NUCLEIC ACID MOLECULES ENCODING ISOPENTENYL MONOPHOSPHATE KINASE, AND METHODS OF USE

Abstract: A cDNA encoding isopentenyl monophosphate kinase (IPK) from peppermint (<i>Mentha x piperita</i>) has been isolated and sequenced, and the corresponding amino acid sequence has been determined. Accordingly, an isolated DNA sequence (SEQ ID NO:1) is provided which codes for the expression of isopentenyl monophosphate kinase (SEQ ID NO:2), from peppermint (<i>Mentha x piperita</i>). In other aspects, replicable recombinant cloning vehicles are provided which code for isopentenyl monophosphate kinase, or for a base sequence sufficiently complementary to at least a portion of isopentenyl monophosphate kinase DNA or RNA to enable hybridization therewith. In yet other aspects, modified host cells are provided that have been transformed, transfected, infected and/or injected with a recombinant cloning vehicle and/or DNA sequence encoding isopentenyl monophosphate kinase. Thus, systems and methods are provided for the recombinant expression of the aforementioned recombinant isopentenyl monophosphate kinase that may be used to facilitate its production, isolation and purification in significant amounts. Recombinant isopentenyl monophosphate kinase may be used to obtain expression or enhanced expression of isopentenyl monophosphate kinase in plants in order to enhance the production of isopentenyl monophosphate kinase, or isoprenoids derived therefrom, or may be otherwise employed for the regulation or expression of isopentenyl monophosphate kinase, or the production of its products.

Description: NUCLEIC ACID MOLECULES ENCODING ISOPENTENYL MONOPHOSPHATE KINASE, AND METHODS OF USE Government Rights This invention was funded in part by United States Department of Energy grant number DE-FG03-96ER20212. The government has certain rights in this invention. Field of the Invention This invention relates to nucleic acid sequences encoding isopentenyl monophosphate kinase, in particular to nucleic acid sequences encoding isopentenyl monophosphate kinase from peppermint. Background of the Invention Isopentenyl diphosphate (IPP) is the central intermediate in the biosynthesis of isoprenoids in all organisms. In higher plants, the formation of IPP is compartmentalized. The mevalonate (MVA) pathway, the enzymes of which are localized to the cytosolic compartment, produces the precursor of triterpenes (sterols) and certain sesquiterpenes (Newman, J.D. & Chappell, J., Crit. Rev. Biochem. Mol. Biol., 34:95-106 [1999]). In plastids, the deoxyxylulose-5-phosphate (DXP) pathway operates to supply IPP for the synthesis of monoterpenes and diterpenes (Eisenreich, W. et al., Tetrahedron Lett., 38:3889-3892 [1997]; Eisenreich, W. et al., Proc. Natl. Acad. Sci. USA, 93:6431-6436 [1996]), several sequiterpenes (McCaskill, D. & Croteau, R., Planta, 197:49-56 [1995]), tetraterpenes (carotenoids), and the prenyl side-chains of chlorophyll and plastoquinone (Lichtenthaler, H.K. et al., FEBS Lett., 400:271-274 [1997]). In addition, there are examples of cooperation between the cytosolic and plastidial pathways in the biosynthesis of stress-induced and constitutively emitted volatile terpenoids from a variety of plants (Piel, J. et al., Angew. Chem. Int. Ed., 37:2478-2481 [1998]), and constitutive sesquiterpenes of chamomile (Adam, K.-P. & Zapp, J., Phytochemistry, 48:953-959 [1998]). In mammals, where the DXP pathway is not known to operate, and in plants, the individual biosynthetic steps of the MVA pathway have been well-characterized (Goldstein, J.L. & Brown, M.S., Nature (London), 343:425-430 [1990]; Bach, T.J., Crit. Rev. Biochem. Mol. Biol., 34:107-122 [1999]). However, for the recently discovered DXP pathway, which also occurs in many eubacteria (Rohmer, M., Prog. Drug Res., 50:135-154 [1998]), the biosynthetic sequence leading to the formation of IPP is still incompletely defined (The FIGURE). The initial step of the pathway involves a condensation of pyruvate (C2 and C3) with D-glyceraldehyde-3-phosphate (GAP) to yield 1 -deoxy-D-xylulose-5- phosphate (Rohmer, M., Biochem. J., 295:517-524 [1993]; Broers, S.T.J., Ph.D. thesis, Eidgenossische Technische Hochschule, Zurich, Switzerland [1994]; Schwarz, M.K., Ph.D. thesis, Eidgenossische Technische Hochschule, Zurich, Switzerland [1994]; Rohmer, M. et al., J. Am. Chem. Soc, 118:2564-2566 [1996]). The enzyme which catalyzes this reaction belongs to a novel family of transketolases, and the corresponding gene has been isolated from Escherichia coli (Sprenger, G.A. et al., Proc. Natl. Acad. Sci. USA, 94:12857-12862 [1997]; Lois, L.M. et al., Proc. Natl. Acad. Sci. USA, 95:2105-2110 [1997]), peppermint (Lange, B.M. et al., Proc. Natl. Acad. Sci. USA, 95:2100-2104 [1998]) and pepper (Bouvier, F. et al., Plant Physiol., 117:1423-1431 [1998]). In the second step of this pathway, rearrangement and reduction of DXP yield 2-C-methyl-D-erythritol (MEP) (Duvold, T. et al., Tetrahedron Lett., 38:4769-4772 [1997]; Duvold, T. et al., Tetrahedron Lett., 38:6181-6184 [1997]; Sagner, S. et al., Tetrahedron Lett., 39:2091-2094 [1998]) (The FIGURE). Recently, genes encoding this DXP reductoisomerase (DXR) have been cloned from E. coli (Takahashi, S. et al., Proc. Natl. Acad. Sci. USA, 95:9879- 9884 [1998]), peppermint (Lange, B.M. & Croteau R., Arch. Biochem. Biophys., 365:170-174 [1999]), and Arabidopsis thaliana (Lange, B.M. & Croteau R., Arch. Biochem. Biophys., 365:170-174 [1999]; Schwender, J. et al., FEBS Lett., 455:140- 144 [1999]). To date, no other intermediates on the route to IPP, the terminal product of the DXP pathway (McCaskill, D. & Croteau R., Tetrahedron Lett., 40:653-656 [1999]; Arigoni, D. et al., Proc. Natl. Acad. Sci. USA, 96:1309-1314 [1999]), have been identified. As disclosed herein, sequencing of 1300 anonymous clones (expressed sequence tags, ESTs) from a cDNA library constructed from mRNA isolated from the oil gland secretory cells of peppermint (Mentha x piperita) (McCaskill, D. & Croteau, R., Planta, 197:49-56 [1995]), afforded, after extensive database comparisons, two clones having homologues of unknown function in plants and eubacteria, the sequences of which contained a motif with homology to the putative ATP -binding domain of the GHMP (galactokinase, homoserine kinase, mevalonate kinase, and phosphomevalonate kinase) family of metabolite kinases. This putative kinase gene from peppermint and its E. coli orthologue, when overexpressed in E. coli, yielded a recombinant enzyme that catalyzes the ATP-dependent phosphorylation of isopentenol monophosphate (IP) to IPP. Feeding experiments with IP and several other isoprenoid precursors, using isolated peppermint secretory cells, confirmed the phosphorylation of IP to IPP to be the last step in the DXP pathway. Summary of the Invention In accordance with the foregoing, a cDNA encoding isopentenyl monophosphate kinase (IPK) from peppermint (Mentha x piperita) has been isolated and sequenced, and the corresponding amino acid sequence has been deduced. Accordingly, the present invention relates to isolated DNA sequences which code for the expression of isopentenyl monophosphate kinase, such as the sequence designated SEQ ID NO:l which encodes an isopentenyl monophosphate kinase protein (SEQ ID NO:2) from peppermint (Mentha x piperita). Additionally, the present invention relates to isolated, recombinant isopentenyl monophosphate kinase proteins, such as the isolated, recombinant isopentenyl monophosphate kinase protein from peppermint (Mentha x piperita) (SEQ ID NO:2). In other aspects, the present invention is directed to rephcable recombinant cloning vehicles comprising a nucleic acid sequence, e.g., a DNA sequence which codes for an isopentenyl monophosphate kinase, or for a base sequence sufficiently complementary to at least a portion of DNA or RNA encoding isopentenyl monophosphate kinase to enable hybridization therewith (e.g., antisense RNA or fragments of DNA complementary to a portion of DNA or RNA molecules encoding isopentenyl monophosphate kinase which are useful as polymerase chain reaction primers or as probes for isopentenyl monophosphate kinase or related genes). In yet other aspects of the invention, modified host cells are provided that have been transformed, transfected, infected and/or injected with a recombinant cloning vehicle and/or DNA sequence of the invention. Thus, the present invention provides for the recombinant expression of isopentenyl monophosphate kinase, and the inventive concepts may be used to facilitate the production, isolation and purification of significant quantities of recombinant isopentenyl monophosphate kinase (or of its primary enzyme products) for subsequent use, to obtain expression or enhanced expression of isopentenyl monophosphate kinase in plants, microorganisms or animals, or may be otherwise employed in an environment where the regulation or expression of isopentenyl monophosphate kinase is desired for the production of this kinase, or its enzyme product, or derivatives thereof. Brief Description of the Drawings The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: The FIGURE shows the DXP pathway for the biosynthesis of IPP, and the proposed role of IPK. The circled P denotes the phosphate moiety. The large open arrow indicates several as yet unidentified steps. Detailed Description of the Preferred Embodiment As used herein, the terms "amino acid" and "amino acids" refer to all naturally occurring L-α-amino acids or their residues. The amino acids are identified by either the single-letter or three-letter designations: Asp D aspartic acid He I isoleucine Thr T threonine Leu L leucine Ser S serine Tyr Y tyrosine Glu E glutamic acid Phe F phenylalanine Pro P proline His H histidine Gly G glycine Lys K lysine Ala A alanine Arg R arginine Cys C cysteine Trp W tryptophan Val V valine Gin Q glutamine Met M methionine Asn N asparagine As used herein, the term "nucleotide" means a monomeric unit of DNA or RNA containing a sugar moiety (pentose), a phosphate and a nitrogenous heterocyclic base. The base is linked to the sugar moiety via the glycosidic carbon (V carbon of pentose) and that combination of base and sugar is called a nucleoside. The base characterizes the nucleotide with the four bases of DNA being adenine ("A"), guanine ("G"), cytosine ("C") and thymine ("T"). Inosine ("I") is a synthetic base that can be used to substitute for any of the four, naturally-occurring bases (A, C, G or T). The four RNA bases are A,G,C and uracil ("U"). The nucleotide sequences described herein comprise a linear array of nucleotides connected by phosphodiester bonds between the 3' and 5' carbons of adjacent pentoses. Abbreviations used herein include: DMA, dimethylallyl alcohol; DMAP, dimefhylallyl monophosphate; DMAPP, dimethylallyl diphosphate; DX, deoxyxylulose; DXP, deoxyxylulose-5-phosphate; DXPS, l-deoxyxylulose-5- phosphate synthase; DXR, deoxyxylulose-5-phosphate reductoisomerase; EST, expressed sequence tag; GAP, glyceraldehyde-3-phosphate; IP, isopentenyl monophosphate; IPK, isopentenyl monophosphate kinase; IPP, isopentenyl diphosphate; ISO, isopentenol; MEP, 2-C-methylerythritol-4-phosphate; MVA, mevalonate; PCR, polymerase chain reaction. The term "percent identity" (%I) means the percentage of amino acids or nucleotides that occupy the same relative position when two amino acid sequences, or two nucleic acid sequences, are aligned side by side. The term "percent similarity" (%S) is a statistical measure of the degree of relatedness of two compared protein sequences. The