IN VITRO SYNTHESIS OF VIRAL RNA BY THE POLIOVIRUS RNA POLYMERASE

Regulation of host factor γ-H2AX level and location by enterovirus A71 for viral replication

Numerous viruses manipulate host factors for viral production. We demonstrated that human enterovirus A71 (EVA71), a primary causative agent for hand, foot, and mouth disease (HFMD), increased the level of the DNA damage response (DDR) marker γ-H2AX. DDR is primarily mediated by the ataxia telangiectasia mutated (ATM), ATM and Rad3-related (ATR), or DNA-dependent protein kinase (DNA-PK) pathways. Upregulation of γ-H2AX by EVA71 was dependent on the ATR but not the ATM or DNA-PK pathway. As a nuclear factor, there is no previous evidence of cytoplasmic distribution of γ-H2AX. However, the present findings demonstrated that EVA71 encouraged the localization of γ-H2AX to the cytoplasm. Of note, γ-H2AX formed a complex with structural protein VP3, non-structural protein 3D, and the viral genome. Treatment with an inhibitor or CRISPR/Cas9 technology to decrease or silence the expression of γ-H2AX decreased viral genome replication in host cells; this effect was accompanied by decreased viral protein expression and virions. In animal experiments, caffeine was used to inhibit DDR; the results revealed that caffeine protected neonatal mice from death after infection with EVA71, laying the foundation for new therapeutic applications of caffeine. More importantly, in children with HFMD, γ-H2AX was upregulated in peripheral blood lymphocytes. The consistent in vitro and in vivo data on γ-H2AX from this study suggested that caffeine or other inhibitors of DDR might be novel therapeutic agents for HFMD.

Coxsackievirus A6 Induces Necroptosis for Viral Production

Hand, foot, and mouth disease (HFMD) is a febrile exanthematous disease with typical or atypical symptoms. Typical HFMD is usually caused by enterovirus 71 (EV71) or coxsackievirus A16, while atypical HFMD is usually caused by coxsackievirus A6 (CA6). In recent years, worldwide outbreaks of CA6-associated HFMD have dramatically increased, although the pathogenic mechanism of CA6 is still unclear. EV71 has been established to induce caspase-dependent apoptosis, but in this study, we demonstrate that CA6 infection promotes a distinct pathway of cell death that involves loss of cell membrane integrity. Necrostatin-1, an inhibitor of necroptosis, blocks the cell death induced by CA6 infection, but Z-DEVD-FMK, an inhibitor of caspase-3, has no effect on CA6-induced cell death. Furthermore, CA6 infection up-regulates the expression of the necroptosis signaling molecule RIPK3. Importantly, necrostatin-1 inhibits CA6 viral production, as assessed by its ability to inhibit levels of VP1 protein and genomic RNA and infectious particles. CA6-induced necroptosis is not dependent on the generation of reactive oxygen species; however, viral 3D protein can directly bind RIPK3, which is suggestive of a direct mechanism of necroptosis induction. Therefore, these results indicate that CA6 induces a mechanism of RIPK3-dependent necroptosis for viral production that is distinct from the mechanism of apoptosis induced by typical HFMD viruses.

Flavors of Flaviviral RNA Structure: towards an Integrated View of RNA Function from Translation through Encapsidation

For many viruses, RNA is the holder of genetic information and serves as the template for both replication and translation. While host and viral proteins play important roles in viral decision‐making, the extent to which viral RNA (vRNA) actively participates in translation and replication might be surprising. Here, the focus is on flaviviruses, which include common human scourges such as dengue, West Nile, and Zika viruses, from an RNA‐centric viewpoint. In reviewing more recent findings, an attempt is made to fill knowledge gaps and revisit some canonical views of vRNA structures involved in replication. In particular, alternative views are offered on the nature of the flaviviral promoter and genome cyclization, and the feasibility of refining in vitro‐derived models with modern RNA probing and sequencing methods is pointed out. By tracing vRNA structures from translation through encapsidation, a dynamic molecule closely involved in the self‐regulation of viral replication is revealed.

