Essential and nonessential noncapsid reovirus proteins

Detection of mammalian orthoreovirus type-3 (Reo-3) infections in mice based on serotype-specific hemagglutination protein sigma-1

Background

Reovirus type-3 infections cause severe pathologies in young mice and thus influence animal experiments in many ways. Therefore, the Federation of Laboratory Animal Science Associations (FELASA) recommends an annual screening in laboratory mice as part of a thorough health monitoring program. Based on the high protein sequence homology among the different reovirus serotypes, immunofluorescence antibody assay and other indirect methods relying on the whole virus are presumably cross-reactive to antibodies triggered by mammalian orthoreovirus infections independent of the serotype.

Methods

The serotype-specific protein σ-1 was expressed in Escherichia coli with an N-terminal Strep-tag and a C-terminal His-tag. The purified Strep-rσ-1-His-construct was used to develop an indirect ELISA by testing defined positive and negative sera obtained by experimental infection of mice as well as field sera.

Results

The Strep-rσ-1-His-ELISA provided high sensitivity and specificity during validation. Notably, a high selectivity was also observed for sera positively tested for other relevant FELASA-listed pathogens. Screening of field samples indicated that a commercial reovirus type-3-based ELISA might be cross-reactive to other murine reovirus serotypes and thus produces false-positive results.

Conclusions

The prevalence of reovirus type-3 might be overestimated in German animal facilities and most likely in other countries as well. The occurrence of other reovirus serotypes, however, raises the question if murine health monitoring programs should be extended to these pathogens.

Electronic supplementary material

The online version of this article (10.1186/s12985-018-1021-8) contains supplementary material, which is available to authorized users.

Reovirus Nonstructural Protein σNS Acts as an RNA Stability Factor Promoting Viral Genome Replication

Viral nonstructural proteins, which are not packaged into virions, are essential for the replication of most viruses. Reovirus, a nonenveloped, double-stranded RNA (dsRNA) virus, encodes three nonstructural proteins that are required for viral replication and dissemination in the host. The reovirus nonstructural protein σNS is a single-stranded RNA (ssRNA)-binding protein that must be expressed in infected cells for production of viral progeny. However, the activities of σNS during individual steps of the reovirus replication cycle are poorly understood. We explored the function of σNS by disrupting its expression during infection using cells expressing a small interfering RNA (siRNA) targeting the σNS-encoding S3 gene and found that σNS is required for viral genome replication. Using complementary biochemical assays, we determined that σNS forms complexes with viral and nonviral RNAs. We also discovered, using in vitro and cell-based RNA degradation experiments, that σNS increases the RNA half-life. Cryo-electron microscopy revealed that σNS and ssRNAs organize into long, filamentous structures. Collectively, our findings indicate that σNS functions as an RNA-binding protein that increases the viral RNA half-life. These results suggest that σNS forms RNA-protein complexes in preparation for genome replication.IMPORTANCE Following infection, viruses synthesize nonstructural proteins that mediate viral replication and promote dissemination. Viruses from the family Reoviridae encode nonstructural proteins that are required for the formation of progeny viruses. Although nonstructural proteins of different viruses in the family Reoviridae diverge in primary sequence, they are functionally homologous and appear to facilitate conserved mechanisms of dsRNA virus replication. Using in vitro and cell culture approaches, we found that the mammalian reovirus nonstructural protein σNS binds and stabilizes viral RNA and is required for genome synthesis. This work contributes new knowledge about basic mechanisms of dsRNA virus replication and provides a foundation for future studies to determine how viruses in the family Reoviridae assort and replicate their genomes.

Genetic Analysis of the Nonstructural (NS) Genes of H9N2 Chicken Influenza Viruses Isolated in China During 1998–2002