percent similarity is calculated by a computer program that assigns a numerical value to each compared pair of amino acids based on observed amino acid replacements in closely related sequences. Calculations are made after a best fit alignment of the two sequences has been made empirically by iterative comparison of all possible alignments. (Henikoff, S. and Henikoff, J.G., Proc. Nat'l Acad Sci USA 89:10915-10919 [1992]). "Oligonucleotide" refers to short length single or double stranded sequences of deoxyribonucleotides linked via phosphodiester bonds. The oligonucleotides are chemically synthesized by known methods and purified, for example, on polyacrylamide gels. The term "isopentenyl monophosphate kinase" is used herein to mean an enzyme capable of catalyzing the ATP-dependent phosphorylation of isopentenol monophosphate (IP) to isopentenyl diphosphate IPP. The term "essential oil plant," or "essential oil plants," refers to a group of plant species that produce high levels of monoterpenoid and/or sesquiterpenoid and/or diterpenoid oils, and/or high levels of monoterpenoid and/or sesquiterpenoid and/or diterpenoid resins. The foregoing oils and/or resins account for greater than about 0.005% of the fresh weight of an essential oil plant that produces them. The essential oils and/or resins are more fully described, for example, in E. Guenther, The Essential Oils, Vols. I-VI, R.E. Krieger Publishing Co., Huntington N.Y., 1975, incorporated herein by reference. The essential oil plants include, but are not limited to: Lamiaceae, including, but not limited to, the following species: Ocimum (basil), Lavandula (Lavender), Origanum (oregano), Mentha (mint), Salvia (sage), Rosmarinus (rosemary), Thymus (thyme), Satureja and Monarda. Umhelliferae, including, but not limited to, the following species: Carum (caraway), Anethum (dill), feniculum (fennel) and Daucus (carrot). Asteraceae (Compositae), including, but not limited to, the following species: Artemisia (tarragon, sage brush), Tanacetum (tansy). Rutaceae (e.g., citrus plants); Rosaceae (e.g., roses); Myrtaceae (e.g., eucalyptus, Melaleuca); the Gramineae (e.g., Cymbopogon (citronella)); Geranaceae (Geranium) and certain conifers including Abies (e.g., Canadian balsam), Cedrus (cedar), Thuja, Pinus (pines) and Juniperus. The range of essential oil plants is more fully set forth in E. Guenther, The Essential Oils, Vols. I-VI, R.E. Krieger Publishing Co., Huntington N.Y., 1975, which is incorporated herein by reference. The term "angiosperm" refers to a class of plants that produce seeds that are enclosed in an ovary. The term "gymnosperm" refers to a class of plants that produce seeds that are not enclosed in an ovary. The terms "alteration", "amino acid sequence alteration", "variant" and "amino acid sequence variant" refer to isopentenyl monophosphate kinase molecules with some differences in their amino acid sequences as compared to native isopentenyl monophosphate kinase. Ordinarily, the variants will possess at least about 70% homology with native isopentenyl monophosphate kinase, and preferably they will be at least about 80% homologous with native isopentenyl monophosphate kinase. The amino acid sequence variants of isopentenyl monophosphate kinase falling within this invention possess substitutions, deletions, and/or insertions at certain positions. Sequence variants of isopentenyl monophosphate kinase may be used to attain desired enhanced or reduced enzymatic activity, or altered substrate utilization or product distribution of isopentenyl monophosphate kinase. Substitutional isopentenyl monophosphate kinase variants are those that have at least one amino acid residue in the native isopentenyl monophosphate kinase sequence removed and a different amino acid inserted in its place at the same position. The substitutions may be single, where only one amino acid in the molecule has been substituted, or they may be multiple, where two or more amino acids have been substituted in the same molecule. Substantial changes in the activity of the isopentenyl monophosphate kinase molecule may be obtained by substituting an amino acid with a side chain that is significantly different in charge and/or structure from that of the native amino acid. This type of substitution would be expected to affect the structure of the polypeptide backbone and/or the charge or hydrophobicity of the molecule in the area of the substitution. Moderate changes in the activity of the isopentenyl monophosphate kinase molecule would be expected by substituting an amino acid with a side chain that is similar in charge and/or structure to that of the native molecule. This type of substitution, referred to as a conservative substitution, would not be expected to substantially alter either the structure of the polypeptide backbone or the charge or hydrophobicity of the molecule in the area of the substitution. Insertional isopentenyl monophosphate kinase variants are those with one or more amino acids inserted immediately adjacent to an amino acid at a particular position in the native isopentenyl monophosphate kinase molecule. Immediately adjacent to an amino acid means connected to either the α-carboxy or α-amino functional group of the amino acid. The insertion may be one or more amino acids. Ordinarily, the insertion will consist of one or two conservative amino acids. Amino acids similar in charge and/or structure to the amino acids adjacent to the site of insertion are defined as conservative. Alternatively, this invention includes insertion of an amino acid with a charge and/or structure that is substantially different from the amino acids adjacent to the site of insertion. Deletional variants are those where one or more amino acids in the native isopentenyl monophosphate kinase molecule have been removed. Ordinarily, deletional variants will have one or two amino acids deleted in a particular region of the isopentenyl monophosphate kinase molecule. The terms "biological activity", "biologically active", "activity" and "active," when used with reference to isopentenyl monophosphate kinase, refer to the ability of isopentenyl monophosphate kinase to catalyze the ATP -dependent phosphorylation of isopentenol monophosphate (IP) to isopentenyl diphosphate IPP, as measured in an enzyme activity assay, such as the assay described in Example 1 below. Amino acid sequence variants of isopentenyl monophosphate kinase may have desirable, altered biological activity including, for example, altered reaction kinetics, substrate utilization product distribution or other characteristics. The terms "DNA sequence encoding", "DNA encoding" and "nucleic acid encoding" refer to the order or sequence of deoxyribonucleotides along a strand of deoxyribonucleic acid. The order of these deoxyribonucleotides determines the order of amino acids along the translated polypeptide chain. The DNA sequence thus codes for the amino acid sequence. The terms "rephcable expression vector" and "expression vector" refer to a piece of DNA, usually double-stranded, which may have inserted into it a piece of DNA called the insert DNA, which is usually foreign DNA. Foreign DNA is defined as heterologous DNA, which is DNA not naturally found in the host. The vector is used to transport the insert DNA into a suitable host cell. Once in the host cell, the vector can replicate independently of or coincidental with the host chromosomal DNA, and several copies of the vector and its inserted DNA may be generated. In addition, the vector contains the necessary elements that permit translating the insert DNA into a polypeptide. Many molecules of the polypeptide encoded by the insert DNA can thus be rapidly synthesized. The terms "transformed host cell," "transformed" and "transformation" refer to the introduction of DNA into a cell. The cell is termed a "host cell", and it may be a prokaryotic or a eukaryotic cell. Typical prokaryotic host cells include various strains of E. coli. Typical eukaryotic host cells are plant cells, such as maize cells, yeast cells, insect cells or animal cells. The introduced DNA is usually in the form of a vector containing an inserted piece of DNA. The introduced DNA sequence may be from the same species as the host cell or from a different species from the host cell, or it may be a hybrid DNA sequence, containing some foreign DNA and some DNA derived from the host species. In accordance with the present invention, a cDNA encoding isopentenyl monophosphate kinase was isolated and sequenced in the following manner. A λZAP cDNA library was constructed from mRNA obtained from isolated peppermint oil gland secretory cells. Randomly picked and purified clones were excised in vivo and inserts of the resulting phagemids were partially sequenced. An apparently full- length IP kinase clone (designated ml 100) (SEQ ID NO:l) was acquired by this means and was used as a template to amplify by PCR a 1218 base pair fragment (corresponding to nucleic acid residues 3-1217 of SEQ ID NO:l) using the primers 5 , -ATGGCTTCCTCCT-CCCATTTCCTC-3 , (forward) (SEQ ID NO:3) and 5'-TTCAGCATCCTGAGGAAAAGACGG-3' (reverse) (SEQ ID NO:4), which was subsequently cloned into the expression vector pBAD TOPO TA (Invitrogen). Clone ml 100 (SEQ ID NO: l) shared a region of high sequence similarity to the putative ATP -binding domain of the GHMP family of kinases. The expressed enzyme strongly tended to form inclusion bodies, and soluble protein could not be obtained in sufficient quantity for affinity-based purification. Thus, crude extracts of sonicated, transformed cells served as the enzyme source for kinase assays. Consistently, the expressed recombinant peppermint kinase gave detectably elevated levels of activity with isopentenol monophosphate and isopentenol as substrates, when compared to the E. coli background. No phosphokinase activity was detected with dimethylallyl monophosphate (DMAP) or MVA as substrate. Kinase activity with dimethyallyl alcohol (DMA) as a substrate, under the standard assay conditions, was always detectable, but never exceeded 0.01 pmol (s g protein) '1 . With DXP, deoxylylulose (DX) and MEP as substrates, kinase activity of less than 0.2 pmol (s g protein) '1 was occasionally detected but in most experiments no activity was observed. A putative E. coli IP kinase gene (SEQ ID NO:5), encoding the protein having the amino acid sequence set forth in (SEQ ID NO: 6) was amplified by PCR using the primers 5'-ATGCGGACACAGTGGCCCTC-3' (forward) (SEQ ID NO:7) and 5'-AAGCATGGCTCTGTGCAATG-3' (reverse) (SEQ ID NO: 8), and genomic DNA from the strain K-12 MG1655 (wild-type) as a template. For expression, the amplicon was cloned into pBAD TOPO TA (Invitrogen) and transformed into E. coli strain TOP 10 One Shot. Assay of crude extracts gave results similar to those obtained with the recombinant peppermint enzyme. However, in this instance, the expressed, soluble enzyme, produced upon induction with 0.02% arabinose, allowed a useful one-step affinity-based purification with a Ni 2+ -binding column. This partially purified protein readily catalyzed the conversion of IP to IPP, ISO to IP, and DMA to DMAP; however, no phosphorylating activity was detectable with DMAP, DXP, DX, MEP or MVA as substrate. This kinase activity (with IP as a substrate) was dependent upon the presence of MgCl 2 as divalent cation and ATP as phosphate donor, whereas CTP, UTP and GTP did not serve as alternate phosphate donors. These results with the enzyme products of the peppermint ml 100 clone (SEQ ID NO:l) and the E. coli ychB