Cucumber mosaic cucumovirus antibodies from a synthetic phage display library

Antibody fragments (scFv) that bind specifically to particles of cucumber mosaic cucumovirus (CMV) were obtained from a library which encodes a diverse array of synthetic antibody fragments, each displayed on the surface of filamentous bacteriophage. After four rounds of selection and enrichment, several clones were obtained which produced scFv that bound specifically to purified particles of CMV in ELISA. BstNI digestion of phagemid DNA resulted in the same restriction pattern for all clones. The nucleotide sequences of three of the clones showed that they belonged to the human VH1 family and that they had a complementarity determining region loop of 7 amino acids. Phage-displayed antibodies and soluble scFv secreted by these clones reacted with particles of CMV in sap from infected plants in ELISA. In immunoblotting tests, soluble scFv preparations reacted with SDS-denatured coat protein extracted from purified preparations of CMV isolates belonging to either subgroup I or II and also with protein extracted by SDS treatment of seeds harvested from naturally infected lupin plants. The results demonstrate the feasibility, and potential applicability, of recombinant antibody methods in plant pathology.

A novel naturally occurring hybrid gene encoded by a plant RNA virus facilitates long distance virus movement

We recently identified a new cucumovirus-specific gene (2b) which is encoded by RNA 2 of the cucumber mosaic cucumovirus (CMV) tripartite RNA genome and whose coding sequence overlaps the C-terminal 69 codons of ORF 2a encoding the RNA polymerase protein. We have now found that although a CMV mutant lacking ORF 2b accumulated in the inoculated cotyledons of cucumber plants, it was unable to spread systemically, demonstrating involvement of 2b in long distance movement. The same mutant infected tobacco systemically with a much reduced virulence and delayed appearance of symptoms, indicating that 2b may contribute to long distance movement in this host. Deletion of the overlapping C-terminal part of ORF 2a did not change infectivity of the mutant in either host species, ruling out 2a mutation as the reason for the change of phenotype. Further infectivity studies with mutants containing partial deletions in ORF 2b further supported the conclusion that 2b encodes a host-specific long distance movement function. Sequence analysis revealed that 2b may represent a novel naturally occurring hybrid gene important to the evolutionary formation of the cucumovirus group and that it could provide a genetic basis for the wide host range of these viruses.

The conserved, hydrophilic and arginine-rich N-terminal domain of cucumovirus coat proteins contributes to their anomalous electrophoretic mobilities in sodium dodecylsulfate-polyacrylamide gels

Although the Mr values of the coat proteins (CPs) of several cucumoviruses have been calculated from their deduced amino acid sequences to be approximately 24,000, the experimentally determined M(r) values using the Laemmli SDS-PAGE system were 30,000-31,000. Examination of the amino acid composition revealed that these CPs are neither highly acidic nor highly basic. Post-translational glycosylation or phosphorylation were also ruled out as contributing factors to the observed anomalous electrophoretic mobility because the products of in vitro translation of cucumovirus RNA 4 and in vivo bacterial expression of the cloned CP gene co-migrated with authentic cucumovirus CPs. Comparison of the hydropathy profiles of the CPs revealed the presence in each of a strikingly similar, highly hydrophilic N-terminal domain of 30-32 amino acid residues that contains a cluster of basic amino acids, mainly arginine. Selective chemical cleavage at tryptophan residues in the CPs of cucumoviruses, known to contain single tryptophan residues, yielded two peptides; an N-terminal peptide that contained the conserved hydrophilic domain and a C-terminal peptide. SDS-PAGE analysis showed that the N-terminal, but not the C-terminal, peptide exhibited the anomalous electrophoretic mobility.

Complete genomic RNA sequences of cucumber mosaic virus strain NT9 from Taiwan

The nucleotide sequences of the RNAs 1, 2, and 3 of the cucumber mosaic virus (CMV) Taiwan isolate NT9 were determined and compared at both the nucleotide and amino acid levels with those of CMV strains Fny, Y, O from subgroup I and strain Q from subgroup II. NT9-CMV has an unique feature at the C-terminus of the 3a protein which contains four extra-amino acids. All three RNAs and their encoded proteins, except 2b, of NT9-CMV share more than 90% identity with those of strains in subgroup I, and 72%-85% identity with Q-CMV. The results indicated the conservation of sequences of CMV derived from different geographical locations.

