PAPAYA RINGSPOT VIRUS NIa PROTEASE GENE

Abstract: NIa protease genes of papaya ringspot virus strains FLA83 W and USA P-type (HA attenuated) strain are provided.

Description: TITLE PAPAYA RINGSPOT VIRUS NIa PROTEASE GENE Field of the Invention This invention relates to a protease gene derived from papaya ringspot virus. More specifically, the invention relates to the genetic engineering of plants and to a method for conferring viral resistance to a plant using an expression cassette encoding papaya ringspot virus PRV FLA83 W or PRV USA P-type (HA attenuated) protease. Background of the Invention Many agriculturally important crops are susceptible to infection by plant viruses, particularly papaya ringspot virus, which can seriously damage a crop, reduce its economic value to the grower, and increase its cost to the consumer. Attempts to control or prevent infection of a crop by a plant virus such as papaya ringspot virus have been made, yet viral pathogens continue to be a significant problem in agriculture. Scientists have recently developed means to produce virus resistant plants using genetic engineering techniques. Such an approach is advantageous in that the genetic material which provides the protection is incorporated into the genome of the plant itself and can be passed on to its progeny. A host plant is resistant if it possesses the ability to suppress or retard the multiplication of a virus, or the development of pathogenic symptoms. "Resistant" is the opposite of "susceptible, " and may be divided into: (1) high, (2) moderate, or (3) low resistance, depending upon its effectiveness. Essentially, a resistant plant shows reduced or no symptom expression, and virus multiplication within it is reduced or negligible. Several different types of host resistance to viruses are recognized. The host may be resistant to: (1) establishment of infection, (2) virus multiplication, or (3) viral movement. Potyviruses are a distinct group of plant viruses which are pathogenic to various crops, and which demonstrate cross-infectivity between plant members of different families. Generally, a potyvirus is a single-stranded RNA virus that is surrounded by a repeating protein monomer, which is termed the coat protein (CP) . The majority of the potyviruses are transmitted in a nonpersistent manner by aphids. As can be seen from the wide range of crops affected by potyviruses, the host range includes such diverse families of plants as Solanaceae. Chenopodiaceae. Gramineae. Compositae. Lecruminosae. Dioscroeaceae. Cucurbitaceae. and Caricaceae. Potyviruses include watermelon mosaic virus II (WMVII) , zucchini yellow mosaic virus (ZYMV) , potato virus Y, tobacco etch and many others. Another potyvirus of economic significance is papaya ringspot virus (PRV) . Two groups of PRV have been identified: the "P" or "papaya ringspot" type infects papayas; and the " " or "watermelon" type infects cucurbits, e.g., squash, but it is unable to infect papaya. Thus, these two groups can be distinguished by host range differences. The potyviruses consist of flexous, filamentous particles of dimensions approximately 780 x 12 nanometers. The viral particles contain a single- stranded positive polarity RNA genome containing about 10,000 nucleotides. Translation of the RΝA genome of potyviruses shows that the RΝA encodes a single large polyprotein of about 330 kD. This polyprotein contains several proteins; these include the coat protein, nuclear inclusion proteins Νla and Νlb, cytoplasmic inclusion protein (Cl) , and other proteases and movement proteins (see Figure 1) . These proteins are found in the infected plant cell and form the necessary components for viral replication. One of the proteins contained in the polyprotein is a 35 kD capsid or coat protein which coats and protects the viral RΝA from degradation. One of the nuclear inclusion proteins, Νlb, is an RΝA replicase component and is thought to have polymerase activity. Cl, a second inclusion protein, is believed to participate in the replicase complex and have a helicase activity. Νla, a third inclusion protein, has a protease activity. In the course of potyvirus infection, Νla and Νlb are translationally transported across the nuclear membrane into the nucleus of the infected plant cell at the later stages of infection and accumulate to high levels. The location of the protease gene appears to be conserved in these viruses. In the tobacco etch virus, the protease cleavage site has been determined to be the dipeptide Gln-Ser, Gln-Gly, or Gin-Ala. Conservation of these dipeptides at the cleavage sites in these viral polyproteins is apparent from the sequences of the above-listed potyviruses. Expression of the coat protein genes from tobacco mosaic virus, alfalfa mosaic virus, cucumber mosaic virus, and potato virus X, among others, in transgenic plants has resulted in plants which are resistant to infection by the respective virus. For reviews, see Fitchen et al. , Annu. Rev. Microbiol.. 4., 739 (1993) and Wilson, Proc. Natl. Acad. Sci. USA. 90. 3134 (1993) . For papaya ringspot virus, Ling et al. (Bio/Technology. £, 752 (1991) ) found that transgenic tobacco plants expressing the PRV coat protein (CP) gene isolated from the PRV strain HA 5-1 (mild) showed delayed symptom development and attenuation of symptoms after infection by a number of potyviruses, including tobacco etch (TEV) , potato virus Y (PVY) , and pepper mottle virus (PeMV) . PRV does not infect tobacco, however. Thus, PRV CP transgenic tobacco plants cannot be used to evaluate protection against PRV. Fitch et al. (Bio/Technology. 10. 1466 (1992)), Gonsalves (American J. of Bot.. 79. 88 (1992)), and Lius et al. (9lst Annual Meeting of the American Society for Horticultural Science Hortscience. 29. 483 (1994)) reported that four R c papaya plants made transgenic for a PRV coat protein gene taken from strain HA 5-1 (mild) displayed varying degrees of resistance against PRV infection, and one line (S55-1) appeared completely resistant to PRV. This appears to be the only papaya line that shows complete resistance to PRV infection. Even though coat protein mediated viral resistance has proven to be useful in a variety of situations, it may not always be the most effective or desirable means for providing viral resistance. In such instances, it would be advantageous to have other methods for conferring viral resistance to plants. Expression of the protease gene (NIa) from tobacco vein mottle virus (TVMV) and potato virus Y (PVY) in transgenic plants has shown the feasibility of using protease gene constructs to produce transgenic plants protected against potyvirus infection (Maiti et al., J. Cell. Biochem.. Suppl. 16F. 217(1992); Vardi et al. , Proc. Natl. Acad. Sci. U.S.A.. 9_0, 7513 (1993); Maiti et al. , Proc. Natl. Acad. Sci. U.S.A. 90. 6110 (1993)). Maiti et al. (1993) showed that the expression of the NIa gene of TVMV in tobacco plants rendered these plants highly resistant to TVMV challenge. In addition, Maiti et al. showed that the NIa gene expressed in these plants was proteolytically active. Vardi et al. transformed tobacco plants with PVY NIa constructs. R j progeny from two lines derived from these transformed plants were resistant to challenge with virus. There is a continuing need for the transgenic expression of genes derived from potyviruses at levels which confer resistance to infection by these viruses. Summary of the Invention This invention provides an isolated and purified DNA molecule that encodes the protease for the FLA83 W-type strain of papaya ringspot virus (PRV) or the protease for the PRV USA P-type (HA attenuated) strain. This invention also provides an isolated and purified DNA molecule that encodes the protease and flanking gene segments for the FLA83 W-type strain of papaya ringspot virus (PRV) or the protease and flanking gene segments for the PRV USA P-type (HA attenuated) strain. The invention also provides a chimeric expression cassette comprising at least one of these DNA molecules, a promoter which functions in plant cells to cause the production of an RNA molecule, and at least one polyadenylation signal comprising 3' nontranslated DNA which functions in plant cells to cause the termination of transcription and the addition of polyadenylated ribonucleotides to the 3' end of the transcribed mRNA sequences, wherein the promoter is operably linked to the DNA molecule, and the DNA molecule is operably linked to the polyadenylation signal. Another embodiment of the invention is exemplified by the insertion of multiple virus gene expression cassettes into one purified DNA molecule, e.g., a plasmid. Preferably, these cassettes include the promoter of the 35S gene of cauliflower mosaic virus and the polyade¬ nylation signal of the cauliflower mosaic virus 35S gene. Also provided are bacterial cells, and transformed plant cells, containing the chimeric expression cassettes comprising the protease gene derived from the FLA83 W-type strain of papaya ringspot virus (referred to herein as PRV FLA83 W) or from the USA P-type (HA attenuated) strain of PRV, and preferably the 35S promoter of cauliflower mosaic virus and the