Near identity of 3- RNA secondary structure in bromoviruses and cucumber mosaic virus

Diversity and modularity of tyrosine-accepting tRNA-like structures

Certain positive-sense single-stranded RNA viruses contain elements at their 3' termini that structurally mimic tRNAs. These tRNA-like structures (TLSs) are classified based on which amino acid is covalently added to the 3' end by host aminoacyl-tRNA synthetase. Recently, a cryoEM reconstruction of a representative tyrosine-accepting tRNA-like structure (TLSTyr) from brome mosaic virus (BMV) revealed a unique mode of recognition of the viral anticodon-mimicking domain by tyrosyl-tRNA synthetase. Some viruses in the hordeivirus genus of Virgaviridae are also selectively aminoacylated with tyrosine, yet these TLS RNAs have a different architecture in the 5' domain that comprises the atypical anticodon loop mimic. Herein, we present bioinformatic and biochemical data supporting a distinct secondary structure for the 5' domain of the hordeivirus TLSTyr compared to those in Bromoviridae Despite forming a different secondary structure, the 5' domain is necessary to achieve robust in vitro aminoacylation. Furthermore, a chimeric RNA containing the 5' domain from the BMV TLSTyr and the 3' domain from a hordeivirus TLSTyr are aminoacylated, illustrating modularity in these structured RNA elements. We propose that the structurally distinct 5' domain of the hordeivirus TLSTyrs performs the same role in mimicking the anticodon loop as its counterpart in the BMV TLSTyr Finally, these structurally and phylogenetically divergent types of TLSTyr provide insight into the evolutionary connections between all classes of viral tRNA-like structures.

Chemical and Enzymatic Probing of Viral RNAs: From Infancy to Maturity and Beyond

RNA molecules are key players in a variety of biological events, and this is particularly true for viral RNAs. To better understand the replication of those pathogens and try to block them, special attention has been paid to the structure of their RNAs. Methods to probe RNA structures have been developed since the 1960s; even if they have evolved over the years, they are still in use today and provide useful information on the folding of RNA molecules, including viral RNAs. The aim of this review is to offer a historical perspective on the structural probing methods used to decipher RNA structures before the development of the selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) methodology and to show how they have influenced the current probing techniques. Actually, these technological breakthroughs, which involved advanced detection methods, were made possible thanks to the development of next-generation sequencing (NGS) but also to the previous works accumulated in the field of structural RNA biology. Finally, we will also discuss how high-throughput SHAPE (hSHAPE) paved the way for the development of sophisticated RNA structural techniques.

Heterologous Replicase from Cucumoviruses can Replicate Viral RNAs, but is Defective in Transcribing Subgenomic RNA4A or Facilitating Viral Movement

Interspecific exchange of RNA1 or RNA2 between the cucumoviruses cucumber mosaic virus (CMV) and tomato aspermy virus (TAV) was reported to be non-viable in plants previously. Here we investigated viability of the reassortants between CMV and TAV in Nicotiana benthamiana plants by Agrobacterium-mediated viral inoculation. The reassortants were composed of CMV RNA1 and TAV RNA2 plus RNA3 replicated in the inoculated leaves, while they were defective in viral systemic movement at the early stage of infection. Interestingly, the reassortant containing TAV RNA1 and CMV RNA2 and RNA3 infected plants systemically, but produced RNA4A (the RNA2 subgenome) at an undetectable level. The defect in production of RNA4A was due to the 1a protein encoded by TAV RNA1, and partially restored by replacing the C-terminus (helicase domain) in TAV 1a with that of CMV 1a. Collectively, exchange of the replicase components between CMV and TAV was acceptable for viral replication, but was defective in either directing transcription of subgenomic RNA4A or facilitating viral long-distance movement. Our finding may shed some light on evolution of subgenomic RNA4A in the family Bromoviridae.

Probing of RNA structures in a positive sense RNA virus reveals selection pressures for structural elements

In single stranded (+)-sense RNA viruses, RNA structural elements (SEs) play essential roles in the infection process from replication to encapsidation. Using selective 2'-hydroxyl acylation analyzed by primer extension sequencing (SHAPE-Seq) and covariation analysis, we explore the structural features of the third genome segment of cucumber mosaic virus (CMV), RNA3 (2216 nt), both in vitro and in plant cell lysates. Comparing SHAPE-Seq and covariation analysis results revealed multiple SEs in the coat protein open reading frame and 3' untranslated region. Four of these SEs were mutated and serially passaged in Nicotiana tabacum plants to identify biologically selected changes to the original mutated sequences. After passaging, loop mutants showed partial reversion to their wild-type sequence and SEs that were structurally disrupted by mutations were restored to wild-type-like structures via synonymous mutations in planta. These results support the existence and selection of virus open reading frame SEs in the host organism and provide a framework for further studies on the role of RNA structure in viral infection. Additionally, this work demonstrates the applicability of high-throughput chemical probing in plant cell lysates and presents a new method for calculating SHAPE reactivities from overlapping reverse transcriptase priming sites.

Development of a molecular assay for the detection of Cucumber mosaic virus and the discrimination of its subgroups I and II

A nucleic acid based test for the detection of the economically important plant virus Cucumber mosaic virus (CMV) based on the Luminex xTAG technology was developed. This technology has the advantage of allowing the simultaneous detection of various targets. Applying this method, we prove the presence of CMV in general and differentiate between its two subgroups I and II for which significant differences concerning severity of symptoms and virulence have been reported. For the development of the test procedure the coat protein gene sequences of 29 CMV isolates were cloned, sequenced and classified into subgroups. Sequences from GenBank were used to design primers. Additionally, a subgroup specific ELISA was conducted for comparison. This work is part of a project which aims to develop a test for the simultaneous detection of various plant pathogens (viral, bacterial and fungal) in plant material.