The complete sequence of a cucumber mosaic virus from Ixora that is deficient in the replication of satellite RNAs

A cucumber mosaic virus (CMV-Ix) from Ixora is unusual in that it does not support the accumulation of some well-characterized CMV satellite RNAs in plants. CMV-Ix can support a particular satellite RNA variant which causes lethal tomato necrosis when inoculated with other CMV strains but not when inoculated with CMV-Ix. This difference in ability to support accumulation of specific satellite variants is apparent even when their sequences differ by only 10 nucleotides. Electroporation of tomato protoplasts with combinations of CMV-Ix or CMV-1 RNA plus the same satellite variants showed similar differences in accumulation, indicating a defect in satellite RNA replication and not movement or encapsidation. Pseudorecombinant virus infections between CMV-1 and CMV-Ix indicated that the genomic determinants responsible for this phenotype reside on RNA 1 since only combinations with CMV-Ix RNA 1 failed to replicate satellite RNA. The complete genome of CMV-Ix was cloned, sequenced and compared with the genomes of other cucumoviruses. CMV-Ix is most similar in RNA and protein sequence to subgroup 1 CMV-Fny and CMV-Y but slightly less similar than they are to each other. CMV-Ix and all cucumovirus strains sequenced thus far share a domain in the 3' untranslated portion of their genomic RNAs in which 39 of 40 bases are completely conserved.

RNA 4 sequences from cucumber mosaic virus subgroups I and II

The complete sequences of RNA 4 from cucumber mosaic virus (CMV) strains Ny (subgroup I) and Sn (subgroup II) have been determined and compared to all other known complete CMV RNA 4 sequences. The identification of a unique EcoRI site, present only in subgroup-II RNA 4 sequences, provides the basis for a simple method of classifying CMV isolates.

Characterization of defective RNAs derived from RNA 3 of the Fny strain of cucumber mosaic cucumovirus

Two defective RNAs (designated D RNA 3 alpha and D RNA 3 beta) were found to be associated with the Fny strain of cucumber mosaic cucumovirus but not with the Sny strain after serial passages in a tobacco host. The D RNAs were derived from RNA 3 by single, in-frame deletions within the 3a open reading frame. A full-length cDNA clone from which biologically active transcripts can be produced in vitro has been constructed for D RNA 3 beta. This transcript can be replicated in tobacco plants infected with subgroup I and II cucumber mosaic cucumovirus strains and with peanut stunt cucumovirus. Translation of D RNA 3 beta in vitro produced a 20-kDa peptide, which was consistent with the predicted coding capacity of the deleted 3a open reading frame. D RNA 3 beta was also associated with polyribosomes isolated from infected tobacco plants. The presence of the D RNAs had no apparent effect upon helper virus yield or symptom production.

Experimental evaluation of the ribonuclease protection assay method for the assessment of genetic heterogeneity in populations of RNA viruses

The ribonuclease (RNase) protection assay (RPA) was evaluated as a method to estimate genetic distances among sequence variants of RNA viruses. The patterns of fragments generated, under different RPA conditions, by three sets of RNA sequence variants of known nucleotide sequence, were analyzed. Both the effectiveness of cleavage (i.e. the probability of cleavage in a certain heteroduplex) and its degree (i.e. in all the molecules in the assay or in a part of them) varied largely according to the nature of the mismatch. Probability and degree of cleavage were also dependent on distant sequence context effects. No correlation could be established between context and cleavage, so that the pattern of fragments in RPA cannot be unequivocally predicted from sequence information. Accordingly, nucleotide sequence differences between two sequence variants cannot be directly derived from RPA data. For all three sequence sets linear relationships were found between the number of non-shared fragments in the RPAs of two variants and their nucleotide sequence differences. Nevertheless, both linearity and the linear regression parameters varied largely according to the sequence set and according to RPA conditions, in a non-predictable way. Thus, under experimental conditions, RPA may not be as appropriate a method to estimate genetic distances between RNA sequences as simulation under an ideal model suggested. Possible ways to diminish the gap between the ideal model and the experimental procedure are proposed.

