The present invention relates to a DMA sequence, a polypeptide encoded by this sequence, and to the use of said DSh sequence and polypeptide in the production of amorphadiene. i Human malaria is a commonly occurring widespread infectious disease, caused in 85% of the cases by Plaatpo^ium falciparum. This parasite is responsible for the most lethal form of malaria, malaria tropicana. Each year, malaria causes clinical illness, often very 1 severe, in over 100 million people of which eventually over 1 million individuals will die. Approximately 40% of the world's population is at risk of malaria infection (as estimated by the World Health Organization). Malaria has traditionally been treated with quinolines. such as quinine, chloroquine, mefloquine and primaquine, and with antifolates. Unfortunately, most P.falciparum strains have become resistant to chloroquine, and some have developed resistance to mefloquine and halofantrine as well. Thus, novel antimalarial drugs to which resistant parasites are sensitive are urgently needed. Artemisinin, as well as its semisynthetic derivatives are promising candidates here. is a sesquiterpene lactone endoperoxide isolated from the aerial parts of the plant Artemisia annua L. Artemisia annua also known aa quinghao (Chinese), annual or sweet wormwood, or sweet annie is an annual herb native to Asia. A.annuar a member of the Asteraceae, belongs to the tribe Anthemideae of the Asteroideae, and is a large herb often reaching more than 2.0 m in height, it is usually single-stemmed with alternating branches. The aromatic leaves are deeply dissected and range from 2.5 to 5 cm in length. Artemisinin is mainly produced in the leaves as a secondary metabolite at a concentration of 0.01 - 0.6% on a dry weight base in natural populations. Artemlsinin is unique Co the plant A.annua with one possible exception of A.apiacea L. The A. annua used in this invention is of Vietnamese origin. Because of its low concentration in plants, arcemisinin la a relatively expensive resource for a drug. Current research has thus been aimed at producing artemiainin at a larger scale by organic synthesis. However, because artemlsinin consist of seven chiral carbon atoms, theoretically 2 = 126 isomers can be formed of which only one is identical to artemlsinin. Because of this complex structure of artemisinin, production of this compound by organic synthesis is not profitable from a commercial point of view. Genetic engineering of the biosynthetie pathway o£ artemisinin may give rise to higher artsmieinin levels in plants. To be able to interfere in the biosynthesis of artemisinin, the biosynthetie pathway has to be known, either completely or partially. Several attempts to elucidate the entire biosynthetie pathway have been undertaken. Until now, however, the exact pathway has remained largely unknown. In the research that led to the present invention, a unique pathway has been discovered which has not been published before. This pathway involves inter alia the formation of the artemisinin precursors amorpha-4,H-diene {IS, 6S, 7l3,10aH-amorpha-4, ll-diene) and the hydroperoxide of dihydroarteannuic acid. Tiiese precursors that were found in A. annua have not been described before in literature. From literature it is known that terpene cyclases (synthases} are branch point enzymes, which likely play an important role in terpenoid biosynthesis. The working hypothesis for this invention is thus that over-expression of such a branch point enzyme (terpene cyclase! may Increase terpenoid production in an organism. Factors that may influence Che success of such an approach are, in the case of ar-temisinin, the number and nature of the subsequent biosynthetic steps leading to artemisinin. Fig. 2 shows the biosynthetic pathway of artemisinin as postulated by the present inventors. This pathway is divided into three parts: The first part (Part I) represents the terpenoid (Isoprenoid) pathway. This pathway is a general pathway, Farnesyl diphosphate (famesyl pyrophosphate) (FPP), for example, is present in every living organism and it is the precursor of a large number of primary and secondary metabolites. It has been established that FPP is the precursor of all sesquiterpenes. Thus, by definition FPP is the precursor of artemisinin. Part II displays the cyclization of the general precursor FPP into the highly specific precursor amorpha-4,11-diene (also referred to as amorphadiene], the first specific precursor of artemisinin. In this pathway amorphadiene synthase is a branch point enzyme, having a key position in the biosynthetic pathway of artemisinin. In part III, dihydroarteannulc