Single-Molecule FRET Reveals Three Conformations for the TLS Domain of Brome Mosaic Virus Genome

Metabolite-dependent conformational switching in RNA riboswitches is now widely accepted as a critical regulatory mechanism for gene expression in bacterial systems. More recently, similar gene regulation mechanisms have been found to be important for viral systems as well. One of the most abundant and best-studied systems is the tRNA-like structure (TLS) domain, which has been found to occur in many plant viruses spread across numerous genera. In this work, folding dynamics for the TLS domain of Brome Mosaic Virus have been investigated using single-molecule fluorescence resonance energy transfer techniques. In particular, burst fluorescence methods are exploited to observe metal-ion ([M(n+)])-induced folding in freely diffusing RNA constructs resembling the minimal TLS element of brome mosaic virus RNA3. The results of these experiments reveal a complex equilibrium of at least three distinct populations. A stepwise, or consecutive, thermodynamic model for TLS folding is developed, which is in good agreement with the [M(n+)]-dependent evolution of conformational populations and existing structural information in the literature. Specifically, this folding pathway explains the metal-ion dependent formation of a functional TLS domain from unfolded RNAs via two consecutive steps: 1) hybridization of a long-range stem interaction, followed by 2) formation of a 3'-terminal pseudoknot. These two conformational transitions are well described by stepwise dissociation constants for [Mg(2+)] (K1 = 328 ± 30 μM and K2 = 1092 ± 183 μM) and [Na(+)] (K1 = 74 ± 6 mM and K2 = 243 ± 52 mM)-induced folding. The proposed thermodynamic model is further supported by inhibition studies of the long-range stem interaction using a complementary DNA oligomer, which effectively shifts the dynamic equilibrium toward the unfolded conformation. Implications of this multistep conformational folding mechanism are discussed with regard to regulation of virus replication.

Fixation of emerging interviral recombinants in cucumber mosaic virus populations

Interstrain recombinants were observed in the progenies of the Cucumber mosaic virus (CMV) reassortant L(1)L(2)F(3) containing RNAs 1 and 2 from LS-CMV and RNA 3 from Fny-CMV. We characterized these recombinants, and we found that their fixation was controlled by the nature of the replicating RNAs 1 and 2. We demonstrate that the 2b gene partially affects this fixation process, but only in the context of homologous RNAs 1 and 2.

Pyrophosphorolysis of CCA addition: implication for fidelity

In nucleic acid polymerization reaction, pyrophosphorolysis is the reversal of nucleotide addition, in which the terminal nucleotide is excised in the presence of inorganic pyrophosphate (PPi). The CCA enzymes are unusual RNA polymerases, which catalyze CCA addition to positions 74-76 at the tRNA 3' end without using a nucleic acid template. To better understand the reaction mechanism of CCA addition, we tested pyrophosphorolysis of CCA enzymes, which are divided into two structurally distinct classes. Here, we show that only class II CCA enzymes catalyze pyrophosphorolysis and that the reaction can initiate from all three CCA positions and proceed processively until the removal of nucleotide C74. Pyrophosphorolysis of class II enzymes establishes a fundamental difference from class I enzymes, and it is achieved only with the tRNA structure and with specific divalent metal ions. Importantly, pyrophosphorolysis enables class II enzymes to efficiently remove an incorrect A75 nucleotide from the 3' end, at a rate much faster than the rate of A75 incorporation, suggesting the ability to perform a previously unexpected quality control mechanism for CCA synthesis. Measurement of kinetic parameters of the class II Escherichia coli CCA enzyme reveals that the enzyme catalyzes pyrophosphorolysis slowly relative to the forward nucleotide addition and that it exhibits weak binding affinity to PPi relative to NTP, suggesting a mechanism in which PPi is rapidly released after each nucleotide addition as a driving force to promote the forward synthesis of CCA.

Viral tRNAs and tRNA-like structures

Viruses commonly exploit or modify some aspect of tRNA biology. Large DNA viruses, especially bacteriophages, phycodnaviruses, and mimiviruses, produce their own tRNAs, apparently to adjust translational capacity during infection. Retroviruses recruit specific host tRNAs for use in priming the reverse transcription of their genome. Certain positive-strand RNA plant viral genomes possess 3'-tRNA-like structures (TLSs) that are built quite differently from authentic tRNAs, and yet efficiently recapitulate several properties of tRNAs. The structures and roles of these TLSs are discussed, emphasizing the variety in both structure and function.

Bacterial gene inserted in an engineered RNA virus: efficient expression in monocotyledonous plant cells

Brome mosaic virus (BMV) is a plant virus whose genome consists of three RNA components. A previously described viral complementary DNA expression system has been used to express both wild-type and altered genomic RNA's in barley protoplasts. Variants of BMV RNA3 were constructed in which the coat gene had been removed or replaced with sequences encoding chloramphenicol acetyltransferase (CAT). CAT sequences were also inserted near the 5' end of the intact coat gene. When inoculated on protoplasts together with transcripts of BMV RNA's 1 and 2, all of these RNA3 derivatives were replicated and produced subgenomic RNA's analogous to the normal subgenomic coat protein messenger RNA. RNA3 derivatives in which the CAT coding sequences were oriented with the same polarity as viral genes produced significant CAT activity in protoplasts. CAT expression was improved by inserting the CAT gene in frame with the upstream coat protein initiation codon, and exceeded expression in plant cells transformed with Ti plasmid-based vectors.

Multicomponent RNA plant virus infection derived from cloned viral cDNA

In vitro transcripts from mixtures of appropriate brome mosaic virus (BMV) cDNA clones are infectious when inoculated onto barley plants. Infectivity depends on in vitro transcription and on the presence of transcripts from clones of all three BMV genetic components. Infectivity is destroyed by RNase after transcription, but it is insensitive to RNase before or to DNase after transcription. Virion RNAs from plants infected with cDNA transcripts hybridize to BMV-specific probes and coelectrophorese with virion RNAs propagated from conventional inoculum. Direct RNA sequencing shows that a deletion in the noncoding region of one infectious BMV clone is preserved in viral RNA from plants systemically infected with transcript mixtures representing that clone.

Analysis of recombination between viral RNAs and transgene mRNA under conditions of high selection pressure in favour of recombinants

One possible environmental risk related to the utilization of virus-resistant transgenic plants expressing viral sequences is the emergence of new viruses generated by recombination between the viral transgene mRNA and the RNA of an infecting virus. This hypothesis has been tested recently for cucumber mosaic virus (CMV) by comparing the recombinant populations in transgenic and non-transgenic plants under conditions of minimal selection pressure in favour of the recombinants. Equivalent populations were observed in transgenic and non-transgenic plants but, in both, there was a strongly dominant hotspot recombinant which was shown recently to be nonviable alone in planta, suggesting that its predominance could be reduced by applying an increased selection pressure in favour of viable recombinants. Partially disabled I17F-CMV mutants were created by engineering 6 nt deletions in five sites in the RNA3 3'-non-coding region (3'-NCR). One mutant was used to inoculate transgenic tobacco plants expressing the coat protein and 3'-NCR of R-CMV. A total of 22 different recombinant types were identified, of which 12 were, as expected, between the transgene mRNA and the mutated I17F-CMV RNA3, while 10 resulted from recombination between the mutated RNA3 and I17F-CMV RNA1. Twenty recombinants were of the aberrant type, while two, including the dominant one detected previously under conditions of minimal selection pressure, were homologous recombinants. All recombinants detected were very similar to ones observed in nature, suggesting that the deployment of transgenic lines similar to the one studied here would not lead to the emergence of new viruses.

