Patent Publication Number: US-2019194698-A1

Title: Host cells and methods for producing hydroxytyrosol

Description:
RELATED PATENT APPLICATIONS 
     This application is a continuation of international Application No. PCT/US2017/039329, filed Jun. 26, 2017, which claims the benefit of U.S. Provisional Patent Application Ser. No. 62/354,657, filed Jun. 24, 2016, which applications are herein incorporated by reference in their entirety. 
    
    
     STATEMENT OF GOVERNMENTAL SUPPORT 
     The invention described and claimed herein was made utilizing funds supplied by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention is in the field of production of hydroxytyrosol. 
     BACKGROUND OF THE INVENTION 
     Hydroxylation of aromatic rings is an important reaction used for the preparation of many valuable compounds including L-3,4-dihydroxyphenylalanine (L-DOPA) for the treatment of Parkinson&#39;s disease, benzylisoquinoline alkaloids, and melatonin. Compared with chemical reaction which frequently uses metallic oxidants in organic solvent, hydroxylation of aromatic ring by microorganisms is an interesting and promising method to synthesize the desired products in a single-step with a high regioselectivity and under mild conditions. Microbial aromatic hydroxylation is involved in the aerobic metabolism of aromatic compounds and mostly performed by oxygenases and tyrosinases during the degradation process either to relieve the toxicity or to metabolize them into organic acid to use as carbon sources. 
     Tyrosinase is an oxidoreductase belongs to type-3 copper protein which includes hemocyanins as an oxygen carrier. (Olivares, 2009; Robb, 1984) This enzyme involves multiple oxidation reaction of L-tyrosine using molecular oxygen as oxidant; the first oxidation step is o-hydroxylation of L-tyrosine to L-DOPA and is known to be the slowest step, and the second oxidation step is the production of o-quinone from o-diphenol which is fast and followed by non-enzymatic reaction to dopachrome, a colored intermediate to melanin pathway. Microbial conversion of tyrosine to L-DOPA is slow process, and the over-oxidation to ortho-quinone is hard to avoid when tyrosinase is used. The use of reducing agent such as ascorbic acid adds more step for the purification of the product from fermentation broth. 
     L-DOPA is an important compound to living cells, especially in animal since it is used as a precursor for many neurotransmitters, and in animal brain, L-DOPA was synthesized by tyrosine hydroxylase (TH) with tetrahydrobiopterin (BH4) as a cofactor. (Kappock, Chem. Rev. 1996; Fitzpatrick, Ann Rev Biochem 1999; Daubner, Arch Biochem Biophys 2011) The use of pterin cofactor during the oxidation step is unique feature of TH and related enzyme such as phenylalanine hydroxylase (PAH) and tryptophan hydroxylase (TPH), (Pribat, J. Bacteriol. 2010) and this helps to prevent over-oxidation of L-tyrosine to o-quinone product which is a problem in microbial L-DOPA production by tyrosinase (Maass, 2003). However, the application of TH enzyme to microbial metabolic engineering has not been reported due to the unavailability of the coenzyme BH4 in microbes. BH4 is a unique co-factor found in animal and no bacterial system has been reported to use BH4 for biosynthesis of L-DOPA or related metabolites. 
     Hydroxytyrosol (HTy) is a high value compound and there is an increasing demand for the stable and sustainable production of HTy with high purity. HTy is one of the most powerful antioxidants with potential biological function as an anti-tumor, anti-atheragenic, anti-inflammatory and/or anti-platelet aggregation agent. It has a wide range of potential applications in industry, such as functional food, dietary supplement, cosmetics, and animal feed. 
     Currently HTy is produced from enriched olive extracts after chemical or enzymatic hydrolysis. However, this method is difficult to apply for a high purity product, since the extracts are complex mixtures of compounds with similar structures. 
     SUMMARY OF THE INVENTION 
     The present invention provides for a composition comprising: (a) a first host cell capable of producing L-DOPA; and (b) a second host cell capable of converting L-DOPA into hydroxytyrosol (HTy); wherein any one or both of the first host cell and the second host cell is a genetically modified host cell. In some embodiments, the first host cell is a first genetically modified host cell, and the second host cell is a second genetically modified host cell. 
     In some embodiments, the first host cell comprises dihydropteridine reductase (DHPR), pterin-4-alpha-carbinolamine dehydratase (PCD), and tyrosine hydroxylase (TH), or any homologous enzyme thereof. In some embodiments, one or more of DHPR, PCD, and TH are heterologous to the first host cell. 
     In some embodiments, the first host cell is engineered to overproduce tyrosine compared to a non-engineered cell, and the first host cell comprises tyrosine hydroxylase (TH), or a homologous enzyme thereof. In some embodiments, the first host cell is a feedback resistant mutant comprising a means to overexpress AroG, or any homologous enzymes thereof, and a means to overexpress TyrA, or a homologous enzyme thereof. In some embodiments, the means to overexpress AroG is one or more copies of the aroG gene introduced into the host cell, either integrated in a chornosome of the host cell or on a plasmid. In some embodiments, the means to overexpress TyrA is one or more copies of the tyrA gene introduced into the host cell, either integrated in a chornosome of the host cell or on a plasmid. In some embodiments, the aroG and/or tyrA genes are transcribed from a strong constitutive promoter. In some embodiments, the first host cell comprises one or more, or all, of the following enzymes, or a corresponding homologous enzyme thereof, for the synthesis of L-tyrosine: phosphoenolpyruvate synthase (PpsA), transketolase A (TktA), DAHP synthase (AroG), DHQ synthase (AroB), DHQ dehydratase (AroD), quinate/shikimate dehydrogenase (YdiB), shikimate dehydrogenase (AroE), shikimate kinase I/II (AroK/L), EPSP synthase (AroA), chorismate synthase (AroC), chorismate mutase/prephenate dehydrogenase (TyrA), and tyrosine aminotransferase (TyrB). In some embodiments, the first host cell is engineered to overproduce tyrosine is capable of producing 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, or 2.6, or more, mM of L-tyrosine when the first host cell is grown or cultured in a M9Y defined medium (1% glucose). 
     In some embodiments, the second host cell comprises L-DOPA decarboxylase (DDC), tyramine oxidase (TYO), and alcohol dehydrogenase (ADH), or any homologous enzyme thereof. In some embodiments, one or more of DDC, TYO, and ADH are heterologous to the second host cell. 
     The present invention provides for a method for producing HTy comprising: (a) providing a first host cell, (b) culturing the first host cell to produce a first culture, (c) providing a second host cell, (d) optionally culturing the second host cell to produce a second culture, (e) combining or mixing the first and second cultures to produce a co-culture comprising the composition of the present invention, and (f) culturing the first and second host cells in the composition such that HTy is produced, and (g) optionally extracting or separating the HTy from the co-culture. 
     The present invention provides for a production method to produce HTy with significant improvement in titer from what was previously reported (Satoh et al. Metabolic Engineering 14 (2012) 603-610). In some embodiments, there are four metabolic and process engineering approaches to improve the efficiency of the synthetic HTy pathway ( FIG. 1 ). 
     The HTy pathway comprises five heterologous enzymes (i.e., heterologous to each other) for tyrosine hydroxylation and downstream conversion of L-DOPA to HTy ( FIG. 2 ). 
     The present invention also provides for a bioreactor design for industrial application comprising the composition of the present invention. 
