Vesicular Stomatitis Virus mRNA Research Articles

Identification of a minimal size requirement for termination of vesicular stomatitis virus mRNA: implications for the mechanism of transcription.

The nonsegmented negative-strand RNA (NNS) viruses have a single-stranded RNA genome tightly encapsidated by the viral nucleocapsid protein. The viral polymerase transcribes the genome responding to specific gene-start and gene-end sequences to yield a series of discrete monocistronic mRNAs. These mRNAs are not produced in equimolar amounts; rather, their abundance reflects the position of the gene with respect to the single 3'-proximal polymerase entry site. Promoter-proximal genes are transcribed in greater abundance than more distal genes due to a localized transcriptional attenuation at each gene junction. In recent years, the application of reverse genetics to the NNS viruses has allowed an examination of the role of the gene-start and gene-end sequences in regulating mRNA synthesis. These studies have defined specific sequences required for initiation, 5' modification, termination, and polyadenylation of the viral mRNAs. In the present report, working with Vesicular stomatitis virus, the prototypic Rhabdovirus, we demonstrate that a gene-end sequence must be positioned a minimal distance from a gene-start sequence for the polymerase to efficiently terminate transcription. Gene-end sequences were almost completely ignored in transcriptional units less than 51 nucleotides. Transcriptional units of 51 to 64 nucleotides allowed termination at the gene-end sequence, although the frequency with which polymerase failed to terminate and instead read through the gene-end sequence to generate a bicistronic transcript was enhanced compared to the observed 1 to 3% for wild-type viral mRNAs. In all instances, failure to terminate at the gene end prevented initiation at the downstream gene start site. In contrast to this size requirement, we show that the sequence between the gene-start and gene-end signals, or its potential to adopt an RNA secondary structure, had only a minor effect on the efficiency with which polymerase terminated transcription. We suggest three possible explanations for the failure of polymerase to terminate transcription in response to a gene-end sequence positioned close to a gene-start sequence which contribute to our emerging picture of the mechanism of transcriptional regulation in this group of viruses.

Antiapoptotic and oncogenic potentials of hepatitis C virus are linked to interferon resistance by viral repression of the PKR protein kinase.

Hepatitis C virus (HCV) is prevalent worldwide and has become a major cause of liver dysfunction and hepatocellular carcinoma. The high prevalence of HCV reflects the persistent nature of infection and the large frequency of cases that resist the current interferon (IFN)-based anti-HCV therapeutic regimens. HCV resistance to IFN has been attributed, in part, to the function of the viral nonstructural 5A (NS5A) protein. NS5A from IFN-resistant strains of HCV can repress the PKR protein kinase, a mediator of the IFN-induced antiviral and apoptotic responses of the host cell and a tumor suppressor. Here we examined the relationship between HCV persistence and resistance to IFN therapy. When expressed in mammalian cells, NS5A from IFN-resistant HCV conferred IFN resistance to vesicular stomatitis virus (VSV), which normally is sensitive to the antiviral actions of IFN. NS5A blocked viral double-stranded RNA (dsRNA)-induced PKR activation and phosphorylation of eIF-2alpha in IFN-treated cells, resulting in high levels of VSV mRNA translation. Mutations within the PKR-binding domain of NS5A restored PKR function and the IFN-induced block to viral mRNA translation. The effects due to NS5A inhibition of PKR were not limited to the rescue of viral mRNA translation but also included a block in PKR-dependent host signaling pathways. Cells expressing NS5A exhibited defective PKR signaling and were refractory to apoptosis induced by exogenous dsRNA. Resistance to apoptosis was attributed to an NS5A-mediated block in eIF-2alpha phosphorylation. Moreover, cells expressing NS5A exhibited a transformed phenotype and formed solid tumors in vivo. Disruption of apoptosis and tumorogenesis required the PKR-binding function of NS5A, demonstrating that these properties may be linked to the IFN-resistant phenotype of HCV.

Polyadenylation of vesicular stomatitis virus mRNA dictates efficient transcription termination at the intercistronic gene junctions.

