The reovirus replicative cycle: conservation of parental RNA and protein

Early Events in Reovirus Infection Influence Induction of Innate Immune Response

Mammalian orthoreovirus (reovirus) is a double-stranded RNA (dsRNA) virus which encapsidates its 10 genome segments within a double-layered viral particle. Reovirus infection triggers an antiviral response in host cells which serves to limit viral replication. This antiviral response is initiated by recognition of the incoming viral genome by host sensors present in the cytoplasm. However, how host sensors gain access to the reovirus genome is unclear, as this dsRNA is protected by the viral particle proteins throughout infection. To initiate infection, reovirus particles are endocytosed and the outer viral particle layer is disassembled through the action of host proteases. This disassembly event is required for viral escape into the cytoplasm to begin replication. We show that endosomal proteases are required even late in infection, when disassembly is complete, to induce an immune response to reovirus. Additionally, counter to dogma, our data demonstrate that at least some viral dsRNA genome is exposed and detectable during entry. We hypothesize that some proportion of reovirus particles remain trapped within endosomes, allowing for the breakdown of these particles and release of their genome. We show that rapidly uncoating mutants escape the endosome more rapidly and induce a diminished immune response. Further, we show that particles entering through dynamin-independent pathways evade detection by host sensors. Overall, our data provide new insight into how genomes from entering reovirus particles are detected by host cells. IMPORTANCE Viruses must infect host cells to replicate, often killing the host cell in the process. However, hosts can activate defenses to limit viral replication and protect the organism. To trigger these host defenses to viral infections, host cells must first recognize that they are infected. Mammalian orthoreovirus (reovirus) is a model system used to study host-virus interactions. This study identifies aspects of host and virus biology which determine the capacity of host cells to detect infection. Notably, entry of reovirus into host cells plays a critical role in determining the magnitude of immune response triggered during infection. Mutants of reovirus which can enter cells more rapidly are better at avoiding detection by the host. Additionally, reovirus can enter cells through multiple routes. Entry through some of these routes also helps reovirus evade detection.

Bluetongue Research at a Crossroads: Modern Genomics Tools Can Pave the Way to New Insights

Bluetongue virus (BTV) is an arthropod-borne, segmented double-stranded RNA virus that can cause severe disease in both wild and domestic ruminants. BTV evolves via several key mechanisms, including the accumulation of mutations over time and the reassortment of genome segments.Additionally, BTV must maintain fitness in two disparate hosts, the insect vector and the ruminant. The specific features of viral adaptation in each host that permit host-switching are poorly characterized. Limited field studies and experimental work have alluded to the presence of these phenomena at work, but our understanding of the factors that drive or constrain BTV's genetic diversification remains incomplete. Current research leveraging novel approaches and whole genome sequencing applications promises to improve our understanding of BTV's evolution, ultimately contributing to the development of better predictive models and management strategies to reduce future impacts of bluetongue epizootics.

Viral Capsid and Polymerase in Reoviridae

The members of the family Reoviridae (reoviruses) consist of 9-12 discrete double-stranded RNA (dsRNA) segments enclosed by single, double, or triple capsid layers. The outer capsid proteins of reoviruses exhibit the highest diversity in both sequence and structural organization. By contrast, the conserved RNA-dependent RNA polymerase (RdRp) structure in the conserved innermost shell in all reoviruses suggests that they share common transcriptional regulatory mechanisms. After reoviruses are delivered into the cytoplasm of a host cell, their inner capsid particles (ICPs) remain intact and serve as a stable nanoscale machine for RNA transcription and capping performed using enzymes in ICPs. Advances in cryo-electron microscopy have enabled the reconstruction at near-atomic resolution of not only the icosahedral capsid, including capping enzymes, but also the nonicosahedrally distributed complexes of RdRps within the capsid at different transcriptional stages. These near-atomic resolution structures allow us to visualize highly coordinated structural changes in the related enzymes, genomic RNA, and capsid protein during reovirus transcription. In addition, reoviruses encode their own enzymes for nascent RNA capping before RNA releasing from their ICPs.

Recognition of Reovirus RNAs by the Innate Immune System

Mammalian orthoreovirus (reovirus) is a dsRNA virus, which has long been used as a model system to study host-virus interactions. One of the earliest interactions during virus infection is the detection of the viral genomic material, and the consequent induction of an interferon (IFN) based antiviral response. Similar to the replication of related dsRNA viruses, the genomic material of reovirus is thought to remain protected by viral structural proteins throughout infection. Thus, how innate immune sensor proteins gain access to the viral genomic material, is incompletely understood. This review summarizes currently known information about the innate immune recognition of the reovirus genomic material. Using this information, we propose hypotheses about host detection of reovirus.