Point mutations in the coat protein of cucumber mosaic virus affect symptom expression and virion accumulation in tobacco

We examined the correlation of the amino acid at position 129 in the coat protein (CP) of cucumber mosaic virus (CMV) with the phenotype of the viral pathology in tobacco by using CP mutants in which several amino acid substitutions had been introduced. An exchange between Ser129 in CMV-Y, a chlorosis-inducing strain, and Pro129 of CMV-O, a green-mosaic-inducing strain, reciprocally altered the phenotypes of those virus strains on tobacco. Replacement of either Ser129 in CMV-Y or Pro129 in CMV-O with a Leu, as is found in a chlorosis-inducing strain, CMV-M, resulted in veinal necrosis. Furthermore, we created mutants that have a Phe or a Gly at position 129. Two Phe129 mutants induced necrotic lesions on the inoculated leaves, and a Gly129 mutant induced green mosaic symptoms. In inoculated protoplasts, the mutant viruses and the wild-type virus all replicated RNA well, and accumulated CP; however, infection with the Leu129 and Phe129 mutants yielded few virions. The Phe129 mutants lacked the capacity to move systemically in tobacco; by 2 weeks post-inoculation, the Phe129 mutants occasionally gave rise to revertants that elicited chlorosis, green mosaic or veinal necrosis. Sequence analysis revealed that one had reverted to the parental Y strain, and the others had additional single amino acid changes (positions 138, 144 or 147). We suggest that amino acids at specific sites affect the whole structure of the CP and affect virus assembly, virus transport and symptom expression.

Host effects and sequences essential for accumulation of defective interfering RNAs of cucumber necrosis and tomato bushy stunt tombusviruses

Passage of cucumber necrosis virus (CNV) containing defective interfering (DI) RNAs through cucumber plants decreased the accumulation of DI RNAs to undetectable levels. Subsequent passages in two Nicotiana species (Nicotiana benthamiana or N. clevelandii) resulted in the appearance of DI RNA species that were larger than the DI RNAs observed during exclusive serial passages of CNV through the Nicotiana species. Sequence analysis of cloned cDNAs corresponding to the two DI RNA populations indicated that the smaller CNV-DI RNAs contained the four conserved regions (I through IV) of the genome typical of tombusvirus DI RNAs, whereas the larger DI RNAs were of similar organization but had a direct repeat of the middle portion of the molecule. This result suggests that the host has an influence on the type of DI RNA that accumulates during consecutive high multiplicity of infection passages. A comparative analysis of deletions targeting the individual conserved regions in both CNV and tomato bushy stunt virus (TBSV) DI RNAs revealed that only region III was completely dispensable for accumulation of either DI RNA species. More refined deletion analyses in regions I and II indicated that smaller segments of 75 and 35 nucleotides (nt), respectively, could be deleted without abolishing infectivity. The dispensable sequences in region II of both TBSV and CNV DI RNAs mapped to the top portion of a putative stem-loop structure. These studies indicate that both essential and nonessential sequences are conserved in DI RNAs. The essential sequences in regions I, II, and IV likely contain important cis-acting elements, whereas nonessential regions such as region III may play secondary roles such as optimally spacing cis-acting elements or maintaining the DI RNA at an overall size that is stable.

Complementation of virus movement in transgenic tobacco expressing the cucumber mosaic virus 3a gene

Tobacco plants were transformed with the cucumber mosaic cucumovirus (CMV) 3a gene and the in planta-expressed 3a protein was detected immunologically. The 3a protein was predominantly localized in a subcellular fraction corresponding to the cytosol. Two frameshift and four deletion mutants were created within the 3a open reading frame of CMV RNA 3. Five of these mutants, containing an N-terminal, large central, or C-terminal 70-amino-acid deletion could not infect nontransformed tobacco plants, but could infect the 3a transgenic tobacco plants, and generally accumulated to wild-type levels. The sixth mutant, lacking the C-terminal 43 amino acids of the 3a protein, was able to infect nontransformed tobacco plants. A delay in accumulation of viral RNA in both the inoculated and the systemically infected leaves was demonstrated for one of the mutants. Thus, the CMV 3a protein is a virus movement protein, the functions of which can be complemented in a transgenic plant. The CMV 3a transgenic plants were able to complement the long-distance movement of a pseudorecombinant cucumovirus defective for this function in tobacco, as well as the cell-to-cell, but not the long-distance, movement of two other related viruses. However, these transgenic plants were unable to complement the long-distance movement of viruses from several other taxonomic groups that could move cell to cell but not long distance in tobacco.

