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

The brome mosaic virus 3' untranslated sequence regulates RNA replication, recombination, and virion assembly

The 3' untranslated region in each of the three genomic RNAs of Brome mosaic virus (BMV) is highly homologous and contains a sequence that folds into a tRNA-like structure (TLS). Experiments performed over the past four decades revealed that the BMV 3' TLS regulates many important steps in BMV infection. This review summarizes in vitro and in vivo studies of the roles of the BMV 3' TLS functioning as a minus-strand promoter, in RNA recombination, and to nucleate virion assembly.

Replication of Plant Viruses

Viruses replicate using both their own genetic information and host cell components and machinery. The different genome types have different replication pathways which contain controls on linking the process with translation and movement around the cell as well as not compromising the infected cell. This chapter discusses the replication mechanisms, faults in replication and replication of viruses co-infecting cells.

Viruses replicate using both their own genetic information and host cell components and machinery. The different genome types have different replication pathways which contain controls on linking the process with translation and movement around the cell as well as not compromising the infected cell. This chapter discusses the replication mechanisms, faults in replication and replication of viruses coinfecting cells.

Reinitiated viral RNA-dependent RNA polymerase resumes replication at a reduced rate

RNA-dependent RNA polymerases (RdRP) form an important class of enzymes that is responsible for genome replication and transcription in RNA viruses and involved in the regulation of RNA interference in plants and fungi. The RdRP kinetics have been extensively studied, but pausing, an important regulatory mechanism for RNA polymerases that has also been implicated in RNA recombination, has not been considered. Here, we report that RdRP experience a dramatic, long-lived decrease in its elongation rate when it is reinitiated following stalling. The rate decrease has an intriguingly weak temperature dependence, is independent of both the nucleotide concentration during stalling and the length of the RNA transcribed prior to stalling; however it is sensitive to RNA structure. This allows us to delineate the potential factors underlying this irreversible conversion of the elongation complex to a less active mode.

Unravelling the complexities of respiratory syncytial virus RNA synthesis

Human respiratory syncytial virus (RSV) is the leading cause of paediatric respiratory disease and is the focus of antiviral- and vaccine-development programmes. These goals have been aided by an understanding of the virus genome architecture and the mechanisms by which it is expressed and replicated. RSV is a member of the order Mononegavirales and, as such, has a genome consisting of a single strand of negative-sense RNA. At first glance, transcription and genome replication appear straightforward, requiring self-contained promoter regions at the 3' ends of the genome and antigenome RNAs, short cis-acting elements flanking each of the genes and one polymerase. However, from these minimal elements, the virus is able to generate an array of capped, methylated and polyadenylated mRNAs and encapsidated antigenome and genome RNAs, all in the appropriate ratios to facilitate virus replication. The apparent simplicity of genome expression and replication is a consequence of considerable complexity in the polymerase structure and its cognate cis-acting sequences; here, our understanding of mechanisms by which the RSV polymerase proteins interact with signals in the RNA template to produce different RNA products is reviewed.

Efficient in vitro system of homologous recombination in brome mosaic bromovirus

Recent in vivo studies have revealed that the subgenomic promoter (sgp) in brome mosaic bromovirus (BMV) RNA3 supports frequent homologous recombination events (R. Wierzchoslawski, A. Dzianott, and J. Bujarski, J. Virol. 78:8552-8564, 2004). In this paper, we describe an sgp-driven in vitro system that supports efficient RNA3 crossovers. A 1:1 mixture of two (-)-sense RNA3 templates was copied with either a BMV replicase (RdRp) preparation or recombinant BMV protein 2a. The BMV replicase enzyme supported a lower recombination frequency than 2a, demonstrating a role of other viral and/or host factors. The described in vitro system will allow us to study the mechanism of homologous RNA recombination.

Recombinant viral RdRps can initiate RNA synthesis from circular templates

The crystal structure of the recombinant hepatitis C virus (HCV) RNA-dependent RNA polymerase (RdRp) revealed extensive interactions between the fingers and the thumb subdomains, resulting in a closed conformation with an established template channel that should specifically accept single-stranded templates. We made circularized RNA templates and found that they were efficiently used by the HCV RdRp to synthesize product RNAs that are significantly longer than the template, suggesting that RdRp could exist in an open conformation prior to template binding. RNA synthesis using circular RNA templates had properties similar to those previously documented for linear RNA, including a need for higher GTP concentration for initiation, usage of GTP analogs, sensitivity to salt, and involvement of active-site residues for product formation. Some products were resistant to challenge with the template competitor heparin, indicating that the elongation complexes remain bound to template and are competent for RNA synthesis. Other products were not elongated in the presence of heparin, indicating that the elongation complex was terminated. Lastly, recombinant RdRps from two other flaviviruses and from the Pseudomonas phage phi6 also could use circular RNA templates for RNA-dependent RNA synthesis, although the phi6 RdRp could only use circular RNAs made from the 3'-terminal sequence of the phi6 genome.

