Molecular cloning and functional expression of O-methyltransferases common to isoquinoline alkaloid and phenylpropanoid biosynthesis

Alkaloid biosynthesis: metabolism and trafficking

Alkaloids represent a highly diverse group of compounds that are related only by the occurrence of a nitrogen atom in a heterocyclic ring. Plants are estimated to produce approximately 12,000 different alkaloids, which can be organized into groups according to their carbon skeletal structures. Alkaloid biosynthesis in plants involves many catalytic steps, catalyzed by enzymes that belong to a wide range of protein families. The characterization of novel alkaloid biosynthetic enzymes in terms of structural biochemistry, molecular and cell biology, and biotechnological applications has been the focus of research over the past several years. The application of genomics to the alkaloid field has accelerated the discovery of cDNAs encoding previously elusive biosynthetic enzymes. Other technologies, such as large-scale gene expression analyses and metabolic engineering approaches with transgenic plants, have provided new insights into the regulatory architecture of alkaloid metabolism.

The genetic manipulation of medicinal and aromatic plants

Medicinal and aromatic plants have always been intimately linked with human health and culture. Plant-derived medicines constitute a substantial component of present day human healthcare systems in industrialized as well as developing countries. They are products of plant secondary metabolism and are involved in many other aspects of a plant's interaction with its immediate environment. The genetic manipulation of plants together with the establishment of in vitro plant regeneration systems facilitates efforts to engineer secondary product metabolic pathways. Advances in the cloning of genes involved in relevant pathways, the development of high throughput screening systems for chemical and biological activity, genomics tools and resources, and the recognition of a higher order of regulation of secondary plant metabolism operating at the whole plant level facilitate strategies for the effective manipulation of secondary products in plants. Here, we discuss advances in engineering metabolic pathways for specific classes of compounds in medicinal and aromatic plants and we identify remaining constraints and future prospects in the field. In particular we focus on indole, tropane, nicotine, isoquinoline alcaloids, monoterpenoids such as menthol and related compounds, diterpenoids such as taxol, sequiterpenoids such as artemisinin and aromatic amino acids.

Methyl allyl ether formation in plants: novel S-adenosyl L-methionine:coniferyl alcohol 9-O-methyltransferase from suspension cultures of three Linum species

A novel 41 kDa methyltransferase displaying high regiospecificity towards the allylic hydroxyl moiety of coniferyl alcohol was cloned from suspension cultures of Linum nodiflorum L. and expressed in E. coli. The apparent K (m) for coniferyl alcohol is 7.23 microM with a V (max) of 707.5 pkat mg(-1) protein at 30 degrees C, whereas the K (m) for the co-substrate S-adenosyl-L-methionine is 18.5 microM. Structure-function relationship studies revealed stringent structure requirements. Even minor substructure deviations as the side-chain saturation or changes in the phenyl ring substitution result in activities decreased by 75-90%. Crotyl and allyl alcohols are not substrates, confirming that the aromatic ring itself is indispensable, and solely the derivatives with a C(3) side-chain are accepted. The enzyme shares only similarities under 46% on amino acid level with other known methyltransferases. The designated reaction product, coniferyl alcohol 9-methyl ether, could be detected in suspension cells. The highest content of up to 0.02% of the dry mass is concurrent with an increase of the specific enzyme activity that reaches its maximum of 3.94 pkat mg(-1) on day 6 of the culture period. Transcript levels estimated by semi-quantitative RT-PCR remain constant until day 6 and recede thereafter. The corresponding methyltransferase from Linum flavum L. differs mainly by one short variable fragment. Biochemical characterization revealed a higher catalytic efficiency and a slightly broader substrate plasticity together with a lower sensitivity to the presence of Zn(2+), Cu(2+) and Co(2+). This is to our knowledge the first report of a regiospecific allylic O-methylation of phenylpropanoids in plants.

Aryltetralin-lignan formation in two different cell suspension cultures of Linum album: deoxypodophyllotoxin 6-hydroxylase, a key enzyme for the formation of 6-methoxypodophyllotoxin

Suspension cultures initiated from two different Linum album seedlings accumulate either podophyllotoxin (PTOX, 2.6 mg/g DW) or 6-methoxypodophyllotoxin (6MPTOX, 5.4 mg/g DW) as main lignans. Two molecules of coniferyl alcohol are dimerized to pinoresinol which is converted via several steps into deoxypodophyllotoxin (DOP) which seems to be the branching point to PTOX or 6MPTOX biosynthesis. DOP is hydroxylated at position 7 to give PTOX by deoxypodophyllotoxin 7-hydroxylase (DOP7H). In contrast, 6MPTOX biosynthesis is achieved by DOP hydroxylation at position 6 to beta-peltatin by the cytochrome P450 enzyme deoxypodophyllotoxin 6-hydroxylase (DOP6H). The following methylation to beta-peltatin-A-methylether is catalyzed by beta-peltatin 6-O-methyltransferase (betaP6OMT) from which 6MPTOX is formed by hydroxylation at position 7 by beta-peltatin-A-methylether 7-hydroxylase (PAM7H). DOP6H and betaP6OMT could be characterized in protein extracts from cell cultures of L. flavum and L. nodiflorum, respectively, and here in L. album for the first time. DOP7H and PAM7H activities could not yet be detected with protein extracts. Experiments of feeding DOP together with inhibitors of cytochrome P450 depending as well as dioxygenase enzymes were performed in order to shed light on the type of DOP7H and PAM7H. Growth parameters and specific activities of enzymes from the phenylpropane as well as the lignan specific biosynthetic pathway were measured during a culture period of 16 days. From the enzymes studied only the DOP6H showed a differential activity sustaining the hypothesis that this enzyme is responsible for the differential lignan accumulation in both cell lines.

MAM3 catalyzes the formation of all aliphatic glucosinolate chain lengths in Arabidopsis

Chain elongated, methionine (Met)-derived glucosinolates are a major class of secondary metabolites in Arabidopsis (Arabidopsis thaliana). The key enzymatic step in determining the length of the chain is the condensation of acetyl-coenzyme A with a series of omega-methylthio-2-oxoalkanoic acids, catalyzed by methylthioalkylmalate (MAM) synthases. The existence of two MAM synthases has been previously reported in the Arabidopsis ecotype Columbia: MAM1 and MAM3 (formerly known as MAM-L). Here, we describe the biochemical properties of the MAM3 enzyme, which is able to catalyze all six condensation reactions of Met chain elongation that occur in Arabidopsis. Underlining its broad substrate specificity, MAM3 also accepts a range of non-Met-derived 2-oxoacids, e.g. converting pyruvate to citramalate and 2-oxoisovalerate to isopropylmalate, a step in leucine biosynthesis. To investigate its role in vivo, we identified plant lines with mutations in MAM3 that resulted in a complete lack or greatly reduced levels of long-chain glucosinolates. This phenotype could be complemented by reintroduction of a MAM3 expression construct. Analysis of MAM3 mutants demonstrated that MAM3 catalyzes the formation of all glucosinolate chain lengths in vivo as well as in vitro, making this enzyme the major generator of glucosinolate chain length diversity in the plant. The localization of MAM3 in the chloroplast suggests that this organelle is the site of Met chain elongation.

