The interplay between capsid dynamics and pathogenesis in tripartite bromoviruses

Infectious virus capsids or virions are considered static structures and undergo various conformational transitions to replicate and infect a wide range of eukaryotic cells. Therefore, virus capsids must be stable enough to overcome the physicochemical environment and flexible enough to reorganize their biologically relevant surface peptides for optimal interaction with the host machinery. Although viral capsid fluctuations, referred to as dynamics or breathing, have been well studied in RNA viruses pathogenic to animals, such information is limited among plant viruses. However, more recent attempts have been made in characterizing the capsid dynamics in the plant virus genus bromovirus characterized by having a tripartite, positive-sense RNA genome. Using the available research data on the genus bromovirus members, this review is focused on updating the readers on the interrelationships between the viral capsid dynamics and host-pathogen interactions.

Effects of Artificial Codon Changes in the Movement Protein Gene on Adaptation of a Hybrid Bromovirus to Cowpea

A hybrid Cowpea chlorotic mottle virus(CCMV) with the movement protein (MP) gene replaced with that of the closely related Brome mosaic virus cannot infect cowpea systemically. Twenty-nine spontaneous mutants from the hybrid CCMV capable of systemic infection in cowpea appeared through biased codon changes that resulted in Lys or Arg at five specific positions in the MP gene. In this study, we report that systemic infection of cowpea with the hybrid CCMV can be achieved by artificial codon changes that do not result in Lys or Arg. We discuss mechanisms that restrict the occurrence of cowpea-adapted mutants in nature.

Cell-penetrating peptides derived from viral capsid proteins

Cell-penetrating peptides (CPP) can translocate across the cell membrane and have been extensively studied for the delivery of proteins, nucleic acids, and therapeutics in mammalian cells. However, characterizations of CPP in plants have only recently been initiated. We showed that the intact virion and a recombinant capsid protein (CaP) from a plant-infecting nonenveloped icosahedral RNA virus, Brome mosaic virus (BMV), can penetrate the membranes of plant protoplasts but are trapped by the extracellular matrix. Furthermore, a 22-residue peptide derived from the N-terminal region of the CaP (CPNT) can enter barley protoplasts and cells of intact barley and Arabidopsis roots. An inhibitor of the macropinocytosis reduced CPNT entry, while treatment with NiCl(2) changed the cellular localization of CPNT. CPNT increased uptake of the green flourescent protein (GFP) into the cell when covalently fused to GFP or when present in trans of GFP. The BMV CPNT overlaps with the sequence known to bind BMV RNA, and it can deliver BMV RNAs into cells, resulting in viral replication, as well as deliver double-stranded RNAs that can induce gene silencing.

Are Sunflower chlorotic mottle virus infection symptoms modulated by early increases in leaf sugar concentration?

Symptom development in a susceptible sunflower line inoculated with Sunflower chlorotic mottle virus (SuCMoV) was followed in the second pair of leaves at different post-inoculation times: before symptom expression (BS), at early (ES) and late (LS) symptom expression. Sugar and starch increases and photoinhibition were observed as early effects BS, and were maintained or enhanced later on, however, chlorophyll loss was detected only at LS. Photoinhibition correlated with a drastic decrease in D1 protein level. The progress of infection was accompanied by decreasing levels of apoplastic reactive oxygen species (ROS). In infected leaves, higher antioxidant enzyme activities (superoxide dismutase, SOD; ascorbate peroxidase, APX; glutathione reductase, GR) were observed from BS. The purpose of this work was to evaluate whether the early increases in carbohydrate accumulation may participate in SuCMoV symptom expression. Similar effects on photoinhibition, apoplastic ROS generation and antioxidant activity were generated when healthy leaves were treated with sugars. These results suggest that photoinhibitory processes and lower apoplastic superoxide levels induced by SuCMoV infection may be modulated by sugar increases.

Tobacco mosaic virus 126-kDa protein increases the susceptibility of Nicotiana tabacum to other viruses and its dosage affects virus-induced gene silencing

The Tobacco mosaic virus (TMV) 126-kDa protein is a suppressor of RNA silencing previously shown to delay the silencing of transgenes in Nicotiana tabacum and N. benthamiana. Here, we demonstrate that expression of a 126-kDa protein-green fluorescent protein (GFP) fusion (126-GFP) in N. tabacum increases susceptibility to a broad assortment of viruses, including Alfalfa mosaic virus, Brome mosaic virus, Tobacco rattle virus (TRV), and Potato virus X. Given its ability to enhance TRV infection in tobacco, we tested the effect of 126-GFP expression on TRV-mediated virus-induced gene silencing (VIGS) and demonstrate that this protein can enhance silencing phenotypes. To explain these results, we examined the poorly understood effect of suppressor dosage on the VIGS response and demonstrated that enhanced VIGS corresponds to the presence of low levels of suppressor protein. A mutant version of the 126-kDa protein, inhibited in its ability to suppress silencing, had a minimal effect on VIGS, suggesting that the suppressor activity of the 126-kDa protein is indeed responsible for the observed dosage effects. These findings illustrate the sensitivity of host plants to relatively small changes in suppressor dosage and have implications for those interested in enhancing silencing phenotypes in tobacco and other species through VIGS.