Melandrium yellow fleck bromovirus infects Arabidopsis thaliana and has genomic RNA sequence characteristics that are unique among bromoviruses

Melandrium yellow fleck bromovirus (MYFV) systemically infected Arabidopsis thaliana, although the susceptibility of several A. thaliana accessions to MYFV differed from their susceptibility to the other two bromoviruses infecting A. thaliana. We constructed full-length cDNA clones of MYFV genomic RNAs 1, 2, and 3 and determined their complete nucleotide sequences. Similar to Broad bean mottle bromovirus, (1) the 5'-terminal nucleotide of the MYFV genomic RNAs was adenine, and (2) the "D-arm" was absent from the tRNA-like structure in the 3' untranslated regions (UTRs) of MYFV RNAs. As unique characteristics, MYFV RNA3 lacked the poly(A) tract in the intercistronic region and contained a directly repeated sequence of about 200 nucleotides and polypyrimidine tracts of heterogeneous lengths in the 5' UTR. Co-infection experiments using RNA3 clones with or without the duplicated sequence demonstrated that the duplication contributed to the competitive fitness of the virus in Nicotiana benthamiana.

Depurination of Brome mosaic virus RNA3 in vivo results in translation-dependent accelerated degradation of the viral RNA

Pokeweed antiviral protein (PAP) is a ribosome-inactivating protein isolated from the pokeweed plant (Phytolacca americana) that exhibits antiviral activity against several plant and animal viruses. We have shown previously that PAP depurinates Brome mosaic virus (BMV) RNAs in vitro and that prior incubation of these RNAs with PAP reduced their synthesis in barley protoplasts. To investigate the post-transcriptional effect of PAP on viral RNA in vivo, we transcribed BMV RNA3 and expressed PAP in the yeast, Saccharomyces cerevisiae, which is a surrogate host for BMV. With an inducible transcription system, we show that the half-life of RNA3 in PAP-expressing cells was significantly less than in cells expressing PAPx, its enzymatically inactive form. PAP bound to RNA3 and depurinated the RNA within open reading frames 3 and 4 and within untranslated regions of the RNA. The depurinated RNA was associated with polysomes, caused ribosomes to stall at the point of depurination, and was targeted for accelerated degradation by components of the No-go decay pathway. As a consequence of translation elongation arrest and increased RNA degradation, expression of PAP in yeast also decreased the level of protein 3a, encoded by the 5'-proximal open reading frame 3 of BMV RNA3. These data provide the first evidence of viral RNA depurination in vivo by any ribosome-inactivating protein and support our hypothesis that depurination contributes to the antiviral activity of PAP, by enhancing viral RNA degradation and reducing translation of viral protein product.

Structural and functional characterization of the 5' region of subgenomic RNA5 of cucumber mosaic virus

The uncapped and ORF-less subgenomic RNA5 is produced in subgroup II strains of cucumber mosaic virus (CMV), but not in subgroup I strains. Its initiation nucleotide (nt 1903) is in a 21 nt conserved sequence (Box1) that is absent in CMV subgroup I. Putative non-coding RNA structural elements surrounding Box1 in the plus and minus strand were identified in silico and by in vitro RNase probing. Four main stem-loop structures (SLM, SLL, SLK and SLJ) were identified between nt 1887 and 1999 of isolate R-CMV (subgroup II), with notable differences within SLM and SLL between the two strands. Mutation of a stem-loop within SLM, even when the predicted wild-type structure was maintained, showed significant reduction in RNA5 levels in planta. Three mutants containing 3-4 nt substitutions between positions -39 and +49 showed significantly reduced levels of RNA5, while another similar mutant at positions 80-83 had RNA5 levels comparable to wild-type. Deletion of Box1 resulted in similar levels of RNA3 and 4 as wild-type, while eliminating RNA5. Insertion of Box1 into a subgroup I isolate was not sufficient to produce RNA5. However, in a mutant with an additional 21 nt of R-CMV 3' of Box1 (positions -1 to +41), low levels of RNA5 were detected. Taken together, these results have identified regions of the viral genome responsible for RNA5 production and in addition provide strong evidence for the existence of newly identified conserved structural elements in the 5' part of the 3' untranslated region.

Role of tRNA-like structures in controlling plant virus replication

Transfer RNA-like structures (TLSs) that are sophisticated functional mimics of tRNAs are found at the 3'-termini of the genomes of a number of plant positive strand RNA viruses. Three natural aminoacylation identities are represented: valine, histidine, and tyrosine. Paralleling this variety in structure, the roles of TLSs vary widely between different viruses. For Turnip yellow mosaic virus, the TLS must be capable of valylation in order to support infectivity, major roles being the provision of translational enhancement and down-regulation of minus strand initiation. In contrast, valylation of the Peanut clump virus TLS is not essential. An intermediate situation seems to exist for Brome mosaic virus, whose RNAs 1 and 2, but not RNA 3, need to be capable of tyrosylation to support infectivity. Other known roles for certain TLSs include: (i) the recruitment of host CCA nucleotidyltransferase as a telomerase to maintain intact 3' CCA termini, (ii) involvement in the encapsidation of viral RNAs, and (iii) presentation of minus strand promoter elements for replicase recognition. In the latter role, the promoter elements reside within the TLS but are not functionally dependent on tRNA mimicry. The phylogenetic distribution of TLSs indicates that their evolutionary history includes frequent horizontal exchange, as has been observed for protein-coding regions of plant positive strand RNA viruses.

Viral protein synthesis in barley protoplasts inoculated with native and fractionated brome mosaic virus RNA

When barley protoplasts were inoculated with brome mosaic virus (BMV) RNAs 1 and 2, there was a pronounced synthesis of the 110,000- and 100,000-dalton virally coded proteins. In contrast, there was no detectable synthesis of any viral proteins following inoculation with RNA 3 alone or RNA 4 alone. When RNAs 1 and 2 were recombined with RNA 3 in the inoculum, the profile of proteins synthesized was identical to that following inoculation with similar quantities of unfractionated BMV RNA; i.e., the 35,000-dalton virally coded protein and coat protein were synthesized in addition to the two high-molecular-weight viral polypeptides. RNAs 1 and 2 were shown not to be selectively bound (in preference to RNAs 3 or 4); hence, these data reveal that one or both of these RNAs encode proteins involved in early events of infection, perhaps replication.