acid {DHAAl, slso called dihydroartemiainic acid, is photo-oxidatively converted into its hydroperoxide (DHAA-OOH). This lydroperoxide of DHAA will spontaneously oxidize into irtemisinin. No enzymes are involved in this part of the pathway and therefore it is impossible to alter irtemisinin production by over-expression of genes involved in this part of the pathway. Cytochrome p-450 catalyzed enzymes and an snoate reductase are probably involved in the conversion if amorphadiene Into DHAA, the transition state between >art XI and part III (see Fig. 3). Because no ntermediates of this part of the pathway are known or >resent [accumulated] in detectable amounts, in the dant, (except arteannuic acid, also called artemlsinic aid or 4,11(13)-amorphadien-l2-oic acid) it is likely hat these precursors are very rapidly converted into HAA. A rate limiting step in this part of the pathway is ot very likely. Talcing al these aspects into account the inventors concluded that the most logical step to be altered by genetic interfering, is the conversion {cyclization) of FPP into amorphadiene by aiaoxphadiene synthase. The object of the present invention ia therefore to provide a way in which artemisinin can be obtained via an at least partially biological route. This object is achieved by the provision of a DKA sequence which exhibits at least a 70V homology to the sequence (SEQ ID NO: 13) as shown in Pig. 12, and which codes for a polypeptide having the biological activity of the enzyme amorphadiene synthase. The biological activity of the eii*yme amorphadiene synthase relates to the conversion of the general precursor famesyl pyrophosphate (PPP) into the specific artemisinin precursor amorpha-4, il-diene, which, in A. annua further converted to artemisinin. Suitable genes according to the invention can be selected by testing the expression product of the gene for its ability to convert PPP into amorpha-4,11-diene. By transforming a suitable host cell with the DNA seqiuence of the invention, the conversion of famesyl pyrophosphate (FPP) into the highly specific precursor amorphadiene can be increased or induced if this conversion route is not naturally present in the organism. In the latter case, the organism should comprise or be able to produce PPP. Suitable host cells are for exanqple bacterial cells, such as E. coil. yeast cells like Saccharomyces eereviaie or Pichia pastoris and in particular oleaginous yeasts, like Yarrowis lipolyltica. or plant cells such as those of A.annua. Several plants are capable of producing large amounts of PPP making them potential organisms for amorphadiene production. The potential oleaginous yeast host cells, like, for example, Yarrowia lipolytica snd Cryptococcus curvatus. have the capacity to accumulate up to about 50 (dry weight) of acorage carbohydrates in oil bodies, making them very interesting candidates as production organisms for large quantities of terpenes. According to the invention, a way to obtain high levels of terpens accumulation is for example by means of re-direction of the metabolic flux in favor of the formation of amorpha-4,11-dlene. In analogy to the approach of an increased carotenoid production by the food yeast Candida utilis through metabolic engineering of the isoprenoid pathway as done by Shimada et al. (Appl. Environ. Microbiol. 64, 2676-2680 11998)) ctoe target genes according to the invention are acetyl CoA carboxylase (ACC, disruption) , hydroxy-methyl-glutaryl CoA reductase (HMGR, over-express ion) , and scalene synthase (SQS, disruption) Co obtain an inciraase of the precursor supplies, and amorpha~4,11-diene synthase over-expreasion to obtain accumulation of amorphadiene in such yeast cells. Because several expression systems (for example Muller et al., ' Yeast 14, 1267-1283 (1998); Park et al., The Journal of Biological Chemistry 272, 6876-6881 (1997); Tharaud et al., Gene 121, 111-119 (1992)) and transformation systems (for example Chen et al., Appl. Microbiol. Biotechnol. 48, 232-235 (1997)) are known for Y.lipolvtica in I literature, transformation and expression of the previously mentioned target genes in Y.lipolytica is possible without serious technical problerm. By adding FPP to a culture medium further comprising the enzyme of the invention (isolated as I described in example 1), or trsmfiformed cells, e.g. E.cpj.