Patent Publication Number: US-2023134936-A1

Title: Compositions and methods for the biosynthesis of 1,4-butanediol and its precursors

Description:
This application is a continuation of U.S. application Ser. No. 15/270,578, filed Sep. 20, 2016, which is a continuation of U.S. application Ser. No. 14/603,174, filed Jan. 22, 2015, now U.S. Pat. No. 9,487,803, which is a continuation of U.S. application Ser. No. 13/717,350, filed Dec. 17, 2012, now U.S. Pat. No. 8,969,054, which is a continuation of U.S. application Ser. No. 13/348,564, filed Jan. 11, 2012, now U.S. Pat. No. 8,357,520, which is a continuation of U.S. application Ser. No. 13/286,135, filed Oct. 31, 2011, now U.S. Pat. No. 8,889,399, which is a continuation of U.S. application Ser. No. 12/049,256, filed Mar. 14, 2008, now U.S. Pat. No. 8,067,214, which claims the benefit of U.S. Provisional Application No. 60/918,463, filed Mar. 16, 2007, the entire contents of each of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Incorporated herein by reference is the Sequence Listing being submitted via EFS-Web as an ASCII text file named 199683-999406US_Sequence_Listing.TXT, created May 4, 2022, and being 1,098 bytes in size. 
     This invention relates generally to in silico design of organisms and, more particularly to organisms having 1,4-butanediol biosynthesis capability. 
     The compound 4-hydroxybutanoic acid (4-hydroxybutanoate, 4-hydroxybutyrate, 4-HB) is a 4-carbon carboxylic acid that has industrial potential as a building block for various commodity and specialty chemicals. In particular, 4-HB has the potential to serve as a new entry point into the 1,4-butanediol family of chemicals, which includes solvents, resins, polymer precursors, and specialty chemicals. 1,4-Butanediol (BDO) is a polymer intermediate and industrial solvent with a global market of about 3 billion lb/year. BDO is currently produced from petrochemical precursors, primarily acetylene, maleic anhydride, and propylene oxide. 
     For example, acetylene is reacted with 2 molecules of formaldehyde in the Reppe synthesis reaction (Kroschwitz and Grant,  Encyclopedia of Chem. Tech ., John Wiley and Sons, Inc., New York (1999)), followed by catalytic hydrogenation to form 1,4-butanediol. It has been estimated that 90% of the acetylene produced in the U.S. is consumed for butanediol production. Alternatively, it can be formed by esterification and catalytic hydrogenation of maleic anhydride, which is derived from butane. Downstream, butanediol can be further transformed; for example, by oxidation to γ-butyrolactone, which can be further converted to pyrrolidone and N-methyl-pyrrolidone, or hydrogenolysis to tetrahydrofuran ( FIG.  1   ). These compounds have varied uses as polymer intermediates, solvents, and additives, and have a combined market of nearly 2 billion lb/year. 
     It is desirable to develop a method for production of these chemicals by alternative means that not only substitute renewable for petroleum-based feedstocks, and also use less energy- and capital-intensive processes. The Department of Energy has proposed 1,4-diacids, and particularly succinic acid, as key biologically-produced intermediates for the manufacture of the butanediol family of products (DOE Report, “Top Value-Added Chemicals from Biomass”, 2004). However, succinic acid is costly to isolate and purify and requires high temperatures and pressures for catalytic reduction to butanediol. 
     Thus, there exists a need for alternative means for effectively producing commercial quantities of 1,4-butanediol and its chemical precursors. The present invention satisfies this need and provides related advantages as well. 
     SUMMARY OF THE INVENTION 
     The invention provides a non-naturally occurring microbial biocatalyst including a microbial organism having a 4-hydroxybutanoic acid (4-HB) biosynthetic pathway having at least one exogenous nucleic acid encoding 4-hydroxybutanoate dehydrogenase, succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase, or α-ketoglutarate decarboxylase, wherein the exogenous nucleic acid is expressed in sufficient amounts to produce monomeric 4-hydroxybutanoic acid (4-HB). Also provided is a non-naturally occurring microbial biocatalyst including a microbial organism having 4-hydroxybutanoic acid (4-HB) and 1,4-butanediol (BDO) biosynthetic pathways, the pathways include at least one exogenous nucleic acid encoding 4-hydroxybutanoate dehydrogenase, succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase, 4-hydroxybutyrate:CoA transferase, 4-butyrate kinase, phosphotransbutyrylase, α-ketoglutarate decarboxylase, aldehyde dehydrogenase, alcohol dehydrogenase or an aldehyde/alcohol dehydrogenase, wherein the exogenous nucleic acid is expressed in sufficient amounts to produce 1,4-butanediol (BDO). Additionally provided is a method for the production of 4-HB. The method includes culturing a non-naturally occurring microbial organism having a 4-hydroxybutanoic acid (4-HB) biosynthetic pathway including at least one exogenous nucleic acid encoding 4-hydroxybutanoate dehydrogenase, succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase or α-ketoglutarate decarboxylase under substantially anaerobic conditions for a sufficient period of time to produce monomeric 4-hydroxybutanoic acid (4-HB). Further provided is a method for the production of BDO. The method includes culturing a non-naturally occurring microbial biocatalyst, comprising a microbial organism having 4-hydroxybutanoic acid (4-HB) and 1,4-butanediol (BDO) biosynthetic pathways, the pathways including at least one exogenous nucleic acid encoding 4-hydroxybutanoate dehydrogenase, succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase, 4-hydroxybutyrate:CoA transferase, 4-hydroxybutyrate kinase, phosphotranshydroxybutyrylase, α-ketoglutarate decarboxylase, aldehyde dehydrogenase, alcohol dehydrogenase or an aldehyde/alcohol dehydrogenase for a sufficient period of time to produce 1,4-butanediol (BDO). The 4-HB and/or BDO products can be secreted into the culture medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram showing an entry point of 4-hydroxybutanoic acid (4-HB) into the product pipeline of the 1,4-butanediol (BDO) family of chemicals, and comparison with chemical synthesis routes from petrochemical feedstocks. Solid black arrows show chemical synthesis routes; dashed blue arrows show a biosynthetic route to 4-HB and subsequent conversion steps to BDO family chemicals. 
         FIG.  2    is a schematic diagram showing biochemical pathways to 4-hydroxybutyrate (4-HB) and to 1,4-butanediol production. The first 5 steps are endogenous to  E. coli , while the remainder can be expressed heterologously. Enzymes catalyzing the biosynthetic reactions are: (1) succinyl-CoA synthetase; (2) CoA-independent succinic semialdehyde dehydrogenase; (3) α-ketoglutarate dehydrogenase; (4) glutamate:succinate semialdehyde transaminase; (5) glutamate decarboxylase; (6) CoA-dependent succinic semialdehyde dehydrogenase; (7) 4-hydroxybutanoate dehydrogenase; (8) α-ketoglutarate decarboxylase; (9) 4-hydroxybutyryl CoA:acetyl-CoA transferase; (10) butyrate kinase; (11) phosphotransbutyrylase; (12) aldehyde dehydrogenase; (13) alcohol dehydrogenase. 
         FIG.  3    is a schematic diagram showing homoserine biosynthesis in  E. coli.    
         FIG.  4    shows a schematic diagram of a predicted homoserine biopathway from L-homoserine to 4-HB. Step  1  is a deduced ammonia-lyase (EC class 4.3.1) with an estimated ΔrxnG of 12 kJ/mol. Step  2  is a deduced oxidoreductase (EC class 1.3.1) with an estimated ΔrxnG of −59 kJ/mol. 
         FIG.  5    shows a schematic diagram for the endogenous  E. coli  pathway for aspartate conversion to succinate via fumarate. This pathway exhibits similar chemistry to the predicted homoserine biopathway. 
         FIG.  6    shows a schematic diagram illustrating the parallels between (A) homoserine and (B) succinyl-CoA biosynthetic pathways to BDO. 
         FIG.  7    is a schematic diagram showing biochemical pathways to acetoacetate in  E. coli.    
         FIG.  8    is a schematic diagram showing a biochemical pathway from acetoacetate to BDO via succinic semialdehyde. 
         FIG.  9    is a schematic diagram showing a reaction scheme of D-lysine-5,6-aminomutase. 
         FIG.  10    is a schematic diagram showing a pathway to acetoacetate from acetyl-CoA. Enzymes are: (1) pyruvate formate-lyase, (2) pyruvate dehydrogenase, (3) acetyl-CoA:acetoacetyl-CoA transferase, (4) acetyl-CoA C-acetyltransferase, (5) phosphotransacetylase, and (6) Acetate kinase. Enzyme 7 represents the multistep acetoacetate to BDO pathway in  FIG.  8   . 
         FIGS.  11 A- 11 C  show the production of 4-HB in glucose minimal medium using  E. coli  strains harboring plasmids expressing various combinations of 4-HB pathway genes.  FIG.  11 A , 4-HB concentration in culture broth;  FIG.  11 B , succinate concentration in culture broth;  FIG.  11 C , culture OD, measured at 600 nm. Clusters of bars represent the 24 hour, 48 hour, and 72 hour (if measured) timepoints. The codes along the x-axis indicate the strain/plasmid combination used. The first index refers to the host strain: 1, MG1655 lacI Q ; 2, MG1655 ΔgabD lacI Q ; 3, MG1655 ΔgabD ΔaldA lacI Q . The second index refers to the plasmid combination used: 1, pZE13-0004-0035 and pZA33-0036; 2, pZE13-0004-0035 and pZA33-0010n; 3, pZE13-0004-0008 and pZA33-0036; 4, pZE13-0004-0008 and pZA33-0010n; 5, Control vectors pZE13 and pZA33. 
         FIG.  12    shows the production of 4-HB from glucose in  E. coli  strains expressing α-ketoglutarate decarboxylase from  Mycobacterium tuberculosis . Strains 1-3 contain pZE13-0032 and pZA33-0036. Strain 4 expresses only the empty vectors pZE13 and pZA33. Host strains are as follows: 1 and 4, MG1655 lacI Q ; 2, MG1655 ΔgabD lacI Q ; 3, MG1655 ΔgabD ΔaldA lacI Q . The bars refer to concentration at 24 and 48 hours. 
         FIG.  13    shows the production of BDO from 10 mM 4-HB in recombinant  E. coli  strains. Numbered positions correspond to experiments with MG1655 lacI Q  containing pZA33-0024, expressing cat2 from  P. gingivalis , and the following genes expressed on pZE13: 1, none (control); 2, 0002; 3, 0003; 4, 0003n; 5, 0011; 6, 0013; 7, 0023; 8, 0025; 9, 0008n; 10, 0035. Gene numbers are defined in Table 6. For each position, the bars refer to aerobic, microaerobic, and anaerobic conditions, respectively. Microaerobic conditions were created by sealing the culture tubes but not evacuating them. 
         FIGS.  14 A- 14 H  show the mass spectrum of 4-HB and BDO produced by MG1655 lacI Q  pZE13-0004-0035-0002 pZA33-0034-0036 grown in M9 minimal medium supplemented with 4 g/L unlabeled glucose ( FIGS.  14 A,  14 C,  14 E and  14 G ) uniformly labeled  13 C-glucose ( FIGS.  14 B,  14 D,  14 F and  14 H ).  FIGS.  14 A and  14 B , mass 116 characteristic fragment of derivatized BDO, containing 2 carbon atoms;  FIGS.  14 C and  14 D , mass 177 characteristic fragment of derivatized BDO, containing 1 carbon atom;  FIGS.  14 E and  14 F , mass 117 characteristic fragment of derivatized 4-HB, containing 2 carbon atoms;  FIGS.  14 G and  14 H , mass 233 characteristic fragment of derivatized 4-HB, containing 4 carbon atoms. 
         FIG.  15    is a schematic process flow diagram of bioprocesses for the production of γ-butyrolactone. Upper panel illustrates fed-batch fermentation with batch separation and lower panel illustrates fed-batch fermentation with continuous separation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention is directed to the design and production of cells and organisms having biosynthetic production capabilities for 4-hydroxybutanoic acid (4-HB), γ-butyrolactone and 1,4-butanediol. In one embodiment, the invention utilizes in silico stoichiometric models of  Escherichia coli  metabolism that identify metabolic designs for biosynthetic production of 4-hydroxybutanoic acid (4-HB) and 1,4-butanediol (BDO). The results described herein indicate that metabolic pathways can be designed and recombinantly engineered to achieve the biosynthesis of 4-HIB and downstream products such as 1,4-butanediol in  Escherichia coli  and other cells or organisms. Biosynthetic production of 4-HB, for example, for the in silico designs can be confirmed by construction of strains having the designed metabolic genotype. These metabolically engineered cells or organisms also can be subjected to adaptive evolution to further augment 4-HB biosynthesis, including under conditions approaching theoretical maximum growth. 
     In certain embodiments, the 4-HB biosynthesis characteristics of the designed strains make them genetically stable and particularly useful in continuous bioprocesses. Separate strain design strategies were identified with incorporation of different non-native or heterologous reaction capabilities into  E. coli  leading to 4-HB and 1,4-butanediol producing metabolic pathways from either CoA-independent succinic semialdehyde dehydrogenase, succinyl-CoA synthetase and CoA-dependent succinic semialdehyde dehydrogenase, or glutamate:succinic semialdehyde transaminase. In silico metabolic designs were identified that resulted in the biosynthesis of 4-HB in both  E. coli  and yeast species from each of these metabolic pathways. The 1,4-butanediol intermediate γ-butyrolactone can be generated in culture by spontaneous cyclization under conditions at pH&lt;7.5, particularly under acidic conditions, such as below pH 5.5, for example, pH&lt;7, pH&lt;6.5, pH&lt;6, and particularly at pH&lt;5.5 or lower. 
     Strains identified via the computational component of the platform can be put into actual production by genetically engineering any of the predicted metabolic alterations which lead to the biosynthetic production of 4-HB, 1,4-butanediol or other intermediate and/or downstream products. In yet a further embodiment, strains exhibiting biosynthetic production of these compounds can be further subjected to adaptive evolution to further augment product biosynthesis. The levels of product biosynthesis yield following adaptive evolution also can be predicted by the computational component of the system. 
     In other specific embodiments, microbial organisms were constructed to express a 4-HB biosynthetic pathway encoding the enzymatic steps from succinate to 4-HB and to 4-HB-CoA. Co-expression of succinate coenzyme A transferase, CoA-dependent succinic semialdehyde dehydrogenase, NAD-dependent 4-hydroxybutyrate dehydrogenase and 4-hydroxybutyrate coenzyme A transferase in a host microbial organism resulted in significant production of 4-HB compared to host microbial organisms lacking a 4-HB biosynthetic pathway. In a further specific embodiment, 4-HB-producing microbial organisms were generated that utilized α-ketoglutarate as a substrate by introducing nucleic acids encoding α-ketoglutarate decarboxylase and NAD-dependent 4-hydroxybutyrate dehydrogenase. 
     In another specific embodiment, microbial organisms containing a 1,4-butanediol (BDO) biosynthetic pathway were constructed that biosynthesized BDO when cultured in the presence of 4-HB. The BDO biosynthetic pathway consisted of a nucleic acid encoding either a multifunctional aldehyde/alcohol dehydrogenase or nucleic acids encoding an aldehyde dehydrogenase and an alcohol dehydrogenase. To support growth on 4-HB substrates, these BDO-producing microbial organisms also expressed 4-hydroxybutyrate CoA transferase or 4-butyrate kinase in conjunction with phosphotranshydroxybutyrlase. In yet a further specific embodiment, microbial organisms were generated that synthesized BDO through exogenous expression of nucleic acids encoding a functional 4-HB biosynthetic pathway and a functional BDO biosynthetic pathway. The 4-HB biosynthetic pathway consisted of succinate coenzyme A transferase, CoA-dependent succinic semialdehyde dehydrogenase, NAD-dependent 4-hydroxybutyrate dehydrogenase and 4-hydroxybutyrate coenzyme A transferase. The BDO pathway consisted of a multifunctional aldehyde/alcohol dehydrogenase. 
     As used herein, the term “non-naturally occurring” when used in reference to a microbial organism or microorganism of the invention is intended to mean that the microbial organism has at least one genetic alteration not normally found in a naturally occurring strain of the referenced species, including wild-type strains of the referenced species. Genetic alterations include, for example, modifications introducing expressible nucleic acids encoding metabolic polypeptides, other nucleic acid additions, nucleic acid deletions and/or other functional disruption of the microbial genetic material. Such modification include, for example, coding regions and functional fragments thereof, for heterologous, homologous or both heterologous and homologous polypeptides for the referenced species. Additional modifications include, for example, non-coding regulatory regions in which the modifications alter expression of a gene or operon. Exemplary metabolic polypeptides include enzymes within a 4-HB biosynthetic pathway and enzymes within a biosynthetic pathway for a BDO family of compounds. 
     A metabolic modification refers to a biochemical reaction that is altered from its naturally occurring state. Therefore, non-naturally occurring microorganisms having genetic modifications to nucleic acids encoding metabolic polypeptides or, functional fragments thereof. Exemplary metabolic modifications are described further below for both  E. coli  and yeast microbial organisms. 
     As used herein, the term “isolated” when used in reference to a microbial organism is intended to mean an organism that is substantially free of at least one component as the referenced microbial organism is found in nature. The term includes a microbial organism that is removed from some or all components as it is found in its natural environment. The term also includes a microbial organism that is removed from some or all components as the microbial organism is found in non-naturally occurring environments. Therefore, an isolated microbial organism is partly or completely separated from other substances as it is found in nature or as it is grown, stored or subsisted in non-naturally occurring environments. Specific examples of isolated microbial organisms include partially pure microbes, substantially pure microbes and microbes cultured in a medium that is non-naturally occurring. 
     As used herein, the terms “microbial,” “microbial organism” or “microorganism” is intended to mean any organism that exists as a microscopic cell that is included within the domains of archaea, bacteria or eukarya. Therefore, the term is intended to encompass prokaryotic or eukaryotic cells or organisms having a microscopic size and includes bacteria, archaea and eubacteria of all species as well as eukaryotic microorganisms such as yeast and fungi. The term also includes cell cultures of any species that can be cultured for the production of a biochemical. 
     As used herein, the term “4-hydroxybutanoic acid” is intended to mean a 4-hydroxy derivative of butyric acid having the chemical formula C 4 H 8 O 3  and a molecular mass of 104.11 g/mol (126.09 g/mol for its sodium salt). The chemical compound 4-hydroxybutanoic acid also is known in the art as 4-HB, 4-hydroxybutyrate, gamma-hydroxybutyric acid or GHB. The term as it is used herein is intended to include any of the compound&#39;s various salt forms and include, for example, 4-hydroxybutanoate and 4-hydroxybutyrate. Specific examples of salt forms for 4-HB include sodium 4-HB and potassium 4-HB. Therefore, the terms 4-hydroxybutanoic acid, 4-HB, 4-hydroxybutyrate, 4-hydroxybutanoate, gamma-hydroxybutyric acid and GHB as well as other art recognized names are used synonymously herein. 
     As used herein, the term “monomeric” when used in reference to 4-HB is intended to mean 4-HB in a non-polymeric or underivatized form. Specific examples of polymeric 4-HB include poly-4-hydroxybutanoic acid and copolymers of, for example, 4-HB and 3-HB. A specific example of a derivatized form of 4-HB is 4-HB-CoA. Other polymeric 4-HB forms and other derivatized forms of 4-HB also are known in the art. 
     As used herein, the term “γ-butyrolactone” is intended to mean a lactone having the chemical formula C 4 H 6 O 2  and a molecular mass of 86.089 g/mol. The chemical compound γ-butyrolactone also is know in the art as GBL, butyrolactone, 1,4-lactone, 4-butyrolactone, 4-hydroxybutyric acid lactone, and gamma-hydroxybutyric acid lactone. The term as it is used herein is intended to include any of the compound&#39;s various salt forms. 
     As used herein, the term “1,4-butanediol” is intended to mean an alcohol derivative of the alkane butane, carrying two hydroxyl groups which has the chemical formula C 4 H 10 O 2  and a molecular mass of 90.12 g/mol. The chemical compound 1,4-butanediol also is known in the art as BDO and is a chemical intermediate or precursor for a family of compounds referred to herein as BDO family of compounds, some of which are exemplified in  FIG.  1   . 
     As used herein, the term “tetrahydrofuran” is intended to mean a heterocyclic organic compound corresponding to the fully hydrogenated analog of the aromatic compound furan which has the chemical formula C 4 H 8 O and a molecular mass of 72.11 g/mol. The chemical compound tetrahydrofuran also is known in the art as THF, tetrahydrofuran, 1,4-epoxybutane, butylene oxide, cyclotetramethylene oxide, oxacyclopentane, diethylene oxide, oxolane, furanidine, hydrofuran, tetra-methylene oxide. The term as it is used herein is intended to include any of the compound&#39;s various salt forms. 
     As used herein, the term “CoA” or “coenzyme A” is intended to mean an organic cofactor or prosthetic group (nonprotein portion of an enzyme) whose presence is required for the activity of many enzymes (the apoenzyme) to form an active enzyme system. Coenzyme A functions in certain condensing enzymes, acts in acetyl or other acyl group transfer and in fatty acid synthesis and oxidation, pyruvate oxidation and in other acetylation. 
     As used herein, the term “substantially anaerobic” when used in reference to a culture or growth condition is intended to mean that the amount of oxygen is less than about 10% of saturation for dissolved oxygen in liquid media. The term also is intended to include sealed chambers of liquid or solid medium maintained with an atmosphere of less than about 1% oxygen. 
     The non-naturally occurring microbal organisms of the invention can contain stable genetic alterations, which refers to microorganisms that can be cultured for greater than five generations without loss of the alteration. Generally, stable genetic alterations include modifications that persist greater than 10 generations, particularly stable modifications will persist more than about 25 generations, and more particularly, stable genetic modificatios will be greater than 50 generations, including indefinitely. 
     Those skilled in the art will understand that the genetic alterations, including metabolic modifications exemplified herein are described with reference to  E. coli  and yeast genes and their corresponding metabolic reactions. However, given the complete genome sequencing of a wide variety of organisms and the high level of skill in the area of genomics, those skilled in the art will readily be able to apply the teachings and guidance provided herein to essentially all other organisms. For example, the  E. coli  metabolic alterations exemplified herein can readily be applied to other species by incorporating the same or analogous encoding nucleic acid from species other than the referenced species. Such genetic alterations include, for example, genetic alterations of species homologs, in general, and in particular, orthologs, paralogs or nonorthologous gene displacements. 
     An ortholog is a gene or genes that are related by vertical descent and are responsible for substantially the same or identical functions in different organisms. For example, mouse epoxide hydrolase and human epoxide hydrolase can be considered orthologs for the biological function of hydrolysis of epoxides. Genes are related by vertical descent when, for example, they share sequence similarity of sufficient amount to indicate they are homologous, or related by evolution from a common ancestor. Genes can also be considered orthologs if they share three-dimensional structure but not necessarily sequence similarity, of a sufficient amount to indicate that they have evolved from a common ancestor to the extent that the primary sequence similarity is not identifiable. Genes that are orthologous can encode proteins with sequence similarity of about 25% to 100% amino acid sequence identity. Genes encoding proteins sharing an amino acid similarity less that 25% can also be considered to have arisen by vertical descent if their three-dimensional structure also shows similarities. Members of the serine protease family of enzymes, including tissue plasminogen activator and elastase, are considered to have arisen by vertical descent from a common ancestor. 
     Orthologs include genes or their encoded gene products that through, for example, evolution, have diverged in structure or overall activity. For example, where one species encodes a gene product exhibiting two functions and where such functions have been separated into distinct genes in a second species, the three genes and their corresponding products are considered to be orthologs. For the growth-coupled production of a biochemical product, those skilled in the art will understand that the orthologous gene harboring the metabolic activity to be disrupted is to be chosen for construction of the non-naturally occurring microorganism. An example of orthologs exhibiting separable activities is where distinct activities have been separated into distinct gene products between two or more species or within a single species. A specific example is the separation of elastase proteolysis and plasminogen proteolysis, two types of serine protease activity, into distinct molecules as plasminogen activator and elastase. A second example is the separation of mycoplasma 5′-3′ exonuclease and Drosophila DNA polymerase III activity. The DNA polymerase from the first species can be considered an ortholog to either or both of the exonuclease or the polymerase from the second species and vice versa. 
     In contrast, paralogs are homologs related by, for example, duplication followed by evolutionary divergence and have similar or common, but not identical functions. Paralogs can originate or derive from, for example, the same species or from a different species. For example, microsomal epoxide hydrolase (epoxide hydrolase I) and soluble epoxide hydrolase (epoxide hydrolase II) can be considered paralogs because they represent two distinct enzymes, co-evolved from a common ancestor, that catalyze distinct reactions and have distinct functions in the same species. Paralogs are proteins from the same species with significant sequence similarity to each other suggesting that they are homologous, or related through co-evolution from a common ancestor. Groups of paralogous protein families include HipA homologs, luciferase genes, peptidases, and others. 
     A nonorthologous gene displacement is a nonorthologous gene from one species that can substitute for a referenced gene function in a different species. Substitution includes, for example, being able to perform substantially the same or a similar function in the species of origin compared to the referenced function in the different species. Although generally, a nonorthologous gene displacement will be identifiable as structurally related to a known gene encoding the referenced function, less structurally related but functionally similar genes and their corresponding gene products nevertheless will still fall within the meaning of the term as it is used herein. Functional similarity requires, for example, at least some structural similarity in the active site or binding region of a nonorthologous gene compared to a gene encoding the function sought to be substituted. Therefore, a nonorthologous gene includes, for example, a paralog or an unrelated gene. 
     Therefore, in identifying and constructing the non-naturally occurring microbial organisms of the invention having 4-HB, GBL and/or BDO biosynthetic capability, those skilled in the art will understand with applying the teaching and guidance provided herein to a particular species that the identification of metabolic modifications can include identification and inclusion or inactivation of orthologs. To the extent that paralogs and/or nonorthologous gene displacements are present in the referenced microorganism that encode an enzyme catalyzing a similar or substantially similar metabolic reaction, those skilled in the art also can utilize these evolutionally related genes. 
     Orthologs, paralogs and nonorthologous gene displacements can be determined by methods well known to those skilled in the art. For example, inspection of nucleic acid or amino acid sequences for two polypeptides will reveal sequence identity and similarities between the compared sequences. Based on such similarities, one skilled in the art can determine if the similarity is sufficiently high to indicate the proteins are related through evolution from a common ancestor. Algorithms well known to those skilled in the art, such as Align, BLAST, Clustal W and others compare and determine a raw sequence similarity or identity, and also determine the presence or significance of gaps in the sequence which can be assigned a weight or score. Such algorithms also are known in the art and are similarly applicable for determining nucleotide sequence similarity or identity. Parameters for sufficient similarity to determine relatedness are computed based on well known methods for calculating statistical similarity, or the chance of finding a similar match in a random polypeptide, and the significance of the match determined. A computer comparison of two or more sequences can, if desired, also be optimized visually by those skilled in the art. Related gene products or proteins can be expected to have a high similarity, for example, 25% to 100% sequence identity. Proteins that are unrelated can have an identity which is essentially the same as would be expected to occur by chance, if a database of sufficient size is scanned (about 5%). Sequences between 5% and 24% may or may not represent sufficient homology to conclude that the compared sequences are related. Additional statistical analysis to determine the significance of such matches given the size of the data set can be carried out to determine the relevance of these sequences. 
     Exemplary parameters for determining relatedness of two or more sequences using the BLAST algorithm, for example, can be as set forth below. Briefly, amino acid sequence alignments can be performed using BLASTP version 2.0.8 (Jan. 5, 1999) and the following parameters: Matrix: 0 BLOSUM62; gap open: 11; gap extension: 1; x_dropoff: 50; expect: 10.0; wordsize: 3; filter: on. Nucleic acid sequence alignments can be performed using BLASTN version 2.0.6 (Sep. 16, 1998) and the following parameters: Match: 1; mismatch: −2; gap open: 5; gap extension: 2; x_dropoff: 50; expect: 10.0; wordsize: 11; filter: off. Those skilled in the art will know what modifications can be made to the above parameters to either increase or decrease the stringency of the comparison, for example, and determine the relatedness of two or more sequences. 
     The invention provides a non-naturally occurring microbial biocatalyst including a microbial organism having a 4-hydroxybutanoic acid (4-HB) biosynthetic pathway that includes at least one exogenous nucleic acid encoding 4-hydroxybutanoate dehydrogenase, CoA-independent succinic semialdehyde dehydrogenase, succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase, glutamate:succinic semialdehyde transaminase, alpha-ketoglutarate decarboxylase, or glutamate decarboxylase, wherein the exogenous nucleic acid is expressed in sufficient amounts to produce monomeric 4-hydroxybutanoic acid (4-HB). 4-hydroxybutanoate dehydrogenase is also referred to as 4-hydroxybutyrate dehydrogenase or 4-HB dehydrogenase. Succinyl-CoA synthetase is also referred to as succinyl-CoA synthase or succinyl-CoA ligase. 
     Also provided is a non-naturally occurring microbial biocatalyst including a microbial organism having a 4-hydroxybutanoic acid (4-HB) biosynthetic pathway having at least one exogenous nucleic acid encoding 4-hydroxybutanoate dehydrogenase, succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase, or α-ketoglutarate decarboxylase, wherein the exogenous nucleic acid is expressed in sufficient amounts to produce monomeric 4-hydroxybutanoic acid (4-HB). 
     The non-naturally occurring microbial biocatalysts of the invention include microbial organisms that employ combinations of metabolic reactions for biosynthetically producing the compounds of the invention. The biosynthesized compounds can be produced intracellularly and/or secreted into the culture medium. Exemplary compounds produced by the non-naturally occurring microorganisms include, for example, 4-hydroxybutanoic acid, 1,4-butanediol and γ-butyrolactone. The relationships of these exemplary compounds with respect to chemical synthesis or biosynthesis are exemplified in  FIG.  1   . 
     In one embodiment, a non-naturally occurring microbial organism is engineered to produce 4-HB. This compound is one useful entry point into the 1,4-butanediol family of compounds. The biochemical reactions for formation of 4-HB from succinate, from succinate through succinyl-CoA or from α-ketoglutarate are shown in steps  1 - 8  of  FIG.  2   . 
     The invention is described herein with general reference to the metabolic reaction, reactant or product thereof, or with specific reference to one or more nucleic acids or genes encoding an enzyme associated with or catalyzing the referenced metabolic reaction, reactant or product. Unless otherwise expressly stated herein, those skilled in the art will understand that reference to a reaction also constitutes reference to the reactants and products of the reaction. Similarly, unless otherwise expressly stated herein, reference to a reactant or product also references the reaction and that reference to any of these metabolic constitutes also references the gene or genes encoding the enzymes that catalyze the referenced reaction, reactant or product. Likewise, given the well known fields of metabolic biochemistry, enzymology and genomics, reference herein to a gene or encoding nucleic acid also constitutes a reference to the corresponding encoded enzyme and the reaction it catalyzes as well as the reactants and products of the reaction. 
     The production of 4-HB via biosynthetic modes using the microbial organisms of the invention is particularly useful because it can produce monomeric 4-HB. The non-naturally occurring microbial organisms of the invention and their biosynthesis of 4-HB and BDO family compounds also is particularly useful because the 4-HB product is (1) secreted; (2) can be devoid of any derivatizations such as Coenzyme A; (3) avoids thermodynamic changes during biosynthesis; (4) allows direct biosynthesis of BDO, and (5) allows for the spontaneous chemical conversion of 4-HB to γ-butyrolactone (GBL) in acidic pH medium. This latter characteristic also is particularly useful for efficient chemical synthesis or biosynthesis of BDO family compounds such as 1,4-butanediol and/or tetrahydrofuran (THF), for example. 
     Microbial organisms generally lack the capacity to synthesize 4-HB and therefore, any of the compounds shown in  FIG.  1    are known to be within the 1,4-butanediol family of compounds or known by those in the art to be within the 1,4-butanediol family of compounds. Moreover, organisms having all of the requisite metabolic enzymatic capabilities are not known to produce 4-HB from the enzymes described and biochemical pathways exemplified herein. Rather, with the possible exception of a few anaerobic microorganisms described further below, the microorganisms having the enzymatic capability use 4-HB as a substrate to produce, for example, succinate. In contrast, the non-naturally occurring microbial organisms of the invention generate 4-HB as a product. As described above, the biosynthesis of 4-HB in its monomeric form is not only particularly useful in chemical synthesis of BDO family of compounds, it also allows for the further biosynthesis of BDO family compounds and avoids altogether chemical synthesis procedures. 
     The non-naturally occurring microbial organisms of the invention that can produce 4-HB are produced by ensuring that a host microbial organism includes functional capabilities for the complete biochemical synthesis of at least one 4-HB biosynthetic pathway of the invention. Ensuring at least one requisite 4-HB biosynthetic pathway confers 4-HB biosynthesis capability onto the host microbial organism. 
     Five requisite 4-HB biosynthetic pathways are exemplified herein and shown for purposes of illustration in  FIG.  2   . One requisite 4-HB biosynthetic pathway includes the biosynthesis of 4-HB from succinate (the succinate pathway). The enzymes participating in this 4-HB pathway include CoA-independent succinic semialdehyde dehydrogenase and 4-hydroxybutanoate dehydrogenase. In this pathway, CoA-independent succinic semialdehyde dehydrogenase catalyzes the reverse reaction to the arrow shown in  FIG.  2   . Another requisite 4-HB biosynthetic pathway includes the biosynthesis from succinate through succinyl-CoA (the succinyl-CoA pathway). The enzymes participating in this 4-HB pathway include succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase and 4-hydroxybutanoate dehydrogenase. Three other requisite 4-HB biosynthetic pathways include the biosynthesis of 4-HB from α-ketoglutarate (the α-ketoglutarate pathways). Hence, a third requisite 4-HB biosynthetic pathway is the biosynthesis of succinic semialdehyde through glutamate:succinic semialdehyde transaminase, glutamate decarboxylase and 4-hydroxybutanoate dehydrogenase. A fourth requisite 4-HB biosynthetic pathway also includes the biosynthesis of 4-HB from α-ketoglutarate, but utilizes α-ketoglutarate decarboxylase to catalyze succinic semialdehyde synthesis. 4-hydroxybutanoate dehydrogenase catalyzes the conversion of succinic semialdehyde to 4-HB. A fifth requisite 4-HB biosynthetic pathway includes the biosynthesis from α-ketoglutarate through succinyl-CoA and utilizes α-ketoglutarate dehydrogenase to produce succinyl-CoA, which funnels into the succinyl-CoA pathway described above. Each of these 4-HB biosynthetic pathways, their substrates, reactants and products are described further below in the Examples. 
     The non-naturally occurring microbial organisms of the invention can be produced by introducing expressible nucleic acids encoding one or more of the enzymes participating in one or more 4-HB biosynthetic pathways. Depending on the host microbial organism chosen for biosynthesis, nucleic acids for some or all of a particular 4-HB biosynthetic pathway can be expressed. For example, if a chosen host is deficient in both enzymes in the succinate to 4-HB pathway and this pathway is selected for 4-HB biosynthesis, then expressible nucleic acids for both CoA-independent succinic semialdehyde dehydrogenase and 4-hydroxybutanoate dehydrogenase are introduced into the host for subsequent exogenous expression. Alternatively, if the chosen host exhibits endogenous CoA-independent succinic semialdehyde dehydrogenase, but is deficient in 4-hydroxybutanoate dehydrogenase then an encoding nucleic acid is needed for this enzyme to achieve 4-HB biosynthesis. 
     In like fashion, where 4-HB biosynthesis is selected to occur through the succinate to succinyl-CoA pathway (the succinyl-CoA pathway), encoding nucleic acids for host deficiencies in the enzymes succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase and/or 4-hydroxybutanoate dehydrogenase are to be exogenously expressed in the recipient host. Selection of 4-HB biosynthesis through the α-ketoglutarate to succinic semialdehyde pathway (the α-ketoglutarate pathway) can utilize exogenous expression for host deficiencies in one or more of the enzymes for glutamate:succinic semialdehyde transaminase, glutamate decarboxylase and/or 4-hydroxybutanoate dehydrogenase, or α-ketoglutarate decarboxylase and 4-hydroxybutanoate dehydrogenase. 
     Depending on the 4-HB biosynthetic pathway constituents of a selected host microbial organism, the non-naturally occurring microbial 4-HB biocatalysts of the invention will include at least one exogenously expressed 4-HB pathway-encoding nucleic acid and up to all encoding nucleic acids for one or more 4-HB biosynthetic pathways. For example, 4-HB biosynthesis can be established from all five pathways in a host deficient in 4-hydroxybutanoate dehydrogenase through exogenous expression of a 4-hydroxybutanoate dehydrogenase encoding nucleic acid. In contrast, 4-HB biosynthesis can be established from all five pathways in a host deficient in all eight enzymes through exogenous expression of all eight of CoA-independent succinic semialdehyde dehydrogenase, succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase, glutamate:succinic semialdehyde transaminase, glutamate decarboxylase, α-ketoglutarate decarboxylase, α-ketoglutarate dehydrogenase and 4-hydroxybutanoate dehydrogenase. 
     Given the teachings and guidance provided herein, those skilled in the art will understand that the number of encoding nucleic acids to introduce in an expressible form will, at least, parallel the 4-HB pathway deficiencies of the selected host microbial organism. Therefore, a non-naturally occurring microbial organism of the invention can have one, two, three, four, five, six, seven or eight nucleic acids encoding the above enzymes constituting one or more 4-HB biosynthetic pathways. In some embodiments, the non-naturally occurring microbial organisms also can include other genetic modifications that facilitate or optimize 4-HB biosynthesis or that confer other useful functions onto the host microbial organism. One such other functionality can include, for example, augmentation of the synthesis of one or more of the 4-HB pathway precursors such as succinate, succinyl-CoA and/or α-ketoglutarate. 
     In some embodiments, a non-naturally occurring microbial organism of the invention is generated from a host that contains the enzymatic capability to synthesize 4-HB. In this specific embodiment it can be useful to increase the synthesis or accumulation of a 4-HB pathway product to, for example, drive 4-HB pathway reactions toward 4-HB production. Increased synthesis or accumulation can be accomplished by, for example, overexpression of nucleic acids encoding one or more of the above-described 4-HB pathway enzymes. Over expression of the 4-HB pathway enzyme or enzymes can occur, for example, through exogenous expression of the endogenous gene or genes, or through exogenous expression of the heterologous gene or genes. Therefore, naturally occurring organisms can be readily generated to be non-naturally 4-HB producing microbial organisms of the invention through overexpression of one, two, three, four, five or all six nucleic acids encoding 4-HB biosynthetic pathway enzymes. In addition, a non-naturally occurring organism can be generated by mutagenesis of an endogenous gene that results in an increase in activity of an enzyme in the 4-HB biosynthetic pathway. 
     In particularly useful embodiments, exogenous expression of the encoding nucleic acids is employed. Exogenous expression confers the ability to custom tailor the expression and/or regulatory elements to the host and application to achieve a desired expression level that is controlled by the user. However, endogenous expression also can be utilized in other embodiments such as by removing a negative regulatory effector or induction of the gene&#39;s promoter when linked to an inducible promoter or other regulatory element. Thus, an endogenous gene having a naturally occurring inducible promoter can be up-regulated by providing the appropriate inducing agent, or the regulatory region of an endogenous gene can be engineered to incorporate an inducible regulatory element, thereby allowing the regulation of increased expression of an endogenous gene at a desired time. Similarly, an inducible promoter can be included as a regulatory element for an exogenous gene introduced into a non-naturally occurring microbial organism (see Examples II and IV, for example). 
     “Exogenous” as it is used herein is intended to mean that the referenced molecule or the referenced activity is introduced into the host microbial organism including, for example, introduction of an encoding nucleic acid into the host genetic material such as by integration into a host chromosome. Therefore, the term as it is used in reference to expression of an encoding nucleic acid refers to introduction of the encoding nucleic acid in an expressible form into the microbial organism. When used in reference to a biosynthetic activity, the term refers to an activity that is introduced into the host reference organism. The source can be, for example, a homologous or heterologous encoding nucleic acid that expresses the referenced activity following introduction into the host microbial organism. Therefore, the term “endogenous” refers to a referenced molecule or activity that is present in the host. Similarly, the term when used in reference to expression of an encoding nucleic acid refers to expression of an encoding nucleic acid contained within the microbial organism. The term “heterologous” refers to a molecule or activity derived from a source other than the referenced species whereas “homologous” refers to a molecule or activity derived from the host microbial organism. Accordingly, exogenous expression of an encoding nucleic acid of the invention can utilize either or both a heterologous or homologous encoding nucleic acid. 
     Sources of encoding nucleic acids for a 4-HB pathway enzyme can include, for example, any species where the encoded gene product is capable of catalyzing the referenced reaction. Such species include both prokaryotic and eukaryotic organisms including, but not limited to, bacteria, including archaea and eubacteria, and eukaryotes, including yeast, plant, insect, animal, and mammal, including human. Exemplary species for such sources include, for example,  E. coli, Saccharomyces cerevisiae, Clostridium kluyveri, Clostridium acetobutylicum, Clostridium beijerinckii, Clostridium saccharoperbutylacetonicum, Clostridium perfringens, Clostridium difficile, Ralstonia eutropha, Mycobacterium bovis, Mycobacterium tuberculosis  and  Porphyromonas gingivalis . For example, the microbial organisms having 4-HB biosynthetic production are exemplified herein with reference to  E. coli  and yeast hosts. However, with the complete genome sequence available for now more than 550 species (with more than half of these available on public databases such as the NCBI), including 395 microorganism genomes and a variety of yeast, fungi, plant, and mammalian genomes, the identification of genes encoding the requisite 4-HB biosynthetic activity for one or more genes in related or distant species, including for example, homologues, orthologs, paralogs and nonorthologous gene displacements of known genes, and the interchange of genetic alterations between organisms is routine and well known in the art. Accordingly, the metabolic alterations enabling biosynthesis of 4-HB and other compounds of the invention described herein with reference to a particular organism such as  E. coli  or yeast can be readily applied to other microorganisms, including prokaryotic and eukaryotic organisms alike. Given the teachings and guidance provided herein, those skilled in the art will know that a metabolic alteration exemplified in one organism can be applied equally to other organisms. 
     In some instances, such as when an alternative 4-HB biosynthetic pathway exists in an unrelated species, 4-HB biosynthesis can be conferred onto the host species by, for example, exogenous expression of a paralog or paralogs from the unrelated species that catalyzes a similar, yet non-identical metabolic reaction to replace the referenced reaction. Because certain differences among metabolic networks exist between different organisms, those skilled in the art will understand that the actual genes usage between different organisms may differ. However, given the teachings and guidance provided herein, those skilled in the art also will understand that the teachings and methods of the invention can be applied to all microbial organisms using the cognate metabolic alterations to those exemplified herein to construct a microbial organism in a species of interest that will synthesize monomeric 4-HB. 
     Host microbial organisms can be selected from, and the non-naturally occurring microbial organisms generated in, for example, bacteria, yeast, fungus or any of a variety of other microorganisms applicable to fermentation processes. Exemplary bacteria include species selected from  E. coli, Klebsiella oxytoca, Anaerobiospirillum succiniciproducens, Actinobacillus succinogenes, Mannheimia succiniciproducens, Rhizobium etli, Bacillus subtilis, Corynebacterium glutamicum, Gluconobacter oxydans, Zymomonas mobilis, Lactococcus lactis, Lactobacillus plantarum, Streptomyces coelicolor, Clostridium acetobutylicum, Pseudomonas fluorescens , and  Pseudomonas putida . Exemplary yeasts or fungi include species selected from  Saccharomyces cerevisiae, Schizosaccharomyces pombe, Kluyveromyces lactis, Kluyveromyces marxianus, Aspergillus terreus, Aspergillus niger  and  Pichia pastoris.    
     Methods for constructing and testing the expression levels of a non-naturally occurring 4-HB-producing host can be performed, for example, by recombinant and detection methods well known in the art. Such methods can be found described in, for example, Sambrook et al.,  Molecular Cloning: A Laboratory Manual, Third Ed ., Cold Spring Harbor Laboratory, New York (2001); Ausubel et al.,  Current Protocols in Molecular Biology , John Wiley and Sons, Baltimore, Md. (1999). 4-HB and GBL can be separated by, for example, HPLC using a Spherisorb 5 ODS1 column and a mobile phase of 70% 10 mM phosphate buffer (pH=7) and 30% methanol, and detected using a UV detector at 215 nm (Hennessy et al. 2004, J. Forensic Sci. 46(6):1-9). BDO is detected by gas chromatography or by HPLC and refractive index detector using an Aminex HPX-87H column and a mobile phase of 0.5 mM sulfuric acid (Gonzalez-Pajuelo et al.,  Met. Eng.  7:329-336 (2005)). 
     For example, an expression vector or vectors can be constructed to harbor one or more 4-HB biosynthetic pathway and/or one or more BDO biosynthetic encoding nucleic acids as exemplified herein operably linked to expression control sequences functional in the host organism. Expression vectors applicable for use in the microbial host organisms of the invention include, for example, plasmids, phage vectors, viral vectors, episomes and artificial chromosomes, including vectors and selection sequences or markers operable for stable integration into a host chromosome. Selectable marker genes also can be included that, for example, provide resistance to antibiotics or toxins, complement auxotrophic deficiencies, or supply critical nutrients not in the culture media. Expression control sequences can include constitutive and inducible promoters, transcription enhancers, transcription terminators, and the like which are well known in the art. When two or more exogenous encoding nucleic acids are to be co-expressed, both nucleic acids can be inserted, for example, into a single expression vector or in separate expression vectors. For single vector expression, the encoding nucleic acids can be operationally linked to one common expression control sequence or linked to different expression control sequences, such as one inducible promoter and one constitutive promoter. The transformation of exogenous nucleic acid sequences involved in a metabolic or synthetic pathway can be confirmed using methods well known in the art. 
     The non-naturally occurring microbial organisms of the invention are constructed using methods well known in the art as exemplified above to exogenously express at least one nucleic acid encoding a 4-HB pathway enzyme in sufficient amounts to produce monomeric 4-HB. Exemplary levels of expression for 4-HB enzymes in each pathway are described further below in the Examples. Following the teachings and guidance provided herein, the non-naturally occurring microbial organisms of the invention can achieve biosynthesis of monomeric 4-HB resulting in intracellular concentrations between about 0.1-25 mM or more. Generally, the intracellular concentration of monomeric 4-HB is between about 3-20 mM, particularly between about 5-15 mM and more particularly between about 8-12 mM, including about 10 mM or more. Intracellular concentrations between and above each of these exemplary ranges also can be achieved from the non-naturally occurring microbial organisms of the invention. 
     As described further below, one exemplary growth condition for achieving biosynthesis of 4-HB includes anaerobic culture or fermentation conditions. In certain embodiments, the non-naturally occurring microbial organisms of the invention can be sustained, cultured or fermented under anaerobic or substantially anaerobic conditions. Briefly, anaerobic conditions refers to an environment devoid of oxygen. Substantially anaerobic conditions include, for example, a culture, batch fermentation or continuous fermentation such that the dissolved oxygen concentration in the medium remains between 0 and 10% of saturation. Substantially anaerobic conditions also includes growing or resting cells in liquid medium or on solid agar inside a sealed chamber maintained with an atmosphere of less than 1% oxygen. The percent of oxygen can be maintained by, for example, sparging the culture with an N 2 /CO 2  mixture or other suitable non-oxygen gas or gases. 
     The invention also provides a non-naturally occurring microbial biocatalyst including a microbial organism having 4-hydroxybutanoic acid (4-HB) and 1,4-butanediol (BDO) biosynthetic pathways that include at least one exogenous nucleic acid encoding 4-hydroxybutanoate dehydrogenase, CoA-independent succinic semialdehyde dehydrogenase, succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase, 4-hydroxybutyrate:CoA transferase, glutamate:succinic semialdehyde transaminase, glutamate decarboxylase, CoA-independent aldehyde dehydrogenase, CoA-dependent aldehyde dehydrogenase or alcohol dehydrogenase, wherein the exogenous nucleic acid is expressed in sufficient amounts to produce 1,4-butanediol (BDO). 4-Hydroxybutyrate:CoA transferase also is known as 4-hydroxybutyryl CoA:acetyl-CoA transferase. 
     The invention further provides non-naturally occurring microbial biocatalyst including a microbial organism having 4-hydroxybutanoic acid (4-HB) and 1,4-butanediol (BDO) biosynthetic pathways, the pathways include at least one exogenous nucleic acid encoding 4-hydroxybutanoate dehydrogenase, succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase, 4-hydroxybutyrate:CoA transferase, 4-butyrate kinase, phosphotransbutyrylase, α-ketoglutarate decarboxylase, aldehyde dehydrogenase, alcohol dehydrogenase or an aldehyde/alcohol dehydrogenase, wherein the exogenous nucleic acid is expressed in sufficient amounts to produce 1,4-butanediol (BDO). 
     Non-naturally occurring microbial organisms also can be generated which biosynthesize BDO. As with the 4-HB producing microbial organisms of the invention, the BDO producing microbial organisms also can produce intracellularly or secret the BDO into the culture medium. Following the teachings and guidance provided previously for the construction of microbial organisms that synthesize 4-HB, additional BDO pathways can be incorporated into the 4-HB producing microbial organisms to generate organisms that also synthesize BDO and other BDO family compounds. The chemical synthesis of BDO and its downstream products are illustrated in  FIG.  1   . The non-naturally occurring microbial organisms of the invention capable of BDO biosynthesis circumvent these chemical synthesis using 4-HB as an entry point as illustrated in  FIG.  2   . As described further below, the 4-HB producers also can be used to chemically convert 4-HB to GBL and then to BDO or THF, for example. Alternatively, the 4-HB producers can be further modified to include biosynthetic capabilities for conversion of 4-HB and/or GBL to BDO. 
     The additional BDO pathways to introduce into 4-HB producers include, for example, the exogenous expression in a host deficient background or the overexpression of one or more of the enzymes exemplified in  FIG.  2    as steps  9 - 13 . One such pathway includes, for example, the enzyme activities necessary to carryout the reactions shown as steps  9 ,  12  and  13  in  FIG.  2   , where the aldehyde and alcohol dehydrogenases can be separate enzymes or a multifunctional enzyme having both aldehyde and alcohol dehydrogenase activity. Another such pathway includes, for example, the enzyme activities necessary to carryout the reactions shown as steps  10 ,  11 ,  12  and  13  in  FIG.  2   , also where the aldehyde and alcohol dehydrogenases can be separate enzymes or a multifunctional enzyme having both aldehyde and alcohol dehydrogenase activity. Accordingly, the additional BDO pathways to introduce into 4-HB producers include, for example, the exogenous expression in a host deficient background or the overexpression of one or more of a 4-hydroxybutyrate:CoA transferase, butyrate kinase, phosphotransbutyrylase, CoA-independent aldehyde dehydrogenase, CoA-dependent aldehyde dehydrogenase or an alcohol dehydrogenase. In the absence of endogenous acyl-CoA synthetase capable of modifying 4-HB, the non-naturally occurring BDO producing microbial organisms can further include an exogenous acyl-CoA synthetase selective for 4-HB, or the combination of multiple enzymes that have as a net reaction conversion of 4-HB into 4-HB-CoA. As exemplified further below in the Examples, butyrate kinase and phosphotransbutyrylase exhibit BDO pathway activity and catalyze the conversions illustrated in  FIG.  2    with a 4-HB substrate. Therefore, these enzymes also can be referred to herein as 4-hydroxybutyrate kinase and phosphotranshydroxybutyrylase respectively. 
     Exemplary alcohol and aldehyde dehydrogenases that can be used for these in vivo conversions from 4-HB to BDO are listed below in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Alcohol and Aldehyde Dehydrogenases for Conversion of 4-HB to BDO. 
               
