Functional long-range RNA-RNA interactions in positive-strand RNA viruses

The RNA Base-Pairing Problem and Base-Pairing Solutions

RNA molecules are folded into structures and complexes to perform a wide variety of functions. Determination of RNA structures and their interactions is a fundamental problem in RNA biology. Most RNA molecules in living cells are large and dynamic, posing unique challenges to structure analysis. Here we review progress in RNA structure analysis, focusing on methods that use the "cross-link, proximally ligate, and sequence" principle for high-throughput detection of base-pairing interactions in living cells. Beginning with a comparison of commonly used methods in structure determination and a brief historical account of psoralen cross-linking studies, we highlight the important features of cross-linking methods and new biological insights into RNA structures and interactions from recent studies. Further improvement of these cross-linking methods and application to previously intractable problems will shed new light on the mechanisms of the "modern RNA world."

Structural and functional analysis of the roles of the HCV 5' NCR miR122-dependent long-range association and SLVI in genome translation and replication

The hepatitis C virus RNA genome possesses a variety of conserved structural elements, in both coding and non-coding regions, that are important for viral replication. These elements are known or predicted to modulate key life cycle events, such as translation and genome replication, some involving conformational changes induced by long-range RNA–RNA interactions. One such element is SLVI, a stem-loop (SL) structure located towards the 5′ end of the core protein-coding region. This element forms an alternative RNA–RNA interaction with complementary sequences in the 5′ untranslated regions that are independently involved in the binding of the cellular microRNA 122 (miR122). The switch between ‘open’ and ‘closed’ structures involving SLVI has previously been proposed to modulate translation, with lower translation efficiency associated with the ‘closed’ conformation. In the current study, we have used selective 2′-hydroxyl acylation analysed by primer extension to validate this RNA–RNA interaction in the absence and presence of miR122. We show that the long-range association (LRA) only forms in the absence of miR122, or otherwise requires the blocking of miR122 binding combined with substantial disruption of SLVI. Using site-directed mutations introduced to promote open or closed conformations of the LRA we demonstrate no correlation between the conformation and the translation phenotype. In addition, we observed no influence on virus replication compared to unmodified genomes. The presence of SLVI is well-documented to suppress translation, but these studies demonstrate that this is not due to its contribution to the LRA. We conclude that, although there are roles for SLVI in translation, the LRA is not a riboswitch regulating the translation and replication phenotypes of the virus.

Functional and structural characterization of the chikungunya virus translational recoding signals

Climate change and human globalization have spurred the rapid spread of mosquito-borne diseases to naïve populations. One such emerging virus of public health concern is chikungunya virus (CHIKV), a member of the Togaviridae family, genus Alphavirus CHIKV pathogenesis is predominately characterized by acute febrile symptoms and severe arthralgia, which can persist in the host long after viral clearance. CHIKV has also been implicated in cases of acute encephalomyelitis, and its vertical transmission has been reported. Currently, no FDA-approved treatments exist for this virus. Recoding elements help expand the coding capacity in many viruses and therefore represent potential therapeutic targets in antiviral treatments. Here, we report the molecular and structural characterization of two CHIKV translational recoding signals: a termination codon read-through (TCR) element located between the nonstructural protein 3 and 4 genes and a programmed -1 ribosomal frameshift (-1 PRF) signal located toward the 3' end of the CHIKV 6K gene. Using Dual-Luciferase and immunoblot assays in HEK293T and U87MG mammalian cell lines, we validated and genetically characterized efficient TCR and -1 PRF. Analyses of RNA chemical modification data with selective 2'-hydroxyl acylation and primer extension (SHAPE) assays revealed that CHIKV -1 PRF is stimulated by a tightly structured, triple-stem hairpin element, consistent with previous observations in alphaviruses, and that the TCR signal is composed of a single large multibulged hairpin element. These findings illuminate the roles of RNA structure in translational recoding and provide critical information relevant for design of live-attenuated vaccines against CHIKV and related viruses.

A Novel Method for Assessing the Statistical Significance of RNA-RNA Interactions Between Two Long RNAs

RNA-RNA interactions are key mechanisms through which noncoding RNA (ncRNA) regions exert biological functions. Computational prediction of RNA-RNA interactions is an essential method for detecting novel RNA-RNA interactions because their comprehensive detection by biological experimentation is still quite difficult. Many RNA-RNA interaction prediction tools have been developed, but they tend to produce many false positives. Accordingly, assessment of the statistical significance of computationally predicted interactions is an important task. However, there is no method to evaluate the statistical significance of RNA-RNA interactions that is applicable to interactions between two long RNA sequences. We developed a method to calculate the p-value for the minimal interaction energy between two long RNA sequences. The developed method depends on the fact that minimum interaction energies of RNA-RNA interactions between long RNAs follow a Gumbel distribution when repeat sequences in RNAs are masked. To show the usefulness of the developed method, we applied it to whole human 5'-untranslated region (UTR) and 3'-UTR sequences to detect novel 5'-UTR-3'-UTR interactions. We thus identified two significant 5'-UTR-3'-UTR interactions. Specifically, the human small proline-rich repeat protein 3 shows conserved 5'-UTR-3'-UTR interactions with some nucleotide variations preserving base pairings among primates. Our developed method enables us to detect statistically significant RNA-RNA interactions between long RNAs such as long ncRNAs. Statistical significance estimates help in identification of interactions for experimental validation and provide novel insights into the function of ncRNA regions.

Towards Long-Range RNA Structure Prediction in Eukaryotic Genes

The ability to form an intramolecular structure plays a fundamental role in eukaryotic RNA biogenesis. Proximate regions in the primary transcripts fold into a local secondary structure, which is then hierarchically assembled into a tertiary structure that is stabilized by RNA-binding proteins and long-range intramolecular base pairings. While the local RNA structure can be predicted reasonably well for short sequences, long-range structure at the scale of eukaryotic genes remains problematic from the computational standpoint. The aim of this review is to list functional examples of long-range RNA structures, to summarize current comparative methods of structure prediction, and to highlight their advances and limitations in the context of long-range RNA structures. Most comparative methods implement the “first-align-then-fold” principle, i.e., they operate on multiple sequence alignments, while functional RNA structures often reside in non-conserved parts of the primary transcripts. The opposite “first-fold-then-align” approach is currently explored to a much lesser extent. Developing novel methods in both directions will improve the performance of comparative RNA structure analysis and help discover novel long-range structures, their higher-order organization, and RNA⁻RNA interactions across the transcriptome.