     The present invention also provides for a genetically modified host cell useful for the methods of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and others will be readily appreciated by the skilled artisan from the following description of illustrative embodiments when read in conjunction with the accompanying drawings. 
         FIG. 1 . Summary of hydroxytyrosol producer engineering. 
         FIG. 2 . Hydroxytyrosol production pathway. TH: tyrosine hydroxylase from mouse, synthesized. DDC: L-DOPA decarboxylase from pig, synthesized. MAO: monoamine oxidase from  Micrococcus luteus,  cloned. 
         FIG. 3 . JW1380 (feaB knock out), pBbE1k-TH-DHPR-PCD (top part), pBbS2c-(co-factor gene). Media M9Y (1 mM L-tyrosine, Ascorbic acid), 37 mL flask culture, Induction OD 0.5. Top part under pTrc (IPTG): 500 μM. Co-factor gene under pTet (aTc): 40 nM. Tyrosine and Ascorbic acid were supplemented at the induction. The supernatant is analyzed by LC-MS at 6, 18, 32 or 39, 53 hours after induction. Error bars indicate standard deviations from triplicate biological replicates. 
         FIG. 4 . Top: JW1380, pBbE1k-TH-reg, pBbS1a (empty vector). Media M9Y (1 mM L-tyrosine), 5 mL tube culture, Induction OD 0.4. Top part under pTrc (IPTG): 500 μM. Tyrosine or downstream chemical is supplemented at the induction. The supernatant is analyzed by LC-MS at 25, 42, 76 hours after induction. Error bars indicate standard deviations from triplicate biological replicates. 
         FIG. 5A . Co-culture scheme version 1 from tyrosine. 
         FIG. 5B . Products profile of co-culture from 1 mM tyrosine. Co-culture: Top-folE: JW1390, pBbE1k-TH-reg-folE, pBbS1a (empty vector). Bottom: JW1380, pBbE2k-TYO, pBbA1a-DDC-RFP. Bottom cells are collected 4 hours after induction, suspended with top strain and further cultured for 40 h (Total 65 h). Single strain: Top-folE+DDC-TYO: JW1380, pBbE1k-TH-reg-folE, pBbS1a-DDC-TYO. The supernatant is analyzed by LC-MS at 40 hours after induction. 
         FIG. 6A . Co-culture scheme version 2 from glucose. 
         FIG. 6B . Products profile of co-culture from 5 g/L glucose. Top: DK176, pBbE1k-TH-reg-folE, pBbS1a (empty vector). Bottom: JW1380, pBbE2k-TYO, pBbA1a-DDC-RFP. Media M9Y (5 g/L glucose), 10 mL flask culture, induction OD 0.4. 
         FIG. 7A . Co-culture scheme for HTy production (from Tyrosine). 
         FIG. 7B . Products profile (single strain culture vs co-culture). Top+Bottom (single strain): JW1380, pBbE1k-TH-reg, pBbS1a-DDC-TYO. Top-folE+Bottom (single strain): JW1380, pBbE1k-TH-reg-folE, pBbS1a-DDC-TYO. Top: JW1380, pBbE1k-TH-reg, pBbS1a (empty vector). Top: JW1380, pBbE1k-TH-reg-folE, pBbS1a (empty vector). Bottom A: JW1380, pBbE2k-TYO, pBbA1a-DDC-RFP. Bottom B: JW1380, pBbS1a-DDC-TYO, pBbE1k (empty vector). Media M9Y (1 mM L-tyrosine), 5 mL tube culture, Induction OD 0.4. Top part under pTrc (IPTG): 500 μM. Tyrosine is supplemented at the induction. The supernatant is analyzed by LC-MS at 73 hours after induction. Error bars indicate standard deviations from triplicate biological replicates. 
         FIG. 8A . Co-culture scheme (from glucose). 
         FIG. 8B . HTy production in co-culture from glucose. Top: DK176, pBbE1k-TH-reg-folE, pBbS1a (empty vector). Bottom A: JW1380, pBbE2k-TYO, pBbA1a-DDC-RFP. Media M9Y (5 g/L glucose). Error bars indicate standard deviations from triplicate biological replicates. 
         FIG. 8C . Simplified product profile by extraction with ethyl acetate. Supernatant at each time point is purified with ethyl acetate extraction. 
         FIG. 9 . Schematic drawing of industrial process. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before the invention is described in detail, it is to be understood that, unless otherwise indicated, this invention is not limited to particular sequences, expression vectors, enzymes, host microorganisms, or processes, as such may vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. 
     As used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to an “expression vector” includes a single expression vector as well as a plurality of expression vectors, either the same (e.g., the same operon) or different; reference to “cell” includes a single cell as well as a plurality of cells; and the like. 
     In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings: 
     The terms “optional” or “optionally” as used herein mean that the subsequently described feature or structure may or may not be present, or that the subsequently described event or circumstance may or may not occur, and that the description includes instances where a particular feature or structure is present and instances where the feature or structure is absent, or instances where the event or circumstance occurs and instances where it does not. 
     The terms “host cell” and “host microorganism” are used interchangeably herein to refer to a living biological cell, such as a microbe, that can be transformed via insertion of an expression vector. Thus, a host organism or cell as described herein may be a prokaryotic organism (e.g., an organism of the kingdom Eubacteria) or a eukaryotic cell. As will be appreciated by one of ordinary skill in the art, a prokaryotic cell lacks a membrane-bound nucleus, while a eukaryotic cell has a membrane-bound nucleus. 
     The term “heterologous DNA” as used herein refers to a polymer of nucleic acids wherein at least one of the following is true: (a) the sequence of nucleic acids is foreign to (i.e., not naturally found in) a given host microorganism; (b) the sequence may be naturally found in a given host microorganism, but in an unnatural (e.g., greater than expected) amount; or (c) the sequence of nucleic acids comprises two or more subsequences that are not found in the same relationship to each other in nature. For example, regarding instance (c), a heterologous nucleic acid sequence that is recombinantly produced will have two or more sequences from unrelated genes arranged to make a new functional nucleic acid. Specifically, the present invention describes the introduction of an expression vector into a host microorganism, wherein the expression vector contains a nucleic acid sequence coding for an enzyme that is not normally found in a host microorganism. With reference to the host microorganism&#39;s genome, then, the nucleic acid sequence that codes for the enzyme is heterologous. 
     The terms “expression vector” or “vector” refer to a compound and/or composition that transduces, transforms, or infects a host microorganism, thereby causing the cell to express nucleic acids and/or proteins other than those native to the cell, or in a manner not native to the cell. An “expression vector” contains a sequence of nucleic acids (ordinarily RNA or DNA) to be expressed by the host microorganism. Optionally, the expression vector also comprises materials to aid in achieving entry of the nucleic acid into the host microorganism, such as a virus, liposome, protein coating, or the like. The expression vectors contemplated for use in the present invention include those into which a nucleic acid sequence can be inserted, along with any preferred or required operational elements. Further, the expression vector must be one that can be transferred into a host microorganism and replicated therein. Preferred expression vectors are plasmids, particularly those with restriction sites that have been well documented and that contain the operational elements preferred or required for transcription of the nucleic acid sequence. Such plasmids, as well as other expression vectors, are well known to those of ordinary skill in the art. 
     The term “transduce” as used herein refers to the transfer of a sequence of nucleic acids into a host microorganism or cell. Only when the sequence of nucleic acids becomes stably replicated by the cell does the host microorganism or cell become “transformed.” As will be appreciated by those of ordinary skill in the art, “transformation” may take place either by incorporation of the sequence of nucleic acids into the cellular genome, i.e., chromosomal integration, or by extrachromosomal integration. In contrast, an expression vector, e.g., a virus, is “infective” when it transduces a host microorganism, replicates, and (without the benefit of any complementary virus or vector) spreads progeny expression vectors, e.g., viruses, of the same type as the original transducing expression vector to other microorganisms, wherein the progeny expression vectors possess the same ability to reproduce. 