clone (SEQ ID NO:5) indicate that this gene (SEQ ID NO:l) encodes an isopentenyl monophosphate kinase (IPK) that is involved in the DXP pathway to isoprenoids. The isolation of a cDNA encoding isopentenyl monophosphate kinase permits the development of an efficient expression system for this protein, provides a useful tool for examining the developmental regulation of isoprenoid biosynthesis and permits the isolation of other isopentenyl monophosphate kinases. The isolation of an isopentenyl monophosphate kinase cDNA also permits the transformation of a wide range of organisms, for example to modify endogenous isoprenoid biosynthesis. Although the isopentenyl monophosphate kinase protein set forth in SEQ ID NO:2 directs the enzyme to plastids, substitution of the putative targeting sequence (SEQ ID NO:2, amino acids 1 to 98) with other transport sequences well known in the art (see, e.g., von Heijne G et al., Eur. J. Biochem 180:535-545 [1989]; Stryer, Biochemistry W.H. Freeman and Company, New York, NY, p. 769 [1988]) may be employed to direct the isopentenyl monophosphate kinase to other cellular or extracellular locations. In addition to native isopentenyl monophosphate kinase amino acid sequences, such as the native isopentenyl monophosphate kinase amino acid sequence of SEQ ID NO:2, sequence variants produced by deletions, substitutions, mutations and/or insertions are intended to be within the scope of the invention except insofar as limited by the prior art. Isopentenyl monophosphate kinase amino acid sequence variants may be constructed by mutating the DNA sequence that encodes wild-type isopentenyl monophosphate kinase, such as by using techniques commonly referred to as site-directed mutagenesis. Various polymerase chain reaction (PCR) methods now well known in the field, such as a two primer system like the Transformer Site-Directed Mutagenesis kit from Clontech, may be employed for this purpose. Following denaturation of the target plasmid in this system, two primers are simultaneously annealed to the plasmid; one of these primers contains the desired site-directed mutation, the other contains a mutation at another point in the plasmid resulting in elimination of a restriction site. Second strand synthesis is then carried out, tightly linking these two mutations, and the resulting plasmids are transformed into a mutS strain of E. coli. Plasmid DNA is isolated from the transformed bacteria, restricted with the relevant restriction enzyme (thereby linearizing the unmutated plasmids), and then retransformed into E. coli. This system allows for generation of mutations directly in an expression plasmid, without the necessity of subcloning or generation of single-stranded phagemids. The tight linkage of the two mutations and the subsequent linearization of unmutated plasmids results in high mutation efficiency and allows minimal screening. Following synthesis of the initial restriction site primer, this method requires the use of only one new primer type per mutation site. Rather than prepare each positional mutant separately, a set of "designed degenerate" oligonucleotide primers can be synthesized in order to introduce all of the desired mutations at a given site simultaneously. Transformants can be screened by sequencing the plasmid DNA through the mutagenized region to identify and sort mutant clones. Each mutant DNA can then be restricted and analyzed by electrophoresis on Mutation Detection Enhancement gel (J.T. Baker) to confirm that no other alterations in the sequence have occurred (by band shift comparison to the unmutagenized control). The verified mutant duplexes can be cloned into a rephcable expression vector, if not already cloned into a vector of this type, and the resulting expression construct used to transform E. coli, such as strain E. coli BL21(DE3)pLysS, for high level production of the mutant protein, and subsequent purification thereof. The method of FAB-MS mapping can be employed to rapidly check the fidelity of mutant expression. This technique provides for sequencing segments throughout the whole protein and provides the necessary confidence in the sequence assignment. In a mapping experiment of this type, protein is digested with a protease (the choice will depend on the specific region to be modified since this segment is of prime interest and the remaining map should be identical to the map of unmutagenized protein). The set of cleavage fragments is fractionated by microbore HPLC (reversed phase or ion exchange, again depending on the specific region to be modified) to provide several peptides in each fraction, and the molecular weights of the peptides are determined by FAB-MS. The masses are then compared to the molecular weights of peptides expected from the digestion of the predicted sequence, and the correctness of the sequence quickly ascertained. Since this mutagenesis approach to protein modification is directed, sequencing of the altered peptide should not be necessary if the MS agrees with prediction. If necessary to verify a changed residue, CAD-tandem MS/MS can be employed to sequence the peptides of the mixture in question, or the target peptide purified for subtractive Edman degradation or carboxypeptidase Y digestion depending on the location of the modification. In the design of a particular site directed mutant, it is generally desirable to first make a non-conservative substitution (e.g., Ala for Cys, His or Glu) and determine if activity is greatly impaired as a consequence. The properties of the mutagenized protein are then examined with particular attention to the kinetic parameters of K m and k cat as sensitive indicators of altered function, from which changes in binding and/or catalysis per se may be deduced by comparison to the native enzyme. If the residue is by this means demonstrated to be important by activity impairment, or knockout, then conservative substitutions can be made, such as Asp for Glu to alter side chain length, Ser for Cys, or Arg for His. For hydrophobic segments, it is largely size that will be altered, although aromatics can also be substituted for alkyl side chains. Changes in the normal product formation can indicate which step(s) of the reaction sequence have been altered by the mutation. Other site directed mutagenesis techniques may also be employed with the nucleotide sequences of the invention. For example, restriction endonuclease digestion of DNA followed by ligation may be used to generate isopentenyl monophosphate kinase deletion variants, as described in section 15.3 of Sambrook et al. (Molecular Cloning: A Laboratory Manual, 2nd Ed., Cold Spring Harbor Laboratory Press, New York, NY [1989]). A similar strategy may be used to construct insertion variants, as described in section 15.3 of Sambrook et al., supra. Oligonucleotide-directed mutagenesis may also be employed for preparing substitution variants of this invention. It may also be used to conveniently prepare the deletion and insertion variants of this invention. This technique is well known in the art as described by Adelman et al. (DNA 2:183 [1983]). Generally, oligonucleotides of at least 25 nucleotides in length are used to insert, delete or substitute two or more nucleotides in the isopentenyl monophosphate kinase molecule. An optimal oligonucleotide will have 12 to 15 perfectly matched nucleotides on either side of the nucleotides coding for the mutation. To mutagenize the wild-type isopentenyl monophosphate kinase, the oligonucleotide is annealed to the single-stranded DNA template molecule under suitable hybridization conditions. A DNA polymerizing enzyme, usually the Klenow fragment of E. coli DNA polymerase I, is then added. This enzyme uses the oligonucleotide as a primer to complete the synthesis of the mutation-bearing strand of DNA. Thus, a heteroduplex molecule is formed such that one strand of DNA encodes the wild-type isopentenyl monophosphate kinase inserted in the vector, and the second strand of DNA encodes the mutated form of isopentenyl monophosphate kinase inserted into the same vector. This heteroduplex molecule is then transformed into a suitable host cell. Mutants with more than one amino acid substituted may be generated in one of several ways. If the amino acids are located close together in the polypeptide chain, they may be mutated simultaneously using one oligonucleotide that codes for all of the desired amino acid substitutions. If however, the amino acids are located some distance from each other (separated by more than ten amino acids, for example) it is more difficult to generate a single oligonucleotide that encodes all of the desired changes. Instead, one of two alternative methods may be employed. In the first method, a separate oligonucleotide is generated for each amino acid to be substituted. The oligonucleotides are then annealed to the single-stranded template DNA simultaneously, and the second strand of DNA that is synthesized from the template will encode all of the desired amino acid substitutions. An alternative method involves two or more rounds of mutagenesis to produce the desired mutant. The first round is as described for the single mutants: wild-type isopentenyl monophosphate kinase DNA is used for the template, an oligonucleotide encoding the first desired amino acid substitution(s) is annealed to this template, and the heteroduplex DNA molecule is then generated. The second round of mutagenesis utilizes the mutated DNA produced in the first round of mutagenesis as the template. Thus, this template already contains one or more mutations. The oligonucleotide encoding the additional desired amino acid substitution(s) is then annealed to this template, and the resulting strand of DNA now encodes mutations from both the first and second rounds of mutagenesis. This resultant DNA can be used as a template in a third round of mutagenesis, and so on. The gene, or other nucleic acid molecule, encoding isopentenyl monophosphate kinase may be incorporated into any organism (intact plant, animal, microbe), or cell culture derived therefrom, that produces isopentenol monophosphate and ATP to effect the ATP-dependent conversion of these primary substrates to isopentenol diphosphate and its subsequent metabolic products, depending on the organism. By way of non-limiting example, an isopentenyl monophosphate kinase gene (or other nucleic acid molecule encoding isopentenyl monophosphate kinase) may be introduced into a plant in order to increase flux through the isoprenoid biosynthetic pathway that produces carotenoids, chlorophyll, plastoquinone, essential oils, resins, phytoalexins (such as casbene). The resulting transgenic plants can be selected for such improved characteristics as: improved plant fitness, improved defense capabilities against pests and pathogens, improved quality traits (such as color, flavor, vitamin content, antioxidants, nutrients and nutraceuticals) and improved yield of useful chemicals (such as pigments, vitamins, essential oils, resins, waxes and synthetic intermediates). Moreover, and by way of non-limiting example, a nucleic acid molecule encoding an isopentenyl monophosphate kinase protein can be subjected to mutagenesis in order to create isopentenyl monophosphate kinase mutant proteins that are resistant to isopentenyl monophosphate kinase-specific herbicides. Additionally, the isolated, recombinant isopentenyl monophosphate kinase proteins of the present invention can be used, for example, in studies to identify novel antibiotics, herbicides and anti-malarial drugs directed