Abstract:
The present invention provides for the manipulation of carbon flux in a recombinant host cell to increase the formation of desirable products. The invention relates to cellulose-digesting organisms that have been genetically modified to allow the production of ethanol at a high yield by redirecting carbon flux at key steps of central metabolism.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage of International Application Number PCT/US2012/057952, filed Sep. 28, 2012, which claims the benefit of U.S. Provisional Application No. 61/542,082, filed Sep. 30, 2011, which are incorporated by reference herein. 
    
    
     STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     This invention was funded, in part, by the United States government under a Department of Energy Biomass Program award # DE-FC36-07G017057. This invention was also funded, in part, by the BioEnergy Science Center (BESC) under the DOE Office of Science through award number DE-POS2-06ER64304. The U.S. Government has certain rights in this invention. 
    
    
     REFERENCE TO SEQUENCE LISTING SUBMITTED ELECTRONICALLY VIA EFS-WEB 
     The content of the electronically submitted sequence listing (Name: 2608_0640001_CORRECTED_SequenceListing_ascii.txt; 253,390 bytes; and Date of Creation: May 12,2014)is herein incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Fuel and energy production have emerged as one of the great challenges of the 21 st  century, and solving these problems touches upon an arena of issues that range from security to poverty to the environment. New approaches to providing for the world&#39;s energy needs are required to address these mounting concerns. 
     Among forms of plant biomass, lignocellulosic biomass (“biomass”) is particularly well-suited for energy applications because of its large-scale availability, low cost, and environmentally benign production. In particular, many energy production and utilization cycles based on cellulosic biomass have very low greenhouse gas emissions on a life-cycle basis. The primary obstacle impeding the more widespread production of energy from biomass feedstocks is the general absence of low-cost technology for overcoming the recalcitrance of these materials to conversion into useful products. Lignocellulosic biomass contains carbohydrate fractions (e.g., cellulose and hemicellulose) that can be converted into ethanol or other end-products including lactic acid and acetic acid. In order to convert these fractions, the cellulose or hemicellulose must ultimately be converted or hydrolyzed into monosaccharides. This hydrolysis has historically proven to be problematic. 
     Cellulose digesting anaerobic bacteria are of great potential utility because they can be used to produce ethanol or other fuels from abundant substrates such as forestry, municipal and agricultural waste. However, it has been challenging to realize this potential utility because of difficulty in the genetic manipulation of these organisms and lack of understanding of their metabolic biochemistry. Genome sequence data and recent advances in biotechnological tools for genetic modification of  Clostridium thermocellum  and other similar organisms have made it possible to make progress in this area, but the great complexity of metabolism makes it difficult to achieve efficiently a desired outcome such as near theoretical ethanol yield from cellulosic substrates. 
     Many microorganisms can metabolize glucose, cellulose or cellodextrins anaerobically, but they vary in the pathways utilized and the products generated. It has been demonstrated in genetically modified  Thermoanaerobacterium saccharolyticum  that glucose and cellobiose can be fermented to ethanol at very close to theoretical yield, but similar genetic manipulations in  Clostridium thermocellum  have not had the same outcome. Argyros, D A, Tripathi S A, Barrett T F, Rogers S R, Feinberg L F, Olson D G, Foden J M, Miller B B, Lynd L R, Hogsett D A, Caiazza N C, High ethanol titers from cellulose using metabolically engineered thermophilic, anaerobic microbes.  Appl. Env. Microbiol.  2011. 77(23):8288-94. 
       Clostridium thermocellum  has both cellulolytic and ethanologenic fermentation capabilities and can directly convert a cellulose-based substrate into ethanol. However,  C. thermocellum  possesses a branched carbon utilization pathway that generates products other than ethanol and is not as amenable to manipulation for ethanol production as that of  T. saccharolyticum . This is exemplified more clearly when the carbon utilization pathways from the two organisms are compared. In homoethanologenic  T. saccharolyticum , the carbon atoms from glucose flow down a linear central metabolic pathway to ethanol ( FIG. 1A ). In  C. thermocellum , a different set of enzymes is present and thus the carbon utilization pathway ( FIG. 1B ) is different than in  T. saccharolyticum . This difference in the carbon-utilization pathways in these organisms makes it infeasible to produce ethanol at theoretical yield with the same modifications. 
     The invention relates to cellulose-digesting organisms that have been genetically modified to allow the production of ethanol at a high yield by redirecting carbon flux at key steps of central metabolism. Redirection means altering the flux of carbon from the normally prevailing routes to alternate routes by means of genetic modification. 
     BRIEF SUMMARY OF THE INVENTION 
     Aspects of the invention are directed to increasing ethanol production by a microorganism through redirecting carbon flux and eliminating the pathways for alternate end-products. In some embodiments, the present invention allows more ethanol to be produced for the same amount of substrate, allowing more profit to be gained from the same substrate, and thus requires less cell mass in exchange for higher fermentation end-products, such as ethanol. 
     In one embodiment, the invention relates to a recombinant microorganism capable of fermenting biomass and producing ethanol. In some embodiments, the microorganism is a prokaryote. 
     As recently as 2010 it was published that  C. thermocellum  contains a pyruvate kinase gene. (Roberts S B, Gowen, C M, Brooks, J P, and Fong, S S, Genome-scale metabolic analysis of  Clostridium thermocellum  for bioethanol production.  BMC Systems Biology.  2010. 4:31). However, contrary to this assertion, the present invention demonstrates that endogenous pyruvate kinase activity is either not present, or is sub-optimal in  C. thermocellum.    
     In one embodiment, the invention relates to a recombinant prokaryotic microorganism comprising a heterologous nucleic acid encoding pyruvate kinase wherein the polynucleotide has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NO: 1, and a genetic modification that leads to the down-regulation of an enzyme in a lactic acid and/or acetic acid pathway wherein the polynucleotide encoding for the down-regulated enzyme has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NOs: 3, 5, 7 or 53. 
     In one embodiment, the invention relates to a recombinant prokaryotic microorganism comprising a heterologous nucleic acid encoding pyruvate kinase wherein the polynucleotide has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NO: 1, and a genetic modification that leads to the down-regulation of an enzyme in a pathway for the conversion of phosphoenolpyruvate to pyruvate wherein the polynucleotide encoding for the down-regulated enzyme has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NOs: 9, 11, 13, 15, 17, or 51. 
     In one embodiment, the invention relates to a recombinant prokaryotic microorganism comprising a heterologous nucleic acid encoding pyruvate kinase wherein the polynucleotide has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NO: 1; a genetic modification that leads to the down-regulation of an enzyme in a lactic acid or acetic acid pathway wherein the polynucleotide encoding for the down-regulated enzyme has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NOs: 3, 5, 7, or 53; and a genetic modification that leads to the down-regulation of an enzyme in a pathway for conversion of phosphoenolpyruvate to pyruvate wherein the polynucleotide encoding for the down-regulated enzyme has a nucleotide sequence at least about 80% identical to the nucleotide sequences of SEQ ID NOs: 9, 11, 13, 15, 17, or 51. 
     In one embodiment, the invention relates to a recombinant prokaryotic microorganism comprising a heterologous nucleic acid encoding pyruvate formate lyase wherein the polynucleotide has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NO: 19, and activating enzymes wherein the polynucleotides encoding for them have a nucleotide sequence at least about 80% identical to the nucleotide sequences of SEQ ID NO: 21. 
     In one embodiment, the invention relates to a recombinant prokaryotic microorganism comprising a heterologous nucleic acid encoding pyruvate formate lyase wherein the polynucleotide has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NO: 19, PFL-activating enzymes wherein the polynucleotides encoding for them have a nucleotide sequence at least about 80% identical to the nucleotide sequences of SEQ ID NO: 21, and a genetic modification that leads to the down-regulation of an enzyme in a pyruvate metabolism pathway wherein the polynucleotide encoding for the down-regulated enzyme has a nucleotide sequence at least about 80% identical to the nucleotide sequences of SEQ ID NOs: 23, 25, 27, 29, 31, 33, 35, 37, 39, or 41. In some embodiments, the enzyme in the pathway is pyruvate oxidoreductase or NADH-dependent reduced ferredoxin:NADP+ oxidoreductase. 
     In one embodiment, the invention relates to a recombinant prokaryotic microorganism comprising a heterologous nucleic acid encoding pyruvate formate lyase wherein the polynucleotide has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NO: 19, PFL-activating enzymes wherein the polynucleotides encoding for them have a nucleotide sequence at least about 80% identical to the nucleotide sequences of SEQ ID NO: 21, and a polynucleotide encoding for the enzyme has a nucleotide sequence at least about 80% identical to the nucleotide sequences of SEQ ID NOs: 43, 45, or 47 or at least about 80% identical to the polypeptide sequences of 49 and 50. In some embodiments, the recombinant prokaryotic microorganism further comprises a genetic modification that leads to the down-regulation of an enzyme in a lactic acid or acetic acid pathway wherein the polynucleotide encoding for the down-regulated enzyme has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NOs: 3, 5, 7, or 53. 
     In one embodiment, the invention relates to a recombinant prokaryotic microorganism comprising a heterologous nucleic acid encoding pyruvate formate lyase wherein the polynucleotide has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NO: 19; PFL-activating enzymes wherein the polynucleotides encoding for them have a nucleotide sequence at least about 80% identical to the nucleotide sequences of SEQ ID NO: 21; a genetic modification that leads to the down-regulation of an enzyme in a pyruvate metabolism pathway wherein the polynucleotide encoding for the down-regulated enzyme has a nucleotide sequence at least about 80% identical to the nucleotide sequences of SEQ ID NOs: 23, 25, 27, 29, 31, 33, 35, 37, 39, or 41; and a genetic modification that leads to the down-regulation of an enzyme in a lactic acid and/or acetic acid pathway wherein the polynucleotide encoding for the down-regulated enzyme has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NOs: 3, 5, 7, or 53. 
     In one embodiment, the invention relates to a recombinant prokaryotic microorganism comprising a heterologous nucleic acid encoding pyruvate kinase wherein the polynucleotide has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NO: 1; a genetic modification that leads to the down-regulation of an enzyme in a lactic acid or acetic acid pathway wherein the polynucleotide encoding for the down-regulated enzyme has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NOs: 3 or 53; a genetic modification that leads to the down-regulation of an enzyme in a pathway for conversion of phosphoenolpyruvate to pyruvate wherein the polynucleotide encoding for the down-regulated enzyme has a nucleotide sequence at least about 80% identical to the nucleotide sequences of SEQ ID NO: 13, and a heterologous nucleic acid encoding a bifunctional acetaldehyde-alcohol dehydrogenase wherein the polynucleotide has a nucleotide sequence at least about 80% identical to the nucleotide sequence of SEQ ID NO: 67. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
