Abstract:
The present invention relates to a plant virus vector comprising a foreign gene linked downstream of a coat protein gene of tobacco mosaic virus via a nucleotide sequence which cause the readthrough, and a plasmid which is transcribed to provide the vector, as well as a process for expression of a foreign gene in a plant by inoculating the plant with the vector. 
     In addition, the present invention relates to a process for efficiently recovering a foreign gene product produced in a plant as virions.

Description:
FIELD OF THE INVENTION 
     The present invention relates to introduction of a foreign gene into a plant using a plant virus vector, improvement of plant properties and expression of a foreign gene throughout a whole plant, and a process for simply and efficiently obtaining a foreign gene product. 
     BACKGROUND OF THE INVENTION 
     It is known that plant viruses represented by tobamovirus, after infecting a plant, proliferate in the plant and rapidly spread systemically throughout the plant while producing viral coat protein in large amounts. 
     So far, some gene engineering systems for these plant viruses have been constructed and some attempts have been carried out to introduce a foreign gene into a plant using said systems. For example, there is a process wherein a coat protein gene is replaced with a foreign gene (Takamatsu et al, EMBO J., 6:307-311 (1987)), and a process wherein a coat protein gene and a foreign gene are directly joined so as to produce a fused protein (Takamatsu et al, FEBS Lett., 269:73-76 (1990 )). 
     However, all of the plant viruses used in these known processes have a drawback in that they do not spread systemically in a plant (i.e., they do not have systemic infectivity), and thus, it is impossible to introduce a useful property and to produce a useful protein throughout a whole plant. 
     For a plant virus to exhibit systemic infectivity, particle formation with wild type coat protein is essential (Saito et al, Virology, 176:329 (1990)). As to existing plant virus vectors, since the coat protein is not produced (replacement-type vector), or the coat protein is in the form of a fused protein, resulting in a big change in properties (direct-joining type vector); then particles cannot be formed and systemic infectivity is not exhibited. 
     In addition, where existing plant virus vectors are used to express a foreign gene, it is very difficult to isolate and purify the foreign gene product from the plant into which the foreign gene was introduced. 
     The reason is that to establish (1) simple apparatus and economy, (2) high recovery, (3) high purity and (4) good reproduction, which are the goals of isolation and purification, a combination of operations, such as differential precipitation, desalting, concentration, and various chromatographies is essential; however, a series of these operations usually takes one-half to one month to complete, and they are often time- and labor-consuming. Even assuming these operations are simple and rapid, a plant includes proteins whose properties, such as molecular weight, isoelectric point, affinity to a solvent, are similar to those of the foreign gene product; and therefore it is difficult to prevent loss of the foreign gene product during the isolation and purification process. Thus, high recovery of the foreign gene product is not expected. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to provide plant virus vectors having the ability to systemically infect a whole plant; plasmids, which are transcribed to provide said plant virus vectors; and a process for expressing a foreign gene throughout a whole plant by inoculating said plant virus vector into a plant. 
     In addition, a process for simply and efficiently obtaining a foreign gene product produced in a plant is provided. 
     To accomplish the above-mentioned purposes, the present inventors use plant virus vector wherein a foreign gene is linked downstream of a coat protein gene of a plant virus via a nucleotide sequence that causes readthrough (Skuzeski et al, J. Mol. Biol., 218:365-373 (1991)); a plasmid that is transcribed to provide said plant virus vector; and infection throughout a whole plant (systemic infectivity) by inoculating said plant virus vector into a plant so as to express the foreign gene throughout the whole plant. It was also found that introduction of a useful property into a whole plant and the production of a useful protein in a whole plant are possible, and thus, the present invention has been achieved. 
     In addition the present inventors found that when a foreign gene product is recovered from a plant, the foreign gene product can be easily and efficiently recovered as viral particles. 
     Prior to explaining the constituent features of the present invention, the terms used in the specification are explained as follows: 
     Plant virus vector: a replicable recombinant product obtainable by introducing a foreign gene into a DNA or RNA sequence in a plant virus. 
     Readthrough: In translation of an RNA to a protein, a phenomenon wherein the translation does not stop at a stop codon, and sometimes continues to the next stop codon. This readthrough phenomenon is caused by a nucleotide sequence near the stop codon. 
     Plant: in addition to an individual plant (a plant consisting of leaves, stem and roots); plant cells, callus, protoplast, as well as adventitious buds, adventitious roots and somatic embryos, derived from callus, are included, which are used as hosts for a plant virus. 
     Wild coat protein: A protein essential for formation of virions. Although it is usually called coat protein, in the present invention, to distinguish it from a coat protein in a fused protein, it is described as wild coat protein. 
     Fused protein: A protein wherein a foreign gene product is joined to the C-terminal of a coat protein, or a protein wherein a part of a coat protein is replaced with a foreign gene product. 
     Virion: A viral genome covered with a coat protein. 
     As plant viruses used in the present invention, there are mentioned a group belonging to DNA viruses, such as Caulimovirus and Geminivirus; and a group belonging to RNA viruses, such as Tobamovirus, Bromovirus and Cucumovirus. Particular virus species include Cauliflower mosaic virus belonging to Caulimovirus; Tomato golden mosaic virus belonging to Geminivirus; Tobacco mosaic virus belonging to Tobamovirus; Bromo mosaic virus belonging to Bromovirus; and Cucumber mosaic virus belonging to Cucumovirus. 
     Foreign genes used in the present invention include genes for peptides having a pharmacological or physiological activity; genes for proteins which provide stress resistance or pest resistance to a plant; and genes for proteins which change flower shape or color; more particularly, genes coding for enkephalins, calcitonins, corticotropins, human epidermal growth factor, and angiotensin converting enzyme inhibitor (ACEI) peptides having hypotensive action, as shown in the following Table 1. 
     