to isopentenyl monophosphate kinase. Eukaryotic expression systems may be utilized for isopentenyl monophosphate kinase production since they are capable of carrying out any required posttranslational modifications and of directing the enzyme to the proper membrane location. A representative eukaryotic expression system for this purpose uses the recombinant baculovirus, Autographa californica nuclear polyhedrosis virus (AcNPV; M.D. Summers and G.E. Smith, A Manual of Methods for Baculovirus Vectors and Insect Cell Culture Procedures [1986]; Luckow et al., Bio-technology 6:47-55 [1987]) for expression of the isopentenyl monophosphate kinase of the invention. Infection of insect cells (such as cells of the species Spodoptera frugiperda) with the recombinant baculoviruses allows for the production of large amounts of the isopentenyl monophosphate kinase protein. In addition, the baculovirus system has other important advantages for the production of recombinant isopentenyl monophosphate kinase. For example, baculoviruses do not infect humans and can therefore be safely handled in large quantities. In the baculovirus system, a DNA construct is prepared including a DNA segment encoding isopentenyl monophosphate kinase and a vector. The vector may comprise the polyhedron gene promoter region of a baculovirus, the baculovirus flanking sequences necessary for proper cross-over during recombination (the flanking sequences comprise about 200-300 base pairs adjacent to the promoter sequence) and a bacterial origin of replication which permits the construct to replicate in bacteria. The vector is constructed so that (i) the DNA segment is placed adjacent (or operably linked or "downstream" or "under the control of) to the polyhedron gene promoter and (ii) the promoter/isopentenyl monophosphate kinase combination is flanked on both sides by 200-300 base pairs of baculovirus DNA (the flanking sequences). To produce the isopentenyl monophosphate kinase DNA construct, a cDNA clone encoding the full length isopentenyl monophosphate kinase is obtained using methods such as those described herein. The DNA construct is contacted in a host cell with baculovirus DNA of an appropriate baculovirus (that is, of the same species of baculovirus as the promoter encoded in the construct) under conditions such that recombination is effected. The resulting recombinant baculoviruses encode the full isopentenyl monophosphate kinase. For example, an insect host cell can be cotransfected or transfected separately with the DNA construct and a functional baculovirus. Resulting recombinant baculoviruses can then be isolated and used to infect cells to effect production of the isopentenyl monophosphate kinase. Host insect cells include, for example, Spodoptera frugiperda cells, that are capable of producing a baculovirus-expressed isopentenyl monophosphate kinase. Insect host cells infected with a recombinant baculovirus of the present invention are then cultured under conditions allowing expression of the baculovirus-encoded isopentenyl monophosphate kinase. Isopentenyl monophosphate kinase thus produced is then extracted from the cells using methods known in the art. Other eukaryotic microbes such as yeasts may also be used to practice this invention. The baker's yeast Saccharomyces cerevisiae, is a commonly used yeast, although several other types are available. The plasmid YRp7 (Stinchcomb et al., Nature 282:39 [1979]; Kingsman et al., Gene 7:141 [1979]; Tschemper et al., Gene 10:157 [1980]) is commonly used as an expression vector in Saccharomyces. This plasmid contains the trpl gene that provides a selection marker for a mutant strain of yeast lacking the ability to grow in the absence of tryptophan, such as strains ATCC No. 44,076 and PEP4-1 (Jones, Genetics 85:12 [1977]). The presence of the trpl lesion as a characteristic of the yeast host cell genome then provides an effective environment for detecting transformation by growth in the absence of tryptophan. Yeast host cells are generally transformed using the polyethylene glycol method, as described by Hinnen (Proc. Natl. Acad. Sci. USA 75:1929 [1978]. Additional yeast transformation protocols are set forth in Gietz et al., N.A.R. 20(17): 1425, 1992; Reeves et al., FEMS 99:193-197, 1992. Suitable promoting sequences in yeast vectors include the promoters for 3-phosphoglycerate kinase (Hitzeman et al., J. Biol. Chem. 255:2073 [1980]) or other glycolytic enzymes (Hess et al., J. Adv. Enzyme Reg. 7:149 [1968]; Holland et al., Biochemistry 17:4900 [1978]), such as enolase, glyceraldehyde-3-phosphate dehydrogenase, hexokinase, pyruvate decarboxylase, phosphofructokinase, glucose-6-phosphate isomerase, 3-phosphoglycerate mutase, pyruvate kinase, triose- phosphate isomerase, phosphoglucose isomerase, and glucokinase. In the construction of suitable expression plasmids, the termination sequences associated with these genes are also ligated into the expression vector 3' of the sequence desired to be expressed to provide polyadenylation of the mRNN and termination. Other promoters that have the additional advantage of transcription controlled by growth conditions are the promoter region for alcohol dehydrogenase 2, isocytochrome C, acid phosphatase, degradative enzymes associated with nitrogen metabolism, and the aforementioned glyceraldehyde-3-phosphate dehydrogenase, and enzymes responsible for maltose and galactose utilization. Any plasmid vector containing yeast-compatible promoter, origin of replication and termination sequences is suitable. Cell cultures derived from multicellular organisms, such as plants, may be used as hosts to practice this invention. Transgenic plants can be obtained, for example, by transferring plasmids that encode isopentenyl monophosphate kinase and a selectable marker gene, e.g., the kan gene encoding resistance to kanamycin, into Agrobacterium tumifaciens containing a helper Ti plasmid as described in Hoeckema et al., Nature 303:179-181 [1983] and culturing the Agrobacterium cells with leaf slices of the plant to be transformed as described by An et al., Plant Physiology 81:301-305 [1986]. Transformation of cultured plant host cells is normally accomplished through Agrobacterium tumifaciens, as described above. Cultures of mammalian host cells and other host cells that do not have rigid cell membrane barriers are usually transformed using the calcium phosphate method as originally described by Graham and Van der Eb (Virology 52:546 [1978]) and modified as described in sections 16.32-16.37 of Sambrook et al., supra. However, other methods for introducing DΝA into cells such as Polybrene (Kawai and Νishizawa, Mol. Cell. Biol. 4:1172 [1984]), protoplast fusion (Schaffher, Proc. Natl. Acad. Sci. USA 77:2163 [1980]), electroporation (Neumann et al., EMBO J. 1:841 [1982]), and direct microinjection into nuclei (Capecchi, Cell 22:479 [1980]) may also be used. Additionally, animal transformation strategies are reviewed in Monastersky G.M. and Robl, J.M., Strategies in Transgenic Animal Science, ASM Press, Washington, DC, 1995. Transformed plant calli may be selected through the selectable marker by growing the cells on a medium containing, e.g., kanamycin, and appropriate amounts of phytohormone such as naphthalene acetic acid and benzyladenine for callus and shoot induction. The plant cells may then be regenerated and the resulting plants transferred to soil using techniques well known to those skilled in the art. In addition, a gene regulating isopentenyl monophosphate kinase production can be incorporated into the plant along with a necessary promoter which is inducible. In the practice of this embodiment of the invention, a promoter that only responds to a specific external or internal stimulus is fused to the target cDNA. Thus, the gene will not be transcribed except in response to the specific stimulus. As long as the gene is not being transcribed, its gene product is not produced. An illustrative example of a responsive promoter system that can be used in the practice of this invention is the glutathione-S-transferase (GST) system in maize. GSTs are a family of enzymes that can detoxify a number of hydrophobic electrophilic compounds that often are used as pre-emergent herbicides (Weigand et al., Plant Molecular Biology 7:235-243 [1986]). Studies have shown that the GSTs are directly involved in causing this enhanced herbicide tolerance. This action is primarily mediated through a specific 1.1 kb mRNA transcription product. In short, maize has a naturally occurring quiescent gene already present that can respond to external stimuli and that can be induced to produce a gene product. This gene has previously been identified and cloned. Thus, in one embodiment of this invention, the promoter is removed from the GST responsive gene and attached to a isopentenyl monophosphate kinase gene that previously has had its native promoter removed. This engineered gene is the combination of a promoter that responds to an external chemical stimulus and a gene responsible for successful production of isopentenyl monophosphate kinase. In addition to the methods described above, several methods are known in the art for transferring cloned DNA into a wide variety of plant species, including gymnosperms, angiosperms, monocots and dicots (see, e.g., Glick and Thompson, eds., Methods in Plant Molecular Biology, CRC Press, Boca Raton, Florida [1993], incorporated by reference herein). Representative examples include electroporation- facilitated DNA uptake by protoplasts in which an electrical pulse transiently permeabilizes cell membranes, permitting the uptake of a variety of biological molecules, including recombinant DNA (Rhodes et al., Science, 240:204-207 [1988]); treatment of protoplasts with polyethylene glycol (Lyznik et al., Plant Molecular Biology, 13:151-161 [1989]); and bombardment of cells with DNA-laden microprojectiles which are propelled by explosive force or compressed gas to penetrate the cell wall (Klein et al., Plant Physiol. 