               
                   
               
             
            
               
                 ALCOHOL DEHYDROGENASES 
               
            
           
           
               
               
            
               
                 ec: 1.1.1.1 
                 alcohol dehydrogenase 
               
               
                 ec: 1.1.1.2 
                 alcohol dehydrogenase (NADP+) 
               
               
                 ec: 1.1.1.4 
                 (R,R)-butanediol dehydrogenase 
               
               
                 ec: 1.1.1.5 
                 acetoin dehydrogenase 
               
               
                 ec: 1.1.1.6 
                 glycerol dehydrogenase 
               
               
                 ec: 1.1.1.7 
                 propanediol-phosphate 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.8 
                 glycerol-3-phosphate 
               
               
                   
                 dehydrogenase (NAD+) 
               
               
                 ec: 1.1.1.11 
                 D-arabinitol 4-dehydrogenase 
               
               
                 ec: 1.1.1.12 
                 L-arabinitol 4-dehydrogenase 
               
               
                 ec: 1.1.1.13 
                 L-arabinitol 2-dehydrogenase 
               
               
                 ec: 1.1.1.14 
                 L-iditol 2-dehydrogenase 
               
               
                 ec: 1.1.1.15 
                 D-iditol 2-dehydrogenase 
               
               
                 ec: 1.1.1.16 
                 galactitol 2-dehydrogenase 
               
               
                 ec: 1.1.1.17 
                 mannitol-1-phosphate 5- 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.18 
                 inositol 2-dehydrogenase 
               
               
                 ec: 1.1.1.21 
                 aldehyde reductase 
               
               
                 ec: 1.1.1.23 
                 histidinol dehydrogenase 
               
               
                 ec: 1.1.1.26 
                 glyoxylate reductase 
               
               
                 ec: 1.1.1.27 
                 L-lactate dehydrogenase 
               
               
                 ec: 1.1.1.28 
                 D-lactate dehydrogenase 
               
               
                 ec: 1.1.1.29 
                 glycerate dehydrogenase 
               
               
                 ec: 1.1.1.30 
                 3-hydroxybutyrate dehydrogenase 
               
               
                 ec: 1.1.1.31 
                 3-hydroxyisobutyrate 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.35 
                 3-hydroxyacyl-CoA dehydrogenase 
               
               
                 ec: 1.1.1.36 
                 acetoacetyl-CoA reductase 
               
               
                 ec: 1.1.1.37 
                 malate dehydrogenase 
               
               
                 ec: 1.1.1.38 
                 malate dehydrogenase 
               
               
                   
                 (oxaloacetate-decarboxylating) 
               
               
                 ec: 1.1.1.39 
                 malate dehydrogenase 
               
               
                   
                 (decarboxylating) 
               
               
                 ec: 1.1.1.40 
                 malate dehydrogenase 
               
               
                   
                 (oxaloacetate-decarboxylating) (NADP+) 
               
               
                 ec: 1.1.1.41 
                 isocitrate dehydrogenase (NAD+) 
               
               
                 ec: 1.1.1.42 
                 isocitrate dehydrogenase (NADP+) 
               
               
                 ec: 1.1.1.54 
                 allyl-alcohol dehydrogenase 
               
               
                 ec: 1.1.1.55 
                 lactaldehyde reductase (NADPH) 
               
               
                 ec: 1.1.1.56 
                 ribitol 2-dehydrogenase 
               
               
                 ec: 1.1.1.59 
                 3-hydroxypropionate 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.60 
                 2-hydroxy-3-oxopropionate 
               
               
                   
                 reductase 
               
               
                 ec: 1.1.1.61 
                 4-hydroxybutyrate dehydrogenase 
               
               
                 ec: 1.1.1.66 
                 omega-hydroxydecanoate 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.67 
                 mannitol 2-dehydrogenase 
               
               
                 ec: 1.1.1.71 
                 alcohol dehydrogenase [NAD(P)+] 
               
               
                 ec: 1.1.1.72 
                 glycerol dehydrogenase (NADP+) 
               
               
                 ec: 1.1.1.73 
                 octanol dehydrogenase 
               
               
                 ec: 1.1.1.75 
                 (R)-aminopropanol dehydrogenase 
               
               
                 ec: 1.1.1.76 
                 (S,S)-butanediol dehydrogenase 
               
               
                 ec: 1.1.1.77 
                 lactaldehyde reductase 
               
               
                 ec: 1.1.1.78 
                 methylglyoxal reductase (NADH- 
               
               
                   
                 dependent) 
               
               
                 ec: 1.1.1.79 
                 glyoxylate reductase (NADP+) 
               