Bluetongue virus capsid assembly and maturation

Maturation is an intrinsic phase of the viral life cycle and is often intertwined with egress. In this review we focus on orbivirus maturation by using Bluetongue virus (BTV) as a representative. BTV, a member of the genus Orbivirus within the family Reoviridae, has over the last three decades been subjected to intense molecular study and is thus one of the best understood viruses. BTV is a non-enveloped virus comprised of two concentric protein shells that encapsidate 10 double-stranded RNA genome segments. Upon cell entry, the outer capsid is shed, releasing the core which does not disassemble into the cytoplasm. The polymerase complex within the core then synthesizes transcripts from each genome segment and extrudes these into the cytoplasm where they act as templates for protein synthesis. Newly synthesized ssRNA then associates with the replicase complex prior to encapsidation by inner and outer protein layers of core within virus-triggered inclusion bodies. Maturation of core occurs outside these inclusion bodies (IBs) via the addition of the outer capsid proteins, which appears to be coupled to a non-lytic, exocytic pathway during early infection. Similar to the enveloped viruses, BTV hijacks the exocytosis and endosomal sorting complex required for trafficking (ESCRT) pathway via a non-structural glycoprotein. This exquisitely detailed understanding is assembled from a broad array of assays, spanning numerous and diverse in vitro and in vivo studies. Presented here are the detailed insights of BTV maturation and egress.

Bluetongue virus: dissection of the polymerase complex

Bluetongue is a vector-borne viral disease of ruminants that is endemic in tropical and subtropical countries. Since 1998 the virus has also appeared in Europe. Partly due to the seriousness of the disease, bluetongue virus (BTV), a member of genus Orbivirus within the family Reoviridae, has been a subject of intense molecular study for the last three decades and is now one of the best understood viruses at the molecular and structural levels. BTV is a complex non-enveloped virus with seven structural proteins arranged in two capsids and a genome of ten double-stranded (ds) RNA segments. Shortly after cell entry, the outer capsid is lost to release an inner capsid (the core) which synthesizes capped mRNAs from each genomic segment, extruding them into the cytoplasm. This requires the efficient co-ordination of a number of enzymes, including helicase, polymerase and RNA capping activities. This review will focus on our current understanding of these catalytic proteins as derived from the use of recombinant proteins, combined with functional assays and the in vitro reconstitution of the transcription/replication complex. In some cases, 3D structures have complemented this analysis to reveal the fine structural detail of these proteins. The combined activities of the core enzymes produce infectious transcripts necessary and sufficient to initiate BTV infection. Such infectious transcripts can now be synthesized wholly in vitro and, when introduced into cells by transfection, lead to the recovery of infectious virus. Future studies thus hold the possibility of analysing the consequence of mutation in a replicating virus system.

Peptides released from reovirus outer capsid form membrane pores that recruit virus particles

Nonenveloped animal viruses must disrupt or perforate a cell membrane during entry. Recent work with reovirus has shown formation of size-selective pores in RBC membranes in concert with structural changes in capsid protein mu1. Here, we demonstrate that mu1 fragments released from reovirus particles are sufficient for pore formation. Both myristoylated N-terminal fragment mu1N and C-terminal fragment phi are released from particles. Both also associate with RBC membranes and contribute to pore formation in the absence of particles, but mu1N has the primary and sufficient role. Particles with a mutant form of mu1, unable to release mu1N or form pores, lack the ability to associate with membranes. They are, however, recruited by pores preformed with peptides released from wild-type particles or with synthetic mu1N. The results provide evidence that docking to membrane pores by virus particles may be a next step in membrane penetration after pore formation by released peptides.

Thermolabilizing pseudoreversions in reovirus outer-capsid protein micro 1 rescue the entry defect conferred by a thermostabilizing mutation

Heat-resistant mutants selected from infectious subvirion particles of mammalian reoviruses have determinative mutations in the major outer-capsid protein micro 1. Here we report the isolation and characterization of intragenic pseudoreversions of one such thermostabilizing mutation. From a plaque that had survived heat selection, a number of viruses with one shared mutation but different second-site mutations were isolated. The effect of the shared mutation alone or in combination with second-site mutations was examined using recoating genetics. The shared mutation, D371A, was found to confer (i) substantial thermostability, (ii) an infectivity defect that followed attachment but preceded viral protein synthesis, and (iii) resistance to micro 1 rearrangement in vitro, with an associated failure to lyse red blood cells. Three different second-site mutations were individually tested in combination with D371A and found to wholly or partially revert these phenotypes. Furthermore, when tested alone in recoated particles, each of these three second-site mutations conferred demonstrable thermolability. This and other evidence suggest that pseudoreversion of micro 1-based thermostabilization can occur by a general mechanism of micro 1-based thermolabilization, not requiring a specific compensatory mutation. The thermostabilizing mutation D371A as well as 9 of the 10 identified second-site mutations are located near contact regions between micro 1 trimers in the reovirus outer capsid. The availability of both thermostabilizing and thermolabilizing mutations in micro 1 should aid in defining the conformational rearrangements and mechanisms involved in membrane penetration during cell entry by this structurally complex nonenveloped animal virus.