Overexpression of Coptis japonica norcoclaurine 6-O-methyltransferase overcomes the rate-limiting step in Benzylisoquinoline alkaloid biosynthesis in cultured Eschscholzia californica

Benzylisoquinoline alkaloids are one of the most important secondary metabolite groups, and include the economically important analgesic morphine and the antimicrobial agent berberine. To improve the production of these alkaloids, we investigated the effect of the overexpression of putative rate-limiting step enzymes in benzylisoquinoline alkaloid biosynthesis. We introduced two O-methyltransferase [Coptis japonica norcoclaurine 6-O-methyltransferase (6OMT) and 3'-hydroxy-N-methylcoclaurine 4'-O-methyltransferase (4'OMT)] expression vectors into cultured California poppy cells to avoid the gene silencing effect of endogenous genes. We established 20 independent lines for 6OMT transformants and 15 independent lines for 4'OMT transformants. HPLC/liquid chromatography-mass spectrometry (LC-MS) analysis revealed that the overexpression of C. japonica 6OMT was associated with an average alkaloid content 7.5 times greater than that in the wild type, whereas the overexpression of C. japonica 4'OMT had only a marginal effect. Further characterization of 6OMT in California poppy cells indicated that a 6OMT-specific gene is missing and 4OMT catalyzes the 6OMT reaction with low activity in California poppy, which supports the notion that the 6OMT reaction is important for alkaloid biosynthesis in this plant species. We discuss the importance of 6OMT in benzylisoquinoline alkaloid biosynthesis and the potential for using a rate-limiting step gene to improve alkaloid production.

Quaternary protoberberine alkaloids

This contribution reviews some general aspects of the quaternary iminium protoberberine alkaloids. The alkaloids represent a very extensive group of secondary metabolites with diverse structures, distribution in nature, and biological effects. The quaternary protoberberine alkaloids (QPA), derived from the 5,6-dihydrodibenzo[a,g]quinolizinium system, belong to a large class of isoquinoline alkaloids. Following a general introduction, the plant sources of QPA, their biosynthesis, and procedures for their isolation are discussed. Analytical methods and spectral data are summarized with emphasis on NMR spectroscopy. The reactivity of QPA is characterized by the sensitivity of the iminium bond CN(+) to nucleophilic attack. The addition of various nucleophiles to the protoberberine skeleton is discussed. An extended discussion of the principal chemical reactivity is included since this governs interactions with biological targets. Quaternary protoberberine alkaloids and some related compounds exhibit considerable biological activities. Recently reported structural studies indicate that the QPA interact with nucleic acids predominantly as intercalators or minor groove binders. Currently, investigations in many laboratories worldwide are focused on the antibacterial and antimalarial activity, cytotoxicity, and potential genotoxicity of QPA.

Functional analysis of members of the isoflavone and isoflavanone O-methyltransferase enzyme families from the model legume Medicago truncatula

Previous studies have identified two distinct O-methyltransferases (OMTs) implicated in isoflavonoid biosynthesis in Medicago species, a 7-OMT methylating the A-ring 7-hydroxyl of the isoflavone daidzein and a 4'-OMT methylating the B-ring 4'-hydroxyl of 2,7,4'-trihydroxyisoflavanone. Genes related to these OMTs from the model legume Medicago truncatula cluster as separate branches of the type I plant small molecule OMT family. To better understand the possible functions of these related OMTs in secondary metabolism in M. truncatula, seven of the OMTs were expressed in E. coli, purified, and their in vitro substrate preferences determined. Many of the enzymes display promiscuous activities, and some exhibit dual regio-specificity for the 4' and 7-hydroxyl moieties of the isoflavonoid nucleus. Protein structure homology modeling was used to help rationalize these catalytic activities. Transcripts encoding the different OMT genes exhibited differential tissue-specific and infection- or elicitor-induced expression, but not always in parallel with changes in expression of confirmed genes of the isoflavonoid pathway. The results are discussed in relation to the potential in vivo functions of these OMTs based on our current understanding of the phytochemistry of M. truncatula, and the difficulties associated with gene annotation in plant secondary metabolism.

Comparative transcript and alkaloid profiling in Papaver species identifies a short chain dehydrogenase/reductase involved in morphine biosynthesis

Plants of the order Ranunculales, especially members of the species Papaver, accumulate a large variety of benzylisoquinoline alkaloids with about 2500 structures, but only the opium poppy (Papaver somniferum) and Papaver setigerum are able to produce the analgesic and narcotic morphine and the antitussive codeine. In this study, we investigated the molecular basis for this exceptional biosynthetic capability by comparison of alkaloid profiles with gene expression profiles between 16 different Papaver species. Out of 2000 expressed sequence tags obtained from P. somniferum, 69 show increased expression in morphinan alkaloid-containing species. One of these cDNAs, exhibiting an expression pattern very similar to previously isolated cDNAs coding for enzymes in benzylisoquinoline biosynthesis, showed the highest amino acid identity to reductases in menthol biosynthesis. After overexpression, the protein encoded by this cDNA reduced the keto group of salutaridine yielding salutaridinol, an intermediate in morphine biosynthesis. The stereoisomer 7-epi-salutaridinol was not formed. Based on its similarities to a previously purified protein from P. somniferum with respect to the high substrate specificity, molecular mass and kinetic data, the recombinant protein was identified as salutaridine reductase (SalR; EC 1.1.1.248). Unlike codeinone reductase, an enzyme acting later in the pathway that catalyses the reduction of a keto group and which belongs to the family of the aldo-keto reductases, the cDNA identified in this study as SalR belongs to the family of short chain dehydrogenases/reductases and is related to reductases in monoterpene metabolism.

Sequential O-methylation of tricetin by a single gene product in wheat

Flavonoid compounds are ubiquitous in nature. They constitute an important part of the human diet and act as active principles of many medicinal plants. Their O-methylation increases their lipophilicity and hence, their compartmentation and functional diversity. We have isolated and characterized a full-length flavonoid O-methyltransferase cDNA (TaOMT2) from a wheat leaf cDNA library. The recombinant TaOMT2 protein was purified to near homogeneity and tested for its substrate preference against a number of phenolic compounds. Enzyme assays and kinetic analyses indicate that TaOMT2 exhibits a pronounced preference for the flavone, tricetin and gives rise to three methylated enzyme reaction products that were identified by TLC, HPLC and ESI-MS/MS as its mono-, di- and trimethyl ether derivatives. The sequential order of tricetin methylation by TaOMT2 is envisaged to proceed via its 3'-mono--->3',5'-di--->3',4',5'-trimethyl ether derivatives. To our knowledge, this is the first report of a gene product that catalyzes three sequential O-methylations of a flavonoid substrate.