A plant RNA virus suppresses RNA silencing through viral RNA replication

RNA interference (RNAi) is a post-transcriptional gene-regulatory mechanism that operates in many eukaryotes. RNAi is induced by double-stranded RNA (dsRNA) and is mainly involved in defence against transposons and viruses. To counteract RNAi, viruses have RNAi suppressors. Here we show a novel mechanism of RNAi suppression by a plant virus Red clover necrotic mosaic virus (RCNMV). To suppress RNAi, RCNMV needs multiple viral components, which include viral RNAs and putative RNA replicase proteins. A close relationship between the RNA elements required for negative-strand RNA synthesis and RNAi suppression suggests a strong link between the viral RNA replication machinery and the RNAi machinery. In a transient assay, RCNMV interferes with the accumulation of small-interfering RNA (siRNAs) in RNAi induced by a hairpin dsRNA and it also interferes with microRNA (miRNA) biogenesis. An Arabidopsis dcl1 mutant showed reduced susceptibility to RCNMV infection. Based on these results, we propose a model in which, to replicate, RCNMV deprives the RNAi machinery of Dicer-like enzymes that are involved in both siRNA and miRNA biogenesis.

Pokeweed antiviral protein inhibits brome mosaic virus replication in plant cells

Pokeweed antiviral protein (PAP) is a ribosome-inactivating protein isolated from the pokeweed plant (Phytolacca americana) that inhibits the proliferation of several plant and animal viruses. We have shown previously that PAP and nontoxic mutants of PAP can directly depurinate brome mosaic virus (BMV) RNA in vitro, resulting in reduced viral protein translation. Here we expand on these initial studies and, using a barley protoplast system, demonstrate that recombinant PAP and nontoxic mutants isolated from E. coli are able to reduce the accumulation of BMV RNAs in vivo. Pretreatment of only BMV RNA3 with PAP prior to transfection of barley protoplasts reduced the accumulation of all BMV RNAs, with a more severe effect on subgenomic RNA4 levels. Using in vitro RNA synthesis assays, we show that a depurinated template causes the BMV replicase to stall at the template nucleotide adjacent to the missing base. These results provide new insight into the antiviral mechanism of PAP, namely that PAP depurination of BMV RNA impedes both RNA replication and subgenomic RNA transcription. These novel activities are distinct from the PAP-induced reduction of viral RNA translation and represent new targets for the inhibition of viral infection.

Repair of the tRNA-like CCA sequence in a multipartite positive-strand RNA virus

The 3' portions of plus-strand brome mosaic virus (BMV) RNAs mimic cellular tRNAs. Nucleotide substitutions or deletions in the 3'CCA of the tRNA-like sequence (TLS) affect minus-strand initiation unless repaired. We observed that 2-nucleotide deletions involving the CCA 3' sequence in one or all BMV RNAs still allowed RNA accumulation in barley protoplasts at significant levels. Alterations of CCA to GGA in only BMV RNA3 also allowed RNA accumulation at wild-type levels. However, substitutions in all three BMV RNAs severely reduced RNA accumulation, demonstrating that substitutions have different repair requirements than do small deletions. Furthermore, wild-type BMV RNA1 was required for the repair and replication of RNAs with nucleotide substitutions. Results from sequencing of progeny viral RNA from mutant input RNAs demonstrated that RNA1 did not contribute its sequence to the mutant RNAs. Instead, the repaired ends were heterogeneous, with one-third having a restored CCA and others having sequences with the only commonality being the restoration of one cytidylate. The role of BMV RNA1 in increased repair was examined.

