Patent Publication Number: US-11034949-B2

Title: Chimeric polypeptides having xylose isomerase activity

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of U.S. Ser. No. 14/820,695 filed on Aug. 7, 2015 which claims priority from U.S. patent application Ser. No. 62/035,752 filed on Aug. 11, 2014, each application of which is incorporated by reference in its entirety. 
    
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was funded, in part, by the Bioenergy Science Center, Oak Ridge National Laboratory, a U.S. Department of Energy Bioenergy Research Center supported by the Office of Biological and Environmental Research, under contract DE-P502-06ER64304. The government has certain rights in the invention. 
    
    
     REFERENCE TO SEQUENCE LISTING SUBMITTED ELECTRONICALLY VIA EFS-WEB 
     The content of the electronically submitted sequence listing (Name: 115235-269 Sequence.txt; Size: 78 kilo bytes; Date of Creation: Feb. 7, 2020) is in accordance with 37 C.F.R. § 1.821-1.825, and is incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The field of the invention generally relates to chimeric polypeptides capable of converting xylose to xylulose, engineered host cells that express the chimeric polypeptide, methods of creating chimeric polypeptides, and methods of fermenting cellulosic biomass to produce biofuels, including ethanol. 
     BACKGROUND OF THE INVENTION 
       Saccharomyces cerevisiae  is the primary biocatalyst used in the commercial production of “first generation” fuel ethanol from sugar based substrates such as corn, sugarcane, and sugarbeet. Second generation ethanol production, also known as cellulosic ethanol production, extends the carbohydrate source to more complex polysaccharides, such as cellulose and hemicellulose, which make up a significant portion of most plant cell walls and therefore most plant material. 
     Feedstocks commercially considered for second generation ethanol production include wood, agriculture residues such as corn stover and wheat straw, sugarcane bagasse and purpose grown materials such as switchgrass. The cellulose and hemicellose must be hydrolyzed to monomeric sugars before fermentation using either mechanical/chemical means and/or enzymatic hydrolysis. The liberated monomeric sugars include glucose, xylose, galactose, mannose, and arabinose with glucose and xylose constituting more than 75% of the monomeric sugars in most feedstocks. For cellulosic ethanol production to be economically viable and compete with first generation ethanol, the biocatalyst must be able to convert the majority, if not all, of the available sugars into ethanol. 
       S. cerevisiae  is the preferred organism for first generation ethanol production due to its robustness, high yield, and many years of safe use. However, naturally occurring  S. cerevisiae  is unable to ferment xylose into ethanol. For  S. cerevisiae  to be a viable biocatalyst for second generation ethanol production, it must be able to ferment xylose. 
     There are two metabolic pathways of xylose fermentation that have been demonstrated in  S. cerevisiae . The pathways differ primarily in the conversion of xylose to xylulose. In the first pathway, the XR-XDH pathway, a xylose reductase (XR) converts xylose to xylitol, which is subsequently converted to xylulose by a xylitol dehydrogenase (XDH). The XR and XDH enzyme pairs tested to date differ in required cofactor, NADH and NADPH, leading to difficulties achieving redox balance. The second commonly tried pathway converts xylose directly to xylulose using a xylose isomerase (XI) with no redox cofactor requirements. XIs from both bacterial and fungal systems have been successfully utilized in  S. cerevisiae . Both pathways utilize the same downstream metabolic engineering: up regulation of the native xylulose kinase (XKS1) and four genes of the pentose phosphate pathway, specifically ribulose-phosphate 3-epimerase (RPE1), ribose-5-phosphate ketol-isomerase (RKI1), transaldolase (TAL1), and transketolase (TKL1) ( FIG. 1 ). Use of the XI pathway also commonly entails deletion of the native aldose reductase gene (GRE3) to eliminate product lost to xylitol formation. 
     BRIEF SUMMARY OF THE INVENTION 
     Aspects of the invention are directed to novel chimeric polypeptides capable of converting xylose directly to xylulose and methods of their creation and use are described herein. In some embodiments, the invention is directed to host cells that express one or more chimeric polypeptide. 
     In some embodiments of the present invention, the chimeric polypeptide has at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 1, 3, 5, 7, and/or 9. In some embodiments, the chimeric polypeptide has an amino acid sequence of SEQ ID NOs: 1, 3, 5, 7, or 9. 
     In some embodiments of the present invention, the chimeric polypeptide has at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with SEQ ID NO: 25. In some embodiments, the chimeric polypeptide has an amino acid sequence of SEQ ID NO: 25. 
     In some embodiments of the present invention, the chimeric polypeptide has at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with SEQ ID NO: 27. In some embodiments, the chimeric polypeptide has an amino acid sequence of SEQ ID NO: 27. 
     In some embodiments, the chimeric polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 2, 4, 6, 8, and/or 10. In some embodiments, the chimeric polypeptide is encoded by a polynucleotide sequence of SEQ ID NOs: 2, 4, 6, 8, or 10. In some embodiments, the polynucleotide sequence is contained in a vector. 
     In some embodiments, the chimeric polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with SEQ ID NO: 26. In some embodiments, the chimeric polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 26. In some embodiments, the polynucleotide sequence is contained in a vector. 
     In some embodiments, the chimeric polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with SEQ ID NO: 28. In some embodiments, the chimeric polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 28. In some embodiments, the polynucleotide sequence is contained in a vector. 
     In some aspects of the present invention, the chimeric polypeptide is created by combining two or more fragments from existing XIs. In some embodiments the donor XI sequences are isolated from nucleotide sequences from such as  Bacteroides thetaiotaomicron, Abiotrophia defectiva, Parabacteroides distasonis, Chitinophaga pinensis, Cyllamyces aberensis, Prevotella ruminicola , and/or a related bacteria. In some embodiments, the nucleotide sequences are codon-optimized, e.g. for expression in yeast. In some embodiments, one or more of the donor XI sequences has a nucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 12, 14, 16, 18, 20, 22, or 24. In some embodiments, one or more of the donor XI sequences has a nucleotide sequence of SEQ ID NOs: 12, 14, 16, 18, 20, 22, or 24. In some embodiments, at least one donor XI sequence has a nucleotide sequence encoding a polypeptide having an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 11, 13, 15, 17, 19, 21, and/or 23. In some embodiments, at least one donor XI sequence has a nucleotide sequence encoding a polypeptide having an amino acid sequence of SEQ ID NOs: 11, 13, 15, 17, 19, 21, and/or 23. 
     In some embodiments the chimeric polypeptides are created by utilizing homologous recombination in yeast cells. In some embodiments, the chimeric polypeptides are created using  S. cerevisiae  cells. In some embodiments, the boundaries of the combined DNA fragments are not specifically chosen, but instead recombination of the fragments occurs in the yeast via its native homologous recombination machinery. In some embodiments, a chimeric XI can be identified and selected for by growing recombinant yeast cells on a medium having xylose as the only carbon source. In some embodiments, colonies capable of growing on xylose as the sole carbon source are selected and characterized as expressing a functional chimeric polypeptide capable of converting xylose to xylulose. 
     In some embodiments, the donor XI sequence encodes an XI polypeptide capable of conferring xylose isomerase activity onto a host cell, e.g. a  S. cerevisiae  cell. In some embodiments, the donor XI sequence encodes an XI polypeptide that is not capable of conferring xylose isomerase activity onto a host cell, e.g. a  S. cerevisiae  cell. In some embodiments, the chimeric polypeptide is comprised of at least two donor sequences, wherein at least one donor sequence encodes an XI capable of conferring xylose isomerase activity onto a host cell, e.g. a  S. cerevisiae  cell, and at least one donor sequence encodes an XI not capable of conferring xylose isomerase activity onto a host cell, e.g. a  S. cerevisiae  cell. 
     In some embodiments, a host cell is engineered to express one or more of the chimeric polypeptides. In some embodiments, the host cell is a yeast cell, e.g. a  S. cerevisiae  cell. In some embodiments the host cell is further modified to have mutations affecting at least one gene encoding a protein involved in the pentose phosphate pathway. In some embodiments, the host cell has at least one mutation that increases the expression or causes the up-regulation of XKS1, RKI1, RPE1, TKL1, and/or TAL1. In some embodiments, the host cell has a modification of one or more aldose reductase genes. In some embodiments, the aldose reductase gene is GRE3. In some embodiments, the host cell has a deletion or disruption of all or part of the endogenous GRE3 gene. In some embodiments, the aldose reductase gene is YPR1. In some embodiments, the host cell has a deletion or disruption of all or part of the endogenous YPR1 gene. In some embodiments, the host cell has a deletion or disruption of all or part of both the endogenous GRE3 gene and the endogenous YPR1 gene. In some embodiments, the host cell has a modification of PGM1 (phosphoglucomutase 1) and/or PGM2. In some embodiments, the host cell overexpresses PGM1 and/or PGM2. In some embodiments, the host cell has increased levels of Pgm1 and/or Pgm2 polypeptide and/or mRNA relative to a comparable host cell lacking a modification of PGM1 and/or PGM2. 
     In some embodiments, the host cell comprises a deletion or disruption of one or more endogenous enzymes that function to produce glycerol and/or regulate glycerol synthesis. In some embodiments, the host cell produces less glycerol than a control recombinant microorganism without deletion or disruption of said one or more endogenous enzymes that function to produce glycerol and/or regulate glycerol synthesis. In some embodiments, the one or more endogenous enzymes that function to produce glycerol are encoded by a GPD1 polynucleotide, a GPD2 polynucleotide, or both a GPD1 polynucleotide and a GPD2 polynucleotide. In some embodiments, one or both of the endogenous GPD1 and/or GPD2 genes are modified by mutation or deletion. In some embodiments, the host cell comprises a heterologous ADHE sequence. In some embodiments, the heterologous ADHE is from  Bifidobacterium adolescentis . In some embodiments the native STL1 gene is upregulated by either modifying the promoter of the native copies or by introducing additional copies of STL1. In some embodiments the host cell comprises an ortholog of the native STL1. In some embodiments the native ACS2 gene is upregulated by either modifying the promoter of the native copies or by introducing additional copies of ACS2. In some embodiments, a yeast strain is used as the host cell. In some embodiments, the background of the yeast strain is an industrial yeast strain. One having ordinary skill in the art would be aware of many potential known yeast strains that can be modified according to the present invention, and this invention contemplates all such potential background yeast strains. 
     In some embodiments of the invention, the recombinant host cell is used to produce a fermentation product from a cellulosic or lignocellulosic material. In some embodiments, the fermentation product is ethanol, lactic acid, 3-hydroxy-propionic acid, hydrogen, butyric acid, acrylic acid, acetic acid, succinic acid, citric acid, malic acid, fumaric acid, an amino acid, 1,3-propane-diol, ethylene, glycerol, acetone, isopropyl alcohol, butanol, a β-lactam, an antibiotic, a cephalosporin, or a combination thereof. In some embodiments, the cellulosic or lignocellulosic material is insoluble cellulose, crystalline cellulose, pretreated hardwood, paper sludge, pretreated corn stover, pretreated sugar cane bagasse, pretreated corn cobs, pretreated switchgrass, pretreated municipal solid waste, pretreated distiller&#39;s dried grains, pretreated wheat straw, corn fiber, agave, or a combination thereof. 
     In some embodiments, one or more of the chimeric polypeptides are purified. In some embodiments, the one or more chimeric polypeptides are purified from a recombinant yeast host cell of the invention, a composition of the invention, a media supernatant of the invention, or a yeast strain of the invention. In some embodiments, one or more chimeric polypeptides in the composition are from a crude extract. In some embodiments, the crude extract is from a recombinant yeast host cell of the invention, a composition of the invention, a media supernatant of the invention, or a yeast strain of the invention. 
     One aspect of the invention is directed to a composition comprising a lignocellulosic material and a recombinant yeast host cell comprising as least one chimeric polypeptide having xylose isomerase activity. Another aspect of the invention is directed to a media supernatant generated by incubating a recombinant yeast host comprising as least one chimeric polypeptide having xylose isomerase activity with a medium containing xylose as the only carbon source. In some embodiments, the medium comprises a cellulosic or lignocellulosic material. In some embodiments, the cellulosic or lignocellulosic material is insoluble cellulose, crystalline cellulose, pretreated hardwood, paper sludge, saw mill or paper mill discards, pretreated corn stover, pretreated sugar cane bagasse, pretreated corn cobs, pretreated switchgrass, pretreated municipal solid waste, pretreated distiller&#39;s dried grains, pretreated wheat straw, corn fiber, agave, or a combination thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic representation of xylose fermentation in genetically engineered  S. cerevisiae.    
         FIG. 2  depicts a schematic representation of the vector used to create the XI library. 
         FIG. 3  depicts a schematic representation of an example of the pieces used to assemble a chimeric XI library.  FIG. 3A  illustrates the process of assembling a vector backbone comprising 5′ and 3′ fragments of the  B. thetaiotaomicron  (Bt) XI coding sequence together with the yeast HSP150 promoter and HXT7 terminator regulatory elements. The vector does not contain the complete Bt XI sequence and therefore does not contain a functional XI and also cannot circularize via homologous recombination resulting in a linear non-replicating DNA fragment. The black bar representing the vector backbone would contain DNA sequences required for selection and replication of the plasmid in  S. cerevisiae  (e.g. 2 micron or CEN origin of replication, antibiotic selection marker such as ZEO)  FIG. 3B  provides representative drawings of individual examples of XI donor sequences to be recombined with the vector DNA depicted in  FIG. 3A  via yeast native homologous recombination resulting in a circular plasmid and in some cases a functional XI. The examples shown in  FIG. 3  are not exhaustive of the XI donor sequence libraries contemplated or described herein. 
         FIG. 4  depicts a schematic representation of XI fragments used in the chimeric XI library assembly.  FIG. 4A  shows methodology and primers used for the amplification of functional parent XIs.  FIG. 4B  shows the methodology and primers used for the amplification of non-functional parent XIs. 
         FIG. 5  depicts a schematic representation of homologous recombination, creating chimeric polypeptide encoding sequences resulting from at least two donor sequences, designated here as “A” and “B”. Solid black segments represent the vector backbone. 
         FIG. 6  depicts a schematic representation of example chimeric polypeptides having xylose isomerase activity created using the methods described herein. Figures A-E represent the chimeric polynucleotides having the nucleotide sequences represented by SEQ ID NOs: 2, 4, 6, 8, and 10 respectively, and the chimeric polypeptides having the amino acid sequences represented by SEQ ID NOs: 1, 3, 5, 7, and 9, respectively. Figure F represents the chimeric polynucleotides having the nucleotide sequences represented by SEQ ID NOs: 26 or 28, and the chimeric polypeptides having the amino acid sequences represented by SEQ ID NOs: 25 or 27. 
         FIG. 7  provides an example of the relative growth of yeast cells expressing a chimeric polypeptide having xylose isomerase activity, having an amino acid sequence of SEQ ID NO:1, on xylose as compared to  B. thetaiotaomicron  xylose isomerase, in two different host strains. 
         FIGS. 8A , B and C provide examples of the relative growth of yeast cells expressing various chimeric polypeptides having xylose isomerase activity, wherein the selected chimeric sequences are integrated within the xylose screening background. “Chimeric XI 1” refers to the chimeric XI represented by the amino acid sequence of SEQ ID NO:1, “Chimeric XI 2” refers to SEQ ID NO:3, “Chimeric XI 3” refers to SEQ ID NO:5, “Chimeric XI 4” refers to SEQ ID NO:7, “Chimeric XI 5” refers to SEQ ID NO:9, “Chimeric XI 6” refers to the chimeric XI represented by the amino acid sequence of SEQ ID NO:25, and “Chimeric XI 7” refers to the chimeric XI represented by the amino acid sequence of SEQ ID NO:7. 
         FIG. 9  provides an example of ethanol production by yeast cells expressing a chimeric polypeptide having xylose isomerase activity, having an amino acid sequence of SEQ ID NO:1, on xylose, wherein the cells optionally have a deletion of the endogenous YPR1 aldose reductase gene (“ypr1Δ”). 
         FIG. 10  provides an example of the relative growth of yeast cells expressing a chimeric polypeptide having xylose isomerase activity, having an amino acid sequence of SEQ ID NO:1, on xylose, wherein the cells optionally over express (“o/e”) PGM1. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     I. Definitions 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art of microbial metabolic engineering. Although methods and materials similar or equivalent to those described herein can be used in the practice of the disclosed methods and compositions, exemplary methods, devices and materials are described herein. 
     The embodiment(s) described, and references in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment(s) described can include a particular feature, structure, or characteristic, but every embodiment does not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     The description of “a” or “an” item herein refers to a single item or multiple items. It is understood that wherever embodiments are described herein with the language “comprising,” otherwise analogous embodiments described in terms of “consisting of” and/or “consisting essentially of” are also provided. Thus, for example, reference to “a polynucleotide” includes a plurality of such polynucleotides and reference to “the microorganism” includes reference to one or more microorganisms, and so forth. 