Methylation of sulfhydryl groups: a new function for a family of small molecule plant O-methyltransferases

In plants, type I and II S-adenosyl-l-methionine-dependent O-methyltransferases (OMTs) catalyze most hydroxyl group methylations of small molecules. A homology-based RT-PCR strategy using Catharanthus roseus (Madagascar periwinkle) RNA previously identified six new type I plant OMT family members. We now describe the molecular and biochemical characterization of a seventh protein. It shares 56-58% identity with caffeic acid OMTs (COMTs), but it failed to methylate COMT substrates, and had no activity with flavonoids. However, the in vitro incubations revealed unusually high background levels without added substrates. A search for the responsible component revealed that the enzyme methylated dithiothreitol (DTT), the reducing agent added for enzyme stabilization. Unexpectedly, product analysis revealed that the methylation occurred on a sulfhydryl moiety, not on a hydroxyl group. Analysis of 34 compounds indicated a broad substrate range, with a preference for small hydrophobic molecules. Benzene thiol (Km 220 microm) and furfuryl thiol (Km 60 microm) were the best substrates (6-7-fold better than DTT). Small isosteric hydrophobic substrates with hydroxyl groups, like phenol and guaiacol, were also methylated, but the activities were at least 5-fold lower than with thiols. The enzyme was named C. roseus S-methyltransferase 1 (CrSMT1). Models based on the COMT crystal structure suggest that S-methylation is mechanistically identical to O-methylation. CrSMT1 so far is the only recognized example of an S-methyltransferase in this protein family. Its properties indicate that a few changes in key residues are sufficient to convert an OMT into a S-methyltransferase (SMT). Future functional investigations of plant methyltransferases should consider the possibility that the enzymes may direct methylation at sulfhydryl groups.

Crystal structure of prephenate dehydrogenase from Aquifex aeolicus. Insights into the catalytic mechanism

The enzyme prephenate dehydrogenase catalyzes the oxidative decarboxylation of prephenate to 4-hydroxyphenylpyruvate for the biosynthesis of tyrosine. Prephenate dehydrogenases exist as either monofunctional or bifunctional enzymes. The bifunctional enzymes are diverse, since the prephenate dehydrogenase domain is associated with other enzymes, such as chorismate mutase and 3-phosphoskimate 1-carboxyvinyltransferase. We report the first crystal structure of a monofunctional prephenate dehydrogenase enzyme from the hyper-thermophile Aquifex aeolicus in complex with NAD+. This protein consists of two structural domains, a modified nucleotide-binding domain and a novel helical prephenate binding domain. The active site of prephenate dehydrogenase is formed at the domain interface and is shared between the subunits of the dimer. We infer from the structure that access to the active site is regulated via a gated mechanism, which is modulated by an ionic network involving a conserved arginine, Arg250. In addition, the crystal structure reveals for the first time the positions of a number of key catalytic residues and the identity of other active site residues that may participate in the reaction mechanism; these residues include Ser126 and Lys246 and the catalytic histidine, His147. Analysis of the structure further reveals that two secondary structure elements, beta3 and beta7, are missing in the prephenate dehydrogenase domain of the bifunctional chorismate mutase-prephenate dehydrogenase enzymes. This observation suggests that the two functional domains of chorismate mutase-prephenate dehydrogenase are interdependent and explains why these domains cannot be separated.

Flavonoid 3'-O-methyltransferase from rice: cDNA cloning, characterization and functional expression

Plant O-methyltransferases (OMTs) are known to be involved in methylation of plant secondary metabolites, especially phenylpropanoid and flavonoid compounds. An OMT, ROMT-9, was cloned and characterized from rice using a reverse transcriptase polymerase chain reaction (RT-PCR). The blast results for ROMT-9 showed a 73% identity with caffeic acid OMTs from maize and Triticum aestivum. ROMT-9 was expressed in Escherichia coli and its recombinant protein was purified using affinity chromatography. It was then tested for its ability to transfer the methyl group of S-adenosyl-l-methionine to the flavonoid substrates, eriodictyol, luteolin, quercetin, and taxifolin, all of which have a 3'-hydroxyl functional group. The reaction products were analyzed using TLC, HPLC, HPLC/MS, and NMR spectroscopy. The NMR analysis showed that ROMT-9 transferred the methyl group specifically to the 3'-hydroxyl group of quercetin, resulting in the formation of its methoxy derivative. Furthermore, ROMT-9 converted flavonoids containing the 3'-hydroxy functional group such as eriodictyol, luteolin, quercetin and taxifolin into the corresponding methoxy derivatives, suggesting that ROMT-9 is an OMT with strict specificity for the 3'-hydroxy group of flavonoids.

Metabolomics and its role in understanding cellular responses in plants

A natural shift is taking place in the approaches being adopted by plant scientists in response to the accessibility of systems-based technology platforms. Metabolomics is one such field, which involves a comprehensive non-biased analysis of metabolites in a given cell at a specific time. This review briefly introduces the emerging field and a range of analytical techniques that are most useful in metabolomics when combined with computational approaches in data analyses. Using cases from Arabidopsis and other selected plant systems, this review highlights how information can be integrated from metabolomics and other functional genomics platforms to obtain a global picture of plant cellular responses. We discuss how metabolomics is enabling large-scale and parallel interrogation of cell states under different stages of development and defined environmental conditions to uncover novel interactions among various pathways. Finally, we discuss selected applications of metabolomics.

Multiple Regiospecific Methylations of a Flavonoid by Plant O-Methyltransferases Expressed in E. coli

Quercetin was methylated with two O-methyltransferases (OMTs) expressed in E. coli. A construct (RSOMT) was designed to express two OMTs: ROMT-9, which methylates specifically at the 3'-hydroxyl group of quercetin and SOMT-2, which methylates at the 4'-hydroxyl group. Both OMT genes were driven by T7 promoters and had ribosome binding sites. Both ROMT-9 and SOMT-2 were successfully expressed in E. coli transformant harboring RSOMT. Reaction products of quercetin with E. coli transformant containing RSOMT showed two methylation products that corresponded to the 3'-methylated and the 3',4'-dimethylated quercetin, which were confirmed by NMR. More than 90% of quercetin was converted into the 3',4'-dimethylated quercetin after 24 h incubation with E. coli transformant harboring RSOMT.

Unusual pseudosubstrate specificity of a novel 3,5-dimethoxyphenol O-methyltransferase cloned from Ruta graveolens L

A cDNA was cloned from Ruta graveolens cells encoding a novel O-methyltransferase (OMT) with high similarity to orcinol or chavicol/eugenol OMTs, but containing a serine-rich N-terminus and a 13 amino acid insertion between motifs IV and V. Expression in Escherichia coli revealed S-adenosyl-l-methionine-dependent OMT activity with methoxylated phenols only with an apparent Km of 20.4 for the prime substrate 3,5-dimethoxyphenol. The enzyme forms a homodimer of 84 kDa, and the activity was insignificantly affected by 2.0 mM Ca2+ or Mg2+, whereas Fe2+, Co2+, Zn2+, Cu2+ or Hg2+ were inhibitory (78-100%). Dithiothreitol (DTT) suppressed the OMT activity. This effect was examined further, and, in the presence of Zn2+ as a potential thiol methyltransferase (TMT) cofactor, the recombinant OMT methylated DTT to DTT-monomethylthioether. Sets of kinetic OMT experiments with 3,5-dimethoxyphenol at various Zn2+/DTT concentrations revealed the competitive binding of DTT with an apparent Ki of 52.0 microM. Thus, the OMT exhibited TMT activity with almost equivalent affinity to the thiol pseudosubstrate which is structurally unrelated to methoxyphenols.