polyaden¬ ylation signal of the cauliflower mosaic virus 35S gene. Plants are also provided, wherein the plants comprise a plurality of transformed cells transformed with an expression cassette containing the protease gene derived from the PRV FLA83 W strain or from the USA P-type (HA attenuated) strain of PRV, and preferably the cauliflower mosaic virus 35S promoter and the polyadenylation signal of the cauliflower mosaic virus gene. Transformed plants of this invention include tobacco, corn, cucumber, peppers, potatoes, soybean, squash, and tomatoes. Especially preferred are members of the CucurJbitaceae (e.g., squash and cucumber) family. Another aspect of the present invention is a method of preparing a PRV-resistant plant, such as a dicot, comprising: transforming plant cells with a chimeric expression cassette comprising a promoter functional in plant cells operably linked to a DNA molecule that encodes a protease as described above; regenerating the plant cells to provide a differentiated plant; and identifying a transformed plant that expresses the PRV protease at a level sufficient to render the plant resistant to infection by the specific strains of PRV disclosed herein. As used herein, with respect to a DNA molecule or "gene," the phrase "isolated and purified" is defined to mean that the molecule is either extracted from its context in the viral genome by chemical means and purified and/or modified to the extent that it can be introduced into the present vectors in the appropriate orientation, i.e., sense or indecency. As used herein, the term "chimeric" refers to the linkage of two or more DNA molecules which are derived from different sources, strains or species (e.g., from bacteria and plants) , or the linkage of two or more DNA molecules, which are derived from the same species and which are linked in a way that does not occur in the native genome. As used herein, the term "heterologous" is defined to mean not identical, e.g. different in nucleotide and/or amino acid sequence, phenotype or an independent isolate. As used herein, the term "expression" is defined to mean transcription or transcription followed by translation of a particular DNA molecule. As used herein, the term "flanking gene segments" means nucleotide sequences 5' to the proteolytically derived N-terminus of the NIa coding sequence and 3' to the proteolytically derived C- ter inus of the NIa coding sequence, which include about 1000 nucleotides each. Brief Description of the Drawings Fig. 1. Schematic representation of the genomic organization of potyviruses. Fig. 2. The nucleotide sequence of the nuclear inclusion body A (NIa) protease gene and flanking gene segments of PRV FLA83 W [SEQ ID N0:1] . The amino acid sequence of the encoded open reading frame is shown below the nucleotide sequence [SEQ ID NO:2] . The nucleotide sequences of oligonucleotides RMM354 [SEQ ID NO:3] and RMM355 [SEQ ID NO:4] are shown above the nucleotide sequence, at the 5' and 3' ends of the nucleotide sequence, respectively. The viral-specific sequences in RMM354 and RMM355 are homologous to sequences in PRV HA (attenuated) USA P (Quemada et al., J. Gen. Virol.. 71. 203 (1990)). In addition RMM354 has novel restriction endonuclease cleavage sites for EcoRl and Ncol while RMM355 has novel restriction endonuclease cleavage sites for BamHl and Ncol. Fig. 3. The alignment of the nucleotide sequences of the nuclear inclusion body A (NIa) and flanking gene segments from PRV isolates: HA-P (Yeh et al. , J. Gen. Virol.. 21, 2531 (1992)) [SEQ ID NO. 7]; USA P (Quemada et al., J. Gen. Virol.. 71, 203 (1990) [SEQ ID NO. 8] ; FLA83. Sequence alignments were generated using the UWGCG program Pileup. The dots represent either the lack of sequence information at the ends of the protease gene or gaps in homology in sequences relative to others in the alignment. Fig. 4. The alignment of the amino acid sequences from papaya ringspot virus isolates described in Figure 3. Sequence differences between virus strains are underlined. The predicted cylindrical inclusion (CI)/VPg, VPg/NIa, and NIa/NIb cleavage sites are shown above the aligned amino acids (Q/G and Q/3) . Alignments were generated using the UWGCG Pileup program. The dots represent either the lack of sequence information at the 5' end of the protease gene or gaps in homology in sequences relative to others in the alignment. Fig. 5. The nucleotide sequence of the nuclear inclusion body A (NIa) protease gene and flanking gene segments of PRV USA P-type strain [SEQ ID NO:5] . The amino acid sequence of the encoded open reading frame is shown below the nucleotide sequence [SEQ ID NO:6] . The nucleotide sequences of oligonucleotides RMM354 [SEQ ID NO:3] and RMM355 [SEQ ID NO:4] are shown at the 5' and 3' ends of the nucleotide sequence, respectively. Fig. 6. A schematic representation of the cloning strategy for FLA83 gene expression cassettes. Fig. 7. A schematic representation of the cloning strategy for PRV USA P-type (HA attenuated) gene expression cassettes. Fig. 8. A schematic representation of the cloning strategy for PRV USA P-type (HA attenuated) gene expression cassettes with a stop codon near the 5' end of the coding sequences. Oligonucleotide primers RMM333 and RMM334 (see Figure 5 for nucleotide sequence of these primers [SEQ ID NO:9 and 10, respectively]) were used to amplify PRV USA P-type (HA attenuated) NIa sequences and introduce a stop codon into the 5' portion of the NIa gene. Detailed Description of the Invention Papaya ringspot virus (PRV) is a single-stranded (+) RNA plant virus that is translated into a single polyprotein. The viral RNA genome is approximately 10,000 bases in length. The expression strategy of potyviruses includes translation of a complete polyprotein from the positive sense viral genomic RNA. Translation of the genomic RNA produces a 330 kD protein which is subsequently cleaved into at least seven smaller viral proteins by a virally encoded protease. The virally encoded proteins include a 35 kD protein at the amino terminal end of the 330 kD protein which is thought to be involved in cell to cell transmission, H C protein is 56 kD in size and is believed to be involved in insect transmission and possess proteolytic activity, a 50 kD protein, a 90 kD cylindrical inclusion protein (Cl) , which is part of the replicase complex and possesses helicase activity, a 6 kD VPg protein which is covalently attached to the 5' end of the viral genomic RNA, a 49 kD NIa protein which functions as a protease, a 60 kD Nib protein which functions as a polymerase, and the coat protein (36 kD) . Two types of PRV have been established based on host range. One type is designated "P type"; it infects Cari cacae (e.g., papaya) , Cucurbi taceae (e.g. , cucurbitis) , and Chenopodiaceae (e.g., Chenopodi urn) (Wang et al. , Phytopathology. £4, 1205 (1994)). A second type is designated "W type"; it infects only Cucurbi taceae and Chenopodiaceae (Wang et al., Phytopathology. 84. 1205 (1994)). Isolates of the P type include HA-severe, called HA-P herein (Wang et al., Phvtooath.. 127. 349 (1992)), HA5-1, called USA P herein, YK (Wang et al., Phytopathology. 84. 1205 (1994) ) , and other isolates as described in Tennant et al. (Phytopathology. 84. 1359 (1994)). Isolates of the W type include FLA83, disclosed herein, PRV-W type (Yeh et al., Phytopathology. 74, 1081 (1984)) and PRV-W (Aust) (Bateson et al., Arch-Viol.. 123. 101 (1992)). To practice the present invention, the protease (NIa) gene of a virus must be isolated from the viral genome and inserted into a vector. Thus, the present invention provides isolated and purified DNA molecules that encode either an NIa protease of PRV FLA83 or an NIa protease of PRV USA P-type (HA attenuated) . As used herein, a DNA molecule that encodes a protease gene includes nucleotides of the coding strand, also referred to as the "sense" strand, as well as nucleotides of the noncoding strand, complementary strand, also referred to as the "antisense" strand, either alone or in their base-paired configuration. For example, a DNA molecule that encodes the protease of PRV FLA83, for example, includes the DNA molecule having the nucleotide sequence of Figure 2 [SEQ ID NO:l], a DNA molecule complementary to the nucleotide sequence of Figure 2 [SEQ ID N0:1], as well as a DNA molecule which also encodes a PRV FLA83 NIa protease and its complement which hybridizes with a PRV FLA83- specific DNA probe in hybridization buffer with 6XSSC, 5X Denhardt's reagent, 0.5% SDS and 100 μg/ml denatured, fragmented salmon sperm DNA and remains bound when washed at 68°C in 0.1XSSC and 0.5% SDS (Sambrook et al., Molecular Cloning: A Laboratory Manual. 