Coxsackievirus A6 Induces Cell Cycle Arrest in G0/G1 Phase for Viral Production

Recent epidemiological data indicate that outbreaks of hand, foot, and mouth disease (HFMD), which can be categorized according to its clinical symptoms as typical or atypical, have markedly increased worldwide. A primary causative agent for typical HFMD outbreaks, enterovirus 71 (EV71), has been shown to manipulate the cell cycle in S phase for own replication; however, it is not clear whether coxsackievirus (CVA6), the main agent for atypical HFMD, also regulates the host cell cycle. In this study, we demonstrate for the first time that CVA6 infection arrests the host cell cycle in G0/G1-phase. Furthermore, synchronization in G0/G1 phase, but not S phase or G2/M phase, promotes viral production. To investigate the mechanism of cell cycle arrest induced by CVA6 infection, we analyzed cell cycle progression after cell cycle synchronization at G0/G1 or G2/M. Our results demonstrate that CVA6 infection promotes G0/G1 phase entry from G2/M phase, and inhibits G0/G1 exit into S phase. In line with its role to arrest cells in G0/G1 phase, the expression of cyclinD1, CDK4, cyclinE1, CDK2, cyclinB1, CDK1, P53, P21, and P16 is regulated by CVA6. Finally, the non-structural proteins of CVA6, RNA-dependent RNA polymerase 3D and protease 3C , are demonstrated to be responsible for the G0/G1-phase arrest. These findings suggest that CVA6 infection arrested cell cycle in G0/G1-phase via non-structural proteins 3D and 3C, which may provide favorable environments for virus production.

Enterovirus 71 mediates cell cycle arrest in S phase through non-structural protein 3D

Many viruses disrupt the host cell cycle to facilitate their own growth. We assessed the mechanism and function of enterovirus 71 (EV71), a primary causative agent for recent hand, foot, and mouth disease outbreaks, in manipulating cell cycle progression. Our results suggest that EV71 infection induces S-phase arrest in diverse cell types by preventing the cell cycle transition from the S phase into the G2/M phase. Similar results were observed for an alternate picornavirus, Coxsackievirus A16. Synchronization in S phase, but not G0/G1 phase or G2/M phase, promotes viral replication. Consistent with its ability to arrest cells in S phase, the expression of cyclin A2, CDK 2, cyclin E1, and cyclin B1 was regulated by EV71 through increasing transcription of cyclin E1, promoting proteasome-mediated degradation of cyclin A2 and regulating the phosphorylation of CDK 2. Finally, a non-structural protein of EV71, the RNA-dependent RNA polymerase 3D, was demonstrated to mediate S-phase cell cycle arrest. These findings suggest that EV71 induces S-phase cell cycle arrest in infected cells via non-structural protein 3D, which may provide favorable conditions for virus production.

Structural properties of double-stranded RNAs associated with biological control of chestnut blight fungus

Double-stranded RNAs (ds RNAs) are thought to be the cytoplasmic determinants responsible for the phenomenon of transmissible hypovirulence in the chestnut blight fungus Endothia parasitica [Murr.] Anderson. The three major ds RNA components associated with the North American hypovirulent strain, Grand Haven 2, were characterized with respect to molecular-hybridization specificity and RNase T1-digestion patterns. The large (L-RNA; approximately 9 kilobase pairs) and middle-sized (M-RNA; approximately 3.5 kilobase pairs) ds RNA components cross-hybridized under stringent conditions and exhibited indistinguishable partial and complete RNase T1 digestion patterns relative to their 5' and 3' termini. These results suggest that M-RNA was derived from L-RNA by an internal deletion event. The small (S-RNA; approximately 1 kilobase pair) RNA was unrelated to L- and M-RNA by these criteria. However, all three ds RNA components contained RNase T1-resistant oligonucleotides at one 5' terminus and at the corresponding 3' terminus of the complementary strand. These RNase T1-resistant species exhibited properties consistent with stretches of poly(uridylic acid) and poly(adenylic acid), respectively. The combined results are discussed in terms of the structural organization of hypovirulence-associated ds RNA molecules and their similarities to "double-stranded" RNA molecules observed in plant and animal cells infected with single-stranded RNA viruses.

VIRUS-SPECIFIC DOUBLE-STRANDED RNA IN POLIOVIRUS-INFECTED CELLS

Poliovirus-infected HeLa cells contain a small amount of an RNA that sediments slightly faster than 16S. This RNA is less dense than viral RNA and resists digestion by ribonuclease. It is therefore, considered to be a double-stranded form of poliovirus RNA. A significant amount of this material is found in association with a membranous virus-specific particulate.

