# Patent

## Title
Long-chain trans-prenyl diphosphate synthase gene

## Abstract

A method for increasing the amount of trans-1,4-polyisoprene contained in a plant, and a method for effectively producing trans-1,4-polyisoprene using a plant are provided. A long-chain trans-prenyl diphosphate synthase gene that comprises DNA having at least one base sequence selected from the group consisting of a base sequence from positions 88 to 1134 of the base sequence of SEQ ID NO: 1 or a complementary sequence thereof, a base sequence from positions 42 to 1088 of the base sequence of SEQ ID NO: 3 or a complementary sequence thereof, and a base sequence from positions 91 to 1140 of the base sequence of SEQ ID NO: 5 or a complementary sequence thereof are disclosed; as well as a plant transformed with an expression vector containing the gene.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a long-chain trans-prenyl diphosphate synthase gene, a plant transformed with an expression vector containing the gene, and a method for producing trans-1,4-polyisoprene using the plant.
2. Description of the Related Art
Polyisoprene (rubber), which is one of isoprenoid compounds, is classified into the cis-form and the trans-form according to the way in which isoprene units are polymerized. It is known that there are many plants that produce long-chain cis-polyisoprene (cis-1,4-polyisoprene), such as Hevea brasiliensis belonging to the family Euphorbiaceae, and Taraxacum and Lactuca indica belonging to the family Asteraceae. Among these, cis-1,4-polyisoprene produced by Hevea brasiliensis is commonly used commercially as a natural rubber (N. Ohya and T. Koyama, “Biosynthesis of natural rubber and other natural polyisoprenoids”, Biopolymers, (Germany), WILEY-VCH, 2001, vol. 2, p. 73-109). On the other hand, it is known that there are a small number of plants that naturally produce long-chain trans-polyisoprene (trans-1,4-polyisoprene), such as Eucommia ulmoides belonging to the family Eucommiaceae, Periploca sepium belonging to the family Apocynaceae, and Mimusops balata and Palaquium gutta belonging to the family Sapotaceae, but they are not commercially used (T. Bamba et al., “In-situ chemical analyses of trans-polyisoprene by histochemical staining and Fourier transform infrared microspectroscopy in a rubber-producing plant, Eucommia ulmoides Oliver”, Planta, 2002, vol. 215, p. 934-939). Among these, Eucommia ulmoides, which is a woody plant native to China, produces fibrous trans-1,4-polyisoprene. The leaves, bark, and peel of Eucommia ulmoides contain a large amount of trans-1,4-polyisoprene (T. Bamba et al.). However, currently, trans-1,4-polyisoprene is chemically synthesized, and used for outer layers of golf ball, plaster casts, sports protectors, and the like. Trans-1,4-polyisoprene is a thermoplastic elastomer having a low-melting point and high elasticity, and is also useful as an insulating material. Here, the word “natural rubber” commonly refers to a natural product-derived rubber in general, but may industrially refer to only a cis-rubber obtained from Hevea brasiliensis. It is not rare for higher plants to produce a rubber, and approximately 500 types of plants are confirmed to contain a rubber (“Regarding Rubber”, (online), Nihonkai Rubber Co., Ltd., (accessed on Sep. 2, 2008), the Internet <http://www.nihonkair.co.jp/semi02.htm>).
All natural isoprenoid compounds are biosynthesized using, as an intermediate, prenyl diphosphate in which isoprene units having five carbon atoms (C5) are continuously linked, and all prenyl diphosphates are biosynthesized by prenyl diphosphate synthases (K. Wang and S. Ohnuma, “Chain-length determination mechanism of isoprenyl diphosphate synthases and implications for molecular evolution”, TIBS, 1999, vol. 24, p. 445-4.51, and Tonetoshi Koyama and Kyozo Ogura, “Unlocking the mystery of natural rubber biosynthesis—Mechanism of construction of isoprene chains inside a living body”, Chemistry Today, 1990, vol. 237, p. 42-49). Prenyl diphosphate synthase is a general term for enzymes that produce prenyl diphosphate having a larger number of isoprene units (a longer chain length) than that of a primer substrate, by catalyzing a reaction that condenses isopentenyl diphosphate (IPP), which is a compound having five carbon atoms (C5), to prenyl diphosphate (an allyl substrate) functioning as a primer substrate (Japanese Laid-Open Patent Publication No. 2004-24275). The prenyl diphosphate synthase is also referred to as a prenyl transferase or a prenyl chain-elongating enzyme (Seiji Takahashi and Tanetoshi Koyama, “Molecular analysis of the enzymes participating in isoprenoid biosynthesis”, Kagaku To Seibutsu (Chemistry and Biology), 2005, vol. 43, p. 296-304).
IPP, which is a substrate for a prenyl diphosphate synthase, is biosynthesized by the mevalonate pathway or the like. Parts of the gene clusters for the enzymes participating in the mevalonate pathway have been clarified in various plants, such as Eucommia ulmoides. 
Prenyl diphosphate synthases can be classified into enzymes that catalyze a condensation reaction that forms an E form (trans-form) double bond, and enzymes that catalyze a condensation reaction that forms a Z form (cis-form) double bond, during condensation of IPP. Furthermore, prenyl diphosphate synthases may catalyze a reaction that further condenses IPP to prenyl diphosphate produced by a condensation reaction. The maximum length of isoprene chain that can be produced by such a condensation polymerization reaction of IPP (the maximum degree of IPP polymerization) is inherent to each prenyl diphosphate synthase. The hydrophobicity of a product changes depending on the isoprene chain length of the product, and, thus, the manner of expression of enzymatic activity significantly varies.
More specifically, prenyl diphosphate synthases can be classified into four types, a prenyl diphosphate synthase I (E-form short-chain prenyl diphosphate synthase), a prenyl diphosphate synthase II (E-form medium-chain prenyl diphosphate synthase), a prenyl diphosphate synthase III (E-form long-chain prenyl diphosphate synthase), and a prenyl diphosphate synthase IV (Z-form long-chain prenyl diphosphate synthase) (Japanese Laid-Open Patent Publication No. 2004-24275).
Examples of the prenyl diphosphate synthase I (E-form short-chain prenyl diphosphate synthase) include a geranyl diphosphate (GPP) synthase (C5→C10), a farnesyl diphosphate (FPP) synthase (C10→C15), and a geranyl geranyl diphosphate (GGPP) synthase (C15→C20). Here, for example, “C5→C10” refers to catalyzing a reaction that produces prenyl diphosphate having ten carbon atoms (C10) by condensing IPP having five carbon atoms to prenyl diphosphate functioning as a primer substrate having five carbon atoms (C5).
Examples of the prenyl diphosphate synthase II (E-form medium-chain prenyl diphosphate synthase) include a hexaprenyl diphosphate (HexPP) synthase (C15→C30) and a heptaprenyl diphosphate (HepPP) synthase (C16→C35).
Examples of the prenyl diphosphate synthase III (E-form long-chain prenyl diphosphate synthase) include an octaprenyl diphosphate (OctPP) synthase (C15→C40), a nonaprenyl diphosphate (NonPP) synthase (C10→C45), and a decaprenyl diphosphate (DecPP) synthase (C15→C50).
Examples of the prenyl diphosphate synthase IV (Z-form long-chain prenyl diphosphate synthase) include a Z-nonaprenyl diphosphate synthase (C15→C45), an undecaprenyl diphosphate (UPP) synthase (C15→C55), and a dehydrodolichyl diphosphate (deDolPP) synthase (C15→C85 to 105).
A rubber transferase gene (HRT2) is isolated from Hevea brasiliensis, which is a plant that produces a cis-rubber, and a protein that is encoded by the HRT2 gene is confirmed to have a cis-prenyl diphosphate-synthesizing activity that condenses IPP to rubber particles. Furthermore, the HRT2 gene is confirmed to complement the functional deficiencies of the dehydrodolichyl diphosphate synthases of a budding yeast (K. Asawatreratanakul et al., “molecular cloning, expression and characterization of cDNA encoding cis-prenyltransferases from Hevea brasiliensis”, Eur. J. Biochem., 2003, vol. 270, p. 4671-4680). However, it has not been reported that transformation of Hevea brasiliensis with an expression vector containing the HRT2 gene results in an increased content of cis-1,4-polyisoprene (cis-rubber) produced by Hevea brasiliensis. 
On the other hand, genes for long-chain trans-prenyl diphosphate synthases participating in the biosynthesis of a trans-rubber have not been isolated and identified from Eucommia ulmoides, Periploca sepium, Mimusops balata, and Palaquium gutta, which are plants that produce a trans-rubber. The inventors of the present invention isolated a prenyl transferase gene from Eucommia ulmoides (base sequence: GenBank Accession Number AB041626, and amino acid sequence: GenBank Accession Number BAB16687), but have not yet identified whether this gene encodes a cis-prenyl diphosphate synthase or a trans-prenyl diphosphate synthase, and whether this gene encodes a short-chain prenyl diphosphate synthase or a long-chain prenyl diphosphate synthase.
