Properties of the tobacco mosaic virus intermediate length RNA-2 and its translation

Differential use of 3'CITEs by the subgenomic RNA of Pea enation mosaic virus 2

The genomic RNA (gRNA) of Pea enation mosaic virus 2 (PEMV2) is the template for p33 and -1 frameshift product p94. The PEMV2 subgenomic RNA (sgRNA) encodes two overlapping ORFs, p26 and p27, which are required for movement and stability of the gRNA. Efficient translation of p33 requires two of three 3' proximal cap-independent translation enhancers (3'CITEs): the kl-TSS, which binds ribosomes and engages in a long-distance interaction with the 5'end; and the adjacent eIF4E-binding PTE. Unlike the gRNA, all three 3'CITEs were required for efficient translation of the sgRNA, which included the ribosome-binding 3'TSS. A hairpin in the 5' proximal coding region of p26/p27 supported translation by the 3'CITEs by engaging in a long-distance RNA:RNA interaction with the kl-TSS. These results strongly suggest that the 5' ends of PEMV2 gRNA and sgRNA connect with the 3'UTR through similar long-distance interactions while having different requirements for 3'CITEs.

Plant-based vaccines: novel and low-cost possible route for Mediterranean innovative vaccination strategies

A plant bioreactor has enormous capability as a system that supports many biological activities, that is, production of plant bodies, virus-like particles (VLPs), and vaccines. Foreign gene expression is an efficient mechanism for getting protein vaccines against different human viral and nonviral diseases. Plants make it easy to deal with safe, inexpensive, and provide trouble-free storage. The broad spectrum of safe gene promoters is being used to avoid risk assessments. Engineered virus-based vectors have no side effect. The process can be manipulated as follows: (a) retrieve and select gene encoding, use an antigenic protein from GenBank and/or from a viral-genome sequence, (b) design and construct hybrid-virus vectors (viral vector with a gene of interest) eventually flanked by plant-specific genetic regulatory elements for constitutive expression for obtaining chimeric virus, (c) gene transformation and/or transfection, for transient expression, into a plant-host model, that is, tobacco, to get protocols processed positively, and then moving into edible host plants, (d) confirmation of protein expression by bioassay, PCR-associated tests (RT-PCR), Northern and Western blotting analysis, and serological assay (ELISA), (e) expression for adjuvant recombinant protein seeking better antigenicity, (f) extraction and purification of expressed protein for identification and dosing, (g) antigenicity capability evaluated using parental or oral delivery in animal models (mice and/or rabbit immunization), and (h) growing of construct-treated edible crops in protective green houses. Some successful cases of heterologous gene-expressed protein, as edible vaccine, are being discussed, that is, hepatitis C virus (HCV). R9 mimotope, also named hypervariable region 1 (HVR1), was derived from the HVR1 of HCV. It was used as a potential neutralizing epitope of HCV. The mimotope was expressed using cucumber mosaic virus coat protein (CP), alfalfa mosaic virus CP P3/RNA3, and tobacco mosaic virus (TMV) CP-tobacco mild green mosaic virus (TMGMV) CP as expression vectors into tobacco plants. Expressed recombinant protein has not only been confirmed as a therapeutic but also as a diagnostic tool. Herpes simplex virus 2 (HSV-2), HSV-2 gD, and HSV-2 VP16 subunits were transfected into tobacco plants, using TMV CP-TMGMV CP expression vectors.

GAPDH--a recruits a plant virus movement protein to cortical virus replication complexes to facilitate viral cell-to-cell movement

The formation of virus movement protein (MP)-containing punctate structures on the cortical endoplasmic reticulum is required for efficient intercellular movement of Red clover necrotic mosaic virus (RCNMV), a bipartite positive-strand RNA plant virus. We found that these cortical punctate structures constitute a viral replication complex (VRC) in addition to the previously reported aggregate structures that formed adjacent to the nucleus. We identified host proteins that interacted with RCNMV MP in virus-infected Nicotiana benthamiana leaves using a tandem affinity purification method followed by mass spectrometry. One of these host proteins was glyceraldehyde 3-phosphate dehydrogenase-A (NbGAPDH-A), which is a component of the Calvin-Benson cycle in chloroplasts. Virus-induced gene silencing of NbGAPDH-A reduced RCNMV multiplication in the inoculated leaves, but not in the single cells, thereby suggesting that GAPDH-A plays a positive role in cell-to-cell movement of RCNMV. The fusion protein of NbGAPDH-A and green fluorescent protein localized exclusively to the chloroplasts. In the presence of RCNMV RNA1, however, the protein localized to the cortical VRC as well as the chloroplasts. Bimolecular fluorescence complementation assay and GST pulldown assay confirmed in vivo and in vitro interactions, respectively, between the MP and NbGAPDH-A. Furthermore, gene silencing of NbGAPDH-A inhibited MP localization to the cortical VRC. We discuss the possible roles of NbGAPDH-A in the RCNMV movement process.