A simple technique for separation of Cowpea chlorotic mottle virus from Cucumber mosaic virus in natural mixed infections

A simple technique was developed to separate Cowpea chlorotic mottle virus (CCMV) from Cucumber mosaic virus (CMV) in natural mixed infections. Sap from cowpea leaves infected naturally with a mixture of CCMV and CMV was inoculated mechanically on the first tri-foliolate leaf of cowpea seedlings. Both inoculated and non-inoculated upper leaves were sampled 3 or 8 days post-inoculation and tested by reverse transcription polymerase chain reaction (RT-PCR) using primers specific to CCMV and CMV. RT-PCR analysis showed the presence of only CCMV in the inoculated leaf and both viruses in the non-inoculated systemically infected upper leaves. Total RNA from the inoculated leaves positive to CCMV only was further confirmed upon re-inoculation to cowpea seedlings. Typical CCMV symptoms were produced within 1 week and RT-PCR analysis showed only the presence of CCMV in both inoculated and non-inoculated systemically infected upper leaves. Systemically infected upper leaves of the same plants were used for CCMV purification. RT-PCR analysis of the purified virion and RNA extracted from the virion further confirmed the absence of CMV contamination. To our knowledge, this is the first report of a method separating CCMV directly from mixed infections with CMV in cowpea.

Analysis of gene function in rice through virus-induced gene silencing

Virus-induced gene silencing (VIGS) is a powerful RNA-silencing based technology adapted for the study of host-gene function. VIGS functions through the expression of a host gene from a virus vector. Both the virus-encoded host sequence and the homologous host target messenger RNA are destroyed or made inactive through a host surveillance system. Here, we describe procedures for the use of a new virus vector for VIGS in monocotyledonous hosts and, in particular, in rice (Oryza sativa), a species for which no VIGS vector was previously available.

Characterization of a Brome mosaic virus strain and its use as a vector for gene silencing in monocotyledonous hosts

Virus-induced gene silencing (VIGS) is used to analyze gene function in dicotyledonous plants but less so in monocotyledonous plants (particularly rice and corn), partially due to the limited number of virus expression vectors available. Here, we report the cloning and modification for VIGS of a virus from Festuca arundinacea Schreb. (tall fescue) that caused systemic mosaic symptoms on barley, rice, and a specific cultivar of maize (Va35) under greenhouse conditions. Through sequencing, the virus was determined to be a strain of Brome mosaic virus (BMV). The virus was named F-BMV (F for Festuca), and genetic determinants that controlled the systemic infection of rice were mapped to RNAs 1 and 2 of the tripartite genome. cDNA from RNA 3 of the Russian strain of BMV (R-BMV) was modified to accept inserts from foreign genes. Coinoculation of RNAs 1 and 2 from F-BMV and RNA 3 from R-BMV expressing a portion of a plant gene to leaves of barley, rice, and maize plants resulted in visual silencing-like phenotypes. The visual phenotypes were correlated with decreased target host transcript levels in the corresponding leaves. The VIGS visual phenotype varied from maintained during silencing of actin 1 transcript expression to transient with incomplete penetration through affected tissue during silencing of phytoene desaturase expression. F-BMV RNA 3 was modified to allow greater accumulation of virus while minimizing virus pathogenicity. The modified vector C-BMV(A/G) (C for chimeric) was shown to be useful for VIGS. These BMV vectors will be useful for analysis of gene function in rice and maize for which no VIGS system is reported.

