Four base-specific nucleoside 5'-triphosphatases in the subviral core of reovirus

Captivating Perplexities of Spinareovirinae 5' RNA Caps

RNAs with methylated cap structures are present throughout multiple domains of life. Given that cap structures play a myriad of important roles beyond translation, such as stability and immune recognition, it is not surprising that viruses have adopted RNA capping processes for their own benefit throughout co-evolution with their hosts. In fact, that RNAs are capped was first discovered in a member of the Spinareovirinae family, Cypovirus, before these findings were translated to other domains of life. This review revisits long-past knowledge and recent studies on RNA capping among members of Spinareovirinae to help elucidate the perplex processes of RNA capping and functions of RNA cap structures during Spinareovirinae infection. The review brings to light the many uncertainties that remain about the precise capping status, enzymes that facilitate specific steps of capping, and the functions of RNA caps during Spinareovirinae replication.

Effects of viscogens on RNA transcription inside reovirus particles

The dsRNA genome of mammalian reovirus (MRV), like the dsDNA genomes of herpesviruses and many bacteriophages, is packed inside its icosahedral capsid in liquid-crystalline form, with concentrations near or more than 400 mg/ml. Viscosity in such environments must be high, but the relevance of viscosity for the macromolecular processes occurring there remains poorly characterized. Here, we describe the use of simple viscogens, glycerol and sucrose, to examine their effects on RNA transcription inside MRV core particles. Transcription inside MRV cores was strongly inhibited by these agents and to a greater extent than either predicted by theory or exhibited by a nonencapsidated transcriptase, suggesting that RNA transcription inside MRV cores is unusually sensitive to viscogen effects. The elongation phase of transcription was found to be a primary target of this inhibition. Similar results were obtained with particles of a second dsRNA virus, rhesus rotavirus, from a divergent taxonomic subfamily. Polymeric viscogens such as polyethylene glycol also inhibited RNA transcription inside MRV cores, but in a size-limited manner, suggesting that diffusion through channels in the MRV core is required for their activity. Modeling of the data suggested that the inherent intracapsid viscosity of both reo- and rotavirus is indeed high, two to three times the viscosity of water. The capacity for quantitative comparisons of intracapsid viscosity and effects of viscogens on macromolecular processes in confined spaces should be similarly informative in other systems.

Assignment of avian reovirus temperature-sensitive mutant recombination groups E, F, and G to genome segments

Avian reoviruses (ARV) are less well understood than their mammalian counterparts. ARV are ubiquitous in commercial poultry and frequently isolated from acutely infected chickens. We previously described isolation of ARV temperature-sensitive (ts) mutants after nitrosoguanidine mutagenesis of wild-type ARV138, their assignment to 7 recombination groups (A-G), and genetic mapping of mutants in groups A-D to specific gene segments. For this study, wild-type serotype ARV176 was crossed with ts mutants tsE158 (Group E), tsF206 (Group F), or tsG247 (Group G) and reassortant progenies analyzed. Reassortant temperature-sensitivities were determined by efficiency of plating at permissive and non-permissive temperatures. Mapping results indicated tsE158, tsF206, and tsG247 mapped to the L1, S4, and L3 genes, respectively, which encode the lambdaA core shell, sigmaNS non-structural, and lambdaC core spike proteins, respectively. Specific amino acid substitutions in each mutant were determined and locations of structural protein alterations were placed within the 3-dimensional structure of homologous mammalian reovirus proteins. Mapping recombination groups E-G marks completion of gene assignments for all seven ts mutant groups previously generated.