     As used herein, the terms “nucleic acid sequence,” “sequence of nucleic acids,” and variations thereof shall be generic to polydeoxyribonucleotides (containing 2-deoxy-D-ribose), to polyribonucleotides (containing D-ribose), to any other type of polynucleotide that is an N-glycoside of a purine or pyrimidine base, and to other polymers containing nonnucleotidic backbones, provided that the polymers contain nucleobases in a configuration that allows for base pairing and base stacking, as found in DNA and RNA. Thus, these terms include known types of nucleic acid sequence modifications, for example, substitution of one or more of the naturally occurring nucleotides with an analog; intemucleotide modifications, such as, for example, those with uncharged linkages (e.g., methyl phosphonates, phosphotriesters, phosphoramidates, carbamates, etc.), with negatively charged linkages (e.g., phosphorothioates, phosphorodithioates, etc.), and with positively charged linkages (e.g., arninoalklyphosphoramidates, aminoalkylphosphotriesters); those containing pendant moieties, such as, for example, proteins (including nucleases, toxins, antibodies, signal peptides, poly-L-lysine, etc.); those with intercalators (e.g., acridine, psoralen, etc.); and those containing chelators (e.g., metals, radioactive metals, boron, oxidative metals, etc.). As used herein, the symbols for nucleotides and polynucleotides are those recommended by the IUPAC-IUB Commission of Biochemical Nomenclature ( Biochem.  9:4022, 1970). 
     The term “operably linked” refers to a functional linkage between a nucleic acid expression control sequence (such as a promoter) and a second nucleic acid sequence, wherein the expression control sequence directs transcription of the nucleic acid corresponding to the second sequence. 
     In some embodiments, the method comprises culturing the genetically modified host cell with exogenously provided tyrosine, or a suitable carbon source. When the method comprises culturing the genetically modified host cell with a suitable carbon source, the genetically modified host cell is capable of synthesizing tyrosine using a native biosynthetic pathway or a heterologous biosynthetic pathway residing on one or more nucleic acids in the host cell, wherein the one or more nucleic acids are on one or more vectors or stably integrated into a host cell chromosome. Suitable carbon sources which the host cell is capable of uptaking and metabolizing. Such carbon sources include but are not limited to sugars, such as monosaccharides, such as glucose. 
     In some embodiments, the method comprises: (a) introducing a nucleic acid construct encoding an enzyme capable of catalyzing the oxidation of the aromatic amino acid into a genetically modified host cell; and (b) culturing the genetically modified host cell under a suitable condition such that the enzyme is expressed in the host cell; such that the culturing results in the genetically modified host cell producing the desired products. 
     In some embodiments, the one or more enzymes are capable of catalyzing the oxidation of tyrosine into L-DOPA, dopamine, 3,4-dihydroxyphenylacetaldehyde, 3,4-dihydroxypehylethanol (hydroxytyrosol), reticuline, thebaine, and/or morphine, such that the culturing the host cell results in the host cell producing L-DOPA, dopamine, 3,4-dihydroxyphenylacetaldehyde, 3,4-dihydroxypehylethanol (hydroxytyrosol), reticuline, thebaine, and/or morphine. 
     In some embodiments, the host cell is capable of endogenously producing tyrosine, either by native enzymes of the tyrosine biosynthetic pathway, or a heterologous tyrosine biosynthetic pathway introduced into the host cell. In some embodiments, the tyrosine biosynthetic pathway comprises one or more of the following enzymes, or any corresponding homologous enzymes thereof: PpsA, TktA, AroG, AroB, AroD, YdiB, AroE, AroK/L, AroA, AroC, TyrA, and TyrB. In some embodiments, the host cell over produces AroB and/or TyrA, or any corresponding homologous enzymes thereof. 
     In some embodiments, the host cell comprises or is capable of expressing TH, DDC, MAO, and/or alcohol dehydrogenase, or homologous enzymes thereof, wherein one or more of the enzymes are overproduced compared to the unmodified host cell or one or more of the enzymes is heterologous to the host cell. In some embodiments, the host cell is capable of endogenously producing tyrosine, either by native enzymes of the tyrosine biosynthetic pathway, or a heterologous tyrosine biosynthetic pathway introduced into the host cell. 
     In some embodiments, the host cell comprises or is capable of expressing heterologous TH (such as mouse TH), heterologous DDC (such as pig DDC), and/or heterologous MAO (such as  M. luteus  MAO), or homologous enzymes thereof. 
     In some embodiments, the host cell natively comprises a nucleic acid encoding an enzyme capable of catalyzing phenylacetaldehyde dehydrogenase into 3,4-dihydroxyphenyl acetate (3,4-DHPA), such as the enzyme phenylacetaldehyde dehydrogenase, wherein the host cell is reduced in the expression of the enzyme. When the host cell is  E. coli,  the enzyme is phenylacetaldehyde dehydrogenase encoded by the feaB gene. The reduced expression can be the result of a mutation that reduced expression or reduces enzymatic activity of the enzyme. An example of such a mutation is a truncated or deleted gene, such as a knock out mutation. 
     One means to have a host cell synthesize MH4 is to have the host cell comprise the enzymes GTP cyclohydrolase I (folE), folX, P-ase, and folM, or homologous enzymes thereof. 
     Tyrosine hydroxylase is an enzyme that uses tetrahydrobiopterin (BH4) or MH4 in the catalysis of tyrosine and tryptophan into L-DOPA and 5-hydroxy-tryptophan, respectively. Pterin-4-alpha-carbinolamine dehydratase (PCD) and dihydropteridine reductase (DHPR) are capable of catalyzing the reactions for BH4 regeneration. In some embodiments, when the genetically modified host cell comprises tyrosine hydroxylase (TH), the host cell further comprises pterin-4-alpha-carbinolamine dehydratase (PCD), or a homologous enzyme thereof, and dihydropteridine reductase (DHPR), or a homologous enzyme thereof. In some embodiments, when the genetically modified host cell does not naturally synthesize BH4, the host cell further comprises GTP cyclohydrolase I (folE), 6-pyruvoyl-tetrahydropterin synthase (PTPS), and sepiapterin reductase (SR), or one or more homologous enzymes thereof. 
     In some embodiments of invention, the method further comprises the step of recovering the produced one or more oxidation products, wherein the recovering step is concurrent or subsequent to the culturing step. 
     Enzymes, and Nucleic Acids Encoding Thereof 
     A homologous enzyme is an enzyme that has a polypeptide sequence that is at least 70%, 75%, 80%, 85%, 90%, 95% or 99% identical to any one of the enzymes described in this specification or in an incorporated reference. The homologous enzyme retains amino acids residues that are recognized as conserved for the enzyme. The homologous enzyme may have non-conserved amino acid residues replaced or found to be of a different amino acid, or amino acid(s) inserted or deleted, but which does not affect or has insignificant effect on the enzymatic activity of the homologous enzyme. The homologous enzyme has an enzymatic activity that is identical or essentially identical to the enzymatic activity any one of the enzymes described in this specification or in an incorporated reference. The homologous enzyme may be found in nature or be an engineered mutant thereof. 
     A suitable tyrosine hydroxylase or tyrosine 3-monooxygenase is mouse tyrosine hydroxylase (NP_033403), or a homologous enzyme thereof, which has the following amino acid sequence: 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 1) 
                   
