Reconstructing the origin and evolution of land plants and their algal relatives is a fundamental problem in plant phylogenetics, and is essential for understanding how critical adaptations arose, including the embryo, vascular tissue, seeds, and flowers. Despite advances in molecular systematics, some hypotheses of relationships remain weakly resolved. Inferring deep phylogenies with bouts of rapid diversification can be problematic; however, genome-scale data should significantly increase the number of informative characters for analyses. Recent phylogenomic reconstructions focused on the major divergences of plants have resulted in promising but inconsistent results. One limitation is sparse taxon sampling, likely resulting from the difficulty and cost of data generation. To address this limitation, transcriptome data for 92 streptophyte taxa were generated and analyzed along with 11 published plant genome sequences. Phylogenetic reconstructions were conducted using up to 852 nuclear genes and 1,701,170 aligned sites. Sixty-nine analyses were performed to test the robustness of phylogenetic inferences to permutations of the data matrix or to phylogenetic method, including supermatrix, supertree, and coalescent-based approaches, maximum-likelihood and Bayesian methods, partitioned and unpartitioned analyses, and amino acid versus DNA alignments. Among other results, we find robust support for a sister-group relationship between land plants and one group of streptophyte green algae, the Zygnematophyceae. Strong and robust support for a clade comprising liverworts and mosses is inconsistent with a widely accepted view of early land plant evolution, and suggests that phylogenetic hypotheses used to understand the evolution of fundamental plant traits should be reevaluated.
Unresolved questions about evolution of the large and diverse legume family include the timing of polyploidy (whole-genome duplication; WGDs) relative to the origin of the major lineages within the Fabaceae and to the origin of symbiotic nitrogen fixation. Previous work has established that a WGD affects most lineages in the Papilionoideae and occurred sometime after the divergence of the papilionoid and mimosoid clades, but the exact timing has been unknown. The history of WGD has also not been established for legume lineages outside the Papilionoideae. We investigated the presence and timing of WGDs in the legumes by querying thousands of phylogenetic trees constructed from transcriptome and genome data from 20 diverse legumes and 17 outgroup species. The timing of duplications in the gene trees indicates that the papilionoid WGD occurred in the common ancestor of all papilionoids. The earliest diverging lineages of the Papilionoideae include both nodulating taxa, such as the genistoids (e.g., lupin), dalbergioids (e.g., peanut), phaseoloids (e.g., beans), and galegoids (=Hologalegina, e.g., clovers), and clades with nonnodulating taxa including Xanthocercis and Cladrastis (evaluated in this study). We also found evidence for several independent WGDs near the base of other major legume lineages, including the Mimosoideae-Cassiinae-Caesalpinieae (MCC), Detarieae, and Cercideae clades. Nodulation is found in the MCC and papilionoid clades, both of which experienced ancestral WGDs. However, there are numerous nonnodulating lineages in both clades, making it unclear whether the phylogenetic distribution of nodulation is due to independent gains or a single origin followed by multiple losses.
The plant hormone auxin is a conserved regulator of development which has been implicated in the generation of morphological novelty. PIN-FORMED1 (PIN) auxin efflux carriers are central to auxin function by regulating its distribution. PIN family members have divergent structures and cellular localizations, but the origin and evolutionary significance of this variation is unresolved. To characterize PIN family evolution, we have undertaken phylogenetic and structural analyses with a massive increase in taxon sampling over previous studies. Our phylogeny shows that following the divergence of the bryophyte and lycophyte lineages, two deep duplication events gave rise to three distinct lineages of PIN proteins in euphyllophytes. Subsequent independent radiations within each of these lineages were taxonomically asymmetric, giving rise to at least 21 clades of PIN proteins, of which 15 are revealed here for the first time. Although most PIN protein clades share a conserved canonical structure with a modular central loop domain, a small number of noncanonical clades dispersed across the phylogeny have highly divergent protein structure. We propose that PIN proteins underwent sub- and neofunctionalization with substantial modification to protein structure throughout plant evolution. Our results have important implications for plant evolution as they suggest that structurally divergent PIN proteins that arose in paralogous radiations contributed to the convergent evolution of organ systems in different land plant lineages.