     A “fragment” refers to any portion of a nucleic or amino acid sequence that is less than the entire sequence. A fragment of a nucleotide or an amino acid sequence can be any length of nucleotides or amino acids that is less than the entire length of the cited sequence and more than two nucleotides or amino acids in length. In some embodiments, the fragment can be from a donor sequence. 
     A “vector,” e.g., a “plasmid” or “YAC” (yeast artificial chromosome) refers to an extrachromosomal element often carrying one or more genes that are not part of the central metabolism of the cell, and can be in the form of a linear or circular double-stranded DNA molecule. Vectors and plasmids can be autonomously replicating sequences, genome integrating sequences, phage or nucleotide sequences, linear, circular, or supercoiled, of a single- or double-stranded DNA or RNA, derived from any source, in which a number of nucleotide sequences have been joined or recombined into a unique construction which is capable of introducing a promoter fragment and DNA sequence for a selected gene product along with appropriate 3′ untranslated sequence into a cell. 
     An “expression vector” is a vector that is capable of directing the expression of genes to which it is operably associated. 
     The term “integrated” as used herein refers to genetic elements that are placed, through molecular biology techniques, into the genome of a host cell. For example, genetic elements can be placed into the chromosomes of the host cell as opposed to in a plasmid carried by the host cell. Methods for integrating genetic elements into the genome of a host cell are well known in the art and include homologous recombination. In some embodiments, more than one copy of the genetic elements are placed into the genome of a host cell. In some embodiments, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more copies of the genetic elements are placed into the genome of a host cell. 
     The term “heterologous” when used in reference to a polynucleotide, a gene, a polypeptide, or an enzyme refers to a polynucleotide, gene, polypeptide, or an enzyme not normally found in the host organism. “Heterologous” also includes a native coding region, or portion thereof, that is removed from the source organism and subsequently reintroduced into the source organism in a form that is different from the corresponding native gene, e.g., not in its natural location in the organism&#39;s genome. The heterologous polynucleotide or gene can be introduced into the host organism by, e.g., gene transfer. A heterologous gene can include a native coding region that is a portion of a chimeric gene including non-native regulatory regions that is reintroduced into the native host. Foreign genes can comprise native genes inserted into a non-native organism, or chimeric genes. A heterologous polynucleotide, gene, polypeptide, or an enzyme can be derived from any source, e.g., eukaryotes, prokaryotes, viruses, or synthetic polynucleotide fragments. The term “heterologous” as used herein also refers to an element of a vector, plasmid or host cell that is derived from a source other than the endogenous source. Thus, for example, a heterologous sequence could be a sequence that is derived from a different gene or plasmid from the same host, from a different strain of host cell, or from an organism of a different taxonomic group (e.g., different kingdom, phylum, class, order, family, genus, or species, or any subgroup within one of these classifications). The term “heterologous” is also used synonymously herein with the term “exogenous.” The term “heterologous expression” refers to the expression of a heterologous polynucleotide or gene by a host. 
     The term “domain” as used herein refers to a part of a molecule or structure that shares common physical or chemical features, for example hydrophobic, polar, globular, helical domains or properties, e.g., a DNA binding domain or an ATP binding domain. Domains can be identified by their homology to conserved structural or functional motifs. Examples of cellobiohydrolase (CBH) domains include the catalytic domain (CD) and the cellulose binding domain (CBD). 
     A “nucleic acid,” “polynucleotide,” or “nucleic acid molecule” is a polymeric compound comprised of covalently linked subunits called nucleotides. Nucleic acid includes polyribonucleic acid (RNA) and polydeoxyribonucleic acid (DNA), both of which can be single-stranded or double-stranded. DNA includes cDNA, genomic DNA, synthetic DNA, and semi-synthetic DNA. 
     An “isolated nucleic acid molecule” or “isolated nucleic acid fragment” refers to the phosphate ester polymeric form of ribonucleosides (adenosine, guanosine, uridine, or cytidine; “RNA molecules”) or deoxyribonucleosides (deoxyadenosine, deoxyguanosine, deoxythymidine, or deoxycytidine; “DNA molecules”), or any phosphoester analogs thereof, such as phosphorothioates and thioesters, in either single stranded form, or a double-stranded helix. Double stranded DNA-DNA, DNA-RNA and RNA-RNA helices are possible. The term nucleic acid molecule, and in particular DNA or RNA molecule, refers only to the primary and secondary structure of the molecule, and does not limit it to any particular tertiary forms. Thus, this term includes double-stranded DNA found, inter alfa, in linear or circular DNA molecules (e.g., restriction fragments), plasmids, and chromosomes. In discussing the structure of particular double-stranded DNA molecules, sequences are described herein according to the normal convention of giving only the sequence in the 5′ to 3′ direction along the non-transcribed strand of DNA (i.e., the strand having a sequence homologous to the mRNA). 
     A “gene” refers to an assembly of nucleotides that encode a polypeptide, and includes cDNA and genomic DNA nucleic acids. “Gene” also refers to a nucleic acid fragment that expresses a specific protein, including intervening sequences (introns) between individual coding segments (exons), as well as regulatory sequences preceding (5′ non-coding sequences) and following (3′ non-coding sequences) the coding sequence. “Native gene” refers to a gene as found in nature with its own regulatory sequences. The terms “gene(s)” or “polynucleotide” or “nucleic acid” or “polynucleotide sequence(s)” are intended to include nucleic acid molecules, e.g., polynucleotides which include an open reading frame encoding a polypeptide, and can further include non-coding regulatory sequences, and introns. In addition, the terms are intended to include one or more genes that map to a functional locus. Also, the terms are intended to include a specific gene for a selected purpose. The gene can be endogenous to the host cell or can be recombinantly introduced into the host cell, e.g., as a plasmid maintained episomally or a plasmid (or fragment thereof) that is stably integrated into the genome. In addition to the plasmid form, a gene can, for example, be in the form of linear DNA or RNA. The term “gene” is also intended to refer to multiple copies of a particular gene, e.g., all of the DNA sequences in a cell encoding a particular gene product. 
     A nucleic acid molecule is “hybridizable” to another nucleic acid molecule, such as a cDNA, genomic DNA, or RNA, when a single stranded form of the nucleic acid molecule can anneal to the other nucleic acid molecule under the appropriate conditions of temperature and solution ionic strength. Hybridization and washing conditions are well known and exemplified, e.g., in Sambrook, J., Fritsch, E. F. and Maniatis, T. MOLECULAR CLONING: A LABORATORY MANUAL, Second Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor (1989), particularly Chapter 11 and Table 11.1 therein (hereinafter “Maniatis”, entirely incorporated herein by reference). The conditions of temperature and ionic strength determine the “stringency” of the hybridization. Stringency conditions can be adjusted to screen for moderately similar fragments, such as homologous sequences from distantly related organisms, to highly similar fragments, such as genes that duplicate functional enzymes from closely related organisms. Post-hybridization washes determine stringency conditions. One set of conditions uses a series of washes starting with 6×SSC, 0.5% SDS at room temperature for 15 min, then repeated with 2×SSC, 0.5% SDS at 45° C. for 30 min, and then repeated twice with 0.2×SSC, 0.5% SDS at 50° C. for 30 min. For more stringent conditions, washes are performed at higher temperatures in which the washes are identical to those above except for the temperature of the final two 30 min washes in 0.2×SSC, 0.5% SDS are increased to 60° C. Another set of highly stringent conditions uses two final washes in 0.1×SSC, 0.1% SDS at 65° C. An additional set of highly stringent conditions are defined by hybridization at 0.1×SSC, 0.1% SDS, 65° C. and washed with 2×SSC, 0.1% SDS followed by 0.1×SSC, 0.1% SDS. 
     Hybridization requires that the two nucleic acids contain complementary sequences, although depending on the stringency of the hybridization, mismatches between bases are possible. The appropriate stringency for hybridizing nucleic acids depends on the length of the nucleic acids and the degree of complementation, variables well known in the art. The greater the degree of similarity or homology between two nucleotide sequences, the greater the value of Tm for hybrids of nucleic acids having those sequences. The relative stability (corresponding to higher Tm) of nucleic acid hybridizations decreases in the following order: RNA:RNA, DNA:RNA, DNA:DNA. For hybrids of greater than 100 nucleotides in length, equations for calculating Tm have been derived (see, e.g., Maniatis at 9.50-9.51). For hybridizations with shorter nucleic acids, i.e., oligonucleotides, the position of mismatches becomes more important, and the length of the oligonucleotide determines its specificity (see, e.g., Maniatis, at 11.7-11.8). In one embodiment the length for a hybridizable nucleic acid is at least about 10 nucleotides. Preferably a minimum length for a hybridizable nucleic acid is at least about 15 nucleotides; more preferably at least about 20 nucleotides; and most preferably the length is at least 30 nucleotides. Furthermore, the skilled artisan will recognize that the temperature and wash solution salt concentration can be adjusted as necessary according to factors such as length of the probe. 
     As used herein the term “codon-optimized” means that a nucleic acid coding region has been adapted for expression in the cells of a given organism by replacing one, or more than one, or a significant number, of codons with one or more codons that are more frequently used in the genes of that organism. 
     The term “percent identity”, as known in the art, is a relationship between two or more polypeptide sequences or two or more polynucleotide sequences, as determined by comparing the sequences. In the art, “identity” also means the degree of sequence relatedness between polypeptide or polynucleotide sequences, as the case can be, as determined by the match between strings of such sequences. 
     As known in the art, “similarity” between two polypeptides is determined by comparing the amino acid sequence and conserved amino acid substitutes thereto of the polypeptide to the sequence of a second polypeptide. Similarity can be between two full sequences, or between a fragment of one sequence and a fragment of a second sequence wherein the fragments are of comparable length or size, or between a fragment of one sequence and the entirety of a second sequence. 
     “Identity” and “similarity” can be readily calculated by known methods, including but not limited to those described in: Computational Molecular Biology (Lesk, A. M, ed.) Oxford University Press, NY (1988); Biocomputing: Informatics and Genome Projects (Smith, D. W., ed.) Academic Press, NY (1993); Computer Analysis of Sequence Data, Part I (Griffin, A. M., and Griffin, H. G., eds.) Humana Press, NJ (1994); Sequence Analysis in Molecular Biology (von Heinje, G., ed.) Academic Press (1987); and Sequence Analysis Primer (Gribskov, M. and Devereux, J., eds.) Stockton Press, NY (1991). Preferred methods to determine identity are designed to give the best match between the sequences tested. Methods to determine identity and similarity are codified in publicly available computer programs. Sequence alignments and percent identity calculations can be performed using the Megalign program of the LASERGENE bioinformatics computing suite (DNASTAR Inc., Madison, Wis.). Multiple alignments of the sequences disclosed herein were performed using the Clustal method of alignment (Higgins and Sharp (1989) CABIOS. 5:151-153) with the default parameters (GAP PENALTY=10, GAP LENGTH PENALTY=10). Default parameters for pairwise alignments using the Clustal method were KTUPLE 1, GAP PENALTY=3, WINDOW=5 and DIAGONALS SAVED=5. 
     Suitable nucleic acid sequences or fragments thereof (isolated polynucleotides of the present invention) encode polypeptides that are at least about 70% to about 75% identical to the amino acid sequences reported herein, at least about 80%, at least about 85%, at least about 86%, at least about 87%, at least about 88%, at least about 89%, or at least about 90% identical to the amino acid sequences reported herein, at least about 91%, at least about 92%, at least about 93%, at least about 94%, or at least about 95% identical to the amino acid sequences reported herein, or at least about 96%, at least about 97%, at least about 98%, at least about 99%, or about 100% identical to the amino acid sequences reported herein. Suitable nucleic acid fragments are at least about 50%, at least about 55%, at least about 60%, at least about 65%, at least about 70%, at least about 75%, at least about 76%, at least about 77%, at least about 78%, at least about 79%, at least about 80%, at least about 81%, at least about 82%, at least about 83%, at least about 84%, at least about 85%, at least about 86%, at least about 87%, at least about 88%, at least about 89%, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, or about 100% identical to the nucleic acid sequences reported herein. Suitable nucleic acid fragments not only have the above identities/similarities but typically encode a polypeptide having at least 50 amino acids, at least 100 amino acids, at least 150 amino acids, at least 200 amino acids, or at least 250 amino acids. 
     A DNA or RNA “coding region” is a DNA or RNA molecule which is transcribed and/or translated into a polypeptide in a cell in vitro or in vivo when placed under the control of appropriate regulatory sequences. “Suitable regulatory regions” refer to nucleic acid regions located upstream (5′ non-coding sequences), within, or downstream (3′ non-coding sequences) of a coding region, and which influence the transcription, RNA processing or stability, or translation of the associated coding region. Regulatory regions include promoters, translation leader sequences, RNA processing site, effector binding site and stem-loop structure. The boundaries of the coding region are determined by a start codon at the 5′ (amino) terminus and a translation stop codon at the 3′ (carboxyl) terminus. A coding region can include, but is not limited to, prokaryotic regions, cDNA from mRNA, genomic DNA molecules, synthetic DNA molecules, or RNA molecules. If the coding region is intended for expression in a eukaryotic cell, a polyadenylation signal and transcription termination sequence will usually be located 3′ to the coding region. 
     An “isoform” is a protein that has the same function as another protein but which is encoded by a different gene and can have small differences in its sequence. 
     A “paralogue” is a protein encoded by a gene related by duplication within a genome. 
     An “orthologue” is gene from a different species that has evolved from a common ancestral gene by speciation. Normally, orthologues retain the same function in the course of evolution as the ancestral gene. 
     “Open reading frame” is abbreviated ORF and means a length of nucleic acid, either DNA, cDNA or RNA, that comprises a translation start signal or initiation codon, such as an ATG or AUG, and a termination codon and can be potentially translated into a polypeptide sequence. 
     “Promoter” refers to a DNA fragment capable of controlling the expression of a coding sequence or functional RNA. In general, a coding region is located 3′ to a promoter. Promoters can be isolated in their entirety from a native gene, or be composed of different elements derived from different promoters found in nature, or even comprise synthetic DNA segments. It is understood by those skilled in the art that different promoters can direct the expression of a gene in different tissues or cell types, or at different stages of development, or in response to different environmental or physiological conditions. Promoters which cause a gene to be expressed in most cell types at most times are commonly referred to as “constitutive promoters”. It is further recognized that since in most cases the exact boundaries of regulatory sequences have not been completely defined, DNA fragments of different lengths can have identical promoter activity. A promoter is generally bounded at its 3′ terminus by the transcription initiation site and extends upstream (5′ direction) to include the minimum number of bases or elements necessary to initiate transcription at levels detectable above background. Within the promoter will be found a transcription initiation site (conveniently defined for example, by mapping with nuclease SI), as well as protein binding domains (consensus sequences) responsible for the binding of RNA polymerase. Several promoters are specifically identified by the present invention, however, one having ordinary skill in the art would understand that any number of additional promoters capable of driving the expression in yeast would be included in the present invention. 
     The term “linker” as used herein refers to a series of nucleotides or amino acids that connect one section of the chimeric polynucleotide or polypeptide to another section of the chimeric polynucleotide of polypeptide. In some embodiments, the linker serves a structural function. 
     A coding region is “under the control” of transcriptional and translational control elements in a cell when RNA polymerase transcribes the coding region into mRNA, which is then trans-RNA spliced (if the coding region contains introns) and translated into the protein encoded by the coding region. 
     “Transcriptional and translational control regions” are DNA regulatory regions, such as promoters, enhancers, terminators, and the like, that provide for the expression of a coding region in a host cell. In eukaryotic cells, polyadenylation signals are control regions. 
     As used herein the term “N-terminal region” refers to the portion of the amino acid sequence consisting of the most N-terminal amino acid residue up to the amino acid residue at position n/2, wherein n is the total number of residues in the sequence. As used herein the term “C-terminal region” refers to the portion of the amino acid sequence consisting of the most C-terminal amino acid residue up to the amino acid residue at position n/2, wherein n is the total number of residues in the sequence. 
     The term “operably associated” refers to the association of nucleic acid sequences on a single nucleic acid fragment so that the function of one is affected by the other. For example, a promoter is operably associated with a coding region when it is capable of affecting the expression of that coding region (i.e., that the coding region is under the transcriptional control of the promoter). Coding regions can be operably associated to regulatory regions in sense or antisense orientation. 