Nucleoside and RNA triphosphatase activities of orthoreovirus transcriptase cofactor mu2

The mammalian Orthoreovirus (mORV) core particle is an icosahedral multienzyme complex for viral mRNA synthesis and provides a delimited system for mechanistic studies of that process. Previous genetic results have identified the mORV mu2 protein as a determinant of viral strain differences in the transcriptase and nucleoside triphosphatase activities of cores. New results in this report provided biochemical and genetic evidence that purified mu2 is itself a divalent cation-dependent nucleoside triphosphatase that can remove the 5' gamma-phosphate from RNA as well. Alanine substitutions in a putative nucleotide binding region of mu2 abrogated both functions but did not affect the purification profile of the protein or its known associations with microtubules and mORV microNS protein in vivo. In vitro microtubule binding by purified mu2 was also demonstrated and not affected by the mutations. Purified mu2 was further demonstrated to interact in vitro with the mORV RNA-dependent RNA polymerase, lambda3, and the presence of lambda3 mildly stimulated the triphosphatase activities of mu2. These findings confirm that mu2 is an enzymatic component of the mORV core and may contribute several possible functions to viral mRNA synthesis.

Characterization of the reovirus lambda1 protein RNA 5'-triphosphatase activity

Characterization of the phosphohydrolytic activities of recombinant reovirus lambda1 protein demonstrates that, in addition to the previously reported nucleoside triphosphate phosphohydrolase and helicase activities, the protein also possesses RNA 5'-triphosphatase activity. This activity was absolutely dependent on the presence of a divalent cation, Mg2+ or Mn2+, and specifically removes the 5'-gamma-phosphate at the end of triphosphate-terminated RNAs. Kinetic competition analysis showed that nucleoside triphosphate phosphohydrolase and RNA 5'-triphosphatase reactions are carried out at a common active site. These results strongly support the idea that, in addition to its role as an RNA helicase during transcription of the viral genome, lambda1 also participates during formation of the cap structure at the 5' end of newly synthesized reovirus mRNAs. The lambda1 protein represents only the third RNA triphosphatase whose primary structure is known and the first described in a double-stranded RNA virus.

Core protein mu2 is a second determinant of nucleoside triphosphatase activities by reovirus cores

NTPase activities in mammalian reovirus cores were examined under various conditions that permitted several new differences to be identified between strains type 1 Lang (T1L) and type 3 Dearing (T3D). One difference concerned the ratio (at pH 8.5) of ATP hydrolysis at 50 degrees C to that at 35 degrees C. A genetic analysis using T1L x T3D reassortant viruses implicated the L3 and M1 gene segments in this difference, with M1 influencing ATPase activity most strongly at high temperatures. L3 and M1 encode the core proteins lambda1 and mu2, respectively. Another difference concerned the absolute levels of GTP hydrolysis by cores at 45 degrees C and pH 6.5. A genetic analysis using T1L x T3D reassortants implicated the M1 gene as the sole determinant of this difference. The results of these experiments, coupled with previous findings (S. Noble and M. L. Nibert, J. Virol. 71:2182-2191, 1997), suggest either that a single type of NTPase in cores is strongly influenced by two different core proteins--lambda1 and mu2--or that cores contain two different types of NTPase influenced by the two proteins. The findings appear relevant for understanding the complex functions of reovirus cores in RNA synthesis and capping.

Characterization of the nucleoside triphosphate phosphohydrolase and helicase activities of the reovirus lambda1 protein

Previous studies have shown that the reovirus lambda1 core protein harbors a putative nucleotide-binding motif and exhibits an affinity for nucleic acids. In addition, a nucleoside triphosphate phosphohydrolase activity present in reovirus cores has been recently assigned to lambda1 using gene reassortment analysis. In this study, it was demonstrated that the recombinant lambda1 protein, expressed in the yeast Pichia pastoris, is able to hydrolyze nucleoside 5'-triphosphates or deoxynucleoside 5'-triphosphates. This activity was absolutely dependent on the presence of a divalent cation, Mg2+ or Mn2+. The protein can also unwind double-stranded nucleic acid molecules in the presence of a nucleoside 5'-triphosphate or deoxynucleoside 5'-triphosphate. These results provide the first biochemical evidence that the reovirus lambda1 protein is a nucleoside triphosphate phosphohydrolase/helicase and strongly support the idea that lambda1 participates in transcription of the viral genome.