Disruption of a stem-loop structure located upstream of pseudoknot domain in Tobacco mosaic virus enhanced its infectivity and viral RNA accumulation

A predicted stem-loop structure of 25 nucleotides, located in the coat protein (CP) gene and 3'-UTR sequences of Tobacco mosaic virus (TMV), was validated previously (Guo et al., 2015). In this study, both disrupted stem-loop and nucleotide deletion mutants of TMV replicated more rapidly in Nicotiana benthamiana protoplasts. The TMV mutant with a complete mirrored stem-loop structure showed similar level of viral RNA accumulation as TMV. Recovering the stem-loop structure also resulted in a similar replication level as TMV. All these mutants induced necrosis in N. benthamiana and assembled into typical rigid rod-shaped virions. TMV mutant without the stem-loop structure induced more local lesions in Chenopodium quinoa. When the putative stem-loop structure in Tomato mosaic virus (ToMV) was disrupted, the mutant also showed an enhanced virus replication. This suggests that the stem-loop structure of TMV is a new cis-acting element with a role in virus replication.

Expression of T7-based constructs in tobacco cells

Bacteriophage T7 promoter and RNA polymerase (T7-Pol) are widely used for recombinant protein expression in bacteria. In plants, there exists conflicting results regarding the efficacy of protein expression from T7-Pol-derived mRNAs. To reconcile these contradictory observations, the expression of green fluorescent protein (GFP) from T7 constructs was evaluated in tobacco protoplasts. T7 constructs transcribed by a nuclearly targeted T7-Pol did not express GFP in plant protoplasts, however T7-Pol lacking a nuclear targeting signal was able to translate cytosolically transcribed mRNAs, but only if the messages contained a viral translation enhancer. GFP expression was further evaluated at the plant level by using agroinfiltration-mediated transient expression system. Unlike for cytosolic expression, nuclear T7 transcripts containing a viral translation enhancer element did not express GFP, and modifications designed to stabilize and facilitate export of T7 transcripts to the cytosol did not improve the expression. We conclude that expression of nuclear T7 constructs is not feasible in tobacco cells, but cytosolic transcription provides an alternative means to over-express RNAs directly in the cytosol.

Intragenomic Long-Distance RNA-RNA Interactions in Plus-Strand RNA Plant Viruses

Plant viruses that contain positive-strand RNA genomes represent an important class of pathogen. The genomes of these viruses harbor RNA sequences and higher-order RNA structures that are essential for the regulation of viral processes during infections. In recent years, it has become increasingly evident that, in addition to locally positioned RNA structures, long-distance intragenomic interactions, involving nucleotide base pairing over large distances, also contribute significantly to the control of various viral events. Viral processes that are modulated by such interactions include genome replication, translation initiation, translational recoding, and subgenomic mRNA transcription. Here, we review the structure and function of different types of long-distance RNA–RNA interactions, herein termed LDRIs, present in members of the family Tombusviridae and other plus-strand RNA plant viruses.

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.

Nucleotide resolution mapping of influenza A virus nucleoprotein-RNA interactions reveals RNA features required for replication

Influenza A virus nucleoprotein (NP) association with viral RNA (vRNA) is essential for packaging, but the pattern of NP binding to vRNA is unclear. Here we applied photoactivatable ribonucleoside enhanced cross-linking and immunoprecipitation (PAR-CLIP) to assess the native-state of NP-vRNA interactions in infected human cells. NP binds short fragments of RNA (~12 nucleotides) non-uniformly and without apparent sequence specificity. Moreover, NP binding is reduced at specific locations within the viral genome, including regions previously identified as required for viral genome segment packaging. Synonymous mutations designed to alter the predicted RNA structures in these low-NP-binding regions impact genome packaging and result in virus attenuation, whereas control mutations or mutagenesis of NP-bound regions have no effect. Finally, we demonstrate that the sequence conservation of low-NP-binding regions is required in multiple genome segments for propagation of diverse mammalian and avian IAV in host cells.

Distinct roles for the IIId2 sub-domain in pestivirus and picornavirus internal ribosome entry sites

Viral internal ribosomes entry site (IRES) elements coordinate the recruitment of the host translation machinery to direct the initiation of viral protein synthesis. Within hepatitis C virus (HCV)-like IRES elements, the sub-domain IIId(1) is crucial for recruiting the 40S ribosomal subunit. However, some HCV-like IRES elements possess an additional sub-domain, termed IIId2, whose function remains unclear. Herein, we show that IIId2 sub-domains from divergent viruses have different functions. The IIId2 sub-domain present in Seneca valley virus (SVV), a picornavirus, is dispensable for IRES activity, while the IIId2 sub-domains of two pestiviruses, classical swine fever virus (CSFV) and border disease virus (BDV), are required for 80S ribosomes assembly and IRES activity. Unlike in SVV, the deletion of IIId2 from the CSFV and BDV IRES elements impairs initiation of translation by inhibiting the assembly of 80S ribosomes. Consequently, this negatively affects the replication of CSFV and BDV. Finally, we show that the SVV IIId2 sub-domain is required for efficient viral RNA synthesis and growth of SVV, but not for IRES function. This study sheds light on the molecular evolution of viruses by clearly demonstrating that conserved RNA structures, within distantly related RNA viruses, have acquired different roles in the virus life cycles.

Probing the Structures of Viral RNA Regulatory Elements with SHAPE and Related Methodologies

Viral RNAs were selected by evolution to possess maximum functionality in a minimal sequence. Depending on the classification of the virus and the type of RNA in question, viral RNAs must alternately be replicated, spliced, transcribed, transported from the nucleus into the cytoplasm, translated and/or packaged into nascent virions, and in most cases, provide the sequence and structural determinants to facilitate these processes. One consequence of this compact multifunctionality is that viral RNA structures can be exquisitely complex, often involving intermolecular interactions with RNA or protein, intramolecular interactions between sequence segments separated by several thousands of nucleotides, or specialized motifs such as pseudoknots or kissing loops. The fluidity of viral RNA structure can also present a challenge when attempting to characterize it, as genomic RNAs especially are likely to sample numerous conformations at various stages of the virus life cycle. Here we review advances in chemoenzymatic structure probing that have made it possible to address such challenges with respect to cis-acting elements, full-length viral genomes and long non-coding RNAs that play a major role in regulating viral gene expression.

Insights into Structural and Mechanistic Features of Viral IRES Elements

Internal ribosome entry site (IRES) elements are cis-acting RNA regions that promote internal initiation of protein synthesis using cap-independent mechanisms. However, distinct types of IRES elements present in the genome of various RNA viruses perform the same function despite lacking conservation of sequence and secondary RNA structure. Likewise, IRES elements differ in host factor requirement to recruit the ribosomal subunits. In spite of this diversity, evolutionarily conserved motifs in each family of RNA viruses preserve sequences impacting on RNA structure and RNA–protein interactions important for IRES activity. Indeed, IRES elements adopting remarkable different structural organizations contain RNA structural motifs that play an essential role in recruiting ribosomes, initiation factors and/or RNA-binding proteins using different mechanisms. Therefore, given that a universal IRES motif remains elusive, it is critical to understand how diverse structural motifs deliver functions relevant for IRES activity. This will be useful for understanding the molecular mechanisms beyond cap-independent translation, as well as the evolutionary history of these regulatory elements. Moreover, it could improve the accuracy to predict IRES-like motifs hidden in genome sequences. This review summarizes recent advances on the diversity and biological relevance of RNA structural motifs for viral IRES elements.

