Vaccinia virus replication. I. Requirement for the host-cell nucleus

Mapping the genomic location of the gene encoding alpha-amanitin resistance in vaccinia virus mutants

To facilitate the determination of the genomic location of the vaccinia virus gene(s) encoding alpha-amanitin resistance (alpha r) (Villarreal et al., J. Virol. 51:359-366, 1984), a collection of alpha r, temperature-sensitive (ts) mutants were isolated. The premise of these experiments was that mutants might be found whose dual phenotypes were the result of a single or two closely linked mutations. Genetic analyses of the alpha rts mutant library revealed two mutants, alpha rts7 and alpha rts12, that apparently fit this criterion; in alpha rts7 the two lesions were indistinguishable, whereas in alpha rts12 the two mutations were closely linked but separable. Cloned vaccinia virus HindIII DNA fragments were used to marker rescue the temperature-sensitive phenotype of these two dual mutants. The temperature-sensitive lesion of alpha rts7 was rescued by the HindIII N fragment (1.5 kilobases), whereas alpha rts12 was rescued by the neighboring HindIII M fragment (2.0 kilobases). The progeny virions of the alpha rts7 HindIII-N rescue reverted to an alpha-amanitin-sensitive phenotype, whereas the alpha rts12 HindIII-M progeny were still resistant to the drug. Taken together, these data indicate that the gene encoding alpha-amanitin resistance maps to the HindIII N fragment and provides evidence for the existence of essential vaccinia virus genes in a region of the genome previously believed to be nonessential for replication in tissue culture. Biochemical analyses revealed that both mutants were capable of synthesizing DNA as well as early and late viral proteins at the permissive and nonpermissive temperatures. At the nonpermissive temperature alpha rts12 and alpha rts7 were unable to process the major core precursors P94 and P65 into VP62 and VP60.

Polyamines stimulate DNA-dependent RNA synthesis catalyzed by vaccinia virus

The RNA synthesis in purified vaccinia virus can occur in the presence of either Mg2+ or Mn2+ if polyamine (spermidine or spermine) is present in the assay system. Under our assay conditions transcription was linear up to 30 min and the RNAs synthesized had a sedimentation coefficient of about 8 to 12 S. We also prepared a virus extract from purified vaccinia virus and tested for in vitro transcription. The soluble transcription system was dependent on the addition of exogenous DNA and single-stranded DNA was a more effective template than double-stranded. In the presence of polyamine and Mg2+ or Mn2+ the viral RNA polymerase was active in the transcription of total native vaccinia DNA and a small fragment cloned in pBR322.

Specific transcription of orthopox virus DNA by HeLa cell RNA polymerase II

A HeLa cell extract was used to transcribe DNA isolated from cowpox virus. Truncated templates generate accurately initiated run-off transcripts of discrete sizes and whose sensitivity to inhibition by alpha-amanitin indicates synthesis by cell RNA polymerase II. A mapped restriction fragment of wild-type cowpox DNA contains specific sites of initiation which are not detected in the geographically equivalent fragment from a cowpox mutant having a defined sequence rearrangement in this region.

Expression of Sindbis virus structural proteins via recombinant vaccinia virus: synthesis, processing, and incorporation into mature Sindbis virions

We have obtained a vaccinia virus recombinant which contains a complete cDNA copy of the 26S RNA of Sindbis virus within the thymidine kinase gene of the vaccinia virus genome. This recombinant constitutively transcribed the Sindbis sequences throughout the infectious cycle, reflecting the dual early-late vaccinia promoter used in this construction. The Sindbis-derived transcripts were translationally active, giving rise to both precursor and mature structural proteins of Sindbis virus, including the capsid protein (C), the precursor of glycoprotein E2 (PE2), and the two mature envelope glycoproteins (E1 and E2). These are the same products translated from the 26S mRNA during Sindbis infection, and thus these proteins were apparently cleaved, glycosylated, and transported in a manner analogous to that seen during authentic Sindbis infections. By using epitope-specific antibodies, it was possible to demonstrate that recombinant-derived proteins were incorporated into Sindbis virions during coinfections with monoclonal antibody-resistant Sindbis variants. These results suggest that all the information necessary to specify the proper biogenesis of Sindbis virus structural proteins resides within the 26S sequences and that vaccinia may provide an appropriate system for using DNA molecular genetic manipulations to unravel a variety of questions pertinent to RNA virus replication.