H9N2 subtype avian influenza viruses are widespread in domestic poultry. Genetic analysis indicated that three lineages of H9N2 viruses have been established in Eurasia and only one lineage is present on chicken farms in mainland China. Here, NS1 genes of eight H9N2 chicken influenza viruses, isolated in mainland China during 1998-2002, were completely sequenced and phylogenetically analyzed. By comparison, the homology of the NS1 of the A/chicken/Neimenggu/ZH/02 (Ck/NM/ZH/02) strain had a high identity (93.8%) with that of A/chicken/Korea/323/96 (Ck/Kor/323/96), which is an A/duck/Hong Kong/Y439/97 (Dk/HK/Y439/97)-like virus. NS1 peptides of seven strains possessed 217 amino acids, while that of the strain Ck/NM/ZH/02 coded 230 amino acids. Except for the amino acid at position 225, the additional amino acid sequence (13 AAs) of NS1 of Ck/NM/Zh/02 at the carboxy-terminus is identical with that of Ck/Kor/323/96. Phylogenetic analysis showed that seven of the tested strains belong to the A/duck/Hong Kong/Y280/97 (DK/HK/Y280/97)-like lineage, while the NS1 gene of Ck/NM/Zh/02 belongs to the Dk/HK/Y439/97-like lineage and has a close relationship with that of the Ck/Kor/323/96-like viruses. Therefore, although most of the H9N2 influenza viruses circulating on chicken farms in mainland China belong to the DK/HK/Y280/97-like lineage, the present results indicate that the other two of the three H9N2 lineage viruses also circulate in the chicken population in mainland China.

Carboxyl-proximal regions of reovirus nonstructural protein muNS necessary and sufficient for forming factory-like inclusions

Mammalian orthoreoviruses are believed to replicate in distinctive, cytoplasmic inclusion bodies, commonly called viral factories or viroplasms. The viral nonstructural protein muNS has been implicated in forming the matrix of these structures, as well as in recruiting other components to them for putative roles in genome replication and particle assembly. In this study, we sought to identify the regions of muNS that are involved in forming factory-like inclusions in transfected cells in the absence of infection or other viral proteins. Sequences in the carboxyl-terminal one-third of the 721-residue muNS protein were linked to this activity. Deletion of as few as eight residues from the carboxyl terminus of muNS resulted in loss of inclusion formation, suggesting that some portion of these residues is required for the phenotype. A region spanning residues 471 to 721 of muNS was the smallest one shown to be sufficient for forming factory-like inclusions. The region from positions 471 to 721 (471-721 region) includes both of two previously predicted coiled-coil segments in muNS, suggesting that one or both of these segments may also be required for inclusion formation. Deletion of the more amino-terminal one of the two predicted coiled-coil segments from the 471-721 region resulted in loss of the phenotype, although replacement of this segment with Aequorea victoria green fluorescent protein, which is known to weakly dimerize, largely restored inclusion formation. Sequences between the two predicted coiled-coil segments were also required for forming factory-like inclusions, and mutation of either one His residue (His570) or one Cys residue (Cys572) within these sequences disrupted the phenotype. The His and Cys residues are part of a small consensus motif that is conserved across muNS homologs from avian orthoreoviruses and aquareoviruses, suggesting this motif may have a common function in these related viruses. The inclusion-forming 471-721 region of muNS was shown to provide a useful platform for the presentation of peptides for studies of protein-protein association through colocalization to factory-like inclusions in transfected cells.

Reovirus sigma NS and mu NS proteins form cytoplasmic inclusion structures in the absence of viral infection

Reovirus replication occurs in the cytoplasm of infected cells and culminates in the formation of crystalline arrays of progeny virions within viral inclusions. Two viral nonstructural proteins, sigma NS and micro NS, and structural protein sigma 3 form protein-RNA complexes early in reovirus infection. To better understand the minimal requirements of viral inclusion formation, we expressed sigma NS, mu NS, and sigma 3 alone and in combination in the absence of viral infection. In contrast to its concentration in inclusion structures during reovirus replication, sigma NS expressed in cells in the absence of infection is distributed diffusely throughout the cytoplasm and does not form structures that resemble viral inclusions. Expressed sigma NS is functional as it complements the defect in temperature-sensitive, sigma NS-mutant virus tsE320. In both transfected and infected cells, mu NS is found in punctate cytoplasmic structures and sigma 3 is distributed diffusely in the cytoplasm and the nucleus. The subcellular localization of mu NS and sigma 3 is not altered when the proteins are expressed together or with sigma NS. However, when expressed with micro NS, sigma NS colocalizes with mu NS to punctate structures similar in morphology to inclusion structures observed early in viral replication. During reovirus infection, both sigma NS and mu NS are detectable 4 h after adsorption and colocalize to punctate structures throughout the viral life cycle. In concordance with these results, sigma NS interacts with mu NS in a yeast two-hybrid assay and by coimmunoprecipitation analysis. These data suggest that sigma NS and mu NS are the minimal viral components required to form inclusions, which then recruit other reovirus proteins and RNA to initiate viral genome replication.