                       TABLE 1______________________________________CEI.sub.12 : Phe--Phe--Val--Ala--Pro--Phe--                       (SEQ ID NO: 9) Pro--Glu--Val--Phe--Gly--LysCEI.sub.β7 : Ala--Val--Pro--Tyr--Pro--Gin--                       (SEQ ID NO: 10) ArgCEI.sub.6 : Thr--Thr--Met--Pro--Leu--Trp                       (SEQ ID NO: 11)CEI.sub.5 : Phe--Phe--Val--Ala--Pro                       (SEQ ID NO: 12)______________________________________ 
    
     These genes may be derived from genomic DNA and cDNA of organisms, as well as plasmid DNAs, although they can also be synthesized by a DNA synthesizer. 
     Nucleotide sequences used in the present invention, which cause readthrough, may be any nucleotide sequences which cause the readthrough phenomenon, and include, for example, naturally occurring nucleotide sequences, such as a nucleotide sequence encoding the 130/180K protein of tobacco mosaic virus, and those nucleotide sequences wherein a part of the nucleotide sequence is replaced with another nucleotide sequence. 
     Particularly, there are mentioned UAGCAAUUA present in the 130/180K dalton protein gene of tobacco mosaic virus, or a corresponding DNA type TAGCAATTA (wherein the underlined portions are a stop codon); UAACAAUUA (TAACAATTA) and UGACAAUUA (TGACAATTA) wherein the stop codon is replaced with another stop codon; and UAGCARYYA (TAGCARYYA) (wherein R represents A or G, Y represents C or U or T) wherein a portion other than the stop codon is replaced, although among them UAGCAAUUA (TAGCAATTA) is preferable because it has high readthrough efficiency. 
     Coat protein genes of plant viruses used in the present invention include naturally occurring genes, genes wherein a part of the naturally occurring gene is deleted, and genes wherein a part of the naturally occurring gene is replaced with another nucleotide sequence, and particularly, coat protein genes whose nucleotide just before the readthrough-causing nucleotide sequence is A, are preferable because of their high readthrough efficiency. 
     Examples of nucleotide replacement sequences are, in the case of tobacco mosaic virus, replacement of the U(T) of the 3&#39;-terminus of the coat protein gene with A, and replacement of the UCU(TCT) of the 3&#39;-terminus with CAA. These replacements are preferable in that they remarkably increase readthrough efficiency. 
     Plasmids used in the present invention include, in the case where the plant virus is an RNA virus, pLFW3 (Meshi et al, Proc. Natl. Acad. Sci. USA, 83:5043 (1986)) having a PM promoter (Ahlquist et al, Mol. Cell. Biol., 4:2876-2882 (1984)) as a promotor for in vitro transcription; pTLW3 (Meshi et al, In: Genetic Engineering with Plant Viruses, Wilson, T. M. A. and Davis, J. W. (Eds)., CRC, Florida, U.S.A., page 154 (1992)) having a T7 promotor (Rosa, Cell, 16:815-825 (1979)), although in the case where the plant virus is a DNA virus, plasmids not having a promotor for in vitro transcription can be used. 
     Plant virus vectors, in the case where the virus is an RNA virus, can be constructed, for example, as follows. 
     First, cDNA of a plant RNA virus is introduced downstream of a promotor of a plasmid having a promotor for in vitro transcription. Next, according to a conventional procedure used in gene engineering, a desired foreign gene is introduced downstream of a coat protein gene of the plant virus via a readthrough-causing nucleotide sequence. 
     In this case, it is preferable to insert a nucleotide sequence encoding amino acid(s) recognized by protease between the readthrough-causing nucleotide sequence and the foreign gene, so that a foreign gene-derived protein (or peptide) can be easily isolated. Amino acids recognized by protease include, for example, arginine and lysine (cleaved with trypsin), phenylalanine and tyrosine (cleaved with chymotrypsin), as well as methionine (chemically cleaved with cyanogen bromide). 
     Next, RNA is prepared from the plasmid as constructed above by in vitro transcription, and is used as a plant virus vector. In addition, the present plant virus vector may be those prepared by forming particles with the coat protein of a wild type virus (plant virus vector particles). 
     Where the virus is DNA, the DNA of the virus is directly introduced into a plasmid for gene engineering. A plant virus vector can be prepared by cutting out the DNA virus portion from the engineered plasmid, or a similar method. 
     The plant virus vector, obtained as described above, can be easily infected into a plant in the form of RNA or DNA, or preferably in the form of a virion if it is RNA, for example, by rubbing it on a leaf with carborundum, or by spraying it on a leaf as a mixture with carborundum. 
     In this case, the plants may be any plant which the present plant virus vector can infect. For example, where the virus used as a vector is Tobacco mosaic virus, plants belonging to the family Solanaceae, such as tobacco, tomato, red pepper, may be used; where the virus is Brome mosaic virus, plants belonging to the family Graminales, such as barley and wheat, may be used; where the virus is cauliflower mosaic virus, plants or cultured cells belonging to the family Cruciferae, such as cauliflower and turnip, may be used. 
     An infected plant simultaneously produces both the wild type coat protein and a fused protein throughout the whole plant. Namely, a foreign gene product is produced as a fused protein. 