Localization by immunogold cytochemistry of the virus-coded 30K protein in plasmodesmata of leaves infected with tobacco mosaic virus

The 30K protein of tobacco mosaic virus (TMV) was localized to the plasmodesmata of infected tobacco leaves by immunogold cytochemistry. This protein has been reported to be in the nuclear fraction of TMV-infected protoplasts, but as it has been proposed to function in cell-to-cell transport of virus, probably via the plasmodesmata, intact tissue was investigated with particular attention directed to plasmodesmata and nuclei. Thin sections were made from leaves mechanically inoculated with TMV at different times. Affinity-purified antibodies against a synthetic peptide corresponding to the C-terminal sequence of the 30K protein were used in the incubations, and parallel sections were incubated with antibodies against TMV. The 30K protein label accumulated inside the plasmodesmata, with a maximum 24 hr after inoculation. No specific label was found in the nuclei or at any other site in the cells.

Replication of tobacco mosaic virus. VIII. Characterization of a third subgenomic TMV RNA

In an earlier study we concluded that tobacco mosaic virus (TMV) infections engender a third subgenomic RNA in infected tissue (P. Palukaitis, F. Garcia-Arenal, M. A. Sulzinski, and M. Zaitlin (1983), Virology 131, 533-545). This RNA of approximate MW of 1.1 x 10(6), termed I1-RNA, was shown to be polyribosome-associated and thus was presumed to serve as a messenger RNA in vivo. Upon in vitro translation of I1-RNA in a rabbit reticulocyte lysate system, a major product of MW approximately 50K was generated. When RNA isolated from polyribosomes of infected tissues was analyzed with clones representing distinct regions of the TMV genome, the I1-RNA was shown to be a subset of the TMV genome, representing the 3'-half of the molecule. A TMV-specific DNA fragment (from a phage M13 clone) containing sequences overlapping the 5' end of I1-RNA was used in nuclease S1-mapping experiments with TMV-RNAs isolated from polyribosomes. I1-RNA was thus shown to be a distinct RNA species and not a class of heterogeneous molecules of approximately the same size. The I1-RNA 5' terminus is residue 3405 in the genome. Based on these findings and on consideration of the TMV-RNA sequence, we propose a model for the translation of I1-RNA: after an untranslated sequence of 90 bases, an AUG codon at residues 3495-3497 initiates a protein of MW 54K, terminating at residue 4915. Thus, the amino acid sequence of the 54K protein is coincident with those residues of the carboxy terminus of the well-known 183K TMV protein.

Identification of the 30K protein of TMV by immunoprecipitation with antibodies directed against a synthetic peptide

A synthetic hexadecapeptide corresponding to the predicted C-terminal sequence of the 30K protein of TMV has been coupled to bovine serum albumin and used to raise antibodies in rabbits. The resulting antiserum reacted with the 30K protein translated in vitro. We report the use of this antiserum in the first detection of the 30K protein in vivo, in TMV-infected tobacco protoplasts. Several proteins, the so called family of 30K-related peptides, were immunoprecipitated among in vitro translation products, but only the 30K protein was immunoprecipitated from TMV-infected protoplasts.

Synthesis of TMV-specific RNAs and proteins at the early stage of infection in tobacco protoplasts: transient expression of the 30K protein and its mRNA

All four TMV-coded proteins (180K, 130K, 30K, and coat) and corresponding mRNAs were detected in TMV-infected protoplasts. The 30K protein and its mRNA were synthesized between 2 and 9 hr postinoculation, while the other proteins and their mRNAs (the CP mRNA, the genomic RNA) were synthesized continuously. The results indicated that the synthesis of the two subgenomic RNAs (the CP mRNA, the 30K protein mRNA) is regulated by different mechanisms.

Nonstructural alfalfa mosaic virus RNA-coded proteins present in tobacco leaf tissue

The proteins synthesized under the direction of alfalfa mosaic virus RNAS in tobacco leaves have been examined under conditions of suppressed host protein synthesis. Besides the coat protein we could detect a 22K (K = apparent molecular weight in thousands), a 35K, and a set of 54K proteins. The 22K protein is serologically related to the coat protein. The 35K protein comigrated with the 35K protein whose synthesis is directed by RNA 3 in vitro The 54K proteins are serologically related to the 35K protein produced in vitro. Readthrough products of the 35K protein cistron into the coat protein cistron have been found previously in wheat germ extracts programmed with RNA 3. Two of these proteins comigrate with the 54K proteins synthesized in vivo. Since the 35K and the coat protein cistrons are read in different reading frames the formation of readthrough products is puzzling. In viruses with a tripartite genome the subgenomic mRNA for coat protein, RNA 4, is not known to be replicated as a separate genome entity. This might indicate that proteins synthesized by readthrough into the coat protein cistron play an essential role during replication, especially in the earliest phases.

