Identification of a protein covalently linked to the 5′ terminus of tobacco vein mottling virus RNA

The mystery remains: How do potyviruses move within and between cells?

The genus Potyvirus is considered as the largest among plant single-stranded (positive-sense) RNA viruses, causing considerable economic damage to vegetable and fruit crops worldwide. Through the coordinated action of four viral proteins and a few identified host factors, potyviruses exploit the endomembrane system of infected cells for their replication and for their intra- and intercellular movement to and through plasmodesmata (PDs). Although a significant amount of data concerning potyvirus movement has been published, no synthetic review compiling and integrating all information relevant to our current understanding of potyvirus transport is available. In this review, we highlight the complexity of potyvirus movement pathways and present three potential nonexclusive mechanisms based on (1) the use of the host endomembrane system to produce membranous replication vesicles that are targeted to PDs and move from cell to cell, (2) the movement of extracellular viral vesicles in the apoplasm, and (3) the transport of virion particles or ribonucleoprotein complexes through PDs. We also present and discuss experimental data supporting these different models as well as the aspects that still remain mostly speculative.

Protein Nucleotidylylation in +ssRNA Viruses

Nucleotidylylation is a post-transcriptional modification important for replication in the picornavirus supergroup of RNA viruses, including members of the Caliciviridae, Coronaviridae, Picornaviridae and Potyviridae virus families. This modification occurs when the RNA-dependent RNA polymerase (RdRp) attaches one or more nucleotides to a target protein through a nucleotidyl-transferase reaction. The most characterized nucleotidylylation target is VPg (viral protein genome-linked), a protein linked to the 5' end of the genome in Caliciviridae, Picornaviridae and Potyviridae. The nucleotidylylation of VPg by RdRp is a critical step for the VPg protein to act as a primer for genome replication and, in Caliciviridae and Potyviridae, for the initiation of translation. In contrast, Coronaviridae do not express a VPg protein, but the nucleotidylylation of proteins involved in replication initiation is critical for genome replication. Furthermore, the RdRp proteins of the viruses that perform nucleotidylylation are themselves nucleotidylylated, and in the case of coronavirus, this has been shown to be essential for viral replication. This review focuses on nucleotidylylation within the picornavirus supergroup of viruses, including the proteins that are modified, what is known about the nucleotidylylation process and the roles that these modifications have in the viral life cycle.

The Biological Impact of the Hypervariable N-Terminal Region of Potyviral Genomes

Potyviridae is the largest family of plant-infecting RNA viruses, encompassing over 30% of known plant viruses. The family is closely related to animal picornaviruses such as enteroviruses and belongs to the picorna-like supergroup. Like all other picorna-like viruses, potyvirids employ polyprotein processing as a gene expression strategy and have single-stranded, positive-sense RNA genomes, most of which are monopartite with a long open reading frame. The potyvirid polyproteins are highly conserved in the central and carboxy-terminal regions. In contrast, the N-terminal region is hypervariable and contains position-specific mutations resulting from transcriptional slippage during viral replication, leading to translational frameshift to produce additional viral proteins essential for viral infection. Some potyvirids even lack one of the N-terminal proteins P1 or helper component-protease and have a genus-specific or species-specific protein instead. This review summarizes current knowledge about the conserved and divergent features of potyvirid genomes and biological relevance and discusses future research directions.

Characterization of Soybean yellow shoot virus, a New Member of the Family Potyviridae Infecting Soybean Plants in Brazil