2nd ed. (1989)). Moreover, the DNA molecules of the present invention can include non-PRV NIa protease nucleotides that do not interfere with expression, such as the nucleotide sequences in the flanking gene segments. The PRV protease gene does not contain the signals necessary for its expression once transferred and integrated into a plant genome. Accordingly, a vector must be constructed to provide the regulatory sequences such that they will be functional upon inserting a desired gene. When the expression vector/insert construct is assembled, it is used to transform plant cells which are then used to regenerate plants. These transgenic plants carry the viral gene in the expression vector/insert construct. The gene is expressed in the plant and increased resistance to viral infection is conferred thereby. Several different methods exist to isolate the protease gene. To do so, one having ordinary skill in the art can use information about the genomic organization of potyviruses to locate and isolate the protease gene. The protease gene is located in the 3' half of the genome, between the VPg gene and the Nib gene (Figure 1) . Additionally, the information related to proteolytic cleavage sites is used to determine the N- terminus of the protease gene. The protease recognition sites are conserved in the potyviruses and have been determined to be either the dipeptide Gln- Ser, Gln-Gly, or Gin-Ala. The nucleotide sequences which encode these dipeptides can be determined. Using methods well known in the art, a quantity of virus is grown and harvested. The viral RNA is then separated and the protease gene isolated using a number of known procedures. A cDNA library is created using the viral RNA, by methods known to the art. The viral RNA is incubated with primers that hybridize to the viral RNA and reverse transcriptase, and a complementary DNA molecule is produced. A DNA complement of the complementary DNA molecule is produced and that sequence represents a DNA copy (cDNA) of the original viral RNA molecule. The DNA complement can be produced in a manner that results in a single double stranded cDNA or polymerase chain reactions can be used to amplify the DNA encoding the cDNA with the use of oligomer primers specific for the protease gene. These primers can include novel restriction sites used in subsequent cloning steps. Thus, a double stranded DNA molecule is generated which contains the sequence information of the viral RNA. These DNA molecules can be cloned in E. coli plasmid vectors after the additions of restriction enzyme linker molecules by DNA ligase. The various fragments are inserted into cloning vectors, such as well- characterized plasmids, which are then used to transform E. coli and create a cDNA library. Previously identified PRV NIa genes can be used as hybridization probes to screen the cDNA library to determine if any of the transformed bacteria contain DNA fragments with sequences coding for the protease region. The cDNA inserts in any bacterial colonies which contain this region can be sequenced. The protease gene is present in its entirety in colonies which have sequences that extend 5' to sequences which encode a N-terminal proteolytic cleavage site and 3' to sequences which encode a C-terminal proteolytic cleavage site for NIa. Alternatively, cDNA fragments can be inserted in the sense orientation into expression vectors. Antibodies against the protease can be used to screen the cDNA expression library and the gene can be isolated from colonies which express the protein. Another molecular strategy to provide virus resistance in transgenic plants is based on antisense RNA. As is well known, a cell manufactures protein by transcribing the DNA of the gene encoding that protein to produce RNA, which is then processed to messenger RNA (mRNA) (e.g., by the removal of introns) and finally translated by ribosomes into protein. This process may be inhibited in the cell by the presence of antisense RNA. The term antisense RNA means an RNA sequence which is complementary to a sequence of bases in the mRNA in question in the sense that each base (or the majority of bases) in the antisense sequence (read in the 3' to 5' sense) is capable of pairing with the corresponding base (G with C, A with U) in the mRNA sequence read in the 5' to 3' sense. It is believed that this inhibition takes place by formation of a complex between the two complementary strands of RNA, thus preventing the formation of protein. How this works is uncertain: the complex may interfere with further transcription, processing, transport or translation, or degrade the mRNA, or have more than one of these effects. This antisense RNA may be produced in the cell by transformation of the cell with an appropriate DNA construct arranged to transcribe the non-template strand (as opposed to the template strand) of the relevant gene (or of a DNA sequence showing substantial homology therewith) . The use of antisense RNA to downregulate the expression of specific plant genes is well known. Reduction of gene expression has led to a change in the phenotype of the plant: either at the level of gross visible phenotypic difference, e.g., lack of anthocyanin production in flower petals of petunia leading to colorless instead of colored petals (van der Krol et al., Nature. 333:866-869 (1988)); or at a more subtle biochemical level, e.g., change in the amount of polygalacturonase and reduction in depolymerization of pectin during tomato fruit ripening (Smith et al., Nature. 334:724-726 (1988)). Another more recently described method of inhibiting gene expression in transgenic plants is the use of sense RNA transcribed from an exogenous template to downregulate the expression of specific plant genes (Jorgensen, Keystone Symposium "Improved Crop and Plant Products through Biotechnology," Abstract Xl-022 (1994) ) . Thus, both antisense and sense RNA have been proven to be useful in achieving downregulation of gene expression in plants. In the present invention, the DNA molecules encoding the protease genes of PRV FLA83 W strain and the USA P- Type (HA attenuated) strain have been determined and the genes have been inserted into expression vectors. These expression cassettes can be individually placed into a vector that can be transmitted into plants, preferably a binary vector. Alternatively, two or more of the PRV protease genes can each be present in an expression cassette which can be placed into the same binary vector, or any one of the PRV NIa expression cassettes of the present invention can be placed into a binary vector with one or more viral gene expression cassettes. The expression vectors contain the necessary genetic regulatory sequences for expression of an inserted gene. The protease gene is inserted such that those regulatory sequences are functional and the genes can be expressed when incorporated into a plant genome. For example, vectors of the present invention can contain combinations of expression cassettes that include DNA from a papaya ringspot virus (PRV) coat protein gene or a heterologous PRV protease gene (e.g., one that is from a different strain), a cucumber mosaic virus coat protein gene, a zucchini yellow mosaic virus coat protein gene, and a watermelon mosaic virus-2 coat protein gene. Moreover, when combinations of viral gene expression cassettes are placed in the same binary plasmid, and that multigene cassette containing plasmid is transformed into a plant, the multiple viral genes all preferably exhibit substantially the same degrees of efficacy when present in transgenic plants. For example, if one examines numerous transgenic lines containing two different intact viral gene expression cassettes, the transgenic line will be immune to infection by both viruses. Similarly, if a line exhibits a delay in symptom development to one virus, it will also exhibit a delay in symptom development to the second virus. Finally, if a line is susceptible to one of the viruses it will be susceptible to the other. This phenomenon is unexpected. If there were not a correlation between the performance of each gene in these multiple gene constructs, this approach as a tool in plant breeding would probably be prohibitively difficult to use. Even with single gene constructs, one must test numerous transgenic plant lines to find one that displays the appropriate level of efficacy. The probability of finding a line with useful levels of expression can range from 10-50% (depending on the species involved) . For further information refer to Applicants' Assignees copending Patent Application Serial No. 08/366,991 entitled "Transgenic Plants Expressing DNA Constructs Containing a Plurality of Genes to Impart Virus Resistance" filed on December 30, 1994, incorporated by reference herein. In order to express the viral gene, the necessary genetic regulatory sequences must be provided. Since the protease of a potyvirus is produced by the post- translational processing of a polyprotein. the protease gene isolated from viral RNA does not contain transcription and translation signals necessary for its expression once transferred and integrated into a plant genome. It must, therefore, be engineered to contain a plant expressible promoter, a translation initiation codon (ATG), and a plant functional poly(A) addition signal (AATAAA) 3' of its translation termination codon. In the present invention, the protease genes are inserted into vectors which contain cloning sites for insertion 3' of the initiation codon and 5' of the poly(A) signal. The promoter is 5' of the initiation codon such that when structural genes are inserted at the cloning site, a functional