Cap-independent translation of plant viral RNAs

The RNAs of many plant viruses lack a 5' cap and must be translated by a cap-independent mechanism. Here, we discuss the remarkably diverse cap-independent translation elements that have been identified in members of the Potyviridae, Luteoviridae, and Tombusviridae families, and genus Tobamovirus. Many other plant viruses have uncapped RNAs but their translation control elements are uncharacterized. Cap-independent translation elements of plant viruses differ strikingly from those of animal viruses: they are smaller (<200 nt), some are located in the 3' untranslated region, some require ribosome scanning from the 5' end of the mRNA, and the 5' UTR elements are much less structured than those of animal viruses. We discuss how these elements may interact with host translation factors, and speculate on their mechanism of action and their roles in the virus replication cycle. Much remains to be learned about how these elements enable plant viruses to usurp the host translational machinery.

Evidence for contribution of an internal ribosome entry site to intercellular transport of a tobamovirus

Previously, it has been shown that tobacco mosaic virus (TMV) U1 and crucifer-infecting TMV contain a 75 nt internal ribosome entry site (IRES) upstream of movement protein (MP) gene (IRES(U1)(MP,75) and IRES(CR)(MP,75), respectively). A movement-deficient TMV mutant, KK6, has been constructed previously [Lehto, K., Grantham, G. L. & Dawson, W. O. (1990). Virology 174, 145-157] by insertion of the second coat protein subgenomic promoter (CP SGP-2) upstream of the MP gene, in addition to the natural CP SGP-1. Here, the authors compare the efficiency of movement function expression by KK6 and a derivative, K86, obtained by insertion of IRES(CR)(MP,75) between the CP SGP-2 and MP genes resulting in restoration of IRES(CR)(MP,75) function in the 5'-untranslated sequence of the I(2) subgenomic RNA of K86. The data indicate that the efficiency of K86 movement was largely restored by this insertion, which was apparently due to the translation-enhancing ability of IRES(CR)(MP,75).

Tobacco mosaic virus, not just a single component virus anymore

Summary Taxonomy: Tobacco mosaic virus (TMV) is the type species of the Tobamovirus genus and a member of the alphavirus-like supergroup. Historically, many tobamoviruses are incorrectly called strains of TMV, although they can differ considerably in sequence similarities and host range from each other and from TMV. Physical properties: TMV virions are 300 x 18 nm rods with a central hollow cavity (Fig. 1) and are composed of 95% capsid protein (CP), and 5% RNA. Each CP subunit interacts with 3-nts in a helical arrangement around the RNA. Virions are stable for decades; infectivity in sap survives heating to 90 degrees C. Hosts: The natural host range of TMV is limited; however, a broad range of weed and crop species, mostly Solanaceae that includes tobacco, pepper and tomato can be infected experimentally [Holmes, F.O. (1946) A comparison of the experimental host ranges of tobacco etch and tobacco mosaic viruses. Phytopathology, 36, 643-657]. TMV distribution is worldwide. No biological vectors are known. Useful website: http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/71010001.htm.

Detection and subcellular localization of the turnip yellow mosaic virus 66K replication protein in infected cells

Turnip yellow mosaic virus (TYMV) encodes a 206-kDa (206K) polyprotein with domains of methyltransferase, proteinase, NTPase/helicase, and RNA-dependent RNA polymerase (RdRp). In vitro, the 206K protein has been shown to undergo proteolytic processing, giving rise to the synthesis of 140-kDa (140K) and 66-kDa (66K) proteins, the latter comprising the RdRp protein domain. Antibodies were raised against the 66K protein and were used to detect the corresponding viral protein in infected cells; both leaf tissues and protoplasts were examined. The antiserum specifically recognized a protein of approximately 66 kDa, indicating that the cleavage observed in vitro is also functional in vivo. The 66K protein accumulates transiently during protoplast infection and localizes to cellular membrane fractions. Indirect immunofluorescence assays and electron microscopy of immunogold-decorated ultrathin sections of infected leaf tissue using anti-66K-specific antibody revealed labeling of membrane vesicles located at the chloroplast envelope.