Natural isolates of Brome mosaic virus with the ability to move from cell to cell independently of coat protein

Brome mosaic virus (BMV) requires encapsidation-competent coat protein (CP) for cell-to-cell movement and the 3a movement protein (MP) is involved in determining the CP requirement for BMV movement. However, these conclusions have been drawn by using BMV strain M1 (BMV-M1) and a related strain. Here, the ability of the MPs of five other natural BMV strains to mediate the movement of BMV-M1 in the absence of CP was tested. The MP of BMV M2 strain (BMV-M2) efficiently mediated the movement of CP-deficient BMV-M1 and the MPs of two other strains functioned similarly to some extent. Furthermore, BMV-M2 itself moved between cells independently of CP, demonstrating that BMV-M1 and -M2 use different movement modes. Reassortment between CP-deficient BMV-M1 and -M2 showed the involvement of RNA3 in determining the CP requirement for cell-to-cell movement and the involvement of RNAs 1 and 2 in movement efficiency and symptom induction in the absence of CP. Spontaneous BMV MP mutants generated in planta that exhibited CP-independent movement were also isolated and analysed. Comparison of the nucleotide differences of the MP genes of BMV-M1, the natural strains and mutants capable of CP-independent movement, together with further mutational analysis of BMV-M1 MP, revealed that single amino acid differences at the C terminus of MP are sufficient to alter the requirement for CP in the movement of BMV-M1. Based on these findings, a possible virus strategy in which a movement mode is selected in plant viruses to optimize viral infectivity in plants is discussed.

Synthesis of infectious in vitro transcripts from Cassia yellow blotch bromovirus cDNA clones and a reassortment analysis with other bromoviruses in protoplasts

Cassia yellow blotch virus (CYBV), genus Bromovirus, was isolated from the Australian native legume, Cassia pleurocarpa, in western Queensland, and its host range was found to be distinct from other bromoviruses. In this study, CYBV was shown to infect systemically and efficiently a model plant species, Arabidopsis thaliana, as we recently reported for another bromovirus, Spring beauty latent virus (SBLV). We constructed full-length cDNA clones of CYBV genomic RNAs from which infectious in vitro transcripts can be transcribed, and determined their complete nucleotide sequences. CYBV RNA3 contains the box B motif in the intercistronic region, but lacks the subgenomic promoter-like sequence in the 5' noncoding region, as does Brome mosaic virus (BMV). To understand relationships among bromoviruses, we generated reassortants between CYBV and three other bromoviruses, BMV, SBLV and Cowpea chlorotic mottle virus. We found that all reassortants between BMV and CYBV accumulated viral RNAs to detectable levels in protoplasts of Nicotiana benthamiana, even when RNAs 1 and 2, which encode the replication proteins 1a and 2a, respectively, were heterologous. Sequence comparison and reassortment experiments of CYBV and other bromoviruses demonstrated that CYBV is closely related to BMV.

Coat protein-independent cell-to-cell movement of bromoviruses expressing brome mosaic virus movement protein with an adaptation-related amino acid change in the central region

The movement protein (MP) of Brome mosaic virus (BMV) depends on the coat protein (CP) to mediate the cell-to-cell movement of BMV and CCMV(B3a), a recombinant Cowpea chlorotic mottle virus (CCMV) expressing BMV MP. Previous studies identified gain-of-function mutations in the central region of BMV MP that enable CCMV(B3a) to adapt to a resistant host. This study demonstrates that all adaptation-related MPs can partially or almost fully mediate the cell-to-cell movement of CCMV(B3a) and BMV without CP. Based on these results, we discuss adaptation mechanisms of CCMV(B3a) and the role of the central region of MP in the determination of virus movement mode.

Identification and characterization of the SSB1 locus involved in symptom development by Spring beauty latent virus infection in Arabidopsis thaliana

The natural variation of Arabidopsis thaliana in response to a bromovirus, Spring beauty latent virus (SBLV), was examined. Of 63 Arabidopsis accessions tested, all were susceptible when inoculated with SBLV, although there was a large degree of variation in symptom development. Most accessions, including Columbia (Col-0), were symptomless or developed only mild symptoms, but four accessions, including S96, showed severe symptoms of SBLV infection. Genetic analysis suggested that the difference in the responses of Col-0 and S96 to SBLV was controlled by a single semidominant locus. We have designated this locus SSB1 (symptom development by SBLV infection). By using genetic markers, SSB1 was mapped to chromosome IV. The patterns of distribution and accumulation of SBLV in sensitive accessions were similar to those in the insensitive accessions. In addition, symptom development in S96 by SBLV infection was critically interrupted by the presence of the NahG gene, which encodes salicylic acid (SA) hydroxylase. These data suggest that symptom development in A. thaliana controlled by SSB1 is independent of the efficiency of SBLV multiplication and is dependent on SA signaling.