By the way, 3-hydroxy-3-methyl glutaryl coenzyme A reductase (HMGR) is considered to be a key enzyme in the biosynthesis system of IPP functioning as a substrate for prenyl diphosphate synthases. When Arabidopsis thaliana is transformed with an expression vector containing DNA that encodes the catalyst domain of the HMGR (HMGR-CD), the transformed Arabidopsis thaliana has the total content of sterols that is approximately 3.6 times as large as that of the wild-type (D. Manzano et al., “The metabolic imbalance underlying lesion formation in Arabidopsis thaliana overexpressing farnesyl diphosphate synthase (isoform 1S) leads to oxidative stress and is triggered by the developmental decline of endogenous HMGR activity”, Planta, 2004, vol. 219, p. 982-992). Here, sterols are one of the isoprenoid compounds that are biosynthesized using IPP as a substrate.
For example, coenzyme Q10 is also known as one of the trans-form isoprenoid compounds. Wild-type Oryza sativa produces coenzyme Q9 using solanesyl diphosphate (in which nine isoprene units are polymerized) as an intermediate. When Oryza sativa is transformed with an expression vector containing DNA that encodes a Gluconobacter suboxydans-derived decaprenyl diphosphate (in which ten isoprene units are polymerized) synthase, the transformed Oryza sativa does not produce coenzyme Q9, but produces coenzyme Q10 using decaprenyl diphosphate as an intermediate (S. Takahashi et al., “Metabolic engineering of coenzyme Q by modification of isoprenoid side chain in plant”, FEBS Lett., 2006, vol. 580, p. 955-959). When Escherichia coli is transformed with an expression vector containing DNA that encodes a fungus-derived decaprenyl diphosphate synthase, the transformed Escherichia coli effectively produces coenzyme Q10 using decaprenyl diphosphate as an intermediate (International Publication Nos. 2002/092811 and 2002/040682).
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for increasing the content of trans-1,4-polyisoprene in a plant, and a method for effectively producing trans-1,4-polyisoprene using a plant.
The inventors of the present invention conducted an in-depth study in order to solve the above-described problems, and found that the content of trans-1,4-polyisoprene in a plant can be increased by isolating and identifying a long-chain trans-prenyl diphosphate synthase gene from Eucommia ulmoides, and transforming the plant with an expression vector containing the gene, and trans-1,4-polyisoprene can be effectively produced by means of using the plant, and, thus, the present invention has been achieved.
The present invention provides a long-chain trans-prenyl diphosphate synthase gene comprising at least one of:
(a) DNA having at least one base sequence selected from the group consisting of a base sequence from positions 88 to 1134 of the base sequence of SEQ ID NO: 1 or a complementary sequence thereof, a base sequence from positions 42 to 1088 of the base sequence of SEQ ID NO: 3 or a complementary sequence thereof, and a base sequence from positions 91 to 1140 of the base sequence of SEQ ID NO: 5 or a complementary sequence thereof;
(b) DNA that forms a hybrid with the (a) DNA under stringent conditions and that encodes a protein having long-chain trans-prenyl diphosphate-synthesizing activity;
(c) DNA that encodes a protein having an E-value of 10−80 or less with respect to at least one amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO: 2, the amino acid sequence of SEQ ID NO: 4, and the amino acid sequence of SEQ ID NO: 6, and having long-chain trans-prenyl diphosphate-synthesizing activity; and
(d) DNA that encodes a protein having an amino acid sequence in which one or several amino acids are substituted, added, deleted or inserted in at least one amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO: 2, the amino acid sequence of SEQ ID NO: 4, and the amino acid sequence of SEQ ID NO: 6, and having long-chain trans-prenyl diphosphate-synthesizing activity.
The present invention further provides a long-chain trans-prenyl diphosphate synthase comprising at least one of:
(A) a protein having at least one amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO: 2, the amino acid sequence of SEQ ID NO: 4, and the amino acid sequence of SEQ ID NO: 6;
(B) a protein having an E-value of 10−80 or less with respect to the amino acid sequence of the (A) protein, and having long-chain trans-prenyl diphosphate-synthesizing activity; and
(C) a protein having an amino acid sequence in which one or several amino acids are substituted, added, deleted or inserted in the amino acid sequence of the (A) protein, and having long-chain trans-prenyl diphosphate-synthesizing activity.
The present invention further provides an expression vector containing the long-chain trans-prenyl diphosphate synthase gene.
The present invention further provides a plant transformed with the expression vector.
In one embodiment, the plant is Eucommia ulmoides. 
In another embodiment, the plant is Nicotiana tabacum. 
The present invention further provides a method for increasing the amount of trans-1,4-polyisoprene contained in a plant, comprising the step of transforming the plant using the expression vector.
The present invention further provides a method for producing trans-1,4-polyisoprene, comprising the steps of
cultivating the plant; and
recovering the trans-1,4-polyisoprene from the cultivated plant.
According to the present invention, a plant that has an increase content of trans-1,4-polyisoprene can be provided by transforming the plant with an expression vector containing a long-chain trans-prenyl diphosphate synthase gene. Trans-1,4-polyisoprene can be effectively produced by cultivating such a plant.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram showing the structure of pCold-TPL1, pCold-TPL3 and pCold-TPL5.
FIG. 2 is an electrophoretogram showing the results of SDS-PAGE and Western blotting of TPL proteins.
FIG. 3 is a graph showing the prenyl diphosphate-synthesizing activity of TPL proteins.
FIG. 4 is a schematic diagram showing the structure of plant-transforming TPL expression vector pBIsGFP-TPL1.
FIG. 5 is a graph showing the amount of TPL1 mRNA.
FIG. 6 is an image obtained using a real spectral imaging microscope showing the distribution of rubber produced inside a cultured root of transformed Eucommia ulmoides. 
FIG. 7 is an image obtained using a real spectral imaging microscope showing the distribution of rubber produced inside a cultured root of wild-type Eucommia ulmoides. 
FIG. 8 is a schematic diagram showing the structure of plant-transforming TPL expression vector pHis-TPL1.
FIG. 9 is a graph showing the amount of TPL1 mRNA and GFP mRNA.
FIG. 10 are graphs showing the results of SEC analysis for wild-type Nicotiana tabacum (A), GFP transformed Nicotiana tabacum GFP #1 (B) and TPL1 transformed Nicotiana tabacum TPL1 #8 (C), with insertion of photographs showing the status of leaves of Nicotiana tabacum subjected to analysis.
FIG. 11 are spectra showing the results of 1H-NMR analysis of mature leaf extracts for Eucommia ulmoides (upper) and TPL1 transformed Nicotiana tabacum (lower), with insertion of a structural formula of trans-1,4-polyisoprene.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides a long-chain trans-prenyl diphosphate synthase gene. The long-chain trans-prenyl diphosphate synthase of the present invention is preferably derived from Eucommia ulmoides. The long-chain trans-prenyl diphosphate synthase of the present invention has an activity of synthesizing long-chain trans-polyisoprene (trans-1,4-polyisoprene) having a molecular weight of 104 to 105 as substantially described in, for example, Examples 4 to 6 below. The long-chain trans-prenyl diphosphate synthase of the present invention catalyzes the synthesis of trans-prenyl diphosphate, being different from Hevea brasiliensis-derived cis-prenyl diphosphate synthase.
The long-chain trans-prenyl diphosphate synthase gene of the present invention may comprise (a) DNA having at least one base sequence selected from the group consisting of a base sequence from positions 88 to 1134 of the base sequence of SEQ ID NO: 1 or a complementary sequence thereof, a base sequence from positions 42 to 1088 of the base sequence of SEQ ID NO: 3 or a complementary sequence thereof, and a base sequence from positions 91 to 1140 of the base sequence of SEQ ID NO: 5 or a complementary sequence thereof.
The long-chain trans-prenyl diphosphate synthase gene of the present invention may comprise (b) DNA that forms a hybrid with the (a) DNA under stringent conditions and that encodes a protein having long-chain trans-prenyl diphosphate-synthesizing activity.
The long-chain trans-prenyl diphosphate synthase gene of the present invention may comprise (c) DNA that encodes a protein having an E-value of 10−80 or less with respect to at least one amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO: 2, the amino acid sequence of SEQ ID NO: 4, and the amino acid sequence of SEQ ID NO: 6, and having long-chain trans-prenyl diphosphate-synthesizing activity.
The long-chain trans-prenyl diphosphate synthase gene of the present invention may comprise (d) DNA that encodes a protein having an amino acid sequence in which one or several amino acids are substituted, added, deleted or inserted in at least one amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO: 2, the amino acid sequence of SEQ ID NO: 4, and the amino acid sequence of SEQ ID NO: 6, and having long-chain trans-prenyl diphosphate-synthesizing activity.