         FIG. 1  depicts a simplified metabolic pathway from cellobiose to ethanol in  T. saccharolyticum  (A) and  C. thermocellum  (B). Only reduced nicotinamide cofactors are shown; the oxidized forms are implied. The cofactors involved in acetate and lactate production are not shown. The multiple steps from cellobiose to PEP are represented by a dotted line, but all other arrows represent single biochemical reactions. Abbreviations are PEP=phosphoenolpyruvate, Pyr=pyruvate, Oxa=oxaloacetate, Mal=malate, Ac-CoA=acetyl-CoA, Aceald=acetaldehyde, Etoh=ethanol, Ac-P=acetyl phosphate, Fdred=reduced ferredoxin, Fdox=oxidized ferredoxin. The names of the genes encoding the enzymes that catalyze each step are shown in italics. 
         FIG. 2  depicts a gene inactivation plasmid for the malate dehydrogenase gene. 
         FIG. 3  depicts the shift in size of the mdh locus after the disruption cassette has been inserted into the mdh gene. 
         FIG. 4  depicts a gene inactivation plasmid for the lactate dehydrogenase gene. 
         FIG. 5 , in Part A, depicts an electrophoretic gel containing DNA samples from the mdh locus. The results of the assay indicate that all 8 strains contain a disruption cassette at the mdh locus, signified by an increased size of the PCR product. Part B depicts the status of the ldh locus in the same eight strains. The gel image demonstrates that the strain represented in lane 6 has a disruption cassette inserted at the ldh locus, disrupting the ldh gene. Strains represented in lanes 5 and 7 are a mixed culture of mutant and wild type cells, while the remainder are just wild type at the ldh locus. Thus, the strain represented in lane 6 has a disruption cassette at both the ldh and mdh loci. 
         FIG. 6 , in part A, shows the growth as a measure of time, measured by optical density (OD), in wild type strains, mdh mutant strains, and mdh, ldh double mutant strains. Part B depicts the lactic acid production as a function of time in wild type strains, mdh mutant strains, and mdh, ldh double mutant strains. Part C depicts the ethanol production as a measure of time in wild type strains, mdh mutant strains, and mdh, ldh double mutant strains. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice of the disclosed methods and compositions, the 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 may 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 may refer 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. 
     The term “heterologous” is used in reference to a polynucleotide or a gene not normally found in the host organism. “Heterologous” includes up-regulated endogenous genes. “Heterologous” also includes a native coding region, or portion thereof, that is 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. A heterologous gene may include a native coding region that is a portion of a chimeric gene including a non-native regulatory region that is reintroduced into the native host or modifications to the native regulatory sequences that affect the expression level of the gene. Foreign genes can comprise native genes inserted into a non-native organism, or chimeric genes. A heterologous polynucleotide, gene, polypeptide, or an enzyme may be derived from any source, e.g., eukaryotes, prokaryotes, viruses, or synthetic polynucleotide fragments, and includes up-regulated endogenous genes. 
     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 may be endogenous to the host cell or may 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 may, for example, be in the form of linear DNA or RNA. The term “gene” is also intended to cover multiple copies of a particular gene, e.g., all of the DNA sequences in a cell encoding a particular gene product. 
     The term “expression” is intended to include the expression of a gene at least at the level of mRNA production, generally subsequently translated into a protein product. 
     As used herein, an “expression vector” is a vector capable of directing the expression of genes to which it is operably linked. 
     In some embodiments, the microorganisms contain enzymes involved in cellulose digestion, metabolism and/or hydrolysis. A “cellulolytic enzyme” can be any enzyme involved in cellulose digestion, metabolism, and/or hydrolysis. The term “cellulase” refers to a class of enzymes produced chiefly by fungi, bacteria, and protozoans that catalyze cellulolysis (i.e. the hydrolysis) of cellulose. However, there are also cellulases produced by other types of organisms such as plants and animals. Several different kinds of cellulases are known, which differ structurally and mechanistically. There are general types of cellulases based on the type of reaction catalyzed: endocellulase breaks internal bonds to disrupt the crystalline structure of cellulose and expose individual cellulose polysaccharide chains; exocellulase cleaves 2-4 units from the ends of the exposed chains produced by endocellulase, resulting in the tetrasaccharides or disaccharide such as cellobiose. There are two main types of exocellulases (or cellobiohydrolases, abbreviate CBH)—one type working processively from the reducing end, and one type working processively from the non-reducing end of cellulose; cellobiase or beta-glucosidase hydrolyses the exocellulase product into individual monosaccharides; oxidative cellulases that depolymerize cellulose by radical reactions, as for instance cellobiose dehydrogenase (acceptor); cellulose phosphorylases that depolymerize cellulose using phosphates instead of water. In the most familiar case of cellulase activity, the enzyme complex breaks down cellulose to beta-glucose. A “cellulase” can be any enzyme involved in cellulose digestion, metabolism and/or hydrolysis, including, for example, an endoglucanase, glucosidase, cellobiohydrolase, xylanase, glucanase, xylosidase, xylan esterase, arabinofuranosidase, galactosidase, cellobiose phosphorylase, cellodextrin phosphorylase, mannanase, mannosidase, xyloglucanase, endoxylanase, glucuronidase, acetylxylanesterase, arabinofuranohydrolase, swollenin, glucuronyl esterase, expansin, pectinase, and feruoyl esterase protein. 
     A “plasmid” or “vector” refers to an extrachromosomal element often carrying one or more genes, and is usually in the form of a circular double-stranded DNA molecule. Plasmids and vectors may also contain additional genetic elements such as autonomously replicating sequences, genome integrating sequences, phage or nucleotide sequences. They may also be linear, circular, or supercoiled, of a single- or double-stranded DNA or RNA, derived from any source. Plasmids and vectors may be constructed by known techniques in which a number of nucleotide sequences have been joined or recombined into a unique construction. Plasmids and vectors generally also include a promoter fragment and DNA sequence for a selected gene product along with appropriate 3′ untranslated sequence. Generally, the plasmids of the present invention are stable and self-replicating. 
     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 oxygen. 
     “Anaerobic conditions” are defined as conditions under which the oxygen concentration in the fermentation medium is too low for the microorganism to use as a terminal electron acceptor. Anaerobic conditions may 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 may be achieved by the microorganism consuming the available oxygen of the 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 make energy, typically in the form of ATP, from carbohydrates. Aerobic metabolism typically occurs, for example, via glycolysis and the TCA cycle, 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 may also occur under aerobic conditions, under conditions where NADH is not fully oxidized via the respiratory chain. 
     As used herein, the term “flux” is the rate of flow of molecules through a metabolic pathway, akin to the flow of material in a process. 
     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, a “pathway” is a group of biochemical reactions that together can convert one compound into another compound in a multi-step process. A product of the first step in a pathway may be a substrate for the second step, and a product of the second step may be a substrate for the third, and so on. Pathways of the present invention include, but are not limited to, the lactate production pathway, the ethanol production pathway, and the acetate production pathway. 
     The term “recombination” or “recombinant” refers to the physical exchange of DNA between two identical (homologous), or nearly identical, DNA molecules. Recombination is used for targeted gene deletion to modify the sequence of a gene. The term “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 “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 or substitution of nucleic acid sequences that disrupt the function and/or expression of the gene. 
     As used herein, the term “down-regulate” includes the deletion or mutation of a genetic sequence, or insertion of a disrupting genetic element, coding or non-coding, such that the production of a gene product is lessened by the deletion, mutation, or insertion. “Delete” or “Deletion” as used herein refers to a removal of a genetic element such that a corresponding gene is completely prevented from being expressed. In some embodiments, deletion refers to a complete gene deletion. Down-regulation can also occur by causing the repression of genetic elements by chemical or other environmental means, for example by engineering a chemically-responsive promoter element to control the expression of a desired gene product. 
     As used herein, the term “up-regulate” includes the insertion, reintroduction, mutation or increased expression of a genetic sequence, such that the production of a gene product is increased by the insertion, reintroduction, or mutation. “Insert” or “Insertion” as used herein refers to an introduction of a genetic element such that a corresponding gene is expressed. Up-regulation can also occur by causing the increased expression of genetic elements through an alteration of the associated regulatory sequence. 
     As used herein, the term “lactic acid pathway” refers to the biochemical pathway that converts carbon-containing substrates, such as pyruvate, from glycolysis into the production of lactic acid. Components of the pathway consist of all substrates, cofactors, byproducts, end-products, and enzymes in the pathway. 
     As used herein, the term “acetic acid pathway” refers to the biochemical pathway that converts carbon-containing substrates, such as pyruvate, from glycolysis into the production of acetic acid or other compounds. Components of the pathway consist of all substrates, cofactors, byproducts, intermediates, end-products, and enzymes in the pathway. 
     As used herein, the term “ethanol pathway” refers to the canonical pathway of ethanol production from pyruvate generated by glycolysis. Components of the pathway consist of all substrates, cofactors, byproducts, intermediates, end-products, and enzymes in the pathway. 
     As used herein, the term “glycolysis” or “glycolytic pathway” refers to the canonical pathway of basic metabolism in which a sugar such as glucose is broken down into more oxidized products, generating energy and/or compounds required for cell growth. The pathway consists of all substrates, cofactors, byproducts, end-products, and enzymes in the pathway. 
     As used herein, the term “pyruvate kinase” is intended to include the enzymes capable of converting phosphoenolpyruvate (PEP) to pyruvate. Pyruvate kinase includes those enzymes that correspond to Enzyme Commission Number (EC) EC 2.7.1.40 and exemplified by SEQ ID NO:1 and SEQ ID NO: 2. 
     As used herein, the term “lactate dehydrogenase” or “LDH” is intended to include the enzymes capable of converting pyruvate to lactate. LDH includes those enzymes that correspond to EC 1.1.1.27 and EC 1.1.1.28 and exemplified by SEQ ID NOs: 3-4 and SEQ ID NOs: 53-54. 
