Takeshi Sassa

Identification of a Biosynthetic Gene Cluster in Rice for Momilactones

Rice diterpenoid phytoalexins such as momilactones and phytocassanes are produced in suspension-cultured rice cells treated with a chitin oligosaccharide elicitor and in rice leaves irradiated with UV light. The common substrate geranylgeranyl diphosphate is converted into diterpene hydrocarbon precursors via a two-step sequential cyclization and then into the bioactive phytoalexins via several oxidation steps. It has been suggested that microsomal cytochrome P-450 monooxygenases (P-450s) are involved in the downstream oxidation of the diterpene hydrocarbons leading to the phytoalexins and that a dehydrogenase is involved in momilactone biosynthesis. However, none of the enzymes involved in the downstream oxidation of the diterpene hydrocarbons have been identified. In this study, we found that a putative dehydrogenase gene (AK103462) and two functionally unknown P-450 genes (CYP99A2 and CYP99A3) form a chitin oligosaccharide elicitor- and UV-inducible gene cluster, together with OsKS4 and OsCyc1, the diterpene cyclase genes involved in momilactone biosynthesis. Functional analysis by heterologous expression in Escherichia coli followed by enzyme assays demonstrated that the AK103462 protein catalyzes the conversion of 3β-hydroxy-9βH-pimara-7,15-dien-19,6β-olide into momilactone A. The double knockdown of CYP99A2 and CYP99A3 specifically suppressed the elicitor-inducible production of momilactones, strongly suggesting that CYP99A2, CYP99A3, or both are involved in momilactone biosynthesis. These results provide strong evidence for the presence on chromosome 4 of a gene cluster involved in momilactone biosynthesis.

ent-Kaurene Synthase from the FungusPhaeosphaeria sp. L487 cDNA ISOLATION, CHARACTERIZATION, AND BACTERIAL EXPRESSION OF A BIFUNCTIONAL DITERPENE CYCLASE IN FUNGAL GIBBERELLIN BIOSYNTHESIS

ent-Kaurene is the first cyclic diterpene intermediate of gibberellin biosynthesis in both plants and fungi. In plants, ent-kaurene is synthesized from geranylgeranyl diphosphate via copalyl diphosphate in a two-step cyclization catalyzed by copalyl diphosphate synthase andent-kaurene synthase. A cell-free system of the fungusPhaeosphaeria sp. L487 converted labeled geranylgeranyl diphosphate to ent-kaurene. A cDNA fragment, which possibly encodes copalyl diphosphate synthase, was isolated by reverse transcription-polymerase chain reaction using degenerate primers based on the consensus motifs of plant enzymes. Translation of a full-length cDNA sequence isolated from the fungal cDNA library revealed an open reading frame for a 106-kDa polypeptide. The deduced amino acid sequence shared 24 and 21% identity with maize copalyl diphosphate synthase and pumpkin ent-kaurene synthase, respectively. A fusion protein produced by expression of the cDNA inEscherichia coli catalyzed the two-step cyclization of geranylgeranyl diphosphate to ent-kaurene. Amo-1618 completely inhibited the copalyl diphosphate synthase activity of the enzyme at 10−6 m, whereas it did not inhibit the ent-kaurene synthase activity even at 10−4 m. These results indicate that the fungus has a bifunctional diterpene cyclase that can convert geranylgeranyl diphosphate into ent-kaurene. They may be separate catalytic sites for the two cyclization reactions.

Fusicoccins are biosynthesized by an unusual chimera diterpene synthase in fungi

Fusicoccins are a class of diterpene glucosides produced by the plant-pathogenic fungus Phomopsis amygdali. As modulators of 14-3-3 proteins, fusicoccins function as potent activators of plasma membrane H+-ATPase in plants and also exhibit unique biological activity in animal cells. Despite their well studied biological activities, no genes encoding fusicoccin biosynthetic enzymes have been identified. Cyclic diterpenes are commonly synthesized via cyclization of a C20 precursor, geranylgeranyl diphosphate (GGDP), which is produced through condensation of the universal C5 isoprene units dimethylallyl diphosphate and isopentenyl diphosphate by prenyltransferases. We found that (+)-fusicocca-2,10 (14)-diene, a tricyclic hydrocarbon precursor for fusicoccins, is biosynthesized from the C5 isoprene units by an unusual multifunctional enzyme, P. amygdali fusicoccadiene synthase (PaFS), which shows both prenyltransferase and terpene cyclase activities. The functional analysis of truncated mutants and site-directed mutagenesis demonstrated that PaFS consists of two domains: a terpene cyclase domain at the N terminus and a prenyltransferase domain at the C terminus. These findings suggest that fusicoccadiene can be produced efficiently in the fungus by using the C5 precursors, irrespective of GGDP availability. In fact, heterologous expression of PaFS alone resulted in the accumulation of fusicocca-2,10 (14)-diene in Escherichia coli cells, whereas no product was detected in E. coli cells expressing Gibberella fujikuroi ent-kaurene synthase, another fungal diterpene cyclase that also uses GGDP as a substrate but does not contain a prenyltransferase domain. Genome walking suggested that fusicoccin biosynthetic enzymes are encoded as a gene cluster near the PaFS gene.