               
                 ec: 1.1.1.80 
                 isopropanol dehydrogenase 
               
               
                   
                 (NADP+) 
               
               
                 ec: 1.1.1.81 
                 hydroxypyruvate reductase 
               
               
                 ec: 1.1.1.82 
                 malate dehydrogenase (NADP+) 
               
               
                 ec: 1.1.1.83 
                 D-malate dehydrogenase 
               
               
                   
                 (decarboxylating) 
               
               
                 ec: 1.1.1.84 
                 dimethylmalate dehydrogenase 
               
               
                 ec: 1.1.1.85 
                 3-isopropylmalate dehydrogenase 
               
               
                 ec: 1.1.1.86 
                 ketol-acid reductoisomerase 
               
               
                 ec: 1.1.1.87 
                 homoisocitrate dehydrogenase 
               
               
                 ec: 1.1.1.88 
                 hydroxymethylglutaryl-CoA 
               
               
                   
                 reductase 
               
               
                 ec: 1.1.1.90 
                 aryl-alcohol dehydrogenase 
               
               
                 ec: 1.1.1.91 
                 aryl-alcohol dehydrogenase 
               
               
                   
                 (NADP+) 
               
               
                 ec: 1.1.1.92 
                 oxaloglycolate reductase 
               
               
                   
                 (decarboxylating) 
               
               
                 ec: 1.1.1.94 
                 glycerol-3-phosphate dehydrogenase 
               
               
                   
                 [NAD(P)+] 
               
               
                 ec: 1.1.1.95 
                 phosphoglycerate dehydrogenase 
               
               
                 ec: 1.1.1.97 
                 3-hydroxybenzyl-alcohol 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.101 
                 acylglycerone-phosphate reductase 
               
               
                 ec: 1.1.1.103 
                 L-threonine 3-dehydrogenase 
               
               
                 ec: 1.1.1.104 
                 4-oxoproline reductase 
               
               
                 ec: 1.1.1.105 
                 retinol dehydrogenase 
               
               
                 ec: 1.1.1.110 
                 indolelactate dehydrogenase 
               
               
                 ec: 1.1.1.112 
                 indanol dehydrogenase 
               
               
                 ec: 1.1.1.113 
                 L-xylose 1-dehydrogenase 
               
               
                 ec: 1.1.1.129 
                 L-threonate 3-dehydrogenase 
               
               
                 ec: 1.1.1.137 
                 ribitol-5-phosphate 2- 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.138 
                 mannitol 2-dehydrogenase 
               
               
                   
                 (NADP+) 
               
               
                 ec: 1.1.1.140 
                 sorbitol-6-phosphate 2- 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.142 
                 D-pinitol dehydrogenase 
               
               
                 ec: 1.1.1.143 
                 sequoyitol dehydrogenase 
               
               
                 ec: 1.1.1.144 
                 perillyl-alcohol dehydrogenase 
               
               
                 ec: 1.1.1.156 
                 glycerol 2-dehydrogenase 
               
               
                   
                 (NADP+) 
               
               
                 ec: 1.1.1.157 
                 3-hydroxybutyryl-CoA 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.163 
                 cyclopentanol dehydrogenase 
               
               
                 ec: 1.1.1.164 
                 hexadecanol dehydrogenase 
               
               
                 ec: 1.1.1.165 
                 2-alkyn-1-ol dehydrogenase 
               
               
                 ec: 1.1.1.166 
                 hydroxycyclohexanecarboxylate 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.167 
                 hydroxymalonate dehydrogenase 
               
               
                 ec: 1.1.1.174 
                 cyclohexane-1,2-diol 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.177 
                 glycerol-3-phosphate 1- 
               
               
                   
                 dehydrogenase (NADP+) 
               
               
                 ec: 1.1.1.178 
                 3-hydroxy-2-methylbutyryl-CoA 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.185 
                 L-glycol dehydrogenase 
               
               
                 ec: 1.1.1.190 
                 indole-3-acetaldehyde reductase 
               
               
                   
                 (NADH) 
               
               
                 ec: 1.1.1.191 
                 indole-3-acetaldehyde reductase 
               
               
                   
                 (NADPH) 
               
               
                 ec: 1.1.1.192 
                 long-chain-alcohol dehydrogenase 
               
               
                 ec: 1.1.1.194 
                 coniferyl-alcohol dehydrogenase 
               
               
                 ec: 1.1.1.195 
                 cinnamyl-alcohol dehydrogenase 
               
               
                 ec: 1.1.1.198 
                 (+)-borneol dehydrogenase 
               
               
                 ec: 1.1.1.202 
                 1,3-propanediol dehydrogenase 
               
               
                 ec: 1.1.1.207 
                 (−)-menthol dehydrogenase 
               
               
                 ec: 1.1.1.208 
                 (+)-neomenthol dehydrogenase 
               
               
                 ec: 1.1.1.216 
                 farnesol dehydrogenase 
               
               
                 ec: 1.1.1.217 
                 benzyl-2-methyl-hydroxybutyrate 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.222 
                 (R)-4-hydroxyphenyllactate 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.223 
                 isopiperitenol dehydrogenase 
               
               
                 ec: 1.1.1.226 
                 4-hydroxycyclohexanecarboxylate 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.229 
                 diethyl 2-methyl-3-oxosuccinate 
               
               
                   
                 reductase 
               
               
                 ec: 1.1.1.237 
                 hydroxyphenylpyruvate reductase 
               
               
                 ec: 1.1.1.244 
                 methanol dehydrogenase 
               
               
                 ec: 1.1.1.245 
                 cyclohexanol dehydrogenase 
               
               
                 ec: 1.1.1.250 
                 D-arabinitol 2-dehydrogenase 
               
               
                 ec: 1.1.1.251 
                 galactitol 1-phosphate 5- 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.255 
                 mannitol dehydrogenase 
               
               
                 ec: 1.1.1.256 
                 fluoren-9-ol dehydrogenase 
               
               
                 ec: 1.1.1.257 
                 4- 
               
               
                   
                 (hydroxymethyl)benzenesulfonate dehydrogenase 
               
               
                 ec: 1.1.1.258 
                 6-hydroxyhexanoate 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.259 
                 3-hydroxypimeloyl-CoA 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.261 
                 glycerol-1-phosphate 
               
               
                   
                 dehydrogenase [NAD(P)+] 
               
               
                 ec: 1.1.1.265 
                 3-methylbutanal reductase 
               
               
                 ec: 1.1.1.283 
                 methylglyoxal reductase (NADPH- 
               
               
                   
                 dependent) 
               
               
                 ec: 1.1.1.286 
                 isocitrate-homoisocitrate 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.1.1.287 
                 D-arabinitol dehydrogenase 
               
               
                   
                 (NADP+) 
               
               
                   
                 butanol dehydrogenase 
               
               
                 ec: 1.2.1.2 
                 formate dehydrogenase 
               
               
                 ec: 1.2.1.3 
                 aldehyde dehydrogenase (NAD+) 
               
               
                 ec: 1.2.1.4 
                 aldehyde dehydrogenase (NADP+) 
               
               
                 ec: 1.2.1.5 
                 aldehyde dehydrogenase 
               
               
                   
                 [NAD(P)+] 
               
               
                 ec: 1.2.1.7 
                 benzaldehyde dehydrogenase 
               
               
                   
                 (NADP+) 
               
               
                 ec: 1.2.1.8 
                 betaine-aldehyde dehydrogenase 
               
               
                 ec: 1.2.1.9 
                 glyceraldehyde-3-phosphate 
               
               
                   
                 dehydrogenase (NADP+) 
               
               
                 ec: 1.2.1.10 
                 acetaldehyde dehydrogenase 
               
               
                   
                 (acetylating) 
               
               
                 ec: 1.2.1.11 
                 aspartate-semialdehyde 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.2.1.12 
                 glyceraldehyde-3-phosphate 
               
               
                   
                 dehydrogenase (phosphorylating) 
               
               
                 ec: 1.2.1.13 
                 glyceraldehyde-3-phosphate 
               
               
                   
                 dehydrogenase (NADP+) (phosphorylating) 
               
               
                 ec: 1.2.1.15 
                 malonate-semialdehyde 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.2.1.16 
                 succinate-semialdehyde 
               
               
                   
                 dehydrogenase [NAD(P)+] 
               
               
                 ec: 1.2.1.17 
                 glyoxylate dehydrogenase 
               
               
                   
                 (acylating) 
               
               
                 ec: 1.2.1.18 
                 malonate-semialdehyde 
               
               
                   
                 dehydrogenase (acetylating) 
               
               
                 ec: 1.2.1.19 
                 aminobutyraldehyde 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.2.1.20 
                 glutarate-semialdehyde 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.2.1.21 
                 glycolaldehyde dehydrogenase 
               
               
                 ec: 1.2.1.22 
                 lactaldehyde dehydrogenase 
               
               
                 ec: 1.2.1.23 
                 2-oxoaldehyde dehydrogenase 
               
               
                   
                 (NAD+) 
               
               
                 ec: 1.2.1.24 
                 succinate-semialdehyde 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.2.1.25 
                 2-oxoisovalerate dehydrogenase 
               
               
                   
                 (acylating) 
               
               
                 ec: 1.2.1.26 
                 2,5-dioxovalerate dehydrogenase 
               
               
                 ec: 1.2.1.27 
                 methylmalonate-semialdehyde 
               
               
                   
                 dehydrogenase (acylating) 
               
               
                 ec: 1.2.1.28 
                 benzaldehyde dehydrogenase 
               
               
                   
                 (NAD+) 
               
               
                 ec: 1.2.1.29 
                 aryl-aldehyde dehydrogenase 
               
               
                 ec: 1.2.1.30 
                 aryl-aldehyde dehydrogenase 
               
               
                   
                 (NADP+) 
               
               
                 ec: 1.2.1.31 
                 L-aminoadipate-semialdehyde 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.2.1.32 
                 aminomuconate-semialdehyde 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.2.1.36 
                 retinal dehydrogenase 
               
               
                 ec: 1.2.1.39 
                 phenylacetaldehyde dehydrogenase 
               
               
                 ec: 1.2.1.41 
                 glutamate-5-semialdehyde 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.2.1.42 
                 hexadecanal dehydrogenase 
               
               
                   
                 (acylating) 
               
               
                 ec: 1.2.1.43 
                 formate dehydrogenase (NADP+) 
               
               
                 ec: 1.2.1.45 
                 4-carboxy-2-hydroxymuconate-6- 
               
               
                   
                 semialdehyde dehydrogenase 
               
               
                 ec: 1.2.1.46 
                 formaldehyde dehydrogenase 
               
               
                 ec: 1.2.1.47 
                 4-trimethylammoniobutyraldehyde 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.2.1.48 
                 long-chain-aldehyde 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.2.1.49 
                 2-oxoaldehyde dehydrogenase 
               
               
                   
                 (NADP+) 
               
               
                 ec: 1.2.1.51 
                 pyruvate dehydrogenase (NADP+) 
               
               
                 ec: 1.2.1.52 
                 oxoglutarate dehydrogenase 
               
               
                   
                 (NADP+) 
               
               
                 ec: 1.2.1.53 
                 4-hydroxyphenylacetaldehyde 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.2.1.57 
                 butanal dehydrogenase 
               
               
                 ec: 1.2.1.58 
                 phenylglyoxylate dehydrogenase 
               
               
                   
                 (acylating) 
               
               
                 ec: 1.2.1.59 
                 glyceraldehyde-3-phosphate 
               
               
                   
                 dehydrogenase (NAD(P)+) (phosphorylating) 
               
               
                 ec: 1.2.1.62 
                 4-formylbenzenesulfonate 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.2.1.63 
                 6-oxohexanoate dehydrogenase 
               
               
                 ec: 1.2.1.64 
                 4-hydroxybenzaldehyde 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.2.1.65 
                 salicylaldehyde dehydrogenase 
               
               
                 ec: 1.2.1.66 
                 mycothiol-dependent formaldehyde 
               
               
                   
                 dehydrogenase 
               
               
                 ec: 1.2.1.67 
                 vanillin dehydrogenase 
               
               
                 ec: 1.2.1.68 
                 coniferyl-aldehyde dehydrogenase 
               
               
                 ec: 1.2.1.69 
                 fluoroacetaldehyde dehydrogenase 
               
               
                 ec: 1.2.1.71 
                 succinylglutamate-semialdehyde 
               
               
                   
                 dehydrogenase 
               
               
                   
               
            
           
         
       
     