     In addition, it can be found by isolating plant virons from infected plants and purifying and lysing the plant virions that both the wild type coat protein and the fused protein are used for formation of virons. 
     This fact means that the fused protein is also used for formation of virions in the copresence of the wild type and is obtained as a fused protein by recovering the plant virions from the plant. The fused protein thus obtained may be cleaved by an appropriate means to obtain a desired foreign gene product. 
     The recovery of virions does not require expensive reagents, is not time-consuming, and can be finished in approximately one day by using centrifugation. In addition, centrifugation is highly reproducible, and the purity of the virions obtained is high. 
     Since the present plant virus vector has a nucleotide sequence which causes readthrough, it simultaneously produces both a wild type coat protein and a fused protein (i.e., a fused protein comprising a desired protein or peptide derived from a foreign gene and the coat protein). Therefore, virions normally result in systemic infection and expression of the foreign gene throughout a whole plant. 
     In addition, according to the present invention, a foreign gene product with high purity can be reproducibly, easily and cheaply isolated and purified. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows plasmid pTLW3. 
     FIG. 2 shows a nucleotide sequence of a synthetic DNA used for construction of plant virus vectors in the Examples and Comparative Examples (A and A&#39; were used in Comparative Example 1; B and B&#39; were used in Example 1; C and C&#39; were used in Example 2; and D and D&#39; were used in Example 3). 
     FIG. 3 shows, in plasmid pTLW3, the positions of DNA fragments used for construction of plasmids in the Examples and Comparative Examples. 
     FIGS. 4(a)-4(b.) FIG. 4(a) shows the plasmid constructed in Comparative Example 1 and 4(b) shows the plasmid constructed in Example 1. 
     FIG. 5 is a diagrammatic sketch that shows the presence or absence of the virus vector in infected and non-infected leaves in experiments infecting individual plants, carried out in Comparative Example 1 and Examples 1 to 3. 
     FIG. 6 is a diagrammatic sketch that shows the presence or absence of ACEI in an infected leaf and a non-infected leaf, in experiments infecting an individual plant, carried out in Comparative Example 1 and Examples 1 to 3. 
     FIG. 7 is a diagrammatic sketch that shows the ability of the present readthrough vectors (Examples 1 to 3) to produce a fused protein. 
     FIG. 8 is a diagrammatic sketch that shows the presence or absence of ACEI in a tomato fruit in an experiment infecting a mini tomato plant, carried out in Example 4. 
     FIG. 9 is a diagrammatic sketch that shows the purity of recovered virions carried out in Example 5. 
     FIGS. 10(a)-10(b) are diagrammatic sketches that represent the presence or absence of a fused protein and an ACEI peptide in virions obtained in an experiment recovering virions, carried out in Example 5; 10(a) shows the result of antibody staining with anti-TMV rabbit serum, and 10(b) shows the result of antibody staining with anti-ACEI rabbit serum. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is further explained in detail in the following Examples. Note that plasmid pTLW3 used in the Examples is shown in FIG. 1. 
     EXAMPLES 1 TO 3 AND COMPARITIVE EXAMPLE 1 
     1. Construction of Template Plasmid Incorporating ACEI Gene Sequence 
     (1) Construction of Synthetic DNAs Comprising ACEI Gene Sequence 
     Eight synthetic DNAs shown in FIG. 2 were prepared in 0.2 μmol scale using a 380A Type DNA synthesizer from Applied Biosystems, Inc. (ABI Inc.). A and A&#39; were used for a conventional, direct, joined type vector (Comparative Example 1), B and B&#39; were used for a readthrough type vector (Example 1), C and C&#39; were used for a readthrough type vector wherein one nucleotide at the 3-terminus of a coat protein gene has been replaced (Example 2), and D and D&#39; were used for a readthrough type vector where three nucleotides at the 3&#39;-terminus of a coat protein gene were replaced (Example 3). All the vectors contain a sequence coding for an arginine at the 5&#39;-side of an ACEI gene sequence so that an ACEI peptide can be isolated from an expressed fused protein by trypsin digestion. The synthesized DNAs were purified using an OPC cartridge from ABI Inc. 
     (2) Phosphorylation and Annealing of Synthetic DNAs 
     The DNAs synthesized in (1) above were adjusted to a concentration of 0.1 μg/μl, phosphorylated with 0.2 mM ATP, purified by treatment with phenol and washed with ethyl ether. The purified DNA was adjusted to a concentration of 0.02 μg/μl, and A and A&#39;, B and B&#39;, C and C&#39;, as well as D and D&#39; were mixed, and each mixture was incubated at 65° C. for 5 minutes. After the incubation, the mixture was allowed to gradually cool to a room temperature for annealing. 
     (3) Preparation of Fragments for Construction of Plasmids 
     The plasmid pTLW3 shown in FIG. 1, which was constructed according to published literature (Meshi (1992), supra), was digested with a combination of restriction enzymes, shown in Table 2, to prepare fragments having different sizes. Note, the relationship between the fragments in pTLW3 is shown in FIG. 3. 
     