     As used herein, the term “phosphotransacetylase” or “PTA” is intended to include the enzymes capable of converting acetyl-CoA to acetylphosphate. PTA includes those enzymes that correspond to EC 2.3.1.8 and exemplified by SEQ ID NO: 5 and SEQ ID NO: 6. 
     As used herein, the term “acetate kinase” or “ACK” is intended to include the enzymes capable of converting acetylphosphate to acetate. ACK includes those enzymes that correspond to EC 2.7.2.1 and exemplified by SEQ ID NO: 7 and SEQ ID NO: 8. 
     As used herein, the term “pyruvate-phosphate dikinase” or “PPDK” is intended to include the enzymes capable of converting pyruvate to PEP. PPDK includes those enzymes that correspond to EC 2.7.9.1 and exemplified by SEQ ID NOs: 9-12. 
     As used herein, the term “phosphoenolpyruvate carboxykinase” or “PEPCK” is intended to include the enzymes capable of converting PEP to oxaloacetate. PEPCK includes those enzymes that correspond to EC 4.1.1.31, EC 4.1.1.32, EC 4.1.1.38, and EC 4.1.1.49 and exemplified by SEQ ID NO: 13 and SEQ ID NO: 14. 
     As used herein, the term “malic enzyme” is intended to include the enzymes capable of converting malate to pyruvate. Malic enzyme includes those enzymes that correspond to EC 1.1.1.38, EC 1.1.1.39, and EC 1.1.1.40 and exemplified by SEQ ID NO: 15 and SEQ ID NO: 16. 
     As used herein, the term “malate dehydrogenase” or “MDH” is intended to include the enzymes capable of converting oxaloacetate to malate. MDH includes those enzymes that correspond to EC 1.1.1.37, EC 1.1.1.82, EC 1.1.1.299, EC 1.1.5.4, EC 1.1.3.3, and EC 1.1.99.7, and exemplified by SEQ ID NOs: 17-18 and SEQ ID NO: 51-52. 
     As used herein, the term “pyruvate formate lyase” or “PFL” is intended to include the enzymes capable of converting pyruvate to formate and acetyl-CoA. PFL includes those enzymes that correspond to EC 2.3.1.54 and exemplified by SEQ ID NO: 19 and SEQ ID NO: 20. 
     As used herein, the term “PFL-activating enzymes” is intended to include those enzymes capable of aiding in the activation of PFL. PFL-activating enzymes include those enzymes that correspond to EC 1.97.1.4 and exemplified by SEQ ID NO: 21 and SEQ ID NO: 22. 
     As used herein, the term “pyruvate oxidoreductase” or “POR” is intended to include those enzymes capable of converting pyruvate and oxidized ferredoxin to acetyl CoA and reduced ferredoxin. POR includes those enzymes that correspond to EC 1.2.7.1 and exemplified by SEQ ID NOs: 23-38. 
     As used herein, the term “NADH-dependent reduced ferredoxin:NADP+ oxidoreductase” or “NfnAB” is intended to include any enzyme that “couples the exergonic reduction of NADP+ with reduced ferredoxin and the endergonic reduction of NADP+ with NADH in a reversible reaction.” Wang S, Huang H, Moll J, Thauer R K. NADP+ reduction with reduced ferredoxin and NADP+ reduction with NADH are coupled via an electron-bifurcating enzyme complex in  Clostridium kluyveri. J. Bacteriol.  2010 October; 192(19):5115-23. NfnAB includes those enzymes that are exemplified by SEQ ID NOs: 39-42. 
     As used herein, the term “formate dehydrogenase” is intended to include those enzymes capable of converting formate to bicarbonate (carbon dioxide). Formate dehydrogenase includes those enzymes that correspond to EC 1.2.1.43 (NAD+-specific) and EC 1.2.1.2 (NADP+-specific) and exemplified by SEQ ID NOs: 43-50. 
     As used herein, the term “alcohol dehydrogenase” or “ADH” is intended to include the enzymes that catalyze the conversion of ethanol into acetylaldehyde. Very commonly, the same enzyme catalyzes the reverse reaction from acetaldehyde to ethanol, which is the direction most relevant to fermentation. Alcohol dehydrogenase includes those enzymes that correspond to EC 1.1.1.1 and EC 1.1.1.2 and exemplified by the enzymes disclosed in GenBank Accession # U49975. 
     As used herein, the term “acetaldehyde dehydrogenase” or “ALDH” is intended to include the enzymes that catalyze the conversion of acetaldehyde into acetyl-CoA. Very commonly, the same enzyme catalyzes the reverse reaction from acetyl-CoA to acetaldehyde, which is the direction most relevant to fermentation. Acetaldehyde dehydrogenase includes those enzymes that correspond to EC 1.2.1.4 and EC 1.2.1.10. 
     As used herein, the term “bifunctional” is intended to include enzymes that catalyze more than one biochemical reaction step. Specific examples of a bifunctional enzyme used herein are enzymes (AdhE and AdhB) that catalyze both the alcohol dehydrogenase and acetaldehyde dehydrogenase reactions ( FIG. 1 ) and those enzymes encoded by SEQ ID NO: 67 and SEQ ID NO: 68 (bifunctional acetaldehyde-alcohol dehydrogenase from  T. saccharolyticum ). Bifunctional acetaldehyde-alcohol dehydrogenase includes those enzymatic reactions that correspond to EC 1.2.1.4, EC 1.2.1.10, EC 1.1.1.2 and EC 1.1.1.1. 
     The term “feedstock” is defined as a raw material or mixture of raw materials supplied to a microorganism or fermentation process from which other products can be made. For example, a carbon source, such as biomass or the carbon compounds derived from biomass are a feedstock for a microorganism that produces a product in a fermentation process. A feedstock can contain nutrients other than a carbon source. 
     Biomass can include any type of biomass known in the art or described herein. The terms “lignocellulosic material,” “lignocellulosic substrate,” and “cellulosic biomass” mean any type of carbon containing feed stock selected from the group consisting of woody biomass, such as recycled wood pulp fiber, sawdust, hardwood, softwood, grasses, sugar-processing residues, agricultural wastes, such as but not limited to rice straw, rice hulls, barley straw, corn cobs, cereal straw, wheat straw, canola straw, oat straw, oat hulls, corn fiber, stover, succulents, agave, or any combination thereof. 
     The term “yield” is defined as the amount of product obtained per unit weight of raw material and may be expressed as g product per g substrate (g/g). Yield may be expressed as a percentage of the theoretical yield. “Theoretical yield” is defined as the maximum amount of product that can be generated per a given amount of substrate as dictated by the stoichiometry of the metabolic pathway used to make the product. For example, the theoretical yield for one typical conversion of glucose to ethanol is 0.51 g EtOH per 1 g glucose. As such, a yield of 4.8 g ethanol from 10 g of glucose would be expressed as 94% of theoretical or 94% theoretical yield. 
     The term “titer” is defined as the strength of a solution or the concentration of a substance in solution. For example, the titer of a product in a fermentation broth is described as g of product in solution per liter of fermentation broth (g/L) or as g/kg broth. 
     “Bacteria”, or “eubacteria”, refers to a domain of prokaryotic organisms. Bacteria include Gram-positive (gram+) bacteria and Gram-negative (gram-) bacteria. 
     In some embodiments of the invention, the host cell is a prokaryotic microorganism. In some embodiments, the host cell is a bacterium. In some embodiments, the host cell is able to digest and ferment cellulose. In some embodiments, the host cell is a thermophilic bacterium. In some embodiments, the microorganism is from the genus  Clostridium . In some embodiments the microorganism is from the genus  Caldicellulosiruptor . In some embodiments, the bacterium is  Clostridium thermocellum . In some embodiments, the bacterium is  Clostridium cellulolyticum . In some embodiments, the bacterium is  Clostridium clariflavum . In some embodiments, the bacterium is  Clostridium phytofermentans . In some embodiments, the bacterium is  Clostridium acetobutylicum . In some embodiments, the bacterium is  Caldicellulosiruptor bescii . In some embodiments, the bacterium is  Caldicellulosiruptor saccharolyticus.    
     In some embodiments, the host cell is a thermotolerant host cell. Thermotolerant host cells can be particularly useful in simultaneous saccharification and fermentation processes by allowing externally produced cellulases and ethanol-producing host cells to perform optimally in similar temperature ranges. 
     In some embodiments, the host cells of the invention are cultured at a temperature above 25° C., above 27° C., above 30° C., above 33° C., above 35° C., above 37° C., above 40° C., above 43° C., above 45° C., or above 47° C. 
     In some embodiments, the host cells of the invention contain genetic constructs that lead to the down-regulation to one or more genes encoding a polypeptide at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to one or more of the polypeptides encoded by SEQ ID NOS: 1-54, 57, 60, 67, 68. 
     In some embodiments, the host cells of the invention contain genetic constructs that lead to the expression or up-regulation of one or more genes encoding a polypeptide at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identical to one or more of the polypeptides encoded by SEQ ID NOS: 1-8, 13-54, 57, 60, 67, 68. 
     In some embodiments, the host cells of the invention are subjected to adaptation to improve their performance. In some embodiments, the host cells are adapted for faster growth by culturing them repeatedly on a growth medium or in a continuous culture device such as a chemostat. 
     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 may 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. 
     “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 may 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 75% identical to the amino acid sequences reported herein, at least about 80%, 85%, or 90% identical to the amino acid sequences reported herein, or at least about 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequences reported herein. Suitable nucleic acid fragments are at least about 70%, 75%, or 80% identical to the nucleic acid sequences reported herein, at least about 80%, 85%, or 90% identical to the nucleic acid sequences reported herein, or at least about 95%, 96%, 97%, 98%, 99%, or 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. 
     Codon Optimization 
     In some embodiments of the present invention, exogenous genes may be codon-optimized in order to express the polypeptide they encode most efficiently in the host cell. Methods of codon optimization are well known in the art. (Welch, M., Villalobos, A., Gustafsson, C., Minshull, J. Designing genes for successful protein expression.  Methods Enzymol.  2011. 498:43-66. 
     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 “CAI,” 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 et al., “The Codon Adaptation Index: a Measure of Directional Synonymous Codon Usage Bias, and Its Potential Applications.”  Nucleic Acids Research  1987. 15: 1281-1295, which is incorporated by reference herein in its entirety. 
     A codon optimized sequence may 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 3, 4, 5, 6, 7, 8, 9, or 10 consecutive bases) can effect transcription negatively. Therefore, it can be useful to remove a run by, for example, replacing at least one nucleotide in the run with another nucleotide. Furthermore, specific restriction enzyme sites may be removed for molecular cloning purposes by replacing at least one nucleotide in the restriction site with another nucleotide. Examples of such restriction enzyme sites include PacI, AscI, BamHI, BglII, EcoRI and XhoI. Additionally, the DNA sequence can be checked for direct repeats, inverted repeats and mirror repeats with lengths of about 5, 6, 7, 8, 9 or 10 bases or longer. Runs of “As” or “Ts”, restriction sites and/or repeats can be modified by replacing at least one codon within the sequence with the “second best” codons, i.e., the codon that occurs at the second highest frequency for a particular amino acid 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 reproduced herein as Table 1. 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 triplets each, whereas tryptophan and methionine are coded for 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. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 The Standard Genetic Code 
               
