Virus multiplication in tobacco protoplasts inoculated with tobacco mosaic virus RNA encapsulated in large unilamellar vesicle liposomes

Viruses roll the dice: the stochastic behavior of viral genome molecules accelerates viral adaptation at the cell and tissue levels

Recent studies on evolutionarily distant viral groups have shown that the number of viral genomes that establish cell infection after cell-to-cell transmission is unexpectedly small (1-20 genomes). This aspect of viral infection appears to be important for the adaptation and survival of viruses. To clarify how the number of viral genomes that establish cell infection is determined, we developed a simulation model of cell infection for tomato mosaic virus (ToMV), a positive-strand RNA virus. The model showed that stochastic processes that govern the replication or degradation of individual genomes result in the infection by a small number of genomes, while a large number of infectious genomes are introduced in the cell. It also predicted two interesting characteristics regarding cell infection patterns: stochastic variation among cells in the number of viral genomes that establish infection and stochastic inequality in the accumulation of their progenies in each cell. Both characteristics were validated experimentally by inoculating tobacco cells with a library of nucleotide sequence-tagged ToMV and analyzing the viral genomes that accumulated in each cell using a high-throughput sequencer. An additional simulation model revealed that these two characteristics enhance selection during tissue infection. The cell infection model also predicted a mechanism that enhances selection at the cellular level: a small difference in the replication abilities of coinfected variants results in a large difference in individual accumulation via the multiple-round formation of the replication complex (i.e., the replication machinery). Importantly, this predicted effect was observed in vivo. The cell infection model was robust to changes in the parameter values, suggesting that other viruses could adopt similar adaptation mechanisms. Taken together, these data reveal a comprehensive picture of viral infection processes including replication, cell-to-cell transmission, and evolution, which are based on the stochastic behavior of the viral genome molecules in each cell.

In vitro viral RNA synthesis by a subcellular fraction of TMV-inoculated tobacco protoplasts

A subcellular fraction which can synthesize viral RNA and subgenomic RNA in vitro was prepared from tobacco mosaic virus (TMV)-inoculated tobacco protoplasts. S(1)-Resistant fragment analysis with strand specific TMV cDNA showed that a large amount of plus-stranded and a small amount of minus-stranded, genome-size RNA was synthesized by this subcellular fraction. Plus-stranded subgenomic RNA of coat protein mRNA size was also synthesized. The time course of the appearance of viral RNA synthetic activity was consistent with that of the appearance of TMV infectivity in vivo.

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.

Aggregation of TMV CP plays a role in CP functions and in coat-protein-mediated resistance

Tobacco mosaic virus (TMV) coat protein (CP) in absence of RNA self-assembles into several different structures depending on pH and ionic strength. Transgenic plants that produce self-assembling CP are resistant to TMV infection, a phenomenon referred to as coat-protein-mediated resistance (CP-MR). The mutant CP Thr42Trp (CP(T42W)) produces enhanced CP-MR compared to wild-type CP. To establish the relationship between the formation of 20S CP aggregates and CP-MR, virus-like particles (VLPs) produced by TMV variants that yield high levels of CP-MR were characterized. We demonstrate that non-helical structures are found in VLPs formed in vivo by CP(T42W) but not by wild-type CP and suggest that the mutation shifts the intracellular equilibrium of aggregates from low to higher proportions of non-helical 20S aggregates. A similar shift in equilibrium of aggregates was observed with CP(D77R), another mutant that confers high level of CP-MR. The mutant CP(D50R) confers a level of CP-MR similar to wild-type CP and aggregates in a manner similar to wild-type CP. We conclude that increased CP-MR is correlated with a shift in intracellular equilibrium of CP aggregates, including aggregates that interfere with virus replication.

