Host-dependent roles of the viral 5' untranslated region (UTR) in RNA stabilization and cap-independent translational enhancement mediated by the 3' UTR of Red clover necrotic mosaic virus RNA1

Structure-Based Regulatory Role for the 5′UTR of RCNMV RNA2

Red clover necrotic mosaic virus (RCNMV) is a segmented positive-strand RNA virus consisting of RNA1 and RNA2. Previous studies demonstrated that efficient translation of RCNMV RNA2 requires de novo synthesis of RNA2 during infections, suggesting that RNA2 replication is required for its translation. We explored a potential mechanism underlying the regulation of replication-associated translation of RNA2 by examining RNA elements in its 5′ untranslated region (5′UTR). Structural analysis of the 5′UTR suggested that it can form two mutually exclusive configurations: a more thermodynamically stable conformation, termed the 5′-basal stem structure (5′BS), in which 5′-terminal sequences are base paired, and an alternative conformation, where the 5′-end segment is single stranded. Functional mutational analysis of the 5′UTR structure indicated that (i) 43S ribosomal subunits enter at the very 5′-end of RNA2; (ii) the alternative conformation, containing unpaired 5′-terminal nucleotides, mediates efficient translation; (iii) the 5′BS conformation, with a paired 5′-end segment, supresses translation; and (iv) the 5′BS conformation confers stability to RNA2 from 5′-to-3′ exoribonuclease Xrn1. Based on our results, we suggest that during infections, newly synthesized RNA2s transiently adopt the alternative conformation to allow for efficient translation, then refold into the 5′BS conformation, which supresses translation and promotes efficient RNA2 replication. The potential advantages of this proposed 5′UTR-based regulatory mechanism for coordinating RNA2 translation and replication are discussed.

Non-Canonical Translation Initiation Mechanisms Employed by Eukaryotic Viral mRNAs

Viruses exploit the translation machinery of an infected cell to synthesize their proteins. Therefore, viral mRNAs have to compete for ribosomes and translation factors with cellular mRNAs. To succeed, eukaryotic viruses adopt multiple strategies. One is to circumvent the need for m7G-cap through alternative instruments for ribosome recruitment. These include internal ribosome entry sites (IRESs), which make translation independent of the free 5' end, or cap-independent translational enhancers (CITEs), which promote initiation at the uncapped 5' end, even if located in 3' untranslated regions (3' UTRs). Even if a virus uses the canonical cap-dependent ribosome recruitment, it can still perturb conventional ribosomal scanning and start codon selection. The pressure for genome compression often gives rise to internal and overlapping open reading frames. Their translation is initiated through specific mechanisms, such as leaky scanning, 43S sliding, shunting, or coupled termination-reinitiation. Deviations from the canonical initiation reduce the dependence of viral mRNAs on translation initiation factors, thereby providing resistance to antiviral mechanisms and cellular stress responses. Moreover, viruses can gain advantage in a competition for the translational machinery by inactivating individual translational factors and/or replacing them with viral counterparts. Certain viruses even create specialized intracellular "translation factories", which spatially isolate the sites of their protein synthesis from cellular antiviral systems, and increase availability of translational components. However, these virus-specific mechanisms may become the Achilles' heel of a viral life cycle. Thus, better understanding of the unconventional mechanisms of viral mRNA translation initiation provides valuable insight for developing new approaches to antiviral therapy.