             
          
           
               
                   
                 T 
                 C 
                 A 
                 G 
               
               
                   
               
             
          
           
               
                 T 
                 TTT Phe (F) 
                 TCT Ser (S) 
                 TAT Tyr (Y) 
                 TGT Cys (C) 
               
               
                   
                 TTC Phe (F) 
                 TCC Ser (S) 
                 TAC Tyr (Y) 
                 TGC 
               
               
                   
                 TTA Leu (L) 
                 TCA Ser (S) 
                 TAA  Ter   
                 TGA  Ter   
               
               
                   
                 TTG Leu (L) 
                 TCG Ser (S) 
                 TAG  Ter   
                 TGG Trp (W) 
               
               
                   
               
               
                 C 
                 CTT Leu (L) 
                 CCT Pro (P) 
                 CAT His (H) 
                 CGT Arg (R) 
               
               
                   
                 CTC Leu (L) 
                 CCC Pro (P) 
                 CAC His (H) 
                 CGC Arg (R) 
               
               
                   
                 CTA Leu (L) 
                 CCA Pro (P) 
                 CAA Gln (Q) 
                 CGA Arg (R) 
               
               
                   
                 CTG Leu (L) 
                 CCG Pro (P) 
                 CAG Gln (Q) 
                 CGG Arg (R) 
               
               
                   
               
               
                 A 
                 ATT Ile (L) 
                 ACT Thr (T) 
                 AAT Asn (N) 
                 AGT Ser (S) 
               
               
                   
                 ATC Ile (I) 
                 ACC Thr (T) 
                 AAC Asn (N) 
                 AGC Ser (S) 
               
               
                   
                 ATA Ile (I) 
                 ACA Thr (T) 
                 AAA Lys (K) 
                 AGA Arg (R) 
               
               
                   
                   ATG  Met (M) 
                 ACG Thr (T) 
                 AAG Lys (K) 
                 AGG Arg (R) 
               
               
                   
               
               
                 G 
                 GTT Val (V) 
                 GCT Ala (A) 
                 GAT Asp (D) 
                 GGT Gly (G) 
               
               
                   
                 GTC Val (V) 
                 GCC Ala (A) 
                 GAC Asp (D) 
                 GGC Gly (G) 
               
               
                   
                 GTA Val (V) 
                 GCA Ala (A) 
                 GAA Glu (E) 
                 GGA Gly (G) 
               
               
                   
                 GTG Val (V) 
                 GCG Ala (A) 
                 GAG Glu (E) 
                 GGG Gly (G) 
               
               
                   
               
             
          
         
       
     
