Abstract:
Nucleic acid comprising or encoding an RNA replica comprising, in this order, the following segments (i) to (iii): i) a nucleic acid sequence encoding a potexvirus RNA-dependent RNA polymerase or a function-conservative variant thereof; ii) a nucleic acid sequence comprising: a) a potexvirus triple gene block or a function-conservative variant thereof and b) a sequence encoding a potexviral coat protein or a function-conservative variant thereof; or a sequence encoding a tobago viral movement protein; and iii) a heterologous nucleic acid sequence expressible from said replica in a plant or in plant tissue.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a process of high-yield expression of a gene of interest in a plant, in plant tissue or in plant cells using a potexvirus-derived replicon and to a nucleic acid comprising or encoding a potexviral RNA replicon usable for expressing a gene of interest. The invention further relates to a kit of parts comprising two or more vectors that together encode the RNA replicon of the invention. 
       BACKGROUND OF THE INVENTION 
       [0002]    High-yield expression of heterologous proteins in plants can be achieved using viral vectors. Viral vector systems were predominantly developed for transient expression followed by infection (Donson at al., 1991,  Proc Natl Acad Sci USA,  88:7204-7208; Chapman, Kavanagh &amp; Baulcombe, 1992,  Plant J.,  2:549-557) or transfection (Marillonnet et al., 2005,  Nat Biotechnol.,  23:718-723; Santi et al., 2006,  Proc Natl Acad Sci USA.  103:861-866; WO2005/049839) of a plant host. The best-established and commercially viable systems are based on plus-sense single-stranded RNA viruses, preferably on Tobacco Mosaic Virus (TMV)-derived vectors (Kumagai at al., 1994,  Proc. Natl. Acad. Sci. USA,  90, 427-430; Mallory et al., 2002,  Nature Biotechnol.  20, 622-625; U.S. Pat. No. 5,316,931; U.S. Pat. No. 5,589,367; U.S. Pat. No. 5,866,785; U.S. Pat. No. 5,977,438; WO02088369; WO02097080; WO0229068; U.S. Pat. No. 5,491,076). 
         [0003]    Another group of RNA virus-based vectors derived from potexvirus PVX (potato virus X) can also provide for reasonably high yield of recombinant proteins, albeit noticeably lower yield than TMV-derived vectors (Chapman, Kavanagh &amp; Baulcombe, 1992,  Plant J.,  2:549-557; Baulcombe, Chapman &amp; Santa Cruz, 1995,  Plant J.,  7:1045-1053; Zhou et al., 2006,  Appl. Microbiol. Biotechnol., April  13, epub ahead of print; Zelada at al., 2006,  Tuberculosis,  86:263-267). Obviously, such system needs further improvement in order to increase the yield of recombinant protein of interest. 
         [0004]    In the first generation of systemic viral vectors, a large proportion of plant resources was wasted for the production of viral coat protein that is necessary for systemic movement of a viral replicon. For TMV-derived vectors this problem was solved by removing the coat protein gene and by using agro-infiltration for efficient systemic delivery of replicons, thus significantly boosting the yield of recombinant proteins of interest (WO2005/049839; Marillonnet et al., (2005),  Nat. Biotechnol.,  23:718-723). However, unlike TMV-derived replicons, potexvirus-derived replicons require viral coat protein not only for systemic, but also for short distance (cell-to-cell) movement. Therefore, the coat protein gene of potexvirus-derived viral vectors cannot be removed without a severe loss of protein expression efficiency. Further, engineering of plant host providing viral coat protein in trans is rarely a good solution because of gene silencing. Also, transgenic plants expressing coat protein might exhibit coat protein-mediated resistance to challenges by plant viruses (Beachy, R N., 1999,  Philos. Trans. R. Soc. Lend B Biol. Sci.,  354:659-664; Wisniewski et al., 1990,  Plant Cell,  2:559-567). Another similar phenomenon called heterologous CP-mediated resistance can be a problem, for example when a transgenic plant expressing PVX CP reduces cell-to-cell spread of TMV RNA (Bazzini et al., 2006,  J. Gen. Virol.,  87:1005-1012). In addition, expression of PVX coat protein in transgenic tobacco plants rescues movement-deficient PVX, but compromises the efficiency of cell-to-cell movement and viral replication (Spillane et al., 1997,  Virology,  236:76-84). This could be an issue when the use of two viral vectors (e.g. TMV- and PVX-based vectors) in the same plant cell is required for expression of hetero-oligomeric proteins (e.g. for the co-expression of the heavy and light chains of a monoclonal antibody) (WO2006/079546; Giritch et al., 2006,  Proc. Natl. Acad. Sci. USA,  in press). 
       GENERAL DESCRIPTION OF THE INVENTION 
       [0005]    Therefore, it is an object of the invention to provide a potexvirus-based viral vector for high yield expression of a protein of interest in plants, in plant tissue or in plant cells. The viral vector should be capable of cell-to-cell movement and should waste as little resources of a plant host for producing viral coat protein as possible. 
         [0006]    This object is solved by a nucleic acid comprising or encoding an RNA replicon comprising, in this order, the following segments:
   a nucleic acid sequence encoding an RNA-dependent RNA polymerase or a function-conservative derivative thereof;   (ii) a nucleic acid sequence encoding one or more proteins required for cell-to-cell movement of said replicon in a plant or in plant tissue;   (iii) a heterologous nucleic acid sequence expressible from said replicon in a plant or in plant tissue;   
 
         [0010]    or a complementary sequence thereof. 
         [0011]    This object is further solved by a nucleic acid comprising or encoding an RNA replicon comprising, in this order, the following segments:
   (i) a nucleic acid sequence encoding an RNA-dependent RNA polymerase or a function-conservative derivative thereof;   (ii) a nucleic acid sequence comprising a potexvirus triple gene block or a function-conservative variant thereof and a potexviral coat protein or a function-conservative variant thereof;   (iii) a heterologous nucleic acid sequence expressible from said replicon in a plant or in plant tissue;
 
or a complementary sequence thereof.
   
 
         [0015]    This object is further solved by a nucleic acid comprising or encoding an RNA replicon comprising, in this order, the following segments:
   (i) a nucleic acid sequence encoding an RNA-dependent RNA polymerase or a function-conservative derivative thereof;   (ii) a nucleic acid sequence comprising a potexvirus triple gene block or a function-conservative variant thereof and a tobamoviral movement protein; and   (iii) a heterologous nucleic acid sequence expressible from said replicon in a plant or in plant tissue;
 
or a complementary sequence thereof.
   
 
         [0019]    Said RNA replicon may be a replicon for replicating and expressing said heterologous nucleic acid sequence in a plant or in plant tissue. Said RNA replicon may be built on a potexvirus and uses the replication and expression system of a potexvirus for expressing a heterologous nucleic acid sequence. 
         [0020]    The nucleic acid of the invention may be an isolated, or an isolated and purified, nucleic acid. The nucleic acid of the invention is typically used as a vector for transfecting or transforming a plant or plant cells. 