     The term “expression,” as used herein, refers to the transcription and stable accumulation of sense (mRNA) or antisense RNA derived from the nucleic acid fragment of the invention. Expression can also refer to translation of mRNA into a polypeptide. 
     The term “lignocellulose” refers to material that is comprised of lignin and cellulose. Examples of lignocelluloses are provided herein and are known in the art. Examples of lignocellulosic materials include but are not limited to corn stover, straw, bagasse, switchgrass, paper, and wood. 
     The “pentose phosphate pathway” or “PPP” refers to a biochemical pathway that creates NADPH from glucose-6-P. The PPP has both an oxidative phase and a non-oxidative phase. There are several enzymes that have been identified to play a role in the PPP, including but not limited to glucose-6-P dehydrogenase, gluconolactonase, 6-phosphogluconate dehydrogenase, ribulose-5-phosphate isomerase, ribose-5-phosphate ketol-isomerase (RKI1), ribulose-5-phosphate 3-epimerase (RPE1), transketolase (TKL1), and transaldolase (TAL1). 
     As used herein “xylose isomerase activity” refers to the ability of an enzyme to directly convert xylose to xylulose. A “xylose isomerase” or “XI” as used herein refers to a protein having xylose isomerase activity. 
     The term “chimeric” or “chimera” refers to a polynucleotide or polypeptide having a nucleotide or polypeptide sequence derived from two or more distinct parent sequences. A “parent sequence” or “donor sequence” is a nucleotide or amino acid sequence used as a source sequence to create the chimeric polynucleotide or polypeptide. 
     As used herein the term “XYM1” or “XYM2” refers to a xylose isomerase coding sequence or polypeptide isolated from an uncultured bacterium as described by Parachin and Gorwa-Grauslund, “Isolation of xylose isomerase by sequence- and function-based screening from a soil metagenome library,”  Biotechnology Biofuels  4(1):9 (2011). 
     As used herein, the term “anaerobic” refers to an organism, biochemical reaction, or process that is active or occurs under conditions of an absence of gaseous O 2 . 
     “Anaerobic conditions” are defined as conditions under which the oxygen concentration in the fermentation medium is too low for the microorganism to use it as a terminal electron acceptor. Anaerobic conditions can be achieved by sparging a fermentation medium with an inert gas such as nitrogen until oxygen is no longer available to the microorganism as a terminal electron acceptor. Alternatively, anaerobic conditions can be achieved by the microorganism consuming the available oxygen of fermentation until oxygen is unavailable to the microorganism as a terminal electron acceptor. 
     “Aerobic metabolism” refers to a biochemical process in which oxygen is used as a terminal electron acceptor to convert energy, typically in the form of ATP, from carbohydrates. Aerobic metabolism typically occurs, for example, via the electron transport chain in mitochondria in eukaryotes, wherein a single glucose molecule is metabolized completely into carbon dioxide in the presence of oxygen. 
     In contrast, “anaerobic metabolism” refers to a biochemical process in which oxygen is not the final acceptor of electrons generated. Anaerobic metabolism can be divided into anaerobic respiration, in which compounds other than oxygen serve as the terminal electron acceptor, and substrate level phosphorylation, in which no exogenous electron acceptor is used and products of an intermediate oxidation state are generated via a “fermentative pathway.” 
     In “fermentative pathways”, the amount of NAD(P)H generated by glycolysis is balanced by the consumption of the same amount of NAD(P)H in subsequent steps. For example, in one of the fermentative pathways of certain yeast strains, NAD(P)H generated through glycolysis donates its electrons to acetaldehyde, yielding ethanol. Fermentative pathways are usually active under anaerobic conditions but can also occur under aerobic conditions, under conditions where NADH is not fully oxidized via the respiratory chain. 
     As used herein, the term “end-product” refers to a chemical compound that is not or cannot be used by a cell, and so is excreted or allowed to diffuse into the extracellular environment. Common examples of end-products from anaerobic fermentation include, but are not limited to, ethanol, acetic acid, formic acid, lactic acid, hydrogen, and carbon dioxide. 
     As used herein, “cofactors” are compounds involved in biochemical reactions that are recycled within the cells and remain at approximately steady state levels. Common examples of cofactors involved in anaerobic fermentation include, but are not limited to, NAD +  and NADP + . In metabolism, a cofactor can act in oxidation-reduction reactions to accept or donate electrons. When organic compounds are broken down by oxidation in metabolism, their energy can be transferred to NAD +  by its reduction to NADH, to NADP +  by its reduction to NADPH, or to another cofactor, FAD + , by its reduction to FADH 2 . The reduced cofactors can then be used as a substrate for a reductase. 
     As used herein, a “pathway” is a group of biochemical reactions that together can convert one compound into another compound in a step-wise process. A product of the first step in a pathway can be a substrate for the second step, and a product of the second step can be a substrate for the third, and so on. Pathways of the present invention include, but are not limited to, the pentose phosphate pathway, the xylose utilization pathway, the ethanol production pathway, and the glycerol production pathway. The term “recombination” or “recombinant” refers to the physical exchange of DNA between two identical (homologous), or nearly identical, DNA molecules. Recombination can be used for targeted gene deletion or to modify the sequence of a gene. The terms “recombinant microorganism” and “recombinant host cell” are used interchangeably herein and refer to microorganisms that have been genetically modified to express or over-express endogenous polynucleotides, or to express heterologous polynucleotides, such as those included in a vector, or which have a modification in expression of an endogenous gene. 
     By “expression modification” it is meant that the expression of the gene, or level of a RNA molecule or equivalent RNA molecules encoding one or more polypeptides or polypeptide subunits, or activity of one or more polypeptides or polypeptide subunits is up regulated or down-regulated, such that expression, level, or activity, is greater than or less than that observed in the absence of the modification. 
     In one aspect of the invention, genes or particular polynucleotide sequences are partially, substantially, or completely deleted, silenced, inactivated, or down-regulated in order to inactivate the enzymatic activity they encode. Complete deletions provide maximum stability because there is no opportunity for a reverse mutation to restore function. Alternatively, genes can be partially, substantially, or completely deleted, silenced, inactivated, or down-regulated by insertion, deletion, removal, or substitution of nucleic acid sequences that disrupt the function and/or expression of the gene. 
     II. Chimeric Polypeptides 
     The present invention provides polypeptides having xylose isomerase activity and the making and use thereof. In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 1, 3, 5, 7, or 9. In some embodiments, the polypeptide has an amino acid sequence of SEQ ID NOs: 1, 3, 5, 7, or 9. In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 2, 4, 6, 8, and/or 10. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NOs: 2, 4, 6, 8, or 10. 
     In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with SEQ ID NO: 25. In some embodiments, the polypeptide has an amino acid sequence of SEQ ID NO: 25. In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with SEQ ID NO: 26. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 26. 
     In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with SEQ ID NO: 27. In some embodiments, the polypeptide has an amino acid sequence of SEQ ID NO: 27. In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with SEQ ID NO: 28. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 28. 
     In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 1. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 1. 
     In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 3. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 3. 
     In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 5. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 5. 
     In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 7. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 7. 
     In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 9. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 9. 
     In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 25. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 25. 
     In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 27. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 27. 
     In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 2. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 2. 
     In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 4. 
     In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 9′7%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 6. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 6. 
     In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 8. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 8. 
     In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 10. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 10. 
     In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 26. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 26. 
     In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 28. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 28. 
     In some embodiments, the C-terminal region of the polypeptide and the N-terminal region of the polypeptide comprise one or more fragments that share sequence homology with one or more donor sequences. In some embodiments, the fragment is between about 25 and about 275, between about 50 and about 275, between about 75 and about 275, between about 100 and about 275, between about 125 and about 275, between about 150 and about 275, between about 175 and about 275, between about 200 and about 275, between about 225 and about 275, between about 250 and about 275 amino acids in length. In some embodiments, the fragment is between about 25 and about 50, between about 25 and about 75, between about 25 and about 100, between about 25 and about 125, between about 25 and about 150, between about 25 and about 175, between about 25 and about 200, between about 25 and about 225, between about 25 and about 250 amino acids in length. In some embodiments, the fragment is between about 50 and about 250, between about 75 and about 225, between about 100 and about 200, between about 125 and about 175, between about 25 and about 100, between about 50 and about 125, between about 75 and about 150, between about 100 and about 175, between about 125 and about 200, between about 150 and about 225, between about 175 and about 250, between about 200 and about 275 amino acids in length. In some embodiments, the C-terminal fragment comprises the most C-terminal at least 25, at least 50, at least 75, at least 100, at least 125, at least 150, at least 175, at least 200, at least 225, at least 250, or at least 275 amino acids of the donor polypeptide. In some embodiments, the C-terminal fragment comprises at least 25, at least 50, at least 75, at least 100, at least 125, at least 150, at least 175, at least 200, at least 225, at least 250, or at least 275 amino acids but does not comprise the most C-terminal amino acid of the donor polypeptide. In some embodiments, the N-terminal fragment comprises the most N-terminal at least 25, at least 50, at least 75, at least 100, at least 125, at least 150, at least 175, at least 200, at least 225, at least 250, or at least 275 amino acids of the donor polypeptide. In some embodiments, the N-terminal fragment comprises at least 25, at least 50, at least 75, at least 100, at least 125, at least 150, at least 175, at least 200, at least 225, at least 250, or at least 275 amino acids but does not comprise the most N-terminal amino acid of the donor polypeptide. 
     In some embodiments, the C-terminal region of the polypeptide comprises at least 5, at least 10, at least 15, at least 20, at least 25, at least 30, at least 40, at least 45, or at least 50 contiguous amino acids from the amino acid sequence of SEQ ID NO: 13. In some embodiments, the polypeptide comprises residues 186-442 of SEQ ID NO: 13 or residues 142-442 of SEQ ID NO:13, wherein the positions in the polypeptide align with the positions in SEQ ID NO: 13. In some embodiments, a fragment of the C-terminal region of the polypeptide comprises an amino acid sequence that is at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 100% identical to a comparably sized fragment of SEQ ID NO:13. In some embodiments, the C-terminal fragment of the polypeptide is at least 25, at least 50, at least 75, at least 100, at least 125, at least 150, at least 175, at least 200, at least 225, at least 250, or at least 275 amino acids in length. 
     In some embodiments, the C-terminal region of the polypeptide comprises at least 5, at least 10, at least 15, at least 20, at least 25, at least 30, at least 40, at least 45, or at least 50 contiguous amino acids from the amino acid sequence of SEQ ID NO: 15. In some embodiments, the polypeptide comprises residues 269-437 of SEQ ID NO: 15, wherein the positions in the polypeptide align with the positions in SEQ ID NO: 15. In some embodiments, a fragment of the C-terminal region of the polypeptide comprises an amino acid sequence that is at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 100% identical to a comparably sized fragment of SEQ ID NO:15. In some embodiments, the C-terminal fragment of the polypeptide is at least 25, at least 50, at least 75, at least 100, at least 125, at least 150, or at least 169 amino acids in length. 
     In some embodiments, the C-terminal region of the polypeptide comprises at least 5, at least 10, at least 15, at least 20, at least 25, at least 30, at least 40, at least 45, or at least 50 contiguous amino acids from the amino acid sequence of SEQ ID NO: 21. In some embodiments, the polypeptide comprises residues 190-285 of SEQ ID NO: 21, wherein the positions in the polypeptide align with the positions in SEQ ID NO: 21. In some embodiments, a fragment of the C-terminal region of the polypeptide comprises an amino acid sequence that is at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 100% identical to a comparably sized fragment of SEQ ID NO:21. In some embodiments, the C-terminal region of the fragment of the polypeptide having identity to SEQ ID NO:21 is at least 25, at least 50, at least 60, or at least 69 amino acids in length. 
     In some embodiments, the N-terminal region comprises at least 5, at least 10, at least 15, at least 20, at least 25, at least 30, at least 40, at least 45, at least 50, at least 60, or at least 66 contiguous amino acids from the amino acid sequence of SEQ ID NO: 11. In some embodiments, the polypeptide comprises residues 137-202 of SEQ ID NO: 11 or residues 134-202 of SEQ ID NO:11, wherein the positions in the polypeptide align with the positions in SEQ ID NO: 11. In some embodiments, a fragment of the N-terminal region of the polypeptide comprises an amino acid sequence that is at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 100% identical to a comparably sized fragment of SEQ ID NO:11. In some embodiments, the N-terminal region fragment of the polypeptide is at least 25, at least 50, at least 66, or at least 69 amino acids in length. 
     In some embodiments, the N-terminal region comprises at least 5, at least 10, at least 15, at least 20, at least 25, at least 30, at least 40, at least 45, or at least 50 contiguous amino acids from the amino acid sequence of SEQ ID NO: 15. In some embodiments, the polypeptide comprises residues 1-155 or residues 24-140 of SEQ ID NO: 15, wherein the positions in the polypeptide align with the positions in SEQ ID NO: 15, wherein the positions in the polypeptide align with the positions in SEQ ID NO:15. In some embodiments, a fragment of the N-terminal region of the polypeptide comprises an amino acid sequence that is at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 100% identical to a comparably sized fragment of SEQ ID NO:15. In some embodiments, the N-terminal fragment of the polypeptide is at least 25, at least 50, at least 75, at least 100, at least 117, at least 125, at least 150, or at least 155 amino acids in length. 
     In some embodiments, the N-terminal region comprises at least 5, at least 10, at least 15, at least 20, at least 25, at least 30, at least 40, at least 45, or at least 50 contiguous amino acids from the amino acid sequence of SEQ ID NO: 17. In some embodiments, the polypeptide comprises residues 1-31 of SEQ ID NO: 17, wherein the positions in the polypeptide align with the positions in SEQ ID NO: 17. In some embodiments, a fragment of the N-terminal region of the polypeptide comprises an amino acid sequence that is at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 100% identical to a comparably sized fragment of SEQ ID NO:17. 
     In some embodiments, the N-terminal region comprises at least 5, at least 10, at least 15, at least 20, or at least 24 contiguous amino acids from the amino acid sequence of SEQ ID NO: 19. In some embodiments, the polypeptide comprises residues 1-24 of SEQ ID NO: 19, wherein the positions in the polypeptide align with the positions in SEQ ID NO: 19. In some embodiments, a fragment of the N-terminal region of the polypeptide comprises an amino acid sequence that is at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 100% identical to a comparably sized fragment of SEQ ID NO:19. In some embodiments, the N-terminal fragment of the polypeptide is at least 10, at least 15, at least 20, at least 24 amino acids in length. 
     In some embodiments, the N-terminal region comprises at least 5, at least 10, at least 15, at least 20, at least 25, at least 30, at least 40, at least 45, or at least 50 contiguous amino acids from the amino acid sequence of SEQ ID NO: 21. In some embodiments, a fragment of the N-terminal region of the polypeptide comprises an amino acid sequence that is at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 100% identical to a comparably sized fragment of SEQ ID NO:21. 
     In some embodiments, the N-terminal region comprises at least 5, at least 10, at least 15, at least 20, at least 25, at least 30, at least 40, at least 45, or at least 50 contiguous amino acids from the amino acid sequence of SEQ ID NO: 23. In some embodiments, the polypeptide comprises residues 1-197 of SEQ ID NO: 23 or residues 13-197 of SEQ ID NO: 23, wherein the positions in the polypeptide align with the positions in SEQ ID NO: 23. In some embodiments, a fragment of the N-terminal region of the polypeptide comprises an amino acid sequence that is at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 100% identical to a comparably sized fragment of SEQ ID NO:23. In some embodiments, the N-terminal fragment of the polypeptide is at least 25, at least 50, at least 75, at least 100, at least 125, at least 150, at least 175, at least 184, or at least 197 amino acids in length. 