unit is formed in which the inserted genes are expressed under the control of the various genetic regulatory sequences. The segment of DNA referred to as the promoter is responsible for the regulation of the transcription of DNA into mRNA. A number of promoters which function in plant cells are known in the art and can be employed in the practice of the present invention. These promoters can be obtained from a variety of sources such as plants or plant viruses, and can include, but are not limited to, promoters isolated from the caulimovirus group such as the cauliflower mosaic virus 35S promoter (CaMV35S) , the enhanced cauliflower mosaic virus 35S promoter (enh CaMV35S) , the figwort mosaic virus full- length transcript promoter (FMV35S) , and the promoter isolated from the chlorophyll a/b binding protein. Other useful promoters include promoters which are capable of expressing the potyvirus proteins in an inducible manner or in a tissue-specific manner in certain cell types in which the infection is known to occur. For example, the inducible promoters from phenylalanine ammonia lyase, chalcone synthase, hydroxyproline rich glycoprotein, extension, pathogenesis-related proteins (e.g. PR-la) , and wound- inducible protease inhibitor from potato may be useful. Preferred promoters for use in the present protease- containing cassettes include the constitutive promoters from CaMV, the Ti genes nopaline synthase (Bevan et al., Nucleic Acids Res. II. 369 (1983)) and octopine synthase (Depicker et al., J. Mol. Appl. Genet.. 1 , 561 (1982)), and the bean storage protein gene phaseolin. The poly(A) addition signals from these genes are also suitable for use in the present cassettes. The particular promoter selected is preferably capable of causing sufficient expression of the DNA coding sequences to which it is operably linked, to result in the production of amounts of the proteins or RNAs effective to provide viral resistance, but not so much as to be detrimental to the cell in which they are expressed. The promoters selected should be capable of functioning in tissues including, but not limited to, epidermal, vascular, and mesophyll tissues. The actual choice of the promoter is not critical, as long as it has sufficient transcriptional activity to accomplish the expression of the preselected proteins or sense and/or antisense RNAs and subsequent conferral of viral resistance to the plants. The nontranslated leader sequence can be derived from any suitable source and can be specifically modified to increase the translation of the mRNA. The 5' nontranslated region can be obtained from the promoter selected to express the gene, an unrelated promoter, the native leader sequence of the gene or coding region to be expressed, viral RNAs, suitable eucaryotic genes, or a synthetic gene sequence. The present invention is not limited to the constructs presented in the following examples. The termination region or 3' nontranslated region which is employed is one which will cause the termination of transcription and the addition of polyadenylated ribonucleotides to the 3' end of the transcribed mRNA sequence. The termination region can be native with the promoter region, native with the structural gene, or can be derived from another source, and preferably include a terminator and a sequence coding for polyadenylation. Suitable 3' nontranslated regions of the chimeric plant gene include but are not limited to: (1) the 3' transcribed, nontranslated regions containing the polyadenylation signal of AgrroJbacterium tumor-inducing (Ti) plasmid genes, such as the nopaline synthase (NOS) gene; and (2) plant genes like the soybean 7S storage protein genes. Preferably, the expression cassettes of the present invention are engineered to contain a constitutive promoter 5' to its translation initiation codon (ATG) and a poly(A) addition signal (AATAAA) 3' to its translation termination codon. Several promoters which function in plants are available, however, the preferred promoter is the 35S constitutive promoters from cauliflower mosaic virus (CaMV) . The poly(A) signal can be obtained from the CaMV 35S gene or from any number of well characterized plant genes, i.e., nopaline synthase, octopine synthase, and the bean storage protein gene phaseolin. The constructions are similar to that used for the expression of the CMV C coat protein in PCT Patent Application PCT/US88/04321, published on June 29, 1989 as WO 89/05858, claiming the benefit of U.S.S.N. 135,591, filed December 21, 1987, entitled "Cucumber Mosaic Virus Coat Protein Gene, " and the CMV WL coat protein in PCT Patent Application PCT/US89/03288, published on March 8, 1990 as WO 90/02185, claiming the benefit of U.S.S.N, 234,404, filed August 19, 1988, entitled "Cucumber Mosaic Virus Coat Protein Gene." Selectable marker genes can be incorporated into the present expression cassettes and used to select for those cells or plants which have become transformed. The marker gene employed may express resistance to an antibiotic, such as kanamycin, gentamycin, G418, hygromycin, streptomycin, spectinomycin, tetracycline, chloramphenicol, and the like. Other markers could be employed in addition to or in the alternative, such as, for example, a gene coding for herbicide tolerance such as tolerance to glyphosate, sulfonylurea, phosphinothricin, or bromoxynil. Additional means of selection could include resistance to methotrexate, heavy metals, complementation providing prototropy to an auxotrophic host, and the like. The particular marker employed will be one which will allow for the selection of transformed cells as opposed to those cells which are not transformed. Depending on the number of different host species one or more markers can be employed, where different conditions of selection would be useful to select the different host, and would be known to those of skill in the art. A screenable marker such as the j3-glucuronidase gene can be used in place of, or with, a selectable marker. Cells transformed with this gene can be identified by the production of a blue product on treatment with 5- bromo-4-chloro-3-indoyl-3-D-glucuronide (X-Gluc) . In developing the present expression construct, i.e., expression cassette, the various components of the expression construct such as the DNA molecules, linkers, or fragments thereof will normally be inserted into a convenient cloning vector, such as a plasmid or phage, which is capable of replication in a bacterial host, such as E. coli . Numerous cloning vectors exist that have been described in the literature. After each cloning, the cloning vector can be isolated and subjected to further manipulation, such as restriction, insertion of new fragments, ligation, deletion, resection, insertion, in vi tro mutagenesis, addition of polylinker fragments, and the like, in order to provide a vector which will meet a particular need. For AgroJacterium-mediated transformation, the expression cassette will be included in a vector, and flanked by fragments of the Agrobacterium Ti or Ri plasmid, representing the right and, optionally the left, borders of the Ti or Ri plasmid transferred DNA (T-DNA) . This facilitates integration of the present chimeric DNA sequences into the genome of the host plant cell. This vector will also contain sequences that facilitate replication of the plasmid in Agrobacterium cells, as well as in E. coli cells. All DNA manipulations are typically carried out in E. coli cells, and the final plasmid bearing the potyvirus gene expression cassette is moved into AgrroJbacterium cells by direct DNA transformation, conjugation, and the like. These Agrobacterium cells will contain a second plasmid, also derived from Ti or Ri plasmids. This second plasmid will carry all the viral genes required for transfer of the foreign DNA into plant cells. Suitable plant transformation cloning vectors include those derived from a Ti plasmid of AgrroJbacterium tumefaciens, as generally disclosed in Glassman et al. (U.S. Pat. No. 5,258,300), or Agrobacterium rhizogenes. A variety of techniques are available for the introduction of the genetic material into or transformation of the plant cell host. However, the particular manner of introduction of the plant vector into the host is not critical to the practice of the present invention, and any method which provides for efficient transformation can be employed. In addition to transformation using plant transformation vectors derived from the tumor-inducing (Ti) or root-inducing (Ri) plasmids of Agrobacterium, alternative methods could be used to insert the DNA constructs of the present invention into plant cells. Such methods may include, for example, the use of liposomes, electroporation, chemicals that increase the free uptake of DNA (Paszkowski et al., EMBO J.. 2_, 2717 (1984)), microinjection (Crossway et al. , Mol. Gen. Genet.. 202. 179 (1985)), electroporation (Fromm et al., Proc. Natl. Acad. Sci. USA. £2., 824 (1985)), or high-velocity microprojectiles (Klein et al. , Nature. 327. 