               
               
                   1 mptpsasspq pkgfrrayse qdtkqaeavt sprfigrrqs liedarkere aaaaaaaaav 
                   
               
               
                   
               
               
                  61 asaepgnple avvfeerdgn avlnllfslr gtkpsslsra lkvfetfeak ihhletrpaq 
               
               
                   
               
               
                 121 rplagsphle yfvrfevpsg dlaallssyr rvsddyrsar edkvpwfprk vseldkchhl 
               
               
                   
               
               
                 181 vtkfdpdldl dhpgfsdqay rqrrkliaei afqykqgepi phveytkeei atwkevyatl 
               
               
                   
               
               
                 241 kglyathacr ehleafqlle rycgyredsi pqledvshfl kertgfqlrp vagllsardf 
               
               
                   
               
               
                 301 laslafrvfq ctqyirhass pmhspepdcc hellghvpml adrtfaqfsq diglaslgas 
               
               
                   
               
               
                 361 deeieklstv ywftvefglc kqngelkayg agllssygel lhslseepev rafdpdtaav 
               
               
                   
               
               
                 421 qpyqdqtyqp vyfvsesfsd akdklrnyas riqrpfsvkf dpytlaidvl dsphtirrsl 
               
               
                   
               
               
                 481 egvqdelhtl tqalsais  
               
            
           
         
       
     
     A suitable pterin-4-alpha-carbinolamine dehydratase (PCD) is human PCD (NP_000272), or a homologous enzyme thereof, which has the following amino acid sequence: 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 2) 
                   
               
               
                  1 magkahrlsa eerdqllpnl ravgwneleg rdaifkqfhf kdfnrafgfm trvalqaekl 
                   
               
               
                   
               
               
                 61 dhhpewfnvy nkvhitlsth ecaglserdi nlasfieqva vsmt  
               
            
           
         
       
     
     A suitable dihydropteridine reductase (DHPR) is human DHPR (P09417), or a homologous enzyme thereof, which has the following amino acid sequence: 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 3) 
                   
               
               
                   1 maaaaaagea rrvlvyggrg algsrcvgaf rarnwwvasv dvveneeasa siivkmtdsf 
                   
               
               
                   
               
               
                  61 teqadqvtae vgkllgeekv dailcvaggw aggnaksksl fkncdlmwkg siwtstissh 
               
               
                   
               
               
                 121 latkhlkegg lltlagakaa ldgtpgmigy gmakgavhql cqslagknsg mppgaaaiav 
               
               
                   
               
               
                 181 lpvtldtpmn rksmpeadfs swtpleflve tfhdwitgkn rpssgsliqv vttegrtelt 
               
               
                   
               
               
                 241 payf  
               
            
           
         
       
     
     A suitable L-DOPA decarboxylase (DDC) is pig DDC, or a homologous enzyme thereof, which has the following nucleotide (SEQ ID NO:4) and amino acid (SEQ ID NO:5) sequences: 
     
       
         
           
               
               
               
               
            
               
                 −10 
                                                 GAATTCACATATGAATGCCAGCGATTTC 
                   
                   
               
               
                   
                                                            M  N  A  S  D  F 
                 6 
               
               
                   
               
               
                 19 
                 CGTCGACGCGGCAAAGAAATGGTGGATTACATGGCGGATTACCTGGAAGGCATCGAAGGT 
                   
               
               
                   
                  R  R  R  G  K  E  M  V  D  Y  M  A  D  Y  L  E  G  I  E  G 
                 26 
               
               
                   
               
               
                 79 
                 CGTCAGGTGTACCCGGATGTGCAGCCGGGGTACCTGCGTCCGCTGATCCCGGCGACCGCC 
                   
               
               
                   
                  R  Q  V  Y  P  D  V  Q  P  G  Y  L  R  P  L  I  P  A  T  A 
                 46 
               
               
                   
               
               
                 139 
                 CCGCAGGAACCGGATACCTTCGAAGATATCCTGCAGGATGTGGAAAAAATCATCATGCCG 
                   
               
               
                   
                  P  Q  R  P  D  T  F  E  D  I  L  Q  D  V  E  K  I  I  M  P 
                 66 
               
               
                   
               
               
                 199 
                 GGGGTGACCCACTGGCACAGCCCGTACTTCTTCGCGTACTTCCCGACCGCCAGCAGCTAC 
                   
               
               
                   
                  G  V  T  H  W  H  S  P  Y  F  F  A  Y  A  P  T  A  S  S  Y 
                 86 
               
               
                   
               
               
                 259 
                 CCGGCGATGCTGGCGGATATGCTGTGCGGTGCGATCGGATGCATCGGTTTCAGCTGGGCG 
                   
               
               
                   
                  P  A  M  L  A  D  M  L  C  G  A  I  G  C  I  G  F  S  W  A 
                 106 
               
               
                   
               
               
                 319 
                 GCTAGCCCGGCGTGCACCGAACTCGAGACCGTGATGATGGATTGGCTGGGCAAAATGCTC 
                   
               
               
                   
                  A  S  P  A  C  T  E  L  E  T  V  N  M  D  W  L  G  K  M  L 
                 126 
               
               
                   
               
               