Thermostabilizing mutations in reovirus outer-capsid protein mu1 selected by heat inactivation of infectious subvirion particles

The 76-kDa mu1 protein of nonfusogenic mammalian reovirus is a major component of the virion outer capsid, which contains 200 mu1 trimers arranged in an incomplete T=13 lattice. In virions, mu1 is largely covered by a second major outer-capsid protein, sigma3, which limits mu1 conformational mobility. In infectious subvirion particles, from which sigma3 has been removed, mu1 is broadly exposed on the surface and can be promoted to rearrange into a protease-sensitive and hydrophobic conformer, leading to membrane perforation or penetration. In this study, mutants that resisted loss of infectivity upon heat inactivation (heat-resistant mutants) were selected from infectious subvirion particles of reovirus strains Type 1 Lang and Type 3 Dearing. All of the mutants were found to have mutations in mu1, and the heat-resistance phenotype was mapped to mu1 by both recoating and reassortant genetics. Heat-resistant mutants were also resistant to rearrangement to the protease-sensitive conformer of mu1, suggesting that heat inactivation is associated with mu1 rearrangement, consistent with published results. Rate constants of heat inactivation were determined, and the dependence of inactivation rate on temperature was consistent with the Arrhenius relationship. The Gibbs free energy of activation was calculated with reference to transition-state theory and was found to be correlated with the degree of heat resistance in each of the analyzed mutants. The mutations are located in upper portions of the mu1 trimer, near intersubunit contacts either within or between trimers in the viral outer capsid. We propose that the mutants stabilize the outer capsid by interfering with unwinding of the mu1 trimer.

Mammalian reovirus, a nonfusogenic nonenveloped virus, forms size-selective pores in a model membrane

During cell entry, reovirus particles with a diameter of 70-80 nm must penetrate the cellular membrane to access the cytoplasm. The mechanism of penetration, without benefit of membrane fusion, is not well characterized for any such nonenveloped animal virus. Lysis of RBCs is an in vitro assay for the membrane perforation activity of reovirus; however, the mechanism of lysis has been unknown. In this report, osmotic-protection experiments using PEGs of different sizes revealed that reovirus-induced lysis of RBCs occurs osmotically, after formation of small size-selective lesions or "pores." Consistent results were obtained by monitoring leakage of fluorophore-tagged dextrans from the interior of resealed RBC ghosts. Gradient fractionations showed that whole virus particles, as well as the myristoylated fragment mu1N that is released from particles, are recruited to RBC membranes in association with pore formation. We propose that formation of small pores is a discrete, intermediate step in the reovirus membrane-penetration pathway, which may be shared by other nonenveloped animal viruses.

The delta region of outer-capsid protein micro 1 undergoes conformational change and release from reovirus particles during cell entry

Cell entry by reoviruses requires a large, transcriptionally active subvirion particle to gain access to the cytoplasm. The features of this particle have been the subject of debate, but three primary candidates-the infectious subvirion particle (ISVP), ISVP*, and core particle forms-that differ in whether putative membrane penetration protein micro 1 and adhesin sigma1 remain particle bound have been identified. Experiments with antibody reagents in this study yielded new information about the steps in particle disassembly during cell entry. Monoclonal antibodies specific for the delta region of micro 1 provided evidence for a conformational change in micro 1 and for release of the delta proteolytic fragment from entering particles. Antiserum raised against cores provided evidence for entry-related changes in particle structure and identified entering particles that largely lack the delta fragment inside cells. Antibodies specific for sigma1 showed that it is also largely shed from entering particles. Limited coimmunostaining with markers for late endosomes and lysosomes indicated the particles lacking delta and sigma1 did not localize to those subcellular compartments, and other observations suggested that both the particles and free delta were released into the cytoplasm. Essentially equivalent findings were obtained with native ISVPs and highly infectious recoated particles containing wild-type proteins. Poorly infectious recoated particles containing a hyperstable mutant form of micro 1, however, showed no evidence for the in vitro and intracellular changes in particle structure normally detected by antibodies, and these particles instead accumulated in late endosomes or lysosomes. Recoated particles with hyperstable micro 1 were also ineffective at mediating erythrocyte lysis in vitro and promoting alpha-sarcin coentry and intoxication of cells in cultures. Based on these and other findings, we propose that ISVP* is a transient intermediate in cell entry which mediates membrane penetration and is then further uncoated in the cytoplasm to yield particles, resembling cores, that largely lack the delta fragment of micro 1.