Synthesis of minus-strand copies of a viral transgene during viral infections of transgenic plants

Viral transgenes designed to provide resistance to specific plant viruses frequently consist of the coat protein gene and a contiguous 3' untranslated region (3'UTR) of viral origin. In many RNA viruses the viral 3'UTR establishes a recognition and initiation site for viral RNA replication. Thus the transgenic transcript may contain a functional virus replication site. Experiments were designed to determine if a challenging virus would recognize this replication site on a nuclear derived transcript and synthesize the complementary RNA. These data demonstrate that upon infection by a virus that recognizes the viral replication site, a full-length complement of the transgenic transcript is produced. In these experiments the replication complex of Brome Mosaic bromovirus recognized the transgenic transcript derived from a Cowpea Chlorotic Mottle bromovirus transgene. The resulting RNA may contribute to RNA recombination events.

tRNA-like structure regulates translation of Brome mosaic virus RNA

For various groups of plant viruses, the genomic RNAs end with a tRNA-like structure (TLS) instead of the 3' poly(A) tail of common mRNAs. The actual function of these TLSs has long been enigmatic. Recently, however, it became clear that for turnip yellow mosaic virus, a tymovirus, the valylated TLS(TYMV) of the single genomic RNA functions as a bait for host ribosomes and directs them to the internal initiation site of translation (with N-terminal valine) of the second open reading frame for the polyprotein. This discovery prompted us to investigate whether the much larger TLSs of a different genus of viruses have a comparable function in translation. Brome mosaic virus (BMV), a bromovirus, has a tripartite RNA genome with a subgenomic RNA4 for coat protein expression. All four RNAs carry a highly conserved and bulky 3' TLS(BMV) (about 200 nucleotides) with determinants for tyrosylation. We discovered TLS(BMV)-catalyzed self-tyrosylation of the tyrosyl-tRNA synthetase but could not clearly detect tyrosine incorporation into any virus-encoded protein. We established that BMV proteins do not need TLS(BMV) tyrosylation for their initiation. However, disruption of the TLSs strongly reduced the translation of genomic RNA1, RNA2, and less strongly, RNA3, whereas coat protein expression from RNA4 remained unaffected. This aberrant translation could be partially restored by providing the TLS(BMV) in trans. Intriguingly, a subdomain of the TLS(BMV) could even almost fully restore translation to the original pattern. We discuss here a model with a central and dominant role for the TLS(BMV) during the BMV infection cycle.

Interaction with capsid protein alters RNA structure and the pathway for in vitro assembly of cowpea chlorotic mottle virus

Viruses use sophisticated mechanisms to allow the specific packaging of their genome over that of host nucleic acids. We examined the in vitro assembly of the Cowpea chlorotic mottle virus (CCMV) and observed that assembly with viral RNA follows two different mechanisms. Initially, CCMV capsid protein (CP) dimers bind RNA with low cooperativity and form virus-like particles of 90 CP dimers and one copy of RNA. Longer incubation reveals a different assembly path. At a stoichiometry of about ten CP dimers per RNA, the CP slowly folds the RNA into a compact structure that can be bound with high cooperativity by additional CP dimers. This folding process is exclusively a function of CP quaternary structure and is independent of RNA sequence. CP-induced folding is distinct from RNA folding that depends on base-pairing to stabilize tertiary structure. We hypothesize that specific encapsidation of viral RNA is a three-step process: specific binding by a few copies of CP, RNA folding, and then cooperative binding of CP to the "labeled" nucleoprotein complex. This mechanism, observed in a plant virus, may be applicable to other viruses that do not halt synthesis of host nucleic acid, including HIV.

Yeast Lsm1p-7p/Pat1p deadenylation-dependent mRNA-decapping factors are required for brome mosaic virus genomic RNA translation

Previously, we used the ability of the higher eukaryotic positive-strand RNA virus brome mosaic virus (BMV) to replicate in yeast to show that the yeast LSM1 gene is required for recruiting BMV RNA from translation to replication. Here we extend this observation to show that Lsm1p and other components of the Lsm1p-Lsm7p/Pat1p deadenylation-dependent mRNA decapping complex were also required for translating BMV RNAs. Inhibition of BMV RNA translation was selective, with no effect on general cellular translation. We show that viral genomic RNAs suitable for RNA replication were already distinguished from nonreplication templates at translation, well before RNA recruitment to replication. Among mRNA turnover pathways, only factors specific for deadenylated mRNA decapping were required for BMV RNA translation. Dependence on these factors was not only a consequence of the nonpolyadenylated nature of BMV RNAs but also involved the combined effects of the viral 5' and 3' noncoding regions and 2a polymerase open reading frame. High-resolution sucrose density gradient analysis showed that, while mutating factors in the Lsm1p-7p/Pat1p complex completely inhibited viral RNA translation, the levels of viral RNA associated with ribosomes were only slightly reduced in mutant yeast. This polysome association was further verified by using a conditional allele of essential translation initiation factor PRT1, which markedly decreased polysome association of viral genomic RNA in the presence or absence of an LSM7 mutation. Together, these results show that a defective Lsm1p-7p/Pat1p complex inhibits BMV RNA translation primarily by stalling or slowing the elongation of ribosomes along the viral open reading frame. Thus, factors in the Lsm1p-7p/Pat1p complex function not only in mRNA decapping but also in translation, and both translation and recruitment of BMV RNAs to viral RNA replication are regulated by a cell pathway that transfers mRNAs from translation to degradation.

Stable RNA structures can repress RNA synthesis in vitro by the brome mosaic virus replicase

A 15-nucleotide (nt) unstructured RNA with an initiation site but lacking a promoter could direct the initiation of RNA synthesis by the brome mosaic virus (BMV) replicase in vitro. However, BMV RNA with a functional initiation site but a mutated promoter could not initiate RNA synthesis either in vitro or in vivo. To explain these two observations, we hypothesize that RNA structures that cannot function as promoters could prevent RNA synthesis by the BMV RNA replicase. We documented that four different nonpromoter stem-loops can inhibit RNA synthesis from an initiation-competent RNA sequence in vitro. Destabilizing these structures increased RNA synthesis. However, RNA synthesis was restored in full only when a BMV RNA promoter element was added in cis. Competition assays to examine replicase-RNA interactions showed that the structured RNAs have a lower affinity for the replicase than do RNAs lacking stable structures or containing a promoter element. The results characterize another potential mechanism whereby the BMV replicase can specifically recognize BMV RNAs.