REPLICATION OF THE RNA OF BACTERIOPHAGE R17

Escherichia coli cells were irradiated with ultraviolet light to stop ribosomal RNA synthesis. After infection of such cells with the single-stranded RNA phage R17, the RNA most rapidly labeled by a pulse of tritiated uridine sedimented in a broad band in the 16S region of sucrose gradients. The effect of ribonuclease on this material and its behavior during a "chase" period in nonradioactive medium suggest that it consists of a core of double-stranded RNA, one strand of which-the viral strand-is continually displaced by a growing strand forming single-stranded tails and ultimately free 27S viral RNA.

Association of poliovirus proteins with the endoplasmic reticulum

Poliovirus proteins, except P3-7c, are associated with the endoplasmic reticulum after extraction of the cytoplasm and centrifugation of membranes to equilibrium in sucrose gradients. Proteins P3-2, P2-X, and P3-9 are found preferentially among the rough endoplasmic reticulum, whereas P3-7c is located in smooth endoplasmic reticulum fractions. P3-7c is probably not membrane associated, since it can be separated from membranes after centrifugation in buffer. However, P3-4a, P2-5b, P2-X, and P3-9 are avidly bound to membranes and cannot be dislodged with high-ionic-strength buffer containing EDTA or 4 M urea. These proteins are digested by trypsin, indicating peripheral rather than internal localization.

Inhibition of poliovirus polymerase by guanidine in vitro

Extracts from poliovirus-infected HeLa cells synthesized 35s viral RNA, replicative intermediate, and double-stranded RNA in vitro. Guanidine inhibited the synthesis of all three species of RNA; production of 35s RNA was most sensitive to the inhibitor. Pulse-chase experiments with [3H]UTP indicated that guanidine had no detectable effect on elongation of polynucleotide chains or the release of completed RNA chains from the viral replication complex. Experiments in which short pulses of precursor were used suggest that guanidine blocked the initiation step of RNA synthesis in vitro.

Effect of cordycepin triphosphate on in vitro RNA synthesis by picornavirus polymerase complexes

Cordycepin triphosphate inhibited in vitro [3H]GMP incorporation by pricornavirus-specific polymerase complexes isolated from infected HeLa cells. The inhibition of [3H]GMP incorporation could be reversed with ATP added to the reaction mixture along with the inhibitor, but not with GTP so added or with ATP added 10 min after the inhibitor. Products synthesized in vitro in the presence of cordycepin triphosphate lacked full-length single-stranded viral RNA. These results support RNA chain termination by specific competition with ATP as the mechanism of inhibition of picornavirus-specific RNA synthesis by cordycepin triphosphate.

Poliovirus-specific primer-dependent RNA polymerase able to copy poly(A)

A template-dependent RNA polymerase has been isolated from poliovirus-infected cells by assaying for the ability of the enzyme to copy poly(A) complexed to an oligo(U) primer. The polymerase was solubilized with detergent, and RNA was removed by precipitation with 2 M LiCl. The solubilized polymerase required both poly(A) and oligo(U) for activity and was stimulated by Mg2+ but was inhibited by Mn2+. Poly(A)-oligo(U)-dependent poly(U) polymerase was not found in extracts of HeLa cells until about 2 hr after poliovirus infection, and then there was a linear increase in activity until about 5 hr. Analysis of the polymerase by glycerol gradient centrifugation showed that the majority of the activity sedimented at about 4 S, indicating that it was no longer complexed with high-molecular-weight RNA or cellular membranes. This poly(A)-oligo(U)-dependent polymerase activity could represent an important component of the poliovirus RNA-dependent RNA polymerase.