Initiation of viral RNA-dependent RNA polymerization

This review summarizes the combined insights from recent structural and functional studies of viral RNA-dependent RNA polymerases (RdRPs) with the primary focus on the mechanisms of initiation of RNA synthesis. Replication of RNA viruses has traditionally been approached using a combination of biochemical and genetic methods. Recently, high-resolution structures of six viral RdRPs have been determined. For three RdRPs, enzyme complexes with metal ions, single-stranded RNA and/or nucleoside triphosphates have also been solved. These advances have expanded our understanding of the molecular mechanisms of viral RNA synthesis and facilitated further RdRP studies by informed site-directed mutagenesis. What transpires is that the basic polymerase right hand shape provides the correct geometrical arrangement of substrate molecules and metal ions at the active site for the nucleotidyl transfer catalysis, while distinct structural elements have evolved in the different systems to ensure efficient initiation of RNA synthesis. These elements feed the template, NTPs and ions into the catalytic cavity, correctly position the template 3′ terminus, transfer the products out of the catalytic site and orchestrate the transition from initiation to elongation.

Template sequence near the initiation nucleotide can modulate brome mosaic virus RNA accumulation in plant protoplasts

Bromoviral templates for plus-strand RNA synthesis are rich in A or U nucleotides in comparison to templates for minus-strand RNA synthesis. Previous studies demonstrated that plus-strand RNA synthesis by the brome mosaic virus (BMV) RNA replicase is more efficient if the template contains an A/U-rich template sequence near the initiation site (K. Sivakumaran and C. C. Kao, J. Virol. 73:6415-6423, 1999). These observations led us to examine the effects of nucleotide changes near the template's initiation site on the accumulation of BMV RNA3 genomic minus-strand, genomic plus-strand, and subgenomic RNAs in barley protoplasts transfected with wild-type and mutant BMV transcripts. Mutations in the template for minus-strand synthesis had only modest effects on BMV replication in barley protoplasts. Mutants with changes to the +3, +5, and +7 template nucleotides accumulated minus-strand RNA at levels similar to the the wild-type level. However, mutations at positions adjacent to the initiation cytidylate in the templates for genomic and subgenomic plus-strand RNA synthesis significantly decreased RNA accumulation. For example, changes at the third template nucleotide for plus-strand RNA3 synthesis resulted in RNA accumulation at between 18 and 24% of the wild-type level, and mutations in the third template nucleotide for subgenomic RNA4 resulted in accumulations at between 7 and 14% of the wild-type level. The effects of the mutations generally decreased as the mutations occurred further from the initiation nucleotide. These findings demonstrate that there are different requirements of the template sequence near the initiation nucleotide for BMV RNA accumulation in plant cells.

Specific inhibition of bovine viral diarrhea virus replicase

Compound-1453 was identified and characterized as a specific inhibitor of bovine viral diarrhea virus (BVDV). The concentration of compound-1453 which results in 50% protection from virus-induced cytopathic effect is approximately 2.2 microM, with a therapeutic index of 60, and it is not active against a panel of RNA and DNA viruses. A time-of-addition experiment suggested that compound-1453 targets a stage of the viral life cycle after viral entry. To determine the target of compound-1453, resistant virus was generated. Resistant variants grew efficiently in the presence or absence of 33 micro M compound-1453 and exhibited replication efficiency in the presence of compound-1453 approximately 1,000-fold higher than that of the wild-type (wt) virus. Functional mapping and sequence analysis of resistant cDNAs revealed a single amino acid substitution (Glu to Gly) at residue 291 in the NS5B polymerase in all eight independently generated cDNA clones. Recombinant virus containing this single mutation retained the resistance phenotype and a replication efficiency similar to that of the original isolated resistant virus. Since compound-1453 did not inhibit BVDV polymerase activity in vitro (50% inhibitory concentration > 300 microM), we developed a membrane-based assay that consisted of a BVDV RNA replicase complex isolated from virus-infected cells. Compound-1453 inhibited the activity of the wt, but not the drug-resistant, replicase in the membrane assay at concentrations similar to those observed in the viral infection assay. This work presents a novel inhibitor of a viral RNA-dependent RNA replicase.

Arresting initiation of hepatitis C virus RNA synthesis using heterocyclic derivatives