Know Your Enemy: Successful Bioinformatic Approaches to Predict Functional RNA Structures in Viral RNAs

Structured RNA elements may control virus replication, transcription and translation, and their distinct features are being exploited by novel antiviral strategies. Viral RNA elements continue to be discovered using combinations of experimental and computational analyses. However, the wealth of sequence data, notably from deep viral RNA sequencing, viromes, and metagenomes, necessitates computational approaches being used as an essential discovery tool. In this review, we describe practical approaches being used to discover functional RNA elements in viral genomes. In addition to success stories in new and emerging viruses, these approaches have revealed some surprising new features of well-studied viruses e.g., human immunodeficiency virus, hepatitis C virus, influenza, and dengue viruses. Some notable discoveries were facilitated by new comparative analyses of diverse viral genome alignments. Importantly, comparative approaches for finding RNA elements embedded in coding and non-coding regions differ. With the exponential growth of computer power we have progressed from stem-loop prediction on single sequences to cutting edge 3D prediction, and from command line to user friendly web interfaces. Despite these advances, many powerful, user friendly prediction tools and resources are underutilized by the virology community.

Structural and functional conservation of cis-acting RNA elements in coronavirus 5'-terminal genome regions

Structure predictions suggest a partial conservation of RNA structure elements in coronavirus terminal genome regions. Here, we determined the structures of stem-loops (SL) 1 and 2 of two alphacoronaviruses, human coronavirus (HCoV) 229E and NL63, by RNA structure probing and studied the functional relevance of these putative cis-acting elements. HCoV-229E SL1 and SL2 mutants generated by reverse genetics were used to study the effects on viral replication of single-nucleotide substitutions predicted to destabilize the SL1 and SL2 structures. The data provide conclusive evidence for the critical role of SL1 and SL2 in HCoV-229E replication and, in some cases, revealed parallels with previously characterized betacoronavirus SL1 and SL2 elements. Also, we were able to rescue viable HCoV-229E mutants carrying replacements of SL2 with equivalent betacoronavirus structural elements. The data obtained in this study reveal a remarkable degree of structural and functional conservation of 5′-terminal RNA structural elements across coronavirus genus boundaries.

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Conservation of 5'-terminal SL1 and SL2 elements in alpha- and betacoronaviruses.

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HCoV-229E reverse genetics data suggest critical role for SL1/2 in viral replication.

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Replacement of SL2 in HCoV-229E with betacoronavirus equivalents is tolerated.

Dual function of a cis-acting RNA element that acts as a replication enhancer and a translation repressor in a plant positive-stranded RNA virus

The genome of red clover necrotic mosaic virus is divided into two positive-stranded RNA molecules of RNA1 and RNA2, which have no 5' cap structure and no 3' poly(A) tail. Previously, we showed that any mutations in the cis-acting RNA replication elements of RNA2 abolished its cap-independent translational activity, suggesting a strong link between RNA replication and translation. Here, we investigated the functions of the 5' untranslated region (UTR) of RNA2 and revealed that the basal stem-structure (5'BS) predicted in the 5' UTR is essential for robust RNA replication. Interestingly, RNA2 mutants with substitution or deletion in the right side of the 5'BS showed strong translational activity, despite their impaired replication competency. Furthermore, nucleotide sequences other than the 5'BS of the 5' UTR were essential to facilitate the replication-associated translation. Overall, these cis-acting RNA elements seem to coordinately regulate the balance between RNA replication and replication-associated translation.

In-cell SHAPE uncovers dynamic interactions between the untranslated regions of the foot-and-mouth disease virus RNA

The genome of RNA viruses folds into 3D structures that include long-range RNA–RNA interactions relevant to control critical steps of the viral cycle. In particular, initiation of translation driven by the IRES element of foot-and-mouth disease virus is stimulated by the 3΄UTR. Here we sought to investigate the RNA local flexibility of the IRES element and the 3΄UTR in living cells. The SHAPE reactivity observed in vivo showed statistically significant differences compared to the free RNA, revealing protected or exposed positions within the IRES and the 3΄UTR. Importantly, the IRES local flexibility was modified in the presence of the 3΄UTR, showing significant protections at residues upstream from the functional start codon. Conversely, presence of the IRES element in cis altered the 3΄UTR local flexibility leading to an overall enhanced reactivity. Unlike the reactivity changes observed in the IRES element, the SHAPE differences of the 3΄UTR were large but not statistically significant, suggesting multiple dynamic RNA interactions. These results were supported by covariation analysis, which predicted IRES-3΄UTR conserved helices in agreement with the protections observed by SHAPE probing. Mutational analysis suggested that disruption of one of these interactions could be compensated by alternative base pairings, providing direct evidences for dynamic long-range interactions between these distant elements of the viral genome.

Concerted action of two 3' cap-independent translation enhancers increases the competitive strength of translated viral genomes

Several families of plant viruses evolved cap-independent translation enhancers (3'CITE) in the 3' untranslated regions of their genomic (g)RNAs to compete with ongoing cap-dependent translation of cellular mRNAs. Umbravirus Pea enation mosaic virus (PEMV)2 is the only example where three 3'CITEs enhance translation: the eIF4E-binding Panicum mosaic virus-like translational enhancer (PTE) and ribosome-binding 3' T-shaped structure (TSS) have been found in viruses of different genera, while the ribosome-binding kl-TSS that provides a long-distance interaction with the 5' end is unique. We report that the PTE is the key translation promoting element, but inhibits translation in cis and in trans in the absence of the kl-TSS by sequestering initiation factor eIF4G. PEMV2 strongly outcompeted a cellular mRNA mimic for translation, indicating that the combination of kl-TSS and PTE is highly efficient. Transferring the 3'-5' interaction from the kl-TSS to the PTE (to fulfill its functionality as found in other viruses) supported translationin vitro, but gRNA did not accumulate to detectable levels in protoplasts in the absence of the kl-TSS. It was shown that the PTE in conjunction with the kl-TSS did not markedly affect the translation initiation rate but rather increased the number of gRNAs available for translation. A model is proposed to explain how 3'CITE-based regulation of ribosome recruitment enhances virus fitness.

Cellular DEAD-box RNA helicase DDX6 modulates interaction of miR-122 with the 5' untranslated region of hepatitis C virus RNA

Hepatitis C virus (HCV) subverts the cellular DEAD-box RNA helicase DDX6 to promote virus infection. Using polysome gradient analysis and the subgenomic HCV Renilla reporter replicon genome, we determined that DDX6 does not affect HCV translation. Rather expression of the subgenomic HCV Renilla luciferase reporter at late times, as well as labeling of newly synthesized viral RNA with 4-thiouridine showed that DDX6 modulates replication. Because DDX6 is an effector protein of the microRNA pathway, we also investigated its role in miR-122-directed HCV gene expression. Similar to sequestering miR-122, depletion of DDX6 modulated HCV RNA stability. Interestingly, miR-122-HCV RNA interaction assays with mutant HCV genomes sites and compensatory exogenous miR-122 showed that DDX6 affects the function of miR-122 at one particular binding site. We propose that DDX6 facilitates the miR-122 interaction with HCV 5' UTR, which is necessary for stabilizing the viral genome and the switch between translation and replication.

