Patent Publication Number: US-2023142611-A1

Title: Process for producing a glucuronide and genetically modified microorganisms useful in this process

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This disclosure claims the benefit, including the filing date, of U.S. Provisional Patent No. 63/273,136, entitled “Process for Producing a Glucuronide and Genetically Modified Microorganisms Useful in this Process” which was filed on Oct. 28, 2021, the entire disclosure of which is hereby incorporated herein by this reference. 
    
    
     STATEMENT OF GOVERNMENT SUPPORT 
     This invention was made with government support under Grant No. CBET-2044558 and Grant No. CHE-1429195 awarded by the National Science Foundation. The government has certain rights in the invention.s 
    
    
     INCORPORATION-BY-REFERENCE OF MATERIAL ELECTRONICALLY FILED 
     Incorporated by reference in its entirety herein is a computer-readable nucleotide/amino acid sequence listing submitted concurrently herewith and identified as follows: One 13,218 byte XML file named “SeqList” created on Aug. 25, 2022. 
     TECHNICAL FIELD 
     The present invention relates to in vitro and in vivo processes for producing a glucuronide; expression vectors, nucleic acids, polypeptides, and recombinant microbial cells useful in carrying out the process; and prodrugs produced by the process 
     BACKGROUND 
     Plant polyphenols are an important group of compounds with diverse chemical structures (Harborne and Baxter 1999; Tsao 2010). Stilbenoids and flavonoids are representative examples of polyphenols which are rich in commonly consumed fruits and vegetables, and both animal and human clinical studies showed their health-promoting effects and antioxidant characteristics (Rice-Evans 2001; Thilakarathna and Rupasinghe 2013). However, low water solubility and poor bioavailability often limit the beneficial effects of polyphenols. Structural modification is extensively attempted to improve the utility of these natural products. Introduction of polar groups such as sugar moieties into the structure is a widely used approach to improve the water solubility of bioactive molecules. Glycosyltransferases can be used as an effective tool to create glycosides from natural products. As such, discovery of novel and versatile glycosyltransferases is important for adding new enzymes into the biocatalytic toolbox. 
     Sugar moieties often play important roles in binding of drugs to biological targets. They may also be involved in other biochemical processes such as distribution, metabolism, and excretion properties of drugs, which can affect the efficacy of its oral administration (Thorson et al. 2004; Weymouth-Wilson 1997; Yu et al. 2002). Meanwhile, glycosylated compounds exhibit increased water-solubility, intestinal absorption, biological half-life, physicochemical stability, and bioavailability for medical and cosmetic applications while also reducing the toxicity compared to their aglycon forms (Bowles et al. 2005; Cai et al. 2013; Gachon et al. 2005; Imai et al. 2012; Kaminaga et al. 2003). Digitoxin, amphotericin, vancomycin, streptomycin, and daunomycin are some examples of the most biologically active natural products and commonly used therapeutics, with one or more sugar moieties (Pandey et al. 2014). Sugar moieties can also contribute to other biological properties of natural products. For instance, the glucuronic acid moieties of glycyrrhizin are essential for its sweet taste (Chung et al. 2020). 
     Glycosyltransferases are the enzymes responsible for transferring sugar moieties from sugar donors to acceptors to yield corresponding glycosides. Microorganisms are a rich source of both natural products and natural product biosynthetic enzymes. Some of these enzymes may be developed into useful biocatalysts for desired reactions. Actinomycetes, the most ubiquitous group of gram-positive filamentous bacteria, are well characterized for their metabolic versatility (Nawani et al. 2013; Prakash et al. 2013). For example, they can decompose organic matter (such as cellulose) which is important for the carbon cycle and maintaining the soil structure (Kim 2016; Priyadharsini and Dhanasekaran 2015). In addition, actinomycetes can also produce various commercial products, such as pharmaceuticals, antibiotics, antitumor agents, and nutraceuticals (Prakash et al. 2013; Remya and Vij ayakumar 2008). Furthermore, many actinomycete genera produce industrially important enzymes applied in biotechnological applications and biomedical fields, such as amylases, cellulases, chitinases, xylanases, and proteases (Mehnaz et al. 2017; Nawani et al. 2013). 
     Glucuronidation is one of the common glycosylation reactions in nature. Due to the polarity of the glucuronic acid moiety, glucuronides often show much better water solubility than their aglycons. Therefore, glucuronidation is a useful approach to create water-soluble glycosides of natural products. UDP-Glucuronyltransferases (UGTs) are the enzymes involved in glucuronidation, which are often observed in metabolism of xenobiotics and endogenous components (bilirubin, bile acids, and certain hormones) during phase II metabolism, which is the common detoxification pathway in the human body (de Wildt et al. 1999; Wilkinson et al. 2008). There are mainly two phases for drug metabolism, namely functionalization reactions and conjugation reactions. Glucuronidation is one of the phase II conjugation reactions. It was predicted that about 10% of the top 200 prescribed drugs recorded in USA are fully or partially metabolized by UGTs. UGTs transfer the glucuronic acid moiety from uridine 5′-diphosphoglucuronic acid (UDP-glucuronic acid) to various exogenous and endogenous compounds. They often work with cytochrome P450 enzymes (CYPs) to metabolize most hepatically cleared drugs. 
     UGTs have been commonly found in plants and animals. However, there are relatively few studies on microbial UGTs. Plant UGTs are usually involved in catalyzing the last step of plant secondary metabolism, specifically, the glucuronic acid group is transferred from the sugar donor to the phenolic hydroxyl group of the substrates, such as flavonoids. O-Glucuronidation makes secondary metabolites of plants possess better solubility and chemical stability, as well as lower toxicity for storage which is helpful for its accumulation in cells. O-Glucuronides are widely present in plants. They can not only resist the invasion of pathogenic bacteria, but also play an important role in the metabolic balance of endogenous hormones. Therefore, they are essential in regulating the physiological and ecological functions of plants (Li et al. 2001). O-Glucuronides of many drugs and their phase I metabolites are produced by human (or animal) UGTs, which can be prepared by UGT catalysis in animal tissues. Compared to plant UGTs, UGTs are more widely distributed in animal tissues, and their functions are similar to those of plant UGTs (Stachulski and Jenkins 1998). However, it is technically challenging and cost-ineffective to obtain UGTs from plants and animals for large scale preparation of desired glucuronides. No UGTs have been discovered from microorganisms to date, which has limited our understanding of this type of enzyme and their applications in industry. 
     Many fields such as drug development, sports drug testing, and the detection of agricultural residues often require the identification, quantification, and pharmacological evaluation of the glucuronidated metabolites (Wilkinson et al. 2011). However, chemical approaches like the Koenigs-Knorr reaction used for glucuronidation often suffer from poor yields and side reactions, as well as tedious protection-deprotection of the hydroxyl groups of sugar moieties (Engstrom et al. 2006; Stachulski et al. 2006). Enzymatic preparation of glucuronides represents a “green” alternative because of the selectivity, mild conditions and elimination of the need of toxic chemical reagents. Discovery of efficient and versatile UGTs is critical for the development of a feasible and viable production process of glucuronides. 
     SUMMARY 
     In our ongoing effort of discovering novel enzymes for natural product glycosylation, we found  Streptomyces chromofuscus  ATCC 49982 can convert resveratrol and several other natural products to corresponding glucuronides. We then analyzed the genome of this strain and discovered a putative UGT gene flanked by two UDP-glucuronic acid biosynthetic genes. The UGT gene was then cloned and heterologously expressed in Escherichia coli BL21(DE3). The enzyme was functionally characterized through in vitro reactions and its optimal reaction conditions were investigated. This enzyme is highly versatile and can convert a variety of substrates into monoglucuronides or diglucuronides. Therefore, this UGT represents a useful tool for the synthesis of glucuronidated metabolites. We also used the engineered  E. coli  strain to produce the two resveratrol glucuronides and the optimal bioconversion conditions were studied. This technology can be applied to a variety of molecules to yield corresponding glucuronides. Furthermore, a glucuronide was prepared from 2′-hydroxyflavone, which showed significantly improved water solubility and antioxidant activity. To our best knowledge, it is the first time that a highly versatile UGT was cloned and characterized from microorganisms, which lays the foundation for the microbial production of valuable glucuronides. 
     In certain aspects, the present invention relates to a process for producing a glucuronide comprising a glucuronic acid moiety bound to a phenolic hydroxyl group or a phenolic carboxyl group, the process comprising the steps of: (i) culturing in a medium a genetically modified microorganism capable of producing a glucuronide comprising a glucuronic acid moiety bound to a phenolic hydroxyl group or a phenolic carboxyl group, wherein the genetically modified microorganism compared to a non-modified control microorganism of the same strain has increased activity of a gene product of a UDP-glucuronyltransferase (UGT) with a SEQ ID NO: 7 or a polypeptide sequence which is at least 95% identical to SEQ ID NO: 7 and which has at least 80% of the enzymatic activity of the polypeptide according to SEQ ID NO: 7 under conditions allowing the microorganism to produce the glucuronide; and (ii) optionally recovering the glucuronide from the medium. 
     In one aspect, the genetically modified microorganism compared to a non-modified control microorganism of the same strain has increased activity of at least one gene product selected from the group consisting of: a UDP-glucose dehydrogenase with a SEQ ID NO: 6 or a polypeptide sequence which is at least 95% identical to SEQ ID NO: 6 and which has at least 80% of the enzymatic activity of the polypeptide according to SEQ ID NO: 6; and a UDP-glucose pyrophosphorylase with a SEQ ID NO: 8 or a polypeptide sequence which is at least 95% identical to SEQ ID NO: 8 and which has at least 80% of the enzymatic activity of the polypeptide according to SEQ ID NO: 8. 
     In another aspect, an increased activity of the respective gene product is achieved by introducing into the microorganism one or more copies of the respective gene product or a nucleotide sequence encoding the respective gene product. 
     In some aspects, the microorganism is selected from the group consisting of bacteria, yeast, filamentous fungi, and microalgae. In one aspect, the microorganism is selected from the group consisting of  Aspergillus, Trichoderma, Saccharomyces, Pichia, Phaffia, Candida, Hansenula, Salmonella, Bacillus, Acinectorhacter, Zymomonas, Agrobacterium, Erythrobacter, Chloroborium, Chlorella, Chromatium, Flavobacterium, Cytophaga, Rhodobacter, Rhodococcus, Streptomyces, Brevibacterium, Corynebacteria, Mycobacterium, Deinococcus, Escherichia, Erwinia, Pantoea, Pseudomonas, Sphingomonas, Methylomonas, Methylobacter, Methylococcus, Methylosinus, Methylomicrobium, Methylocystis, Alcaligenes, Synechocystis, Methanomonas, Synechococcus, Anabeana, Thiobacillus, Methanobacterium, Klebsiella,  and  Myxococcus.    
     In other aspects, the medium comprises a phenolic compound. In one embodiment, the phenolic compound is selected from the group consisting of resveratrol, quercetin, ferulic acid, vanillic acid, curcumin, vanillin, chrysin, zearalenone, apigenin, doxorubicin, etoposide, morphine, ezetimibe, 2′-hydroxyflavone, and combinations thereof. In another embodiment, the phenolic compound is at a concentration of between about 0.25 mM and about 0.75 mM. 
     In yet other aspects, the medium is maintained: a) at a pH of between about 5.5 and about 7.5; b) at a temperature of between about 35° C. and about 45° C.; and/or c) for a duration of between about 2.5 hour and 3.5 hours. 
     In certain aspects, the present invention relates to an expression vector comprising a gene encoding a UGT operably linked to a promoter suitable to drive expression of the UGT in microbial cells, wherein the UGT gene has a nucleotide sequence having at least 90% sequence identity to the nucleotide sequence shown in SEQ ID NO: 4, and wherein the UGT has an amino acid sequence having at least 90% sequence identity to the amino acid sequence shown in SEQ ID NO: 7. 
     In other aspects, the present invention provides a nucleic acid comprising a gene encoding a UGT, wherein the UGT gene has a nucleotide sequence having at least 90% sequence identity to the nucleotide sequence shown in SEQ ID NO: 4, and wherein the UGT has an amino acid sequence having at least 90% sequence identity to the amino acid sequence shown in SEQ ID NO: 7. 
     In yet other aspects, the present invention provides a polypeptide comprising an amino acid sequence having at least 90% sequence identity to the amino acid sequence shown in SEQ ID NO: 7. 
     In certain aspects, the present invention relates to a microbial cell comprising an expression vector, a nucleic acid, or a polypeptide as described herein, wherein the microbial cell is optionally a bacterial, yeast, filamentous fungi, or microalgal cell. 
     In certain aspects, the present disclosure concerns microbial cells comprising at least one of (a) expression vector comprising a gene encoding a UGT operably linked to a promoter suitable to drive expression of the UGT in microbial cells, wherein the UGT gene has a nucleotide sequence having at least 90% sequence identity to the nucleotide sequence shown in SEQ ID NO: 4, and wherein the UGT has an amino acid sequence having at least 90% sequence identity to the amino acid sequence shown in SEQ ID NO: 7; (b) nucleic acid comprising a gene encoding a UGT, wherein the UGT gene has a nucleotide sequence having at least 90% sequence identity to the nucleotide sequence shown in SEQ ID NO: 4, and wherein the UGT has an amino acid sequence having at least 90% sequence identity to the amino acid sequence shown in SEQ ID NO: 7, and (c) polypeptide comprising an amino acid sequence having at least 90% sequence identity to the amino acid sequence shown in SEQ ID NO: 7; wherein the microbial cell is optionally a bacterial, yeast, filamentous fungi, or microalgal cell. 
     In one aspect, the present invention provides an in vitro process for producing a glucuronide comprising a glucuronic acid moiety bound to a phenolic hydroxyl group or a phenolic carboxyl group, the process comprising the steps of: (i) purifying or partially purifying a gene product of a UDP-glucuronyltransferase (UGT) with a SEQ ID NO: 7 or a polypeptide sequence which is at least 95% identical to SEQ ID NO: 7 and which has at least 80% of the enzymatic activity of the polypeptide according to SEQ ID NO: 7; (ii) combining a phenolic compound and UDP-glucuronic acid with the UGT under conditions suitable to allow the production of the glucuronide; and (iii) optionally recovering the glucuronide. 
     In certain aspects, conditions suitable to allow the production of the glucuronide comprise: a) a pH of between about 6.5 and about 8.5; b) a temperature of between about 25° C. and about 35° C.; and/or c) with Ca 2+ , Mg 2+ , and/or Mn 2+  at a concentration of between about 5 mM and 15 mM. 
     In other aspects, the present invention provides a prodrug comprising a phenolic compound with a glucuronic acid moiety bound to a phenolic hydroxyl group or a phenolic carboxyl group, wherein the prodrug is produced with a process disclosed herein. 
     In one aspect, the prodrug is produced with a phenolic compound. Examples of phenolic compounds include resveratrol, quercetin, ferulic acid, vanillic acid, curcumin, vanillin, chrysin, zearalenone, apigenin, doxorubicin, etoposide, morphine, ezetimibe, and combinations thereof. 
     In other aspects, glucuronidation of the phenolic compound results in a monoglucuronide, a diglucuronide, or a combination thereof. In yet another aspect, the prodrug is cleaved by a β-glucuronidase after administration to a subject in need thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A- 1 E  depict screening of five actinomycete strains for the ability to biotransform resveratrol.  FIG.  1 A  depicts an HPLC analysis (300 nm) of biotransformation of resveratrol by five actinomycete strains. (i) resveratrol+YM medium; (ii) resveratrol+ S. roseum  No. 79089; (iii) resveratrol+ S. reseiscleroticus  ATCC 53903; (iv) resveratrol+ A. hibisca  P157-2; (v) resveratrol+ Streptomyces  sp. FERM BP-2474; (vi) resveratrol+ S. chromofuscus  ATCC 49982.  FIG.  1 B  depicts a UV spectra comparison of the substrate and product 1.  FIG.  1 C  depicts a UV spectra comparison of the substrate and product 2.  FIG.  1 D  depicts an ESI-MS (−) spectrum of product 1.  FIG.  1 E  depicts an ESI-MS (−) spectrum of product 2. 
         FIG.  2    depicts key HMBC correlations of compounds 1-7. 
         FIGS.  3 A- 3 C  depict an HPLC analysis of glucuronidation of different substrates by  S. chromofuscus  ATCC 49982.  FIG.  3 A  depicts glucuronidation of quercetin (350 nm);  FIG.  3 B  depicts glucuronidation of ferulic acid (300 nm);  FIG.  3 C  depicts glucuronidation of vanillic acid (300 nm). (i) substrate+YM medium; (ii) substrate+ S. chromofuscus  ATCC 49982. 
         FIGS.  4 A and  4 B  depict a putative glucuronidation (gca) gene cluster discovered in  S. chromofuscus  ATCC 49982.  FIG.  4 A  depicts the organization of the gca gene cluster.  FIG.  4 B  depicts proposed pathways for UDP-glucuronic acid biosynthesis and glucuronidation of resveratrol. 
         FIGS.  5 A- 5 D  depict heterologous expression and in vitro functional characterization of GcaC.  FIG.  5 A  depicts N-His6-tagged GcaC (˜43.4 kDa) purified from cell lysate analyzed by SDS-PAGE.  FIG.  5 B  depicts an HPLC analysis (300 nm) of the in vitro reaction of GcaC with resveratrol. (i) resveratrol+reaction buffer without GcaC; (ii) resveratrol+GcaC.  FIG.  5 C  depicts an ESI-MS (−) spectrum of in vitro product 1.  FIG.  5 D  depicts an ESI-MS (−) spectrum of in vitro product 2. 
         FIGS.  6 A -6H depict HPLC analyses of in vitro glucuronidation of different sugar acceptor substrates by GcaC.  FIG.  6 A  depicts glucuronidation of quercetin (350 nm).  FIG.  6 B  depicts glucuronidation of ferulic acid (300 nm).  FIG.  6 C  depicts glucuronidation of vanillic acid (300 nm).  FIG.  6 D  depicts glucuronidation of curcumin (420 nm).  FIG.  6 E  depicts glucuronidation of vanillin (300 nm).  FIG.  6 F  depicts glucuronidation of chrysin (350 nm).  FIG.  6 G  depicts glucuronidation of zearalenone (250 nm).  FIG.  6 H  depicts glucuronidation of apigenin (350 nm). For each figure: (i) substrate incubated with the reaction buffer without GcaC; (ii) substrate incubated with the reaction buffer with GcaC. 
         FIGS.  7 A- 7 C  depict the determination of the optimum in vitro reaction conditions for GcaC.  FIG.  7 A  depicts the effect of reaction temperature on the GcaC glucuronidation activity.  FIG.  7 B  depicts the effect of reaction pH on the GcaC glucuronidation activity.  FIG.  7 C  depicts the effect of metal ions on the GcaC glucuronidation activity. Data are presented as the mean±SD from three independent experiments. 
         FIGS.  8 A- 8 F  depict optimization of in vivo production of resveratrol glucuronides by  E. coli  BL21(DE3)/pJR36.  FIG.  8 A  depicts an HPLC analysis (300 nm) of resveratrol glucuronidation by GcaC in  E. coli  BL21(DE3). (i) Commercial standard of resveratrol; (ii) resveratrol+ E. coli  BL21(DE3)/pET28a; (iii) resveratrol+ E. coli  BL21(DE3)/pJR36.  FIG.  8 B  depicts the effect of cell density on resveratrol glucuronidation.  FIG.  8 C  depicts the effect of reaction pH on resveratrol glucuronidation by  E. coli  BL21(DE3)/pJR36.  FIG.  8 D  depicts the effect of temperature on resveratrol glucuronidation by  E. coli  BL21(DE3)/pJR36.  FIG.  8 E  depicts the effect of reaction time on resveratrol glucuronidation by  E. coli  BL21(DE3)/pJR36.  FIG.  8 F  depicts the effect of substrate concentration on resveratrol glucuronidation by  E. coli  BL21(DE3)/pJR36. Data are presented as the mean±SD from three independent experiments. 
         FIG.  9    shows HPLC analysis of glucuronidation of 2′-hydroxyflavone at 300 nm. Top:  E. coli -GcaC+2′-hydroxyflavone; Bottom:  S. chromofuscus  ATCC 49982+2′-hydroxyflavone. 
         FIG.  10    shows HMBC correlations for hydroxyflavone-2′-O-β-D-glucuronide. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. For example, “a” or “an” means “at least one” or “one or more.” 
     Throughout this disclosure, various aspects of the claimed subject matter are presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the claimed subject matter. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, where a range of values is provided, it is understood that each intervening value, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the claimed subject matter. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges, and are also encompassed within the claimed subject matter, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the claimed subject matter. This applies regardless of the breadth of the range. 
     As used herein, a “biologically pure” strain is intended to mean the strain separated from materials with which it is normally associated in nature. A strain associated with other strains, or with compounds or materials that it is not normally found with in nature, is still defined as “biologically pure.” A monoculture of a particular strain is, of course, “biologically pure.” In different embodiments, a “biologically pure” culture has been purified at least 2× or 5× or 10× or 50× or 100× or 1000× or higher (to the extent considered feasible by a skilled person in the art) from the material with which it is normally associated in nature. As a non-limiting example, if a culture is normally associated with soil, the organism can be biologically pure to an extent that its concentration in a given quantity of purified or partially purified material with which it is normally associated (e.g. soil) is at least 2× or 5× or 10× or 50× or 100× or 1000× or higher (to the extent considered feasible by a skilled person in the art) that in the original unpurified material. 
     An “expression vector” as used herein is a DNA molecule used to transfer and express foreign genetic material in a cell. Such vectors include a promoter sequence operably linked to the gene encoding the protein to be expressed. “Promoter” means a minimal DNA sequence sufficient to direct transcription of a DNA sequence to which it is operably linked. “Promoter” is also meant to encompass those promoter elements sufficient for promoter-dependent gene expression controllable for cell type specific expression; such elements may be located in the 5′ or 3′ regions of the native gene. 
     An expression vector may also include a termination codon and expression enhancers. Any suitable vectors, promoters, enhancers, and termination codons may be used to express the enzymes according to the invention. Suitable vectors include plasmids, binary vectors, phages, phagemids, viral vectors and artificial chromosomes (e.g., yeast artificial chromosomes or bacterial artificial chromosomes). 
     Generally speaking, those skilled in the art are well able to construct vectors and design protocols for recombinant gene expression. Suitable vectors can be chosen or constructed, containing, in addition to the elements of the invention described above, appropriate regulatory sequences, including promoter sequences, terminator fragments, polyadenylation sequences, marker genes and other sequences as appropriate. Molecular biology techniques suitable for the expression of polypeptides in cells are well known in the art. For further details see, for example, Molecular Cloning: a Laboratory Manual: 2nd edition, Sambrook et al, 1989, Cold Spring Harbor Laboratory Press or Current Protocols in Molecular Biology, Second Edition, Ausubel et al. eds., John Wiley &amp; Sons, (1995, and periodic supplements). 
     The term “operably linked” used herein includes the situation where a selected gene and promoter are covalently linked in such a way as to place the expression of the gene (i.e., polypeptide coding) under the influence or control of the promoter. Thus, a promoter is operably linked to a gene if the promoter is capable of effecting transcription of the gene into RNA in a cell. Where appropriate, the resulting RNA transcript may then be translated into a desired protein or polypeptide. The promoter is suitable to effect expression of the operably linked gene in a microbial cell. 
     The invention also provides a nucleic acid comprising a gene encoding a UGT. The UGT gene present in the nucleic acid can have the requisite features and sequence identity as described herein in relation to the expression vectors. 
     Alignment and calculation of percentage amino acid or nucleotide sequence identity can be achieved in various ways know to a person of skill in the art, for example, using publicly available computer software such as ClustalW 1.82, T-coffee or Megalign (DNASTAR) software. When using such software, the default parameters, e.g., for gap penalty and extension penalty, are preferably used. The default parameters of ClustalW 1.82 are: Protein Gap Open Penalty=10.0, Protein Gap Extension Penalty=0.2, Protein matrix=Gonnet, Protein/DNA ENDGAP-1, Protein/DNA GAPDIST=4. 
     The percentage identity can then be calculated from the multiple alignment as (N/T)*100, where N is the number of positions at which the two sequences share an identical residue, and T is the total number of positions compared. Alternatively, percentage identity can be calculated as (N/S)*100 where S is the length of the shorter sequence being compared. The amino acid/polypeptide/nucleic acid sequences may be synthesized de novo or may be native amino acid/polypeptide/nucleic acid sequence, or a derivative thereof. 
     Due to the degeneracy of the genetic code, it is clear that any nucleic acid sequence could be varied or changed without substantially affecting the sequence of the protein encoded thereby, to provide a functional variant thereof. Suitable nucleotide variants are those having a sequence altered by the substitution of different codons that encode the same amino acid within the sequence, thus producing a silent change. Other suitable variants are those having homologous nucleotide sequences but comprising all, or portions of, sequence which are altered by the substitution of different codons that encode an amino acid with a side chain of similar biophysical properties to the amino acid it substitutes, to produce a conservative change. For example, small non-polar, hydrophobic amino acids include glycine, alanine, leucine, isoleucine, valine, proline, and methionine. Large non-polar, hydrophobic amino acids include phenylalanine, tryptophan, and tyrosine. The polar neutral amino acids include serine, threonine, cysteine, asparagine, and glutamine. The positively charged (basic) amino acids include lysine, arginine, and histidine. The negatively charged (acidic) amino acids include aspartic acid and glutamic acid. As described herein, suitable variants of the UGT can contain amino acid substitutions at any amino acid. 
     In some aspects, the present invention relates to a polypeptide that functions as a UGT. A preferred polypeptide sequence for a UGT is disclosed in SEQ ID NO: 7; as well as polypeptide sequences which are at least 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, and 99% identical to SEQ ID NO: 7 and which have at least 50%, preferred 80%, 90%, 100% and most preferred more than 100%, more than 120% , more than 150% of the enzymatic activity of the polypeptide according to SEQ ID NO: 7. 
     In other aspects, the present invention relates to nucleic acid comprising a gene encoding a UGT. A preferred nucleotide sequence for a UGT is disclosed in SEQ ID NO: 4; as well as nucleotide sequences which are at least 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, and 99% identical to SEQ ID NO: 4. 
     A genetically modified microorganism having a modification which confers an increased activity of a gene products means a microorganism which is treated by endogenous or exogenous factors in order to increase at the end the activity of the corresponding gene product. 
     This is possible at different levels, e.g., at the level of the DNA, at the level of the RNA and at the level of the protein. At the level of the DNA an increased activity of the respective gene product can be achieved by introducing into the microorganism genes in one or multiple copies for the respective gene product or one or multiple copies of mutated genes coding for gene products with a higher enzymatic activity or for more stable gene products enzymes, which will not be degraded in the microorganism as rapidly as the unmutated gene products. These additional copies of the respective genes can be introduced into the genome of the microorganism or in an extrachromosomal way. Also, they can be introduced in a permanent or a transient way. 
     At the level of RNA, it is possible to increase the gene expression by using genetic elements such as effective promoters, enhancers, terminators, and other regulatory elements which allow a higher or a permanent gene expression of the respective gene. 
     Another possibility is to stabilize the RNA transcripts made of the respective gene in order to effect a higher number of transcripts available for translation per time unit. 
     Another possibility is to deregulate a strongly regulated gene expression, e.g., feedback inhibition, by altering the regulatory elements on the polynucleotide site, such as operators, or on the polypeptide site such as repressors. By this means a highly regulated gene expression (“bottleneck”) can be deregulated in order to allow a higher activity of the gene product. Another possibility to interfere with at the protein side is the codon usage of the gene. A codon usage optimized for the respective host organism allows a high translation rate resulting in an increase of active protein. 
     The possibilities to intervene at can be combined in various ways, e.g., multiple copies of a codon optimized gene can be expressed under a strong promotor. 
     In the context of this invention the term “genetically modified microorganism” should be understood in a broad sense; not only “genes” and “genetic elements” such as promotors, are encompassed by the term “genetically modified microorganism”, but also a derepression of gene regulation by using molecules binding tightly to a repressor and thus inactivating the repressor is understood as a genetically modified microorganism according to this invention. In the context of this invention also combinations of gene products are used. 
     The recovering of the glucuronide from the fermentation product can be performed by a number of routine techniques known in biotechnology such as precipitation and centrifugation. 
     Glycosylation is an effective way to increase the polarity of natural products. UDP-Glucuronyltransferases (UGTs) are commonly observed and extensively studied in phase II drug metabolism. However, UGTs in microorganisms are not well studied, which hampered the utilization of this type of enzyme in microbial glucuronidation of natural products. Screening of five actinomycete strains showed that  Streptomyces chromofuscus  ATCC 49982 can convert diverse plant polyphenols into more polar products, which were characterized as various glucuronides based on their spectral data. Analysis of the genome of this strain revealed a putative glucuronidation gene cluster that contains a UGT gene (gcaC) and two UDP-glucuronic acid biosynthetic genes (gcaB and gcaD). The gcaC gene was cloned and heterologously expressed in  Escherichia coli.  Incubation of the purified enzyme with resveratrol and UDP-glucuronic acid led to the production of resveratrol-4′-O-β-D-glucuronide and resveratrol-3-O-β-D-glucuronide, allowing GcaC to be characterized as a flexible UGT. The optimal in vitro reaction pH and temperature for GcaC are 7.5 and 30° C., respectively. Its activity can be stimulated by Ca 2+ , Mg 2+  and Mn 2+ , whereas Z n   2+ , Cu 2+  and Fe 2+  showed inhibitory effects. Furthermore, GcaC has a broad substrate specificity, which can glucuronidate various substrates besides resveratrol, including quercetin, ferulic acid, vanillic acid, curcumin, vanillin, chrysin, zearalenone, and apigenin. The titers of resveratrol-4′-O-β-D-glucuronide and resveratrol-3-O-β-D-glucuronide in  E. coli/ GcaC were 78.381±0.366 mg/L and 14.991±0.248 mg/L from 114.125 mg/L resveratrol within 3 hours. In some aspects, the present invention provides an effective way to produce glucuronides of resveratrol and other health-benefitting natural products. Non-limiting examples of health-benefitting natural products that can act as substrates for GcaC are presented in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Examples of plant phenolic compounds that act as substrates for GcaC. 
               