A protein covalently linked to poliovirus genome RNA

Poliovirion [32P]RNA, after digestion with RNase T2, yields mononucleotides and a labeled compound "X," which is not negatively charged at pH 5. X contains, relative to the label in virion RNA, one to two phosphates and is partially acid insoluble. It can be labeled with tritiated amino acids 3 hr after infection, is insoluble in chloroform/methanol, and can be digested with Pronase. These observations suggest that X is a protein. The protein cannot be removed from the polio genome when the RNA is (i) sedimented through a sucrose gradient in 0.5 M NaCl, (ii) heated to 100 degrees in the presence of sodium dodecyl sulfate followed by sedimentation through a sucrose gradient in 80% dimethylsulfoxide, or (iii) banded in 4 M cesium trichloroacetate. Digestion of the 32P-labeled protein with Pronase yields one major 32P-labeled product, which contains pUp. The protein migrates faster than capsid protein VP4 in a polyacrylamide gel. Our data show that the genome of poliovirus, but not poliovirus mRNA [A. Nomoto, Y. F. Lee, and E. Wimmer (1976) Proc. Natl. Acad. Sci. USA 73, 375-380], is covalently attached to a small virus-coded protein (molecular weight less than 7000), which we call VPg. VPg is probably linked to the 5' end of the polio genome. Possible functions of VPg in viral replication are discussed.

In vitro replication of cowpea mosaic virus RNA. II. Solubilization of membrane-bound replicase and the partial purification of the solubilized enzyme

A method for the solubilization of membrane-bound Cowpea mosaic virus RNA replicase has been developed by bypassing the use of detergents. Solubilization has been achieved by washing the 31,000 x g-pellet containing the bound replicase with a Mg2+-deficient buffer. This procedure had several advantages as compared to treatments with nonionic or ionic detergents: (i) the solubilized enzyme was stable at 4 C, (ii) more than 80% of the replicase could be solubilized without loss of total enzyme activity, (iii) the replicase was rather selectively released resulting in a two- to threefold increase in specific activity per se, and (iv) most of the green color from chloroplast fragments present in the crude replicase fraction remained membrane bound resulting in only slightly colored preparations of solubilized enzyme. The solubilized replicase has been further purified by DEAE-Bio Gel column chromatography. RNA synthesis directed by the DEAE-purified enzyme was template dependent and proceeded at a linear rate for at least 9 h.

Replication of picornaviruses. I. Evidence from in vitro RNA synthesis that poly(A) of the poliovirus genome is genetically coded

A crude replication complex has been isolated from poliovirus-infected HeLa cells and used for synthesis of poliovirus replicative intermediate (RI) RNA, replicative form (RF) RNA, and single-stranded (SS) RNA in vitro. All three classes of virus-specific RNA synthesized in vitro are shown to contain poly(A). Poly(A) of RF and of SS RNA [RF-poly(A) and SS-poly(A)] has a chain length (50 to 70 nucleotides) that is shorter than that of poly(A) of in vivo-synthesized RNAs. Poly(A) of RI [RI-poly(A),] however, is at least 200 nucleotides long and, therefore, larger than poly(A) of RI isolated from HeLa cells 4 h after infection. The crude membrane-bound replication complex contains a terminal adenylate transferase activity that is stimulated by Mn2+ and the addition of an (Ap)2AOH primer. This transferase activity is found also in extracts of mock-infected cells. Partial purificaiton of the replication complex in a stepwise sucrose gradient, in which the viral replicase is associated with the smooth cytoplasmic membrane fraction, does not remove the terminal transferase. However, when the partially purified replication complex is treated with deoxycholate and sedimented through a sucrose gradient, a soluble replication complex can be isolated that is free from terminal adenylate transferase. This soluble replication complex was found to synthesize viral RNA-linked poly(A) longer in chain length than that synthesized by the crude replication complex. Taking into account the 5'-terminal poly(U) in poliovirus minus strands, our data suggest that polyadenylation of poliovirus RNA occurs by transcription and not by end addition. When compared to other viral systems, poliovirus and, probably, all picornaviruses appear to be unique in that the poly(A) of their genome is genetically coded.