Atypical RNA Elements Modulate Translational Readthrough in Tobacco Necrosis Virus D

Tobacco necrosis virus, strain D (TNV-D), is a positive-strand RNA virus in the genus Betanecrovirus and family Tombusviridae The production of its RNA-dependent RNA polymerase, p82, is achieved by translational readthrough. This process is stimulated by an RNA structure that is positioned immediately downstream of the recoding site, termed the readthrough stem-loop (RTSL), and a sequence in the 3' untranslated region of the TNV-D genome, called the distal readthrough element (DRTE). Notably, a base pairing interaction between the RTSL and the DRTE, spanning ∼3,000 nucleotides, is required for enhancement of readthrough. Here, some of the structural features of the RTSL, as well as RNA sequences and structures that flank either the RTSL or DRTE, were investigated for their involvement in translational readthrough and virus infectivity. The results revealed that (i) the RTSL-DRTE interaction cannot be functionally replaced by stabilizing the RTSL structure, (ii) a novel tertiary RNA structure positioned just 3' to the RTSL is required for optimal translational readthrough and virus infectivity, and (iii) these same activities also rely on an RNA stem-loop located immediately upstream of the DRTE. Functional counterparts for the RTSL-proximal structure may also be present in other tombusvirids. The identification of additional distinct RNA structures that modulate readthrough suggests that regulation of this process by genomic features may be more complex than previously appreciated. Possible roles for these novel RNA elements are discussed.IMPORTANCE The analysis of factors that affect recoding events in viruses is leading to an ever more complex picture of this important process. In this study, two new atypical RNA elements were shown to contribute to efficient translational readthrough of the TNV-D polymerase and to mediate robust viral genome accumulation in infections. One of the structures, located close to the recoding site, could have functional equivalents in related genera, while the other structure, positioned 3' proximally in the viral genome, is likely limited to betanecroviruses. Irrespective of their prevalence, the identification of these novel RNA elements adds to the current repertoire of viral genome-based modulators of translational readthrough and provides a notable example of the complexity of regulation of this process.

Regulation Mechanisms of Viral IRES-Driven Translation

Internal ribosome entry sites (IRESs) can be found in the mRNA of many viruses as well as in cellular genes involved in the stress response, cell cycle, and apoptosis. IRES-mediated translation can occur when dominant cap-dependent translation is inhibited, and viruses can take advantage of this to subvert host translation machinery. In this review, we focus on the four major types of IRES identified in RNA viruses, and outline their distinct structural properties and requirements of translational factors. We further discuss auxiliary host factors known as IRES trans-acting factors (ITAFs), which are involved in the modulation of optimal IRES activity. Currently known strategies employed by viruses to harness ITAFs and regulate IRES activity are also highlighted.

Non-canonical Translation in Plant RNA Viruses

Viral protein synthesis is completely dependent upon the host cell's translational machinery. Canonical translation of host mRNAs depends on structural elements such as the 5' cap structure and/or the 3' poly(A) tail of the mRNAs. Although many viral mRNAs are devoid of one or both of these structures, they can still translate efficiently using non-canonical mechanisms. Here, we review the tools utilized by positive-sense single-stranded (+ss) RNA plant viruses to initiate non-canonical translation, focusing on cis-acting sequences present in viral mRNAs. We highlight how these elements may interact with host translation factors and speculate on their contribution for achieving translational control. We also describe other translation strategies used by plant viruses to optimize the usage of the coding capacity of their very compact genomes, including leaky scanning initiation, ribosomal frameshifting and stop-codon readthrough. Finally, future research perspectives on the unusual translational strategies of +ssRNA viruses are discussed, including parallelisms between viral and host mRNAs mechanisms of translation, particularly for host mRNAs which are translated under stress conditions.

Viral RNA switch mediates the dynamic control of flavivirus replicase recruitment by genome cyclization

Viral replicase recruitment and long-range RNA interactions are essential for RNA virus replication, yet the mechanism of their interplay remains elusive. Flaviviruses include numerous important human pathogens, e.g., dengue virus (DENV) and Zika virus (ZIKV). Here, we revealed a highly conserved, conformation-tunable cis-acting element named 5′-UAR-flanking stem (UFS) in the flavivirus genomic 5′ terminus. We demonstrated that the UFS was critical for efficient NS5 recruitment and viral RNA synthesis in different flaviviruses. Interestingly, stabilization of the DENV UFS impaired both genome cyclization and vRNA replication. Moreover, the UFS unwound in response to genome cyclization, leading to the decreased affinity of NS5 for the viral 5′ end. Thus, we propose that the UFS is switched by genome cyclization to regulate dynamic RdRp binding for vRNA replication. This study demonstrates that the UFS enables communication between flavivirus genome cyclization and RdRp recruitment, highlighting the presence of switch-like mechanisms among RNA viruses.

DOI:http://dx.doi.org/10.7554/eLife.17636.001

Flaviviruses include a large family of viruses that are harmful to human health, such as dengue virus, West Nile virus and Zika virus. Understanding the details of the life cycle of these viruses is important for better controlling and treating the diseases that they cause.

The genetic information of flaviviruses is stored in single-stranded molecules of RNA. To form new copies of a virus, the RNA must be replicated in a process that involves two critical steps. First, an enzyme called viral RNA polymerase NS5 must be recruited to a specific end of the RNA strand (known as the 5′ end). Then, the ends of the RNA strand bind together to form a circular loop. However, little is known about whether these two processes are linked, or how they are regulated.

Using bioinformatics, biochemical and reverse genetics approaches, Liu et al. have now identified a new section of RNA in the 5′ end of the flavivirus RNA, named the 5′-UAR-flanking stem (or UFS for short), which is critical for viral replication. The UFS plays an important role in efficiently recruiting the NS5 viral RNA polymerase to the 5′ end of the flavivirus RNA.

After the RNA forms a circle, the UFS unwinds. This makes the NS5 polymerase less likely to bind to the 5′ end of the RNA. Stabilizing the structure of the UFS impairs the ability of the RNA strand to form a circle, and hence reduces the ability of the RNA to replicate. Thus, the UFS links and enables communication between the processes that form the flavivirus RNA into a circle and that recruit the viral RNA polymerase to the RNA.

The structural basis of the interaction between the flavivirus RNA 5′ end, including the UFS element, and the viral RNA polymerase now deserves further investigation. It will be also important to explore whether other types of viruses regulate their replication via a similar mechanism.