            
           
           
               
               
               
            
               
                 Common name 
                 Structure 
                 IUPAC name 
               
               
                   
               
               
                 p-Hydroxybenzoic acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 4-Hydroxybenzoic acid 
               
               
                   
               
               
                 p-Coumaric acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 3-(4-Hydroxyphenyl)-2- propenoic acid 
               
               
                   
               
               
                 Protocatechuic acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 3,4-Dihydroxybenzoic acid 
               
               
                   
               
               
                 Caffeic acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 3-(3,4-Dihydroxyphenyl)-2- propenoic acid 
               
               
                   
               
               
                 Gallic acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 3,4,5-Trihydroxybenzoic acid 
               
               
                   
               
               
                 Vanillic acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 4-Hydroxy-3-methoxybenzoic acid 
               
               
                   
               
               
                 Isovanillic acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 3-Hydroxy-4-methoxybenzoic acid 
               
               
                   
               
               
                 Syringic acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 4-Hydroxy-3,5- dimethoxybenzoic acid 
               
               
                   
               
               
                 Ferulic acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 3-(4-Hydroxy-3-methoxy- phenyl)prop-2-enoic acid 
               
               
                   
               
               
                 Chlorogenic acid (3- Caffeoylquinic acid) 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 3-{[(2E)-3-(3,4- Dihydroxyphenyl)prop-2- enoyl]oxy}-1,4,5- trihydroxycyclohexanecarboxylic acid 
               
               
                   
               
               
                 Di-caffeoylquinic acid (Cynarine) 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 (1R,3R,4S,5R)-1,3-Bis[[(E)-3- (3,4-dihydroxyphenyl)prop-2- enoyl]oxy]-4,5- dihydroxycyclohexane-1- carboxylic acid 
               
               
                   
               
            
           
         
       
     