     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. 
     Redirection of Carbon Flux 
     One aspect of the present invention relates to a recombinant microorganism comprising a heterologous nucleic acid sequence encoding a pyruvate kinase and a genetic modification that leads to the down-regulation of an enzyme in an acetic acid and/or lactic acid pathway. In some embodiments, the host organism lacks an endogenous pyruvate kinase. In other embodiments, endogenous pyruvate kinase may be supplemented by up-regulation of the endogenous enzyme or the expression of one or more additional copies of the pyruvate kinase by introducing the copies into a host cell of the invention. Alternately, in other embodiments of the invention, a gene encoding PEP synthase (EC 2.7.9.2), PEP phosphatase (EC 3.1.3.60), or a PEP phosphotransferase (EC 2.7.3.9, EC 2.7.1.12) can be expressed in place or in addition to a pyruvate kinase. 
     In some embodiments, the enzyme in the acetic acid pathway or lactic acid pathway is selected from the group encoded by a lactate dehydrogenase polynucleotide, a phosphotransacetylase polynucleotide, or an acetate kinase polynucleotide. In some embodiments, the microorganism is from the genus  Clostridium . In some embodiments the microorganism is the bacterium  Clostridium thermocellum . The redirected flux can then be optimized by growth-coupled selection. Specifically, continuous culture or serial dilution cultures can be performed to select for cells that grow faster and, by necessity, produce ethanol faster. Methods for selection of microorganisms are known in the art and described, for example, in U.S. Appl. Pub. Nos. 2011/0189744 and 2011/0059485 which are incorporated herein by reference. 
     In some aspects, this invention relates to a recombinant microorganism comprising a heterologous nucleic acid sequence encoding a pyruvate kinase and a genetic modification that leads to the down-regulation of an enzyme in a pathway which converts phosphoenolpyruvate (PEP) to pyruvate. In some embodiments, the enzyme that is down-regulated is encoded by a polynucleotide selected from the group consisting of a pyruvate-phosphate dikinase polynucleotide, a phosphoenolpyruvate carboxykinase-encoding polynucleotide, a malate dehydrogenase-encoding polynucleotide, or a malic enzyme-encoding polynucleotide. In some embodiments, the down-regulated enzyme is encoded by the nucleic acid sequence of SEQ ID NOs: 9-18 and SEQ ID NOs: 51-52. 
     One aspect of this invention relates to a recombinant microorganism comprising a heterologous nucleic acid sequence comprising a pyruvate kinase, a genetic modification that leads to the down-regulation of an enzyme in an acetic acid and/or a lactic acid pathway, and a genetic modification that leads to the down-regulation of an enzyme in a pathway for the conversion of phosphoenolpyruvate to pyruvate through methods known in the art or described herein. In some embodiments, the enzyme in the acetic acid pathway or lactic acid pathway is from the group encoded by a lactate dehydrogenase polynucleotide, a phosphotransacetylase polynucleotide, or an acetate kinase polynucleotide. In some embodiments the enzyme in the phosphoenolpyruvate to pyruvate pathway is from the group encoded by a pyruvate-phosphate dikinase polynucleotide, a phosphoenolpyruvate carboxykinase polynucleotide, a malate dehydrogenase polynucleotide, or a malic enzyme polynucleotide. 
     In some embodiments, the present invention relates to a microorganism comprising a heterologous nucleic acid sequence encoding a pyruvate formate lyase enzyme and heterologous nucleic acids encoding PFL-activating enzymes. In other embodiments, in organisms that already possess these enzymes, the genes can be up-regulated or one or more additional copies of the desired genes can be introduced to give higher expression of the desired enzymes. In another embodiment, the present invention relates to a microorganism comprising a heterologous nucleic acid sequence encoding a pyruvate formate lyase enzyme, heterologous nucleic acids encoding PFL-activating enzymes, and a genetic modification that leads to the down-regulation of the enzymes pyruvate oxidoreductase or NADH-dependent reduced ferredexin:NADP+ oxidoreductase through methods known in the art, e.g., (Berrios-Rivera 2002, Hatrongjit 2010, Popov 1994, and U.S. Pat. Nos. 7,709,261 and 7,256,016) or described herein. 
     In one embodiment, the invention relates to a microorganism comprising a heterologous nucleic acid sequence encoding a pyruvate formate lyase, heterologous nucleic acids encoding PFL-activating enzymes, a genetic modification that leads to the down-regulation of the enzymes pyruvate oxidoreductase or NADH-dependent reduced ferredoxin:NADP+ oxidoreductase. In some embodiments, the microorganism is from the genus  Clostridium . In some embodiments the microorganism is the bacterium  Clostridium thermocellum.    
     In another embodiment, the invention relates to a microorganism comprising a heterologous nucleic acid sequence encoding a pyruvate formate lyase, heterologous nucleic acids encoding PFL-activating enzymes, and a genetic modification that leads to the down-regulation of an enzyme in an ethanol pathway through methods known in the art or described herein. 
     In another embodiment, the invention relates to a microorganism comprising a heterologous nucleic acid sequence encoding a pyruvate formate lyase or the up-regulation of endogenous pyruvate formate lyase, heterologous nucleic acids encoding PFL-activating enzymes, and a heterologous nucleic acid sequence encoding formate dehydrogenase. In other embodiments, endogenous formate dehydrogenase may be supplemented by up-regulation of the endogenous enzyme or the expression of one or more additional copies of the formate dehydrogenase by introducing the copies into a host cell of the invention. 
     In some embodiments, the microorganism further comprises a genetic modification that leads to the down-regulation of an enzyme in an acetic acid and/or lactic acid pathway through methods known in the art or described herein. In some embodiments, the enzyme in the acetic acid pathway or lactic acid pathway is from the group encoded by a lactate dehydrogenase polynucleotide, a phosphotransacetylase polynucleotide, or an acetate kinase polynucleotide. 
     One embodiment of the present invention relates to a recombinant microorganism comprising a genetic modification that leads to the down-regulation of the enzyme encoding malate dehydrogenase through methods known in the art or described herein. One embodiment of the invention relates to a recombinant microorganism comprising a genetic modification that leads to the down-regulation of the enzyme encoding lactate dehydrogenase through methods known in the art or described herein. Another embodiment of the present invention relates to a recombinant microorganism comprising a genetic modification that leads to the down-regulation of the enzyme encoding malate dehydrogenase and a genetic modification that leads to the down-regulation of the enzyme encoding lactate dehydrogenase through methods known in the art or described herein. In some embodiments, the microorganism is from the genus  Clostridium . In some embodiments the microorganism is the bacterium  Clostridium  cellulolyticum. 
     One embodiment relates to a recombinant prokaryotic microorganism comprising a genetic modification that leads to the down-regulation of an enzyme encoding malate dehydrogenase wherein said microorganism in capable of producing ethanol at a higher rate than an otherwise identical microorganism in which the enzyme encoding malate dehydrogenase is not down-regulated. In some embodiments, the microorganism is from the genus  Clostridium . In some embodiments the microorganism is the bacterium  Clostridium cellulolyticum . In some embodiments, the organism contains genetic modifications that lead to the down regulation of malate dehydrogenase and lactate dehydrogenase. 
     In some embodiments, the microorganism further comprises a genetic modification that leads to the down-regulation of an enzyme containing phosphotransacetylase. In some embodiments, the microorganism further comprises a bifunctional acetaldehyde-alcohol dehydrogenase. In some embodiments, the bifunctional acetaldehyde-alcohol dehydrogenase is AdhE or AdhB. 
     In some embodiments, the invention relates to a microorganism comprising a heterologous nucleic acid sequence encoding a pyruvate kinase, a heterologous nucleic acid sequence encoding PEPCK, a heterologous nucleic acid sequence encoding a bifunctional acetaldehyde-alcohol dehydrogenase and additionally comprises a genetic modification that leads to the down-regulation of an enzyme encoding lactate dehydrogenase. In some embodiments, the invention relates to a microorganism comprising a heterologous nucleic acid sequence encoding a pyruvate kinase, a heterologous nucleic acid sequence encoding PEPCK, a heterologous nucleic acid sequence encoding AdhB and additionally comprises a genetic modification that leads to the down-regulation of an enzyme encoding lactate dehydrogenase. In some embodiments, the invention relates to a microorganism comprising a heterologous nucleic acid sequence encoding a pyruvate kinase, a heterologous nucleic acid sequence encoding PEPCK, a heterologous nucleic acid sequence encoding AdhE, and a genetic modification that leads to the down-regulation of an enzyme encoding lactate dehydrogenase. In some embodiments, the invention relates to a microorganism comprising a heterologous nucleic acid sequence encoding a pyruvate kinase, a heterologous nucleic acid sequence encoding PEPCK, a heterologous nucleic acid sequence encoding AdhE, a genetic modification that leads to the down-regulation of an enzyme encoding lactate dehydrogenase, and a genetic modification that leads to the down-regulation of PTA. In some embodiments, the invention relates to a microorganism comprising a heterologous nucleic acid sequence encoding a pyruvate kinase, a heterologous nucleic acid sequence encoding PEPCK, a heterologous nucleic acid sequence encoding AdhB, a heterologous nucleic acid sequence encoding AdhE, and additionally comprises a genetic modification that leads to the down-regulation of an enzyme encoding lactate dehydrogenase. In some embodiments, the AdhB is from  T. pseudethanolicus . In some embodiments, the pyruvate kinase is from  T. saccharolyticum . In some embodiments, the AdhE is from  T. saccharolyticum . In some embodiments, PEPCK is down-regulated in the microorganism. 