High-throughput evolutionary optimization of the induction medium towards recombinant protein production in BY-2 tobacco

Bright yellow (BY-2) tobacco cells combined with the XVE chemically inducible system are one of the most promising plant-based platforms for recombinant protein production. This offers a range of benefits, including the separation of the cell growth and heterologous gene expression, lack of risk of infecting the end product with prions and human viruses or appropriate protein glycosylation and folding. However, low protein productivity remains a major obstacle that limits the extensive commercialization of bioproduction in plants. A number of molecular, cell culture and down processing approaches have been made to overcome this problem. Media development for the specific nutritional and hormonal requirements of transgenic plant cells is one of the most efficient cell-culture approaches. We optimized the induction medium towards recombinant protein production in BY-2 and demonstrated the usefulness of evolutionary medium optimization for high-yield protein production in liquid plant cultures. A reliable XVE/GFP model, parallel conducting experiments in a microscale on 96-well plates, and dedicated Gene Game evolutionary optimization software allowed for an effective search of 7611 possible solutions of 11-component media. Within the 4608 formulations tested, the Induct X medium was found with a significant 107.14% increase in protein expression in relation to the standard BY-2 medium.

5'-Terminal stem-loop of carnation ringspot virus RNA1 is required for the efficient amplification of viral RNAs

Carnation ringspot virus (CRSV) is the prototype virus of the genus Dianthovirus. Full-length cDNAs of CRSV strainsPV-0097 and PV-21 were constructed and the infectivity of in vitro transcripts was analyzed. Infectivity of PV-0097 and PV-21 to several plants was markedly higher than that of 1.30, a previously reported infectious CRSV clone. Overall RNA sequences of these viruses were similar, but PV-0097 and PV-21 contained additional nucleotides at the 5' end of RNA1. Stem-loop structures were predicted in the 5'-terminal region of PV-0097 and PV-21 RNA1 but not in 1.30 RNA1. Mutant CRSV 1.30 RNA1 that contains the terminal 4 nucleotides of PV-0097, predicted to fold a 5'-terminal stem-loop structure, recovered higher level accumulation of viral RNAs in the inoculated protoplasts and leaves of Nicotiana benthamiana. These results suggest that the 5'-terminal stem-loop structure of CRSV RNA1 plays an important role in efficient amplification of the virus.

Inter- and Intra-Host Nucleotide Variations in Hepatitis A Virus in Culture and Clinical Samples Detected by Next-Generation Sequencing

The accurate virus detection, strain discrimination, and source attribution of contaminated food items remains a persistent challenge because of the high mutation rates anticipated to occur in foodborne RNA viruses, such as hepatitis A virus (HAV). This has led to predictions of the existence of more than one sequence variant between the hosts (inter-host) or within an individual host (intra-host). However, there have been no reports of intra-host variants from an infected single individual, and little is known about the accuracy of the single nucleotide variations (SNVs) calling with various methods. In this study, the presence and identity of viral SNVs, either between HAV clinical specimens or among a series of samples derived from HAV clone1-infected FRhK4 cells, were determined following analyses of nucleotide sequences generated using next-generation sequencing (NGS) and pyrosequencing methods. The results demonstrate the co-existence of inter- and intra-host variants both in the clinical specimens and the cultured samples. The discovery and confirmation of multi-viral RNAs in an infected individual is dependent on the strain discrimination at the SNV level, and critical for successful outbreak traceback and source attribution investigations. The detection of SNVs in a time series of HAV infected FRhK4 cells improved our understanding on the mutation dynamics determined probably by different selective pressures. Additionally, it demonstrated that NGS could potentially provide a valuable investigative approach toward SNV detection and identification for other RNA viruses.

A Dual Interaction Between the 5'- and 3'-Ends of the Melon Necrotic Spot Virus (MNSV) RNA Genome Is Required for Efficient Cap-Independent Translation