A new virus species, belonging to the family Potyviridae and capable of infecting most of the soybean cultivars grown in Brazil, was collected in Lavras, Minas Gerais, Brazil, and named Soybean yellow shoot virus (SoyYSV). In this study, the complete 9,052-nucleotide genome of SoyYSV was determined and the structural, biological, and molecular properties of the virus were investigated. The SoyYSV genome encoded a single polyprotein that could be subsequently cleaved, generating 11 proteins. The SoyYSV genome shared 49% nucleotide and 36% amino acid sequence identity with Blackberry virus Y. However, the P1 protein of SoyYSV was much smaller and lacked the ALK1 domain characteristic of the genus Brambyvirus. Electron microscopy revealed flexuous filamentous virus particles, 760 to 780 nm in length, and cytoplasmic inclusions typical of those found in plant cells infected with Potyviridae species. In addition to soybean, SoyYSV infected species in the Amaranthaceae, Caricaceae, Fabaceae, and Solanaceae families. Among the most common potyviruses present in Brazil, only SoyYSV induced local necrotic lesions in Carica papaya L. SoyYSV was transmissible by Myzus persicae and Aphis gossypii but lacked the HC-Pro domain required for aphid transmission in other potyviruses. No seed transmission in soybean was observed.

Variability in eukaryotic initiation factor iso4E in Brassica rapa influences interactions with the viral protein linked to the genome of Turnip mosaic virus

Plant potyviruses require eukaryotic translation initiation factors (eIFs) such as eIF4E and eIF(iso)4E to replicate and spread. When Turnip mosaic virus (TuMV) infects a host plant, its viral protein linked to the genome (VPg) needs to interact with eIF4E or eIF(iso)4E to initiate translation. TuMV utilizes BraA.eIF4E.a, BraA.eIF4E.c, BraA.eIF(iso)4E.a, and BraA.eIF(iso)4E.c of Brassica rapa to initiate translation in Arabidopsis thaliana. In this study, the BraA.eIF4E.a, BraA.eIF4E.c, BraA.eIF(iso)4E.a, and BraA.eIF(iso)4E.c genes were cloned and sequenced from eight B. rapa lines, namely, two BraA.eIF4E.a alleles, four BraA.eIF4E.c alleles, four BraA.eIF(iso)4E.a alleles, and two BraA.eIF(iso)4E.c alleles. Yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) analyses indicated that TuMV VPg could not interact with eIF4E, but only with eIF(iso)4E of B. rapa. In addition, the VPgs of the different TuMV isolates interacted with various eIF(iso)4E copies in B. rapa. In particular, TuMV-UK1/CDN1 VPg only interacted with BraA.eIF(iso)4E.c, not with BraA.eIF(iso)4E.a. Some single nucleotide polymorphisms (SNPs) were identified that may have affected the interaction between eIF(iso)4E and VPg such as the SNP T₁₀₆C in BraA.eIF(iso)4E.c and the SNP A₁₅₄C in VPg. Furthermore, a three-dimensional structural model of the BraA.eIF(iso)4E.c-1 protein was constructed to identify the specific conformation of the variable amino acids from BraA.eIF(iso)4E.c. The 36^th amino acid in BraA.eIF(iso)4E.c is highly conserved and may play an important role in establishing protein structural stability. The findings of the present study may lay the foundation for future investigations on the co-evolution of TuMV and eIF(iso)4E.

Protein intrinsic disorder within the Potyvirus genus: from proteome-wide analysis to functional annotation

Within proteins, intrinsically disordered regions (IDRs) are devoid of stable secondary and tertiary structures under physiological conditions and rather exist as dynamic ensembles of inter-converting conformers. Although ubiquitous in all domains of life, the intrinsic disorder content is highly variable in viral genomes. Over the years, functional annotations of disordered regions at the scale of the whole proteome have been conducted for several animal viruses. But to date, similar studies applied to plant viruses are still missing. Based on disorder prediction tools combined with annotation programs and evolutionary studies, we analyzed the intrinsic disorder content in Potyvirus, using a 10-species dataset representative of this genus diversity. In this paper, we revealed that: (i) the Potyvirus proteome displays high disorder content, (ii) disorder is conserved during Potyvirus evolution, suggesting a functional advantage of IDRs, (iii) IDRs evolve faster than ordered regions, and (iv) IDRs may be associated with major biological functions required for the Potyvirus cycle. Notably, the proteins P1, Coat protein (CP) and Viral genome-linked protein (VPg) display a high content of conserved disorder, enriched in specific motifs mimicking eukaryotic functional modules and suggesting strategies of host machinery hijacking. In these three proteins, IDRs are particularly conserved despite their high amino acid polymorphism, indicating a link to adaptive processes. Through this comprehensive study, we further investigate the biological relevance of intrinsic disorder in Potyvirus biology and we propose a functional annotation of potyviral proteome IDRs.