     Pathways other than those exemplified above also can be employed to generate the biosynthesis of BDO in non-naturally occurring microbial organisms. In one embodiment, biosynthesis can be achieved using a L-homoserine to BDO pathway. This pathway has a molar yield of 0.90 mol/mol glucose, which appears restricted by the availability of reducing equivalents. A second pathway synthesizes BDO from acetoacetate and is capable of achieving the maximum theoretical yield of 1.091 mol/mol glucose. Implementation of either pathway can be achieved by introduction of two exogenous enzymes, and both pathways can additionally complement BDO production via succinyl-CoA. Pathway enzymes, thermodynamics, theoretical yields and overall feasibility are described further below. 
     A homoserine pathway also can be engineered to generate BDO-producing microbial organisms. Homoserine is an intermediate in threonine and methionine metabolism, formed from oxaloacetate via aspartate. The conversion of oxaloacetate to homoserine requires one NADH, two NADPH, and one ATP ( FIG.  3   ). Once formed, homoserine feeds into biosynthetic pathways for both threonine and methionine. In most organisms, high levels of threonine or methionine feedback to repress the homoserine biosynthesis pathway (Caspi et al.,  Nucleic Acids Res.  34:D511-D516 (1990)). 
     The transformation of homoserine to 4-hydroxybutyrate (4-HB) can be accomplished in two enzymatic steps as shown in  FIG.  4   . The first step of this pathway is deamination of homoserine by a putative ammonia lyase. This reaction has an estimated thermodynamic barrier of 12 kJ/mol, but can likely be driven in the forward direction by a concentration gradient. In step  2 , the product alkene, 4-hydroxybut-2-enoate is reduced to 4-HB by a putative reductase at the cost of one NADH. This reaction step is highly thermodynamically favorable in the direction of 4-HB synthesis, with an estimated ΔrxnG of −59 kJ/mol. 4-HB can then be converted to BDO as in  FIG.  2    above. 
     Enzymes available for catalyzing the above transformations are shown in  FIG.  5   . For example, the ammonia lyase in step  1  of the pathway closely resembles the chemistry of aspartate ammonia-lyase (aspartase). Aspartase is a widespread enzyme in microorganisms, and has been characterized extensively (Viola, R. E.,  Mol. Biol.  74:295-341 (2008)). The crystal structure of the  E. coli  aspartase has been solved (Shi et al.,  Biochemistry  36:9136-9144 (1997)), so it is therefore possible to directly engineer mutations in the enzyme&#39;s active site that would alter its substrate specificity to include homoserine. The oxidoreductase in step  2  has chemistry similar to several well-characterized enzymes including fumarate reductase in the  E. coli  TCA cycle. Since the thermodynamics of this reaction are highly favorable, an endogenous reductase with broad substrate specificity will likely be able to reduce 4-hydroxybut-2-enoate. The yield of this pathway under anaerobic conditions is 0.9 mol BDO per mol glucose although, when compared to the pathway in  FIG.  2    (1.09 mol/mol glucose), both pathways appear to have similar energetic and reductive requirements from the metabolic precursor oxaloacetate ( FIG.  6   ). 
     The succinyl-CoA pathway was found to have a higher yield due to the fact that it is more energetically efficient. The conversion of one oxaloacetate molecule to BDO via the homoserine pathway will require the expenditure of 2 ATP equivalents. Because the conversion of glucose to two oxaloacetate molecules can generate a maximum of 3 ATP molecules assuming PEP carboxykinase to be reversible, the overall conversion of glucose to BDO via homoserine has a negative energetic yield. As expected, if we assume that energy can be generated via respiration, the maximum yield of the homoserine pathway increases to 1.05 mol/mol glucose which is 96% of the succinyl-CoA pathway yield. The succinyl-CoA pathway can channel some of the carbon flux through pyruvate dehydrogenase and the oxidative branch of the TCA cycle to generate both reducing equivalents and succinyl-CoA without an energetic expenditure. Thus, it does not encounter the same energetic difficulties as the homoserine pathway because not all of the flux is channeled through oxaloacetate to succinyl-CoA to BDO. Overall, the homoserine pathway demonstrates a moderately high-yielding route to BDO. One particularly useful characteristic is that it involves minimal engineering, with only two non-native steps. The pathway is likely to be thermodynamically favorable in the direction of BDO synthesis. 
     An acetoacetate pathway also can be engineered to generate BDO-producing microbial organisms. In  E. coli  acetoacetate is produced from acetone and leucine degradation. Acetoacetate also can be formed from acetyl-CoA by enzymes involved in fatty acid metabolism, including acetyl-CoA acetyltransferase and acetoacetyl-CoA transferase ( FIG.  7   ). Biosynthetic routes through acetoacetate are also particularly useful in microbial organisms that can metabolize single carbon compounds to form acetyl-CoA. 
     A three step route from acetoacetate to succinic semialdehyde ( FIG.  8   ) can be used to synthesize BDO through acetoacetate. Succinic semialddehyde, which is one reduction step removed from succinyl-CoA or one decarboxylation step removed from α-ketoglutarate, can be converted to BDO following three reductions steps ( FIG.  2   ). Briefly, step  1  of the acetoacetate biopathway entails conversion of acetoacetate to 3-aminobutanoate by an ω-aminotransferase. The ω-amino acid:pyruvate aminotransferase (ω-APT) from  Alcaligens denitrificans  was overexpressed in  E. coli  and shown to have a high activity toward 3-aminobutanoate in vitro (Yun et al.,  Appl. Environ. Microbiol.  70:2529-2534 (2004)). The activity of ω-APT in the direction required here was not measured in this study, due to spontaneous decomposition of acetoacetate to acetone in the reaction mixture. However, the thermodynamics indicate that it is feasible. 
     In step  2 , a putative aminomutase shifts the amine group from the 3- to the 4-position of the carbon backbone. An aminomutase performing this function on 3-aminobutanoate has not been characterized, but an enzyme from  Clostridium sticklandii  has a very similar mechanism ( FIG.  9   ). The enzyme, D-lysine-5,6-aminomutase, is involved in lysine biosynthesis. 
     The synthetic route to BDO from acetoacetate passes through 4-aminobutanoate, a metabolite in  E. coli  that&#39;s normally formed from decarboxylation of glutamate. Once formed, 4-aminobutanoate can be converted to succinic semialdehyde by 4-aminobutanoate transaminase (2.6.1.19), an enzyme which has been biochemically characterized. The thermodynamics of this enzyme and other steps of the pathway are close to equilibrium, so the operation of enzymes in the direction of interest is likely to be driven by substrate and product concentrations 
     One consideration for selecting candidate enzymes in this pathway is the stereoselectivity of the enzymes involved in the first two steps. The ω-ABT in  Alcaligens denitrificans  is specific to the L-stereoisomer of 3-aminobutanoate, while D-lysine-5,6-aminomutase likely requires the D-stereoisomer. If enzymes with complementary stereoselectivity can&#39;t be found or engineered, it would be necessary to add a third enzyme to the pathway with racemase activity that can convert L-3-aminobutanoate to D-3-aminobutanoate. While amino acid racemases are widespread, whether these enzymes can function on ω-amino acids is not known. 
     The maximum theoretical molar yield of this pathway under anaerobic conditions is 1.091 mol/mol glucose. In order to generate flux from acetoacetate to BDO it was necessary to assume that acetyl-CoA:acetoacetyl-CoA transferase (enzyme 3 in  FIG.  10   ) is reversible. The function of this enzyme in  E. coli  is to metabolize short-chain fatty acids by first converting them into thioesters. 
     While the operation of acetyl-CoA:acetoacetyl-CoA transferase in the acetate-consuming direction has not been demonstrated experimentally in  E. coli , studies on similar enzymes in other organisms support the assumption that this reaction is reversible. The enzyme butyryl-CoA:acetate:CoA transferase in gut microbes  Roseburia  sp. and  F. prasnitzii  operates in the acetate utilizing direction to produce butyrate (Duncan et al.,  Appl. Environ. Microbiol  68:5186-5190 (2002)). Another very similar enzyme, acetyl:succinate CoA-transferase in  Trypanosoma brucei , also operates in the acetate utilizing direction. This reaction has a ΔrxnG close to equilibrium, so high concentrations of acetate can likely drive the reaction in the direction of interest. At the maximum theoretical BDO production rate of 1.09 mol/mol glucose simulations predict that  E. coli  can generate 1.098 mol ATP per mol glucose with no fermentation byproducts. This ATP yield should be sufficient for cell growth, maintenance, and production. The acetoacetate biopathway is a high-yielding route to BDO from acetyl-CoA. Like the homoserine pathway, this pathway requires minimal strain engineering, with only two non-native steps in addition to the BDO pathway. 
     Therefore, in addition to any of the various modifications exemplified previously for establishing 4-HB biosynthesis in a selected host, the BDO producing microbial organisms can include any of the previous combinations and permutations of 4-HB pathway metabolic modifications as well as any combination of expression for CoA-independent aldehyde dehydrogenase, CoA-dependent aldehyde dehydrogenase or an alcohol dehydrogenase to generate biosynthetic pathways for GBL and/or BDO. Therefore, the BDO producers of the invention can have exogenous expression of, for example, one, two, three, four, five, six, seven, eight, nine or all 10 enzymes corresponding to any of the six 4-HB pathway and/or any of the 4 BDO pathway enzymes. 
     Design and construction of the genetically modified microbial organisms is carried out using methods well known in the art to achieve sufficient amounts of expression to produce BDO. In particular, the non-naturally occurring microbial organisms of the invention can achieve biosynthesis of BDO resulting in intracellular concentrations between about 0.1-25 mM or more. Generally, the intracellular concentration of BDO is between about 3-20 mM, particularly between about 5-15 mM and more particularly between about 8-12 mM, including about 10 mM or more. Intracellular concentrations between and above each of these exemplary ranges also can be achieved from the non-naturally occurring microbial organisms of the invention. As with the 4-HB producers, the BDO producers also can be sustained, cultured or fermented under anaerobic conditions. 
     The invention further provides a method for the production of 4-HB. The method includes culturing a non-naturally occurring microbial organism having a 4-hydroxybutanoic acid (4-HB) biosynthetic pathway comprising at least one exogenous nucleic acid encoding 4-hydroxybutanoate dehydrogenase, CoA-independent succinic semialdehyde dehydrogenase, succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase, glutamate:succinic semialdehyde transaminase, α-ketoglutarate decarboxylase, or glutamate decarboxylase under substantially anaerobic conditions for a sufficient period of time to produce monomeric 4-hydroxybutanoic acid (4-TB). The method can additionally include chemical conversion of 4-HB to GBL and to BDO or THF, for example. 
     Additionally provided is a method for the production of 4-HB. The method includes culturing a non-naturally occurring microbial organism having a 4-hydroxybutanoic acid (4-HB) biosynthetic pathway including at least one exogenous nucleic acid encoding 4-hydroxybutanoate dehydrogenase, succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase or α-ketoglutarate decarboxylase under substantially anaerobic conditions for a sufficient period of time to produce monomeric 4-hydroxybutanoic acid (4-HB). The 4-HB product can be secreted into the culture medium. 
     Further provided is a method for the production of BDO. The method includes culturing a non-naturally occurring microbial biocatalyst, comprising a microbial organism having 4-hydroxybutanoic acid (4-HB) and 1,4-butanediol (BDO) biosynthetic pathways, the pathways including at least one exogenous nucleic acid encoding 4-hydroxybutanoate dehydrogenase, succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase, 4-hydroxybutyrate:CoA transferase, 4-hydroxybutyrate kinase, phosphotranshydroxybutyrylase, α-ketoglutarate decarboxylase, aldehyde dehydrogenase, alcohol dehydrogenase or an aldehyde/alcohol dehydrogenase for a sufficient period of time to produce 1,4-butanediol (BDO). The BDO product can be secreted into the culture medium. 
     It is understood that, in methods of the invention, any of the one or more exogenous nucleic acids can be introduced into a microbial organism to produce a non-naturally occurring microbial organism of the invention. The nucleic acids can be introduced so as to confer, for example, a 4-HB, BDO, THE or GBL biosynthetic pathway onto the microbial organism. Alternatively, encoding nucleic acids can be introduced to produce an intermediate microbial organism having the biosynthetic capability to catalyze some of the required reactions to confer 4-HB, BDO, THE or GBL biosynthetic capability. For example, a non-naturally occurring microbial organism having a 4-HB biosynthetic pathway can comprise at least two exogenous nucleic acids encoding desired enzymes, such as the combination of 4-hydroxybutanoate dehydrogenase and α-ketoglutarate decarboxylase; 4-hydroxybutanoate dehydrogenase and CoA-independent succinic semialdehyde dehydrogenase; 4-hydroxybutanoate dehydrogenase and CoA-dependent succinic semialdehyde dehydrogenase; CoA-dependent succinic semialdehyde dehydrogenase and succinyl-CoA synthetase; succinyl-CoA synthetase and glutamate decarboxylase, and the like. Thus, it is understood that any combination of two or more enzymes of a biosynthetic pathway can be included in a non-naturally occurring microbial organism of the invention. Similarly, it is understood that any combination of three or more enzymes of a biosynthetic pathway can be included in a non-naturally occurring microbial organism of the invention, for example, 4-hydroxybutanoate dehydrogenase, α-ketoglutarate decarboxylase and CoA-dependent succinic semialdehyde dehydrogenase; CoA-independent succinic semialdehyde dehydrogenase and succinyl-CoA synthetase; 4-hydroxybutanoate dehydrogenase, CoA-dependent succinic semialdehyde dehydrogenase and glutamate:succinic semialdehyde transaminase, and so forth, as desired, so long as the combination of enzymes of the desired biosynthetic pathway results in production of the corresponding desired product. 
     Similarly, for example, with respect to any one or more exogenous nucleic acids introduced to confer BDO production, a non-naturally occurring microbial organism having a BDO biosynthetic pathway can comprise at least two exogenous nucleic acids encoding desired enzymes, such as the combination of 4-hydroxybutanoate dehydrogenase and α-ketoglutarate decarboxylase; 4-hydroxybutanoate dehydrogenase and 4-hydroxybutyryl CoA:acetyl-CoA transferase; 4-hydroxybutanoate dehydrogenase and butyrate kinase; 4-hydroxybutanoate dehydrogenase and phosphotransbutyrylase; 4-hydroxybutyryl CoA:acetyl-CoA transferase and aldehyde dehydrogenase; 4-hydroxybutyryl CoA:acetyl-CoA transferase and alcohol dehydrogenase; 4-hydroxybutyryl CoA:acetyl-CoA transferase and an aldehyde/alcohol dehydrogenase, and the like. Thus, it is understood that any combination of two or more enzymes of a biosynthetic pathway can be included in a non-naturally occurring microbial organism of the invention. Similarly, it is understood that any combination of three or more enzymes of a biosynthetic pathway can be included in a non-naturally occurring microbial organism of the invention, for example, 4-hydroxybutanoate dehydrogenase, α-ketoglutarate decarboxylase and 4-hydroxybutyryl CoA:acetyl-CoA transferase; 4-hydroxybutanoate dehydrogenase, butyrate kinase and phosphotransbutyrylase; 4-hydroxybutanoate dehydrogenase, 4-hydroxybutyryl CoA:acetyl-CoA transferase and aldehyde dehydrogenase; 4-hydroxybutyryl CoA:acetyl-CoA transferase, aldehyde dehydrogenase and alcohol dehydrogenase; butyrate kinase, phosphotransbutyrylase and an aldehyde/alcohol dehydrogenase, and the like. Similarly, any combination of four, five or more enzymes of a biosynthetic pathway as disclosed herein can be included in a non-naturally occurring microbial organism of the invention, as desired, so long as the combination of enzymes of the desired biosynthetic pathway results in production of the corresponding desired product. 
     Any of the non-naturally occurring microbial organisms described previously can be cultured to produce and/or secrete the biosynthetic products of the invention. For example, the 4-HB producers can be cultured for the biosynthetic production of 4-HB. The 4-HB can be isolated or be treated as described below to generate GBL, THE and/or BDO. Similarly, the BDO producers can be cultured for the biosynthetic production of BDO. The BDO can be isolated or subjected to further treatments for the chemical synthesis of BDO family compounds such as those downstream compounds exemplified in  FIG.  1   . 
     The growth medium can be, for example, any carbohydrate source which can supply a source of carbon to the non-naturally occurring microorganism. Such sources include, for example, sugars such as glucose, xylose, arabinose, galactose, mannose, fructose and starch. Other sources of carbohydrate include, for example, renewable feedstocks and biomass. Exemplary types of biomasses that can be used as feedstocks in the methods of the invention include cellulosic biomass, hemicellulosic biomass and lignin feedstocks or portions of feedstocks. Such biomass feedstocks contain, for example, carbohydrate substrates useful as carbon sources such as glucose, xylose, arabinose, galactose, mannose, fructose and starch. Given the teachings and guidance provided herein, those skilled in the art will understand that renewable feedstocks and biomass other than those exemplified above also can be used for culturing the microbial organisms of the invention for the production of 4-HB and other compounds of the invention. 
     Accordingly, given the teachings and guidance provided herein, those skilled in the art will understand that a non-naturally occurring microbial organism can be produced that secretes the biosynthesized compounds of the invention when grown on a carbon source such as a carbohydrate. Such compounds include, for example, 4-HB, BDO and any of the intermediates metabolites in the 4-HB pathway, the BDO pathway and/or the combined 4-HB and BDO pathways. All that is required is to engineer in one or more of the enzyme activities shown in  FIG.  2    to achieve biosynthesis of the desired compound or intermediate including, for example, inclusion of some or all of the 4-HB and/or BDO biosynthetic pathways. Accordingly, the invention provides a non-naturally occurring microbial organism that secretes 4-HB when grown on a carbohydrate, secretes BDO when grown on a carbohydrate and/or secretes any of the intermediate metabolites shown in  FIG.  2    when grown on a carbohydrate. The BDO producing microbial organisms of the invention can initiate synthesis from, for example, succinate, succinyl-CoA, α-ketogluterate, succinic semialdehyde, 4-HB, 4-hydroxybutyrylphosphate, 4-hydroxybutyryl-CoA (4-HB-CoA) and/or 4-hydroxybutyraldehyde. 
     In some embodiments, culture conditions include anaerobic or substantially anaerobic growth or maintenance conditions. Exemplary anaerobic conditions have been described previously and are well known in the art. Exemplary anaerobic conditions for fermentation processes are described below in the Examples. Any of these conditions can be employed with the non-naturally occurring microbial organisms as well as other anaerobic conditions well known in the art. Under such anaerobic conditions, the 4-HB and BDO producers can synthesize monomeric 4-HB and BDO, respectively, at intracellular concentrations of 5-10 mM or more as well as all other concentrations exemplified previously. 
     A number of downstream compounds also can be generated for the 4-HB and BDO producing non-naturally occurring microbial organisms of the invention. With respect to the 4-HB producing microbial organisms of the invention, monomeric 4-HB and GBL exist in equilibrium in the culture medium. The conversion of 4-HB to GBL can be efficiently accomplished by, for example, culturing the microbial organisms in acid pH medium. A pH less than or equal to 7.5, in particular at or below pH 5.5, spontaneously converts 4-HB to GBL as illustrated in  FIG.  1   . 
     The resultant GBL can be separated from 4-HB and other components in the culture using a variety of methods well known in the art. Such separation methods include, for example, the extraction procedures exemplified in the Examples as well as methods which include continuous liquid-liquid extraction, pervaporation, membrane filtration, membrane separation, reverse osmosis, electrodialysis, distillation, crystallization, centrifugation, extractive filtration, ion exchange chromatography, size exclusion chromatography, adsorption chromatography, and ultrafiltration. All of the above methods are well known in the art. Separated GBL can be further purified by, for example, distillation. 
     Another down stream compound that can be produced from the 4-HB producing non-naturally occurring microbial organisms of the invention includes, for example, BDO. This compound can be synthesized by, for example, chemical hydrogenation of GBL. Chemical hydrogenation reactions are well known in the art. One exemplary procedure includes the chemical reduction of 4-HB and/or GBL or a mixture of these two components deriving from the culture using a heterogeneous or homogeneous hydrogenation catalyst together with hydrogen, or a hydride-based reducing agent used stoichiometrically or catalytically, to produce 1,4-butanediol. 
     Other procedures well known in the art are equally applicable for the above chemical reaction and include, for example, WO No. 82/03854 (Bradley, et al.), which describes the hydrogenolysis of gamma-butyrolactone in the vapor phase over a copper oxide and zinc oxide catalyst. British Pat. No. 1,230,276, which describes the hydrogenation of gamma-butyrolactone using a copper oxide-chromium oxide catalyst. The hydrogenation is carried out in the liquid phase. Batch reactions also are exemplified having high total reactor pressures. Reactant and product partial pressures in the reactors are well above the respective dew points. British Pat. No. 1,314,126, which describes the hydrogenation of gamma-butyrolactone in the liquid phase over a nickel-cobalt-thorium oxide catalyst. Batch reactions are exemplified as having high total pressures and component partial pressures well above respective component dew points. British Pat. No. 1,344,557, which describes the hydrogenation of gamma-butyrolactone in the liquid phase over a copper oxide-chromium oxide catalyst. A vapor phase or vapor-containing mixed phase is indicated as suitable in some instances. A continuous flow tubular reactor is exemplified using high total reactor pressures. British Pat. No. 1,512,751, which describes the hydrogenation of gamma-butyrolactone to 1,4-butanediol in the liquid phase over a copper oxide-chromium oxide catalyst. Batch reactions are exemplified with high total reactor pressures and, where determinable, reactant and product partial pressures well above the respective dew points. U.S. Pat. No. 4,301,077, which describes the hydrogenation to 1,4-butanediol of gamma-butyrolactone over a Ru—Ni—Co—Zn catalyst. The reaction can be conducted in the liquid or gas phase or in a mixed liquid-gas phase. Exemplified are continuous flow liquid phase reactions at high total reactor pressures and relatively low reactor productivities. U.S. Pat. No. 4,048,196, which describes the production of 1,4-butanediol by the liquid phase hydrogenation of gamma-butyrolactone over a copper oxide-zinc oxide catalyst. Further exemplified is a continuous flow tubular reactor operating at high total reactor pressures and high reactant and product partial pressures. And U.S. Pat. No. 4,652,685, which describes the hydrogenation of lactones to glycols. 
     A further downstream compound that can be produced form the 4-HB producing microbial organisms of the invention includes, for example, THF. This compound can be synthesized by, for example, chemical hydrogenation of GBL. One exemplary procedure well known in the art applicable for the conversion of GBL to THE includes, for example, chemical reduction of 4-HB and/or GBL or a mixture of these two components deriving from the culture using a heterogeneous or homogeneous hydrogenation catalyst together with hydrogen, or a hydride-based reducing agent used stoichiometrically or catalytically, to produce tetrahydrofuran. Other procedures well know in the art are equally applicable for the above chemical reaction and include, for example, U.S. Pat. No. 6,686,310, which describes high surface area sol-gel route prepared hydrogenation catalysts. Processes for the reduction of maleic acid to tetrahydrofuran (THF) and 1,4-butanediol (BDO) and for the reduction of gamma butyrolactone to tetrahydrofuran and 1,4-butanediol also are described. 
     The culture conditions can include, for example, liquid culture procedures as well as fermentation and other large scale culture procedures. As described further below in the Examples, particularly useful yields of the biosynthetic products of the invention can be obtained under anaerobic or substantially anaerobic culture conditions. 
     The invention further provides a method of manufacturing 4-HB. The method includes fermenting a non-naturally occurring microbial organism having a 4-hydroxybutanoic acid (4-HB) biosynthetic pathway comprising at least one exogenous nucleic acid encoding 4-hydroxybutanoate dehydrogenase, CoA-independent succinic semialdehyde dehydrogenase, succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase, glutamate:succinic semialdehyde transaminase, α-ketoglutarate decarboxylase, or glutamate decarboxylase under substantially anaerobic conditions for a sufficient period of time to produce monomeric 4-hydroxybutanoic acid (4-HB), the process comprising fed-batch fermentation and batch separation; fed-batch fermentation and continuous separation, or continuous fermentation and continuous separation. 
     The culture and chemical hydrogenations described above also can be scaled up and grown continuously for manufacturing of 4-HB, GBL, BDO and/or THF. Exemplary growth procedures include, for example, fed-batch fermentation and batch separation; fed-batch fermentation and continuous separation, or continuous fermentation and continuous separation. All of these processes are well known in the art. Employing the 4-HB producers allows for simultaneous 4-HB biosynthesis and chemical conversion to GBL, BDO and/or THF by employing the above hydrogenation procedures simultaneous with continuous cultures methods such as fermentation. Other hydrogenation procedures also are well known in the art and can be equally applied to the methods of the invention. 
     Fermentation procedures are particularly useful for the biosynthetic production of commercial quantities of 4-HB and/or BDO. Generally, and as with non-continuous culture procedures, the continuous and/or near-continuous production of 4-HB or BDO will include culturing a non-naturally occurring 4-HB or BDO producing organism of the invention in sufficient nutrients and medium to sustain and/or nearly sustain growth in an exponential phase. Continuous culture under such conditions can be include, for example, 1 day, 2, 3, 4, 5, 6 or 7 days or more. Additionally, continuous culture can include 1 week, 2, 3, 4 or 5 or more weeks and up to several months. Alternatively, organisms of the invention can be cultured for hours, if suitable for a particular application. It is to be understood that the continuous and/or near-continuous culture conditions also can include all time intervals in between these exemplary periods. 
     Fermentation procedures are well known in the art. Briefly, fermentation for the biosynthetic production of 4-HB, BDO or other 4-HB derived products of the invention can be utilized in, for example, fed-batch fermentation and batch separation; fed-batch fermentation and continuous separation, or continuous fermentation and continuous separation. Examples of batch and continuous fermentation procedures well known in the art are exemplified further below in the Examples. 
     In addition, to the above fermentation procedures using the 4-HB or BDO producers of the invention for continuous production of substantial quantities of monomeric 4-HB and BDO, respectively, the 4-HB producers also can be, for example, simultaneously subjected to chemical synthesis procedures as described previously for the chemical conversion of monomeric 4-HB to, for example, GBL, BDO and/or THF. The BDO producers can similarly be, for example, simultaneously subjected to chemical synthesis procedures as described previously for the chemical conversion of BDO to, for example, THF, GBL, pyrrolidones and/or other BDO family compounds. In addition, the products of the 4-HB and BDO producers can be separated from the fermentation culture and sequentially subjected to chemical conversion, as disclosed herein. 
     Briefly, hydrogenation of GBL in the fermentation broth can be performed as described by Frost et al.,  Biotechnology Progress  18: 201-211 (2002). Another procedure for hydrogenation during fermentation include, for example, the methods described in, for example, U.S. Pat. No. 5,478,952. This method is further exemplified in the Examples below. 
     Therefore, the invention additionally provides a method of manufacturing γ-butyrolactone (GBL), tetrahydrofuran (TIF) or 1,4-butanediol (BDO). The method includes fermenting a non-naturally occurring microbial organism having 4-hydroxybutanoic acid (4-HB) and/or 1,4-butanediol (BDO) biosynthetic pathways, the pathways comprise at least one exogenous nucleic acid encoding 4-hydroxybutanoate dehydrogenase, CoA-independent succinic semialdehyde dehydrogenase, succinyl-CoA synthetase, CoA-dependent succinic semialdehyde dehydrogenase, 4-hydroxybutyrate:CoA transferase, glutamate:succinic semialdehyde transaminase, α-ketoglutarate decarboxylase, glutamate decarboxylase, 4-hydroxybutanoate kinase, phosphotransbutyrylase, CoA-independent 1,4-butanediol semialdehyde dehydrogenase, CoA-dependent 1,4-butanediol semialdehyde dehydrogenase, CoA-independent 1,4-butanediol alcohol dehydrogenase or CoA-dependent 1,4-butanediol alcohol dehydrogenase, under substantially anaerobic conditions for a sufficient period of time to produce 1,4-butanediol (BDO), GBL or THF, the fermenting comprising fed-batch fermentation and batch separation; fed-batch fermentation and continuous separation, or continuous fermentation and continuous separation. 
     In addition to the biosynthesis of 4-HB, BDO and other products of the invention as described herein, the non-naturally occurring microbial organisms and methods of the invention also can be utilized in various combinations with each other and with other microbial organisms and methods well known in the art to achieve product biosynthesis by other routes. For example, one alternative to produce BDO other than use of the 4-HB producers and chemical steps or other than use of the BDO producer directly is through addition of another microbial organism capable of converting 4-HB or a 4-HB product exemplified herein to BDO. 
     One such procedure includes, for example, the fermentation of a 4-HB producing microbial organism of the invention to produce 4-HB, as described above and below. The 4-HB can then be used as a substrate for a second microbial organism that converts 4-HB to, for example, BDO, GBL and/or THF. The 4-HB can be added directly to another culture of the second organism or the original culture of 4-HB producers can be depleted of these microbial organisms by, for example, cell separation, and then subsequent addition of the second organism to the fermentation broth can utilized to produce the final product without intermediate purification steps. One exemplary second organism having the capacity to biochemically utilize 4-HB as a substrate for conversion to BDO, for example, is  Clostridium acetobutylicum  (see, for example, Jewell et al.,  Current Microbiology,  13:215-19 (1986)). 
     In other embodiments, the non-naturally occurring microbial organisms and methods of the invention can be assembled in a wide variety of subpathways to achieve biosynthesis of, for example, 4-HB and/or BDO as described. In these embodiments, biosynthetic pathways for a desired product of the invention can be segregated into different microbial organisms and the different microbial organisms can be co-cultured to produce the final product. In such a biosynthetic scheme, the product of one microbial organism is the substrate for a second microbial organism until the final product is synthesized. For example, the biosynthesis of BDO can be accomplished as described previously by constructing a microbial organism that contains biosynthetic pathways for conversion of a substrate such as endogenous succinate through 4-HB to the final product BDO. Alternatively, BDO also can be biosynthetically produced from microbial organisms through co-culture or co-fermentation using two organisms in the same vessel. A first microbial organism being a 4-HB producer with genes to produce 4-HB from succinic acid, and a second microbial organism being a BDO producer with genes to convert 4-HB to BDO. 
     Given the teachings and guidance provided herein, those skilled in the art will understand that a wide variety of combinations and permutations exist for the non-naturally occurring microbial organisms and methods of the invention together with other microbial organisms, with the co-culture of other non-naturally occurring microbial organisms having subpathways and with combinations of other chemical and/or biochemical procedures well known in the art to produce 4-HB, BDO, GBL and THF products of the invention. 
     One computational method for identifying and designing metabolic alterations favoring biosynthesis of a product is the OptKnock computational framework, Burgard et al.,  Biotechnol Bioeng,  84: 647-57 (2003). OptKnock is a metabolic modeling and simulation program that suggests gene deletion strategies that result in genetically stable microorganisms which overproduce the target product. Specifically, the framework examines the complete metabolic and/or biochemical network of a microorganism in order to suggest genetic manipulations that force the desired biochemical to become an obligatory byproduct of cell growth. By coupling biochemical production with cell growth through strategically placed gene deletions or other functional gene disruption, the growth selection pressures imposed on the engineered strains after long periods of time in a bioreactor lead to improvements in performance as a result of the compulsory growth-coupled biochemical production. Lastly, when gene deletions are constructed there is a negligible possibility of the designed strains reverting to their wild-type states because the genes selected by OptKnock are to be completely removed from the genome. Therefore, this computational methodology can be used to either identify alternative pathways that lead to biosynthesis of 4-HB and/or BDO or used in connection with the non-naturally occurring microbial organisms for further optimization of 4-HB and/or BDO biosynthesis. 
     Briefly, OptKnock is a term used herein to refer to a computational method and system for modeling cellular metabolism. The OptKnock program relates to a framework of models and methods that incorporate particular constraints into flux balance analysis (FBA) models. These constraints include, for example, qualitative kinetic information, qualitative regulatory information, and/or DNA microarray experimental data. OptKnock also computes solutions to various metabolic problems by, for example, tightening the flux boundaries derived through flux balance models and subsequently probing the performance limits of metabolic networks in the presence of gene additions or deletions. OptKnock computational framework allows the construction of model formulations that enable an effective query of the performance limits of metabolic networks and provides methods for solving the resulting mixed-integer linear programming problems. The metabolic modeling and simulation methods referred to herein as OptKnock are described in, for example, U.S. patent application Ser. No. 10/043,440, filed Jan. 10, 2002, and in International Patent No. PCT/US02/00660, filed Jan. 10, 2002. 
     Another computational method for identifying and designing metabolic alterations favoring biosynthetic production of a product is metabolic modeling and simulation system termed SimPheny®. This computational method and system is described in, for example, U.S. patent application Ser. No. 10/173,547, filed Jun. 14, 2002, and in International Patent Application No. PCT/US03/18838, filed Jun. 13, 2003. 
     SimPheny® is a computational system that can be used to produce a network model in silico and to simulate the flux of mass, energy or charge through the chemical reactions of a biological system to define a solution space that contains any and all possible functionalities of the chemical reactions in the system, thereby determining a range of allowed activities for the biological system. This approach is referred to as constraints-based modeling because the solution space is defined by constraints such as the known stoichiometry of the included reactions as well as reaction thermodynamic and capacity constraints associated with maximum fluxes through reactions. The space defined by these constraints can be interrogated to determine the phenotypic capabilities and behavior of the biological system or of its biochemical components. Analysis methods such as convex analysis, linear programming and the calculation of extreme pathways as described, for example, in Schilling et al.,  J. Theor. Biol.  203:229-248 (2000); Schilling et al.,  Biotech. Bioeng.  71:286-306 (2000) and Schilling et al.,  Biotech. Prog.  15:288-295 (1999), can be used to determine such phenotypic capabilities. As described in the Examples below, this computation methodology was used to identify and analyze the feasible as well as the optimal 4-HB biosynthetic pathways in 4-HB non-producing microbial organisms. 
     As described above, one constraints-based method used in the computational programs applicable to the invention is flux balance analysis. Flux balance analysis is based on flux balancing in a steady state condition and can be performed as described in, for example, Varma and Palsson,  Biotech. Bioeng.  12:994-998 (1994). Flux balance approaches have been applied to reaction networks to simulate or predict systemic properties of, for example, adipocyte metabolism as described in Fell and Small,  J. Biochem.  138:781-786 (1986), acetate secretion from  E. coli  under ATP maximization conditions as described in Majewski and Domach,  Biotech. Bioeng.  35:732-738 (1990) or ethanol secretion by yeast as described in Vanrolleghem et al.,  Biotech. Prog.  12:434-448 (1996). Additionally, this approach can be used to predict or simulate the growth of  E. coli  on a variety of single-carbon sources as well as the metabolism of  H. influenzae  as described in Edwards and Palsson,  Proc. Natl. Acad. Sci.  97:5528-5533 (2000), Edwards and Palsson,  J. Bio. Chem.  274:17410-17416 (1999) and Edwards et al.,  Nature Biotech.  19:125-130 (2001). 
     Once the solution space has been defined, it can be analyzed to determine possible solutions under various conditions. This computational approach is consistent with biological realities because biological systems are flexible and can reach the same result in many different ways. Biological systems are designed through evolutionary mechanisms that have been restricted by fundamental constraints that all living systems must face. Therefore, constraints-based modeling strategy embraces these general realities. Further, the ability to continuously impose further restrictions on a network model via the tightening of constraints results in a reduction in the size of the solution space, thereby enhancing the precision with which physiological performance or phenotype can be predicted. 
     Given the teachings and guidance provided herein, those skilled in the art will be able to apply various computational frameworks for metabolic modeling and simulation to design and implement biosynthesis of 4-HB, BDO, GBL, THE and other BDO family compounds in host microbial organisms other than  E. coli  and yeast. Such metabolic modeling and simulation methods include, for example, the computational systems exemplified above as SimPheny® and OptKnock. For illustration of the invention, some methods are described herein with reference to the OptKnock computation framework for modeling and simulation. Those skilled in the art will know how to apply the identification, design and implementation of the metabolic alterations using OptKnock to any of such other metabolic modeling and simulation computational frameworks and methods well known in the art. 
     The ability of a cell or organism to biosynthetically produce a biochemical product can be illustrated in the context of the biochemical production limits of a typical metabolic network calculated using an in silico model. These limits are obtained by fixing the uptake rate(s) of the limiting substrate(s) to their experimentally measured value(s) and calculating the maximum and minimum rates of biochemical production at each attainable level of growth. The production of a desired biochemical generally is in direct competition with biomass formation for intracellular resources. Under these circumstances, enhanced rates of biochemical production will necessarily result in sub-maximal growth rates. The knockouts suggested by the above metabolic modeling and simulation programs such as OptKnock are designed to restrict the allowable solution boundaries forcing a change in metabolic behavior from the wild-type strain. Although the actual solution boundaries for a given strain will expand or contract as the substrate uptake rate(s) increase or decrease, each experimental point will lie within its calculated solution boundary. Plots such as these enable accurate predictions of how close the designed strains are to their performance limits which also indicates how much room is available for improvement. 
     The OptKnock mathematical framework is exemplified herein for pinpointing gene deletions leading to product biosynthesis and, particularly, growth-coupled product biosynthesis. The procedure builds upon constraint-based metabolic modeling which narrows the range of possible phenotypes that a cellular system can display through the successive imposition of governing physico-chemical constraints, Price et al.,  Nat Rev Microbiol,  2: 886-97 (2004). As described above, constraint-based models and simulations are well known in the art and generally invoke the optimization of a particular cellular objective, subject to network stoichiometry, to suggest a likely flux distribution. 
     Briefly, the maximization of a cellular objective quantified as an aggregate reaction flux for a steady state metabolic network comprising a set N={1, . . . , N} of metabolites and a set M={1, . . . , M} of metabolic reactions is expressed mathematically as follows: 
     
       
         
           
             	 
             
               
                 maximize 
                 ⁢ 
                     
                 
                   v 
                   
                     cellular 
                     ⁢ 
                         
                     objective 
                   
                 
               
               ⁢ 
               
 
               	 
               
                 
                   
                     subject 
                     ⁢ 
                         
                     to 
                     ⁢ 
                         
                     
                       
                         ∑ 
                         
                           j 
                           = 
                           1 
                         
                         M 
                       
                       
                         
                           S 
                           ij 
                         
                         ⁢ 
                         
                           v 
                           j 
                         
                       
                     
                   
                   = 
                   0 
                 
                 , 
                 
                   ∀ 
                   
                     i 
                     ∈ 
                   
                 
               
             
           
         
       
       
         
           
             
               v 
               substrate 
             
             = 
             
               
                 v 
                 
                   substrate 
                   ⁢ 
                   _ 
                   ⁢ 
                   uptake 
                 
               
               ⁢ 
                   
               m 
               ⁢ 
               mol 
               / 
               
                 gDW 
                 · 
                 hr 
               
               ⁢ 
                   
               
                 ∀ 
                 
                   i 
                   ∈ 
                   
                     { 
                     
                       limiting 
                       ⁢ 
                           
                       substrate 
                       ⁢ 
                           
                       
                         ( 
                         s 
                         ) 
                       
                     
                     } 
                   
                 
               
             
           
         
       
       
         
           
             	 
             
               
                 
                   v 
                   atp 
                 
                 ≥ 
                 
                   
                     v 
                     
                       atp 
                       ⁢ 
                       _ 
                       ⁢ 
                       main 
                     
                   
                   ⁢ 
                   m 
                   ⁢ 
                   mol 
                   / 
                   
                     gDW 
                     · 
                     hr 
                   
                 
               
               ⁢ 
               
 
               	 
               
                 
                   
                     v 
                     j 
                   
                   ≥ 
                   0 
                 
                 , 
                 
                   ∀ 
                   
                     j 
                     ∈ 
                     
                       { 
                       
                         irrev 
                         . 
                             
                         reactions 
                       
                       } 
                     
                   
                 
               
             
           
         
       
     
     where S ij  is the stoichiometric coefficient of metabolite i in reaction j, v j  is the flux of reaction j, v substrate_uptake  represents the assumed or measured uptake rate(s) of the limiting substrate(s), and v atp_main  is the non-growth associated ATP maintenance requirement. The vector v includes both internal and external fluxes. In this study, the cellular objective is often assumed to be a drain of biosynthetic precursors in the ratios required for biomass formation, Neidhardt, F. C. et al.,  Escherichia coli and Salmonella: Cellular and Molecular Biology,  2nd ed. 1996, Washington, D.C.: ASM Press. 2 v. (xx, 2822, lxxvi). The fluxes are generally reported per 1 gDW·hr (gram of dry weight times hour) such that biomass formation is expressed as g biomass produced/gDW·hr or 1/hr. 
     The modeling of gene deletions, and thus reaction elimination, first employs the incorporation of binary variables into the constraint-based approach framework, Burgard et al.,  Biotechnol Bioeng,  74: 364-375 (2001), Burgard et al.,  Biotechnol Prog,  17: 791-797 (2001). These binary variables, 
     
       
         
           
             
               y 
               j 
             
             = 
             
               { 
               
                 
                   
                     
                       
                         1 
                         , 
                         
                           if 
                           ⁢ 
                               
                           reaction 
                           ⁢ 
                               
                           flux 
                           ⁢ 
                               
                           
                             v 
                             j 
                           
                           ⁢ 
                               
                           is 
                           ⁢ 
                               
                           active 
                         
                       
                     
                   
                   
                     
                       
                         0 
                         , 
                         
                           if 
                           ⁢ 
                               
                           reaction 
                           ⁢ 
                               
                           flux 
                           ⁢ 
                               
                           
                             v 
                             j 
                           
                           ⁢ 
                               
                           is 
                           ⁢ 
                               
                           not 
                           ⁢ 
                               
                           active 
                         
                       
                     
                   
                 
                 , 
                 
                   ∀ 
                   
                     j 
                     ∈ 
                     M 
                   
                 
               
             
           
         
       
     
     assume a value of 1 if reaction j is active and a value of 0 if it is inactive. The following constraint, 
     
       
      
       v 
       j 
       min 
       ·y 
       j 
       ≤v 
       j 
       ≤v 
       j 
       max 
       ·y 
       j 
       , ∀j∈ 
       
         
      
     
     ensures that reaction flux v j  is set to zero only if variable y j  is equal to zero. Alternatively, when y j  is equal to one, v j  is free to assume any value between a lower v j   min  and an upper v j   max  bound. Here, v j   min  and v j   max  are identified by minimizing and maximizing, respectively, every reaction flux subject to the network constraints described above, Mahadevan et al.,  Metab Eng,  5: 264-76 (2003). 
     Optimal gene/reaction knockouts are identified by solving a bilevel optimization problem that chooses the set of active reactions (y j =1) such that an optimal growth solution for the resulting network overproduces the chemical of interest. Mathematically, this bilevel optimization problem is expressed as the following bilevel mixed-integer optimization problem: 
     
       
         
           
             
               
                 maximize 
                 
                   y 
                   j 
                 
               
               ⁢ 
                   
               
                 v 
                 chemical 
               
               ⁢ 
                   
               
                 ( 
                 OptKnock 
                 ) 
               
             
             ⁢ 
             
 
             
               ( 
               
                 
                   
                     
                       
                         subject 
                         ⁢ 
                             
                         to 
                       
                       
                         v 
                         j 
                       
                     
                   
                   
                     maximize 
                   
                   
                     
                       v 
                       biomass 
                     
                   
                   
                       
                   
                 
                 
                   
                       
                   
                   
                     
                       subject 
                       ⁢ 
                           
                       to 
                     
                   
                   
                     
                       
                         
                           
                             ∑ 
                             
                               j 
                               = 
                               1 
                             
                             M 
                           
                           
                             
                               S 
                               ij 
                             
                             ⁢ 
                             
                               v 
                               j 
                             
                           
                         
                         = 
                         0 
                       
                       , 
                     
                   
                   
                     
                       ∀ 
                       
                         i 
                         ∈ 
                       
                     
                   
                 
                 
                   
                       
                   
                   
                       
                   
                   
                     
                       
                         v 
                         substrate 
                       
                       = 
                       
                         v 
                         
                           substrate 
                           ⁢ 
                           _ 
                           ⁢ 
                           uptake 
                         