     One embodiment of the present invention relates to a composition comprising a microorganism described herein and a carbon-containing feedstock comprising woody biomass, such as recycled wood pulp fiber, sawdust, hardwood, softwood, grasses, sugar processing residues, agricultural wastes, such as but not limited to rise straw, rice hulls, barley straw, corn cobs, cereal straw, wheat straw, canola straw, oat straw, oat hulls, corn fiber, stover, succulents, agave, or any combination thereof. 
     Ethanol Production 
     For a microorganism to produce ethanol most economically, it is desired to produce a high yield. In one embodiment, the only product produced is ethanol. Extra products lead to a reduction in product yield and an increase in capital and operating costs, particularly if the extra products have little or no value. Extra products also require additional capital and operating costs to separate these products from ethanol. 
     Ethanol production can be measured using any method known in the art. For example, the quantity of ethanol in fermentation samples can be assessed using HPLC analysis. Many ethanol assay kits are commercially available that use, for example, alcohol oxidase enzyme based assays. Methods of determining ethanol production are within the scope of those skilled in the art from the teachings herein. 
     In some embodiments, the host cell is able to digest and ferment cellulose. In some embodiments, the host cell is a thermophilic bacterium. In some embodiments, the microorganism of the invention is from the genus  Clostridium . In some embodiments the microorganism is from the genus  Caldicellulosiruptor . In some embodiments, the bacterium is  Clostridium thermocellum . In some embodiments, the bacterium is  Clostridium cellulolyticum . In some embodiments, the bacterium is  Clostridium clariflavum . In some embodiments, the bacterium is  Clostridium phytofermentans . In some embodiments, the bacterium is  Clostridium acetobutylicum . In some embodiments, the bacterium is  Caldicellulosiruptor bescii . In some embodiments, the bacterium is  Caldicellulosiruptor saccharolyticus.    
     In some embodiments of the invention where redirected carbon flux generates increased ethanol production, the ethanol output can be improved by growth-coupled selection. For example, continuous culture or serial dilution cultures can be performed to select for cells that grow faster and/or produce ethanol (or any desired product) more efficiently on a desired feedstock. 
     One embodiment of the present invention relates to a method of producing ethanol using a microorganism described herein wherein said microorganism is cultured in the presence of a carbon containing feedstock for sufficient time to produce ethanol and, optionally, extracting the ethanol. 
     Ethanol may be extracted by methods known in the art. See, e.g., U.S. Appl. Pub. No. 2011/0171709, which is incorporated herein by reference. 
     Another embodiment of the present invention relates to a method of producing ethanol using a co-culture composed of at least two microorganisms in which at least one of the organisms is an organism described herein, and at least one of the organisms is a genetically distinct microorganism. In some embodiments, the genetically distinct microorganism is a yeast or bacterium. In some embodiments the genetically distinct microorganism is any organism from the genus  Issatchenkia, Pichia, Clavispora, Candida, Hansenula, Kluyveromyces, Trichoderma, Thermoascus, Escherichia, Clostridium, Thermoanaerobacter  and  Thermoanaerobacterium.    
     In some embodiments, the recombinant microorganism produces about 2 to about 3 times more ethanol than a wildtype, non-recombinant organism; about 1.5 to about 2 times more ethanol than a wildtype, non-recombinant organism; about 1.5 to about 5 times more ethanol than a wildtype, non-recombinant organism; about 1.5 to about 7 times more ethanol than a wildtype, non-recombinant organism; about 1.5 to about 10 times more ethanol than a wildtype, non-recombinant organism; about 1.5 to about 15 times more ethanol than a wildtype, non-recombinant organism; about 1.5 to about 20 times more ethanol than a wildtype, non-recombinant organism; about 1.5 to about 30 times more ethanol than a wildtype, non-recombinant organism; about 1.5 to about 50 times more ethanol than a wildtype, non-recombinant organism; about 1.5 to about 75 times more ethanol than a wildtype, non-recombinant organism; about 1.5 to about 100 times more ethanol than a wildtype, non-recombinant organism. 
     In some embodiments, the recombinant microorganism produces about 2 to about 3% more ethanol than a wildtype, non-recombinant organism; at least about 1.5 to at least about 2% more ethanol than a wildtype, non-recombinant organism; at least about 1.5 to at least about 5% more ethanol than a wildtype, non-recombinant organism; at least about 1.5 to at least about 7% more ethanol than a wildtype, non-recombinant organism; at least about 1.5 to at least about 10% more ethanol than a wildtype, non-recombinant organism; at least about 1.5 to at least about 15% more ethanol than a wildtype, non-recombinant organism; at least about 1.5 to at least about 20% more ethanol than a wildtype, non-recombinant organism; at least about 1.5 to at least about 30% more ethanol than a wildtype, non-recombinant organism; at least about 1.5 to at least about 50% more ethanol than a wildtype, non-recombinant organism; at least about 1.5 to at least about 75% more ethanol than a wildtype, non-recombinant organism; at least about 1.5 to at least about 100% more ethanol than a wildtype, non-recombinant organism. 
     In some embodiments, the recombinant microorganism produces about 0.5 g/L ethanol to about 2 g/L ethanol, about 0.5 g/L ethanol to about 3 g/L ethanol, about 0.5 g/L ethanol to about 5 g/L ethanol, about 0.5 g/L ethanol to about 7 g/L ethanol, about 0.5 g/L ethanol to about 10 g/L ethanol, about 0.5 g/L ethanol to about 15 g/L ethanol, about 0.5 g/L ethanol to about 20 g/L ethanol, about 0.5 g/L ethanol to about 30 g/L ethanol, about 0.5 g/L ethanol to about 40 g/L ethanol, about 0.5 g/L ethanol to about 50 g/L ethanol, about 0.5 g/L ethanol to about 75 g/L ethanol, or about 0.5 g/L ethanol to about 99 g/L ethanol per 24 hour incubation on a carbon-containing feed stock. 
     In some embodiments, the recombinant microorganism produces ethanol at about 55% to about 75% of theoretical yield, about 50% to about 80% of theoretical yield, about 45% to about 85% of theoretical yield, about 40% to about 90% of theoretical yield, about 35% to about 95% of theoretical yield, about 30% to about 99% of theoretical yield, or about 25% to about 99% of theoretical yield. 
     In some embodiments, methods of producing ethanol can comprise contacting a biomass feedstock with a host cell or co-culture of the invention and additionally contacting the biomass feedstock with externally produced saccharolytic enzymes. Exemplary externally produced saccharolytic enzymes are commercially available and are known to those of skill in the art. 
     EXAMPLES 
     Example 1 
     Insertion of Pyruvate Kinase on a Replicating Plasmid 
     The gene for pyruvate kinase was introduced into  C. thermocellum  on a replicating plasmid. The gene for pyruvate kinase was amplified by PCR from  T. saccharolyticum  and cloned into plasmid pMU102 (described in Tripathi S A, Olson D G, Argyros D A, Miller B B, Barrett T F, Murphy D M, McCool J D, Warner A K, Rajgarhia V B, Lynd L R, Hogsett D A, Caiazza N C. Development of pyrF-based genetic system for targeted gene deletion in  Clostridium thermocellum  and creation of a pta mutant.  Appl Environ Microbiol.  2010. 76(19):6591-9.), creating plasmid pMU2106. SEQ ID NO: 55. This plasmid was transformed into  C. thermocellum  WT strain DSM1313 [available from the public repository DSMZ] followed by selection for thiamphenicol resistance. The created strain was designated M1716. 
     Example 2 
     Insertion of Pyruvate Kinase into the Genome 
     The gene for pyruvate kinase was introduced into the chromosome of  C. thermocellum . The pyruvate kinase gene was amplified by PCR from  T. saccharolyticum  and cloned downstream from the native  C. thermocellum  enolase promoter to generate plasmid pDGO-05. SEQ ID NO: 56 and SEQ ID NO: 57. This plasmid was transformed into strain M1354 (hpt deletion strain) (Argyros, D A, Tripathi S A, Barrett T F, Rogers S R, Feinberg L F, Olson D G, Foden J M, Miller B B, Lynd L R, Hogsett D A, Caiazza N C, High ethanol titers from cellulose using metabolically engineered thermophilic, anaerobic microbes.  Appl. Env. Microbiol.  2011. 77(23):8288-94; use of hpt deletion strains is also described in U.S. application Ser. No. 13/393,093, which is incorporated herein by reference.), and selected for thiamphenicol and FuDR resistance, resulting in insertion of the pyruvate kinase gene at the ldh locus of  C. thermocellum . Those cells were then subjected to AZH selection to remove the hpt and antibiotic resistance genes. The resulting strain was designated DS8. 
     Example 3 
     Redirection of Carbon Flux by Reducing PEPCK Expression 
     The expression of PEPCK was reduced dramatically in strain DS8 by altering the start codon from ATG to GTG. The plasmid pYD01 was built for this purpose by yeast-mediated recombination using methods described previous and known in the art. SEQ ID NOs: 58-60. (Shanks R M, Caiazza N C, Hinsa S M, Toutain C M, O&#39;Toole G A.  Saccharomyces cerevisiae -based molecular tool kit for manipulation of genes from gram-negative bacteria.  Appl Environ Microbiol.  2006 July; 72(7):5027-36). The plasmid pYD01 contains two fragments from the upstream region of the pckA gene. One of the fragments contains a modified start codon that has been changed to GTG. The cat and hpt genes are positioned between the two fragments. The resulting plasmid was transformed into  C. thermocellum , and integrants were selected with thiamphenicol selection, then integrants were selected with thiamphenicol plus FuDR. Next, by selecting for resistance to AZH, clones were selected which had undergone recombination between the two copies of the pckA upstream region, thus eliminating the cat and hpt genes. Colonies were then screened for those carrying the GTG mutation by PCR amplification. Once such colony was saved as strain YD01. As shown in the table below, PEPCK activity was greatly reduced in the mutant strain. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 PEPCK activity is greatly reduced in strain YD01. 
               