Intracellular location of rabbit poxvirus nucleic acid within infected cells as determined by in situ hybridization

The intracellular location of rabbit poxvirus DNA within cells during the course of infection has been determined by the hybridization in situ of labeled viral DNA probes to uninfected and infected cells under various conditions. Extensive control experiments were performed to demonstrate that DNA could be detected selectively and accurately within the cell. Our results suggest that rabbit poxvirus DNA is located only within the cytoplasm during the reproductive cycle, and we found no evidence that viral DNA enters the cell nucleus. The pattern of hybridization of viral DNA at early times (1 and 2 h postinfection) and in the presence of inhibitors of viral DNA synthesis suggests that there may be an association between the input viral DNA and some structural component of the host cell. A number of observations support the hypothesis that the host cell nucleus is required for a productive poxvirus infection. Our results are discussed in terms of the possible role of the nucleus in the replication of poxviruses.

Neomycin resistance as a dominant selectable marker for selection and isolation of vaccinia virus recombinants

The antibiotic G418 was shown to be an effective inhibitor of vaccinia virus replication when an appropriate concentration of it was added to cell monolayers 48 h before infection. Genetic engineering techniques were used in concert with DNA transfection protocols to construct vaccinia virus recombinants containing the neomycin resistance gene (neo) from transposon Tn5. These recombinants contained the neo gene linked in either the correct or incorrect orientation relative to the vaccinia virus 7.5-kilodalton gene promoter which is expressed constitutively throughout the course of infection. The vaccinia virus recombinant containing the chimeric neo gene in the proper orientation was able to grow and form plaques in the presence of G418, whereas both the wild-type and the recombinant virus with the neo gene in the opposite polarity were inhibited by more than 98%. The effect of G418 on virus growth may be mediated at least in part by selective inhibition of the synthesis of a subset of late viral proteins. These results are discussed with reference to using this system, the conferral of resistance to G418 with neo as a positive selectable marker, to facilitate constructing vaccinia virus recombinants which contain foreign genes of interest.

Transcriptional and translational analysis of the vaccinia virus late gene L65

Among the products of vaccinia virus genes which are expressed late in infection is a major polypeptide (Mr, 65,000) designated L65. Pulse-chase analyses indicated that L65 is not subject to posttranslational cleavage as is the core polypeptide p4b which migrates to a similar position in sodium dodecyl sulfate-polyacrylamide gels. A polypeptide of 65,000 molecular weight produced in reticulocyte lysates programmed with viral mRNA isolated late in infection was identified as L65 by peptide mapping. L65 mRNA was purified by hybridization selection to restriction fragments of the viral genome and translated in vitro. This allowed the gene encoding L65 to be mapped to the rightmost 4.5 kilobase pairs of the HindIII D fragment. Transcriptional mapping of this region of the genome detected a late mRNA which was initiated at 450 base pairs to the right of the HindIII D-A junction, was transcribed in the leftward direction, and was terminated in the nondescript manner typical of vaccinia virus late mRNAs.

In vitro transcription of a cloned vaccinia virus gene by a soluble extract prepared from vaccinia virus-infected HeLa cells

Faithful transcription of a vaccinia virus gene was accomplished in vitro by using a soluble extract prepared from vaccinia virus-infected HeLa cells. Specific transcription of the cloned vaccinia virus gene was detected by using template DNA restricted within the transcribed region. The vaccinia virus gene was not transcribed by extracts prepared from uninfected HeLa cells even with supplementation by purified vaccinia virus RNA polymerase, nor was a clone of adenovirus 2 DNA bearing the major late promoter transcribed by the extract from vaccinia virus-infected HeLa cells. Thus, infection by vaccinia virus altered cellular transcriptional specificity to favor expression of vaccinia virus genes. RNA synthesis by the infected cell extract was resistant to alpha-amanitin but strongly inhibited by beta, gamma-imido ATP and novobiocin.