                       TABLE 2______________________________________KpnI/M1uI Fragment  7.1        kbpKpnI/StuI Fragment  1.2        kbpStuI/AvaII Fragment 0.56       kbpNsiI/M1uI Fragment  0.2        kbp______________________________________ 
    
     After separation by agarose electrophoresis, each fragment was prepared. using a GENE CLEAN kit (BIO 101), according to the instructions attached to the kit. 
     Among the above-mentioned fragments, the KpnI-MluI 7.1 kbp fragment was dephosphorylated at its end by alkaline phosphatase treatment, and used in the following reaction. 
     (4) Construction and Preparation of Plasmid 
     Each of 4 solutions of the annealed synthetic DNAs prepared in (2) above was reacted with a solution of 4 DNA fragments derived from pTLW3 prepared in (3) above respectively at 15° C. for 15 hours in the following composition so as to construct 4 plasmids. Among the plasmids thus obtained, two plasmids are shown in FIG. 4 (a: Comparative Example 1; and b: Example 1). 
     
                       TABLE 3______________________________________Solution of annealed synthetic               (0.02   μg/μl)                               1.0  μlDNAKpnI/M1uI Fragment  (0.001  μg/μl)                               9.0  μlKpnI/StuI Fragment  (0.01   μg/μl)                               1.5  μlStuI/AvaII Fragment (0.015  μg/μl)                               0.5  μlNsiI/M1uI Fragment  (0.005  μg/μl)                               0.5  μl× 10 binding buffer           1.5  μlT4 Ligase [Takara Shuzo 350 Units/  1.0  μlμl]Total                               15.0 μl______________________________________ 
    
     Note, the composition of the ×10 binding buffer is shown in Table 4. 
     
                       TABLE 4______________________________________Composition of × 10 binding buffer______________________________________Tris-HC1 (pH = 7.6)             660 mMMgCl.sub.2         66 mMDTT               100 mMATP                1 mM______________________________________ 
    
     The reaction mixture as such was used to transform competent cells of E. coli HB101 (Takara Shuzo), and colonies resistant to carbenicilin were selected. A desired plasmid was extracted and purified from E. coli cells of selected colonies. 
     2. Construction of Plant Virus Vectors by in Vitro Transcription 
     Four plasmids constructed in (1) above (i.e., a plasmid having a sequence of a conventional vector, and plasmids having a sequence of a vector of the present invention) were digested with a restriction enzyme, purified by phenol treatment and ethanol precipitation, and used as template DNAs. In vitro transcription reaction was carried out using the template DNA in the reaction system shown in Table 5, at 37° C. for 2 hours, to construct four plant virus vectors (RNA). The plant virus vectors (RNA) thus constructed were purified by phenol treatment and ethanol precipitation. 
     
                       TABLE 5______________________________________1M Tris-HCl (pH = 8.0)   4 μl1M MgCl.sub.2            2 μl1M DTT                   1 μl10 mg/ml BSA             1 μl100 mM ATP               1 μl100 mM UTP               1 μl100 mM CTP               1 μl10 mM GTP                2 μl10 Mm CAP                20 μlTemplate DNA (0.5 mg/ml) 10 μlT7 Polymerase [Takara Shuzo 50 Units/μl]                    1 μlRNase Inhibitor [Takara Shuzo 50 Units/μl]                    1 μlH.sub.2 O                55 μlTotal                    100 μl______________________________________ 
    
     The plant virus vector (RNA) prepared as above and the wild type coat protein of tobacco mosaic virus purified according to a known procedure (Frankel-Contrat, Virology, 4:1-4 (1957)), were reacted in the composition shown in Table 6, at 20° C. for 15 hours to form virions artificially in vitro. The reaction mixture as such was used as the plant virus vector particle solution. 
     
                       TABLE 6______________________________________Plant virus vector (RNA)               (0.3 μg/μl)                          8 μlWild type coat protein               (10 μg/μl)                         10 μl0.2M Phosphate buffer               (pH = 7.0)                         18 μl______________________________________ 
    
     3. Introduction of Foreign Gene into Individual Plant by Inoculation of Plant Virus Vector Particles 
     The solution of the plant virus vector particles prepared in the above-mentioned 2) was diluted 100 fold with 10 mM phosphate buffer (pH 7.0), and 100 μl of the diluted solution was rubbed on each leaf of a 6 week old Nicotiana tabacum cv. Samsun seedling using carborundum (Nakarai Tesk, Carborundum, 600 mesh). 
     4. Confirmation of Replication of Plant Virus Vector in Individual Plant and Movement to Upper Leaves 
     (1) Preparation of Sample 
     15 days after the plants&#39; infection with plant virus vector particles, 10 mg of sections of infected leaf and non-infected leaves from the same individual plant were cut off. To each leaf section, 50 μl of an SDS sampling buffer having the composition shown in Table 7 was added, and after homogenizing, the homogenate was treated at 100° C. for 3 minutes. After cooling, the homogenate was centrifuged at 12,000 rpm for 5 minutes, and the supernatant was used as an SDS-protein sample. 
     
                       TABLE 7______________________________________Composition of SDS sampling buffer______________________________________Tris-HCl (pH = 6.8)    120 mMSDS                     4%2-Mercaptoethanol       10%Glycerol                20%BPB                     0.002%______________________________________ 
    