The gene of the present invention may comprise DNA that forms a hybrid, under stringent conditions, with DNA having at least one base sequence selected from the group consisting of a base sequence from positions 88 to 1134 of the base sequence of SEQ ID NO: 1 or a complementary sequence thereof, a base sequence from positions 42 to 1088 of the base sequence of SEQ ID NO: 3 or a complementary sequence thereof, and a base sequence from positions 91 to 1140 of the base sequence of SEQ ID NO: 5 or a complementary sequence thereof, as long as the gene encodes a protein having the long-chain trans-prenyl diphosphate-synthesizing activity.
The long-chain trans-prenyl diphosphate-synthesizing activity can be confirmed using a method commonly used by those skilled in the art, such as the methods substantially described in, for example, Examples 4 to 6 below.
In the present invention, the stringent conditions refer to the conditions in which only DNA that encodes a protein having the long-chain trans-prenyl diphosphate-synthesizing activity forms a hybrid (a so-called specific hybrid) with DNA that encodes the long-chain trans-prenyl diphosphate synthase, and DNA that encodes a protein not having the synthesizing activity does not form a hybrid (a so-called non-specific hybrid) with DNA that encodes the long-chain trans-prenyl diphosphate synthase. Those skilled in the art can easily determine such conditions by appropriately selecting the salt levels of a reaction liquid and a washing liquid used for forming a hybrid, the temperatures during reaction and washing, and the like. More specifically, the conditions may be applicable in which 6×SSC (0.9M NaCl, 0.09M trisodium citrate) or 6×SSPE (3M NaCl, 0.2M NaH2PO4, 20 mM EDTA-2Na, pH 7.4) is used as a reaction liquid to form a hybrid at 42° C., and then 0.5×SSC is used as a washing liquid to perform washing at 42° C., but there is no limitation to this.
The gene of the present invention may comprise DNA that encodes a protein having an E-value of 10−80 or less, preferably 10−100 or less, more preferably 10−120 or less, even more preferably 10−140 or less, and even more preferably 10−160 or less, with respect to an amino acid sequence of a protein having at least one amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO: 2, the amino acid sequence of SEQ ID NO: 4, and the amino acid sequence of SEQ ID NO: 6, as long as the gene encodes a protein having the long-chain trans-prenyl diphosphate-synthesizing activity.
In the present invention, the E-value refers to an expected value that is an index of homology, that is, an expected value of the number of homologous sequences that are found completely accidentally in a database. In the NCBI homology search program BLAST, the E-value refers to a value of a comparison result (Expect) displayed for one of two sequences that are to be compared when the other sequence is input as a query sequence using default parameters. A smaller E-value indicates a higher homology.
The gene of the present invention may comprise DNA that encodes a protein having an amino acid sequence in which one or several (20 or less, preferably ten or less, more preferably five or less, and even more preferably three or less) amino acids are substituted, added, deleted or inserted in an amino acid sequence of the long-chain trans-prenyl diphosphate synthase, as long as the gene encodes a protein having the long-chain trans-prenyl diphosphate-synthesizing activity. Such a mutation of the amino acid sequence may result from substitution, addition, deletion or insertion of a base on the DNA, and may result from either natural induction of mutation or artificial induction of mutation (e.g., use of a site-directed mutagenesis).
The gene of the present invention can be obtained by preparing a probe or a primer based on the sequence information described in this specification and performing PCR that uses the chromosome DNA or cDNA of Eucommia ulmoides as a template, using a method commonly used by those skilled in the art. It will be appreciated that the gene of the present invention can also be obtained via reverse transcription-PCR that uses RNA as a template. The gene of the present invention may be not only natural polynucleotides of DNA, RNA or the like, but also artificial molecules containing artificial nucleotide derivatives. Furthermore, the gene of the present invention may also be a chimera molecule of DNA-RNA.
The long-chain trans-prenyl diphosphate synthase of the present invention may comprise (A) a protein having at least one amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO: 2, the amino acid sequence of SEQ ID NO: 4, and the amino acid sequence of SEQ ID NO: 6.
The long-chain trans-prenyl diphosphate synthase of the present invention may comprise (B) a protein having an E-value of 10−80 or less with respect to the amino acid sequence of the (A) protein, and having long-chain trans-prenyl diphosphate-synthesizing activity.
The long-chain trans-prenyl diphosphate synthase of the present invention may comprise (C) a protein having an amino acid sequence in which one or several amino acids are substituted, added, deleted or inserted in the amino acid sequence of the (A), and having long-chain trans-prenyl diphosphate-synthesizing activity.
The enzyme of the present invention may comprise a protein having an E-value of 10−80 or less, preferably 10−100 or less, more preferably 10−120 or less, even more preferably 10−140 or less, and even more preferably 10−160 or less, with respect to an amino acid sequence of a protein having at least one amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO: 2, the amino acid sequence of SEQ ID NO: 4, and the amino acid sequence of SEQ ID NO: 6, as long as the enzyme has the long-chain trans-prenyl diphosphate-synthesizing activity.
The enzyme of the present invention may comprise a protein having an amino acid sequence in which one or several (20 or less, preferably ten or less, more preferably five or less, and even more preferably three or less) amino acids are substituted, added, deleted or inserted in an amino acid sequence of a protein having at least one amino acid sequence selected from the group consisting of the amino acid sequence of SEQ ID NO: 2, the amino acid sequence of SEQ ID NO: 4, and the amino acid sequence of SEQ ID NO: 6, as long as the enzyme has the long-chain trans-prenyl diphosphate-synthesizing activity.
The enzyme of the present invention taxonomically belongs to transferase (EC2.5.1), and is classified as the long-chain prenyl diphosphate synthase III (E-form long-chain prenyl diphosphate synthase). The enzyme of the present invention catalyzes a condensation reaction that uses prenyl diphosphate (C5n) having 5n carbon atoms (n is an integer) and isopentenyl diphosphate (IPP, C5) having five carbon atoms as a substrate and transfers an isoprenyl group (C5) to the prenyl diphosphate (C5n), thereby producing prenyl diphosphate (C5(n+1)) having 5(n+1) carbon atoms and a by-product of diphosphate. This condensation reaction is characterized in that the isoprenyl group is transferred to the prenyl diphosphate (C5n) at the trans-position. The maximum number of isoprene units (isoprene chain length) or the maximum degree of IPP polymerization that can be produced by a condensation polymerization reaction that repeats the condensation reaction is 11 or more, preferably 20 or more, more preferably 30 or more, and even more preferably 40 or more. The enzyme of the present invention can be purified according to a common method, and can be purified according to, for example, the method described in Example 4 below.
The enzyme of the present invention has high homology (E-value <10−80) with an FPP synthase of an eukaryote on the amino acid sequence. The amino acid sequence of a prenyl diphosphate synthase of an eukaryote has two aspartate-rich motifs (first aspartate-rich motif (FARM) and second aspartate-rich motif (SARM)). In the enzyme of the present invention, the FARM sequence refers to an amino acid sequence comprising aspartic acid-aspartic acid-isoleucine-methionine seen in, for example, an amino acid sequence from positions 99 to 102 of the amino acid sequence of SEQ ID NO: 2, an amino acid sequence from positions 99 to 102 of the amino acid sequence of SEQ ID NO: 4, or an amino acid sequence from positions 100 to 103 of the amino acid sequence of SEQ ID NO: 6. In the amino acid sequence of an FPP synthase of an eukaryote, an amino acid sequence consisting of tyrosine immediately followed by phenylalanine, or an amino acid sequence consisting of phenylalanine immediately followed by phenylalanine is present near the amino terminal end of the FARM. These amino acid sequences are considered to participate in the degree of IPP polymerization in the condensation polymerization reaction catalyzed by a prenyl diphosphate synthase. In the enzyme of the present invention, these amino acid sequences are replaced by an amino acid sequence consisting of cysteine immediately followed by alanine. It seems that this substitution provides the enzyme of the present invention with a long-chain prenyl diphosphate-synthesizing activity.
Accordingly, the gene of the present invention may comprise DNA that encodes an amino acid sequence consisting of cysteine immediately followed by alanine near the amino terminal end of the FARM in the amino acid sequence, for example, DNA having a base sequence from positions 367 to 372 of the base sequence of SEQ ID NO: 1, a base sequence from positions 321 to 326 of the base sequence of SEQ ID NO: 3, or a base sequence from positions 373 to 378 of the base sequence of SEQ ID NO: 5, as long as the gene encodes a protein having the long-chain trans-prenyl diphosphate-synthesizing activity.