In eukaryotes, the formation of a 5'-cap and 3'-poly(A) dependent protein-protein bridge is required for translation of its mRNAs. In contrast, several plant virus RNA genomes lack both of these mRNA features, but instead have a 3'-CITE (for cap-independent translation enhancer), a RNA element present in their 3'-untranslated region that recruits translation initiation factors and is able to control its cap-independent translation. For several 3'-CITEs, direct RNA-RNA long-distance interactions based on sequence complementarity between the 5'- and 3'-ends are required for efficient translation, as they bring the translation initiation factors bound to the 3'-CITE to the 5'-end. For the carmovirus melon necrotic spot virus (MNSV), a 3'-CITE has been identified, and the presence of its 5'-end in cis has been shown to be required for its activity. Here, we analyze the secondary structure of the 5'-end of the MNSV RNA genome and identify two highly conserved nucleotide sequence stretches that are complementary to the apical loop of its 3'-CITE. In in vivo cap-independent translation assays with mutant constructs, by disrupting and restoring sequence complementarity, we show that the interaction between the 3'-CITE and at least one complementary sequence in the 5'-end is essential for virus RNA translation, although efficient virus translation and multiplication requires both connections. The complementary sequence stretches are invariant in all MNSV isolates, suggesting that the dual 5'-3' RNA:RNA interactions are required for optimal MNSV cap-independent translation and multiplication.

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.

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.

Differential use of 3'CITEs by the subgenomic RNA of Pea enation mosaic virus 2

The genomic RNA (gRNA) of Pea enation mosaic virus 2 (PEMV2) is the template for p33 and -1 frameshift product p94. The PEMV2 subgenomic RNA (sgRNA) encodes two overlapping ORFs, p26 and p27, which are required for movement and stability of the gRNA. Efficient translation of p33 requires two of three 3' proximal cap-independent translation enhancers (3'CITEs): the kl-TSS, which binds ribosomes and engages in a long-distance interaction with the 5'end; and the adjacent eIF4E-binding PTE. Unlike the gRNA, all three 3'CITEs were required for efficient translation of the sgRNA, which included the ribosome-binding 3'TSS. A hairpin in the 5' proximal coding region of p26/p27 supported translation by the 3'CITEs by engaging in a long-distance RNA:RNA interaction with the kl-TSS. These results strongly suggest that the 5' ends of PEMV2 gRNA and sgRNA connect with the 3'UTR through similar long-distance interactions while having different requirements for 3'CITEs.

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.

Structural and Functional Diversity of Plant Virus 3'-Cap-Independent Translation Enhancers (3'-CITEs)

Most of the positive-strand RNA plant viruses lack the 5'-cap and/or the poly(A)-tail that act synergistically to stimulate canonical translation of cellular mRNAs. However, they have RNA elements in the 5'- or 3'-untranslated regions of their RNAs that are required for their cap-independent translation. Cap-independent translation enhancers (CITEs) have been identified in the genomic 3'-end of viruses belonging to the family Tombusviridae and the genus Luteovirus. Seven classes of 3'-CITEs have been described to date based on their different RNA structures. They generally control the efficient formation of the translation initiation complex by varying mechanisms. Some 3'-CITEs bind eukaryotic translation initiation factors, others ribosomal subunits, bridging these to the 5'-end by different mechanisms, often long-distance RNA-RNA interactions. As previously proposed and recently found in one case in nature, 3'-CITEs are functionally independent elements that are transferable through recombination between viral genomes, leading to potential advantages for virus multiplication. In this review, the knowledge on 3'-CITEs and their functioning is updated. We also suggest that there is local structural conservation in the regions interacting with eIF4E of 3'-CITEs belonging to different classes.

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.

Base-paired structure in the 5' untranslated region is required for the efficient amplification of negative-strand RNA3 in the bromovirus melandrium yellow fleck virus

Melandrium yellow fleck virus belongs to the genus Bromovirus, which is a group of tripartite plant RNA viruses. This virus has an approximately 200-nucleotide direct repeat sequence in the 5' untranslated region (UTR) of RNA3 that encodes the 3a movement protein. In the present study, protoplast assays suggested that the duplicated region contains amplification-enhancing elements. Deletion analyses of the 5' UTR of RNA3 showed that mutations in the short base-paired region, which is located dozens of bases upstream of the initiation codon of the 3a gene, greatly reduced the accumulation of RNA3. Disruption and restoration of the base-paired structure caused the accumulation of RNA3 to be decreased and restored, respectively. In vitro translation/replication assays demonstrated that the base-paired structure is important for the efficient amplification of negative-stand RNA3. A similar base-paired structure in RNA3 of another bromovirus, brome mosaic virus (BMV), also facilitated the efficient amplification of BMV RNA3, but only in combination with melandrium yellow fleck virus (MYFV) replicase and not with BMV replicase, thereby suggesting specific interactions between base-paired structures and MYFV replicase.