70 (1987) and transformation using viruses or pollen. The choice of plant tissue source or cultured plant cells for transformation will depend on the nature of the host plant and the transformation protocol. Useful tissue sources include callus, suspension culture cells, protoplasts, leaf segments, stem segments, tassels, pollen, embryos, hypocotyls, tuber segments, meristematic regions, and the like. The tissue source is regenerable, in that it will retain the ability to regenerate whole, fertile plants following transformation. The transformation is carried out under conditions directed to the plant tissue of choice. The plant cells or tissue are exposed to the DNA carrying the present potyvirus multi-gene expression cassette for an effective period of time. This can range from a less- than-one-second pulse of electricity for electroporation, to a two-to-three day co-cultivation in the presence of plasmid-bearing AgrroJbacterium cells. Buffers and media used will also vary with the plant tissue source and transformation protocol. Many transformation protocols employ a feeder layer of suspended culture cells (tobacco or Black Mexican Sweet Corn, for example) on the surface of solid media plates, separated by a sterile filter paper disk from the plant cells or tissues being transformed. Following treatment with DNA, the plant cells or tissue may be cultivated for varying lengths of time prior to selection, or may be immediately exposed to a selective agent such as those described hereinabove. Protocols involving exposure to Agrobacterimr. will also include an agent inhibitory to the growth of the AgrroJbacterium cells. Commonly used compounds are antibiotics such as cefotaxime and carbenicillin. The media used in the selection may be formulated to maintain transformed callus or suspension culture cells in an undifferentiated state, or to allow production of shoots from callus, leaf or stem segments, tuber disks, and the like. Cells or callus observed to be growing in the presence of normally inhibitory concentrations of the selective agents are presumed to be transformed and may be subcultured several additional times on the same medium to remove nonresistant sections. The cells or callus can then be assayed for the presence of the viral gene cassette, or can be subjected to known plant regeneration protocols. In protocols involving the direct production of shoots, those shoots appearing on the selective media are presumed to be transformed and can be excised and rooted, either on selective medium suitable for the production of roots, or by simply dipping the excised shoot in a root-inducing compound and directly planting it in vermiculite. In order to produce transgenic plants exhibiting viral resistance, the viral genes must be taken up into the - 24 - plant cell and stably integrated within the plant genome. Plant cells and tissues selected for their resistance to an inhibitory agent are presumed to have acquired the selectable marker gene encoding this resistance during the transformation treatment. Since the marker gene is commonly linked to the viral genes, it can be assumed that the viral genes have similarly been acquired. Southern blot hybridization analysis using a probe specific to the viral genes can then be used to confirm that the foreign genes have been taken up and integrated into the genome of the plant cell. This technique may also give some indication of the number of copies of the gene that have been incorporated. Successful transcription of the foreign gene into mRNA can likewise be assayed using Northern blot hybridization analysis of total cellular RNA and/or cellular RNA that has been enriched in a polyadenylated region. mRNA molecules encompassed within the scope of the invention are those which contain viral specific sequences derived from the viral genes present in the transformed vector which are of the same polarity as that of the viral genomic RNA such that they are capable of base pairing with viral specific RNA of the opposite polarity to that of viral genomic RNA under conditions described in Chapter 7 of Sambrook et al. (1989) . Moreover, mRNA molecules encompassed within the scope of the invention are those which contain viral specific sequences derived from the viral genes present in the transformed vector which are of the opposite polarity to that of the viral genomic RNA such that they are capable of base pairing with viral genomic RNA under conditions described in Chapter 7 in Sambrook et al. (1989). The presence of a viral protease gene can also be detected by indirect assays, such as the Western blot assay described by Maiti et al. (Proc. Natl. Acad. Sci. U.S.A. 90. 6110 (1993)). Maiti et al. constructed a fusion protein containing the TVMV NIa protease and the E. coli glnH gene and transformed the construct into tobacco. Transgenic plants were assayed by Western blot analysis for the glnH gene product with an antibody to glnH. Not only was a glnH protein expressed in these plants, the glnH product was cleaved out of the fusion protein, presumably by NIa. Potyvirus resistance can also be assayed via infectivity studies as generally disclosed by Namba et al. (Gene. 107. 181 (1991)) wherein plants are scored as symptomatic when any inoculated leaf shows veinclearing, mosaic or necrotic symptoms. Seed from plants regenerated from tissue culture is grown in the field and self-pollinated to generate true breeding plants. The progeny from these plants become true breeding lines which are evaluated for viral resistance in the field under a range of environmental conditions. The commercial value of viral-resistant plants is greatest if many different hybrid combinations with resistance are available for sale. Additionally, hybrids adapted to one part of a country are not adapted to another part because of differences in such traits as maturity, disease and insect tolerance. Because of this, it is necessary to breed viral resistance into a large number of parental lines so that many hybrid combinations can be produced. Adding viral resistance to agronomically elite lines is most efficiently accomplished when the genetic control of viral resistance is understood. This requires crossing resistant and sensitive plants and studying the pattern of inheritance in segregating generations to ascertain whether the trait is expressed as dominant or recessive, the number of genes involved, and any possible interaction between genes if more than one is required for expression. With respect to transgenic plants of the type disclosed herein, the transgenes exhibit dominant, single gene Mendelian behavior. This genetic analysis can be part of the initial efforts to convert agronomically elite, yet sensitive lines to resistant lines. A conversion process (backcrossing) is carried out by crossing the original transgenic resistant line with a sensitive elite line and crossing the progeny back to the sensitive parent. The progeny from this cross will segregate such that some plants carry the resistance gene(s) whereas some do not. Plants carrying the resistance gene(s) will be crossed again to the sensitive parent resulting in progeny which segregate for resistance and sensitivity once more. This is repeated until the original sensitive parent has been converted to a resistant line, yet possesses all of the other important attributes originally found in the sensitive parent. A separate backcrossing program is implemented for every sensitive elite line that is to be converted to a virus resistant line. Subsequent to the backcrossing, the new resistant lines and the appropriate combinations of lines which make good commercial hybrids are evaluated for viral resistance, as well as for a battery of important agronomic traits. Resistant lines and hybrids are produced which are true to type of the original sensitive lines and hybrids. This requires evaluation under a range of environmental conditions under which the lines or hybrids will be grown commercially. Parental lines of hybrids that perform satisfactorily are increased and utilized for hybrid production using standard hybrid production practices. The invention will be further described by reference to the following detailed examples. Enzymes were obtained from commercial sources and were used according to the vendor's recommendations or other variations known in the art. Other reagents, buffers, etc., were obtained from commercial sources, such as GIBCO-BRL, Bethesda, MD, and Sigma Chemical Co., St. Louis, MO, unless otherwise specified. Most of the recombinant DNA methods employed in practicing the present invention are standard procedures, well known to those skilled in the art, and described in detail in, for example, European Patent Application Publication Number 223,452, published November 29, 1986, which is incorporated herein by reference. General references containing such standard techniques include the following: R. Wu, ed., Methods in Enzymology. Vol. 68 (1979); J.H. Miller, Experiments in Molecular Genetics (1972) ; J. Sambrook et al., Molecular Cloning: A Laboratory Manual. 