Furthermore, the enzyme of the present invention may comprise a protein having an amino acid sequence consisting of cysteine immediately followed by alanine near the amino terminal end of the FARM in the amino acid sequence, for example, an amino acid sequence from positions 94 to 95 of the amino acid sequence of SEQ ID NO: 2, an amino acid sequence from positions 94 to 95 of the amino acid sequence of SEQ ID NO: 4, or an amino acid sequence from positions 95 to 96 of the amino acid sequence of SEQ ID NO: 6, as long as the enzyme has the long-chain trans-prenyl diphosphate-synthesizing activity.
Furthermore, the enzyme of the present invention has an insertion sequence of 6 amino acids that is not present in an FPP synthase of an eukaryote, between the amino terminal end and the FARM of the amino acid sequence. Analysis of an expected three-dimensional structural model suggests that this insertion sequence forms a protruding structure on the surface of a protein, and this insertion sequence may function as a localization signal.
Accordingly, the gene of the present invention may comprise DNA that encodes an insertion sequence of 6 amino acids that is not present in the FPP synthase, between the amino terminal end and the FARM of the amino acid sequence, for example, DNA having a base sequence from positions 277 to 294 of the base sequence of SEQ ID NO: 1, a base sequence from positions 231 to 248 of the base sequence of SEQ ID NO: 3, or a base sequence from positions 283 to 300 of the base sequence of SEQ ID NO: 5, as long as the gene encodes a protein having the long-chain trans-prenyl diphosphate-synthesizing activity.
Furthermore, the enzyme of the present invention may comprise a protein having an insertion sequence of 6 amino acids that is not present in the FPP synthase, between the amino terminal end and the FARM of the amino acid sequence, for example, an amino acid sequence from positions 64 to 69 of the amino acid sequence of SEQ ID NO: 2, an amino acid sequence from positions 64 to 69 of the amino acid sequence of SEQ ID NO: 4, or an amino acid sequence from positions 65 to 70 of the amino acid sequence of SEQ ID NO: 6, as long as the enzyme has the long-chain trans-prenyl diphosphate-synthesizing activity.
The expression vector of the present invention includes the long-chain trans-prenyl diphosphate synthase gene. The expression vector of the present invention can be constructed using a method commonly used by those skilled in the art. Various base vectors can be used depending on a host that is to be transformed. For example, in the case of Escherichia coli, examples of the vector include pUC19, pMAL-p2, pCold I, pGEX, pET, pMalc2, pTrc99A, and the like. In the case of a yeast, examples of the vector include pYES, pYC, pYI, pYL, pYEUra3TM, and the like. In the case of a plant, examples of the vector include pIG121-Hm, pBI12, pBI221, pBIN19, pCAMBIA2301, pCC22, pGA482, pPCV001, pCGN1647, pJJ1881, pPZP111, pGreen0029, pBI101, pBI121, pYLTAC7, and the like. The vectors appropriately include not only a target gene, but also DNA participating in the regulation of gene expression, such as a promoter and a terminator for expressing the gene, and a selection marker for selecting a transformant. Various promoters can be used depending on a host that is to be transformed. For example, in the case of Escherichia coli, examples of the promoter include T7 promoter, lac promoter, tac promoter, trp promoter, cspA promoter, and the like. In the case of a yeast, examples of the promoter include PMA1 promoter, ADH1 promoter, GAL1 promoter, PGK promoter, PHO5 promoter, GAPDH promoter, and the like. In the case of a plant, examples of the promoter include CaMV35S promoter, NOS promoter, CAB promoter, UBI promoter, and the like.
In the present invention, examples of the host that is to be transformed include microorganisms, such as Escherichia coli and yeasts, and plants. The host is preferably a plant, and more preferably Eucommia ulmoides belonging to the family Eucommiaceae and Periploca sepium belonging to the family Apocynaceae. Various transformation methods can be used depending on the host that is to be transformed. For example, in the case of Escherichia coli, examples of the method include competent cell methods, electroporation methods, and the like. In the case of a yeast, examples of the method include lithium acetate methods, spheroplast methods, and the like. In the case of a plant, examples of the method include Agrobacterium methods, particle gun bombardment methods, electroporation methods, and the like. The transformant can be selected and separated from wild-type, which have not been transformed, by using a selection marker contained in the vector. Examples of the selection marker that can be commonly used include an antibiotic resistance gene, a green fluorescent protein (GFP) gene, and the like, and various selection markers can be used depending on the host that is to be transformed. For example, in the case of Escherichia coli, examples of the selection marker include an ampicillin resistance gene, a chloramphenicol resistance gene, and the like. In the case of a yeast, examples of the selection marker include an aureobasidin A resistance gene and a gene for a nutritional auxotrophy such as various amino acid auxotrophies, and the like. In the case of a plant, examples of the selection marker include a kanamycin resistance gene, a hygromycin resistance gene, and the like.
In order to produce a transformed plant (e.g., Eucommia ulmoides, Periploca sepium, Nicotiana tabacum, and the like, preferably Eucommia ulmoides) that has an increased content of trans-1,4-polyisoprene, the long-chain trans-prenyl diphosphate synthase gene of the present invention can be preferably used for transforming the plant.
The transformed plant (preferably transformed Eucommia ulmoides) of the present invention can be cultivated using a method commonly used by those skilled in the art. The trans-1,4-polyisoprene can be recovered and purified using a method commonly used by those skilled in the art from leaves, bark, peel, and the like of a mature tree of the transformed Eucommia ulmoides. 
EXAMPLES
Hereinafter, the present invention will be described further in detail by way of examples, but the present invention is not limited thereto.
Example 1
Isolation of TPL1-cDNA (SEQ ID NO: 1)
Preparation of the Total RNA of Eucommia ulmoides: 1
Leaves of a young current year's branch collected in late May from an Eucommia ulmoides sample tree grown in Ikina, Ehime (Japan) were used as an Eucommia ulmoides plant body sample. While being cooled with liquid nitrogen, the Eucommia ulmoides plant body sample (the leaves of the current year's branch) was crushed with a mortar and pestle, and suspended in 2×CTAB solution (2% (w/v) hexadecyltrimethylammonium bromide (CTAB), 1% (w/v) β-mercaptoethanol, 0.1M Tris-HCl (pH 9.5), 1.4M NaCl, and 20 mM EDTA) with a volume ten times (w/v) the sample. The resultant was incubated at 65° C. for ten minutes, and then treated (washed) with chloroform/isoamyl alcohol (repeated twice). Subsequently, to the recovered aqueous layer, 10M LiCl with a volume 1⁄4 times (w/v) the aqueous layer was added, and the mixture was allowed to stand at −20° C. for two hours, thereby performing RNA-selective precipitation. The resultant was centrifuged, the precipitate was dissolved in an appropriate volume of tris-EDTA (TE) buffer solution, subsequently the resultant was centrifuged, and the supernatant was recovered, while polysaccharides were removed. The recovered supernatant was treated with phenol, phenol/chloroform, and chloroform/isoamyl alcohol, and RNA-selective precipitation with LiCl was performed again. The precipitate was washed with 70% ethanol, dried under reduced pressure, and then dissolved in diethylpyrocarbonate (DEPC)-treated water to give the total RNA.
Preparation of Eucommia ulmoides-Derived cDNA
A reverse transcription reaction was performed using the Eucommia ulmoides leaf-derived total RNA as a template, and AMV Reverse Transcriptase XL (manufactured by Takara Bio Inc.) to give Eucommia ulmoides-derived cDNA. Oligo dT Adaptor Primer (manufactured by Takara Bio Inc.) was used as a primer for the reverse transcription reaction.
Determination of TPL1-cDNA Partial Sequence Using Degenerate PCR
A first PCR was performed using the Eucommia ulmoides-derived cDNA as a template, and TaKaRa Ex Taq (manufactured by Takara Bio Inc.). Next, a second PCR was performed using the first PCR product as a template, and TaKaRa Ex Taq. In both PCRs, degenerate primers were used as primers, and the base sequences were as follows. In both PCRs, a cycle of five minutes at 94° C. was performed once, a cycle of one minute at 94° C., one minute at 54° C. and then two minutes at 74° C. was performed 30 times, and then a cycle of seven minutes at 74° C. was performed once. The amplified fragments obtained by the second PCR were cloned into the pUC18 vector, and sequenced for a plurality of the plasmid clones. As a result, it was found that some of the clones had a partial sequence of the base sequence of SEQ ID NO: 1.
Primer Set for the First PCR