3' cap-independent translation enhancers of plant viruses

In the absence of a 5' cap, plant positive-strand RNA viruses have evolved a number of different elements in their 3' untranslated region (UTR) to attract initiation factors and/or ribosomes to their templates. These 3' cap-independent translational enhancers (3' CITEs) take different forms, such as I-shaped, Y-shaped, T-shaped, or pseudoknotted structures, or radiate multiple helices from a central hub. Common features of most 3' CITEs include the ability to bind a component of the translation initiation factor eIF4F complex and to engage in an RNA-RNA kissing-loop interaction with a hairpin loop located at the 5' end of the RNA. The two T-shaped structures can bind to ribosomes and ribosomal subunits, with one structure also able to engage in a simultaneous long-distance RNA-RNA interaction. Several of these 3' CITEs are interchangeable and there is evidence that natural recombination allows exchange of modular CITE units, which may overcome genetic resistance or extend the virus's host range.

ADP ribosylation factor 1 plays an essential role in the replication of a plant RNA virus

Eukaryotic positive-strand RNA viruses replicate using the membrane-bound replicase complexes, which contain multiple viral and host components. Virus infection induces the remodeling of intracellular membranes. Virus-induced membrane structures are thought to increase the local concentration of the components that are required for replication and provide a scaffold for tethering the replicase complexes. However, the mechanisms underlying virus-induced membrane remodeling are poorly understood. RNA replication of red clover necrotic mosaic virus (RCNMV), a positive-strand RNA plant virus, is associated with the endoplasmic reticulum (ER) membranes, and ER morphology is perturbed in RCNMV-infected cells. Here, we identified ADP ribosylation factor 1 (Arf1) in the affinity-purified RCNMV RNA-dependent RNA polymerase fraction. Arf1 is a highly conserved, ubiquitous, small GTPase that is implicated in the formation of the coat protein complex I (COPI) vesicles on Golgi membranes. Using in vitro pulldown and bimolecular fluorescence complementation analyses, we showed that Arf1 interacted with the viral p27 replication protein within the virus-induced large punctate structures of the ER membrane. We found that inhibition of the nucleotide exchange activity of Arf1 using the inhibitor brefeldin A (BFA) disrupted the assembly of the viral replicase complex and p27-mediated ER remodeling. We also showed that BFA treatment and the expression of dominant negative Arf1 mutants compromised RCNMV RNA replication in protoplasts. Interestingly, the expression of a dominant negative mutant of Sar1, a key regulator of the biogenesis of COPII vesicles at ER exit sites, also compromised RCNMV RNA replication. These results suggest that the replication of RCNMV depends on the host membrane traffic machinery.

Molecular biology and epidemiology of dianthoviruses

The genus Dianthovirus is one of eight genera in the family Tombusviridae. All the genera have monopartite positive-stranded RNA genomes, except the dianthoviruses which have bipartite genomes. The dianthoviruses are distributed worldwide. Although they share common structural features with the other Tombusviridae viruses in their virions and the terminal structure of the genomic RNAs, the bipartite nature of the dianthovirus genome offers an ideal experimental system with which to study basic issues of virology. The two genomic RNAs seem to use distinct strategies to regulate their translation, transcription, genome replication, genome packaging, and cell-to-cell movement during infection. This review summarizes the current state of our knowledge of the dianthoviruses, with its main emphasis on the molecular biology of the virus, including the viral and host factors required for its infection of host plants. The epidemiology of the virus and the possible viral impacts on agriculture and the environment are also discussed.