Cellular targets of functional and dysfunctional mutants of tobacco mosaic virus movement protein fused to green fluorescent protein

Intercellular transport of tobacco mosaic virus (TMV) RNA involves the accumulation of virus-encoded movement protein (MP) in plasmodesmata (Pd), in endoplasmic reticulum (ER)-derived inclusion bodies, and on microtubules. The functional significance of these interactions in viral RNA (vRNA) movement was tested in planta and in protoplasts with TMV derivatives expressing N- and C-terminal deletion mutants of MP fused to the green fluorescent protein. Deletion of 55 amino acids from the C terminus of MP did not interfere with the vRNA transport function of MP:GFP but abolished its accumulation in inclusion bodies, indicating that accumulation of MP at these ER-derived sites is not a requirement for function in vRNA intercellular movement. Deletion of 66 amino acids from the C terminus of MP inactivated the protein, and viral infection occurred only upon complementation in plants transgenic for MP. The functional deficiency of the mutant protein correlated with its inability to associate with microtubules and, independently, with its absence from Pd at the leading edge of infection. Inactivation of MP by N-terminal deletions was correlated with the inability of the protein to target Pd throughout the infection site, whereas its associations with microtubules and inclusion bodies were unaffected. The observations support a role of MP-interacting microtubules in TMV RNA movement and indicate that MP targets microtubules and Pd by independent mechanisms. Moreover, accumulation of MP in Pd late in infection is insufficient to support viral movement, confirming that intercellular transport of vRNA relies on the presence of MP in Pd at the leading edge of infection.

Mapping of the Tobacco mosaic virus movement protein and coat protein subgenomic RNA promoters in vivo

The Tobacco mosaic virus movement protein (MP) and coat protein (CP) are expressed from 3'-coterminal subgenomic RNAs (sgRNAs). The transcription start site of the MP sgRNA, previously mapped to positions 4838 (Y. Watanabe, T. Meshi, and Y. Okada (1984), FEBS Lett. 173, 247-250) and 4828 (K. Lehto, G. L. Grantham, and W. O. Dawson (1990), Virology 174, 145-157) for the TMV OM and U1 strains, respectively, has been reexamined and mapped to position 4838 for strain U1. Sequences of the MP and CP sgRNA promoters were delineated by deletion analysis. The boundaries for minimal and full MP sgRNA promoter activity were localized between -35 and +10 and -95 and +40, respectively, relative to the transcription start site. The minimal CP sgRNA promoter was mapped between -69 and +12, whereas the boundaries of the fully active promoter were between -157 and +54. Computer analysis predicted two stem-loop structures (SL1 and SL2) upstream of the MP sgRNA transcription start site. Deletion analysis and site-directed mutagenesis suggested that SL1 secondary structure, but not its sequence, was required for MP sgRNA promoter activity, whereas a 39-nt deletion removing most of the SL2 region increased MP sgRNA accumulation fourfold. Computer-predicted folding of the fully active CP sgRNA promoter revealed one long stem-loop structure. Deletion analysis suggested that the upper part of this stem-loop, located upstream of the transcription start site, was essential for transcription and that the lower part of the stem had an enhancing role.

Internal initiation of translation directed by the 5'-untranslated region of the tobamovirus subgenomic RNA I(2)

Previously we reported that, unlike RNA of typical tobamoviruses, the translation of the coat protein (CP) gene of a crucifer-infecting tobamovirus (crTMV) in vitro occurred by an internal ribosome entry mechanism mediated by the 148-nt region that contained an internal ribosome entry site (IRES(CP,148)(CR)). The equivalent 148-nt sequence from TMV U1 RNA (U1(CP,148)(SP)) was incapable of promoting internal initiation. In the present work, we have found that the 228-nt region upstream of the movement protein (MP) gene of crTMV RNA (IRES(MP,228)(CR)) contained an IRES element that directed in vitro translation of the 3'-proximal reporter genes from chimeric dicistronic transcripts. Surprisingly, the equivalent 228-nt sequence upstream from the MP gene of TMV U1 directed translation of the downstream gene of a dicistronic transcripts as well. Consequently this sequence was termed IRES(MP,228)(U1). It was shown that IRES(MP,228)(CR), IRES(MP,228)(U1), and IRES(CP,148)(CR) could mediate expression of the 3'-proximal GUS gene from dicistronic 35S promoter-based constructs in vivo in experiments on transfection of tobacco protoplasts and particle bombardment of Nicotiana benthamiana leaves. The results indicated that an IRES element was located within the 75-nt region upstream of MP gene (IRES(MP,75)), which corresponded closely to the length of the 5'UTR of TMV subgenomic RNA (sgRNA) I(2). The RNA transcripts structurally equivalent to I(2) sgRNAs of TMV U1 and crTMV, but containing a hairpin structure (H) immediately upstream of IRES(MP,75) (HIRES(MP), (75)(CR)-MP-CP-3'UTR; HIRES(MP,75)(U1)-MP-CP-3'UTR), were able to express the MP gene in vitro. The capacity of HIRES(MP,75)(CR) sequence for mediating internal translation of the 3'-proximal GUS gene in vivo, in tobacco protoplasts, was demonstrated. We suggested that expression of the MP gene from I(2) sgRNAs might proceed via internal ribosome entry pathway mediated by IRES(MP) element contained in the 75-nt 5'UTR. Our results admit that a ribosome scanning mechanism of the MP gene expression from I(2) sgRNA operates concurrently.