Infection and RNA recombination of Brome mosaic virus in Arabidopsis thaliana

Ecotypes of Arabidopsis thaliana supported the replication and systemic spread of Brome mosaic virus (BMV) RNAs. Infection was induced either by manual inoculation with viral RNA or by BMV virions, demonstrating that virus disassembly did not prevent infection. When in vitro-transcribed BMV RNAs 1-3 were used, production of subgenomic RNA4 was observed, showing that BMV RNA replication and transcription had occurred. Furthermore, inoculations of the transgenic Arabidopsis line that expressed a suppressor of RNA interference (RNAi) pathway markedly increased the BMV RNA concentrations. Inoculations with designed BMV RNA3 recombination vectors generated both homologous and nonhomologous BMV RNA-RNA recombinants. Thus, all cellular factors essential for BMV RNA replication, transcription, and RNA recombination were shown to be present in Arabidopsis. The current scope of understanding of the model Arabidopsis plant system should facilitate the identification of these factors governing the BMV life cycle.

Use of Spring beauty latent virus to identify compatible interactions between bromovirus components required for virus infection

Spring beauty latent virus (SBLV) is a member of the genus Bromovirus, and is closely related to Brome mosaic virus (BMV) and Cowpea chlorotic mottle virus (CCMV). Compatible interactions between viral components are required for successful infection of plants by BMV and CCMV. To further our understanding of interactions between bromovirus components, we used SBLV to produce reassortants among the three bromoviruses. We found that SBLV RNA 2 functioned with heterologous bromovirus RNA 1 in infections of whole plants and protoplasts of Nicotiana benthamiana, although SBLV RNA 1 did not function with heterologous bromovirus RNA 2. A DNA-based transient assay for 1a and 2a proteins, which are encoded by RNAs 1 and 2, respectively further suggested that SBLV 2a protein may function in combination with heterologous bromovirus 1a protein. Moreover, analysis of the ability of reassortants to spread locally revealed that an RNA 2-mediated interaction between viral components may be required for efficient cell-to-cell movement of bromoviruses.

A transcriptionally active subgenomic promoter supports homologous crossovers in a plus-strand RNA virus

Genetic RNA recombination plays an important role in viral evolution, but its molecular mechanism is not well understood. In this work we describe homologous RNA recombination activity that is supported by a subgenomic promoter (sgp) region in the RNA3 segment of brome mosaic bromovirus (BMV), a tripartite plus-strand RNA virus. The crossover frequencies were determined by coinoculations with pairs of BMV RNA3 variants that carried a duplicated sgp region flanked by marker restriction sites. A region composed of the sgp core, a poly(A) tract, and an upstream enhancer supported homologous exchanges in 25% of the analyzed RNA3 progeny. However, mutations in the sgp core stopped both the transcription of the sgp RNA and homologous recombination. These data provide evidence for an association of RNA recombination with transcription.

Complete nucleotide sequence of spring beauty latent virus, a bromovirus infectious to Arabidopsis thaliana

Spring beauty latent virus (SBLV), a bromovirus, systemically and efficiently infected Arabidopsis thaliana, whereas the well-studied bromoviruses brome mosaic virus (BMV) and cowpea chlorotic mottle virus (CCMV) did not infect and poorly infected A. thaliana, respectively. We constructed biologically active cDNA clones of SBLV genomic RNAs and determined their complete nucleotide sequences. Interestingly, SBLV RNA3 contains both the box B motif in the intercistronic region, as does BMV, and the subgenomic promoter-like sequence in the 5' noncoding region, as does CCMV. Sequence comparisons of SBLV, BMV, CCMV, and broad bean mottle virus demonstrated that SBLV is closely related to BMV and CCMV.

Studies on functional interaction between brome mosaic virus replicase proteins during RNA recombination, using combined mutants in vivo and in vitro

Two viral proteins, 1a and 2a, direct replication of brome mosaic bromovirus (BMV) RNAs as well as they participate in BMV RNA recombination. To study the relationship between replication and recombination, double BMV variants that carried mutations in 1a and 2a genes were tested. The observed effects revealed that the 1a helicase and 2a N-terminal or core domains were functionally linked during both processes in vivo. The use of a series of mutant BMV replicase (RdRp) preparations demonstrated in vitro the participation of the 1a and 2a domains in BMV RNA copying and in template switching during minus-strand synthesis. The observed effects support previous observations that the characteristics of homologous and nonhomologous recombination can be modified separately by mutations at different sites on BMV replicase proteins.

The C terminus of brome mosaic virus coat protein controls viral cell-to-cell and long-distance movement

To investigate the functional domains of the coat protein (CP; 189 amino acids) of Brome mosaic virus, a plant RNA virus, 19 alanine-scanning mutants were constructed and tested for their infectivity in barley and Nicotiana benthamiana. Despite its apparent normal replicative competence and CP production, the C-terminal mutant F184A produced no virions. Furthermore, virion-forming C-terminal mutants P178A and D182A failed to move from cell to cell in both plant species, and mutants D181A and V187A showed host-specific movement. These results indicate that the C-terminal region of CP plays some important roles in virus movement and encapsidation. The specificity of certain mutations for viral movement in two different plant species is evidence for the involvement of host-specific factors.