Comparative macroarray analysis of morphine containing Papaver somniferum and eight morphine free Papaver species identifies an O-methyltransferase involved in benzylisoquinoline biosynthesis

Benzylisoquinoline alkaloids constitute a group of about 2,500 structures and are mainly produced by plants of the order Ranunculales. But only the opium poppy, Papaver somniferum, and Papaver setigerum are able to produce morphine. In this study, we started to investigate by gene expression analysis the molecular basis for this exceptional biosynthetic ability. A sequencing project from P. somniferum seedlings was initiated using a method based on the amplified fragment length polymorphism technique that resulted in 849 UniGenes. These cDNAs were analysed on macroarrays for differential expression between morphine-containing P. somniferum plants and eight other Papaver species, which accumulate other benzylisoquinolines instead of morphine. Three cDNAs showing increased expression in P. somniferum compared to all the other Papaver species were identified. Whereas two showed no significant homology to any known protein, one putatively encoded an O-methyltransferase. Analysis of substrate specificity of the heterologously expressed protein and mass spectrometric identification of the enzymatic products identified this protein as S-adenosyl-L-methionine:(R,S)-3'-hydroxy-N-methylcoclaurine 4'-O-methyltransferase (EC 2.1.1.116). Unlike other O-methyltransferases of different positional specificities implicated in benzylisoquinoline metabolism, the enzyme only accepted tetrahydroxylated tetrahydrobenzylisoquinolines as substrates; methylation was tolerated only at the 6-hydroxy position.

Regiospecific methylation of naringenin to ponciretin by soybean O-methyltransferase expressed in Escherichia coli

Flavonoids found in plants most likely undergo a variety of modification reactions such as hydroxylation, glycosylation, and/or methylation. Among these, O-methylation has an effect on the solubility and thus on the antimicrobial activity of the flavonoids. We analyzed the conversion of naringenin with a methyltransferase, SOMT-2, from Glycine max. SOMT-2 was expressed in Escherichia coli as a glutathion S-transferase fusion protein. E. coli harboring SOMT-2 was grown with daidzein, geninstein, apigenin, naringenin, and quercetin, respectively, and reaction products were analyzed with thin layer chromatography and HPLC. SOMT-2 could convert apigenin, daidzein, genistein, and quercetin into the corresponding 4'-O-methylated compounds such as acacetin, formononetin, biochanine A, and 4'-methylated quercetin whereas naringenin turned out to be the best substrate tested. SOMT-2 stoichiometically converted naringenin (4',5,7-trihyroxyflavanone) into a ponciretin (4'-methoxy-5,7-dihydroxyflavanone), whose structure was determined by NMR and LC/mass spectral analyses. Considering the reactions, SOMT-2 may have a regiospecific methylation activity, resulting in transforming 4'-hydroxyl group of flavonoids B-ring to 4'-methyl group.

Metabolon formation and metabolic channeling in the biosynthesis of plant natural products

Metabolon formation and metabolic channeling in plant secondary metabolism enable plants to effectively synthesize specific natural products and to avoid metabolic interference. Channeling can involve different cell types, take advantage of compartmentalization within the same cell or proceed directly within a metabolon. New experimental approaches document the importance of channeling in the synthesis of isoprenoids, alkaloids, phenylpropanoids, flavonoids and cyanogenic glucosides. Metabolon formation and metabolic channeling in natural-product synthesis facilitate attempts to genetically engineer new pathways into plants to improve their content of valuable natural products. They also offer the opportunity to introduce new traits by genetic engineering to produce plant cultivars that adhere to the principle of substantial equivalence.

O-Methylation of benzaldehyde derivatives by "lignin specific" caffeic acid 3-O-methyltransferase

Although S-adenosyl-l-methionine (SAM) dependent caffeic acid/5-hydroxyferulic acid 3/5-O-methyltransferase (COMT) is one of the key enzymes in lignin biosynthesis, the present work demonstrates that alfalfa COMT methylates benzaldehyde derivatives more efficiently than lignin pathway intermediates. 3,4-Dihydroxy, 5-methoxybenzaldehyde and protocatechuic aldehyde were the best in vitro substrates for OMT activity in extracts from developing alfalfa stems, and these compounds were preferred over lignin pathway intermediates for 3-O-methylation by recombinant alfalfa COMT expressed in Escherichia coli. OMT activity with benzaldehydes was strongly reduced in extracts from stems of transgenic alfalfa down-regulated in COMT. However, although COMT down-regulation drastically affects lignin composition, it does not appear to significantly impact metabolism of benzaldehyde derivatives in alfalfa. Structurally designed site-directed mutants of COMT showed altered relative substrate preferences for lignin precursors and benzaldehyde derivatives. Taken together, these results indicate that COMT may have more than one role in phenylpropanoid metabolism (but probably not in alfalfa), and that engineered COMT enzymes could be useful for metabolic engineering of both lignin and benzaldehyde-derived flavors and fragrances.

Furanocoumarin biosynthesis in Ammi majus L

Plants belonging to the Apiaceae or Rutaceae accumulate methoxylated psoralens, such as bergapten or xanthotoxin, as the final products of their furanocoumarin biosynthesis, and the rate of accumulation depends on environmental and other cues. Distinct O-methyltransferase activities had been reported to methylate bergaptol to bergapten and xanthotoxol to xanthotoxin, from induced cell cultures of Ruta graveolens, Petroselinum crispum and Ammi majus. Bergaptol 5-O-methyltransferase (BMT) cDNA was cloned from dark-grown Ammi majus L. cells treated with a crude fungal elicitor. The translated polypeptide of 38.7 kDa, composed of 354 amino acids, revealed considerable sequence similarity to heterologous caffeic acid 3-O-methyltransferases (COMTs). For homologous comparison, COMT was cloned from A. majus plants and shown to share 64% identity and about 79% similarity with the BMT sequence at the polypeptide level. Functional expression of both enzymes in Escherichia coli revealed that the BMT activity in the bacterial extracts was labile and rapidly lost on purification, whereas the COMT activity remained stable. Furthermore, the recombinant AmBMT, which was most active in potassium phosphate buffer of pH 8 at 42 degrees C, showed narrow substrate specificity for bergaptol (Km SAM 6.5 micro m; Km Bergaptol 2.8 micro m) when assayed with a variety of substrates, including xanthotoxol, while the AmCOMT accepted 5-hydroxyferulic acid, esculetin and other substrates. Dark-grown A. majus cells expressed significant BMT activity which nevertheless increased sevenfold within 8 h upon the addition of elicitor and reached a transient maximum at 8-11 h, whereas the COMT activity was rather low and did not respond to the elicitation. Complementary Northern blotting revealed that the BMT transcript abundance increased to a maximum at 7 h, while only a weak constitutive signal was observed for the COMT transcript. The AmBMT sequence thus represents a novel database accession specific for the biosynthesis of psoralens.