Molecular biology of potyviruses

Potyvirus is the largest genus of plant viruses causing significant losses in a wide range of crops. Potyviruses are aphid transmitted in a nonpersistent manner and some of them are also seed transmitted. As important pathogens, potyviruses are much more studied than other plant viruses belonging to other genera and their study covers many aspects of plant virology, such as functional characterization of viral proteins, molecular interaction with hosts and vectors, structure, taxonomy, evolution, epidemiology, and diagnosis. Biotechnological applications of potyviruses are also being explored. During this last decade, substantial advances have been made in the understanding of the molecular biology of these viruses and the functions of their various proteins. After a general presentation on the family Potyviridae and the potyviral proteins, we present an update of the knowledge on potyvirus multiplication, movement, and transmission and on potyvirus/plant compatible interactions including pathogenicity and symptom determinants. We end the review providing information on biotechnological applications of potyviruses.

Structures of the compact helical core domains of feline calicivirus and murine norovirus VPg proteins

We report the solution structures of the VPg proteins from feline calicivirus (FCV) and murine norovirus (MNV), which have been determined by nuclear magnetic resonance spectroscopy. In both cases, the core of the protein adopts a compact helical structure flanked by flexible N and C termini. Remarkably, while the core of FCV VPg contains a well-defined three-helix bundle, the MNV VPg core has just the first two of these secondary structure elements. In both cases, the VPg cores are stabilized by networks of hydrophobic and salt bridge interactions. The Tyr residue in VPg that is nucleotidylated by the viral NS7 polymerase (Y24 in FCV, Y26 in MNV) occurs in a conserved position within the first helix of the core. Intriguingly, given its structure, VPg would appear to be unable to bind to the viral polymerase so as to place this Tyr in the active site without a major conformation change to VPg or the polymerase. However, mutations that destabilized the VPg core either had no effect on or reduced both the ability of the protein to be nucleotidylated and virus infectivity and did not reveal a clear structure-activity relationship. The precise role of the calicivirus VPg core in virus replication remains to be determined, but knowledge of its structure will facilitate future investigations.

Electroporetic transfection of pepper protoplasts with plant potyviruses

Potyviruses are a persistent threat to bell pepper (Capsicum annuum L.) production worldwide. Much effort has been expended to study the resistance response of pepper cultivars at whole plant levels but with only limited effort at the cellular level using protoplasts. A pepper protoplast isolation procedure is available but an inoculation procedure is needed that provides consistent and highly efficient infection. An electroporation-based procedure for inoculation of potyviruses was developed using a base procedure developed for Cucumber mosaic virus (CMV). The final parameters identified for efficient potyvirus infection of pepper protoplasts involves two 25ms pulses, 200V each pulse with a 10s interval between pulses. Depending on the method of detection, e.g., ELISA versus RT-PCR, potyvirus RNA inoculum ranged from 10 to 40μg with infection detection occurring with samples of 50,000-100,000 protoplasts.

Microsatellites in different Potyvirus genomes: survey and analysis

Simple sequence repeats (SSRs) have been extensively used for various genetic and evolutionary studies in eukaryotic and prokaryotic organisms, while few relevant researches have been made in viruses. The Potyvirus is a fine system to study roles and evolution of SSRs in viruses. The densities, relative abundances, compositions and evolutionary inferences of SSRs in 45 different Potyvirus genomes have been analyzed in this study. Results showed that the densities and relative abundances of SSRs are similar in all those Potyvirus genomes. The number of SSRs decreases with an increase in the length of repeat unit. Dinucleotide repeats are the most abundant and followed by trinucleotide repeats, and the numbers of tetra-, penta- and hexanucleotide repeats are very small. Repeats of AC/CA, AG/GA and AAG/GAA predominate, whereas repeats of CG/GC, ATA and CAC are rare. The genome sizes of the Potyvirus species have little influence on the total number and relative abundance of SSRs. Our study suggested that the variety of SSRs may be related to the genome diversity of Potyvirus. Maybe Potyvirus and HIV genomes have the similar evolution mode and parallel evolution level.