                 379 
                 CAGCTTCCGGAAGCGTTCCTGGCGGGCGAAGCCGGTGAAGGCGGCGGCGTGATCCAGGGT 
                   
               
               
                   
                  Q  L  P  E  A  F  L  A  G  E  A  G  E  G  G  G  V  I  Q  G 
                 146 
               
               
                   
               
               
                 439 
                 AGCGCCAGCGAAGCCACCCTGGTGGCGCTGCTGGCGGCGCGTACCAAAGTGGTGCGACGT 
                   
               
               
                   
                  S  A  S  E  A  T  L  V  A LL  L  A  A  R  T  K  V  V  R  R 
                 166 
               
               
                   
               
               
                 499 
                 CTGCAAGCGGCGAGCCCGGGCCTGACCCAGGGCGCGGTGCTGGAAAAACTAGTGGCGTAC 
                   
               
               
                   
                  L  Q  A  A  S  P  G  L  T  Q  G  A  V  L  E  K  L  V  A  Y 
                 186 
               
               
                   
               
               
                 559 
                 GCGAGTGATCAGGCGCACAGCAGCGTGGAACGTGCCGGCCTGATCGGCGGCGTGAAACTG 
                   
               
               
                   
                  A  S  D  Q  A  H  S  S  V  E  R  A  G  L  I  G  G  V  K  L 
                 206 
               
               
                   
               
               
                 619 
                 AAAGCGATCCCGAGCGATGGCAAATTCGCGATGCGTGCGAGCGCGCTGCAGGAGGCCCTG 
                   
               
               
                   
                  K  A  I  P  S  D  G  K  F  A  M  R  A  S  A  L  Q  E  A  L 
                 226 
               
               
                   
               
               
                 679 
                 GAGAGAGACAAGGCTGCCGGCCTGATTCCTTTCTTCGTGGTGGCTACGCTGGGGACCACA 
                   
               
               
                   
                  E  R  D  K  A  A  G  L  I  P  F  F  V  V  A  T  L  G  T  T 
                 246 
               
               
                   
               
               
                 739 
                 TCGTGCTGCTCCTTTGACAATCTCTTAGAAGTGGGACCCATCTGTCACGAAGAGGACATA 
                   
               
               
                   
                  S  C  C  S  F  D  N  L  L  E  V  G  P  I  C  H  E  E  D  I 
                 266 
               
               
                   
               
               
                 799 
                 TGGCTGCACGTGGATGCTGCCTACGCAGGCAGTGCCTTCATCTGCCCTGAGTTCCGGCAC 
                   
               
               
                   
                  W  L  H  V  D  A  A  Y  A  H  S  A  F  I  C  P  E  F  R  H 
                 286 
               
               
                   
               
               
                 859 
                 CTGCTGAATGGAGTGGAGTTTGCAGATTCATTTAACTTTAATCCCCACAAATGGCTCTTG 
                   
               
               
                   
                  L  L  N  G  V  E  F  A  D  S  F  N  F  N  P  H  K  W  L  L 
                 306 
               
               
                   
               
               
                 919 
                 GTGAATTTTGACTGCTCGGCTATGTGGGTGAAAAGGAGAACGGACCTGACTGGAGCCTTC 
                   
               
               
                   
                  V  N  F  D  C  S  A  M  W  V  K  R  R  T  D  L  T  G  A  F 
                 326 
               
               
                   
               
               
                 979 
                 AAATTGGACCCCGTGTACTTAAAGCACAGCCACCAGGGCTCGGGGCTTATCACGGACTAC 
                   
               
               
                   
                  K  L  D  P  V  Y  L  K  H  S  H  Q  G  S  G  L  I  T  D  Y 
                 346 
               
               
                   
               
               
                 1039 
                 AGGCACTGGCAGCTGCCACTGGGTCGGCGATTCCGGTCCCTGAAAATGTGGTTTGTTTTT 
                   
               
               
                   
                  R  H  W  Q  L  P  L  G  R  R  F  R  S  L  K  M  W  F  V  F 
                 366 
               
               
                   
               
               
                 1099 
                 AGGATGTACGGAGTCAAGGGACTGCAGGCCTATATCCGCAAGCACGTGCAGCTGTCTCAT 
                   
               
               
                   
                  R  M  Y  G  V  K  G  L  Q  A  Y  I  R  K  H  V  Q  L  S  H 
                 386 
               
               
                   
               
               
                 1159 
                 GAGTTTGAGGCATTTGTGCTTCAGGATCCACGCTTTGAAGTCTGTGCCGAAGTCACCCTG 
                   
               
               
                   
                  E  F  E  A  F  V  L  Q  D  P  R  F  E  V  C  A  E  V  T  L 
                 406 
               
               
                   
               
               
                 1219 
                 GGGCTGGTGTGTTTCCGGCTGAAGGGCTCCGACGGACTGAATGAAGCGCTTCTGGAAAGG 
                   
               
               
                   
                  G  L  V  C  F  R  L  K  G  S  D  G  L  N  E  A  L  L  E  R 
                 426 
               
               
                   
               
               
                 1279 
                 ATAAACAGCGCCAGGAAAATCCACTTGGTTCCCTGTCGCCTGAGGGGCCAGTTCGTGCTG 
                   
               
               
                   
                  I  N  S  A  R  K  I  H  L  V  P  C  R  L  R  G  Q  F  V  L 
                 446 
               
               
                   
               
               
                 1339 
                 CGGTTCGCCATCTGCTCGCGCAAGGTGGAGTCGGGCCACGTGCGGCTGGCCTGGGAGCAC 
                   
               
               
                   
                  R  F  A  I  C  S  R  K  V  E  S  G  H  V  R  L  A  W  E  H 
                 465 
               
               
                   
               
               
                 1399 
                 ATCCGAGGGCTGGCGGCCGAGCTGCTGGCCGCGGAGGAGGGAAAGGCAGAGATCAAAAGT 
                   
               
               
                   
                  I  R  G  L  A  A  E  L  L  A  A  E  E  G  K  A  E  I  K  S 
                 486 
               
               
                   
               
               
                 1459 
                 TGAAGTGCCCTGAAGAGCAGAATCGGAGGAGACGCGTCGTCCCCGCTCCGAGGCGTAGAG 
                   
               
               
                   
                  * 
                   
               
               
                   
               
               
                 1519 
                 CCTGCAATGGTCCCCCCAGTTCTTTAGCCCACGTTCTCCAGAAAGAAGCTTGTGCCTACG 
                   
               
               
                   
               
               
                 1579 
                 TCTGACCAGCCTCTCAGCAATGAAGAAGTATTATTTGCTCTTTGAAAAGTTAATCCCAGT 
                   
               
               
                   
               
               
                 1639 
                 GGAGACAGCTTTTACTCTTTATTTGGCTGTGATTGTTTGTTGATTAAAACATCATAGGTT 
                   
               
               
                   
               