Differential roles of Hsp70 and Hsp90 in the assembly of the replicase complex of a positive-strand RNA plant virus

Assembly of viral replicase complexes of eukaryotic positive-strand RNA viruses is a regulated process: multiple viral and host components must be assembled on intracellular membranes and ordered into quaternary complexes capable of synthesizing viral RNAs. However, the molecular mechanisms underlying this process are poorly understood. In this study, we used a model virus, Red clover necrotic mosaic virus (RCNMV), whose replicase complex can be detected readily as the 480-kDa functional protein complex. We found that host heat shock proteins Hsp70 and Hsp90 are required for RCNMV RNA replication and that they interact with p27, a virus-encoded component of the 480-kDa replicase complex, on the endoplasmic reticulum membrane. Using a cell-free viral translation/replication system in combination with specific inhibitors of Hsp70 and Hsp90, we found that inhibition of p27-Hsp70 interaction inhibits the formation of the 480-kDa complex but instead induces the accumulation of large complexes that are nonfunctional in viral RNA synthesis. In contrast, inhibition of p27-Hsp90 interaction did not induce such large complexes but rendered p27 incapable of binding to a specific viral RNA element, which is a critical step for the assembly of the 480-kDa replicase complex and viral RNA replication. Together, our results suggest that Hsp70 and Hsp90 regulate different steps in the assembly of the RCNMV replicase complex.

Poly(A)-binding protein facilitates translation of an uncapped/nonpolyadenylated viral RNA by binding to the 3' untranslated region

Viruses employ an alternative translation mechanism to exploit cellular resources at the expense of host mRNAs and to allow preferential translation. Plant RNA viruses often lack both a 5' cap and a 3' poly(A) tail in their genomic RNAs. Instead, cap-independent translation enhancer elements (CITEs) located in the 3' untranslated region (UTR) mediate their translation. Although eukaryotic translation initiation factors (eIFs) or ribosomes have been shown to bind to the 3'CITEs, our knowledge is still limited for the mechanism, especially for cellular factors. Here, we searched for cellular factors that stimulate the 3'CITE-mediated translation of Red clover necrotic mosaic virus (RCNMV) RNA1 using RNA aptamer-based one-step affinity chromatography, followed by mass spectrometry analysis. We identified the poly(A)-binding protein (PABP) as one of the key players in the 3'CITE-mediated translation of RCNMV RNA1. We found that PABP binds to an A-rich sequence (ARS) in the viral 3' UTR. The ARS is conserved among dianthoviruses. Mutagenesis and a tethering assay revealed that the PABP-ARS interaction stimulates 3'CITE-mediated translation of RCNMV RNA1. We also found that both the ARS and 3'CITE are important for the recruitment of the plant eIF4F and eIFiso4F factors to the 3' UTR and of the 40S ribosomal subunit to the viral mRNA. Our results suggest that dianthoviruses have evolved the ARS and 3'CITE as substitutes for the 3' poly(A) tail and the 5' cap of eukaryotic mRNAs for the efficient recruitment of eIFs, PABP, and ribosomes to the uncapped/nonpolyadenylated viral mRNA.

Untranslated regions of diverse plant viral RNAs vary greatly in translation enhancement efficiency

BACKGROUND

Whole plants or plant cell cultures can serve as low cost bioreactors to produce massive amounts of a specific protein for pharmacological or industrial use. To maximize protein expression, translation of mRNA must be optimized. Many plant viral RNAs harbor extremely efficient translation enhancers. However, few of these different translation elements have been compared side-by-side. Thus, it is unclear which are the most efficient translation enhancers. Here, we compare the effects of untranslated regions (UTRs) containing translation elements from six plant viruses on translation in wheat germ extract and in monocotyledenous and dicotyledenous plant cells.

RESULTS

The highest expressing uncapped mRNAs contained viral UTRs harboring Barley yellow dwarf virus (BYDV)-like cap-independent translation elements (BTEs). The BYDV BTE conferred the most efficient translation of a luciferase reporter in wheat germ extract and oat protoplasts, while uncapped mRNA containing the BTE from Tobacco necrosis virus-D translated most efficiently in tobacco cells. Capped mRNA containing the Tobacco mosaic virus omega sequence was the most efficient mRNA in tobacco cells. UTRs from Satellite tobacco necrosis virus, Tomato bushy stunt virus, and Crucifer-infecting tobamovirus (crTMV) did not stimulate translation efficiently. mRNA with the crTMV 5' UTR was unstable in tobacco protoplasts.

CONCLUSIONS

BTEs confer the highest levels of translation of uncapped mRNAs in vitro and in vivo, while the capped omega sequence is most efficient in tobacco cells. These results provide a basis for understanding mechanisms of translation enhancement, and for maximizing protein synthesis in cell-free systems, transgenic plants, or in viral expression vectors.

3' Cap-independent translation enhancers of positive-strand RNA plant viruses

Positive-strand RNA plant viruses that are neither 5'-capped nor 3'-polyadenylated use nontraditional mechanisms to recruit ribosomes to the 5'-end of their viral genomes. One strategy employed by some of these viruses involves a type of RNA element, termed the 3' cap-independent translation enhancer (3'CITE), located in or near the 3'-untranslated region of viral RNA genomes. 3'CITEs function to mediate efficient translation of 5'-proximally encoded viral proteins and function by recruiting either translation initiation factors or the 60S ribosomal subunit to the viral RNA. Recent mechanistic and structural studies have revealed important new insights and details of how 3'CITEs are able to facilitate viral translation and allow these viruses to compete efficiently against cellular mRNAs for the host translational machinery.

A long-distance RNA-RNA interaction plays an important role in programmed -1 ribosomal frameshifting in the translation of p88 replicase protein of Red clover necrotic mosaic virus

Programmed -1 ribosomal frameshifting (-1 PRF) is one viral translation strategy to express overlapping genes in positive-strand RNA viruses. Red clover necrotic mosaic virus (RCNMV) uses this strategy to express its replicase component protein p88. In this study, we used a cell-free translation system to map cis-acting RNA elements required for -1 PRF. Our results show that a small stem-loop structure adjacent to the cap-independent translation element in the 3' untranslated region (UTR) of RCNMV RNA1 is required for -1 PRF. Site-directed mutagenesis experiments suggested that this stem-loop regulates -1 PRF via base-pairing with complementary sequences in a bulged stem-loop adjacent to the shifty site. The existence of RNA elements responsible for -1 PRF and the cap-independent translation of replicase proteins in the 3' UTR of RNA1 might be important for switching translation to replication and for regulating the ratio of p88 to p27.

Arabidopsis cell-free extract, ACE, a new in vitro translation system derived from Arabidopsis callus cultures

The analysis of post-transcriptional regulatory mechanisms in plants has benefited greatly from the use of cell-free extract systems. Arabidopsis as a model system provides extensive genetic resources; however, to date a suitable cell-free translation system from Arabidopsis has not been available. In this study, we devised an Arabidopsis cell-free extract (ACE) to be used for in vitro translation studies. Protoplasts were prepared from callus cultures derived from Arabidopsis seedlings, and cell-free extracts were prepared after evacuolation of the protoplasts by Percoll gradient centrifugation. The new ACE system exhibits translation activity comparable with that of the wheat germ extract system. We demonstrated that ACE prepared from the 5'-3' exoribonuclease-deficient mutant of Arabidopsis, xrn4-5, exhibited increased stability of an uncapped mRNA as compared with that from wild-type Arabidopsis. We applied the ACE system to study post-transcriptional regulation of AtCGS1. AtCGS1 codes for cystathionine γ-synthase (CGS) that catalyzes the first committed step of methionine and S-adenosyl-l-methionine (AdoMet) biosynthesis in plants, and is feedback regulated by mRNA degradation coupled with translation elongation arrest. The ACE system was capable of reproducing translation elongation arrest and subsequent AtCGS1 mRNA degradation that are induced by AdoMet. The ACE system described here can be prepared in a month after seed sowing and will make it possible to study post-transcriptional regulation of plant genes while taking advantage of the genetics of Arabidopsis.