Mixed-subunit capsids can be assembled in vitro with coat protein subunits from two cucumoviruses

Virus particles were reassembled in vitro from tomato aspermy virus strain V (V-TAV) RNA and a mixture of subunits prepared from V-TAV and 35S-labelled cucumber mosaic virus strain T (T-CMV). Immunodiffusion tests showed that the reassembled particles reacted with polyclonal antisera raised against both V-TAV and T-CMV. Radioactivity was found in the precipitin line formed between the reassembled particles and antiserum raised against T-CMV as well as in the precipitin line formed between the reassembled particles and antiserum raised against V-TAV. This shows that 35S-labelled T-CMV protein subunits were incorporated with V-TAV protein subunits into the same particles. Thus, coat proteins of V-TAV and T-CMV can co-assemble and form mixed-subunit capsids in vitro.

Cucumber mosaic virus 3a protein potentiates cell-to-cell trafficking of CMV RNA in tobacco plants

Contrary to a previous report, electron microscopic studies on the Fny strain of cucumber mosaic virus (CMV)-infected tobacco tissues revealed that plasmodesmata were not structurally modified during CMV infection, nor were virions ever observed in plasmodesmata connecting infected cells. To further explore the basis of CMV infection, experiments were performed on the CMV 3a ORF. The 3a protein of CMV was expressed in and purified from Escherichia coli. The purified protein was labeled with fluorescein isothiocyanate (FITC) and subsequently microinjected into mesophyll cells of mature leaves of Nicotiana tabacum cv. Turkish Samsun NN. Within a brief period (as little as 1 sec), the microinjected FITC-labeled CMV 3a protein moved into neighboring cells. Co-injection of unlabeled CMV 3a protein with 9.4-kDa fluorescein-conjugated dextran (F-dextran) resulted in extensive cell-to-cell movement (diffusion) of the F-dextran, indicating that the 3a protein can interact with and dilate plasmodesmata. Furthermore, co-injection of unlabeled 3a protein with fluorescently labeled infectious CMV RNA molecules resulted in rapid and extensive cell-to-cell transport. In contrast, a mutant form of the 3a protein was unable to traffic from cell to cell, to increase the size exclusion limit of plasmodesmata, or to potentiate cell-to-cell trafficking of CMV RNA molecules. Microinjection studies performed on transgenic tobacco plants expressing the CMV 3a protein indicated that fluorescently labeled CMV RNA moved out of the target cell into the surrounding mesophyll tissue. In addition, expression of the CMV 3a protein also potentiated the cell-to-cell movement of 9.4-kDa F-dextran. Collectively, these results provide direct experimental evidence that the CMV 3a protein functions as the movement protein of CMV. These findings are consistent with the hypothesis that CMV moves from cell-to-cell in the form of a ribonucleoprotein complex.

Interspecific reassortment of genomic segments in the evolution of cucumoviruses

Segmented genomes of RNA viruses are thought to evolve and be maintained in analogy to sexual recombination and reassortment in eukaryotic systems. If reassortment among genomes is an important event in cucumoviral evolution, then such events should be detectable among extant viruses. In this study, phylogenetic analyses of cucumoviruses were performed using aligned amino acid sequences. The results reveal different relationships among species when the three genomic segments are compared, suggesting that reassortment events have given rise to extant forms. In addition, we describe a cucumovirus isolate that is composed of genomic segments from two distinct viral species. These results indicate that reassortment events may provide a mechanism for speciation in cucumoviruses.

Efficient infection from cDNA clones of cucumber mosaic cucumovirus RNAs in a new plasmid vector

Full-length cucumber mosaic cucumovirus (CMV) cDNAs were cloned into a new plasmid vector containing a modified plant virus promoter designed to transcribe the inserted sequence from its first nucleotide. cDNA copies of CMV strain Q (Q-CMV) genomic RNAs 1, 2 and 3 cloned into this vector were infectious when inoculated together, producing symptoms indistinguishable from those caused by wildtype Q-CMV infection. The infectivity of the clones could be substantially increased by excision of the viral insert together with the transcriptional promoter and terminator prior to inoculation. A diagnostic but silent mutation was introduced into the RNA 2 cDNA and found to be stably maintained in viral infection, allowing distinction of the recombinant virus from native contaminants. The infectious cDNA clones supported the replication of CMV satellite RNA when co-inoculated with biologically active Q-CMV satellite RNA transcripts. Using the infectious cDNAs described, it was found that a newly-identified overlapping gene (2b) encoded by Q-CMV RNA 2 was not essential for either systemic viral infection of Nicotiana glutinosa or replication of the satellite RNAs.