               
                 1699 
                 TCTGCATCCTTGAAGTTGTCAGCGGTGGTCCACTTTCCGGGGCAACCTATGCTGATGGGA 
                   
               
               
                   
               
               
                 1759 
                 TTTGAGATGATACCCGTGGTCTTTAAATTACTCTGTCCTGTGGCTTATGCTT AATAAA TG 
                   
               
               
                   
               
               
                 1819 
                 ATGTGAAGTGTAAAAAAAAAAAAAAAAAAAA 
                   
               
            
           
         
       
     
     A suitable monoamine oxidase (MAO) is Micrococcus luteus MAO (ACS30544.1), or a homologous enzyme thereof, which has the following amino acid sequence: 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 6) 
                   
               
               
                   1 mttapatagr errtsdvvvi gagpaglmaa rtakagglsv tvlearrrvg grtwnglveg 
                   
               
               
                   
               
               
                  61 adgkdhfiei ggqwispdqt rlislveelg lptfsrfrdg rnvyvdprge rhvydgldfp 
               
               
                   
               
               
                 121 vaektdremd rliakidelt aeidaaapwe hpraaeldti sfrhwleqes ddpeaidnvs 
               
               
                   
               
               
                 181 iyiasgmltk pshtfsmlqa llmaasagsf rnlvdedfil dkrveggmqs vsltmaaelg 
               
               
                   
               
               
                 241 ddvvlgqpvr tlrwaepdps tadekngvaa dvrngvandg aagdvvaltd dyevharyav 
               
               
                   
               
               
                 301 lavppnlysr isfeppmpre ggiahqhism glvikvhavy etpfwreegl sgtcfgggrl 
               
               
                   
               
               
                 361 vqeiydntnr genlaggapg eedphgtivg fvsdvyaeqm walpeeerka ailgamaeyl 
               
               
                   
               
               
                 421 gprtlepiaf flsdmaaeew trgayatsyd lgglsrwghl qnrptgpihy acsdiaaegy 
               
               
                   
               
               
                 481 qhvdgairmg eaaalaiaer eatdagutg  
               
            
           
         
       
     
     A suitable AroG is  E. coli  AroG, or a homologous enzyme thereof, which has the following amino acid sequence: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 7) 
               
               
                   
                 MNYQNDDLRI KEIKELLPPV ALLEKFPATE NAANTVAHAR  
               
               
                   
                   
               
               
                   
                 KAIHKILKGN DDRLLVVIGP CSIHDPVAAK EYATRLLALR  
               
               
                   
                   
               
               
                   
                 EELKDELEIV MRVYFEKPRT TVGWKGLIND PHMDNSFQIN  
               
               
                   
                   
               
               
                   
                 DGLRIARKLL LDINDSGLPA AGEFLDMITP QYLADLMSWG  
               
               
                   
                   
               
               
                   
                 AIGARTTESQ VHRELASGLS CPVGFKNGTD GTIKVAIDAI  
               
               
                   
                   
               
               
                   
                 NAAGAPHCFL SVTKWGHSAI VNTSGNGDCH IILRGGKEPN  
               
               
                   
                   
               
               
                   
                 YSAKHVAEVK EGLNKAGLPA QVMIDFSHAN SSKQFKKQMD  
               
               
                   
                   
               
               
                   
                 VCADVCQQIA GGEKAIIGVM VESHLVEGNQ SLESGEPLAY  
               
               
                   
                   
               
               
                   
                 GKSITDACIG WEDTDALLRQ LANAVKARRG  
               
            
           
         
       
     
     A suitable TyrA is  E. coli  TyrA, or a homologous enzyme thereof, which has the following amino acid sequence: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 8) 
               
               
                   
                 MVAELTALRD QIDEVDKALL NLLAKRLELV AEVGEVKSRF  
               
               
                   
                   
               
               
                   
                 GLPIYVPERE ASMLASRRAE AEALGVPPDL IEDVLRRVMR  
               
               
                   
                   
               
               
                   
                 ESYSSENDKG FKTLCPSLRP VVIVGGGGQM GRLFEKMLTL  
               
               
                   
                   
               
               
                   
                 SGYQVRILEQ HDWDRAADIV ADAGMVIVSV PIHVTEQVIG  
               
               
                   
                   
               
               
                   
                 KLPPLPKDCI LVDLASVKNG PLQAMLVAHD GPVLGLHPMF  
               
               
                   
                   
               
               
                   
                 GPDSGSLAKQ VVVWCDGRKP EAYQWFLEQI QVWGARLHRI  
               
               
                   
                   
               
               
                   
                 SAVEHDQNMA FIQALRHFAT FAYGLHLAEE NVQLEQLLAL  
               
               
                   
                   
               
               
                   
                 SSPIYRLELA MVGRLFAQDP QLYADIIMSS ERNLALIKRY  
               
               
                   
                   
               
               
                   
                 YKRFGEAIEL LEQGDKQAFI DSFRKVEHWF GDYAQRFQSE  
               
               
                   
                   
               
               
                   
                 SRVLLRQAND NRQ  
               
            
           
         
       