DOI:http://dx.doi.org/10.7554/eLife.17636.002

A Cap-to-Tail Guide to mRNA Translation Strategies in Virus-Infected Cells

Although viruses require cellular functions to replicate, their absolute dependence upon the host translation machinery to produce polypeptides indispensable for their reproduction is most conspicuous. Despite their incredible diversity, the mRNAs produced by all viruses must engage cellular ribosomes. This has proven to be anything but a passive process and has revealed a remarkable array of tactics for rapidly subverting control over and dominating cellular regulatory pathways that influence translation initiation, elongation, and termination. Besides enforcing viral mRNA translation, these processes profoundly impact host cell-intrinsic immune defenses at the ready to deny foreign mRNA access to ribosomes and block protein synthesis. Finally, genome size constraints have driven the evolution of resourceful strategies for maximizing viral coding capacity. Here, we review the amazing strategies that work to regulate translation in virus-infected cells, highlighting both virus-specific tactics and the tremendous insight they provide into fundamental translational control mechanisms in health and disease.

Saturation of recognition elements blocks evolution of new tRNA identities

Understanding the principles that led to the current complexity of the genetic code is a central question in evolution. Expansion of the genetic code required the selection of new transfer RNAs (tRNAs) with specific recognition signals that allowed them to be matured, modified, aminoacylated, and processed by the ribosome without compromising the fidelity or efficiency of protein synthesis. We show that saturation of recognition signals blocks the emergence of new tRNA identities and that the rate of nucleotide substitutions in tRNAs is higher in species with fewer tRNA genes. We propose that the growth of the genetic code stalled because a limit was reached in the number of identity elements that can be effectively used in the tRNA structure.

Nucleolin promotes in vitro translation of feline calicivirus genomic RNA

Feline calicivirus depends on host-cell proteins for its replication. We previously showed that knockdown of nucleolin (NCL), a phosphoprotein involved in ribosome biogenesis, resulted in the reduction of FCV protein synthesis and virus yield. Here, we found that NCL may not be involved in FCV binding and entry into cells, but it binds to both ends of the FCV genomic RNA, and stimulates its translation in vitro. AGRO100, an aptamer that specifically binds and inactivates NCL, caused a strong reduction in FCV protein synthesis. This effect could be reversed by the addition of full-length NCL but not by a ΔrNCL, lacking the N-terminal domain. Consistent with this, FCV infection of CrFK cells stably expressing ΔrNCL led to a reduction in virus protein translation. These results suggest that NCL is part of the FCV RNA translational complex, and that the N-terminal part of the protein is required for efficient FCV replication.

A Viral mRNA Motif at the 3'-Untranslated Region that Confers Translatability in a Cell-Specific Manner. Implications for Virus Evolution

Sindbis virus (SINV) mRNAs contain several motifs that participate in the regulation of their translation. We have discovered a motif at the 3′ untranslated region (UTR) of viral mRNAs, constituted by three repeated sequences, which is involved in the translation of both SINV genomic and subgenomic mRNAs in insect, but not in mammalian cells. These data illustrate for the first time that an element present at the 3′-UTR confers translatability to mRNAs from an animal virus in a cell-specific manner. Sequences located at the beginning of the 5′-UTR may also regulate SINV subgenomic mRNA translation in both cell lines in a context of infection. Moreover, a replicon derived from Sleeping disease virus, an alphavirus that have no known arthropod vector for transmission, is much more efficient in insect cells when the repeated sequences from SINV are inserted at its 3′-UTR, due to the enhanced translatability of its mRNAs. Thus, these findings provide a clue to understand, at the molecular level, the evolution of alphaviruses and their host range.

Standing your ground to exoribonucleases: Function of Flavivirus long non-coding RNAs

Members of the Flaviviridae (e.g., Dengue virus, West Nile virus, and Hepatitis C virus) contain a positive-sense RNA genome that encodes a large polyprotein. It is now also clear most if not all of these viruses also produce an abundant subgenomic long non-coding RNA. These non-coding RNAs, which are called subgenomic flavivirus RNAs (sfRNAs) or Xrn1-resistant RNAs (xrRNAs), are stable decay intermediates generated from the viral genomic RNA through the stalling of the cellular exoribonuclease Xrn1 at highly structured regions. Several functions of these flavivirus long non-coding RNAs have been revealed in recent years. The generation of these sfRNAs/xrRNAs from viral transcripts results in the repression of Xrn1 and the dysregulation of cellular mRNA stability. The abundant sfRNAs also serve directly as a decoy for important cellular protein regulators of the interferon and RNA interference antiviral pathways. Thus the generation of long non-coding RNAs from flaviviruses, hepaciviruses and pestiviruses likely disrupts aspects of innate immunity and may directly contribute to viral replication, cytopathology and pathogenesis.

TMV mutants with poly(A) tracts of different lengths demonstrate structural variations in 3'UTR affecting viral RNAs accumulation and symptom expression

The upstream pseudoknots domain (UPD) of Tobacco mosaic virus (TMV) is located at the 3'-untranslated region (UTR). It plays an important role in virus replication and translation. To determine the importance of UPD and 3'-UTR, and the effects of introduced RNA elements in TMV 3'-UTR, a series of TMV mutants with internal poly(A) tract upstream of UPD was constructed for structural analysis by selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE). TMV(24A+UPD) and TMV(42A+UPD) formed a similar structure as that of TMV 3'-UTR, but TMV(62A+UPD) structures altered by the introduced poly(A) tract. In addition, TMV(24A+UPD) had a higher viral RNAs accumulation than TMV in N. benthamiana protoplasts, and induced lethal symptoms in the infected plants. TMV(62A+UPD) showed a drastically reduced accumulation, its coat protein was undetectable in protoplasts, and the inoculated plants remained symptomless. This study analyzed the structures of 3'-UTR of TMV and found that the longer poly(A) tract introduced upstream of UPD reduced viral RNAs accumulation and induced milder symptoms in N. benthamiana. In conclusion, different lengths of the internal poly(A) tract introduced into the TMV 3'UTR lead to structural variations that affect virus accumulation and symptom expression.

Continuous and Discontinuous RNA Synthesis in Coronaviruses

Replication of the coronavirus genome requires continuous RNA synthesis, whereas transcription is a discontinuous process unique among RNA viruses. Transcription includes a template switch during the synthesis of subgenomic negative-strand RNAs to add a copy of the leader sequence. Coronavirus transcription is regulated by multiple factors, including the extent of base-pairing between transcription-regulating sequences of positive and negative polarity, viral and cell protein-RNA binding, and high-order RNA-RNA interactions. Coronavirus RNA synthesis is performed by a replication-transcription complex that includes viral and cell proteins that recognize cis-acting RNA elements mainly located in the highly structured 5' and 3' untranslated regions. In addition to many viral nonstructural proteins, the presence of cell nuclear proteins and the viral nucleocapsid protein increases virus amplification efficacy. Coronavirus RNA synthesis is connected with the formation of double-membrane vesicles and convoluted membranes. Coronaviruses encode proofreading machinery, unique in the RNA virus world, to ensure the maintenance of their large genome size.