Brome mosaic virus RNA replication: revealing the role of the host in RNA virus replication

The replication of positive-strand RNA viruses is a complex multi-step process involving interactions between the viral genome, virus-encoded replication factors, and host factors. The plant virus brome mosaic virus (BMV) has served as a model for positive-strand RNA virus replication, recombination, and virion assembly. This review addresses recent findings on the identification and characterization of host factors in BMV RNA replication. To date, all characterized host factors facilitate steps that lead to assembly of a functional BMV RNA replication complex. Some of these host factors are required for regulation of viral gene expression. Others are needed to co-regulate BMV RNA translation and recruitment of BMV RNAs from translation to viral RNA replication complexes on the endoplasmic reticulum. Other host factors provide essential lipid modifications in the endoplasmic reticulum membrane or function as molecular chaperones to activate the replication complex. Characterizing the functions of these host factors is revealing basic aspects of virus RNA replication and helping to define the normal functions of these factors in the host.

tRNA elements mediate the assembly of an icosahedral RNA virus

tRNAs, the adapter molecules in protein synthesis, also serve as metabolic cofactors and as primers for viral RNA-directed DNA synthesis. The genomic and subgenomic RNAs of some plant viruses have a 3'-terminal tRNA-like structure (TLS) that can accept a specific amino acid and serve as a site for initiation of replication and as a simple telomere. We report a previously undescribed role for the TLS of brome mosaic virus (BMV), and potentially for cellular tRNA, in mediating the assembly of its icosahedral virions. BMV genomic RNAs and subgenomic RNA lacking the TLS failed to assemble into virions when incubated with purified BMV coat protein. Assembly was restored by addition of a 201-nt RNA containing the BMV TLS. TLSs from two other plant viruses as well as tRNAs from wheat germ and yeast were similarly active in the BMV virion assembly reaction, but ribosomal RNA and polyadenylate did not facilitate assembly. Surprisingly, virions assembled from TLS-less BMV RNA in the presence of tRNAs or TLS-containing short RNA did not incorporate the latter molecules. Consistent with a critical role for the BMV TLS in virion assembly, mutations in the BMV genomic RNAs that were designed to disrupt the folding of the TLS also abolished virion assembly. We discuss the likely roles of the TLS in early stages of virion assembly.

Establishment of an infectious RNA transcription system for Striped jack nervous necrosis virus, the type species of the betanodaviruses

A system has been established to produce infectious RNA transcripts for Striped jack nervous necrosis virus (SJNNV), the type species of the betanodaviruses, which infect fish. An enzymological analysis suggested that both RNA1 and RNA2 of SJNNV have a 5' cap. Both RNAs were largely resistant to 3' polyadenylation and ligation, suggesting the presence of an interfering 3' structure, while a small quantity of viral RNAs were polyadenylated in vitro. The complete 5' and 3' non-coding sequences of both segments were determined using the rapid amplification of cDNA ends method. Based on the terminal sequences obtained, RT-PCR was carried out and plasmid clones containing full-length cDNA copies of both RNAs, positioned downstream of a T7 promoter, were constructed. These plasmids were cleaved at a unique restriction site just downstream of the 3' terminus of each SJNNV sequence and were transcribed in vitro into RNA with a cap structure analogue. A mixture of the transcripts was transfected into the fish cell line E-11. Using indirect immunofluorescence staining with anti-SJNNV serum, fluorescence was observed specifically in these transfected cells; this culture supernatant exhibited pathogenicity to striped jack larvae. Northern blot analysis of E-11 cells infected with the recombinant virus or SJNNV showed small RNA (ca. 0.4 kb) that was newly synthesized and corresponded to the 3'-terminal region of RNA1. Finally, the complete nucleotide sequences of these functional cDNAs (RNA1, 3107 nt; RNA2, 1421 nt) were determined. This is the first report of betanodavirus cDNA clones from which infectious genomic RNAs can be transcribed.

Specific tyrosylation of the bulky tRNA-like structure of brome mosaic virus RNA relies solely on identity nucleotides present in its amino acid-accepting domain

Residues specifying aminoacylation by yeast tyrosyl-tRNA synthetase (TyrRS) of the tRNA-like structure present at the 3'-end of brome mosaic virus (BMV) RNA were determined by the in vitro approach using phage T7 transcripts. They correspond to nucleotides equivalent to base-pair C1-G72 and discriminator base A73 in the amino acid-acceptor branch of the molecule. No functional equivalents of the tyrosine anticodon residues, shown to be weakly involved in tyrosine identity of canonical tRNA(Tyr), were found in the BMV tRNA-like structure. This indicates a behaviour of this large and intricate molecule reminiscent of that of a minihelix derived from an amino acid-acceptor branch. Furthermore, iodine footprinting experiments performed on a tyrosylable BMV RNA transcript of 196 nt complexed to yeast TyrRS indicate that the amino acid-acceptor branch of the viral RNA is protected against cleavages as well as a hairpin domain, which is possibly located perpendicularly to its accepting branch. This domain without the canonical anticodon loop or the tyrosine anticodon acts as an anchor for TyrRS interaction leading to a better efficiency of tyrosylation.

Structural and thermodynamic studies on mutant RNA motifs that impair the specificity between a viral replicase and its promoter

The 3'-end region of the genomic RNA of brome mosaic virus forms a tRNA-like structure that is critical for its replication. Previous studies have shown that in this region, a stem-loop structure, called SLC, is necessary and sufficient for the binding of the RNA replicase, and for RNA replication. Recently, we determined the high-resolution NMR structure of SLC, which demonstrated that a 5'-AUA-3' triloop region is an important structural element for the enzymatic recognition. We proposed that the 5'-adenine of the triloop, which is rigidly fixed ("clamped") to the stem, is a key recognition element for the replicase. To elucidate the role of this "clamped base motif" for the enzymatic recognition, we have now investigated the solution conformations of several stem-loop molecules with mutant triloops, 5'-UUA-3', 5'-GUA-3', 5'-CUA-3' and 5'-UUU-3', that destroy the enzymatic recognition. For the GUA and UUA mutants, we have obtained high-resolution solution structures using 2D NMR. All four mutants have very similar thermodynamic stabilities, and all have the same secondary structures, a triloop with a five base-paired stem helix. In addition, they have quite similar sugar puckering patterns in the triloop region. The NMR structures of the GUA and UUA show that the 5' nucleotide of the triloop (G6 in GUA or U6 in UUA) lacks the strong interactions that hold its base in a fixed position. In particular, the U6 of UUA is found in two different conformations. Neither of these two mutants has the clamped base motif that was observed in the wild-type. While UUA also shows global change in the overall triloop conformation, GUA shows a very similar triloop conformation to the wild-type except for the lack of this motif. The absence of the clamped base motif is the only common structural difference between these two mutants and the wild-type. These results clearly indicate that the loss of function of the UUA and GUA mutants comes mainly from the destruction of a small key recognition motif rather than from global changes in their triloop conformations. Based on this study, we conclude that the key structural motif in the triloop recognized by the replicase is a solution-exposed, 5'-adenine base in the triloop that is clamped to the stem helix, which is called a clamped adenine motif.

