Casbene synthetase: regulation of phytoalexin biosynthesis in Ricinus communis L. seedlings. Purification of casbene synthetase and regulation of its biosynthesis during elicitation

Tigliane and daphnane diterpenoids from Thymelaeaceae family: chemistry, biological activity, and potential in drug discovery

Tigliane and daphnane diterpenoids are characteristically distributed in plants of the Thymelaeaceae family as well as the Euphorbiaceae family and are structurally diverse due to the presence of polyoxygenated functionalities in the polycyclic skeleton. These diterpenoids are known as toxic components, while they have been shown to exhibit a wide variety of biological activities, such as anti-cancer, anti-HIV, and analgesic activity, and are attracting attention in the field of natural product drug discovery. This review focuses on naturally occurring tigliane and daphnane diterpenoids from plants of the Thymelaeaceae family and provides an overview of their chemical structure, distribution, isolation, structure determination, chemical synthesis, and biological activities, with a prime focus on the recent findings.

Production of C20, C30 and C40 terpenes in the engineered phototrophic bacterium Rhodobacter capsulatus

Terpenes constitute one of the largest groups of secondary metabolites that are used, for example, as food-additives, fragrances or pharmaceuticals. Due to the formation of an intracytoplasmic membrane system and an efficient intrinsic tetraterpene pathway, the phototrophic α-proteobacterium Rhodobacter capsulatus offers favorable properties for the production of hydrophobic terpenes. However, research efforts have largely focused on sesquiterpene production. Recently, we have developed modular tools allowing to engineer the biosynthesis of terpene precursors. These tools were now applied to boost the biosynthesis of the diterpene casbene, the triterpene squalene and the tetraterpene β-carotene in R. capsulatus SB1003. Selected enzymes of the intrinsic isoprenoid pathway and the heterologous mevalonate (MVA) pathway were co-expressed together with the respective terpene synthases in various combinations. Remarkably, co-expression of genes ispA, idi and dxs enhanced the synthesis of casbene and β-carotene. In contrast, co-expression of precursor biosynthetic genes with the squalene synthase from Arabidopsis thaliana reduced squalene titers. Therefore, we further employed four alternative pro- and eukaryotic squalene synthases. Here, the synthase from Methylococcus capsulatus enabled highest product levels of 90 mg/L squalene upon co-expression with ispA. In summary, we demonstrate the applicability of R. capsulatus for the heterologous production of diverse terpene classes and provide relevant insights for further development of such platforms.

(+)-Germacrene A biosynthesis . The committed step in the biosynthesis of bitter sesquiterpene lactones in chicory

The leaves and especially the roots of chicory (Cichorium intybus L. ) contain high concentrations of bitter sesquiterpene lactones such as the guianolides lactupicrin, lactucin, and 8-deoxylactucin. Eudesmanolides and germacranolides are present in smaller amounts. Their postulated biosynthesis through the mevalonate-farnesyl diphosphate-germacradiene pathway has now been confirmed by the isolation of a (+)-germacrene A synthase from chicory roots. This sesquiterpene cyclase was purified 200-fold using a combination of anion-exchange and dye-ligand chromatography. It has a Km value of 6. 6 &mgr;M, an estimated molecular mass of 54 kD, and a (broad) pH optimum around 6.7. Germacrene A, the enzymatic product, proved to be much more stable than reported in literature. Its heat-induced Cope rearrangement into (-)-beta-elemene was utilized to determine its absolute configuration on an enantioselective gas chromatography column. To our knowledge, until now in sesquiterpene biosynthesis, germacrene A has only been reported as an (postulated) enzyme-bound intermediate, which, instead of being released, is subjected to additional cyclization(s) by the same enzyme that generated it from farnesyl diphosphate. However, in chicory germacrene A is released from the sesquiterpene cyclase. Apparently, subsequent oxidations and/or glucosylation of the germacrane skeleton, together with a germacrene cyclase, determine whether guaiane- or eudesmane-type sesquiterpene lactones are produced.