     
     The nucleic acid constructs of the present invention comprise nucleic acid sequences encoding one or more of the subject enzymes. The nucleic acid of the subject enzymes are operably linked to promoters and optionally control sequences such that the subject enzymes are expressed in a host cell cultured under suitable conditions. The promoters and control sequences are specific for each host cell species. In some embodiments, expression vectors comprise the nucleic acid constructs. Methods for designing and making nucleic acid constructs and expression vectors are well known to those skilled in the art. 
     Sequences of nucleic acids encoding the subject enzymes are prepared by any suitable method known to those of ordinary skill in the art, including, for example, direct chemical synthesis or cloning. For direct chemical synthesis, formation of a polymer of nucleic acids typically involves sequential addition of 3′-blocked and 5′-blocked nucleotide monomers to the terminal 5′-hydroxyl group of a growing nucleotide chain, wherein each addition is effected by nucleophilic attack of the terminal 5′-hydroxyl group of the growing chain on the 3′-position of the added monomer, which is typically a phosphorus derivative, such as a phosphotriester, phosphoramidite, or the like. Such methodology is known to those of ordinary skill in the art and is described in the pertinent texts and literature (e.g., in Matteuci et al. (1980)  Tet. Lett.  521:719; U.S. Pat. Nos. 4,500,707; 5,436,327; and 5,700,637). In addition, the desired sequences may be isolated from natural sources by splitting DNA using appropriate restriction enzymes, separating the fragments using gel electrophoresis, and thereafter, recovering the desired nucleic acid sequence from the gel via techniques known to those of ordinary skill in the art, such as utilization of polymerase chain reactions (PCR; e.g., U.S. Pat. No. 4,683,195). 
     Each nucleic acid sequence encoding the desired subject enzyme can be incorporated into an expression vector. Incorporation of the individual nucleic acid sequences may be accomplished through known methods that include, for example, the use of restriction enzymes (such as BamHI, EcoRI, Hhal, XhoI, XmaI, and so forth) to cleave specific sites in the expression vector, e.g., plasmid. The restriction enzyme produces single stranded ends that may be annealed to a nucleic acid sequence having, or synthesized to have, a terminus with a sequence complementary to the ends of the cleaved expression vector. Annealing is performed using an appropriate enzyme, e.g., DNA ligase. As will be appreciated by those of ordinary skill in the art, both the expression vector and the desired nucleic acid sequence are often cleaved with the same restriction enzyme, thereby assuring that the ends of the expression vector and the ends of the nucleic acid sequence are complementary to each other. In addition, DNA linkers may be used to facilitate linking of nucleic acids sequences into an expression vector. 
     A series of individual nucleic acid sequences can also be combined by utilizing methods that are known to those having ordinary skill in the art (e.g., U.S. Pat. No. 4,683,195). 
     For example, each of the desired nucleic acid sequences can be initially generated in a separate PCR. Thereafter, specific primers are designed such that the ends of the PCR products contain complementary sequences. When the PCR products are mixed, denatured, and reannealed, the strands having the matching sequences at their 3′ ends overlap and can act as primers for each other Extension of this overlap by DNA polymerase produces a molecule in which the original sequences are “spliced” together. In this way, a series of individual nucleic acid sequences may be “spliced” together and subsequently transduced into a host microorganism simultaneously. Thus, expression of each of the plurality of nucleic acid sequences is effected. 
     Individual nucleic acid sequences, or “spliced” nucleic acid sequences, are then incorporated into an expression vector. The invention is not limited with respect to the process by which the nucleic acid sequence is incorporated into the expression vector. Those of ordinary skill in the art are familiar with the necessary steps for incorporating a nucleic acid sequence into an expression vector. A typical expression vector contains the desired nucleic acid sequence preceded by one or more regulatory regions, along with a ribosome binding site, e.g., a nucleotide sequence that is 3-9 nucleotides in length and located 3-11 nucleotides upstream of the initiation codon in  E. coli.  See Shine et al. (1975)  Nature  254:34 and Steitz, in Biological Regulation and Development: Gene Expression (ed. R. F. Goldberger), vol. 1, p. 349, 1979, Plenum Publishing, N.Y. 
     Regulatory regions include, for example, those regions that contain a promoter and an operator. A promoter is operably linked to the desired nucleic acid sequence, thereby initiating transcription of the nucleic acid sequence via an RNA polymerase enzyme. An operator is a sequence of nucleic acids adjacent to the promoter, which contains a protein-binding domain where a repressor protein can bind. In the absence of a repressor protein, transcription initiates through the promoter. When present, the repressor protein specific to the protein-binding domain of the operator binds to the operator, thereby inhibiting transcription. In this way, control of transcription is accomplished, based upon the particular regulatory regions used and the presence or absence of the corresponding repressor protein. An example includes lactose promoters (LacI repressor protein changes conformation when contacted with lactose, thereby preventing the LacI repressor protein from binding to the operator). Another example is the tac promoter. (See deBoer et al. (1983)  Proc. Natl. Acad. Sci. USA,  80:21-25.) As will be appreciated by those of ordinary skill in the art, these and other expression vectors may be used in the present invention, and the invention is not limited in this respect. 
     Although any suitable expression vector may be used to incorporate the desired sequences, readily available expression vectors include, without limitation: plasmids, such as pSC101, pBR322, pBBR1MCS-3, pUR, pEX, pMR100, pCR4, pBAD24, pUC19; bacteriophages, such as M13 phage and λ phage. Of course, such expression vectors may only be suitable for particular host cells. One of ordinary skill in the art, however, can readily determine through routine experimentation whether any particular expression vector is suited for any given host cell. For example, the expression vector can be introduced into the host cell, which is then monitored for viability and expression of the sequences contained in the vector. In addition, reference may be made to the relevant texts and literature, which describe expression vectors and their suitability to any particular host cell. 
     The expression vectors of the invention must be introduced or transferred into the host cell. Such methods for transferring the expression vectors into host cells are well known to those of ordinary skill in the art. For example, one method for transforming  E. coli  with an expression vector involves a calcium chloride treatment wherein the expression vector is introduced via a calcium precipitate. Other salts, e.g., calcium phosphate, may also be used following a similar procedure. In addition, electroporation (i.e., the application of current to increase the permeability of cells to nucleic acid sequences) may be used to transfect the host microorganism. Also, microinjection of the nucleic acid sequencers) provides the ability to transfect host microorganisms. Other means, such as lipid complexes, liposomes, and dendrimers, may also be employed. Those of ordinary skill in the art can transfect a host cell with a desired sequence using these or other methods. 
     For identifying a transfected host cell, a variety of methods are available. For example, a culture of potentially transfected host cells may be separated, using a suitable dilution, into individual cells and thereafter individually grown and tested for expression of the desired nucleic acid sequence. In addition, when plasmids are used, an often-used practice involves the selection of cells based upon antimicrobial resistance that has been conferred by genes intentionally contained within the expression vector, such as the amp, gpt, neo, and hyg genes. 
     The host cell is transformed with at least one expression vector. When only a single expression vector is used (without the addition of an intermediate), the vector will contain all of the nucleic acid sequences necessary. 
     Once the host cell has been transformed with the expression vector, the host cell is allowed to grow. For microbial hosts, this process entails culturing the cells in a suitable medium. It is important that the culture medium contain an excess carbon source, such as a sugar (e.g., glucose) when an intermediate is not introduced. In this way, cellular production of aromatic amino acid ensured. When added, the intermediate is present in an excess amount in the culture medium. 
     As the host cell grows and/or multiplies, expression of the enzymes necessary for producing the oxidation product(s) is affected. Once expressed, the enzymes catalyze the steps necessary for carrying out the steps of optionally aromatic amino acid production, BH4 production, and oxidation product production. If an intermediate has been introduced, the expressed enzymes catalyze those steps necessary to convert the intermediate into the respective oxidation product. Any means for recovering the oxidation product from the host cell may be used. For example, the host cell may be harvested and subjected to hypotonic conditions, thereby lysing the cells. The lysate may then be centrifuged and the supernatant subjected to high performance liquid chromatography (HPLC) or gas chromatography (GC). Once the oxidation product is recovered, modification, as desired, may be carried out on the oxidation product. 
     Host Cells 
     The host cells of the present invention are genetically modified in that heterologous nucleic acid have been introduced into the host cells, and as such the genetically modified host cells do not occur in nature. The suitable host cell is one capable of expressing a nucleic acid construct encoding one or more enzymes described herein. The gene(s) encoding the enzyme(s) may be heterologous to the host cell or the gene may be native to the host cell but is operatively linked to a heterologous promoter and one or more control regions which result in a higher expression of the gene in the host cell. 
     The enzyme can be native or heterologous to the host cell. Where the enzyme is native to the host cell, the host cell is genetically modified to modulate expression of the enzyme. This modification can involve the modification of the chromosomal gene encoding the enzyme in the host cell or a nucleic acid construct encoding the gene of the enzyme is introduced into the host cell. One of the effects of the modification is the expression of the enzyme is modulated in the host cell, such as the increased expression of the enzyme in the host cell as compared to the expression of the enzyme in an unmodified host cell. 
     In some embodiments, the host cell natively comprises a nucleic acid encoding an enzyme capable of phenylacetaldehyde dehydrogenase into 3,4-dihydroxyphenyl acetate (3,4-DHPA), such as the enzyme phenylacetaldehyde dehydrogenase, wherein the host cell is reduced in the expression of the enzyme. When the host cell is  E. coli,  the enzyme is phenylacetaldehyde dehydrogenase encoded by the feaB gene. The reduced expression can be the result of a mutation that reduced expression or reduces enzymatic activity of the enzyme. An example of such a mutation is a truncated or deleted gene, such as a knock out mutation. 
     Any prokaryotic or eukaryotic host cell may be used in the present method so long as it remains viable after being transformed with a sequence of nucleic acids. Generally, although not necessarily, the host microorganism is bacterial. In some embodiments, the host cell is a Gram negative bacterium. In some embodiments, the host cell is of the phylum Proteobactera. In some embodiments, the host cell is of the class Gammaproteobacteria. In some embodiments, the host cell is of the order Enterobacteriales. In some embodiments, the host cell is of the family Enterobacteriaceae. Examples of bacterial host cells include, without limitation, those species assigned to the  Escherichia, Enterobacter, Azotobacter, Erwinia, Bacillus, Pseudomonas, Klebsielia, Proteus, Salmonella, Serratia, Shigella, Rhizobia, Vitreoscilla,  and  Paracoccus  taxonomical classes. Preferably, the host cell is not adversely affected by the transduction of the necessary nucleic acid sequences, the subsequent expression of the proteins (i.e., enzymes), or the resulting intermediates required for carrying out the steps associated with the mevalonate pathway. For example, it is preferred that minimal “cross-talk” (i.e., interference) occur between the host cell&#39;s own metabolic processes and those processes involved with the mevalonate pathway. Suitable eukaryotic cells include, but are not limited to, fungal, insect or mammalian cells. Suitable fungal cells are yeast cells, such as yeast cells of the  Saccharomyces  genus. 
     It is to be understood that, while the invention has been described in conjunction with the preferred specific embodiments thereof, the foregoing description is intended to illustrate and not limit the scope of the invention. Other aspects, advantages, and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains. 
     All patents, patent applications, and publications mentioned herein are hereby incorporated by reference in their entireties. 
     The invention having been described, the following examples are offered to illustrate the subject invention by way of illustration, not by way of limitation. 
     EXAMPLE 1 
     Engineering of Hydroxytyrosol Production in  Escherichia coli    
     Hydroxytyrosol (HTy) is one of the most powerful antioxidants with potential applications in industry, such as functional food, dietary supplement, cosmetics, and animal feed. An engineered  E. coli  capable of producing HTy with the synthetic pathway consists of five heterologous genes has been previously reported. Described herein are three metabolic engineering efforts to improve the efficiency of the synthetic HTy pathway. 
     First, the co-factor biosynthetic pathway for tyrosine hydroxylation is engineered. Next, inhibition of the tyrosine hydroxylation by downstream chemicals is identified and a co-culture strategy designed to overcome it is applied. With these engineering, the product yield from tyrosine is improve about 3-fold. 
     Lastly, a host strain is engineered to overproduce tyrosine. The production of L-DOPA from glucose without any external supplementation of tyrosine is confirmed, and the resulting L-DOPA is further converted into HTy by co-culture strategy with 25-fold improvement from the previous result. 
     A microbial HTy production process is engineered that has a potential for industrialization. Currently, HTy is produced from enriched olive extracts after chemical or enzymatic hydrolysis. There is an increasing demand for stable and sustainable production, and we believe microbial fermentation can be a promising solution. 
       FIG. 3  shows a co-factor biosynthetic pathway engineered for tyrosine hydroxylation.  FIG. 3  shows FolE introduction increases L-DOPA production from tyrosine by 2-folds. Inhibition of the tyrosine hydroxylation by downstream compounds is identified ( FIG. 4 ).  FIG. 4  shows TH is inhibited by hydroxytyrosol and dopamine. By applying a co-culture strategy, the inhibition of the tyrosine hydroxylation by downstream compounds is overcome, and product yield from tyrosine is improve more than about 3-fold ( FIG. 5 ). The gene expression of pathway enzymes is optimized using metabolomics and proteomic data. The host strain is engineered to overproduce tyrosine. With these modifications, the product from glucose is improved more than about 10-fold using the co-culture strategy ( FIG. 6 ). HTy production increases more than 3-fold with the co-culture strategy shown in  FIGS. 7A and 7B .  FIGS. 8A to 8C  show high purity HTy can be obtained from glucose. 
     This work demonstrates a potential industrial application of microbial HTy production and provides a good renewable microbial platform for the production of a wide range of chemicals that involve the hydroxylation of aromatic amino acid. Table 1 compares the previously reported hydroxytyrosol yields (Satoh et al. Metabolic Engineering 14 (2012) 603-610) and hydroxytyrosol yields reported herein. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Comparison of previously reported hydroxytyrosol 
               