         [0021]    The invention further provides a kit of parts comprising at least two vectors (pro-vectors) that are capable of assembling said nucleic acid of the invention in plant cells by site-directed recombination. 
         [0022]    The invention also provides a process of expressing a heterologous nucleic acid sequence of interest in a plant or in plant tissue, comprising providing a plant or plant tissue with said nucleic acid of the invention. The invention also provides a process of expressing a heterologous nucleic acid sequence of interest in a plant or in plant tissue, comprising providing a plant or plant tissue with two or more vectors that are capable of assembling said nucleic acid of the invention in plant cells by site-directed recombination. 
         [0023]    The inventors have surprisingly identified a way to increase the expression yield of a protein of interest expressed in a plant or in plant tissue from a potexviral vector by a vector design wherein the sequences as defined in item (ii) are positioned after (downstream in 5 to 3′ direction) the RNA-dependent RNA polymerase coding sequence (RdRp or RdRP) of item (i) and precede said heterologous nucleic acid sequence of item (iii). In the special case of potexviral vectors, this vector design leads to a cell-to-cell movement capability of the RNA replicon and, at the same time, to higher expression levels of the heterologous nucleic acid compared to conventional potexviral vectors where a heterologous nucleic acid was placed upstream of the potexviral coat protein gene. The effect identified by the inventors may be due to the fact that the position of the sequences of item (ii) leads to lower expression levels of these proteins and to less consumption of plant resources for the expression of these proteins, allowing higher expression levels of the 3′ protein of interest. Importantly, the limited expression level of potexviral coat protein in the invention is sufficient for supporting efficient cell-to-cell movement for high-yield expression of the heterologous sequence of interest. 
         [0024]    Potexviruses are plant RNA viruses with a plus-sense single-stranded genome. Thus, said nucleic acid of the invention may be RNA being or comprising said RNA replicon or may be DNA encoding said RNA replicon. Herein, the terms “potexviral vector” or “potexviral replicon” mean that the vector or replicon make use of the replication and protein expression system of potexviruses. Said nucleic acid may be built on a natural potexvirus e.g. by using genetic components from a potexvirus. Said nucleic acid of the invention may be obtainable by inserting said heterologous nucleic acid sequence into a nucleic acid construct encoding a potexvirus, whereby said heterologous nucleic acid sequence of interest is inserted downstream of a sequence enocoding the coat protein of said potexvirus. However, various modifications may be made to the various genetic components of a natural potexvirus, such as to the RdRP gene, the triple gene block, the coat protein gene, or to the 5′ or 3′ non-translated regions of a potexvirus, examples for which are described below. 
         [0025]    Said RNA replicon of the invention comprises, in the order from the 5′ end to the 3′ end, said segments (i) to (iii) of the invention. Further genetic elements will typically be present on said replicon for replication and expression. For being an RNA replicon, i.e. for autonomous replication in a plant cell, said RNA replicon encodes an RdRp or a function-conservative derivative thereof. Said RNA replicon may further have potexviral 5′- or 3′-untranslated regions and promoter-sequences in the 5′- or 3′-untranslated region of said RNA replicon for binding said RdRp and for replicating said RNA replicon. Said RNA replicon further may have sub-genomic promoters in segments of item (ii) and (iii) for generating sub-genomic RNAs for the expression of the proteins encoded by the segments of items (ii) and (iii). If said nucleic acid is DNA, it will typically have a transcription promoter for allowing production by transcription of said RNA replicon in vitro or in planta. An example of a transcription promoter allowing transcription of said RNA replicon from a DNA nucleic acid in planta is the 35S promoter that is widely used in plant biotechnology. 
         [0026]    Said segment (i) may encode an RdRp of a potexvirus such as potato virus X, or a function-conservative variant of said potexvirus RdRp. The RdRp used in said RNA replicon may be considered a function-conservative variant of a potexviral RdRp if said sequence of item (i) encodes a protein having a sequence identity of at least 36% to a protein encoded by SEQ ID NO:4. In another embodiment, said sequence identity is at least 45%, in a further embodiment at least 55%, in another embodiment at least 65% and in an even further embodiment at least 75% to a protein encoded by SEQ ID NO:4. These sequence identities may be present over the entire sequence of SEQ ID NO:4. Alternatively, these sequence identities may be present within a protein sequence segment of at least 300 amino acid residues, within a protein sequence segment of at least 500 amino acid residues, within a protein sequence segment of at least 900 amino acid residues, or within a protein sequence segment of at least 1400 amino acid residues. 
         [0027]    Herein, amino acid sequence identities may be determined using BLASTX 2.2.14 using the standard settings. The standard settings allow, for example, for sequence gaps in alignments. 
         [0028]    In one example, said sequence identity between a protein encoded by SEQ ID NO: 4 and a function-conservative variant of a potexvirus RdRp is 45% in a protein sequence segment of at least 900 amino acid residues. In another example, said sequence identity between a protein encoded by SEQ ID NO: 4 and a function-conservative variant of a potexvirus RdRp is 55% in a protein sequence segment of at least 900 amino acid residues. 
         [0029]    Alternatively, the RdRp used in said RNA replicon may be considered a function-conservative variant of a potexviral RdRp if said sequence of item (i) encodes a protein having a sequence homology of at least 50% to a protein encoded by SEQ ID NO:4. In another embodiment, said sequence homology is at least 60%, in a further embodiment at least 70%, and in another embodiment at least 80% to a protein encoded by SEQ ID NO:4. These sequence homologies may be present over the entire sequence of SEQ ID NO:4. Alternatively, these sequence homologies may be present within a protein sequence segment of at least 300 amino acid residues, at least 500 amino acid residues, at least 900 amino acid residues, or at least 1400 amino acid residues. Amino acid sequence homologies may be determined using BLASTX 2.2.14 using the standard settings. The standard settings allow, for example, for sequence gaps in alignments. 
         [0030]    In one example, said sequence homology between a protein encoded by SEQ ID NO: 4 and a function-conservative variant of a potexvirus RdRp is 70% in a protein sequence segment of at least 900 amino acid residues. In another example, said sequence identity between a protein encoded by SEQ ID NO: 4 and a function-conservative variant of a potexvirus RdRp is 80% in a protein sequence segment of at least 900 amino acid residues. 
         [0031]    Alternatively, the RdRp used in said RNA replicon may be considered a function-conservative variant of a potexviral RdRp if said sequence of item (i) has a sequence identity of at least 55%, of at least 60%, or of at least 70% to SEQ ID NO: 4. Said sequence identities may be present within SEQ ID NO:4, or within a sequence segment of at least 900 nucleotides, within a sequence segment of at least 1500 nucleotides, within a sequence segment of at least 2000 nucleotides, or within a sequence segment of at least 4200 nucleotides of SEQ ID NO:4. Nucleotide sequence identities may be determined using BLASTN 2.2.14 using the standard settings. The standard settings allow, for example, for sequence gaps in the alignments. 