Tobacco mosaic virus (TMV) and potato virus X (PVX) coat proteins confer heterologous interference to PVX and TMV infection, respectively

Replication of Potato virus X (PVX) was reduced in transgenic protoplasts that accumulated wild-type coat protein (CPWT) of Tobacco mosaic virus (TMV) or a mutant CP, CP(T42W), that produced highly ordered states of aggregation, including pseudovirions. This reaction is referred to as heterologous CP-mediated resistance. However, protoplasts expressing a CP mutant that abolished aggregation and did not produce pseudovirions, CPT28W, did not reduce PVX replication. Similarly, in transgenic tobacco plants producing TMV CPWT or CP(T42W), there was a delay in local cell-to-cell spread of PVX infection that was not observed in CP(T28W) plants or in non-transgenic plants. The results suggest that the quaternary structure of the TMV CP regulates the mechanism(s) of heterologous CP-mediated resistance. Similarly, transgenic protoplasts that produced PVX CP conferred transient protection against infection by TMV RNA. Transgenic plants that accumulated PVX CP reduced the cell-to-cell spread of infection and resulted in a delay in systemic infection following inoculation with TMV or TMV RNA. Heterologous CP-mediated resistance was characterized by a brief delay in systemic infection, whilst homologous CP-mediated resistance conferred reduced or no systemic infection.

Replication of plant RNA virus genomes in a cell-free extract of evacuolated plant protoplasts

The replication of eukaryotic positive-strand RNA virus genomes occurs through a complex process involving multiple viral and host proteins and intracellular membranes. Here we report a cell-free system that reproduces this process in vitro. This system uses a membrane-containing extract of uninfected plant protoplasts from which the vacuoles had been removed by Percoll gradient centrifugation. We demonstrate that the system supported translation, negative-strand RNA synthesis, genomic RNA replication, and subgenomic RNA transcription of tomato mosaic virus and two other plant positive-strand RNA viruses. The RNA synthesis, which depended on translation of the genomic RNA, produced virus-related RNA species similar to those that are generated in vivo. This system will aid in the elucidation of the mechanisms of genome replication in these viruses.

Coat protein regulates formation of replication complexes during tobacco mosaic virus infection

The genome of tobacco mosaic virus (TMV) encodes replicase protein(s), movement protein (MP), and capsid protein (CP). On infection, one or more viral proteins direct the assembly of virus replication complexes (VRCs), in association with host-derived membranes. The impact of CP-mediated resistance on the structures of the replication complexes was examined in nontransgenic and transgenic BY-2 cell lines that produce wild-type CP, mutant CP(T42W), and Ds-Red, which was targeted to endoplasmic reticulum by using immunofluorescence and 3D microscopy. We developed a model of VRCs that shows a clear association of MP with and surrounding the endoplasmic reticulum. Replicase is located within the MP bodies, as well as isolated sites throughout the cell. CP surrounds the VRCs. CP enhances the production of MP and increases the size of the VRC; however, the mutant CP(T42W) reduces the amount of MP and interferes with the formation of VRCs. We propose a regulatory role of the CP in the establishment of the VRC. We suggest that the lack of formation of VRCs restricts the efficiency of virus replication and the formation of virus movement complexes, resulting in restriction of cell-cell spread of infection. This results in higher levels of plant CP-mediated protection provided by CP(T42W).

Sequence elements controlling expression of Barley stripe mosaic virus subgenomic RNAs in vivo

Barley stripe mosaic virus (BSMV) contains three positive-sense, single-stranded genomic RNAs, designated alpha, beta, and gamma, that encode seven major proteins and one minor translational readthrough protein. Three proteins (alphaa, betaa, and gammaa) are translated directly from the genomic RNAs and the remaining proteins encoded on RNAbeta and RNAgamma are expressed via three subgenomic messenger RNAs (sgRNAs). sgRNAbeta1 directs synthesis of the triple gene block 1 (TGB1) protein. The TGB2 protein, the TGB2' minor translational readthrough protein, and the TGB3 protein are expressed from sgRNAbeta2, which is present in considerably lower abundance than sgRNAbeta1. A third sgRNA, sgRNAgamma, is required for expression of the gammab protein. We have used deletion analyses and site-specific mutations to define the boundaries of promoter regions that are critical for expression of the BSMV sgRNAs in infected protoplasts. The results reveal that the sgRNAbeta1 promoter encompasses positions -29 to -2 relative to its transcription start site and is adjacent to a cis-acting element required for RNAbeta replication that maps from -107 to -74 relative to the sgRNAbeta1 start site. The core sgRNAbeta2 promoter includes residues -32 to -17 relative to the sgRNAbeta2 transcriptional start site, although maximal activity requires an upstream hexanucleotide sequence residing from positions -64 to -59. The sgRNAgamma promoter maps from -21 to +2 relative to its transcription start site and therefore partially overlaps the gammaa gene. The sgRNAbeta1, beta2, and gamma promoters also differ substantially in sequence, but have similarities to the putative homologous promoters of other Hordeiviruses. These differences are postulated to affect competition for the viral polymerase, coordination of the temporal expression and abundance of the TGB proteins, and constitutive expression of the gammab protein.