Vaccinia virus induces ribonucleotide reductase in primate cells

Infection of monkey kidney (BSC-40) cells with vaccinia virus strain WR resulted in a marked increase in ribonucleoside diphosphate reductase (EC 1.17.4.1) activity as measured by CDP reduction in cell-free extracts. After a synchronous infection, increased activity was detected at 2 h, peaked at 4 to 5 h, and then declined between 6 and 8 h to the endogenous cellular level. The induction, detectable at 0.5 PFU/cell, correlated strongly with multiplicity of infection to 10 PFU/cell and continued to increase to 50 PFU/cell. It paralleled the previously described induction of viral DNA polymerase and thymidine kinase, suggesting that the reductase may also be a product of early transcription of the viral genome. The inhibition of DNA synthesis throughout infection resulted in prolonged accumulation of reductase activity and delayed and incomplete down-regulation at 8 h, suggesting that repression involves late functions. Rescue of fluorodeoxyuridine-inhibited DNA synthesis with exogenous thymidine restored the normal pattern. Preferential association of the induced reductase with the cytoplasmic sites of vaccinia virus DNA replication (virosomes) was not detected. The induced enzyme is similar in several respects to other eucaryotic ribonucleotide reductases, but is distinct from host cell reductase in response to certain modulators of reductase activity (M. B. Slabaugh and Christopher K. Mathews, J. Virol. 52:501-506, 1984). Full activity required an activator, exogenous reducing equivalents, and iron. Hydroxyurea, EDTA, dATP, and dTTP inhibited CDP reduction, setting this reductase apart from T4 reductase, which is not inhibited by dATP, and from herpesvirus reductase, which requires no activation and is insensitive to deoxyribonucleoside triphosphate inhibition.

Isolation of vaccinia virus mutants capable of replicating independently of the host cell nucleus

alpha-Amanitin-resistant vaccinia virus mutants were isolated after serial viral passages in BSC-40 cells that were carried out in the presence of inhibitory levels (6 micrograms/ml) of alpha-amanitin. One such mutant, alpha-27, was highly refractory (greater than 95%) to alpha-amanitin-mediated inhibition and was selected for further study. In the absence of drug, the phenotypes of alpha-27 and wild-type vaccinia virus were indistinguishable with respect to growth kinetics. DNA synthesis, protein synthesis, and morphogenesis. Infections in the presence of alpha-amanitin revealed two striking differences, however. First, wild-type virus was unable to catalyze proteolytic processing of the two major capsid proteins VP62 and VP60, whereas alpha-27 was most efficient at this process. Second, wild-type viral morphogenesis within the infected cells was arrested by alpha-amanitin at an apparently analogous step to that previously described for enucleated cells. This observation was supported by the ability of alpha-27 virus to replicate in enucleated BSC-40 cells. Restriction enzyme analyses of alpha-27 versus wild-type genomes revealed that a XhoI cleavage site was altered in the alpha-27 DNA molecule, suggesting a possible location for the alpha-amanitin resistance locus.

Interferon-mediated, double-stranded RNA-dependent protein kinase is inhibited in extracts from vaccinia virus-infected cells

The interferon-inducible, double-stranded RNA (dsRNA)-dependent protein kinase which phosphorylates an endogenous HeLa 69 kilodalton polypeptide or exogenous initiation factor eIF2 was inhibited during vaccinia virus infection. High interferon doses (20,000 reference units per ml) did not prevent this inhibition. The inhibition required protein synthesis but not viral DNA synthesis during infection, suggesting that an early vaccinia virus gene function was responsible. An active dsRNA-dependent protein kinase could be recovered from an inactive extract by purification on polyinosinate X polycytidylate-cellulose. An inhibitor of the protein kinase, therefore, must be present in the inactive extract. Similar results have been obtained with mouse L929 cells. At early time points of infection, the protein kinase in cell extracts required exogenous dsRNA for activity. This argues against endogenous viral dsRNA and activation of the kinase in the intact cell. At late time points of infection (when vaccinia virus dsRNA was almost certainly formed), the inhibitor of the kinase is present. Accordingly, it seems unlikely that the kinase played any role in the interferon-mediated inhibition of virus growth observed in these cells under these particular conditions.