Involvement of the plant nucleolus in virus and viroid infections: parallels with animal pathosystems

The nucleolus is a dynamic subnuclear body with roles in ribosome subunit biogenesis, mediation of cell-stress responses, and regulation of cell growth. An increasing number of reports reveal that similar to the proteins of animal viruses, many plant virus proteins localize in the nucleolus to divert host nucleolar proteins from their natural functions in order to exert novel role(s) in the virus infection cycle. This chapter will highlight studies showing how plant viruses recruit nucleolar functions to facilitate virus translation and replication, virus movement and assembly of virus-specific ribonucleoprotein (RNP) particles, and to counteract plant host defense responses. Plant viruses also provide a valuable tool to gain new insights into novel nucleolar functions and processes. Investigating the interactions between plant viruses and the nucleolus will facilitate the design of novel strategies to control plant virus infections.

Cellular remodeling during plant virus infection

This review focuses on the extensive membrane and organelle rearrangements that have been observed in plant cells infected with RNA viruses. The modifications generally involve the formation of spherules, vesicles, and/or multivesicular bodies associated with various organelles such as the endoplasmic reticulum and peroxisomes. These virus-induced organelles house the viral RNA replication complex and are known as virus factories or viroplasms. Membrane and organelle alterations are attributed to the action of one or two viral proteins, which additionally act as a scaffold for the assembly of a large complex of proteins of both viral and host origin and viral RNA. Some virus factories have been shown to align with and traffic along microfilaments. In addition to viral RNA replication, the factories may be involved in other processes such as viral RNA translation and cell-to-cell virus transport. Confining the process of RNA replication to a specific location may also prevent the activation of certain host defense functions.

Interaction of genome-linked protein (VPg) of turnip mosaic virus with wheat germ translation initiation factors eIFiso4E and eIFiso4F

The interaction between VPg of turnip mosaic virus and wheat germ eukaryotic translation initiation factors eIFiso4E and eIFiso4F (the complex of eIFiso4E and eIFiso4G) were measured and compared. The fluorescence quenching data showed the presence of one binding site on eIFiso4E for VPg. Scatchard analysis revealed the binding affinity (K(a)) and average binding sites (n) for VPg were (8.51 +/- 0.21) x 10(6) M(-1) and 1.0, respectively. The addition of eIFiso4G to the eIFiso4E increased the binding affinity 1.5-fold for VPg as compared with eIFiso4E alone. However, eIFiso4G alone did not bind with VPg. The van't Hoff analyses showed that VPg binding is enthalpy-driven and entropy-favorable with a large negative DeltaH degrees (-29.32 +/- 0.13 kJmol(-1)) and positive DeltaS degrees (36.88 +/- 0.25 Jmol(-1)K(-1)). A Lineweaver-Burk plot indicates mixed-type competitive ligand binding between VPg and anthraniloyl-7-methylguanosine triphosphate for eIFiso4E. Fluorescence stopped-flow studies of eIFiso4E and eIFiso4F with VPg show rapid binding, suggesting kinetic competition between VPg and m(7)G cap. The VPg protein binds much faster than cap analogs. The activation energies for binding of eIFiso4E and eIFiso4F with VPg were 50.70 +/- 1.27 and 75.37 +/- 2.95 kJmol(-1) respectively. Enhancement of eIFiso4F-VPg binding with the addition of a structured RNA derived from tobacco etch virus suggests that translation initiation involving VPg occurs at internal ribosomal entry sites. Furthermore, the formation of a protein-RNA complex containing VPg suggests the possibility of direct participation of VPg in the translation of the viral genome.