Structure of the reovirus membrane-penetration protein, Mu1, in a complex with is protector protein, Sigma3

Cell entry by nonenveloped animal viruses requires membrane penetration without membrane fusion. The reovirus penetration agent is the outer-capsid protein, Mu1. The structure of Mu1, complexed with its "protector" protein, Sigma3, and the fit of this Mu1(3)Sigma3(3) heterohexameric complex into the cryoEM image of an intact virion, reveal molecular events essential for viral penetration. Autolytic cleavage divides Mu1 into myristoylated Mu1N and Mu1C. A long hydrophobic pocket can receive the myristoyl group. Dissociation of Mu1N, linked to a major conformational change of the entire Mu1 trimer, must precede myristoyl-group insertion into the cellular membrane. A myristoyl switch, coupling exposure of the fatty acid chain, autolytic cleavage of Mu1N, and long-range molecular rearrangement of Mu1C, thus appears to be part of the penetration mechanism.

Structure of the reovirus outer capsid and dsRNA-binding protein sigma3 at 1.8 A resolution

The crystallographically determined structure of the reovirus outer capsid protein sigma3 reveals a two-lobed structure organized around a long central helix. The smaller of the two lobes includes a CCHC zinc-binding site. Residues that vary between strains and serotypes lie mainly on one surface of the protein; residues on the opposite surface are conserved. From a fit of this model to a reconstruction of the whole virion from electron cryomicroscopy, we propose that each sigma3 subunit is positioned with the small lobe anchoring it to the protein mu1 on the surface of the virion, and the large lobe, the site of initial cleavages during entry-related proteolytic disassembly, protruding outwards. The surface containing variable residues faces solvent. The crystallographic asymmetric unit contains two sigma3 subunits, tightly associated as a dimer. One broad surface of the dimer has a positively charged surface patch, which extends across the dyad. In infected cells, sigma3 binds dsRNA and inhibits the interferon response. The location and extent of the positively charged surface patch suggest that the dimer is the RNA-binding form of sigma3.

Mutant cells selected during persistent reovirus infection do not express mature cathepsin L and do not support reovirus disassembly

Persistent reovirus infections of murine L929 cells select cellular mutations that inhibit viral disassembly within the endocytic pathway. Mutant cells support reovirus growth when infection is initiated with infectious subvirion particles (ISVPs), which are intermediates in reovirus disassembly formed following proteolysis of viral outer-capsid proteins. However, mutant cells do not support growth of virions, indicating that these cells have a defect in virion-to-ISVP processing. To better understand mechanisms by which viruses use the endocytic pathway to enter cells, we defined steps in reovirus replication blocked in mutant cells selected during persistent infection. Subcellular localization of reovirus after adsorption to parental and mutant cells was assessed using confocal microscopy and virions conjugated to a fluorescent probe. Parental and mutant cells did not differ in the capacity to internalize virions or distribute them to perinuclear compartments. Using pH-sensitive probes, the intravesicular pH was determined and found to be equivalent in parental and mutant cells. In both cell types, virions localized to acidified intracellular organelles. The capacity of parental and mutant cells to support proteolysis of reovirus virions was assessed by monitoring the appearance of disassembly intermediates following adsorption of radiolabeled viral particles. Within 2 h after adsorption to parental cells, proteolysis of viral outer-capsid proteins was observed, consistent with formation of ISVPs. However, in mutant cells, no proteolysis of viral proteins was detected up to 8 h postadsorption. Since treatment of cells with E64, an inhibitor of cysteine-containing proteases, blocks reovirus disassembly, we used immunoblot analysis to assess the expression of cathepsin L, a lysosomal cysteine protease. In contrast to parental cells, mutant cells did not express the mature, proteolytically active form of the enzyme. The defect in cathepsin L maturation was not associated with mutations in procathepsin L mRNA, was not complemented by procathepsin L overexpression, and did not affect the maturation of cathepsin B, another lysosomal cysteine protease. These findings indicate that persistent reovirus infections select cellular mutations that affect the maturation of cathepsin L and suggest that alterations in the expression of lysosomal proteases can modulate viral cytopathicity.

Reovirus virion-like particles obtained by recoating infectious subvirion particles with baculovirus-expressed sigma3 protein: an approach for analyzing sigma3 functions during virus entry