Characterization of an ATPase activity in reovirus cores and its genetic association with core-shell protein lambda1

A previously identified nucleoside triphosphatase activity in mammalian reovirus cores was further characterized by comparing two reovirus strains whose cores differ in their efficiencies of ATP hydrolysis. In assays using a panel of reassortant viruses derived from these strains, the difference in ATPase activity at standard conditions was genetically associated with viral genome segment L3, encoding protein lambda1, a major constituent of the core shell that possesses sequence motifs characteristic of other ATPases. The ATPase activity of cores was affected by several other reaction components, including temperature, pH, nature and concentration of monovalent and divalent cations, and nature and concentration of anions. A strain difference in the response of core ATPase activity to monovalent acetate salts was also mapped to L3/lambda1 by using reassortant viruses. Experiments with different nucleoside triphosphates demonstrated that ATP is the preferred ribonucleotide substrate for cores of both strains. Other experiments suggested that the ATPase is latent in reovirus virions and infectious subviral particles but undergoes activation during production of cores in close association with the protease-mediated degradation of outer-capsid protein mu1 and its cleavage products, suggesting that mu1 may play a role in regulating the ATPase.

Protein P4 of the bacteriophage phi 6 procapsid has a nucleoside triphosphate-binding site with associated nucleoside triphosphate phosphohydrolase activity

Bacteriophage phi 6 contains three segments of double-stranded RNA. The procapsid consists of proteins P1, P2, P4, and P7, which are encoded by the viral L segment. cDNA copies of this segment have been cloned into plasmids that direct the production of these proteins, which assemble into polyhedral procapsids. These procapsids are capable of packaging plus-sense phi 6 RNA in the presence of nucleoside triphosphate and synthesizing the complementary minus strand to form double-stranded RNA. In this article, we report the presence of a nucleotide-binding site in protein P4. The viral procapsid and nucleocapsid exhibit a nucleoside triphosphate phosphohydrolase activity that converts nucleoside triphosphates into nucleoside diphosphates.

Isolation and enzymatic characterization of protein lambda 2, the reovirus guanylyltransferase

Protein lambda 2 of reovirus serotype 3 has been purified to homogeneity from extracts of cells infected with hybrid vaccinia virus strain WR into whose TK gene of the reovirus L2 genome segment under the control of the CPV ATI protein gene promoter had been inserted. Protein lambda 2 is formed in large amounts (final purification factor about 180) as a monomer that shows no tendency to pentamerize into the reovirus core projections/spikes. Isolated protein lambda 2 is reversibly guanylylated by GTP (that is, it carries out the GTP-PPi exchange reaction) and can transfer the -GMP moiety to GTP to form GppppG, to GDP to form GpppG, and to 5'-pp-terminated RNA to form GpppG- caps. These studies confirm previous studies on reovirus cores that indicated that protein lambda 2 is the reovirus guanylyltransferase. Protein lambda 2 possesses neither nucleoside nor RNA triphosphatase activities, nor methyltransferase activities; thus it is the reovirus capping enzyme, but provides neither the required 5'-ppG-terminated substrate nor does it methylate the cap structure. These must be functions of lambda 2 pentamers or of other individual or complexed components of reovirus cores.

In vitro packaging of the bacteriophage phi 6 ssRNA genomic precursors

Bacteriophage phi 6 contains three segments of double-stranded RNA within a nucleocapsid. Plasmids containing cDNA copies of the large genomic segment direct the synthesis of viral proteins that assemble into procapsids in Escherichia coli or Pseudomonas phaseolicola. These structures are dodecahedral assemblages of proteins P1, P2, P4, and P7. We report in this paper that these particles are capable of packaging viral single-stranded plus-sense RNA in vitro. The packaging reaction requires the presence of ATP or dATP. Synthesis of minus strands takes place within this filled procapsid in the presence of all four nucleoside triphosphates. Packaged ssRNA is found to be protected from added ribonuclease.

Viral RNA polymerases

Recent progress in molecular biological techniques revealed that genomes of animal viruses are complex in structure, for example, with respect to the chemical nature (DNA or RNA), strandedness (double or single), genetic sense (positive or negative), circularity (circle or linear), and so on. In agreement with this complexity in the genome structure, the modes of transcription and replication are various among virus families. The purpose of this article is to review and bring up to date the literature on viral RNA polymerases involved in transcription of animal DNA viruses and in both transcription and replication of RNA viruses. This review shows that the viral RNA polymerases are complex in both structure and function, being composed of multiple subunits and carrying multiple functions. The functions exposed seem to be controlled through structural interconversion.

