Metabolic engineering of Saccharomyces cerevisiae for production of germacrene A, a precursor of beta-elemene

Beta-elemene, a sesquiterpene and the major component of the medicinal herb Curcuma wenyujin, has antitumor activity against various types of cancer and could potentially serve as a potent antineoplastic drug. However, its current mode of production through extraction from plants has been inefficient and suffers from limited natural resources. Here, we engineered a yeast cell factory for the sustainable production of germacrene A, which can be transformed to beta-elemene by a one-step chemical reaction in vitro. Two heterologous germacrene A synthases (GASs) converting farnesyl pyrophosphate (FPP) to germacrene A were evaluated in yeast for their ability to produce germacrene A. Thereafter, several metabolic engineering strategies were used to improve the production level. Overexpression of truncated 3-hydroxyl-3-methylglutaryl-CoA reductase and fusion of FPP synthase with GAS, led to a sixfold increase in germacrene A production in shake-flask culture. Finally, 190.7 mg/l of germacrene A was achieved. The results reported in this study represent the highest titer of germacrene A reported to date. These results provide a basis for creating an efficient route for further industrial application re-placing the traditional extraction of beta-elemene from plant sources.

Discovery of germacrene A synthases in Barnadesia spinosa: The first committed step in sesquiterpene lactone biosynthesis in the basal member of the Asteraceae

The Andes-endemic Barnadesioideae lineage is the oldest surviving and phylogenetically basal subfamily of the Asteraceae (Compositae), a prolific group of flowering plants with world-wide distribution (∼24,000 species) marked by a rich diversity of sesquiterpene lactones (STLs). Intriguingly, there is no evidence that members of the Barnadesioideae produce STLs, specialized metabolites thought to have contributed to the adaptive success of the Asteraceae family outside South America. The biosynthesis of STLs requires the intimate expression and functional integration of germacrene A synthase (GAS) and germacrene A oxidase (GAO) to sequentially cyclize and oxidize farnesyl diphosphate into the advanced intermediate germacrene A acid leading to diverse STLs. Our previous discovery of GAO activity conserved across all major subfamilies of Asteraceae, including the phylogenetically basal lineage of Barnadesioideae, prompted further investigation of the presence of the gateway GAS in Barnadesioideae. Herein we isolated two terpene synthases (BsGAS1/BsGAS2) from the basal Barnadesia spinosa (Barnadesioideae) that displayed robust GAS activity when reconstituted in yeast and characterized in vitro. Despite the apparent lack of STLs in the Barnadesioideae, this work unambiguously confirms the presence of GAS in the basal genera of the Asteraceae. Phylogenetic analysis reveals that the two BsGASs fall into two distinct clades of the Asteraceae's GASs, and BsGAS1 clade is only retained in the evolutionary closer Cichorioideae subfamily, implicating BsGAS2 is likely the ancestral base of most GASs found in the lineages outside the Barnadesioideae. Taken together, these results show the enzymatic capacities of GAS and GAO emerged prior to the subsequent radiation of STL-producing Asteraceae subfamilies.

Cytochrome P450s from Cynara cardunculus L. CYP71AV9 and CYP71BL5, catalyze distinct hydroxylations in the sesquiterpene lactone biosynthetic pathway

Cynara cardunculus (Asteraceae) is a cross pollinated perennial crop which includes the two cultivated taxa globe artichoke and cultivated cardoon. The leaves of these plants contain high concentrations of sesquiterpene lactones (STLs) among which cynaropicrin is the most represented, and has recently attracted attention because of its therapeutic potential as anti-tumor and anti-photoaging agent. Costunolide is considered the common precursor of the STLs and three enzymes are involved in its biosynthetic pathway: i.e. the germacrene A synthase (GAS), the germacrene A oxidase (GAO) and the costunolide synthase (COS). Here we report on the isolation of two P450 genes, (i.e. CYP71AV9 and CYP71BL5), in a set of ∼19,000 C. cardunculus unigenes, and their functional characterization in yeast and in planta. The metabolite analyses revealed that the co-expression of CYP71AV9 together with GAS resulted in the biosynthesis of germacra-1(10),4,11(13)-trien-12-oic acid in yeast. The co-expression of CYP71BL5 and CYP71AV9 with GAS led to biosynthesis of the free costunolide in yeast and costunolide conjugates in Nicotiana benthamiana, demonstrating their involvement in STL biosynthesis as GAO and COS enzymes. The substrate specificity of CYP71AV9 was investigated by testing its ability to convert amorpha-4,11-diene, (+)-germacrene D and cascarilladiene to their oxidized products when co-expressed in yeast with the corresponding terpene synthases.