Role of cucumovirus capsid protein in long-distance movement within the infected plant

Direct evidence is presented for a host-specific role of the cucumovirus capsid protein in long-distance movement within infected plants. Cucumber (Cucumis sativus L.) is a systemic host for cucumber mosaic cucumovirus (CMV). Tomato aspermy cucumovirus, strain 1 (1-TAV), multiplied to the levels of CMV (i.e., replicated, moved from cell to cell, and formed infectious particles) in the inoculated leaves of cucumbers but was completely unable to spread systemically. The defective long-distance systemic movement of 1-TAV was complemented by CMV in mixed infections. Coinfection of cucumbers with 1-TAV RNA with various combinations of transcripts from full-length cDNA clones of CMV genomic RNA 1, RNA2, and RNA3 showed that CMV RNA3 alone complemented 1-TAV long-distance movement. We obtained mutants containing mutations in the two open reading frames in CMV RNA3 encoding the 3a protein and the capsid protein (CP), both of which are necessary for cell-to-cell movement of CMV. Complementation experiments with mutant CMV RNA3 showed that only 3a protein mutants, i.e., those with an intact CP, complemented the long-distance movement of 1-TAV in cucumbers. Since CMV and TAV have common systemic host plants, the results presented here are strong evidence for an active, host-specific function of the CPs of these two cucumoviruses for long-distance spread in the phloem. The results also suggest that the plasmodesmata in the vascular system and/or at the boundary between the mesophyll and the vascular system, involved in long-distance movement through the phloem, and those in the mesophyll, involved in cell-to-cell movement, differ functionally.

Variation in the hypervariable region of cucumber mosaic virus satellite RNAs is affected by the helper virus and the initial sequence context

The D satellite RNA (sat RNA) of cucumber mosaic virus (CMV) was previously shown to contain a region of hypervariability around nucleotide 230, in wild-type populations and in cDNA clones and progeny of one such clone (pDsat4) after passage with the subgroup I strain Fny-CMV. This hypervariable region (HVR) consists of a series of consecutive A and/or U residues. We found that variability is also generated in the HVR of transcript derived from pDsat4 after passage with the subgroup II strain LS-CMV and with tomato aspermy virus (TAV). However, the progeny differ with respect to the sequence of the HVR after passage with both LS-CMV and TAV. Another D-sat RNA cDNA clone that contains a C residue in the HVR, pDsat1, was previously shown not to develop variability in the HVR upon passage with Fny-CMV. However, when the C (position 231) was changed to an A residue, variability developed by the third passage with Fny-CMV. An additional cDNA clone derived from the B1-sat RNA, pBsat5, also contains a C residue in the region analogous to the D-sat RNA HVR and did not develop variability upon passage with either Fny- or LS-CMV. Changing this C to a U residue did not result in the development of hypervariability in the progeny of transcript from this mutant. Models to explain the generation of hypervariability are discussed.

Genetic analysis of helper virus-specific selective amplification of cucumber mosaic virus satellite RNAs

Satellite RNAs (sat-RNAs) are small molecular parasites associated with a number of plant RNA viruses. The cucumber mosaic virus (CMV) sat-RNAs are ca. 335 nucleotides and have evolved to produce a large number of closely related sat-RNAs. Different cucumoviruses can act as helper viruses in the amplification of CMV sat-RNAs. We have found that different helper viruses show a preference for a particular sat-RNA in a mixed infection. In this study the specificity of WL47 sat-RNA amplification by LS-CMV and of D4 sat-RNA amplification by tomato aspermy virus in mixed infections was examined. Recombinant cDNA clones of D4 sat-RNA and WL47 sat-RNA were used to map the sat-RNA sequences responsible for the helper virus selection of a specific sat-RNA for amplification.