Structure-function studies with mammalian reoviruses have been limited by the lack of a reverse-genetic system for engineering mutations into the viral genome. To circumvent this limitation in a partial way for the major outer-capsid protein sigma3, we obtained in vitro assembly of large numbers of virion-like particles by binding baculovirus-expressed sigma3 protein to infectious subvirion particles (ISVPs) that lack sigma3. A level of sigma3 binding approaching 100% of that in native virions was routinely achieved. The sigma3 coat in these recoated ISVPs (rcISVPs) appeared very similar to that in virions by electron microscopy and three-dimensional image reconstruction. rcISVPs retained full infectivity in murine L cells, allowing their use to study sigma3 functions in virus entry. Upon infection, rcISVPs behaved identically to virions in showing an extended lag phase prior to exponential growth and in being inhibited from entering cells by either the weak base NH4Cl or the cysteine proteinase inhibitor E-64. rcISVPs also mimicked virions in being incapable of in vitro activation to mediate lysis of erythrocytes and transcription of the viral mRNAs. Last, rcISVPs behaved like virions in showing minor loss of infectivity at 52 degrees C. Since rcISVPs contain virion-like levels of sigma3 but contain outer-capsid protein mu1/mu1C mostly cleaved at the delta-phi junction as in ISVPs, the fact that rcISVPs behaved like virions (and not ISVPs) in all of the assays that we performed suggests that sigma3, and not the delta-phi cleavage of mu1/mu1C, determines the observed differences in behavior between virions and ISVPs. To demonstrate the applicability of rcISVPs for genetic studies of protein functions in reovirus entry (an approach that we call recoating genetics), we used chimeric sigma3 proteins to localize the primary determinants of a strain-dependent difference in sigma3 cleavage rate to a carboxy-terminal region of the ISVP-bound protein.

No role for pepstatin-A-sensitive acidic proteinases in reovirus infections of L or MDCK cells

Strong evidence indicates that virions of mammalian reoviruses undergo proteolytic processing by acid-dependent cellular proteinases as an essential step in productive infection. Proteolytic processing takes the form of a series of cleavages of outer-capsid proteins final sigma3 and mu1/mu1C. Previous studies showed an effect of both NH4Cl and E-64 on these cleavages, indicating that one or more of the acid-dependent cysteine proteinases in mammalian cells (cathepsins B and L, for example) is required; however, these studies did not address whether acid-dependent aspartic proteinases in those cells (cathepsin D, for example) may also be required. To determine the role of aspartic proteinases in reovirus entry, studies with pepstatin A, a specific inhibitor of aspartic proteinases, were performed. The results showed that pepstatin A neither blocks nor slows reovirus infection of L or MDCK cells. Experiments using ribonuclease A and other proteins as cleavable substrates showed that cathepsin-D-like proteinases from these cells are inhibited within the tested range of pepstatin A concentrations both in vitro and within living cells. In other experiments, virion-bound final sigma3 protein was shown to be a poor substrate for cleavage by cathepsin D in vitro, consistent with the findings with inhibitors. In sum, the data indicate that cathepsin-D-like aspartic proteinases provide little or no activity toward proteolytic events required for infection of L or MDCK cells with reovirus virions.

Protease cleavage of reovirus capsid protein mu1/mu1C is blocked by alkyl sulfate detergents, yielding a new type of infectious subvirion particle

Mammalian reovirus virions undergo partial disassembly of the outer capsid upon exposure to proteases in vitro, producing infectious subvirion particles (ISVPs) that lack protein sigma3 and contain protein mu1/mu1C as endoprotease-generated fragments mu1delta/delta and phi. ISVPs are thought to be required for two early steps in reovirus infection: membrane penetration and activation of the particle-bound viral transcriptase complexes. Genetic and biochemical evidence implicates outer-capsid protein mu1 in both these steps. To determine whether the cleavage of mu1/mu1C is relevant to the unique properties of ISVPs, we analyzed the properties of novel subvirion particles that lacked sigma3 yet retained mu1/mu1C in an uncleaved but cleavable form. These detergent-plus-protease subvirion particles (dpSVPs) were produced by treating virions with chymotrypsin in the presence of micelle-forming concentrations of alkyl sulfate detergents. Infections with dpSVPs in murine L or canine MDCK cells provided evidence that the cleavage of mu1/mu1C during viral entry into these cells is dispensable for reovirus infection. Additionally, dpSVPs behaved like ISVPs in their capacity to permeabilize lipid bilayers and to undergo transcriptase activation in vitro, supporting the conclusion that cleavage of mu1/mu1C to mu1delta/delta and phi during viral entry is not required for either membrane penetration or transcriptase activation in cells. The capacity of alkyl sulfate detergents to inhibit the cleavage of mu1/mu1C in a reversible fashion suggests a specific association between virus particle and detergent micelles that may mimic virus particle-phospholipid membrane interactions during reovirus entry into cells.

Identification and characterization of a double-stranded RNA- reovirus temperature-sensitive mutant defective in minor core protein mu2

A newly identified temperature-sensitive mutant whose defect was mapped to the reovirus M1 gene (minor core protein mu2) was studied to better understand the functions of this virion protein. Sequence determination of the Ml gene of this mutant (tsH11.2) revealed a predicted methionine-to-threonine alteration at amino acid 399 and a change from proline to histidine at amino acid 414. The mutant made normal amounts of single-stranded RNA, both in in vitro transcriptase assays and in infected cells, and normal amounts of progeny viral protein at early times in a restrictive infection. However, tsH11.2 produced neither detectable progeny protein nor double-stranded RNA at late times in a restrictive infection. These studies indicate that mu2 plays a role in the conversion of reovirus mRNA to progeny double-stranded RNA.