Mapping the molecular determinant of pathogenicity in a hammerhead viroid: a tetraloop within the in vivo branched RNA conformation

Chrysanthemum chlorotic mottle viroid (CChMVd) is an RNA of 398-399 nt that can adopt hammerhead structures in both polarity strands. We have identified by Northern-blot hybridization a nonsymptomatic strain (CChMVd-NS) that protects against challenge inoculation with the symptomatic strain (CChMVd-S). Analysis of CChMVd-NS cDNA clones has revealed a size and sequence very similar to those of the CChMVd-S strain. Some of the mutations observed in CChMVd-NS molecular variants were previously identified in CChMVd-S RNA, but others were never found in this RNA. When bioassayed in chrysanthemum, cDNA clones containing the CChMVd-NS specific mutations were infectious but nonsymptomatic. Site-directed mutagenesis showed that one of the CChMVd-NS-specific mutations, a UUUC --> GAAA substitution, was sufficient to change the symptomatic phenotype into the nonsymptomatic one without altering the final accumulation level of the viroid RNA. The pathogenicity determinant-to our knowledge, a determinant of this class has not been described previously in hammerhead viroids-is located in a tetraloop of the computer-predicted branched conformation for CChMVd RNA. Analysis of the sequence heterogeneity found in CChMVd-S and -NS variants strongly supports the existence of such a conformation in vivo, showing that the rod-like or quasi-rod-like secondary structure is not a universal paradigm for viroids.

Characterization of the brome mosaic virus movement protein expressed in E. coli

The biochemical and functional properties of the movement protein (MP) of brome mosaic virus (BMV) were investigated. Expression and purification of the BMV MP from Escherichia coli resulted in a pure and soluble protein preparation. Sucrose gradient centrifugation revealed that BMV MP forms oligomers consisting of two or more copies but no higher order multimers even when different ionic strengths and pHs were applied. Nitro-cellulose filter binding and gel retardation studies showed that in vitro the BMV MP preferentially bound to ss nucleic acids (RNA and DNA); the affinity to ssRNA was lower compared to BMV coat protein. The binding to ss nucleic acid was cooperative and not sequence specific and the hypothetical binding site was calculated to be around three to six nucleotides per MP monomer. The nucleic acid binding properties of the BMV MP are discussed in relation to the recent finding that this protein is also able to form tubular structures in infected protoplasts.

Molecular studies on bromovirus capsid protein. IV. Coat protein exchanges between brome mosaic and cowpea chlorotic mottle viruses exhibit neutral effects in heterologous hosts

Two members of the bromovirus group, brome mosaic virus (BMV) and cowpea chlorotic mottle virus (CCMV), selectively infect barley and cowpea, respectively, and also differ in their ability to systemically infect a common permissive host, Chenopodium quinoa. CCMV is confined to inoculated leaves of C. quinoa, whereas BMV causes rapid systemic mottling. To examine whether host-specific determinants for systemic movement of BMV and CCMV in each of these hosts are localized in the coat protein (CP), sequences encoding this gene were exchanged between biologically active clones of BMV RNA3 (B3) and CCMV RNA3 (C3) to create chimera expressing heterologous CP genes (B3/CCP and C3/BCP). Inoculation of each chimera with its respective wild-type (wt) RNAs 1 and 2 to barley or cowpea or C. quinoa plants resulted in symptom phenotype and long distance movement characteristics similar to those of the parental virus donating RNAs 1 and 2. These observations suggest that neither BMV CP nor CCMV CP has host-specific determinants for long distance movement. Inoculation of additional recombinant viruses, constructed by reassorting wt genomic RNAs 1 and 2 of BMV and CCMV with either heterologous wt RNA3 (i.e., B1 + B2 + C3 and C1 + C2 + B3) or heterologous chimeric RNA3 (i.e., B1 + B2 + C3/BCP and C1 + C2 + B3/CCP), to susceptible hosts resulted only in localized infections. The significance of these observations in relation to bromovirus movement is discussed.