     The present invention is further illustrated by the following examples that should not be construed as limiting. The contents of all references, patents, and published patent applications cited throughout this application, as well as the Figures, are incorporated herein by reference in their entirety for all purposes. 
     EXAMPLES 
     Example 1. Materials and Methods 
     General Equipment and Experimental Materials 
     Agilent 1200 HPLC instrument with an Agilent Eclipse Plus-C 18  column (5 μm, 250 mm×4.6 mm) was used to analyze and purify the products. The samples were eluted with methanol-water (5:95 to 95:5 over 30 minutes, v/v, containing 0.1% formic acid) at a flow rate of 1 mL/min. ESI-MS spectra were obtained on an Agilent 6130 single quadrupole LC-MS in the negative mode. All purified compounds were dissolved in in deuterated dimethyl sulfoxide (DMSO-d 6 ) to collect the NMR spectra on a Bruker Avance III HD Ascend-500 NMR instrument (500 MHz for  1 H NMR and 125 MHz for  13 C NMR) located in the Department of Chemistry and Biochemistry, Utah State University. The chemical shift (δ) values are given in parts per million (ppm). The coupling constants values) are reported in hertz (Hz). Low-resolution ESI-MS spectra were obtained on an Agilent 6130 LC-MS to confirm the molecular weights of glucuronides. 
     Phusion High-Fidelity DNA polymerase, restriction enzymes and T4 DNA ligase were purchased from New England Biolabs. PCR reactions were conducted with an Arktik Thermal Cycler (Thermo Scientific). Genomic DNA extraction was performed using the Quick-DNA™ Fungal/Bacteria DNA Miniprep Kit (Zymo Research, USA). Plasmid extraction was performed using the Thermo Scientific GeneJET Plasmid Miniprep Kit (Thermo Scientific). Primers were ordered from Thermo Scientific and dissolved in Tris-EDTA (TE) buffer to the concentration of 100 ng/mL. Standard compounds like resveratrol, quercetin, curcumin, vanillic acid, ferulic acid, vanillin, chrysin, zearalenone, and apigeninwere purchased from Sigma-Aldrich (USA). HisPur™ Ni-NTA resin, Luria-Bertani (LB) medium, yeast and malt extracts were purchased from Fisher Scientific (Rockford, Ill., USA). Bradford assay solution was purchased from TCI America (Portland, Oreg., USA). Solvents and all other chemicals were purchased from Fisher Scientific. Milli-Q water was used throughout this study. 
     Strains, Vectors, Media, and Culture Conditions 
     Streptosporangium roseum No. 79089 (NRRL 2505) was provided by the ARS Culture Collection (NRRL) of the United States Department of Agriculture.  Streptomyces reseiscleroticus  ATCC 53903, Actinomadura hibisca P157-2 (ATCC 53557), and  S. chromofuscus  ATCC 49982 were obtained from the American Type Culture Collection (ATCC).  Streptomyces  sp. FERM BP-2474 was acquired from Patent and Bio-Resource Center, National Institute of Advanced Industrial Science and Technology, Japan. 
       E. coli  XL1-Blue and BL21(DE3) were both purchased from Agilent.  E. coli  XL1-Blue was used for routine gene cloning and plasmid propagation.  E. coli  BL21(DE3) was used for protein expression and in vivo biotransformation. The pJET1.2 (Thermo Fisher Scientific, USA) and pET28a (+) (Millipore Sigma, USA) vectors were used, respectively, to clone and express UGT. The  E. coli  strains were routinely grown at 37° C. on LB agar plates or in liquid LB medium (Fisher Scientific, USA). When necessary, carbenicillin (50 μg/mL) and kanamycin (50 μg/mL) were supplemented into the culture media for selecting correct clones.  S. chromofuscus  ATCC 49982 was grown on YM plate (4 g/L yeast extract, 10 g/L malt extract, 4 g/L glucose, and 20 g/L agar) or in YM broth (4 g/L yeast extract, 10 g/L malt extract, and 4 g/L glucose) at 28° C. Recombinant UGT was expressed in  E. coli  BL21(DE3) at 28° C. Isopropyl-1-thio-β-D-galactopyranoside (IPTG) (Gold Biotechnology, USA) was used at 200 μM to induce protein expression in  E. coli  BL21(DE3). 
     Screening of Glucuronidating Actinomycete Strains 
     To test the ability of actinomyces to glucuronidate resveratrol, we screened five strains, namely,  S. roseum  No. 79089,  S. reseiscleroticus  ATCC 53903,  A. hibisca  ATCC 53557,  Streptomyces  sp. FERM BP-2474, and  S. chromofuscus  ATCC 49982. These bacteria were grown in 50 mL of YM medium in a rotary shaker at 250 rpm and 28° C. for 3 days. 
     Resveratrol (4 mg) was dissolved in dimethyl sulfoxide (DMSO) and added into each culture. The cultures were incubated under the same conditions for an additional 4 days. After that, 1 mL of fermentation broth was sampled and centrifuged at 15,000×g for 10 min. The supernatant was analyzed by HPLC on an Agilent Eclipse Plus C 18  column (5 μm, 250 mm×4.6 mm) at 300 nm. A gradient of methanol-water (5-95%, v/v) containing 0.1% (v/v) formic acid was programmed over 35 min at a flow rate of 1 mL/min. 
     In order to check whether  S. chromofuscus  ATCC 49982 can glucuronidate other substrates, quercetin, ferulic acid, and vanillic acid were incubated with the culture of this strain in a similar way. The supernatant was analyzed by HPLC on an Agilent Eclipse Plus C 18  column (5 μm, 250 mm×4.6 mm) at 350 nm (quercetin) or 300 nm (ferulic acid and vanillic acid) using the same HPLC conditions for resveratrol. 
     Extraction and Purification of Compounds 
     To isolate the biotransformation products of resveratrol for structure elucidation,  S. chromofuscus  ATCC 49982 was cultivated in 1-L Erlenmeyer flasks, containing 250 mL of YM medium, to biotransform 20 mg of resveratrol. After 4 days, the fermentation broth was centrifuged at 4,000×g for 10 min. After water in the supernatant was evaporated, the residue was dissolved in 2 mL of methanol. The sample was subjected to Sephadex LH-20 column chromatography, and eluted with methanol-water (1:1, v/v). The products-containing fraction was further separated by reverse phase HPLC, and eluted with methanol-water (10-50%, 0-17 min; 50-95, 17-22 min) containing 0.1% formic acid (v/v) to yield 1 (3.6 mg) and 2 (2.8 mg). Products 1 and 2 were also used to prepare a standard curve to quantify the formation of these products in the in vitro and in vivo reactions. 
     A similar procedure was used to isolate the biotransformation products of quercetin, ferulic acid and vanillic acid for structure elucidation except the HPLC methods for final compound purification. The gradient elution method with methanol-water (20-40%, 0-2 min; 40%, 2-15 min; 40-95%, 15-20 min) was used to yield product 3 (7.4 mg) and 4 (5.8 mg). The gradient elution method with methanol-water (5-60%, 0-15 min; 60-95%, 15-16 min; 95%, 16-18 min) was used to yield product 5 (4.8 mg), 6 (3.0 mg) and 7 (1.2 mg). All purified products were subjected to NMR analysis. The purified products were dissolved in DMSO-d 6  and their chemical structures were characterized by NMR spectra. 
     Resveratrol-4′-O-β-D-glucuronide (1):  1 H NMR (500 MHz, DMSO-d 6 ): δ 7.52 (d, J=8.8 Hz, 2H, H-2′ and H-6′), 7.01 (d, J=8.8 Hz, 2H, H-3′ and H-5′), 6.96 (d, J=17.2 Hz, 2H, H-7, 8), 6.41 (d, J=2.1 Hz, 2H, H-2 and H-6), 6.14 (t, J=2.1 Hz, 1H, H-4), 5.07 (d, J=7.5 Hz, 1H, H-1″), 3.93 (m, 1H, H-5″), 3.41-3.28 (overlapped, 3H, H-2′, H-3′, and H-4′).  13 C NMR (125 MHz, DMSO-d 6 ): δ 170.5 (C-6″), 159.0 (C-3 and C-5), 156.9 (C-4′), 139.4 (C-1), 131.6 (C-1′), 128.1 (C-2′ and C-6′), 127.8 (C-8), 127.7 (C-7), 116.8 (C-3′ and C-5′), 104.9 (C-2 and C-6), 102.5 (C-4), 100.3 (C-1″), 76.2 (C-3″), 75.9 (C-5″), 73.4 (C-2″), 71.8 (C-4″). 
     Resveratrol-3-O-β-D-glucuronide (2):  1 H NMR (500 MHz, DMSO-d 6 ): δ 7.40 (d, J=8.6 Hz, 2H, H-2′ and H-6′), 7.01 (d, J=16.2 Hz, 1H, H-7), 6.87 (d, J=16.4 Hz, 1H, H-8), 6.76 (d, J=8.6 Hz, 2H, H-3′ and H-5′), 6.66 (s, 1H, H-2), 6.62 (s, 1H, H-6), 6.32 (t, J=2.1 Hz, 1H, H-4), 4.99 (d, J=7.7 Hz, 1H, H-1″), 3.89 (m, 1H, H-5″), 3.40-3.27 (overlapped, 3H, H-2′, H-3′, and H-4′).  13 C NMR (125 MHz, DMSO-d 6 ): δ 170.1 (C-6″), 158.4 (C-3 and C-5), 157.9 (C-4′), 139.4 (C-1), 132.2 (C-1′), 128.6 (C-8), 127.9 (C-2′ and C-6′), 126.4 (C-7), 115.5 (C-3′ and C-5′), 104.8 (C-2 and C-6), 102.1 (C-4), 100.5 (C-1″), 77.2 (C-3″), 75.8 (C-5″), 72.9 (C-2″), 71.4 (C-4″). 
     Quercetin-7-O-β-D-glucuronide (3):  1 H NMR (500 MHz, DMSO-d 6 ): δ 7.72 (d, J=2.2 Hz, 1H, H-2′), 7.56 (dd, J=8.5, 2.2 Hz, 1H, H-6′), 6.91 (d, J=8.5 Hz, 1H, H-5′), 6.80 (d, J=2.0 Hz, 1H, H-8), 6.44 (d, J=2.0 Hz, 1H, H-6), 5.29 (d, J=7.3 Hz, 1H, H-1″), 4.06 (d, J=9.6 Hz, 1H, H-5″), 3.41 (m, 1H, H-4″), 3.35-3.31 (overlapped, 2H, H-2″ and H-3″).  13 C NMR (125 MHz, DMSO-d 6 ): δ 176.0 (C-4), 170.1 (C-6″), 162.2 (C-7), 160.4 (C-5), 155.7 (C-9), 147.9 (C-4′), 147.6 (C-2), 145.1 (C-3′), 136.1 (C-3), 121.8 (C-1′), 120.0 (C-6′), 115.6 (C-5′), 115.4 (C-2′), 104.8 (C-10), 99.1 (C-1″), 98.6 (C-6), 94.1 (C-8), 75.6 (C-3″), 75.4 (C-5″), 72.8 (C-2″), 71.2 (C-4″). 
     Quercetin-3-O-β-D-glucuronide (4):  1 H NMR (500 MHz, DMSO-d 6 ): δ 7.61 (dd, J=8.5, 2.2 Hz, 1H, H-6′), 7.53 (d, J=2.2 Hz, 1H, H-2′), 6.84 (d, J=8.5 Hz, 1H, H-5′), 6.41 (d, J=2.0 Hz, 1H, H-8), 6.21 (d, J=2.0 Hz, 1H, H-6), 5.50 (d, J=7.5 Hz, 1H, H-1″), 3.57 (d, J=9.7 Hz, 1H, H-5″), 3.39-3.25 (overlapped, 3H, H-2′, H-3′, and H-4′).  13 C NMR (125 MHz, DMSO-d 6 ): δ 177.1 (C-4), 169.7 (C-6″), 164.2 (C-7), 161.2 (C-5), 156.2 (C-2 and C-9), 148.6 (C-4′), 144.9 (C-3′), 133.0 (C-3), 121.7 (C-1′), 120.8 (C-6′), 116.0 (C-2′), 115.2 (C-5′), 104.8 (C-10), 101.0 (C-1″), 98.8 (C-6), 93.6 (C-8), 76.0 (C-5″), 75.8 (C-2″), 73.8 (C-4″), 71.3 (C-3″). 
     Ferulic acid-4-O-β-D-glucuronide (5):  1 H NMR (500 MHz, DMSO-d 6 ): δ 7.52 (d, J=15.9 Hz, 1H, H-7), 7.34 (d, J=1.3 Hz, 1H, H-2), 7.19 (dd, J=8.5, 1.3 Hz, 1H, H-6), 7.09 (d, J=8.5 Hz, 1H, H-5), 6.47 (d, J=15.9 Hz, 1H, H-8), 5.13 (d, J=7.1 Hz, 1H, H-1′), 3.86 (s, 3H, OCH3), 3.86 (d, J=9.5 Hz, 1H, H-5′), 3.37 (m, 1H, H-4′), 3.32-3.29 (overlapped, 2H, H-2′ and H-3′).  13 C NMR (125 MHz, DMSO-d 6 ): 8 170.2 (C-6′), 167.8 (C-9), 149.2 (C-3), 147.8 (C-4), 143.9 (C-7), 128.4 (C-1), 122.1 (C-6), 117.4 (C-8), 114.9 (C-5), 111.3 (C-2), 99.2 (C-1′), 76.1 (C-3′), 75.3 (C-5′), 72.9 (C-2′), 71.4 (C-4′), 55.7 (OCH3). 
     Vanillic acid-4-O-β-D-glucuronide (6):  1 H NMR (500 MHz, DMSO-d 6 ): δ 7.51 (dd, J=8.5, 1.6 Hz, 1H, H-6), 7.48 (d, J=1.6 Hz, 1H, H-2), 7.15 (d, J=8.5 Hz, 1H, H-5), 5.11 (d, J=6.1 Hz, 1H, H-1′), 3.81 (s, 3H, OCH3), 3.71 (m, 1H, H-5′), 3.32-3.26 (overlapped, H-2′, H-3′, and H-4′).  13 C NMR (125 MHz, DMSO-d 6 ): δ 170.8 (C-6′), 167.0 (C-7), 150.0 (C-4), 148.5 (C-3), 124.4 (C-1), 122.7 (C-6), 114.3 (C-5), 112.7 (C-2), 99.2 (C-1′), 76.4 (C-3′), 74.7 (C-5′), 72.9 (C-2′), 71.6 (C-4′), 55.6 (OCH 3 ). 
     Vanillic acid-7-O-β-D-glucuronide (7):  1 H NMR (500 MHz, DMSO-d 6 ): δ 7.54 (dd, J=8.3, 2.0 Hz, 1H, H-6), 7.49 (d, J=2.0 Hz, 1H, H-2), 6.89 (d, J=8.3 Hz, 1H, H-5), 5.54 (d, J=7.5 Hz, 1H, H-1′), 3.83 (s, 3H, OCH3), 3.82 (m, 1H, H-5′), 3.40-3.35 (overlapped, 3H, H-2′, H-3′, and H-4′).  13 C NMR (125 MHz, DMSO-d 6 ): δ 170.4 (C-6′), 164.4 (C-7), 152.1 (C-4), 147.4 (C-3), 124.1 (C-6), 119.7 (C-1), 115.2 (C-5), 112.9 (C-2), 94.6 (C-1′), 75.9 (C-3′), 75.7 (C-5′), 72.7 (C-2′), 71.6 (C-4′), 55.7 (OCH 3 ). 
     Genome Analysis and Amplification of the Putative UGT Gene from  S. chromofuscus  ATCC 49982 
     The genomic DNA was extracted from  S. chromofuscus  ATCC 49982 using the Quick-DNA™ Fungal/Bacterial Microprep Kit (Zymo Research, USA) by following the manufacturer&#39;s standard procedure. The 454 next-generation sequencing system and Rapid Annotation using Subsystem Technology (RAST) were used respectively to sequence and annotate the genomic DNA of  S. chromofuscus.  The glucuronidation (gca) gene cluster was deposited into GenBank under the accession number MZ666424. The detailed functions of each gene in the gene cluster was predicted based on BLAST analysis of the corresponding amino acid sequences. 
     Plasmids were extracted from  E. coli  via the GeneJET Plasmid Miniprep Kit (Thermo Fisher Scientific). Primers were synthesized by Thermo Scientific (5′-AATTGTTTAAACCATATGCGAGTACTGTTCACCAC-3′ (SEQ ID NO: 1) and 5′-AATTGCTAGCAAGCTTTCAGACGATCTTCTGCAGGTC-3′ (SEQ ID NO: 2)). The primers (1 μM), genomic DNA (0.2 μL), dNTP mix (200 μM), 5× buffer (4 μL), DMSO (0.4 μL), Phusion High-Fidelity DNA Polymerase (0.2 μL at 2 U/μL) and nuclease-free water were mixed to 20 μL for amplification of the UGT (gcaC) gene (1,209 bp). The PCR program began with an initial denaturation at 95° C. for 5 min, and then 20 cycles of touchdown program (95° C. for 30 s, annealing at 70° C. for 40 s, decreasing 0.5° C. per cycle, and extension at 72° C. for 100 s), followed by 20 cycles of regular program (95° C. for 30 s, annealing at 60° C. for 40 s, and extension at 72° C. for 100 s), and finally, 68° C. for 15 min of extension. 
     Construction of Cloning and Expression Plasmids 
     After purification with a GeneJET Gel Extraction Kit, the target PCR product was ligated into the pJET1.2 cloning vector to yield pJR34 (pJET1.2-gcaC). After the pJR34 plasmid was confirmed by digestion check and sequencing using the Sanger method, the gene was excised from pJR34 and introduced to the pET28a expression vector between the NdeI and HindIII restriction sites to yield expression plasmid pJR36 (pET28a-gcaC). The ligation product was transferred into  E. coli  XL1-Blue competent cells through chemical transformation, and the transformants were selected on LB agar with 50 μg/mL kanamycin. The correct plasmids were confirmed by digestion check with Ndel and HindIII. 
     Heterologous Expression of GcaC and In Vivo Biotransformation of Resveratrol in  E. coli  BL21(DE3) 
     The expression plasmid pJR36 was transferred into  E. coli  BL21(DE3) through chemical transformation. The correct transformant of  E. coli  BL21(DE3)/pJR36 was picked from LB agar into 5 mL of LB medium supplemented with kanamycin (50 μg/mL), incubating at 37° C. with shaking (250 rpm) for about 12 hours. Then 500 μL of the seed culture was inoculated into 50 mL of LB broth with kanamycin (50 μg/mL) with shaking at 250 rpm and 37° C. When the OD 600  reached between 0.4 and 0.6, protein expression was induced with 200 μM IPTG, and the culture was maintained at 28° C. for an additional 16 hours with shaking at 250 rpm. After protein expression, 0.35 mM resveratrol and 0.11 M glucose were added as the substrates. The culture was incubated under the same conditions for an additional 48 hours. After fermentation, 1 mL of the culture was sampled and centrifuged at 15,000×g for 10 min. 100 μL of the supernatant was subjected to analysis on an Agilent 1200 HPLC (at 300 nm) coupled with an Agilent 6130 single quadrupole mass spectrometer. 
     Purification of Recombinant GcaC from  E. coli BL 21(DE3)/pJR36 
     The  E. coli  BL21(DE3)/pJR36 cells were centrifuged at 3,600×g and 4° C. for 10 min. The harvested cells were resuspended in lysis buffer that consisted of 20 mM Tris-HCl buffer (pH 7.9), 0.5 M NaCl and 1 mM dithiothreitol (DTT). Subsequently, the cells were disrupted by ultrasonication (Misonix Sonicator 3000, Misonix Inc., USA) on ice (20 seconds on, 40 seconds off, and in total 15 min). The cell lysate was centrifuged at 15,000×g and 4° C. for 10 min and the supernatant was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) as soluble fraction. A premixed protein marker (EZ-Run™ Pre-Stained Rec Protein Ladder, molecular range: 11-170 kDa, Fisher BioReagents) was used for SDS-PAGE analysis as the reference. After electrophoresis, the gel was stained with 0.1% Coomassie Brilliant Blue R250, followed by destaining in 20% (v/v) acetic acid-water. 