Galanthamine is an Amaryllidaceae alkaloid used to treat the symptoms of Alzheimer's disease. This compound is primarily isolated from daffodil (Narcissus spp.), snowdrop (Galanthus spp.), and summer snowflake (Leucojum aestivum). Despite its importance as a medicine, no genes involved in the biosynthetic pathway of galanthamine have been identified. This absence of genetic information on biosynthetic pathways is a limiting factor in the development of synthetic biology platforms for many important botanical medicines. The paucity of information is largely due to the limitations of traditional methods for finding biochemical pathway enzymes and genes in non-model organisms. A new bioinformatic approach using several recent technological improvements was applied to search for genes in the proposed galanthamine biosynthetic pathway, first targeting methyltransferases due to strong signature amino acid sequences in the proteins. Using Illumina sequencing, a de novo transcriptome assembly was constructed for daffodil. BLAST was used to identify sequences that contain signatures for plant O-methyltransferases in this transcriptome. The program HAYSTACK was then used to identify methyltransferases that fit a model for galanthamine biosynthesis in leaf, bulb and inflorescence tissues. One candidate gene for the methylation of norbelladine to 4'-O-methylnorbelladine in the proposed galanthamine biosynthetic pathway was identified. This methyltransferase cDNA was expressed in E. coli and the protein purified by affinity chromatography. The resulting protein was found to be a norbelladine 4'-O-methyltransferase (NpN4OMT) of the proposed galanthamine biosynthetic pathway.
Noni has been used in traditional medicine and as food for thousands of years. While the fruits serve as food and internal medicine, leaves were traditionally used only topically. In recent years, concern regarding the possible content of anthraquinones in noni has led to scrutiny by the European Food Safety Authority. Little research existed on the content of anthraquinones in different noni preparations, with no information about the potential effect of harvest and preparation methods. Our research focused on lucidin, alizarin, and rubiadin, the most important anthraquinones from a health perspective. We found that the production process (fermentation/juice production versus drying/lyophilization) has no effect on the anthraquinone content. The source product, however, does have implications: noni fruit puree from which seeds had been removed as well as consumer products produced from such puree had no detectable amounts of any anthraquinones. Products that did contain seed or leaf material in all cases did contain partly significant amounts of anthraquinones. To alleviate safety concerns, we suggest that noni products, whether fermented or unfermented juice or powder, should be derived only from fully ripe noni fruits, and that any seed material needs to be removed during the production process.
We report the development and testing of an accurate mass-time (AMT) tag approach for the LC/MS-based identification of plant natural products (PNPs) in complex extracts. An AMT tag library was developed for approximately 500 PNPs with diverse chemical structures, detected in electrospray and atmospheric pressure chemical ionization modes (both positive and negative polarities). In addition, to enable peak annotations with high confidence, MS/MS spectra were acquired with three different fragmentation energies. The LC/MS and MS/MS data sets were integrated into online spectral search tools and repositories (Spektraris and MassBank), thus allowing users to interrogate their own data sets for the potential presence of PNPs. The utility of the AMT tag library approach is demonstrated by the detection and annotation of active principles in 27 different medicinal plant species with diverse chemical constituents.
Berberine, palmatine and dehydrocoreximine are end products of protoberberine biosynthesis. These quaternary protoberberines are elicitor inducible and, like other phytoalexins, are highly oxidized. The oxidative potential of these compounds is derived from a diverse array of biosynthetic steps involving hydroxylation, intra-molecular C-C coupling, methylenedioxy bridge formation and a dehydrogenation reaction as the final step in the biosynthesis. For the berberine biosynthetic pathway, the identification of the dehydrogenase gene is the last remaining uncharacterized step in the elucidation of the biosynthesis at the gene level. An enzyme able to catalyze these reactions, (S)-tetrahydroprotoberberine oxidase (STOX, EC 188.8.131.52), was originally purified in the 1980s from suspension cells of Berberis wilsoniae and identified as a flavoprotein (Amann et al. 1984). We report enzymatic activity from recombinant STOX expressed in Spodoptera frugiperda Sf9 insect cells. The coding sequence was derived successively from peptide sequences of purified STOX protein. Furthermore, a recombinant oxidase with protoberberine dehydrogenase activity was obtained from a cDNA library of Argemone mexicana, a traditional medicinal plant that contains protoberberine alkaloids. The relationship of the two enzymes is discussed regarding their enzymatic activity, phylogeny and the alkaloid occurrence in the plants. Potential substrate binding and STOX-specific amino acid residues were identified based on sequence analysis and homology modeling.