         [0032]    Said RNA replicon comprises said nucleic acid sequence of item (ii) for allowing cell-to-cell movement of said RNA replicon in a plant or in plant tissue. Cell-to-cell movement of said RNA replicon is important for achieving expression of the sequence of item (iii) in as many cells of said plant or said tissue as possible. Said nucleic acid sequence of item (ii) may comprise the potexviral triple gene block (abbreviated “TGB” herein) or a function-conservative variant thereof (a review on the TGB is found in  J. Gen. Virol.  (2003) 84, 1351-1366). The potexviral triple gene block encodes three proteins necessary to provide the capability of cell-to-cell movement to a potexvirus. Thus, a variant of said TGB is considered to be a function-conservative variant of the TGB if the variant can provide, optionally with other necessary components, the RNA replicon of the invention with the capability of cell-to-cell movement in a plant or in plant tissue. 
         [0033]    An example of a potexviral TGB is the TGB of potato virus X (referred to as “PVX TGB” herein). The PVX TGB consists of three genes encoding three proteins designated 25K, 12K, and 8K according to their approximate molecular weight. The gene sequences encoding the PVX 25K, the PVX 12 K protein, and the PVX 8K protein are given in SEQ ID NO: 9, SEQ ID NO: 11, and SEQ ID NO: 13, respectively. Protein sequences of the PVX 25 K protein, the PVX 12K protein, and the PVX BK protein are given in SEQ ID NO: 10, SEQ ID NO: 12, and SEQ ID NO: 14, respectively. 
         [0034]    In one embodiment, said function-conservative variant of a potexvirus TGB is a block of three genes, said block encoding three proteins one of which having a sequence identity of at least 33% to the PVX 25K protein, one having a sequence identity of at least 36% to the PVX 12K protein and one having a sequence identity of at least 30% to the PVX 8K protein. In another embodiment, said function-conservative variant of a potexvirus TGB encodes three proteins one of which having a sequence identity of at least 40% to the PVX 25K protein, one having a sequence identity of at least 40% to the PVX 12K protein, and one having a sequence identity of at least 40% to the PVX 8K protein. In a further embodiment, said function-conservative variant of a potexvirus TGB encodes three proteins one of which having a sequence identity of at least 50% to the PVX 25K protein, one having a sequence identity of at least 50% to the PVX 12K protein and one having a sequence identity of at least 50% to the PVX 8K protein. In a further embodiment, the corresonding sequence identity values are 60% for each protein. In a further embodiment, the corresonding sequence identity values are 70% for each protein. 
         [0035]    In another embodiment, said function-conservative variant of a potexvirus TGB encodes three proteins as follows: a first protein comprising a protein sequence segment of at least 200 amino acid residues, said segment having a sequence identity of at least 40% to a sequence segment of the PVX 25K protein; a second protein comprising a protein sequence segment of at least 100 amino acid residues, said sequence segment having a sequence identity of at least 40% to a sequence segment of the PVX 12K protein; and a third protein comprising a protein sequence segment of at least 55 amino acid residues, said sequence segment having a sequence identity of at least 40% to a sequence segment of the PVX 8K protein. In a further embodiment, the corresponding sequence identity values are 50% for each protein. In a further embodiment, the corresponding sequence identity values are 60% for each of said first, second, and third protein. 
         [0036]    Said nucleic acid sequence of item (ii) may comprise a further sequence encoding a protein necessary for cell-to-cell movement of said RNA replicon such as a potexviral coat protein or a function-conservative variant thereof. A variant of said potexviral coat protein is considered a function-conservative variant of said coat protein if it is capable of providing said RNA replicon, together with other necessary components such as the TGB, with the capability of cell-to-cell movement in a plant or in plant tissue. In one embodiment where said RNA replicon comprises a potexviral coat protein (or a function-conservative variant thereof), said RNA replicon does not have an origin of viral particle assembly for avoiding spread of said RNA replicon from plant to plant in the form of an assembled plant virus. If said RNA replicon comprises a potexviral coat protein gene (or a function-conservative variant thereof) and a TGB (or a function-conservative variant thereof), it is possible that said TGB is located upstream of said coat protein gene or vice versa. Thus, said potexviral coat protein gene (or a function-conservative variant thereof) and said TGB (or a function-conservative variant thereof) may be present in any order in said nucleic acid sequence of item (ii). 
         [0037]    The coding sequence of a PVX coat protein is given as SEQ ID NO: 7, and the amino acid sequence of the PVX coat protein is given as SEQ ID NO: 8. A protein can be considered a function-conservative variant of a potexviral coat protein if it comprises a protein sequence segment of at least 200, alternatively at least 220, further alternatively 237 amino acid residues, said sequence segment having a sequence identity of at least 35% to a sequence segment of SEQ ID NO: 8. In another embodiment, a protein is considered a function-conservative variant of a potexviral coat protein if it comprises a protein sequence segment of at least 200, alternatively at least 220, further alternatively 237 amino acid residues, said sequence segment having a sequence identity of at least 45% to a sequence segment of SEQ ID NO: 8. In alternative embodiments, the corresponding sequence identity values are 55% or 65%. 
         [0038]    Alternatively, said nucleic acid sequence of item (ii) may comprise, optionally instead of said sequence encoding said potexviral coat protein, a sequence encoding a plant viral movement protein (MP). An example of a suitable MP is a tobamoviral MP such as an MP of tobacco mosaic virus or an MP of turnip vein clearing virus. Said sequence encoding a plant viral movement protein and said potexvirus TGB (or a function-conservative variant thereof) may be present in any order in said nucleic acid sequence of item (ii). 
         [0039]    Said heterologous nucleic acid sequence of item (iii) typically comprises the coding sequence of a protein of interest to be expressed in a plant or in plant tissue. Such coding sequence of item (iii) is also referred to herein as gene of interest. Said sequence of item (iii) is heterologous to said plant virus on which said RNA replicon is based. In many cases, said sequence is also heterologous to said plant or said plant tissue in which it is to be expressed. For being expressible from said RNA replicon in a plant or in plant tissue, said sequence of item (iii) typically comprises a sub-genomic promoter and other sequences required for expression such as ribosome binding site and/or an internal ribosome entry site (IRES). In a preferred embodiment, said heterologous nucleic acid sequence of item (iii) has one gene of interest that codes for one protein of interest. 
         [0040]    Said nucleic acid of the invention may comprise a potexvirus 5′-nontranslated region (5′-NTR) and a potexvirus 3′-nontranslated region (3′-NTR). 
         [0041]    The process of the invention can be used for producing one protein of interest or more than one protein of interest in a plant or in plant tissue. Said process comprises providing a plant, plant tissue or plant cells with said nucleic acid of the invention. The process of the invention is preferably performed in plants or in plant tissue. In one embodiment, said process is a transient expression process, whereby incorporation of the nucleic acid of the invention into chromosomal DNA of the plant host is not necessary. 