Mammalian reovirus nonstructural protein microNS forms large inclusions and colocalizes with reovirus microtubule-associated protein micro2 in transfected cells

Cells infected with mammalian orthoreoviruses contain large cytoplasmic phase-dense inclusions believed to be the sites of viral replication and assembly, but the morphogenesis, structure, and specific functions of these "viral factories" are poorly understood. Using immunofluorescence microscopy, we found that reovirus nonstructural protein microNS expressed in transfected cells forms inclusions that resemble the globular viral factories formed in cells infected with reovirus strain type 3 Dearing from our laboratory (T3D(N)). In the transfected cells, the formation of microNS large globular perinuclear inclusions was dependent on the microtubule network, as demonstrated by the appearance of many smaller microNS globular inclusions dispersed throughout the cytoplasm after treatment with the microtubule-depolymerizing drug nocodazole. Coexpression of microNS and reovirus protein micro2 from a different strain, type 1 Lang (T1L), which forms filamentous viral factories, altered the distributions of both proteins. In cotransfected cells, the two proteins colocalized in thick filamentous structures. After nocodazole treatment, many small dispersed globular inclusions containing microNS and micro2 were seen, demonstrating that the microtubule network is required for the formation of the filamentous structures. When coexpressed, the micro2 protein from T3D(N) also colocalized with microNS, but in globular inclusions rather than filamentous structures. The morphology difference between the globular inclusions containing microNS and micro2 protein from T3D(N) and the filamentous structures containing microNS and micro2 protein from T1L in cotransfected cells mimicked the morphology difference between globular and filamentous factories in reovirus-infected cells, which is determined by the micro2-encoding M1 genome segment. We found that the first 40 amino acids of microNS are required for colocalization with micro2 but not for inclusion formation. Similarly, a fusion of microNS amino acids 1 to 41 to green fluorescent protein was sufficient for colocalization with the micro2 protein from T1L but not for inclusion formation. These observations suggest a functional difference between microNS and microNSC, a smaller form of the protein that is present in infected cells and that is missing amino acids from the amino terminus of microNS. The capacity of microNS to form inclusions and to colocalize with micro2 in transfected cells suggests a key role for microNS in forming viral factories in reovirus-infected cells.

Reovirus nonstructural protein muNS binds to core particles but does not inhibit their transcription and capping activities

Previous studies provided evidence that nonstructural protein muNS of mammalian reoviruses is present in particle assembly intermediates isolated from infected cells. Morgan and Zweerink (Virology 68:455-466, 1975) showed that a subset of these intermediates, which can synthesize the viral plus strand RNA transcripts in vitro, comprise core-like particles plus large amounts of muNS. Given the possible role of muNS in particle assembly and/or transcription implied by those findings, we tested whether recombinant muNS can bind to cores in vitro. The muNS protein bound to cores, but not to two particle forms, virions and intermediate subvirion particles, that contain additional outer-capsid proteins. Incubating cores with increasing amounts of muNS resulted in particle complexes of progressively decreasing buoyant density, approaching the density of protein alone when very large amounts of muNS were bound. Thus, the muNS-core interaction did not exhibit saturation or a defined stoichiometry. Negative-stain electron microscopy of the muNS-bound cores revealed that the cores were intact and linked together in large complexes by an amorphous density, which we ascribe to muNS. The muNS-core complexes retained the capacity to synthesize the viral plus strand transcripts as well as the capacity to add methylated caps to the 5' ends of the transcripts. In vitro competition assays showed that mixing muNS with cores greatly reduced the formation of recoated cores by stoichiometric binding of outer-capsid proteins mu1 and sigma3. These findings are consistent with the presence of muNS in transcriptase particles as described previously and suggest that, by binding to cores in the infected cell, muNS may block or delay outer-capsid assembly and allow continued transcription by these particles.

Mammalian reovirus M3 gene sequences and conservation of coiled-coil motifs near the carboxyl terminus of the microNS protein

Nucleotide sequences of the mammalian orthoreovirus (reovirus) type 1 Lang and type 2 Jones M3 gene segments were newly determined. The nucleotide sequence of the reovirus type 3 Dearing M3 segment also was determined to compare with a previously reported M3 sequence for that isolate. Comparisons showed Lang and Dearing M3 to be more closely related than either was to Jones M3, consistent with previous findings for other reovirus gene segments. The microNS protein sequences deduced from each M3 segment were shown to be related in a similar pattern as the respective nucleotide sequences and to contain several regions of greater or less than average variability among the three isolates. Identification of conserved methionine codons near the 5' ends of the Lang, Jones, and Dearing M3 plus strands lent support to the hypothesis that microNSC, a smaller protein also encoded by M3, arises by translation initiation from a downstream methionine codon within the same open reading frame as microNS. Other analyses of the deduced protein sequences indicated that regions within the carboxyl-terminal third of microNS and microNSC from each isolate have a propensity to form alpha-helical coiled coils, most likely coiled-coil dimers. The new sequences will augment further studies on microNS and microNSC structure and function.