2nd ed. (1989); and D.M. Glover, ed. , DNA Cloning Vol. II (1982) . Figures 6-8 are presented to illustrate the constructions of this invention. Example I To isolate and engineer the PRV NIa gene, the following steps can be used: 1) Purify PRV virions and isolate PRV viral RNA from the virion preparation; 2) construct single-stranded cDNAs of PRV viral RNA; 3) amplify NIa sequences by PCR amplification using viral sequence specific primers; 4) clone the PCR product into a plant expression cassette placed into an appropriate binary vector; 5) produce PRV NIa transgenic plants; and 6) challenge the progeny of RQ transgenic plants to identify lines which confer the desired properties. A. Isolation of PRV FLA83-W Viral RNA 7-day-old yellow crookneck squash plants grown in the greenhouse were inoculated with PRV strain W (watermelon) Florida-83; 21 days post inoculation leaves were harvested and PRV virus isolated. The procedure used was based on a modified method used by Purcifull et al. (Phytopathology. 69. 112 (1979)) for PRV type W isolation. Approximately 50 grams of fresh leaf tissue was homogenized in 100 ml 0.5 M potassium phosphate buffer (pH 7.5 "PB") containing 0.1% sodium sulphate, 25 ml chloroform, and 25 ml carbon tetrachloride. After centrifugation of the extract at 1,000 x g for 5 minutes the pellet was resuspended in 50 ml of PB buffer and centrifuged again at 1,000 x g for 5 minutes. The supernatants from each centrifugation are pooled then centrifuged at 13,000 x g for 15 minutes. To the resulting supernatant, Triton X-100 was added to a final concentration of 1% (v/v) , polyethyleneglycol (PEG) 8,000 (Reagent grade from Sigma Chemical Co.) to a final concentration of 4% (w/v) and NaCl to a final concentration of 100 mM. The suspension was stirred for l hour at 0-4° C. This suspension was centrifuged at 10,000 x g for 10 minutes. The pellet was resuspended in 40 ml of PB. After centrifugation at 12,000 x g for 10 minutes the pellet was discarded and virus was precipitated from the supernatant by adding PEG to a final concentration of 8% (w/v) and NaCl to a final concentration of 100 mM, and stirring for 0.5 hour at 0-4° C. After centrifugation at 12,000 x g for 10 minutes the pellets were resuspended with the aid of a tissue grinder in 5 ml of 20 mM PB and layered over a 30% Cs 2 S0 4 cushion. This suspension was centrifuged in a Beckman Ti75 at 140,000 x g for 18 hours at 5°C. After centrifugation, the virus band was harvested and dialyzed against 20 mM PB overnight at 4°C. The dialyzed virus preparation was lysed and viral RNA precipitated by the addition of LiCl to a final concentration of 2M. The viral RNA was recovered by centrifugation. Viral RNA was dissolved and precipitated by ethanol and resuspended in water. B. Cloning and Engineering PRV Protease Genes (a) FLA83 W PRV FLA83 W RNA was prepared as described above. Subsequently, the first cDNA strand was synthesized using PRV FLA83 W RNA template in a reaction that included the following: approximately 3-5 μg PRV FLA83 W RNA, 1 x buffer for Superscript Reverse Transcriptase (supplied by BRL-GIBCO, Grand Island, NY) , 2 mM dNTPs, oligomer primer RMM355 (37.5 μg/mL, SEQ ID NO:4) , 2.0 μL RNasin (Promega, Madison, WI) , and 2.5 μL Superscript Reverse Transcriptase (BRL-GIBCO) in a 20-μL reaction. After this reaction was allowed to proceed for 30 minutes at 37°C, an aliquot of the first strand reaction was used as a template in a polymerase chain reaction with RMM354 and RMM355 [SEQ ID NO: 3 and 4, respectively] to amplify a region of the FLA83 W genome (Figure 6) . The RMM354 primer supplies an ATG translation initiation codon. This region includes 189 base pairs of the 3' end of the Cl gene, the entire VPg gene, the entire NIa protease gene, and 146 base pairs of the 5' end of the Nib gene. The 1835 bp PCR amplified product was cloned into the pCRII vector included in the TA Cloning™ Kit supplied by Invitrogen Corp. A clone was recovered that contained PRV sequences (PRVNIaFLA TA-4) . This clone was sequenced with the use of a kit (Sequenase 2 purchased from USB, Cleveland, Ohio) . The 1789 bp Ncol fragment of PRVNIaFLA TA-4 containing PRV sequences was excised from PRVNIaFLA TA-4, isolated and inserted into the plant expression cassette pUC1318cpexpress. Cassettes containing the insert of PRV sequences in the sense orientation were isolated by a partial BamHl digestion (PRVFla83 NIa424) and inserted into the Bglll site of pEPGlll to give pEPG250 (for further information on parental binary vectors shown in Table 1, see Applicants' Assignees copending Patent Application Serial No. 08/366,991 entitled "Transgenic Plants Expressing DNA Constructs Containing a Plurality of Genes to Impart Virus Resistance" filed on December 30, 1994, incorporated by reference herein. For further information on PRV coat protein genes, see Applicants' Assignees copending Patent Application Serial No. 08/366,881 entitled "Papaya Ringspot Virus Coat Protein Gene" filed on December 30, 1994, incorporated by reference herein. For further information on ZYMV and WMV2 coat protein genes, see Applicants' Assignees copending Patent Application Serial No. 08/232,846 filed on April 25, 1994 entitled "Potyvirus Coat Protein Genes and Plants Transformed Therewith," incorporated by reference herein. For further information on CMV-C and CMV-wl coat protein genes, see Quemada et al. , J. Gen. Virol.. 70. 1065 (1989) . The binary plasmids were transformed into Agrobacterium tumefaciens strain C58Z707 and Mog301 (Table 1) (b) USA Type P (HA attenuated) A cDNA clone (#99) obtained from D. Gonsalves at Cornell University of the 3' end of PRV USA P-type (HA attenuated) strain served as a PCR template to amplify a PRV region that included 189 bp of the 3' end of the Cl gene, the entire VpG gene, the entire NIa protease gene, and 146 bp of the 5' end of Nib gene (Figure 5) . Primers RMM354 and RMM355 [SEQ ID NO: 3 and 4, respectively] were used during the PCR amplification to introduce novel restriction sites at each end of the PRV segment engineered. The resulting PCR-amplified segment was digested with EcoRl and BamHl (see Figures 2 and 5) and cloned by inserting it into the vector pGEMX-l (Promega, Madison, WI) . The PRV sequences in a resulting clone (pGEMX-l NIa-1) were nucleotide- sequenced (Figure 5; [SEQ ID NO: 5]). There were no sequence differences between the sequence of clone 99 and clone pGEMX-l NIa-1. pGEMX-l NIa-1 was digested with Ncol and the resulting Ncol fragment isolated for insertion into the expression cassette pUC18cpexpress. Both sense and antisense clones of expression cassettes (the expression cassettes are designated cpexpress PRV NIa 1-4 for the sense orientation and cpexpress PRV NIa 1-5 for the antisense orientation) containing the Ncol fragment of PRV were isolated. The plasmid containing the antisense orientation cassette is known as pUC18cpexpressPRVNIal-5. The plasmid containing the sense orientation cassette is known as pUC18cpexpressPRVNIal-4. Subsequently, the Hindlll fragments containing expression cassettes from each pUClθ plasmid containing either expression cassette were inserted into the Hindlll site of pUC1318 (clone pUCl318cpexpressPRVNIal-4 and pUC1318cpexpressPRVNIal- 5) to provide additional sites for installing cassettes into binary plasmids (Figure 7) . Subsequently, both Xbal and BamHl fragments were isolated from pUC1318cpexpressPRVNIal - 4 and pUC1318cpexpressPRVΝIal- 5. These fragments were inserted into the corresponding Xbal or Bglll sites of pGA482G, pEPGlll, PEPG106, pEPG109, pEPG120, or pEGG252 (Table 1). Resulting binary plasmids were transformed into Agrobacteria tumefaciens strains Mog30l and C58Z707. A PRV USA P-type (HA attenuated) NIa gene cassette was prepared that included an introduced stop codon (Figure 8) . To prepare the NIa coding sequence for insertion into the expression cassette pUClβcpexpress, novel restriction sites were introduced with oligomer primers RMM333 and RMM334 (see Figure 4 [SEQ ID NO: 9 and 10, respectively] ) . In addition, RMM333 introduced a single base pair deletion which results in a stop codon near the translation start site. The fragment amplified by oligomer pair RMM333 and RMM334 is 1339 base pairs in length and is shown in Figure 8. After PCR amplification, the fragment was engineered to obtain the cassette PRVNIa-1 (stop) (HA) as shown in Figure 8. The cassette was inserted into binary plasmids as described in Table 1. Table 1 Binary Parental Plasmid Site PRV NIa Cassette pEPG# PGA482G pEPG120 (CMV l62G) Xbal PRVNIal-4 (s) (HA)200 pGA482G PEPG120 (CMVwl62G) Xbal PRVNIal-5(asdouble) (HA) 201 pGA482G pGA482G Xbal PRVNIal-5 (as) (HA) 202 pPRBN pEPG109 (CWL41/Z/W) Xbal PRVNIal-4 (s) (HA) 113 pPRBN pEPG109 (CWL41/Z/W) Xbal PRVNIal-5 (as) (HA)114 pPRBN pEPGlll (C/Z/W) Bglll PRVNIal-4 (s) (HA)224 pPRBN pEPGlll (C/Z/ ) Bglll PRVNIal-5 (as) (HA)225 pPRBN pEPG106 (Z ) Bglll PRVNIal-4 (s) (HA)226 pPRBN pEPG106 (Z ) Bglll PRVNIal-5 (as) (HA)227 C. Transfer of PRV Protease Genes to Plants AgrroJacteriu-π-mediated transfer of the plant expressible PRV protease genes described herein was done using the methods described in PCT published application WO 89/05859, entitled "Agrobacterium Mediated Transformation of Germinating Plant Seeds.' Transgenic cucumber lines have been produced with the USA P-type (HA-attenuated) PRV NIa gene construct described above. Progeny