Polyadenylic acid on poliovirus RNA. III. In vitro addition of polyadenylic acid to poliovirus RNAs

A crude RNA polymerase preparation was made from HeLa cells infected for 3 h with poliovirus. All virus-specific RNA species labeled in vitro (35S RNA, replicative intermediate RNA [RI], and double-stranded RNA [dsRNA]) would bind to poly(U) filters and contained RNase-resistant stretches of poly(A) which could be analyzed by electrophoresis in polyacrylamide gels. After incubation for 45 min with [3-H]ATP in the presence of the other three nucleoside triphosphates, the labeled poly(A) on the RI and dsRNA migrated on gels as relatively homogenous peaks approximately 200 nucleotides in length. In contrast, the poly(A) from the 35S RNA had a heterogeneous size distribution ranging from 50 to 250 nucleotides. In the absence of UTP, CTP, and GTP, the size of the newly labeled poly(A) on the dsRNA and RI RNA was the same as it was in the presence of all four nucleoside triphosphates. However the poly(A) on the 35S RNA lacked the larger sequences seen when the other three nucleoside triphosphates were present. When [3-H]ATP was used as the label in infected and uninfected extracts, heterogeneous single-stranded RNA sedimenting at less than 28S was also labeled. This heterogeneous RNA probably represents HeLa cytoplasmic RNA to which small lengths of poly(A) (approximately 15 nucleotides) had been added. These results indicate that in the in vitro system poly(A) can be added to both newly synthesized and preexisting RNA molecules. Furthermore, an enzyme capable of terminal addition of poly(A) exists in both infected and uninfected extracts.

Isolation of a viral polypeptide associated with poliovirus RNA polymerase

Poliovirus-infected HeLa cells were labeled with radioactive methionine or phenylalanine and subjected to a new purification procedure for the viral induced RNA polymerase activity. Detergent-solubilized polymerase activity was purified by precipitation with 2 M LiCl and sedimentation through sucrose gradients. Approximately 0.001% of the incorporated amino acid radio-activity sediments with the peak of polymerase activity. Gradient fractions comprising the polymerase activity peak were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and found to contain predominantly one virus-specific polypeptide. Polyacrylamide gel electrophoresis also reveals that this purified polypeptide migrates with a 58,000 molecular weight noncapsid polio-virus polypeptide.

Degradation of single- and double-stranded RNA by frog virus 3

Purified preparations of frog virus 3 possess ribonuclease activities directed against single-and double-stranded RNA. Double-stranded RNAs isolated from purified reovirus type 3 and from HeLa cells infected with poliovirus and single-stranded poliovirus RNA from purified virus are readily degraded by incubation with frog virus 3. The mode of action of the nucleases is endonucleolytic. Under the assay conditions used for the viral enzyme, crude extracts of uninfected HeLa, L, and baby hamster kidney cells did not show enzyme activity against double-stranded RNA but exhibited activity against single-stranded RNA. The dependence of the viral nucleases on divalent cations for optimal activity and the inhibition of the cleavage of single-stranded RNA by 0.2 M NaCl suggests that the enzymes are either virus-coded or virus-induced.

Effect of mengovirus replication on choline metabolism and membrane formation in novikoff hepatoma cells

Infection of Novikoff rat hepatoma cells (subline NlSL-67) with mengovirus resulted in a two- to threefold increase in the rate of choline incorporation into membrane phosphatidylcholine at about 3 hr after infection, without affecting the rate of transport of choline into the cell or its phosphorylation. The time course of virus-stimulated phosphatidylcholine synthesis was compared with the time courses of other virus-induced processes during a single cycle of replication. The formation of viral ribonucleic acid (RNA) polymerase and of viral RNA commenced about 1 hr earlier than the virus-stimulated choline incorporation. Further, isopycnic centrifugation of cytoplasmic extracts indicated that the excess of phosphatidylcholine synthesized by infected cells is not located in the membrane structures associated with the viral RNA replication complex, but with structures of a lower density (1.08 to 1.14 g/cc). These membrane structures probably represent the smooth vesicles which accumulate in the cytoplasm of infected cells during the period of increased phosphatidylcholine synthesis between 3 and 5 hr after infection. They are formed with both newly synthesized phosphatidylcholine and phosphatidylcholine present prior to infection. However, concomitant protein synthesis is not required for the stimulated synthesis of membranes; the effect was not inhibited by treating the cells with inhibitors of protein synthesis at 3 hr after infection, although virus production was inhibited about 90% and virus-induced cell degeneration was markedly reduced and delayed. Production of mature virus began normally at about the same time as the stimulation of phosphatidylcholine synthesis. Treatment of infected cells with puromycin at 2 hr, on the other hand, completely inhibited the stimulation of phosphatidylcholine synthesis.