Requirement for Host RNA-Silencing Components and the Virus-Silencing Suppressor when Second-Site Mutations Compensate for Structural Defects in the 3' Untranslated Region

Turnip crinkle virus (TCV) contains a structured 3' region with hairpins and pseudoknots that form a complex network of noncanonical RNA:RNA interactions supporting higher-order structure critical for translation and replication. We investigated several second-site mutations in the p38 coat protein open reading frame (ORF) that arose in response to a mutation in the asymmetric loop of a critical 3' untranslated region (UTR) hairpin that disrupts local higher-order structure. All tested second-site mutations improved accumulation of TCV in conjunction with a partial reversion of the primary mutation (TCV-rev1) but had neutral or a negative effect on wild-type (wt) TCV or TCV with the primary mutation. SHAPE (selective 2'-hydroxyl acylation analyzed by primer extension) structure probing indicated that these second-site mutations reside in an RNA domain that includes most of p38 (domain 2), and evidence for RNA:RNA interactions between domain 2 and 3'UTR-containing domain 1 was found. However, second-site mutations were not compensatory in the absence of p38, which is also the TCV silencing suppressor, or in dcl-2/dcl4 or ago1/ago2 backgrounds. One second-site mutation reduced silencing suppressor activity of p38 by altering one of two GW motifs that are required for p38 binding to double-stranded RNAs (dsRNAs) and interaction with RNA-induced silencing complex (RISC)-associated AGO1/AGO2. Another second-site mutation substantially reduced accumulation of TCV-rev1 in the absence of p38 or DCL2/DCL4. We suggest that the second-site mutations in the p38 ORF exert positive effects through a similar downstream mechanism, either by enhancing accumulation of beneficial DCL-produced viral small RNAs that positively regulate the accumulation of TCV-rev1 or by affecting the susceptibility of TCV-rev1 to RISC loaded with viral small RNAs.

IMPORTANCE

Genomes of positive-strand RNA viruses fold into high-order RNA structures. Viruses with mutations in regions critical for translation and replication often acquire second-site mutations that exert a positive compensatory effect through reestablishment of canonical base pairing with the altered region. In this study, two distal second-site mutations that individually arose in response to a primary mutation in a critical 3' UTR hairpin in the genomic RNA of turnip crinkle virus did not directly interact with the primary mutation. Although different second-site changes had different attributes, compensation was dependent on the production of the viral p38 silencing suppressor and on the presence of silencing-required DCL and AGO proteins. Our results provide an unexpected connection between a 3' UTR primary-site mutation proposed to disrupt higher-order structure and the RNA-silencing machinery.

Selective 2'-hydroxyl acylation analyzed by primer extension and mutational profiling (SHAPE-MaP) for direct, versatile and accurate RNA structure analysis

Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) chemistries exploit small electrophilic reagents that react with 2'-hydroxyl groups to interrogate RNA structure at single-nucleotide resolution. Mutational profiling (MaP) identifies modified residues by using reverse transcriptase to misread a SHAPE-modified nucleotide and then counting the resulting mutations by massively parallel sequencing. The SHAPE-MaP approach measures the structure of large and transcriptome-wide systems as accurately as can be done for simple model RNAs. This protocol describes the experimental steps, implemented over 3 d, that are required to perform SHAPE probing and to construct multiplexed SHAPE-MaP libraries suitable for deep sequencing. Automated processing of MaP sequencing data is accomplished using two software packages. ShapeMapper converts raw sequencing files into mutational profiles, creates SHAPE reactivity plots and provides useful troubleshooting information. SuperFold uses these data to model RNA secondary structures, identify regions with well-defined structures and visualize probable and alternative helices, often in under 1 d. SHAPE-MaP can be used to make nucleotide-resolution biophysical measurements of individual RNA motifs, rare components of complex RNA ensembles and entire transcriptomes.

Biologically-supported structural model for a viral satellite RNA

Satellite RNAs (satRNAs) are a class of small parasitic RNA replicon that associate with different viruses, including plus-strand RNA viruses. Because satRNAs do not encode a polymerase or capsid subunit, they rely on a companion virus to provide these proteins for their RNA replication and packaging. SatRNAs recruit these and other required factors via their RNA sequences and structures. Here, through a combination of chemical probing analysis of RNA structure, phylogenetic structural comparisons, and viability assays of satRNA mutants in infected cells, the biological importance of a deduced higher-order structure for a 619 nt long tombusvirus satRNA was assessed. Functionally-relevant secondary and tertiary RNA structures were identified throughout the length of the satRNA. Notably, a 3′-terminal segment was found to adopt two mutually-exclusive RNA secondary structures, both of which were required for efficient satRNA accumulation. Accordingly, these alternative conformations likely function as a type of RNA switch. The RNA switch was also found to engage in a required long-range kissing-loop interaction with an upstream sequence. Collectively, these results establish a high level of conformational complexity within this small parasitic RNA and provide a valuable structural framework for detailed mechanistic studies.

Properties of satellite tobacco mosaic virus phenotypes expressed in the presence and absence of helper virus

In this study, we assembled an Agrobacterium-based transient expression system for the ectopic expression of Satellite tobacco mosaic virus (STMV) (+) or (-) transcripts and their biological activity was confirmed when Nicotiana benthamiana plants were co-expressed with helper Tobacco mosaic virus replicase. Characterization of STMV in the presence and absence of its HV revealed: (i) HV-dependent expression of STMV (+) in N. benthamiana, but not in N. tabacum, generated a replication-deficient but translation and encapsidation competent variant lacking the highly conserved 3' 150 nucleotides (nt) (STMVΔ150); (ii) mutational analysis demonstrated that a conserved 3' stem-loop structure in wild type and STMVΔ150 located between nt 874 and 897 is essential for translation of CP; (iii) helper virus-independent expression of CP from wt STMV was competent for the assembly of empty aberrant virion-like particles; whereas, CP translated from STMVΔ150 resulted in disorganized CP aggregates suggesting a role for the 3'tRNA-like structure in STMV assembly.