of R 0 transgenic plants were challenged in the greenhouse. Challenge results indicate that PRV USA P-type NIa transgenic R, plants are protected to a significant extent against both homologous PRV challenge and heterologous PRV challenge. Compared with non-transgenic controls, transgenic R, progeny show delayed onset and reduced symptoms on cucumber leaves and fruits. The transgenic status of these plants was verified by their growth and development on media containing kanamycin and polymerase chain reaction amplification of the integrated transgenes. All squash and cucumber transgenic lines tested were positive for the NPTII gene by PCR. Most cantaloupe lines (>90%) were positive for NPTII by PCR. Most (>85%) plants positive for NPTII by PCR also tested positive for NIa sequences. Some cucumber plants transgenic for the constructs in pEPG200 PRV NIa showed delayed onset and reduced PRV symptom severity (Tables I, II, and V) , while other cucumber lines transgenic for pEPG200 showed no delay in the onset of symptoms or reduction of severity (Tables III and IV) . Cucumber lines transgenic for PRV coat protein genes showed a reduction of symptom severity (for examples see Tables VI-VIII) . However, if both NIa and CP genes are introduced into the same plant by sexual hybridization, resulting progeny are highly resistant to PRV infection. Consider progeny from the cross: MS642-93321-212-56-7 X MS642-93190- 200-1-13. The construct pEPG212 includes PRVwmcplδ (s) /Cwl4l/ ZYE72/WMBn22, and the construct pEPG200 includes PRVhNIaI-4 (s)/CMVwl62-G. Progeny that include both constructs are essentially resistant to PRV. Progeny show resistance to PRV isolated from Florida (FLA83) and California. Accordingly, both coat protein and protease genes incorporated into the same plant by sexual hybridization provide high levels of protection to PRV. Numerous binary plasmids were prepared that included both PRV coat protein and protease genes for transformation into plants. By this procedure both genes were incorporated into the same plant without the need for sexual hybridization. Instead, both genes were introduced by a single gene transfer (transformation) experiment. PCR Results: Ro plants were tested for PRV NIa transgene integration by PCR amplifying genes positioned at either end of the inserted blocks of construct. Left BorderINPTII PRV NIa[CMV C |Right Border For transgenic plants (squash, cucumber, and melon) the presence of PRV NIa was established by PCR detection of NPTII and CMV CP (see map, above) . These results indicate that if both NPTII and CMV CP genes are detected, genes internal to these two are also present. 34 / 1 In some cases PRV NIa was tested for directly, however, in all cases both NPTII and CMV CP were assayed by PCR. This table shows squash, melon, and cantaloupe plants transformed with binary plasmid listed in the first column. These PRV NIa constructs. The second column indicates numbers of plants in which both NPTII and CMV coast protein genes were identified by PCR. The third column indicates numbers of plants which were positive for the NPTII gene and negative for CMV coat protein. Eighty six percent of plants testing NPTII * were positive for the CMV CP gene. Details on the cucumber cross MS642-93321-212-56-7 X MS 642-93190-200-1-13. Below we give protection data for progeny of this cross. The challenge virus indicates which isolate of PRV was used to challenge transgenic progeny of the indicated cross. Plant No. indicates the progeny number of the cross. Each of the three challenges listed below is a separate experiment. Each challenge experiment included separate individual progeny from the above cross. For example, plant number "1" in the PRV-FLA83 challenge is a different plant than plant number "1": in the PRV-CA challenge. PCR results are summarized in the CP/NIa column. Both PRV coat protein and PRV NIa were assayed by PCR. A "+" indicates that the gene was identified, and a "-" indicates that by PCR no gene was identified. Disease rating was carried out 17, 14 and 31 days post inoculation (DPI) . Disease ratings range from 0 (no symptoms) to 7 (severe symptoms) . NT indicates that the progeny challenged were not tested by PCR. 34/2 - 35 - Table I PRVH-NIa 1-4 (s) /CMVWL62-G Symptomatic Plants Line Virus DPI NPT-II Ratio (%) Disease Rating MS642-93130- PRV-FL 12 + 0/6 (0) 0.0 200-8 6/7 (87) 3.1 PRV-TX + 0/6 (0) 0.0 4/5 (80) 2.4 PRV-NC + 6/12 (50) 0.5 3/3 (100) 1.0 PRV-FL 16 + 0/6 (0) 0.0 6/7 (87) 4.3 PRV-TX + 1/6 (17) 0.2 4/5 (80) 2.4 PRV-NC + 12/12 (100) 5.9 3/3 (100) 7.0 PRV-FL 19 + 0/6 (0) 0.0 6/7 (87) 4.0 PRV-TX + 1/6 (17) 0.2 4/5 (80) 2.4 PRV-NC + 12/12 (100) 1.0 3/3 (100) 7.0 - 36 Table II PRVH-NIa 1-4(S)/CMVWL62-G Symptomatic Plants Line Virus DPI NPT-II Ratio (%) Disease Rating GA715-93190- PRV-FL 10 + 0/9 (0) 0.0 200-70 6/6 (100 5.0 PRV-CA + 1/9 (9) 0.6 5/6 (83) 4.5 PRV-FL 16 + 3/9 (33) 1.0 5/5 (100) 5.0 PRV-CA + 8/9 (89) 3.3 6/6 (100) 4.7 PRV-FL 23 + 8/9 (89) 4.4 5/5 (100) 5.0 PRV-CA + 9/9 (100) 3.4 6/6 (100) 5.0 Table III PRVH-NIa 1-4(S)/CMVWL62-G Symptomatic Plants Line Virus DPI NPT-II Ratio (%) Disease Rating MSCY2-93190- PRV-CA 8 + 0/10 (0) 0.0 200-1 3/3 (100) 5.0 PRV-CA 12 + 10/10 (100) 3.0 3/3 (100) 5.0 PRV-CA 17 + 10/10 (100) 3.0 3/3 (100) 5.0 37 Table IV PRVh NIa 1-4(S)/CMVWL62-G Symptomatic Plants Line Virus DPI NPT-II Ratio (%) Disease Rating GA715-93190- PRV-FL 12 + 0/5 (0) 0.0 200-68 9/9 (100) 3.0 PRV-CA + 0/6 (0) 0.0 7/7 (100) 6.7 PRV-TX + 0/4 (0) 0.0 8/9 (88) 2.7 PRV-FL 19 + 0/5 (0) 0.0 9/9 (100) 5.8 PRV-CA + 0/6 (0) 0.0 7/7 (100) 7.0 PRV-TX + 0/4 (0) 0.0 8/9 (88) 4.1 PRV-FL 28 + 0/5 (0) 0.0 9/9 (100) 5.7 PRV-CA + 0/6 (0) 0.0 7/7 (100) 7.0 PRV-TX + 0/4 (0) 0.0 8/9 (88) 3.9 Table V PNIa22/CWL41/ZY72/WMBW22 Symptomatic Plants Line Virus DPI NPT-II Ratio (%) Disease Rating GA14A-93281- PRV-FL 12 + 0/9 (0) 0.0 113-6 4/4 (100) 3.0 PRV-CA + 0/7 (0) 0.0 6/6 (100) 6.3 PRV-TX + 0/8 (0) 0.0 4/6 (67) 2.0 PRV-NC + 0/7 (0) 0.0 4/8 (50) 2.4 PRV-FL 19 + 7/9 (77) 2.7 4/4 (100) 4.5 PRV-CA + 5/7 (71) 2.7 6/6 (100) 5.0 38 PRV-TX + 2/8 (16) 0.5 4/5 (80) 3.6 PRV-NC + 3/7 (43) 1.1 5/5 (100) 5.0 PRV-FL 21 + 7/9 (77) 3.2 4/4 (100) 5.0 PRV-CA + 5/7 (71) 3.3 6/6 (100) 5.0 PRV-TX + 4/8 (50) 1.8 6/6 (100) 4.3 PRV-NC + 6/7 (86) 4.3 5/5 (100) 7.0 Table VI PRVWMCP16 (S) /CWL/ZY72/WMBN22 Symptomatic Plants Line Virus DPI NPT-II Ratio (%) Disease Rating MS642-93316- PRV-FL 4 + 0/8 (0) 0.0 212-12 6/7 (86) 4.3 PRV-CA + 0/4 (0) 0.0 11/11 (100) 4.5 PRV-TX + 0/9 (0) 0.0 0/6 (0) 0.0 PRV-NC + 0/9 (0) 0.0 0/3 (0) 0.0 PRV-FL 12 + 8/8 (100) 3.5 7/7 (100) 4.4 PRV-CA + 4/4 (100) 5.0 11/11 (100) 5.0 PRV-TX + 8/9 (89) 2.7 6/6 (100) 3.0 PRV-NC + 0/9 (0) 0.0 0/6 (0) 0.0 PRV-FL 20 + 8/8 (100) 3.7 7/7 (100) 4.3 PRV-CA + 4/4 (100) 3.5 11/11 (100) 5.0 PRV-TX + 9/9 (100) 4.8 6/6 (100) 4.3 - 39 PRV-NC + 9/9 (100) 4.8 3/3 (100) 3.7 Table VII PRVWMCP16 (S) /CWL/ZY72/WMBN22 Symptomatic Plants Line Virus DPI NPT-II Ratio (%) Disease Rating MSGY2-93321- PRV-FL 14 + 6/6 (100) 3.3 212-56 8/8 (100) 5.0 PRV-CA + 8/8 (100) 5.3 5/5 (100) 5.4 PRV-FL 17 + 6/6 (100) 3.0 8/8 (100) 5.0 PRV-CA + 7/7 (100) 3.0 5/5 (100) 5.0 PRV-FL 21 + 6/6 (100) 3.0 8/8 (100) 5.0 PRV-CA + 7/7 (100) 3.9 5/5 (100) 5.4 40 - Table VIII PRVwmcplδ (S) /CWL/ZY72/WMBN22 Symptomatic Plants Line Virus DPI NPT-II Ratio (%) Disease Rating GA715-93293- PRV-FL 13 + 1/3 (33) 1.0 212-21 7/7 (100) 4.7 PRV-CA + 2/4 (50) 0.5 5/6 (83) 4.2 PRV-TX + 0/2 (0) 0.0 0/8 (0) 0.0 PRV-NC + 0/3 (0) 0.0 0/7 (0) 0.0 PRV-FL 16 + 3/3 (100) 3.0 7/7 (100) 5.0 PRV-CA + 4/4 (100) 4.5 6/6 (100) 6.7 PRV-TX + 2/2 (100) 1.0 8/8 (100) 1.0 PRV-NC + 3/3 (100) 3.0 7/7 (100) 3.0 PRV-FL 29 + 3/3 (100) 5.0 7/7 (100) 5.9 PRV-CA + 4/4 (100) 5.0 6/6 (100) 7.0 PRV-TX + 2/2 (100) 3.0 8/8 (100) 3.0 PRV-CA + 3/3 (100) 5.0 7/7 (100) 5.9 - 41 - TABLE 9A INBRED CONSTRUCT PRV-W-FL83 PRV-W-CA CA40 - 3/3 + 12/12 + 14/15 + 7/7 - 4/4 2/2 + 10/13 GA715 - 3/3 + 4/4 - 7/7 + 8/8 2/2 + 6/6 7/7 + 0/8 7/7 + 11/11 8/8 GP14A + 3/3 + 10/10 3/3 5/5 + 9/9 5/5 + 7/8 MS642 - 1/1 3/3 + 3/3 10/10 11/11 4/4 + 2/2 6/8 9/9 + 5/5 1/5 13/13 8/8 + 2/2 7/7 6/6 5/5 + 6/6 6/6 + 8/8 5/5 + 6/6 7/7 + 10/10 5/5 5/5 10/10 5/5 5/5 7/7 7/7 2/2 12/12 - 42 - INBRED CONSTRUCT PRV-W-FL83 PRV-W-CA V1151 + V3893 + 8/8 - 6/6 V4306 + 8/8 - 3/3 - 10/10 + 5/5 + _ 1/1 + 12/13 + 4/4 - 7/7 + + 7/7 - 7/7 - 6/8 + 2/2 + 11/11 - 4/4 YC77E CA95 0/2 + + 0/12 - 0/3 + GA715 + + - 5/5 8/8 + 7/7 4/5 + 4/4 4/4 - - 8 8//88 10/10 - 5/5 6/6 + 5/5 4/4 - 7/7 4/4 + 3/3 6/6 GP14A + 5/5 - 7/7 + + 5/5 5/5 - - 8 8//88 7/7 - 43 INBRED CONSTR .UCT PRV-W- FL83 PRV-W-CA + 7/8 4/4 2/2 5/5 1/1 3/3 + 11/11 9/9 + MP56 + 1/6 3/3 + 3/3 7/7 1/8 6/6 5/15 6/6 9/9 CA10 8/11 2/2 8/8 CA100 + + CA111 CA14 10/10 4/4 + CA24 + 15/15 CA40 6/6 4/4 8/8 5/5 CA40/100 - 44 - INBRED CONSTRUCT PRV-W-FL83 PRV-W-CA CA76 + + CA95 0/4 + 0/11 + 0/12 - 45 - All publications, patents and patent documents are incorporated by reference herein, as though individually incorporated by reference. The invention has been described with reference to various specific and preferred embodiments and techniques. However, it should be understood that many variations and modifications may be made while remaining within the spirit and scope of the invention.