The intercistronic gene junctions of vesicular stomatitis virus (VSV) contain conserved sequence elements that are important for polyadenylation and transcription termination of upstream transcript as well as reinitiation of transcription of downstream transcript. To examine the role of the putative polyadenylation signal 3'AUACU(7)5' at the gene junctions in polyadenylation and transcription termination, we constructed plasmids encoding antigenomic minireplicons containing one or two transcription units. In plasmid-transfected cells, analyses of the bicistronic minireplicon containing the wild-type or mutant intercistronic gene junctions for the ability to direct synthesis of polyadenylated upstream, downstream, and readthrough mRNAs showed that the AUACU(7) sequence element is required for polyadenylation of VSV mRNA. Deletion of AUAC or U(7) resulted in templates that did not support polyadenylation of upstream mRNA. Interestingly, we found that the loss of polyadenylation function led to antitermination of the upstream transcript and resulted in a readthrough transcript that contained the upstream and downstream mRNA sequences. Mutations that blocked polyadenylation also blocked transcription termination and generated mostly readthrough transcript. Reverse transcription-PCR of readthrough transcripts and subsequent nucleotide sequencing of the amplified product revealed no extra adenosine residues at the junction of the readthrough transcript. These results indicate that polyadenylation is required for transcription termination of VSV mRNA. The intergenic dinucleotide GA did not appear to be necessary for transcription termination. Furthermore, we found that insertion of the polyadenylation signal sequence AUACU(7) alone was sufficient to direct polyadenylation and efficient transcription termination at the inserted site. Taken together, the data presented here support the conclusion that polyadenylation is the major determinant of transcription termination at the intercistronic gene junctions of VSV.

Resistance to virus infection conferred by the interferon-induced promyelocytic leukemia protein.

The interferon (IFN)-induced promyelocytic leukemia (PML) protein is specifically associated with nuclear bodies (NBs) whose functions are yet unknown. Two of the NB-associated proteins, PML and Sp100, are induced by IFN. Here we show that overexpression of PML and not Sp100 induces resistance to infections by vesicular stomatitis virus (VSV) (a rhabdovirus) and influenza A virus (an orthomyxovirus) but not by encephalomyocarditis virus (a picornavirus). Inhibition of viral multiplication was dependent on both the level of PML expression and the multiplicity of infection and reached 100-fold. PML was shown to interfere with VSV mRNA and protein synthesis. Compared to the IFN mediator MxA protein, PML had less powerful antiviral activity. While nuclear body localization of PML did not seem to be required for the antiviral effect, deletion of the PML coiled-coil domain completely abolished it. Taken together, these results suggest that PML can contribute to the antiviral state induced in IFN-treated cells.

Cis-Acting signals involved in termination of vesicular stomatitis virus mRNA synthesis include the conserved AUAC and the U7 signal for polyadenylation.

We investigated the cis-acting sequences involved in termination of vesicular stomatis virus mRNA synthesis by using bicistronic genomic analogs. All of the cis-acting signals necessary for termination reside within the first 13 nucleotides of the 23-nucleotide conserved gene junction. This 13-nucleotide termination sequence at the end of the upstream gene comprises the tetranucleotide AUAC, the tract containing seven uridines (U7 tract), and the intergenic dinucleotide (GA), but it does not include the downstream gene start sequence. Data presented here show that upstream mRNA termination is independent of downstream mRNA initiation. Alteration of any nucleotide in the 13-nucleotide sequence decreased the termination activity of the gene junction and resulted in increased synthesis of a bicistronic readthrough RNA. This finding indicated that the wild-type gene junction has evolved to achieve the maximum termination efficiency. The most critical position of the AUAC sequence was the C, which could not be altered without complete loss of mRNA termination. Reducing the length of the wild-type U7 tract to zero, five, or six U residues also totally abolished mRNA termination, resulting in exclusive synthesis of the bicistronic readthrough mRNA. Shortening the wild-type U7 tract to either five or six U residues abolished VSV polymerase slippage during readthrough RNA synthesis. Since neither the U5 nor U6 template was able to direct mRNA termination, these data imply that polymerase slippage is a prerequisite for termination. Evidence is also presented to show that in addition to causing polymerase slippage, the U7 tract itself or its poly(A) product constitutes an essential signal for mRNA termination.