Bio-fermentation of modified flavonoids: an example of in vivo diversification of secondary metabolites

A bio-fermentation technique was used for the in vivo diversification of flavonoid structures based on expression in Escherichia coli of six O-methyltransferases (OMTs) from Mentha x piperita and one O-glucosyltransferase (GT) each from Arabidopsis thaliana and Allium cepa. Enzymes were shown to be regio-specific in in vitro experiments and modified a broad range of flavonoid substrates at various positions. Using the flavonol quercetin as a model substrate, we show that the product spectrum produced with the in vivo approach is identical to that found in vitro. Additionally, using mixed cultures of E. coli expressing different classes of modifying genes (OMTs and GTs), the production of polymethylated flavonoid glucosides was observed. This report demonstrates the potential to increase the structural diversity of plant secondary metabolites using a multi-enzyme, bio-fermentation approach.

Transcriptional regulation of sorghum defense determinants against a phloem-feeding aphid

When attacked by a phloem-feeding greenbug aphid (Schizaphis graminum), sorghum (Sorghum bicolor) activates jasmonic acid (JA)- and salicylic acid (SA)-regulated genes, as well as genes outside known wounding and SA signaling pathways. A collection of 672 cDNAs was obtained by differential subtraction with cDNAs prepared from sorghum seedlings infested by greenbug aphids and those from uninfested seedlings. Subsequent expression profiling using DNA microarray and northern-blot analyses identified 82 transcript types from this collection responsive to greenbug feeding, methyl jasmonate (MeJA), or SA application. DNA sequencing analyses indicated that these encoded proteins functioning in direct defense, defense signaling, oxidative burst, secondary metabolism, abiotic stress, cell maintenance, and photosynthesis, as well as proteins of unknown function. In response to insect feeding, sorghum increased transcript abundance of numerous defense genes, with some SA-dependent pathogenesis-related genes responding to greenbug more strongly than to SA. In contrast, only weak induction of MeJA-regulated defense genes was observed after greenbug treatment. However, infestation tests confirmed that JA-regulated pathways were effective in plant defense against greenbugs. Activation of certain transcripts exclusively by greenbug infestation was observed, and may represent unique signal transduction events independent of JA- and SA-regulated pathways. Results indicate that plants coordinately regulate defense gene expression when attacked by phloem-feeding aphids, but also suggest that aphids are able to avoid triggering activation of some otherwise potentially effective plant defensive machinery, possibly through their particular mode of feeding.

(R,S)-Reticuline 7-O-methyltransferase and (R,S)-norcoclaurine 6-O-methyltransferase of Papaver somniferum - cDNA cloning and characterization of methyl transfer enzymes of alkaloid biosynthesis in opium poppy

S-Adenosyl-L-methionine:(R,S)-reticuline 7-O-methyltransferase converts reticuline to laudanine in tetrahydrobenzylisoquinoline biosynthesis in the opium poppy Papaver somniferum. This enzyme activity has not yet been detected in plants. A proteomic analysis of P. somniferum latex identified a gel spot that contained a protein(s) whose partial amino acid sequences were homologous to those of plant O-methyltransferases. cDNA was amplified from P. somniferum RNA by reverse transcription PCR using primers based on these internal amino acid sequences. Recombinant protein was then expressed in Spodoptera frugiperda Sf9 cells in a baculovirus expression vector. Steady-state kinetic measurements with one heterologously expressed enzyme and mass spectrometric analysis of the enzymatic products suggested that this unusual enzyme is capable of carrying through sequential O-methylations on the isoquinoline and on the benzyl moiety of several substrates. The tetrahydrobenzylisoquinolines (R)-reticuline (4.2 sec(-1) mm(-1)), (S)-reticuline (4.5 sec(-1) mm(-1)), (R)-protosinomenine (1.7 sec(-1) mm(-1)), and (R,S)-isoorientaline (1.4 sec(-1) mm(-1)) as well as guaiacol (5.9 sec(-1) mm(-1)) and isovanillic acid (1.2 sec(-1) mm(-1)) are O-methylated by the enzyme with the ratio kcat/K m shown in parentheses. A P. somniferum cDNA encoding (R,S)-norcoclaurine 6-O-methyltransferase was similarly isolated and characterized. This enzyme was less permissive, methylating only (R,S)-norcoclaurine (7.4 sec(-1) mm(-1)), (R)-norprotosinomenine (4.1 sec(-1) mm(-1)), (S)-norprotosinomenine (4.0 sec(-1) mm(-1)) and (R,S)-isoorientaline (1.0 sec(-1) mm(-1)). A phylogenetic comparison of the amino acid sequences of these O-methyltransferases to those from 28 other plant species suggests that these enzymes group more closely to isoquinoline biosynthetic O-methyltransferases from Coptis japonica than to those from Thalictrum tuberosum that can O-methylate both alkaloid and phenylpropanoid substrates.

An Arabidopsis thaliana gene for methylsalicylate biosynthesis, identified by a biochemical genomics approach, has a role in defense

Emission of methylsalicylate (MeSA), and occasionally of methylbenzoate (MeBA), from Arabidopsis thaliana leaves was detected following the application of some forms of both biotic and abiotic stresses to the plant. Maximal emission of MeSA was observed following alamethicin treatment of leaves. A gene (AtBSMT1) encoding a protein with both benzoic acid (BA) and salicylic acid (SA) carboxyl methyltransferase activities was identified using a biochemical genomics approach. Its ortholog (AlBSMT1) in A. lyrata, a close relative of A. thaliana, was also isolated. The AtBSMT1 protein utilizes SA more efficiently than BA, whereas AlBSMT1 catalyzes the methylation of SA less effectively than that of BA. The AtBSMT1 and AlBSMT1 genes showed expression in leaves under normal growth conditions and were more highly expressed in the flowers. In A. thaliana leaves, the expression of AtBSMT1 was induced by alamethicin, Plutella xylostella herbivory, uprooting, physical wounding, and methyl jasmonate. SA was not an effective inducer. Using a beta-glucuronidase (GUS) reporter approach, the promoter activity of AtBSMT1 was localized to the sepals of flowers, and also to leaf trichomes and hydathodes. Upon thrip damage to leaves, AtBSMT1 promoter activity was induced specifically around the lesions.

A novel Mg(2+)-dependent O-methyltransferase in the phenylpropanoid metabolism of Mesembryanthemum crystallinum

Upon irradiation with elevated light intensities, the ice plant (Mesembryanthemum crystallinum) accumulates a complex pattern of methylated and glycosylated flavonol conjugates in the upper epidermal layer. Identification of a flavonol methylating activity, partial purification of the enzyme, and sequencing of the corresponding peptide fragments revealed a novel S-adenosyl-l-methionine-dependent O-methyltransferase that was specific for flavonoids and caffeoyl-CoA. Cloning and functional expression of the corresponding cDNA verified that the new methyltransferase is a multifunctional 26.6-kDa Mg(2+)-dependent enzyme, which shows a significant sequence similarity to the cluster of caffeoyl coenzyme A-methylating enzymes. Functional analysis of highly homologous members from chickweed (Stellaria longipes), Arabidopsis thaliana, and tobacco (Nicotiana tabacum) demonstrated that the enzymes from the ice plant, chickweed, and A. thaliana possess a broader substrate specificity toward o-hydroquinone-like structures than previously anticipated for Mg(2+)-dependent O-methyltransferases, and are distinctly different from the tobacco enzyme. Besides caffeoyl-CoA and flavonols, a high specificity was also observed for caffeoylglucose, a compound never before reported to be methylated by any plant O-methyltransferase. Based on phylogenetic analysis of the amino acid sequence and differences in acceptor specificities among both animal and plant O-methyltransferases, we propose that the enzymes from the Centrospermae, along with the predicted gene product from A. thaliana, form a novel subclass within the caffeoyl coenzyme A-dependent O-methyltransferases, with potential divergent functions not restricted to lignin monomer biosynthesis.