The hepatitis C virus (HCV) NS5B protein encodes an RNA-dependent RNA polymerase (RdRp), the primary catalytic enzyme of the HCV replicase complex. Recently, two benzo-1,2,4-thiadiazine compounds were shown to be potent, highly specific inhibitors of the genotype 1b HCV RdRp containing a carboxyl-terminal 21 residue truncation (delta21 HCV RdRp) (Dhanak, D., Duffy, K., Johnston, V. K., Lin-Goerke, J., Darcy, M., Shaw, A. N. G. B., Silverman, C., Gates, A. T., Earnshaw, D. L., Casper, D. J., Kaura, A., Baker, A., Greenwood, C., Gutshall, L. L., Maley, D., DelVecchio, A., Macarron, R., Hofmann, G. A., Alnoah, Z., Cheng, H.-Y., Chan, G., Khandekar, S., Keenan, R. M., and Sarisky, R. T. (2002) J. Biol. Chem. 277, 38322-38327). Compound 4 (C(21)H(21)N(3)O(4)S) reduces viral replication by virtue of its direct interaction with the viral polymerase rather than by nonspecific titration of nucleic acid template. In this study, we present several lines of evidence to demonstrate that this inhibitor interferes with the initiation step of RNA synthesis rather than acting as an elongation inhibitor. Inhibition of initial phosphodiester bond formation occurred regardless of whether replication was initiated by primer-dependent or de novo mechanisms. Filter binding studies using increasing concentrations of compound 4 did not interfere with the ability of delta21 HCV RdRp to interact with nucleic acid. Furthermore, varying the order of reagent addition in the primer extension assay showed no distinct differences in inhibition profile. Finally, surface plasmon resonance analyses provided evidence that a ternary complex is capable of forming between the RNA template, RdRp, and compound 4. Together, these data suggest that this heterocyclic agent interacts with the apoenzyme, as well as with the RNA-bound form of delta21 HCV RdRp, and therefore does not directly interfere with the RdRp-RNA interaction to mediate inhibition.

Structural analysis of the hepatitis C virus RNA polymerase in complex with ribonucleotides

We report here the results of a systematic high-resolution X-ray crystallographic analysis of complexes of the hepatitis C virus (HCV) RNA polymerase with ribonucleoside triphosphates (rNTPs) and divalent metal ions. An unexpected observation revealed by this study is the existence of a specific rGTP binding site in a shallow pocket at the molecular surface of the enzyme, 30 A away from the catalytic site. This previously unidentified rGTP pocket, which lies at the interface between fingers and thumb, may be an allosteric regulatory site and could play a role in allowing alternative interactions between the two domains during a possible conformational change of the enzyme required for efficient initiation. The electron density map at 1.7-A resolution clearly shows the mode of binding of the guanosine moiety to the enzyme. In the catalytic site, density corresponding to the triphosphates of nucleotides bound to the catalytic metals was apparent in each complex with nucleotides. Moreover, a network of triphosphate densities was detected; these densities superpose to the corresponding moieties of the nucleotides observed in the initiation complex reported for the polymerase of bacteriophage phi6, strengthening the proposal that the two enzymes initiate replication de novo by similar mechanisms. No equivalent of the protein stacking platform observed for the priming nucleotide in the phi6 enzyme is present in HCV polymerase, however, again suggesting that a change in conformation of the thumb domain takes place upon template binding to allow for efficient de novo initiation of RNA synthesis.

Lessons learned from the core RNA promoters of Brome mosaic virus and Cucumber mosaic virus

summary RNA core promoters are nucleotide sequences needed to direct proper initiation of viral RNA synthesis by the viral replicase. Minimal length core promoter-templates that can direct accurate initiation of the genomic plus-, genomic minus-, and subgenomic RNAs of Brome mosaic virus and Cucumber mosaic virus were characterized in previous works. Several common themes and differences were observed in how each of the core promoters directed the initiation of viral RNA synthesis in vitro. These observations are summarized and compared in this short review.

De novo synthesis of RNA by the dengue virus RNA-dependent RNA polymerase exhibits temperature dependence at the initiation but not elongation phase

Replication of positive strand flaviviruses is mediated by the viral RNA-dependent RNA polymerases (RdRP). To study replication of dengue virus (DEN), a flavivirus family member, an in vitro RdRP assay was established using cytoplasmic extracts of DEN-infected mosquito cells and viral subgenomic RNA templates containing 5'- and 3'-terminal regions (TRs). Evidence supported that an interaction between the TRs containing conserved stem-loop, cyclization motifs, and pseudoknot structural elements is required for RNA synthesis. Two RNA products, a template size and a hairpin, twice that of the template, were formed. To isolate the function of the viral RdRP (NS5) from that of other host or viral factors present in the cytoplasmic extracts, the NS5 protein was expressed and purified from Escherichia coli. In this study, we show that the purified NS5 alone is sufficient for the synthesis of the two products and that the template-length RNA is the product of de novo initiation. Furthermore, the incubation temperature during initiation, but not elongation phase of RNA synthesis modulates the relative amounts of the hairpin and de novo RNA products. A model is proposed that a specific conformation of the viral polymerase and/or structure at the 3' end of the template RNA is required for de novo initiation.