Replication of tobacco mosaic virus RNA

The replication of tobacco mosaic virus (TMV) RNA involves synthesis of a negative-strand RNA using the genomic positive-strand RNA as a template, followed by the synthesis of positive-strand RNA on the negative-strand RNA templates. Intermediates of replication isolated from infected cells include completely double-stranded RNA (replicative form) and partly double-stranded and partly single-stranded RNA (replicative intermediate), but it is not known whether these structures are double-stranded or largely single-stranded in vivo. The synthesis of negative strands ceases before that of positive strands, and positive and negative strands may be synthesized by two different polymerases. The genomic-length negative strand also serves as a template for the synthesis of subgenomic mRNAs for the virus movement and coat proteins. Both the virus-encoded 126-kDa protein, which has amino-acid sequence motifs typical of methyltransferases and helicases, and the 183-kDa protein, which has additional motifs characteristic of RNA-dependent RNA polymerases, are required for efficient TMV RNA replication. Purified TMV RNA polymerase also contains a host protein serologically related to the RNA-binding subunit of the yeast translational initiation factor, eIF3. Study of Arabidopsis mutants defective in RNA replication indicates that at least two host proteins are needed for TMV RNA replication. The tomato resistance gene Tm-1 may also encode a mutant form of a host protein component of the TMV replicase. TMV replicase complexes are located on the endoplasmic reticulum in close association with the cytoskeleton in cytoplasmic bodies called viroplasms, which mature to produce 'X bodies'. Viroplasms are sites of both RNA replication and protein synthesis, and may provide compartments in which the various stages of the virus mutiplication cycle (protein synthesis, RNA replication, virus movement, encapsidation) are localized and coordinated. Membranes may also be important for the configuration of the replicase with respect to initiation of RNA synthesis, and synthesis and release of progeny single-stranded RNA.

Historical overview of research on the tobacco mosaic virus genome: genome organization, infectivity and gene manipulation

Early in the development of molecular biology, TMV RNA was widely used as a mRNA [corrected] that could be purified easily, and it contributed much to research on protein synthesis. Also, in the early stages of elucidation of the genetic code, artificially produced TMV mutants were widely used and provided the first proof that the genetic code was non-overlapping. In 1982, Goelet et al. determined the complete TMV RNA base sequence of 6395 nucleotides. The four genes (130K, 180K, 30K and coat protein) could then be mapped at precise locations in the TMV genome. Furthermore it had become clear, a little earlier, that genes located internally in the genome were expressed via subgenomic mRNAs. The initiation site for assembly of TMV particles was also determined. However, although TMV contributed so much at the beginning of the development of molecular biology, its influence was replaced by that of Escherichia coli and its phages in the next phase. As recombinant DNA technology developed in the 1980s, RNA virus research became more detached from the frontier of molecular biology. To recover from this setback, a gene-manipulation system was needed for RNA viruses. In 1986, two such systems were developed for TMV, using full-length cDNA clones, by Dawson's group and by Okada's group. Thus, reverse genetics could be used to elucidate the basic functions of all proteins encoded by the TMV genome. Identification of the function of the 30K protein was especially important because it was the first evidence that a plant virus possesses a cell-to-cell movement function. Many other plant viruses have since been found to encode comparable 'movement proteins'. TMV thus became the first plant virus for which structures and functions were known for all its genes. At the birth of molecular plant pathology, TMV became a leader again. TMV has also played pioneering roles in many other fields. TMV was the first virus for which the amino acid sequence of the coat protein was determined and first virus for which cotranslational disassembly was demonstrated both in vivo and in vitro. It was the first virus for which activation of a resistance gene in a host plant was related to the molecular specificity of a product of a viral gene. Also, in the field of plant biotechnology, TMV vectors are among the most promising. Thus, for the 100 years since Beijerinck's work, TMV research has consistently played a leading role in opening up new areas of study, not only in plant pathology, but also in virology, biochemistry, molecular biology, RNA genetics and biotechnology.