New genes in alkaloid metabolism and transport

The biosynthetic pathway of plant alkaloids is composed of several distinct enzymes of varying substrate specificities. Homology-based cloning of candidate genes and their subsequent functional testing in heterologous expression systems are accelerating the pace at which the gene catalogues of alkaloid biosynthesis are expanding. Availability of diverse genes involved in the biosynthesis, catabolism, transport, and regulation of pharmaceutically important alkaloids should steadily advance our molecular understanding of alkaloid biology and will enable us to devise more rational strategies for metabolic engineering.

Metabolome diversity: too few genes, too many metabolites?

The multitude of metabolites found in living organisms and the calculated, unexpected small number of genes identified during genome sequencing projects discomfit biologists. Several processes on the transcription and translation level lead to the formation of isoenzymes and can therefore explain at least parts of this surprising result. However, poor enzyme specificity may also contribute to metabolome diversity. In former studies, when enzymes were isolated from natural sources, impure protein preparations were hold responsible for broad enzyme specificity. Nowadays, highly purified enzymes are available by molecular biological methods such as heterologous expression in host organisms and they can be thoroughly analyzed. During biochemical analysis of heterologously expressed enzymes poor specificity was observed for enzymes involved in fruit ripening, e.g. in flavour and color formation. Surprisingly broad specificity was shown for the reactants in the case of alcohol acyl-CoA transferase, O-methyltransferase, glucosyltransferase, P450 monooxygenases as well as polyketide synthases and for the product in the case of monoterpene synthases. Literature data confirm the assumption of limited specificity for enzymes involved in metabolism and bioformation of secondary metabolites. It is concluded that metabolome diversity is caused by low enzyme specificity but availability of suitable substrates due to compartmentation has also taken into account.

cDNA cloning and biochemical characterization of S-adenosyl-L-methionine: 2,7,4'-trihydroxyisoflavanone 4'-O-methyltransferase, a critical enzyme of the legume isoflavonoid phytoalexin pathway

Formononetin (7-hydroxy-4'-methoxyisoflavone, also known as 4'-O-methyldaidzein) is an essential intermediate of ecophysiologically active leguminous isoflavonoids. The biosynthetic pathway to produce 4'-methoxyl of formononetin has been unknown because the methyl transfer from S-adenosyl-L-methionine (SAM) to 4'-hydroxyl of daidzein has never been detected in any plants. A hypothesis that SAM: daidzein 7-O-methyltransferase (D7OMT), an enzyme with a different regiospecificity, is involved in formononetin biosynthesis through its intracellular compartmentation with other enzymes recently prevails, but no direct evidence has been presented. We proposed a new scheme of formononetin biosynthesis involving 2,7,4'-trihydroxyisoflavanone as the methyl acceptor and subsequent dehydration. We now cloned a cDNA encoding SAM: 2,7,4'-trihydroxyisoflavanone 4'-O-methyltransferase (HI4'OMT) through the screening of functionally expressed Glycyrrhiza echinata (Fabaceae) cDNAs. The reaction product, 2,7-dihydroxy-4'-methoxyisoflavanone, was unambiguously identified. Recombinant G. echinata D7OMT did not show HI4'OMT activity, and G. echinata HI4'OMT protein free from D7OMT was partially purified. HI4'OMT is thus concluded to be distinct from D7OMT, and their distant phylogenetic relationship was further presented. HI4'OMT may be functionally identical to (+)-6a-hydroxymaackiain 3-OMT of pea. Homologous cDNAs were found in several legumes, and the catalytic function of the Lotus japonicus HI4'OMT was verified, indicating that HI4'OMT is the enzyme of formononetin biosynthesis in general legumes.

Novel S-adenosyl-L-methionine:salicylic acid carboxyl methyltransferase, an enzyme responsible for biosynthesis of methyl salicylate and methyl benzoate, is not involved in floral scent production in snapdragon flowers

Using a functional genomic approach we have isolated and characterized a cDNA that encodes a salicylic acid carboxyl methyltransferase (SAMT) from Antirrhinum majus. The sequence of the protein encoded by SAMT has higher amino acid identity to Clarkia breweri SAMT than to snapdragon benzoic acid carboxyl methyltransferase (BAMT) (55 and 40% amino acid identity, respectively). Escherichia coli-expressed SAMT protein catalyzes the formation of the volatile ester methyl salicylate from salicylic acid with a K(m) value of 83 microM. It can also methylate benzoic acid to form methyl benzoate, but its K(m) value for benzoic acid is 1.72 mM. Snapdragon flowers do not emit methyl salicylate. The potential involvement of SAMT in production and emission of methyl benzoate in snapdragon flowers was analyzed by RNA gel blot analysis. SAMT mRNA was not detected in floral tissues by RNA blot hybridization, but low levels of SAMT gene expression were detected after real-time RT-PCR in the presence of SAMT-specific primers, indicating that this gene does not contribute significantly, if at all, in methyl benzoate production and emission in snapdragon flowers. Expression of SAMT in petal tissue was found to be induced by salicylic and jasmonic acid treatments.

Isolation, cloning and expression of a multifunctional O-methyltransferase capable of forming 2,5-dimethyl-4-methoxy-3(2H)-furanone, one of the key aroma compounds in strawberry fruits

Strawberry fruits contain an uncommon group of key aroma compounds with a 2,5-dimethyl-3(2H)-furanone structure. Here, we report on the methylation of 2,5-dimethyl-4-hydroxy-3(2H)-furanone (DMHF) to 2,5-dimethyl-4-methoxy-3(2H)-furanone (DMMF) by a S-adenosyl-L-methionine dependent O-methyltransferase, the cloning of the corresponding cDNA and characterization of the encoded protein. Northern-hybridization indicated that the Strawberry-OMT specific transcripts accumulated during ripening in strawberry fruits and were absent in root, petiole, leaf and flower. The protein was functionally expressed in E. coli and exhibited a substrate specificity for catechol, caffeic acid, protocatechuic aldehyde, caffeoyl CoA and DMHF. A common structural feature of the accepted substrates was a o-diphenolic structure also present in DMHF in its dienolic tautomer. FaOMT is active as a homodimer and the native enzyme shows optimum activity at pH 8.5 and 37 degrees C. It does not require a cofactor for enzymatic activity. Due to the expression pattern of FaOMT and the enzymatic activity in the different stages of fruit ripening we suppose that FaOMT is involved in lignification of the achenes and the vascular bundles in the expanding fruit. In addition, it is concluded that the Strawberry-OMT plays an important role in the biosynthesis of strawberry volatiles such as vanillin and DMMF.