Completion of RNA synthesis by viral RNA replicases

How the 5'-terminus of the template affects RNA synthesis by viral RNA replicases is poorly understood. Using short DNA, RNA and RNA-DNA chimeric templates that can direct synthesis of replicase products, we found that DNA templates tend to direct the synthesis of RNA products that are shorter by 1 nt in comparison to RNA templates. Template-length RNA synthesis was also affected by the concentration of nucleoside triphosphates, the identity of the bases at specific positions close to the 5'-terminus and the C2'-hydroxyl of a ribose at the third nucleotide from the 5'-terminal nucleotide. Similar requirements are observed with two bromoviral replicases, but not with a recombinant RNA-dependent RNA polymerase. These results begin to define the interactions needed for the viral replicase to complete synthesis of viral RNA.

De novo initiation of viral RNA-dependent RNA synthesis

RNA viruses use several initiation strategies to ensure that their RNAs are synthesized in appropriate amounts, have correct termini, and can be translated efficiently. Many viruses with genomes of single-stranded positive-, negative-, and double-stranded RNA initiate RNA synthesis by a de novo (primer-independent) mechanism. This review summarizes biochemical features and variations of de novo initiation in viral RNA replication.

Characterization of the cis-acting elements controlling subgenomic mRNAs of citrus tristeza virus: production of positive- and negative-stranded 3'-terminal and positive-stranded 5'-terminal RNAs

Citrus tristeza virus (CTV), a member of the Closteroviridae, has an approximately 20-kb positive-sense RNA genome with two 5' ORFs translated from the genomic RNA and 10 3' genes expressed via nine or ten 3'-terminal subgenomic (sg) RNAs. The expression of the 3' genes appears to have properties intermediate between the smaller viruses of the "alphavirus supergroup" and the larger viruses of the Coronaviridae. The sgRNAs are contiguous with the genome, without a common 5' leader, and are associated with large amounts of complementary sgRNAs. Production of the different sgRNAs is regulated temporally and quantitatively, with the highly expressed genes having noncoding regions (NCR) 5' of the ORFs. The cis-acting elements that control the highly expressed major coat protein (CP) gene and the intermediately expressed minor coat protein (CPm) gene were mapped and compared. Mutational analysis showed that the CP sgRNA controller element mapped within nts -47 to -5 upstream of the transcription start site, entirely within the NCR, while the CPm control region mapped within a 57 nt sequence within the upstream ORF. Although both regions were predicted to fold into two stem-loop structures, mutagenesis suggested that primary structure might be more important than the secondary structure. Because each controller element produced large amounts of 3'-terminal positive- and negative-stranded sgRNAs, we could not differentiate whether the cis-acting element functioned as a promoter or terminator, or both. Reversal of the control element unexpectedly produced large amounts of a negative-stranded sgRNA apparently by termination of negative-stranded genomic RNA synthesis. Further examination of controller elements in their native orientation showed normal production of abundant amounts of positive-stranded sgRNAs extending to near the 5'-terminus, corresponding to termination at each controller element. Thus, each controller element produced three sgRNAs, a 5'-terminal positive strand and both positive- and negative-stranded 3'-terminal RNAs. Therefore, theoretically CTV could produce 30-33 species of RNAs in infected cells.

Role of the 3'-untranslated regions of alfalfa mosaic virus RNAs in the formation of a transiently expressed replicase in plants and in the assembly of virions

Alfalfa mosaic virus (AMV) RNAs 1 and 2 encode the replicase proteins P1 and P2, respectively, whereas RNA 3 encodes the movement protein and the coat protein (CP). When RNAs 1 and 2 were transiently expressed from a T-DNA vector (R12 construct) by agroinfiltration of Nicotiana benthamiana, the infiltrated leaves accumulated minus-strand RNAs 1 and 2 and relatively small amounts of plus-strand RNAs. In addition, RNA-dependent RNA polymerase (RdRp) activity could be detected in extracts of the infiltrated leaves. After transient expression of RNAs 1 and 2 with the 3'-untranslated regions (UTRs) of both RNAs deleted (R1Delta/2Delta construct), no replication of RNAs 1 and 2 was observed, while the infiltrated leaves supported replication of RNA 3 after inoculation of the leaves with RNA 3 or expression of RNA 3 from a T-DNA vector (R3 construct). No RdRp activity could be isolated from leaves infiltrated with the R1Delta/2Delta construct, although P1 and P2 sedimented in a region of a glycerol gradient where active RdRp was found in plants infiltrated with R12. RdRp activity could be isolated from leaves infiltrated with constructs R1Delta/2 (3'-UTR of RNA 1 deleted), R1/2Delta (3'-UTR of RNA 2 deleted), or R1Delta/2Delta plus R3. This demonstrates that the 3'-UTR of AMV RNAs is required for the formation of a complex with in vitro enzyme activity. RNAs 1 and 2 with the 3'-UTRs deleted were encapsidated into virions by CP expressed from RNA 3. This shows that the high-affinity binding site for CP at the 3'-termini of AMV RNAs is not required for assembly of virus particles.