Cellular Proteins Act as Bridge Between 5' and 3' Ends of the Coxsackievirus B3 Mediating Genome Circularization During RNA Translation

The positive single-stranded RNA genome of the Coxsackievirus B3 (CVB3) contains a 5' untranslated region (UTR) which hosts the internal ribosome entry site (IRES) element that governs cap-independent translation initiation and a polyadenylated 3' UTR which is required for stimulating the IRES activity. Viral RNA genomes could circularize to regulate initiation of translation and RNA synthesis at 5' and 3' ends. Interactions could either take place by direct RNA-RNA contacts, through cellular protein bridges mediating RNA circularization or both. Accordingly, we aimed to assess the nature of molecular interactions between these two regions and to evaluate cellular factors required for mRNA 3' end-mediated stimulation of CVB3 IRES-driven translation. By gel shift assays, we have showed that combining, in vitro, 5' and 3' UTR fragments had no discernible effect on the structures of RNAs, arguing against the presence of specific canonical RNA-RNA cyclization sequences between these two regions. Competitive UV crosslinking assays using BHK-21 cell extract showed common cellular proteins eIF3b, PTB, and La binding to both 5'- and 3' end RNAs. PCBP 1-2 and PABP were shown to bind, respectively, to 5' and 3' UTR probes. Taking together, these data suggest that CVB3 5'-3' end bridging occurs through 5' UTR-protein-protein-3' UTR interactions and not through RNA-RNA direct contact. The dual involvement of the 3' and 5' UTRs in controlling viral translation and RNA synthesis highlights the relevance of these regions in the infectious virus life cycle, making them suitable candidates for targeted CVB3 antiviral therapy.

RNA Structures as Mediators of Neurological Diseases and as Drug Targets

RNAs adopt diverse folded structures that are essential for function and thus play critical roles in cellular biology. A striking example of this is the ribosome, a complex, three-dimensionally folded macromolecular machine that orchestrates protein synthesis. Advances in RNA biochemistry, structural and molecular biology, and bioinformatics have revealed other non-coding RNAs whose functions are dictated by their structure. It is not surprising that aberrantly folded RNA structures contribute to disease. In this Review, we provide a brief introduction into RNA structural biology and then describe how RNA structures function in cells and cause or contribute to neurological disease. Finally, we highlight successful applications of rational design principles to provide chemical probes and lead compounds targeting structured RNAs. Based on several examples of well-characterized RNA-driven neurological disorders, we demonstrate how designed small molecules can facilitate the study of RNA dysfunction, elucidating previously unknown roles for RNA in disease, and provide lead therapeutics.

Giant viruses: enfants terribles in the microbal world

ABSTRACT 

In the past decade knowledge about Megaviruses, also denoted as nucleocytoplasmic large DNA viruses (NCLDVs) has been growing quickly. In the current paper, we present a general description of NCLDVs including their genome size, gene functions and homologies to other forms of life and viruses. Evolution of giant viruses from the fourth domain of life (now extinct) and a genomic complexification from smaller DNA viruses are described. Undeniable is the fact that Megaviruses in terms of their size and genomic capacity belong in between cellular and viral worlds breaking several ‘viral dogmas.’ The host range of NCLDVs goes far beyond unicellular organisms and a major challenge in the future is to establish their pathogenicity in human population.

Exploring the architecture of viral RNA genomes

The genomes of RNA viruses contain local structural elements and long-range interactions that control various steps in virus replication. While many individual RNA elements have been characterized, it remains less clear how the structure and activity of such elements are integrated and regulated within the complex context of complete viral genomes. Recent technical advances, particularly the development of high-throughput solution structure mapping methods, have made secondary structural analysis of entire viral RNA genomes feasible. As a consequence, whole-genome structural models have been deduced for a number of plus-strand RNA viruses and retroviruses and these structures have provided intriguing functional and evolutionary insights into global genome architecture.

Cis- and trans-regulation of luteovirus gene expression by the 3' end of the viral genome

Translation of the 5.7 kb luteovirus genome is controlled by the 3' untranslated region (UTR). Base pairing between regions of the 3' UTR and sequences kilobases upstream is required for cap-independent translation and ribosomal frameshifting needed to synthesize the viral replicase. Luteoviruses produce subgenomic RNAs, which can serve as mRNA, but one sgRNA also regulates translation initiation in trans. As on all viruses, the 3' and 5' ends contain structures that are presumed to facilitate RNA synthesis. This review describes the structures and interactions of barley yellow dwarf virus RNA that facilitate the complex interplay between the above events and result in a successful virus infection. We also present surprising results on the apparent lack of need for some subgenomic RNAs for the virus to infect cells or whole plants. In summary, the UTRs of luteoviruses are highly complex entities that control and fine-tune many key events of the virus replication cycle.

Challenges associated with research on RNA viruses of insects

Dicistroviridae and Iflaviridae (part of the group formerly identified as picorna-like viruses) are rapidly growing families within the order Picornavirales. Work on these emerging groups of arthropod viruses offers a unique and exciting opportunity for virologist, but this task comes with particular challenges. The lack of cell culture systems and infectious clones has imposed limitations on the advancement of study of these viruses. Here we discuss the goals and challenges regarding the establishment of controlled systems as well as some issues associated with insect RNA virology at the organismal level. These concerns apply to RNA viruses affecting other organisms for which basic research tools are limited. A list of pitfalls associated with RNA virus research along with recommendations is provided.

Functions of the 5'- and 3'-untranslated regions of tobamovirus RNA

The tobamovirus genome is a 5'-m(7)G-capped RNA that carries a tRNA-like structure at its 3'-terminus. The genomic RNA serves as the template for both translation and negative-strand RNA synthesis. The 5'- and 3'-untranslated regions (UTRs) of the genomic RNA contain elements that enhance translation, and the 3'-UTR also contains the elements necessary for the initiation of negative-strand RNA synthesis. Recent studies using a cell-free viral RNA translation-replication system revealed that a 70-nucleotide region containing a part of the 5'-UTR is bound cotranslationally by tobacco mosaic virus (TMV) replication proteins translated from the genomic RNA and that the binding leads the genomic RNA to RNA replication pathway. This mechanism explains the cis-preferential replication of TMV by the replication proteins. The binding also inhibits further translation to avoid a fatal ribosome-RNA polymerase collision, which might arise if translation and negative-strand synthesis occur simultaneously on a single genomic RNA molecule. Therefore, the 5'- and 3'-UTRs play multiple important roles in the life cycle of tobamovirus.

3'UTRs of carmoviruses

Carmovirus is a genus of small, single-stranded, positive-strand RNA viruses in the Tombusviridae. One member of the carmoviruses, Turnip crinkle virus (TCV), has been used extensively as a model for examining the structure and function of RNA elements in 3'UTR as well as in other regions of the virus. Using a variety of genetic, biochemical and computational methods, a structure for the TCV 3'UTR has emerged where secondary structures and tertiary interactions combine to adopt higher order 3-D structures including an internal, ribosome-binding tRNA-shaped configuration that functions as a 3' cap-independent translation enhancer (3'CITE). The TCV 3'CITE also serves as a scaffold for non-canonical interactions throughout the 3'UTR and extending into the upstream open reading frame, interactions that are significantly disrupted upon binding by the RNA-dependent RNA polymerase. Long-distance interactions that connect elements in the 3'UTR with both the 5' end and the internal ribosome recoding site suggest that 3'UTR of carmoviruses are intimately involved in multiple functions in the virus life cycle. Although carmoviruses share very similar genome organizations, lengths of 5' and 3'UTRs, and structural features at the 3' end, the similarity rapidly breaks down the further removed from the 3' terminus revealing different 3'CITEs and unique virus-specific structural features. This review summarizes 20 years of work dissecting the structure and function of the 3'UTR of TCV and other carmoviruses. The astonishing structural complexity of the 3'UTRs of these simple carmoviruses provides lessons that are likely applicable to many other plant and animal RNA viruses.