The opium poppy (Papaver somniferum L.) is one of the oldest known medicinal plants. In the biosynthetic pathway for morphine and codeine, salutaridine is reduced to salutaridinol by salutaridine reductase (SalR; EC 184.108.40.206) using NADPH as coenzyme. Here, we report the atomic structure of SalR to a resolution of ?1.9 ? in the presence of NADPH. The core structure is highly homologous to other members of the short chain dehydrogenase/reductase family. The major difference is that the nicotinamide moiety and the substrate-binding pocket are covered by a loop (residues 265-279), on top of which lies a large "flap"-like domain (residues 105-140). This configuration appears to be a combination of the two common structural themes found in other members of the short chain dehydrogenase/reductase family. Previous modeling studies suggested that substrate inhibition is due to mutually exclusive productive and nonproductive modes of substrate binding in the active site. This model was tested via site-directed mutagenesis, and a number of these mutations abrogated substrate inhibition. However, the atomic structure of SalR shows that these mutated residues are instead distributed over a wide area of the enzyme, and many are not in the active site. To explain how residues distal to the active site might affect catalysis, a model is presented whereby SalR may undergo significant conformational changes during catalytic turnover.
The biosynthesis of morphine, a stereochemically complex alkaloid, has been shown to occur in plants and animals. A search in the human genome for methyltransferases capable of catalyzing the N-methylation of benzylisoquinoline alkaloids, as biosynthetic precursors of morphine, yielded two enzymes, PNMT (EC 220.127.116.11) and NMT (EC 18.104.22.168). Introduction of an N-terminal poly-histidine tag enabled purification of both proteins by immobilized metal affinity chromatography. Recombinant PNMT and NMT were characterized for their catalytic activity towards four benzylisoquinolines: tetrahydropapaveroline (THP), 6-O-methyl-THP, 4-O-methyl-THP and norreticuline. Human PNMT accepted none of the offered alkaloids and was only active with its established substrate, phenylethanolamine. The second enzyme, human NMT, converted all four benzylisoquinolines, however, with a strict preference for (R)-configured morphine precursors. Determination of kinetic parameters of NMT for the four (R)-configured benzylisoquinoline alkaloids by LC-MS/MS revealed (R)-norreticuline to be the best substrate with an even higher catalytic activity as compared to the previously reported natural substrate tryptamine. In addition, isolation of the morphine precursor salutaridine from urine of mice injected (i.p.) with (R)-THP provides new evidence that the initial steps of morphine biosynthesis in mammals occur stereochemically and sequentially differently than in plants and suggests an involvement of the herein characterized (R)-specific NMT.
Formation of the methylenedioxy bridge is an integral step in the biosynthesis of benzo[c]phenanthridine and protoberberine alkaloids in the Papaveraceae family of plants. This reaction in plants is catalyzed by cytochrome P450-dependent enzymes. Two cDNAs that encode cytochrome P450 enzymes belonging to the CYP719 family were identified upon interrogation of an EST dataset prepared from 2-month-old plantlets of the Mexican prickly poppy Argemone mexicana that accumulated the benzo[c]phenanthridine alkaloid sanguinarine and the protoberberine alkaloid berberine. CYP719A13 and CYP719A14 are 58% identical to each other and 77% and 60% identical, respectively, to stylopine synthase CYP719A2 of benzo[c]phenanthridine biosynthesis in Eschscholzia californica. Functional heterologous expression of CYP719A14 and CYP719A13 in Spodoptera frugiperda Sf9 cells produced recombinant enzymes that catalyzed the formation of the methylenedioxy bridge of (S)-cheilanthifoline from (S)-scoulerine and of (S)-stylopine from (S)-cheilanthifoline, respectively. Twenty-seven potential substrates were tested with each enzyme. Whereas CYP719A14 transformed only (S)-scoulerine to (S)-cheilanthifoline (K(m) 1.9±0.3; k(cat)/K(m) 1.7), CYP719A13 converted (S)-tetrahydrocolumbamine to (S)-canadine (K(m) 2.7±1.3; k(cat)/K(m) 12.8), (S)-cheilanthifoline to (S)-stylopine (K(m) 5.2±3.0; k(cat)/K(m) 2.6) and (S)-scoulerine to (S)-nandinine (K(m) 8.1±1.9; k(cat)/K(m) 0.7). These results indicate that although CYP719A14 participates in only sanguinarine biosynthesis, CYP719A13 can be involved in both sanguinarine and berberine formation in A. mexicana.