Synthesis in Escherichia coli of the reovirus nonstructural protein sigma NS

The coding region of reovirus type 3 genomic segment S3, encoding the nonstructural protein sigma NS, was placed under the control of the bacteriophage lambda pL promoter in the Escherichia coli expression plasmid pRC23 (J.C. Lacal, E. Santos, V. Notario, M. Barbacid, S. Yamazaki, H.-F. Kung, C. Seamans, S. McAndrew, and R. Crowl, Proc. Natl. Acad. Sci. USA 81:5305-5309). Derepression of the pL promoter led to the synthesis of a protein of the same molecular weight as sigma NS produced in reovirus-infected L cells. The expressed protein was indistinguishable from authentic sigma NS by peptide mapping with Staphylococcus aureus V8 protease and by immunoblot analysis. Most importantly, the purified protein had nucleic acid-binding properties similar to that previously shown for sigma NS obtained from infected cells. Binding of single-stranded RNAs by recombinant sigma NS protein was inhibited by GTP.

Biosynthesis of reovirus-specified polypeptides. The s1 mRNA synthesized in vivo is structurally and functionally indistinguishable from in vitro-synthesized s1 mRNA and encodes two polypeptides, sigma 1a and sigma 1bNS

The structural and functional properties of the reovirus serotype 1 (Lang strain) s1 mRNA were examined. Reovirus s-class mRNAs, synthesized either in vivo within infected mouse L cells or in vitro by chymotrypsin-derived cores of purified virions, were purified by filter-hybridization using cDNA clones of the S-class genome segments. S1 cDNA-selected mRNA encoded the synthesis of the Mr approximately 12,000 nonstructural polypeptide designated sigma 1bNS in addition to the well-established structural polypeptide sigma 1, now designated sigma 1a. The coding properties of in vivo- and in vitro-synthesized s1 mRNA were equivalent: both encoded sigma 1a and sigma 1bNS. Primer extension analysis of s1 mRNA revealed a single major 5' terminus for both in vivo- and in vitro-synthesized s1 mRNA. These results suggest that there is a single transcript of the reovirus S1 genome segment which is functionally dicistronic, and likely encodes both sigma 1a and sigma 1bNS.

Identification of a new polypeptide coded by reovirus gene S1

The reovirus S1 gene has recently been shown potentially to encode two polypeptides (from two overlapping reading frames) having predicted molecular weights of 49,071 and 16,143 (Nagata et al., Nucleic Acids Res. 12:8699-8710, 1984; Bassel-Duby et al., Nature [London], in press). The larger polypeptide is reovirus protein sigma 1, but synthesis of the smaller polypeptide has not been described to date. A truncated clone of the S1 gene in which the first ATG is deleted was expressed in an in vitro protein synthesis system to yield a approximately 13-kilodalton polypeptide, as determined from migration on sodium dodecyl sulfate-polyacrylamide gels. A polypeptide with a similar migration pattern on sodium dodecyl sulfate-polyacrylamide gels was present in reovirus-infected cells and absent from mock-infected cells. Comparative tryptic peptide analysis of the 13-kilodalton polypeptides produced in vivo and in vitro showed them to be identical. Thus, the s1 mRNA of reovirus type 3 is apparently bicistronic, and we suggest that the approximately 13-kilodalton polypeptide be called sigma s (standing for sigma small).