Factors regulating template switch in vitro by viral RNA-dependent RNA polymerases: implications for RNA-RNA recombination

Copy-choice RNA recombination occurs during viral RNA synthesis when the viral transcription complex switches templates. We demonstrate that RNA-dependent RNA polymerase from bovine viral diarrhea virus and the replicases from three plant-infecting RNA viruses can produce easily detectable recombination products in vitro by switching templates during elongative RNA synthesis. Template sequence and/or structure, and NTP availability affected the frequency of template switch by the transcription complex. Our results provide biochemical support for copy-choice recombination and establish assays for mechanistic analyses of intermolecular RNA recombination in vitro.

Template nucleotide moieties required for de novo initiation of RNA synthesis by a recombinant viral RNA-dependent RNA polymerase

The recombinant RNA-dependent RNA polymerase of the bovine viral diarrhea virus specifically requires a cytidylate at the 3' end for the de novo initiation of RNA synthesis (C. C. Kao, A. M. Del Vecchio, and W. Zhong, Virology 253:1-7, 1999). Using RNAs containing nucleotide analogs, we found that the N3 and C4-amino group at the initiation cytidine were required for RNA synthesis. However, the ribose C2'-hydroxyl of the initiating cytidylate can accept several modifications and retain the ability to direct synthesis. The only unacceptable modification is a protonated C2'-amino group. Quite strikingly, the recognition of the functional groups for the initiation cytidylate and other template nucleotides are different. For example, a C5-methyl group in cytidine can direct RNA synthesis at all template positions except at the initiation cytidylate and C2'-amino modifications are tolerated better after the +11 position. When a 4-thiouracil (4sU) base analog that allows only imperfect base pairing with the nascent RNA is placed at different positions in the template, the efficiency of synthesis is correlated with the calculated stability of the template-nascent RNA duplex adjacent to the position of the 4sU. These results define the requirements for the specific interactions required for the initiation of RNA synthesis and will be compared to the mechanisms of initiation by other RNA-dependent and DNA-dependent RNA polymerases.

Brome mosaic virus, good for an RNA virologist's basic needs

Abstract Taxonomic relationship: Type member of the Bromovirus genus, family Bromoviridae. A member of the alphavirus-like supergroup of positive-sense single-stranded RNA viruses. Physical properties: Virions are nonenveloped icosahedrals made up of 180 coat protein subunits (Fig. 1). The particles are 26 nm in diameter and contain 22% nucleic acid and 78% protein. The BMV genome is composed of three positive-sense, capped RNAs: RNA1 (3.2 kb), RNA2 (2.9 kb), RNA3 (2.1 kb) (Fig. 2). Viral proteins: RNA1 encodes protein 1a, containing capping and putative RNA helicase activities. RNA2 encodes protein 2a, a putative RNA-dependent RNA polymerase. RNA3 codes for two proteins: 3a, which is required for cell-to-cell movement, and the capsid protein. The capsid is translated from a subgenomic RNA, RNA4 (1.2 kb). Hosts: Monocots in the Poacea family, including Bromus inermis, Zea mays and Hordeum vulgare, in which BMV causes brown streaks. BMV can also infect the dicots Nicotiana benthamiana and several Chenopodium species. In N. benthamiana, the infection is asymptomatic while infection of Chenopodium can cause either necrotic or chlorotic lesions. Useful website:http://www4.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/10030001.htm.