     The supernatant of the cell lysate was loaded onto a HisPur™ Ni-NTA affinity column (Thermo Scientific, Rockford, USA) to purify the enzymes based on the manufacturer&#39;s protocol. After eluting the column with cold (4° C.) washing buffers (50 mM Tris-HCl, 2 mM EDTA, pH 7.9) containing 0, 10 mM and 25 mM imidazole, the recombinant UDPGT was finally eluted with elution buffer (50 mM Tris-HCl, 2 mM EDTA, pH 7.9) containing 250 mM imidazole. The purified His6-tagged protein was concentrated and desalted with buffer A (50 mM Tris-HCl, 2 mM EDTA, pH 7.9) to 1 mL, using the 30 K Macrosep Advance Centrifugal Device (Pall Corporation, New York, USA). This purified GcaC enzyme was stored in 50% (v/v) glycerol at −20° C. The protein concentration was determined using the Bradford assay. All protein purification steps were carried out at 4° C. 
     Functional Characterization of GcaC Through In Vitro Enzymatic Reactions 
     We characterized the function of GcaC by reacting the enzyme with the substrate resveratrol. Enzymatic assays were conducted in a 100-μL reaction system, which included 20 mM Tris-HCl (pH 8.0), 2.2 mM substrate, 1 mM MgCl 2 , 2 mM sugar donor (UDP-glucuronic acid), and 23.7 μg purified recombinant GcaC protein. The mixtures were thoroughly mixed and incubated at 30° C. for 6 hours, and then 200 μL of HPLC-grade methanol was added to terminate the reaction. The reaction mixtures were centrifuged at 13,000×g for 10 min, and supernatants were collected to analyze the products by LC-MS. Compounds were eluted with a gradient of methanol-water containing 0.1% formic acid (0-30 min, 5:95-95:5, v/v), programming over 30 min at a flow rate of 1 mL/min with detection at 300 nm. Agilent 6130 single quadruple mass spectrometer was used to collect the ESI-MS spectrum. 
     The substrate specificity of GcaC was investigated using additional sugar-acceptor substrates, including curcumin, quercetin, ferulic acid, vanillic acid, vanillin, chrysin, apigenin, and zearalenone. In terms of sugar-donor substrates, another structurally similar sugar-donor substrate UDP-glucose was also reacted with resveratrol, besides UDP-glucuronic acid. Reaction products were analyzed by LC-MS after centrifugation. 
     Determination of the Optimal In Vitro Reaction Conditions 
     Purified resveratrol glucuronides were used to establish standard curves for quantifying product formation. The effects of reaction temperature, pH, and metal ions on the catalytic activity of GcaC were examined. To determine the optimum temperature, the reaction mixtures with resveratrol as the substrate were incubated at different temperatures (20, 25, 30, 35, 40, 45, and 50° C.) for 6 hours. Glucuronidation reactions were conducted in a 100-μL reaction system consisting of reaction buffer 20 mM Tris-HCl (pH 8.0), 23.7 μg of purified GcaC, 2 mM UDP-glucuronic acid, 1 mM MgCl 2  and 1.0 mM resveratrol. Reactions were terminated by adding 200 μL of methanol followed by centrifugation at 13,000×g for 10 min. The supernatants were sampled and analyzed by HPLC. In order to find out the optimum pH, GcaC was reacted with resveratrol at 30° C. in 200 mM phosphate buffer with different pH values (pH 4.5, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, and 9.0). The conversion rate of resveratrol was quantified by HPLC. Control reaction was conducted under the same conditions, but without GcaC. Different metal ions such as CaCl 2 , CuCl 2 , FeSO 4 , MgCl 2 , MnCl 2 , and ZnCl 2  were also tested. Control reaction was performed under the same conditions without adding these metal ions. The experiments were carried out with individually divalent metal ion at the final concentration of 10 mM, and UDP-glucuronic acid and resveratrol were used as sugar donor and sugar acceptor, respectively. All reactions were performed in triplicate and conversion rates were expressed as the mean±standard deviation. 
     Whole-Cell Bioconversion of Resveratrol Into Resveratrol Glucuronides by  E. coli  BL21(DE3)/p1R36 
       E. coli  BL21(DE3)/pJR16 was grown and induced as described above. The cells were collected by centrifugation for whole-cell bioconversion after IPTG induction at 28° C. for 16 hours. Phosphate buffer was used to re-suspend the cells, and cell density was determined using a UV-Vis spectrophotometer (Thermo Scientific, Rockford, USA) by recording the OD 600  value. In order to determine the whole-cell bioconversion conditions, different pH values (pH 4.5, 5.5, 6.0, 6.5, 7.0, 7.5, and 8.5), temperatures (25, 30, 35, 40, and 45° C.), cell densities (OD 600  2.5, 5, 7.5, 10, 12.5, and 15), reaction times (1, 2, 3, 4, and 5 h) and substrate concentrations (0.25, 0.5, 1.0, 1.5, 2.0 and 2.5 mM) were investigated. Product formation was quantified by HPLC. Finally, the whole-cell biotransformation experiment was performed in a 1-L reaction system under the optimal conditions. The reaction consisted of  E. coli  BL21(DE3)/pJR36 cells (OD 600  10.0) and 0.5 mM resveratrol, and the bioconversion process was performed at 40° C., pH 6.5, and 250 rpm for 3 hours. 
     Example 2. Screening of Different Actinomycete Strains for the Ability to Glycosylate Resveratrol 
     Actinomycetes are known to produce a variety of bioactive natural products and contain abundant biosynthetic enzymes. We hypothesized that some of these strains may have versatile UGTs that can introduce the glucuronic acid moiety to plant polyphenols such as resveratrol. To this end, resveratrol was incubated with five different strains, including  S. roseum  No. 79089,  S. reseiscleroticus  ATCC 53903,  A. hibisca  P157-2,  Streptomyces  sp. FERM BP-2474, and  S. chromofuscus  ATCC 49982. HPLC analysis revealed that two more polar metabolites, at 15.5 min for product 1 and 17.0 min for product 2 respectively, were synthesized from resveratrol by  S. chromofuscus  ATCC 49982 in 4 days ( FIG.  1 A ). The UV absorption patterns of the products were both similar to that of resveratrol, suggesting that these two polar products are derivatives of the substrate ( FIGS.  1 B and  1 C ). However, no products were detected in the other four strains. 
     Example 3. Characterization of the Two Biotransformed Products of Resveratrol by  S. chromofuscus  ATCC 49982 
     ESI-MS spectra of both 1 ( FIG.  1 D ) and 2 ( FIG.  1 E ) showed the same corresponding quasimolecular ions [M-H] −  at m/z 402.8 and [2M-H] −  at m/z 806.7, respectively. Therefore, products 1 and 2 have the same molecular weight of 404, which is 176 mass units larger than the substrate resveratrol, suggesting that a glucuronic acid moiety was added to different hydroxyl groups of resveratrol. 
     In order to elucidate the chemical structure, two purified products 1 and 2 were subjected to NMR analysis. The  13 C NMR spectra of 1 and 2 both revealed 20 carbon signals. In addition to the 14 signals belonging to resveratrol, six additional carbon signals at δ C  170.5, 100.3, 76.2, 75.9, 73.4, and 71.8 for 1 and δ C  170.1,100.5, 77.2, 75.8, 72.9, and 71.4 for 2 were found in the spectra, further suggesting that a sugar moiety was added to resveratrol at one of its hydroxyl groups. Unlike glucose and other common sugars, this sugar moiety has a carbon signal at around δ C  170, indicating the presence of a carboxyl group. Both the  1 H and  13 C signals of this sugar moiety supported the presence of a glucuronic acid moiety. As shown in the  1 H NMR spectra, the anomeric proton signals at δ H  5.07 (d, J=7.5 Hz), and 4.99 (d, J=7.7 Hz) of 1 and 2, respectively, indicated the β-configuration of these two compounds for the glucuronic acid moiety. HMBC spectrum of 1 revealed the correlation of H-1″ at δ H  5.07 to C-4′ at δ C  156.9 ( FIG.  2   ), which confirmed that 1 has a glucuronic acid moiety at C-4′. Similarly, H-1″ at δ H  4.99 of 2 had HMBC correlation to C-3 at δ C  158.4, indicating that the glucuronic acid moiety was introduced at C-3 ( FIG.  2   ). Therefore, products 1 and 2 were characterized as resveratrol-4′-O-β-D-glucuronide and resveratrol-3-O-β-D-glucuronide, respectively. The NMR data of 1 and 2 are consistent with those reported in literature (Wang et al. 2004). 
     Example 4. Characterization of the Biotransformed Products of Quercetin, Ferulic Acid and Vanillic Acid by  S. chromofuscus  ATCC 49982 
     To check whether  S. chromofuscus  can also glucuronidate other natural products, quercetin, ferulic acid and vanillic acid were incubated with  S. chromofuscus  respectively. We found that all these three substrates can be glucuronidated into one or two glucuronides ( FIG.  3   ). 
     When quercetin was used as substrate, HPLC analysis ( FIG.  3 A ) showed that two polar products at 18.0 and 19.2 min, respectively, were formed from quercetin. Two products showed the similar UV spectra to the substrate. ESI-MS spectra of both 3 and 4 showed the corresponding quasi-molecular ion [M-H] −  at m/z 476.9 and 476.6, respectively. Therefore, products 3 and 4 have the same molecular weight of 478, which is 176 mass units larger than the substrate quercetin, indicating that a glucuronic acid moiety was added to different hydroxyl groups of quercetin. 
     When the ferulic acid was used as the substrate, a polar product 5 at 12.8 min was detected by HPLC ( FIG.  3 B ). Two more polar products 6 and 7 appeared at 9.7 and 10.8 min when vanillic acid was incubated with  S. chromofuscus  ATCC 49982 ( FIG.  3 C ). These products showed the similar UV spectra to the corresponding substrates. The ESI-MS spectra of 5-7 showed a [M-H] −  ion peak at m/z 368.9, 342.9, and 342.9, respectively, indicating that their molecular weights are 370, 344 and 344 Da, which are consistent with the addition of a glucuronic acid moiety to the substrates ferulic acid and vanillic acid. Therefore,  S. chromofuscus  was found to be able to glucuronidate various phenolic natural products. 
     The NMR spectra of products 3-7 were collected. The  13 C NMR spectra of compounds 3 and 4 both showed 21 carbon signals. In addition to the signals from quercetin, six additional carbon signals at δ C  170.1, 99.1, 75.6, 75.4, 72.8, and 71.2 for 3 and δ C  169.7, 101.0, 76.0, 75.8, 73.8, and 71.3 for 4 were found in the spectra. These signals were similar to those from the glucuronic acid moiety of 1 and 2, thus suggesting that a glucuronic acid moiety was added to one of the hydroxyl groups in quercetin. The anomeric proton signals at δ H  5.29 (d, J=7.3 Hz), and 5.50 (d, J=7.5 Hz) of 3 and 4 in the  1 H NMR spectra, respectively, indicated the β-configuration of the sugar moiety in these two compounds. HMBC spectrum of 3 revealed the correlation of H-1″ at δ H  5.29 to C-7 at δ C  162.2 ( FIG.  2   ), which confirmed that 3 has a glucuronic acid moiety at C-7. Similarly, H-1″ of 4 at δ H  5.50 had HMBC correlation to C-3 at δ C  133.0, indicating that the glucuronic acid moiety was introduced at C-3 ( FIG.  2   ). Therefore, products 3 and 4 were characterized as quercetin-7-O-β-D-glucuronide and quercetin-3-O-β-D-glucuronide, respectively. The NMR data of 3 and 4 are consistent with those reported in literature (Marvalin and Azerad 2011a). 
     The  13 C NMR spectrum of compounds 5 revealed 16 carbon signals. Compared to the substrate ferulic acid, there were six additional carbon signals at δ C  170.2, 99.2, 76.1, 75.3, 72.9, and 71.4, further suggesting that a glucuronic acid moiety was added to ferulic acid. As shown in the  1 H NMR spectrum, the anomeric proton signal of 5 at δ H  δ 5.13 (d, J=7.1 Hz) indicated the β-configuration of the glucuronic acid moiety. HMBC spectrum of 5 revealed the correlation of H-1″ (δ H  5.13, d, J=7.1 Hz) to C-4 at δ C  147.8 ( FIG.  2   ), which confirmed that the position of glucuronic acid moiety is at C-4 of 5. Therefore, 5 was characterized as ferulic acid-4-O-β-D-glucuronide. The NMR data were assigned based on the 1D and 2D NMR spectra for the first time. 
     The  13 C NMR spectra of both 6 sand 7 showed 14 carbon signals. In addition to the eight carbon signals from vanillic acid, six additional carbon signals at δ C  170.8, 99.2, 76.4, 74.7, 72.9, and 71.6 were observed for 6 and δ C  170.4, 94.6, 75.9, 75.7, 72.3, and 71.6 for 7, indicating that a glucuronic acid moiety was added to vanillic acid at different positions to yield the two products. In the  1 H NMR spectra, the anomeric proton signals of 6 and 7 at δ H  5.11 (d, J=6.1 Hz) and 5.54 (d, J=7.5 Hz), respectively, indicated the (3-configuration of the glucuronic acid moiety in these compounds. HMBC spectrum of 6 revealed the correlation of H-1″ at δ H  5.11 to C-4 at δ C  150.0 ( FIG.  2   ), which confirmed that 6 has a glucuronic acid moiety at C-4. Similarly, H-1″ of 7 at δ H  5.54 had HMBC correlation to C-7 at δ C  164.4, indicating that the glucuronic acid moiety was introduced at C-7 ( FIG.  2   ). Therefore, 6 and 7 were characterized as vanillic acid-4-O-β-D-glucuronide and vanillic acid-7-O-β-D-glucuronide, respectively. The NMR data of compound 6 is consistent with those reported in literature (Almeida et al. 2017) and the NMR data of compound 7 were assigned based on the 1D and 2D NMR spectra for the first time. 
     Example 5. Discovery of a Putative Glucuronidation Gene Cluster from  S. chromofuscus  ATCC 49982 
     To find out which enzyme is responsible for the glucuronidation in S. chromofuscus ATCC 49982, we analyzed the genome of this strain that was previously sequenced by our group. Annotation of the genome indicated that there are more than 30 glycosyltransferases in  S. chromofuscus  ATCC 49982. To narrow down, we looked at the genes adjacent to these glycosyltransferase genes. A gene cluster ( FIG.  4 A ) putatively involved in the glucuronidation was discovered, and the predicted functions of the genes are shown in Table 2. The five genes, named gcaA-E, were predicted to be TetR transcriptional regulator, UDP-glucose dehydrogenase, UDP-glucuronosyltransferase, UDP-glucose pyrophosphorylase, and MFS transporter, respectively. While gcaA and gcaE might be involved in the regulation of the gene cluster and sugar transport, respectively, gcaB, gcaC and gcaD were proposed to be involved in the biosynthesis and transfer of UDP-glucuronic acid ( FIG.  4   b   ). More specifically, GcaD (containing 320 amino acids or aa) generates UDP-glucose from glucose-1-phosphate as a UDP-glucose pyrophosphorylase, GcaB (484 aa) functions as a UDP-glucose dehydrogenase to synthesize UDP-glucuronic acid, and finally GcaC (402 aa) transfers the UDP-glucuronic acid moiety to a sugar acceptor such as resveratrol. The nucleotide sequences and amino acid sequences of GcaB, GcaC, and GcaD are presented in Table 3 and Table 4, respectively. 
     BLAST analysis of the amino acid sequence of GcaC revealed that it is homologous to a number of glycosyltransferases including a few uncharacterized UDP-glucuronyl/UDP-glucosyltransferase, such as those from  Streptomyces malaysiensis  (GenBank accession number NIY63294.1, 408 aa, 70% identity) and  Streptomyces violaceusniger  (GenBank accession number AEM85024.1, 404 aa, 68% identity). Although both genes are not functionally characterized so far, they still provide useful information to predict the function of GcaC. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 The glucuronidation (gca) gene cluster in  S. chromofuscus  ATCC 49982. 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Identity/ 
               