Effects of coat protein mutations and reduced movement protein expression on infection spread by cowpea chlorotic mottle virus and its hybrid derivatives

Previously we have reported that the essential 3a movement gene of icosahedral cowpea chlorotic mottle virus (CCMV) can be functionally replaced by the 30-kDa movement gene of rod-shaped sunn-hemp mosaic virus (SHMV). Because plant RNA viruses differ in requiring or not requiring coat protein for systemic infection, we have now investigated whether systemic spread by this CCMV/SHMV hybrid is dependent on its CCMV coat protein as well as its SHMV movement protein. We find that either deletion or frameshift mutations in the coat protein gene block systemic spread. Thus, like wild-type CCMV, systemic infection by the hybrid is dependent on both movement protein and coat protein. These results further support the conclusion that the required functions of the coat and movement proteins in CCMV spread do not depend on sequence-specific interaction between these proteins. Additional features of the hybrid also motivated testing the effects of modulating movement protein expression. Creating an extra, out-of-frame translational start codon (AUG) shortly upstream of the 3a movement protein gene in CCMV downregulated its expression 18-fold. Nevertheless, for CCMV derivatives bearing either the CCMV 3a gene or the SHMV 30-kDa gene, the extra AUG resulted in only a minor delay in the onset of viral spread and little or no effect on the subsequent rate of cell-to-cell spread. Thus, under normal circumstances, the rate of CCMV cell-to-cell spread in cowpea plants appears to be limited primarily by factors other than movement protein synthesis.

Characterization of a disassembly deficient mutant of cowpea chlorotic mottle virus

An understanding of virus disassembly requires a detailed understanding of the protein-protein and protein-nucleic acid interactions which stabilize the virion. We have characterized a mutant of cowpea chlorotic mottle virus [cpR26C (coat protein R26C)] that displays increased virion stability and is abnormal in virion disassembly when purified under nonreducing conditions. Reduced virions are infectious, whereas nonreduced virions are noninfectious. The cpR26C mutant virions purified under nonreducing conditions resist disassembly in 0.5 M CaCl2, pH 7.5. The nonreduced cpR26C mutant virions swell in neutral pH conditions (pH 7.5) but do not disassociate when the ionic strength is increased. In contrast, wild-type virions or cpR26C mutant virions isolated under reducing conditions completely disassociate into the RNA and capsid protein components at pH 7.5 and high ionic strength (i > 1.0). Sequence analysis of the cpR26C mutant identified a single C to U nucleotide change at position 1435 of RNA 3 (position 86 of RNA 4), which results in a arginine to cysteine change at position 26 of the coat protein. The cpR26C mutant provides an ideal chemical switch for examining virion assembly and disassembly.

Molecular studies on bromovirus capsid protein. II. Functional analysis of the amino-terminal arginine-rich motif and its role in encapsidation, movement, and pathology

The N-terminal region of the brome mosaic bromovirus (BMV) coat protein (CP) contains an arginine-rich motif that is conserved among plant and nonplant viruses and implicated in binding the RNA during encapsidation. To elucidate the functional significance of this conserved motif in the BMV CP, a series of deletions encompassing the arginine-rich motif was introduced into a biologically active clone of BMV RNA3, and their effect on replication, encapsidation, and infection in plants was examined. Analysis of infection phenotypes elicited on Chenopodium quinoa revealed the importance of the first 19 N-proximal amino acids of BMV CP in encapsidation and pathogenicity. Inoculation of C. quinoa with three viable variants of BMV RNA3 lacking the first 11, 14, and 18 N-terminal amino acids of the CP resulted in the development of necrotic local lesions and restricted the spread of infection to inoculated leaves. Progeny analysis from symptomatic leaves revealed that, in each case, virus accumulation was severely affected by the introduced mutations and each truncated CP differed in its ability to package genomic RNA. In contrast to these observations in C. quinoa, none of the CP variants was able to establish either local or systemic infections in barley plants. The intrinsic role played by the N-terminal arginine-rich motif of BMV CP in packaging viral RNAs and the interactions between the host and the truncated CPs in modulating symptom expression and movement are discussed.

A single codon change in a conserved motif of a bromovirus movement protein gene confers compatibility with a new host