     In some embodiments, the polypeptide comprises a C-terminal region and an N-terminal region, wherein the C-terminal region of the polypeptide comprises at least 5 contiguous amino acids from the amino acid sequence of SEQ ID NO:13 and the N-terminal region of the polypeptide comprises at least 5 contiguous amino acids from an amino acid sequence selected from the group consisting of SEQ ID NOs: 11, 15, 17, 23, and combinations thereof. In some embodiments, the polypeptide comprises a C-terminal region and an N-terminal region, wherein the C-terminal region of the polypeptide comprises at least 10 contiguous amino acids from the amino acid sequence of SEQ ID NO:13 and the N-terminal region of the polypeptide comprises at least 10 contiguous amino acids from an amino acid sequence selected from the group consisting of SEQ ID NOs: 11, 15, 17, 23, and combinations thereof. In some embodiments, the polypeptide comprises a C-terminal region and an N-terminal region, wherein the C-terminal region of the polypeptide comprises at least 15 contiguous amino acids from the amino acid sequence of SEQ ID NO:13 and the N-terminal region of the polypeptide comprises at least 15 contiguous amino acids from an amino acid sequence selected from the group consisting of SEQ ID NOs: 11, 15, 17, 23, and combinations thereof. In some embodiments, the polypeptide comprises a C-terminal region and an N-terminal region, wherein the C-terminal region of the polypeptide comprises at least 20 contiguous amino acids from the amino acid sequence of SEQ ID NO:13 and the N-terminal region of the polypeptide comprises at least 20 contiguous amino acids from an amino acid sequence selected from the group consisting of SEQ ID NOs: 11, 15, 17, 23, and combinations thereof. 
     In some embodiments, the polypeptide comprises a C-terminal region and an N-terminal region, wherein the C-terminal region comprises at least 5 contiguous amino acids from the amino acid sequence of SEQ ID NO:15 and the N-terminal region comprises at least 5 contiguous amino acids from an amino acid sequence selected from the group consisting of SEQ ID NOs: 19, 21, 23, and combinations thereof. In some embodiments, the polypeptide comprises a C-terminal region and an N-terminal region, wherein the C-terminal region comprises at least 10 contiguous amino acids from the amino acid sequence of SEQ ID NO:15 and the N-terminal region comprises at least 10 contiguous amino acids from an amino acid sequence selected from the group consisting of SEQ ID NOs: 19, 21, 23, and combinations thereof. In some embodiments, the polypeptide comprises a C-terminal region and an N-terminal region, wherein the C-terminal region comprises at least 15 contiguous amino acids from the amino acid sequence of SEQ ID NO:15 and the N-terminal region comprises at least 15 contiguous amino acids from an amino acid sequence selected from the group consisting of SEQ ID NOs: 19, 21, 23, and combinations thereof. In some embodiments, the polypeptide comprises a C-terminal region and an N-terminal region, wherein the C-terminal region comprises at least 20 contiguous amino acids from the amino acid sequence of SEQ ID NO:15 and the N-terminal region comprises at least 20 contiguous amino acids from an amino acid sequence selected from the group consisting of SEQ ID NOs: 19, 21, 23, and combinations thereof. 
     In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 90% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 90% identity with a comparably sized fragment of SEQ ID NO:13, and a third fragment thereof has at least 90% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 91% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 91% identity with a comparably sized fragment of SEQ ID NO:13, and a third fragment thereof has at least 91% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 92% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 92% identity with a comparably sized fragment of SEQ ID NO:13, and a third fragment thereof has at least 92% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 93% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 93% identity with a comparably sized fragment of SEQ ID NO:13, and a third fragment thereof has at least 93% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 94% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 94% identity with a comparably sized fragment of SEQ ID NO:13, and a third fragment thereof has at least 94% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 95% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 95% identity with a comparably sized fragment of SEQ ID NO:13, and a third fragment thereof has at least 95% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 96% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 96% identity with a comparably sized fragment of SEQ ID NO:13, and a third fragment thereof has at least 96% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 97% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 97% identity with a comparably sized fragment of SEQ ID NO:13, and a third fragment thereof has at least 97% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 98% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 98% identity with a comparably sized fragment of SEQ ID NO:13, and a third fragment thereof has at least 98% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 99% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 99% identity with a comparably sized fragment of SEQ ID NO:13, and a third fragment thereof has at least 99% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has 100% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 100% identity with a comparably sized fragment of SEQ ID NO:13, and a third fragment thereof has at least 100% identity with a comparably sized fragment of SEQ ID NO:23. 
     In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 90% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 90% identity with a comparably sized fragment of SEQ ID NO:13, a third fragment thereof has at least 90% identity with a comparably sized fragment of SEQ ID NO:15, and a fourth fragment thereof has at least 90% identity with SEQ ID NO:17. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 91% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 91% identity with a comparably sized fragment of SEQ ID NO:13, a third fragment thereof has at least 91% identity with a comparably sized fragment of SEQ ID NO:15, and a fourth fragment thereof has at least 91% identity with a comparably sized fragment of SEQ ID NO:17. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 92% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 92% identity with a comparably sized fragment of SEQ ID NO:13, a third fragment thereof has at least 92% identity with a comparably sized fragment of SEQ ID NO:15, and a fourth fragment thereof has at least 92% identity with a comparably sized fragment of SEQ ID NO:17. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 93% sequence identity with a comparably long fragment of SEQ ID NO:11, a second fragment thereof has at least 93% identity with a comparably long fragment of SEQ ID NO:13, a third fragment thereof has at least 93% identity with a comparably long fragment of SEQ ID NO:15, and a fourth fragment thereof has at least 93% identity with a comparably long fragment of SEQ ID NO:17. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 94% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 94% identity with a comparably sized fragment of SEQ ID NO:13, a third fragment thereof has at least 94% identity with a comparably sized fragment of SEQ ID NO:15, and a fourth fragment thereof has at least 94% identity with a comparably sized fragment of SEQ ID NO:17. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 95% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 95% identity with a comparably sized fragment of SEQ ID NO:13, a third fragment thereof has at least 95% identity with a comparably sized fragment of SEQ ID NO:15, and a fourth fragment thereof has at least 95% identity with a comparably sized fragment of SEQ ID NO:17. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 96% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 96% identity with a comparably sized fragment of SEQ ID NO:13, a third fragment thereof has at least 96% identity with a comparably sized fragment of SEQ ID NO:15, and a fourth fragment thereof has at least 96% identity with a comparably sized fragment of SEQ ID NO:17. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 97% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 97% identity with a comparably sized fragment of SEQ ID NO:13, a third fragment thereof has at least 97% identity with a comparably sized fragment of SEQ ID NO:15, and a fourth fragment thereof has at least 97% identity with a comparably sized fragment of SEQ ID NO:17. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 98% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 98% identity with a comparably sized fragment of SEQ ID NO:13, a third fragment thereof has at least 98% identity with a comparably sized fragment of SEQ ID NO:15, and a fourth fragment thereof has at least 98% identity with a comparably sized fragment of SEQ ID NO:17. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 99% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has at least 99% identity with a comparably sized fragment of SEQ ID NO:13, a third fragment thereof has at least 99% identity with a comparably sized fragment of SEQ ID NO:15, and a fourth fragment thereof has at least 99% identity with a comparably sized fragment of SEQ ID NO:17. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has 100% sequence identity with a comparably sized fragment of SEQ ID NO:11, a second fragment thereof has 100% identity with a comparably sized fragment of SEQ ID NO:13, a third fragment thereof has 100% identity with a comparably sized fragment of SEQ ID NO:15, and a fourth fragment thereof has 100% identity with a comparably sized fragment of SEQ ID NO:17. 
     In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 90% sequence identity with a comparably sized fragment of SEQ ID NO:13 and a second fragment thereof has at least 90% identity with a comparably sized fragment of SEQ ID NO:15. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 91% sequence identity with a comparably sized fragment of SEQ ID NO:13 and a second fragment thereof has at least 91% identity with a comparably sized fragment of SEQ ID NO:15. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 92% sequence identity with a comparably sized fragment of SEQ ID NO:13 and a second fragment thereof has at least 92% identity with a comparably sized fragment of SEQ ID NO:15. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 93% sequence identity with a comparably sized fragment of SEQ ID NO:13 and a second fragment thereof has at least 93% identity with a comparably sized fragment of SEQ ID NO:15. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 94% sequence identity with a comparably sized fragment of SEQ ID NO:13 and a second fragment thereof has at least 94% identity with a comparably sized fragment of SEQ ID NO:15. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 95% sequence identity with a comparably sized fragment of SEQ ID NO:13 and a second fragment thereof has at least 95% identity with a comparably sized fragment of SEQ ID NO:15. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 96% sequence identity with a comparably sized fragment of SEQ ID NO:13 and a second fragment thereof has at least 96% identity with a comparably sized fragment of SEQ ID NO:15. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 97% sequence identity with a comparably sized fragment of SEQ ID NO:13 and a second fragment thereof has at least 97% identity with a comparably sized fragment of SEQ ID NO:15. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 98% sequence identity with a comparably sized fragment of SEQ ID NO:13 and a second fragment thereof has at least 98% identity with a comparably sized fragment of SEQ ID NO:15. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 99% sequence identity with a comparably sized fragment of SEQ ID NO:13 and a second fragment thereof has at least 99% identity with a comparably sized fragment of SEQ ID NO:15. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has 100% sequence identity with a comparably sized fragment of SEQ ID NO:13 and a second fragment thereof has 100% identity with a comparably sized fragment of SEQ ID NO:15. 
     In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 90% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 90% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 90% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 91% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 91% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 91% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 92% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 92% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 92% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 93% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 93% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 93% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 94% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 94% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 94% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 95% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 95% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 95% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 96% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 96% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 96% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 97% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 97% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 97% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 98% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 98% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 98% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 99% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 99% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 99% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has 100% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 100% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 100% identity with a comparably sized fragment of SEQ ID NO:23. 
     In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 90% sequence identity with a comparably sized fragment of SEQ ID NO:13, a second fragment thereof has at least 90% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 90% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 91% sequence identity with a comparably sized fragment of SEQ ID NO:13, a second fragment thereof has at least 91% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 91% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 92% sequence identity with a comparably sized fragment of SEQ ID NO:13, a second fragment thereof has at least 92% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 92% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 93% sequence identity with a comparably sized fragment of SEQ ID NO:13, a second fragment thereof has at least 93% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 93% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 94% sequence identity with a comparably sized fragment of SEQ ID NO:13, a second fragment thereof has at least 94% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 94% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 95% sequence identity with a comparably sized fragment of SEQ ID NO:13, a second fragment thereof has at least 95% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 95% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 96% sequence identity with a comparably sized fragment of SEQ ID NO:13, a second fragment thereof has at least 96% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 96% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 97% sequence identity with a comparably sized fragment of SEQ ID NO:13, a second fragment thereof has at least 97% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 97% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 98% sequence identity with a comparably sized fragment of SEQ ID NO:13, a second fragment thereof has at least 98% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 98% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 99% sequence identity with a comparably sized fragment of SEQ ID NO:13, a second fragment thereof has at least 99% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 99% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has 100% sequence identity with a comparably sized fragment of SEQ ID NO:13, a second fragment thereof has at least 100% identity with a comparably sized fragment of SEQ ID NO:21, and a third fragment thereof has at least 100% identity with a comparably sized fragment of SEQ ID NO:23. 
     In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 90% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 90% identity with a comparably sized fragment of SEQ ID NO:19, a third fragment thereof has at least 90% identity with a comparably sized fragment of SEQ ID NO:21, and a fourth fragment thereof has at least 90% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 91% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 91% identity with a comparably sized fragment of SEQ ID NO:19, a third fragment thereof has at least 91% identity with a comparably sized fragment of SEQ ID NO:21, and a fourth fragment thereof has at least 91% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 92% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 92% identity with a comparably sized fragment of SEQ ID NO:19, a third fragment thereof has at least 92% identity with a comparably sized fragment of SEQ ID NO:21, and a fourth fragment thereof has at least 92% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 93% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 93% identity with a comparably sized fragment of SEQ ID NO:19, a third fragment thereof has at least 93% identity with a comparably sized fragment of SEQ ID NO:21, and a fourth fragment thereof has at least 93% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 94% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 94% identity with a comparably sized fragment of SEQ ID NO:19, a third fragment thereof has at least 94% identity with a comparably sized fragment of SEQ ID NO:21, and a fourth fragment thereof has at least 94% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 95% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 95% identity with a comparably sized fragment of SEQ ID NO:19, a third fragment thereof has at least 95% identity with a comparably sized fragment of SEQ ID NO:21, and a fourth fragment thereof has at least 95% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 96% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 96% identity with a comparably sized fragment of SEQ ID NO:19, a third fragment thereof has at least 96% identity with a comparably sized fragment of SEQ ID NO:21, and a fourth fragment thereof has at least 96% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 97% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 97% identity with a comparably sized fragment of SEQ ID NO:19, a third fragment thereof has at least 97% identity with a comparably sized fragment of SEQ ID NO:21, and a fourth fragment thereof has at least 97% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 98% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 98% identity with a comparably sized fragment of SEQ ID NO:19, a third fragment thereof has at least 98% identity with a comparably sized fragment of SEQ ID NO:21, and a fourth fragment thereof has at least 98% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has at least 99% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has at least 99% identity with a comparably sized fragment of SEQ ID NO:19, a third fragment thereof has at least 99% identity with a comparably sized fragment of SEQ ID NO:21, and a fourth fragment thereof has at least 99% identity with a comparably sized fragment of SEQ ID NO:23. In some embodiments, the polypeptide comprises an amino acid sequence wherein a fragment thereof has 100% sequence identity with a comparably sized fragment of SEQ ID NO:15, a second fragment thereof has 100% identity with a comparably sized fragment of SEQ ID NO:19, a third fragment thereof has 100% identity with a comparably sized fragment of SEQ ID NO:21, and a fourth fragment thereof has 100% identity with a comparably sized fragment of SEQ ID NO:23. 