Reovirus guanylyltransferase is L2 gene product lambda 2

Reovirus guanylyltransferase, studied as a covalent enzyme-GMP intermediate, was used to guanylate appropriate acceptor molecules in vitro to produce authentic cap structures. Guanylyltransferase activity was associated with lambda 2, the 140-kilodalton product of the L2 gene segment of reovirus serotypes 1 and 3.

Structure and function of the reovirus genome

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mRNA capping enzymes are masked in reovirus progeny subviral particles

We examined the enzyme activities associated with progeny subviral particles isolated from L-cells infected with reovirus at 12 h postinfection. Activities normally present in reovirus cores were also found to be present in the progeny subviral particles, with the exception of the capping enzymes. The methylase and guanyl transferase activities, which constitute the capping system, were present in a masked form that could be activated by chymotrypsin digestion. The appearance of these progeny subviral particles in infected cells coincided with the time when mRNA synthesis was maximal, suggesting that viral mRNA synthesized at later times is uncapped.

A simple method for the preparation of [beta-32P]purine nucleoside triphosphase

A rapid, simple and inexpensive procedure is described for the preparation of purine ribo-and deoxyribonucleoside triphosphates specifically and highly radiolabeled with [32P]phosphate in the beta position. The method involves two successive enzymatic reactions: conversion of donor [gamma-32P]ATP in the presence of an excess of acceptor 5'-mononucleotide to the 5'-diphosphates by myokinase or guanosine 5'-monophosphate kinase followed by phosphorylation with pyruvate kinase to yield 5'-triphosphates.

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.

m7G5'ppp5'GmptcpUp at the 5' terminus of reovirus messenger RNA

In the presence of S-adenosyl methionine the 5' terminal guanosine residue of in vitro synthesized reovirus mRNA becomes methylated at the 2'-OH position. In addition, 7-methyl guanylic acid is condensed covalently at the 5' terminus resulting in the formation of a 5' to 5' triphosphate bridge. Analysis of the 5' terminal sequence of methylated reovirus mRNA revealed that it has the structure m7G5'ppp5'GmpCpUp.

Reovirus messenger RNA contains a methylated, blocked 5'-terminal structure: m-7G(5')ppp(5')G-MpCp-

Reovirus mRNA synthesized in vitro by the virus-associated RNA polymerase in the presence of S-adenosylmethionine contains blocked, methylated 5'-termini with the structure, m-7G(5')ppp(5')G-MpCp. The functional significance and possible mechanism of formation of this novel 5'-5' terminal nucleotide linkage are discussed.

In vitro methylation of nascent reovirus mRNA by a virion-associated methyl transferase

Chymotrypsin-derived cores, but not virions, catalyze the transfer of methyl groups from S-adenosyl methionine (SAM) to nascent mRNA synthesized in vitro by the core polymerase. The reaction requires Mg(++), and is dependent on the presence of all 4 ribonucleoside triphosphates (rNTPs). Methylation proceeds optimally at 51 degrees C. All ten species of mRNA become methylated during transcription and it is estimated that one methyl residue is incorporated per RNA chain. Experiments designed to determine the location of the methylated nucleotide clearly demonstrate that methylation occurs exclusively at the 5' ends of nascent mRNA.

Poly(A) polymerase activity in reovirus

An enzymatic activity which synthesized oligo(A) in vitro was found in highly purified reovirus. The poly(A) polymerase activity was dependent on Mn(2+) and utilized only ATP, whereas the virion-associated RNA polymerase required all four ribonucleoside triphosphates and Mg(2+). Oligo(A) synthesis was demonstrated with complete virions and infectious subviral particles derived from virus by limited chymotrypsin digestion but not with cores, a product of extensive chymotrypsin digestion of virus. The enzymatic product and the oligo(A) from purified virions were isolated by binding to oligo(dT)-cellulose columns. Most of the in vitro product was similar in size and structure to the oligo(A) from purified virions by the criteria of gel electrophoresis, DEAE-cellulose chromatography, end-group analysis, and sensitivity to RNase. The evidence suggests that oligo(A) synthesis is mediated by the poly(A) polymerase during a late step in viral morphogenesis and may result from an alternative activity of the virion-associated transcriptase.