Ipecac alkaloids produced in the medicinal plant Psychotria ipecacuanha such as emetine and cephaeline possess a monoterpenoid-tetrahydroisoquinoline skeleton, which is formed by condensation of dopamine and secologanin. The condensation products are deglucosylated, and the resulting aglycon is further processed to protoemetine, which is condensed with the second molecule of dopamine, followed by conversion into cephaeline and emetine. Although four hydroxy groups derived from two molecules of dopamine need to be methylated to form emetine, the order of O-methylation reactions had been veiled. We recently identified three Ipecac alkaloid O-methyltransferases (IpeOMT1-IpeOMT3) that are sufficient for catalyzing O-methylations of all four hydroxy groups. Detailed characterization of their catalytic properties with integration of that of the previously identified Ipecac alkaloid ?-glucosidase (IpeGlu1) revealed a large portion of the biosynthetic pathway of Ipecac alkaloids. The results provide proof-of-concept to the significance and the usefulness of the biosynthetic pathway strategy by EST analysis coupled with recombinant enzyme characterization. At the same time, however, the results raised an intriguing question about the subcellular network between the biosynthetic enzymes and intermediates. Here, we provide additional discussion about this point, and indicate what remains to be elucidated.
It has been firmly established that humans excrete a small but steady amount of the isoquinoline alkaloid morphine in their urine. It is unclear whether it is of dietary or endogenous origin. There is no doubt that a simple isoquinoline alkaloid, tetrahydropapaveroline (THP), is found in human and rodent brain as well as in human urine. This suggests a potential biogenetic relationship between both alkaloids. Unlabeled THP or [1,3,4-D(3)]-THP was injected intraperitoneally into mice and the urine was analyzed. This potential precursor was extensively metabolized (96%). Among the metabolites found was the phenol-coupled product salutaridine, the known morphine precursor in the opium poppy plant. Synthetic [7D]-salutaridinol, the biosynthetic reduction product of salutaridine, injected intraperitoneally into live animals led to the formation of [7D]-thebaine, which was excreted in urine. [N-CD(3)]-thebaine was also administered and yielded [N-CD(3)]-morphine and the congeners [N-CD(3)]-codeine and [N-CD(3)]-oripavine in urine. These results show for the first time that live animals have the biosynthetic capability to convert a normal constituent of rodents, THP, to morphine. Morphine and its precursors are normally not found in tissues or organs, presumably due to metabolic breakdown. Hence, only that portion of the isoquinoline alkaloids excreted in urine unmetabolized can be detected. Analysis of urine by high resolution-mass spectrometry proved to be a powerful method for tracking endogenous morphine and its biosynthetic precursors.
Papaverine is one of the earliest opium alkaloids for which a biosynthetic hypothesis was developed on theoretical grounds. Norlaudanosoline (=tetrahydropapaveroline) was claimed as the immediate precursor alkaloid for a multitude of nitrogen containing plant metabolites. This tetrahydroxylated compound was proposed to be fully O-methylated. The resulting tetrahydropapaverine should then aromatize to papaverine. In view of experimental data, this pathway has to be revised. Precursor administration to 8-day-old seedlings of Papaver followed by direct examination of the metabolic fate of the stable-isotope-labeled precursors in the total plant extract, without further purification of the metabolites, led to elucidation of the papaverine pathway in vivo. The central and earliest benzylisoquinoline alkaloid is not the tetraoxygenated norlaudanosoline, but instead the trihydroxylated norcoclaurine that is further converted into (S)-reticuline, the established precursor for poppy alkaloids. The papaverine pathway is opened by the methylation of (S)-reticuline to generate (S)-laudanine. A second methylation at the 3 position of laudanine leads to laudanosine, both known alkaloids from the opium poppy. Subsequent N-demethylation of laudanosine yields the known precursor of papaverine: tetrahydropapaverine. Inspection of the subsequent aromatization reaction established the presence of an intermediate, 1,2-dihydropapaverine, which has been characterized. The final step to papaverine is dehydrogenation of the 1,2-bond, yielding the target compound papaverine. We conclusively show herein that the previously claimed norreticuline does not play a role in the biosynthesis of papaverine.