     In some embodiments, the chimeric polypeptide is an enzyme. In some embodiments, the chimeric polypeptide is an enzyme capable of converting xylose to xylulose. In some embodiments, the chimeric polypeptide is a xylose isomerase (XI). In some embodiments, at least one parent sequence used as donor material for the chimeric polynucleotide is derived from a polynucleotide that encodes an XI. In some embodiments, when expressed heterologously in a yeast cell, the parent XI sequence is capable of conferring xylose isomerase activity to the host yeast cell. In some embodiments, when expressed heterologously in a yeast cell, the parent XI sequence is not capable of conferring xylose isomerase activity to the host yeast cell. When the donor XI does not function in the yeast host cell, it is referred to herein as a non-functional parent XI. 
     It is understood that nucleotide sequences encoding engineered forms of any of the chimeric polypeptides defined above are expressly included in the present invention. Further, any nucleotide sequence that comprises one or more amino acid substitutions, insertions and/or deletions as compared to the corresponding naturally occurring donor sequences, but that are within the ranges of identity or similarity as defined herein are expressly included in the invention. However, the chimeric polypeptides having xylose isomerase activity maintain certain conserved motifs. In one embodiment, the chimeric nucleotide sequence of the invention encodes a chimeric xylose isomerase amino acid sequence comprising a xylose isomerase signature sequence as defined, e.g., by Meaden et al. (1994, Gene, 141: 97-101): VXW[GP]GREG[YSTA] (present at positions 188-196, relative to SEQ ID NO: 11) and [LIVM]EPKPX[EQ]P (present at positions 233-240, relative to SEQ ID NO: 11), wherein “X” can be any amino acid and wherein amino acids in brackets indicates that one of the bracketed amino acids can be present at that position in the signature sequence. A xylose isomerase amino acid sequence of the invention can further comprise the conserved amino acid residues His-103, Asp-106, and Asp-341, which constitute a triad directly involved in catalysis, Lys-236 plays a structural as well as a functional catalytic role, and Glu-234 (relative to SEQ ID NO: 11), which is involved in magnesium binding (Vangrysperre et al., “Localization of the essential histidine and carboxylate group in D-xylose isomerases,”  Biochem. J.  265: 699-705(1990); Henrick et al., “Structures of D-xylose isomerase from Arthrobacter strain B3728 containing the inhibitors xylitol and D-sorbitol at 2.5 A and 2.3 A resolution, respectively,”  J. Mol. Biol.  208: 129-157 (1989); Bhosale et al., “Molecular and industrial aspects of glucose isomerase,”  Microbiol. Rev.  60: 280-300 (1996)). Amino acid positions of the above signature sequences and conserved residues refer to positions in the reference amino acid sequence of the  B. thetaiotaomicron  xylose isomerase of SEQ ID NO: 11. In amino acid sequences of the invention other than SEQ ID NO: 11, the amino acid positions of the above signature sequences and conserved residues are present in amino acid positions corresponding to the positions of the signature sequences and conserved residues in SEQ ID NO: 11, for example in a ClustalW (1.83 or 1.81) sequence alignment using default settings. The skilled person will know how to identify corresponding amino acid positions in xylose isomerase amino acid sequences other than SEQ ID NO: 11 using amino acid sequence alignment algorithms as defined hereinabove. These regions and positions will tolerate no or only conservative amino acid substitutions. One having ordinary skill in the art would understand that even conserved motifs can remain functional with conservative amino acid substitutions, and such substitutions are envisioned by the present invention. Amino acid substitutions outside of these regions and positions are unlikely to greatly affect xylose isomerase activity. 
     Additional structural features common to XIs have been described, e.g., by Chang et al., “Crystal Structures of Thermostable Xylose Isomerases from  Thermus caldophilus  and  Thermus thermophiles : Possible Structural Determinants of Thermostability,”  J. Mol. Biol.  288:623-34 (1999), which is incorporated by reference in its entirety, and RCSB Protein Data Bank, “Xylose Isomerase From  Thermotoga neapolitana ,” (PDB code-1A0E), see rcsb.org/pdb/explore/explore.do?structureId=1A0E, last accessed Jun. 29, 2014, at 5:15 pm. There are several known metal binding sits in the XI sequence, including at residues Glu-234, Glu-270, His-273, Asp-298, Asp-309, Asp-311, and Asp-341. One having ordinary skill in the art would understand that any deletions or non-conservative substitutions at any one or more of these residues may lead to a decreased functionability of the resulting XI. 
     In some embodiments, a host cell is engineered to express one or more of the chimeric polypeptides. In some embodiments, the host cell is a fungal cell, e.g. a yeast cell, e.g. a  S. cerevisiae  cell. In some embodiments the host cell is modified to have mutations affecting at least one gene encoding a protein of the pentose phosphate pathway. In some embodiments, the host cell has at least one mutation affecting the expression of at least one of XKS1, RKI1, RPE1, TKL1, TAL1, or a combination thereof. In some embodiments, the host cell has one or more mutations that correlate with an increase in the expression or an up-regulation of one or more of XKS1, RKI1, RPE1, TKL1, and/or TAL1. In some embodiments the host cell can be modified through the heterologous expression of one or more polynucleotides encoding XKS1, RKH, RPE1, TKL1, and/or TAL1. In some embodiments, the host cell has one or more mutations that correlate with a decrease in the expression or down-regulation of one or more of XKS1, RKI1, RPE1, TKL1, and/or TAL1. In some embodiments, the host cell has a modification of one or more endogenous aldose reductases. In some embodiments, the aldose reductase is GRE3. In some embodiments, the host cell has a deletion or disruption of all or part of the endogenous GRE3 gene. In some embodiments, the aldose reductase gene is YPR1. In some embodiments, the host cell has a deletion or disruption of all or part of the endogenous YPR1 gene. In some embodiments, the host cell has a deletion or disruption of all or part of both the endogenous GRE3 gene and the endogenous YPR1 gene. In some embodiments, the host cell has a modification of PGM1 and/or PGM2. In some embodiments, the host cell overexpresses PGM1 and/or PGM2. In some embodiments, the host cell has increased levels of Pgm1 and/or Pgm2 polypeptide and/or mRNA relative to a comparable host cell lacking a modification of PGM1 and/or PGM2. In some embodiments, the host cell is a modified industrial yeast strain. 
     In some embodiments, the host cell comprises a deletion or disruption of one or more native enzymes that function to produce glycerol and/or regulate glycerol synthesis as described, e.g., in U.S. Patent Application Publication No. 2014/0186930, which is incorporated by reference herein in its entirety. In some embodiments, the host cell produces less glycerol than a control recombinant microorganism without deletion or disruption of said one or more endogenous enzymes that function to produce glycerol and/or regulate glycerol synthesis. In some embodiments, the one or more endogenous enzymes that function to produce glycerol are encoded by a GPD1 polynucleotide, a GPD2 polynucleotide, or both a GPD1 polynucleotide and a GPD2 polynucleotide. In some embodiments, one or both of the endogenous GPD1 and/or GPD2 genes are modified by mutation or deletion. In some embodiments, the host cell comprises a heterologous ADHE sequence. In some embodiments, the heterologous ADHE is from  Bifidobacterium adolescentis . In some embodiments the native STL1 gene is upregulated by either modifying the promoter of the native copies or by introducing additional copies of STL1. In some embodiments the host cell comprises an ortholog of the native STL1. In some embodiments the native ACS2 gene is upregulated by either modifying the promoter of the native copies or by introducing additional copies of ACS2. 
     In some embodiments, the host cell comprises more than one copy of the chimeric polynucleotide. In some embodiments, the host cell comprises two copies, three copies, four copies, five copies, six copies, seven copies, eight copies, nine copies, ten copies, eleven copies, at least twelve copies, at least fifteen copies, or at least twenty copies of the chimeric polynucleotide. 
     In some embodiments, the chimeric polynucleotide can be present in a vector. In some embodiments, the host cell can comprise the chimeric polynucleotide within a vector. In some embodiments, the vector is a plasmid. In some embodiments, the host cell can express the chimeric polynucleotide from the vector. In some embodiments, the chimeric polynucleotide can be incorporated into the genome of the host cell. In some embodiments, the host cell is a fungal cell. In some embodiments, the host cell is a yeast cell. In some embodiments, the host cell is a  S. cerevisiae  cell. 
     Certain embodiments of the present invention describe methods for producing a fermentation product. In certain embodiments, the recombinant host cell comprising the chimeric polynucleotide or the recombinant polypeptide is contacted with a carbon source. In certain embodiments, the carbon source comprises xylose. In certain embodiments, xylose is the sole source of carbon in the carbon source. In certain embodiments, a fermentation product is produced by contacting the host cell with the carbon source. In certain embodiments, the fermentation product is recovered. In certain embodiments, the fermentation product is selected from the group consisting of ethanol, lactic acid, 3-hydroxy-propionic acid, hydrogen, butyric acid, acrylic acid, acetic acid, succinic acid, citric acid, malic acid, fumaric acid, an amino acid, 1,3-propane-diol, ethylene, glycerol, acetone, isopropyl alcohol, butanol, a β-lactam, an antibiotic, cephalosporin, or a combination thereof. In certain embodiments, the fermentation product is ethanol. 
     III. Methods of Synthesis 
     Certain embodiments of the present invention describe methods of synthesizing a chimeric polynucleotide or chimeric polypeptide. In some embodiments, the chimeric polypeptide is synthesized by providing two or more parent xylose isomerase-encoding DNA sequences; providing a linearized vector capable of replication in yeast; providing a yeast cell; transforming the yeast cell with the at least two parent DNA sequences and the linearized vector; and screening the transformed yeast cells for recombination of the parent DNA sequences. 
     In some embodiments, the chimeric polypeptide is synthesized by providing at least two or more parent polynucleotide sequences; inserting each parent polynucleotide into a vector backbone creating a parent vector library; transforming host cells with at least two parent vectors; screening the transformed yeast cells for recombination of the parent polynucleotide sequence; and identifying transformed host cells wherein recombination of the at least two parent vectors has led to a chimeric polynucleotide sequence encoding a chimeric polypeptide. 
     In some embodiments, the method further comprises isolating the chimeric polynucleotide. 
     In some embodiments, at least two parent DNA sequences are used to create the chimeric polynucleotide or chimeric polypeptide. In some embodiments, at least three parent DNA sequences are used to create the chimeric polynucleotide or chimeric polypeptide. In some embodiments, at least four parent DNA sequences are used to create the chimeric polynucleotide or chimeric polypeptide. 
     In some embodiments, each parent DNA sequence used to create the chimeric polynucleotide or chimeric polypeptide is unique, such that no two parent sequences used in the same chimeric polynucleotide or polypeptide are derived from the same species. In some embodiments, at least one parent DNA sequence used to create the chimeric polynucleotide or chimeric polypeptide is derived from the same species as at least one other parent DNA sequence. 
     In some embodiments, at least one of the full length parent DNA sequences used to create the chimeric polynucleotide or the chimeric polypeptide encodes a polypeptide having xylose isomerase activity. In some embodiments, each full length parent DNA sequence used to create the chimeric polynucleotide or the chimeric polypeptide encodes a polypeptide having xylose isomerase activity. 
     In some embodiments, the donor parent sequences are codon optimized. In some embodiments, the donor parent sequences are synthesized as two independent but overlapping DNA fragments. In some embodiments, the donor parent sequences are synthesized as two independent and non-overlapping DNA fragments. In some embodiments, the two fragments together encompass the entire parent sequence. In some embodiments, the two fragments together encompass an incomplete portion of the entire parent sequence. In some embodiments, at least one independent DNA fragment comprises a region homologous to either the  S. cerevisiae  HSP150 promoter (HSP150p) or the  S. cerevisiae  ADH1 promoter (ADH1p). In some embodiments, at least one independent DNA fragment comprises a region homologous to HSP150p. In some embodiments, at least one independent DNA fragment comprises a region homologous to at least one of the  S. cerevisiae  HXT7 (HXT7t), PDC1 (PDC1t), RPL41B (RPL41Bt), RPL15A (RPL15At), DIT1 (DIT10, RPL3 (RPL3t), IDP1 (IDP1t), YHI9 (YHI9t), EFM1 (EFM1t), or VMA16 (VMA16t) terminator sequences. In some embodiments, at least one independent DNA fragment comprises a region homologous to HXT7t. In some embodiments, one independent DNA fragment from a parent DNA sequence comprises a region homologous to the  S. cerevisiae  HSP150 promoter (HSP150p) and a second independent DNA fragment from the same parent DNA sequence comprises a region homologous to the  S. cerevisiae  HXT7 terminator sequence (HXT7t). In some embodiments, a promoter that is native to the host cell is used. In some embodiments, a promoter that is not native to the host cell is used. In some embodiments, a synthetic promoter is used. 
     In some embodiments, the vector backbone comprises one or more regions homologous to the  S. cerevisiae  HSP150 promoter (HSP150p). In some embodiments, the vector backbone comprises one or more regions homologous to the  S. cerevisiae  HXT7 (HXT7t) terminator sequence. In some embodiments, the vector backbone comprises one or more regions homologous to the  S. cerevisiae  HSP150 promoter (HSP150p) and comprises one or more regions homologous to the  S. cerevisiae  HXT7 terminator sequence (HXT7t). In some embodiments, the vector backbone is comprised of all or a fragment of pMU2116. In some embodiments the vector is circular. In some embodiments, the vector is linear. In certain embodiments, the vector comprises a single parent DNA fragment. In some embodiments, the vector comprises two independent and overlapping parent DNA fragments. In some embodiments, the vector comprises two independent and non-overlapping parent DNA fragments. In some embodiments, the vector is linear with a 5′ fragment of the parent polynucleotide ligated to one end of the linear vector and a 3′ fragment of the parent polynucleotide ligated to the opposite end of the linear vector, wherein the 5′ and 3′ fragments are independent and overlap. In some embodiments, the vector is linear with a 5′ fragment of the parent polynucleotide ligated to one end of the linear vector and a 3′ fragment of the parent polynucleotide ligated to the opposite end of the linear vector, wherein the 5′ and 3′ fragments are independent and do not overlap. In some embodiments, the circular vector comprises the entire parent XI coding region. In some embodiments, the circular vector comprises one or more fragments of the parent XI coding region. 
     In some embodiments the parent DNA is ligated directly with the vector backbone. In some embodiments a linker is used. In some embodiments the DNA fragments and the vector backbone fragments are transformed into the same host cell. In some embodiments, the host cell is a yeast. In some embodiments, the yeast is  S. cerevisiae . In some embodiments, the  S. cerevisiae  is an industrial yeast strain. In some embodiments, the industrial yeast strain is genetically modified. In some embodiments, the formation of the chimeric sequence occurs in the host cell. In some embodiments, the chimeric sequence occurs through natural homologous recombination. In some embodiments, the chimeric polynucleotide or polypeptide is created through in vitro assembly methods. In some embodiments, the chimeric polynucleotide or polypeptide is created by in vitro homologous recombination. In some embodiments, the chimeric polynucleotide or polypeptide is created by sequence and ligase independent cloning (SLIC). In some embodiments, the chimeric polynucleotide or polypeptide is created by successive hybridization assembling (SHA). 
     In some embodiments, a chimeric polynucleotide library is created. In some embodiments, the chimeric library is created by transforming host cells with two or more vector backbone fragments; one or more full length non-functional parent DNA sequences, wherein the non-functional parent DNA sequence encodes a polypeptide unable to confer function normally in the host cell; and at least two fragments from functional parent DNA sequences, wherein the functional DNA sequences are able to function normally in the host cell. In some embodiments, the donor sequences encode for a polypeptide having xylose isomerase activity (e.g. an XI). In some embodiments, the donor DNA sequences include but are not limited to DNA sequences derived from the XI coding sequences of  Clostridium difficile  (CdXI),  C. aberensis  (CaXI),  Abiotrophia defectiva  (AdXI),  Lachnoanaerobaculum saburreum  (LsXI),  Reticulitermes speratus  (RsXI),  Prevotella ruminicola  (PrXI),  Lactobacillus xylosus  (LxXI),  B. thetaiotaomicron  (BtXI),  Piromyces  (PiXI),  Clostridium phytofermentans  (CpXI),  Ciona intestinalis  (CiXI),  Fusobacterium morfiferum  (FmXI),  Thermotoga maritima  (TmXI),  Mastotermes darwiniensis  (MdXI),  Bacillus stearothermophilus  (BsXI),  Hordeum vulgare  (HvXI),  Zea mays  (ZmXI),  Arabidopsis thaliana  (AtXI),  Xanthomonas campestris  (XcXI),  C. pinensis  (CpiXI), and  P. distasonis  (PdXI) and the DNA sequences encoding XYM1 and XYM2. 