Sense primer:
CIYTIGGITGGTGYRTNGARTGG
(SEQ ID NO: 7)



 


Antisense
GTYCANGTYCTRCTIATIIAICTIAC
(SEQ ID NO: 8)


primer:






Primer Set for the Second PCR










Sense primer:
GGTGIRTIGARTGGYTNCARGC
(SEQ ID NO: 9)



 


Antisense
CCCNTRNATRAAIGTICAIGTIC
(SEQ ID NO: 10)


primer:






Preparation of the Total RNA of Eucommia ulmoides: 2
Total RNA was prepared as in the preparation of the total RNA of Eucommia ulmoides 1, excerpt that phloem (bark) and xylem of a young current year's branch collected in late May from an Eucommia ulmoides sample tree grown in Ikina, Ehime (Japan) were used as an Eucommia ulmoides plant body sample. The obtained total RNA was quantified by measuring optical density (OD), and confirmed by electrophoresis. Here, 2 mg of total RNA was obtained from approximately 4 g of phloem, and 0.84 mg of total RNA was obtained from approximately 4 g of xylem (the ratios of OD at 260 nm to OD at 280 nm were 1.991 and 1.956, respectively).
Preparation of cDNA Library of Eucommia ulmoides 
Then, mRNA was purified from the Eucommia ulmoides phloem and xylem-derived total RNA sample using Oligotex-dT30<Super> (manufactured by Takara Bio Inc.). Next, a cDNA library was prepared from the mRNA using the Lambda ZAP II XR Library Construction Kit (manufactured by Stratagene).
Preparation of Probe for cDNA Library Screening
PCR was performed using the plasmid clones obtained in the degenerate PCR and containing a partial sequence of the base sequence of SEQ ID NO: 1 as a template, and TaKaRa Ex Taq. The base sequences of the primers used were as follows. In the PCR, a cycle of five minutes at 94° C. was performed once, a cycle of one minute at 94° C., one minute at 50° C. and then one minute at 72° C. was performed 30 times, and then a cycle of seven minutes at 72° C. was performed once. The PCR reaction product was labeled with alkaline phosphatase using AlkPhos Direct Labelling and Detection System with CDP-Star (manufactured by GE Healthcare) to give a probe for screening.
Primer Set for PCR










Sense primer:
GTGCTCTTGTTCTTGATGATA
(SEQ ID NO: 11)



 


Antisense
CAAGAAGTATGTCCTTCATGT
(SEQ ID NO: 12)


primer:






Screening of Eucommia ulmoides cDNA Library
From the Eucommia ulmoides cDNA library, phage plaque lifting was performed onto an Hybond N+ membrane (manufactured by GE Healthcare) according to a common method. Next, this membrane was subjected to hybridization, washing and signal detection using the probe for screening and the AlkPhos Direct Labelling and Detection System with CDP-Star. The hybridization was performed at 55° C. for 16 hours, primary washing was performed twice at 55° C. for ten minutes, and secondary washing was performed twice at room temperature for five minutes. As a result of the screening, it was found that 23 positive phage plaques were obtained. Next, these phage clones were converted to plasmid clones by in vivo excision using the Lambda ZAP II XR Library Construction Kit. The 23 plasmid clones were sequenced. As a result, it was found that ten clones had the same base sequence as the base sequence of SEQ ID NO: 1 or a base sequence considered to be a splicing variant thereof.
Example 2
Isolation of TPL3-cDNA (SEQ ID NO: 3)
TPL3-cDNA was isolated as in Example 1. The obtained full-length cDNA (TPL3-cDNA) had the base sequence of SEQ ID NO: 3. The base sequence from positions 37 to 1089 of the base sequence of SEQ ID NO: 3 had a homology of 76% with the base sequence of TPL1-cDNA (SEQ ID NO: 1: GenBank Accession Number AB041626). The base sequence from positions 42 to 1088 of the base sequence of SEQ ID NO: 3 had an open reading frame. The deduced amino acid sequence encoded by this cDNA is as shown in SEQ ID NO: 4. The complete amino acid sequence (positions 1 to 348) of SEQ ID NO: 4 has an E-value of 10−169 (a homology of 77%) with respect to the amino acid sequence of TPL1 (SEQ ID NO: 2: GenBank Accession Number BAB16687).
Example 3
Isolation of TPL5-cDNA (SEQ ID NO: 5)
Preparation of cDNA Library of Eucommia ulmoides 
From an Eucommia ulmoides phloem and xylem-derived total RNA sample prepared as in Example 1, a cDNA library was prepared by the G-capping method at Hitachi Instruments Service Co., Ltd. The phloem-derived cDNA library had a library size of 3.8×105, an insertion ratio of 88% (24 samples/agarose gel electrophoresis), and a full-length ratio of 86% (with respect to clones carrying inserts). The xylem-derived cDNA library had a library size of 2.2×105, an insertion ratio of 79% (24 samples/agarose gel electrophoresis), and a full-length ratio of 63% (with respect to clones carrying inserts).
EST Analysis of Eucommia ulmoides 
Approximately 20000 clones of each of the Eucommia ulmoides phloem and xylem-derived cDNA libraries were subjected to base sequence analysis at the Laboratory of Genome Informatics of the Kitasato Institute for Life Sciences, Kitasato University. Based on the sequence information obtained by the sequence analysis, clones not carrying inserts and clones for which the sequence could not be read were removed, and precise sequence information was obtained. Here, 16567 and 16113 precise EST sequences were obtained for the phloem and xylem libraries (total 32680), respectively. Next, the obtained sequences were subjected to clustering and annotation. “Clustering” refers to processing that clusters the same sequences and similar sequences among the EST sequences. NTT software VISUALBIO clustering was used for the clustering. “Annotation” refers to processing that annotates the EST sequences based on comparison with known genes. A homology search using NCBI BLAST was used for the annotation. The database used for the search was nr (All non-redundant GenBank CDS translations+PDB+SwissProt+PIR (Peptide Sequence Database)).
Isolation of TPL5-cDNA
Based on the information obtained by the clustering and the annotation, a sequence on the 5′-terminal end side of TPL5 (SEQ ID NO: 5) having an extremely high homology with TPL1 (SEQ ID NO: 1) was found. A sequence on the 3′-terminal end side of TPL5 (SEQ ID NO: 5) was determined by 3′-RACE (Rapid Amplification of cDNA Ends, RACE). Here, a 3′-Full RACE Core Set (manufactured by Takara Bio Inc.) was used for the 3′-RACE. First, a reverse transcription reaction was performed using the total RNA obtained in Example 1 as a template, and Oligo dT-3 sites Adaptor Primer attached to the 3′-Full RACE Core Set as a primer. Next, a first PCR was performed using the reverse transcription reaction product as a template, and, then, a second PCR was performed using the first PCR product as a template. The base sequences of the primers used were as follows. In both of the first and the second PCRs, a cycle of 60 seconds at 94° C., 60 seconds at 54° C. and then two minutes at 74° C. was performed 30 times. The amplified fragments obtained by the second PCR were TA cloned into the pT7Blue vector (manufactured by Takara Bio Inc.), and sequenced.
Primer Set for the First PCR