Isoprenoid biosynthesis in Artemisia annua: cloning and heterologous expression of a germacrene A synthase from a glandular trichome cDNA library

Artemisia annua (Asteraceae) is the source of the anti-malarial compound artemisinin. To elucidate the biosynthetic pathway and to isolate and characterize genes involved in the biosynthesis of terpenoids including artemisinin in A. annua, glandular trichomes were used as an enriched source for biochemical and molecular biological studies. The sequencing of 900 randomly selected clones from a glandular trichome plasmid cDNA library revealed the presence of many ESTs involved in isoprenoid biosynthesis such as enzymes from the methylerythritol phosphate pathway and the mevalonate pathway, amorpha-4,11-diene synthase and other sesquiterpene synthases, monoterpene synthases and two cDNAs showing high similarity to germacrene A synthases. Full-length sequencing of the latter two ESTs resulted in a 1686-bp ORF encoding a protein of 562 aa. Upon expression in Escherichia coli, the recombinant protein was inactive with geranyl diphosphate, but catalyzed the cyclization of farnesyl diphosphate to germacrene A. These results demonstrate the potential of the use of A. annua glandular trichomes as a starting material for studying isoprenoid biosynthesis in this plant species.

Biosynthesis of the irregular monoterpene artemisia ketone, the sesquiterpene germacrene D and other isoprenoids in Tanacetum vulgare L. (Asteraceae)

The incorporation of [1-13C]-labeled glucose into the irregular monoterpene artemisia ketone, the regular monoterpenes camphor and beta-thujone, the sesquiterpene germacrene D, the diterpene trans-phytol and beta-sitosterol and isofucosterol has been studied in axenic cultures of Tanacetum vulgare L. (Asteraceae). Quantitative 13C NMR spectroscopic analysis of the resulting labeling patterns showed that the isoprene units of the monoterpenes and the diterpene are formed via the methylerythritol phosphate (MEP) pathway, whereas the isoprene building blocks of the sesquiterpene and the sterols originate from the mevalonic acid (MVA) pathway.

Mechanism and Stereochemistry of the Germacradienol/Germacrene D Synthase of Streptomyces coelicolor A3(2)

Incubation of farnesyl diphosphate (1, FPP) with recombinant germacradienol synthase from Streptomyces coelicolor A3(2) gave, in addition to (4S,7R)-germacra-1(10)E,5E-diene-11-ol (2), 15% of (-)-germacrene D (5). Incubations of [1,1-2H2]FPP (1a), (1R)-[1-2H]FPP (1b), and (1S)-[1-2H]FPP (1c) with germacradienol/germacrene D synthase and analysis of the resulting samples of germacradienol (2) and germacrene D (5) by a combination of 1H, 2H, and 13C NMR and mass spectrometry established that it is H-1si of FPP that is lost in the formation of germacradienol (2) and that undergoes 1,3-hydride transfer in the formation of (-)-germacrene D (5). The proportion of the two products was also sensitive to isotopic labeling, with cyclization of (1S)-[1-2H]FPP (1c) giving an increased proportion (35%) of 5. These results could be explained by a mechanism involving partitioning of a common helminthogermacradienyl cation intermediate 7.

Forest tent caterpillars (Malacosoma disstria) induce local and systemic diurnal emissions of terpenoid volatiles in hybrid poplar (Populus trichocarpa x deltoides): cDNA cloning, functional characterization, and patterns of gene expression of (-)-germacrene D synthase, PtdTPS1

Feeding forest tent caterpillars (FTCs) induced local and systemic diurnal emissions of (-)-germacrene D, along with (E)-beta-ocimene, linalool, (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), benzene cyanide, and (E,E)-alpha-farnesene, from leaves of hybrid poplar. FTC feeding induced substantially higher levels of volatiles in local and systemic leaves than did mechanical wounding. A full-length poplar sesquiterpene synthase cDNA (PtdTPS1) was isolated and functionally identified as (-)-germacrene D synthase. Expression of PtdTPS1, expression of genes of early, intermediate and late steps in terpenoid biosynthesis, and expression of a lipoxygenase gene (PtdLOX1) were analyzed in local FTC-infested and systemic leaves. Transcript levels of PtdTPS1 and PtdLOX1 were strongly increased in response to herbivory. PtdTPS1 was also induced by mechanical wounding or by methyl jasmonate (MeJA) treatment. FTC feeding did not affect transcript levels of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR), and isoprene synthase (IPS). Two other TPS genes, PtdTPS2 and PtTPS3, and farnesyl diphosphate synthase were only very transiently induced. These results illustrate differential expression of terpenoid pathway genes in response to insect feeding and a key function of (-)-germacrene D synthase PtdTPS1 for herbivore-induced local and systemic volatile emissions in hybrid poplar. FTC-induced transcripts of PtdTPS1 followed diurnal rhythm. Spatial patterns of FTC-induced PtdTPS1 transcript accumulation revealed acropetal but not basipetal direction of the systemic response. Implications for tritrophic poplar-FTC-predator/parasitoid interactions are discussed.