In vitro transcription by the turnip yellow mosaic virus RNA polymerase: a comparison with the alfalfa mosaic virus and brome mosaic virus replicases

Recently, we showed that the main determinant in the tRNA-like structure of turnip yellow mosaic virus RNA to initiate minus-strand synthesis in vitro is the 3' ACCA end. By mutational analysis of the 3'-terminal hairpin, we show here that only a non-base-paired ACCA end is functional and that the stability of the wild-type 3'-proximal hairpin is the most favorable, in that it has the lowest DeltaG value and a high transcription efficiency. With a nested set of RNA fragments, we show that the minimum template length is 9 nucleotides and that transcription is improved with increasing the length of the template. The results also suggest that proper base stacking contributes to efficient transcription initiation. Internal initiation is shown to take place on every NPyCPu sequence of a nonstructured template. However, the position of the internal initiation site in the template is important, and competition between the different sites takes place. Internal initiation was also studied with the RNA-dependent RNA polymerase of brome mosaic virus (BMV) and alfalfa mosaic virus (AlMV). The BMV polymerase can start internally on ACCA sequences, though inefficiently. Unexpectedly, the polymerases of both AlMV and BMV can start efficiently on an internal AUGC sequence.

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.

Initiation of genomic plus-strand RNA synthesis from DNA and RNA templates by a viral RNA-dependent RNA polymerase

In contrast to the synthesis of minus-strand genomic and plus-strand subgenomic RNAs, the requirements for brome mosaic virus (BMV) genomic plus-strand RNA synthesis in vitro have not been previously reported. Therefore, little is known about the biochemical requirements for directing genomic plus-strand synthesis. Using DNA templates to characterize the requirements for RNA-dependent RNA polymerase template recognition, we found that initiation from the 3' end of a template requires one nucleotide 3' of the initiation nucleotide. The addition of a nontemplated nucleotide at the 3' end of minus-strand BMV RNAs led to initiation of genomic plus-strand RNA in vitro. Genomic plus-strand initiation was specific since cucumber mosaic virus minus-strand RNA templates were unable to direct efficient synthesis under the same conditions. In addition, mutational analysis of the minus-strand template revealed that the -1 nontemplated nucleotide, along with the +1 cytidylate and +2 adenylate, is important for RNA-dependent RNA polymerase interaction. Furthermore, genomic plus-strand RNA synthesis is affected by sequences 5' of the initiation site.

The N-terminal half of the brome mosaic virus 1a protein has RNA capping-associated activities: specificity for GTP and S-adenosylmethionine

The N-terminal half of the brome mosaic virus (BMV) 1a replication-associated protein contains sequence motifs found in RNA methyltransferases. We demonstrate that recombinant BMV methyltransferase-like (MT) domain expressed in Escherichia coli forms an adduct with a guanine nucleotide in a reaction that requires S-adenosylmethionine (AdoMet) and divalent cations. Moieties in GTP and AdoMet required for adduct formation were determined using a competition assay and chemical analogues. In the guanine nucleotide the ribose 2' hydroxyl, the triphosphates, the base C6 keto group, and possibly the N1 imine are required. In AdoMet, the methyl group and the ability to transfer a methyl group to guanine nucleotide were demonstrated to be required for adduct formation. The effects of methyltransferase inhibitors on viral RNA synthesis was determined using an in vitro RNA synthesis assay. These results are consistent with the previously reported activities of alphaviral nsP1 methyltransferase protein and identify the chemical moieties required for the BMV methyltransferase activity.

Selective stimulation of hepatitis C virus and pestivirus NS5B RNA polymerase activity by GTP

NS5B of the hepatitis C virus is an RNA template-dependent RNA polymerase and therefore the key player of the viral replicase complex. Using a highly purified enzyme expressed with recombinant baculoviruses in insect cells, we demonstrate a stimulation of RNA synthesis up to 2 orders of magnitude by high concentrations of GTP but not with ATP, CTP, UTP, GDP, or GMP. Enhancement of RNA synthesis was found with various heteropolymeric RNA templates, with poly(C)-oligo(G)12 but not with poly(A)-oligo(U)12. Several amino acid substitutions in polymerase motifs B, C, and D previously shown to be crucial for RdRp activity were tested for GTP stimulation of RNA synthesis. Most of these mutations, in particular those affecting the GDD motif (motif C) strongly reduced or completely abolished activation by GTP, suggesting that the same NTP-binding site is used for stimulation and RNA synthesis. Since GTP did not affect the overall RNA binding properties or the elongation rate, high concentrations of GTP appear to accelerate a rate-limiting step at the level of initiation of RNA synthesis. Finally, enhancement of RNA synthesis by high GTP concentrations was also found with NS5B of the pestivirus classical swine fever virus, but not with the 3D polymerase of poliovirus. Thus, stimulation of RdRp activity by GTP is evolutionarily conserved between the closely related hepaciviruses and pestiviruses but not between these and the more distantly related picornaviruses.