               
                 yields and hydroxytyrosol yields reported herein. 
               
            
           
           
               
               
               
            
               
                 Substrate 
                 Previously reported 
                 Hydroxytyrosol yields 
               
               
                 (concentration in 
                 hydroxytyrosol yields 
                 by co-culture reported 
               
               
                 media) 
                 (mM) 
                 herein by (mM) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 L-DOPA (1 mM) 
                 0.74 
                 0.74 
               
               
                 Tyrosine (1 mM) 
                 0.19 
                 0.64 
               
               
                 Glucose (55 mM) 
                 0.08 
                 0.98 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 2 
     Engineering of an  Escherichia Coli  L-tyrosine Overproducer 
     For some hydroxytyrosol producing strains, an L-tyrosine overproducer is required. Modular system for tyrosine producer is available (Juminaga et al., 2012), however, this system already utilizes two plasmids, thus making it difficult for combining with other modules. Instead of integrating full pathway for tyrosine biosynthesis, an L-tyrosine overproducer is constructed by integrating feedback resistant mutant tyrA and aroG at the pykF locus of  E. coli  MG1655 (DE3). AroG is located at the entrance of the shikimate pathway and tyrA is located at the very last reaction of the shikimate pathway for tyrosine synthesis. These two genes are known for being the first and second limiting reactions of tyrosine synthesis, thus enhancing these two genes is a good strategy for making a tyrosine overproducing strain. A strain, named DK176, is constructed by of introducing the aroG and tyrA genes into the parent strain. When the DK176 strain is cultured in M9Y medium (1% glucose), it achieves a yield of 2.68 mM of L-tyrosine, which is a significant improvement for L-tyrosine production. 
     While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.