                       
                     
                   
                   
                     
                       ∀ 
                       
                         i 
                         ∈ 
                         
                           { 
                           
                             limiting 
                             ⁢ 
                                 
                             substrate 
                             ⁢ 
                                 
                             
                               ( 
                               s 
                               ) 
                             
                           
                           } 
                         
                       
                     
                   
                 
                 
                   
                       
                   
                   
                       
                   
                   
                     
                       
                         v 
                         atp 
                       
                       ≥ 
                       
                         v 
                         
                           atp 
                           ⁢ 
                           _ 
                           ⁢ 
                           main 
                         
                       
                     
                   
                   
                       
                   
                 
               
               ) 
             
             ⁢ 
             
 
             
               
                 v 
                 biomass 
               
               ≥ 
               
                 v 
                 biomass 
                 target 
               
             
             ⁢ 
             
 
             
               
                 
                   
                     v 
                     j 
                     
                       m 
                       ⁢ 
                       i 
                       ⁢ 
                       n 
                     
                   
                   · 
                   
                     y 
                     j 
                   
                 
                 ≤ 
                 
                   v 
                   j 
                 
                 ≤ 
                 
                   
                     v 
                     j 
                     
                       m 
                       ⁢ 
                       a 
                       ⁢ 
                       x 
                     
                   
                   · 
                   
                     y 
                     j 
                   
                 
               
               , 
               
                 ∀ 
                 
                   j 
                   ∈ 
                 
               
             
             ⁢ 
             
 
             
               
                 
                   ∑ 
                   
                     j 
                     ∈ 
                     
                       𝒻ℴ𝓇𝓌𝒶𝓇𝒹 
                     
                   
                 
                 
                   ( 
                   
                     1 
                     - 
                     
                       y 
                       j 
                     
                   
                   ) 
                 
               
               = 
               K 
             
             ⁢ 
             
 
             
               
                 
                   y 
                   j 
                 
                 ∈ 
                 
                   { 
                   
                     0 
                     , 
                     1 
                   
                   } 
                 
               
               , 
               
                 ∀ 
                 
                   j 
                   ∈ 
                 
               
             
           
         
       
     
     where v chemical  is the production of the desired target product, for example succinate or other biochemical product, and K is the number of allowable knockouts. Note that setting K equal to zero returns the maximum biomass solution of the complete network, while setting K equal to one identifies the single gene/reaction knockout (y j =0) such that the resulting network involves the maximum overproduction given its maximum biomass yield. The final constraint ensures that the resulting network meets a minimum biomass yield. Burgard et al.,  Biotechnol Bioeng,  84: 647-57 (2003), provide a more detailed description of the model formulation and solution procedure. Problems containing hundreds of binary variables can be solved in the order of minutes to hours using CPLEX 8.0 , GAMS: The Solver Manuals.  2003: GAMS Development Corporation, accessed via the GAMS, Brooke et al.,  GAMS Development Corporation  (1998), modeling environment on an IBM RS6000-270 workstation. The OptKnock framework has already been able to identify promising gene deletion strategies for biochemical overproduction, Burgard et al.,  Biotechnol Bioeng,  84: 647-57 (2003), Pharkya et al.,  Biotechnol Bioeng,  84: 887-899 (2003), and establishes a systematic framework that will naturally encompass future improvements in metabolic and regulatory modeling frameworks. 
     Any solution of the above described bilevel OptKnock problem will provide one set of metabolic reactions to disrupt. Elimination of each reaction within the set or metabolic modification can result in 4-HB or BDO as an obligatory product during the growth phase of the organism. Because the reactions are known, a solution to the bilevel OptKnock problem also will provide the associated gene or genes encoding one or more enzymes that catalyze each reaction within the set of reactions. Identification of a set of reactions and their corresponding genes encoding the enzymes participating in each reaction is generally an automated process, accomplished through correlation of the reactions with a reaction database having a relationship between enzymes and encoding genes. 
     Once identified, the set of reactions that are to be disrupted in order to achieve 4-HB or BDO production are implemented in the target cell or organism by functional disruption of at least one gene encoding each metabolic reaction within the set. One particularly useful means to achieve functional disruption of the reaction set is by deletion of each encoding gene. However, in some instances, it can be beneficial to disrupt the reaction by other genetic aberrations including, for example, mutation, deletion of regulatory regions such as promoters or cis binding sites for regulatory factors, or by truncation of the coding sequence at any of a number of locations. These latter aberrations, resulting in less than total deletion of the gene set can be useful, for example, when rapid assessments of the succinate coupling are desired or when genetic reversion is less likely to occur. 
     To identify additional productive solutions to the above described bilevel OptKnock problem which lead to further sets of reactions to disrupt or metabolic modifications that can result in the biosynthesis, including growth-coupled biosynthesis of 4-HB or other biochemical product, an optimization method, termed integer cuts, can be implemented. This method proceeds by iteratively solving the OptKnock problem exemplified above with the incorporation of an additional constraint referred to as an integer cut at each iteration. Integer cut constraints effectively prevent the solution procedure from choosing the exact same set of reactions identified in any previous iteration that obligatory couples product biosynthesis to growth. For example, if a previously identified growth-coupled metabolic modification specifies reactions 1, 2, and 3 for disruption, then the following constraint prevents the same reactions from being simultaneously considered in subsequent solutions: y 1 +y 2 +y 3 ≥1. The integer cut method is well known in the art and can be found described in, for example, reference, Burgard et al.,  Biotechnol Prog,  17: 791-797 (2001). As with all methods described herein with reference to their use in combination with the OptKnock computational framework for metabolic modeling and simulation, the integer cut method of reducing redundancy in iterative computational analysis also can be applied with other computational frameworks well known in the art including, for example, SimPheny®. 
     Constraints of the above form preclude identification of larger reaction sets that include previously identified sets. For example, employing the integer cut optimization method above in a further iteration would preclude identifying a quadruple reaction set that specified reactions 1, 2, and 3 for disruption since these reactions had been previously identified. To ensure identification of all possible reaction sets leading to biosynthetic production of a product, a modification of the integer cut method can be employed. 
     Briefly, the modified integer cut procedure begins with iteration ‘zero’ which calculates the maximum production of the desired biochemical at optimal growth for a wild-type network. This calculation corresponds to an OptKnock solution with K equaling 0. Next, single knockouts are considered and the two parameter sets, objstore iter  and ystore iter,j , are introduced to store the objective function (v chemical ) and reaction on-off information (y j ), respectively, at each iteration, iter. The following constraints are then successively added to the OptKnock formulation at each iteration. 
     
       
         
           
             
               v 
               chemical 
             
             ≥ 
             
               
                 objstore 
                 iter 
               
               + 
               ε 
               - 
               
                 M 
                 · 
                 
                   
                     ∑ 
                     
                       
                         j 
                         ∈ 
                         
                           ystore 
                           
                             iter 
                             , 
                             j 
                           
                         
                       
                       = 
                       0 
                     
                   
                   
                     y 
                     j 
                   
                 
               
             
           
         
       
     
     In the above equation, ε and M are a small and a large numbers, respectively. In general, ε can be set at about 0.01 and M can be set at about 1000. However, numbers smaller and/or larger then these numbers also can be used. M ensures that the constraint can be binding only for previously identified knockout strategies, while ε ensures that adding knockouts to a previously identified strategy must lead to an increase of at least ε in biochemical production at optimal growth. The approach moves onto double deletions whenever a single deletion strategy fails to improve upon the wild-type strain. Triple deletions are then considered when no double deletion strategy improves upon the wild-type strain, and so on. The end result is a ranked list, represented as desired biochemical production at optimal growth, of distinct deletion strategies that differ from each other by at least one knockout. This optimization procedure as well as the identification of a wide variety of reaction sets that, when disrupted, lead to the biosynthesis, including growth-coupled production, of a biochemical product. Given the teachings and guidance provided herein, those skilled in the art will understand that the methods and metabolic engineering designs exemplified herein are equally applicable to identify new biosynthetic pathways and/or to the obligatory coupling of cell or microorganism growth to any biochemical product. 
     The methods exemplified above and further illustrated in the Examples below enable the construction of cells and organisms that biosynthetically produce, including obligatory couple production of a target biochemical product to growth of the cell or organism engineered to harbor the identified genetic alterations. In this regard, metabolic alterations have been identified that result in the biosynthesis of 4-HB and 1,4-butanediol. Microorganism strains constructed with the identified metabolic alterations produce elevated levels of 4-HB or BDO compared to unmodified microbial organisms. These strains can be beneficially used for the commercial production of 4-HB, BDO, THF and GBL, for example, in continuous fermentation process without being subjected to the negative selective pressures. 
     Therefore, the computational methods described herein enable the identification and implementation of metabolic modifications that are identified by an in silico method selected from OptKnock or SimPheny. The set of metabolic modifications can include, for example, addition of one or more biosynthetic pathway enzymes and/or functional disruption of one or more metabolic reactions including, for example, disruption by gene deletion. 
     It is understood that modifications which do not substantially affect the activity of the various embodiments of this invention are also included within the definition of the invention provided herein. Accordingly, the following examples are intended to illustrate but not limit the present invention. 
     Example I 
     Biosynthesis of 4-Hydroxybutanoic Acid 
     This Example describes the biochemical pathways for 4-HB production. 
     Previous reports of 4-HB synthesis in microbes have focused on this compound as an intermediate in production of the biodegradable plastic poly-hydroxyalkanoate (PHA) (U.S. Pat. No. 6,117,658). The use of 4-HB/3-HB copolymers over poly-3-hydroxybutyrate polymer (PHB) can result in plastic that is less brittle (Saito and Doi,  Intl. J. Biol. Macromol.  16:99-104 (1994)). The production of monomeric 4-HB described herein is a fundamentally distinct process for several reasons: (1) the product is secreted, as opposed to PHA which is produced intracellularly and remains in the cell; (2) for organisms that produce hydroxybutanoate polymers, free 4-HB is not produced, but rather the Coenzyme A derivative is used by the polyhydroxyalkanoate synthase; (3) in the case of the polymer, formation of the granular product changes thermodynamics; and (4) extracellular pH is not an issue for production of the polymer, whereas it will affect whether 4-HB is present in the free acid or conjugate base state, and also the equilibrium between 4-HB and GBL. 
     4-HB can be produced in two enzymatic reduction steps from succinate, a central metabolite of the TCA cycle, with succinic semialdehyde as the intermediate ( FIG.  2   ). The first of these enzymes, succinic semialdehyde dehydrogenase, is native to many organisms including  E. coli , in which both NADH- and NADPH-dependent enzymes have been found (Donnelly and Cooper,  Eur. J. Biochem.  113:555-561 (1981); Donnelly and Cooper,  J. Bacteriol.  145:1425-1427 (1981); Marek and Henson,  J. Bacteriol.  170:991-994 (1988)). There is also evidence supporting succinic semialdehyde dehydrogenase activity in  S. cerevisiae  (Ramos et al.,  Eur. J. Biochem.  149:401-404 (1985)), and a putative gene has been identified by sequence homology. However, most reports indicate that this enzyme proceeds in the direction of succinate synthesis, as shown in  FIG.  2    (Donnelly and Cooper, supra; Lutke-Eversloh and Steinbuchel,  FEMS Microbiol. Lett.  181:63-71 (1999)), participating in the degradation pathway of 4-HB and gamma-aminobutyrate. Succinic semialdehyde also is natively produced by certain microbial organisms such as  E. coli  through the TCA cycle intermediate α-ketogluterate via the action of two enzymes: glutamate:succinic semialdehyde transaminase and glutamate decarboxylase. An alternative pathway, used by the obligate anaerobe  Clostridium kluyveri  to degrade succinate, activates succinate to succinyl-CoA, then converts succinyl-CoA to succinic semialdehyde using an alternative succinic semialdehyde dehydrogenase which is known to function in this direction (Sohling and Gottschalk,  Eur. J. Biochem.  212:121-127 (1993)). However, this route has the energetic cost of ATP required to convert succinate to succinyl-CoA. 
     The second enzyme of the pathway, 4-hydroxybutanoate dehydrogenase, is not native to  E. coli  or yeast but is found in various bacteria such as  C. kluyveri  and  Ralstonia eutropha  (Lutke-Eversloh and Steinbuchel, supra; Sohling and Gottschalk,  J. Bacteriol.  178:871-880 (1996); Valentin et al.,  Eur. J. Biochem.  227:43-60 (1995); Wolff and Kenealy,  Protein Expr. Purif  6:206-212 (1995)). These enzymes are known to be NADH-dependent, though NADPH-dependent forms also exist. An additional pathway to 4-HB from alpha-ketoglutarate was demonstrated in  E. coli  resulting in the accumulation of poly(4-hydroxybutyric acid) (Song et al.,  Wei Sheng Wu Xue. Bao.  45:382-386 (2005)). The recombinant strain required the overexpression of three heterologous genes, PHA synthase ( R. eutropha ), 4-hydroxybutyrate dehydrogenase ( R. eutropha ) and 4-hydroxybutyrate:CoA transferase ( C. kluyveri ), along with two native  E. coli  genes: glutamate:succinic semialdehyde transaminase and glutamate decarboxylase. Steps  4  and  5  in  FIG.  2    can alternatively be carried out by an alpha-ketoglutarate decarboxylase such as the one identified in  Euglena gracilis  (Shigeoka et al.,  Biochem. J.  282(Pt2):319-323 (1992); Shigeoka and Nakano,  Arch. Biochem. Biophys.  288:22-28 (1991); Shigeoka and Nakano,  Biochem J.  292(Pt 2):463-467 (1993)). However, this enzyme has not previously been applied to impact the production of 4-HB or related polymers in any organism. 
     The reported directionality of succinic semialdehyde dehydrogenase led to the investigation of the thermodynamics of 4-HB metabolism. Specifically, this study investigated whether or not the reactions involved in the conversion of succinate or succinyl-CoA to 4-HB are thermodynamically favorable (i.e., ΔG r &lt;0) under the typical physiological conditions present in  E. coli  and  S. cerevisiae . All oxidation/reduction reactions were assumed to utilize NADH, although the results for assuming NADPH utilization would be similar. Standard Gibbs free energies of formation (ΔG f   o ) were calculated for each compound in the succinate and succinyl-CoA pathways shown in  FIG.  2    based on the group contribution method (Mavrovouniotis, M. L.,  J. Biol. Chem.  266:14440-14445 (1991)). Each standard Gibbs energy of formation was then transformed in order to obtain a criterion of spontaneous change at specified pressure, temperature, pH, and ionic strength (Alberty, R. A.,  Biochem. Biophys. Acta  1207:1-11 (1994)) (equation 1). 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                       
                         G 
                         f 
                         ′ 
                       
                       ( 
                       
                         I 
                         , 
                         pH 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       Δ 
                       ⁢ 
                       
                         
                           G 
                           f 
                           o 
                         
                         ( 
                         
                           I 
                           = 
                           0 
                         
                         ) 
                       
                     
                     + 
                     
                       
                         N 
                         H 
                       
                       ⁢ 
                       RT 
                       ⁢ 
                       
                         ln 
                         ⁡ 
                         ( 
                         
                           10 
                           
                             p 
                             ⁢ 
                             H 
                           
                         
                         ) 
                       
                     
                     - 
                     
                       2.915 
                       
                         I 
                       
                       ⁢ 
                       
                         
                           
                             z 
                             2 
                           
                           - 
                           
                             N 
                             H 
                           
                         
                         
                           1 
                           + 
                           
                             B 
                             ⁢ 
                             
                               I 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Where ΔG f   o  is the standard Gibbs energy of formation, N H  is the number of hydrogen atoms in the compound, R is the universal gas constant, T is constant at 298K, z is the charge of the molecule at the pH of interest, I is the ionic strength in M, and B is a constant equal to 1.6 L 0.5 /mol 0.5 . 
     Equation 1 reveals that both intracellular pH and ionic strength play a role in determining thermodynamic feasibility. Normally, intracellular pH of cells is very well regulated, even when there are large variations in the culture pH. The intracellular pH of  E. coli  and  S. cerevisiae  have both been reported in the literature.  E. coli  maintains an intracellular pH of 7.4-7.7 during typical growth conditions in neutral buffers, but can drop to 7.2 in pH 6 medium, and even go as low as 6.9 for external pH of 5 (Riondet et al.,  Biotechnology Tech.  11:735-738 (1997)). However, growth of  E. coli  is severely inhibited at external pH below 6. Yeast pH exhibits more variation. During exponential growth phase,  S. cerevisiae  internal pH has been measured to be in the range of 6.7-7.0 with external pH controlled at 5.0 (Dombek and Ingram,  Appl. Environ. Microbiol.  53:1286-1291 (1987)). On the other hand, in resting cells the internal pH drops to below 6 when the external pH is 6 or less (Imai and Ohno,  J. Biotechnol.  38:165-172 (1995)). This analysis assumes an intracellular pH of 7.4 for  E. coli  and 6.8 for  S. cerevisiae . An ionic strength of 0.15 also was assumed (Valenti et al., supra). 
     Transformed Gibbs energies of formation were calculated at the standard state (pH=7.0, I=0) and at physiological states of  E. coli  (pH=7.4, I=0.15) and  S. cerevisiae  (pH=6.8, I=0.15). Transformed Gibbs energies of reaction (ΔG r ′) were then calculated by taking the difference in ΔG f ′ between the products and reactants. The transformed Gibbs energies of the reactions necessary to convert succinate or succinyl-CoA to 4-HB are provided in Table 2. Although some of the steps have calculated positive delta G values, the standard errors for these calculations and concentration gradients indicate that any of the steps are feasible. Note that the standard error, U f,est , on ΔG f  calculated by the group contribution theory is 4 kcal/mol. The uncertainty in ΔG r , U r,est , can be calculated as the Euclidean norm of the uncertainty for ΔG f  of each compound (Equation). 
     
       
         
           
             
               
                 
                   
                     U 
                     
                       r 
                       , 
                       est 
                     
                   
                   = 
                   
                     
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           m 
                         
                         
                           
                             n 
                             i 
                             2 
                           
                           * 
                           
                             U 
                             
                               f 
                               , 
                               est 
                             
                             2 
                           
                         
                       
                     
                     = 
                     
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           m 
                         
                         
                           16 
                           ⁢ 
                           
                             n 
                             i 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Where n is the stoichiometric coefficient and i is the compound. For the examined reactions, this uncertainty is on the order of 8 kcal/mol. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Gibbs free energy of reaction (kcal/mole) at different pH and ionic 
               
               
                 strength values. The first column is under standard conditions, while 
               
               
                 the others are adjusted according to equation 1. Temperature is constant 
               
               
                 at 298 K. Error bars for these values are on the order of 8 kcal/mol, 
               
               
                 as calculated by equation 2. Abbreviations: suc, succinate; sucsa, succinic 
               
               
                 semialdehyde; succoa, succinyl-CoA; Pi, inorganic phosphate. 
               
            
           
           
               
               
               
               
            
               
                   
                 ΔG r °′ 
                 ΔG r ′ 
                 ΔG r ′ 
               
               
                   
                 pH = 7.0 
                 pH = 7.4 
                 pH = 6.8 
               
               
                 Reaction 
                 IS = 0 
                 IS = 0.15M 
                 IS = 0.15M 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 succ + NADH + 2 H+ → 
                 12.0 
                 14.4 
                 12.8 
               
               
                 sucsa + NAD + h2o 
                   
                   
                   
               
               
                 succ + coa + ATP → 
                 0.30 
                 −0.03 
                 −0.03 
               
               
                 succoa + ADP + Pi 
                   
                   
                   
               
               
                 succoa + NADH + H+ → 
                 4.4 
                 7.0 
                 6.2 
               
               
                 sucsa + NAD + coa 
                   
                   
                   
               
               
                 sucsa + NADH + H+ → 
                 −5.0 
                 −3.8 
                 −4.6 
               
               
                 4-HB + NAD 
               
               
                   
               
            
           
         
       
     
     Table 2 reveals that the reaction most likely to encounter a thermodynamic barrier after considering potential uncertainty in our calculations is succinic semialdehyde dehydrogenase (step  1  in  FIG.  2   ). Whether this reaction can be driven closer to thermodynamic feasibility by varying the assumed concentrations of the participating metabolites also was studied. For example, the standard Gibbs energies assume concentrations of 1 M for all participating compounds (except water). In an anaerobic environment, NADH will be present at a several-fold higher concentration than NAD. Assuming [NADH]=5×[NAD], we calculated the effect on ΔG r ′ using the equation 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                       G 
                       f 
                       ′ 
                     
                   
                   = 
                   
                     
                       Δ 
                       ⁢ 
                       
                         G 
                         f 
                         
                           0 
                           ′ 
                         
                       
                     
                     + 
                     
                       RT 
                       ⁢ 
                       ln 
                       ⁢ 
                       
                         
                           ∏ 
                           
                             [ 
                             prod 
                             ] 
                           
                         
                         
                           ∏ 
                           
                             [ 
                             react 
                             ] 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     This change results in a difference of about 1 kcal/mol in the delta G values for succinic semialdehyde dehydrogenase. Equation 3 was also used to calculate other effects on ΔG r , such as high succinate concentration to drive the reactions. A 1000-fold difference in the concentrations of succinate and succinic semialdehyde will contribute about 5 kcal/mol to delta G. Taken together with an assumed uncertainty of 8 kcal/mol, the possibility that succinic semialdehyde dehydrogenase will operate in the direction towards succinic semialdehyde under some set of physiological conditions cannot be eliminated. Thus, the direct route from succinate to 4-HB remains a consideration in subsequent analysis. 
     The microbial production capabilities of 4-hydroxybutyrate were explored in two microbes,  Escherichia coli  and  Saccharomyces cerevisiae , using in silico metabolic models of each organism. Potential pathways to 4-HB proceed via a succinate, succinyl-CoA, or alpha-ketoglutarate intermediate as shown in  FIG.  2   . 
     A first step in the 4-HB production pathway from succinate involves the conversion of succinate to succinic semialdehyde via an NADH- or NADPH-dependent succinic semialdehyde dehydrogenase. In  E. coli , gabD is an NADP-dependent succinic semialdehyde dehydrogenase and is part of a gene cluster involved in 4-aminobutyrate uptake and degradation (Niegemann et al.,  Arch. Microbiol.  160:454-460 (1993); Schneider et al.,  J. Bacteriol.  184:6976-6986 (2002)). sad is believed to encode the enzyme for NAD-dependent succinic semialdehyde dehydrogenase activity (Marek and Henson, supra).  S. cerevisiae  contains only the NADPH-dependent succinic semialdehyde dehydrogenase, putatively assigned to UGA2, which localizes to the cytosol (Huh et al.,  Nature  425:686-691 (2003)). The maximum yield calculations assuming the succinate pathway to 4-HB in both  E. coli  and  S. cerevisiae  require only the assumption that a non-native 4-HB dehydrogenase has been added to their metabolic networks. 
     The pathway from succinyl-CoA to 4-hydroxybutyrate was described in U.S. Pat. No. 6,117,658 as part of a process for making polyhydroxyalkanoates comprising 4-hydroxybutyrate monomer units.  Clostridium kluyveri  is one example organism known to possess CoA-dependent succinic semialdehyde dehydrogenase activity (Sohling and Gottschalk, supra; Sohling and Gottschalk, supra). In this study, it is assumed that this enzyme, from  C. kluyveri  or another organism, is expressed in  E. coli  or  S. cerevisiae  along with a non-native or heterologous 4-HB dehydrogenase to complete the pathway from succinyl-CoA to 4-HB. The pathway from alpha-ketoglutarate to 4-HB was demonstrated in  E. coli  resulting in the accumulation of poly(4-hydroxybutyric acid) to 30% of dry cell weight (Song et al., supra). As  E. coli  and  S. cerevisiae  natively or endogenously possess both glutamate:succinic semialdehyde transaminase and glutamate decarboxylase (Coleman et al.,  J. Biol. Chem.  276:244-250 (2001)), the pathway from AKG to 4-HB can be completed in both organisms by assuming only that a non-native 4-HB dehydrogenase is present. 
     Example II 
     Production of 4-Hydroxybutanoic Acid in  E. coli    
     This Example describes the biosynthetic yields for 4-hydroxybutanoic acid resulting from each biochemical pathway. 
     In this section, the maximum theoretical yields of 4-HB from glucose are calculated assuming that each of the three metabolic pathways depicted in  FIG.  2    are functional in  E. coli . A genome-scale metabolic model of  E. coli , similar to the one described in Reed et al.,  Genome Biol.  4:R54 (2003), was used as the basis for the analysis. The energetic gain, in terms of ATP molecules produced, of each maximum yielding pathway is calculated assuming anaerobic conditions, unless otherwise stated. 4-Hydroxybutyrate is assumed to exit in  E. coli  via proton symport, as is the case with most organic acids. It is also possible that GBL is secreted by simple diffusion, and in this case the energetics would be more favorable than in the case considered here. The impact of cofactor specificity (i.e., NADH or NADPH-dependence) of the participating enzymes on the maximum yield and energetics of each pathway also was investigated. 
     The results from the analysis are shown in Tables 3 A-C. From an energetic and yield standpoint, the succinate to 4-HB pathway is the most promising. Specifically, the calculations reveal that the maximum theoretical yield of 4-HB from glucose is 1.33 mol/mol (0.77 g/g; 0.89 Cmol/Cmol) assuming the succinate to 4-HB pathway is functional. In addition, the anaerobic production of 4-HB via succinate would result in the net production of either 1.8, 1.5, or 1.1 mol of ATP per glucose depending upon the assumed cofactor specificity of the participating enzymes. These energetic yields are comparable to the 2.0 ATP per glucose that can be obtained via substrate level phosphorylation by the production of ethanol or lactate suggesting the potential for anaerobic homo-4-HB production in  E. coli.    
     The succinyl-CoA route to 4-HB is another promising pathway when considering maximum yield and energetics. A 1.33 mol/mol yield of 4-HB is achievable in  E. coli  if at least one of the pathway steps is assumed NADH-dependent. However, because this pathway requires the formation of succinyl-CoA, its energetic yield is lower than that of the succinate pathway. An oxygen requirement is anticipated at high 4-HB yields if both the CoA-dependent succinic semialdehyde dehydrogenase and 4-HB dehydrogenase steps are assumed NADPH-dependent. In this case, the production of 4-HB at the maximum yield would result in no net ATP gain and possibly not support the energetic maintenance demands needed for  E. coli  survival. Thus, some energy would have to originate from oxidative phosphorylation to enable homo-fermentative 4-HB production. The alpha-ketoglutarate pathway utilizing glutamate:succinate semialdehyde transaminase and glutamate decarboxylase toward 4-HB is the least favorable of the three potential routes with a maximum achievable yield of 1.0 mol 4-HB per mol of glucose. In addition to the lower maximum yield, this pathway requires the utilization of 1.5 moles of oxygen per mol of glucose converted to 4-HB. The energetics of this pathway are unaffected by the assumed cofactor specificity of 4-HB dehydrogenase. 
     
       
         
           
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 The overall substrate conversion stoichiometry to 4-HB assuming the A) 
               
               
                 succinate, B) succinyl-CoA, or C) alpha-ketoglutarate production routes 
               
               
                 are functional in  E. coli . Glucose and oxygen are taken 
               
               
                 up while all other molecules are produced. 
               