     In some embodiments, the transformed host cells are allowed to recover following transformation. In some embodiments, the transformed host cells are incubated overnight to allow for recovery following transformation. In some embodiments, the transformed host cells are plated onto plates comprising media comprising xylose as a carbon source. In some embodiments, the host cells are plated onto plates comprising media comprising xylose as the only carbon source. In some embodiments, the cells are cultured in a liquid media comprising xylose as the only carbon source. In some embodiments, colonies of transformed host cells are selected, wherein the selected colonies are able to grown on media comprising xylose or comprising xylose as the sole source of carbon. In some embodiments, the selected colonies are re-struck to fresh plates or cultured in liquid media. In some embodiments, a chimeric XI can be selected by culturing the transformed host cells in the presence of xylose and xylitol to select for a xylose isomerase with decreased xylitol inhibition. In some embodiments, the host cells can be cultured at varying temperatures. In some embodiments, the host cells are cultured at a temperature selected from the group consisting of about 25, about 26, about 27, about 28, about 29, about 30, about 31, about 32, about 33, about 34, about 35, about 36, about 37, about 38, about 39, and about 40 degrees C. In some embodiments, the host cells are cultured at a temperature of from about 25 to about 40 degrees C., from about 25 to about 35 degrees C., from about 25 to about 30 degrees C., from about 30 to about 40 degrees C., from about 30 to about 35 degrees C., from about 20 to about 45 degrees C., or from about 35 to about 40 degrees C. 
     In some embodiments, DNA from the selected transformed host cells is isolated, and the DNA is subjected to PCR amplification. In some embodiments, PCR is used to amplify all or a fragment of the chimeric polynucleotide. In some embodiments, primers specific to the HSP150p/HXT7t sequences are used in a PCR reaction to amplify the chimeric polynucleotide. In some embodiments, the amplified chimeric polynucleotide or a fragment thereof is sequenced. In some embodiments, the amplified chimeric polynucleotide or fragment thereof is sequenced using the Sanger method. In some embodiments, the resulting sequence of the chimeric polynucleotide or fragment thereof is aligned with one or more of the parent DNA sequences to determine the origin of the chimeric polynucleotide. In some embodiments, the amplified chimeric polynucleotide or fragment thereof is re-cloned into a vector backbone and rescreened for activity on xylose. 
     In some embodiments, a selected chimeric XI can be used in subsequent rounds of generating additional chimeric XIs. Any chimeric XI can be further modified by directed modifications of the sequence. In some embodiments, the chimeric polynucleotide is subjected to mutagenesis. In some embodiments, site-directed mutagenesis is used to introduce mutations to the chimeric polynucleotide. In some embodiments, site-directed mutagenesis is performed using PCR-based methods. In some embodiments, site-directed mutagenesis is performed using traditional PCR. In some embodiments, site-directed mutagenesis is performed using primer extension. In some embodiments, site-directed mutagenesis is performed using inverse PCR. In some embodiments, random mutagenesis is used to introduce mutations to the chimeric polynucleotide. In some embodiments, random mutagenesis is performed using PCR-based methods. In some embodiments, random mutagenesis is performed using error-prone PCR. In some embodiments, random mutagenesis is performed using insertion mutagenesis (e.g. through the use of transposons that randomly insert into the DNA sequence). In some embodiments, random mutagenesis is performed using chemical exposure (e.g. ethyl methanesulfonate (EMS) or nitrous acid. In some embodiments, random mutagenesis is performed using radiation exposure (e.g. exposure to UV light). In some embodiments, mutated chimeric polynucleotides are screened for growth and/or activity on xylose. 
     IV. Codon-Optimization 
     In some embodiments, the nucleotide sequence of the chimeric polynucleotide is codon-optimized for expression in a fungal host cell. In some embodiments, one or more of the parent polynucleotide sequences used to create the chimeric polynucleotide is codon optimized for expression in a fungal host cell. In some embodiments, the nucleotide sequence of the chimeric polynucleotide or the nucleotide sequence of one or more of the parent polynucleotide sequences is codon-optimized for expression in a yeast host cell. In some embodiments the nucleotide sequence of the chimeric polynucleotide or the nucleotide sequence of one or more of the parent polynucleotide sequences is codon-optimized for expression in  S. cerevisiae . Codon-optimized polynucleotides can have a codon adaptation index (CAI) of about 0.8 to 1.0, about 0.9 to 1.0, or about 0.95 to 1.0. 
     In general, highly expressed genes in an organism are biased towards codons that are recognized by the most abundant tRNA species in that organism. One measure of this bias is the “codon adaptation index” or “CM,” which measures the extent to which the codons used to encode each amino acid in a particular gene are those which occur most frequently in a reference set of highly expressed genes from an organism. The Codon Adaptation Index is described in more detail in Sharp and Li,  Nucleic Acids Research  15:1281-1295 (1987), which is incorporated by reference herein in its entirety. 
     The CAI of codon-optimized sequences used in the present invention corresponds to from about 0.6 to about 1.0, from about 0.7 to about 1.0, from about 0.8 to about 1.0, from about 0.9 to about 1.0, from about 9.5 to about 1.0, or about 1.0. A codon-optimized sequence can be further modified for expression in a particular organism, depending on that organism&#39;s biological constraints. For example, large runs of “As” or “Ts” (e.g., runs greater than 4, 5, 6, 7, 8, 9, or 10 consecutive bases) can be removed from the sequences if these are known to effect transcription negatively. Furthermore, specific restriction enzyme sites can be removed for molecular cloning purposes. Examples of such restriction enzyme sites include Pad, Ascl, BamHI, Bglll, EcoRJ and Xhol. Additionally, the DNA sequence can be checked for direct repeats, inverted repeats and mirror repeats with lengths of ten bases or longer, which can be modified manually by replacing codons with “second best” codons, i.e., codons that occur at the second highest frequency within the particular organism for which the sequence is being optimized. 
     Deviations in the nucleotide sequence that comprise the codons encoding the amino acids of any polypeptide chain allow for variations in the sequence coding for the gene. Since each codon consists of three nucleotides, and the nucleotides comprising DNA are restricted to four specific bases, there are 64 possible combinations of nucleotides, 61 of which encode amino acids (the remaining three codons encode signals ending translation). The “genetic code” which shows which codons encode which amino acids is well known to one of skill in the art. As a result, many amino acids are designated by more than one codon. For example, the amino acids alanine and proline are coded for by four triplets, serine and arginine by six, whereas tryptophan and methionine are coded by just one triplet. This degeneracy allows for DNA base composition to vary over a wide range without altering the amino acid sequence of the proteins encoded by the DNA. 
     Many organisms display a bias for use of particular codons to code for insertion of a particular amino acid in a growing peptide chain. Codon preference or codon bias, differences in codon usage between organisms, is afforded by degeneracy of the genetic code, and is well documented among many organisms. Codon bias often correlates with the efficiency of translation of messenger RNA (mRNA), which is in turn believed to be dependent on, inter alia, the properties of the codons being translated and the availability of particular transfer RNA (tRNA) molecules. The predominance of selected tRNAs in a cell is generally a reflection of the codons used most frequently in peptide synthesis. Accordingly, genes can be tailored for optimal gene expression in a given organism based on codon optimization. 
     Given the large number of gene sequences available for a wide variety of animal, plant and microbial species, it is possible to calculate the relative frequencies of codon usage. Codon usage tables and codon-optimizing programs are readily available, for example, see kazusa.or.jp/codon/ (visited Jul. 15, 2014), and these tables can be adapted in a number of ways. See, e.g., Nakamura, Y., et al. “Codon usage tabulated from the international DNA sequence databases: status for the year 2000, ” Nucl. Acids Res.  28:292 (2000). 
     By utilizing one or more available tables, one of ordinary skill in the art can apply the frequencies to any given polypeptide sequence, and produce a nucleic acid fragment of a codon-optimized coding region which encodes the polypeptide, but which uses codons optimal for a given species. Codon-optimized coding regions can be designed by various different methods known to one having ordinary skill in the art. 
     In certain embodiments, an entire polypeptide sequence, or fragment, variant, or derivative thereof is codon-optimized by any method known in the art. Various desired fragments, variants or derivatives are designed, and each is then codon-optimized individually. In addition, partially codon-optimized coding regions of the present invention can be designed and constructed. For example, the invention includes a nucleic acid fragment of a codon-optimized coding region encoding a polypeptide in which at least about 1%, 2%, 3%, 4%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100% of the codon positions have been codon-optimized for a given species. That is, they contain a codon that is preferentially used in the genes of a desired species, e.g., a yeast species such as  S. cerevisiae , in place of a codon that is normally used in the native nucleic acid sequence. 
     In additional embodiments, a full-length polypeptide sequence is codon-optimized for a given species resulting in a codon-optimized coding region encoding the entire polypeptide, and then nucleic acid fragments of the codon-optimized coding region, which encode fragments, variants, and derivatives of the polypeptide are made from the original codon-optimized coding region. As would be well understood by those of ordinary skill in the art, if codons have been randomly assigned to the full-length coding region based on their frequency of use in a given species, nucleic acid fragments encoding fragments, variants, and derivatives would not necessarily be fully codon-optimized for the given species. However, such sequences are still much closer to the codon usage of the desired species than the native codon usage. The advantage of this approach is that synthesizing codon-optimized nucleic acid fragments encoding each fragment, variant, and derivative of a given polypeptide, although routine, would be time consuming and would result in significant expense. 
     In some embodiments, one or more of the donor parent polynucleotide sequences are codon-optimized for expression in yeast. In some embodiments, the chimeric polynucleotide is codon-optimized for expression in yeast. 
     V. Methods of Producing Ethanol 
     Certain aspects of the present invention are directed to methods of producing a fermentation product. In some embodiments of the invention, the recombinant host cell is used to produce a fermentation product from a cellulosic or lignocellulosic material. In some embodiments, the fermentation product is ethanol, lactic acid, 3-hydroxy-propionic acid, hydrogen, butyric acid, acrylic acid, acetic acid, succinic acid, citric acid, malic acid, fumaric acid, an amino acid, 1,3-propane-diol, ethylene, glycerol, acetone, isopropyl alcohol, butanol, a β-lactam, an antibiotic, a cephalosporin, or a combination thereof. In some embodiments, the cellulosic or lignocellulosic material is insoluble cellulose, crystalline cellulose, pretreated hardwood, paper sludge, pretreated corn stover, pretreated sugar cane bagasse, pretreated corn cobs, pretreated switchgrass, pretreated municipal solid waste, pretreated distiller&#39;s dried grains, pretreated wheat straw, corn fiber, agave, or a combination thereof. 
     In some embodiments, one or more of the chimeric polypeptides are purified. In some embodiments, the one or more chimeric polypeptides are purified from a recombinant yeast host cell of the invention, a composition of the invention, a media supernatant of the invention, or a yeast strain of the invention. In some embodiments, one or more chimeric polypeptides in the composition are from a crude extract. In some embodiments, the crude extract is from a recombinant yeast host cell of the invention, a composition of the invention, a media supernatant of the invention, or a yeast strain of the invention. 
     One aspect of the invention is directed to a composition comprising a lignocellulosic material and a recombinant yeast host cell comprising at least one chimeric polypeptide having xylose isomerase activity. Another aspect of the invention is directed to a media supernatant generated by incubating a recombinant yeast host comprising as least one chimeric polypeptide having xylose isomerase activity with a medium containing xylose as the only carbon source. In some embodiments, the medium comprises a cellulosic or lignocellulosic material. In some embodiments, the cellulosic or lignocellulosic material is insoluble cellulose, crystalline cellulose, pretreated hardwood, paper sludge, saw mill or paper mill discards, pretreated corn stover, pretreated sugar cane bagasse, pretreated corn cobs, pretreated switchgrass, pretreated municipal solid waste, pretreated distiller&#39;s dried grains, pretreated wheat straw, corn fiber, agave, or a combination thereof. 
     In some embodiments, a fermentation product is produced by a method comprising contacting a recombinant host cell of the present invention with a carbon source, wherein the carbon source comprises xylose. In some embodiments, the fermentation product is selected from the group consisting of ethanol, lactic acid, 3-hydroxy-propionic acid, hydrogen, butyric acid, acrylic acid, acetic acid, succinic acid, citric acid, malic acid, fumaric acid, an amino acid, 1,3-propane-diol, ethylene, glycerol, acetone, isopropyl alcohol, butanol, a β-lactam, an antibiotic, and a cephalosporin. In some embodiments, the fermentation product is ethanol. In some embodiments, the fermentation product is recovered. 
     Certain aspects of the present invention are directed to a method of producing ethanol comprising contacting a source material comprising xylose with a polypeptide of the present invention. In some embodiments the polypeptide is a chimeric polypeptide. In some embodiments, the chimeric polypeptide has xylose isomerase activity (e.g. is an XI). In some embodiments, the source material is contacted by the polypeptide. In some embodiments, the polypeptide is isolated. In some embodiments, the source material is contacted by a produced by a transformed host cell. In some embodiments, the XI is not isolated. In some embodiments, the source material is contacted by a host cell of the present invention. 
     In some embodiments, the source material is a cellulosic biomass. In some embodiments, the source material is a lignocellulosic biomass. In some embodiments, the source material is selected from the group consisting of insoluble cellulose, crystalline cellulose, pretreated hardwood, softwood, paper sludge, newspaper, sweet sorghum, pretreated corn stover, pretreated sugar cane bagasse, pretreated corn cobs, pretreated switchgrass, pretreated municipal solid waste, pretreated distiller&#39;s dried grains, pretreated wheat straw, rice straw, nut shells, banana waste, sponge gourd fibers, corn fiber, agave, trees, corn stover, wheat straw, sugar cane bagasse, switchgrass, and combinations thereof. In some embodiments, the source material is corn stover. 
     EXAMPLES 
     The invention now being generally described, it will be more readily understood by reference to the following examples, which are included merely for purposes of illustration of certain aspect and embodiments of the present invention, and are not intended to limit the invention. 
     Example 1 —S. cerevisiae  Background Strain 
     A strain of  S. cerevisiae  was created that was suitable for the testing of functional xylose isomerases. The GRE3 locus of an industrial yeast strain was replaced with expression cassettes for the pentose phosphate pathway genes RPE1, RKI1, TKL1, and TAL1 as well as the native  S. cerevisiae  xyulokinase XKS1 ( FIG. 1 ). 
     Example 2—Production of a Parent Vector Library 
     A total of 22 Donor XI sequences were codon optimized and synthesized by Integrated DNA Technologies (IDT gblocks) in two independent but overlapping DNA fragments. These XI fragments also contained homologous regions to either the  S. cerevisiae  HSP150 promoter (HSP150p) or the  S. cerevisiae  HXT7 terminator sequence (HXT7t), the regulatory regions used to drive the expression of the XIs ( FIG. 3 ). The overlapping glocks for each XI were assembled via in vitro assembly (See, e.g., Gibson and Russello, “Gibson Assembly®—Building a Synthetic Biology Toolset,” see neb.com/tools-and-resources/feature-articles/gibson-assembly-building-a-synthetic-biology-toolset (last viewed Jul. 17, 2014)) along with the full length HSP150p and HXT7t. The in vitro assembly reaction was used as template for the PCR amplification of the full length expression cassette using Phusion polymerase. Parent XIs were tested for independent functionality by cloning them into the vector shown in  FIG. 2  using YML within the  S. cerevisiae  strain described in Example 1. The resulting transformations were plated to YNBX plates and observed for the formation of colonies. 
     For use in the XI library, the vector shown in  FIG. 2  was amplified in two over lapping fragments which resulted in the absence of the fragment spanning from the ADH1p through the PDC1t. This region was instead replaced with the HSP150p, the XIs, and the HXT7t via homologous recombination. Sequences enabling replication in yeast (2 micron ori, CEN/ARS ori) or  E. coli  (PBM1 ori, pBR322 ori, Col E1 ori, pUC ori) are shown. Protein coding sequences include the antibiotic resistance genes (ZEO R, Amp R) and the  S. cerevisiae  URA3 (Sc URA3) for selection in ura auxotrophs. 
     Nine of the 22 parent XIs screened were found to generate colonies when expressed within yeast; Cd XI, Ca XI, Ad XI, Ls XI, Rs XI, Pr XI, Lx XI, Bt XI, and Cp XI. For the generation of the XI chimera library, these donors were amplified in three overlapping segments using Phusion polymerase and gel extracted and purified using the nucleospin gel and PCR clean-up kit from Macherey-Nagel. In order to prevent the assembly of any functional parent XI the three fragments for any given XI were never included in the same transformation (Table 1). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Summary of XI fragments added to each transformation. In addition to the fragments 
               