Defective tobamovirus movement protein lacking wild-type phosphorylation sites can be complemented by substitutions found in revertants

We reported previously that the movement protein (MP) of tomato mosaic tobamovirus is phosphorylated, and we proposed that MP phosphorylation is important for viral pathogenesis. Experimental data indicated that phosphorylation enhances the stability of MP in vivo and enables the protein to assume the correct intracellular location to perform its function. A mutant virus designated 37A238A was constructed; this virus lacked two serine residues within the MP, which prevented its phosphorylation. In the present study, we inoculated plants with the 37A238A mutant, and as expected, it was unable to produce local lesions on the leaves. However, after an extended period, we found that lesions did occur, which were due to revertant viruses. Several revertants were isolated, and the genetic changes in their MPs were examined together with any changes in their in vivo characteristics. We found that reversion to virulence was associated first with increased MP stability in infected cells and second with a shift in MP intracellular localization over time. In one case, the revertant MP was not phosphorylated in vivo, but it was functional.

Interactions of the TGB1 protein during cell-to-cell movement of Barley stripe mosaic virus

We have recently used a green fluorescent protein (GFP) fusion to the gammab protein of Barley stripe mosaic virus (BSMV) to monitor cell-to-cell and systemic virus movement. The gammab protein is involved in expression of the triple gene block (TGB) proteins encoded by RNAbeta but is not essential for cell-to-cell movement. The GFP fusion appears not to compromise replication or movement substantially, and mutagenesis experiments demonstrated that the three most abundant TGB-encoded proteins, betab (TGB1), betac (TGB3), and betad (TGB2), are each required for cell-to-cell movement (D. M. Lawrence and A. O. Jackson, Mol. Plant Pathol. 2:65-75, 2001). We have now extended these analyses by engineering a fusion of GFP to TGB1 to examine the expression and interactions of this protein during infection. BSMV derivatives containing the TGB1 fusion were able to move from cell to cell and establish local lesions in Chenopodium amaranticolor and systemic infections of Nicotiana benthamiana and barley. In these hosts, the GFP-TGB1 fusion protein exhibited a temporal pattern of expression along the advancing edge of the infection front. Microscopic examination of the subcellular localization of the GFP-TGB1 protein indicated an association with the endoplasmic reticulum and with plasmodesmata. The subcellular localization of the TGB1 protein was altered in infections in which site-specific mutations were introduced into the six conserved regions of the helicase domain and in mutants unable to express the TGB2 and/or TGB3 proteins. These results are compatible with a model suggesting that movement requires associations of the TGB1 protein with cytoplasmic membranes that are facilitated by the TGB2 and TGB3 proteins.

Tobamovirus replicase coding region is involved in cell-to-cell movement

Tobacco mosaic virus (TMV) encodes a 30-kDa movement protein (MP) which enables viral movement from cell to cell. It is, however, unclear whether the 126- and 183-kDa replicase proteins are involved in the cell-to-cell movement of TMV. In the course of our studies into TMV-R, a strain with a host range different from that of TMV-U1, we have obtained an interesting chimeric virus, UR-hel. The amino acid sequence differences between UR-hel and TMV-U1 are located only in the helicase-like domain of the replicase. Interestingly, UR-hel has a defect in its cell-to-cell movement. The replication of UR-hel showed a level of replication of the genome, synthesis, and accumulation of MP similar to that observed in TMV-U1-inoculated protoplasts. Such observations support the hypothesis that the replicase coding region may in some fashion be involved in cell-to-cell movement of TMV.