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.

Polypeptide composition of Influenza B viruses and enzymes associated with the purified virus particles

Influenza B/LEE/40, B/Rome/1/67, B/Hong Kong/8/73, and B/Victoria/98926/70 viruses have a similar polypeptide composition as analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. These viruses are composed of six or seven polypeptides, depending on whether one or two high-molecular-weight polypeptides are resolved, ranging in molecular weights from 27,000 to 90,400. Three of these polypeptides, namely the heavy and light hemagglutinin chains and the neuraminidase, have attached carbohydrate. Highly purified influenza B/LEE/40 and B/Rome/1/67 virus preparations have RNA-dependent RNA polymerase activity equivalent to the incorporation of 100 and 30 pmol, respectively, of (3)H-UMP per mg of virus protein per h at 37 C, which is demonstrated only in detergent-treated virus suspensions. However, no RNA-dependent DNA polymerase enzyme activity was detected in the two viruses although virus suspensions were "activated" by heat, alpha-chymotrypsin, and detergents. Other enzymatic activities were associated with purified preparations of influenza B virus and were attributed to minor contamination of virus with host cell enzymes. Thus, nucleoside and deoxynucleoside phosphohydrolase enzymes were active in the absence of detergents and catalyzed the release of 1,200 and 1,800 nmol of P(i) per mg of virus protein in 30 min at 37 C from ATP and dATP substrates. Thin-layer chromatography indicated that the products of the phosphohydrolase enzymes of influenza B/LEE/40 were mainly nucleoside diphosphate and monophosphate. The latter enzymes were tightly bound to influenza B/LEE/40 virus and could not be removed completely by repeated centrifugation, including centrifugation of the virus to equilibrium in density gradients of 25 to 40% (wt/vol) cesium chloride. A low degree of RNase (approximately 0.01 mug% contamination) and phosphatase (10-30 nmol of P(i) released per mg of virus protein per 30 min) activity was detected in some, but not all, influenza B/LEE/40 virus preparations.

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.

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.

Deoxyribonucleic acid-dependent nucleotide phosphohydrolase activity in purified vaccinia virus

A nucleotide phosphohydrolase (adenosine triphosphatase), which is associated with vaccinia virus cores, has been solubilized and shown to be deoxyribonucleic acid dependent.

Proteins of a polyhedral cytoplasmic deoxyvirus. 3. Structure of frog virus 3 and location ov virus-associated adenosine triphosphate phosphohydrolase

The adenosine triphosphatase associated with frog virus 3 shows high specificity for ATP or deoxyadenosine triphosphate and appears to be distinct from the corresponding activity in host cells, poxvirus, or reovirus. The enzyme activity is probably integrated into virus particles since it is firmly associated with subviral particles produced when approximately 50 to 60% of the outer viral protein is removed by detergent treatment. The occurrence of adenosine triphosphatase activity within three unrelated viruses suggests that adenosine tryphosphatase might be a necessary function for most viruses that replicate in the cell cytoplasm.

Enzymes and nucleotides in virions of Rous sarcoma virus

In addition to the previously described deoxyribonucleic acid (DNA) polymerase, DNA ligase, DNA exonuclease, and DNA endonuclease activities, purified virions of Schmidt-Ruppin strain of Rous sarcoma virus (SRV) have nucleotides and nucleotide kinase, phosphatase, hexokinase, and lactate dehydrogenase activities. The SRV virions have no glucose-6-phosphate dehydrogenase activity. All enzyme activities, but glucose-6-phosphate dehydrogenase and adenosine triphosphatase, were increased by disruption of the virions. The DNA polymerase, DNA ligase, and hexokinase activities had a higher specific activity in purified virion cores. It is suggested that during assembly virions of SRV may pick up cytoplasmic components which bind to virion proteins. The role of these components in viral replication is not known at present.