prokaryotic xylose isomerase in yeast.   The xylose isomerase according to the invention is the first xylose isomerase of cluster II (of three clusters) of xylose isomerases (see  FIG. 3 ), which could be expressed functionally in yeasts.   The xylose isomerase according to the invention is only slightly inhibited by xylitol.       
 
         [0104]    Several reports about the difficulties with regard to the functional expression of xylose isomerases in yeast exist (Gárdonyi and Hahn-Hägerdahl, 2003; as well as reference cited therein). 
         [0105]    The inventors have succeeded for the first time to express a prokaryotic xylose isomerase in functional form in yeasts such that they are enabled to metabolize xylose under physiological conditions and in significant quantities and to convert it to products (e.g. ethanol). As described in the prior art, this is not trivial. Numerous attempts were made and all of them were so far unsuccessful (see Sarthy et al., 1987; Amore et al., 1989; Moes et al., 1996, U.S. Pat. No. 6,475,768). The inventors have now succeeded to demonstrate that especially the  C. phytofermentans  xylose isomerase, in contrast to all the other, hitherto known prokaryotic enzymes, enable the yeast to metabolize xylose under physiological conditions and in significant quantities and to make products out of it. 
         [0106]    Examples of lignocellulosic hydrolysates having a significant proportion of xylan (Hayn et al., 1993):
       Grass: 16%   Wheat bran: 19%   Corn waste: 19%       
 
       BRIEF DESCRIPTION OF THE SEQUENCES 
       [0110]    The present invention is clarified further in the following figures, sequences and examples, however, without being limited to these. The cited references are fully incorporated by reference herein. The sequences and figures show: 
         [0111]    SEQ ID NO: 1 the protein sequence of the xylose isomerase ORF (open reading frame) of  C. phytofermentans,  
   (see also GenBank Accession Nos. ABX41597 and CP000885 (from 19 th  Nov. 2007)),   
 
         [0113]    SEQ ID NO: 2 the nucleic acid sequence of the open reading frame (ORF) of the xylose isomerase from  C. phytofermentans,  
   (see also GenBank Accession No. CP000885 (from 19 th  Nov. 2007)),   
 
         [0115]    SEQ ID NO: 3 the nucleic acid sequence of the open reading frame (ORF) of the xylose isomerase from  C. phytofermentans  in a codon-optimized form. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0116]      FIG. 1 . Composition of biomass. Biomass consists of cellulose, hemicellulose and lignin. The second most occurring hemicellulose is a highly branched polymer consisting of pentoses, uronic acids and hexoses. To a large proportion, the hemicellulose consists of the pentoses xylose and arabinose. 
           [0117]      FIG. 2 . Diagram of the conversion of D-xylose in recombinant  S. cerevisiae  by means of direct isomerization 
           [0118]      FIG. 3 . Genealogical tree of the different xylose isomerases 
           [0119]    The genealogical tree of the tested xylose isomerases is depicted. Comparisons with regard to the similarity of the xylose isomerases were performed with the program “MEGA version 4”. 
           [0120]      FIGS. 4A-4C . Used vectors. The starting plasmid for the construction of p426H7-XI-Clos (4B) or p426H7-opt.XI-Clos (4C) was the plasmid p426HXT7-6HIS (4A). Vector p426HXT7-6HIS is a 2μ expression plasmid, which has a URA3 marker. The open reading frame (ORF) and its codon-optimized form of the xylose isomerase from  C. phytofermentans  according to the invention, respectively, was cloned behind the truncated strong HXT7 promoter and the CYC1 terminator of the plasmid p426HXT7-6HIS. 
           [0121]      FIG. 5A-5C . Growth behaviour on medium containing xylose using the different xylose isomerase genes 
           [0122]    Growth tests of recombinant  S. cerevisiae  strains, which include the bacterial D-xylose metabolism with the xylose isomerase from  C. phytofermentans . Growth tests were performed on agar plates with SC medium and 2% xylose as the only carbon source. The native (5B) and the codon-optimized form (5C) of the xylose isomerase from  C. phytofermentans  were tested. The empty vector p426HXT7-6HIS (5A) served as the negative control. 
           [0123]      FIG. 6 . Xylose conversion in recombinant yeast strains using a bacterial xylose isomerase 
           [0124]    The xylose conversion of recombinant yeast cells MKY09, which contained the native and the codon-optimized form of the xylose isomerase from  C. phytofermentans  was tested. The empty vector p426HXT7-6HIS served as a comparison. Growth curves were performed in liquid SC medium with 1.4% xylose under aerobic conditions. HPLC samples were taken in parallel to measure the optical density at 600 nm. See also table 2, example 3. 
           [0125]      FIG. 7 . Enzyme kinetics 
           [0126]    Eadie-Hofstee plot of the xylose conversion of the native and the codon-optimized xylose isomerase from C. phytofermentans 
           [0127]    The strain CEN.PK2-1C transformed with the plasmid p426H7-XI-Clos and p426H7-opt.XI-Clos, respectively, was grown over night in synthetic complete medium with 2% glucose and no uracil. Raw extracts were prepared and quantitative enzyme tests were performed. A representative result is shown. The values indicated in table 3 are average values from at least 3 independent measurements. 
       