Effects of Psoralen-Derivatized Oligonucleoside Methylphosphonates on Vesicular Stomatitis Virus (VSV) Protein SynthesisIn Vitroand in VSV-Infected Cells

Abstract Oligodeoxyribonucleoside methylphosphonates (16-mers) targeted to VSV mRNAs were derivatized with 4′-{[N-(aminoethyl)amino]methyl}-4,5′8-trimethylpsoralen. These oligomers specifically inhibit translation of their targeted mRNAs in vitro following UV irradiation of the oligomer-mRNA complexes. Psoralen-derivatized oligonucleoside methylphosphonates are stable in cells and can inhibit VSV protein synthesis in culture following UV-irradiation of VSV-infected cells.

Progression of a vesicular stomatitis virus infection in primary lymphocytes is restricted at multiple levels during B cell activation.

Small resting B cells do not support a productive vesicular stomatitis virus (VSV) infection, but are induced by B cell activators to become fully permissive for VSV replication. Nonpermissive B cell populations restrict VSV expression at multiple points: transcript levels, translation, and maturation. Unstimulated resting G0 B cells can be infected by VSV and support the synthesis of all VSV mRNAs. Steady-state levels of viral transcripts are selectively enhanced by T cell-derived cytokines to an extent comparable with that seen for cytokine-regulated cellular mRNAs. However, viral proteins are not detected in immunoprecipitates from unstimulated or cytokine-stimulated B cells despite the fact that viral mRNAs are associated with polysomes and can be translated in vitro. This translational block is released by stimulation of infected B cells with mitogenic anti-lg or LPS, or non-mitogenic PMA. VSV virion maturation is also regulated by activation signals, because neither anti-lg nor PMA-stimulated B cells produce high levels of infectious VSV particles. Because anti-lg stimulation supports viral genome replication, maturational arrest is apparently at virus assembly or release. PMA and ionomycin induces changes beyond those seen with anti-lg, because these B cells produce PFUs at levels comparable with those seen with LPS-activated B cells and VSV-permissive cell lines. Activation-dependent regulation of virus expression provides a new paradigm for assessing activator-induced events in B cell differentiation not revealed by previous assessments of proliferation of Ab synthesis.

Vesicular stomatitis virus transcription inhibited by purified MxA protein

MxA is a GTPase encoded by an interferon-inducible human gene. Its constitutive expression renders transfected mammaliancells resistant to infections with several different RNA viruses, including vesicular stomatitis virus (VSV). Differences in viral RNA levels of VSV-infected cells either expressing or lacking MxA indicated that VSV mRNA synthesis is the principal target of MxA action. We now used purified histidine-tagged MxA (His-MxA) that we produced in Escherichia coli to successfully inhibit VSV in vitro transcription, a reaction catalyzed by VSV ribonucleoprotein complexes isolated from virus-infected cells or from purified virions. MxA was inactive when added to preformed VSV mRNAs, arguing against the possibility that it has a negative effect on viral RNA stability. MxA inhibited both leader RNA and mRNA synthesis of VSV, suggesting that it interfered with transcription initiation. The degree of VSV inhibition correlated directly with the specific GTPase activities of the various wild-type MxA preparations. No inhibition of viral mRNA synthesis was observed when a C-terminally truncated, GTPase-inactive variant of His-MxA was added to the transcription reactions. Purified His-MxA-E645R, a mutant of MxA with normal GTPase activity whose range of antiviral activity in vivo is altered so that it no longer inhibits VSV, showed no inhibitory effect on VSV in vitro transcription. Since MxA inhibited VSV RNA synthesis in the presence of GMP-PNP or GTP γS, GTP analogs that are readily accepted by the viral polymerase but cannot be hydrolyzed by MxA, the possibility was excluded that MxA acts by depleting the viral polymerase for its nucleotide substrates. Thus, binding of GTP rather than its hydrolysis seems of importance for the anti-VSV activity of MxA.