Distribution of TMV movement protein in single living protoplasts immobilized in agarose

Recent studies of the tobacco mosaic virus (TMV) P30 movement protein (MP) fused with green fluorescent protein (GFP) during TMV infection described the involvement of elements of the cytoskeleton and components of the endoplasmic reticulum (ER) in the intracellular trafficking of MP:GFP from the sites of synthesis in the cytoplasm to plasmodesmata. To examine in real-time the pattern of synthesis, accumulation and degradation of MP:GFP, we developed a method to immobilize protoplasts in agarose such that they are maintained alive for extended periods of time. The pattern of MP:GFP accumulation in single living protoplasts visualized by confocal laser scanning microscopy (CLSM) was parallel to that previously described in a population of protoplasts harvested at different times post-infection. Additionally, a network of weakly fluorescent filaments, which are apparently different from microtubules, was observed to surround the nucleus and these filaments were associated with fluorescent bodies (previously identified as ER-derived structures). Later in infection, the fluorescent bodies increased in size and coalesced to form larger structures that accumulated near the periphery of the cells while highly fluorescent non-cortical filaments were observed distributed in the cytoplasm. The putative involvement of these filaments in targeting the fluorescent bodies to the periphery of the cell is discussed. Studies of single, embedded protoplasts make it possible to observe changes in amount and subcellular localization of viral and other proteins.

Gene expression from viral RNA genomes

This review is centered on the major strategies used by plant RNA viruses to produce the proteins required for virus multiplication. The strategies at the level of transcription presented here are synthesis of mRNA or subgenomic RNAs from viral RNA templates, and 'cap-snatching'. At the level of translation, several strategies have been evolved by viruses at the steps of initiation, elongation and termination. At the initiation step, the classical scanning mode is the most frequent strategy employed by viruses; however in a vast number of cases, leaky scanning of the initiation complex allows expression of more than one protein from the same RNA sequence. During elongation, frameshift allows the formation of two proteins differing in their carboxy terminus. At the termination step, suppression of termination produces a protein with an elongated carboxy terminus. The last strategy that will be described is co- and/or post-translational cleavage of a polyprotein precursor by virally encoded proteinases. Most (+)-stranded RNA viruses utilize a combination of various strategies.

In vivo phosphorylation of the 30-kDa protein of tobacco mosaic virus

The 30-kDa protein of tobacco mosaic virus, which is involved in cell-to-cell movement function, is phosphorylated in tobacco protoplasts. To investigate which portion of the protein is phosphorylated we inoculated several truncated 30-kDa protein mutants into protoplasts and determined whether or not those truncated proteins are phosphorylated. The results showed that amino acid residues 234-261 of the 30-kDa protein are required for this phosphorylation.

The P30 movement protein of tobacco mosaic virus is a single-strand nucleic acid binding protein

The P30 protein of tobacco mosaic virus (TMV) is required for cell to cell movement of viral RNA, which presumably occurs through plant intercellular connections, the plasmodesmata. The mechanism by which P30 mediates transfer of TMV RNA molecules through plasmodesmata channels is unknown. We have identified P30 as an RNA and single-stranded (ss) DNA binding protein. Binding of purified P30 to ss nucleic acids is strong, highly cooperative, and sequence nonspecific with a minimal binding site of 4-7 nucleotides per P30 monomer. In-frame deletions across P30 were used to localize the ss nucleic acid binding domain to within amino acid residues 65-86 of the protein. We propose that binding of P30 to TMV RNA creates an unfolded protein-RNA complex that functions as an intermediate in virus cell to cell movement through plasmodesmata.

Insertion of sequences containing the coat protein subgenomic RNA promoter and leader in front of the tobacco mosaic virus 30K ORF delays its expression and causes defective cell-to-cell movement

The regulation of the internal open reading frame (ORF) of tobacco mosaic virus (TMV) that encodes the 30K movement protein was examined by constructing mutants in vitro with the putative coat protein subgenomic RNA promoter and leader sequences inserted upstream of the 30K ORF. A mutant with a 49-nucleotide fragment of the promoter region inserted replicated only transiently before being overtaken by a progeny wild-type virus with the insert deleted. A mutant with a 253-nucleotide promoter region fragment inserted replicated stably, and the inserted promoter was active in its new location. The production of 30K protein was not enhanced by this promoter/leader insertion to a level similar to that of coat protein. However, the accumulation of 30K protein was delayed, suggesting that different promoters/leader sequences determine the time of expression of the genes. This mutant was deficient in movement. A similar mutant, but with increased production of 30K protein, overcame the movement deficiency, suggesting that 30K protein is needed during the early stages of infection for efficient cell-to-cell movement of the virus.