The opium poppy Papaver somniferum is the source of the narcotic analgesics morphine and codeine. Salutaridine reductase (SalR; EC 22.214.171.124) reduces the C-7 keto group of salutaridine to the C-7 (S)-hydroxyl group of salutaridinol in the biosynthetic pathway that leads to morphine in the opium poppy plant. P. somniferum SalR was overproduced in Escherichia coli and purified using cobalt-affinity and size-exclusion chromatography. Hexagonal crystals belonging to space group P6(4)22 or P6(2)22 were obtained using ammonium sulfate as precipitant and diffracted to a resolution of 1.9 A.
The medicinal plant Psychotria ipecacuanha produces ipecac alkaloids, a series of monoterpenoid-isoquinoline alkaloids such as emetine and cephaeline, whose biosynthesis derives from condensation of dopamine and secologanin. Here, we identified three cDNAs, IpeOMT1-IpeOMT3, encoding ipecac alkaloid O-methyltransferases (OMTs) from P. ipecacuanha. They were coordinately transcribed with the recently identified ipecac alkaloid beta-glucosidase Ipeglu1. Their amino acid sequences were closely related to each other and rather to the flavonoid OMTs than to the OMTs involved in benzylisoquinoline alkaloid biosynthesis. Characterization of the recombinant IpeOMT enzymes with integration of the enzymatic properties of the IpeGlu1 revealed that emetine biosynthesis branches off from N-deacetylisoipecoside through its 6-O-methylation by IpeOMT1, with a minor contribution by IpeOMT2, followed by deglucosylation by IpeGlu1. The 7-hydroxy group of the isoquinoline skeleton of the aglycon is methylated by IpeOMT3 prior to the formation of protoemetine that is condensed with a second dopamine molecule, followed by sequential O-methylations by IpeOMT2 and IpeOMT1 to form cephaeline and emetine, respectively. In addition to this central pathway of ipecac alkaloid biosynthesis, formation of all methyl derivatives of ipecac alkaloids in P. ipecacuanha could be explained by the enzymatic activities of IpeOMT1-IpeOMT3, indicating that they are sufficient for all O-methylation reactions of ipecac alkaloid biosynthesis.
Morphine is a powerful analgesic natural product produced by the opium poppy Papaver somniferum. Although formal syntheses of this alkaloid have been reported, the morphine molecule contains five stereocenters and a C-C phenol linkage that to date render a total synthesis of morphine commercially unfeasible. The C-C phenol-coupling reaction along the biosynthetic pathway to morphine in opium poppy is catalyzed by the cytochrome P450-dependent oxygenase salutaridine synthase. We report herein on the identification of salutaridine synthase as a member of the CYP719 family of cytochromes P450 during a screen of recombinant cytochromes P450 of opium poppy functionally expressed in Spodoptera frugiperda Sf9 cells. Recombinant CYP719B1 is a highly stereo- and regioselective enzyme; of forty-one compounds tested as potential substrates, only (R)-reticuline and (R)-norreticuline resulted in formation of a product (salutaridine and norsalutaridine, respectively). To date, CYP719s have been characterized catalyzing only the formation of a methylenedioxy bridge in berberine biosynthesis (canadine synthase, CYP719A1) and in benzo[c]phenanthridine biosynthesis (stylopine synthase, CYP719A14). Previously identified phenol-coupling enzymes of plant alkaloid biosynthesis belong only to the CYP80 family of cytochromes. CYP719B1 therefore is the prototype for a new family of plant cytochromes P450 that catalyze formation of a phenol-couple.
A cytochrome P450 (P450) enzyme in porcine liver that catalyzed the phenol-coupling reaction of the substrate (R)-reticuline to salutaridine was previously purified to homogeneity (Amann, T., Roos, P. H., Huh, H., and Zenk, M. H. (1995) Heterocycles 40, 425-440). This reaction was found to be catalyzed by human P450s 2D6 and 3A4 in the presence of (R)-reticuline and NADPH to yield not a single product, but rather (-)-isoboldine, (-)-corytuberine, (+)-pallidine, and salutaridine, the para-ortho coupled established precursor of morphine in the poppy plant and most likely also in mammals. (S)-Reticuline, a substrate of both P450 enzymes, yielded the phenol-coupled alkaloids (+)-isoboldine, (+)-corytuberine, (-)-pallidine, and sinoacutine; none of these serve as a morphine precursor. Catalytic efficiencies were similar for P450 2D6 and P450 3A4 in the presence of cytochrome b(5) with (R)-reticuline as substrate. The mechanism of phenol coupling is not yet established; however, we favor a single cycle of iron oxidation to yield salutaridine and the three other alkaloids from (R)-reticuline. The total yield of salutaridine formed can supply the 10 nm concentration of morphine found in human neuroblastoma cell cultures and in brain tissues of mice.
Berberine bridge enzyme catalyzes the stereospecific oxidation and carbon-carbon bond formation of (S)-reticuline to (S)-scoulerine. In addition to this type of reactivity the enzyme can further oxidize (S)-scoulerine to the deeply red protoberberine alkaloid dehydroscoulerine albeit with a much lower rate of conversion. In the course of the four electron oxidation, no dihydroprotoberberine species intermediate was detectable suggesting that the second oxidation step leading to aromatization proceeds at a much faster rate. Performing the reaction in the presence of oxygen and under anoxic conditions did not affect the kinetics of the overall reaction suggesting no strict requirement for oxygen in the oxidation of the unstable dihydroprotoberberine intermediate. In addition to the kinetic characterization of this reaction we also present a structure of the enzyme in complex with the fully oxidized product. Combined with information available for the binding modes of (S)-reticuline and (S)-scoulerine a possible mechanism for the additional oxidation is presented. This is compared to previous reports of enzymes ((S)-tetrahydroprotoberberine oxidase and canadine oxidase) showing a similar type of reactivity in different plant species.
Papaver somniferum L. was transformed with an RNAi construct designed to reduce transcript levels of the gene encoding the morphine biosynthetic enzyme, salutaridinol 7-O-acetyltransferase (SalAT). RNA interference of salAT led to accumulation of the intermediate compounds, salutaridine and salutaridinol, in a ratio ranging from 2:1 to 56:1. Along the morphine biosynthetic pathway, salutaridine is stereospecifically reduced by salutaridine reductase (SalR) to salutaridinol, which is subsequently acetylated by SalAT. SalAT transcript was shown by quantitative PCR to be diminished, while salR transcript levels remained unaffected. Yeast two-hybrid and co-immunoprecipitation analyses indicated an interaction between SalR and SalAT, which suggested the occurrence of an enzyme complex and provided an explanation for the unexpected accumulation of salutaridine. Decreased concentrations of thebaine and codeine in latex were also observed, while the morphine levels remained constant compared to concentrations found in untransformed control plants.
Next-generation sequencing plays a central role in the characterization and quantification of transcriptomes. Although numerous metrics are purported to quantify the quality of RNA, there have been no large-scale empirical evaluations of the major determinants of sequencing success. We used a combination of existing and newly developed methods to isolate total RNA from 1115 samples from 695 plant species in 324 families, which represents >900 million years of phylogenetic diversity from green algae through flowering plants, including many plants of economic importance. We then sequenced 629 of these samples on Illumina GAIIx and HiSeq platforms and performed a large comparative analysis to identify predictors of RNA quality and the diversity of putative genes (scaffolds) expressed within samples. Tissue types (e.g., leaf vs. flower) varied in RNA quality, sequencing depth and the number of scaffolds. Tissue age also influenced RNA quality but not the number of scaffolds ? 1000 bp. Overall, 36% of the variation in the number of scaffolds was explained by metrics of RNA integrity (RIN score), RNA purity (OD 260/230), sequencing platform (GAIIx vs HiSeq) and the amount of total RNA used for sequencing. However, our results show that the most commonly used measures of RNA quality (e.g., RIN) are weak predictors of the number of scaffolds because Illumina sequencing is robust to variation in RNA quality. These results provide novel insight into the methods that are most important in isolating high quality RNA for sequencing and assembling plant transcriptomes. The methods and recommendations provided here could increase the efficiency and decrease the cost of RNA sequencing for individual labs and genome centers.
Geranylgeraniol (GGOH), a bioactive acyclic diterpene with apoptotic induction activity, is the immediate precursor of the commercial anti-peptic, plaunotol (18-hydroxy geranylgeraniol), which is found in Croton stellatopilosus (Ohba). From this plant, a cDNA encoding a prenyl diphosphate phosphatase (CsPDP), which catalyses the dephosphorylation of geranylgeranyl diphosphate (GGPP) to GGOH, was isolated using a PCR approach. The full-length cDNA contained 888bp and encoded a 33.6 kDa protein (295 amino acids) that was phylogenetically grouped into the phosphatidic acid phosphatase (PAP) enzyme family. The deduced amino acid sequence showed 6 hydrophobic transmembrane regions with 57-85% homology to the sequences of other plant PAPs. The recombinant CsPDP and its 4 truncated constructs exhibited decreasing dephosphorylation activities relative to the lengths of the N-terminal deletions. While the full-length CsPDP successfully performed the two sequential monodephosphorylation steps on GGPP to form GGOH, the larger N-terminal deletion in the truncated enzymes appeared to specifically decrease the catalytic efficiency of the second monodephosphorylation step. The information presented here on the CsPDP cDNA and factors affecting the dephosphorylation activity of its recombinant protein may eventually lead to the discovery of the specific GGPP phosphatase gene and enzyme that are involved in the formation of GGOH in the biosynthetic pathway of plaunotol in C. stellatopilosus.
The bicyclic diterpene (-)-sclareol is accumulated in glandular trichomes in Salvia sclarea (Schmiderer et al., 2008), and is a major terpenoid component of this plant species. It is used as the starting material for Ambrox synthesis, a synthetic ambergris analog used in the flavor and fragrance industry. In order to investigate the formation of sclareol, cDNA prepared from secretory cells of glandular trichomes from S. sclarea inflorescence were randomly sequenced. A putative copalyl diphosphate synthase encoding EST, SscTPS1, was functionally expressed in Escherichia coli. Whereas reaction of geranylgeranyl diphosphate with the putative copalyl diphosphate synthase followed by hydrolysis with alkaline phosphatase yielded a diastereomeric mixture of (13R)- and (13S)-manoyl oxide, HCl hydrolysis yielded (-)-sclareol (1) and 13-epi-sclareol as products. The product of the reaction of SscTPS1 with geranylgeranyl diphosphate was subjected to analysis by LC-negative ion ESI-MS/MS without prior hydrolysis. EPI scans were consistent with copalyl diphosphate to which 18 mass units had added (m/z 467 [M+H](-)). The enzymatic reaction was also carried out in the presence of 60% H2(18)O. LC-negative ion ESI-MS/MS analysis established an additional reaction product consistent with the incorporation of (18)O. Incubation in the presence of 60% (2)H2O resulted in the incorporation of one deuterium atom. These results suggest water capture of the carbocation intermediate, which is known to occur in reactions catalyzed by monoterpene synthases, but has been described only several times for diterpene synthases.
The assumption that CYP2D1 is the corresponding rat cytochrome to human CYP2D6 has been revisited using recombinant proteins in direct enzyme assays. CYP2D1 and 2D2 were incubated with known CYP2D6 substrates, the three morphine precursors thebaine, codeine and (R)-reticuline. Mass spectrometric analysis showed that rat CYP2D2, not 2D1, catalyzed the 3-O-demethylation reaction of thebaine and codeine. In addition, CYP2D2 incubated with (R)-reticuline generated four products corytuberine, pallidine, salutaridine and isoboldine while rat CYP2D1 was completely inactive. This intramolecular phenol-coupling reaction follows the same mechanism as observed for CYP2D6. Michaelis-Menten kinetic parameters revealed high catalytic efficiencies for rat CYP2D2. These findings suggest a critical evaluation of other commonly accepted, however untested, CYP2D1 substrates.
Although it is agreed that a major polyploidy event, gamma, occurred within the eudicots, the phylogenetic placement of the event remains unclear.
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