         [0042]    If said nucleic acid of the invention is RNA, it may be used for infecting a plant or plant tissue, preferably in combination with mechanical injury of infected plant tissue such as leaves. In another embodiment, said nucleic acid of the invention is DNA and said DNA may be introduced into cells of a plant or plant tissue, e.g. by particle bombardment or by  Agrobacterium -mediated transformation.  Agrobacterium -mediated transformation is the method of choice if several plants are to be provided with said nucleic acid of the invention, i.e. for large scale protein production methods. 
         [0043]    The process of the invention may be performed using the pro-vector approach (WO02088369; Marillonnet et al., 2004,  Proc. Natl. Acad. Sci. USA,  101:6852-6857) by providing a plant or plant tissue with said kit or parts of the invention. In cells of said plant, the nucleic acid of the invention is then produced by site-specific recombination between pro-vectors. In one embodiment, a first vector (pro-vector) comprising or encoding segments of items (i) and (ii) and a second vector (pro-vector) comprising or encoding the segment of item (iii) is provided to a plant or plant tissue, wherein said first and said second pro-vector each has a recombination site for allowing assembly of a nucleic acid of the invention by site-specific recombination between said first and said second pro-vector. Two or more vectors may be provided to a plant or to plant tissue by providing mixtures of the vectors or mixtures of  Agrobacterium  strains, each strain containing one of said vectors, to a plant or to plant tissue. 
         [0044]    Said one or more than one protein of interest may be purified after production in said plant or plant tissue. Methods or purifying proteins from plants or plant cells are known in the art. In one method, a protein of interest may be directed to a plant apoplast and purified therefrom as described in WO 03/020938. 
         [0045]    If one protein of interest has to be produced, a nucleotide sequence coding for said protein of interest may be included in said nucleotide sequence encoding said RNA replicon. If two or more proteins of interest are to be produced in the same plant or in the same plant tissue, said plant or plant cells may be provided with a second nucleic acid comprising or encoding a second or further RNA replicon. Said further RNA replicon may then encode one or more further proteins of interest. In one embodiment, a first and a second nucleic acid of the invention may comprise or encode non-competing RNA replicons as described in WO2006/079546. 
         [0046]    The present invention may in principle be applied to any plants for which infectious potexviruses exist and for which viral vector systems were established. In one embodiment, dicotyledonous plants or tissue or cells thereof are used. In another embodiment,  Nicotiana  species like  Nicotiana benthamiana  and  Nicotiana tabacum  are used; preferred plant species other than  Nicotiana  species are  Petunia hybrida, Brassica campestris, B. juncea,  cress, arugula, mustard, Strawberry spinach,  Chenopodium capitatum,  alfalfa, lettuce, sunflower, potato and cucumber. The most preferred plant RNA viruses the RNA replicons of the invention may be based on are Potexviruses such as potato virus X (PVX), papaya mosaic potexvirus or bamboo mosaic potexvirus. 
         [0047]    The major application of the present invention is the production of a protein of interest in plants, plant leaves or plant tissue or cell culture. If the process of the invention is performed in plants, plants that do not enter the human or animal food chain are preferred, like  Nicotiana  species. Plants that do not enter the human or animal food chain can be cultivated in an open field and harvested within a certain period after infection with said RNA replicon. Preferably, whole plants or plant parts shall be confined to a contained environment, e.g. a glasshouse or a designed chamber for the incubation period necessary to provide for desired level of expression. 
         [0048]    The efficiency of the production process of the present invention is such that a new dimension in plant expression systems is attained. The expression levels achievable with the present invention are such that expenditures for downstream processing (including separation and purification of the protein of interest) are low enough to make the process of the invention competitive with other large-scale expression systems. The invention provides the first high-yield potexviral plant expression system that can be used on a large scale. 
       ADVANTAGEOUS EMBODIMENTS OF THE INVENTION 
       [0049]    Preferred embodiments as described in dependent claims or as described herein can be combined. For example, the invention provides a nucleic acid comprising or encoding an RNA replicon comprising, in this order, the following segments (i) to (iii):
   (i) a nucleic acid sequence encoding a potexvirus RNA-dependent RNA polymerase or a function-conservative variant thereof;   (ii) a nucleic acid sequence comprising:   
 
         [0052]    (a) a potexvirus triple gene block or a function-conservative variant thereof and 
         [0053]    (b) a sequence encoding a potexviral coat protein or a function-conservative variant thereof; and
   (iii) a heterologous nucleic acid sequence expressible from said replicon in a plant or in plant tissue;
 
wherein said function-conservative variant of said potexvirus triple gene block encodes three proteins as follows: a first protein comprising a protein sequence segment of at least 200 amino acid residues, said protein sequence segment having a sequence identity of at least 40% to a sequence segment of SEQ ID NO: 10; a second protein comprising a protein sequence segment of at least 100 amino acid residues, said protein sequence segment having a sequence identity of at least 40% to a sequence segment of SEQ ID NO: 12; and a third protein comprising a protein sequence segment of at least 55 amino acid residues, said protein sequence segment having a sequence identity of at least 40% to a sequence segment of SEQ ID NO: 14;
   wherein said function-conservative variant of said potexviral coat protein comprises a protein sequence segment of at least 200 amino acid residues, said sequence segment having a sequence identity of at least 35% to a sequence segment of SEQ ID NO: 8; and wherein said sequence of item (i) encodes a protein having a sequence identity of at least 36% to a protein encoded by SEQ ID NO:4 within a protein sequence segment of at least 300 amino acid residues, preferably at least 500 amino acid residues, more preferably at least 900 amino acid residues, and most preferably at least 1400 amino acid residues.   
 
         [0056]    Sequence identity values given in the above embodiment can be exchanged by more specific identity values and/or larger sequence segments as disclosed in the general description of the invention. Further, the above embodiment can be combined with preferred embodiments as defined in the dependent claims or as described in this description. The above nucleic acid may be used in a process of expressing a heterologous nucleic acid sequence of interest in a plant or in plant tissue such as in Nicotiana plants or Nicotiana plant tissue or cells thereof. 
         [0057]    In another embodiment, the invention provides a nucleic acid comprising or encoding an RNA replicon comprising, in this order, the following segments (i) to (iii):
   (i) a nucleic acid sequence encoding a potexvirus RNA-dependent RNA polymerase or a function-conservative variant thereof;   (ii) a nucleic acid sequence comprising:   
 
         [0060]    (a) a function-conservative variant of a potexvirus triple gene block and 
         [0061]    (b) a sequence encoding a function-conservative variant of a potexviral coat protein; and
   (iii) a heterologous nucleic acid sequence expressible from said replicon in a plant or in plant tissue;
 
wherein said function-conservative variants are as defined above.