Overexpression, single-step purification, and site-directed mutagenetic analysis of casbene synthase

Casbene synthase is a diterpene cyclase isolated from castor bean (Ricinus communis L), which catalyzes the cyclization of geranylgeranyl diphosphate to form the phytoalexin casbene. We here report the overexpression of casbene synthase in Escherichia coli in soluble form using a thioredoxin fusion system. The amplified DNA by PCR carried on pCS7 was inserted into the expression vector pET32b(+) to form pCAS.2. The resulting transformants of pCAS. 2/BL21(DE3) produced a thioredoxin casbene synthase fusion protein (20-30% of total soluble protein) when induced with isopropyl beta-d-thiogalactopyranoside at 20 degrees C. Recombinant casbene synthase was purified to homogeneity in a single step with a His-binding metal-affinity column. Casbene synthase has a conserved aspartate-rich region [amino acids 355-359 (DDTID)], one cysteine, and three histidines with several prenyl transferases and terpene cyclases. Seven mutants were constructed by site-directed mutagenesis. The importance of Asp 355 and Asp 356 for catalysis was established by an increase in Km as well as a reduction in kcat in the corresponding glutamate mutants. These results indicate that the first and the second aspartate are involved in catalysis, while the third aspartate and the conserved cysteine and histidine residues selected for mutagenesis appear not to be involved in catalysis.

Regulation of oleoresinosis in grand fir (Abies grandis). Differential transcriptional control of monoterpene, sesquiterpene, and diterpene synthase genes in response to wounding

Grand fir (Abies grandis Lindl.) has been developed as a model system for the study of wound-induced oleoresinosis in conifers as a response to insect attack. Oleoresin is a roughly equal mixture of turpentine (85% monoterpenes [C10] and 15% sesquiterpenes [C15]) and rosin (diterpene [C20] resin acids) that acts to seal wounds and is toxic to both invading insects and their pathogenic fungal symbionts. The dynamic regulation of wound-induced oleoresin formation was studied over 29 d at the enzyme level by in vitro assay of the three classes of synthases directly responsible for the formation of monoterpenes, sesquiterpenes, and diterpenes from the corresponding C10, C15, and C20 prenyl diphosphate precursors, and at the gene level by RNA-blot hybridization using terpene synthase class-directed DNA probes. In overall appearance, the shapes of the time-course curves for all classes of synthase activities are similar, suggesting coordinate formation of all of the terpenoid types. However, closer inspection indicates that the monoterpene synthases arise earlier, as shown by an abbreviated time course over 6 to 48 h. RNA-blot analyses indicated that the genes for all three classes of enzymes are transcriptionally activated in response to wounding, with the monoterpene synthases up-regulated first (transcripts detectable 2 h after wounding), in agreement with the results of cell-free assays of monoterpene synthase activity, followed by the coordinately regulated sesquiterpene synthases and diterpene synthases (transcription beginning on d 3-4). The differential timing in the production of oleoresin components of this defense response is consistent with the immediate formation of monoterpenes to act as insect toxins and their later generation at solvent levels for the mobilization of resin acids responsible for wound sealing.

Prospects for the bioengineering of isoprenoid biosynthesis

Over the last decade, our understanding of isoprenoid biosynthesis has progressed to the stage where specific strategies for the bioengineering of essential oil production can be considered. This review provides a current overview of the enzymology and regulation of essential oil isoprenoid biosynthesis. The reaction mechanisms of the synthases which produce many of the basic isoprenoid skeletons are described in detail. Coverage is also provided of the regulation of isoprenoid biosynthesis, including the roles played by tissue and subcellular compartmentation, and by partitioning of intermediates between different branches of isoprenoid metabolism. This provides necessary context for rationally targeting specific enzymes of metabolic pathways for bioengineering essential oil production. Wherever possible, emphasis is placed on research specific to essential oil isoprenoid biosynthesis, although relevant work related to other isoprenoids is also considered when it can provide useful insights. Finally, building upon this understanding of essential oil isoprenoid biosynthesis, several approaches to the bioengineering of isoprenoid metabolism are considered.