Requirements for cell-to-cell movement of Barley stripe mosaic virus in monocot and dicot hosts

Summary The Barley stripe mosaic virus (BSMV) RNAss genome contains a series of overlapping open reading frames termed the triple gene block. The three most abundant proteins, betab, betac and betad, have been shown to have essential roles in infectivity, but their function in cell-to-cell movement has not previously been unambiguously defined, nor has the role of a minor translational read-through protein, betad' been characterized. We have now examined the direct involvement of each of these proteins in cell-to-cell movement in planta by engineering fusions of the green fluorescent protein (GFP) to a cysteine-rich regulatory protein designated gammab. Microscopic examination of inoculated and systemically infected barley and oat leaves revealed high levels of fluorescence that moved rapidly through the compact striate vascular tissue without infecting epidermal cells. In contrast, a radial pattern of fluorescence spread through a large number of epidermal and mesophyll cells before entry into the reticulate vascular tissue of the dicot hosts Nicotiania benthamiana and Chenopodium amaranticolor. Mutational analyses indicated that the betab, betac and betad proteins are each essential for cell-to-cell movement in local lesion and systemic hosts, whereas the betad' protein is dispensable. Collectively, these results demonstrate conclusively that the three major triple gene block-encoded proteins act in concert to mediate cell-to-cell movement of BSMV.

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.

Nuclear localization of turnip crinkle virus movement protein p8

Turnip crinkle virus (TCV) is a single-stranded positive-sense RNA virus of the Carmovirus genus. Two of its five open reading frames (ORFs), encoding proteins of 8 and 9 kDa, are required for cell-to-cell movement of the virus. These movement proteins (MPs) were fused to green fluorescent protein (GFP) to determine their cellular localization. In protoplasts, p9-GFP, like GFP itself, is found throughout the cytoplasm, as well as in cell nuclei. In contrast, p8-GFP was confined to the cell nucleus. Similar localization patterns were observed when specific small peptide epitopes were fused to p8 and p9 proteins instead of GFP. The cytoplasmic localization of p9-GFP and nuclear localization of p8-GFP were also detected in leaves after particle bombardment of DNA encoding these fusion proteins or after overexpression of p8-GFP in transgenic Arabidopsis seedlings. The expression of the GFP fusion proteins by recombinant TCV viruses in infected protoplasts or on inoculated Arabidopsis leaves produced similar patterns. Unlike TMV-MP and other MPs studied to date, no obvious punctuate expression in the cell wall or association with the cytoskeleton was detected. The sequence analysis of p8 revealed two unique nuclear localization signals (NLSs), which were not conserved within p8 homologues of other viruses in the genus Carmovirus. Mutation in either of these NLSs did not disrupt the nuclear localization of p8-GFP. However, when both NLSs were mutated, p8-GFP expression was no longer restricted to cell nuclei. The NLSs are not required for cell-to-cell movement; TCV recombinant viruses mutated in one or both NLSs could still facilitate cell-to-cell movement of the virus. The nuclear localization of p8 suggests a novel function for this protein in the cell nucleus.

Arabinogalactan protein and wall-associated kinase in a plasmalemmal reticulum with specialized vertices

Arabinogalactan protein and wall-associated kinase (WAK) are suspected to be regulatory players at the interface between cytoplasm and cell wall. Both WAK(s) and arabinogalactan shown likely to represent arabinogalactan protein(s) have been visualized there with computational optical-sectioning microscopy. The arabinogalactan occurs in a polyhedral array at the external face of the cell membrane. WAK, and other proteins as yet unidentified, appear to fasten the membrane to the wall at vertices of the array. Evidence is presented that the array bears an important part of the mechanical stress experienced by the membrane, and it is speculated that the architectural organization of arabinogalactan protein, WAK, and other components of the array is critical for coordination of endomembrane activities, growth, and differentiation. The array has been named the plasmalemmal reticulum.