^. comprising the DKA sequence of the invention (as described in examples 3 and 4), which is ejcpressed, FPP is converted into amorphadiene. Amorphadiene can then be used as a starting material for Che production of > artemisinin. Transformed cells in which amorphadiene is produced as a result of the expression of amorphadiene synthase of the invention can be used either in disrupted form, by for example sonication, or as incact cells, as a source of amorphadiene. Over-expression of the amorphadiene synthase encoding gene in A.annua will increase arcemisinin production, because the terpene cyclase is expected to be the rate limiting step. The results o£ the present research {postulated biosynthetic pathway of artemisinin) make the presence of a single major rate limiting step at the place of the amorphadiene synthase clear. Over-expression of the amorphadiene synthase encoding gene can increase the production of artemisinin in R.annua. The chemical structure of the first specific precursor of artemisinin, a cyclization product of FPP was not known in literature. Neither has anyone so far detected such a compound in A.annua. Nevertheless it was poeeible to predict a likely structure for this cyclization product, based on the structure of DHAA and arteannuic acid (Fig. 3), The structure predicted in this way was consistent with a compound which is known in literature as 4,11-amorphadiene (J.D. Connelly & R.A. Hill in; Dictionary of terpenoids, chapmann and Hill, London, England}, as depicted in Fig. 4. This compound, isolated from Viquiera oblonifolia has previously been described by Bohlmann et al. under the incorrect name cadina-4,11-diene (Phytochemistry 23(5) Il83-1184 (19S4)]. Starting from arteannuic acid (isolated from A.annua. it was possible to synthesize amorphadiene. Amorphadiene obtained in this way was in all chemical and physical aspects identical to amorphadiene as described by Bohlmann et al,, and this standard was used to show the presence of amorphadiene in a terpene extract of A■annua. A further object of the present invention is to provide a polypeptide having the biological activity of the enzyme amorphadiene synthase, obtainable by a process as described in example 1. This polypeptide can be used to convert FPP into amorphadiene which subsequently can be converted into artemisinin. Conversion can take place either in planta. when the polypeptide amorphadiene synthase is expressed in a plant that contains the necessary enzymes to further convert amorphadiene into artemiainin, or in vitro when FPP and the polypeptide (either in isolated form or as an expression product in a cell) are brought together In an incubation mixture. Ainorphadiene, produced by a sultiOsle host organism transformed with the DNA sequence of the invention as precursor, can subsequently be chemically converted to dihydroarteannuic acid. Dihydroarteannuic acid per ae can be used or in the production of artemiainin. The chemical conversion of amorphadiene into dihydroarteannuic acid (Fig. 15) starts with the enantio-, stereo- and regioselective (anti-markownilcoff) hydroboration of amorphadiene with BH3, yielding a trialkylbcrane, followed by an oxidation of the trialkylborane with NaOH/HjO, yielding the alcohol (Advanced Organic Chemistry, Jerry March, 4th Edition, Wiley, 1992) . A mild oxidation of the alcohol to the acid can be obtained by PDC (pyridinium dichromate] without attacking the second double bond (Fig, IS) (Organic Synthesis, M.B. Smith, 1st Edition, McGraw-Hill, 1994). Many genes encoding enzymes involved in the biosynthetic pathway of famesyl diphosphate are cloned and known In literature. For A.annua, for example, the sequence of the famesyl diphosphate synthase encoding gene ia known in literature (Y. Matsushita, W-K. Kang and V. Charlwood Gene, 172 (199S) 2Q7-209). A further approach to introduce or increase the amorphadiene production in an organism, is to transform such an organism (for example A.annua simultaneously with the DNA sequence of the invention with one or more genes involved in the biosynthesis of farnesyl diphosphate. The expression of a fusion protein of amorphadiene synthase and farnesyl diphosphate synthase may be an example here. (Seagui)terpenes, such as amorphadiene, are also known as flavor and fragrance compounds in the food and perfume Industry. In addition, terpanese play a role in plant-insect interactions, such as the attraction or repulsion of insecta by plants. Furthermore, dihydro-arteannuic acid, which is an intermediate in the metabolic route from amorphadiene into artemisinin in A. annua can be used aa an antioxidant. Amorphadiene, obtained by (over)expression of the DHA sequence of the invention, or by using the polypeptide {amorphadiene synthase) of the invention, can be applied for these purposes as well. The plants that can be used for this invention are preferably plants already producing artemisinin. A