Identification of amino acids in auxiliary replicase protein p27 critical for its RNA-binding activity and the assembly of the replicase complex in Red clover necrotic mosaic virus

The specific recognition of genomic RNAs by viral replicase proteins is a key regulatory step during the early replication process in positive-strand RNA viruses. In this study, we characterized the RNA-binding activity of the auxiliary replicase protein p27 of Red clover necrotic mosaic virus (RCNMV), which has a bipartite genome consisting of RNA1 and RNA2. Aptamer pull-down assays identified the amino acid residues of p27 involved in its specific interaction with RNA2. The RNA-binding activity of p27 correlated with its activity in recruiting RNA2 to membranes. We also identified the amino acids required for the formation of the 480-kDa replicase complex, a key player of RCNMV RNA replication. These amino acids are not involved in the functions of p27 that bind viral RNA or replicase proteins, suggesting an additional role for p27 in the assembly of the replicase complex. Our results demonstrate that p27 has multiple functions in RCNMV replication.

Template recognition mechanisms by replicase proteins differ between bipartite positive-strand genomic RNAs of a plant virus

Recognition of RNA templates by viral replicase proteins is one of the key steps in the replication process of all RNA viruses. However, the mechanisms underlying this phenomenon, including primary RNA elements that are recognized by the viral replicase proteins, are not well understood. Here, we used aptamer pulldown assays with membrane fractionation and protein-RNA coimmunoprecipitation in a cell-free viral translation/replication system to investigate how viral replicase proteins recognize the bipartite genomic RNAs of the Red clover necrotic mosaic virus (RCNMV). RCNMV replicase proteins bound specifically to a Y-shaped RNA element (YRE) located in the 3' untranslated region (UTR) of RNA2, which also interacted with the 480-kDa replicase complexes that contain viral and host proteins. The replicase-YRE interaction recruited RNA2 to the membrane fraction. Conversely, RNA1 fragments failed to interact with the replicase proteins supplied in trans. The results of protein-RNA coimmunoprecipitation assays suggest that RNA1 interacts with the replicase proteins coupled with their translation. Thus, the initial template recognition mechanisms employed by the replicase differ between RCNMV bipartite genomic RNAs and RNA elements are primary determinants of the differential replication mechanism.

Structural plasticity of Barley yellow dwarf virus-like cap-independent translation elements in four genera of plant viral RNAs

The 3' untranslated regions (UTRs) of many plant viral RNAs contain cap-independent translation elements (3' CITEs). Among the 3' CITEs, the Barley yellow dwarf virus (BYDV)-like translation elements (BTEs) form a structurally variable and widely distributed group. Viruses in three genera were known to harbor 3' BTEs, defined by the presence of a 17-nt consensus sequence. To understand BTE function, knowledge of phylogenetically conserved structure is essential, yet the secondary structure has been determined only for the BYDV BTE. Here we show that Rose spring dwarf-associated luteovirus, and two viruses in a fourth genus, Umbravirus, contain functional BTEs, despite deviating in the 17nt consensus sequence. Structure probing by selective 2'-hydroxyl acylation and primer extension (SHAPE) revealed conserved and highly variable structures in BTEs in all four genera. We conclude that BTEs tolerate striking evolutionary plasticity in structure, while retaining the ability to stimulate cap-independent translation.