Degradation of tobacco mosaic virus movement protein by the 26S proteasome

Cell-to-cell spread of tobacco mosaic virus is facilitated by the virus-encoded 30-kDa movement protein (MP). This process involves interaction of viral proteins with host components, including the cytoskeleton and the endoplasmic reticulum (ER). During virus infection, high-molecular-weight forms of MP were detected in tobacco BY-2 protoplasts. Inhibition of the 26S proteasome by MG115 and clasto-lactacystin-beta-lactone enhanced the accumulation of high-molecular-weight forms of MP and led to increased stability of the MP. Such treatment also increased the apparent accumulation of polyubiquitinated host proteins. By fusion of MP with the jellyfish green fluorescent protein (GFP), we demonstrated that inhibition of the 26S proteasome led to accumulation of the MP-GFP fusion preferentially on the ER, particularly the perinuclear ER. We suggest that polyubiquitination of MP and subsequent degradation by the 26S proteasome may play a substantial role in regulation of virus spread by reducing the damage caused by the MP on the structure of cortical ER.

Phosphorylation and/or presence of serine 37 in the movement protein of tomato mosaic tobamovirus is essential for intracellular localization and stability in vivo

The P30 movement protein (MP) of tomato mosaic tobamovirus (ToMV) is synthesized in the early stages of infection and is phosphorylated in vivo. Here, we determined that serine 37 and serine 238 in the ToMV MP are sites of phosphorylation. MP mutants in which serine was replaced by alanine at positions 37 and 238 (LQ37A238A) or at position 37 only (LQ37A) were not phosphorylated, and mutant viruses did not infect tobacco or tomato plants. By contrast, mutation of serine 238 to alanine did not affect the infectivity of the virus (LQ238A). To investigate the subcellular localization of mutant MPs, we constructed viruses that expressed each mutant MP fused with the green fluorescent protein (GFP) of Aequorea victoria. Wild-type and mutant LQ238A MP fusion proteins showed distinct temporally regulated patterns of MP-GFP localization in protoplasts and formation of fluorescent ring-shaped infection sites on Nicotiana benthamiana. However mutant virus LQ37A MP-GFP did not show a distinct pattern of localization or formation of fluorescent rings. Pulse-chase experiments revealed that MP produced by mutant virus LQ37A was less stable than wild-type and LQ238A MPs. MP which contained threonine at position 37 was phosphorylated, but the stability of the MP in vivo was very low. These studies suggest that the presence of serine at position 37 or phosphorylation of serine 37 is essential for intracellular localization and stability of the MP, which is necessary for the protein to function.

Interaction of tobamovirus movement proteins with the plant cytoskeleton

The movement protein of tobacco mosaic tobamovirus and related viruses is essential for the cell-to-cell spread of infection and, in part, determines the host range of the virus. Movement protein (MP) was fused with the jellyfish green fluorescent protein (GFP), and a modified virus that contained this MP:GFP fusion protein retained infectivity. In protoplasts and leaf tissues, the MP:GFP fusion protein was detected as long filaments shortly after infection. Double-labeling fluorescence microscopy suggests that the MP interacts and coaligns with microtubules. The distribution of the MP is disrupted by treatments that disrupt microtubules, but not by cytochalasin B, which disrupts filamentous F-actin. Microtubules may target the MP to plasmodesmata, the intercellular channels that connect adjacent cells.

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.

Biological activities of hybrid RNAs generated by 3'-end exchanges between tobacco mosaic and brome mosaic viruses

Sequences within the conserved, aminoacylatable 3' noncoding regions of brome mosaic virus (BMV) genomic RNAs 1, 2, and 3 direct initiation of negative-strand synthesis by BMV polymerase extracts and, like sequences at the structurally divergent but aminoacylatable 3' end of tobacco mosaic virus (TMV) RNA, are required in cis for RNA replication in vivo. A series of chimeric RNAs in which selected 3' segments were exchanged between the tyrosine-accepting BMV and histidine-accepting TMV RNAs were constructed and their amplification was examined in protoplasts inoculated with or without other BMV and TMV RNAs. TMV derivatives whose 3' noncoding region was replaced by sequences from BMV RNA3 were independently replication competent when the genes for the TMV 130,000-M(r) and 180,000-M(r) replication factors remained intact. TMV replicase can thus utilize the BMV-derived 3' end, though at lower efficiency than the wild-type (wt) TMV 3' end. Providing functional BMV RNA replicase by coinoculation with BMV genomic RNAs 1 and 2 did not improve the amplification of these hybrid genomic RNAs. By contrast, BMV RNA3 derivatives carrying the 3' noncoding region of TMV were not amplified when coinoculated with wt BMV RNA1 and RNA2, wt TMV RNA, or all three. Thus, BMV replicase appeared to be unable to utilize the TMV 3' end, and there was no evidence of intervirus complementation in the replication of any of the hybrid RNAs. In protoplasts coinoculated with BMV RNA1 and RNA2, the nonamplifiable RNA3 derivatives bearing TMV 3' sequences gave rise to diverse new rearranged or recombined RNA species that were amplifiable.