prime example is Artemisia annua- as this species contains the remainder of the pathway leading to artemisinin. However, this invention may also be used for the production of amorphadiene in plants, which, as mentioned before, can be used as a flavor or fragrance conipound or biocide, or can be converted to artemisinin, either chemically or by bioconveraion using microorganisms, yeasts or plant cells. The plant that can be used for the production of amorphadiene is preferably a plant already producing sesquiterpenes, as these plants already have the basic pathway and storage compartments available, or a plant in which the biosynthesis of sesquiterpenoids can be induced by elicitation. The methods of this invention are readily applicable via conventional techniques to numerous plant species, including for example species from the genera Carum, glBhorimp. Daucus. Junlperue. Chamomilla. LactUSa., Poqpostemon and Vetiveria. and species of the inducible (by elicitation) sesquiterpenoid phytoalexin producing genera Capsicum. Goasypium. Lvcopersicon. Nicotiana Phleum Bolanum and Ulmus- However, also common agricultural crops like soybean, sunflower and rapeseed are interesting candidates here. The invention will be further illustrated by the following examples, but will not be limited thereto. In the example reference is made to the following figures: rig. 1: Structural formula of arternisinln. Fig. 3: Postulated biosynthetic pathway of artemisinin in A.annua. Fig. 3; Transition between part II and III of Fig. 2: hypothetical conversion of amorphadiene into dihydroarteannuic acid in A. annua Fig. 4: Structural formula of amorpha-4,11-diene. Fig. 5: Radio-OC chromatograms of the [3H]-PPP-assays. A. Flame Ionization Detector (FID) signal of amorphadiene (reference). B. Radio signals of the ^H labeled assay products amorphadiene (retention time 14 min.) and famesol (as a product of aspecific phosphohydroiase activity, retention time 28 min.) obtained with crude enzyme extract. C. Radio signal of the 'H labeled assay product amorphadiene obtained with Mono-Q purified enzyme extract. Fig. 6: Mass epectrum of reference amorphadiene compared with mass spectrum of the FPP assay with terpene cyclases (synthases} purified from A. annua This comparison yielded a quality score of 99%, corresponding with a maximum score of identicalness. Fig. 7: Probe generated by PCR and cloned into pOEM 7Zf*. Fig. 8 : Nucleotide sequence (SEQ ID N0:9) and deduced amino acid sequence (SEQ ID NO:10) of the probe (538 bp) generated by PCR with primers A and B. rig. 9: Released plasmid of a positive clone isolated from the cDHA library of induced Ai flilPUa ■ Fig. 10 : Nucleotide sequence (SEQ IDN0:11) and deduced amino acid sequence (SEQ ID N0:12) of a positive clone (amorphadiene synthase encoding gene) isolated from the cDNA library of induced A.annua. The sequence (SEQ ID N0:11) is flanked with EcgRI (NotI) adapters (Gibco BRL). Fig . 11: Part, between start and stop codon (flanked by Mpol and BamHI Bites, respectively) , o£ the amorphadiane synthase encoding gene cloned in the NcoI/BamHI site of the expression vector pET lid. Fig. 12 : Nucleotide sequence (SEQ ID NO: 13) and deduced amino acid sequence (SEQ ID NO:14) of the amorphadlene synthase encoding gene, between start and stop codon (flanked by NcoI and BamHI sites, respectively) , obtained by PCR with primers C and D. Fig. 13: SDS-PAGE gel: lanes 1 and 2 show pellet and supernatant of pET lid, respectively (negative control) ; lanes 3 and 4 show pellet and supernatant of tobacco 5-epi-aristoloehene synthase (TEAS) gene in pET lid (positive control), lanes 5, 7, 9 and 6, 8, lO, respectivaly show pellet and supernatant of amorphadiene synthase in pET lid. All constructs were expressed in E,poll BL21 (DE3). The lanes with the pellet fractions of TEAS in pET lid (positive controls) and anorphadiene synthase in pGT lid show a clear spot which was not present in the negative control pET lid. Mw is low Molecular Weight marker (Pharmacia Biotech). rig. 14: A. Flame Ionization Detector (FID) signals of amorpha-4,ll-dlene and farnesol (references); E. Radio-GC chromstograms of the [1H]-ppp-assays with Intact BL21 (DEB) cells, transformed with the amorphadiene synthase encoding gene in the expression vector pET lid; C. Radio-GC chromatograms of the ['H]-FPP-assays with the supernatant of sonicated BL21 {DE3) cells, transformed with the amorphadiene synthase encoding gene in the ei^ression vector pET lid. Fig. 1$: Hypothetical chemical synthesis of dihydroarteannuic acid using amorpha-4,ii-diene as a precursor. The reaction consists of an enantio-, stereo-and