Diterpene Cyclases Responsible for the Biosynthesis of Phytoalexins, Momilactones A, B, and Oryzalexins A–F in Rice

Rice (Oryza sativa L.) produces diterpene phytoalexins, such as momilactones, oryzalexins, and phytocassanes. Using rice genome information and in vitro assay with recombinant enzymes, we identified genes (OsKS4 and OsKS10) encoding the type-A diterpene cyclases 9β-pimara-7,15-diene synthase and ent-sandaracopimaradiene synthase which are involved in the biosynthesis of momilactones A, B and oryzalexins A–F respectively. Transcript levels of these two genes increased remarkably after ultraviolet (UV) treatment, which is consistent with elevated production of phytoalexins by UV. These two genes might prove powerful tools for understanding plant defense mechanisms in rice.

Stemar-13-ene synthase, a diterpene cyclase involved in the biosynthesis of the phytoalexin oryzalexin S in rice

In suspension‐cultured rice cells, diterpenoid phytoalexins are produced in response to exogenously applied elicitors. We isolated a cDNA encoding a diterpene cyclase, OsDTC2, from suspension‐cultured rice cells treated with a chitin elicitor. The OsDTC2 cDNA was overexpressed in Escherichia coli as a fusion protein with glutathione S‐transferase, and the recombinant OsDTC2 was indicated to function as stemar‐13‐ene synthase that converted syn‐copalyl diphosphate to stemar‐13‐ene, a putative diterpene hydrocarbon precursor of the phytoalexin oryzalexin S. The level of OsDTC2 mRNA in suspension‐cultured rice cells began to increase 3 h after addition of the elicitor and reached the maximum after 8 h. The expression of OsDTC2 was also induced in UV‐irradiated rice leaves. In addition, we indicated that stemar‐13‐ene accumulated in the chitin‐elicited suspension‐cultured rice cells and the UV‐irradiated rice leaves.

Functional Analysis of the Two Interacting Cyclase Domains in ent-Kaurene Synthase from the FungusPhaeosphaeria sp. L487 and a Comparison with Cyclases from Higher Plants

We report here kinetic analysis and identification of the two cyclase domains in a bifunctional diterpene cyclase, Phaeosphaeria ent-kaurene synthase (FCPS/KS). Kinetics of a recombinant FCPS/KS protein indicated that the affinity for copalyl diphosphate is higher than that for geranylgeranyl diphosphate (GGDP). ent-Kaurene production from GGDP by FCPS/KS was enhanced by the addition of a plant ent-kaurene synthase (KS) but not by plant CDP synthase (CPS), suggesting that the rate of ent-kaurene production of FCPS/KS may be limited by the KS activity. Site-directed mutagenesis of aspartate-rich motifs in FCPS/KS indicated that the 318DVDD motif near the N terminus and the 656DEFFE motif near the C terminus may be part of the active site for the CPS and KS reactions, respectively. The other aspartate-rich 132DDVLD motif near the N terminus is thought to be involved in both reactions. Functional analysis of the N- and C-terminal truncated mutants revealed that a N-terminal 59-kDa polypeptide catalyzed the CPS reaction and a C-terminal 66-kDa polypeptide showed KS activity. A 101-kDa polypeptide lacking the first 43 amino acids of the N terminus reduced KS activity severely without CPS activity. These results indicate that there are two separate interacting domains in the 106-kDa polypeptide of FCPS/KS.

Erinacine Q, a New Erinacine from Hericium erinaceum, and its Biosynthetic Route to Erinacine C in the Basidiomycete

Erinacines as cyathane-xylosides are known to have potent stimulating activity for nerve-growth-factor synthesis. Our search for new cyathane metabolites from a liquid culture of Hericium erinaceum YB4-6237 resulted in the isolation of a new erinacine named erinacine Q (1). NMR spectrometry and a chemical derivation from erinacine P (2) determined the compound to be a derivative in which the formyl group of erinacine P had been reduced to the hydroxymethyl group. To clarify the biosynthetic relationship between erinacine Q and the others, [1′-13C]erinacine Q ([1′-13C]-1) was chemically derived from [1′-13C]erinacine P ([1′-13C]-2) which had been prepared by feeding [1-13C]-D-glucose to the basidiomycete. The biotransformation of labeled erinacine Q into [1′-13C]erinacine C ([1′-13C]-5) via [1′-13C]erinacine P in this basidiomycete was demonstrated by NMR spectrometry.