Aroma biosynthesis in strawberry: s-adenosylmethionine:furaneol o-methyltransferase activity in ripening fruits

Among the most important volatile compounds in the aroma of strawberries are 2,5-dimethyl-4-hydroxy-3(2H)-furanone (Furaneol) and its methoxy derivative (methoxyfuraneol, mesifuran). Three strawberry varieties, Malach, Tamar, and Yael, were assessed for total volatiles, Furaneol, and methoxyfuraneol. The content of these compounds sharply increased during fruit ripening, with maximum values at the ripe stage. An enzymatic activity that transfers a methyl group from S-adenosylmethionine (SAM) to Furaneol sharply increases during ripening of strawberry fruits. The in vitro generated methoxyfuraneol was identified by radio-TLC and GC-MS. The partially purified enzyme had a native molecular mass of approximately 80 kDa, with optimum activity at pH 8.5 and 37 degrees C. A high apparent K(m) of 5 mM was calculated for Furaneol, whereas this enzyme preparation apparently accepted as substrates other o-dihydroxyphenol derivatives (such as catechol, caffeic acid, and protocatechuic aldehyde) with much higher affinities (K(m) approximately 105, 130, and 20 microM, respectively). A K(m) for SAM was found to be approximately 5 microM, regardless of the acceptor used. Substrates that contained a phenolic group with only one OH group, such as p-coumaric and trans-ferulic acid, as well as trans-anol and coniferyl alcohol, were apparently not accepted by this activity. It is suggested that Furaneol methylation is mediated by an O-methyltransferase activity and that this activity increases during fruit ripening.

Molecular cloning and characterization of coclaurine N-methyltransferase from cultured cells of Coptis japonica

S-adenosyl-L-methionine:coclaurine N-methyltransferase (CNMT) converts coclaurine to N-methylcoclaurine in isoquinoline alkaloid biosynthesis. The N-terminal amino acid sequence of Coptis CNMT was used to amplify the corresponding cDNA fragment and later to isolate full-length cDNA using 5'- and 3'-rapid amplification of cDNA ends (RACE). The nucleotide sequence and predicted amino acid sequence showed that the cDNA encoded 358 amino acids, which contained a putative S-adenosyl-L-methionine binding domain and showed relatively high homology to tomato phosphoethanolamine-N-methyltransferase. A recombinant protein was expressed in Escherichia coli, and its CNMT activity was confirmed. Recombinant CNMT was purified to homogeneity, and enzymological characterization confirmed that Coptis CNMT has quite broad substrate specificity, i.e. not only for 6-O-methylnorlaudanosoline and norreticuline but also for 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline. The evolution of N-methyltransferases in secondary metabolism is discussed based on sequence similarity.

Predicting the substrates of cloned plant O-methyltransferases

Plant O-methyltransferases (OMTs) have important roles in secondary metabolite biosynthesis. Sequencing projects and homology-based cloning strategies yield sequences for proteins with similarities to known OMTs, but the identification of the physiological substrates is not trivial. We investigated with a cDNA cloned from Catharanthus roseus the possibilities for predicting the substrates of OMTs, using the information from previous work and two newly identified motifs that were based on information from the crystal structures of two plant OMTs. The results, confirmed by functional analysis of the recombinant protein, indicated that a careful analysis of the deduced protein sequence can provide clues for predicting the substrates of cloned OMTs.

Chemical syntheses of caffeoyl and 5-OH coniferyl aldehydes and alcohols and determination of lignin O-methyltransferase activities in dicot and monocot species

To investigate the substrate preferences of O-methyltransferases in the monolignol biosynthetic pathways, caffeoyl and 5-hydroxy coniferyl aldehydes were synthesized by a new procedure involving a Wittig reaction with the corresponding hydroxybenzaldehydes. The same procedure can also be used to synthesize caffeoyl and 5-hydroxyconiferyl alcohols. Relative O-methyltransferase activities against these substrates were determined using crude extracts and recombinant caffeic acid O-methyltransferase from alfalfa (Medicago sativa), and crude extracts from the model legume Medicago truncatula, tobacco, wheat and tall fescue. Extracts from all these species catalyzed methylation of the various monolignol aldehydes and alcohols more effectively than the corresponding hydroxycinnamic acids.

Identification of the enzymatic active site of tobacco caffeoyl-coenzyme A O-methyltransferase by site-directed mutagenesis

Animal catechol O-methyltransferases and plant caffeoyl-coenzyme A O-methyltransferases share about 20% sequence identity and display common structural features. The crystallographic structure of rat liver catechol O-methyltransferase was used as a template to construct a homology model for tobacco caffeoyl-coenzyme A O-methyltransferase. Integrating substrate specificity data, the three-dimensional model identified several amino acid residues putatively involved in substrate binding. These residues were mutated by a polymerase chain reaction method and wild-type and mutant enzymes were each expressed in Escherichia coli and purified. Substitution of Arg-220 with Thr resulted in the total loss of enzyme activity, thus indicating that Arg-220 is involved in the electrostatic interaction with the coenzyme A moiety of the substrate. Changes of Asp-58 to Ala and Gln-61 to Ser were shown to increase K(m) values for caffeoyl coenzyme A and to decrease catalytic activity. Deletions of two amino acid sequences specific for plant enzymes abolished activity. The secondary structures of the mutants, as measured by circular dichroism, were essentially unperturbed as compared with the wild type. Similar changes in circular dichroism spectra were observed after addition of caffeoyl coenzyme A to the wild-type enzyme and the substitution mutants but not in the case of deletion mutants, thus revealing the importance of these sequences in substrate-enzyme interactions.

Identification of 4-coumarate:coenzyme A ligase (4CL) substrate recognition domains

4-coumarate:CoA ligase (4CL), the last enzyme of the general phenylpropanoid pathway, provides precursors for the biosynthesis of a large variety of plant natural products. 4 CL catalyzes the formation of CoA thiol esters of 4-coumarate and other hydroxycinnamates in a two step reaction involving the formation of an adenylate intermediate. 4 CL shares conserved peptide motifs with diverse adenylate-forming enzymes such as firefly luciferases, non-ribosomal peptide synthetases, and acyl:CoA synthetases. Amino acid residues involved in 4 CL catalytic activities have been identified, but domains involved in determining substrate specificity remain unknown. To address this question, we took advantage of the difference in substrate usage between the Arabidopsis thaliana 4 CL isoforms At4CL1 and At4CL2. While both enzymes convert 4-coumarate, only At4CL1 is also capable of converting ferulate. Employing a domain swapping approach, we identified two adjacent domains involved in substrate recognition. Both substrate binding domain I (sbd I) and sbd II of At4CL1 alone were sufficient to confer ferulate utilization ability upon chimeric proteins otherwise consisting of At4CL2 sequences. In contrast, sbd I and sbd II of At4CL2 together were required to abolish ferulate utilization in the context of At4CL1. Sbd I corresponds to a region previously identified as the substrate binding domain of the adenylation subunit of bacterial peptide synthetases, while sbd II centers on a conserved domain of so far unknown function in adenylate-forming enzymes (GEI/LxIxG). At4CL1 and At4CL2 differ in nine amino acids within sbd I and four within sbd II, suggesting that these play roles in substrate recognition.