region selective (anti-markownikoff) hydroboration of amorphadiene with BH^ followed by an oxidation of the formed trialkylboranes with NaOH/H^O, yielding the alcohol. A mild oxidation of the alcohol to the acid can be obtained with PDC (pyridinium dichromate) without attacking the second double bond. Fig. 16 Determination of the molecular weight of ainorpha-4,ll-diene synthase by size-exclusion chromatography (gel filtration). -*- is activity curve; -*- ie molecular weight marker is molecular weight calibration line. EXAMPLE 1 Conversion of farnesyl pyrophosphate into amorphadiene bv amorphadiene synthase A. Isolation, partial purification and identification of amorphadiene synthase from A■annua During enzyme isolation and preparation of the assays, all operations were carried out on ice or at 4''C. Ten grams of frozen young leaves from greenhouse-grown A.annua were ground in a pre-chilled mortar and pestle in 40 ml of pre-chilled buffer containing 25 mM MES (pH 5.5), 20% (v/v) glycerol, 25 mM sodium ascorbate, 2S mM NaHSO3, 10 mM MgCl, and 5 mM DTT (buffer A) slurried with 1 g polyvinylpblypyrrolidone [PVPP) and a spatula tip of purified sea sand. Ten grams of polystyrene resin (Amberlite XAD-4, ServaJ were added and the slurry was stirred carefully for 10 min and then filtered through cheesecloth. The filtrate was centrifuged at 20,000g for 20 min (pellet discarded), and then at lOO.DOOg for 90 min. A 3-ml subsample of the supernatant was desalted to a buffer containing 15 mM MOPSO (pH 7.0), 10* (v/v) glycerol, 1 mM sodium ascorbate, 10 mM MgCl, and 2 mM DTT {buffer B) and used for enzyme assays/product identification (see below at 'B'). The remainder of the supernatant was added to 12.5 g DEAE anion exchanger {Whatman DE-52), which had been rinsed several times with buffer A, and stirred carefully for 10 min. After cencrifugation at IB.OOOg for 20 min, the supernatant was decanted and the DE-52 pellet discarded. Protein in Che supernatant were precipitated by adding (HH,);so, to a final concentration of 70%, careful stirring for 3D rain, and centrifugation at 2D,000g for 10 min. The resulting pellet was resuapended in 6 ml buffer A and desalted to buffer B. After addition of glycerol up to 30% (v/v) this enzyme preparation could be frozen in liquid N2 and stored at -80C without loss of activity. 0.5 ml of this enzyme preparation was applied to a Mono-Q FPLC column (HH5/5, Pharmacia Biotech) , previously equilibrated with buffer B without sodium ascorbate, with 0.1% Tween-20. The enzyme was eluced with 3 gradient of 0-2.0 M KCl in the same buffer. For determination of enzyme activities, 50 μ1 of the 0.75~ml fractions were diluted 2-fold in an Eppendorf tube with buffer B and 20 μM [3H]FPP was added. The reaction mixture was overlaid with 1 ml of hexane to trap volatile products and the contents mixed. After incubation for 30 min at 30C, the vials were vigorously mixed, and centrifuged briefly to separate phases. A portion of the hexane phase (750 μ1} was transferred to a new Eppendorf tube containing 40 mg of silica gel (0.035-0.07 mm, pore diameter 6 nm, Janssen Chimica) to bind terpenols produced by phoaphohydrolases, and, after mixing and centrifugation, 500 fi.1 of the hexane layer was removed for liquid scintillation counting in 4.5 ml of Ultima Gold cocktail (Packard). The active fractions were combined, and an assay carried out to determine product identity (see below]. After the Mono-Q step, the enzyme was separated from all other FPP-converting activities (Fig. 5C). This enzyme preparation was used for the measurement of enzyme characteristics such as molecular weight and K4. The molecular weight was determined using size-exclusion chromatography. 200 μl of the Mono-Q sluent was loaded on a Superdex 75 {H/RlD/30, Pharmacia Biotech! and eluted in the same buffer as used for Hono-2- Enzyme activities in 0.5 ml fractions were determined as described for Mono-Q, but using undiluted eluent. The ::olumn was calibrated using cytochrome C, ribonuclease A, o-chymotrypsinogen, ovalbumin and BSA (all from Sigma). The estiraatod molecular weight was 56 kDa (Fig. 16) . Enzywie-kinetics were determined using 5- and 10-fold diluted Mono-Q eluted enzyme preparation and [^H] -PPP concentrations ranging from 0.25-100μM. Km for amorphadiene synthase was 0.6 (M. B, Determination of product identity For determination of product identity, 20 μM [3H]-FPP (Amersham; for radio-GC analysis) or 50 μH unlabeiled FPP {Sigma,- for GC-MS analysis) were added to 1 ml of the enzyme preparations. After the addition of a 1 ml redistilled pentane overlay to Crap volatile products, the tubes