               
                 Gene 
                 Size (aa) 
                 Predicted function 
                 Homolog/source/NCBI accession no. 
                 Similarity 
               
               
                   
               
               
                 gcaA 
                 230 
                 TetR family transcriptional regulator 
                 TetR family transcriptional regulator/ Streptomyces  sp. 
                 71%/81% 
               
               
                   
                   
                   
                 CRXT-G-22]/WP_187751940.1 
                   
               
               
                   
                   
                   
                 UDP-glucose/GDP-mannose dehydrogenase family 
                   
               
               
                 gcaB 
                 484 
                 UDP-Glucose dehydrogenase 
                 protein/ Streptomyces  sp. SolWspMP-5a-2/ 
                 82%/86% 
               
               
                   
                   
                   
                 WP_093832645.1 
                   
               
               
                 gcaC 
                 402 
                 UDP-Glucuronyltransferase 
                 UDP-glucuronyl/UDP-glucosyltransferase/ Streptomyces   
                 70%/80% 
               
               
                   
                   
                   
                   malaysiensis /NIY63294.1 
                   
               
               
                 gcaD 
                 320 
                 UDP-Glucose pyrophosphorylase 
                 GalU/ Streptococcus pneumonia /AJ004869.1 
                 45%/62% 
               
               
                 gcaE 
                 540 
                 MFS transporter 
                 MFS transporter/ Streptomyces   
                 83%/90% 
               
               
                   
                   
                   
                   hygroscopicus /WP_060945424.1 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Nucleotide sequences of gcaB, gcaC, and gcaD. 
               
            
           
           
               
               
               
            
               
                 Gene 
                 Nucleotide Sequence 
                 SEQ ID NO: 
               
               
                   
               
               
                 gcaB 
                 ATGCGGGCCGTTGATCTCCGACACCCGCCCTATGTGTTCAGGTCCGACGACGGCGTCGGACCTGTTCCCCTGC 
                 3 
               
               
                   
                 TGGTGATACGGCCGAGGCGCCGGCCCTCGGTGAGGCTGAAACTCGAACAAGCGGAATCGGTGATTCTCATGGG 
                   
               
               
                   
                 ACTTCGACATATAACCGTCATTGGAACCGGCTATGTCGGTCTGACCACCGGCGCCTGTCTGGCCTCCCTCGGA 
                   
               
               
                   
                 CACCGGGTGGTGTGCGCGGATGCCGACGAGGGCAAGGTCGAACGGCTCCGGCGGGCGGAGGTCGACATCCTCG 
                   
               
               
                   
                 AACCGGGCCTTACGGAAGTCGTCCGCGAGGGGCTGGAATCCGGCCGTCTCCAGTTCGTGAGGGACACCCGGGC 
                   
               
               
                   
                 GGCCGTCGAGGAGGCCGAGGTCGTCTTCCTCTGCCTGCCCACCCCGATGGGCGTCGGAGGCGCCGCCGACCTG 
                   
               
               
                   
                 GCCGCCGTCGAGGCGGTCGCCGACGAGGTCCGCGACCGCCTGCCGCGTGGCTGCACGGTGGTCAACAAGTCCA 
                   
               
               
                   
                 CCGTGCCGGTCGGCACCGCCGAGCGCGTCGCGGCCCTGCTCGGCCGCCCCGACGTGACCGTGGTGAGCAACCC 
                   
               
               
                   
                 GGAGTTCCTCCGCGAGGGACACGCGGTGGACGACTTCCTCCACCCCGACCGGATCGTGGTGGGCGCCGCCGAC 
                   
               
               
                   
                 GCGGACGCGGCCCGGCTGGTGGCCGACCTGTACCTGGACATCGACGCACCGCGCGTGGTCACCGACACCGCCG 
                   
               
               
                   
                 GCGCCGAACTCGCCAAGTACGCCGCGAACTTCTTCCTCGCGATGAAGCTGTCGTTCGCGAACAACCTGGCCAC 
                   
               
               
                   
                 GCTCTGCGAAAGTCTCGGCGCGAACATCGACGACGTGGTCGCCGGCATCGGCCACGACCCACGGATCGGCGGC 
                   
               
               
                   
                 GCCTTCCTCAAGCCCGGGCCCGGATGGGGCGGTTCCTGCCTGCCCAAGGACACGCACGCGCTGCTGCGCGTCT 
                   
               
               
                   
                 GCGAGGAGTCAGGCGTCGAGTTCCCGCTGCTCCGGGCCACCATCGAGACCAACGTCGAGCACCAGCGCCGGCT 
                   
               
               
                   
                 CGTCGAGCGGGTGACCGCCGGATGCGCGGGGGCGGACGGTTCACTGCGCGGCGTCCGGATCGGGCTGCTCGGT 
                   
               
               
                   
                 CTCACCTTCAAGGCCGGCACCTTCGACCTGCGCGACTCGCCCGCCCTCGCCATCGCCCGCCTGCTGCGGGAGC 
                   
               
               
                   
                 GGGGCGCCGAACTGCGCGCCTACGACCCGGCCCTCAGCGAGCTCCGCCCCGACCTCGGCGATCTGCTCACCAT 
                   
               
               
                   
                 CACCGGTACCCCCCTCGAAGCGGTCGACGGGGCCCGCGCCTGCGTCGTACTCACCGAGTGGCCGCAATTCCGC 
                   
               
               