Deoxyribonucleic acid polymerase associated with Rous sarcoma virus and avian myeloblastosis virus: properties of the enzyme and its product

Deoxyribonucleic acid (DNA) polymerase activity can be elicited in purified preparations of avian myeloblastosis virus and Rous sarcoma virus (Schmidt-Ruppin strain) by treatment with nonionic detergent. The enzyme(s) and its synthetic products appear to be virion-associated. Enzymatic activity can be inhibited by pretreatment with either ribonuclease (8- to 10-fold inhibition) or actinomycin D (twofold inhibition). By contrast, rifampin has little, if any effect. The enzyme(s) synthesizes two primary products, a ribonucleic acid (RNA):DNA hybrid and DNA which is free of RNA. The results of both zonal and equilibrium centrifugation indicate that nascent chains of DNA are associated with the 70S viral RNA. It is concluded that at least two enzymatic activities are under study: transcription of DNA from viral RNA, and subsequent, additional synthesis of DNA, utilizing product of the initial reaction as template.

SV40-specific RNA in the nucleus and polyribosomes of transformed cells

Cells transformed by the oncogenic virus SV40 are known to contain viral DNA integrated into cellular DNA and to produce virus-specific RNA. It has been shown that nuclear molecules containing virus-specific sequences are considerably longer than presumed virus-specific mRNA molecules from cytoplasmic polyribosomes. This finding suggests the possibility that cytoplasmic mRNA is derived by the specific cleavage of larger nuclear RNA.

Encephalomyocarditis virus ribonucleic acid polymerase associated with 150S cytoplasmic particles

Cytoplasmic particles which sedimented at 150S were the smallest structures containing detectable viral ribonucleic acid polymerase in mouse cells infected with encephalomyocarditis virus.

In vitro synthesis of poliovirus ribonucleic acid: role of the replicative intermediate

Poliovirus ribonucleic acid (RNA) polymerase crude extracts could be stored frozen in liquid nitrogen without loss of activity or specificity. The major in vitro product of these extracts was viral single-stranded RNA. However, after short periods of incubation with radioactive nucleoside triphosphates, most of the incorporated label was found in replicative intermediate. When excess unlabeled nucleoside triphosphate was added, the label was displaced from the replicative intermediate and accumulated as viral RNA. It is concluded from this experiment that the replicative intermediate is the precursor to viral RNA. In addition, some of the label was chased into double-stranded RNA. The implications of this finding are discussed.

Synthesis of ribonucleic acid in cells infected with LSc poliovirus at elevated temperatures

The synthesis of viral ribonucleic acid (RNA) was detected within 2 hr after infection with LSc poliovirus at 35 C. This RNA eluted as a single peak with 0.9 m NaCl on methylated albumin celite columns, was sensitive to ribonuclease, precipitated in the presence of 2 m LiCl, and had an S(20) value at 34 +/- 2 in linear sucrose gradients. When cells were infected at 39 to 40 C, there was also early synthesis of RNA. However, 2 hr after infection this synthesis was drastically inhibited. The absence of net RNA synthesis at 39 to 40 C during the late stages of infection was not caused by rapid degradation of newly formed RNA, since the RNA produced between 1 and 2 hr at 39 to 40 C was still present 3.5 hr after infection. There was a 3 log(10) inhibition in the production of infectious virus when p-fluorophenylalanine was present in the medium at a concentration of 25 mug/ml. This concentration of analogue had little effect upon the production of viral polymerase and viral RNA. Virus grown in the presence of analogue at a concentration of 10 mug/ml exhibited increased heat sensitivity compared to control virus. However, viral polymerase exhibited no change in sensitivity to heat or manganese when cells were exposed to 25 mug of p-fluorophenylalanine per ml during infection. p-Fluorophenylalanine had a relatively selective effect on viral capsid protein but did not reverse the inhibition of synthesis of viral RNA at 39 to 40 C.