91:440-444 [1989] and Boynton et al., Science, 240:1534-1538 [1988]). Transformation of Taxus species can be achieved, for example, by employing the methods set forth in Han et al., Plant Science, 95:187-196 [1994], incorporated by reference herein. N method that has been applied to Rye plants (Secale cereale) is to directly inject plasmid DΝN, including a selectable marker gene, into developing floral tillers (de la Pena et al., Nature 325:274-276 [1987]). Further, plant viruses can be used as vectors to transfer genes to plant cells. Examples of plant viruses that can be used as vectors to transform plants include the Cauliflower Mosaic Virus (Brisson et al., Nature 310:511-514 [1984]). Additionally, plant transformation strategies and techniques are reviewed in Birch, R.G., Ann Rev Plant Phys Plant Mol Biol, 48:297 [1997]; Forester et al., Exp. Agric, 33:15-33 [1997]. The aforementioned publications disclosing plant transformation techniques are incorporated herein by reference, and minor variations make these technologies applicable to a broad range of plant species. Each of these techniques has advantages and disadvantages. In each of the techniques, DΝA from a plasmid is genetically engineered such that it contains not only the gene of interest, but also selectable and screenable marker genes. A selectable marker gene is used to select only those cells that have integrated copies of the plasmid (the construction is such that the gene of interest and the selectable and screenable genes are transferred as a unit). The screenable gene provides another check for the successful culturing of only those cells carrying the genes of interest. A commonly used selectable marker gene is neomycin phosphotransferase II (ΝPT II). This gene conveys resistance to kanamycin, a compound that can be added directly to the growth media on which the cells grow. Plant cells are normally susceptible to kanamycin and, as a result, die. The presence of the ΝPT II gene overcomes the effects of the kanamycin and each cell with this gene remains viable. Another selectable marker gene which can be employed in the practice of this invention is the gene which confers resistance to the herbicide glufosinate (Basta). A screenable gene commonly used is the β-glucuronidase gene (GUS). The presence of this gene is characterized using a histochemical reaction in which a sample of putatively transformed cells is treated with a GUS assay solution. After an appropriate incubation, the cells containing the GUS gene turn blue. Preferably, the plasmid will contain both selectable and screenable marker genes. The plasmid containing one or more of these genes is introduced into either plant protoplasts or callus cells by any of the previously mentioned techniques. Ifthe marker gene is a selectable gene, only those cells that have incorporated the DNA package survive under selection with the appropriate phytotoxic agent. Once the appropriate cells are identified and propagated, plants are regenerated. Progeny from the transformed plants must be tested to insure that the DNA package has been successfully integrated into the plant genome. Mammalian host cells may also be used in the practice of the invention. Examples of suitable mammalian cell lines include monkey kidney CVI line transformed by SV40 (COS-7, ATCC CRL 1651); human embryonic kidney line 293S (Graham et al., J. Gen. Virol. 36:59 [1977]); baby hamster kidney cells (BHK, ATCC CCL 10); Chinese hamster ovary cells (Urlab and Chasin, Proc. Natl. Acad. Sci USA 77:4216 [1980]); mouse sertoli cells (TM4, Mather, Biol. Reprod. 23:243 [1980]); monkey kidney cells (CVI-76, ATCC CCL 70); African green monkey kidney cells (VERO-76, ATCC CRL- 1587); human cervical carcinoma cells (HELA, ATCC CCL 2); canine kidney cells (MDCK, ATCC CCL 34); buffalo rat liver cells (BRL 3A, ATCC CRL 1442); human lung cells (W138, ATCC CCL 75); human liver cells (Hep G2, HB 8065); mouse mammary tumor cells (MMT 060562, ATCC CCL 51); rat hepatoma cells (HTC, MI.54, Baumann et al., J. Cell Biol. 85:1 [1980]); and TRI cells (Mather et al., Annals N.Y. Acad. Sci. 383:44 [1982]). Expression vectors for these cells ordinarily include (if necessary) DNA sequences for an origin of replication, a promoter located in front of the gene to be expressed, a ribosome binding site, an RNA splice site, a polyadenylation site, and a transcription terminator site. Promoters used in mammalian expression vectors are often of viral origin. These viral promoters are commonly derived from polyoma virus, Adenovirus 2, and most frequently Simian Virus 40 (SV40). The SV40 virus contains two promoters that are termed the early and late promoters. These promoters are particularly useful because they are both easily obtained from the virus as one DNA fragment that also contains the viral origin of replication (Fiers et al., Nature 273:113 [1978]). Smaller or larger SV40 DNA fragments may also be used, provided they contain the approximately 250-bp sequence extending from the Hindlll site toward the Bgll site located in the viral origin of replication. Alternatively, promoters that are naturally associated with the foreign gene (homologous promoters) may be used provided that they are compatible with the host cell line selected for transformation. An origin of replication may be obtained from an exogenous source, such as SV40 or other virus (e.g., Polyoma, Adeno, VSV, BPV) and inserted into the cloning vector. Alternatively, the origin of replication may be provided by the host cell chromosomal replication mechanism. If the vector containing the foreign gene is integrated into the host cell chromosome, the latter is often sufficient. The use of a secondary DNA coding sequence can enhance production levels of isopentenyl monophosphate kinase in transformed cell lines. The secondary coding sequence typically comprises the enzyme dihydrofolate reductase (DHFR). The wild-type form of DHFR is normally inhibited by the chemical methotrexate (MTX). The level of DHFR expression in a cell will vary depending on the amount of MTX added to the cultured host cells. An additional feature of DHFR that makes it particularly useful as a secondary sequence is that it can be used as a selection marker to identify transformed cells. Two forms of DHFR are available for use as secondary sequences, wild-type DHFR and MTX-resistant DHFR. The type of DHFR used in a particular host cell depends on whether the host cell is DHFR deficient (such that it either produces very low levels of DHFR endogenously, or it does not produce functional DHFR at all). DHFR-deficient cell lines such as the CHO cell line described by Urlaub and Chasin, supra, are transformed with wild- type DHFR coding sequences. After transformation, these DHFR-deficient cell lines express functional DHFR and are capable of growing in a culture medium lacking the nutrients hypoxanthine, glycine and thymidine. Nontransformed cells will not survive in this medium. The MTX-resistant form of DHFR can be used as a means of selecting for transformed host cells in those host cells that endogenously produce normal amounts of functional DHFR that is MTX sensitive. The CHO-K1 cell line (ATCC No. CL 61) possesses these characteristics, and is thus a useful cell line for this purpose. The addition of MTX to the cell culture medium will permit only those cells transformed with the DNA encoding the MTX-resistant DHFR to grow. The nontransformed cells will be unable to survive in this medium. Prokaryotes may also be used as host cells for the initial cloning steps of this invention. They are particularly useful for rapid production of large amounts of DNA, for production of single-stranded DNA templates used for site-directed mutagenesis, for screening many mutants simultaneously, and for DNA sequencing of the mutants generated. Suitable prokaryotic host cells include E. coli K12 strain 294 (ATCC No. 31,446), E. coli strain W3110 (ATCC No. 27,325) E. coli XI 776 (ATCC No. 31,537), and E. coli B; however many other strains of E. coli, such as HB101, JM101, NM522, NM538, NM539, and many other species and genera of prokaryotes including bacilli such as Bacillus subtilis, other enterobacteriaceae such as Salmonella typhimurium or Serratia marcesans, and various Pseudomonas species may all be used as hosts. Prokaryotic host cells or other host cells with rigid cell walls are preferably transformed using the calcium chloride method as described in section 1.82 of Sambrook et al., supra. Alternatively, electroporation may be used for transformation of these cells. Prokaryote transformation techniques are set forth in Dower, W. J., Genetic Engineering, Principles and Methods, 12:275-296, Plenum Publishing Corp. [1990]; Hanahan et al., Meth. Enxymol, 204:63 [1991]. As will be apparent to those skilled in the art, any plasmid vectors containing replicon and control sequences that are derived from species compatible with the host cell may also be used in the practice of the invention. The vector usually has a replication site, marker genes that provide phenotypic selection in transformed cells, one or more promoters, and a polylinker region containing several restriction sites for insertion of foreign DNA. Plasmids typically used for transformation of E. coli include pBR322, pUC18, pUC19, pUCIlδ, pUC119, and Bluescript Ml 3, all of which are described in sections 1.12-1.20 of Sambrook et al., supra. However, many other suitable vectors are available as well. These vectors contain genes coding for ampicillin and/or tetracycline resistance which enables cells transformed with these vectors to grow in the presence of these antibiotics. The promoters most commonly used in prokaryotic vectors include the β-lactamase (penicillinase) and lactose promoter systems (Chang et al. Nature 375:615 [1978]; Itakura et al., Science 198:1056 [1977]; Goeddel et al., Nature 281:544 [1979]) and a tryptophan (tip) promoter system (Goeddel et al., Nucl. Acids Res. 8:4057 [1980]; ΕPO Appl. Publ. No. 36,776), and the alkaline phosphatase systems. While these are the most commonly used, other microbial promoters have been utilized, and details concerning their nucleotide sequences have been published, enabling a skilled worker to ligate them functionally into plasmid vectors (see Siebenlist et al., Cell 20:269 [1980]). Many eukaryotic proteins normally secreted from the cell contain an endogenous secretion signal sequence as part of the amino acid sequence. Thus, proteins normally found in the cytoplasm can be targeted for secretion by linking a signal sequence to the protein. This is readily accomplished by ligating DNA encoding a signal sequence to the 5' end of the DNA encoding the protein and then expressing this fusion protein in an appropriate host cell. The DNA encoding the signal sequence may be obtained as a restriction fragment from any gene encoding a protein with a signal sequence. Thus, prokaryotic, yeast, and eukaryotic signal sequences may be used herein, depending on the type of host cell utilized to practice the invention. The DNA and amino acid sequence encoding the signal sequence portion of several eukaryotic genes including, for example, human growth hormone, proinsulin, and proalbumin are known (see Stryer, Biochemistry W.H. Freeman and Company, New York, NY, p. 769 [1988]), and can be used as signal sequences in appropriate eukaryotic host cells. Yeast signal sequences, as for example acid phosphatase (Arima et al., Nuc. Acids Res. 11:1657 [1983]), alpha-factor, alkaline phosphatase and invertase may be used to direct secretion from yeast host cells. Prokaryotic signal sequences from genes encoding, for example, LamB or OmpF (Wong et al., Gene 68:193 [1988]), MalE, PhoA, or beta-lactamase, as well as other genes, may be used to target proteins from prokaryotic cells into the culture medium. The isopentenyl monophosphate kinase protein having the sequence set forth in SEQ ID NO:2 includes a putative amino terminal membrane insertion sequence at residues 1 through 98, and in the embodiment shown in SEQ ID NO:2 directs the enzyme to plastids. Alternative trafficking sequences from plants, animals and microbes can be employed in the practice of the invention to direct the gene product to the cytoplasm, endoplasmic reticulum, mitochondria or other cellular components, or to target the protein for export to the medium. These considerations apply to the overexpression of isopentenyl monophosphate kinase, and to direction of expression within cells or intact organisms to permit gene product function in any desired location. The construction of suitable vectors containing DNA encoding replication sequences, regulatory sequences, phenotypic selection genes and the isopentenyl monophosphate kinase DNA of interest are prepared using standard recombinant DNA procedures. Isolated plasmids and DNA fragments are cleaved, tailored, and ligated together in a specific order to generate the desired vectors, as is well known in the art (see, for example, Sambrook et al., supra). As discussed above, isopentenyl monophosphate kinase variants are preferably produced by