Recognition of the core RNA promoter for minus-strand RNA synthesis by the replicases of Brome mosaic virus and Cucumber mosaic virus

Replication of viral RNA genomes requires the specific interaction between the replicase and the RNA template. Members of the Bromovirus and Cucumovirus genera have a tRNA-like structure at the 3' end of their genomic RNAs that interacts with the replicase and is required for minus-strand synthesis. In Brome mosaic virus (BMV), a stem-loop structure named C (SLC) is present within the tRNA-like region and is required for replicase binding and initiation of RNA synthesis in vitro. We have prepared an enriched replicase fraction from tobacco plants infected with the Fny isolate of Cucumber mosaic virus (Fny-CMV) that will direct synthesis from exogenously added templates. Using this replicase, we demonstrate that the SLC-like structure in Fny-CMV plays a role similar to that of BMV SLC in interacting with the CMV replicase. While the majority of CMV isolates have SLC-like elements similar to that of Fny-CMV, a second group displays sequence or structural features that are distinct but nonetheless recognized by Fny-CMV replicase for RNA synthesis. Both motifs have a 5'CA3' dinucleotide that is invariant in the CMV isolates examined, and mutational analysis indicates that these are critical for interaction with the replicase. In the context of the entire tRNA-like element, both CMV SLC-like motifs are recognized by the BMV replicase. However, neither motif can direct synthesis by the BMV replicase in the absence of other tRNA-like elements, indicating that other features of the CMV tRNA can induce promoter recognition by a heterologous replicase.

RNA sequence and secondary structural determinants in a minimal viral promoter that directs replicase recognition and initiation of genomic plus-strand RNA synthesis

Viral RNA replication provides a useful system to study the structure and function of RNAs and the mechanism of RNA synthesis from RNA templates. Previously we demonstrated that a 27 nt RNA from brome mosaic virus (BMV) can direct correct initiation of genomic plus-strand RNA synthesis by the BMV replicase. In this study, using biochemical, nuclear magnetic resonance, and thermodynamic analyses, we determined that the secondary structure of this 27 nt RNA can be significantly altered and retain the ability to direct RNA synthesis. In contrast, we find that position-specific changes in the RNA sequence will affect replicase recognition, modulate the polymerization process, and contribute to the differential accumulation of viral RNAs. These functional results are in agreement with the phylogenetic analysis of BMV and related viral sequences and suggest that a similar mechanism of RNA synthesis takes place for members of the alphavirus superfamily.

A minimal RNA promoter for minus-strand RNA synthesis by the brome mosaic virus polymerase complex

The approximately 150 nt tRNA-like structure present at the 3' end of each of the brome mosaic virus (BMV) genomic RNAs is sufficient to direct minus-strand RNA synthesis. RNAs containing mutations in the tRNA-like structure that decrease minus-strand synthesis were tested for their ability to interact with RdRp (RNA-dependent RNA polymerase) using a template competition assay. Mutations that are predicted to disrupt the pseudoknot and stem B1 do not affect the ability of the tRNA-like structure to interact with RdRp. Similarly, the +1 and +2 nucleotides are not required for stable template-RdRp interaction. Mutations in the bulge and hairpin loops of stem C decreased the ability of the tRNA-like structure to interact with RdRp. Furthermore, in the absence of the rest of the BMV tRNA, stem C is able to interact with RdRp. The addition of an accessible initiation sequence containing ACCA3' to stem C created an RNA capable of directing RNA synthesis. Synthesis from this minimal minus-strand template is dependent on sequences in the hairpin and bulged loops.

Sequences 5' of the conserved tRNA-like promoter modulate the initiation of minus-strand synthesis by the brome mosaic virus RNA-dependent RNA polymerase

Each of the brome mosaic virus (BMV) genomic RNAs contains a conserved tRNA-like structure that is sufficient to direct minus-strand RNA synthesis in vitro. The tRNA-like promoters, tB1 and tB3, direct approximately equal amounts of synthesis in vitro. However, 5' sequences were found to affect the amount of minus-strand synthesis, suggesting that sequences beyond the tRNA-like structure are important in moderating minus-strand synthesis. Consistent with this, sequences upstream the tRNA-like structure are able to partially suppress mutations at or near the initiation site. This activity is observed in the 5' sequences of both BMV and CMV (cucumber mosaic virus) templates. However, a chimeric RNA containing the CMV tRNA-like promoter fused to the 5' sequences of BMV was not able to suppress mutations at the initiation site, suggesting that homologous 5' and 3' sequences are required to affect initiation. The ability to suppress mutations at the initiation site was correlated with a slight increase in the ability of the BMV RNA-dependent RNA polymerase to interact with the RNA.

Resected RNA pseudoknots and their recognition by histidyl-tRNA synthetase

Duplexes constituted by closed or open RNA circles paired to single-stranded oligonucleotides terminating with 3'-CCAOH form resected pseudoknots that are substrates of yeast histidyl-tRNA synthetase. Design of this RNA fold is linked to the mimicry of the pseudoknotted amino acid accepting branch of the tRNA-like domain from brome mosaic virus, known to be charged by tyrosyl-tRNA synthetases, with RNA minihelices recapitulating accepting branches of canonical tRNAs. Prediction of the histidylation function of the new family of minimalist tRNA-like structures relates to the geometry of resected pseudoknots that allows proper presentation to histidyl-tRNA synthetase of analogues of the histidine identity determinants N-1 and N73 present in tRNAs. This geometry is such that the analogue of the major N-1 histidine determinant in the RNA circles faces the analogue of the discriminator N73 nucleotide in the accepting oligonucleotides. The combination of identity elements found in tRNAHis species from archaea, eubacteria, and organelles (G-1/C73) is the most efficient for determining histidylation of the duplexes. The inverse combination (C-1/G73) leads to the worst histidine acceptors with charging efficiencies reduced by 2-3 orders of magnitude. Altogether, these findings open new perspectives for understanding evolution of tRNA identity and serendipitous RNA functions.