               
                 highlighted below each transformation also received the two plasmid backbone 
               
               
                 fragments and each of the full length non-functional donor Xis. The boxes with 
               
               
                 a “Y” indicate DNA fragments which were added to each transformation. 
               
            
           
           
               
               
               
               
            
               
                   
                 Transformation 1 
                 Transformation 2 
                 Transformation 3 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Frag A 
                 Frag B 
                 Frag C 
                 Frag A 
                 Frag B 
                 Frag C 
                 Frag A 
                 Frag B 
                 Frag C 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Cd XI 
                 Y 
                 Y 
                   
                   
                 Y 
                 Y 
                 Y 
                   
                 Y 
               
               
                 Ca XI 
                 Y 
                 Y 
                   
                   
                 Y 
                 Y 
                 Y 
                   
                 Y 
               
               
                 Ad XI 
                 Y 
                 Y 
                   
                   
                 Y 
                 Y 
                 Y 
                   
                 Y 
               
               
                 Ls XI 
                   
                 Y 
                 Y 
                 Y 
                   
                 Y 
                 Y 
                 Y 
               
               
                 Rs1 XI 
                   
                 Y 
                 Y 
                 Y 
                   
                 Y 
                 Y 
                 Y 
               
               
                 Pr XI 
                   
                 Y 
                 Y 
                 Y 
                   
                 Y 
                 Y 
                 Y 
               
               
                 Lx XI 
                 Y 
                   
                 Y 
                 Y 
                 Y 
                   
                   
                 Y 
                 Y 
               
               
                 Bt XI 
                 Y 
                   
                 Y 
                 Y 
                 Y 
                   
                   
                 Y 
                 Y 
               
               
                 Cp XI 
                 Y 
                   
                 Y 
                 Y 
                 Y 
                   
                   
                 Y 
                 Y 
               
               
                   
               
            
           
         
       
     