2-5A accumulates to high levels in interferon-treated, vaccinia virus-infected cells in the absence of any inhibition of virus replication

We investigated the effects of interferon treatment on virus yield, protein synthesis, and the 2-5A system in vaccinia virus-infected HeLa, L929, and CV1 cells. Under the culture conditions used, vaccinia virus replication was relatively insensitive to the antiviral effects of interferon. In L929 and HeLa cells, interferon at 400 reference units (r.u.) per ml had little effect on viral protein synthesis, the virus-induced inhibition of host protein synthesis, or virus yield: 2,000 to 20,000 r.u./ml were required to inhibit these. Despite this, high levels (up to 5 microM) of 2-5A [ppp(A2'p)nA; n greater than or equal to 2] were found during vaccinia infection of all of these types of cells treated with 400 r.u. of interferon per ml, i.e., at interferon concentrations too low to inhibit significantly virus growth. High levels (up to 5 microM) were also found in non-interferon-treated HeLa cells (which have a high constitutive level of 2-5A synthetase) in which vaccinia virus replicates perfectly well. It can be concluded that high levels of 2-5A per se have no necessary antiviral effect on vaccinia virus in these systems. These results are in marked contrast to those obtained here and previously with encephalomyocarditis virus. For example, in HeLa cells less than 20 nM 2-5A accumulated, but virus replication was inhibited by 50 r.u. of interferon per ml (Silverman et al., Eur. J. Biochem. 124:131-138, 1982). The characteristic cleavage of rRNA by the 2-5A-dependent RNase was delayed relative to 2-5A accumulation in the vaccinia virus-infected cells. This delay was not the result of either defective 2-5A or of a stable virus-induced inhibition of the 2-5A-dependent RNase; 2-5A extracted from the cells had full biological activity when assayed by activation of the 2-5A-dependent RNase in cell extract, and the 2-5A-dependent RNase extracted from the vaccinia virus-infected cells was fully active in vitro. The basis for the delay remains to be determined. High levels of 2-5A were not observed when late (DNA synthesis-dependent) vaccinia transcription was inhibited by either cycloheximide or cytosine arabinoside. The only known activator of the 2-5A synthetase is double-stranded RNA. The presence of 2-5A therefore implies the natural occurrence of double-stranded structures in late viral RNA in intact vaccinia virus-infected cells.

Transcriptional and translational analysis of a strongly expressed early region of the vaccinia virus genome

A transcriptional and translational map was obtained for a 7-kilobase pair EcoRI fragment of vaccinia virus DNA containing 4% of the viral genome. This particular region contains a cluster of early genes which are transcribed in viral cores in vitro and in infected cells before uncoating of the viral DNA. Expression of this region was characterized by using vaccinia virus DNA sequences cloned in phage and plasmid vectors. Polypeptides encoded on the 7-kilobase pair fragment were identified by cell-free translation of viral mRNA selected by hybridization to immobilized DNA fragments. Early RNA programmed the synthesis of six proteins having apparent molecular weights of 63,000, 38,000, 37,000, 25,000, 15,000, and 13,000. The same result was obtained with RNA synthesized in vitro. A new species of 40 kilodaltons was synthesized in response to late RNA. Of the six "early" polypeptides, only the 13-kilodalton component was synthesized from late mRNA. RNA derived from the 7-kilobase pair region was analyzed by a variety of methods including hybridization blot, in vitro recapping, and S1 nuclease techniques. A surprisingly complex pattern of early transcription was found, indicating the existence of families of overlapping RNA species which share common 5'-proximal sequences. In addition, larger RNAs were identified spanning two or more adjacent genes. These RNAs appear to possess a common 5' terminus with transcripts derived from the first gene and are coterminal at the 3' end with RNAs from the downstream gene. Late RNA encoding the 40-kilodalton protein was shown to be heterogeneous in size. A single 5' terminus but no unique 3' terminus was identified for this class of transcripts. RNA species synthesized by cores in vitro were of similar size to authentic transcripts isolated from infected cells at early times.

Effects of ATP and inhibitory factors on the activity of vaccinia virus type I topoisomerase