   
 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0063]      FIG. 1 . A— N. benthamiana  plants (left picture) and plant leaves (right picture) monitored under UV light after agro-infiltration with different PVX vectors. 35S-PVX-CP: vector providing PVX coat protein (CP) under control of CaMV 35S promoter; 35S-TVCV MP: vector providing tobamovirus Turnip Vein Clearing Virus (TVCV) movement protein (MP) under control of CaMV 35S promoter. B—schematic presentation of T-DNA regions of constructs pICH20799 and pICH21333. PVX Pol: potato virus X RNA-dependent RNA polymerase; 35S: CaMV 35S promoter; 25K, 12K, 8K: triple gene block; CP: PVX coat protein; PVX ntr: PVX non-translated region; GFP: jellyfish green fluorescent protein; sgc: subgenomic promoter of coat protein gene; RB: T-DNA right border; LB: T-DNA left border. A crossed-out ATG means that an ATG codon was mutated to prevent translation start at the indicated position. 
           [0064]      FIG. 2 . Expression of GFP using different PVX provector modules. A— N. benthamiana  leaves monitored under UV light after agro-infiltration with different 5′ PVX provectors in combination with 3′ provector pICH21282 and in the presence of phage C31 integrase providing for site-specific recombination-mediated assembly of viral replicon-encoding DNA. Only 5′ provectors (e.g. pICH22922; pICH22942, etc.) mark the inoculation spots. 35S-TVCV MP: vector providing tobamovirus Turnip Vein Clearing Virus movement protein (MP) under control of the CaMV 35S promoter. The pictures were taken 7 days post infiltration (7 dpi).  Agrobacterium  suspensions used for infiltration were diluted 10 5  fold for achieving an expression level allowing visual detection of differences in expression levels. 
           [0065]    B depicts T-DNA regions of plasmids pIC22922, pICH22939, pICH22942, pICH22953 and pICH21282. PVX Pol: potato virus X RNA-dependent RNA polymerase; 35S: CaMV 35S promoter; 25K, 12K, 8K: triple gene block; 25K (trunk): truncated gene encoding for 25K protein; CP: PVX coat protein; Pvx ntr: PVX non-translated region; GFP: jellyfish green fluorescent protein; sgc: subgenomic promoter of coat protein gene; int: 5′ end of plant intron; AttP and AttB: sequences recognized by site-specific integrase of phage C31. 
           [0066]      FIG. 3 . Comparison of GFP expression using PVX provector modules that do not (pICH22922) or do provide (pICH-122939 or pICH22988) for cell-to-cell movement in plant tissue. A— N. benthamiana  leaf monitored under UV light after agro-infiltration with different 5′ PVX provectors in combination with 3′ provector pICH21282 and in the presence of phage C31 integrase providing for site-specific recombination mediated assembly of viral replicon-encoding DNA. The picture was taken 7 days post infiltration (7 dpi).  Agrobacterium  suspensions used for infiltration were diluted 10 3  or 10 5  fold as indicated. 
           [0067]    B—depicts T-DNA regions of plasmids pIC22922, pICH22939 and pICH22988 pICH22953 and pICH21282. PVX Pol: potato virus X RNA-dependent RNA polymerase; 35S: CaMV 35S promoter; 25K, 12K, 8K: triple gene block; 25K (trunk): truncated gene encoding for 25K protein; “NOV MP: tobamovirus Turnip Vein Clearing Virus movement protein; CP: PVX coat protein; Pvx ntr: PVX non-translated region; GFP: jellyfish green fluorescent protein; sgc: subgenomic promoter of coat protein gene; int: 5′ end of plant intron; AttP and AttB: sequences recognized by site-specific integrase of phage C31. 
           [0068]      FIG. 4 . Comparison of GFP expression using PVX provector modules that do not (pICH22577) or do provide (pICH22988 or pICH24180) for cell-to-cell movement. A— N. benthamiana  leaf monitored under UV light after agro-infiltration with different 5′ PVX provectors in combination with 3′ provector pICH21282 and in the presence of phage C31 integrase providing for site-specific recombination-mediated assembly of viral replicon-encoding DNA. The picture was taken 7 days post infiltration (7 dpi).  Agrobacterium  suspensions used for infiltration were diluted 10 3  and 10 5  fold as indicated. B—depicts T-DNA regions of plasmids pIC22577, pICH22988 and pICH24180. PVX Pol: potato virus X RNA-dependent RNA polymerase; 35S: CaMV 35Spromoter; 25K, 12K, 8K: triple gene block; CP: PVX coat protein; PVX ntr: PVX non-translated region; sgc: subgenomic promoter of coat protein gene; sg25: subgenomic promoter of 25K gene; int: 5′ end of plant intron; AttP: sequence recognized by site-specific integrase of phage C31. 
           [0069]      FIG. 5 . GFP expression using different PVX expression cassettes (provector modules and assembled vectors) with CP located at different positions in front of the GFP gene. A— N. benthamiana  leaf monitored under UV light after agro-infiltration with assembled viral vectors pICH25491, pICH25488 or different 5′ PVX provectors (pICH22988 or pICH24180) in combination with 3′ provector pICH21282 and in the presence of phage C31 integrase providing for site-specific recombination-mediated assembly of viral replicon-encoding DNA. The picture was taken 7 days post infiltration.  Agrobacterium  suspension used for infiltration were diluted 10 5  fold 
           [0070]    B—depicts T-DNA regions of plasmids pIC22988, pICH25491, pICH25488 and pICH24180. PVX Pol: potato virus X RNA-dependent RNA polymerase; 35S: CaMV 35Spromoter; 25K, 12K, 8K: triple gene block; CP: PVX coat protein; Pvx ntr: PVX non-translated region; sgc: subgenomic promoter of coat protein gene; sg25: subgenomic promoter of 25K gene; int: 5′ end of plant intron; AttP: sequence recognized by site-specific integrase of phage C31. 
           [0071]      FIG. 6 . GFP expression using assembled PVX expression cassettes with CP located at different positions in front of the GFP gene. A— N. benthamiana  leaves monitored under UV light after agro-infiltration with assembled viral vectors pICH25491, pICH25488 and pICH20799 ten (left panel) and sixteen (right panel) days post-infiltration.  Agrobacterium  suspensions used for infiltration were diluted 10 5  fold. 
           [0072]    B—depicts T-DNA regions of plasmids pIC20799, pICH25491, and pICH25488. PVX Pol: potato virus X RNA-dependent RNA polymerase; 35S: CaMV 35Spromoter; 25K, 12K, 8K: triple gene block; CP: PVX coat protein; PVX ntr: PVX non-translated region; sgc: subgenomic promoter of coat protein gene; sg25: subgenomic promoter of 25K gene. 
           [0073]      FIG. 7  depicts PVX cloning vectors with multiple cloning sites (CS). PVX Pol: potato virus X RNA-dependent RNA polymerase; 35S: CaMV 35Spromoter; 25K, 12K, 8K: triple gene block; CP: PVX coat protein; PVX ntr: PVX non-translated region; sgc: subgenomic promoter of coat protein gene; sg25: subgenomic promoter of 25K gene. 