Sequences of the S1 genes of the three serotypes of reovirus

The S1 genes of the three serotypes of reovirus have been cloned and sequenced. The S1 genes encode protein sigma 1, the protein against which serotype-specific neutralizing antibodies are directed; it is also the reovirus hemagglutinin and cell-attachment protein and is a major determinant of host range/tissue specificity and of the nature of the interaction of reovirus with cells of the immune system. The S1 genes of serotypes 1, 2, and 3 are 1458, 1442, and 1416 nucleotides long, respectively. They possess untranslated regions 13, 13, and 12 nucleotides long at their 5' termini and 188, 229, and 36 nucleotides long at their 3' termini. They possess two open reading frames. The first starts with a "weak" initiation codon and extends for 418, 399, and 455 codons, respectively; this is the size expected for the sigma 1 proteins. The other reading frame starts at a "strong" initiation codon about 70 residues downstream from the 5' terminus but extends for only about 120 codons, being terminated by 3 in-phase termination codons in all three genes. The proteins encoded by these short open reading frames are quite basic. The serotype 1 and 2 S1 genes are much more closely related to each other (28% homology) than to the serotype 3 S1 gene (5% and 9% homology, respectively). These figures are based on direct homology calculations, adjusted for 25% random coincidence. Serologic evidence and hydrophobicity profiles agree that the sigma 1 proteins of serotypes 1 and 2 are much more closely related to each other (about 40% homology) than to that of serotype 3 (only about 20% homology). The fact that the serotype 1 and 2 S1 genes are much more closely related to each other than to the serotype 3 S1 gene is remarkable since for all other nine reovirus genes the serotype 1 and 3 genes are much more closely related to each other than to the serotype 2 gene. Mechanisms that may effect this remarkable evolutionary pattern are discussed.

Genetic and molecular mechanisms of viral pathogenesis: implications for prevention and treatment

The pathogenesis of infection of mice by the mammalian reoviruses involves several discrete steps. Each of the three viral outer capsid proteins has a highly distinct and specialized role: one protein (sigma 1) binds to cell surface receptors; a second protein (mu 1C) determines the capacity for viral growth at mucosal surfaces; and the third protein (sigma 3) is responsible for inhibiting cell macromolecular synthesis. A detailed picture of the molecular basis of reovirus virulence and attention is now emerging.

Avian reovirus polypeptides: analysis of intracellular virus-specified products, virions, top component, and cores

Avian reovirus-specified polypeptides can be separated into three size classes: large (lambda), medium (mu), and small (sigma), similar to those of the mammalian reoviruses. A nomenclature has been proposed to indicate the individual polypeptides within each size class by progressive alphabetical subscripts. Three lambda polypeptides (lambda(A), lambda(B), and lambda(C)) are found in infectious viral particles and have molecular weights of 145,000, 130,000, and 115,000, respectively. All are present in core preparations, and two (lambda(A) and lambda(B)) appear to be exposed at the surface of the virion. Two mu polypeptides can be distinguished in purified virus (mu(A), 72,000 daltons; mu(B), 70,000 daltons), and another is occasionally evident by immunoprecipitation from infected-cell extracts (mu(NS)). mu(B) represents the major outer capsid protein and is structurally homologous to mu(1C) of the mammalian reoviruses. No additional mu proteins can be detected, and there is no evidence for a product-precursor relationship among these proteins. Three major sigma proteins are evident in viral particles. sigma(C) has the lowest molecular weight, is part of the outer capsid of the virion, and appears homologous to the mammalian sigma(1) protein. Interestingly, it demonstrates the greatest polymorphism of all the polypeptides among the different avian reoviruses examined. sigma(B) (36,000 daltons) is a major constituent of the outer capsid and, like sigma(C), is exposed to the surface of the virion. sigma(A) (39,000 daltons) appears to be an internal protein. An additional polypeptide band in the sigma class having an apparent molecular weight of 34,000 to 35,000 can be seen under three different conditions: (i) in some S1133 reovirus preparations, particularly after prolonged storage, a new band (sigma(B')) appears with a reduction in intensity of sigma(B), suggesting that sigma(B') is a degradation product of sigma(B); (ii) in polypeptides immunoprecipitated from infected-cell extracts, a major band (sigma(NS)) is apparent migrating just ahead of sigma(B); (iii) in top component preparations from all avian reoviruses examined, a band (sigma(TC)) with mobility identical to that of sigma(NS) represents a major constitutent and appears to be incorporated within the particle itself. The relationship among these three bands is not currently known. Avian reovirus polypeptides are thus in general similar to those found in mammalian reoviruses, but differences do exist which may be important for understanding viral structure and assembly.