                   
                 GACCTGGACTGGGAGGCCGTCGCCGGGCGGCTCGCCGCGCCGCTCGTCTACGACTTCCGTAACATCCTCGATC 
                   
               
               
                   
                 CGGCACGGCTCGACAGGGCCGCGCTGACCTGGGAAGGCATCGGTCGTTCGCTGGCAATGGCCAGTTGA 
                   
               
               
                   
               
               
                 gcaC 
                 GTGCGAGTACTGTTCACCACGCTCGGTAGTCCCTCCCACGGTCGCGCACAGCTTCCGCTGGCCCGGGCGTTCG 
                 4 
               
               
                   
                 CCGCGGCCGGACACGACGTCCTCGTGGCCACCACCCCGACCCTCGCCTCCGTCTTCGAACAGGACGACGTCCG 
                   
               
               
                   
                 GGTGACCGTCTGCATGGGCGATTTCACGCCACAGTCCTTCATCACCCCCGAACTGCTCCAGGAGGCGATGCGG 
                   
               
               
                   
                 CCGGGTCAGGACGGTGAGCCGCAGGACGCCATGGCGCGCCTCATGCCCGAGATCACCTCCGGCCCGATGGCCA 
                   
               
               
                   
                 GGAAGCTGTGGGAAGAGATCCTTCCGGTGGCGCGGGAGTTCGCCCCCGACCTCATCCTGCGCGACGGCATGGA 
                   
               
               
                   
                 CCTGAGCTCGTGCCTGATCGCGGAACACCTCGGCATCCCGCAACTGCCCACCCCTTCGGGCACGAACAACCTC 
                   
               
               
                   
                 ATCGACCCCGCCATGGTGCTGCCCGGCCTGAACGTCCTGCGGAAGGAATTCGGGCTGTCCGCCCAGGAGGACC 
                   
               
               
                   
                 CGCTGTCGCTCGTCCCCCACGGGCGCGTCGACTACGTACCGGCGGCCTTCTCGTTCGCCCAGCACCTGCCCTC 
                   
               
               
                   
                 GTCGTGGTCCTACCGGCAGACCGTGACCGTGGACCGCAGCTCGGTCCTGCCGGAGTGGATCGCCCAACTGCCC 
                   
               
               
                   
                 ACCGACCGCCCCCTGGTGTTCGCCGCGCTCGGCACCGCCCTTCCGATGATCAGGGAGATGGGGGCCGAGGCGA 
                   
               
               
                   
                 CCGGGCCGTCGCTGTTCCCGATGCCGGACCCGGTGGACACGCTGCGGTTGATGATCGAGGCGGTGTCGCGGCT 
                   
               
               
                   
                 CGACGACTGCACCGTGGTCGTCGCCACCTCCGGCATCCCGGCGGACACCGAGGGCCTGCCGCCGCATGTGCAC 
                   
               
               
                   
                 GTCACCGACCGGGTGCCGCAGCCCCTGCTGCTGGAGTCCGTCGACGCGTTCCTCACCCACGGCGGCTTCAACA 
                   
               
               
                   
                 GCATCCGGGAGGCGCTGCGCACGGCCACCCCGATGGCGGTGCTGCCCCAGTTCGGCGACCAGCCCGCCAACGC 
                   
               
               
                   
                 GCGCCGCGTCGAGGAACTCGGCCTCGGCCGGGAGATCACCGACATCACCGCGGACGGCATCACCAAGGCCGTA 
                   
               
               
                   
                 CGCGAGGTGCTGACCGACCCCGGCATCCGGGCCAGGACGCGCGAGGCCCGGCTGGCGATGCTGGCGCTGCCGG 
                   
               
               
                   
                 AGATCGACAGCGCCGTGGCCGACCTGCAGAAGATCGTCTGA 
                   
               
               
                   
               
               
                 gcaD 
                 ATGTCGCCTTCCCCGCCCCCTGCCACCACCGTCACCAAGGCCGTGATACCGGCCGCCGGCCTGGGCACCCGCT 
                 5 
               
               
                   
                 TCCTCCCGGCGACGAAGGCCATGCCCAAGGAGATGCTGCCCGTCGTCGACCGCCCGGCCATCCAGTACGTGGT 
                   
               
               
                   
                 GGAAGAGGCCGCCGGCGCCGGTCTCTCCGATCTGCTCGTGATCACCGGGCGGAACAAGCGCCCGCTGGAGGAC 
                   
               
               
                   
                 CACTTCGACCACGCCTGGGAACTGGAGGAGGCCCTGACCCGCAAGGGCGACGAGGGCAGGCTGCGCAGTGTCC 
                   
               
               
                   
                 GGGAGTCCACCGCGCTCGCCGCGATCCACTACGTCCGGCAGGGCACCCCGGCGGGCCTCGGACACGCCGTGCT 
                   
               
               
                   
                 CTGCGCCCAACAGCACGTGGGGGACGAGCCGTTCGCCGTACTCCTGGGCGACGACCTCATCGACCCGCGCGAT 
                   
               
               
                   
                 CCACTGCTCACCCGGATGATCGAGATACGGGAACGGTTCGGCGGCAGCGTGGTCGCGCTGATGGAGACCGACC 
                   
               
               
                   
                 CCGCCTCGATCCACCTGTACGGCTGCGCGGCGGTCGAACCCACCGCACAGGACGATGTCGTACGCCTGACCGA 
                   
               
               
                   
                 CCTGGTGGAGAAGCCCGCCCCGGGGCAAGCCCCCAGCGCGTACGCGGTCATCGGACGCTATCTGCTGGACCCG 
                   
               
               
                   
                 GCCGTGTTCGAGGTGCTGCGCCGCACCCCGCCCGGCCACGGCGGCGAGATCCAGCTCACCGACGCCCTGCGCG 
                   
               
               
                   
                 AACTGGCGCACGGCGGTGCCACCTCCCCGGGCGGCCCGGTCCACGGCGTGCTGTTCACCGGACGGCGCTACGA 
                   
               
               
                   
                 CACCGGCGACCGCGCCGAGTACCTGCGCACCATCGTCCGACTGGCCTACGAACACGACGACCTCGGCCCCGGG 
                   
               
               
                   
                 TTCCGGGAGTGGCTGACCGCGTTCGTCGACGCCGAACGCGAGGCCCCCACCGCGGCGGCCGACGGCGGCCCGG 
                   
               
               
                   
                 GTGTCGCGGCATGA 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Amino acid sequences of GcaB, GcaC, and GcaD. 
               
            
           
           
               
               
               
            
               
                   
                   
                 SEQ 
               
               
                 Protein 
                 Amino Acid Sequence 
                 ID NO: 
               
               
                   
               
               
                 GcaB 
                 MRAVDLRHPPYVFRSDDGVGPVPLLVIRPRRRPSV 
                   
               
               
                   
                 RLKLEQAESVILMGLRHITVIGTGYVGLTTGACLA 
                   
               
               
                   
                 SLGHRVVCADADEGKVERLRRAEVDILEPGLTEVV 
                   
               
               
                   
                 REGLESGRLQFVRDTRAAVEEAEVVFLCLPTPMGV 
                   
               
               
                   
                 GGAADLAAVEAVADEVRDRLPRGCTVVNKSTVPVG 
                   
               
               
                   
                 TAERVAALLGRPDVTVVSNPEFLREGHAVDDFLHP 
                   
               
               
                   
                 DRIVVGAADADAARLVADLYLDIDAPRVVTDTAGA 
                   
               
               
                   
                 ELAKYAANFFLAMKLSFANNLATLCESLGANIDDV 
                 6 
               
               
                   
                 VAGIGHDPRIGGAFLKPGPGWGGSCLPKDTHALLR 
                   
               
               
                   
                 VCEESGVEFPLLRATIETNVEHQRRLVERVTAGCA 
                   
               
               
                   
                 GADGSLRGVRIGLLGLTFKAGTFDLRDSPALAIAR 
                   
               
               
                   
                 LLRERGAELRAYDPALSELRPDLGDLLTITGTPLE 
                   
               
               
                   
                 AVDGARACVVLTEWPQFRDLDWEAVAGRLAAPLVY 
                   
               
               
                   
                 DFRNILDPARLDRAALTWEGIGRSLAMAS 
                   
               
               
                   
               
               
                 GcaC 
                 VRVLFTTLGSPSHGRAQLPLARAFAAAGHDVLVAT 
                   
               
               
                   
                 TPTLASVFEQDDVRVTVCMGDFTPQSFITPELLQE 
                   
               
               
                   
                 AMRPGQDGEPQDAMARLMPEITSGPMARKLWEEIL 
                   
               
               
                   
                 PVAREFAPDLILRDGMDLSSCLIAEHLGIPQLPTP 
                   
               
               
                   
                 SGTNNLIDPAMVLPGLNVLRKEFGLSAQEDPLSLV 
                   
               
               
                   
                 PHGRVDYVPAAFSFAQHLPSSWSYRQTVTVDRSSV 
                   
               
               
                   
                 LPEWIAQLPTDRPLVFAALGTALPMIREMGAEATG 
                 7 
               
               
                   
                 PSLFPMPDPVDTLRLMIEAVSRLDDCTVVVATSGI 
                   
               
               
                   
                 PADTEGLPPHVHVTDRVPQPLLLESVDAFLTHGGF 
                   
               
               
                   
                 NSIREALRTATPMAVLPQFGDQPANARRVEELGLG 
                   
               
               
                   
                 REITDITADGITKAVREVLTDPGIRARTREARLAM 
                   
               
               
                   
                 LALPEIDSAVADLQKIV 
                   
               
               
                   
               
               
                 GcaD 
                 MSPSPPPATTVTKAVIPAAGLGTRFLPATKAMPKE 
                   
               
               
                   
                 MLPVVDRPAIQYVVEEAAGAGLSDLLVITGRNKRP 
                   
               
               
                   
                 LEDHFDHAWELEEALTRKGDEGRLRSVRESTALAA 
                   
               
               
                   
                 IHYVRQGTPAGLGHAVLCAQQHVGDEPFAVLLGDD 
                   
               
               
                   
                 LIDPRDPLLTRMIEIRERFGGSVVALMETDPASIH 
                   
               
               
                   
                 LYGCAAVEPTAQDDVVRLTDLVEKPAPGQAPSAYA 
                   
               
               
                   
                 VIGRYLLDPAVFEVLRRTPPGHGGEIQLTDALREL 
                 8 
               
               
                   
                 AHGGATSPGGPVHGVLFTGRRYDTGDRAEYLRTIV 
                   
               
               
                   
                 RLAYEHDDLGPGFREWLTAFVDAEREAPTAAADGG 
                   
               
               
                   
                 PGVAA 
                   
               
               
                   
               
            
           
         
       