           [0074]      FIG. 8  shows a map of pICH25491. The nucleotide sequence of the T-DNA region of pICH25491 is given in SEQ ID NO:6. 
           [0075]      FIG. 9  shows a map of pICH25488. The nucleotide sequence of the T-DNA region of pICH25488 is given in SEQ ID NO:5. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0076]    This invention describes a new design for potexvirus-derived RNA replicons that improves yield of a protein of interest to be expressed from said RNA replicons in a plant or in plant tissue. The process of the invention has better biosafety features than conventional potexvirus-derived RNA replicons, as it produces less viral coat protein and consequently, can form less viral particles. 
         [0077]    We have surprisingly found that the position of a coding sequence for a coat protein or heterologous movement protein that precedes said heterologous nucleic acid sequence (iii) encoding a protein of interest increases the yield of said recombinant protein at the expense of viral coat protein. The PVX coat protein is necessary not only for systemic, but also together with three other proteins of triple gene block for cell-to-cell (short distance) movement (Yang et al., 2000,  Mol. Plant Microbe Interact.,  13:599-605; Fedorkin et al., 2001,  J. Gen. Virol.,  82:449-458). Herein, it is shown (see example 1 and  FIG. 1 ) that the absence of PVX coat protein makes PVX-derived RNA replicons deficient in long distance ( FIG. 1A , left panel) and cell-to-cell ( FIG. 1A , right panel) movement. Cell-to cell movement can be restored by providing in trans PVX coat protein (CP) or a tobamoviral movement protein, e.g. tobacco mosaic virus (TMV) movement protein (MP) ( FIG. 1A , right panel). It is also shown (example 2,  FIG. 2 ) that a PVX-derived RNA replicon with a TVCV movement protein is capable of short distance (cell-to-cell) movement. The experiment was carried out using the pro-vector system (Marillonnet et al., 2004,  Proc. Natl. Acad. Sci. USA,  101:6852-6857) that allows assembly in planta of a DNA precursor of an RNA replicon via site-specific recombination e.g. between AttP and AttB sites of provectors ( FIG. 2B ). 
         [0078]    A comparative analysis of PVX-derived RNA replicons with either TVCV MP (pICH22939) or PVX CP is described in example 3 and shown in  FIG. 3 . As in the case described above, the provector system was used. It is evident from  FIG. 3  that PVX CP positioned in front of the gene of interest provides for cell-to-cell movement of said RNA replicon. As it follows from the brightness and density of GFP spots ( FIG. 3A ), the efficiency of cell-to-cell movement is significantly higher than with TVCV MP. 
         [0079]    In another embodiment of this invention, the effect of the position of PVX CP preceding the gene of interest was tested. It is evident from  FIG. 4A  that the location of CP after the triple gene block provides for a more efficient cell-to-cell movement than location of said CP between PVX RNA dependent RNA polymerase and the triple gene block. The provector system was also used in this experiment. No difference in the expression level of the protein of interest (GFP) was found between the cases where provectors or assembled vectors providing for PVX-derived RNA replicons were used (see Example 5,  FIG. 5 ). When we included in comparative studies vectors that provide for RNA replicon containing CP after the gene of interest, the expression level (brightness of GFP expressing area) of said gene of interest from such vector was surprisingly significantly weaker compared with vectors where CP was positioned before the gene of interest (pICH20799 versus pICH25488 or pICH25491,  FIG. 6 , example 6). 
         [0080]    In the examples, we use PVX-based RNA replicons. However, other viruses belonging to potexvirus taxonomic group can be used for the construction of RNA virus-based vectors according to the present invention. Viral species usable for the invention include but are not limited to bamboo mosaic potexvirus, papaya mosaic potexvirus, alternanthera mosaic potexvirus, clover yellow mosaic virus, plantain virus X, white clover mosaic virus and potato aucuba mosaic virus. For details on plant viral movement proteins, see the recent review of W J Lucas (2006,  Virology,  344:169-184). 
         [0081]    One RNA replicon derived from one plant virus may be used in the process of the invention, e.g. from potato virus X (PVX). However, two or more different RNA replicons may be used in a plant or plant tissue for expressing two different proteins of interest, whereby such different RNA replicons are preferably derived from different plant viruses. Such different plant viruses from which said different RNA replicons may be derived are preferably synergistic or non-competing viruses. “Synergistic” and “non-competing” are used herein synonymously. Synergistic viruses can coexist and efficiently amplify in the same plant cells. Similarly, RNA replicons derived from synergistic RNA viruses can co-exist and efficiently amplify in the same plant cells. An example of such a synergistic pair of RNA replicons is a pair of RNA replicons, whereby one RNA replicon is derived from TMV or TVCV and the other RNA replicon is derived from a potexvirus such as PVX. Synergistic RNA replicons may be used for the expression of two or more proteins or protein subunits of interest, such as the heavy and the light chain of a monoclonal antibody, in the same plant cell. Processes of expressing two or more proteins of interest in the same plant or in the same plant cells using different (non-competing) viral vectors is described in EP1686176. 
         [0082]    In the examples, we predominantly used  Agrobacterium -mediated delivery of viral vectors in plant cells. Various other methods usually used for stable transformation of plants may be used for the delivery of vectors into plant cells such as direct introduction of a heterologous nucleotide sequence into cells by means of microprojectile bombardment, electroporation or PEG-mediated transformation of protoplasts.  Agrobacterium -mediated plant transformation is preferred. Thus, a heterologous nucleotide sequence may be transformed into plant cells by various technologies such as by a Ti-plasmid vector carried by  Agrobacterium  (U.S. Pat. No. 5,591,616; U.S. Pat. No. 4,940,838; U.S. Pat. No. 5,464,763), particle or microprojectile bombardment (U.S. Pat. No. 05,100,792; EP 00444882B1; EP 00434616B1). In principle, other plant transformation methods can also be used e.g. microinjection (WO 09209696; WO 09400583A1; EP 175966B1), electroporation (EP00564595B1; EP00290395B1; WO 08706614A1), etc. The choice of the transformation method depends inter alia on the plant species to be transformed. For example, microprojectile bombardment may be preferred for monocot transformation, while for dicots,  Agrobacterium -mediated transformation gives generally better results. 