    
    
     EXAMPLES 
     Methods 
       [0128]    1. Strains and Media 
         [0129]    Bacteria 
         [0130]      E. coli  SURE (Stratagene) 
         [0131]      E. coli  DH5α (Stratagene) 
         [0132]      Bacillus licheniformis  (37° C.) 
         [0133]      Agrobacterium tumefaciens  (26° C.) 
         [0134]      Burkholderia xenovorans  (28° C.) 
         [0135]      Clostridium phytofermentans  (30° C., anaerobic) 
         [0136]      Lactobacillus pentosus  (30° C.) 
         [0137]      Leifsonia xyli  (28° C.) 
         [0138]      Pseudomonas syringae pv. phaseolicola  (28° C.) 
         [0139]      Robiginitalea biformata  (30° C.) 
         [0140]      Saccharophagus degradans  (26° C.) 
         [0141]      Salmonella typhimurium  LT2 (28° C.) 
         [0142]      Staphylococcus xylosus  (37° C.) 
         [0143]      Streptomyces diastaticus  (28° C.) 
         [0144]      Xanthomonas campestris  (26° C.) 
       Other Organisms 
       [0145]      Arabidopsis thaliana  (genomic DNA) 
         [0000]    Media and Cultivation of  E. coli    
         [0146]    Complete Medium LB:
       1% tryptone, 0.5% yeast extract, 0.5% NaCl, pH 7.5 (see Maniatis, 1982).       
 
         [0148]    For the selection for a plasmid-coded antibiotic resistance, 40 pg/ml of ampicillin was added to the medium after autoclaving. Solid culture media additionally contained 2% agar. The cultivation took place at 37° C. 
       Media and Cultivation of Further Bacteria 
       [0149]    Composition of the media and cultivation conditions, see information from the DSMZ (Deutsche Sammlung von Mikroorganismen and Zellkulturen, Brunswick, Germany). 
       Yeast 
       [0150]    Strain CEN.PK2-1C 
         [0151]    CEN.PK2-1C (MATa leu2-3, 112 ura3-52 trpl-289 his3-Δ1MAL2-8c SUC2) 
       Strain MYKO9  
       [0152]    MKY09 is based on the strain CEN.PK2-1C (MATa leu2-3, 112 ura3-52 trpl-289 his3-Δ1MAL2-8C SUC2, PromTKL1::loxP-Prom-vkHXT7, PromRPE1::loxP-Prom-vkHXT7, PromRKI1::loxP-Prom-vkHXT7, Prom GAL2::loxP-Prom-vkHXT7, PromXKS1::loxP-Prom-vkHXT7), including further unknown mutations. 
       Media and Cultivation of Yeasts 
       [0153]    Synthetic complete selective medium SC:
       0.67% yeast nitrogen base w/o amino acids, pH 6.3, amino acid/nucleobase solution, carbon source in the concentration respectively given       
 
         [0155]    Synthetic minimal selective medium SM:
       0.16% yeast nitrogen base w/o amino acid and ammonium sulphate, 0.5% ammonium sulphate, 20 mM of potassium dihydrogenphosphate, pH 6.3, carbon source in the concentration respectively given       
 