Regulation of tobamovirus gene expression

This chapter discusses tobacco mosaic virus (TMV) strains U1, OM, L, CGMMV, 0, and Cc. The production of each TMV protein is regulated differently, both in amounts and times of production. The chapter discusses some of the strategies that tobamoviruses uses to control gene expression: (1) different subgenomic RNA promoter/leader sequences control timing of expression of genes, (2) genes expressed via subgenomic mRNAs are expressed in decreasing amounts with increasing distances from the 3' terminus, and (3) TMV mRNAs appear to be translationally regulated differently from host mRNAs. Genome organization affects gene expression, but it appears to be equally important for the efficiency of replication and the ability of the genomic structure to be stably propagated. Different virus groups have evolved different gene arrangements. Tobamovirus genes expressed via subgenomic mRNAs appear to be expressed in increasing amounts when positioned nearer the 3’ terminus.

Changing the start codon context of the 30K gene of tobacco mosaic virus from "weak" to "strong" does not increase expression

The translation initiation region of the 30K gene of tobacco mosaic virus (TMV) was modified by in vitro mutagenesis to create more optimal start codon contexts. A complicating factor was that modifications in this region also altered the 3' terminus of the 183K ORF that overlaps the 30K ORF. An insertion of GACUCGA between nucleotides 4901 and 4902 resulted in a purine (G) in position -3 relative to the AUG creating a "stronger" start codon context, but this also changed the last four amino acids of the 183K protein. This mutant was infectious, replicated efficiently, but produced reduced amounts of 30K protein. Despite the reduced amount of movement protein, this mutant spread effectively from cell to cell and had a phenotype indistinguishable from that of wild-type virus. A more conservative mutation inserted GAC between TMV nucleotides 4901 and 4902 resulting in a "strong" start codon context (ACGAUGG) and modification of the 183K protein only by insertion of an aspartic acid adjacent to a native aspartic acid. This modification did not enhance the production of 30K protein. These data demonstrate consensus sequences that are optimal for other eukaryotic systems did not cause increased expression of the 30K gene in vivo. The modified sequences of both mutants were stably maintained during relatively long periods of replication. Even though each mutant replicated efficiently, when mixed with wild-type TMV, neither mutant effectively competed with the wild-type virus. Another mutant which removed the native 30K AUG to determine whether subsequent internal start codons with "stronger" contexts would function in its absence was constructed. However, this mutant and a mutant that fused the 183K reading frame to the 30K reading frame did not replicate and move in intact plants.

The expression of the TMV-specific 30-kDa protein in tobacco protoplasts is strongly and selectively enhanced by actinomycin

The TMV-encoded 30-kDa protein has been implicated in the cell-to-cell transport of TMV in the infected plant. The polyethylene glycol-mediated inoculation of tobacco protoplasts with TMV particles and TMV RNA was used to compare the time courses of the viral 30-kDa protein synthesis in vivo. Upon infection of protoplasts with TMV RNA, the synthesis of the viral 30-kDa protein starts after 4 to 6 hr, has its maximum after 8 to 10 hr, and decreases. After inoculation of protoplasts with TMV, however, the start of the viral 30-kDa protein synthesis and its maximum are delayed by 2 hr, followed by the same decrease. We show that actinomycin D dramatically stimulates the synthesis of the 30-kDa protein by up to 2 orders of magnitude, whereas the synthesis of the viral 126 kDa, the 183 kDa, and the coat protein is increased only by a factor of 2. Surprisingly, actinomycin V is twice as active as actinomycin D, whereas actinomycin I is nearly inactive. The specific stimulation of the 30-kDa synthesis by actinomycin D in vivo depends neither on the Nicotiana variety nor on the TMV strain used. Final evidence that the 30-kDa protein is truly TMV-derived is provided by the slightly different electrophoretic mobilities of the 30-kDa proteins encoded by TMV strains vulgare, dahlemense, and U2. The identification of the 30-kDa protein in two-dimensional gels was achieved for the first time by a combination of ionic and nonionic detergents for the solubilization of the 30-kDa protein and by the specific stimulation of its synthesis by actinomycin D. The mechanism of the strong and selective actinomycin effect on the viral 30-kDa protein synthesis in vivo is as yet obscure. Actinomycin does not appear to act directly on viral protein biosynthesis, since it neither stimulates the 30-kDa synthesis upon translation of TMV RNA in vitro nor alters the ratio of the products. Actinomycin may rather act by inhibiting selectively the synthesis of a host factor whose synthesis starts at least 4 hr after TMV infection and which strongly inhibits the expression of the viral 30-kDa transport protein.