Brome mosaic virus (BMV) and cowpea chlorotic mottle virus (CCMV) are closely related bromoviruses with tripartite RNA genomes, but distinct host ranges: BMV systemically infects the monocot barley, while CCMV systemically infects the dicot cowpea. We have previously shown that in approximately 10% of inoculated cowpea plants, a CCMV hybrid [CCMV(B3a)] with the 3a cell-to-cell movement protein gene replaced by that of cowpea-nonadapted BMV directs systemic infections, which are caused by secondary mutation(s) of the hybrid virus. Here, to further analyze the role of RNA3 in adaptation to a new host, RNA3 cDNA clones were constructed from total RNA recovered from the uninoculated upper leaves of systemically infected cowpea plants inoculated with CCMV(B3a). Sequence and mutational analysis of two such RNA3 clones revealed that a single codon change (A776-->C) in a conserved motif of the 3a movement protein gene conferred compatibility for systemic infection of a new host, cowpea, suggesting that this site in the 3a gene is directly or indirectly involved in crucial host interactions associated with host-range specificity. The adaptive hybrid viruses carrying this mutation induced exacerbated symptoms, while wt CCMV appeared nearly symptomless, showing that the bromovirus 3a movement protein gene can significantly contribute to regulating symptom development. However, introducing this cowpea-adaptive mutation into the BMV genome had little effect on the ability of BMV to systemically infect barley.

Coding changes in the 3a cell-to-cell movement gene can extend the host range of brome mosaic virus systemic infection

The M1 and M2 strains of brome mosaic virus (BMV) both systemically infect the monocot host barley, but only the M2 strain systemically infects the dicot cowpea line TVu-612. We have shown previously that this difference in host range maps primarily to RNA3. To further characterize the role of RNA3 in host specificity, a series of RNA3 hybrids were tested, in inoculations with M1 RNA1 and RNA2, for ability to systemically infect TVu-612 cowpea. Although all hybrids were amplified well in cowpea protoplasts and all supported systemic infection in barley plants, only those with the 3a cell-to-cell movement gene of BMV-M2 supported systemic infection of cowpea. The sequences of the M1 and M2 3a proteins differ at four positions. Introducing these four coding differences individually or in various combinations into M1 RNA3 revealed that all four influenced BMV adaptation to cowpea and that these four differences were sufficient to account for the difference in ability between M1 and M2 RNA3s to support systemic infection of this legume. These coding changes were also associated with faster spread of infection in inoculated cowpea leaves, suggesting that they influence the ability to systemically infect TVu-612 cowpea through effects on the rate of cell-to-cell spread.

Inhibition of brome mosaic virus (BMV) amplification in protoplasts from transgenic tobacco plants expressing replicable BMV RNAs

Transgenic tobacco plants (V123 plants) expressing a set of full-length brome mosaic virus (BMV) genomic RNAs from the cauliflower mosaic virus 35S promoter were produced. The accumulation level of BMV RNAs in V123 plant cells was approximately 1% of that in nontransgenic tobacco protoplasts inoculated with BMV RNAs. The level of BMV RNA in V123 protoplasts did not increase after inoculating the protoplasts with BMV RNAs, whereas V123 protoplasts supported the accumulation of cucumber mosaic virus (CMV) RNAs to a level similar to that in non-transgenic tobacco protoplasts after inoculation with CMV RNA. Such BMV-specific resistance was also observed in protoplasts from V12 plants expressing full-length BMV RNA1 and RNA2, both of which are required and sufficient for BMV RNA replication. On the other hand, protoplasts from M12 plants, expressing truncated BMV RNA1 and RNA2 in which the 3' 200 nucleotides required for BMV RNA replication were deleted, exhibited weaker resistance to infection with BMV RNA than V12 protoplasts, although the accumulation level of truncated BMV RNA1 and RNA2 in M12 protoplasts was higher than that of BMV RNA1 and RNA2 in V12 protoplasts. These results suggest that expression of BMV RNA replicons is involved in the induction of resistance, rather than high-level accumulation of BMV RNAs and/or their encoded proteins.

A spontaneous mutation in the movement protein gene of brome mosaic virus modulates symptom phenotype in Nicotiana benthamiana

Brome mosaic virus (BMV) is a positive-strand RNA virus with a multipartite genome that causes symptomless infection in Nicotiana benthamiana. We have isolated and characterized a strain of BMV that produced uniform vein chlorosis in systemically infected N. benthamiana. Analysis of pseudorecombinants constructed by exchanging RNA 1 and 2 and RNA 3 components between wild-type (non-symptom-inducing) and vein chlorosis-inducing strains of BMV indicated that the genetic determinant for the induction of the chlorotic phenotype is located on RNA 3. Sequence analysis of progeny RNA 3 recovered from symptomatic N. benthamiana plants revealed that vein chlorosis is due to the single nucleotide transition 887G-->887A, which changes the codon for Val-266 to Ile-266 in the movement protein gene. The mutation had no detectable effect on the accumulation of virus in either inoculated or systematically infected leaves of N. benthamiana. The vein chlorosis phenotype is the manifestation of the substitution of Ile-266 for Val-266 in the movement protein gene, since additional alterations in this region (a silent mutation, i.e., 887GUU889-->GUC, and an alteration of valine to phenylalanine, i.e., 887GUU889-->887UUU889) resulted in symptomless infections on N. benthamiana. The modulation of the symptom phenotype by the substitution of Ile-266 for Val-266 is specific for N. benthamiana, since neither movement nor the symptom phenotype in barley plants was affected.