were carefully mixed and incubated for I h at 30''C. Boiled samples were used as controls. Following the assay, the tubes were vigorously mixed. The organic layer was removed and passed over a short column of aluminum oxide overlaid with anhydrous MgSO4. The assay was extracted with another 1 ml of diethyl ether which was also passed over the aluminum oxide column, and the column washed with i.s ml of diethyl-ether. For GC-analysis, the combined pentane/diethyl-ether mixture was slowly concentrated under a stream of N2. Radio-GLc was performed on a Carlo-Erba 4160 Series gas chromatograph equipped with a RAGA-9D radioactivity detector (Raytest, Straubenhardt, Germany). Sample components eluting from the column were quantitatively reduced before radioactivity measurement by passage through a conversion reactor filled with platinum chips at 800C. Samples of 1 μl were injected in the cold on-column mode. The column was a fused silica capillary (30 m x 0.32 ram i.d.) coated with a film of 0.25 μm of polyethylene glycol (EconoCap EC-WAX, Alltech Associates) and operated with a He-flow of 1.2 ml min-1 The oven tamperatute was programmed to VCC for 5 min, followed by a ramp of 5" min'' to 210''C and a final time of 5 min. To determine retention times and peak identities (by co-elution of radioactivity with reference scandarde), about 20% of the column effluent was split with an adjustable splitter to an FID (temperature 270C) . The remainder was directed to the conversion reactor and radio detector. H2 was added prior to the reactor at 3 ml min-1, and CH, as a quench gas prior to the radioactivity detector (5 ml counting tube) to give a total flow of 36 ml min2. The major [3H}-labeled product co-eluted with the amorphadiene reference standard (retention time 14 min) (Fig. 5B). The second radiolabeled product ie famesol, the product of aspecific phosphohydrolase activity. After the Mono-Q step, the enzyme was separated from all other FPP-converting activities (Fig. 5C). This enzyme preparation was used for the measurement of enzyme characteristics such as molecular weight and Kn. GC-MS analysis was performed using a HP 5890 series II GC and HP 5972A. Maaa Selective Detector (Hewlett-Packard) equipped with an HP-5MS or HP-Innowax column (both 30 m x 0.25 mm i.d., 0.25 μm df) . The oven was programmed at an initial temperature of 70c for l min, with a ranp of 5C min-1 to 210C and final time of 5 min. The injection port {splitless mode), interface and MS source temperatures were 175, 290 and 180°C, respectively, and the He inlet pressure was controlled by electronic pressure control to achieve a constant column flow of 1.0 ml min'^. Ionization potential was set at 70 ev, and scanning was performed from 30-250 amu. The (NH4)2SO4 precipitated enzyme preparation was free of endogenous sesquiterpenes. QC-MS analysis on the two different GC-columns of sesquiterpene products generated from FPP by this enzyme preparation showed that the main product had a mass spectrum and retention time equal to that of the semi-synthetically produced amorphadiene (Pig. 6) . EXAMPLE 2 Isolation and charaeterization of Che amorphadiene synthase encoding aene A, Induction of transcription As revealed in part III of Fig. 2, DHAA is photo-oxidatively converted into DHRA-CX)H. In this reaction a reactive form of oxygen {singlet o2) is added to DHAA. DHAA plays the role of an anti-oxidant, a scavenger of reactive oxygen species. Artemieinin is Che stable end product of this reaction in which reactive oxygen is stored. Under stress conditions, (for example photo-stresB, frost, drought or mechanical damage) reactive species of oxygen are formed in Che plant. In response to this reactive oxygen generally plants are producing anti-oxidants. it is likely that A.annua will produce DHAA as antl-oxidant in response to this release 3f reactive oxygen. By exposing A.annua to stress conditions the transcription of the gene encoding amorphadiene synthase will be induced. To achieve this situation A. annua plants grown under climate room conditions (23''C, 90% moisture, 3000 lux) were exposed to stress conditions by putting them for one hour at approximately 30% moisture (drought stress and 6000 lux. [photo stress) at 30C, i. Isolation of total RHA Total RNA of stress induced plants (according .o example 2.A) was isolated from young leaves by the method of Verwoerd et al. (Nucleic Acids Research I7(£), 1362 (1989)). DNase 1 (Deoxyribonuclease I, RMase free) 'as used to remove DHA from the RNA isolate. The DNase I 'as inactivated by exposure at 70c during 15 minutes. '. cDNA synthesis The reverse transcription reaction was