Identifying functional domains within terpene cyclases using a domain-swapping strategy

Cyclic terpenes and terpenoids are found throughout nature. They comprise an especially important class of compounds from plants that mediate plant- environment interactions, and they serve as pharmaceutical agents with antimicrobial and anti-tumor activities. Molecular comparisons of several terpene cyclases, the key enzymes responsible for the multistep cyclization of C10, C15, and C20 allylic diphosphate substrates, have revealed a striking level of sequence similarity and conservation of exon position and size within the genes. Functional domains responsible for a terminal enzymatic step were identified by swapping regions approximating exons between a Nicotiana tabacum 5-epi-aristolochene synthase (TEAS) gene and a Hyoscyamus muticus vetispiradiene synthase (HVS) gene and by characterization of the resulting chimeric enzymes expressed in bacteria. While exon 4 of the TEAS gene conferred specificity for the predominant reaction products of the tobacco enzyme, exon 6 of the HVS gene conferred specificity for the predominant reaction products of the Hyoscyamus enzyme. Combining these two functional domains of the TEAS and HVS genes resulted in a novel enzyme capable of synthesizing reaction products reflective of both parent enzymes. The relative ratio of the TEAS and HVS reaction products was also influenced by the source of exon 5 present in the new chimeric enzymes. The association of catalytic activities with conserved but separate exonic domains suggests a general means for generating additional novel terpene cyclases.

Purification of (+)-delta-cadinene synthase, a sesquiterpene cyclase from bacteria-inoculated cotton foliar tissue

A sesquiterpene cyclase whose activity is induced in a glandless, bacterial blight-resistant line of cotton (Gossypium hirsutum L.) catalyses the conversion of (E,E)-farnesyl diphosphate to (+)-delta-cadinene. This enzyme was purified by a combination of salt-induced phase separation, hydroxylapatite fractionation, hydrophobic interaction and strong anion-exchange chromatography, and denaturing polyacrylamide gel electrophoresis, followed by renaturation with Tween 80. The purified enzyme has a molecular weight of 64-65 kDa, and exhibited a single silver-staining band following electrophoresis in analytical denaturing polyacrylamide gels. Amino acid sequences of three tryptic peptides from the enzyme have been determined and are similar to known sequences in other terpene cyclases from plants.

Cloning and bacterial expression of a sesquiterpene cyclase from Hyoscyamus muticus and its molecular comparison to related terpene cyclases

Genomic and cDNA clones for vetispiradiene synthase, a sesquiterpene cyclase found in Hyoscyamus muticus, were isolated using a combination of reverse transcription-polymerase chain reactions and conventional cloning procedures. RNA blot hybridization demonstrated an induction of mRNA consistent with the induction of cyclase enzyme activity in elicitor-treated cells, DNA blot hybridization indicated a gene family of 6 to 8 members, and bacterial expression of 3 cDNA clones indicated that each coded for a vetispiradiene synthase enzyme activity catalyzing the synthesis of a single reaction product. Intron-exon organization of the vetispiradiene synthase gene was identical with that previously described for 5-epi-aristolochene synthase (tobacco sesquiterpene cyclase) and casbene synthase (castor bean diterpene cyclase), and the vetispiradiene synthase amino acid sequence was 77% identical with and 81% similar to the tobacco sesquiterpene cyclase. Regions of the vetispiradiene synthase sequence centered around amino acids 60, 100, and 370 were conspicuously different relative to the tobacco sesquiterpene cyclase. The sequence similarity between the tobacco and H. muticus enzymes is suggested to be reflective of the conservation of several partial reactions common to both enzymes, and the differences may be reflective of a partial reaction unique to each enzyme.

Cloning of casbene synthase cDNA: evidence for conserved structural features among terpenoid cyclases in plants

A near-full-length casbene synthase cDNA clone, pCS7, was isolated by using a partial cDNA clone, pCS4, to probe a lambda gt10 library constructed from poly(A)+ RNA from elicited castor bean seedlings. The cDNA insert had a length of 1983 bases with a polyadenylate tail of 19 bases. Translation of the cDNA sequence revealed an open reading frame encoding a 601-aa protein with a predicted M(r) of 68,960. Search of the GenBank data base with the deduced translation product revealed 42% identity and 65% similarity with 5-epi-aristolochene synthase from tobacco and 31% identity and 53% similarity with limonene synthase from spearmint. Each of the three proteins catalyzes an intramolecular cyclization of a prenyl diphosphate substrate to a specific cyclic terpenoid hydrocarbon product. The proposed reaction mechanisms for the three catalytic processes share common chemical features, even though the products being formed are members of three different classes of terpenoid compounds. Analysis of the alignment of the three proteins suggests that both primary and secondary structural elements are conserved. These similarities suggest that the genes that encode terpenoid cyclization enzymes of this type in angiosperms have undergone divergent evolution from an ancestral progenitor gene. In support of this proposition, the locations of five of the six introns in the casbene synthase gene align very closely with those of the five introns in the 5-epi-aristolochene synthase gene.