Intracellular amplification and expression of a synthetic analog of rotavirus genomic RNA bearing a foreign marker gene: mapping cis-acting nucleotides in the 3'-noncoding region

cDNAs were constructed to encode plus- or minus-sense analogs of gene 9 RNA of porcine rotavirus strain OSU in which the bacterial chloramphenicol acetyltransferase (CAT) reporter gene was flanked by the 5'-terminal 44 nucleotides (nt) and 3'-terminal 35 nt of the authentic rotavirus gene. Transfection of plus-sense gene-9-CAT RNA into rotavirus-infected cells resulted in its amplification and in the efficient expression of CAT; this was greatly enhanced by the presence of a 5' cap structure. Amplification was ablated by omitting the rotavirus superinfection or by removing the 3'-terminal 35-nt rotavirus sequence from the RNA. This result indicated that amplification depended both on rotavirus proteins supplied in trans and on cis-acting rotavirus sequences. Minus-sense or double-stranded gene-9-CAT RNA was essentially inactive, indicating that synthetic RNAs can be introduced into the rotavirus replicative cycle in vivo only when provided in the plus sense. However, incorporation of the CAT-bearing RNA into infectious rotavirus was not detected. Two heterologous rotaviruses, the simian RRV and chicken Ch2 strains, efficiently complemented the OSU-based gene-9-CAT RNA, even though the Ch2 strain was only 50%-66% related in the noncoding regions. Mutational analysis of the 35-nt 3'-noncoding region showed that the 3'-terminal 12 or 17 nt were sufficient for reduced (12% or 23%, respectively) levels of amplification, whereas inclusion of the 3'-terminal 19 nt fully restored amplification. Thus, the 3'-terminal cis-acting signals required for amplification include the 7-nt-terminal consensus sequence together with 12 nt of adjoining, less-well-conserved sequence.

Structure and function of the reovirus genome

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Control of protein synthesis by a temperature-sensitive mutant of reovirus 3. I. Temperature-sensitive function of ts261-b mutant

The ability of a temperature-sensitive (ts) mutant of reovirus, ts261-b, to synthesize virus-specific RNAs and proteins during infection at the nonpermissive temperature (37 degrees C) was investigated. The relative amounts of the mutant virus-specific single-stranded (ss) RNA's and double-stranded (ds) RNA's synthesized in cells at 37 degrees C were 20 to 25% as much as those synthesized in the wild-type virus-infected cells. The 10 segments of the mutant ds RNAs and the three size classes of the ss RNAs were synthesized in the usual proportions. The methylation of the mutant viral mRNA's (ss RNAs) was not blocked at 37 degrees C in infected cells. A striking temperature-sensitive restricted function of the ts261-b mutant was expressed in the synthesis of the viral proteins. This study, which uses an in vitro protein-synthesizing system reconstituted with an endogenous polysomal fraction and a postribosomal supernatant from reovirus-infected cells, has demonstrated that the endogenous polysomes obtained from ts261-b mutant-infected cells at 37 degrees C are not active in the synthesis of the viral polypeptides of known molecular weights, and the amounts of the mutant viral polypeptides synthesized in vitro by these polysomes are 5 to 9% of those synthesized by the corresponding fraction from wild-type-infected cells. The impaired protein-synthesizing capacity of the mutant virus-specific polysomes can be restored during maintenance of the infected cells at 30 degrees C after shift-down from 37 degrees C. The in vitro synthesis of viral polypeptides of known size by the active endogenous polysomes derived from cells infected at the permissive temperature is accelerated by the addition of the postribosomal supernatant obtained from cells infected at the permissive temperature. The postribosomal supernatant from mutant-infected cells at 37 degrees C did not have a stimulatory effect, but rather, it inhibited in vitro viral protein synthesis.

Nonpermissive infection of L cells by an avian reovirus: restricted transcription of the viral genome

Avian reovirus multiples in chicken embryo fibroblasts. Although the avian virus adsorbs to L cells and is uncoated therein, it does not multiply. In the nonpermissive infection of L cells with the avian reovirus only four of the genomic segments of the viral genome are transcribed, L1, M3, S3, and S4, and these are the same segments that have been designated previously as early functions in the permissive infection of L cells with type 3 reovirus. When L cells are co-infected with avian reovirus and type 3 virus all ten segments of the avian viral genome are transcribed, although there is no synthesis of avian viral double-stranded RNA. Type 3 reovirus multiplies almost normally in this mixed infection. The most likely explanation is that a cellular repressor blocks transcription of the six late segments of the avian viral genome and that this repressor is removed by the co-infection with type 3 virus. A second block prevents replication of the viral genome.