Identification of the genome-linked protein in virions of Potato virus A, with comparison to other members in genus Potyvirus

Viruses of the genus Potyvirus, the largest genus of plant-infecting viruses, have a messenger-polarity ssRNA genome encapsidated by approximately 2000 units of the viral coat protein (CP), resulting in filamentous virions. Only few studies have examined potyvirus virions for the presence of other structural proteins. A protein linked covalently to the 5'-end of the genome has been identified in Tobacco vein mottling virus (TVMV) and Tobacco etch virus (TEV). In TEV, it is either the viral NIa protein or only its N-terminal domain (VPg) separated autocatalytically from the C-terminal proteinase domain (NIa-Pro). Virions of TVMV carry only the VPg. We examined virions of Potato virus A (PVA) for the genome-linked protein using immunoblotting or iodination and immunoprecipitation. The VPg ( approximately 25 kDa) only, and not the unprocessed NIa, was detected. Another signal corresponding to approximately 49 kDa was detected in disrupted, RNase-treated virions with anti-VPg antibodies but not with antibodies to NIa-Pro. Since it possibly represented a dimeric form of the VPg, self-interaction of the VPg was tested using the yeast two-hybrid system, which showed that the VPg self-interacts in the absence of viral RNA.

Sequence and phylogenetic analysis of the cytoplasmic inclusion protein gene of zucchini yellow mosaic potyvirus: its role in classification of the Potyviridae

The cytoplasmic inclusion (CI) gene of a Singapore isolate of zucchini yellow mosaic virus (ZYMV-S) was sequenced and compared with CI of 14 other potyviruses. In addition to the consensus sequence GAVGSGKST of nucleotide binding motif (NTBM) which is implicated as a membrane-binding component of the RNA helicase complex, five other conserved motifs were found. Phylogenetic trees were constructed from sequence data for the CI and the coat protein. Similar branching patterns obtained from both CI and coat protein analyses suggests that phylogenetic relationship among potyviruses can be determined using the CI. We propose that phylogenetic analysis of CI gene may be used as an alternative approach for the study of evolution within the family Potyviridae.

Release of a 22-kDa protein derived from the amino-terminal domain of the 49-kDa NIa of turnip mosaic potyvirus in Escherichia coli

The coding region for the precursor 6K-small nuclear inclusion a (NIa) protein and for the NIa protein of turnip mosaic potyvirus (TuMV) were introduced into the plasmid pET-11d for high-level expression in Escherichia coli. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblot analyses of E. coli proteins showed that the NIa protein underwent endoproteolysis and released a 22-kDa polypeptide. NH2-terminal amino acid sequencing of the recombinant 22-kDa protein was performed and was identical to the predicted amino end of the NIa protein. Site-directed mutagenesis confirmed that the hydrolysis was associated with the NIa proteolytic activity and that the proteinase recognized a Glu residue within an amino acid sequence found in the NIa protein which fitted the TuMV consensus cleavage site sequence. Fusion of the 6K protein with the NIa protein partially inhibited the hydrolytic reaction. The recombinant 22-kDa protein is likely the VPg of TuMV.

Characterization of the potyviral HC-pro autoproteolytic cleavage site

The helper component-proteinase (HC-Pro) encoded by potyviruses functions to cleave the viral polyprotein by an autoproteolytic mechanism at the HC-Pro C-terminus. This protein belongs to a group of viral cysteine-type proteinases and has been shown previously to catalyze proteolysis between a Gly-Gly dipeptide. The amino acid sequence requirements surrounding the HC-Pro C-terminal cleavage site of the tobacco etch virus polyprotein have been investigated using site-directed mutagenesis and in vitro expression systems. A total of 51 polyprotein derivatives, each differing by the substitution of a single amino acid between the P5 and P2' positions, were tested for autoproteolytic activity. Substitutions of Tyr (P4), Val (P2), Gly (P1), and Gly (P1') were found to eliminate or nearly eliminate proteolysis. Substitutions of Thr (P5), Asn (P3), and Met (P2'), on the other hand, were permissive for proteolysis, although the apparent processing rates of some polyproteins containing these alterations were reduced. These results suggest that auto-recognition by HC-Pro involves the interaction of the enzymatic binding site with four amino acids surrounding the cleavage site. Comparison of the homologous sequences of five potyviral polyproteins revealed that the residues essential for processing are strictly conserved, whereas the nonessential residues are divergent. The relationship between HC-Pro and other viral and cellular cysteine-type proteinases is discussed.