     (2) Detection of Plant Virus Vector in Sample 
     The presence or absence of plant virus vector was determined by the presence or absence of wild type coat protein and a fused protein. One μl of the SDS-protein sample prepared in the above-mentioned (1) was subjected to SDS-PAGE (12.5% polyacrylamide gel) to separate proteins. The separated proteins were transferred from the gel to a PVDF membrane by electric blotting. Next, an anti-tobacco mosaic virus rabbit serum was diluted 64,000 fold with TBS buffer [20 mM Tris-HCl (pH 7.6) and 137 mM NaCl] to obtain a primary antibody staining solution, and antibody staining was carried out using a rabbit antibody blotting detection kit (Amersham) according to the instructions attached to the kit. A result is shown in FIG. 5. 
     Although a conventional type plant virus vector producing a fused protein alone (Comparative Example 1) was detected in an infected leaf, movement to non-infected leaves was not observed (see FIG. 5, lanes 1 and 2). 
     On the other hand, the present plant virus vector, into which a readthrough sequence was introduced so that both the wild type coat protein and a fused protein are simultaneously produced (see Examples 1 to 3), was detected in both the infected and non-infected leaves, which confirms that the vector was spread from one leaf to another leaf within the plant (i.e., it had systemic infectivity) (see FIG. 5, lanes 3 to 8). 
     5) Confirmation of Production of ACEI in Plant 
     According to the same procedures as described in the above-mentioned 4), the sample of 4) was subjected to electrophoresis and transferred to the membrane, and antibody staining was carried out, using an anti-ACEI rabbit serum diluted 128,000 fold with TBS buffer as a primary antibody staining solution. 
     The result is shown in FIG. 6. Thus, it was confirmed that the conventional type plant virus vector produced ACEI in the form of a fused protein only in an infected leaf, and the present plant virus vectors produce ACEI in the form of a fused protein in both infected non-infected leaves. 
     Comparison of Fused Protein Productivity of Plant Virus Vectors 
     The same procedure as described in 4)(2) was carried out to compare the fused protein productivity of plant virus vectors of Examples 1 to 3. 
     In this case, samples to be subjected to SDS-PAGE were extracted from infected leaves, and the amount of the sample was adjusted to correspond to 0.2 mg of a live leaf. In addition, as a standard, 100, 75, 50, 25, 10, and 5 ng of wild type coat protein of tobacco mosaic virus was simultaneously subjected to SDS-PAGE. Similar to 4)(2) above, detection was carried out by antibody staining with anti-tobacco mosaic virus rabbit serum. The result is shown in FIG. 7. 
     When the amount of fused protein produced by each vector was calculated by comparing the color densities of bands in FIG. 7, the vector of Example. 1 produced 0.025 mg fused protein per lg tobacco leaf, and the vectors of Examples 2 and 3 produced 0.25 mg fused protein per 1 g tobacco leaf. From this result, it was found that replacement of U with A, or UCU with CAA at the 3&#39;-terminus of the coat protein gene enhances the efficiency of the readthrough, and increases fused protein productivity by 10 times. 
     EXAMPLE 4 
     A solution of the present plant virus vector particles having the ACEI gene sequence constructed in Example 1 was infected into a leaf of a 6 month old mini tomato plant [Sugarlamp purchased from SAKATA NO TANE] grown from seed. The infection was carried out as in Example 1. After 14 days, tomato fruit was harvested, and the ACEI in a protein sample extracted from the tomato fruit was detected using an anti-ACEI antibody according to the same procedure as described in Example 1. The result is shown in FIG. 8. 
     As can be seen from FIG. 8, ACEI was produced in the tomato fruit. 
     It is expected that the tomato fruit produced contains a large amount of ACEI which exhibits hypotensive action by oral uptake. 
     EXAMPLE 5 