Regulated transcription of the genomes of defective virions and temperature-sensitive mutants of reovirus

Defective reovirus, which lacks the largest (L1) of the 10 double-stranded (ds) RNA genomic segments, attaches to L cells and is uncoated in the same way as reovirus. The defective genome does not replicate in the cells, but it is transcribed. During the first 5 h after infection, three of the genomic segments, M3, S3, and S4, are more frequently transcribed than the remaining six segments. During the succeeding 5 h, there is a transition to a situation in which all nine segments are transcribed at the same relative frequencies. Since the class C ts mutation has been allocated to the L1 segment (Spandidos and Graham, 1975) the transcription of the C mutant genome was investigated in cells infected with it at the nonpermissive temperature, at which the parental genome does not replicate. Genomic segments L1, M3, S3, and S4 are predominantly transcribed at early times, and later all 10 segments are transcribed with the same relative frequencies. Transcription of the defective viral genome and the C mutant genome is therefore regulated in the same way as previously found for wild-type virus (Nonoyama, Millward, and Graham, 1974), and the regulation is independent of genome replication. Apparently the L1 segment function is involved in dsRNA synthesis but not in regulating the early to late transcription. It is suggested that a cellular repressor may be involved in this regulation and that derepression might be effected by one of the early viral gene products. Virion transcriptase activity was studied in vitro with cores prepared by chymotrypsin digestion of purified defective and standard virions. For both genomes the relative frequencies of transcription of the dsRNA segments are inversely proportional to their molecular weights. These results can be accounted for in a model that postulates each segment to be transcribed independently of the other. The same model with certain restrictions can describe the in vivo transcription of the viral genome.

Synthesis of reovirus-specific polypeptides in cells pretreated with cycloheximide

When L cells are infected with reovirus in the presence of cycloheximide neither virus-specific polypeptides nor viral double-stranded RNA are synthesized. There is some synthesis of viral single-stranded RNA, transcribed mainly from segments L1, M3, S3, and S4 of the 10 viral genomic segments, and in previous work this has been termed the early mRNA pattern. In an attempt to determine whether these early transcripts are functional mRNA's, the transcripts were allowed to accumulate for a period of 17.5 h at 31 C in cycloheximide-treated cells. The cycloheximide was removed and the cells were exposed for various periods to radioactive amino acids to label any virus-specific polypeptides that might be synthesized. An immunoprecipitation technique was used to separate the viral polypeptides from cellular extracts and this precipitate was then analyzed on sodium dodecyl sulfate-polyacrylamide gels. Within 30 min of cycloheximide removal, four major polypeptides (lambda2, mu0, sigma2a, and sigma3) and two minor polypeptides (lambda1 and mu2) were found. In infected cells without cycloheximide eight viral polypeptides (lambda1, lambda2, mu0, mu2, sigma1, sigma2, sigma2a, sigma3) were found at 17.5 h after infection and the same pattern was found between 3 to 4 h after removal of cycloheximide which had been present for 17.5 h after infection. The latter result shows that the cycloheximide inhibition is reversible and that the cells readily recovered and synthesized the normal complement of viral polypeptides. In one set of experiments cordycepin was added to infected cells immediately after the removal of cycloheximide at 17.5 h to inhibit the synthesis of new viral transcripts. During the succeeding 4 h in the presence of cordycepin, the pattern of protein synthesis was the same as that obtained during the 30 min after cycloheximide removal. It is concluded that the polypeptides formed right after removal of cycloheximide are the translation products of transcripts accumulated during cycloheximide treatment and, therefore, that these transcripts are functional viral mRNA's.

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.

Control of transcription of the reovirus genome

The ten double-stranded RNA segments of the reovirus genome are not transcribed at equal frequencies until later times during the course of infection. When an inhibitor of protein synthesis such as cycloheximide was added to infected cells at the beginning of infection, only four of the ten segments were transcribed. By two hr post infection, five and possibly seven of the segments were being transcribed. By four hr post infection all ten segments were being transcribed but not yet at equal relative frequencies. The transcription pattern at intermediate (4 hr) and late (10 hr) times were verified without using cycloheximide. A repressor present in the host cell may be responsible for controlling transcription of the reovirus genome.