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.

Spatial perturbations within an RNA promoter specifically recognized by a viral RNA-dependent RNA polymerase (RdRp) reveal that RdRp can adjust its promoter binding sites

RNA synthesis during viral replication requires specific recognition of RNA promoters by the viral RNA-dependent RNA polymerase (RdRp). Four nucleotides (-17, -14, -13, and -11) within the brome mosaic virus (BMV) subgenomic core promoter are required for RNA synthesis by the BMV RdRp (R. W. Siegel et al., Proc. Natl. Acad. Sci. USA 94:11238-11243, 1997). The spatial requirements for these four nucleotides and the initiation (+1) cytidylate were examined in RNAs containing nucleotide insertions and deletions within the BMV subgenomic core promoter. Spatial perturbations between nucleotides -17 and -11 resulted in decreased RNA synthesis in vitro. However, synthesis was still dependent on the key nucleotides identified in the wild-type core promoter and the initiation cytidylate. In contrast, changes between nucleotides -11 and +1 had a less severe effect on RNA synthesis but resulted in RNA products initiated at alternative locations in addition to the +1 cytidylate. The results suggest a degree of flexibility in the recognition of the subgenomic promoter by the BMV RdRp and are compared with functional regions in other DNA and RNA promoters.

Analysis of RNA-dependent RNA polymerase structure and function as guided by known polymerase structures and computer predictions of secondary structure

RNA-dependent RNA polymerases (RdRps) function as the catalytic subunit of the viral replicase required for the replication of all positive strand RNA viruses. The vast majority of RdRps have been identified solely on the basis of sequence similarity. Structural studies of RdRps have lagged behind those of the DNA-dependent DNA polymerases, DNA-dependent RNA polymerases, and reverse transcriptases until the recent report of the partial crystal structure of the poliovirus RdRp, 3Dpol [Hansen, J. L., et al. (1997). Structure 5, 1109-1122]. We seek to address whether all RdRps will have structures similar to those found in the poliovirus polymerase structure. Therefore, the PHD method of Rost and Sander [Rost, B., and Sander, C. (1993a). J. Mol. Biol. 232, 584-599; Rost, B., and Sander, C. (1994). Protein 19, 55-77] was used to predict the secondary structure of the RdRps from six different viral families: bromoviruses, tobamoviruses, tombusvirus, leviviruses, hepatitis C-like viruses, and picornaviruses. These predictions were compared with the known crystal structure of the poliovirus polymerase. The PHD method was also used to predict picornavirus structures in places in which the poliovirus crystal structure was disordered. All five families and the picornaviruses share a similar order of secondary structure elements present in their polymerase proteins. All except the leviviruses have the unique region observed in the poliovirus 3Dpol that is suggested to be involved in polymerase oligomerization. These structural predictions are used to explain the phenotypes of a collection of mutations that exist in several RNA polymerases. This analysis will help to guide further characterization of RdRps.

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.

Minimal template requirements for initiation of minus-strand synthesis in vitro by the RNA-dependent RNA polymerase of turnip yellow mosaic virus

From mutational analysis of the 3'-terminal hairpin of turnip yellow mosaic virus (TYMV) RNA and use of nonstructured C-rich RNA templates, we conclude that the main determinant in the tRNA-like structure of TYMV RNA for initiation of minus-strand synthesis by the viral RNA-dependent RNA polymerase (RdRp) is the non-base-paired 3' ACC(A) end. Base pairing of this 3' end reduces the transcription efficiency drastically, and deletion of only the 3'-terminal A residue results in a fivefold drop in efficiency. The two C residues of the 3' ACCA end are required for efficient transcription, as shown by substitution mutations. However, the 5' A residue is not specifically involved in initiation of transcription, as shown by substitution mutations. Furthermore, the hairpin stem and loop upstream of the 3' ACCA end also do not interact with the RdRp in a base-specific way. However, for efficient transcription, the hairpin stem should be at least five bp in length, while the calculated deltaG value should be less than -10.5 kcal/mol. Unexpectedly, the use of nonstructured C-rich RNA templates showed that the RdRp can start internally on an NCCN or NUCN sequence. Therefore, a possible function of the tRNA-like structure of TYMV RNA may be to prevent internal initiation of minus-strand synthesis.

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.