Picornavirus IRES elements: RNA structure and host protein interactions

Internal ribosome entry site (IRES) elements were discovered in picornaviruses. These elements are cis-acting RNA sequences that adopt diverse three-dimensional structures and recruit the translation machinery using a 5' end-independent mechanism assisted by a subset of translation initiation factors and various RNA binding proteins termed IRES transacting factors (ITAFs). Many of these factors suffer important modifications during infection including cleavage by picornavirus proteases, changes in the phosphorylation level and/or redistribution of the protein from the nuclear to the cytoplasm compartment. Picornavirus IRES are amongst the most potent elements described so far. However, given their large diversity and complexity, the mechanistic basis of its mode of action is not yet fully understood. This review is focused to describe recent advances on the studies of RNA structure and RNA-protein interactions modulating picornavirus IRES activity.

Conserved motifs in a tombusvirus polymerase modulate genome replication, subgenomic transcription, and amplification of defective interfering RNAs

The replication of plus-strand RNA virus genomes is mediated by virally encoded RNA-dependent RNA polymerases (RdRps). We have investigated the role of the C-proximal region in the RdRp of tomato bushy stunt virus (TBSV) in mediating viral RNA synthesis. TBSV is the prototype species in the genus Tombusvirus, family Tombusviridae, and its RdRp is responsible for replicating the viral genome, transcribing two subgenomic mRNAs, and supporting replication of defective interfering RNAs. Comparative sequence analysis of the RdRps of tombusvirids identified three highly conserved motifs in their C-proximal regions, and these sequences were subsequently targeted for mutational analysis in TBSV. The results revealed that these motifs are important for (i) synthesizing viral genomic RNA and subgenomic mRNAs, (ii) facilitating plus- and/or minus-strand synthesis, and (iii) modulating trans-replication of a defective interfering RNA. These motifs were also found to be conserved in other plant viruses as well as in a fungal and insect virus. The collective findings are discussed in relation to viral RNA synthesis and taxonomy.

IMPORTANCE

Little is currently known about the structure and function of the viral polymerases that replicate the genomes of RNA plant viruses. Tombusviruses, the prototype of the tombusvirids, have been used as model plus-strand RNA plant viruses for understanding many of the steps in the infectious process; however, their polymerases remain poorly characterized. To help address this issue, the function of the C-terminal region of the polymerase of a tombusvirus was investigated. Three conserved motifs were identified and targeted for mutational analysis. The results revealed that these polymerase motifs are important for determining what type of viral RNA is produced, facilitating different steps in viral RNA production, and amplifying subgenomic RNA replicons. Accordingly, the C-terminal region of the tombusvirus polymerase is needed for a variety of fundamental activities. Furthermore, as these motifs are also present in distantly related viruses, the significance of these results extends beyond tombusvirids.

The length of an internal poly(A) tract of hibiscus latent Singapore virus is crucial for its replication

Hibiscus latent Singapore virus (HLSV) mutants were constructed to study roles of its internal poly(A) tract (IPAT) in viral replication and coat protein (CP) expression. Shortening of the IPAT resulted in reduced HLSV RNA accumulation and its minimal length required for HLSV CP expression in plants was 24 nt. Disruption of a putative long range RNA-RNA interacting structure between 5' and 3' untranslated regions of HLSV-22A and -24A resulted in reduced viral RNA and undetectable CP accumulation in inoculated leaves. Replacement of the IPAT in HLSV with an upstream pseudoknot domain (UPD) of Tobacco mosaic virus (TMV) or insertion of the UPD to the immediate downstream of a 24 nt IPAT in HLSV resulted in drastically reduced viral RNA replication. Plants infected with a TMV mutant by replacement of the UPD with 43 nt IPAT exhibited milder mosaic symptoms without necrosis. We have proposed a model for HLSV replication.

Crop immunity against viruses: outcomes and future challenges

Viruses cause epidemics on all major cultures of agronomic importance, representing a serious threat to global food security. As strict intracellular pathogens, they cannot be controlled chemically and prophylactic measures consist mainly in the destruction of infected plants and excessive pesticide applications to limit the population of vector organisms. A powerful alternative frequently employed in agriculture relies on the use of crop genetic resistances, approach that depends on mechanisms governing plant-virus interactions. Hence, knowledge related to the molecular bases of viral infections and crop resistances is key to face viral attacks in fields. Over the past 80 years, great advances have been made on our understanding of plant immunity against viruses. Although most of the known natural resistance genes have long been dominant R genes (encoding NBS-LRR proteins), a vast number of crop recessive resistance genes were cloned in the last decade, emphasizing another evolutive strategy to block viruses. In addition, the discovery of RNA interference pathways highlighted a very efficient antiviral system targeting the infectious agent at the nucleic acid level. Insidiously, plant viruses evolve and often acquire the ability to overcome the resistances employed by breeders. The development of efficient and durable resistances able to withstand the extreme genetic plasticity of viruses therefore represents a major challenge for the coming years. This review aims at describing some of the most devastating diseases caused by viruses on crops and summarizes current knowledge about plant-virus interactions, focusing on resistance mechanisms that prevent or limit viral infection in plants. In addition, I will discuss the current outcomes of the actions employed to control viral diseases in fields and the future investigations that need to be undertaken to develop sustainable broad-spectrum crop resistances against viruses.

Mapping overlapping functional elements embedded within the protein-coding regions of RNA viruses

Identification of the full complement of genes and other functional elements in any virus is crucial to fully understand its molecular biology and guide the development of effective control strategies. RNA viruses have compact multifunctional genomes that frequently contain overlapping genes and non-coding functional elements embedded within protein-coding sequences. Overlapping features often escape detection because it can be difficult to disentangle the multiple roles of the constituent nucleotides via mutational analyses, while high-throughput experimental techniques are often unable to distinguish functional elements from incidental features. However, RNA viruses evolve very rapidly so that, even within a single species, substitutions rapidly accumulate at neutral or near-neutral sites providing great potential for comparative genomics to distinguish the signature of purifying selection. Computationally identified features can then be efficiently targeted for experimental analysis. Here we analyze alignments of protein-coding virus sequences to identify regions where there is a statistically significant reduction in the degree of variability at synonymous sites, a characteristic signature of overlapping functional elements. Having previously tested this technique by experimental verification of discoveries in selected viruses, we now analyze sequence alignments for ∼700 RNA virus species to identify hundreds of such regions, many of which have not been previously described.