Mutational analysis of the coat protein gene of tobacco mosaic virus in relation to hypersensitive response in tobacco plants with the N' gene

Tomato strain L of tobacco mosaic virus (TMV-L) induces a hypersensitive response (necrotic local lesions) on tobacco plants with the N' gene. A factor responsible for induction of the hypersensitive response has been mapped to the coat protein gene. We have constructed several mutants which have insertions or deletions in the coat protein gene. Frame-shift mutants which cause premature termination of translation of the coat protein caused no necrotic local lesions on N' plants. Mutants which result in the expression of coat protein derivatives with one amino acid inserted after residue 56, 101, or 152 caused necrotic local lesions on N' plants. Deletion mutants lacking the coding region for fewer than the C-terminal 13 amino acid residues caused necrotic local lesions, whereas mutants lacking the coding region for the C-terminal 38 residues caused no necrotic local lesions. These results show that modifications of the coat protein gene affect its ability to induce the hypersensitive response in N' plants.

Changes in the integrity of large unilamellar vesicles due to their interaction with tobacco cell suspensions

Negatively charged large unilamellar vesicles (LUV) were incubated with tobacco (Nicotiana tabacum var. xanthi) cell suspensions and with the cell-free medium of the cell suspensions. The extent of cell-LUV interaction was determined by the leakage of the LUV contents. Cells enhanced the leakage of LUV contents and this effect increased with cell age. Addition of polylysine to the reaction mixture increased even further the leakage of the LUV contents. The cell-free medium of the cell suspension also affected the integrity of the LUV. Cell-free medium, by itself, promoted leakage of LUV contents and caused a reduction in the leakage exerted by polylysine. Centrifugation (8000g) of the cell-free medium decreased its effect, heat treatment (122°C) did not alter its effect and sonication enhanced it. The effects of the cell-free medium are attributed to the presence of cell wall debris of disintegrated cells.

Liposome-mediated introduction of the chloramphenicol acetyl transferase (CAT) gene and its expression in tobacco protoplasts

The expression plasmid vector pUC8CaMVCAT, containing the chloramphenicol acetyl transferase (CAT) gene, was encapsulated in large unilamellar vesicles (LUV) and introduced into tobacco protoplasts derived from either cell suspension culture or leaf mesophyll. Treatment with liposomes took place in a buffer containing either NaCl or CaCl2, but no polyethylene glycol. The presence of polylysine in the incubation buffer increased the adsorption of liposomes to protoplasts but decreased the efficiency of CAT gene expression.The expression of the introduced CAT gene could be monitored for at least seven days, following the treatment (about 25% acetylation at day 3 as well as at day 7). Plasmid DNA sequences could be detected, apparently unmodified, for at least nine days in the plant cells, though unintegrated in the host genome.

Attenuated strains of tobacco mosaic virus. Reduced synthesis of a viral protein with a cell-to-cell movement function

Attenuated strains of tobacco mosaic virus (TMV) have been used to protect crops against virulent strains. The synthesis of viral proteins and RNAs was investigated in protoplasts that had been infected separately with three tomato strains of TMV, virulent type L, and attenuated strains L11 and L11A. It was revealed that the mutations, which are responsible for the viral attenuation and have been mapped in the p126 (p184) gene, caused a reduction of the synthesis of the viral-coded p30 protein with a cell-to-cell movement function and its mRNA, but it had no significant effect on the synthesis of other viral proteins and RNAs in virus-infected protoplasts. Thus, it was shown that the attenuated strains can multiply as efficiently as the virulent strain in initially inoculated cells, but they can not spread efficiently outside the infected cells. In addition, it is suggested that a non-structural protein, p126 or p184, of TMV is involved in the synthesis of viral subgenomic p30 mRNA.

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.