             
          
           
               
                   
                   
                 PEPCK specific activity (units/mg 
               
               
                   
                   
                 protein) 
               
               
                   
                   
               
             
          
           
               
                   
                 WT 
                 4.14 
               
               
                   
                 DS8 
                 4.09 
               
               
                   
                 YD01 
                 0.54 
               
               
                   
                   
               
             
          
         
       
     
     Example 4 
     Redirection of Carbon Flux by Down-Regulating Malic Enzyme 
     The gene for malic enzyme was down-regulated in strain DS8 from Example 2 by using plasmid pYD02, based on the protocol described by Olson et al. (“Deletion of the Ce148S cellulase from  Clostridium thermocellum .” PNAS 2010. 107(41):17727-32.) 
     pYD02 was built by yeast mediated recombination. (Shanks R M, Caiazza N C, Hinsa S M, Toutain C M, O&#39;Toole G A. “ Saccharomyces cerevisiae -based molecular tool kit for manipulation of genes from gram-negative bacteria.” Appl Environ Microbiol. 2006 July; 72(7):5027-36). It contains the fused 5′ and 3′ flanking regions of the gene for malic enzyme (gene number Cthe_0344) and an internal fragment of the same gene separated by the hpt and cat genes. SEQ ID NO: 61. The plasmid was transformed into  C. thermocellum  and the transformants were then selected for FuDR resistance to select for integrants. The integrants were further selected on AZH. Surviving clones had undergone recombination between the 5′ flanking region DNA and an identical sequence upstream, thereby eliminating the cat and hpt genes and all of the coding sequence of the gene for malic enzyme. A colony was designated YD02 and saved for future work. SEQ ID NO: 62. 
     In fermentations in minimal carbon medium +1 g/L yeast extract +5 g/L cellobiose,  C. thermocellum  strains YD01 and YD02 produced more ethanol than their parent strain 1313, as shown in the table of HPLC results below. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Ethanol production greatly increased in strains YD01 and YD02. 
               
             
          
           
               
                   
                   
                   
                   
                   
                 % of 
               
               
                   
                   
                   
                   
                   
                 theoretical 
               
               
                   
                 Acetate 
                 Lactate 
                 Formate 
                 Ethanol  
                 ethanol  
               
               
                 Strain 
                 g/L 
                 g/L 
                 g/L 
                 g/L 
                 yield 
               
               
                   
               
             
          
           
               
                 YD01 
                 0.86 
                 0 
                 .92 
                 1.78 
                 70% 
               
               
                 YD02 
                 0.768 
                 0 
                 1.00 
                 1.72 
                 67% 
               
               
                 1313 
                 1.25 
                 0.02 
                 .27 
                 .35 
                 14% 
               
               
                 (no yeast 
                   
                   
                   
                   
                   
               
               
                 extract) 
               
               
                   
               
             
          
         
       
     