Effect of temperature on cucumber mosaic virus satellite-induced lethal tomato necrosis is helper virus strain dependent

The effect of temperature on the response of tomato (Lycopersicon esculentum Mill. cv. Rutgers) to infections with the necrogenic cucumber mosaic virus (CMV) satellite D-CARNA 5 was investigated with each of four CMV strains D, 1, Y and S functioning as helper virus. At 24 degrees C lethal necrosis was observed in all infections. However, at 32 degrees C the response varied from total absence or reduction of necrosis with some strains to accelerated lethal necrosis with others. The total lack of necrotic response with CMV-S and the aggravated necrosis with CMV-Y at the higher temperature both turned out to be independent of the coinfecting satellite, and rather to correlate with the changing rate of viral RNA accumulation in tomato, which probably was responsible for the changes in pathogenic response. However, when CMV-D was helper virus, satellite accumulation decreased, while with CMV-1 it increased, respectively, while viral RNA accumulations were not seriously affected. Although these profound effects of temperature seem to link the necrotic response of tomato to the competitive replication dynamics of the infecting virus/satellite combination in the case of CMV-D/D-CARNA 5, temperature effects at other levels of disease induction probably play an important role as well.

The replication of a necrogenic cucumber mosaic virus satellite is temperature-sensitive in tomato

Lethal necrosis development in tomato plants infected with cucumber mosaic virus (CMV) strain D containing the necrogenic satellite D-CARNA 5 and held at 32 degrees C is shown to be impaired. CARNA 5 accumulation in tomato at 32 degrees C is reduced about 100-fold compared to accumulation in plants held at 24 degrees C, while viral RNA accumulation is reduced about 5-fold. CMV-infected tomato held for 3 days at 24 degrees C prior to shift to 32 degrees C do not develop lethal necrosis. Longer incubations at 24 degrees C prior to shift to 32 degrees C allow necrosis to develop. CMV-infected plants held for up to 4 weeks at 32 degrees C required an additional 8-10 days at 24 degrees C to develop necrosis. Necrogenic CMV-infected plants held at 24 degrees C and analyzed 3 days p.i. contained detectable amounts of ss- and ds-CARNA 5; upon shift to 32 degrees C, such CARNA 5 declined to undetectable levels and lethal necrosis did not occur. There appear to be temperature-sensitive factors that are required for efficient satellite replication which are not required for efficient viral RNA replication. Whether these factor(s) are of host or satellite origin is uncertain.

Mapping helper virus functions for cucumber mosaic virus satellite RNA with pseudorecombinants derived from cucumber mosaic and tomato aspermy viruses

P-TAV is a strain of tomato aspermy virus (TAV) able to efficiently support the systemic accumulation of some (i.e., B2-satRNA) but not of other (i.e., Ix-satRNA) strains of the satellite RNA (satRNA) of cucumber mosaic virus (CMV) in both tobacco and in tomato. As reported for V-TAV, the failure to support the systemic accumulation of Ix-satRNA seems to be due to an inefficient support of its systemic movement. Pseudorecombinants obtained by the exchange of RNAs 1 + 2 between P-TAV and Trk7-CMV, an efficient helper for the systemic accumulation of Ix-satRNA, were assayed for their ability to support the accumulation of CMV-satRNAs in tobacco plants and protoplasts. Pseudorecombinants having RNAs 1 + 2 from CMV supported the systemic movement and accumulation of CMV-satRNA as efficiently as CMV, whereas pseudorecombinants having RNAs 1 + 2 from TAV supported the CMV-satRNA very poorly. Thus, the ability to support the systemic movement and accumulation of CMV-satRNA is determined primarily by RNAs 1 + 2 and not by RNA 3, which is presumed to encode movement functions in the cucumoviruses and only has a minor, modulating effect on the systemic accumulation of satRNA. This suggests that for systemic movement CMV-satRNA has to interact with (the gene products of) RNAs 1 and/or 2 or that these viral RNAs compete with the satRNA for interaction with the coat or other movement proteins.