Claims: - 46 - WHAT IS CLAIMED ISt 1. An isolated and purified DNA molecule consisting essentially of DNA encoding the NIa protease of the FLA83 W-type strain of papaya ringspot virus. 2. The isolated and purified DNA molecule of claim l consisting of DNA encoding the NIa protease of the FLA83 W-type strain of papaya ringspot virus. 3. The isolated and purified DNA molecule of claim 1 from the FLA83 W-type strain of papaya ringspot virus having the nucleotide sequence shown in Figure 2 [SEQ ID N0:1] . 4. A vector comprising a chimeric expression cassette comprising the DNA molecule of claim 1, a promoter and a polyadenylation signal, wherein the promoter is operably linked to the DNA molecule, and the DNA molecule is operably linked to the polyadenylation signal. 5. The vector of claim 4 wherein the promoter is the cauliflower mosaic virus 35S promoter. 6. The vector of claim 5 wherein the polyadenylation signal is the polyadenylation signal of the cauliflower mosaic 35S gene. 7. A bacterial cell comprising the vector of claim 4. 8. The bacterial cell of claim 7 wherein the bacterial cell is selected from the group consisting of an Agrobacterium tumefaciens cell and an Agrobacterium rhizogenes cell. - 47 - 9. A transformed plant cell transformed with the vector of claim 4. 10. The transformed plant cell of claim 9 wherein the promoter is cauliflower mosaic virus 35S promoter and the polyadenylation signal is the polyadenylation signal of the cauliflower mosaic 35S gene. 11. A plant selected from the family Cucurbi taceae comprising a plurality of the transformed cells of claim 10. 12. An isolated and purified DNA molecule consisting essentially of DNA encoding the NIa protease of the USA P-type (HA attenuated) strain of papaya ringspot virus. 13. The isolated and purified DNA molecule of claim 12 consisting of DNA which encodes the NIa protease of the USA P-type (HA attenuated) strain of papaya ringspot virus. 14. The isolated and purified DNA molecule of claim 12 from the USA P-type (HA attenuated) strain of papaya ringspot virus having the nucleotide sequence shown in Figure 5 [SEQ ID NO:5] . 15. A vector comprising a chimeric expression cassette comprising the DNA molecule of claim 12, a promoter and a polyadenylation signal, wherein the promoter is operably linked to the DNA molecule, and the DNA molecule is operably linked to the polyadenylation signal. 16. The vector of claim 15 wherein the promoter is the cauliflower mosaic virus 35S promoter. - 48 - 17. The vector of claim 15 wherein the polyadenylation signal is the polyadenylation signal of the cauliflower mosaic 35S gene. 18. A bacterial cell comprising the vector of claim 15. 19. The bacterial cell of claim 18 wherein the bacterial cell is selected from the group consisting of an AgrroJacteriitm tumefaciens cell and an Agrobacterium rhizogenes cell. 20. A transformed plant cell transformed with the vector of claim 15. 21. The transformed plant cell of claim 20 wherein the promoter is cauliflower mosaic virus 35S promoter and the polyadenylation signal is the polyadenylation signal of the cauliflower mosaic 35S gene. 22. A plant selected from the family Cucurbi taceae comprising a plurality of the transformed cells of claim 21. 23. A method of preparing a papaya ringspot viral resistant plant comprising: (a) transforming plant cells with a chimeric expression cassette comprising a promoter functional in plant cells operably linked to a DNA molecule that encodes a protease; wherein the DNA molecule is derived from a papaya ringspot virus strain selected from the group consisting of FLA83 W-type and USA P-type (HA attenuated) ; (b) regenerating the plant cells to provide a differentiated plant; and (c) identifying a transformed plant that expresses the papaya ringspot protease gene at a level - 49 - sufficient to render the plant resistant to infection by papaya ringspot virus. 24. The method of claim 23 wherein the DNA molecule is derived from a papaya ringspot virus strain having the nucleotide sequence shown in Figure 2 [SEQ ID NO:l] or Figure 5 [SEQ ID NO:5] . 25. The method of claim 23 wherein the plant is a dicot. 26. The method of claim 23 wherein the dicot is selected from the family Cucurbi aceae. 27. A vector comprising a chimeric expression cassette comprising the DNA molecule of claim l and at least one chimeric expression cassette comprising a heterologous PRV protease gene, a PRVcoat protein gene, a cucumber mosaic virus coat protein gene, a zucchini yellow mosaic virus coat protein gene, or a watermelon mosaic virus-2 coat protein gene, wherein each expression cassette comprises a promoter and a polyadenylation signal wherein the promoter is operably linked to the DNA molecule, and the DNA molecule is operably linked to the polyadenylation signal. 28. A bacterial cell comprising the vector of claim 27. 29. A transformed plant cell transformed with the vector of claim 27. 30. The transformed plant cell of claim 29 wherein the promoter is cauliflower mosaic virus 35S promoter and the polyadenylation signal is the polyadenylation signal of the cauliflower mosaic 35S gene. - 50 - 31. A vector comprising a chimeric expression cassette comprising the DNA molecule of claim 12 and at least one chimeric expression cassette comprising a heterologous PRV protease gene, a PRVcoat protein gene, a cucumber mosaic virus coat protein gene, a zucchini yellow mosaic virus coat protein gene, or a watermelon mosaic virus-2 coat protein gene, wherein each expression cassette comprises a promoter and a polyadenylation signal wherein the promoter is operably linked to the DNA molecule, and the DNA molecule is operably linked to the polyadenylation signal. 32. A bacterial cell comprising the vector of claim 31. 33. A transformed plant cell transformed with the vector of claim 31. 34. The transformed plant cell of claim 33 wherein the promoter is cauliflower mosaic virus 35S promoter and the polyadenylation signal is the polyadenylation signal of the cauliflower mosaic 35S gene.

Inventors: BOESHORE MAURY L, CARNEY KIM J, GONSALVES DENNIS, MCMASTER J RUSSELL, REYNOLDS JOHN F, SLIGHTON JERRY L, TRICOLI DAVID M, BOESHORE, MAURY, L., CARNEY, KIM, J., GONSALVES, DENNIS, MCMASTER, J., RUSSELL, REYNOLDS, JOHN, F., SLIGHTON, JERRY, L., TRICOLI, DAVID, M.
Applicants: ASGROW SEED CO, BOESHORE MAURY L, CARNEY KIM J, CORNELL RES FOUNDATION INC, GONSALVES DENNIS, MCMASTER J RUSSELL, REYNOLDS JOHN F, SLIGHTON JERRY L, TRICOLI DAVID M, ASGROW SEED COMPANY, BOESHORE, MAURY, L., CARNEY, KIM, J., CORNELL RESEARCH FOUNDATION, INC., GONSALVES, DENNIS, MCMASTER, J., RUSSELL, REYNOLDS, JOHN, F., SLIGHTON, JERRY, L., TRICOLI, DAVID, M.
Application year: 1995
Main classification: C12N15
IPC codes: C12N9, C12N15