     Example 5 
     Redirection of Carbon Flux by Down-Regulation of PPDK 
     The gene for pyruvate-phosphate dikinase (PPDK) was deleted to improve ethanol yield. To create a deletion construct, plasmid pMU2051 was created using yeast mediated ligation. SEQ ID NO: 63. This plasmid was transformed into strain M1354(Δhpt) and selected for in liquid medium with thiamphenicol. A serial dilution of the transformation was plated to select for isolated colonies. A single colony was PCR screened to confirm presence of plasmid pMU2051 and inoculated into liquid medium and grown overnight. The following day, cells were plated with thiamphenicol (10 ug/ml) plus FUDR(10 ug/ml), with or without pyruvate. Colonies were observed only on the plate supplemented with pyruvate. Seven colonies were screened by PCR for a merodiploid insertion of the drug marker at the PPDK locus using primers X09712 (CCTCATTTGATAATTGCCTCCTCAT(SEQ ID NO: 70)) and X09713(ATCGCATTTTGCCGTTATGTGCCATTGAA(SEQ ID NO: 71)). A ˜4.6 kb band indicated the colony contained only cells where the PPDK gene was replaced with the deletion cassette. A ˜3.87 kb band indicated the presence of a wild type PDDK locus. Of the seven colonies, one carried the desired mutation. This colony was dilution plated on minimal medium containing 300 ug/ml 8-azahypoxanthine to remove the marker and create a clean deletion of PPDK. This strain was subsequently saved as strain M1631. SEQ ID NO: 64. 
     Example 6 
     Redirection of Carbon Flux by Deletion of Malate Dehydrogenase 
     Standard cloning methods were used to generate a gene inactivation plasmid aimed at disrupting the malate dehydrogenase (“mdh”) gene of  C. cellulolyticum . The gene inactivation plasmid was created using a disruption cassette. A map of the plasmid can be seen in  FIG. 2 . The plasmid was transformed into  C. cellulolyticum  and chloramphenicol resistant colonies were screened for insertion at the mdh locus by PCR.  FIG. 3  shows the mdh locus in the wild type strain (lane 3) and one of the chloramphenicol resistant clones (lane 2). The shift in size of the band in lane 2 compared to the wild type (lane 3) is evidence that a disruption cassette is inserted into the mdh gene. 
     Example 7 
     Redirection of Carbon Flux by Deletion of Malate Dehydrogenase and Lactate Dehydrogenase 
     Standard cloning methods were used to generate a gene inactivation plasmid aimed at disrupting the ldh gene of  C. cellulolyticum . The gene inactivation plasmid was created using a disruption cassette. A map of the plasmid can be seen in  FIG. 4 . The above plasmid was transformed into the  C. cellulolyticum  mdh mutant background and 8 erythromycin resistant colonies were screened for insertion at both the mdh and ldh loci by PCR. Part A of the gel image in  FIG. 5  shows the mdh locus in which all 8 strains show insertion of the disruption cassette at the mdh locus, signified by an increase size of the PCR product. Part B of the gel image in  FIG. 5  shows the status of the ldh locus in the same 8 strains. Of the 8 strains assayed, the gel image below clearly demonstrates that the strain represented in lane 6 has a disruption cassette inserted at the ldh locus. Strains represented in lanes 5 and 7 are a mixed culture of mutant and wild type cells, while the remainder are just wild type at the ldh locus. Thus, the strain represented in lane 6 has a disruption cassette inserted at both the ldh and mdh loci. 
     Example 8 
     Ethanol and Lactic Acid Production by mdh and mdh, ldh Mutant Strains 
     Ethanol and lactic acid production were tested on wildtype, the mdh mutant strain and the mdh, ldh double mutant. As seen in  FIG. 6 , a fermentation profile was created for the mdh and mdh, ldh mutant strains in comparison to the wildtype, non-recombinant strain. Strains were incubated in media composed of 0.5035 g/L KH 2 PO 4 , 0.6097 g/L K 2 HPO 4 , 1.9820 g/L urea, 0.5083 g/L MgCl 2 *6H 2 O, 0.0441 g/L CaCl 2 *2H 2 O, 0.0011 g/L FeSO 4 , 6.0491 g/L sodium β-glycerophosphate*5H 2 O, 2.9410 g/L sodium citrate tribasic *2H 2 O, 11.0976 g/L MOPS sodium salt, 1.1063 g/L L-cysteine-HCl*H 2 O, 5 g/L yeast extract, 0.2% resazurin solution and 15 g/L cellobiose. As can be seen in  FIG. 6A , as measured by OD, growth was slightly depressed in both the mdh and mdh, 1 dh mutant cells, with the mdh, ldh mutant (“DB mutant”) serving as a partial rescue of growth over the mdh mutant. As can be seen in  FIG. 6B , both the wildtype and mdh strains had virtually identical levels of lactic acid production, while there was essentially no lactic acid production in the mdh, ldh mutant. As can be seen in  FIG. 6C , both the mutant strains saw an increase in the amount of ethanol produced. The mdh mutant produced almost 2 times as much ethanol as the wildtype strain produced. The mdh, ldh mutant produced almost 3 times as much ethanol as the wildtype strain produced. 
     Example 9 
     Heterologous Expression of adhE with Altered Co-factor Specificity in a Strain with Redirected Carbon Flux 
     U.S. Provisional Appl. No. 61/565,261, which is incorporated herein by reference describes bifunctional enzymes that catalyze both the alcohol dehydrogenase and acetaldehyde dehydrogenase reactions. The bifunctional acetaldehyde-alcohol dehydrogenase, encoded by the gene adhE, was PCR amplified from  Thermoanaerobacterium saccharolyticum  strain ALK2 ( T. saccharolyticum  adhE: SEQ ID NOs: 67 and 68). This strain is described in Shaw A J et al., Metabolic engineering of a thermophilic bacterium to produce ethanol at high yield.  Proc Natl Acad Sci USA.  2008. 105(37):13769-74. The adhE gene was cloned into  Clostridium thermocellum  replicating plasmid pDGO-66 to form plasmid pYD10 (SEQ ID NO: 65). Plasmid pYD10 was transformed into  Clostridium thermocellum  strain YD01 and selected for in liquid medium with thiamphenicol, resulting in strain YD12. This strain was grown in MTC media with 2 g/L yeast extract to test fermentation characteristics. Strain YD 12 produced 6.25 g/L ethanol from 15 g/L cellobiose, and the ethanol yield was 1.55 mole ethanol/mole glucose equivalent, which is equal to 78% of theoretical yield. In another experiment, strain YD12 was grown in medium containing 25 g/L cellobiose. After 72 hours, 24.3 g/L of cellobiose was used and the optical density of the culture was 2.4. The products observed by HPLC are shown in the table below. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Products observed by HPLC. 
               
             
          
           
               
                   
                 Acetate g/L 
                 Lactate g/L 
                 Formate g/L 
                 Ethanol g/L 
               
               
                   
                   
               
               
                   
                 2.2 
                 0.2 
                 0.6 
                 10 
               
               
                   
                   
               
             
          
         
       
     
     Other strains of  C. thermocellum  were generated that contained genes encoding bifunctional alcohol dehydrogenase. The adhB gene from  Thermoanaerobacter pseudoethanolicus  was amplified by PCR and cloned between DNA flanking regions matching the hpt gene from  C. thermocellum , generating plasmid pJLO7 (SEQ ID NO: 69). Insertion of heterologous adhB into the hpt locus of strain YD01 was performed with this plasmid by established methods, generating strain YD06. An additional bifunctional alcohol dehydrogenase was then expressed heterolgously in YD06 by transforming it with the plasmid pYD10 (SEQ ID NO: 65), generating strain YD08. This plasmid carries the adhE gene from  T.saccharolyticum  strain ALK2. 
     Example 10 
     Heterologous Expression of adhE with Altered Co-factor Specificity in a Strain with Redirected Carbon Flux and a pta Knockout 
     The plasmid pMU1817 (SEQ ID NO: 66) was constructed to delete the phosphotransacetylase (pta) gene from  Clostridium thermocellum . It was transformed into strain YD01 and selected for with thiamphenicol (Tm). Cells were plated onto agar medium containing Tm and FUDR and grown for 3 days until colonies appeared. The colonies were then plated onto media with 8-AZH to select clones in which the cat-hpt cassette had been lost by homologous recombination. The resulting strain, called YD05, was grown on MTC media with yeast extract. The ethanol yield was approximately the same as wild type. In order to increase the ethanol yield, the bifunctional acetaldehyde-alcohol dehydrogenase gene adhE from strain ALK2, which is a gene with altered co-factor specificity, was heterologously expressed. Plasmid pYD10 (SEQ ID NO: 65) was transformed into strain YD05, generating strain YD07. This strain, when grown on media containing cellobiose, produced 1.75 mole-ethanol/mole-glucose equivalent, which equals 87.5% of the theoretical yield. 
     Adaptation: The strains YD01, YD02 and YD12 were evolved for faster growth by serial transfer in MTC medium containing 5 g/liter Avicel or cellobiose for 10+ transfers. Transfers were by done by subculturing at a dilution of 1:10 every 48 to 72 h. 
     This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the invention. The scope of the invention is not limited to the disclosed embodiment(s). The invention is defined by the claims appended hereto. 
     In the following description, for purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one having ordinary skill in the art that the invention may be practiced without these specific details. In some instances, well-known features may be omitted or simplified so as not to obscure the present invention. All publications referenced in this specification are incorporated by reference in their entirety.