Interactions of psoralen-derivatized oligodeoxyribonucleoside methylphosphonates with vesicular stomatitis virus messenger RNA.

The ability of oligonucleotides to interact selectively with their targets is an important consideration in the design of antisense oligonucleotides. This is especially important in the case of antisense oligomers, such as psoralen-derivatized oligomers, which can irreversibly bind to their targets. We have studied the interactions of a series of psoralen-derivatized antisense oligonucleoside methylphosphonates with the mRNAs of vesicular stomatitis virus (VSV), mRNAs that have a high degree of sequence homology. Cross-linking reactions were carried out under conditions of low ionic strength in order to reduce mRNA secondary structure. A 12-mer, whose sequence was complementary to VSV M-mRNA and partially complementary to sequences found in N, NS, and G mRNA cross-linked extensively to N-message. On the other hand, 16-mers whose sequences were uniquely complementary to binding sites on N- or M-mRNA specifically and efficiently cross-linked to their targeted mRNAs over the temperature range 0 degree to 37 degrees C. A reverse transcriptase-catalyzed primer extension assay was used to show that one of the N-specific oligomers cross-linked at the expected site on N-mRNA and to estimate the extent of cross-linking. The results demonstrate that psoralen-derivatized oligonucleoside methylphosphonates can cross-link in a sequence-specific manner if the sequences of these oligomers are chosen carefully so as to avoid extensive partial complementarity with other mRNA sequences.

Transcriptional activity and mutational analysis of recombinant vesicular stomatitis virus RNA polymerase.

The 241-kDa large (L) protein of vesicular stomatitis virus (VSV) is the multifunctional catalytic component of the viral RNA polymerase. A protocol has been developed for the synthesis of recombinant L protein that will support viral mRNA synthesis in vitro. COS cells were transfected with a transient expression vector (pSV-VSL1 [M. Schubert, G. G. Harmison, C. D. Richardson, and E. Meier, Proc. Natl. Acad. Sci. USA 82:7984-7988, 1985]) which contains the simian virus 40 late promoter for the transcription of a cDNA copy of the L protein of the Indiana serotype of VSV. Cytoplasmic extracts of these cells efficiently transcribed VSV mRNAs in vitro in conjunction with N protein-RNA template purified from virus and recombinant phosphoprotein synthesized in Escherichia coli. mRNA synthesis was completely dependent upon addition of both bacterial phosphoprotein and extracts from cells transfected with the L gene. Extracts from mock-transfected cells or from cells transfected with the expression vector alone did not support VSV RNA synthesis. RNA synthesis was proportional to the concentration of cell extract used, with an optimum of 0.2 mg/ml. Rhabdoviruses and paramyxoviruses contain a highly conserved GDNQ motif which was mutated in the transfected L gene. All constructs with mutations within the core GDN abrogated transcriptional activity except for the mutant containing GDD, which retained 25% activity. Conserved amino acid changes outside of the core GDN and changes corresponding to other paromyxovirus and rhabdovirus L proteins retained variable transcriptional activity. These findings provide experimental evidence that the GDN of negative-strand, nonsegmented RNA viruses is a variant of the GDD motif of plus-strand RNA viruses and of the XDD motif of DNA viruses and reverse transcriptases.

Inhibition of vesicular stomatitis virus mRNA synthesis by human MxA protein.