Vaccinia virus cores contain a type I topoisomerase which promotes the relaxation of superhelical DNA of either handedness (Bauer et al., Proc. Natl. Acad. Sci. U.S.A. 74:1841-1845, 1977). The activity of partially purified vaccinia virus topoisomerase (VV-Topo I) was determined in the presence of ATP, dATP, GTP, ADP, and ATP analogs in which hydrolysis of the alpha, beta or beta, gamma phosphate bond is restricted. Topoisomerase activity was stimulated 2.5-fold by the addition of 2 to 4 mM ATP or dATP to standard assay mixtures; 2 mM GTP produced no significant effect on enzyme activity. The addition of 2 mM beta, gamma-imido ATP or 2 mM gamma-thiophosphate ATP reduced VV-Topo I activity by 80 and 65%, respectively. In contrast, 4 mM alpha, beta-methylene ATP produced no significant change in topoisomerase activity compared to ATP itself. Assays performed in the presence of 4 mM ADP exhibited an 80% reduction in enzyme activity. The preparations of VV-Topo I used for these studies showed, however, no detectable DNA-dependent or -independent ATPase activity. The activity of VV-Topo I was similarly measured in the presence of the antibiotics novobiocin and coumermycin A1, which inhibited enzyme activity by 50% at concentrations of 180 and 40 microM, respectively. Comparable inhibition of VV-Topo I activity was observed in the presence of 1 mM beta, gamma-imido ATP. We determined that novobiocin inhibits vaccinia core transcription at the same concentrations which inhibit vaccinia core topoisomerase I activity. These results suggest that the vaccinia DNA topoisomerase may play a role in the ATP-dependent transcription of viral genes from intact core particles.

Vaccinia virus induces cellular mRNA degradation

The infection of mouse L cells with vaccinia virus induced a rapid inhibition of cellular polypeptide synthesis and a diversion of protein synthesis to the exclusive production of viral polypeptides. This shutoff of cell-specific protein synthesis was achieved by a novel mechanism by which the virus induced the rapid degradation of cellular mRNAs. Concurrent with the degradation of cellular mRNA, the virus proceeds in the orderly temporal expression of its own genetic information. The effect of vaccinia virus infection upon two abundant L-cell mRNAs was assessed by using the highly conserved cDNA sequences that encode chicken beta-actin and rat alpha-tubulin. Hybridization analyses demonstrated that throughout infection there is a rapid and progressive degradation of both of these mRNAs. In fact, after 3 h of infection they are reduced to less than 50% of their concentration in uninfected L cells, and between 8 to 10 h they are almost entirely degraded. This observation explains in part the mechanism by which vaccinia virus inhibits host cell protein synthesis.

Biochemical and genetic characterization of vaccinia virus temperature-sensitive mutants with DNA- and DNAf-phenotypes

Eighty temperature-sensitive (ts) mutants of vaccinia virus were examined for defects in synthesis of DNA. Nine ts mutants were incapable of synthesizing DNA at the restrictive temperature of 39.5 degrees C (DNA- mutants). Biochemical and genetic data indicate that all 9 DNA- mutants carry mutations in different genes. Temperature shift-up experiments have shown that 6 ts mutants with the DNA- phenotype have mutations in the genes coding for the proteins directly associated with vaccinia DNA synthesis. Temperature shift-down experiments in the presence of cytosine arabinoside revealed 5 ts mutants capable of synthesizing DNA at the elevated temperature, but this DNA failed to form infectious virions. These ts mutants were designated as DNAf- mutants. Pulse-chase experiments for the DNAf- mutant 1877 revealed that viral DNA produced at 39.5 degrees C was incapable of entering into mature virions or any subviral particles. Based on the data for recombination among ts mutants with the DNA- and DNAf- phenotype a tentative genetic map was constructed.

Proteins specified by African Swine Fever virus. IV. Glycoproteins and phosphoproteins

African Swine Fever virus infected MS cells labeled with radioactive 14C-amino acids, 32Pi or [3H]-glucosamine were examined by high resolution sodium dodecylsulfate polyacrylamide gel electrophoresis and showed 43 infected cell polypeptides. Twenty-one of these proteins were present in the nuclear fraction of infected cells. At least 22 of the infected cell polypeptides induced antibodies during natural infections in swine. The pattern of infected cell polypeptides modified by incorporation of showed prosthetic groups that at least 8 polypeptides were phosphorylated and at least three specific viral glycoproteins (A, B and C) were detected by immunoprecipitation. The most highly glycosylated polypeptide corresponds to the structural viral protein VP51.

Animal virus screens for potential teratogens. I. Poxvirus morphogenesis

The growth of poxvirions in cell culture is considered a teratogen screening test, since this virus has a rapid, simple morphogenetic pathway that is dependent upon cell proliferation. Vaccinia WR-infected BSC 40 monolayers were exposed to 42 known teratogens and 9 nonteratogens at dosages from 1 microM to 100 mM. After 24 h of infection, the number of functional virions was determined by plaque assay. Thirty-three of the 42 teratogens inhibited the virus, 3 teratogens stimulated the virus, and 6 teratogens were false-negatives. