Synthesis of reovirus double-stranded RNA within virionlike particles

The multi-functional reovirus σ3 protein is a virulence factor that suppresses stress granule formation and is associated with myocardial injury

The mammalian orthoreovirus double-stranded (ds) RNA-binding protein σ3 is a multifunctional protein that promotes viral protein synthesis and facilitates viral entry and assembly. The dsRNA-binding capacity of σ3 correlates with its capacity to prevent dsRNA-mediated activation of protein kinase R (PKR). However, the effect of σ3 binding to dsRNA during viral infection is largely unknown. To identify functions of σ3 dsRNA-binding activity during reovirus infection, we engineered a panel of thirteen σ3 mutants and screened them for the capacity to bind dsRNA. Six mutants were defective in dsRNA binding, and mutations in these constructs cluster in a putative dsRNA-binding region on the surface of σ3. Two recombinant viruses expressing these σ3 dsRNA-binding mutants, K287T and R296T, display strikingly different phenotypes. In a cell-type dependent manner, K287T, but not R296T, replicates less efficiently than wild-type (WT) virus. In cells in which K287T virus demonstrates a replication deficit, PKR activation occurs and abundant stress granules (SGs) are formed at late times post-infection. In contrast, the R296T virus retains the capacity to suppress activation of PKR and does not mediate formation of SGs at late times post-infection. These findings indicate that σ3 inhibits PKR independently of its capacity to bind dsRNA. In infected mice, K287T produces lower viral titers in the spleen, liver, lungs, and heart relative to WT or R296T. Moreover, mice inoculated with WT or R296T viruses develop myocarditis, whereas those inoculated with K287T do not. Overall, our results indicate that σ3 functions to suppress PKR activation and subsequent SG formation during viral infection and that these functions correlate with virulence in mice.

The σ3 protein of mammalian orthoreoviruses is a double-stranded RNA binding protein that has classically been thought to function by scavenging dsRNA within infected cells and thus prevents activation of cellular sensors of dsRNA such as the kinase PKR. Here we used mutagenesis to identify the region of σ3 responsible for binding dsRNA. Characterization of mutant viruses expressing σ3 proteins incapable of binding dsRNA show that contrary to expectation, dsRNA binding is not required for σ3-mediated inhibition of PKR. We show that one mutant virus (R296T) despite being deficient in dsRNA-binding can inhibit PKR and replicates similar to WT virus. In contrast, another mutant virus (K287T) that bears a σ3 protein that cannot prevent dsRNA-mediated activation of PKR induces stress granules in infected cells and replicates less efficiently than WT virus. In vivo, the K287T mutant is attenuated in its replication and unlike WT virus and the R296T mutant virus does not cause heart disease (myocarditis).

Inter-segment complementarity in orbiviruses: a driver for co-ordinated genome packaging in the Reoviridae?

The process by which eukaryotic viruses with segmented genomes select a complete set of genome segments for packaging into progeny virus particles is not understood. In this study a model based on the association of genome segments through specific RNA-RNA interactions driven by base pairing was formalized and tested in the Orbivirus genus of the Reoviridae family. A strategy combining screening of the genomic sequences for inter-segment complementarity with direct functional testing of inter-segment RNA-RNA interactions using reverse genetics is described in the type species of the Orbivirus genus, Bluetongue virus (BTV). Two examples, involving four of the ten BTV genomic segments, of specific inter-segment interaction motifs whose maintenance is essential for the generation of infectious virus, were identified. Equivalent inter-segment complementarities were found between the identified regions of the orthologous genome segments of all orbiviruses, including phylogenetically distant species. Specific interaction of the participating RNA segments was confirmed in vitro using electrophoretic mobility shift assays, with the interactions inhibited using oligonucleotides complementary to the interaction motif of one of the interacting partners, and also through mutagenesis of the motifs. In each example, the base pairing rather than the absolute sequence was critical to the formation of a functional inter-segment interaction, with mutations only being tolerated in rescued virus if compensating changes were made in the interacting partner to restore uninterrupted base pairing. The absolute sequence of the complementarity motifs varied between species, indicating that this newly identified phenomenon may contribute to the observed lack of reassortment between Orbivirus species.

Reovirus replication protein μ2 influences cell tropism by promoting particle assembly within viral inclusions

The double-stranded RNA virus mammalian reovirus displays broad cell, tissue, and host tropism. A critical checkpoint in the reovirus replication cycle resides within viral cytoplasmic inclusions, which are biosynthetic centers of genome multiplication and new-particle assembly. Replication of strain type 3 Dearing (T3) is arrested in Madin-Darby canine kidney (MDCK) cells at a step subsequent to inclusion development and prior to formation of genomic double-stranded RNA. This phenotype is primarily regulated by viral replication protein μ2. To understand how reovirus inclusions differ in productively and abortively infected MDCK cells, we used confocal immunofluorescence and thin-section transmission electron microscopy (TEM) to probe inclusion organization and particle morphogenesis. Although no abnormalities in inclusion morphology or viral protein localization were observed in T3-infected MDCK cells using confocal microscopy, TEM revealed markedly diminished production of mature progeny virions. T3 inclusions were less frequent and smaller than those formed by T3-T1M1, a productively replicating reovirus strain, and contained decreased numbers of complete particles. T3 replication was enhanced when cells were cultivated at 31°C, and inclusion ultrastructure at low-temperature infection more closely resembled that of a productive infection. These results indicate that particle assembly in T3-infected MDCK cells is defective, possibly due to a temperature-sensitive structural or functional property of μ2. Thus, reovirus cell tropism can be governed by interactions between viral replication proteins and the unique cell environment that modulate efficiency of particle assembly.

Guanidine hydrochloride inhibits mammalian orthoreovirus growth by reversibly blocking the synthesis of double-stranded RNA

Millimolar concentrations of guanidine hydrochloride (GuHCl) are known to inhibit the replication of many plant and animal viruses having positive-sense RNA genomes. For example, GuHCl reversibly interacts with the nucleotide-binding region of poliovirus protein 2C(ATPase), resulting in a specific inhibition of viral negative-sense RNA synthesis. The use of GuHCl thereby allows for the spatiotemporal separation of poliovirus gene expression and RNA replication and provides a powerful tool to synchronize the initiation of negative-sense RNA synthesis during in vitro replication reactions. In the present study, we examined the effect of GuHCl on mammalian orthoreovirus (MRV), a double-stranded RNA (dsRNA) virus from the family Reoviridae. MRV growth in murine L929 cells was reversibly inhibited by 15 mM GuHCl. Furthermore, 15 mM GuHCl provided specific inhibition of viral dsRNA synthesis while sparing both positive-sense RNA synthesis and viral mRNA translation. By using GuHCl to provide temporal separation of MRV gene expression and genome replication, we obtained evidence that MRV primary transcripts support sufficient protein synthesis to assemble morphologically normal viral factories containing functional replicase complexes. In addition, the coordinated use of GuHCl and cycloheximide allowed us to demonstrate that MRV dsRNA synthesis can occur in the absence of ongoing protein synthesis, although to only a limited extent. Future studies utilizing the reversible inhibition of MRV dsRNA synthesis will focus on elucidating the target of GuHCl, as well as the components of the MRV replicase complexes.

Reovirus sigma NS protein localizes to inclusions through an association requiring the mu NS amino terminus

Cells infected with mammalian reoviruses contain phase-dense inclusions, called viral factories, in which viral replication and assembly are thought to occur. The major reovirus nonstructural protein mu NS forms morphologically similar phase-dense inclusions when expressed in the absence of other viral proteins, suggesting it is a primary determinant of factory formation. In this study we examined the localization of the other major reovirus nonstructural protein, sigma NS. Although sigma NS colocalized with mu NS in viral factories during infection, it was distributed diffusely throughout the cell when expressed in the absence of mu NS. When coexpressed with mu NS, sigma NS was redistributed and colocalized with mu NS inclusions, indicating that the two proteins associate in the absence of other viral proteins and suggesting that this association may mediate the localization of sigma NS to viral factories in infected cells. We have previously shown that mu NS residues 1 to 40 or 41 are both necessary and sufficient for mu NS association with the viral microtubule-associated protein mu 2. In the present study we found that this same region of micro NS is required for its association with sigma NS. We further dissected this region, identifying residues 1 to 13 of mu NS as necessary for association with sigma NS, but not with mu 2. Deletion of sigma NS residues 1 to 11, which we have previously shown to be required for RNA binding by that protein, resulted in diminished association of sigma NS with mu NS. Furthermore, when treated with RNase, a large portion of sigma NS was released from mu NS coimmunoprecipitates, suggesting that RNA contributes to their association. The results of this study provide further evidence that mu NS plays a key role in forming the reovirus factories and recruiting other components to them.

The reovirus mutant tsA279 L2 gene is associated with generation of a spikeless core particle: implications for capsid assembly

Previous studies which used intertypic reassortants of the wild-type reovirus serotype 1 Lang and the temperature-sensitive (ts) serotype 3 mutant clone tsA279 identified two ts lesions; one lesion, in the M2 gene segment, was associated with defective transmembrane transport of restrictively assembled virions (P. R. Hazelton and K. M. Coombs, Virology 207:46-58, 1995). In the present study we show that the second lesion, in the L2 gene segment, which encodes the lambda2 protein, is associated with the accumulation of a core-like particle defective for the lambda2 pentameric spike. Physicochemical, biochemical, and immunological studies showed that these structures were deficient for genomic double-stranded RNA, the core spike protein lambda2, and the minor core protein micro2. Core particles with the lambda2 spike structure accumulated after temperature shift-down from a restrictive to a permissive temperature in the presence of cycloheximide. These data suggest the spike-deficient, core-like particle is an assembly intermediate in reovirus morphogenesis. The existence of this naturally occurring primary core structure suggests that the core proteins lambda1, lambda3, and sigma2 interact to initiate the process of virion capsid assembly through a dodecahedral mechanism. The next step in the proposed capsid assembly model would be the association of the minor core protein mu2, either preceding or collateral to the condensation of the lambda2 pentameric spike at the apices of the primary core structure. The assembly pathway of the reovirus double capsid is further elaborated when these observations are combined with structures identified in other studies.

Identification of sequence elements containing signals for replication and encapsidation of the reovirus M1 genome segment

In reovirus the genetic signals that control genome replication and encapsidation are unknown. Serial passage of reovirus results in the accumulation of deletion mutants that contain fragments of genome segments. The smallest fragments found in deletion mutants will consist of the minimum essential sequences for genome replication and assembly. T1 x T3 reassortants containing the L2 segment from T3 and the M3 segment derived from T1 generate deletions in segment M1 on serial passage. Fragments of M1 segments were produced by serial passage, characterized by PAGE and Northern blotting before amplification by PCR, cloning, and sequencing. Thirteen of the smallest deletion fragments were sequenced. All of the smallest fragments contained sequences from both termini of segment M1. The smallest fragment was 344 nucleotides long. The consensus sequences consisted of 132-135 nucleotides from the 5' end of the plus strand and 183-185 nucleotides from the 3' end of the plus strand. It is concluded that these regions contain all the signals necessary for the replication and assembly of the M1 genome segment.

Avian reovirus S1133 can replicate in mouse L cells: effect of pH and cell attachment status on viral infection

Previous reports have suggested that avian reovirus S1133 fails to replicate in mouse L cells. In this article, we report that replication does occur under certain culture conditions. The avian reovirus was found to grow in mouse L cells at pH 6.4 and 7.2 but not at pH 8.2. Culture medium with a basic pH directly inhibited viral transcription and genome replication. As a result, viral protein synthesis was also affected. At permissive pH levels, avian reovirus grew better in monolayers than in suspension cultures of L cells because of the influence of cell attachment status on viral macromolecular synthesis. Our results not only show that avian reovirus can replicate in mouse L cells but also help to explain why it did not in previous studies.

Antibodies to the Giardia lamblia double-stranded RNA virus major protein can block the viral infection

The double-stranded RNA virus-like particles, found among several independent isolates and cloned strains of Giardia lamblia, have previously been reported to be spheres of 35 nm with a genome of 7 kilobase pairs and a major protein of 100 kDa. The virus is capable of infecting certain virus-free isolates of G. lamblia. Antisera raised in mice against the intact virus did not react with the double-stranded RNA, but reacted strongly with the 100 kDa protein in Western blots. Preincubation of the virus with antisera abolished viral infectivity, whereas the antisera against double-stranded RNA showed only a weak blocking effect. Inclusion of the antiviral sera in the cultures of virus-infected G. lamblia at 10(3)-fold dilution resulted in elimination of the virus from the protozoa. Apparently, the 100 kDa protein is necessary for the initiation of viral infection and possibly subsequent assembly or replication of viral progeny particles.

Synthesis of simian rotavirus SA11 double-stranded RNA in a cell-free system

A cell-free system was developed to study the replication of simian rotavirus SA11. The components of the system included (i) subviral particles prepared from infected cells to template the synthesis of viral RNA and (ii) an mRNA-dependent rabbit reticulocyte lysate to support protein synthesis. Based upon nuclease-sensitivity, approximately 20% of the RNA made in vitro was double-stranded (dsRNA) and 80% single-stranded (ssRNA). Electrophoretic analysis of the RNA products on polyacrylamide and low pH agarose gels showed that the system supported the synthesis of 11 dsRNAs and 11 positive-sense ssRNAs that corresponded in size to authentic viral RNAs. The synthesis of dsRNA in vitro was determined to be an asymmetrical process in which a nuclease-sensitive positive-strand RNA acted as a template for the synthesis of negative-strand RNA. The system also supported the initiation of negative-strand RNA using exogenous viral positive-strand RNA as a template. Finally, analysis of subviral particles recovered from reactions suggested that viral proteins made in vitro assembled into nucleoprotein complexes which were similar to those present in infected cells. Together, these results indicate that the cell-free system supported rotavirus RNA replication, transcription and the assembly of subviral particles.

Structure and function of the reovirus genome

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Virus-Specific RNA Synthesis in Cells Infected by Infectious Pancreatic Necrosis Virus

Pulse-labeling experiments with [ 3 H]uridine revealed that the rate of infections pancreatic necrosis virus-specific RNA synthesis was maximal at 8 to 10 h after infection and was completely diminished by 12 to 14 h. Three forms of RNA intermediates were detected: (i) a putative transcription intermediate (TRI) which comigrated in acrylamide gels with virion double-stranded RNA (dsRNA) after RNase treatment; (ii) a 24S genome length mRNA which could be resolved into two bands by polyacrylamide gel electrophoresis; and (iii) a 14S dsRNA component indistinguishable from virion RNA by gradient centrifugation and gel electrophoresis. The TRI (i) was LiCl precipitable; (ii) sedimented slightly faster and broader (14 to 16S) than the 14S virion dsRNA; (iii) had a lower electrophoretic mobility in acrylamide gels than dsRNA, barely entering acrylamide gels as a heterogenous component; (iv) yielded genome-sized pieces of dsRNA after RNase digestion; and (v) was the most abundant RNA form early in the infectious cycle. The 24S single-stranded RNA was thought to be the viral mRNA since it: (i) became labeled during short pulses; (ii) was found in the polysomal fraction of infected cells; and (iii) hybridized to denatured viral RNA, forming two segments of RNase-resistant RNA that comigrated with virion dsRNA in gels. The 24S mRNA component was formed before the synthesis of dsRNA, and radioactivity could be chased from 24S single-stranded RNA to dsRNA, indicating that 24S RNA may serve as template for the synthesis of complementary strands to form dsRNA. Similar to reovirus, infectious pancreatic necrosis viral 24S mRNA contained no polyadenylic acid tracts.

Semi-conservative transcription in particles of a double-stranded RNA mycovirus

During transcription in vitro catalysed by the virion RNA polymerase of Aspergillus foetidus virus AfV-S in the presence of tritiated UTP, the virus double-stranded RNA becomes labelled in one strand, which has the same sequence as the single-stranded RNA transcripts produced. Most of the label incorporated into double-stranded RNA could be chased into single stranded RNA by further reaction with excess unlabelled nucleoside triphosphates. In reactions containing tritiated UTP the single-stranded RNA transcripts released after the first round of transcription were unlabelled. It is concluded that transcription in virions of AfV-S occurs by displacement of one of the strands of double-stranded RNA by the RNA strand being newly synthesised i.e. the reaction is semi-conservative with respect to double-stranded RNA.

Reovirus-specific enzyme(s) associated with subviral particles responds in vitro to polyribocytidylate to yield double-stranded polyribocytidylate-polyriboguanylate

In reovirus-infected cells, virus-specific particles accumulate that have associated with them a polyribocytidylate [poly(C)]-dependent polymerase. This enzyme copies in vitro poly(C) to yield the double-stranded poly(C).polyriboguanylate [poly(G)]. The particles with poly(C)-dependent polymerase were heterogeneous in size, with most sedimenting from 300S to 550S. Exponential increase in these particles began at 23 h, and maximal amounts were present by 31 h, the time of onset of exponential growth of virus at 30 degrees C. Maximal amounts of particles with active transcriptase and replicase were present at 15 and 18 h after infection. Thereafter, there was a marked decrease in particles with active transcriptase and replicase until base line levels were reached at 31 h. Thus, the increase in poly(C)-responding particles occurred coincident with the decrease in particles with active transcriptase and replicase. The requirement for poly(C) as template was specific because no RNA was synthesized in vitro in response to any other homopolymer, including 2'-O-methyl-poly(C). Synthesis was optimal in the presence of Mn(2+) as the divalent cation, and no primer was necessary for synthesis. In contrast, the dinucleotide GpG markedly stimulated synthesis in the presence of 8 mM Mg(2+). The size of the poly(C).poly(G) synthesized in vitro was dependent on the size of the poly(C) used as template. This suggested that the whole template was copied into a complementary strand of similar size. The T(m) of the product was between 100 and 130 degrees C. Hydrolysis of the product labeled in [(32)P]GMP with alkali or RNase T2 yielded GMP as the only labeled mononucleotide. This does indicate that the synthesis of the poly(G) strand in vitro did not proceed by end addition to the poly(C) template, but proceeded on a separate strand.

Recombination between temperature-sensitive and deletion mutants of reovirus

In standard pairwise crosses there was no detectable recombination between defective reovirus lacking the largest genomic segment and prototypes of the seven known classes of ts mutants. However, in such crosses between R2A (201) and the various prototypes frequencies of ts+ recombinants between 2.6 and 6.1% were observed, as others have found (Fields, 1971; Fields and Joklik, 1969). An infectious center assay was devised to measure recombination in this system, and it was found that all mixedly infected cells gave rise to ts+ recombinants in crosses between prototype ts mutants, but no recombination was detectable when the defective virus was crossed with three different ts mutants. The ts mutation of mutant R2A (201) was efficiently rescued when crossed with UV-inactivated wild-type virus but not when crossed with UV-inactivated defective virus. It is concluded from these various experiments that if there is any recombination between these defective reovirions and any known class of ts mutants it is too low to be measured by methods presently available. The kinetics of recombination were measured in cells mixedly infected with R2A (201) and R2B (352) mutants. At the earliest time progeny virus could be found in the cells the frequency of ts+ recombinants was 4.5%, and this frequency remained unchanged despite a subsequent 1,000-fold increase in progeny virus.

Regulation of macromolecular synthesis in reovirus-infected L-929 cells I. Effect of L-histidinol

The histidine analogue L-histidinol, reported by Vaughan and Hansen (1973) to establish a potent, readily reversible inhibition of eukaryotic protein synthesis in vivo, was used to investigate the regulation of macromolecular synthesis in reovirus-infected L-929 cells. The addition of L-histidinol to normal L cells led to a total inhibition of protein synthesis. The inhibition appeared to be a consequence neither of isotope dilution resulting from elevated endogenous amino acids nor of an inability of treated cells to accumulate exogenous amino acids. Addition of L-histidine to histidinol-arrested cells resulted in a complete recovery of protein synthesis. Similarly, protein synthesis in reovirus-infected L cells examined 17 h postinfection (31 C) was totally inhibited by histidinol treatment and was readily reversed by the addition of histidine. Reovirus-infected cells treated with histidinol had an essentially unaltered capacity to synthesize reovirus single-stranded RNA relative to unperturbed cultures but a diminishing ability to maintain genome RNA synthesis. Addition of L-histidine to arrested cultures led to a complete recovery of genome RNA synthesis. The L-histidinol-mediated arrest of protein synthesis was both very effective and easily reversed, suggesting the general applicability of this novel inhibitor to investigations of regulation of macromolecular synthesis in both normal and virus-infected eukaryotic cells.

Fractionation of nucleic acids from Penicillium chrysogenum and associated ribonucleic acid viruses by selective exclusion and retention in agarose gels

Double-stranded nucleic acids from a strain of Penicillium chrysogenum containing RNA viruses were isolated by agarose-gel filtration, and separated into DNA and double-stranded RNA fractions by agarose-gel chromatography in 2.5m-NaCl. The DNA fraction contained less than 1% alkali-labile polynucleotides, and sedimented homogeneously at 8-10S in alkaline sucrose gradients. In CsCl gradients it tended to band in the density region of 1.66-1.72g/ml. It had a ;melting' temperature (T(m)) of 75 degrees C in 0.015m-NaCl-0.0015m-trisodium citrate, corresponding to 51.5mol% of G+C. The double-stranded RNA fraction did not contain detectable DNA. It could not band in CsCl up to a density of 1.78g/ml, and mainly consisted of a 14-15S RNA species with a T(m) of 88.5 degrees C in the above solvent, and a G+C content of 49.3 mol%.

Shythesis of reovirus oligo adenylic acid in vivo and in vitro

The formation of reovirus double-stranded (ds) RNA and of oligo adenylic acid (oligo A) is inhibited by 5 mug of actinomycin D per ml added at the time of viral infection. Viral proteins are synthesized and assembled into dsRNA-deficient particles under these conditions. The addition of cycloheximide to infected cells during the mid-logarithmic phase of viral replication terminates protein and dsRNA synthesis, but allows continued oligo A synthesis for about 1 h. The (3)H-labeled oligo A formed in the presence of cycloheximide is incorporated into particles whose density in CsCl is identical to that of reovirions. Using the large particulate or virus factory-containing cytoplasmic fraction of infected L-cells, we have established an in vitro system for the synthesis of oligo A. The in vitro product migrates slightly faster in sodium dodecyl sulfate acrylamide gels than marker oligo A. Oligo A synthesis in vitro continues for about 1 h, requires, the presence of only one ribonucleoside triphosphate (ATP), is not inhibited by DNase or RNase, but is abruptly terminated by the addition of chymotrypsin to the reaction mixture. Oligo A formed both in vivo and in vitro is released from the factory fraction by chymotrypsin digestion. The enzymes which catalyze the synthesis of oligo A, dsRNA, and single-stranded RNA all exhibit a similar temperature dependence with an optimum of approximately 45 C. These results indicate that oligo A is formed within the core of the nascent virion after the completion of dsRNA synthesis; they suggest that the oligo A polymerase is an alternative activity of the virion-bound transcriptase and that it is regulated by outer capsomere proteins.

RNA polymerase activity associated with bacteriophage phi 6

The Pseudomonas phaseolicola bacteriophage phi6 incorporated labeled UTP into an acid-insoluble precipitate. Incorporation was dependent on the presence of manganese acetate, ATP, GTP, CTP, and a short heat treatment of the phage; the reaction was stimulated by NH(4)Cl. The substitution of (14)C-ATP, -CTP or -GTP for UTP, together with the appropriate unlabeled ribonucleoside triphosphates, disclosed that CMP was incorporated to the greatest extent followed by GMP, UMP, and AMP. Radioactive RNAs formed by the reaction were resistant to RNases A and T(1) in high salt but susceptible to these nucleases in low salt. The labeled RNA co-sedimented and co-electrophoresed with phi6 double-stranded (ds) RNA. However, the distribution of the radioactivity into the three ds-RNA components varied depending on the (14)C-ribonucleoside triphosphate used in the reaction. The incorporation of UMP was primarily into the two smaller ds-RNA segments, GMP primarily into the large ds-RNA segment, and CMP and AMP were about equally distributed into all three ds-RNA segments.