Sense
ACAGTGGCTGGGCAGATGATAG
(SEQ ID NO: 13)



primer:






Antisense primer: 3 sites Adaptor Primer attached to the 3′-Full RACE Core Set

Primer Set for the Second PCR










Sense
TTACCACACTTCTCGGAGAGGC
(SEQ ID NO: 14)



primer:


 


Antisense
CGCTTGCATCCATTCGATACACC
(SEQ ID NO: 15)


primer:






The obtained full-length cDNA (TPL5-cDNA) had the base sequence of SEQ ID NO: 5. The base sequence from positions 113 to 1132 of the base sequence of SEQ ID NO: 5 had a homology of 76% with the base sequence of TPL1-cDNA (SEQ ID NO: 1: GenBank Accession Number AB041626). The base sequence from positions 91 to 1140 of the base sequence of SEQ ID NO: 5 had an open reading frame. The deduced amino acid sequence encoded by this cDNA is as shown in SEQ ID NO: 6. The complete amino acid sequence (positions 1 to 349) of SEQ ID NO: 6 has an E-value of 10−166 (a homology of 79%) with respect to the amino acid sequence of TPL1 (SEQ ID NO: 2: GenBank Accession Number BAB16687).
Example 4
Preparation and Analysis of TPL Protein
Construction of TPL Expression Vector
Total RNA was prepared from leaves of the Eucommia ulmoides sample tree using an RNeasy Plant Mini Kit (manufactured by Qiagen). Buffer RLC provided with the kit was used as a buffer solution for total RNA extraction. Next, cDNA fragments of TPL1, TPL3 and TPL5 were amplified by PCR using the total RNA as a template, and a High Fidelity RNA PCR Kit (manufactured by Takara Bio Inc.). The base sequences of the primers used were as follows. In the PCR, a cycle of 30 seconds at 94° C., 30 seconds at 55° C. and then two minutes at 72° C. was performed 35 times.
Primer Set for TPL1-cDNA Amplification








(SEQ ID NO: 16)












Sense primer:
ACGCTGTCCTTGCACTTG




 








(SEQ ID NO: 17)












Antisense primer:
GGAGAACCAAATATGCAATAAAGCCTG







Primer Set for TPL3-cDNA Amplification








(SEQ ID NO: 18)










Sense primer:
GGCCTTTCGTTCTCTCTCTCTCTCTT



 








(SEQ ID NO: 19)










Antisense primer:
ACGACTACATTTATTCAGGTTCGAAGTC







Primer Set for TPL5-cDNA Amplification








(SEQ ID NO: 20)










Sense primer:
GATCAACACATCCTTGAGCGTTACC



 








(SEQ ID NO: 21)










Antisense primer:
GTTAGTCGTTGCAATTTATTTGTTCCCTC







Each amplified fragment obtained by the PCR was inserted into the restriction enzyme EcoRV site in the multicloning site of plasmid pBluescript II KS—(manufactured by Stratagene), and plasmid clones were constructed. The plasmid clones into which the cDNA of TPL1, TPL3 and TPL5 was inserted were designated as pBluescript-TPL1, pBluescript-TPL3 and pBluescript-TPL5, respectively. Each clone was sequenced and confirmed to have no mutation due to PCR amplification.
Fragments of the sequence encoding the protein of TPL1, TPL3 and TPL5 were amplified by PCR using each plasmid clone as a template, and Pyrobest DNA Polymerase (manufactured by Takara Bio Inc.). The base sequences of the primers used were as follows. The sense primer had a recognition sequence of the restriction enzyme NdeI at the 5′-terminal end, and the antisense primer had a recognition sequence of the restriction enzyme XhoI at the 5′-terminal end. In the PCR, a cycle of 30 seconds at 94° C., 30 seconds at 55° C. and then two minutes at 72° C. was performed 35 times.
Primer Set for Amplification of TPL1 Protein-Encoding Sequence








(SEQ ID NO: 22)










Sense primer:
GAGAGAGCATATGGCGGAACTGAAGAAAGAATTTC



 








(SEQ ID NO: 23)










Antisense:
CCGCTCGAGCTACTTGAGCCTCCTGTGAATCTTAG



primer:






Primer Set for Amplification of TPL3 Protein-Encoding Sequence








(SEQ ID NO: 24)










Sense primer:
GAGAGAGCATATGACCGAGCTGAAGAGCAAATTTG



 








(SEQ ID NO: 25)










Antisense
CCGCTCGAGCTACTTGAGCCTCTTGTGTATCTTAGC



primer:






Primer Set for Amplification of TPL5 Protein-Encoding Sequence








(SEQ ID NO: 26)










Sense primer:
GAGAGAGCATATGGCGGAAACGACCCAA



 








(SEQ ID NO: 27)










Antisense
CCGCTCGAGTCAATAATGCCTCCGATAGATCTTTGC



primer:






Each amplified fragment obtained by the PCR was inserted between the restriction enzyme NdeI site and the restriction enzyme XhoI site in the multicloning site of the cold shock expression vector pCold I (manufactured by Takara Bio Inc.), and TPL expression vectors were constructed. pCold I has a structure in which a protein that is expressed by a gene inserted into the multicloning site and a histidine hexamer tag (His-Tag) can be produced as a fusion protein. The TPL expression vectors into which fragments of the sequence encoding the protein of TPL1, TPL3 and TPL5 were inserted were designated as pCold-TPL1, pCold-TPL3 and pCold-TPL5 (FIG. 1), respectively. Each vector was sequenced and confirmed to have no mutation due to PCR amplification.
Transformation of Escherichia coli 
In order to express the TPL protein as a soluble protein inside an Escherichia coli cell, first, a competent cell of an Escherichia coli BL21 (DE3) strain was transformed with chaperone plasmid pG-Tf2 (manufactured by Takara Bio Inc.). Next, from the transformed Escherichia coli obtained a competent cell was further prepared, and this competent cell was transformed with TPL expression vector pCold-TPL1, pCold-TPL3 or pCold-TPL5. In this manner, transformed Escherichia coli in which a TPL expression vector and a chaperone plasmid coexpressed was obtained.
Preparation of TPL Protein
The coexpressing transformed Escherichia coli was cultured with shaking at 37° C. in 50 mL of LB medium containing ampicillin (50 μg/mL) and chloramphenicol (20 μg/mL), which are agents for selecting transformants, and tetracycline (1 ng/mL), which is an agent for inducing chaperone expression, until OD at 600 nm of the culture medium was approximately 0.5 (before induction of expression). Next, the culture medium was cooled at 15° C. for 30 minutes, and isopropyl-β-D-thiogalactoside (final concentration 0.5 mM) was added thereto. Next, culturing with shaking was continued at 15° C. for another 24 hours (after induction of expression).
Escherichia coli was recovered by centrifugation from 10 mL of the culture medium after induction of expression, and suspended in 1 mL of cell-crushing buffer solution (60 mM NaH2PO4, 300 mM NaCl, 10 mM imidazole, 0.5 mM PMSF, 2% TritonX100, 1 mg/mL lysozyme, pH 8.0). After the suspension was allowed to stand on ice for ten minutes, Escherichia coli in the suspension was crushed on ice using an ultrasonic crusher SONIFIER450 (manufactured by Branson), and then the liquid after crushing was separated into supernatant and precipitate by centrifugation at 10000×g at 4° C. for 30 minutes. A TPL protein fused with a histidine hexamer tag produced was purified from the supernatant using an Ni-NTA spin column (manufactured by Qiagen) under non-denaturing conditions. A washing buffer solution (50 mM NaH2PO4, 300 mM NaCl, 50 mM imidazole, 0.1% TritonX100, pH 8.0) was used to wash the column, and an elution buffer solution (50 mM NaH2PO4, 300 mM NaCl, 500 mM imidazole, 0.1% TritonX100, pH 8.0) was used to elute the TPL protein.
Analysis of TPL Protein
Samples taken during preparation of the TPL protein were analyzed by SDS-PAGE and Western blotting. In the SDS-PAGE, 0.05 mM. Tris-HCl (pH 6.8) containing 2% SDS and 6% β-mercaptoethanol was used as a buffer solution for the gel, a buffer solution for electrophoresis and a buffer solution for samples, and the acrylamide concentration in the separation gel was 10%. The culture medium before induction of expression and the culture medium after induction of expression, and eluted fractions of the TPL proteins were heated in a buffer solution for samples at 100° C. for five minutes, and each 10 μL was subjected to SDS-PAGE. In the Western blotting, transfer was performed from the gel-after SDS-PAGE onto a Hybond-P PVDF membrane (manufactured by GE Healthcare), and this membrane was reacted with His-Tag Monoclonal Antibody (manufactured by Novagen) diluted 1000 times as a primary antibody, and then with Anti-Mouse IgG (H+L) AP Conjugate (manufactured by Promega) diluted 5000 times as a secondary antibody. Using the ProtoBlot II AP System with Stabilized Substrate (manufactured by Promega), signals on the membrane were detected. FIG. 2 shows the results. FIG. 2 shows that the TPL proteins were purified.
Next, the prenyl diphosphate-synthesizing activity of the purified TPL proteins was measured. First, 200 μL of enzyme reaction liquid containing 5% of an eluted fraction of the TPL protein (sample), 10 μM or 4 μM of allyl substrate, and 100 μM of radioactive 32P-labeled IPP (37 GBq/mol) in a buffer solution (100 mM K-MOPS (pH 8.0), 5 mM MgCl2, 0.1% TritonX100) was prepared, and allowed to stand at 30° C. for 16 hours. Next, 200 μL of NaCl-saturated water and 1 mL of butanol saturated with NaCl-saturated water were added to the enzyme reaction liquid, and the mixture was vigorously vortexed for one minute, and, thus, the enzyme reaction was stopped. The resultant was centrifuged at room temperature for three minutes, and separated into an aqueous layer and a butanol layer. Next, 200 μL of the butanol layer was mixed with 3 mL of a liquid scintillation cocktail Clear-sol I (manufactured by Nacalai Tesque, Inc.), and the radioactivity (dpm) of the mixed liquid was measured in a liquid scintillation counter Tri-Carb 2100 (manufactured by Packard). IPP is insoluble in butanol, but a reaction product of IPP and an allyl substrate is soluble in butanol. Thus, the radioactivity of the butanol layer indicates the IPP condensation polymerization reaction activity due to a prenyl diphosphate synthase. FIG. 3 shows the results. FIG. 3 shows, for each sample, the average (bar graph) and standard deviation (error bar) of the values obtained from triplicated experiments by subtracting the radioactivity of an enzyme reaction liquid without a sample (background) from the radioactivity of the enzyme reaction liquid with a sample. It is apparent from FIG. 3 that all of TPL1, TPL3 and TPL5 catalyze an IPP polymerization reaction when using FPP or GGPP as an allyl substrate. However, the IPP polymerization reaction was not catalyzed when using DMAPP (dimethylallyl pyrophosphate) as an allyl substrate.
Example 5
Transformation of Eucommia ulmoides Cultured Root
Construction of Plant-Transforming TPL Expression Vector
Restriction enzyme NdeI-XhoI fragments of the TPL expression vector pCold-TPL1 were inserted between the restriction enzyme XhoI site and the restriction enzyme KpnI site in the multicloning site of the plant-transforming vector pBIsGFP (given by Norihiro Mitsukawa, Biotechnology Division, Toyota Central R&D Labs, Inc.), and a plant-transforming TPL expression vector pBIsGFP-TPL1 was constructed (FIG. 4). The pBIsGFP contains a kanamycin resistance gene and a modified GFP gene.
Transformation of Eucommia ulmoides Cultured Root
Seeds collected from a female strain of Eucommia ulmoides grown in Chengdu, Sichuan Province, China were aseptically disseminated in a germination medium (1⁄2 MS medium, 20 g/L sucrose). A rootlet of a young plant body at 10 to 20 days after dissemination was excised into pieces having a length of 2 to 3 cm, and cultured with reciprocal shaking (120 revolution/min) in a growth medium (1⁄2 MS liquid medium, 1 μM NAA). This Eucommia ulmoides cultured root could be grown for a long period of time by subculturing every four weeks, and used as an experimental material. The grown new root was cut into pieces having a length of approximately 1.5 cm, and transformed with pBIsGFP-TPL1 by an Agrobacterium method. In the Agrobacterium method, the Agrobacterium tumefaciens LBA4404 strain was used. In order to promote infection with Agrobacteria, the cultured root was treated with ultrasonic waves for 20 minutes before infection. The cultured root infected with Agrobacteria was transferred onto a callus induction agar medium (MS medium, 1 μM 2-iP, 1 μM NAA), calluses were induced, and then a transformed Eucommia ulmoides callus was selected. The transformed callus was selected by use of a GFP signal. Next, the selected transformed callus was transferred onto a root induction medium (MS medium, 1 μM NAA), and roots were differentiated and grown from the callus. Here, among the transformed Eucommia ulmoides cultured roots obtained, 20 lines were maintained and used for analysis.
Analysis of Transformed Eucommia ulmoides Cultured Root: 1
The amount of TPL1 mRNA for 20 lines of the transformed Eucommia ulmoides cultured roots was quantified by Realtime PCR (FIG. 5). RNA was extracted from the transformed Eucommia ulmoides cultured roots using an RNeasy Plant Mini Kit (manufactured by Qiagen). First, contaminating DNA was removed using RNase-Free DNase I (manufactured by Qiagen). Next, the concentration of the RNA sample was measured. At the same time, six concentrations of samples (RNA concentration: 400, 100, 25, 6.25, 1.56 and 0.39 ng/μL for a calibration curve were prepared. Then, cDNA was prepared from the RNA sample extracted from the transformed Eucommia ulmoides cultured roots and the calibration curve samples, using a High Capacity Reverse Transcription Kit (manufactured by Applied Biosystems). Realtime PCR was performed using this cDNA as a template, and the ABI Prism 7300 Sequence Detection System (manufactured by Applied Biosystems) by the SYBR Green method. The base sequences of the primers used were as follows. In the Realtime PCR, a cycle of two minutes at 50° C. and then ten minutes at 95° C. was performed once, and then a cycle of 15 seconds at 95° C. and then one minute at 60° C. was performed 40 times. Next, a 15-second reaction at 95° C., a one-minute reaction at 60° C., a 15-second reaction at 95° C., and then a 15-second reaction at 60° C. were performed, and the dissociation status was observed.
Primer Set for TPL1 Quantification










Sense
AAGGAGCTCAACTCACTGAGAGC
(SEQ ID NO: 28)



primer:


 


Antisense
AATGCACCAACCCAACACAG
(SEQ ID NO: 29)


primer:






Primer Set for Calibration Curve (Detection of Internal Reference Gene EF1α).