Characterization of RNA products associated with or aborted by a viral RNA-dependent RNA polymerase

Ternary RNA-dependent RNA polymerase complexes were arrested at various stages of synthesis in vitro by limiting one or more NTPs. The RNAs synthesized prior to/during the arrest of the complex were characterized and the ability of the arrested complexes to continue elongation of these nascent RNAs in the presence of all four NTPs was analyzed. Nascent RNAs of 10 nt and longer remained associated with the RdRp complex and could be extended into full-length RNAs while shorter nascent RNAs were released by the complex. These results establish that previously observed RdRp-synthesized oligoribonucleotides of 8 nt or less are abortive initiation products and confirms that RdRp undergoes a transition from initiation to elongation after the synthesis of 8 nt.

RNA synthesis by the brome mosaic virus RNA-dependent RNA polymerase: transition from initiation to elongation

The initiation and elongation phases of (-)-strand RNA synthesis in vitro by the brome mosaic virus RNA-dependent RNA polymerase (RdRp) are differentially sensitive to inhibitors. In an attempt to characterize further the transition RdRp makes from initiation to elongation, we determined the conditions needed to pause the ternary complex and complete only one round of RNA synthesis. During the transition we were able to discern step-wise increases in the affinity of RdRp for RNA by measuring sensitivity to heparin and competition for RdRp by an alternative template. Three distinct stability levels of RdRp-template interactions were found. The first stable RdRp-RNA complex was observed when RdRp bound to the RNA template. A further increase occurred when RdRp synthesized the first phosphodiester bond. A final increase occurred upon formation of between 3 and 13 phosphodiester bonds. After this last transition, RdRp appeared to be tightly committed to the template RNA. These results are analogous to the mechanism of action of DNA-dependent RNA polymerases and are relevant to protein-RNA interaction and template switching by an RdRp.

Initiation of minus-strand RNA synthesis by the brome mosaicvirus RNA-dependent RNA polymerase: use of oligoribonucleotide primers

Various DNA- and RNA-dependent RNA polymerases have been reported to use oligoribonucleotide primers to initiate nucleic acid synthesis. For the brome mosaic virus RNA-dependent RNA polymerase (RdRp), we determined that in reactions performed with limited GTP concentrations, minus-strand RNA synthesis can be stimulated by the inclusion of guanosine monophosphate or specific oligoribonucleotides. Furthermore, guanylyl-3',5'-guanosine (GpG) was incorporated into minus-strand RNA and increased the rate of minus-strand RNA synthesis. In the presence of GpG, RdRp's Km for GTP decreased from 50 microM to approximately 3 microM while the Kms for other nucleotides were unaffected. These results have implications for the mechanism of initiation by RdRp.

The turnip yellow mosaic virus tRNA-like structure cannot be replaced by generic tRNA-like elements or by heterologous 3' untranslated regions known to enhance mRNA expression and stability

The tRNA-like structure (TLS) at the 3' end of the turnip yellow mosaic virus genome was replaced with heterologous tRNA-like elements, and with a poly(A) tail, in order to assess its role. Replacement with the valylatable TLSs from two closely related tymoviruses resulted in infectious viruses. In contrast, no systemic symptoms on plants, and only low viral accumulations in protoplasts, were observed for three chimeric genomes with 3' sequences known to enhance mRNA stability and translatability. One of these chimeras had a poly(A) tail, and the others had the TLS with associated upstream pseudoknot tracts from the 3' ends of brome mosaic and tobacco mosaic viruses. The latter two chimeric RNAs were shown to be appropriately folded by demonstrating their aminoacylation in vitro with tyrosine and histidine, respectively. The results show that enhancement of genome stability or gene expression is not the major role of the turnip yellow mosaic virus TLS. The major role is likely to be replicational, dependent on features present in tymoviral TLSs but not in generic tRNA-like structures.

Viral RNA as a potential target for two independent mechanisms of replicase-mediated resistance against cucumber mosaic virus

Transgenic tobacco showing replicase-mediated resistance against cucumber mosaic virus (CMV) can be infected by the strain K-CMV. By use of chimeric constructs between full-length cDNA clones of RNA2 of strains Fny-CMV and K-CMV, the existence of two independent mechanisms of replicase-mediated resistance against viral replication and movement of Fny-CMV was demonstrated in these plants. The data indicate that viral RNA may serve as the target for both mechanisms of resistance. A positive correlation was observed between the amount of K-CMV RNA2 sequence present in the chimeric constructs and the ability to overcome the inhibition of replication, whereas a sequence domain was delimited in K-CMV RNA2 responsible for the ability of this strain to break resistance against virus movement.

Targeting of the site of nonhomologous genetic recombination in brome mosaic virus

The genome of brome mosaic virus (BMV) consists of three positive strand RNA segments that share a high degree of sequence homology in the 3' noncoding region. The phenomenon of both homologous and nonhomologous intersegment RNA-RNA recombinant has been demonstrated within the 3' noncoding region of BMV RNAs. It has been postulated that nonhomologous crossovers occur at local heteroduplexes formed between the recombining BMV RNA substrates of the same polarity and that the formation of double-stranded regions facilitates strand switching by the replicase. To test the hypothesis of hybridization-mediated recombination in BMV, RNA-3 constructs carrying short antisense RNA1-derived sequences have been used to induce nonhomologous recombination events between RNA-1 and RNA-3 at or near the site of hybridization. We find that both the incidence of recombination and the location of recombinant junctions depends on the structure and the stability of heteroduplexes. Furthermore, our preliminary results demonstrate that mutations in the helicase-like domain of BMV protein 1a affect the location of recombinant junctions. This provides experimental evidence that BMV replicase protein 1a participates in recombination.

Molecular studies of genetic RNA-RNA recombination in brome mosaic virus

It is well known that DNA-based organisms rearrange and repair their genomic DNA through recombination processes, and that these rearrangements serve as a powerful source of variability and adaptation for these organisms. In RNA viruses' genetic recombination is defined as any process leading to the exchange of information between viral RNAs. There are two types of recombination events: legitimate and illegitimate. While legitimate (homologous) recombination occurs between closely related sequences at corresponding positions, illegitimate (nonhomologous) recombination could happen at any position among the unrelated RNA molecules. In order to differentiate between the symmetrical and asymmetrical homologous crosses, Lai defined the former as homologous recombination and the latter as aberrant homologous recombination. This chapter uses brome mosaic virus (BMV), a multicomponent plant RNA virus, as an example to discuss the progress in studying the mechanism of genetic recombination in positive-stranded RNA viruses. Studies described in this chapter summarize the molecular approaches used to increase the frequency of recombination among BMV RNA segments and, more importantly, to target the sites of crossovers to specific BMV RNA regions. It demonstrates that the latter can be accomplished by introducing local complementarities to the recombining substrates.