               
                   
               
             
            
               
                 A) Succinate Pathway 
               
            
           
           
               
               
               
               
            
               
                 Cofactor 
                   
                 1 NADH step 
                   
               
               
                 Specificity 
                 2 NADH steps 
                 1 NADPH step 
                 2 NADPH steps 
               
               
                   
               
               
                 Glucose 
                 −1.000 
                 −1.000 
                 −1.000 
               
               
                 Oxygen 
                 0.000 
                 0.000 
                 0.000 
               
               
                 Protons 
                 1.333 
                 1.333 
                 1.333 
               
               
                 4HB 
                 1.333 
                 1.333 
                 1.333 
               
               
                 CO2 
                 0.667 
                 0.667 
                 0.667 
               
               
                 H2O 
                 0.667 
                 0.667 
                 0.667 
               
               
                 ATP 
                 1.800 
                 1.510 
                 1.097 
               
               
                   
               
            
           
           
               
            
               
                 B) Succinyl-CoA Pathway 
               
            
           
           
               
               
               
               
               
            
               
                 Cofactor 
                 2 NADH 
                 1 NADH step 
                 2 NADPH 
                 2 NADPH 
               
               
                 Specificity 
                 steps 
                 1 NADPH step 
                 steps 
                 steps 
               
               
                   
               
               
                 Glucose 
                 −1.000 
                 −1.000 
                 −1.000 
                 −1.000 
               
               
                 Oxygen 
                 0.000 
                 0.000 
                 −0.036 
                 0.000 
               
               
                 Protons 
                 1.333 
                 1.333 
                 1.325 
                 1.294 
               
               
                 4HB 
                 1.333 
                 1.333 
                 1.325 
                 1.294 
               
               
                 CO2 
                 0.667 
                 0.667 
                 0.698 
                 0.082 
               
               
                 H2O 
                 0.667 
                 0.667 
                 0.698 
                 0.470 
               
               
                 ATP 
                 0.467 
                 0.177 
                 0.000 
                 0.000 
               
               
                   
               
            
           
           
               
            
               
                 C) Alpha-ketoglutarate Pathway 
               
            
           
           
               
               
               
               
            
               
                   
                 Cofactor 
                   
                   
               
               
                   
                 Specificity 
                 1 NADH step 
                 1 NADPH step 
               
               
                   
               
               
                   
                 Glucose 
                 −1.000 
                 −1.000 
               
               
                   
                 Oxygen 
                 −1.500 
                 −1.500 
               
               
                   
                 Protons 
                 1.000 
                 1.000 
               
               
                   
                 4HB 
                 1.000 
                 1.000 
               
               
                   
                 CO2 
                 2.000 
                 2.000 
               
               
                   
                 H2O 
                 2.000 
                 2.000 
               
               
                   
                 ATP 
                 5.500 
                 5.500 
               
               
                   
               
            
           
         
       
     
     In order to corroborate the computational predictions proposed in this report, the strains expressing a complete pathway to 4-HB can be constructed and tested. Corroboration is performed with both  E. coli  (Examples II and IV) and  S. cerevisiae  (Example III). In  E. coli , the relevant genes are expressed in a synthetic operon behind an inducible promoter on a medium- or high-copy plasmid; for example the P BAD  promoter which is induced by arabinose, on a plasmid of the pBAD series (Guzman et al.,  J. Bacteriol.  177:4121-4130 (1995)). In  S. cerevisiae , genes are integrated into the chromosome behind the PDC1 promoter, replacing the native pyruvate carboxylase gene. It has been reported that this results in higher expression of foreign genes than from a plasmid (Ishida et al.,  Appl. Environ. Microbiol.  71:1964-1970 (2005)), and will also ensure expression during anaerobic conditions. 
     Cells containing the relevant constructs are grown in minimal media containing glucose, with addition of arabinose in the case of  E. coli  containing genes expressed under the P BAD  promoter. Periodic samples are taken for both gene expression and enzyme activity analysis. Enzyme activity assays are performed on crude cell extracts using procedures well known in the art. Alternatively, assays based on the oxidation of NAD(P)H, which is produced in all dehydrogenase reaction steps and detectable by spectrophotometry can be utilized. In addition, antibodies can be used to detect the level of particular enzymes. In lieu of or in addition to enzyme activity measurements, RNA can be isolated from parallel samples and transcript of the gene of interest measured by reverse transcriptase PCR. Any constructs lacking detectable transcript expression are reanalyzed to ensure the encoding nucleic acids are harbored in an expressible form. Where transcripts are detected, this result indicates either a lack of translation or production of inactive enzyme. A variety of methods well known in the art can additionally be employed, such as codon optimization, engineering a strong ribosome binding site, use of a gene from a different species, and prevention of N-glycosylation (for expression of bacterial enzymes in yeast) by conversion of Asn residues to Asp. Once all required enzyme activities are detected, the next step is to measure the production of 4-HP in vivo. Triplicate shake flask cultures are grown either anaerobically or microaerobically, depending on the conditions required (see above), and periodic samples taken. Organic acids present in the culture supernatants are analyzed by HPLC using the Aminex AH-87X column. The elution time of 4-HB will be determined using a standard purchased from a chemical supplier. 
     The CoA-independent pathway can be implemented and tested for corroboration. In this case, the genes overexpressed are the native succinic semialdehyde dehydrogenase from each organism, and the 4-hydroxybutanoate dehydrogenase from  Ralstonia eutropha . Once both enzyme activities are detected as discussed above, the strains are tested for 4-HB production. Corroboration also can be obtained from implementing the CoA-dependent pathway. The CoA-dependent succinic semialdehyde dehydrogenase and the 4-hydroxybutanoate dehydrogenase from  Clostridium kluyveri  are expressed as described above. In addition, overexpression of the native succinyl-CoA synthetase also can be performed, to funnel more succinate into the heterologous pathway. Finally, if 4-HB production is unfavorable, different culture conditions can be tested, such as a change in oxygenation status which can manipulate the NAD(P)H/NAD(P) ratio. 
     Example III 
     Production of 4-Hydroxybutanoic Acid in Yeast 
     This Example describes the biosynthetic yields for 4-hydroxybutanoic acid resulting from each biochemical pathway in  S. cerevisiae.    
     In this section, the maximum theoretical yields of 4-HB from glucose are calculated assuming that each of the three metabolic pathways depicted in  FIG.  2    are functional in  S. cerevisiae . A genome-scale metabolic model of  S. cerevisiae , similar to the one described in Forster et al.  Genome Res.  13:244-253 (2003) was used as the basis for the analysis. The energetic gain of each maximum yielding pathway is calculated assuming anaerobic conditions unless otherwise stated. 4-hydroxybutyrate is assumed to exit  S. cerevisiae  via proton symport, as is the case with most organic acids. The impact of cofactor specificity (i.e., NADH or NADPH-dependence) of the participating enzymes on the maximum yield and energetics of each pathway was also investigated. 
     The results from the analysis are shown in Tables 4 A-C. As with  E. coli , the succinate to 4-HB pathway is the most promising provided that the thermodynamic concerns raised in Example I can be overcome. The calculations reveal that the maximum theoretical yield of 4-HB from glucose is 1.33 mol/mol (0.77 g/g; 0.89 Cmol/Cmol) in  S. cerevisiae . In addition, the anaerobic production of 4-HB via succinate would result in the net production of either 1.4, 1.1, or 0.5 mol of ATP per glucose depending upon the assumed cofactor specificity of the participating enzymes. 
     The succinyl-CoA route to 4-HB is the second most favorable pathway. A maximum yield of 1.33 mol 4-HB/mol glucose is achievable in  S. cerevisiae  regardless of cofactor specificity. However, net energy generation at the maximum theoretical yield is possible only if both the CoA-dependent succinic semialdehyde dehydrogenase and 4-HB dehydrogenase steps are assumed to be NADH-dependent. If either step is NADPH-dependent, no net ATP will be gained from anaerobic 4-HB production and an alternate energy source (e.g., oxidative phosphorylation) would be required to support cell growth and maintenance. The alpha-ketoglutarate route toward 4-HB is the least favorable of the three potential pathways in  S. cerevisiae  although the maximum yield of 1.1-1.2 mol 4-HB per mol glucose is slightly higher than was found in  E. coli . Nevertheless, this pathway requires an oxygen uptake of 0.8-0.9 mol oxygen per mol glucose to become energetically neutral. 
     
       
         
           
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 The overall substrate conversion stoichiometry to 4-HB in  S. cerevisiae ., 
               
               
                 assuming the A) succinate, B) succinyl-CoA, or C) alpha-ketoglutarate 
               
               
                 production routes are functional in  S. cerevisiae . Glucose and oxygen 
               
               
                 are taken up while all other molecules are produced. 
               
               
                   
               
             
            
               
                 A) Succinate Pathway 
               
            
           
           
               
               
               
               
            
               
                 Cofactor 
                   
                 1 NADH step 
                   
               
               
                 Specificity 
                 2 NADH steps 
                 1 NADPH step 
                 2 NADPH steps 
               
               
                   
               
               
                 Glucose 
                 −1.000 
                 −1.000 
                 −1.000 
               
               
                 Oxygen 
                 0.000 
                 0.000 
                 0.000 
               
               
                 Protons 
                 1.333 
                 1.333 
                 1.333 
               
               
                 4HB 
                 1.333 
                 1.333 
                 1.333 
               
               
                 CO2 
                 0.667 
                 0.667 
                 0.667 
               
               
                 H2O 
                 0.667 
                 0.667 
                 0.667 
               
               
                 ATP 
                 1.444 
                 1.067 
                 0.533 
               
               
                   
               
            
           
           
               
            
               
                 B) Succinyl-CoA Pathway 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Cofactor 
                 2 NADH 
                 1 NADH step 
                 2 NADPH 
               
               
                   
                 Specificity 
                 steps 
                 1 NADPH step 
                 steps 
               
               
                   
               
               
                   
                 Glucose 
                 −1.000 
                 −1.000 
                 −1.000 
               
               
                   
                 Oxygen 
                 0.000 
                 0.000 
                 0.000 
               
               
                   
                 Protons 
                 1.333 
                 1.333 
                 1.333 
               
               
                   
                 4HB 
                 1.333 
                 1.333 
                 1.333 
               
               
                   
                 CO2 
                 0.667 
                 0.667 
                 0.667 
               
               
                   
                 H2O 
                 0.667 
                 0.667 
                 0.667 
               
               
                   
                 ATP 
                 0.533 
                 0.000 
                 0.000 
               
               
                   
               
            
           
           
               
            
               
                 C) Alpha-ketoglutarate Pathway 
               
            
           
           
               
               
               
               
            
               
                   
                 Cofactor 
                   
                   
               
               
                   
                 Specificity 
                 1 NADH step 
                 1 NADPH step 
               
               
                   
               
               
                   
                 Glucose 
                 −1.000 
                 −1.000 
               
               
                   
                 Oxygen 
                 −0.785 
                 −0.879 
               
               
                   
                 Protons 
                 1.159 
                 1.138 
               
               
                   
                 4HB 
                 1.159 
                 1.138 
               
               
                   
                 CO2 
                 1.364 
                 1.448 
               
               
                   
                 H2O 
                 1.364 
                 1.448 
               
               
                   
                 ATP 
                 0.000 
                 0.000 
               
               
                   
               
            
           
         
       
     
     Example IV 
     Biosynthesis of 1,4-Butanediol from Succinate and α-Ketogluterate 
     This Example illustrates the construction and biosynthetic production of 4-HB and BDO from microbial organisms. 
     As described previously in Examples I-III, the thermodynamic characteristics of the biotransformation steps from 4-HB to BDO shown in  FIG.  1    also were calculated based on standard Gibbs free energy of formation determined by group contribution. The results are provide in Table 5. Similarly, although some of the steps have calculated positive delta G values, the standard errors for these calculations and concentration gradients indicate that any of the steps are feasible. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Gibbs free energy of reaction (kcal/mole) under standard conditions 
               
               
                 (pH and ionic strength values). Temperature was constant at 298 K. 
               
               
                 Error bars for these values are on the order of 8 kcal/mol, as calculated 
               
               
                 by equation 2. Abbreviations: 4-HBald, 4-hydroxybutyraldehyde. 
               
            
           
           
               
               
            
               
                   
                 ΔG r °′ 
               
               
                   
                 pH = 7.0 
               
               
                 Reaction 
                 IS = 0 
               
               
                   
               
            
           
           
               
               
            
               
                 4-HB + NADH + H+ → 4-HBald + NAD 
                 2.4 
               
               
                 4-HB + acetyl-CoA → 4-HB-CoA + 
                 −5.0 
               
               
                 acetate 
                   
               
               
                 4-HB-CoA + NADH + H+ → 4-HBald + 
                 9.5 
               
               
                 NAD + CoA 
                   
               
               
                 4-HBald + NADH + H+ → bdo + NAD 
                 −5.0 
               
               
                   
               
            
           
         
       
     
     Theoretical yields were calculated assuming all the pathways in  FIG.  2    are incorporated into  E. coli . A genome-scale metabolic model of  E. coli , similar to the one described in Reed et al.,  Genome Biol  4:R54 (2003), was used as the basis for the analysis. The maximum theoretical yield assuming energetic neutrality and no cell growth or maintenance was 1.09 mol BDO/mol glucose under microaerobic conditions. Simulations performed under anaerobic conditions, which can be utilized to drive the pathway toward BDO production, either acetate or ethanol is produced as a co-product. Under these conditions, the maximum yields were 1.04 and 1.00 mol/mol, respectively. One alternative is to add limiting amounts of nitrate as an electron acceptor, thus controlling the amount of respiration that can occur. Under this condition, the maximum yield returns to 1.09 mol/mol. Another alternative is to replace the native  E. coli  phosphoenolpyruvate (PEP) carboxylase with a heterologous or engineered phosphoenolpyruvate carboxykinase that is capable of functioning in the direction of PEP carboxylation. This enzyme produces ATP, whereas the PEP carboxylase does not. Under this assumption, the maximum yield returns to 1.09 mol/mol. 
     In addition, there are several alternative enzymes that can be utilized in the pathway described above. The native or endogenous enzyme for conversion of succinate to succinyl-CoA (Step  1  in  FIG.  2   ) can be replaced by a CoA transferase such as that encoded by the cat1 gene  C. kluyveri  (Sohling, B. and G. Gottschalk,  Eur. J Biochem.  212:121-127 (1993)), which functions in a similar manner to Step  9 . However, the production of acetate by this enzyme may not be optimal, as it might be secreted rather than being converted back to acetyl-CoA. In this respect, it also can be beneficial to eliminate acetate formation in Step  9 . As one alternative to this CoA transferase, a mechanism can be employed in which the 4-HB is first phosphorylated by ATP and then converted to the CoA derivative, similar to the acetate kinase/phosphotransacetylase pathway in  E. coli  for the conversion of acetate to acetyl-CoA. The net cost of this route is one ATP, which is the same as is required to regenerate acetyl-CoA from acetate. The enzymes phosphotransbutyrylase (ptb) and butyrate kinase (bk) are known to carry out these steps on the non-hydroxylated molecules for butyrate production in  C. acetobutylicum  (Cary et al.,  Appl Environ Microbiol  56:1576-1583 (1990); Valentine, R. C. and R. S. Wolfe,  J Biol Chem.  235:1948-1952 (1960)). These enzymes are reversible, allowing synthesis to proceed in the direction of 4-HB. 
     BDO also can be produced via α-ketoglutarate in addition to or instead of through succinate. A described previously, and exemplified further below, one pathway to accomplish product biosynthesis is with the production of succinic semialdehyde via α-ketoglutarate using the endogenous enzymes ( FIG.  2   , Steps  4 - 5 ). An alternative is to use an α-ketoglutarate decarboxylase that can perform this conversion in one step ( FIG.  2   , Step  8 ; Tian et al.,  Proc Natl Acad Sci U.S.A  102:10670-10675 (2005)). 
     For the construction of different strains of BDO-producing microbial organisms, a list of applicable genes was assembled for corroboration. Briefly, one or more genes within the 4-HB and/or BDO biosynthetic pathways were identified for each step of the complete BDO-producing pathway shown in  FIG.  2   , using available literature resources, the NCBI genetic database, and homology searches. The genes cloned and assessed in this study are presented below in in Table 6, along with the appropriate references and URL citations to the polypeptide sequence. As discussed further below, some genes were synthesized for codon optimization while others were cloned via PCR from the genomic DNA of the native or wild-type organism. For some genes both approaches were used, and in this case the native genes are indicated by an “n” suffix to the gene identification number when used in an experiment. Note that only the DNA sequences differ; the proteins are identical. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Genes expressed in host BDO-producting microbial organisms. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Reaction 
                   
                   
                   
                   
                   
               
               
                 Gene ID 
                 number 
                 Gene 
                 Source 
                   
                   
                   
               
               
                 number 
                 (FIG. 1) 
                 name 
                 organism 
                 Enzyme name 
                 Link to protein sequence 
                 Reference 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 0001 
                 9 
                 Cat2 
                 
                   Clostridium 
                 
                 4-hydroxybutyrate 
                 www.ncbi.nlm.nih.gov/entrez/ 
                 (29)d} 
               
               
                   
                   
                   
                 
                   kluyveri 
                 
                 coenzyme A 
                 viewer.fcgi?db=nuccore&amp;id= 
                   
               
               
                   
                   
                   
                 DSM 555 
                 transferase 
                 1228100 
                   
               
               
                 0002 
                 12/13 
                 adhE 
                 
                   Clostridium 
                 
                 Aldehyde/alcohol 
                 www.ncbi.nlm.nih.gov/entrez/ 
                 (22)d} 
               
               
                   
                   
                   
                 
                   acetobutylicum 
                 
                 dehydrogenase 
                 viewer.fcgi?db=protein&amp;val= 
                   
               
               
                   
                   
                   
                 ATCC 824 
                   
                 15004739 
                   
               
               
                 0003 
                 12/13 
                 adhE2 
                 
                   Clostridium 
                 
                 Aldehyde/alcohol 
                 www.ncbi.nlm.nih.gov/entrez/ 
                 (12)d} 
               
               
                   
                   
                   
                 
                   acetobutylicum 
                 
                 dehydrogenase 
                 viewer.fcgi?val=NP_149325.1 
                   
               
               
                   
                   
                   
                 ATCC 824 
                   
                   
                   
               
               
                 0004 
                 1 
                 Cat1 
                 
                   Clostridium 
                 
                 Succinate 
                 www.ncbi.nlm.nih.gov/entrez/ 
                 (29)d} 
               
               
                   
                   
                   
                 
                   kluyveri 
                 
                 coenzyme A 
                 viewer.fcgi?db=nuccore&amp;id= 
                   
               
               
                   
                   
                   
                 DSM 555 
                 transferase 
                 1228100 
                   
               
               
                 0008 
                 6 
                 sucD 
                 
                   Clostridium 
                 
                 Succinic 
                 www.ncbi.nlm.nih.gov/entrez/ 
                 (29)d} 
               
               
                   
                   
                   
                 
                   kluyveri 
                 
                 semialdehyde 
                 viewer.fcgi?db=nuccore&amp;id= 
                   
               
               
                   
                   
                   
                 DSM 555 
                 dehydrogenase 
                 1228100 
                   
               
               
                   
                   
                   
                   
                 (CoA-dependent) 
                   
                   
               
               
                 0009 
                 7 
                 4-HBd 
                 
                   Ralstonia 
                 
                 4-hydroxybutyrate 
                 www.ncbi.nlm.nih.gov/entrez/ 
                 (32)d} 
               
               
                   
                   
                   
                   eutropha  H16 
                 dehydrogenase 
                 viewer.fcgi?val=YP_726053.1 
                   
               
               
                   
                   
                   
                   
                 (NAD-dependent) 
                   
                   
               
               
                 0010 
                 7 
                 4-HBd 
                 
                   Clostridium 
                 
                 4-hydroxybutyrate 
                 www.ncbi.nlm.nih.gov/entrez/ 
                 (29)d} 
               
               
                   
                   
                   
                 
                   kluyveri 
                 
                 dehydrogenase 
                 viewer.fcgi?db=nuccore&amp;id= 
                   
               
               
                   
                   
                   
                 DSM 555 
                 (NAD-dependent) 
                 1228100 
                   
               
               
                 0011 
                 12/13 
                 adhE 
                 
                   E. coli 
                 
                 Aldehyde/alcohol 
                 www.shigen.nig.ac.jp/ecoli/pe 
                   
               
               
                   
                   
                   
                   
                 dehydrogenase 
                 c/genes.List.DetailAction.do?f 
                   
               
               
                   
                   
                   
                   
                   
                 romListFlag=true&amp;featureTyp 
                   
               
               
                   
                   
                   
                   
                   
                 e=1&amp;orfId=1219 
                   
               
               
                 0012 
                 12/13 
                 yqhD 
                 
                   E. coli 
                 
                 Aldehyde/alcohol 
                 www.shigen.nig.ac.jp/ecoli/pe 
                   
               
               
                   
                   
                   
                   
                 dehydrogenase 
                 c/genes.List.DetailAction.do 
                   
               
               
                 0013 
                 13 
                 bdhB 
                 
                   Clostridium 
                 
                 Butanol 
                 www.ncbi.nlm.nih.gov/entrez/ 
                 (35)d} 
               
               
                   
                   
                   
                 
                   acetobutylicum 
                 
                 dehydrogenase II 
                 viewer.fcgi?val=NP_349891.1 
                   
               
               
                   
                   
                   
                 ATCC 824 
                   
                   
                   
               
               
                 0020 
                 11 
                 ptb 
                 
                   Clostridium 
                 
                 Phospho- 
                 www.ncbi.nlm.nih.gov/entrez/ 
                  (4)d} 
               
               
                   
                   
                   
                 
                   acetobutylicum 
                 
                 transbutyrylase 
                 viewer.fcgi?db=protein&amp;id=1 
                   
               
               
                   
                   
                   
                 ATCC 824 
                   
                 5896327 
                   
               
               
                 0021 
                 10 
                 buk1 
                 
                   Clostridium 
                 
                 Butyrate kinase I 
                 www.ncbi.nlm.nih.gov/entrez/ 
                  (4)d} 
               
               
                   
                   
                   
                 
                   acetobutylicum 
                 
                   
                 viewer.fcgi?db=protein&amp;id=2 
                   
               
               
                   
                   
                   
                 ATCC 824 
                   
                 0137334 
                   
               
               
                 0022 
                 10 
                 buk2 
                 
                   Clostridium 
                 
                 Butyrate kinase II 
                 www.ncbi.nlm.nih.gov/entrez/ 
                  (4)d} 
               
               
                   
                   
                   
                 
                   acetobutylicum 
                 
                   
                 viewer.fcgi?db=protein&amp;id=2 
                   
               
               
                   
                   
                   
                 ATCC 824 
                   
                 0137415 
                   
               
               
                 0023 
                 13 
                 adhEm 
                 isolated from 
                 Alcohol 
                   
                 (37)d} 
               
               
                   
                   
                   
                 metalibrary 
                 dehydrogenase 
                   
                   
               
               
                   
                   
                   
                 of anaerobic 
                   
                   
                   
               
               
                   
                   
                   
                 sewage 
                   
                   
                   
               
               
                   
                   
                   
                 digester 
                   
                   
                   
               
               
                   
                   
                   
                 microbial 
                   
                   
                   
               
               
                   
                   
                   
                 consortia 
                   
                   
                   
               
               
                 0024 
                 13 
                 adhE 
                 
                   Clostridium 
                 
                 Alcohol 
                 www.genome.jp/dbget- 
                   
               
               
                   
                   
                   
                 
                   thermocellum 
                 
                 dehydrogenase 
                 bin/www_bget?cth:Cthe_0423 
                   
               
               
                 0025 
                 13 
                 ald 
                 
                   Clostridium 
                 
                 Coenzyme A- 
                 www.ncbi.nlm.nih.gov/entrez/ 
                 (31)d} 
               
               
                   
                   
                   
                 
                   beijerinckii 
                 
                 acylating aldehyde 
                 viewer.fcgi?db=protein&amp;id=4 
                   
               
               
                   
                   
                   
                   
                 dehydrogenase 
                 9036681 
                   
               
               
                 0026 
                 13 
                 bdhA 
                 
                   Clostridium 
                 
                 Butanol 
                 www.ncbi.nlm.nih.gov/entrez/ 
                 (35)d} 
               
               
                   
                   
                   
                 
                   acetobutylicum 
                 
                 dehydrogenase 
                 viewer.fcgi?val=NP_349892.1 
                   
               
               
                   
                   
                   
                 ATCC 824 
                   
                   
                   
               
               
                 0027 
                 12 
                 bld 
                 
                   Clostridium 
                 
                 Butyraldehyde 
                 www.ncbi.nlm.nih.gov/entrez/ 
                 (18)d} 
               
               
                   
                   
                   
                 
                   saccharoperbutyl- 
                 
                 dehydrogenase 
                 viewer.fcgi?db=protein&amp;id=3 
                   
               
               
                   
                   
                   
                 
                   acetonicum 
                 
                   
                 1075383 
                   
               
               
                 0028 
                 13 
                 bdh 
                 
                   Clostridium 
                 
                 Butanol 
                 www.ncbi.nlm.nih.gov/entrez/ 
                 (18)d} 
               
               
                   
                   
                   
                 
                   saccharoperbutyl- 
                 
                 dehydrogenase 
                 viewer.fcgi?db=protein&amp;id=1 
                   
               
               
                   
                   
                   
                 
                   acetonicum 
                 
                   
                 24221917 
                   
               
               
                 0029 
                 12/13 
                 adhE 
                 
                   Clostridium 
                 
                 Aldehyde/alcohol 
                 www.genome.jp/dbget- 
                   
               
               
                   
                   
                   
                 
                   tetani 
                 
                 dehydrogenase 
                 bin/www_bget?ctc:CTC01366 
                   
               
               
                 0030 
                 12/13 
                 adhE 
                 
                   Clostridium 
                 
                 Aldehyde/alcohol 
                 www.genome.jp/dbget- 
                   
               
               
                   
                   
                   
                 
                   perfringens 
                 
                 dehydrogenase 
                 bin/www_bget?cpe:CPE2531 
                   
               
               
                 0031 
                 12/13 
                 adhE 
                 
                   Clostridium 
                 
                 Aldehyde/alcohol 
                 www.genome.jp/dbget- 
                   
               
               
                   
                   
                   
                 
                   difficile 
                 
                 dehydrogenase 
                 bin/www_bget?cdf:CD2966 
                   
               
               
                 0032 
                 8 
                 sucA 
                 
                   Mycobacterium 
                 
                 α-ketoglutarate 
                 www.ncbi.nlm.nih.gov/entrez/ 
                 (30)d} 
               
               
                   
                   
                   
                 
                   bovis 
                 
                 decarboxylase 
                 viewer.fcgi?val=YP_977400.1 
                   
               
               
                   
                   
                   
                 BCG, Pasteur 
                   
                   
                   
               
               
                 0033 
                 9 
                 cat2 
                 
                   Clostridium 
                 
                 4-hydroxybutyrate 
                 www.ncbi.nlm.nih.gov/entrez/ 
                   
               
               
                   
                   
                   
                 
                   aminobutyricum 
                 
                 coenzyme A 
                 viewer.fcgi?db=protein&amp;val= 
                   
               
               
                   
                   
                   
                   
                 transferase 
                 6249316 
                   
               
               
                 0034 
                 9 
                 cat2 
                 
                   Porphyromonas 
                 
                 4-hydroxybutyrate 
                 www.ncbi.nlm.nih.gov/entrez/ 
                   
               
               
                   
                   
                   
                 
                   gingivalis 
                 
                 coenzyme A 
                 viewer.fcgi?db=protein&amp;val= 
                   
               
               
                   
                   
                   
                 W83 
                 transferase 
                 34541558 
                   
               
               
                 0035 
                 6 
                 sucD 
                 
                   Porphyromonas 
                 
                 Succinic 
                 www.ncbi.nlm.nih.gov/entrez/ 
                   
               
               
                   
                   
                   
                 
                   gingivalis 
                 
                 semialdehyde 
                 viewer.fcgi?val=NP_904963.1 
                   
               
               
                   
                   
                   
                 W83 
                 dehydrogenase 
                   
                   
               
               
                   
                   
                   
                   
                 (CoA-dependent) 
                   
                   
               
               
                 0036 
                 7 
                 4-HBd 
                 
                   Porphyromonas 
                 
                 NAD-dependent 
                 www.ncbi.nlm.nih.gov/entrez/ 
                   
               
               
                   
                   
                   
                 
                   gingivalis 
                 
                 4-hydroxybutyrate 
                 viewer.fcgi?val=NP_904964.1 
                   
               
               
                   
                   
                   
                 W83 
                 dehydrogenase 
                   
                   
               
               
                 0037 
                 7 
                 gbd 
                 Uncultured 
                 4-hydroxybutyrate 
                 www.ncbi.nlm.nih.gov/entrez/ 
                 (16)d} 
               
               
                   
                   
                   
                 bacterium 
                 dehydrogenase 
                 viewer.fcgi?db=nuccore&amp;id= 
                   
               
               
                   
                   
                   
                   
                   
                 5916168 
                   
               
               
                 0038 
                 1 
                 sucCD 
                 
                   E. coli 
                 
                 Succinyl-CoA 
                 www.shigen.nig.ac.jp/ecoli/pe 
                   
               
               
                   
                   
                   
                   
                 synthetase 
                 c/genes.List.DetailAction.do 
               
               
                   
               
            
           
         
       
     
     Expression Vector Construction for BDO pathway. Vector backbones and some strains were obtained from Dr. Rolf Lutz of Expressys (www.expressys.de/). The vectors and strains are based on the pZ Expression System developed by Dr. Rolf Lutz and Prof. Hermann Bujard (Lutz, R. and H. Bujard,  Nucleic Acids Res  25:1203-1210 (1997)). Vectors obtained were pZE13luc, pZA33luc, pZS*13luc and pZE22luc and contained the luciferase gene as a stuffer fragment. To replace the luciferase stuffer fragment with a lacZ-alpha fragment flanked by appropriate restriction enzyme sites, the luciferase stuffer fragment was first removed from each vector by digestion with EcoRI and XbaI. The lacZ-alpha fragment was PCR amplified from pUC19 with the following primers: 
     
       
         
           
               
            
               
                 lacZalpha-RI 
               
               
                 (SEQ ID NO: 1) 
               
               
                 5′GACGAATTCGCTAGCAAGAGGAGAAGTCGACATGTCCAATTCACTGGC 
               
               
                   
               
               
                 CGTCGTTTTAC3′ 
               
               
                   
               
               
                 lacZalpha 3′BB 
               
               
                 (SEQ ID NO: 2) 
               
               
                 5′-GACCCTAGGAAGCTTTCTAGAGTCGACCTATGCGGCATCAGAGCAG 
               
               
                   
               
               
                 A-3′. 
               