Sense
CCGAGCGTGAACGTGGTAT
(SEQ ID NO: 30)



primer:


 


Anti
TAGTACTTGGTGGTTTCGAATTTCC
(SEQ ID NO: 31)


primer:






Analysis of Transformed Eucommia ulmoides Cultured Roots: 2
The distributions of rubber produced inside the transformed Eucommia ulmoides cultured root TPL1-9-7 (FIG. 6), which had the largest amount of TPL1 mRNA, and a wild-type Eucommia ulmoides cultured root (FIG. 7) were evaluated using a real spectral imaging microscope (SCLSM). Separated fluorescence images of the cultured roots were acquired using DIGITAL ECLIPSE C1si manufactured by Nikon Corporation as the SCLSM. The fluorescence separation was performed using a reference spectrum with EZ-C1 3.40 software manufactured by Nikon Corporation. The reference spectrum was acquired as follows. Fluorescence spectral images of fibrous trans-polyisoprene taken from Eucommia ulmoides bark for herbal medicine and stained with Nile red, and a cross-sectional piece of the bark of a current year's branch of Eucommia ulmoides stained with Nile red (for staining particulate liposoluble substance) and Fluorescent Brightener 28 (for staining cell walls) were acquired using DIGITAL ECLIPSE C1si, the fluorescence spectra of ten regions of interest (ROI) assigned by EZ-C1 3.40 were measured, and their average value was taken as the reference spectrum. In the acquisition of the Nile red-derived fluorescence spectra, spectra in the range of 418 to 578 nm were acquired using a solid-state laser (488 nm, 20 mW). In the acquisition of the Fluorescent Brightener 28-derived fluorescence spectra, spectra in the wavelength range of 498 to 658 nm were acquired using a BD laser (408 nm, 17 mW). The fluorescence maximum wavelengths of the acquired reference spectra were 545 nm (Nile red-derived, trans-polyisoprene), 575 nm (Nile red-derived, particulate liposoluble substance), and 450 nm (Fluorescent Brightener 28-derived, cell wall). In the acquisition of the separated fluorescence images of the cultured roots, spectra in the range of 423 to 723 nm were acquired using a solid-state laser and a BD laser.
As a result, it was found that the rubber chain of the transformed Eucommia ulmoides cultured root TPL1-9-7 was 1.4 times as long as that of the wild-type Eucommia ulmoides cultured root. Furthermore, while the rubber was accumulated in the form of granules in the wild-type Eucommia ulmoides cultured root, the rubber was changed into the form of fibers in the transformed Eucommia ulmoides cultured root TPL1-9-7. Accordingly, it was found that TPL1 participate in the control of the rubber chain length of Eucommia ulmoides. 
Example 6
Transformation of Nicotiana tabacum 
Construction of Plant-Transforming TPL Expression Vector
A TPL1 expression cassette having a fragment of the sequence encoding TPL1 protein derived from pBluescript-TPL1 described in Example 4, and the CaMV35S promoter fragment and the NOS terminator fragment derived from pBI221 (manufactured by Clontech) was constructed, and inserted into the multicloning site of plant-transforming vector pCAMBIA2301 (manufactured by Cambia), and, thus, plant-transforming TPL expression vector pHis-TPL1 was constructed (FIG. 8). pCAMBIA2301 contains a kanamycin resistance gene and the β-glucuronidase gene.
Transformation of Nicotiana tabacum 
Leaves (leaf disks) of Nicotiana tabacum cv Xanthi were transformed with pHis-TPL1 by the Agrobacterium method. Furthermore, transformation with pBIsGFP was performed as a control. In the Agrobacterium method, the Agrobacterium tumefaciens LBA4404 strain was used.
Analysis of Transformed Nicotiana tabacum 
The amounts of TPL1 and GFP (control) mRNA of the transformed Nicotiana tabacum (TPL1 transformed Nicotiana tabacum) obtained by transformation with pHis-TPL1 and the transformed Nicotiana tabacum (GFP transformed Nicotiana tabacum) obtained by transformation with pBIsGFP were measured by Realtime PCR (FIG. 9). The experimental method and conditions were as in Example 5. Next, mature leaves were collected from the TPL1 transformed Nicotiana tabacum TPL1 #8 and the GFP transformed Nicotiana tabacum GFP #1, which had a large amount of mRNA, and wild-type Nicotiana tabacum before formation of flower buds, and subjected to Soxhlet-extraction with ethanol, and then Soxhlet-extraction with toluene. Polyisoprene was obtained by extraction with toluene. The extract was analyzed by size-exclusion chromatography (SEC). A Hitachi 7000 series liquid chromatograph (manufactured by Hitachi, Ltd.) was used. A PLgel Mini Mixed B (10 μm, 250×internal diameter 4.6 mm, manufactured by Polymer Laboratories, Shropshire, England) was used as the column, and THF was used as the eluent. Analysis was performed at a column temperature of 40° C. and a flow rate of 0.2 mL/min, and the ultraviolet absorption (210 nm) was detected. Seven types of cis-1,4-polyisoprene (Mn=1199400, Mw/Mn=1.10; Mn=138000, Mw/Mn=1.05; Mn=30000, Mw/Mn=1.04; Mn=12000, Mw/Mn=1.04; Mn=6000, Mw/Mn=1.04; Mn=2560, Mw/Mn=1.08; Mn=1150, Mw/Mn=1.11) manufactured by Polymer Source were used as samples for a calibration curve in the SEC analysis. An SIC-480II manufactured by System Instruments and analysis software were used to collect data, generate a calibration curve, and calculate a molecular weight distribution.
As a result, it was confirmed that TPL1 transformed Nicotiana tabacum TPL1 #8 had high-molecular weight components with a molecular weight of 104 to 105 (FIG. 10C). It was also confirmed that the TPL1 transformed Nicotiana tabacum TPL1 #1, #3, #6 and #7, having a large amount of TPL1 mRNA, similarly had high-molecular weight components with a molecular weight of 104 to 105. On the other hand, it was confirmed that the GFP transformed Nicotiana tabacum GFP #1 and the wild-type Nicotiana tabacum did not have a high-molecular weight component (FIGS. 10A and 10B). It was also confirmed that the GFP transformed Nicotiana tabacum GFP #2 and #6 to #8, having a large amount of GFP mRNA, did not have a high-molecular weight component. Next, high-molecular weight fractions with a molecular weight of 104 to 105 according to SEC analysis were separated from the mature leaf extract of the TPL1 transformed Nicotiana tabacum TPL1 #8, and subjected to structure analysis by 1H-NMR analysis using a Varian Unity-INOVA 600 Spectrometer (manufactured by Varian). As a result, it was confirmed that the high-molecular weight fractions with a molecular weight of 104 to 105 contained trans-1,4-polyisoprene as in the rubber of Eucommia ulmoides (FIG. 11). Accordingly, it was found that TPL1 is a trans-1,4-polyisoprene synthase, and is a long-chain trans-prenyl diphosphate synthase.
According to the present invention, when a plant is transformed with an expression vector containing a long-chain trans-prenyl diphosphate synthase gene, a plant that has an increased content of trans-1,4-polyisoprene can be obtained. According to the present invention, the content of trans-1,4-polyisoprene in a plant can be increased, and trans-1,4-polyisoprene can be effectively produced using such a plant. In particular, trans-1,4-polyisoprene can be easily extracted from the peel of transformed Eucommia ulmoides that has a higher content of trans-1,4-polyisoprene, and, thus, trans-1,4-polyisoprene can be easily provided as an industrial raw material.

## Claims

1. An expression vector containing a long-chain trans-prenyl diphosphate synthase gene comprising a DNA having at least one base sequence selected from the group consisting of a base sequence from positions 42 to 1088 of the base sequence of SEQ ID NO: 3 or a complementary sequence thereof, and a base sequence from positions 91 to 1140 of the base sequence of SEQ ID NO: 5 or a complementary sequence thereof.

2. A plant transformed with the expression vector of claim 1.

3. The plant of claim 2, wherein the plant is Eucommia ulmoides.

4. The plant of claim 2, wherein the plant is Nicotiana tabacum.

5. A method for increasing the amount of trans-1,4-polyisoprene contained in a plant, comprising the step of transforming the plant using the expression vector of claim 1.

6. A method for producing trans-1,4-polyisoprene, comprising the steps of:
cultivating the plant of claim 2; and
recovering the trans-1,4-polyisoprene from the cultivated plant.

7. A method for producing trans-1,4-polyisoprene, comprising the steps of:
cultivating the plant of claim 3; and
recovering the trans-1,4-polyisoprene from the cultivated plant.

8. A method for producing trans-1,4-polyisoprene, comprising the steps of:
cultivating the plant of claim 4; and
recovering the trans-1,4-polyisoprene from the cultivated plant.