     For the remaining 13 donors (Ci XI, Fm XI, Tm XI, Md XI, Bs XI, Hv XI, Zm XI, At XI, XYM1, XYM2, Xc XI, Cpi XI, Pd XI), which showed insufficient activity within the modified yeast strain to generate colony formation on xylose, full length XI sequences containing homology to the HSP150p and HXT7t sequences were amplified. PCR products were again gel extracted and purified using the nucleospin gel and PCR clean-up kit from Macherey-Nagel. The plasmid vector for which the XI expression cassette would assemble, as shown in  FIG. 2 , was also amplified using Phusion polymerase in two overlapping segments, which were gel extracted and purified as above. A schematic showing the amplification strategy is contained in  FIG. 4 . 
     Example 3—Chimeric XI Library Transformations 
     The DNA fragments generated above were transformed into the modified yeast strain described in Example 1 in three separate transformations using standard electroporation techniques. All reactions received two plasmid backbone fragments, each of the full length non-functional donor XIs, and two out of the three fragments for each of the nine functional XI parent sequences as outlined in Table 1 and summarized in  FIG. 5 . Transformations were allowed to recover in 1 ml of YPD for 2 hours at 35° C. prior to the addition of 1 ml of YPD+Zeo200 μg/ml (100 μg/ml final concentration) followed by overnight recovery. 1 ml of each transformation was plated to YNBX plates in 100 μl aliquots. Plates were incubated at 35° C. and observed daily for the formation of colonies. Any colonies obtained were re-struck to fresh YNBX plates prior to analysis. 
     Example 4—Amplification, Sequencing, and Screening 
     The plasmid inserts from each of the re-struck colonies were amplified by colony PCR using Phusion polymerase and primers specific to the HSP150p/HXT7t sequences. The products were checked by gel electrophoresis and purified using the gel and PCR clean-up kit from Macherey-Nagel. The purified PCR products were sequenced by the Sanger method at the Dartmouth College Sequencing Facility on the Applied Biosystems Model 3100 sequencer; the resulting reads were assembled using SeqMan Pro from the DNASTAR Lasergene 10 Core Suite. Chimeric sequences were aligned to all parent XIs in order to determine the origin of the DNA sequences. Sample chimeric polynucleotides created and identified using this approach are represented in  FIG. 6 . 
     For select chimeras of interest the amplified inserts were re-cloned into the plasmid backbone by YML and 8 independent colonies were chosen for screening on YNBX liquid media. Transformants were grown overnight in 600 ul of YPD+Zeo100 μg/ml in a 96 well plate along with control cultures grown in either YPD (integrated strains free of antibiotic markers) or YPD+Zeo100 μg/ml (plasmid containing controls, i.e. Bt XI). The overnight cultures were diluted 1/600 in fresh YNBX media and grown for 48 hours at 35° C. with constant shaking aerobically, prior to the measurement of the OD 600 for each culture. 
     Example 5—Growth Analysis 
     Selected chimera strains were inoculated into YPX media (yeast extract, peptone, and xylose) at identical starting cell concentrations in a 96-well plate format. Samples were then loaded into a Bioteck plate reader in an anaerobic chamber. Cell density was tracked using OD 600  measurements. Sample data of one such analysis is shown in  FIGS. 7,8A  and *B. Data is plotted for seven different strains over approximately 36 hours. In  FIG. 7 , the chimeric XI strain expressing the chimeric XI of SEQ ID NO:1 exhibits a better growth rate than the  B. thetaiotaomicron  XI strain in at least one of the strains tested. The data in  FIG. 7  shows strains engineered with the  B. thetaiotaomicron  xylose isomerase (BtXI) and strains engineered with the chimeric XI 1 (SEQ ID NO:1), as provided in the present invention. The BtXI and chimeric strains are genetically identical except for the different XIs. A negative control strain is shown that is unable to grow on xylose.  FIG. 8A  shows the growth rates of various chimeric XI&#39;s integrated at the same copy number within the xylose screening background.  FIG. 8B  shows the growth rates of chimeric XI 1 and chimeric XI 6 integrated at the same copy number within the xylose screening background.  FIGS. 8A  and B also show a negative control strain that is unable to grow on xylose. Cell density was also tracked using CO 2  measurements. As shown in  FIG. 8C , CO 2  production was monitored as an indirect measurement of xylose consumption and growth rate.  FIG. 8C  shows the growth rates of chimeric XI 1 and chimeric XI 7 integrated at the same copy number within the xylose screening background 
     Example 6—Deletion of YPR1 Improves Ethanol Production from Xylose 
     Selected chimera strains and a control strain were inoculated in YPX media (yeast extract, peptone, xylose) at identical starting cell concentrations in sealed pressure bottles. Samples were taken every six to twelve hours, and ethanol concentrations determined. The “chimera 1” strain, as shown in  FIGS. 9 and 10 , was created by stable genomic integration of a chimeric XI into the “host strain,” used in  FIGS. 9 and 10  as a negative control. Chimera 1 expresses a chimeric xylose isomerase comprising the amino acid sequence of SEQ ID NO:1. Chimera 1 further comprises additional copies of the native genes TKL1, TAL1, RPE1, RKI1, and XKS, and a deletion of the GRE3 gene. “Chimera 1+ypr1Δ” is identical to the Chimera 1 strain, except that Chimera 1+yprΔ has a deletion of the YPR1 gene. The “host strain” is a negative control that is unable to ferment xylose and produces no ethanol.  FIG. 9  shows that the deletion of YPR1 enables the yeast cells to ferment xylose more quickly with no decrease in ethanol yield than comparable yeast cells that do not have a deletion of YPR1. 
     Example 7—Overexpression of PGM1 Improves Xylose Fermentation 
     Selected chimera strains and a control strain were inoculated in YPX media (yeast extract, peptone, xylose) at identical starting cell concentrations in a 96-well plate format and loaded into a Biotek plate reader in an anaerobic chamber. Cell density was tracked using OD 600  measurements. The data in  FIG. 10  is plotted for three strains over approximately 48 hours. The “chimera 1” strain was created by stable genomic integration of a chimeric XI into the “host strain.” Chimera 1 expresses a chimeric xylose isomerase comprising the amino acid sequence of SEQ ID NO:1. Chimera 1 further comprises additional copies of the native genes TKL1, TAL1, RPE1, RKI1, and XKS, and a deletion of the GRE3 gene. The “chimera 1+PGM1 o/e” strain is identical to the chimera 1 strain, except that chimera 1+PGM1 o/e has additional copies of the native PGM1 gene expressed under the control of a constitutive native promoter. The “host strain” is a negative control that is unable to grow on xylose.  FIG. 10  shows that overexpression of PGM1 enables the yeast cells to grow more quickly on xylose than comparable yeast cells that do not overexpress PGM1. 
     All documents cited herein, including journal articles or abstracts, published or corresponding U.S. or foreign patent applications, issued or foreign patents, or any other documents, are each entirely incorporated by reference herein, including all data, tables, figures, and text presented in the cited documents. 
     Following are particular embodiments of the disclosed invention: 
     E1. A polypeptide comprising an amino acid sequence having at least 90% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, and 9. 
     E2. The polypeptide of claim  1 , wherein the amino acid sequence has at least 95% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, and 9. 
     E3. The polypeptide of claim  1 , wherein the amino acid sequence has at least 98% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, and 9. 
     E4. The polypeptide of claim  1 , wherein the amino acid sequence has at least 99% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, and 9. 
     E5. The polypeptide of claim  1 , wherein the amino acid sequence has 100% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, and 9. 
     E6. A polypeptide comprising an amino acid sequence having at least 90%, 95%, 98%, 99% or 100% sequence identity with an amino acid sequence of SEQ ID NO: 25 or SEQ ID NO: 27. 
     E7. A polypeptide comprising a C-terminal region and an N-terminal region, wherein the C-terminal region of the polypeptide comprises at least 5 contiguous amino acids from the amino acid sequence of SEQ ID NO:13 and the N-terminal region of the polypeptide comprises at least 5 contiguous amino acids from an amino acid sequence selected from the group consisting of SEQ ID NOs: 11, 15, 17, 23, and combinations thereof. 
     E8. The polypeptide of E7, wherein the N-terminal region of the polypeptide comprises at least 5 contiguous amino acids from the amino acid sequence of SEQ ID NO: 11. 
     E9. The polypeptide of E7, wherein the N-terminal region of the polypeptide comprises at least 5 contiguous amino acids from the amino acid sequence of SEQ ID NO: 15. 
     E10. The polypeptide of E7, wherein the N-terminal region of the polypeptide comprises at least 5 contiguous amino acids from the amino acid sequence of SEQ ID NO: 17. 
     E11. The polypeptide of E7, wherein the N-terminal region of the polypeptide comprises at least 5 contiguous amino acids from the amino acid sequence of SEQ ID NO: 23. 
     E12. The polypeptide of E7, E8, and E11, wherein the N-terminal region of the polypeptide comprises at least 5 contiguous amino acids from an amino acid sequence selected from the group consisting of SEQ ID NOs: 11, 23, and a combination thereof. 
     E13. The polypeptide of any one of E7 to E12, wherein the N-terminal region of the polypeptide comprises at least 5 contiguous amino acids from the amino acid sequence selected from the group consisting of SEQ ID NOs: 11, 17, 15, and combinations thereof. 
     E14. A polypeptide comprising a C-terminal region and an N-terminal region, wherein the C-terminal region comprises at least 5 contiguous amino acids from the amino acid sequence of SEQ ID NO:15 and the N-terminal region comprises at least 5 contiguous amino acids from an amino acid sequence selected from the group consisting of SEQ ID NOs: 19, 21, 23, and combinations thereof. 
     E15. The polypeptide of E14, wherein the C-terminal region further comprises at least 5 contiguous amino acids from SEQ ID NO:21. 
     E16. The polypeptide of E14 or E15, wherein the N-terminal region comprises at least 5 contiguous amino acids from SEQ ID NO:23. 
     E17. The polypeptide of E15 or E16, wherein the N-terminal region further comprises at least 5 contiguous amino acids from SEQ ID NO:21. 
     E18. The polypeptide of E16 or E17, wherein the N-terminal region further comprises at least 5 contiguous amino acids from SEQ ID NO:19. 
     E19. The polypeptide of any one of E1 to E18, wherein the polypeptide comprises: 
     a. the amino acids VXW[GP]GREG[YSTA] present at positions 188-196, wherein “X” is any amino acid and wherein amino acids in brackets indicate that one of the bracketed amino acids can be present at that position; 
     b. the amino acids [LIVM]EPKPX[EQ]P present at positions 233-240, wherein “X” can be any amino acid and wherein amino acids in brackets indicates that one of the bracketed amino acids can be present at that position; and 
     c. a His residue at position 103, an Asp residue at position 106, and an Asp residue at position 341; 
     wherein the positions refer to positions in the reference amino acid sequence of SEQ ID NO: 11. 
     E20. The polypeptide of any one of E7 to E19, wherein the polypeptide is an enzyme. 
     E21. The polypeptide of any one of E7 to E20, wherein the polypeptide has xylose isomerase activity. 
     E22. A polynucleotide having a nucleotide sequence encoding the amino acid sequence of any one of E1 to E18. 
     E23. The polynucleotide of E22, wherein the nucleotide sequence is codon optimized. 
     E24. The polynucleotide of E23, wherein the nucleotide sequence is codon optimized for expression in a fungal cell. 
     E25. The polynucleotide of E24, wherein the fungal cell is a yeast cell. 
     E26. The polynucleotide of E25, wherein the yeast cell is a  Saccharomyces cerevisiae  cell. 
     E27. A vector comprising the polynucleotide of any one of E22 to E26. 
     E28. A recombinant host cell comprising the vector of E27. 
     E29. A recombinant host cell comprising the polypeptide of any one of E1 to E21. 
     E30. A recombinant host cell comprising the polynucleotide of any one of E22 to E26. 
     E31. The recombinant host cell of E30, wherein the host cell has a genome and the polynucleotide is integrated into the genome of the host cell. 
     E32. The recombinant host cell of any one of E28 to E31, wherein the host cell is capable of growing on xylose as the sole carbon source. 
     E33. The recombinant host cell of any one of E28 to E32, wherein the host cell is a yeast cell. 
     E34. The recombinant host cell of any one of E28 to E33, wherein the host cell is a member of a genus selected from the group consisting of  Saccharomyces, Kluyveromyces, Candida, Pichia, Schizosaccharomyces, Hansenula, Kloeckera, Schwanniomyces , and  Yarrowia.    
     E35. The recombinant host cell of any one of E28 to E34, wherein the host cell is a member of a species selected from the group consisting of  Saccharomyces cerevisiae, Saccharomyces bulderi, Saccharomyces exiguus, Saccharomyces uvarum, Saccharomyces diastaticus, Kloeckera lactis, Kloeckera marxianus , and  Kloeckera fragilis.    
     E36. The recombinant host cell of any one of E28 to E35, wherein the host cell is a  S. cerevisiae  cell. 
     E37. The recombinant host cell of any one of E28 to E 36, wherein the host cell further comprises at least one additional genetic modification of one or more genes encoding a protein of the pentose phosphate pathway. 
     E38. The recombinant host cell of E37, wherein the host cell comprises additional genetic modifications in at least one of the endogenous genes selected from the group consisting of XKS1, RKI1, RPE1, TKL1, and TAL1. 
     E39. The recombinant host cell of E38, wherein the host cell comprises genetic modifications that lead to the overexpression of at least one of the endogenous genes selected from the group consisting of XKS1, RKI1, RPE1, TKL1, and TAL1. 
     E40. The recombinant host cell of any one of E28 to E39, wherein the host cell further comprises a modification of one or more aldose reductase genes. 
     E41. The recombinant host cell of E40, wherein the aldose reductase gene is GRE3. 
     E42. The recombinant host cell of E41, wherein the host cell comprises a deletion or disruption of the endogenous GRE3 gene. 
     E43. The recombinant host cell of E40, wherein the aldose reductase gene is YPR1. 
     E44. The recombinant host cell of E43, wherein the host cell comprises a deletion or disruption of the endogenous YPR1 gene. 
     E45. The recombinant host cell of E40, wherein said modification comprises a deletion or disruption of the endogenous aldose reductase genes GRE3 and YPR1. 
     E46. The recombinant host cell of any one of E28 to E45, wherein the host cell further comprises a modification of the endogenous PGM1 gene. 
     E47. The recombinant host cell of E46, wherein the modification of the endogenous PGM1 gene results in the overexpression of PGM1. 
     E48. A method for producing a fermentation product comprising contacting the recombinant host cell of any one of E28 to E47 with a carbon source, wherein said carbon source comprises xylose. 
     E49. The method of E48, wherein the fermentation product is selected from the group consisting of ethanol, lactic acid, 3-hydroxy-propionic acid, hydrogen, butyric acid, acrylic acid, acetic acid, succinic acid, citric acid, malic acid, fumaric acid, an amino acid, 1,3-propane-diol, ethylene, glycerol, acetone, isopropyl alcohol, butanol, a β-lactam, an antibiotic, and a cephalosporin. 
     E50. The method of E49, wherein the fermentation product is ethanol. 
     E51. The method of any one of E48 to E50, further comprising recovering the fermentation product. 
     E52. A method of synthesizing a chimeric polypeptide comprising: 
     a. providing at least two or more parent polynucleotide sequences, a linearized vector capable of replication in yeast, and a yeast cell; 
     b. transforming the yeast cell with the at least two parent polynucleotide sequences and the linearized vector; and 
     c. screening the transformed yeast cells for recombination of the parent polynucleotide sequences. 
     E53. The method of E52 wherein the identified yeast cells comprising the chimeric polynucleotide are isolated. 
     E54. The method of E52 or E53, wherein one or more of the parent polynucleotide sequences are derived from the genomes of a species selected from the group consisting of  Bacteroides thetaiotaomicron, Abiotrophia defectiva, Parabacteroides distasonis, Chitinophaga pinensis, Cyllamyces aberensis, Prevotella ruminicola , and combinations thereof. 
     E55. The method of any one of E52 to E54, wherein at least one of the parent polynucleotide sequences encodes a polypeptide capable of converting xylose to xylulose. 
     E56. The method of any one of E52 to E54, wherein at least one of the polynucleotide sequences is a fragment of a full length polynucleotide sequence, wherein the full length polynucleotide sequence encodes a polypeptide capable of converting xylose to xylulose. 
     E57. The method of any one of E52 to E54, wherein the linearized vector comprises at least one homologous region comprising a polynucleotide sequence that has homology to at least one section of the parent polynucleotide sequence. 
     E58. The method of E57, wherein the at least one homologous region of the linearized vector is positioned at either the 5′ or 3′ end of the linearized vector. 
     E59. The method of E58, wherein the linearized vector comprises a homologous region at the 5′ end and a homologous region at the 3′ end. 
     E60. The method of any one of E52 to E59, wherein the transformed yeast cells undergo homologous recombination to generate a circular vector. 
     E61. The method of any one of E52 to E60, wherein the parent polynucleotide sequence is selected from the group consisting of SEQ ID NOs: 12, 14, 16, 18, 20, 22, and 24, or a fragment thereof. 
     E62. The method of any one of E52 to E61, wherein the synthesized chimeric polypeptide is capable of converting xylose to xylulose. 
     E63. The method of any one of E52 to E62, wherein the transformed yeast cells are screened by growing the transformed cells on xylose as the sole carbon source. 
     E64. The method of any one of E52 to E 62, wherein the identified cells are capable of growth on xylose as the sole carbon source are isolated. 
     E65. The method of any one of E52 to E 62, wherein the identified cells are lysed and DNA is extracted. 
     E66. The method of E65, wherein the extracted DNA is used as a template to amplify the chimeric polynucleotide sequence. 
     E67. A method of producing ethanol comprising contacting a source material comprising xylose with the polypeptide of any one of claims E1 to E 21. 
     E68. The method of E67, wherein the source material is a cellulosic or lignocellulosic biomass. 
     E69. The method of E67, wherein the source material is at least one material selected from the group consisting of insoluble cellulose, crystalline cellulose, pretreated hardwood, paper sludge, pretreated corn stover, pretreated sugar cane bagasse, pretreated corn cobs, pretreated switchgrass, pretreated municipal solid waste, pretreated distiller&#39;s dried grains, pretreated wheat straw, corn fiber, agave, trees, corn stover, wheat straw, sugar cane bagasse, and switchgrass. 
     E70. The method of E69, wherein the source material is corn stover. 
     E71. A method of producing ethanol comprising contacting a source material comprising xylose with the recombinant host cell of any one of E30 to E46. 
     E72. The method of E71, wherein the source material is a cellulosic biomass. 
     E73. The method of E71, wherein the source material is at least one material selected from the group consisting of trees, corn stover, wheat straw, sugarcane bagasse, switchgrass, corn, sugarcane, sugarbeet, and combinations thereof. 
     E74. The method of E73, wherein the source material is corn stover. 
     Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.