The general properties of potyviruses

The criteria used during the past three decades for including viruses in the potyvirus group are briefly discussed and evaluated. The biological and physico-chemical properties of the viruses transmitted by aphids, mites, whiteflies, or the fungus Polymyxa graminis are reviewed, and the taxonomic value of their molecular properties in regrouping the viruses into four groups or genera within the family Potyviridae is discussed.

Two newly detected nonstructural viral proteins in potyvirus-infected cells

The existence of two viral RNA-encoded proteins in cells infected with tobacco vein mottling potyvirus (TVMV) has been demonstrated. One of the proteins (named 34K) maps at the N-terminus of the TVMV polyprotein and the other (42K) between the helper component and cylindrical inclusion proteins; both had previously been predicted in the consensus potyviral genetic map. The 34K and 42K coding regions of TVMV were cloned separately in a bacterial expression vector and the proteins were isolated from transformed Escherichia coli. These were used to raise polyclonal antibodies which reacted specifically with proteins of the expected size in immunoblots of extracts of TVMV-infected tobacco leaves and protoplasts. In addition to 42K, the anti-42K serum detected similar amounts of a second protein of apparent size 37 kDa that was absent in 42K-expressing bacteria. Both 34K and 42K were present predominantly in membrane-enriched fractions of extracts of TVMV-infected tobacco leaves. Computer analysis of the deduced amino acid sequence of 42K suggests that this viral protein may be an integral transmembrane protein.

Identification of the initiation codon of plum pox potyvirus genomic RNA

The expression of plum pox potyvirus (PPV) genomic RNA takes place through translation of its unique long and functional open reading frame (ORF) into a large polyprotein that undergoes extensive proteolytic processing. In this paper we show that the AUG recognized as the initiation codon of the PPV ORF by in vitro translation systems is the one found at nucleotide position 147, in spite of the presence at position 36 of an in-phase AUG that marks the start of the ORF. Deletion of a substantial part of the PPV 5' nontranslated region (5'-NTR), from nucleotide 19 to 101, does not impair the in vitro translation of PPV synthetic transcripts. By introduction of mutations that disrupt either of these two AUGs into a full-length PPV cDNA clone, it is shown that, while alteration of the first AUG does not have any effect on virus viability, growth, or symptom induction, destruction of the second renders the viral RNA noninfectious. This result indicates that the AUG employed in vivo is also the second. The hypothesis that this AUG could be recognized through a ribosomal internal entry mechanism has been tested in vitro using various bicistronic transcripts in which the PPV 5'-NTR was internally placed. The second cistron of these bicistronic RNAs was translated, but only at low levels, indicating that the PPV 5'-NTR is not able to drive in vitro an efficient internal entry of the ribosomes and suggesting that PPV RNA translation might proceed through a conventional leaky scanning mechanism.

The VPg of tobacco etch virus RNA is the 49-kDa proteinase or the N-terminal 24-kDa part of the proteinase

Preparations of tobacco etch virus (TEV) RNA which were purified by sucrose gradient centrifugation, digested with RNase, and analyzed by SDS-polyacrylamide gel electrophoresis contained proteins of 49, 32, and 24 kDa. The 49- and 24-kDa proteins reacted with polyclonal antiserum to the TEV 49-kDa proteinase while the 32-kDa protein reacted with anti-TEV serum. Further purification of the RNA by centrifugation through CsCl removed the coat protein (32 kDa), but not the 49- and 24-kDa proteins. The 49- and 24-kDa proteins did not migrate into a polyacrylamdie gel when the RNA was not digested with RNase. These results indicate that the VPg of TEV is either the 49-kDa proteinase or the 24 kDa that represents the amino-terminal half thereof.