Induction of interferon in L cells by reoviruses

L cell cultures challenged with reoviruses types 1 and 3 produced little to no detectable interferon under conditions which permitted virus replication and under conditions which prevented replication of a temperature-sensitive mutant strain of reovirus type 3 (ts-1). Ultraviolet-irradiated reoviruses and double-stranded ribonucleic acids extracted from purified reovirus type 3 also induced little to no interferon in L cell cultures. Under similar conditions, MM virus proved to be an effective inducer of interferon. Exposure of L cells to interferon prior to challenge with virus (priming) failed to enhance interferon production upon subsequent challenge with reovirus although priming increased the amount of interferon produced following MM virus challenge. L cell cultures that were challenged with reovirus type 3 and subsequently with either MM or Colorado tick fever viruses produced similar titers of interferon as cell cultures that were challenged with either MM or Colorado tick fever virus alone, respectively. These data show that the presence of production of reovirus type 3 double-stranded ribonucleic acid is not sufficient for induction of interferon in L cell cultures and that additional processes which are required for induction of interferon in L cell cultures are not expressed by reovirus type 3.

New intermediate subviral particles in the in vitro uncoating of reovirus virions by chymotrypsin

Reovirus virions, grown in suspension cultures of L cells and extensively purified by density gradient and velocity gradient centrifugation after their release from cell debris by fluorocarbon extraction, are characterized by a mean particle diameter of 73 nm and a density in CsCl of 1.36 to 1.37 g/cm(3). Treatment of intact virions by chymotrypsin (CHT) digestion in vitro converts them to subviral particles (SVP) having characteristics which are determined by the species of monovalent cation present during the digestion. In the presence of Cs(+) ions, CHT converts the virions to SVP of mean diameter 51 nm and density 1.43 to 1.44 g/cm(3). In the presence of K(+) ions, the conversion is to SVP of diameter 51 nm and density 1.39 to 1.40 g/cm(3). The SVP made in the presence of either Cs(+) or K(+) possess an extremely active RNA polymerase and nucleoside triphosphate phosphohydrolase (NTPase) activity in vitro and are resistant to further digestion by CHT. Treatment of intact virions with CHT in the presence of Na(+) or Li(+) ions results in their conversion to SVP of mean diameter 64 nm and density 1.37 to 1.38 g/cm(3). Such SVP are not active in in vitro RNA synthesis or NTP hydrolysis and are resistant to further digestion by CHT even during prolonged exposure to high concentrations of enzyme. Addition of Cs(+) or K(+) ions to the digestion mixture allows conversion of the 64-nm diameter SVP to 51-nm diameter SVP in which the RNA polymerase and NTPase are active in vitro. Analysis of the proteins present in intact virions and in the different SVP reveals clear differences which indicate that the conversions are accomplished by removal or cleavage of particular species of polypeptides.

Reovirus: effect of noninfective viral components on cellular deoxyribonucleic acid synthesis

We determined the effects of noninfective reovirus components on cellular deoxyribonucleic acid (DNA) synthesis. Reovirus inactivated by ultraviolet light inhibited cellular DNA synthesis, whereas reovirus cores and empty capsids did not. Both cores and empty capsids were adsorbed to cells. Adenine-rich ribonucleic acid (RNA) from reovirus, adsorbed to cells in the presence of diethyl-aminoethyl-dextran, produced a partial inhibition of DNA synthesis. RNA was synthesized in the presence of actinomycin D after infection with ultraviolet light-irradiated reovirus, and this RNA synthesis was not due to multiplicity reactivation of virus infectivity. These data suggest that viral structural proteins do not inhibit DNA synthesis and that the inhibition produced by ultraviolet-irradiated virus may be mediated in part or in toto by a newly synthesized viral product.

Extraordinary effects of specific monovalent cations on activation of reovirus transcriptase by chymotrypsin in vitro

Activation of reovirus transcriptase activity, latent in intact virions, by digestion of purified virions with chymotrypsin (CHT) in vitro shows a stringent requirement for specific monovalent cations. Cs(+), Rb(+), or K(+) ions are capable of facilitating activation by chymotryptic digestion. Na(+), Li(+), or NH(4) (+) ions are not capable of facilitating the CHT activation of polymerase activity and are antagonistic towards the effects of the facilitating ions. The data indicate that the effect of the cations is exerted on activation of the polymerase activity by CHT as opposed to an effect on polymerization per se. This effect may be important biologically in that it provides a mechanism whereby the virion can sense whether it is in an intracellular or an extracellular environment and thereby can avoid premature uncoating.

Asynchronous synthesis of the complementary strands of the reovirus genome

The mechanism of replication of the double-stranded RNA genome of reovirus has been analyzed by tracing the fate of the parental double-stranded RNA genome and by determining whether the complementary strands, which comprise the progeny double-stranded RNA, are synthesized simultaneously or sequentially. The results indicate that the parental double-stranded RNA is conserved as the original duplex molecule within a subviral particle throughout the viral replicative cycle. The complementary strands, which form the progeny double-stranded RNA, are produced asynchronously. Minus strands are synthesized on preformed plus-strand templates, whereas plus strands appear to be synthesized on double-stranded RNA templates.