Targeting the site of RNA-RNA recombination in brome mosaic virus with antisense sequences

It has been postulated that local hybridizations between viral RNAs can mediate recombination in brome mosaic virus (BMV) and in poliovirus. To test this model, a 3' fragment of BMV RNA1 was inserted into the 3' noncoding sequence of BMV RNA3 in an antisense orientation. This resulted in high-frequency nonhomologous crossovers at or near the hybridized region. Insertion of the same RNA1 fragment in a positive-sense orientation did not promote recombination. Modification of the antisense insert by deletion of 3' portions did not affect the sites of crossover. However, modification of the 5' portion shifted the crossovers toward the central part of the heteroduplex region. Our results provide experimental evidence that recombinant crosses can be primed by hybridization between viral RNA molecules.

Contributions of the brome mosaic virus RNA-3 3'-nontranslated region to replication and translation

Sequences upstream of the 3'-terminal tRNA-like structure of brome mosaic virus RNAs have been predicted to fold into several stem-loop and pseudoknot structures. To elucidate the functional role of this upstream region, a series of deletions was made in cDNA clones of RNA-3, a genomic component not required for replication. These deletion mutants were transcribed in vitro and cotransfected with RNA-1 and RNA-2 into barley protoplasts. Deletion of single stem-loop structures gave progeny retaining near-wild-type accumulation levels. Constructions representing deletion of two or three stem-loops substantially lowered the accumulation of progeny RNA-3 relative to wild-type levels. RNA-3 mutants bearing deletions of longer sequences or of the entire region (delta PsKs RNA-3) replicated poorly, yielding no detectable RNA-3 or RNA-4 progeny. Levels of RNA-1 and RNA-2, in the presence of a mutant RNA-3, were found to increase relative to the accumulation observed in a complete wild-type transfection. The stability of delta PsKs RNA-3 in protoplasts was somewhat lower than that of wild-type RNA during the first 3 h postinoculation. Little difference in translatability in vitro of wild-type and RNA-3 constructs bearing deletions within the stem-loop region was observed, and Western immunoblot analysis of viral coat protein produced in transfected protoplasts showed that protein accumulation paralleled the amount of RNA-4 message produced from the various sequences evaluated. These results indicate that the RNA-3 pseudoknot region plays a minor role in translational control but contributes substantially to the overall replication of the brome mosaic virus genome.

Non-canonical substrates of aminoacyl-tRNA synthetases: the tRNA-like structure of brome mosaic virus genomic RNA

A 3-D model of the tyrosylable tRNA-like domain of the genomic brome mosaic virus RNAs was built by computer modelling based on solution probing of the molecule with different chemical and enzymatic reagents. This model encompasses four major structural domains, including two peculiar substructures oriented perpendicularly and mimicking a tRNA structure, and a fifth domain which makes the connection with the rest of the viral RNA. After recalling the different steps that led to the present structural knowledge of the BMV tRNA-like domain, we review its novel structural features revealed by the modelling and that did not appear in older versions of 3-D models of this structure. These features comprise additional base-pairs, hairpin loops, new tertiary long-range interactions, and a second pseudoknot. The main goal of this paper is to strengthen the validity of the model by establishing correlations between the putative 3-D conformation and the functional properties of the domain. For that, we show how the present structural model rationalises mutagenic and footprinting data that have established the importance of specific regions of the RNA for its recognition and aminoacylation by yeast tyrosyl-tRNA synthetase. We discuss further how the model corroborates mutational analyses performed to understand recognition of this RNA domain by the (ATP,CTP):tRNA nucleotidyl-transferase and by the viral replicase. The published mutants of the BMV tRNA-like domain fall into two classes. In one class, the mutants leave unchanged the overall architecture of the molecule, thereby affecting functions directly. In the second class, the overall architecture of the mutants is perturbed, and thus functions are affected indirectly.

Genetic recombination in brome mosaic virus: effect of sequence and replication of RNA on accumulation of recombinants

In order to facilitate the isolation of recombinants in brome mosaic virus, a series of duplication mutants with alterations in the RNA3 3' noncoding region has been engineered. The distribution of crossovers, which was observed to be dependent on the parental RNA3 sequence, supported the role of RNA structure in recombination. However, a negative correlation between replication of the parental RNA3 constructs and the accumulation of recombinant progeny confirmed the role of selection.

Heterologous recombination in the double-stranded RNA bacteriophage phi 6

Bacteriophage phi 6 contains three double-stranded RNA genomic segments. We have constructed a virus with an insertion of a kanamycin resistance gene in genomic RNA segment M. The virus forms small, turbid plaques, and its genome is unstable. Virus from a single plaque contained from about 0.1 to 10% large clear-plaque forms of the virus; these were usually missing the kanamycin resistance gene, and in many cases, the resulting segment M was larger or smaller than its normal size. Sequence analysis of the genomic RNA of the apparent deletions showed that they were formed by recombination events between segment M and either segment S or L. These heterologous recombination events resulted in the loss of the kanamycin resistance gene from segment M and the replacement of the 3' end of segment M with the 3' end of segment S or L. Although the 3' ends of the single-stranded RNA transcripts of the genomic segments appear to have extensive secondary structure, the sequences at the 3' ends are not involved in the specificity of genomic packaging.

Unique nucleotide differences in the conserved 3' termini of brome mosaic virus RNAs are maintained through their optimization of genome replication

To explore the functionality and conservation of specific base differences in the 3' 200 nucleotides of brome mosaic virus (BMV) RNA-1 (1t) and RNA-2 (2t) with respect to the 3' end of RNA-3 (3t), all possible permutations were used to exchange these regions among the genomic RNAs. When all RNAs bore the 1t promoter, total RNA accumulation was only 15% of wild type; when the 2t or 3t promoter was present on all three RNAs total RNA accumulation was reduced to 30 or 35% of wild type. Two major processes were found to be involved in these dramatic differences. The first reflects the distinct and competitive strengths of the (-)-strand promoters in these sequences, which were shown to have a 3t greater than 1t greater than 2t hierarchy. The second is the importance of the context of upstream sequences in which the 3' promoter is placed. Important contributions of the 3t promoter in preferential amplification of RNA-3 were apparent from changed RNA 1 + 2: 3 ratios and reduced progeny accumulation from transfections using the RNA-3/1t chimera. These interactions contribute to temporal modulation as well as overall optimization of viral RNA functions, leading to selection and maintenance of the specific base differences present in the otherwise highly conserved 3' 200 nucleotides of each genomic RNA component of BMV.