     1) Introduction of Foreign Gene into Plant by Inoculation of Plant Virus Vector 
     A solution of the present plant virus vector particles having ACEI gene sequence constructed in Example 2 was infected to a leaf of a 6 week old Nicotiana tabacum cv. Samsun seedling. The infection was carried out as in Examples 1 or 2. 
     Recovery of Plant Virions from Plant Incorporating a Foreign Gene 
     1 to 2 months after the infection, virions were purified from 10 g of leaves of the infected plant. 
     More specifically, the following procedures were carried out. Non-infected leaves of the infected tobacco plant were frozen and disrupted with a mixer. Next, to the disrupted non-infected leaves was added the same weight of 0.1M phosphate buffer (pH 7.0) containing 0.1% thioglycolic acid, and after homogenizing the mixture, the homogenate was centrifuged at a low temperature and at a low speed (8000×g, 10 minutes). The precipitate was discarded, and to the supernatant, was added 2M sodium chloride solution in an amount of 0.06 volume relative to the volume of the supernatant, and 20% polyethylene glycol in an amount of 0.2 volume relative to the volume of the supernatant, and the mixture was allowed to stand on ice for one hour. 
     Next, the mixture was centrifuged at low speed to recover the precipitate. The precipitate was thoroughly dispersed in distilled water, and a differential centrifugation comprising low speed centrifugation and high speed centrifugation (100,000×g, 50 minutes) was repeated twice. The precipitate from the final centrifugation was suspended in distilled water to obtain the virions. 
     The concentration of the virions was calculated as 10 mg/ml from absorbance at a wavelength of 280 nm. The isolation and purification procedure described above took about one day. 
     3. Evaluation of the Purity of the Virions 
     To 5 μl of the virions was added 5 μl of an SDS sampling buffer having the composition shown in Table 7, and after treating at 100° C. for 5 minutes, the mixture was used as an SDS-protein sample. Two μl of the SDS-protein sample was subjected to SDS-PAGE (12.5% polyacrylamide gel), and the gel was stained with Coomassie Brilliant Blue, decolored with 7.5% acetic acid/5% methanol, and dried. The result is shown in FIG. 9. 
     As can be seen from FIG. 9, only the wild type coat protein and the fused protein were detected, and other proteins derived from the plant were almost completely eliminated. 
     4) Detection of Fused Protein in Virions 
     An SDS-protein sample was prepared according to the same procedure as described in 3) above, and diluted 10 fold, and 2 μl of the diluted sample was subjected to SDS-PAGE (12.5% polyacrylamide gel) to separate proteins. The separated proteins were transferred from the gel to a PVDF membrane by electro blotting. 
     Next, antibody staining was carried out using an anti-TMV rabbit serum or anti-ACE1 rabbit serum as a primary staining solution, and using a rabbit antibody blotting detection kit (Promega) according to the instructions attached to the kit. The result is shown in FIGS. 10(a) and (b). 
     The ratio of fused protein and wild type coat protein incorporated into the virions was about 1:20, and the ratio substantially conformed to that in proteins isolated and purified from the plant. This fact shows that the fused protein was efficiently incorporated into the virions. 
     In addition, since the virions derived from non-infected leaves, contained the fused protein, it was found that by using a readthrough vector, virions containing a fused protein can be recovered from the whole plant. 
     INDUSTRIAL APPLICABILITY 
     As can be seen from the above, since the present plant virus vectors provide systemic infectivity, introduction of a desired property into a plant and the production of a foreign gene product in a whole plant are possible. 
     In addition, according to the present process, a foreign gene product with high purity can be reproducibly, cheaply and easily isolated and purified. 
     