Characterization of Tm-1 gene action on replication of common isolates and a resistance-breaking isolate of TMV

Tm-1 is a gene which confers resistance to infection, in tomatoes, by tobacco mosaic virus (TMV). To investigate the biochemical mechanism of the resistance, we have established cell suspensions of three lines of tomatoes, i.e., +/+ (susceptible, wild-type, no Tm-1 gene), Tm-1/+ (heterozygous for the Tm-1 gene), and Tm-1/Tm-1 (homozygous for the Tm-1 gene). Protoplasts isolated from these cells were inoculated with RNA of the tomato strain L and Lta1 (a resistance-breaking strain which was recently isolated spontaneously from L) of TMV by means of electrophoration. The syntheses of all viral-coded proteins and TMV-specific RNAs could be detected in L-inoculated +/+ and Lta1-inoculated +/+, Tm-1/+, Tm-1/Tm-1 protoplasts, while their production was markedly reduced in L-inoculated Tm-1/+ protoplasts. L strain could multiply in Tm-1/+ protoplasts to a greater extent with less delay when a large amount of inoculum RNA was used. However, viral production was completely blocked in Tm-1/Tm-1 protoplasts even when a large amount of L-RNA was used for inoculation.

In vitro transcription and translation of cloned cDNAs encoding the 30-kDa protein gene of TMV

A cDNA clone encoding the nonstructural, 30-kDa protein of the common (U1) strain of tobacco mosaic virus (TMV) was isolated and characterized. cDNA clones representing the intact gene as well as deletions from the 5' end of the gene were subcloned into SP6 vectors. Capped RNAs produced by in vitro transcription reactions were translated in a wheat germ cell-free system. The resultant proteins were compared to proteins obtained from the in vitro translation of intermediate length (I2) rods of TMV. Transcripts of the cDNA clones encoded polypeptides of 30, 28, or 18 kDa that were immunoprecipitated by antibody prepared against a synthetic peptide representing the carboxy terminus of the 30-kDa protein. cDNA clones containing the intact 30-kDa sequence coded for 30-kDa polypeptides while clones lacking the 30-kDa initiation codon produced 28-kDa polypeptides. Surprisingly, translation of a transcript from a cDNA clone containing the 30-kDa gene plus 390 nucleotides 5' of the initiator AUG yielded a polypeptide with an approximate molecular mass of 18 kDa. The results indicate that an intact and functional 30-kDa protein gene has been cloned. The significance of these results, with respect to determining the function of the 30-kDa protein, is discussed.

Subcellular localization of the 30K protein in TMV-inoculated tobacco protoplasts

We investigated the intracellular localization of the 30K protein in TMV-inoculated tobacco protoplasts by means of pulse-labeling and pulse-chase experiments with [35S]methionine. Protoplasts were lysed with a nonionic detergent and the extracts were centrifuged to yield soluble and crude nuclear fractions. Most of the 30K protein was found in the crude nuclear fraction. The nuclear fraction was further purified by centrifugation in a step-wise Percoll gradient. Nuclei and the 30K protein were found in the same fractions. The results of pulse-chase experiments indicated that the 30K protein is synthesized in the soluble fraction and then translocated to the crude nuclear fraction. The 30K protein of Ls1, a temperature-sensitive (ts) mutant affecting the cell-to-cell viral transport function, was also found in the nuclei, even at a nonpermissive temperature. These results suggested that the 30K protein has to be localized in the nuclei to function, and that impaired translocation of the 30K protein to the nuclei is not responsible for the ts lesion of mutant Ls1.

The initiation site for transcription of the TMV 30-kDa protein messenger RNA

The initiation site for transcription of the 30-kDa protein mRNA of tobacco mosaic virus was mapped uniquely at residue 1558 from the 3'-terminus on TMV RNA using the primer-extension and the S1-nuclease mapping method.

Analysis of in vitro and in vivo products of the TMV 30kDa open reading frame using antisera raised against a synthetic peptide

The peptide Tyr-Ser-Glu-Ala-Thr-Val-Ala-Glu-Ser-Asp-Ser-Phe (the predicted C-terminal 11 amino acids of the TMV 30kDa open reading frame plus an additional N-terminal Tyr residue) was synthesized by solid phase methods and used to raise antisera in rabbits. These antisera precipitated 4 major proteins (p30, p28, p19 and 18.5kDa) from in vitro translation products of TMV short rod RNA, but only one, of apparent M r = 30500, from TMV-infected tobacco protoplasts. This protein was made between 8 and 16 h post infection, and had [35S]Met-labelled tryptic peptides identical to those of in vitro synthesized p30.