     
     Example 6. Heterologous Expression and Purification of GcaC from  E. coli  BL21(DE3) 
     To identify the function of GcaC, we amplified this gene from the genomic DNA of  S. chromofuscus  and ligated it to pET28a to yield the corresponding expression plasmid, pJR36 (pET28a-gcaC). The plasmid was then expressed in  E. coli  BL21(DE3) with IPTG induction. Ni-NTA column chromatography was used to purify the N-His6-tagged GcaC from the cell lysate and the purified enzyme was analyzed by SDS-PAGE. As shown in  FIG.  5   a   , GcaC (˜43.4 kDa) was expressed and purified from  E. coli  BL21(DE3)/pJR36 to homogeneity and the isolation yield of this enzyme was 11.87 mg/L. 
     Example 7. In Vitro Functional Characterization of GcaC 
     To check the function of purified GcaC, resveratrol was incubated with this enzyme in the presence of UDP-glucuronic acid. HPLC analysis showed that compared to the control (trace i,  FIG.  5 B ), two polar products were formed from resveratrol (trace ii,  FIG.  5 B ). Two products showed the similar UV spectra to the substrates. ESI-MS spectra ( FIGS.  5 C and  5 D ) of these products showed the corresponding quasi-molecular ion [M-H] −  at m/z 402.9, respectively. Therefore, they have the same molecular weight of 404, which is 176 mass units larger than the substrate resveratrol, indicating that a glucuronic acid moiety was added to different positions of resveratrol. Furthermore, the retention times of these two in vitro products were the same as those of resveratrol-4′-O-β-D-glucuronide and resveratrol-3-O-β-D-glucuronide produced in the in vivo biotransformation by  S. chromofuscus.  Formation of the two resveratrol glucuronides in the in vitro reactions allowed the characterization of GcaC as the responsible UGT in  S. chromofuscus.    
     Example 8. Broad Substrate Specificity of GcaC Toward Sugar Acceptor Substrates 
     To explore the substrate specificity of GcaC, this enzyme was reacted with different sugar donors and sugar acceptors. UDP-Glucose is structurally similar to UDP-glucuronic acid and the BLAST analysis of the sequence of GcaC suggested that this enzyme is homologous to a predicted UDP-glucuronyltransferase or UDP-glucosyltransferase from  S. malaysiensis  (Table 2). Therefore, we reacted resveratrol with UDP-glucose in the presence of GcaC. However, no products were formed from resveratrol (data not shown), indicating that GcaC is a specific UDP-glucuronyltransferase that strictly transfers UDP-glucuronic acid. 
     We next investigated the substrate specificity of GcaC to sugar acceptor substrates by reacting various natural products, including quercetin, ferulic acid, vanillic acid, curcumin, vanillin, chrysin, zearalenone and apigenin, with UDP-glucuronic acid in the presence of GcaC. When quercetin was used as the substrate, HPLC analysis ( FIG.  6 A ) showed that six polar products were formed. All products showed the similar UV spectra to the substrate. ESI-MS spectra of Q1-Q6 showed the corresponding quasi-molecular ion [M-H] −  at m/z 653.0, 653.0, 476.9, 476.9, 476.9, and 476.9, respectively. Therefore, products Q3-Q6 have the same molecular weight of 478, which is 176 mass units larger than the substrate quercetin, indicating that a glucuronic acid moiety was added to different hydroxyl groups of quercetin. Since quercetin has five free phenolic hydroxyl groups, we propose that the glucuronic acid moiety was introduced to four of these hydroxyl groups to generate the four monoglucuronides. The molecular weight of both Q1 and Q2 were found to be 654, which is 352 mass units larger than the substrate quercetin or 176 units larger than products Q3-Q6, suggesting that two glucuronic acid moieties were added to quercetin to form diglucuronides. Therefore, this enzyme not only adds the glucuronic acid moiety to different hydroxyl groups of quercetin, but also accepts the resulting monoglucuronides as the substrates to further generate diglucuronides. 
     When ferulic acid was incubated with GcaC, one polar product F1 at 12.8 min was detected by HPLC ( FIG.  6 B ). Similarly, one product VA1 appeared at 9.7 min when vanillic acid was used as a sugar acceptor ( FIG.  6 C ). Both products showed the similar UV spectra to the corresponding substrates. The ESI-MS spectra of F1 and VA1 showed a [M-H] −  ion peak at m/z 368.9 and 342.9, respectively, indicating that their molecular weights are 370 Da and 344 Da, which are consistent with the addition of a glucuronic acid moiety to ferulic acid and vanillic acid, respectively. Furthermore, the retention times of F1 and VA1 are the same as 5 and 6 from the biotransformation by  S. chromofuscus  49982, namely ferulic acid-4-O-β-D-glucuronide and vanillic acid-4-O-β-D-glucuronide. 
     Meanwhile, when curcumin was used as the substrate, HPLC analysis showed that compared to the negative control (trace i,  FIG.  6 D ), GcaC generated two polar products C1 and C2 (trace ii,  FIG.  6 D ), at 21.8 and 25.3 min respectively. The two products had the UV spectra similar to that of the substrate. The ESI-MS spectra of the two products showed the [M-H] −  ion peak at m/z 719.1 and 542.8, indicating that their molecular weights are 720 Da and 544 Da, respectively. These are consistent with a diglucuronide and a monoglucuronide of curcumin, whose molecular weight is 368 Da. This result further indicated that GcaC can transfer one or two glucuronic acid moieties to curcumin. 
     Similarly, we also used vanillin as the substrate, and one polar product VN1 at 10.0 min was detected by HPLC ( FIG.  6 E ). One product CS1 was also observed at 22.0 min when chrysin was incubated with GcaC ( FIG.  6 E ). Both products showed the similar UV spectra to their relative substrates. The ESI-MS spectra of VN1 and CS1 showed a [M-H] −  ion peak at m/z 326.9 and 428.8, respectively, indicating that their molecular weights are 328 Da and 430 Da, which are consistent with the addition of a glucuronic acid moiety to vanillin and chrysin, respectively. Similarly, when zearalenone and apigenin were used as the substrates, HPLC analysis showed that compared to the negative controls (trace i,  FIGS.  6   g  and  6   h   ), GcaC generated a product Z1 (trace ii,  FIG.  6 G ) at 23.0 min from zearalenone and a product A1 (trace ii,  FIG.  6 H ) at 19.0 min from apigenin. The products showed the UV spectra similar to those of their substrates. The ESI-MS spectrum of Z1 exhibited the [M-H] −  ion peak at m/z 493.1, which means the molecular weight is 494 Da and it is in accord with the addition of a glucuronic acid moiety to zearalenone with a molecular weight of 318. The ESI-MS spectrum of A1 showed the [M-H] −  ion peak at m/z 445.1, indicating that molecular weight is 446 Da. This is consistent with a monoglucuronide of apigenin, whose molecular weight is 270 Da. Therefore, GcaC synthesized the corresponding glucuronides from zearalenone and apigenin. 
     Example 9. Determination of the Optimal In Vitro Reaction Conditions of GcaC 
     The effects of reaction temperature and pH on the GcaC-catalyzed glucuronidation were examined. Purified GcaC was incubated with resveratrol and UDP-glucuronic acid under different conditions. First, the reaction was conducted at different temperatures ranging from 20° C. to 50° C. The conversion rates of resveratrol were quantified by HPLC and compared. As shown in  FIG.  7 A , GcaC has the highest glucuronidation activity at 30° C. with the highest conversion rate. When the temperature increased from 20° C. to 30° C., the conversion rate of resveratrol increased from 29.61% to 70.85%. However, when the reaction temperatures were above 30° C., the resveratrol conversion rate gradually decreased. Furthermore, the conversion rate reduced approximately by 53.60% at 50° C. compared to that at 30° C. Therefore, the optimum reaction temperature for GcaC was determined to be 30° C. 
     Next, we determined the pH effect on the glucuronidation activity of GcaC. The enzyme was reacted with resveratrol in 200 mM phosphate buffer at 30° C. but at different pH values, including 4.5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5 and 9. With the increase of pH from 4.5 to 7.5, the conversion rate of resveratrol by GcaC steadily increased from 7.8% to 59.3%. Nevertheless, when the pH value further increased to 9.0, the conversion rate decreased to 37.2% ( FIG.  7 B ). Hence, the optimum pH for GcaC is 7.5. 
     Apart from temperature and pH effects, we also investigated the effect of various metal ions on the activity of GcaC. At 10 mM, we found that Ca 2+ , Mg 2+ , and Mn 2+  had a simulative function on the UGT activity of GcaC ( FIG.  7 C ). With these three metal ions applied in the enzymatic reaction, the catalytic activity was 1.33, 1.27, and 1.09-fold higher than that of the control group, respectively. Among these three metal ions, Ca 2+  has the strongest stimulating effect. By contrast, the activity of GcaC was inhibited when Cu 2+ , Fe 2+ , and Zn 2+  were added into the reaction system. The conversion rate of resveratrol decreased from 52.3% to 39.1%, 48.6%, and 43.8% ( FIG.  7 C ), respectively. In conclusion, the optimal in vitro reaction conditions for purified GcaC are at 30° C., pH 7.5 with 10 mM of Ca 2+ . 
     Example 10. Optimized Production of Resveratrol-4′-O-β-D-Glucuronide and Resveratrol-3-O-β-D-Glucuronide with the Engineered  E. coli  BL21(DE3) 
     We next tested whether  E. coli  BL21(DE3)/pJR36 can convert resveratrol into the two glucuronides. To this end, resveratrol was incubated with the IPTG-induced broth of this engineered strain, with  E. coli  BL21(DE3)/pET28a as the negative control. As shown in  FIG.  8 A , compared to the commercial standard (trace i) and negative control (trace ii), incubation of resveratrol with  E. coli  BL21(DE3)/pJR36 yielded the glucuronidation products 1 and 2. 
     In vivo biotransformation of resveratrol by  E. coli  BL21(DE3)/pJR36 indicated that GcaC can generate resveratrol-4′-O-β-D-glucuronide as the major product from resveratrol in  E. coli,  which provides an alternative approach to biosynthesize this resveratrol glucuronide. Therefore, it is critical to discover the optimal conditions for this whole-cell bioconversion process. The effect of different cell concentrations on the formation of resveratrol glucuronides at 40° C. overnight was first investigated, ranging from OD 600  2.5 to 15.0. In the presence of 1.5 mM resveratrol and 0.11 M glucose, we discovered that the conversion rate of resveratrol steadily increased with increasing cell concentrations within the range of OD 600  2.5-10.0 ( FIG.  8 B ). The conversion rate of resveratrol in the biotransformation system reached 53.8% when the OD 600  value was 10.0, which was 27.3% higher than the titer at OD 600  2.5. However, further increase in cell concentration resulted in a drop of the conversion rate. The conversion rate of resveratrol decreased to 34.4% at OD 600  15.0, indicating that oversaturated cells did not yield more products. 
     The conversion of resveratrol to its glucuronides was also determined by HPLC at different pH values (pH 4.5-8.5). When the reaction pH was 6.5, the conversion rate reached its highest level, namely 38.1% ( FIG.  8 C ), which is 23.2% higher than that at pH 4.5. We next investigated the impact of temperature on the whole-cell conversion efficiency of resveratrol to its glucuronides. Specifically,  E. coli  BL21(DE3)/pJR36 cells (OD 600  10.0) were incubated with resveratrol at different temperatures ranging from 25 to 45° C. in 200 mM phosphate buffer (pH 6.5). It was clearly proved that the higher temperature could effectively improve the conversion of resveratrol into resveratrol-4′-O-β-D-glucuronide and resveratrol-3-O-β-D-glucuronide. The highest conversion rate reached 48.6% at 40° C. ( FIG.  8 D ). Further temperature increase to 45° C. resulted in a lower conversion rate. 
     To determine how the bioconversion time affects the conversion rate of resveratrol in the engineered  E. coli  strain, we conducted a time course analysis for the conversion of 1.0 mM resveratrol to its glucuronides. The experiments were performed with GcaC-expressing  E. coli  BL21(DE3) strain (OD 600  10.0) at 40° C. in 200 mM phosphate buffer (pH 6.5). The reaction was sampled at 1, 2, 3, 4 and 5 hours. Within the first 3 hours, the conversion rate of resveratrol increased from 18.0% to 34.8% ( FIG.  8 E ). After that, the increase in the conversion rate slowed down gradually. Thus, we chose 3 hours as the preferred reaction time. 
     Substrate concentration can also affect the bioconversion rate. Therefore, we next tested different resveratrol concentrations ranging from 0.25 to 2.5 mM, for further optimizing the production of resveratrol-4′-O-β-D-glucuronide and resveratrol-3-O-β-D-glucuronide. The reactions were conducted at pH 6.5, OD 600  10, and 40° C. for 3 hours. The conversion rates of resveratrol to its glucuronides were similar when the substrate concentration were 0.25 mM and 0.5 mM, namely 82.6% to 83.2%. However, the conversion rate fell to 41.7% when the concentration of resveratrol was further increased to 1.0 mM ( FIG.  8 F ). When the concentration of resveratrol was further increased to 2.5 mM, the conversion rate dramatically decreased to 7.9%. As a result, 0.5 mM was deemed to be the optimal substrate concentration according to the productivity of resveratrol glucuronides under the selected reaction conditions. 
     Finally, we scaled up the reaction to 1 L considering all the effects mentioned above. The titer of resveratrol-4′-O-β-D-glucuronide and resveratrol-3-O-β-D-glucuronide were 0.194±0.001 mM (equivalent to 78.381±0.366 mg/L) and 0.037±0.001 mM (equivalent to 14.991±0.248 mg/L) from a total of 0.5 mM (equivalent to 114.125 mg/L) of resveratrol in 3 hours at 40° C. in 200 mM phosphate buffer (pH 6.5) with shaking at 250 rpm. 
     Polyphenols are a large group of structurally diverse bioactive molecules. Many natural products from this group, such as resveratrol, quercetin, and curcumin, have promising biological activities. These molecules are available from certain foods and are consumed as dietary supplements (Sauer and Plauth 2017). However, their health benefits in the human body are often limited due to poor water solubility. Glycosylation is an effective approach to improve the water solubility and bioavailability. Actinomycetes are well-known for their ability to biosynthesize novel and pharmaceutically beneficial secondary metabolites with various bioactivities (Fidan et al. 2019). Among them, Streptomyces is widely used to produce industrially important bioactive molecules, such as oxytetracycline (Yu et al. 2013a).  S. chromofuscus  ATCC 49982 is a typical example which produces the anti-cholesterol polyketide natural product herboxidiene (Yu et al. 2013b). In this study, five actinomycete strains were screened for the ability to biotransform resveratrol, and only  S. chromofuscus  ATCC 49982 was found to generate two glycosides, which were structurally characterized as resveratrol-3-O-β-D-glucuronide and resveratrol-4′-O-β-D-glucuronide, respectively. Resveratrol is a plant-derived stilbenoid with a variety of bioactivities, including antimicrobial, antiviral, anti-inflammatory, antioxidant, antiaging, anticancer, antiplatelet, phytoestrogenic, neuro-protective, and cardio-protective activities (Thuan et al. 2018; Yu et al. 2002). Its ubiquitous presence in diets such as grapes, wine, olive oil, white tea, peanuts and cranberries has attracted considerable research interest (Pervaiz 2003; Wang et al. 2002). Nevertheless, its low bioavailability results in limited absorption after oral administration and impedes the formulation of functional foods. Furthermore, its sensitivity to air and light also hinders the nutraceutical and medicinal applications and exploitation of resveratrol (Francioso et al. 2014; Jeon et al. 2016; Uesugi et al. 2017). 
     Resveratroloside (resveratrol-4′-O-β-D-glucoside) and polydatin (resveratrol-3-O-β-D-glucoside) are two common water-soluble derivatives of resveratrol, and they exhibit anticancer and antioxidant activities. Moreover, they can prevent coronary heart diseases (Shimoda et al. 2013). In the meantime, resveratroloside and polydatin are more resistant to enzymatic oxidation than resveratrol (King et al. 2006). In other words, glycosylation of resveratrol can extend its half-life in the cell and maintains the beneficial antioxidant capacity and biological properties (Regev-Shoshani et al. 2003). As one of the most abundant forms of resveratrol in nature, polydatin is the major bioactive compound of Polygonum cuspidatum root which is used to treat cardiac ailments, including atherosclerosis and inflammation in Japanese and Chinese folk medicine (Romero-Pérez et al. 1999). While the two resveratrol glucuronides obtained in this work are structurally slightly different than those of glucosides, it will be interesting to find out how these molecules perform in vivo. In fact, resveratrol-3-O-β-D-glucuronide and resveratrol-4′-O-β-D-glucuronide were previously reported to be the metabolites of resveratrol in the human body. While resveratrol has cytotoxicity to human peripheral blood mononuclear cells at 30 μM, these glucuronides did not show any cytotoxicity at 300 μM. The ubiquitous human β-glucuronidase can convert the metabolites back to resveratrol locally or systematically in vivo (Wang et al. 2004). Therefore, the two resveratrol glucuronides may represent useful pro-drugs of resveratrol for clinical applications. 
       S. chromofuscus  ATCC 49982 was also found to be able to glucuronidate other phenolic compounds including quercetin, vanillic acid, and ferulic acid. Two products, namely quercetin-7-O-β-D-glucuronide and quercetin-3-O-β-D-glucuronide, were obtained from quercetin. This result indicated that the dedicated UGT in this strain is flexible and can transfer the glucuronic acid moiety to different positions of quercetin. Ferulic acid-4-O-β-D-glucuronide was generated from ferulic acid by  S. chromofuscus  ATCC 49982. Interestingly, vanillic acid-4-O-β-D-glucuronide and vanillic acid-7-O-β-D-glucuronide were synthesized when vanillic acid is used as the substrate. The production of vanillic acid-7-O-β-D-glucuronide is somewhat surprising as the sugar moiety was introduced to the carboxyl group, which is quite chemically different than the phenolic hydroxyl group, further indicating that the UGT in  S. chromofuscus  ATCC 49982 is a highly versatile enzyme. 
     Encouraged by the high flexibility of this enzyme, we sought to find and identify the dedicated the UGT in  S. chromofuscus  ATCC 49982. While there are many glycosyltransferase genes in the genome, we were able to spot the most possible glycosyltransferase responsible for the observed glucuronidation in this strain because that it is flanked by two UDP-glucuronic acid biosynthetic genes (gcaD and gcaB), which encode UDP-glucose pyrophosphorylase and UDP-glucose dehydrogenase, respectively (Table 2). These two enzymes convert cellular glucose-1-phosphate to UDP-glucuronic acid ( FIG.  4 B ). This makes GcaC a strong candidate of the dedicated UGT. This gene was cloned and expressed in  E. coli  BL21(DE3). The purified GcaC was functionally characterized as a UGT as it generated resveratrol-3-O-β-D-glucuronide and resveratrol-4′-O-β-D-glucuronide from resveratrol in the presence of UDP-glucuronic acid. This represents the first UGT fully characterized from microorganisms, providing a reference enzyme for future investigatin of more microbial UGTs. 
     UGTs play important roles in plant growth and development. They can also be a useful tool for structural modification of bioactive molecules in metabolic engineering applications, because it can greatly change the bioactivity, solubility, or stability of metabolites (De Bruyn et al. 2015; Kren and Martínková 2001). For example, the sweetness and solubility were greatly improved from glycyrrhetinic acid to its final product glycyrrhizin by two-step glucuronosylation, which is used as an anti-hepatitis agent and a sweetener worldwide (Xu et al. 2016). In fact, the two glucuronic acid moieties of glycyrrhizin also reduce the side-effects (Yonekura and Hanada 2011). Researchers found that glucuronidated anthocyanins showed improved color stability in response to light compared to its glucosylated form, indicating that UGTs may be used to stabilize natural colorants for industrial use, such as commercial food colorant products (Osmani et al. 2009). Glucuronidation can also improve the bioactivity of natural products. For instance, glucuronidated flavonoids exhibited relatively stronger inhibitory activity of amyloid (β(A(β) and human islet amyloid polypeptide (hIAPP) aggregation than their aglycons (Hmidene et al. 2017). More importantly, the conjugation location of glucuronidation can alter the biological effects. Morphine-6-glucuronide is a much more potent agonist than morphine itself, while morphine-3-glucuronide is an extremely potent antagonist (Paul et al. 1989; Smith et al. 1990). This finding indicates that glucuronides may have very distinctive effects themselves. 
     To explore the pential of GcaC as a useful biocatalytic tool, we investigated the substrate specificity of this enzyme. Although the BLAST analysis suggested that GcaC is homologous to a possible UDP-glucosyltransferase/UDP-glucuronyltransferase from  S. malaysiensis,  our in vitro reaction results revealed that GcaC is a specific UDP-glucuronyltransferase, as it cannot take UDP-glucose as a sugar donor. By contrast, GcaC has showed high flexibilty toward the sugar acceptor substrates. A varitey of substrates, such as quercetin, ferulic acid, vanillic acid, curcumin, vanillin, chrysin, zearalenone and apigenin, can be accepted by GcaC as sugar acceptors to yield various glucuronides. Compared to BpUGAT from plant red daisy ( Bellis perennis ) that is specific for anthocyanin glucuronidation (Sawada et al. 2005), GcaC showed relaxed substrate specificity to sugar acceptors. Interestingly, it was found that different products may be produced from the same substrates in the in vivo biotransformation by  S. chromofuscus  ATCC 49982 and in vitro reactions with purified GcaC, likely due to the availability of UDP-glucuronic acid and other factors in the cells. For example, quercetin-7-O-β-D-glucuronide and quercetin-3-O-β-D-glucuronide were obtained as the major products when quercetin was incubated with  S. chromofuscus  ATCC 49982. However, a total of six glucuronides were detected when quercetin was reacted with UDP-glucuronic acid in the presence of purified GcaC. Interestingly, among the six products, four monoglucuronides and two diglucuronides of quercetin were obtained based on LC-MS analysis. Quercetin has five free hydroxyl groups. In addition to the 3-OH and 7-OH, two other hydroxyl groups were chosen by GcaC to introduce the sugar moiety. More interestingly, GcaC appeared to be able to introduce two glucuronic acid moieties to quercetin to create diglucuronides. Similarly, in vitro reaction of GcaC with curcumin also yielded a monoglucuronide and a diglucuronide. These results indicated that GcaC is a highly versatile enzyme for the synthesis of glucuronides. Compared to the tedious chemical methods to synthesize glucuronides (Sharipova et al. 2017), discovery of GcaC provides an effective tool to prepare various glucuronides from different phenolic compounds in an environmental-friendly and efficient way. 
     The optimal in vitro enzymatic reactions and in vivo bioconversion conditions of GcaC were investigated in this work. The optimum reaction temperature and pH for this enzyme were found to be 7.5 and 30° C. in phosphate buffer, which are both slightly lower than BpUGAT from  B. perennis  (8.0 and 35° C.). However, these two UGTs have different response to metal ions. For example, Ca 2+  had an inhibitory effect on BpUGAT, but stimulated the activity of GcaC. Similarly, Mg 2+  and Mn 2+  , with negligible effects on the catalytic activity of BpUGAT (Sawada et al. 2005), enhanced the activity of GcaC. By contrast, the inhibitory effects of Cu 2+ , Zn 2+  and Fe 2+  on GcaC is similar to the previous discovered UBGAT from  Scutellaria baicalensis  (Nagashima et al. 2000). 
     GcaC represents the first characterized UGT from microorganisms. BLAST analysis showed that this microbial UGT showed little or no homology to UGTs from other sources. For example, GcaC has less than 25% identity to BpUGAT from  B. perennis  (Gene bank accession number AB190262) (Sawada et al. 2005), GuUGAT from  Glycyrrhiza uralensis  (Gene bank accession number KT759000) (Xu et al. 2016), and UBGAT from  Scutellaria baicalensis  (Gene bank accession number BAC98300) (Nagashima et al. 2000). Discovery and characterization of GcaC yielded a reference enzyme for future identification of additional UGTs from microorganisms. More importantly, this highly flexible enzyme may (1) glucuronidate a variety of substrates, (2) introduce the glucuronic acid moiety to different positions of a substrate, and (3) introduce one or two glucuronic acid moieties to a substrate to yield monoglucosides and diglucosides. Therefore,  S. chromofuscus  ATCC 49982, GcaC and  E. coli  BL21(DE3)/GcaC can be used to prepare desired glucuronides from particular substrates. For example, vanillic acid-COOH-glucuronide was only previously detected in human urine after tea intake, and it was only identified by using MS analysis without available NMR data (Ridder et al. 2012; van der Hooft et al. 2012). This work for the first time biosynthesized vanillic acid-7-O-β-D-glucuronide for complete structure elucidation. This work also demonstrated that this novel UGT can be efficiently expressed in  E. coli  BL21(DE3) and the engineered strain can be used as a whole-cell biocatalyst to prepare resveratrol glucuronides. The titers of resveratrol-4′-O-β-D-glucuronide and resveratrol-3-O-β-D-glucuronide were 78.381±0.366 mg/L and 14.991±0.248 mg/L from 114.125 mg/L resveratrol in a 1-L reaction system within 3 hours. Overall, this work provides a highly versatile UGT that can serve as a useful biocatalyst to prepare valuable glucuronides of bioactive molecules. 
     Example 11. Hydroxyflavone-2′-O-β-D-glucuronide was Prepared from 2′-hydroxyflavone by Incubation of the Substrate with  S. chromofuscus  ATCC 49982 or the Engineered  E. coli -GcaC 
     Hydroxyflavone-2′-O-β-D-glucuronide was prepared from 2′-hydroxyflavone by incubation of the substrate with  S. chromofuscus  ATCC 49982 or the engineered  E. coli -GcaC as described above. The HPLC analysis results are shown in  FIG.  9   . 
     The product was isolated from the cultures as a white, amorphous powder using HPLC. Its molecular formula was deduced to be C 21 H 18 O 9  based on the [M-H] +  peak at m/z 415.1013 (415.1029 calcd for C 21 H 19 O 9 ) in high-resolution electrospray ionization mass spectrometry (HRESIMS) spectrum, indicating that this compound has thirteen degrees of unsaturation. To further elucidate the chemical structure, this product was analyzed by NMR. The  13 C NMR analysis presented 21 peaks in the spectrum. In addition to the 15 signals belonging to the substrate, six additional carbon signals at δ C  170.0, 99.6, 76.0, 75.5, 73.0, and 71.3 were found in the spectra, together with the additional proton signals at δ H  3.34-3.98 in the  1 H NMR spectrum, suggesting that a sugar moiety has been added to 2′-hydroxyflavone. Unlike the common sugar glucose, this sugar moiety has a quaternary carbon signal at δ C  170.0, indicating the presence of a carboxyl group in the sugar moiety. Therefore, both the  1 H and  13 C signals of this sugar moiety are consistent with a glucuronic acid moiety. Moreover, the  1 H NMR spectrum showed a doublet at δ H  5.32, indicative of an anomeric proton with a coupling constant of 7.2 Hz, and the chemical shift along with the J-coupling value were consistent with that of β-D-glucuronic acid. The correlation of the anomeric H-1″ signal at δ H  5.55 to C-2′ signal at δ C  154.0 in the HMBC spectrum revealed that the glucuronic acid moiety was located at C-2′ ( FIG.  10   ). The above data together with the detailed analysis of its 2D NMR spectra confirmed its structure as hydroxyflavone-2′-O-β-D-glucuronide and all signals were assigned accordingly (Table 5). 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                   1 H (500 MHz) and  13 C NMR (125 MHz) data for 
               