         [0157]    Concentration of the amino acids and nucleobases in the synthetic complete medium (according to Zimmermann, 1975): adenine (0.08 mM), arginine (0.22 mM), histidine (0.25 mM), isoleucine (0.44 mM), leucine (0.44 mM), lysine (0.35 mM), methionine (0.26 mM), phenylalanine (0.29 mM), tryptophan (0.19 mM), threonine (0.48 mM), tyrosine (0.34 mM), uracil (0.44 mM), valine (0.49 mM). L-arabinose and D-glucose were used as the carbon source. 
       2. Plasmids 
       [0158]    Plasmids used 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                 Plasmid 
                 Source/reference 
                 Description 
               
               
                   
               
             
             
               
                 p426HXT7- 
                 Hamacher et al., 
                 2μ expression plasmid for the 
               
               
                 6HIS 
                 2002 
                 overexpression of genes and for the 
               
               
                 (=p426H7) 
                   
                 production of a His 6  epitope; URA3 
               
               
                   
                   
                 selection marker gene, truncated HXT7 
               
               
                   
                   
                 promoter and CYC1 terminator 
               
               
                   
               
             
          
         
       
     
         [0159]    Plasmids constructed in the course of this work 
         [0000]    
       
         
               
               
             
           
               
                   
               
               
                 Plasmid 
                 Description 
               
               
                   
               
             
             
               
                 p426H7-XI- 
                 Cloning of the XI from  A. tumefaciens  in p426HXT7-6HIS 
               
               
                 Agro 
                 omitting the His 6  epitope 
               
               
                 p426H7-XI- 
                 Cloning of the XI from  A. thaliana  in p426HXT7-6HIS 
               
               
                 Arab 
                 omitting the His 6  epitope 
               
               
                 p426H7-XI- 
                 Cloning of the XI from  B. licheniformi s in p426HXT7- 
               
               
                 BaLi 
                 6HIS omitting the His 6  epitope 
               
               
                 p426H7-XI- 
                 Cloning of the XI from  B. xenovorans  in p426HXT7-6HIS 
               
               
                 Burk 
                 omitting the His 6  epitope 
               
               
                 p426H7-XI- 
                 Cloning of the XI from C. phytofermentans  in p426HXT7- 
               
               
                 Clos 
                 6HIS omitting the His 6  epitope 
               
               
                 p426H7- 
                 Cloning of the codon-optimized XI from  C.   
               
               
                 opt.XI-Clos 
                   phytofermentans  in p426HXT7-6HIS omitting the His 6   
               
               
                   
                 epitope 
               
               
                 p426H7-XI- 
                 Cloning of the XI from  L. pentosus  in p426HXT7-6HIS 
               
               
                 Lacto 
                 omitting the His 6  epitope 
               
               
                 p426H7-XI- 
                 Cloning of the XI from  L. xyli  in p426HXT7-6HIS 
               
               
                 Leif 
                 omitting the His 6  epitope 
               
               
                 p426H7- 
                 Cloning of the codon-optimized XI from  Piromyces  sp. E2 
               
               
                 opt.XI-Piro 
                 in p426HXT7-6HIS omitting the His 6  epitope 
               
               
                 p426H7-XI- 
                 Cloning of the XI from  P. syringae  in p426HXT7-6HIS 
               
               
                 Pseudo 
                 omitting the His 6  epitope 
               
               
                 p426H7-XI- 
                 Cloning of the XI from  R. biformata  in p426HXT7-6HIS 
               
               
                 Robi 
                 omitting the His 6  epitope 
               
               
                 p426H7-XI- 
                 Cloning of the XI from  S. degradans  in p426HXT7-6HIS 
               
               
                 Saccha 
                 omitting the His 6  epitope 
               
               
                 p426H7-XI- 
                 Cloning of the XI from  S. typhimurium  in p426HXT7-6HIS 
               
               
                 Salmo 
                 omitting the His 6  epitope 
               
               
                 p426H7-XI- 
                 Cloning of the XI from  S. xylosus  in p426HXT7-6HIS 
               
               
                 Staph 
                 omitting the His 6  epitope 
               
               
                 p426H7-XI- 
                 Cloning of the XI from  S. diastaticus  in p426HXT7-6HIS 
               
               
                 Strep 
                 omitting the His 6  epitope 
               
               
                 p426H7-XI- 
                 Cloning of the XI from  X. campestri s in p426HXT7-6HIS 
               
               
                 Xantho 
                 omitting the His 6  epitope 
               
               
                   
               
             
          
         
       
     