Tryptic peptide analysis of outer capsid polypeptides of mammalian reovirus serotypes 1, 2, and 3

We studied the structural relationships among the outer capsid polypeptides of prototype strains of mammalian reovirus serotypes 1, 2, and 3 by tryptic peptide mapping. The micron1C polypeptide showed an extraordinary degree of conservation of its methionine-containing tryptic peptides. In contrast, the most abundant viral polypeptide, sigma 3, contained both conserved and unique methionine-containing tryptic peptides. The viral type-specific antigen, the sigma 1 polypeptide, contained both conserved and unique methionine- and tyrosine-containing tryptic peptides. These results suggested that the mammalian reovirus genome segments encoding each of the viral outer capsid polypeptides were derived from common ancestral segments which have diverged to different degrees.

Subunit structure of the reovirus spike

Cross-linking reovirus spike protein with the bifunctional reagent dimethyl suberimidate revealed that each spike was composed of a pentameric aggregate of polypeptide lambda 2.

Genome RNAs and polypeptides of reovirus serotypes 1, 2, and 3

The virus-specific double-stranded genome RNA and polypeptides present in virions and cells infected with the three mammalian reovirus serotypes have been examined by co-electrophoresis in several different polyacrylamide gel systems. The double-stranded RNA and polypeptide species previously described for type 3 Dearing were found to have corresponding species in the other serotypes examined. In each serotype several RNA and polypeptide species were found to have different electrophoretic mobilities from the corresponding RNA or polypeptide species of type 3 Dearing. The combination of electrophoretic variants among the RNAs and polypeptides of the reovirus serotypes gave electrophoretic markers in all 10 of the reovirus genes. The usefulness of these electrophoretic markers in "mapping" the reovirus genome is discussed.

Reovirus-specific polypeptides: analysis using discontinuous gel electrophoresis

The electrophoretic analysis of reovirus-specific polypeptides in infected cells using a discontinuous gel system has allowed the resolution of additional viral-specific polypeptides, including one large-sized gamma3 and two (or possibly three) medium-sized (mu3, mu4, mu5(?)) species. The proteins designated mu0, sigma1, and sigma2 based on electrophoretic mobility in gel systems containing phosphate-urea correspond to mu4, sigma2, and sigma1, respectively, when analyzed in systems containing Tris-glycine. It is likely that protein modifications (phosphorylation and glycosylation) are responsible for at least some of these differences.

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.

Complementation of defective reovirus by ts mutants

Defective reovirions lacking the largest (L-1) of the normal 10 genomic segments grow only in association with helper reovirus. Because of the similarity in properties of defective and infectious virions, separation of the two populations by physical methods has been unseccessful. Controlled digestion of purified virus removes the outer capsomeres of the virions. The resulting core particles containing the viral genome have a buoyant density of 1.43/ml if derived from infectious virions and of 1.415g/ml if they originate in defectives, and this difference permits ready separation of the two types of cores. With the purpose of obtaining a pure population of defective virions, L cells were co-infected with defective cores and a class E temperature-sensitive mutant which has a mutation in an early function. After three serial passages at the permissive temperature (31 C) to build up the defective population, a fourth passage was made at 39 C, the nonpermissive temperature. The virus purified from this passage was predominantly defective; it contained practically no E mutant and had a low background of wild-type virus. Complementation was thus asymmetric; the L-1 function required for growth of defective virus was supplied by the E mutant and is thus a trans-function, while defective virus did not complement the E mutation which is thus in a cis-acting function. Defective virions were indistinguishable from infectious virions except for the absence of the L-1 genomic segment in the defectives. Such defective virions could be complemented at 39 C by class A and B temperature-sensitive mutants, both of which have lesions in late functions.

Synthesis of all the gene products of the reovirus genome in vivo and in vitro

Sixteen virus-specific polypeptides have been resolved in reovirus-infected mouse L cells by using SDS-polyacrylamide slab gel electrophoresis and autoradiography. Of these, ten have been designated as primary products of the genome by the following criteria: they are present in lysates of infected cells labeled for a short time; they co-migrate on SDS-polyacrylamide slab gels with polypeptides synthesized in cell-free-extracts of wheat germ in response to purified viral mRNA; and their molecular weights correspond to the values expected if all ten reovirus mRNA species are monocistronic. Reovirus mRNA species lack 3' poly(A) but are translated into proteins of the expected size. The pattern of synthesis of the primary gene products observed in vitro mimicks that observed in reovirus-infected cells suggesting that the structure of the mRNA may profoundly influence its translation. The results further indicate that there is little, if any, exclusively regulatory information in the reovirus genome since both in vivo and in vitro, transcripts of the ten genome segments direct the synthesis of ten polypeptides that presumably correspond to the primary gene products. The expression of the reovirus genome thus appears to be complete.

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.