               
                 hydroxyflavone-2’-0-D-glucuronide in DMSO-d 6 . 
               
            
           
           
               
               
               
            
               
                   
                   
                 Hydroxyflavone-2‘-O-β-D-glucuronide 
               
            
           
           
               
               
               
               
            
               
                   
                 Position 
                 δ C , type 
                 δ H  (J in Hz) 
               
               
                   
                   
               
               
                   
                  2 
                 160.7, C 
                   
               
               
                   
                  3 
                 112.2, CH  
                 7.04 (1H, s) 
               
               
                   
                  4 
                 177.2, C 
                   
               
               
                   
                  5 
                 124.7, CH 
                 8.06 (1H, dd, J = 7.9, 1.5 Hz) 
               
               
                   
                  6 
                 125.4, CH 
                 7.50 (1H, t, J = 7.5 Hz) 
               
               
                   
                  7 
                 134.2, CH 
                 7.82 (1H, m) 
               
               
                   
                  8 
                 118.6, CH 
                 7.74 (1H, d, J = 8.3 Hz) 
               
               
                   
                  9 
                 156.0, C 
                   
               
               
                   
                 10 
                 123.2, C 
                   
               
               
                   
                  1′ 
                 121.0, C 
                   
               
               
                   
                  2′ 
                 154.9, C 
                   
               
               
                   
                  3′ 
                 115.3, CH 
                 7.37 (1H, d, J = 8.5Hz) 
               
               
                   
                  4′ 
                 132.7, CH 
                 7.57 (1H, m) 
               
               
                   
                  5′ 
                 122.2, CH 
                 7.23 (1H, t, J = 7.6 Hz) 
               
               
                   
                  6′ 
                 129.4, CH 
                 7.93 (1H, dd, J = 7.8, 1.5 Hz) 
               
               
                   
                  1″ 
                  99.6, CH 
                 5.32 (1H, d, J = 7.2 Hz) 
               
               
                   
                  2″ 
                  73.0, CH 
                 3.34 (2H, m, overlapped) 
               
               
                   
                  3″ 
                  76.0, CH 
                 3.34 (2H, m, overlapped) 
               
               
                   
                  4″ 
                  71.3, CH 
                 3.42 (1H, m) 
               
               
                   
                  5″ 
                  75.5, CH 
                 3.98 (1H, d, J = 9.6 Hz) 
               
               
                   
                  6″ 
                 170.0, C 
               
               
                   
                   
               
            
           
         
       
     
     Hydroxyflavone-2′-O-β-D-glucuronide: White, amorphous powder; UVmax 247, 308 nm; IR vmax (film) 3242.9, 2985.9, 1614.8, 1556.0, 1468.1, 1379.7, 1216.3, 1088.8 cm −1 ;  1 H and  13 C NMR data, see Table 5; negative ESIMS m/z 412.8 [M-H] − ; positive HRESIMS m/z 415.1013 [M-H] +  (415.1029 calcd for C 21 H 19 O 9 ). 
     Determination of the Water-Solubility 
     The purified product was tested for their water solubility as previously described (Wang et al. 2014). Purified 2′-hydroxyflavone glycoside was used to establish standard curves for quantifying water solubility. Briefly, the purified compound and the substrate were each mixed with 300 μL of distilled water in an Eppendorf tube at 25° C. An ultrasonic cleaner was used to facilitate the dissolution. After 30 min of sonication and centrifugation at 13,000×g for 10 min, supernatant of each sample was analyzed by HPLC to determine the compound concentrations in the solution. All samples were performed in triplicate and water solubility of each sample are expressed as the mean±standard deviation (SD). 
     The water solubility of hydroxyflavone-2′-O-β-D-glucuronide was computed in triplicate (n=3) and determined to be 34.8±0.9 mg/L, which is around 14 times than that of the substrate 2′-hydroxyflavone (2.5±0.7 mg/L). This result demonstrated that the glucuronidation technology in this invention is an effective approach to enhancing the water solubility. 
     Antioxidant Assay 
     The DPPH radical scavenging assay was performed to evaluate the antioxidant activity of all four new compounds using a reported method with minor modifications (Jiao et al. 2015. Briefly, 180 μt of 150 μM fresh DPPH in MeOH were mixed with 20 μL of sample at different concentrations (62.5, 125, 250, 500, and 1000 μM in DMSO) in the wells of 96-well plates. The natural antioxidant ascorbic acid and the substrate 2′-hydroxyflavone were used as the positive and negative controls, respectively. After shaking in the dark for 30 min at room temperature, the reaction was detected by measuring the absorbance A sample+DPPH  at 517 nm using a microplate reader. A sample  means the blank measurement for each tested compound; specifically, 20 μL of sample was mixed with 180 μL of MeOH. A DPPH  is the absorbance of the mixture of 20 μL of distilled water and 180 μL of DPPH. A blank  indicates the absorbance of the mixture of 20 μL of distilled water and 180 μL of MeOH. All tests were performed in triplicate. The formula DPPH scavenging activity (%)=[1−(A sample+DPPH −A sample )/(A DPPH −A blank )]×100%. The logIC 50  values (the concentration required to scavenge 50% of radicals)] of tested compounds were calculated using the GraphPad Prism software. The results are expressed as the mean±SD. Significant differences were determined by one-way analysis of variance (ANOVA) using the SAS Studio web-based environment, and p&lt;0.05 was considered significant. All samples were performed in triplicate and antioxidant activity of each sample is expressed as the mean±SD. 
     The resulting pairwise comparison analysis demonstrated that hydroxyflavone-2′-O-β-D-glucuronide has significantly improved radical scavenging activity, decreasing the mean logIC 50  from 65.2±8.6 μM (2′-Hydroxyflavone) to 20.4±0.7 μM (p&lt;0.001). This result confirmed that the glucuronidation technology described in this invention can effectively enhance the biological activity of bioactive molecules. 
     Unless defined otherwise, all technical and scientific terms herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials, similar or equivalent to those described herein, can be used in the practice or testing of the present invention, the preferred methods and materials are described herein. All publications, patents, and patent publications cited are incorporated by reference herein in their entirety for all purposes. 
     The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. 
     While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth and as follows in the scope of the appended claims. 
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