            
           
         
       
     
     This generated a fragment with a 5′ end of EcoRI site, NheI site, a Ribosomal Binding Site, a SalI site and the start codon. On the 3′ end of the fragment contained the stop codon, XbaI, HindIII, and AvrII sites. The PCR product was digested with EcoRI and AvrII and ligated into the base vectors digested with EcoRI and XbaI (XbaI and AvrII have compatible ends and generate a non-site). Because NheI and XbaI restriction enzyme sites generate compatible ends that can be ligated together (but generate a NheI/XbaI non-site that is not digested by either enzyme), the genes cloned into the vectors could be “Biobricked” together (http://openwetware.org/wiki/Synthetic_Biology:BioBricks). Briefly, this method enables joining an unlimited number of genes into the vector using the same 2 restriction sites (as long as the sites do not appear internal to the genes), because the sites between the genes are destroyed after each addition. 
     All vectors have the pZ designation followed by letters and numbers indication the origin of replication, antibiotic resistance marker and promoter/regulatory unit. The origin of replication is the second letter and is denoted by E for ColE1, A for p15A and S for pSC101-based origins. The first number represents the antibiotic resistance marker (1 for Ampicillin, 2 for Kanamycin, 3 for Chloramphenicol, 4 for Spectinomycin and 5 for Tetracycline). The final number defines the promoter that regulated the gene of interest (1 for P LetO-1 , 2 for P LlacO-1 , 3 for P AllacO-1 , and 4 for P lac/ara-1 ). The MCS and the gene of interest follows immediately after. For the work discussed here we employed two base vectors, pZA33 and pZE13, modified for the biobricks insertions as discussed above. Once the gene(s) of interest have been cloned into them, resulting plasmids are indicated using the four digit gene codes given in Table 6; e.g., pZA33-XXXX-YYYY- . . . . 
     Host Strain Construction. The parent strain in all studies described here is  E. coli  K-12 strain MG1655. Markerless deletion strains in adhE, gabD, and aldA were constructed under service contract by a third party using the redET method (Datsenko, K. A. and B. L. Wanner,  Proc Natl Acad Sci U.S.A  97:6640-6645 (2000)). Subsequent strains were constructed via bacteriophage P1 mediated transduction (Miller, J. 1973. Experiments in Molecular Genetics. Cold Spring Harbor Laboratories, New York). Strain C600Z1 (laci q , PN25-tetR, SpR, lacYl, leuB6,mcrB+, supE44, thi-1, thr-1, tonA21) was obtained from Expressys and was used as a source of a lacI q  allele for P1 transduction. Bacteriophage Plvir was grown on the C600Z1  E. coli  strain, which has the spectinomycin resistance gene linked to the lacI q . The P1 lysate grown on C600Z1 was used to infect MG1655 with selection for spectinomycin resistance. The spectinomycin resistant colonies were then screened for the linked lacI q  by determining the ability of the transductants to repress expression of a gene linked to a P AllacO-1  promoter. The resulting strain was designated MG1655 lacI q . A similar procedure was used to introduce lacI Q  into the deletion strains. 
     Production of 4-HB From Succinate. For construction of a 4-HB producer from succinate, genes encoding steps from succinate to 4-HB and 4-HB-CoA (1, 6, 7, and 9 in  FIG.  2   ) were assembled onto the pZA33 and pZE13 vectors as described below. Various combinations of genes were assessed, as well as constructs bearing incomplete pathways as controls (Tables 7 and 8). The plasmids were then transformed into host strains containing lacI Q , which allow inducible expression by addition of isopropyl β-D-1-thiogalactopyranoside (IPTG). Both wild-type and hosts with deletions in genes encoding the native succinic semialdehyde dehydrogenase (step  2  in  FIG.  1   ) were tested. 
     Activity of the heterologous enzymes were first tested in in vitro assays, using strain MG1655 lacI Q  as the host for the plasmid constructs containing the pathway genes. Cells were grown aerobically in LB media (Difco) containing the appropriate antibiotics for each construct, and induced by addition of IPTG at 1 mM when the optical density (OD600) reached approximately 0.5. Cells were harvested after 6 hours, and enzyme assays conducted as discussed below. 
     In Vitro Enzyme Assays. To obtain crude extracts for activity assays, cells were harvested by centrifugation at 4,500 rpm (Beckman-Coulter, Allegera X-15R) for 10 min. The pellets were resuspended in 0.3 mL BugBuster (Novagen) reagent with benzonase and lysozyme, and lysis proceeded for 15 minutes at room temperature with gentle shaking. Cell-free lysate was obtained by centrifugation at 14,000 rpm (Eppendorf centrifuge 5402) for 30 min at 4° C. Cell protein in the sample was determined using the method of Bradford et al.,  Anal. Biochem.  72:248-254 (1976), and specific enzyme assays conducted as described below. Activities are reported in Units/mg protein, where a unit of activity is defined as the amount of enzyme required to convert 1 μmol of substrate in 1 min. at room temperature. In general, reported values are averages of at least 3 replicate assays. 
     Succinyl-CoA transferase (Cat1) activity was determined by monitoring the formation of acetyl-CoA from succinyl-CoA and acetate, following a previously described procedure Sohling and Gottschalk,  J. Bacteriol.  178:871-880 (1996). Succinyl-CoA synthetase (SucCD) activity was determined by following the formation of succinyl-CoA from succinate and CoA in the presence of ATP. The experiment followed a procedure described by Cha and Parks,  J. Biol. Chem.  239:1961-1967 (1964). CoA-dependent succinate semialdehyde dehydrogenase (SucD) activity was determined by following the conversion of NAD to NADH at 340 nm in the presence of succinate semialdehyde and CoA (Sohling and Gottschalk,  Eur. J. Biochem.  212:121-127 (1993)). 4-HB dehydrogenase (4-HBd) enzyme activity was determined by monitoring the oxidation of NADH to NAD at 340 nm in the presence of succinate semialdehyde. The experiment followed a published procedure Gerhardt et al.  Arch. Microbiol.  174:189-199 (2000). 4-HB CoA transferase (Cat2) activity was determined using a modified procedure from Scherf and Buckel,  Appl. Environ. Microbiol.  57:2699-2702 (1991). The formation of 4-HB-CoA or butyryl-CoA formation from acetyl-CoA and 4-HB or butyrate was determined using HPLC. 
     Alcohol (ADH) and aldehyde (ALD) dehydrogenase was assayed in the reductive direction using a procedure adapted from several literature sources (Durre et al.,  FEMS Microbiol. Rev.  17:251-262 (1995); Palosaari and Rogers,  J. Bacteriol.  170:2971-2976 (1988) and Welch et al.,  Arch. Biochem. Biophys.  273:309-318 (1989). The oxidation of NADH is followed by reading absorbance at 340 nM every four seconds for a total of 240 seconds at room temperature. The reductive assays were performed in 100 mM MOPS (adjusted to pH 7.5 with KOH), 0.4 mM NADH, and from 1 to 50 μl of cell extract. The reaction is started by adding the following reagents: 100 μl of 100 mM acetaldehyde or butyraldehyde for ADH, or 100 μl of 1 mM acetyl-CoA or butyryl-CoA for ALD. The Spectrophotometer is quickly blanked and then the kinetic read is started. The resulting slope of the reduction in absorbance at 340 nM per minute, along with the molar extinction coefficient of NAD(P)H at 340 nM (6000) and the protein concentration of the extract, can be used to determine the specific activity. 
     The enzyme activity of PTB is measured in the direction of butyryl-CoA to butyryl-phosphate as described in Cary et al.  J. Bacteriol.  170:4613-4618 (1988). It provides inorganic phosphate for the conversion, and follows the increase in free CoA with the reagent 5,5′-dithiobis-(2-nitrobenzoic acid), or DTNB. DTNB rapidly reacts with thiol groups such as free CoA to release the yellow-colored 2-nitro-5-mercaptobenzoic acid (TNB), which absorbs at 412 nm with a molar extinction coefficient of 14,140 M cm −1 . The assay buffer contained 150 mM potassium phosphate at pH 7.4, 0.1 mM DTNB, and 0.2 mM butyryl-CoA, and the reaction was started by addition of 2 to 50 μL cell extract. The enzyme activity of BK is measured in the direction of butyrate to butyryl-phosphate formation at the expense of ATP. The procedure is similar to the assay for acetate kinase previously described Rose et al.,  J. Biol. Chem.  211:737-756 (1954). However we have found another acetate kinase enzyme assay protocol provided by Sigma to be more useful and sensitive. This assay links conversion of ATP to ADP by acetate kinase to the linked conversion of ADP and phosphoenol pyruvate (PEP) to ATP and pyruvate by pyruvate kinase, followed by the conversion of pyruvate and NADH to lactate and NAD+ by lactate dehydrogenase. Substituting butyrate for acetate is the only major modification to enable the assay to follow BK enzyme activity. The assay mixture contained 80 mM triethanolamine buffer at pH 7.6, 200 mM sodium butyrate, 10 mM MgCl2, 0.1 mM NADH, 6.6 mM ATP, 1.8 mM phosphoenolpyruvate. Pyruvate kinase, lactate dehydrogenase, and myokinase were added according to the manufacturer&#39;s instructions. The reaction was started by adding 2 to 50 μL cell extract, and the reaction was monitored based on the decrease in absorbance at 340 nm indicating NADH oxidation. 
     Analysis of CoA Derivatives by HPLC. An HPLC based assay was developed to monitor enzymatic reactions involving coenzyme A (CoA) transfer. The developed method enabled enzyme activity characterization by quantitative determination of CoA, acetyl CoA (AcCoA), butyryl CoA (BuCoA) and 4-hydroxybutyrate CoA (4-HBCoA) present in in-vitro reaction mixtures. Sensitivity down to low μM was achieved, as well as excellent resolution of all the CoA derivatives of interest. 
     Chemical and sample preparation was performed as follows. Briefly, CoA, AcCoA, BuCoA and all other chemicals, were obtained from Sigma-Aldrich. The solvents, methanol and acetonitrile, were of HPLC grade. Standard calibration curves exhibited excellent linearity in the 0.01-1 mg/mL concentration range. Enzymatic reaction mixtures contained 100 mM Tris HCl buffer (pH 7), aliquots were taken at different time points, quenched with formic acid (0.04% final concentration) and directly analyzed by HPLC. 
     HPLC analysis was performed using an Agilent 1100 HPLC system equipped with a binary pump, degasser, thermostated autosampler and column compartment, and diode array detector (DAD), was used for the analysis. A reversed phase column, Kromasil 100 Sum C18, 4.6×150 mm (Peeke Scientific), was employed. 25 mM potassium phosphate (pH 7) and methanol or acetonitrile, were used as aqueous and organic solvents at 1 mL/min flow rate. Two methods were developed: a short one with a faster gradient for the analysis of well-resolved CoA, AcCoA and BuCoA, and a longer method for distinguishing between closely eluting AcCoA and 4-HBCoA. Short method employed acetonitrile gradient (0 min—5%, 6 min—30%, 6.5 min—5%, 10 min—5%) and resulted in the retention times 2.7, 4.1 and 5.5 min for CoA, AcCoA and BuCoA, respectively. In the long method methanol was used with the following linear gradient: 0 min—5%, 20 min—35%, 20.5 min—5%, 25 min—5%. The retention times for CoA, AcCoA, 4-HBCoA and BuCoA were 5.8, 8.4, 9.2 and 16.0 min, respectively. The injection volume was 5 μL, column temperature 30° C., and UV absorbance was monitored at 260 nm. 
     The results demonstrated activity of each of the four pathway steps (Table 6), though activity is clearly dependent on the gene source, position of the gene in the vector, and the context of other genes with which it is expressed. For example, gene 0035 encodes a succinic semialdehyde dehydrogenase that is more active than that encoded by 0008, and 0036 and 0010n are more active 4-HB dehydrogenase genes than 0009. There also seems to be better 4-HB dehydrogenase activity when there is another gene preceding it on the same operon. 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 In vitro enzyme activities in cell extracts from MG1655 lacI Q  containing the plasmids expressing genes 
               
               
                 in the 4-HB-CoA pathway. Activities are reported in Units/mg protein, where a unit of activity is defined 
               
               
                 as the amount of enzyme required to convert 1 μmol of substrate in 1 min. at room temperature. 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Sample # 
                 pZE13 (a) 
                 pZA33 (b) 
                 OD600 
                 Cell Prot (c) 
                 Cat1 
                 SucD 
                 4HBd 
                 Cat2 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 cat1 (0004) 
                   
                 2.71 
                 6.43 
                 1.232 
                 0.00 
                   
                   
               
               
                 2 
                 cat1 (0004)-sucD (0035) 
                   
                 2.03 
                 5.00 
                 0.761 
                 2.57 
                   
                   
               
               
                 3 
                 cat1 (0004)-sucD (0008) 
                   
                 1.04 
                 3.01 
                 0.783 
                 0.01 
                   
                   
               
               
                 4 
                 sucD (0035) 
                   
                 2.31 
                 6.94 
                   
                 2.32 
                   
                   
               
               
                 5 
                 sucD (0008) 
                   
                 1.10 
                 4.16 
                   
                 0.05 
                   
                   
               
               
                 6 
                   
                 4hbd (0009) 
                 2.81 
                 7.94 
                 0.003 
                   
                 0.25 
                   
               
               
                 7 
                   
                 4hbd (0036) 
                 2.63 
                 7.84 
                   
                   
                 3.31 
                   
               
               
                 8 
                   
                 4hbd (0010n) 
                 2.00 
                 5.08 
                   
                   
                 2.57 
                   
               
               
                 9 
                 cat1 (0004)-sucD (0035) 
                 4hbd (0009) 
                 2.07 
                 5.04 
                 0.600 
                 1.85 
                 0.01 
                   
               
               
                 10 
                 cat1 (0004)-sucD (0035) 
                 4hbd (0036) 
                 2.08 
                 5.40 
                 0.694 
                 1.73 
                 0.41 
                   
               
               
                 11 
                 cat1 (0004)-sucD (0035) 
                 4hbd (0010n) 
                 2.44 
                 4.73 
                 0.679 
                 2.28 
                 0.37 
                   
               
               
                 12 
                 cat1 (0004)-sucD (0008) 
                 4hbd (0009) 
                 1.08 
                 3.99 
                 0.572 
                 −0.01 
                 0.02 
                   
               
               
                 13 
                 cat1 (0004)-sucD (0008) 
                 4hbd (0036) 
                 0.77 
                 2.60 
                 0.898 
                 −0.01 
                 0.04 
                   
               
               
                 14 
                 cat1 (0004)-sucD (0008) 
                 4hbd (0010n) 
                 0.63 
                 2.47 
                 0.776 
                 0.00 
                 0.00 
                   
               
               
                 15 
                   
                 cat2 (0034) 
                 2.56 
                 7.86 
                   
                   
                   
                 1.283 
               
               
                 16 
                   
                 cat2(0034)-4hbd(0036) 
                 3.13 
                 8.04 
                   
                   
                 24.86 
                 0.993 
               
               
                 17 
                   
                 cat2(0034)-4hbd(0010n) 
                 2.38 
                 7.03 
                   
                   
                 7.45 
                 0.675 
               
               
                 18 
                   
                 4hbd(0036)-cat2(0034) 
                 2.69 
                 8.26 
                   
                   
                 2.15 
                 7.490 
               
               
                 19 
                   
                 4hbd(0010n)-cat2(0034) 
                 2.44 
                 6.59 
                   
                   
                 0.59 
                 4.101 
               
               
                   
               
               
                 (a) Genes expressed from Plac on pZE13, a high-copy plasmid with colE1 origin and ampicillin resistance. Gene identification numbers are as given in Table 2 
               
               
                 (b) Genes expressed from Plac on pZA33, a medium-copy plasmid with pACYC origin and chloramphenicol resistance. 
               
               
                 (c) Cell protein given as mg protein per mL extract. 
               
            
           
         
       
     
     Recombinant strains containing genes in the 4-HB pathway were then evaluated for the ability to produce 4-HB in vivo from central metabolic intermediates. Cells were grown anaerobically in LB medium to OD600 of approximately 0.4, then induced with 1 mM IPTG. One hour later, sodium succinate was added to 10 mM, and samples taken for analysis following an additional 24 and 48 hours. 4-HB in the culture broth was analyzed by GC-MS as described below. The results indicate that the recombinant strain can produce over 2 mM 4-HB after 24 hours, compared to essentially zero in the control strain (Table 8). 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Production of 4-HB from succinate in  E. coli  strains harboring plasmids expressing various combinations of 4-HB pathway genes. 
               
            
           
           
               
               
               
            
               
                   
                 24 Hours 
                 48 Hours 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                 4HB norm. 
                   
                   
                 4HB norm. 
               
               
                 Sample # 
                 Host Strain 
                 pZE13 
                 pZA33 
                 OD600 
                 4HB, μM 
                 (a) 
                 OD600 
                 4HB, μM 
                 (a) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 MG1655 lacIq 
                 cat1 (0004)-sucD (0035) 
                 4hbd (0009) 
                 0.47 
                 487 
                 1036 
                 1.04 
                 1780 
                 1711 
               
               
                 2 
                 MG1655 lacIq 
                 cat1 (0004)-sucD (0035) 
                 4hbd (0027) 
                 0.41 
                 111 
                 270 
                 0.99 
                 214 
                 217 
               
               
                 3 
                 MG1655 lacIq 
                 cat1 (0004)-sucD (0035) 
                 4hbd (0036) 
                 0.47 
                 863 
                 1835 
                 0.48 
                 2152 
                 4484 
               
               
                 4 
                 MG1655 lacIq 
                 cat1 (0004)-sucD (0035) 
                 4hbd (0010n) 
                 0.46 
                 956 
                 2078 
                 0.49 
                 2221 
                 4533 
               
               
                 5 
                 MG1655 lacIq 
                 cat1 (0004)-sucD (0008) 
                 4hbd (0009) 
                 0.38 
                 493 
                 1296 
                 0.37 
                 1338 
                 3616 
               
               
                 6 
                 MG1655 lacIq 
                 cat1 (0004)-sucD (0008) 
                 4hbd (0027) 
                 0.32 
                 26 
                 81 
                 0.27 
                 87 
                 323 
               
               
                 7 
                 MG1655 lacIq 
                 cat1 (0004)-sucD (0008) 
                 4hbd (0036) 
                 0.24 
                 506 
                 2108 
                 0.31 
                 1448 
                 4672 
               
               
                 8 
                 MG1655 lacIq 
                 cat1 (0004)-sucD (0008) 
                 4hbd (0010n) 
                 0.24 
                 78 
                 324 
                 0.56 
                 233 
                 416 
               
               
                 9 
                 MG1655 lacIq gabD 
                 cat1 (0004)-sucD (0035) 
                 4hbd (0009) 
                 0.53 
                 656 
                 1237 
                 1.03 
                 1643 
                 1595 
               
               
                 10 
                 MG1655 lacIq gabD 
                 cat1 (0004)-sucD (0035) 
                 4hbd (0027) 
                 0.44 
                 92 
                 209 
                 0.98 
                 214 
                 218 
               
               
                 11 
                 MG1655 lacIq gabD 
                 cat1 (0004)-sucD (0035) 
                 4hbd (0036) 
                 0.51 
                 1072 
                 2102 
                 0.97 
                 2358 
                 2431 
               
               
                 12 
                 MG1655 lacIq gabD 
                 cat1 (0004)-sucD (0035) 
                 4hbd (0010n) 
                 0.51 
                 981 
                 1924 
                 0.97 
                 2121 
                 2186 
               
               
                 13 
                 MG1655 lacIq gabD 
                 cat1 (0004)-sucD (0008) 
                 4hbd (0009) 
                 0.35 
                 407 
                 1162 
                 0.77 
                 1178 
                 1530 
               
               
                 14 
                 MG1655 lacIq gabD 
                 cat1 (0004)-sucD (0008) 
                 4hbd (0027) 
                 0.51 
                 19 
                 36 
                 1.07 
                 50 
                 47 
               
               
                 15 
                 MG1655 lacIq gabD 
                 cat1 (0004)-sucD (0008) 
                 4hbd (0036) 
                 0.35 
                 584 
                 1669 
                 0.78 
                 1350 
                 1731 
               
               
                 16 
                 MG1655 lacIq gabD 
                 cat1 (0004)-sucD (0008) 
                 4hbd (0010n) 
                 0.32 
                 74 
                 232 
                 0.82 
                 232 
                 283 
               
               
                 17 
                 MG1655 lacIq 
                 vector only 
                 vector only 
                 0.8 
                 1 
                 2 
                 1.44 
                 3 
                 2 
               
               
                 18 
                 MG1655 lacIq gabD 
                 vector only 
                 vector only 
                 0.89 
                 1 
                 2 
                 1.41 
                 7 
                 5 
               
               
                   
               
               
                 (a) Normalized 4-HB concentration, μM/OD600 units 
               
            
           
         
       
     
     An alternate to using a CoA transferase (cat1) to produce succinyl-CoA from succinate is to use the native  E. coli  sucCD genes, encoding succinyl-CoA synthetase. This gene cluster was cloned onto pZE13 along with candidate genes for the remaining steps to 4-HB to create pZE13-0038-0035-0036. 
     Production of 4-HB from Glucose. Although the above experiments demonstrate a functional pathway to 4-HB from a central metabolic intermediate (succinate), an industrial process would require the production of chemicals from low-cost carbohydrate feedstocks such as glucose or sucrose. Thus, the next set of experiments was aimed to determine whether endogenous succinate produced by the cells during growth on glucose could fuel the 4-HB pathway. Cells were grown anaerobically in M9 minimal medium (6.78 g/L Na 2 HPO 4 , 3.0 g/L KH 2 PO 4 , 0.5 g/L NaCl, 1.0 g/L NH 4 Cl, 1 mM MgSO 4 , 0.1 mM CaCl 2 )) supplemented with 20 g/L glucose, 100 mM 3-(N-morpholino)propanesulfonic acid (MOPS) to improve the buffering capacity, 10 μg/mL thiamine, and the appropriate antibiotics. 0.25 mM IPTG was added when OD600 reached approximately 0.2, and samples taken for 4-HB analysis every 24 hours following induction. In all cases 4-HB plateaued after 24 hours, with a maximum of about 1 mM in the best strains ( FIG.  11   a   ), while the succinate concentration continued to rise ( FIG.  11   b   ). This indicates that the supply of succinate to the pathway is likely not limiting, and that the bottleneck may be in the activity of the enzymes themselves or in NADH availability. 0035 and 0036 are clearly the best gene candidates for CoA-dependent succinic semialdehyde dehydrogenase and 4-HB dehydrogenase, respectively. The elimination of one or both of the genes encoding known (gabD) or putative (aldA) native succinic semialdehyde dehydrogenases had little effect on performance. Finally, it should be noted that the cells grew to a much lower OD in the 4-HB-producing strains than in the controls ( FIG.  11   c   ). 
     An alternate pathway for the production of 4-HB from glucose is via α-ketoglutarate. We explored the use of an α-ketoglutarate decarboxylase from  Mycobacterium tuberculosis  Tian et al.,  Proc. Natl. Acad. Sci. USA  102:10670-10675 (2005) to produce succinic semialdehyde directly from α-ketoglutarate (step  8  in  FIG.  2   ). To demonstrate that this gene (0032) was functional in vivo, we expressed it on pZE13 in the same host as 4-HB dehydrogenase (gene 0036) on pZA33. This strain was capable of producing over 1.0 mM 4-HB within 24 hours following induction with 1 mM IPTG ( FIG.  12   ). Since this strain does not express a CoA-dependent succinic semialdehyde dehydrogenase, the possibility of succinic semialdehyde production via succinyl-CoA is eliminated. It is also possible that the native genes responsible for producing succinic semialdehyde could function in this pathway (steps  4  and  5  in  FIG.  2   ); however, the amount of 4-HB produced when the pZE13-0032 plasmid was left out of the host is the negligible. 
     Production of BDO from 4-HB. The production of BDO from 4-HB required two reduction steps, catalyzed by dehydrogenases. Alcohol and aldehyde dehydrogenases (ADH and ALD, respectively) are NAD+/H and/or NADP+/H-dependent enzymes that together can reduce a carboxylic acid group on a molecule to an alcohol group, or in reverse, can perform the oxidation of an alcohol to a carboxylic acid. This biotransformation has been demonstrated in wild-type  Clostridium acetobutylicum  (Jewell et al.,  Current Microbiology,  13:215-19 (1986)), but neither the enzymes responsible nor the genes responsible were identified. In addition, it is not known whether activation to 4-HB-CoA is first required (step  9  in  FIG.  2   ), or if the aldehyde dehydrogenase (step  12 ) can act directly on 4-HB. We developed a list of candidate enzymes from  C. acetobutylicum  and related organisms based on known activity with the non-hydroxylated analogues to 4-HB and pathway intermediates, or by similarity to these characterized genes (Table 6). Since some of the candidates are multifunctional dehydrogenases, they could potentially catalyze both the NAD(P)H-dependent reduction of the acid (or CoA-derivative) to the aldehyde, and of the aldehyde to the alcohol. Before beginning work with these genes in  E. coli , we first validated the result referenced above using  C. acetobutylicum  ATCC 824. Cells were grown in Schaedler broth (Accumedia, Lansing, Mich.) supplemented with 10 mM 4-HB, in an anaerobic atmosphere of 10% CO 2 , 10% H 2 , and 80% N 2  at 30° C. Periodic culture samples were taken, centrifuged, and the broth analyzed for BDO by GC-MS as described below. BDO concentrations of 0.1 mM, 0.9 mM, and 1.5 mM were detected after 1 day, 2 days, and 7 days incubation, respectively. No BDO was detected in culture grown without 4-HB addition. To demonstrate that the BDO produced was derived from glucose, we grew the best BDO producing strain MG1655 lacI Q  pZE13-0004-0035-0002 pZA33-0034-0036 in M9 minimal medium supplemented with 4 g/L uniformly labeled  13 C-glucose. Cells were induced at OD of 0.67 with 1 mM IPTG, and a sample taken after 24 hours. Analysis of the culture supernatant was performed by mass spectrometry. 
     Gene candidates for the 4-HB to BDO conversion pathway were next tested for activity when expressed in the  E. coli  host MG1655 lacI Q . Recombinant strains containing each gene candidate expressed on pZA33 were grown in the presence of 0.25 mM IPTG for four hours at 37° C. to fully induce expression of the enzyme. Four hours after induction, cells were harvested and assayed for ADH and ALD activity as described above. Since 4-HB-CoA and 4-hydroxybutyraldehyde are not available commercially, assays were performed using the non-hydroxylated substrates (Table 9). The ratio in activity between 4-carbon and 2-carbon substrates for  C. acetobutylicum  adhE2 (0002) and  E. coli  adhE (0011) were similar to those previously reported in the literature a Atsumi et al.,  Biochim. Biophys. Acta.  1207:1-11 (1994). 
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 In vitro enzyme activities in cell extracts from MG1655 lacI Q  containing 
               
               
                 pZA33 expressing gene candidates for aldehyde and alcohol 
               
               
                 dehydrogenases. Activities are expressed in μmol min −1  mg cell protein −1 . 
               