means of mutation(s) that are generated using the method of site-specific mutagenesis. This method requires the synthesis and use of specific oligonucleotides that encode both the sequence of the desired mutation and a sufficient number of adjacent nucleotides to allow the oligonucleotide to stably hybridize to the DNA template. The foregoing may be more fully understood in connection with the following representative examples, in which "Plasmids" are designated by a lower case p followed by an alphanumeric designation. The starting plasmids used in this invention are either commercially available, publicly available on an unrestricted basis, or can be constructed from such available plasmids using published procedures. In addition, other equivalent plasmids are known in the art and will be apparent to the ordinary artisan. "Digestion", "cutting" or "cleaving" of DNA refers to catalytic cleavage of the DNA with an enzyme that acts only at particular locations in the DNA. These enzymes are called restriction endonucleases, and the site along the DNA sequence where each enzyme cleaves is called a restriction site. The restriction enzymes used in this invention are commercially available and are used according to the instructions supplied by the manufacturers. (See also sections 1.60-1.61 and sections 3.38-3.39 of Sambrook et al., supra.) "Recovery" or "isolation" of a given fragment of DNA from a restriction digest means separation of the resulting DNA fragment on a polyacrylamide or an agarose gel by electrophoresis, identification of the fragment of interest by comparison of its mobility versus that of marker DNA fragments of known molecular weight, removal of the gel section containing the desired fragment, and separation of the gel from DNA. This procedure is known generally. For example, see Lawn et al. (Nucleic Acids Res. 9:6103-6114 [1982]), and Goeddel et al. (Nucleic Acids Res., supra). The following examples merely illustrate the best mode now contemplated for practicing the invention, but should not be construed to limit the invention. Example 1 Substrates. Cloning Strategies and Enzymatic Assays Unless stated otherwise, the following substrates, cloning strategies and enzymatic assays were utilized in the cloning, expression and characterization of mint isopentenyl monophosphate kinase (SEQ ID NO:2), and its bacterial ortholog (SEQ ID NO:6). Substrates: [l- 3 H]Dimethylallyl diphosphate (555 GBq/mmol 1 ) was purchased from American Radiolabeled Chemicals and was adjusted to a specific activity of 2.13 GBq/mmol "1 by dilution with the unlabeled compound (purchased from Sigma). [4- 14 C]Isopentenyl diphosphate (2.13 GBq/mmol '1 ) was obtained from DuPont/NEN. [l- 3 H]Dimethylallyl monophosphate and [4- 14 C]isopentenyl monophosphate were generated enzymatically by treatment of the parent diphosphates (0.17 μmol of each) with apyrase (Sigma, catalogue number A6410) under the following conditions: 10 units of apyrase in 200 μl of 0.2 M succinate buffer, pH 6.0, for 5 min at 23°C. Extended incubation (1 h) under the same conditions with a mixture of apyrase and acid phosphatase (Sigma, catalogue number A6535) yielded [l- 3 H]dimethylallyl alcohol and [4- 14 C]isopentenol. Each phosphorylated product was purified by semi-preparative ion-pair HPLC, and identity was established by dephosphorylation and subsequent GC-MS analysis of the derived alcohol using methods modified from a published protocol (Lange, B.M. & Croteau R., Arch. Biochem. Biophys., 365:170-174 [1999]). [l- 14 C]l-Deoxy-D- xylulose-5-phosphate and [2- 14 C]2-C-methyl-D-erythritol were prepared enzymatically from [2- 14 C]pyruvic acid (DuPont/NEN, 0.59 GBq/mmol '1 ) as described previously (Lange, B.M., Wildung, M., McCaskill, D. & Croteau R., Proc. Natl. Acad. Sci. USA, 95:2100-2104 [1998]; Lange, B.M. & Croteau R., Arch. Biochem. Biophys., 365:170-174 [1999]). R,S_-[2- 14 C]Mevalonic acid (2.12 GBq/mmol "1 ) was purchased from DuPont/NEN. Bacterial Strains and Plasmid Constructs: A λZAP cDNA library was constructed from mRNA obtained (Logemann, J., Schell, J. & Willmitzer, L.. Anal. Biochem., 163: 16-20 [1987]) from isolated peppermint oil gland secretory cells (McCaskill, D. & Croteau, R., Planta, 197:49-56 [1995]) according to the manufacturer's instructions (Stratagene). Randomly picked and purified clones were excised in vivo and inserts of the resulting pBluescript SK " phagemids were partially sequenced from both ends. An apparently full-length, peppermint IP kinase clone (designated mll00)(SEQ ID NO:l) was acquired by this means and was used as a template to amplify by PCR the portion of the sequence of SEQ ID NO:l extending from residue 3 to residue 1217, using the primers 5'-ATGGCTTCCTCCT-CCCATTTCCTC-3' (forward) (SEQ ID NO:3) and 5'-TTCAGCATCCTGAGGAAAAGACGG-3* (reverse) (SEQ ID NO:4), which was subsequently cloned into the expression vector pBAD TOPO TA (Invitrogen). E. coli strain BL21-CodonPlus-RIL (F " ompT, hsdS(r B m B " ), dc ø Tet R , gal, endA, Hte, [argU, ileY, leuW, Cam R ]; Invitrogen) served as host in the transformation. The putative E. coli IP kinase gene (SΕQ ID NO:5) was amplified by PCR using the primers 5'-ATGCGGACACAGTGGCCCTC-3' (forward) (SΕQ ID NO:7) and 5'-AAGCATGGCTCTGTGCAATG-3' (reverse) (SΕQ ID NO: 8), and genomic DNA from the strain K-12 MG1655 (wild- type) as a template. For expression, the amplicon was cloned into pBAD TOPO TA (Invitrogen) and transformed into E. coli strain TOP 10 One Shot (F\ mcrA, Δ(mrτ-hsdRMS-mcrBC φ801acZΔM15, ΔkιcX74, recAl, deoR, araD139 Δ(ara-leu)7697, galU, galK, rpsL, (Str ), endAl, nupG: Invitrogen). Kinase Assays and Product Identification: Bacterial cells were grown in LB medium supplemented with appropriate antibiotics (ampicillin and chloramphenicol for the expression of the putative peppermint kinase (SΕQ ID NO:l), ampicillin for the expression of the putative E. coli kinase (SΕQ ID NO:5)) to an OD 600 of 0.2, then treated with either 0.02% arabinose (induction of transgene expression) or 0.02% glucose (repression of transgene expression), and incubated at 20°C for 20 h. After harvest by centrifugation (1800 X g, 5 min), the cells were resuspended in 2 ml of assay buffer (100 mM Tris/HCl (pH 7.5) containing 20 mM MgCl 2 , 20 mM ATP, and 1 mM DTT), and disrupted by sonication at 0-4°C; the protein content of the resulting homogenate was determined using the Bio-Rad protein assay. To aliquots containing 400 μg of total protein, 0.79 nmol of the appropriate substrate was added, and the mixture was incubated at 30°C for 5-60 min. Cell debris was pelleted by centrifugation (13,000 rpm, bench-top centrifuge), protein was removed by filtration through a Nanosep cartridge (10 kDa cut-off; Pall Filtron), and the filtrate was analyzed by reversed-phase ion-pair radio-HPLC using a modification of a previously published method (McCaskill, D. & Croteau, R., Anal. Biochem., 215:142-149 [1993]); column: Adsorbosphere HS C18 (Alltech, 5 μm particle size, 4.6 mm i.d., 250 mm length); solvents: (A) 10 mM tetrabutylammonium acetate (pH 6), (B) 10 mM tetrabutylammonium acetate in 70% aq. methanol (pH 6), (C) 70% aq. methanol; gradient: 100% (A) (hold for 10 min), gradient to 80% (B) / 20% (A) (65 min), hold for 10 min, gradient to 100% (C) (10 min), hold for 10 min; flow rate: 1 ml/min "1 ; retention times: IPP, 72.6 min; DMAPP, 70.4 min; IP, 64.6 min; DMAP, 63.7 min; isopentenol, 32.9 min; dimethylallyl alcohol, 34.6 min; DXP, 49.4 min; DX, 21.2 min; MΕP, 46.8 min; MVA, 32.6 min. Partial Purification of Recombinant E. coli IP Kinase: Bacteria were grown as described herein. After centrifugation (1800 X g, 5 min), cells were resuspended in 1.5 ml of lysis buffer (20 mM sodium phosphate (pH 7.8) containing 500 mM NaCl), incubated with 100 μg egg white lysozyme (15 min on ice), and sonicated using three 10-second bursts at medium intensity. The crude lysate was treated with RNase (5 μg, 10 min, 30°C) and DNase (5 μg, 10 min, 30°C), insoluble debris was removed by centrifugation (15,000 X g, 10 min), and the supernatant was transferred to a tube containing 4 ml of ProBond resin (Invitrogen) preequilibrated with lysis buffer. To bind the recombinant enzyme, which was expressed as a fusion protein containing a 6xHis affinity tag, the suspension was gently agitated for 20 min at 4°C. After brief centrifugation (700 X g, 2 min), the supernatant was carefully decanted and the remaining resin was washed twice with 40 ml of lysis buffer. The resin was transferred to a gravity flow column and washed with an additional 5 ml of lysis buffer, and the recombinant protein was then eluted with 3 ml of elution buffer (20 mM potassium phosphate (pH 6.4) containing 500 mM NaCl and 300 mM imidazole). The eluent was transferred to Econo-Pac 10-DG columns (Bio-Rad) and the IP kinase was desalted by elution with 100 mM Tris/HCl buffer (pH 7.5). Aliquots of this partially purified preparation (15 μg total protein) were assayed for kinase activity as described herein. Isolation of and Feeding Studies with Peppermint Oil Gland Secretory Cells: Leaves (15-20 g; <10 mm in length) were excised from peppermint plants (Mentha x piperita L. cv. Black Mitcham) and the oil gland secretory cells were isolated by the glass bead abrasion method (McCaskill, D. & Croteau, R., Planta, 197:49-56 [1995]). Following isolation, the secretory cells were washed with 25 mM Tris/HCl buffer (pH 7.3) containing 200 mM sorbitol, 10 mM sucrose, 5 mM MgCl 2 , 10 mM KC1, 1 mM ethyleneglycol bis({3-aminoethyl ether), 8.5 mM and 0.1 mM Na 4 P 2 O 7 , and then suspended in the same buffer supplemented with 2 mM ATP, 0.1 mM NADPH, 0.1 mM NAD0 5 mM phosphoenol pyruvate, and 5 mM glucose-6- phosphate. Cell density was determined using a hemocytometer and was adjusted to 1-2 X lO 5 cellular disks (each containing eight secretory cells) per milliliter suspension. Aliquots (1-1.5 ml) were transferred to 15 ml screw-cap glass vials, and the suspended cells were aerated and incubated at 23°C for 2 h. At the end of the incubation period, the suspension was extracted three times with 1 ml diethyl ether. The combined organic extract was washed with 1 ml of 1 M Na 2 CO 3 and dried over Na^O^ An aliquot was removed for liquid scintillation counting and, to the remainder, authentic standards (10-50 μg each) of isopentenol, dimethylallyl alcohol, geraniol, farnesol, limonene, menthone, menthol, pulegone, humulene, and caryophyllene were added. These extracts were then slowly concentrated on ice under a gentle stream of N 2 to ~200 μl, and were then transferred to conical glass vials and further concentrated to 5-10 μl at 20°C in preparation for chromatographic analysis. Radio-GLC Analysis of Peppermint Isoprenoids: Radio-GC was performed according to a published method (Croteau, R. & Satterwhite, D.M. J. Chromatog. 