Characterization of a rice gene family encoding type-a diterpene cyclases

We have previously isolated and characterized the rice (Oryza sativa) cDNAs, OsCyc1/OsCPS4, OsCyc2/OsCPS2, OsKS4, OsDTC1/OsKS7, OsDTC2/OsKS8 and OsKS10, which encode cyclases that are responsible for diterpene phytoalexin biosynthesis. Among the other members of this gene family, OsCPS1 and OsKS1 have been suggested as being responsible for gibberellin biosynthesis, OsKSL11 has recently been shown to encode stemodene synthase, and the functions of the three other diterpene cyclase genes in the rice genome, OsKS3, OsKS5 and OsKS6, have not yet been determined. In this study, we show that recombinant OsKS5 and OsKS6 expressed in E. coli converted ent-copalyl diphosphate into ent-pimara-8(14),15-diene and ent-kaur-15-ene, respectively. Neither product is a hydrocarbon precursor required in the biosynthesis of either gibberellins or phytoalexins. OsKS3 may be a pseudogene from which the translated product is a truncated enzyme. These results suggest that the diterpene cyclase genes responsible for gibberellin and phytoalexin biosynthesis are not functionally redundant.

Effects of combined treatment with rapamycin and cotylenin A, a novel differentiation-inducing agent, on human breast carcinoma MCF-7 cells and xenografts

IntroductionRapamycin, an inhibitor of the serine/threonine kinase target of rapamycin, induces G1 arrest and/or apoptosis. Although rapamycin and its analogues are attractive candidates for cancer therapy, their sensitivities with respect to growth inhibition differ markedly among various cancer cells. Using human breast carcinoma cell line MCF-7 as an experimental model system, we examined the growth-inhibitory effects of combinations of various agents and rapamycin to find the agent that most potently enhances the growth-inhibitory effect of rapamycin.MethodWe evaluated the growth-inhibitory effect of rapamycin plus various agents, including cotylenin A (a novel inducer of differentiation of myeloid leukaemia cells) to MCF-7 cells, using either MTT assay or trypan blue dye exclusion test. The cell cycle was analyzed using propidium iodide-stained nuclei. Expressions of several genes in MCF-7 cells with rapamycin plus cotylenin A were studied using cDNA microarray analysis and RT-PCR. The in vitro results of MCF-7 cells treated with rapamycin plus cotylenin A were further confirmed in vivo in a mouse xenograft model.ResultsWe found that the sensitivity of rapamycin to MCF-7 cells was markedly affected by cotylenin A. This treatment induced growth arrest of the cells at the G1 phase, rather than apoptosis, and induced senescence-associated β-galactosidase activity. We examined the gene expression profiles associated with exposure to rapamycin and cotylenin A using cDNA microarrays. We found that expressions of cyclin G2, transforming growth factor-β-induced 68 kDa protein, BCL2-interacting killer, and growth factor receptor-bound 7 were markedly induced in MCF-7 cells treated with rapamycin plus cotylenin A. Furthermore, combined treatment with rapamycin and cotylenin A significantly inhibited the growth of MCF-7 cells as xenografts, without apparent adverse effects.ConclusionRapamycin and cotylenin A cooperatively induced growth arrest in breast carcinoma MCF-7 cells in vitro, and treatment with rapamycin and cotylenin A combined more strongly inhibited the growth of MCF-7 cells as xenografts in vivo than treatment with rapamycin or cotylenin A alone, suggesting that this combination may have therapeutic value in treating breast cancer. We also identified several genes that were markedly modulated in MCF-7 cells treated with rapamycin plus cotylenin A.

Diterpene Phytoalexins Are Biosynthesized in and Exuded from the Roots of Rice Seedlings

Rice (Oryza sativa L.) produces a variety of diterpene phytoalexins, such as momilactones, phytocassanes, and oryzalexins. Momilactone B was previously identified as an allelopathic substance exuded from the roots of rice. We identified in this present study momilactone A and phytocassanes A–E in extracts of, and exudates from, the roots of rice seedlings. The concentration of each compound was of the same order of magnitude as that of momilactone B. Expression analyses of the diterpene cyclase genes responsible for the biosynthesis of momilactones and phytocassanes suggest that these phytoalexins found in roots are primarily biosynthesized in those roots. None of phytocassanes B–E exhibited allelopathic activity against dicot seedling growth, whereas momilactone A showed much weaker allelopathic activity than momilactone B. The exudation of diterpene phytoalexins from the roots might be part of a system for defense against root-infecting pathogens.