The interferon-induced human MxA protein inhibits the multiplication of influenza virus and vesicular stomatitis virus (VSV) by an unknown mechanism. Here we show that MxA protein interferes with VSV mRNA synthesis. Transfected Swiss 3T3 mouse cells constitutively expressing MxA protein and control cells were infected with VSV, and viral RNA and protein synthesis was monitored. Viral macromolecules were very abundant in control cells at 4 h postinfection, whereas the pools of VSV proteins and RNAs were more than 50-fold reduced in cells expressing MxA. To determine whether MxA inhibited VSV primary transcription, we infected the cells in the presence of the protein synthesis inhibitor cycloheximide and measured the pools of the five viral mRNAs at 4 h postinfection. VSV L mRNA concentration was more than 20-fold reduced, VSV G mRNA concentration was about 10-fold reduced, and the other viral mRNAs were three- to fivefold less abundant in MxA-expressing cells than in control cells. Our results thus indicate that MxA interferes with normal VSV mRNA synthesis either directly by inhibiting the activity of the viral polymerase complex or indirectly by reducing the stability of the VSV mRNAs.

Antiviral activity and possible mechanisms of action of oligonucleotides-poly(L-lysine) conjugates targeted to vesicular stomatitis virus mRNA and genomic RNA

Synthetic oligonucleotides (oligomers) complementary to vesicular stomatitis virus (VSV) N protein mRNA have specific antiviral properties at concentrations lower than 1 microM when they are covalently linked to poly(L-lysine) (PLL). Since it is generally postulated that antisense oligomers act at the translational level, oligomers with potential targets on VSV viral mRNA and/or genomic RNA have been tested here. In vitro translation experiments in reticulocyte lysates, in vitro transcription experiments with permeabilized viruses, measurement of viral RNA transcription and accumulation in VSV infected cells, and antiviral experiments demonstrate in our model that antisense oligomers probably also act at other levels. Difficulties in the choice of the most effective antisense oligomer targets are also discussed.

Poliovirus protease 2A is required for interference with vesicular stomatitis virus-specified protein synthesis.

A subunit of eukaryotic initiation factor-4F (eIF-4F) which is a component of the protein complex which binds to the methylated cap structure at the 5' end of most cellular mRNAs, is proteolytically cleaved in poliovirus-infected cells resulting in the shutoff of cellular protein synthesis. Poliovirus mRNA is selectively translated in infected cells, in part, because translation of the uncapped viral mRNA does not require an intact cap binding protein complex. Wild-type poliovirus also inhibits the translation of vesicular stomatitis virus (VSV) mRNAs in coinfected cells, however, it has been unclear whether similar mechanisms are employed by poliovirus to interfere with cellular and VSV protein synthesis. Degradation of eIF-4F appears to be an indirect function of the poliovirus-encoded protease 2A. A poliovirus mutant in 2A failed to mediate eIF-4F cleavage and selectively terminate translation of capped cellular mRNAs. Unlike wild-type poliovirus, 2A-1 does not interfere with VSV-specified protein synthesis. These results indicate that the same viral protein, 2A protease, is required not only to effectively terminate host protein synthesis, but also to interfere with expression of a heterologous virus, VSV. In addition, 2A-1 specifies a function, heretofore undescribed for poliovirus, which interferes with VSV-induced shutoff of protein synthesis.

Differential effect of ATP concentration on synthesis of vesicular stomatitis virus leader RNAs and mRNAs

Cleavage of the beta-gamma bond of ATP is required for wild-type (wt) vesicular stomatitis virus transcription in vitro. Recent findings have established that a domain-specific phosphorylation of the virus NS protein is necessary for activity. We report here that RNA synthesis catalyzed by purified standard wt virions responded cooperatively to various ATP concentrations, with half-maximal activity at approximately 500 microM. In contrast, mutant polR1 standard virions and wt defective interfering particles both showed conventional Michaelis-Menten kinetic profiles with Km values of approximately 143 and approximately 133 microM, respectively. The former synthesize readthrough products of the leader-N gene junction in addition to plus-strand leader RNA and mRNAs, whereas the latter synthesize only minus-strand leader RNA. The cooperative response of wt virus products, however, was specific to mRNAs; the small fraction of the total products corresponding to plus-strand leader approximated Michaelis-Menten behavior. Since the unique phenotype of the polR mutants correlates with the synthesis of replicationlike products in vitro, the affected ATP-requiring function most likely regulates both transcription and replication. We suggest that this mutated function involves phosphorylation of viral proteins.

Involvement of the 24-kDa cap-binding protein in regulation of protein synthesis in mitosis.

The rate of protein synthesis in metaphase-arrested cells is reduced as compared to interphase cells. The reduction occurs at the translation initiation step. Here, we show that, whereas poliovirus RNA translation is not affected by the mitotic translational block, the translation of vesicular stomatitis virus mRNAs is. In an attempt to elucidate the mechanism by which initiation of protein synthesis is reduced in mitotic cells, we found that the interaction of the mRNA 24-kDa cap-binding protein (CBP) with the mRNA 5' cap structure is reduced in mitotic cell extracts, consistent with their lower translational efficiency. Addition of cap-binding protein complex stimulated the translation of endogenous mRNA in extracts from mitotic but not interphase cells. In addition, we found that the 24-kDa CBP from mitotic cells was metabolically labeled with 32P to a lesser extent than the protein purified from interphase cells. These results are consistent with a hypothesis that the 24-kDa CBP is implicated in the inhibition of protein synthesis in metaphase-arrested cells. Possible mechanisms for this inhibition are offered.

The fates of undermethylated mRNA cap structures of vesicular stomatitis virus (New Jersey) during in Vitro transcription

A dual label pulse-chase analysis employing S-adenosyl- l-[ methyl- 3H]methionine and [β- 32P]GTP has been devised to identigy precursors to the cap structure 7 mG(5′)ppp(5′)Am present on the 5′-terminal of vesicular stomatitis virus mRNAs. Both monomethylated cap structures, 7 mG(5′)ppp(5′)A and G(5′)ppp(5′)Am, have been detected in vitro in New Jersey serotype reactions containing suboptimal concentrations of S-adenosyl- l-methionine. The simultaneous chasing of both radiolabeled substrates allowed the determination of the transcriptive fate of each pulse-labeled cap structure in the total RNA population. Ten percent of the 7 mG(5′)ppp(5′)A cap structure and 27% of the G(5′)ppp(5′)Am cap structure generated during the pulse were chased into 7 mG(5′)ppp(5′)Am. These results suggested that while there may have been a preferred order of 5′-cap methylation, the order was not compulsory. The dual label analysis also revealed that only 34% of the pulse-labeled G(5′)ppp(5′)A cap structure could be chased into methylated cap structures. A nascent RNA chain length-dependent “methylation window” is proposed.

EFFECT OF RIBAVIRIN ON VIRAL TRANSCRIPTION

We have examined the effect of ribavirin on vesicular stomatitis virus (VSV) RNA synthesis using a cell-free transcription system. Our prior data indicated that ribavirin had little effect on primary transcription of VSV mRNAs synthesized intracellularly, but that these RNAs were translated inefficiently. Consistent with these data, VSV cell-free transcription was inhibited only mildly (25%) by ribavirin triphosphate at 100 mg/ml. However, ribavirin diphosphate (RDF) and, to a lesser extent, ribavirin monophosphate, were 2-3 times as inhibitory at the same concentration. Transcripts synthesized in the presence of RDP were full lengthed. The inhibition by RDP could be reversed by increasing the concentration of GTP. However, reversal could also be achieved by increasing the concentration of UTP, CTP, and ATP, but not by the addition of GDP. RDP was added to a LaCrosse virus cell-free transcription assay to determine if an inhibitory effect could be established in a viral system more sensitive to ribavirin than VSV; addition of RDP to this reaction led to profound inhibition of RNA synthesis at concentrations as low as 0.1 ug/ml. These data suggest that in some viruses ribavirin may have a major direct effect on initial steps of transcription and that this effect may be mediated by the di- and monophosphorylated forms.

Stoichiometric association of cap-binding protein I with translated polysomal globin mRNP.