         __________________________________________________________________________SEQUENCE LISTING(1) GENERAL INFORMATION:(iii) NUMBER OF SEQUENCES: 12(2) INFORMATION FOR SEQ ID NO:1:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 66 base pairs(B) TYPE: nucleic acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: DNA (synethetic)(xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:GACCTCTGCACCTGCATCTAGATTCTTCGTTGCTCCTTTTCCTGAAGTAT50TCGGTAAGTAAATGCA66(2) INFORMATION FOR SEQ ID NO:2:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 59 base pairs(B) TYPE: nucleic acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: DNA (synethetic)(xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:TTTACTTACCGAATACTTCAGGAAAAGGAGCAACGAAGAATCTAGATGCA50GGTGCAGAG59(2) INFORMATION FOR SEQ ID NO:3:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 75 base pairs(B) TYPE: nucleic acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: DNA (synethetic)(xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:GACCTCTGCACCTGCATCTTAGCAATTAAGATTCTTCGTTGCTCCTTTTC50CTGAAGTATTCGGTAAGTAAATGCA75(2) INFORMATION FOR SEQ ID NO:4:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 68 base pairs(B) TYPE: nucleic acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: DNA (synethetic)(xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:TTTACTTACCGAATACTTCAGGAAAAGGAGCAACGAAGAATCTTAATTGC50TAAGATGCAGGTGCAGAG68(2) INFORMATION FOR SEQ ID NO:5:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 75 base pairs(B) TYPE: nucleic acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: DNA (synthetic)(xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:GACCTCTGCACCTGCATCATAGCAATTAAGATTCTTCGTTGCTCCTTTTC50CTGAAGTATTCGGTAAGTAAATGCA75(2) INFORMATION FOR SEQ ID NO:6:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 68 base pairs(B) TYPE: nucleic acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: DNA (synthetic)(xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:TTTACTTACCGAATACTTCAGGAAAAGGAGCAACGAAGAATCTTAATTGC50TATGATGCAGGTGCAGAG68(2) INFORMATION FOR SEQ ID NO:7:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 75 base pairs(B) TYPE: nucleic acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: DNA (synthetic)(xi) SEQUENCE DESCRIPTION: SEQ ID NO:7:GACCTCTGCACCTGCACAATAGCAATTAAGATTCTTCGTTGCTCCTTTTC50CTGAAGTATTCGGTAAGTAAATGCA75(2) INFORMATION FOR SEQ ID NO:8:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 68 base pairs(B) TYPE: nucleic acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: DNA (synthetic)(xi) SEQUENCE DESCRIPTION: SEQ ID NO:8:TTTACTTACCGAATACTTCAGGAAAAGGAGCAACGAAGAATCTTAATTGC50TATTGTGCAGGTGCAGAG68(2) INFORMATION FOR SEQ ID NO:9:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 12 amino acids(B) TYPE: amino acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(xi) SEQUENCE DESCRIPTION: SEQ ID NO:9:PhePheValAlaProPheProGluValPheGlyLys510(2) INFORMATION FOR SEQ ID NO:10:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 7 amino acids(B) TYPE: amino acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(xi) SEQUENCE DESCRIPTION: SEQ ID NO:10:AlaValProTyrProGlnArg(2) INFORMATION FOR SEQ ID NO:11:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 6 amino acids(B) TYPE: amino acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(xi) SEQUENCE DESCRIPTION: SEQ ID NO:11:ThrThrMetProLeuTrp5(2) INFORMATION FOR SEQ ID NO:12:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 5 amino acids(B) TYPE: amino acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: peptide(xi) SEQUENCE DESCRIPTION: SEQ ID NO:12:PhePheValAlaPro5__________________________________________________________________________