Determination of polyprotein processing sites by amino terminal sequencing of nonstructural proteins encoded by plum pox potyvirus

Nonstructural proteins of plum pox potyvirus were partially purified following a procedure described for the isolation of tobacco etch virus nuclear inclusion proteins. Plum pox virus proteins with electrophoretic mobilities corresponding to 49, 59 and 68 kDa reacted with antibodies against the 49 kDa and 54 kDa components of the nuclear inclusions and the 70 kDa component of the cylindrical inclusions of tobacco etch virus, respectively. Further purification by size exclusion high performance liquid chromatography or SDS-polyacrylamide gel electrophoresis, and amino terminal amino acid sequencing permitted the location in the plum pox virus polyprotein of the cleavage sites from which the 49 kDa (NIa-type, protease), 59 kDa (NIb-type, putative RNA replicase), and 68 kDa (CI-type) proteins originate. A 110 kDa protein which copurified with the plum pox virus inclusion proteins reacted with both anti-NIa and anti-NIb sera and had the same amino terminus as the plum pox virus 49 kDa protein, indicating that it is a non-processed 49-59 kDa polypeptide.

Expression of a potyvirus non-structural protein in transgenic tobacco

A cDNA fragment encoding the cytoplasmic inclusion protein of tobacco vein mottling virus was inserted into the plant expression cassette of a Ti plasmid-based binary vector. The vector was transferred to Agrobacterium tumifaciens, and following a modified leaf disc procedure, transformed tobacco plants were obtained. Analysis of poly(A)+ RNA from transgenic plants revealed a novel RNA of approximately 2100 nucleotides possessing tobacco vein mottling virus sequences. Also, immunoprecipitation of protein extracts of [35S]methionine-labeled transformed callus using anti-cytoplasmic inclusion protein antiserum revealed a polypeptide of approximately 70 kDa. This size is consistent with that predicted from the inserted tobacco vein mottling virus coding sequences. Together these data demonstrate the expression of the cytoplasmic inclusion protein in the absence of viral infections.

Mapping of the tobacco vein mottling virus VPg cistron

The location of the cistron encoding the genome-linked protein (VPg) in the potyvirus tobacco vein mottling virus (TVMV) was investigated. Precipitation of 125I-labeled VPg with anti-tobacco etch virus 49K nuclear inclusion protein antiserum (which reacts with the NIa nuclear inclusion protein of TVMV) indicated that the TVMV VPg is immunologically related to NIa. Lysyl residues were found to be present at positions 2, 11, and 16 of the amino-terminal region of the VPg. A search of the TVMV polyprotein sequence for this distribution of lysyl residues revealed a unique location beginning at amino acid residue 1801, the proposed amino-terminus of the NIa protein.

On the inhibition of plant virus multiplication by ribavirin

The inhibition of the replication of potato virus X (PVX), belladonna mottle virus, tobacco mosaic virus, potato virus Y (PVY), and tobacco necrosis virus by ribavirin and pyrazofurin is described with emphasis on the inhibition of PVX by ribavirin. Ribavirin inhibits an early step of PVX replication. The inhibition is reversed to different degrees by all ribo- and deoxyribonucleosides, most strongly by thymidine. In tobacco leaves, nucleosides compete with ribavirin for phosphorylation to monophosphate by a nucleoside phosphotransferase. However, the final and main phosphorylation product of ribavirin is triphosphate. It is suggested that ribavirin triphosphate is the antiviral form and that it acts by inhibiting the capping of viral RNAs. This mode of action cannot be applied to the inhibition of PVY, the RNA of which is probably covalently linked to a protein at the 5'-terminus.