carried out in a 20 μl reaction containing 5 (μg total RNA, 0.2 /μg oligo (dT)j, D.5 mM each dATP, dXTP, dCTP and dGTP, 10 mM DTT, 2 U ribonucleaee inhibitor (Gibco BRL), first strand synthesis buffer (Protnega) and catalyzed with 200 U Moloney murine leukemia virus (M-MLV) reverse transcriptase RNase H minus (Promega). After 1 h incubation at 37C the reaction was stopped by storing the reaction mixture at -20'C. D. PCR-based probe generation Based on comparison of sequences of terpenoid synthases, two degenerated primers were designed for two conserved regions. The sequence of the sense primer (primer A) was 5'-GA(C/T) GA(G/A) AA(C/T) GGI AR{G/A) TT(C/T) AA(G/A) GA-3' and the sequence of the anti sense primer (primer Bl vas S'-CC IG/AITA IGC (G/A)TC (G/A)AA IGT Kpnl, gindlll upstream from the terwinator. The orientation of the amorpha-4,11-diene encoding gene in pLV399 was checked by restriction analysis with Pstl and Ndel. After partial digestion of this construct with Rpnl the amorphs-4,ll-diene encoding gene flanked by the 35S promoter and nos terminator was ligated into the Kpnl digested binary vector pCGNI1548. To mobilize the recombinant binary vector to Acrobactarium tumofacians I.AA4404 {Gibco BRL. Life Technologies), a triparental mating procedure was carried out by using B.eoli (DMSa) carrying the recombinant binary vector and a helper E.coli carrying the plasmid pRK2013 to mobilize the recombinant binary vector to fi. tumefaciene.ene LBA44 04. This transformed Aarobaeterium strain was used for transformation of explants from the target plant speciee. Only the transformed tissue carrying a resistance marker (kanamycin-resistance, present between the binary plaamid T-DNA borders) regenerated on a selectable (kanamycin containing) regeneration medium. [According to Rogers SG, Horsch RB, Fraley RT methods Enzymol (1986)118: 621-640). The plants regenerated out of the transformed tissue expressed the araorphadiene synthase gene as followed from the presence therein of amorphadiene as confirmed by GC-MS analyses. EXAMPLE 6 Conversion nf amorphadiene into artemiainin (DHAA) by A.annua This assay was carried out in a way analogous to the method as described by Koepp et al. (The Journal of Biological Chemistry 270, 8686-6690 (1995)). Radioactive (2H-labeled) amorphadiene was fed to leaf discs of A. annua. For the infiltration of amorphadiene into the leaf discs of A-annua the radioactive amorphadiene can be made water soluble by complexation with cyclodextrina, for example. Radioactive amorphadiene is obtained by using the PPP-assay with the transformed E,coli BL21(DE3) cells (carrying the cloned amorphadiene synthetase gene of A.annual, Identification of the product(s) made in this assay was done by radio-GC analysis. The expected intermediates arteannuic acid (AA) , dihydroarteannuic acid (DUAA) and the end product artemieinin were all used as references. A mixture of a-cyclodextrin, S-cyclodextrin, y-cyclodextrin, and partially 'H-labeled amorpha-4,11-diene (20 μm) in a molar ratio of 5:5:5:1 was prepared and A.annua leaf discs were incubated in this mixture. After 120 hours of incubation artemisinic acid and dihydroartemisinic acid could be detected by radio-GC in a way analogous to part B of example 1. EXAMPLE 7 Expression of amorpha-4 11- diene synthanae in transgenic: A .annua and the production of artemisinin Transformed A.nnu plants were prepared as deacribed in example 5. For the regeneration of A. annua the medium for callus, shoot and root induction consisted of Hurashige and Skoog micro and macro elements including modified vitamins (Duchefa Biochemie, Haarlem, The Netherlands), 4% (w/v) sucrose, 0.1 mg/L Indole-3-aceCic acid (ZAA) , 0.1 mg/L 6-benzylaminopurine (BAPl and 0.8% (w/v) agar (Plant agar, Duchefa Blochenie, Haarlem, the Netherlands) . The pH was adjusted to 5.7 with WaOH prior to the addition of agar. The medium was autoclaved at 1 bar for 20 min. Transformed explants were regenerated on this medium to fully regenerated plants, The regenerated plants were found to over-express the enzyme amorpha-4,ll-dien€ synthase which led to production of artenisinic acid, dihydroartemisinic acid, and artemistinin at a level above the natural level in non-transformed plants. EXAMPLE 8 Expression of the amorpha 4 11- diene synthase gene in Saccharomyces eerevisiae and Piehia paatoria Por functional ei^ression the cDHA clone was subcloned into the inducible expression vector pYBS2 (episomal vector. Invitrogen} and the constitutive expression vector (integrating the gene construct into the genome) pOAPZ A (Invitrogen). To introduce suieable restriction sites