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Structures required for poly(A) tail-independent translation overlap with, but are distinct from, cap-independent translation and RNA replication signals at the 3' end of Tobacco necrosis virus RNA.

Tobacco necrosis necrovirus (TNV) RNA lacks both a 5' cap and a poly(A) tail but is translated efficiently, owing in part to a Barley yellow dwarf virus (BYDV)-like cap-independent translation element (BTE) in its 3' untranslated region (UTR). Here, we identify sequence downstream of the BTE that is necessary for poly(A) tail-independent translation in vivo by using RNA encoding a luciferase reporter gene flanked by viral UTRs. Deletions and point mutations caused loss of translation that was restored by adding a poly(A) tail, and not by adding a 5' cap. The two 3'-proximal stem-loops in the viral genome contribute to poly(A) tail-independent translation, as well as RNA replication. For all necroviruses, we predict a conserved 3' UTR secondary structure that includes the BTE at one end of a long helical axis and the stem-loops required for poly(A) tail-independent translation and RNA replication at the other end. This work shows that a viral genome can harbor distinct cap- and poly(A) tail-mimic sequences in the 3' UTR.

3' Untranslated Regions↗

Positive- and negative-acting signals combine to determine differential RNA replication from the paramyxovirus simian virus 5 genomic and antigenomic promoters.

The cis-acting signals found at the 3' ends of the genomic and antigenomic RNAs are a major factor determining the level of paramyxovirus RNA replication from each promoter. Using a minigenome system that reconstitutes SV5 RNA synthesis from cDNA-derived components, we show here that the genomic promoter (GP) for the paramyxovirus SV5 directs RNA replication approximately 14-fold lower than that seen from the antigenomic promoter (AGP). The goal of this study was to identify cis-acting signals responsible for differential levels of RNA replication from the SV5 GP and AGP. We have previously shown that the SV5 AGP contains three sequence-dependent elements (CRI, CRII, and Region III) that are separated by sequence-independent spacer regions. Minigenomes containing chimeric promoters were constructed to test the hypothesis that transfer of discrete cis-acting AGP elements to the GP could confer higher replication properties to the GP. Minigenomes containing a substitution of the AGP CRI, CRII, or Region III elements alone in place of the corresponding GP sequences did not show enhanced levels of RNA replication. However, transfer of both the AGP 3' terminal CRI and Region III elements into the corresponding sites of the GP led to a minigenome which replicated to approximately 40% of the levels seen with the AGP. This enhanced RNA replication from the GP was further increased up to AGP levels by also including the intervening AGP segment (bases 20-50) located between CRI and Region III. Importantly, transfer of nonviral sequences in place of GP bases 20-50 also increased RNA replication to levels approaching that of the AGP, but only in the context of the AGP CRI and Region III substitutions. These data indicate that differential levels of RNA replication from the SV5 GP and AGP are due to a combination of positive-acting signals in the AGP (CRI and Region III) and a negative-acting signal in the GP (bases 20-50). Possible functions for the SV5 promoter elements in determining RNA replication levels are proposed.

Amino Acid Sequence↗

Functional analysis of mosquito-borne flavivirus conserved sequence elements within 3' untranslated region of West Nile virus by use of a reporting replicon that differentiates between viral translation and RNA replication.

We have developed a reporting replicon of West Nile virus (WNV) that could be used to quantitatively distinguish viral translation and RNA replication. A Renilla luciferase (Rluc) gene was fused in-frame with the open reading frame of a subgenomic replicon in the position where the viral structural region was deleted, resulting in RlucRep. Transfection of BHK cells with RlucRep RNA yielded two distinctive Rluc signal peaks, one between 2 and 10 h and the other after 26 h posttransfection. By contrast, only the 2- to 10-h Rluc signal peak was observed in cells transfected with a mutant replicon containing an inactivated viral polymerase NS5 (RlucRep-NS5mt). Immunofluorescence and real-time reverse transcriptase PCR assays showed that the levels of viral protein expression and RNA replication increased in cells transfected with the RlucRep but not in those transfected with the RlucRep-NS5mt. These results suggest that the Rluc signal that occurred at 2 to 10 h posttransfection reflects viral translation of the input replicon, while the Rluc activity after 26 h posttransfection represents RNA replication. Using this system, we showed that mutations of conserved sequence (CS) elements within the 3' untranslated region of the mosquito-borne flaviviruses did not significantly affect WNV translation but severely diminished or completely abolished RNA replication. Mutations of CS1 that blocked the potential base pairing with a conserved sequence in the 5' region of the capsid gene (5'CS) abolished RNA replication. Restoration of the 5'CS-CS1 interaction rescued viral replication. Replicons containing individual deletions of CS2, repeated CS2 (RCS2), CS3, or RCS3 were viable, but their RNA replication was dramatically compromised. These results demonstrate that genome cyclization through the 5'CS-CS1 interaction is essential for WNV RNA replication, whereas CS2, RCS2, CS3, and RCS3 facilitate, but are dispensable for, WNV replication.

3' Untranslated Regions↗

Identification and characterization of a host protein required for efficient template selection in viral RNA replication.

Biochemical studies suggest that positive-strand RNA virus replication involves host as well as viral functions. Brome mosaic virus (BMV) is a member of the alphavirus-like superfamily of animal and plant positive-strand RNA viruses. Yeast expressing the BMV RNA replication proteins 1a and 2a supports BMV RNA replication and mRNA synthesis. Using the ability of BMV to replicate in yeast, we show that efficient BMV RNA replication requires Lsm1p, a yeast protein related to core RNA splicing factors but shown herein to be cytoplasmic. Haploid yeast with an Lsm1p mutation was defective in an early template selection step in BMV RNA replication, involving the helicase-like replication protein 1a and an internal viral RNA element conserved with tRNAs. Lsm1p dependence of this interaction was suppressed by adding 3' poly(A) to the normally unpolyadenylated BMV RNA. Our results show Lsm1p involvement in a specific step of BMV RNA replication and connections between Lsm1p and poly(A) function, possibly through interaction with factors binding mRNA 5' ends.

Amino Acid Sequence↗

Restriction of poliovirus RNA replication in persistently infected nerve cells.

The aetiology of post-polio syndrome may involve persistence of poliovirus (PV) in the CNS. PV persists in the CNS of infected paralysed mice for over a year after the acute phase of paralytic poliomyelitis. However, infectious PV particles cannot be recovered from homogenates of CNS from paralysed mice after the acute phase of disease, indicating that PV replication is restricted. To identify the molecular mechanism by which PV replication is limited, PV RNA synthesis was analysed by estimating the relative level of genomic (plus-strand) and complementary (minus-strand) PV RNA in the CNS of persistently infected mice. PV RNA replication decreased during the 6 months following onset of paralysis, due mainly to inhibition of plus-strand RNA synthesis. Thus, restriction of PV RNA synthesis may contribute to persistence by limiting virus replication in the mouse CNS. Interestingly, viral RNA replication was similarly inhibited in neuroblastoma IMR-32 cell cultures persistently infected with PV. This in vitro model thus shows that cellular factors play a role in the inhibition of viral RNA synthesis.

Animals↗

Minimum protein requirements for transcription and RNA replication of a minigenome of human parainfluenza virus type 3 and evaluation of the rule of six.

A reconstituted transcription and RNA replication system for human parainfluenza virus type 3 (HPIV3) was developed using components expressed intracellularly from transfected plasmids driven by T7 RNA polymerase supplied by a vaccinia virus recombinant. The system is based on a negative-sense analog of HPIV3 genomic RNA in which the viral genes were deleted and replaced with that encoding bacterial chloramphenicol acetyl transferase (CAT). The N, P, and L proteins expressed from cotransfected plasmids were necessary and sufficient to direct efficient transcription and RNA replication. Transcription yielded subgenomic polyadenylated mRNA, which was isolated by oligo(dT) chromatography. RNA replication yielded a mini-antigenome and progeny minigenome, which were shown to be encapsidated based on resistance to digestion with micrococcal nuclease. A panel of cDNAs was constructed to encode minigenomes which differed in length by single-nucleotide increments. Transcription and RNA replication in the reconstituted system were most efficient for the minigenome whose length was an even multiple of six. Both RNA replication and transcription appeared to be governed by the rule. However, minigenomes whose lengths were one nucleotide greater than or less than an even multiple of six also were very active, especially in RNA replication, indicating that the rule was not absolute.

Chloramphenicol O-Acetyltransferase↗

Disruption of hepatitis C virus RNA replication through inhibition of host protein geranylgeranylation.

Hepatitis C virus (HCV) RNA replication depends on viral protein association with intracellular membranes, but the influence of membrane composition on viral replication is unclear. We report that HCV RNA replication and assembly of the viral replication complex require geranylgeranylation of one or more host proteins. In cultured hepatoma cells, HCV RNA replication was disrupted by treatment with lovastatin, an inhibitor of 3-hydroxy-3-methyglutaryl CoA reductase, or with an inhibitor of protein geranylgeranyl transferase I, each of which induced the dissolution of the HCV replication complex. Viral replication was not affected by treatment of cells with an inhibitor of farnesyl transferase. When added to lovastatin-treated cells, geranylgeraniol, but not farnesol, restored replication complex assembly and viral replication. Inasmuch as the HCV genome does not encode a canonical geranylgeranylated protein, the data suggest the involvement of a geranylgeranylated host protein in HCV replication. Inhibition of its geranylgeranylation affords a therapeutic strategy for treatment of HCV infection.

Antiviral Agents↗

Proteomics analysis of the tombusvirus replicase: Hsp70 molecular chaperone is associated with the replicase and enhances viral RNA replication.

Plus-strand RNA virus replication occurs via the assembly of viral replicase complexes involving multiple viral and host proteins. To identify host proteins present in the cucumber necrosis tombusvirus (CNV) replicase, we affinity purified functional viral replicase complexes from yeast. Mass spectrometry analysis of proteins resolved by two-dimensional gel electrophoresis revealed the presence of CNV p33 and p92 replicase proteins as well as four major host proteins in the CNV replicase. The host proteins included the Ssa1/2p molecular chaperones (yeast homologues of Hsp70 proteins), Tdh2/3p (glyceraldehyde-3-phosphate dehydrogenase, an RNA-binding protein), Pdc1p (pyruvate decarboxylase), and an unknown approximately 35-kDa acidic protein. Copurification experiments demonstrated that Ssa1p bound to p33 replication protein in vivo, and surface plasmon resonance measurements with purified recombinant proteins confirmed this interaction in vitro. The double mutant strain (ssa1 ssa2) showed 75% reduction in viral RNA accumulation, whereas overexpression of either Ssa1p or Ssa2p stimulated viral RNA replication by approximately threefold. The activity of the purified CNV replicase correlated with viral RNA replication in the above-mentioned ssa1 ssa2 mutant and in the Ssa overexpression strains, suggesting that Ssa1/2p likely plays an important role in the assembly of the CNV replicase.

Adenosine Triphosphatases↗

Poliovirus RNA replication requires genome circularization through a protein-protein bridge.

The mechanisms and factors involved in the replication of positive stranded RNA viruses are still unclear. Using poliovirus as a model, we show that a long-range interaction between ribonucleoprotein (RNP) complexes formed at the ends of the viral genome is necessary for RNA replication. Initiation of negative strand RNA synthesis requires a 3' poly(A) tail. Strikingly, it also requires a cloverleaf-like RNA structure located at the other end of the genome. An RNP complex formed around the 5' cloverleaf RNA structure interacts with the poly(A) binding protein bound to the 3' poly(A) tail, thus linking the ends of the viral RNA and effectively circularizing it. Formation of this circular RNP complex is required for initiation of negative strand RNA synthesis. RNA circularization may be a general replication mechanism for positive stranded RNA viruses.

Base Sequence↗

Identification of the cis-acting signal for minus-strand RNA synthesis of a murine coronavirus: implications for the role of minus-strand RNA in RNA replication and transcription.

Minus-strand RNA is the first RNA species made by plus-strand RNA viruses, such as mouse hepatitis virus (MHV), and serves as a template for subsequent RNA replication and transcription. The regulation of minus-strand RNA synthesis has been difficult to study because of the paucity of minus-strand RNA. We have optimized a ribonuclease (RNase) protection assay which enabled the detection of minus-strand RNA synthesis from nonreplicating RNAs, thus clearly separating minus-strand from plus-strand RNA synthesis. We used an MHV defective interfering (DI) RNA containing a chloramphenicol acetyltransferase gene as a reporter to determine the cis-acting signal for MHV minus-strand RNA synthesis. It was found that minus-strand RNAs existed in double-stranded RNA form in the cell. By using various deletion clones, we demonstrated that the cis-acting signal for minus-strand RNA synthesis resides in the 55 nucleotides from the 3' end plus poly(A) tail of the MHV genome. This is much shorter than the 436 nucleotides previously reported for the 3'-end replication signal. No specific upstream MHV sequence was required for the initiation of minus-strand RNA synthesis. This finding suggests that the requirement for minus-strand RNA synthesis is much less stringent than that for genomic and subgenomic plus-strand RNA synthesis and that some of the minus-strand RNAs made may not be functional since they may lack the recognition signals for RNA replication or transcription. We further showed that the DI clones which actively transcribed a subgenomic mRNA from an internal intergenic sequence synthesized much less minus-strand RNA than those clones which did not transcribe subgenomic mRNAs, indicating that minus-strand RNA synthesis was inhibited by transcription from an internal promoter of the same DI RNA. This result also suggests that the regulation of the quantities of subgenomic mRNAs is not at the point of minus-strand RNA synthesis but rather at plus-strand RNA synthesis. Furthermore, the finding that the leader sequence was not required for minus-strand RNA synthesis suggests that the leader RNA regulates mRNA transcription during plus-strand RNA synthesis.

Animals↗

Regulation of PKR and IRF-1 during hepatitis C virus RNA replication.

The virus-host interactions that influence hepatitis C virus (HCV) replication are largely unknown but are thought to involve those that disrupt components of the innate intracellular antiviral response. Here we examined cellular antiviral pathways that are triggered during HCV RNA replication. We report that (i) RNA replication of HCV subgenomic replicons stimulated double-stranded RNA (dsRNA) signaling pathways within cultured human hepatoma cells, and (ii) viral RNA replication efficiency corresponded with an ability to block a key cellular antiviral effector pathway that is triggered by dsRNA and includes IFN regulatory factor-1 (IRF-1) and protein kinase R (PKR). The block to dsRNA signaling was mapped to the viral nonstructural 5A (NS5A) protein, which colocalized with PKR and suppressed the dsRNA activation of PKR during HCV RNA replication. NS5A alone was sufficient to block both the activation of IRF-1 and the induction of an IRF-1-dependent cellular promoter by dsRNA. Mutations that clustered in or adjacent to the PKR-binding domain of NS5A relieved the blockade to this IRF-1 regulatory pathway, resulting in induction of IRF-1-dependent antiviral effector genes and the concomitant reduction in HCV RNA replication efficiency. Our results provide further evidence to support a role for PKR in dsRNA signaling processes that activate IRF-1 during virus infection and suggest that NS5A may influence HCV persistence by blocking IRF-1 activation and disrupting a host antiviral pathway that plays a role in suppressing virus replication.

DNA-Binding Proteins↗

Ribozymes: aiming at RNA replication and protein synthesis.

The RNA world hypothesis is founded on the idea of an RNA replicase, or self-replicating RNA molecule, and presupposes the later emergence of ribozymes capable of catalyzing the synthesis of peptides. The recent demonstrations of ribozyme-catalyzed template-directed primer extension, and of ribozyme-catalyzed amide bond synthesis, confirm the plausibility of the RNA world, and highlight the steps that remain to be demonstrated in the laboratory.

Animals↗

Genetic evidence for an interaction between a picornaviral cis-acting RNA replication element and 3CD protein.

Internally located, cis-acting RNA replication elements, termed cres, are essential for replication of the genomes of picornaviruses such as human rhinovirus 14 (HRV-14) and poliovirus because they template uridylylation of the protein primer, VPg, by the polymerase 3D(pol). These cres form stem-loop structures sharing a common loop motif, and the HRV-14 cre can substitute functionally for the poliovirus cre in both uridylylation in vitro and RNA replication in vivo. We show, however, that the poliovirus cre is unable to support HRV-14 RNA replication. This lack of complementation maps to the stem of the poliovirus cre and was reversed by single nucleotide substitutions in the stem as well as the base of the loop. Replication-competent, revertant viruses rescued from dicistronic HRV-14 RNAs containing the poliovirus cre, or a chimeric cre containing the poliovirus stem, contained adaptive amino acid substitutions. These mapped to the surface of both the polymerase 3D(pol), at the tip of the "thumb" domain, and the protease 3C(pro), on the side opposing the active site and near the end of an extended strand segment implicated previously in RNA binding. These mutations substantially enhanced replication competence when introduced into HRV-14 RNAs containing the poliovirus cre, and they were additive in their effects. The data support a model in which 3CD or its derivatives 3C(pro) and 3D(pol) interact directly with the stem of the cre during uridylylation of VPg.

Base Sequence↗

Activation of the interferon-beta promoter during hepatitis C virus RNA replication.

In this study we examined the impact of hepatitis C virus (HCV) RNA replication on the innate antiviral response of the host cell. Replication of an HCV subgenomic replicon stimulated the activation of the interferon (IFN)-beta promoter and the production of IFN in human hepatoma cells. Using a variety of functional assays, we found that HCV RNA replication induced the activation and DNA-binding activity of NFkappaB and interferon regulatory factor (IRF)-1. In addition, microscopy experiments revealed a higher frequency of cells containing the nuclear-localized, active form of IRF-3 in HCV replicon cultures versus control cultures. Consistent with these observations, cells harboring the HCV replicon exhibited high basal level expression of a subset of IFN-stimulated antiviral genes. Our results indicate that HCV RNA replication can stimulate cellular antiviral programs that contribute to the assembly and activation of the IFN-beta enhanceosome complex and initiation of the antiviral state. Stable HCV RNA replication in the face of the host antiviral response suggests that HCV may encode one or more proteins capable of overcoming specific antiviral processes, thereby supporting persistent infection.

2',5'-Oligoadenylate Synthetase↗

Genetic analysis of sequences in the 3' nontranslated region of hepatitis C virus that are important for RNA replication.

The genome of the hepatitis C virus (HCV) is a plus-strand RNA molecule that carries a single long open reading frame. It is flanked at either end by highly conserved nontranslated regions (NTRs) that mediate crucial steps in the viral life cycle. The 3' NTR of HCV has a tripartite structure composed of an about 40-nucleotide variable region, a poly(U/UC) tract that has a heterogeneous length, and a highly conserved 98-nucleotide 3'-terminal sequence designated the X tail or 3'X. Conflicting data as to the role the sequences in the 3' NTR play in RNA replication have been reported. By using the HCV replicon system, which is based on the self-replication of subgenomic HCV RNAs in human hepatoma cell line Huh-7, we mapped in this study the sequences in the 3' NTR required for RNA replication. We found that a mutant with a complete deletion of the variable region is viable but that replication is reduced significantly. Only replicons in which the poly(U/UC) tract was replaced by a homouridine stretch of at least 26 nucleotides were able to replicate, whereas RNAs with homopolymeric guanine, adenine, or cytosine sequences were inactive. Deletions of individual or all stem-loop structures in 3'X were not tolerated, demonstrating that this region is most crucial for efficient RNA replication. Finally, we found that none of these deletions or substitutions within the 3' NTR affected RNA stability or translation, demonstrating that the primary effect of the mutations was on RNA replication. These data represent the first detailed mapping of sequences in the 3' NTR assumed to act as a promoter for initiation of minus-strand RNA synthesis.

3' Untranslated Regions↗

Essential role of cyclization sequences in flavivirus RNA replication.

A possible role in RNA replication for interactions between conserved complementary (cyclization) sequences in the 5'- and 3'-terminal regions of Flavivirus RNA was previously suggested but never tested in vivo. Using the M-fold program for RNA secondary-structure predictions, we examined for the first time the base-pairing interactions between the covalently linked 5' genomic region (first ~160 nucleotides) and the 3' untranslated region (last ~115 nucleotides) for a range of mosquito-borne Flavivirus species. Base-pairing occurred as predicted for the previously proposed conserved cyclization sequences. In order to obtain experimental evidence of the predicted interactions, the putative cyclization sequences (5' or 3') in the replicon RNA of the mosquito-borne Kunjin virus were mutated either separately, to destroy base-pairing, or simultaneously, to restore the complementarity. None of the RNAs with separate mutations in only the 5' or only the 3' cyclization sequences was able to replicate after transfection into BHK cells, while replicon RNA with simultaneous compensatory mutations in both cyclization sequences was replication competent. This was detected by immunofluorescence for expression of the major nonstructural protein NS3 and by Northern blot analysis for amplification and accumulation of replicon RNA. We then used the M-fold program to analyze RNA secondary structure of the covalently linked 5'- and 3'-terminal regions of three tick-borne virus species and identified a previously undescribed additional pair of conserved complementary sequences in locations similar to those of the mosquito-borne species. They base-paired with DeltaG values of approximately -20 kcal, equivalent or greater in stability than those calculated for the originally proposed cyclization sequences. The results show that the base-pairing between 5' and 3' complementary sequences, rather than the nucleotide sequence per se, is essential for the replication of mosquito-borne Kunjin virus RNA and that more than one pair of cyclization sequences might be involved in the replication of the tick-borne Flavivirus species.

3' Untranslated Regions↗

Protein-protein interactions and glycerophospholipids in bromovirus and nodavirus RNA replication.

The plant bromoviruses and animal nodaviruses are distinct groups of positive strand RNA viruses that have proven to be useful models for RNA replication studies. Bromoviruses encode two large proteins required for RNA replication: 1a contains domains implicated in helicase and capping functions, and 2a contains a central polymerase-like domain. Using immunoprecipitation and far-western blotting, we have now shown that 1a and 2a form a specific complex in vitro and have mapped the interacting domains. Molecular genetic data implicate the 1a-2a complex in RNA replication and suggest that it supports coordinate action of the putative helicase, polymerase, and capping domains. The locations of the interacting 1a and 2a domains have implications for replication models and the evolution of virus genomes bearing homologous replication genes in fused vs. divided forms. For the nodavirus Flock house virus (FHV), a true RNA replicase has been isolated that carries out complete, highly active replication of added FHV RNA, producing newly synthesized positive strand RNA in predominantly ssRNA form. Positive strand RNA synthesis in this FHV cell-free system is strongly dependent on the addition of any of several glycerophospholipids. Positive strand RNA synthesis depends on the complete glycerophospholipid structure, including the polar head group and diacyl glycerol lipid portion, and is strongly influenced by acyl chain length.

Bromovirus↗

Brome mosaic virus Protein 1a recruits viral RNA2 to RNA replication through a 5' proximal RNA2 signal.

Brome mosaic virus (BMV), a positive-strand RNA virus in the alphavirus-like superfamily, encodes two RNA replication factors. Membrane-associated 1a protein contains a helicase-like domain and RNA capping functions. 2a, which is targeted to membranes by 1a, contains a central polymerase-like domain. In the absence of 2a and RNA replication, 1a acts through an intergenic replication signal in BMV genomic RNA3 to stabilize RNA3 and induce RNA3 to associate with cellular membrane. Multiple results imply that 1a-induced RNA3 stabilization reflects interactions involved in recruiting RNA3 templates into replication. To determine if 1a had similar effects on another BMV RNA replication template, we constructed a plasmid expressing BMV genomic RNA2 in vivo. In vivo-expressed RNA2 templates were replicated upon expression of 1a and 2a. In the absence of 2a, 1a stabilized RNA2 and induced RNA2 to associate with membrane. Deletion analysis demonstrated that 1a-induced membrane association of RNA2 was mediated by sequences in the 5'-proximal third of RNA2. The RNA2 5' untranslated region was sufficient to confer 1a-induced membrane association on a nonviral RNA. However, sequences in the N-terminal region of the 2a open reading frame enhanced 1a responsiveness of RNA2 and a chimeric RNA. A 5'-terminal RNA2 stem-loop important for RNA2 replication was essential for 1a-induced membrane association of RNA2 and, like the 1a-responsive RNA3 intergenic region, contained a required box B motif corresponding to the TPsiC stem-loop of host tRNAs. The level of 1a-induced membrane association of various RNA2 mutants correlated well with their abilities to serve as replication templates. These results support and expand the conclusion that 1a-induced BMV RNA stabilization and membrane association reflect early, 1a-mediated steps in viral RNA replication.

Base Sequence↗