Characterization of the constitutive and wound-inducible monoterpene cyclases of grand fir (Abies grandis)

Monoterpene cyclase activity is greatly increased in grand fir (Abies grandis) sapling stems in response to wounding and the composition of the cyclic olefin mixture generated differs from that produced constitutively as determined by radio gas-liquid chromatography. Cell-free extracts from wounded stems and from non-wounded controls were systematically compared for monoterpene cyclase activities following partial purification and separation of these enzymes by anion-exchange chromatography (Mono Q FPLC) and native PAGE. The increase in monoterpene cyclase activity following wounding represents both the apparent enhancement of constitutive cyclase activities and the appearance of novel cyclization enzymes that are absent in nonwounded controls. A pinene cyclase was shown to be the major wound-inducible enzyme directly responsible for oleoresin monoterpene formation and was tentatively identified as a 62-kDa protein by SDS-PAGE.

Purification and characterization of the monoterpene cyclase gamma-terpinene synthase from Thymus vulgaris

The monoterpene cyclase, gamma-terpinene synthase, from Thymus vulgaris (thyme) leaves was purified to apparent homogeneity by isoelectric focusing and dye-ligand, anion-exchange, hydrophobic interaction, and gel permeation chromatography. The enzyme has a native molecular weight of 96,000 as determined by gel permeation chromatography, and exhibited a specific activity of 538 nmol/h.mg protein (turnover number of approximately 0.01/s). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed the enzyme to be composed of two apparently identical subunits of Mr approximately 55,000. The protein was very hydrophobic, and possessed a pI value of 4.85 as determined by isoelectric focusing. Maximum activity was observed at pH 6.8 in the presence of 20 mM Mg2+; 5 mM Mn2+ could support catalysis, albeit at a much lower rate. The Km value for the substrate, geranyl pyrophosphate, was 2.6 microM. Cyclase activity was inhibited by cysteine- and histidine-directed reagents. Purified gamma-terpinene synthase also possessed the ability to cyclize geranyl pyrophosphate to small amounts of alpha-thujene and to lesser quantities of myrcene, alpha-terpinene, limonene, linalool, terpinen-4-ol, and alpha-terpineol, all of which appear to be coproducts of the reaction sequence leading to gamma-terpinene. In general properties, the gamma-terpinene synthase from thyme leaves resembles other monoterpene cyclases as well as sesquiterpene and diterpene cyclases.

Dye-ligand and immobilized metal ion interaction chromatography for the purification of enzymes of prenyl pyrophosphate metabolism

Dye-ligand and immobilized metal ion interaction chromatography were shown to be efficient techniques for the rapid batchwise fractionation, from crude plant extracts, of a series of enzymes of prenyl pyrophosphate metabolism. Isopentenyl pyrophosphate isomerase, two prenyltransferases, and a number of terpene cyclases (synthases) were readily adsorbed to Matrex Gel Red A (a dimeric triazine dye coupled to cross-linked agarose beads), and desorbed in good yield with relatively high concentrations of KCl and increasing pH. Although all of these enzymes exhibit the common feature of employing a pyrophosphorylated substrate, selective elution could not be achieved with substrate or substrate analogues bearing a pyrophosphate function. Nor could the strong binding of these enzymes to triazine dyes be attributed solely to metal ion interactions or to hydrophobic effects. In a similar way, the isomerase, the prenyltransferases, and all of the terpene cyclases bound to a column of iminodiacetate-immobilized Ni(II) and were desorbed in relatively high fold purity with 15 mM imidazole. Although all of these enzymes bear accessible histidine residues, the interactions with the chelated metal ion were not sufficiently different to permit selective enzyme desorbtion by imidazole gradient elution. However, the use of columns charged with Zn(II) or Co(II) did allow some separation of the different cyclase and transferase types. While empirical in nature, these techniques offer simple, effective, and high-capacity methods for the preliminary concentration and purification of a group of enzymes that utilize prenyl pyrophosphate intermediates of isoprenoid biosynthesis.