for aubcloning, the gene was amplified by PCR using a sense primer (primer E) 5'-CGA Gftft TTC XTO TCA CTT ACA a-3' (SEQ |D NO:7) (introducing a EcsBI site preceding the start codon ATO) and an anti-sense primer (primer F) 5'-GOAT CTC Gftg TCA TAT ACT CAT-3' (SEQ ID N0;8)( introducing a fiaaHI site directly behind the stop codon TO*). Subcloning of the PCR product into pyES2 and pGAPZ A was done in a way analogue to Example 3. The obtained gene constructs were transformed to respectively Saccharomyces cerevisiae and pichia pastoria using the S perevisiae BasyComp™ transformation kit (Invitrogen) to tranaform S carevisiae and the Pichia Eaaycomp™ traneformation kit (Invitrogen) for transformation of P.pastoris. All transformations were carried out according to the instructions of the manufacturer. Growth, selection and induction were also performed in accordance to the inetructions of the manufacturer. Harvesting and sonication of the yeast cells was done in an analogous way to the method as described in. Example. The FPP assay with the extracts of the yeast cells in which the amorpha-4,11-diene synthase gene was expressed yielded identical GC-RAGA and QC-MS chromatograms as obtained in example 4. WE CLAIM : 1. A recombinant host cell comprising a DNA sequence encoding a polypeptide having the biological activity of amorpha-4,ll-diene synthase which exhibits at least 70%, 80%, 90% or 95% homology to the sequence as shown in SEQ. ID NO: 13 or the complementary strand thereof; or an isolated DNA sequence as shown in SEQ. ID NO: 13 or the complementary strand thereof; or a DNA construct comprising the said DNA sequence operably linked to suitable transcription initiation and termination sequence, wherein the cell is a plant cell. 2. The recombinant host cell as claimed in claim 1, wherein Ihe cell is derived from a plant itself producing sesquiterpenes. 3. The recombinant host cell as claimed in claim 2, wherein the cell is an A. annua cell or a V. oblongifolia cell. 4. The recombinant host cell as claimed in claim 2, wherein the cell is derived from a plant selected from the group consisting of the genera Carum, Cichorium, Daucus, Juniperus, Chamomilla, Lacluca, Pogostemon and Vetiveria. 5. The recombinant host cell as claimed in claim 1, wherein the cell is derived from a plant in which the biosynthesis of sesquiterpenoids can be induced by elicitation. 6. The recombinant host cell as claimed in claim 5, wherem the cell is derived from a plant selected from the group consisting of the genera Capsicum, Gossypium, Lycopersicon, Nicotiana, Phleum, Solanum and Ulmus. 7. The recombinant host cell as claimed in claim 1, wherein the cell is derived from a plant selected from the group of soybean, sunflower and rapeseed. 8. The recombinant host cell as claimed in any of the preceding claims, wherein the cell further comprises the genetic information coding for the enzymes that fimher convert amorpha-4,ll-diene to artemisinin, which host cell has expressed the said DNA sequence. 9. The recombinant host cell as claimed in claim 8, wherein the cells are disrupted. 10. Transgenic organism harboring in its genome more copies of a DNA sequence, that encodes a polypeptide having the biological activity of amorpha-4,11 -diene synthase which exhibits at least 70%, 80%, 90% or 95% homology to the sequence as shown in SEQ. ID NO: 13 or the complementary strand thereof; or an isolated DNA sequence as shown in SEQ. ID NO: 13 or the complementary strand thereof, than are present in a corresponding non-transgenic organism. 11. Transgenic organism as claimed in claim 10, which organism is a plant itself producing sesquiterpenes. 12. Transgenic organism as claimed in claim 11, which organism is A. annua or V. oblongifolia. 13. Transgenic organism as claimed in claim 11, which organism is a plant selected from the group consisting of the genera Carum, Cichorium, Daucus, Juniperus, Chamomilla, Lactuca, Pogostemon and Vetiveria. 14. Transgenic organism as claimed in claim 10, which organism is a plant in which the biosynthesis of sesquiterpenoids can be induced by elicitation. 15. Transgenic organism as claimed in claim 14, which organism is a plant selected from the group consisting of the genera Capsicum, Gossypium, Lycopersicon, Nicotiana, Phleum, Solanum and Ulmus. 16. Transgenic organism as claimed in claim 10, which organism is a plant selected from the group consisting of soybean, sunflower and rapeseed. 17. Source of artemisinin, comprising the host cells as claimed in any of claims 1 to 9. 18. The source as claimed in claim 17, wherein the cells are disrupted. Dated this 18 day of July 2008