Mutational analysis of the coat protein gene of brome mosaic virus: effects on replication and movement in barley and in Chenopodium hybridum

The coat protein (CP) open reading frame (ORF) of brome mosaic virus (BMV) has been mutated to study host-related CP functions in barley, a systemic host, and in Chenopodium hybridum L. which supports both local lesion formation and systemic spread of BMV. To test the role of the N-terminal region of CP, mutants C1 to C3, which synthesized the CP lacking first seven amino acids, and mutant D1, which had Trp 22 and Thr 23 replaced with Phe-Gly-Ser, were generated. C1 to C3 inhibited virus systemic spread in C. hybridum but not in barley while D1 only reduced virus accumulation in noninoculated leaves of C. hybridum. More internal CP regions were tested by mutation of Lys 63 to Leu (mutant SP3) and Lys 129 to Arg (mutant SP1). SP1 behaved similarly to C1 to C3 while SP3 similarly to D1. In addition, SP3 reduced concentrations of RNA3 and RNA4 in both hosts. Apparently, various CP regions differentially affect, either directly or indirectly, virus translocation in different hosts, suggesting both the CP and host factors to be important for virus spread. Larger deletions in the CP ORF (mutants BB4 and SX1) or a decrease of CP production by using a frameshift mutant C, inhibited virus systemic spread in both hosts, and delayed the appearance of smaller local lesions on C. hybridum. Thus, the CP is not required for cell-to-cell movement but is required for systemic translocation of BMV.

Infectious transcripts from PCR-amplified broad bean mottle bromovirus cDNA clones and variable nature of leader regions in RNA 3 segment

The genome of broad bean mottle bromovirus (BBMV) contains three positive-sense ssRNA segments, each capped with m7GpppA. Full-length transcribable cDNA clones for four strains of BBMV were constructed by employing reverse transcriptase-PCR (RT-PCR) and a high fidelity Vent DNA polymerase. The transcribed BBMV RNAs contained a 5' non-viral G residue and, although delayed, produced symptoms similar to those observed in plants infected with authentic virion RNAs. The transcripts replicated inefficiently in protoplasts. In contrast, transcript-derived progeny BBMV RNAs had the repaired termini, were as infectious as the authentic BBMV RNAs and replicated to high levels in protoplasts. In vitro translation of synthetic RNAs confirmed the previously proposed gene expression strategy for BBMV. Sequencing of virion RNAs from the Bawden strain revealed two forms of BBMV RNA3 components, the longer form containing 21 5' extra nucleotides derived by the duplication of two short 5' leader regions. The relative concentration of the two RNA 3 forms was found to be host-dependent, with the longer form prevailing in broad bean and Nictiana clevelandii infections and the shorter form in bean infections.

Second-site mutations in the brome mosaic virus RNA3 intercistronic region partially suppress a defect in coat protein mRNA transcription

An intercistronic oligo(A) tract is present in the genomic RNA3 of all bromoviruses sequenced to date and, for brome mosaic virus (BMV), is known to function as an activating sequence in transcription of the subgenomic coat protein mRNA, RNA4. Mutations able to partially compensate for removal of the oligo(A) from BMV RNA3 were identified by obtaining spontaneous second-site revertants. The starting BMV RNA3 mutant carried a deletion of 17 of 18 residues of the intercistronic oligo(A), resulting in a nearly complete loss of subgenomic coat protein mRNA synthesis and reduced RNA3 accumulation. The responsible suppressor mutations acted in cis and were located in the 244-base RNA3 intercistronic region containing the original deletion. Three mutations associated with the revertant phenotype were characterized: (i) A single U-->A substitution in the core subgenomic mRNA promoter restored 35% of wild-type promoter activity. (ii) A duplication of 8 bases (UAUUAUUA) immediately 5' to the oligo(A) deletion site resulted in higher levels of both RNA3 and RNA4 accumulation. (iii) A point substitution in a conserved cellular motif corresponding to box B of RNA polymerase III promoters reduced RNA3 accumulation. Together with certain intervirally conserved promoter sequences, the spontaneous adaptation of a bromovirus subgenomic promoter to function without its unusual oligo(A) activator suggests that bromovirus subgenomic mRNA transcription may share underlying mechanistic similarities with the many other members of the alphavirus-like superfamily, whose subgenomic promoters all lack an oligo(A) tract.