Mapping determinants in cucumber mosaic virus for transmission by Aphis gossypii

Determinants have been mapped in the coat protein of cucumber mosaic virus (CMV) which influence transmission by the aphid Aphis gossypii. Chimeric RNA 3s were constructed which contain sequences from the efficiently aphid-transmitted strain Fny-CMV and the very poorly aphid-transmissible M-CMV. Analyses of chimeric viruses revealed that two regions in the coat protein were critical for transmission by aphids. The coat proteins of Fny- and M-CMV were previously shown to differ at eight amino acid positions. Amino acid changes were introduced into regions of the coat protein gene of M-CMV shown to be involved in transmission and two amino acid changes were demonstrated as essential for the restoration of aphid transmissibility. Nonconservative changes at positions 129 (Leu to Pro) and 162 (Thr to Ala) modified the coat protein to resemble Fny-CMV at these positions and restored transmissibility by A. gossypii. A change at amino acid position 168 (Cys to Tyr) may influence the efficiency of aphid transmission, but is not essential. A single amino acid change at position 129 (Leu to Pro) did not restore aphid transmissibility to M-CMV. The findings are discussed in relation to the nonpersistent transmission of plant viruses.

Analysis of the determinants of the satellite RNA of cucumber mosaic cucumovirus for high accumulation in squash

Ix-satRNA is a 334-nt cucumber mosaic cucomovirus satellite RNA (CMV-satRNA) with the unusual property of accumulating to high levels in host plants of the family Cucurbitaceae. To identify the determinants for this unusual phenotype, recombinant satRNAs were obtained, by in vitro manipulation of full-length cDNA clones between clone-derived Ix5-satRNA and two other CMV-satRNAs that do not accumulate efficiently in squash: D4-satRNA, which differs in 7 positions from Ix5-satRNA, and B21-satRNA, which differs in 45 positions. The analysis of the accumulation in squash of these hybrid satRNAs showed that determinants for high accumulation in squash occur in three molecular domains: nucleotides 1-46 (a C at position 20), 47-185 (a C at position 102), and 185-334 (not identified). None of these determinants seemed to be necessary or sufficient to confer a phenotype of high accumulation in squash. They resulted, or did not result, in such a phenotype according to the sequences that occur in other parts of the satRNA molecule. Not all of the identified determinants were equally affected by sequence context. These results may suggest that the phenotype in squash is determined by the conformation of the satRNA.

Mapping local and systemic symptom determinants of cucumber mosaic cucumovirus in tobacco

Cucumber mosaic cucumovirus (CMV) can be divided into two subgroups, I and II. LS-CMV and most other subgroup II strains cause a mild, systemic mottle on tobacco and can induce a necrotic etching (necrotic rings) symptom on inoculated tobacco leaves. In contrast, Fny-CMV and most other subgroup I strains cause severe, systemic mosaic symptoms on tobacco, but do not induce the necrotic etching symptom. Full-length cDNA clones of all three genomic RNAs of LS-CMV were constructed and infectious RNAs were generated from these clones. Using pseudorecombinants constructed from the infectious transcripts of LS-CMV and Fny-CMV, we found that both RNAs 1 and 2 of Fny-CMV are involved in determining the severity of systemic symptom on tobacco, and that LS-CMV RNA 3 contains the determinant for the necrotic etching symptom. Chimeras formed between Fny- and LS-CMV RNA 3 were used to demonstrate that the inducer of the necrotic etching symptoms mapped to the 5' 618 nucleotides of LS-CMV RNA 3, and required sequences in both the 5' non-translated region, as well as the 3a gene of CMV.

The 3a protein from cucumber mosaic virus increases the gating capacity of plasmodesmata in transgenic tobacco plants

The 3a protein, encoded by RNA 3 of cucumber mosaic virus (CMV), is the putative movement protein of viral progeny in infected plants. An analysis of transgenic tobacco plants constitutively expressing the CMV 3a protein showed that the protein is accumulated in leaves at every stage of development. In fully expanded leaves the protein is immunodetectable mostly in a cell-wall-enriched fraction. Dye-coupling experiments using fluorescent-dextran probes were performed on fully expanded leaves to study the modifying effect of CMV 3a protein on the gating capacity of plasmodesmata. Movement of fluorescein-isothiocyanate-labelled dextran with a mean molecular mass of 10,000 Da, and an approximate Stokes' radius of 2.3 nm, was detected between cells of the 3a protein transgenic plants, but not in the control plants. These results are consistent with the idea that the CMV 3a protein is involved in the modification of plasmodesmata and, therefore, in the cell-to-cell spread of the virus.