         [0083]    The present invention is preferably carried out with higher, multi-cellular plants. Preferred plants for the use in this invention include any plant species with preference given to agronomically and horticulturally important species. Common crop plants for the use in present invention include alfalfa, barley, beans, canola, cowpeas, cotton, corn, clover, lotus, lentils, lupine, millet, oats, peas, peanuts, rice, rye, sweet clover, sunflower, sweetpea, soybean, sorghum triticale, yam beans, velvet beans, vetch, wheat, wisteria, and nut plants. The plant species preferred for practicing of this invention are including but not restricted to: 
         [0084]    Representatives of Graminae, Compositae, Solanacea and Rosaceae. Additionally, preferred species for use in the invention, as well as those specified above, plants from the genera:  Arabidopsis, Agrostis, Allium, Antirrhinum, Apium, Arachis, Asparagus, Atropa, Avena, Bambusa, Brassica, Bromus, Browaalia, Camellia, Cannabis, Capsicum, Cicer, Chenopodium, Chichorium, Citrus, Coffea, Coix, Cucumis, Curcubita, Cynodon, Dactylis, Datura, Daucus, Digitalis, Dioscorea, Elaeis, Eleusine, Festuca, Fragaria, Geranium, Glycine, Helianthus, Heterocallis, Hevea, Hordeum, Hyoscyamus, Ipomoea, Lactuca, Lens, Lilium, Linum, Lolium, Lotus, Lycopersicon, Majorana, Malus, Mangifera, Manihot, Medicago, Nemesia, Nicotiana, Onobrychis, Oryza, Panicum, Pelargonium, Pennisetum, Petunia, Pisum, Phaseolus, Phleum, Poa, Prunus, Ranunculus, Raphanus, Ribes, Ricinus, Rubus, Saccharum, Salpiglossis, Secale, Senecio, Setaria, Sinapis, Solanum, Sorghum, Stenotaphrum, Theobroma, Trifolium, Trigonella, Triticum, Vicia, Vigna, Vitis, Zea,  and the  Olyreae,  the  Pharoideae  and many others. 
         [0085]    In one specific embodiment of the process of the invention, RNA replicons derived from PVX are used with  Nicotiana  plants. 
         [0086]    Proteins of interest, or fragments thereof, that can be expressed, in sense or antisense orientation, using the invention include: starch modifying enzymes (starch synthase, starch phosphorylation enzyme, debranching enzyme, starch branching enzyme, starch branching enzyme II, granule bound starch synthase), sucrose phosphate synthase, sucrose phosphorylase, polygalacturonase, polyfructan sucrase, ADP glucose pyrophosphorylase, cyclodextrin glycosyltransferase, fructosyl transferase, glycogen synthase, pectin esterase, aprotinin, avidin, bacterial levansucrase,  E. coli  glgA protein, MAPK4 and orthologues, nitrogen assimilation/methabolism enzyme, glutamine synthase, plant osmotin, 2S albumin, thaumatin, site-specific recombinase/integrase (FLP, Cre, R recombinase, Int, SSVI Integrase R, Integrase phiC31, or an active fragment or variant thereof), isopentenyl transferase, Sca M5 (soybean calmodulin), coleopteran type toxin or an insecticidally active fragment, ubiquitin conjugating enzyme (E2) fusion proteins, enzymes that metabolise lipids, amino acids, sugars, nucleic acids and polysaccharides, superoxide dismutase, inactive proenzyme form of a protease, plant protein toxins, traits altering fiber in fiber producing plants, Coleopteran active toxin from  Bacillus thuringiensis  (Bt2 toxin, insecticidal crystal protein (ICP), CryIC toxin, delta endotoxin, polyopeptide toxin, protoxin etc.), insect specific. toxin AaIT, cellulose degrading enzymes, E1 cellulase from  Acidothermus celluloticus,  lignin modifying enzymes, cinnamoyl alcohol dehydrogenase, trehalose-6-phosphate synthase, enzymes of cytokinin metabolic pathway, HMG-CoA reductase,  E. coli  inorganic pyrophosphatase, seed storage protein,  Erwinia herbicola  lycopen synthase, ACC oxidase, pTOM36 encoded protein, phytase, ketohydrolase, acetoacetyl CoA reductase, PHB (polyhydroxybutanoate) synthase, acyl carrier protein, napin, EA9, non-higher plant phytoene synthase, pTOM5 encoded protein, ETR (ethylene receptor), plastidic pyruvate phosphate dikinase, nematode-inducible transmembrane pore protein, trait enhancing photosynthetic or plastid function of the plant cell, stilbene synthase, an enzyme capable of hydroxylating phenols, catechol dioxygenase, catechol 2,3-dioxygenase, chloromuconate cycloisomerase, anthranilate synthase,  Brassica  AGL15 protein, fructose 1,6-biphosphatase (FBPase), AMV RNA3, PVY replicase, PLRV replicase, potyvirus coat protein, CMV coat protein, TMV coat protein, luteovirus replicase, MDMV messenger RNA, mutant geminiviral replicase,  Umbellularia californica  C12:0 preferring acyl-ACP thioesterase, plant C10 or C12:0 preferring acyl-ACP thioesterase, C14:0 preferring acyl-ACP thioesterase (luxD), plant synthase factor A, plant synthase factor B, 6-desaturase, protein having an enzymatic activity in the peroxysomal -oxidation of fatty acids in plant cells, acyl-CoA oxidase, 3-ketoacyl-CoA thiolase, lipase, maize acetyl-CoA-carboxylase, 5-enolpyruvylshikimate-3-phosphate synthase (EPSP), phosphinothricin acetyl transferase (BAR, PAT), CP4 protein, ACC deaminase, ribozyme, protein having posttranslational cleavage site, protein fusion consisting of a DNA-binding domain of Ga14 transcriptional activator and a transcriptional activation domain, a translational fusion of oleosin protein with protein of interest capable of targeting the fusion protein into the lipid phase, DHPS gene conferring sulfonamide resistance, bacterial nitrilase, 2,4-D monooxygenase, acetolactate synthase or acetohydroxyacid synthase (ALS, AHAS), polygalacturonase, bacterial nitrilase, fusion of amino terminal hydrophobic region of a mature phosphate translocator protein residing in the inner envelope membrane of the plastid with protein of interest to be targeted into said membrane etc. 
         [0087]    Any human or animal protein can be expressed using the system of the invention. Examples of such proteins of interest include inter alia the following proteins (pharmaceutical proteins): immune response proteins (monoclonal antibodies, single chain antibodies, T cell receptors etc.), antigens, colony stimulating factors, relaxins, polypeptide hormones, cytokines and their receptors, interferons, growth factors and coagulation factors, enzymatically active lysosomal enzyme, fibrinolytic polypeptides, blood clotting factors, trypsinogen, 1-antitrypsin (AAT), as well as function-conservative proteins like fusions, mutant versions and synthetic derivatives of the above proteins. 
         [0088]    The content of European patent application No. 06 018 713.5, filed on Sep. 6, 2006, the priority of which is claimed by the present patent application is incorporated herein by reference in its entirety. 