The protein composition of a 12S polysomal globin messenger ribonucleoprotein (pmRNP) from rabbit reticulocytes was examined. The pmRNP was released from purified polysomes by puromycin treatment under run-off conditions of protein synthesis. The protein pattern of this pmRNP depends on the potassium ion concentration used during the run-off and the subsequent isolation. Several proteins show a salt-dependent association with the pmRNP while a few are constituents of the pmRNP at all salt concentrations tested. By cross-linking the pmRNP-derived proteins to [3H]methyl-labelled oxidized vesicular stomatitis virus (VSV) mRNA and by immunoblotting against anti-cap-binding protein (CBP I) antibodies, it is demonstrated that the association of the CBP I with the pmRNP depends on the ionic strength. At 65 mM KCl, CBP I shows low affinity for the pmRNP; at 140 mM KCl, the affinity of CBP I for the pmRNP is greatly enhanced. At this ionic strength, equimolar amounts of CBP I and mRNA are found in the pmRNP. At 500 mM KCl, the pmRNP is completely devoid of CBP I. In the non-translated free cytoplasmic mRNP (cmRNP) no CBP can be detected by either the cross-link or the immunoblot technique.

Effect of ribavirin on macromolecular synthesis in vesicular stomatitis virus-infected cells.

Ribavirin at 200 micrograms/ml inhibited vesicular stomatitis virus (VSV) growth in Chinese hamster ovary (CHO) cells by 2.5 logs. To determine the mechanism of this inhibition, viral macromolecular synthesis was examined. VSV primary transcription remained unaffected, but overall VSV RNA synthesis decreased by 40 to 60%. When ribavirin was added 1.5 h after infection, inhibition of progeny production was partially lost, indicating that the antiviral effect was on an early stage after primary transcription. Inhibition of RNA polymerization by premature chain termination was not evident. Viral translation, on the other hand, was reduced by 95% with an inhibition of every protein species. Furthermore, viral RNA synthesized in the presence of ribavirin did not translate well in an in vitro translation system. In contrast, uninfected CHO cells treated with ribavirin showed a greater sensitivity in RNA synthesis than in protein synthesis. This suggests that the cellular translational machinery was not directly affected. Short-term treatment of cells resulted in negligible toxicity, but after 24 h there was marked alteration of cellular integrity. These results, taken together with data on other viruses, suggest that in the presence of ribavirin, dysfunctional VSV mRNA was synthesized, resulting in its failure to be translated. The selective antiviral effects of ribavirin and its relative lack of toxicity for host cells may be predicted on the basis of mRNA turnover and the requirements for de novo functional mRNA.

Poly(riboadenylic acid) preferentially inhibits in vitro translation of cellular mRNAs compared with vaccinia virus mRNAs: possible role in vaccinia virus cytopathology.

Vaccinia virus-induced inhibition of host protein synthesis seems to be mediated by viral transcripts based on their differential inhibition of cellular mRNA translation in a rabbit reticulocyte lysate system. In this study, we demonstrated that the removal of poly(riboadenylic acid) [poly(A)] from the in vitro viral transcripts abolished this inhibition in the same cell-free system. This observation led us to the finding that less than 1 microM poly(A) completely inhibited HeLa cell mRNA translation in the reticulocyte lysate, whereas only 50% inhibition of vaccinia virus mRNA translation was observed at the same concentration. Similar results were also obtained in a wheat germ protein-synthesizing system. This inhibitory effect of poly(A) was totally abrogated by the addition of polydeoxythymidylate. This selective inhibition was highly specific for poly(A) since other homopolymers, including poly(G), poly(C), and poly(dA), were not capable of causing such an inhibition. Poly(U), however, had a moderate selective inhibitory effect. Among the several mRNAs tested, the translation of L-cell, encephalomyocarditis virus, and reovirus RNAs was also sensitive to poly(A). However, vesicular stomatitis virus mRNA translation was strikingly more resistant. These results suggest that poly(A), which is also synthesized by the virion-associated poly(A) polymerase may be involved in vaccinia virus-mediated host cell shutoff.