Studies of the activity of diphtheria toxin. I. Poliovirus replication in intoxicated HeLa cells

It has been demonstrated that a saturating dose of diphtheria toxin produced a 90% inhibition of polio-virus replication in HeLa cells. This inhibition was reflected in infectious viral RNA synthesis and in mature virus production. Toxin had no direct effect on virus particles or I-RNA, and poliovirus adsorption and eclipse appeared to be carried out normally in intoxicated cells. When toxin was given at various time intervals after infection, the amount of inhibition depended on the time of toxin addition. Toxin given before or immediately after infection gave maximum inhibition, while toxin given several hours after infection had little effect. The data suggest that toxin inhibits viral replication through its effect on protein synthesis. It is likely that a critical step in the viral replication cycle, the production of poliovirus-induced RNA polymerase, is inhibited, and possibly the synthesis of capsid protein. Ammonium salts and the aliphatic amines, glycamine and prolamine, prevented the inhibition of viral replication by toxin. The kinetics of the protective action of ammonium chloride and diphtheria antitoxin are remarkably similar.

Ribonucleic acid polymerase catalyzing synthesis of double-stranded arbovirus ribonucleic acid

The large-particle fraction from the cytoplasm of chick embryo fibroblasts infected with Semliki Forest virus was found to catalyze the incorporation of the 5'-triphosphates of guanosine, adenine, cytidine, and uridine into an acid-insoluble alkali-labile product. The conditions affecting the preparation and assay of this enzyme were investigated. The ribonucleic acid (RNA) polymerase was not present in uninfected cells, and it appeared in infected cells at the time of rapid viral RNA synthesis. The polymerase was found to catalyze the synthesis of a species of RNA which was resistant to ribonuclease and which exhibited the sedimentation properties, buoyant density, and thermal transition temperature of the double-stranded RNA found in vivo in chick cells infected with Semliki forest virus. Attempts to demonstrate that the reaction product of this enzyme also included single-stranded viral RNA were not successful. Although other interpretations are possible, these results give some support to the suggestion that more than one enzyme may be involved in the replication of viral RNA.

Alterations of protein synthesis in arbovirus-infected L cells

Lust, George (Fort Detrick, Frederick, Md.). Alterations of protein synthesis in arbovirus-infected L cells. J. Bacteriol. 91:1612-1617. 1966.-Cellular protein synthesis and ribonucleic acid (RNA) synthesis in mouse L cells were markedly depressed 1 hr after infection with Venezuelan equine encephalomyelitis virus. Host RNA and protein synthesis were inhibited more rapidly by the virus infection than by actinomycin D. In cells infected 4 hr, a cytoplasmic RNA polymerase was demonstrated which was absent in uninfected cells. At this time, deoxyribonucleic acid-directed RNA synthesis catalyzed by the nuclear RNA polymerase was inhibited in vitro in enzyme preparations from nuclei of virus-infected cells. For optimal activity, the cytoplasmic RNA polymerase required the four nucleoside triphosphates, Mg(++), and RNA. The enzyme was insensitive to actinomycin D and deoxyribonuclease, indicating that it catalyzed RNA-directed RNA synthesis. Attempts to purify the induced polymerase further were unsuccessful. Fresh preparations had to be used because the enzymatic activity was unstable.

Effect of interferon on polymerization of single-stranded and double-stranded mengovirus ribonucleic acid

Gordon, Irving (University of Southern California, Los Angeles), Sara S. Chenault, Douglas Stevenson, and Jean D. Acton. Effect of interferon on polymerization of single-stranded and double-stranded mengovirus ribonucleic acid. J. Bacteriol. 91:1230-1238. 1966.-The effect of interferon on actinomycin-resistant mengovirus ribonucleic acid (RNA) replication in L cells was investigated to determine whether defective or partially polymerized RNA products were made and whether synthesis of any specific class of virus RNA was prevented. RNA labeled with uridine-C(14) was extracted in hot and cold phenol and analyzed by zonal sucrose density centrifugation. Both single- and double-stranded infectious RNA peaks were identified. Interferon treatment caused almost complete depression of uridine-C(14) incorporation throughout linear sucrose gradients except in the 4S region, and no infectivity was detectable in any fraction. These inhibitory effects are attributable to the action of interferon, because they were reversed when cultures were treated with actinomycin D simultaneously with interferon. The results, with those of other investigators, indicate that the step at which interferon interrupts virus multiplication is between the events immediately after uncoating and the formation of template "minus" strands; under the conditions of our experiments, no partially polymerized virus RNA products were made.