Purification and characterization of the sesquiterpene cyclase patchoulol synthase from Pogostemon cablin

The sesquiterpene cyclase, patchoulol synthase, from Pogostemon cablin (patchouli) leaves was purified to apparent homogeneity by chromatofocusing, anion exchange, gel permeation, and hydroxylapatite chromatography. The enzyme showed a maximum specific activity of about 20 nmol/min/mg protein, and a native molecular weight of 80,000 as determined by gel permeation chromatography. The protein was very hydrophobic, as judged by chromatographic behavior on several matrices, and possessed a pI value of about 5.0, as determined by isoelectric and chromatofocusing. SDS-PAGE showed the enzyme to be composed of two apparently identical subunits of Mr approximately 40,000. Maximum activity was observed at pH 6.7 in the presence of Mg2+ (Km approximately 1.7 mM); other divalent metal ions were ineffective in promoting catalysis. The Km value for the substrate, farnesyl pyrophosphate, was 6.8 microM. Patchoulol synthase copurified with the ability to transform farnesyl pyrophosphate to cyclic olefins (alpha- and beta-patchoulene, alpha-bulnesene, and alpha-guiaene) and this observation, plus evidence based on differential inhibition and inactivation studies, suggested that these structurally related products are synthesized by the same cyclase enzyme. In general properties, the patchoulol synthase from patchouli leaves resembles fungal sesquiterpene olefin cyclases except for the ability to synthesize multiple products, a property more typical of monoterpene cyclases of higher plant origin.

Regulation of expression of the casbene synthetase gene during elicitation of castor bean seedlings with pectic fragments

A castor bean cDNA library has been constructed in the expression vector lambda gt11. Screening of 30,000 plaques with antibodies against casbene synthetase yielded six positive clones, from which three purified clones were obtained after sequential screening. The cDNA inserts in these clones ranged in size from 200 to 500 base pairs and were shown to share homologous sequences. One of the clones (pCS4) was examined in hybrid-selected and hybrid-arrested translation experiments and shown to contain a partial sequence of the casbene synthetase coding region. Northern analysis using the pCS4 clone as a probe revealed that the total hybridizable casbene synthetase mRNA amount increased to a maximum at 6 h after treatment of seedlings with pectic fragment elicitor and then decreased steadily with almost the same kinetics as observed previously for the changes in the translatable casbene synthetase mRNA activity under these same conditions. Run-on transcription experiments were performed with nuclei isolated from castor bean seedlings at various time periods after treatment with pectic fragment elicitors. Transcription of the casbene synthetase gene was first detected at 2 h after elicitation, rose to a maximum at 5 h, and fell rapidly thereafter. These results suggest that the accumulation of casbene synthetase, and hence casbene, is governed primarily by the rate of transcription of the casbene synthetase gene during elicitation by pectic fragments.

Partial purification and characterization of two sesquiterpene cyclases from sage (Salvia officinalis) which catalyze the respective conversion of farnesyl pyrophosphate to humulene and caryophyllene

Humulene cyclase and caryophyllene cyclase, two enzymes which catalyze the cyclization of farnesyl pyrophosphate to the respective sesquiterpene olefins, have been partially purified from the supernatant fraction of a sage (Salvia officinalis) leaf epidermis extract and separated from each other by a combination of hydrophobic interaction, gel filtration, and ion-exchange chromatography. The molecular weight of both cyclases was estimated by gel filtration to be 57,000 and both cyclases exhibited a pH optimum of 6.5 and preferred Mg2+ (Km approximately 1.5 mM) as the required divalent metal cation. Both enzymes possessed a Km of about 1.7 microM for farnesyl pyrophosphate, were strongly inhibited by p-hydroxymercuribenzoate, and exhibited comparable sensitivities to a variety of other potential inhibitors. The properties of the two sesquiterpene olefin cyclases, which are the first from a higher plant source to be examined in detail, were very similar to each other and to other monoterpene, sesquiterpene, and diterpene cyclases previously described.