500:349-354 [1990]) with several modifications; 0.25 mm i.d. x 30 m AT 1000 fused silica column with 0.2 μm film thickness (Alltech); helium carrier gas at 3-6 ml/min ' '; temperature program from 70 to 200°C at 5°C/min "1 ; injector at 200°C; thermal conductivity detector at 220°C and 140 mA; propane quench gas at 10-15 ml/min "1 . The relative peak area corresponding to each biosynthetic product was converted to dpm as a fraction of the total radioactivity determined by liquid scintillation counting prior to concentration. The absolute amount of each component formed was then calculated based on the specific activity of the corresponding radiolabeled substrate. Example 2 Cloning of a Nucleic Acid Molecule Encoding Isopentenyl Monophosphate Kinase from Peppermint The oil glands (glandular trichomes) of mint species are highly specialized for the production of monoterpenes and sesquiterpenes, and the secretory cells of these structures are thus highly enriched in the machinery for terpenoid biosynthesis (Lange, B.M. & Croteau, R., Curr. Opin. Plant Biol., 2:139-144 [1999]). As described in Example 1, 1300 random clones obtained from an enriched cDNA library, constructed specifically from mRNA isolated from peppermint glandular trichome secretory cells as described in Example 1 herein, were analyzed. Since the most advanced, defined intermediate of the plastidial DXP pathway to isoprenoids is 2-C-methyl-D-erythritol-4-phosphate (Duvold, T., Bravo, J.M., Pale-Grosdemange, C. & Rohmer, M., Tetrahedron Lett., 38:4769-4772 [1997]; Duvold, T., Call, P., Bravo, J.M. & Rohmer M., Tetrahedron Lett., 38:6181-6184 [1997]; Sagner, S., Eisenreich, W., Fellermeier, M., Latzel, C., Bacher, A. & Zenk, M.H., Tetrahedron Lett. 39:2091-2094 [1998]), and the end product of the pathway is IPP (McCaskill, D. & Croteau R., Tetrahedron Lett., 40:653-656 [1999]; Arigoni, D., Eisenreich, W., Latzel, C, Sagner, S., Radykewicz, T., Zenk, M.H. & Bacher, A, Proc. Natl. Acad. Sci. USA, 96:1309-1314 [1999]), a phosphorylation step must occur at some point during this reaction sequence. Accordingly, metabolite phosphokinases were sought, but only two clones with similarity to adenylate kinases were noted by searching the common databases. However, a more detailed search of the Prosite database (http://www.expasy.ch/prosite) revealed another more promising clone (designated mllOO) (SEQ ID NO:l) which shared a region of high sequence similarity to the putative ATP -binding domain of the GHMP family of kinases (Tsay, Y.H. & Robinson, G.W., Mol. Cell. Biol., 11:620-631 [1991]). The deduced amino acid sequence of this peppermint clone (SEQ ID NO:2) additionally showed significant homology to a chromoplast-directed protein of unknown function from ripening tomato fruits (Lawrence, S.D. Cline, K. & Moore, G.A., Plant Mol. Biol., 33:483- 492 [1997]) and to a number of hypothetical proteins from several eubacteria. To examine the possible function of the peppermint clone ml 100 (SEQ ID NO:l), the coding region (nucleotides 3 to 1217 of SEQ ID NO:l) was amplified by PCR, transferred into the expression vector pBAD TOPO TA (to yield pBAD-MPK), and, for convenient purification, expressed as a 6xHis fusion protein in E. coli BL21- CodonPlus-RIL cells (which contain an extra plasmid encoding rare tRNAs that specify Arg, Leu and He). However, the expressed enzyme strongly tended to form inclusion bodies, and, even after several attempts to increase the soluble activity by decreasing the amount of arabinose added for induction (0.001-0.01%) and by prolonging the incubation at lower temperatures (8-15°C), soluble protein could not be obtained in sufficient quantity for affinity-based (Ni 2+ -binding) purification. Thus, crude extracts of sonicated, transformed cells served as the enzyme source for kinase assays. As a control, E. coli harboring pBAD-MPK were treated with 0.02% glucose which leads to repression of transgene expression. Consistently, the expressed recombinant peppermint kinase gave detectably elevated levels of activity with isopentenol monophosphate (IP; 1.43 pmol (s g protein) "1 and isopentenol (ISO; 0.10 pmol/ s . g protein as substrates, when compared to the E. coli background (repressed) controls (IP, 0-1.0 pmol/s • g protein; ISO, 0.08 pmol /s • g protein (Table 1). Table Substrate specificity of recombinant IP kinases Peppermint* E. coli* E. coli* E. coli (repressed) (induced) (partially purified) Substrate Rate [pmol /s • g protein] 1 IP 1.43 <1.0* 2.41 178 ± 81 ISO 0.10 0.08 0.10 79 ± 24 DMAP n.d. n.d. n.d. n.d. DMA < 0.01 <0.01 O.01 1 1 ± 5 DXP < 0.1 § < 0.1« < 0.1* n.d. DX < 0.1 § < 0.1 § < 0.1 § n.d. MEP < 0.2 § < 0.2 § < 0.2 § n.d. MVA n.d. n.d. n.d. n.d. * These crude extracts contained high phosphatase activity; the rates given are thus minimums. t Rates are given as averages of two to five separate experiments, with standard deviation where appropπate. t The kinase activity in this assay varied but was always below the rate shown in Table 1. In most of these assays, no kinase activity was observed; in some cases, mmor levels of the apparent product were detected. n.d. not detectable. No phosphokinase activity was detected with dimethylallyl monophosphate (DMAP) or MVA as substrate. Kinase activity with dimethyallyl alcohol (DMA) as a substrate, under the standard assay conditions, was always detectable, but never exceeded 0.01 pmol/s • g protein. With DXP, deoxylylulose (DX) and MEP as substrates, kinase activity of less than 0.2 pmol/s • g protein was occasionally detected but in most experiments no activity was observed. Because these assays with crude extracts were severely compromised by the presence of competing phosphatases (as evidenced by the production of the corresponding dephosphorylated products upon HPLC analysis), the conversion rates observed must be regarded as minimum values. Example 3 Cloning of a Nucleic Acid Molecule Encoding Isopentenyl Monophosphate Kinase from Escherichia coli As an alternative to further expression studies with the recombinant peppermint enzyme (SEQ ID NO:2), such as by truncation of the putative plastidial targeting sequence as an attempt to minimize protein misfolding (Williams, D.C. et al., (1998) Biochemistry 37:12213-12220.), the E. coli orthologue (SEQ ID NO:6) of the peppermint kinase (SEQ ID NO:2) was evaluated. This E. coli gene (ychB) (SEQ ID NO:l), which was found by database homology searching and which is located at 27.2 min of the chromosomal map, was amplified by PCR and transferred into pBAD TOPO TA (to yield pBAD-ECK), for similar expression as a 6xHis fusion protein. Assay of crude extracts gave results similar to those obtained with the recombinant peppermint enzyme (Table 1). However, in this instance, the expressed, soluble enzyme, produced upon induction with 0.02% arabinose, allowed a useful one-step affinity-based purification with the Ni 2+ -binding column. This partially purified protein readily catalyzed the conversion of IP to IPP (178 ± 81 pmol/s • g protein), ISO to IP (79 ± 24 pmol/s • g protein), and DMA to DMAP (11 ± 5 pmol/s . g protein); however, no phosphorylating activity was detectable with DMAP, DXP, DX, MEP or MVA as substrate (Table 1). This kinase activity (with IP as a substrate) was dependent upon the presence of MgCl 2 as divalent cation and ATP as phosphate donor, whereas CTP, UTP and GTP did not serve as alternate phosphate donors. As a control, pBAD-ECK expression in E. coli was repressed by addition of 0.02% glucose, and the extracted proteins were subjected to the same purification step as the peppermint IPK (SEQ ID NO:2) described in Example 2, herein; kinase assays with these enzyme preparations yielded no detectable activity with any of the above substrates. These results with the enzyme products of the peppermint ml 100 clone (SEQ ID NO:l) and the E. coli ychB clone (SEQ ID NO:5) suggest that the peppermint gene (SEQ ID NO:l) encodes an isopentenyl monophosphate kinase (IPK) that is involved in the DXP pathway to isoprenoids. Example 4 Sequence Analysis The peppermint IPK gene (SEQ ID NO:l) contains an open reading frame of 1218 nucleotides. The first 98 deduced amino acid residues display the general characteristics of plastidial targeting sequences (von Heijne, G., Steppuhn, J. & Herrmann, R.G., Eur. J. Biochem., 180:535-545 [1989]), and, when this putative leader peptide is excluded, a mature protein of 308 amino acids with a predicted size of about 33 kDa is obtained. The gene encoding E. coli IPK (SEQ ID NO:5) consists of 852 nucleotides, which corresponds to an enzyme of 283 amino acids with a size of 31 kDa (SEQ ID NO:6). Database sequence comparison of translated, putative IPK genes from several different organisms revealed very high similarity/identity scores within the plant kingdom (>81.6/74.8% for presumptive orthologues found in tomato (SEQ ID NO:9 encoding the protein of SEQ ID NO: 10) and Arabidopsis thaliana (SEQ ID NO:l 1 encoding the protein of SEQ ID NO: 12)), and a high degree of sequence variation among eubacteria (39.0-70.2/25.6-62.5%) and between plants and eubacteria (38.3-48.8/28.5-38.6%). The isopentenyl diphosphate kinases appear to share a conserved, glycine-rich sequence motif (PXGAGLGGGSSNAAX (15 . 16) (K/R) (SEQ ID NO: 13) similar to the conserved sequence PXXXGL(G/S)SS(NG)XX (12 . 25) (K R) (SEQ ID NO: 14) found in the GHMP family of kinases, including galactokinase, homoserine kinase, mevalonate kinase and phosphomevalonate kinase (Tsay, Y.H. & Robinson, G.W., Mol. Cell. Biol., 11:620-631 [1991]). A related motif is also present in protein kinases (Hanks, S.K., Quinn, A.M. & Hunter, T., Science, 241:42-52 [1988]). The gene for the A. thaliana IPK orthologue is located on chromosome 2 (AC005168; BAC F12C20; PID g3426035), near the marker B68, and contains 10 introns. Neither the intron/exon organization nor a phylogenetic analysis reveals a direct evolutionary relationship between different classes of the GHMP kinase family (data not shown). A detailed survey of the available microbial genome project databases did not indicate the IPK gene (SEQ ID NO:l) to be part of a cluster with other (potential) genes of the DXP pathway. Example 5 Incorporation of IP into Isoprenoids of Peppermint Secretory Cells Although IP was shown to be the preferred substrate of both peppermint IPK (SEQ ID NO:2) and E. coli IPK (SEQ ID NO:6), it remained to be directly demonstrated that IP was an intermediate of the DXP pathway. Previous experiments with isolated peppermint oil gland secretory cells had demonstrated that the MVA pathway in these cells is blocked at an early stage, and that IPP utilized for both monoterpene and sesquiterpene biosynthesis is synthesized exclusively in the plastids from pyruvate (McCaskill, D. & Croteau, R., Planta, 197:49-56 [1995]), almost certainly via the DXP pathway (Eisenreich, W. et al., Tetrahedron Lett., 38:3889-3892 [1997]). The high degree of metabolic specialization and the ability to synthesize monoterpenes and sesquiterpenes de novo from basic precursors, including phosphorylated intermediates (McCaskill, D. & Croteau, R., Planta, 197:49-56 [1995]), made the isolated secretory cells an ideal model system to establish that IP was an intermediate of the DXP pathway, and the activity of IP kinase, in vivo. Thus, isolated secretory cells were incubated under comparable conditions with several radiolabeled substrates (ISO, IP, IPP, DMA, DMAP, DMAPP), and, after extraction of products with diethyl ether, the incorporation into monoterpenes and sesquiterpenes was quantified by liquid scintillation counting and radio-GC analysis of these volatile metabolites (Table 2). Table 2. Conversion of potential C 5 -precursors into terpenoids by isolated peppermint oil gland secretory cells Precursor Total Terpenoids* (C,o + C 15 ) IPP 561 IP 43 ISO 304 DMAPP 4 DMAP 6 DMA n.d. * Rates of incorporation into total monoterpenoids and sesquiterpenoids are given in pmol (h 10 5 cell clusters) "1 (average of two experiments), and include geraniol and farnesol released by phosphatases from the intermediates geranyl diphosphate (C ]0 ) and farnesyl diphosphate (C 15 ). n.d. not detectable. Since the IP kinase (SEQ ID NO:2) is plastidial, as is monoterpene biosynthesis, whereas sesquiterpene biosynthesis is cytosolic (McCaskill, D. & Croteau, R., Planta, 197:49-56 [1995]), uptake and partitioning differences between the C 5 precursors influence the distribution between monoterpene and sesquiterpene biosynthetic pathways. In a similar fashion, endogenous phosphatases of both plastidial and cytosolic origin can complicate the partitioning of precursors into the pathways of these compartments. Thus, as a measure of the conversion efficiency of each C 5 precursor, total monoterpenoids (C 10 ) and sesquiterpenoids (C 15 ), including geraniol and farnesol released by phosphatases from the corresponding diphosphate ester intermediates, were recorded. By this measure, IPP was most readily converted to terpenoid end-products as expected (561 pmol (h 10 5 cell clusters) "1 ), followed by ISO (304 pmol (h 10 5 cell clusters) "1 ), most likely reflecting efficient plastidial uptake of this low molecular weight alcohol, and then IP (43 pmol (h 10 5 cell clusters) "1 ). DMAPP and DMAP were not very efficient precursors of terpenoids in secretory cells (<6 pmol (h 10 5 cell clusters) "1 ), and the incorporation of DMA was negligible. Although ISO, likely because of uptake rates, and the more advanced precursor IPP were transformed to terpenoids in vivo at higher rates than was IP, the latter was incorporated at a rate (43 pmol (h 10 5 cell clusters) "1 ) comparable to that observed previously with pyruvate (67 pmol (h 10 5 cell clusters) "1 ) (McCaskill, D. & Croteau, R., Planta, 197:49-56 [1995]), an efficient, established precursor of the DXP pathway. Although IP and ISO were shown to be efficient in vitro substrates for this newly defined kinase (SEQ ID NO:2), and the role of this kinase (SEQ ID NO:2) was demonstrated by in vivo feeding studies, several steps of the mevalonate-independent pathway still remain to be elucidated, and it cannot be ruled out that the required phosphorylation step also may occur with intermediates other than IP. Example 6 Characteristics of Presently Preferred Nucleic Acid Molecules Encoding Isopentenyl Monophosphate Kinase Proteins of the Invention Presently preferred nucleic acid molecules encoding isopentenyl monophosphate kinase proteins of the present invention are capable of hybridizing to the antisense, complementary nucleic acid sequence of the nucleic acid molecule having the sequence set forth in SEQ ID NO: 1 under stringent conditions. Presently preferred, high stringency conditions are: hybridization in 5 X SSC at 65 ° C for 16 hours, followed by two washes in 2 X SSC at room temperature (20 ° C to 25 ° C) for 15 minutes per wash, followed by two washes in 0.5 X SSC at 65 ° C for 20 minutes per wash. Presently preferred, moderate stringency conditions are: hybridization in 5 X SSC at 65 ° C for 16 hours, followed by two washes in 2 X SSC at room temperature (20 ° C to 25 ° C) for 20 minutes per wash, followed by one wash in 1.0 X SSC at 55 ° C for 30 minutes. Presently preferred, low stringency conditions are: hybridization in 6 X SSC at room temperature (20 ° C to 25 C) for 16 hours, followed by two washes in 3.0 X SSC at room temperature (20 ° C to 25 ° C) for 20 minutes per wash. The ability of the nucleic acid molecules of the present invention to hybridize to the complementary sequence of the nucleic acid sequence set forth in SEQ ID NO:l, can be determined utilizing the technique of hybridizing radiolabelled nucleic acid probes to nucleic acids immobilized on nitrocellulose filters or nylon membranes as set forth, for example, at pages 9.52 to 9.55 of Molecular Cloning, A Laboratory Manual (2nd edition), J. Sambrook, E.F. Fritsch and T. Maniatis eds., the cited pages of which are incorporated herein by reference. Example 7 Characteristics of Presently Preferred Isolated. Recombinant Isopentenyl Monophosphate Kinase Proteins of the Invention Presently preferred, isolated, recombinant isopentenyl monophosphate kinase proteins of the present invention have a mature molecular weight (i.e., excluding any amino terminal targeting sequence) of from about 30 kDa to about 36 kDa; utilize Mg 2+ (or other divalent metal ion) and ATP as cofactors; utilize isopentenol, isopentenyl monophosphate or dimethylallyl alcohol as substrates; have a pH optimum in the range of from about 6.0 to about 8.0, and have a Km (utilizing isopentenyl monophosphate as substrate and ATP as cofactor) of less than 1 mM. Example 8 Representative Examples of Nucleic Acid Molecules Useful in the Methods of the Present Invention In one aspect, the present invention provides methods of altering the level of expression of isopentenyl monophosphate kinase in a host cell comprising introducing into the host cell a nucleic acid molecule encoding an isopentenyl monophosphate kinase under conditions that enable expression of the isopentenyl monophosphate kinase in the host cell. Representative examples of nucleic acid molecules, encoding an isopentenyl monophosphate kinase protein, that are useful in the methods of the present invention to alter the level of expression of isopentenyl monophosphate kinase in a host cell include the nucleic acid molecules having the sequences set forth in SEQ ID NO:l (encoding the peppermint isopentenyl monophosphate kinase of SEQ ID NO:2), SEQ ID NO:5 (encoding the E. coli isopentenyl monophosphate kinase of SEQ ID NO:6), SEQ ID NO:9 (encoding the Lycopersicon esculentum isopentenyl monophosphate kinase of SEQ ID NO: 10), SEQ ID NO: 11 (encoding the Arabidopsos isopentenyl monophosphate kinase of SEQ ID NO: 12) and SEQ ID NO: 15 (encoding the Synechocystis sp. isopentenyl monophosphate kinase of SEQ ID NO: 16). In another aspect, the present invention provides methods for reducing the level of expression of isopentenyl monophosphate kinase in a host cell comprising introducing into the host cell a nucleic acid molecule, encoding at least a portion of an isopentenyl monophosphate kinase, oriented in antisense orientation with respect to a promoter sequence under conditions enabling transcription of the antisense

Claims: nucleic acid molecule. Representative examples of nucleic acid molecules, encoding an isopentenyl monophosphate kinase protein, that are useful in the practice of the present invention to reduce the level of expression of isopentenyl monophosphate kinase in a host cell include the nucleic acid molecules having the sequences set forth in SEQ ID NO:l (encoding the peppermint isopentenyl monophosphate kinase of SEQ ID NO:2), SEQ ID NO:5 (encoding the E. coli isopentenyl monophosphate kinase of SEQ ID NO:6), SEQ ID NO:9 (encoding the Lycopersicon esculentum isopentenyl monophosphate kinase of SEQ ID NO: 10), SEQ ID NO:l 1 (encoding the Arabidopsos isopentenyl monophosphate kinase of SEQ ID NO: 12) and SEQ ID NO: 15 (encoding the Svnechocvstis sp. isopentenyl monophosphate kinase of SEQ ID NO: 16). While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 1. An isolated nucleic acid molecule encoding an isopentenyl monophosphate kinase, provided that said isolated nucleic acid molecule does not consist of a nucleic acid sequence selected from the group consisting of SEQ ID NO:5, SEQ ID NO:9, SEQ ID NO:l 1 and SEQ ID NO: 15. 2. An isolated nucleic acid molecule of Claim 1 encoding a prokaryotic isopentenyl monophosphate kinase. 3. An isolated nucleic acid molecule of Claim 1 encoding a eukaryotic isopentenyl monophosphate kinase. 4. An isolated nucleic acid molecule of Claim 3 encoding a eukaryotic plant isopentenyl monophosphate kinase. 5. An isolated nucleic acid molecule of Claim 4 encoding a gymnosperm isopentenyl monophosphate kinase. 6. An isolated nucleic acid molecule of Claim 4 encoding an angiosperm isopentenyl monophosphate kinase. 7. An isolated nucleic acid molecule of Claim 6 encoding an essential oil plant isopentenyl monophosphate kinase. 8. An isolated nucleic acid molecule of Claim 7 encoding an isopentenyl monophosphate kinase from a member of the family Lamiaceae. 9. An isolated nucleic acid molecule of Claim 8 encoding a Mentha isopentenyl monophosphate kinase. 10. An isolated nucleic acid molecule of Claim 9 encoding a Mentha piperita isopentenyl monophosphate kinase. 11. An isolated nucleic acid molecule of Claim 10 encoding an isopentenyl monophosphate kinase protein consisting of the amino acid sequence set forth in SEQ ID NO:2. 12. An isolated nucleic acid molecule of Claim 10 consisting of the nucleic acid sequence set forth in SEQ ID NO:l . 13. An isolated, recombinant isopentenyl monophosphate kinase protein. 14. An isolated, recombinant, prokaryotic isopentenyl monophosphate kinase protein of Claim 13. 15. An isolated, recombinant, eukaryotic isopentenyl monophosphate kinase protein of Claim 13. 16. An isolated, recombinant, plant isopentenyl monophosphate kinase protein of Claim 13. 17. An isolated, recombinant, gymnosperm isopentenyl monophosphate kinase protein of Claim 16. 18. An isolated, recombinant, angiosperm isopentenyl monophosphate kinase protein of Claim 16. 19. An isolated, recombinant, essential oil plant isopentenyl monophosphate kinase protein of Claim 16. 20. An isolated, recombinant, Lamiaceae isopentenyl monophosphate kinase protein of Claim 16. 21. An isolated, recombinant, Mentha isopentenyl monophosphate kinase protein of Claim 16. 22. An isolated, recombinant, Mentha piperita isopentenyl monophosphate kinase protein of Claim 16. 23. An isolated, recombinant, Mentha piperita isopentenyl monophosphate kinase protein of Claim 16 consisting of the amino acid sequence set forth in SEQ ID NO:2. 24. A rephcable expression vector comprising a nucleic acid sequence encoding an isopentenyl monophosphate kinase, provided that said isolated nucleic acid molecule does not consist of a nucleic acid sequence selected from the group consisting of SEQ ID NO:5, SEQ ID NO:9, SEQ ID NO:l 1 and SEQ ID NO: 15. 25. A rephcable expression vector of Claim 24 comprising a nucleic acid sequence encoding a plant isopentenyl monophosphate kinase. 26. A host cell comprising a vector of Claim 24. 27. A host cell comprising a vector of Claim 25. 28. A method of altering the level of expression of isopentenyl monophosphate kinase in a host cell comprising introducing into said host cell a nucleic acid molecule encoding an isopentenyl monophosphate kinase under conditions that enable expression of said isopentenyl monophosphate kinase in said host cell. 29. The method of Claim 28 wherein said host cell is a plant cell. 30. The method of Claim 28 wherein said host cell is an animal cell. 31. The method of Claim 28 wherein said host cell is a prokaryotic cell. 32. The method of Claim 28 wherein said host cell is a yeast cell. 33. The method of Claim 28 wherein said host cell is a fungal cell. 34. The method of Claim 28 wherein the level of expression of isopentenyl monophosphate kinase in said host cell is enhanced. 35. A method for reducing the level of expression of isopentenyl monophosphate kinase in a host cell comprising introducing into the host cell a nucleic acid molecule, encoding at least a portion of an isopentenyl monophosphate kinase, oriented in antisense orientation with respect to a promoter sequence under conditions enabling transcription of the antisense nucleic acid molecule. 36. The method of Claim 35 wherein said host cell is a plant cell.

Inventors: CROTEAU RODNEY B, LANGE BERND M, CROTEAU, RODNEY, B., LANGE, BERND, M.
Applicants: CROTEAU RODNEY B, LANGE BERND M, UNIV WASHINGTON, CROTEAU, RODNEY, B., LANGE, BERND, M., WASHINGTON STATE UNIVERSITY RESEARCH FOUNDATION
Application year: 2000
Main classification: C12P21
IPC codes: C12N9