            
           
           
               
               
               
            
               
                   
                 Aldehyde dehydrogenase 
                 Alcohol dehydrogenase 
               
            
           
           
               
               
            
               
                   
                 Substrate 
               
            
           
           
               
               
               
               
               
            
               
                 Gene 
                 Butyryl-CoA 
                 Acetyl-CoA 
                 Butyraldehyde 
                 Acetaldehyde 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 0002 
                 0.0076 
                 0.0046 
                 0.0264 
                 0.0247 
               
               
                 0003n 
                 0.0060 
                 0.0072 
                 0.0080 
                 0.0075 
               
               
                 0011 
                 0.0069 
                 0.0095 
                 0.0265 
                 0.0093 
               
               
                 0013 
                 N.D. 
                 N.D. 
                 0.0130 
                 0.0142 
               
               
                 0023 
                 0.0089 
                 0.0137 
                 0.0178 
                 0.0235 
               
               
                 0025 
                 0 
                 0.0001 
                 N.D. 
                 N.D. 
               
               
                 0026 
                 0 
                 0.0005 
                 0.0024 
                 0.0008 
               
               
                   
               
               
                 N.D., not determined. 
               
            
           
         
       
     
     For the BDO production experiments, cat2 from  Porphyromonas gingivalis  W83 (gene 0034) was included on pZA33 for the conversion of 4-HB to 4-HB-CoA, while the candidate dehydrogenase genes were expressed on pZE13. The host strain was MG1655 lacI Q . Along with the alcohol and aldehyde dehydrogenase candidates, we also tested the ability of CoA-dependent succinic semialdehyde dehydrogenases (sucD) to function in this step, due to the similarity of the substrates. Cells were grown to an OD of about 0.5 in LB medium supplemented with 10 mM 4-HB, induced with 1 mM IPTG, and culture broth samples taken after 24 hours and analyzed for BDO as described below. The best BDO production occurred using adhE2 from  C. acetobutylicum , sucD from  C. kluyveri , or sucD from  P. gingivalis  ( FIG.  13   ). Interestingly, the absolute amount of BDO produced was higher under aerobic conditions; however, this is primarily due to the lower cell density achieved in anaerobic cultures. When normalized to cell OD, the BDO production per unit biomass is higher in anaerobic conditions (Table 10). 
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                 Absolute and normalized BDO concentrations from cultures of cells 
               
               
                 expressing adhE2 from  C. acetobutylicum , sucD from  C. kluyveri , 
               
               
                 or sucD from  P. gingivalis  (data from experiments 2, 9, and 10 
               
               
                 in FIG. 11), as well as the negative control (experiment 1). 
               
            
           
           
               
               
               
               
               
            
               
                 Gene 
                   
                 BDO 
                 OD 
                   
               
               
                 expressed 
                 Conditions 
                 (μM) 
                 (600 nm) 
                 BDO/OD 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 none 
                 Aerobic 
                 0 
                 13.4 
                 0 
               
               
                 none 
                 Microaerobic 
                 0.5 
                 6.7 
                 0.09 
               
               
                 none 
                 Anaerobic 
                 2.2 
                 1.26 
                 1.75 
               
               
                 0002 
                 Aerobic 
                 138.3 
                 9.12 
                 15.2 
               
               
                 0002 
                 Microaerobic 
                 48.2 
                 5.52 
                 8.73 
               
               
                 0002 
                 Anaerobic 
                 54.7 
                 1.35 
                 40.5 
               
               
                 0008n 
                 Aerobic 
                 255.8 
                 5.37 
                 47.6 
               
               
                 0008n 
                 Microaerobic 
                 127.9 
                 3.05 
                 41.9 
               
               
                 0008n 
                 Anaerobic 
                 60.8 
                 0.62 
                 98.1 
               
               
                 0035 
                 Aerobic 
                 21.3 
                 14.0 
                 1.52 
               
               
                 0035 
                 Microaerobic 
                 13.1 
                 4.14 
                 3.16 
               
               
                 0035 
                 Anaerobic 
                 21.3 
                 1.06 
                 20.1 
               
               
                   
               
            
           
         
       
     
     As discussed in Section 2, it may be advantageous to use a route for converting 4-HB to 4-HB-CoA that does not generate acetate as a byproduct. To this aim, we tested the use of phosphotransbutyrylase (ptb) and butyrate kinase (bk) from  C. acetobutylicum  to carry out this conversion via steps  10  and  11  in  FIG.  2   . The native ptb/bk operon from  C. acetobutylicum  (genes 0020 and 0021) was cloned and expressed in pZA33. Extracts from cells containing the resulting construct were taken and assayed for the two enzyme activities as described herein. The specific activity of BK was approximately 65 U/mg, while the specific activity of PTB was approximately 5 U/mg. One unit (U) of activity is defined as conversion of 1 μM substrate in 1 minute at room temperature. Finally, the construct was tested for participation in the conversion of 4-HB to BDO. Host strains were transformed with the pZA33-0020-0021 construct described and pZE13-0002, and compared to use of cat2 in BDO production using the aerobic procedure used above in  FIG.  13   . The BK/PTB strain produced 1 mM BDO, compared to 2 mM when using cat2 (Table 11). Interestingly, the results were dependent on whether the host strain contained a deletion in the native adhE gene. 
     
       
         
           
               
             
               
                 TABLE 11 
               
             
            
               
                   
               
               
                 Absolute and normalized BDO concentrations from cultures of cells 
               
               
                 expressing adhE2 from  C. acetobutylicum  in pZE13 along with 
               
               
                 either cat2 from  P. gingivalis  (0034) or the PTB/BK genes from 
               
               
                   C. acetobutylicum  on pZA33. Host strains were either MG1655 
               
               
                 lacI Q  or MG1655 ΔadhE lacI Q . 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 BDO 
                 OD 
                   
               
               
                 Genes 
                 Host Strain 
                 (μM) 
                 (600 nm) 
                 BDO/OD 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 0034 
                 MG1655 lacI Q   
                 0.827 
                 19.9 
                 0.042 
               
               
                 0020 + 0021 
                 MG1655 lacI Q   
                 0.007 
                 9.8 
                 0.0007 
               
               
                 0034 
                 MG1655 ΔadhE 
                 2.084 
                 12.5 
                 0.166 
               
               
                   
                 lacI Q   
                   
                   
                   
               
               
                 0020 + 0021 
                 MG1655 ΔadhE 
                 0.975 
                 18.8 
                 0.052 
               
               
                   
                 lacI Q   
               
               
                   
               
            
           
         
       
     
     Production of BDO from Glucose. The final step of pathway corroboration is to express both the 4-HB and BDO segments of the pathway in  E. coli  and demonstrate production of BDO in glucose minimal medium. New plasmids were constructed so that all the required genes fit on two plasmids. In general, cat1, adhE, and sucD genes were expressed from pZE13, and cat2 and 4-HBd were expressed from pZA33. Various combinations of gene source and gene order were tested in the MG1655 lacI Q  background. Cells were grown anaerobically in M9 minimal medium (6.78 g/L Na 2 HPO 4 , 3.0 g/L KH 2 PO 4 , 0.5 g/L NaCl, 1.0 g/L NH 4 Cl, 1 mM MgSO 4 , 0.1 mM CaCl 2 )) supplemented with 20 g/L glucose, 100 mM 3-(N-morpholino)propanesulfonic acid (MOPS) to improve the buffering capacity, 10 μg/mL thiamine, and the appropriate antibiotics. 0.25 mM IPTG was added approximately 15 hours following inoculation, and culture supernatant samples taken for BDO, 4-HB, and succinate analysis 24 and 48 hours following induction. The production of BDO appeared to show a dependency on gene order (Table 12). The highest BDO production, over 0.5 mM, was obtained with cat2 expressed first, followed by 4-HBd on pZA33, and cat1 followed by  P. gingivalis  sucD on pZE13. The addition of  C. acetobutylicum  adhE2 in the last position on pZE13 resulted in slight improvement. 4-HB and succinate were also produced at higher concentrations. 
     
       
         
           
               
             
               
                 TABLE 12 
               
             
            
               
                   
               
               
                 Production of BDO, 4-HB, and succinate in recombinant  E. coli  strains expressing combinations of BDO 
               
               
                 pathway genes, grown in minimal medium supplemented with 20 g/L glucose. Concentrations are given in mM. 
               
            
           
           
               
               
               
            
               
                   
                 24 Hours 
                 48 Hours 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Induc- 
                 OD600 
                   
                   
                   
                 OD600 
                   
                   
                   
               
               
                 Sample 
                 pZE13 
                 pZA33 
                 tion OD 
                 nm 
                 Su 
                 4HB 
                 BDO 
                 nm 
                 Su 
                 4HB 
                 BDO 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 cat1(0004)-sucD(0035) 
                 4hbd (0036)-cat2(0034) 
                 0.92 
                 1.29 
                 5.44 
                 1.37 
                 0.240 
                 1.24 
                 6.42 
                 1.49 
                 0.280 
               
               
                 2 
                 cat1(0004)-sucD(0008N) 
                 4hbd (0036)-cat2(0034) 
                 0.36 
                 1.11 
                 6.90 
                 1.24 
                 0.011 
                 1.06 
                 7.63 
                 1.33 
                 0.011 
               
               
                 3 
                 adhE(0002)-cat1(0004)- 
                 4hbd (0036)-cat2(0034) 
                 0.20 
                 0.44 
                 0.34 
                 1.84 
                 0.050 
                 0.60 
                 1.93 
                 2.67 
                 0.119 
               
               
                   
                 sucD(0035) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 4 
                 cat1(0004)-sucD(0035)- 
                 4hbd (0036)-cat2(0034) 
                 1.31 
                 1.90 
                 9.02 
                 0.73 
                 0.073 
                 1.95 
                 9.73 
                 0.82 
                 0.077 
               
               
                   
                 adhE(0002) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 5 
                 adhE(0002)-cat1(0004)- 
                 4hbd (0036)-cat2(0034) 
                 0.17 
                 0.45 
                 1.04 
                 1.04 
                 0.008 
                 0.94 
                 7.13 
                 1.02 
                 0.017 
               
               
                   
                 sucD(0008N) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 6 
                 cat1(0004)-sucD(0008N)- 
                 4hbd (0036)-cat2(0034) 
                 1.30 
                 1.77 
                 10.47 
                 0.25 
                 0.004 
                 1.80 
                 11.49 
                 0.28 
                 0.003 
               
               
                   
                 adhE(0002) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 7 
                 cat1(0004)-sucD(0035) 
                 cat2(0034)-4hbd(0036) 
                 1.09 
                 1.29 
                 5.63 
                 2.15 
                 0.461 
                 1.38 
                 6.66 
                 2.30 
                 0.520 
               
               
                 8 
                 cat1(0004)-sucD(0008N) 
                 cat2(0034)-4hbd(0036) 
                 1.81 
                 2.01 
                 11.28 
                 0.02 
                 0.000 
                 2.24 
                 11.13 
                 0.02 
                 0.000 
               
               
                 9 
                 adhE(0002)-cat1(0004)- 
                 cat2(0034)-4hbd(0036) 
                 0.24 
                 1.99 
                 2.02 
                 2.32 
                 0.106 
                 0.89 
                 4.85 
                 2.41 
                 0.186 
               
               
                   
                 sucD(0035) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 10 
                 cat1(0004)-sucD(0035)- 
                 cat2(0034)-4hbd(0036) 
                 0.98 
                 1.17 
                 5.30 
                 2.08 
                 0.569 
                 1.33 
                 6.15 
                 2.14 
                 0.640 
               
               
                   
                 adhE(0002) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 11 
                 adhE(0002)-cat1(0004)- 
                 cat2(0034)-4hbd(0036) 
                 0.20 
                 0.53 
                 1.38 
                 2.30 
                 0.019 
                 0.91 
                 8.10 
                 1.49 
                 0.034 
               
               
                   
                 sucD(0008N) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 12 
                 cat1(0004)-sucD(0008N)- 
                 cat2(0034)-4hbd(0036) 
                 2.14 
                 2.73 
                 12.07 
                 0.16 
                 0.000 
                 3.10 
                 11.79 
                 0.17 
                 0.002 
               
               
                   
                 adhE(0002) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 13 
                 vector only 
                 vector only 
                 2.11 
                 2.62 
                 9.03 
                 0.01 
                 0.000 
                 3.00 
                 12.05 
                 0.01 
                 0.000 
               
               
                   
               
            
           
         
       
     
     Analysis of BDO, 4-HB and succinate by GCMS. BDO, 4-HB and succinate in fermentation and cell culture samples were derivatized by silylation and quantitatively analyzed by GCMS using methods adapted from literature reports ((Simonov et al.,  J. Anal Chem.  59:965-971 (2004)). The developed method demonstrated good sensitivity down to 1 M, linearity up to at least 25 mM, as well as excellent selectivity and reproducibility. 
     Sample preparation was performed as follows: 100 μL filtered (0.2 μm or 0.45 μm syringe filters) samples, e.g. fermentation broth, cell culture or standard solutions, were dried down in a Speed Vac Concentrator (Savant SVC-100H) for approximately 1 hour at ambient temperature, followed by the addition of 20 μL 10 mM cyclohexanol solution, as an internal standard, in dimethylformamide. The mixtures were vortexed and sonicated in a water bath (Branson 3510) for 15 min to ensure homogeneity. 100 μL silylation derivatization reagent, N,O-bis(trimethylsilyl)triflouro-acetimide (BSTFA) with 1% trimethylchlorosilane, was added, and the mixture was incubated at 70° C. for 30 min. The derivatized samples were centrifuged for 5 min, and the clear solutions were directly injected into GCMS. All the chemicals and reagents were from Sigma-Aldrich, with the exception of BDO which was purchased from J.T.Baker. 
     GCMS was performed on an Agilent gas chromatograph 6890N, interfaced to a mass-selective detector (MSD) 5973N operated in electron impact ionization (EI) mode has been used for the analysis. A DB-5MS capillary column (J&amp;W Scientific, Agilent Technologies), 30 m×0.25 mm i.d.×0.25 m film thickness, was used. The GC was operated in a split injection mode introducing 1 μL of sample at 20:1 split ratio. The injection port temperature was 250° C. Helium was used as a carrier gas, and the flow rate was maintained at 1.0 mL/min. A temperature gradient program was optimized to ensure good resolution of the analytes of interest and minimum matrix interference. The oven was initially held at 80° C. for 1 min, then ramped to 120° C. at 2° C./min, followed by fast ramping to 320° C. at 100° C./min and final hold for 6 min at 320° C. The MS interface transfer line was maintained at 280° C. The data were acquired using ‘lowmass’ MS tune settings and 30-400 m/z mass-range scan. The total analysis time was 29 min including 3 min solvent delay. The retention times corresponded to 5.2, 10.5, 14.0 and 18.2 min for BSTFA-derivatized cyclohexanol, BDO, 4-HB and succinate, respectively. For quantitative analysis, the following specific mass fragments were selected (extracted ion chromatograms): m/z 157 for internal standard cyclohexanol, 116 for BDO, and 147 for both 4-HB and succinate. Standard calibration curves were constructed using analyte solutions in the corresponding cell culture or fermentation medium to match sample matrix as close as possible. GCMS data were processed using Environmental Data Analysis ChemStation software (Agilent Technologies). 
     The results indicated that most of the 4-HB and BDO produced were labeled with  13 C ( FIG.  14   , right-hand sides). Mass spectra from a parallel culture grown in unlabeled glucose are shown for comparison ( FIG.  14   , left-hand sides). Note that the peaks seen are for fragments of the derivatized molecule containing different numbers of carbon atoms from the metabolite. The derivatization reagent also contributes some carbon and silicon atoms that naturally-occurring label distribution, so the results are not strictly quantitative. 
     Production of BDO from 4-HB using alternate pathways. The various alternate pathways were also tested for BDO production. This includes use of the native  E. coli  SucCD enzyme to convert succinate to succinyl-CoA (Table 13, rows 2-3), use of α-ketoglutarate decarboxylase in the α-ketoglutarate pathway (Table 13, row 4), and use of PTB/BK as an alternate means to generate the CoA-derivative of 4HB (Table 13, row 1). Strains were constructed containing plasmids expressing the genes indicated in Table 13, which encompass these variants. The results show that in all cases, production of 4-HB and BDO occurred (Table 13). 
     
       
         
           
               
             
               
                 TABLE 13 
               
             
            
               
                   
               
               
                 Production of BDO, 4-HB, and succinate in recombinant  E. coli  strains 
               
               
                 genes for different BDO pathway variants, grown anaerobically in 
               
               
                 minimal medium supplemented with 20 g/L glucose, and harvested 24 
               
               
                 hours after induction with 0.1 mM IPTG. Concentrations are given in mM. 
               
            
           
           
               
               
               
               
               
            
               
                 Genes on pZE13 
                 Genes on pZA33 
                 Succinate 
                 4-HB 
                 BDO 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 0002 + 0004 + 0035 
                 0020n − 0021n − 0036 
                 0.336 
                 2.91 
                 0.230 
               
               
                 0038 + 0035 
                 0034 − 0036 
                 0.814 
                 2.81 
                 0.126 
               
               
                 0038 + 0035 
                 0036 − 0034 
                 0.741 
                 2.57 
                 0.114 
               
               
                 0035 + 0032 
                 0034 − 0036 
                 5.01 
                 0.538 
                 0.154 
               
               
                   
               
            
           
         
       
     
     Example V 
     Biosynthesis of 4-Hydroxybutanoic Acid, γ-Butyrolactone and 1,4-Butanediol 
     This Example describes the biosynthetic production of 4-hydroxybutanoic acid, γ-butyrolactone and 1,4-butanediol using fermentation and other bioprocesses. 
     Methods for the integration of the 4-HB fermentation step into a complete process for the production of purified GBL, 1,4-butanediol (BDO) and tetrahydrofuran (THF) are described below. Since 4-HB and GBL are in equilibrium, the fermentation broth will contain both compounds. At low pH this equilibrium is shifted to favor GBL. Therefore, the fermentation can operate at pH 7.5 or less, generally pH 5.5 or less. After removal of biomass, the product stream enters into a separation step in which GBL is removed and the remaining stream enriched in 4-HB is recycled. Finally, GBL is distilled to remove any impurities. The process operates in one of three ways: 1) fed-batch fermentation and batch separation; 2) fed-batch fermentation and continuous separation; 3) continuous fermentation and continuous separation. The first two of these modes are shown schematically in  FIG.  15   . The integrated fermentation procedures described below also are used for the BDO producing cells of the invention for biosynthesis of BDO and subsequent BDO family products. 
     Fermentation protocol to produce 4-HB/GBL (batch): The production organism is grown in a 10 L bioreactor sparged with an N 2 /CO 2  mixture, using 5 L broth containing 5 g/L potassium phosphate, 2.5 g/L ammonium chloride, 0.5 g/L magnesium sulfate, and 30 g/L corn steep liquor, and an initial glucose concentration of 20 g/L. As the cells grow and utilize the glucose, additional 70% glucose is fed into the bioreactor at a rate approximately balancing glucose consumption. The temperature of the bioreactor is maintained at 30 degrees C. Growth continues for approximately 24 hours, until 4-HB reaches a concentration of between 20-200 g/L, with the cell density being between 5 and 10 g/L. The pH is not controlled, and will typically decrease to pH 3-6 by the end of the run. Upon completion of the cultivation period, the fermenter contents are passed through a cell separation unit (e.g., centrifuge) to remove cells and cell debris, and the fermentation broth is transferred to a product separations unit. Isolation of 4-HB and/or GBL would take place by standard separations procedures employed in the art to separate organic products from dilute aqueous solutions, such as liquid-liquid extraction using a water immiscible organic solvent (e.g., toluene) to provide an organic solution of 4-HB/GBL. The resulting solution is then subjected to standard distillation methods to remove and recycle the organic solvent and to provide GBL (boiling point 204-205° C.) which is isolated as a purified liquid. 
     Fermentation protocol to produce 4-HB/GBL (fully continuous): The production organism is first grown up in batch mode using the apparatus and medium composition described above, except that the initial glucose concentration is 30-50 g/L. When glucose is exhausted, feed medium of the same composition is supplied continuously at a rate between 0.5 L/hr and 1 L/hr, and liquid is withdrawn at the same rate. The 4-HB concentration in the bioreactor remains constant at 30-40 g/L, and the cell density remains constant between 3-5 g/L. Temperature is maintained at 30 degrees C., and the pH is maintained at 4.5 using concentrated NaOH and HCl, as required. The bioreactor is operated continuously for one month, with samples taken every day to assure consistency of 4-HB concentration. In continuous mode, fermenter contents are constantly removed as new feed medium is supplied. The exit stream, containing cells, medium, and products 4-HB and/or GBL, is then subjected to a continuous product separations procedure, with or without removing cells and cell debris, and would take place by standard continuous separations methods employed in the art to separate organic products from dilute aqueous solutions, such as continuous liquid-liquid extraction using a water immiscible organic solvent (e.g., toluene) to provide an organic solution of 4-HB/GBL. The resulting solution is subsequently subjected to standard continuous distillation methods to remove and recycle the organic solvent and to provide GBL (boiling point 204-205° C.) which is isolated as a purified liquid. 
     GBL Reduction Protocol: Once GBL is isolated and purified as described above, it will then be subjected to reduction protocols such as those well known in the art (references cited) to produce 1,4-butanediol or tetrahydrofuran (THF) or a mixture thereof. Heterogeneous or homogeneous hydrogenation catalysts combined with GBL under hydrogen pressure are well known to provide the products 1,4-butanediol or tetrahydrofuran (THF) or a mixture thereof. It is important to note that the 4-HB/GBL product mixture that is separated from the fermentation broth, as described above, may be subjected directly, prior to GBL isolation and purification, to these same reduction protocols to provide the products 1,4-butanediol or tetrahydrofuran or a mixture thereof. The resulting products, 1,4-butanediol and THF are then isolated and purified by procedures well known in the art. 
     Fermentation and hydrogenation protocol to produce BDO or THF directly (batch): Cells are grown in a 10 L bioreactor sparged with an N 2 /CO 2  mixture, using 5 L broth containing 5 g/L potassium phosphate, 2.5 g/L ammonium chloride, 0.5 g/L magnesium sulfate, and 30 g/L corn steep liquor, and an initial glucose concentration of 20 g/L. As the cells grow and utilize the glucose, additional 70% glucose is fed into the bioreactor at a rate approximately balancing glucose consumption. The temperature of the bioreactor is maintained at 30 degrees C. Growth continues for approximately 24 hours, until 4-HB reaches a concentration of between 20-200 g/L, with the cell density being between 5 and 10 g/L. The pH is not controlled, and will typically decrease to pH 3-6 by the end of the run. Upon completion of the cultivation period, the fermenter contents are passed through a cell separation unit (e.g., centrifuge) to remove cells and cell debris, and the fermentation broth is transferred to a reduction unit (e.g., hydrogenation vessel), where the mixture 4-HB/GBL is directly reduced to either 1,4-butanediol or THF or a mixture thereof. Following completion of the reduction procedure, the reactor contents are transferred to a product separations unit. Isolation of 1,4-butanediol and/or THF would take place by standard separations procedures employed in the art to separate organic products from dilute aqueous solutions, such as liquid-liquid extraction using a water immiscible organic solvent (e.g., toluene) to provide an organic solution of 1,4-butanediol and/or THF. The resulting solution is then subjected to standard distillation methods to remove and recycle the organic solvent and to provide 1,4-butanediol and/or THE which are isolated as a purified liquids. 
     Fermentation and hydrogenation protocol to produce BDO or THF directly (fully continuous): The cells are first grown up in batch mode using the apparatus and medium composition described above, except that the initial glucose concentration is 30-50 g/L. When glucose is exhausted, feed medium of the same composition is supplied continuously at a rate between 0.5 L/hr and 1 L/hr, and liquid is withdrawn at the same rate. The 4-HB concentration in the bioreactor remains constant at 30-40 g/L, and the cell density remains constant between 3-5 g/L. Temperature is maintained at 30 degrees C., and the pH is maintained at 4.5 using concentrated NaOH and HCl, as required. The bioreactor is operated continuously for one month, with samples taken every day to assure consistency of 4-HB concentration. In continuous mode, fermenter contents are constantly removed as new feed medium is supplied. The exit stream, containing cells, medium, and products 4-HB and/or GBL, is then passed through a cell separation unit (e.g., centrifuge) to remove cells and cell debris, and the fermentation broth is transferred to a continuous reduction unit (e.g., hydrogenation vessel), where the mixture 4-HB/GBL is directly reduced to either 1,4-butanediol or THF or a mixture thereof. Following completion of the reduction procedure, the reactor contents are transferred to a continuous product separations unit. Isolation of 1,4-butanediol and/or THF would take place by standard continuous separations procedures employed in the art to separate organic products from dilute aqueous solutions, such as liquid-liquid extraction using a water immiscible organic solvent (e.g., toluene) to provide an organic solution of 1,4-butanediol and/or THF. The resulting solution is then subjected to standard continuous distillation methods to remove and recycle the organic solvent and to provide 1,4-butanediol and/or THF which are isolated as a purified liquids. 
     Fermentation protocol to produce BDO directly (batch): The production organism is grown in a 10 L bioreactor sparged with an N 2 /CO 2  mixture, using 5 L broth containing 5 g/L potassium phosphate, 2.5 g/L ammonium chloride, 0.5 g/L magnesium sulfate, and 30 g/L corn steep liquor, and an initial glucose concentration of 20 g/L. As the cells grow and utilize the glucose, additional 70% glucose is fed into the bioreactor at a rate approximately balancing glucose consumption. The temperature of the bioreactor is maintained at 30 degrees C. Growth continues for approximately 24 hours, until BDO reaches a concentration of between 20-200 g/L, with the cell density generally being between 5 and 10 g/L. Upon completion of the cultivation period, the fermenter contents are passed through a cell separation unit (e.g., centrifuge) to remove cells and cell debris, and the fermentation broth is transferred to a product separations unit. Isolation of BDO would take place by standard separations procedures employed in the art to separate organic products from dilute aqueous solutions, such as liquid-liquid extraction using a water immiscible organic solvent (e.g., toluene) to provide an organic solution of BDO. The resulting solution is then subjected to standard distillation methods to remove and recycle the organic solvent and to provide BDO (boiling point 228-229° C.) which is isolated as a purified liquid. 
     Fermentation protocol to produce BDO directly (fully continuous): The production organism is first grown up in batch mode using the apparatus and medium composition described above, except that the initial glucose concentration is 30-50 g/L. When glucose is exhausted, feed medium of the same composition is supplied continuously at a rate between 0.5 L/hr and 1 L/hr, and liquid is withdrawn at the same rate. The BDO concentration in the bioreactor remains constant at 30-40 g/L, and the cell density remains constant between 3-5 g/L. Temperature is maintained at 30 degrees C., and the pH is maintained at 4.5 using concentrated NaOH and HCl, as required. The bioreactor is operated continuously for one month, with samples taken every day to assure consistency of BDO concentration. In continuous mode, fermenter contents are constantly removed as new feed medium is supplied. The exit stream, containing cells, medium, and the product BDO, is then subjected to a continuous product separations procedure, with or without removing cells and cell debris, and would take place by standard continuous separations methods employed in the art to separate organic products from dilute aqueous solutions, such as continuous liquid-liquid extraction using a water immiscible organic solvent (e.g., toluene) to provide an organic solution of BDO. The resulting solution is subsequently subjected to standard continuous distillation methods to remove and recycle the organic solvent and to provide BDO (boiling point 228-229° C.) which is isolated as a purified liquid (mpt 20° C.). 
     Throughout this application various publications have been referenced within parentheses. The disclosures of these publications in their entireties are hereby incorporated by reference in this application in order to more fully describe the state of the art to which this invention pertains. 
     Although the invention has been described with reference to the disclosed embodiments, those skilled in the art will readily appreciate that the specific examples and studies detailed above are only illustrative of the invention. It should be understood that various modifications can be made without departing from the spirit of the invention. Accordingly, the invention is limited only by the following claims.