       Examples 
     Example 1 
       [0089]    PVX Constructs Lacking CP Do Not Move from Cell-to-Cell 
         [0090]    A PVX construct containing a GFP gene (pA3151) was obtained from Professor Yuri Dorokhov (Moscow State University, Russia). This construct was made by cloning a NheI-SalI fragment containing the GFP coding sequence into pPVX201 (Baulcombe, D., Chapman, S. &amp; Santa Cruz, S., 1995,  Plant J.,  7:1045-1053) digested with NheI and SalI, and then subcloning from the resulting construct a Hind3-SacI fragment (containing the entire viral insert and GFP gene) into pBIN19. The viral insert was then subcloned from pA3151 into pICBV52 (a small pBIN19-based Kan R  binary vector) as a SacI-SphI fragment, resulting in plasmid pICH20799. This construct contains the PVX construct cloned under the control of the 35S promoter ( FIG. 1B ). The GFP coding sequence is expressed from a duplicated CP subgenomic promoter (sgc), and is located between the triple gene block (TGB) and the CP coding sequence. Since the duplicated subgenomic promoter fragment upstream of GFP also contained the CP start codon, the start codon was eliminated by mutation from ATG to AGG. 
         [0091]    A control construct lacking CP was made from pICH20799 ( FIG. 1B ) by deleting the CP gene and its subgenomic promoter (using PCR with one of the primers overlapping the deletion endpoints). The resulting construct, pICH21333 ( FIG. 1B ), was transformed into  Agrobacterium  and infiltrated in a  Nicotiana benthamiana  leaf. As expected, viral replicons were unable to move from cell-to-cell or systemically. Movement of the replicons was rescued when pICH21333 was coinfiltrated together with constructs pICH10745 or pICH22066, which provide transient expression of TVCV MP or PVX CP, respectively ( FIG. 1A ). pICH10745 and pICH22066 constructs contain the TVCV MP or PVX CP coding sequences, respectively, under control of the 35S promoter. 
       Example 2  
       [0092]    A PVX Replicon with the TVCV MP Gene Cloned Between the TGB and the Gene of Interest is Able to Move from Cell to Cell 
         [0093]    Since TVCV MP is able to provide cell-to-cell movement to PVX replicons that lack PVX CP, the TVCV MP gene was cloned in the PVX construct between the triple gene block and the gene of interest (GFP in this case), under control of a duplicated CP subgenomic promoter. For practical reasons, this construct (pICH22939,  FIG. 2B  and SEQ ID NO:1) was made as a 5′ provector module which contains the 35S promoter fused to 5′ viral vector sequences but lacks the 3′ part of the construct including the coding sequence of the gene of interest and the PVX 3′ NTR. A recombination site (the Streptomyces phage C31 AttP site) and intron sequences follow the viral construct sequences. A 3′ provector module, pICH21282  FIG. 2B ), contains a compatible recombination site (Streptomyces phage C31 AttB recombination site) followed by intron sequences, the GFP gene and the PVX NTR. The 5′ and 3′ parts of the construct are assembled in vivo by site-specific recombination at the recombination sites by coinfiltration of agrobacteria containing the 5′ and 3′ modules and the PhiC31 recombinase (pICH10881, containing the PhiC31 recombinase under control of the  Arabidopsis  ACT2 promoter, see Marillonnet et al., 2005,  Proc. Natl. Acad. Sci. USA,  101: 6852-6857). After T-DNA delivery of the 5′ and 3′ modules to plant cells and recombination at the AttP and AttB sites, the recombined DNA is transcribed from the 35S promoter. The recombination site is excised from the transcript by splicing of the flanking intron sequences, and the spliced transcript exported from the nucleus to the cytoplasm where it is amplified as a normal viral RNA replicon. 
         [0094]    Infiltration of a control provector construct (pICH22922, 5′ provector without TVCV MP sequences) with the 3′ provector pICH21282 led to the formation of replicons incapable of cell-to-cell movement. In contrast, infiltration of pICH22939 led to replicons that were capable to move from cell to cell ( FIG. 2A ). 
         [0095]    Since TMV is able to provide cell-to-cell movement to TMV replicons without the need for other viral proteins, a second PVX construct was made in which the 12K and 8K proteins of the triple-gene block were deleted, resulting in construct pICH22953 ( FIG. 2B  and SEQ ID NO:2). In this case, the TMV MP is expressed from the 12K subgenomic promoter (sg12). Infiltration of pICH22953 with pICH21282 led to replicons that were able to move from cell-to-cell. Cell-to-cell movement and GFP fluorescence were however not better or stronger than when using construct pICH22939 ( FIG. 2 ). 
       Example 3  
       [0096]    A PVX Vector with a PVX CP Gene Cloned Between the TGB and the Gene of Interest is Able to Move from Cell to Cell 
         [0097]    A construct similar to pICH22939 but with PVX CP replacing TVCV MP was made. The sequence of this construct is the same as that of the wild type virus from the 5′ end of the virus up to the end of the CP. The CP is then followed by the duplicated CP subgenomic promoter that is used for expression of the gene of interest. Infiltration of this construct, pICH22988 ( FIG. 3B ), with pICH21282 ( FIG. 2B ) led to the formation of replicons that moved from cell to cell much faster and that produced brighter GFP fluorescence than replicons produced from pICH22939 ( FIG. 3A ). 
       Example 4  
       [0098]    A PVX Vector with a PVX CP Gene Cloned Between the RdRP and the TGB is Able to Move Cell-to-Cell 
         [0099]    The PVX CP gene was cloned between the RdRp and the triple gene block, under control of a duplicated 25K subgenomic promoter (sg25), resulting in construct pICH24180 ( FIG. 4B , SEQ ID NO:3). Infiltration of pICH24180 in combination with pICH21282 and an integrase source (pICH10881) led to the formation of viral replicons that were able to move from cell to cell ( FIG. 4A ). The construct pICH22577 (similar to pICH22922 but differ for a few restriction sites) was infiltrated instead of pICH24180 in control experiment and exhibited no cell-to-cell movement, as expected. As with pICH22988, GFP fluorescence from pICH24180 was stronger than with construct pICH22939. 
       Example 5  
       [0100]    Complete Assembled PVX Viral Vectors Containing CP Also Provide Cell-to-Cell Movement. 
         [0101]    Assembled constructs including a 3′ provector with GFP part (pICH21282,  FIG. 2B ) and corresponding to provectors pICH22988 or pICH24180 were made, yielding plasmids pICH25488 and pICH25491, respectively ( FIG. 5B ). The nucleotide sequences of the T-DNA region of pICH25488 and pICH25491 are given as SEQ ID NO:5 and SEQ ID NO:6, respectively. Both constructs were found to work like the provectors, as expected, and provided cell-to-cell movement and strong GFP fluorescence ( FIG. 5A ). GFP fluorescence was also much stronger than with the standard construct (pICH20799) in which the CP gene is located after the gene of interest suggesting expression of more recombinant protein ( FIG. 6A ). Coomassie-stained protein gels showed that a much higher amount of GFP protein was expressed from pICH25491 and pICH25488 than from pICH20799, and that in contrast, a much higher amount of PVX CP was produced from pICH20799 than from the other two constructs. 
       Example 6 
       [0102]    PVX Cloning Vectors 
         [0103]    A schematic representation of two types of PVX cloning vectors with CP between the RdRP and the TGB, or between the TGB and gene of interest, are shown. CS: cloning sites.