Molecular characterization of the salutaridinol 7-O-acetyltransferase involved in morphine biosynthesis in opium poppy Papaver somniferum

Salutaridinol 7-O-acetyltransferase (EC ) catalyzes the conversion of the phenanthrene alkaloid salutaridinol to salutaridinol-7-O-acetate, the immediate precursor of thebaine along the morphine biosynthetic pathway. We have isolated a cDNA clone that corresponds to the internal amino acid sequences of the native enzyme purified from a cell suspension culture of opium poppy Papaver somniferum. The recombinant enzyme acetylated the 7-hydroxyl moiety of salutaridinol in the presence of acetyl-CoA. The apparent K(m) value for salutaridinol was determined to be 9 microm and 54 microm for acetyl-CoA. The gene transcript was detected in extracts from Papaver orientale and Papaver bracteatum in addition to P. somniferum. Genomic DNA gel blot analysis indicated that there is likely a single copy of this gene in the P. somniferum genome. The amino acid sequence of salutaridinol 7-O-acetyltransferase is most similar (37% identity) to that of deacetylvindoline acetyltransferase of Catharanthus roseus. Salutaridinol 7-O-acetyltransferase is the second enzyme specific to morphine biosynthesis for which we have isolated a cDNA. Taken together with the other cDNAs cloned encoding norcoclaurine 6-O-methyltransferase, (S)-N-methylcoclaurine 3'-hydroxylase, the cytochrome P-450 reductase, and codeinone reductase, significant progress has been made toward accumulating genes of this pathway to enable the end goal of a biotechnological production of morphinan alkaloids.

The expanding universe of alkaloid biosynthesis

Characterization of many of the major gene families responsible for the generation of central intermediates and for their decoration, together with the development of large genomics and proteomics databases, has revolutionized our capability to identify exotic and interesting natural-product pathways. Over the next few years, these tools will facilitate dramatic advances in our knowledge of the biosynthesis of alkaloids, which will far surpass that which we have learned in the past 50 years. These tools will also be exploited for the rapid characterization of regulatory genes, which control the development of specialized cell factories for alkaloid biosynthesis.

ALKALOID BIOSYNTHESIS IN PLANTS: Biochemistry, Cell Biology, Molecular Regulation, and Metabolic Engineering Applications

Recent advances in the cell, developmental, and molecular biology of alkaloid biosynthesis have heightened our appreciation for the complexity and importance of plant secondary pathways. Several biosynthetic genes involved in the formation of tropane, benzylisoquinoline, and terpenoid indole alkaloids have now been isolated. The early events of signal perception, the pathways of signal transduction, and the function of gene promoters have been studied in relation to the regulation of alkaloid metabolism. Enzymes involved in alkaloid biosynthesis are associated with diverse subcellular compartments including the cytosol, vacuole, tonoplast membrane, endoplasmic reticulum, chloroplast stroma, thylakoid membranes, and perhaps unique "biosynthetic" or transport vesicles. Localization studies have shown that sequential alkaloid biosynthetic enzymes can also occur in distinct cell types, suggesting the intercellular transport of pathway intermediates. Isolated genes have also been used to genetically alter the accumulation of specific alkaloids and other plant secondary metabolites. Metabolic modifications include increased indole alkaloid levels, altered tropane alkaloid accumulation, elevated serotonin synthesis, reduced indole glucosinolate production, redirected shikimate metabolism, and increased cell wall-bound tyramine formation. This review discusses the biochemistry, cell biology, molecular regulation, and metabolic engineering of alkaloid biosynthesis in plants.

ORCAnization of jasmonate-responsive gene expression in alkaloid metabolism

Jasmonic acid is an important plant stress signalling molecule. It induces the biosynthesis of defence proteins and protective secondary metabolites. In alkaloid metabolism, jasmonate acts by coordinate activation of the expression of multiple biosynthesis genes. In terpenoid indole alkaloid metabolism and primary precursor pathways, jasmonate induces gene expression and metabolism via ORCAs, which are members of the AP2/ERF-domain family of plant transcription factors. Other jasmonate-regulated (secondary) metabolic pathways might also be controlled by ORCA-like AP2/ERF-domain transcription factors. If so, such regulators could be used to improve plant fitness or metabolite productivity of plants or cell cultures.

Purification and characterization of coclaurine N-methyltransferase from cultured Coptis japonica cells

S-Adenosyl-L-methionine (SAM): coclaurine N-methyltransferase (CNMT), which catalyzes the transfer of a methyl group from S-adenosyl-L-methionine to the amino group of the tetrahydrobenzylisoquinoline alkaloid coclaurine. was purified 340-fold from Coptis japonica cells in 1% yield to give an almost homogeneous protein. The purified enzyme, which occurred as a homotetramer with a native Mr of 160 kDa (gel-filtration chromatography) and a subunit Mr of 45 kDa (SDS-polyacrylamide gel electrophoresis), had an optimum pH of 7.0 and a pI of 4.2. Whereas (R)-coclaurine was the best substrate for enzyme activity, Coptis CNMT had broad substrate specificity and no stereospecificity CNMT methylated norlaudanosoline, 6,7-dimethoxyl-1,2,3,4-tetrahydroisoquinoline and 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline. The enzyme did not require any metal ion. p-Chloromercuribenzoate and iodoacetamide did not inhibit CNMT activity, but the addition of Co2+, Cu2+ or Mn2+ at 5 mM severely inhibited such activity by 75, 47 and 57%, respectively. The substrate-saturation kinetics of CNMT for norreticuline and SAM were of the typical Michaelis-Menten-type with respective Km values of 0.38 and 0.65 mM.

Combinatorial biochemistry in plants: the case of O-methyltransferases

Combinatorial chemistry is common place today in chemical synthesis. Virtually thousands of derivatives of a molecule can be achieved by automated systems. The use of biological systems to exploit combinatorial chemistry (combinatorial biochemistry) now has multiple examples in the polyketide field. The modular functional domain structure of polyketide synthases have been recombined through genetic engineering into unnatural constellations in heterologous hosts in order to produce polyketide structures not yet discovered in nature. We present herein an example for a potential type of combinatorial biochemistry in alkaloidal systems using various combinations of Thalictrum tuberosum (meadow rue) O-methyltransferase subunits that result in heterodimeric enzymes with substrate specificities that differ from those of the homodimeric native enzymes.

Genetics and biochemistry of secondary metabolites in plants: an evolutionary perspective

The evolution of new genes to make novel secondary compounds in plants is an ongoing process and might account for most of the differences in gene function among plant genomes. Although there are many substrates and products in plant secondary metabolism, there are only a few types of reactions. Repeated evolution is a special form of convergent evolution in which new enzymes with the same function evolve independently in separate plant lineages from a shared pool of related enzymes with similar but not identical functions. This appears to be common in secondary metabolism and might confound the assignment of gene function based on sequence information alone.