       3. Transformation: 
       [0160]    Transformation of  E. coli    
         [0161]    The transformation of  E. coli  cells was performed with the electroporation method according to Dower et al. (1988) and Wirth (1993) by means of an Easyject prima instrument (EQUIBO). 
         [0000]    Transformation of  S. cerevisiae    
         [0162]    The transformation of  S. cerevisiae  strains with plasmid DNA or DNA fragments was performed in accordance with the lithium acetate method according to Gietz and Woods (1994). 
       4. Preparation of DNA 
       [0163]    Isolation of plasmid DNA from  E. coli    
         [0164]    The isolation of plasmid DNA from  E. coli  was performed in accordance with the method of alkaline lysis according to Birnboim and Doly (1979), modified according to Maniatis et al. (1982) or alternatively with the “QIAprep Spin Miniprep Kit” from the company Qiagen. 
         [0165]    High-purity plasmid DNA for sequencing was prepared with the “Plasmid Mini Kit” from the company Qiagen according to the manufacturer&#39;s instructions. 
         [0000]    Isolation of Plasmid DNA from  S. cerevisiae    
         [0166]    The cells of a stationary yeast culture (5m1) were harvested by centrifugation, washed and resuspended in 400 μl of buffer B1 (Plasmid Mini Kit, company Qiagen). Following the addition of 400 μl of buffer B2 and ⅔ of a volume of glass beads (Ø0.45 mm), the cell disruption was performed by shaking for 5 minutes on a Vibrax (Vibrax-VXR from Janke &amp; Kunkel or IKA). ½ of a volume of buffer B3 was added to the supernatant, it was mixed and incubated for 10 min on ice. After centrifuging for 10 minutes at 13,000 rpm, the plasmid DNA was precipitated at room temperature by adding 0.75 ml of isopropanol to the supernatant. The DNA pelleted by centrifugation for 30 min at 13,000 rpm was washed with 70% ethanol, dried and resuspended in 20 μl of water. 1 μl of the DNA was used for the transformation in  E. coli.    
         [0000]    Colony PCR of  B. licheniformis  and  S. degradans    
         [0167]    Minor amounts of cells were collected from bacterial cultures growing on a plate by means of a toothpick and transferred into a PCR reaction vessel. Following the addition of H 2 O, 0.2 mM dNTP mix, 1X PCR buffer (contains 1.5 mM MgCl 2 ) and in each case 10 pmol of the corresponding oligonucleotide primer, the cell disruption was performed in a thermocycler from the company Techne at 99° C. for 10 min. This batch was directly used in a PCR reaction as a template. By adding 1 U of polymerase, the polymerase chain reaction was started with a total volume of 50 μl. 
       Determination of the DNA Concentration 
       [0168]    The DNA concentration was measured spectrophotometrically in a wavelength range of 240-300 nm. If the purity of the DNA, determined with the quotient E 260nm /E 280nm , is 1.8, the extinction E 260nm =1.0 corresponds to a DNA concentration of 50 μg of dsDNA/ml (Maniatis et al., 1982). 
       DNA Amplification by Means of PCR 
       [0169]    Use of the Phusion™ High Fidelity Systems 
         [0170]    The polymerase chain reaction was performed in a total volume of 50 μl with the “Phusion™ High Fidelity PCR System” from the company Finnzymes according to the manufacturer&#39;s instructions. Each batch consisted of 1-10 ng of DNA or 1-2 yeast gcolonies as the synthesis template, 0.2 mM of dNTP mix, 1X buffer 2 (contains 1.5 mM of MgCl 2 ), 1 U of polymerase and in each case 100 pmol of the corresponding oligonucleotide primer. The PCR reaction was performed in a thermocycler from the company Techne and the PCR conditions were chosen as follows, as required: 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
               
             
             
               
                 1. 
                  1x 
                 30 sec, 98° C. 
                 Denaturation of the DNA 
               
               
                 2. 
                 30x 
                 10 sec, 98° C. 
                 Denaturation of the DNA 
               
               
                   
                   
                 30 sec, 52-62° C. 
                 Annealing/bonding of the oligonucleotides 
               
               
                   
                   
                   
                 to the DNA 
               
               
                   
                   
                 50 sec, 72 
                 DNA synthesis/elongation 
               
               
                 3. 
                  1x 
                  7 min, 72° C. 
                 DNA synthesis/elongation 
               
               
                   
               
             
          
         
       
     
         [0171]    The polymerase was added after the first denaturation step (“hot-start PCR”). The number of synthesis steps, the annealing temperature and the elongation time were adapted to the specific melting temperatures of the oligonucleotides used or the size of the product to be expected, respectively. The PCR products were examined by means of an agarose gel electrophoresis and subsequently purified. 
       DNA Purification of PCR Products 
       [0172]    The purification of the PCR products was performed with the “QIAquick PCR Purification Kit” from the company Qiagen according to the manufacturer&#39;s instructions. 
       Gel Electrophoretic Separation of DNA Fragments 
       [0173]    The separation of DNA fragments having a size of 0.15-20 kb was performed in 0.5-1% agarose gels with 0.5 μg/ml of ethidium bromide. 1X TAE buffer (40 mM of Tris, 40 mM of acetic acid, 2 mM of EDTA) was used as the gel and running buffer (Maniatis et al., 1982). A lambda phage DNA cut with the restriction endonucleases EcoRI and HindIII served as a size standard. Before application, 1/10 of a volume of blue marker (1X TAE buffer, 10% glycerine, 0.004% bromophenol blue) was added to the DNA samples and they were visualized after the separation by irradiation with UV light (254 nm). 
         [0000]    Isolation of DNA Fragments from Agarose Gels 
         [0174]    The desired DNA fragment was cut out from the TAE agarose gel under long-wave UV light (366 nm) and isolated with the “QIAquick Gel Extraction Kit” from the company Qiagen according to the manufacturer&#39;s instructions. 
       5. Enzymatic Modification of DNA 
       [0175]    DNA restriction 
         [0176]    Sequence-specific cleavage of the DNA with restriction endonucleases was performed for 1 hour with 2-5 U of enzyme per pg of DNA under the incubation conditions recommended by the manufacturer. 
       6. Metabolite Analyses 
       [0177]    Samples were taken at different times and centrifuged at 4° C. for 15 min at 13,000 rpm and 450 μl were collected from the supernatant. The protein precipitation was performed with 50% sulphosalicylic acid. 1/10 of a volume of sulphosalicylic acid was added onto the samples, mixed and centrifuged for 20 min at 13,000 rpm at 4° C. The supernatant was collected and the samples could be used for the measurement after another dilution with water. Samples with D-glucose, D-xylose, xylitol, acetate, glycerine and ethanol served as standards, which were employed in concentrations of 0.05% w/w, 0.1% w/v, 0.5% w/v, 1.0% w/v and 2.0% w/v. 
         [0178]    The sugar concentration and the ethanol concentration were measured by means of BioLC (Dionex). The autosampler “AS50”, the column heater “TCC-100”, the RI detector “RI-101” (Shodex) and the gradient pump “GS50” were used in the measurement. The measurement of the samples was performed with the column VA 300/7.7 Nucleogel Sugar 810 H (Macherey-Nagel). The column was eluted at a temperature of 65° C. with 5 mM H 2   SO   4  as the eluent and at a flow rate of 0.6 ml.min −1 . The evaluation of the data was performed with the program Chromeleon Version 6.50™ (version 6.50, Dionex). 
         [0000]    7. Measurement of Enzyme Activities in  S. cerevisiae    
       Preparation of Raw Extracts 
       [0179]    50 ml of cultures of yeast cells were grown to the exponential phase in synthetic minimal medium with 2% glucose. The cells were harvested, washed twice in Tris-HCl buffer (pH 7.5) and disrupted by means of glass beads (Ø=0.45 nm) for 8 min on a Vibrax (Janke &amp; Kunkel, Vibrax-VBR) at 4° C. Cell debris was removed by centrifugation for 10 min at 13,000 rpm. Subsequently, the supernatant was collected and filled up to 2 ml with cold Tris-HCl buffer (pH 7.5) and used as a raw extract for the protein determination and for the measurement of the enzyme activities or the xylitol inhibition. 
       Protein Determination 
       [0180]    The protein concentration was determined with the kit “Roti-Quant” from the company Carl Roth GmbH+Co. according to the manufacturer&#39;s instructions on the basis of Bradford (1976). In this connection, bovine serum albumin (BSA) in concentrations of 0-100 μg/ml served as the standard. After an incubation time of at least 5 min at room temperature, the samples were measured in microtiter plates with a microtiter plate photometer from the company Molecular Devices at OD 590 . 
       Measurement of the Xylose Isomerase Activity 
       [0181]    To determine the xylose isomerase activity, recombinant yeast cells containing the vector p426H7-XI-Clos or p426H7-opt.XI-Clos, respectively, were grown, harvested and raw extracts were prepared. Recombinant yeast cells containing the empty vector p426HXT7-6HIS served as a comparison. In a total volume of 1 ml, the conversion of 6.25-500 mM of xylose with 100 pl of raw extract, 0.23 mM of NADH, 10 mM of MgCl 2 , 2 U of sorbitol dehydrogenase in 100 mM of Tris-HCl buffer (pH 7.5) was continuously monitored. The acceptance of NADH as a measured variable was determined spectrophotometrically at a wave length of 340 nm. The reaction was started by adding xylose. 
       Measurement of the Xylitol Inhibition 
       [0182]    To determine the xylitol inhibition of the xylose isomerase recombinant yeast cells containing the vector p426H7-XI-Clos were grown, harvested and raw extracts were prepared. Recombinant yeast cells with the vector p426H7-opt.XI-Piro or the vector p426HXT7-6HIS, respectively, served as a comparison. In a total volume of 1 ml, the conversion of 6.25-500 mM of xylose with 100 μl of raw extract, 10-100 mM of xylitol, 0.23 mM of NADH, 10 mM of MgCl 2 , 2 U of sorbitol dehydrogenase in 100 mM of Tris-HCl buffer (pH 7.5) was continuously monitored. The acceptance of NADH as a measured variable was determined spectrophotometrically at a wave length of 340 nm. The reaction was started by adding xylose. 
       Example 1 
     Screen of a (Highly) Functional Prokaryotic Xylose Isomerase 
     A) Construction of MKY09 
       [0183]    In the yeast strain CEN.PK2-1C, all the genes of the non-oxidative pentose phosphate pathway as well as the xylulokinase (XKS1) and GAL2 were overexpressed. To this end, the endogenous promoters were replaced with the truncated HXT7 promoter. This strain was named MKY09 and used for the screen for functional xylose isomerases. 
       B) Selection of the Xylose Isomerases to be Tested 
       [0184]    To make a selection of the xylose isomerases to be tested, protein sequences of xylose isomerases from the database NCBI BLAST were compared. An excerpt of the xylose isomerase obtained is depicted in  FIG. 3 . 14 xylose isomerases from different organisms were selected to be tested on their functionality in yeast. 
       C) Execution of the Screen 
       [0185]    To this end, genomic DNA was isolated from the organisms. The cells were grown, harvested and disrupted (see “Isolation of plasmid DNA from  S. cerevisiae ” and “Colony PCR from  B. licheniformis  and  S. degradans ”, respectively). The open reading frame (ORF) of XI from the mentioned organisms was amplified with primers additionally having homologous regions to the HXT7 promoter or CYC1 terminator. 
         [0186]    The obtained PCR products were together with the vector p426HXT7-6HIS linearized with EcoRI/BamHI transformed in yeast and cloned via in vivo recombination into the plasmid between the HXT7 promoter or CYC1 terminator, respectively ( FIG. 4 ). The sequence of the plasmids obtained was verified by means of restriction analysis. Furthermore, the functionality of the new isomerases and its effect on the xylose conversion in yeast was to be studied. However, it was not possible to amplify the desired PCR product with the xylose isomerase from the organisms  Streptomyces diastaticus  and  Leifsonia xyli.  Both xylose isomerases thus could not be tested on functionality in yeast. 
       D) Growth Behaviour (Plate) 
       [0187]    Out of the 12 different tested xylose isomerases, a xylose isomerase was found, which was functional in yeast strain MKY09. Recombinant yeasts containing the xylose isomerase from  C. phytofermentans  showed good growth on plates containing xylose ( FIG. 5 ). 
       Example 2 
     Codon Optimization of the Gene for Xylose Degradation in Yeast Codon Optimization of Genes According to the Codon Usage of the Glycolysis Genes from  S. cerevisiae     
       [0188]    The preferred codon usage of the glycolysis genes from  S. cerevisiae  was determined and is listed in table 1. The ORF of the gene XI from  C. phytofermentans  was codon-optimized. That is, the sequences of the open reading frame were adapted to the preferred codon usage indicated below. The protein sequence of the enzymes remained unchanged. The genes were synthesized by an external company and supplied in dried form in company-owned vectors. Further details about the synthesis of genes can be found under www.geneart.com. 
         [0000]                                  TABLE 1                   Preferred codon usage of the glycolytic genes from  S. cerevisiae                      Codon usage of           Amino acid   codon-optimized genes                       Ala   GCT           Arg   AGA           Asn   AAC           Asp   GAC           Cys   TGT           Gln   CAA           Glu   GAA           Gly   GGT           His   CAC           Ile   ATT           Leu   TTG           Lys   AAG           Met   ATG           Phe   TTC           Pro   CCA           Ser   TCT           Thr   ACC           Trp   TGG           Tyr   TAC           Val   GTT           Stop   TAA                        
B) Introduction of the Codon-optimized Xylose Isomerase Gene into the Strain MKY09
 
         [0189]    To test the codon-optimized xylose isomerase gene in strain MKY09, the gene had to be subcloned into a yeast vector. To this end, the codon-optimized XI-ORF was amplified with primers and cloned into the linearized vector p426HXT7-6HIS (see “Execution of the screen”). The sequence of the obtained plasmid p426H7-opt.XI-Clos was verified by means of restriction analysis. To test the functionality of the codon-optimized isomerase, the plasmid p426H7-opt.XI-Clos was transformed in the strain MKY09. Recombinant yeast strains showed good growth on plates with medium containing xylose ( FIG. 5 ). Further characterizations of the native and the codon-optimized XI from  C. phytofermentans  followed. 
       Example 3 
     Characterization of the Functional Prokaryotic Xylose Isomerase A) Growth Behaviour and Xylose Conversion 
       [0190]    The growth of the strain MKY09 with the native and the codon-optimized xylose isomerase from  C. phytofermentans  was investigated in growth tests on medium containing xylose under aerobic conditions. The empty vector p426HXT7-6HIS served as a comparison. 
         [0191]    The strains were grown in SC medium with 0.1% glucose and 1.4% xylose and inoculated with an OD 600nm =0.2 in 50 ml of SC medium with 0.1% glucose and 1.4% xylose. The incubation was performed in shaking flasks under aerobic conditions at 30° C. Samples for the determination of the optical density and for the determination of the metabolite composition were taken several times. 
         [0192]    The growth curves showed that all the recombinant yeasts grew on glucose up to an OD 600  of 2.5 (table 2). After another 50 h, the yeast strain containing the native xylose isomerase from  C. phytofermentans  began to grow further on xylose and reached a final OD 600  of 3.5 at a maximum growth rate of 0.0058 h −1  on medium containing xylose. The yeast strain with the codon-optimized xylose isomerase likewise reached a final OD 600  of 3.5. The maximum growth rate was 0.0072 h −1 . Yeast transformants with the empty vector p426HXT7-6HIS showed no growth on xylose and began to die already after 150 h. 
         [0193]    The recombinant yeasts containing the native xylose isomerase from  C. phytofermentans  or the codon-optimized xylose isomerase, respectively, converted more than 2.6 g of xylose in 312 hours ( FIG. 6 ). 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Determination of the maximum growth rate on xylose (μ) 
               
             
          
           
               
                   
                 MKY09 transformed 
                   
               
               
                   
                 with plasmid 
                 Max. growth rate 
               
               
                   
                   
               
               
                   
                 p426H7-XI-Clos 
                 0.0058 
               
               
                   
                 p426H7-opt.XI-Clos 
                 0.0072 
               
               
                   
                   
               
             
          
         
       
     
         [0194]    It could be shown with this experiment that the introduction of the native as well as the codon-optimized xylose isomerase from  C. phytofermentans  allows the recombinant  S. cerevisiae  strains growth on D-xylose and its conversion. By means of the codon optimization of the xylose isomerase, a higher max. growth rate could be achieved. 
       B) Measurement of the Xylose Isomerase Activity 
       [0195]    Enzyme tests were performed directly after the raw extract preparation. The XI activity was performed at 30° C. in a reaction mix (100 mM of Tris-HCl, pH 7,5; 10 mM of MgCl 2 , 0.23 mM of NADH; 2 U of sorbitol dehydrogenase) with different raw extract concentrations. The reaction was started with 6.25-500 mM of xylose. 
         [0196]    The determination of the enzyme kinetics of the native form of the xylose isomerase resulted in a K m  value of 61.85±3.41 mM and for the codon-optimized form a K m  value of 66.01±1 mM ( FIG. 7  and table 3). As expected, the K m  values were thus the same as they do not differ significantly. 
         [0197]    V max  (μmol/min −1  mg protein −1 ) was 0.0076 for the native form of the xylose isomerase and 0.0344 for the codon-optimized form ( FIG. 7 ). Therefore, V max  could be increased by more than 450% by means of the codon optimization of the enzyme. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 CEN.PK2-1C transformed 
                 V max  (μmol/min −1   
                   
               
               
                   
                 with plasmid 
                 protein −1 ) mg 
                 K m  (mM) 
               
               
                   
                   
               
             
             
               
                   
                 p426H7-XI-Clos 
                 0.0076 
                 61.85 ± 3.4 
               
               
                   
                 p426H7-opt.XI-Clos 
                 0.0344 
                 66.01 ± 1     
               
               
                   
                   
               
             
          
         
       
     
         [0198]    The strain CEN.PK2-1C transformed with the plasmid p426H7-XI-Clos and p426H7-opt.XI-Clos, respectively, was grown over night in synthetic complete medium with 2% glucose and no uracil. Raw extracts were prepared and quantitative enzyme tests were performed. 
       C) Measurement of the Xylitol Inhibition 
       [0199]    The determination of the xylitol inhibition of the xylose isomerases was performed directly after the raw extract preparation. The XI activity was performed at 30° C. in a reaction mix (100 mM of Tris-HCl, pH 7.5; 10 mM of MgCl 2 , 0.23 mM of NADH; 2 U of sorbitol dehydrogenase) with different raw extract concentrations. Additionally, different concentrations of xylitol (10-100 mM) were present in the reaction mix. The reaction was started with 6.25-500 mM of xylose. 
         [0200]    K was determined via the equation K m ′=K m ′=K m * (1+i/K i ), i being the xylitol concentration used and K m ′ being the apparent K m  value at the corresponding xylitol concentration. 
         [0201]    The determination of the kinetics of the xylitol inhibition of the xylose isomerase form  C. phytofermentans  resulted in a K i  value of 14.24±1.48 mM (table 4). As already described several times (Yamanaka et al., 1969 and references cited therein), it is a competitive inhibition. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 CEN.PK2-1C transformed 
                   
               
               
                   
                 with plasmid 
                 K i  (mM) 
               
               
                   
                   
               
             
             
               
                   
                 p426H7-opt.XI-Piro 
                  4.67 ± 1.77 
               
               
                   
                 p426H7-opt.XI-Clos 
                 14.51 ± 1.08 
               
               
                   
                   
               
             
          
         
       
     
         [0202]    The strain CEN.PK2-1C transformed with the plasmid p426H7-opt.XI-Clos and p426H7-opt.XI-Piro, respectively, was grown over night in synthetic complete medium with 2% glucose and no uracil. Raw extracts were prepared and quantitative enzyme tests with constant xylitol concentrations of 10-100 mM were performed. 
         [0203]    The xylose isomerase from  Piromyces  sp.E2 and the empty vector p426HXT7-6HIS served as a comparison. The determined K i  value of the xylose isomerase from  Piromyces  sp.E2 was 4.67±1.77 mM. 
         [0204]    It can be seen from the determined K i  values that the xylose isomerase from  C. phytofermentans  is significantly less inhibited by xylitol than the xylose isomerase from  Piromyces  sp.E2. 
       D) Examples of Vectors for the Xylose Isomerase 
       [0205]    The plasmid p426HXT7-6HIS was the starting plasmid for the construction of p426H7-opt.XI-Clos. The vector is a 2μp expression plasmid, which has a URA3 marker. 
         [0206]    Further possible expression vectors are from the series of pRS303X, p3RS305X and p3RS306X. These are integrative vectors, which have a dominant antibiotic marker. Further details about these vectors can be found in Taxis and Knop (2006). 
       REFERENCES 
       [0207]    Amore, R., Wilhelm, M. and Hollenberg, C.P. (1989) 
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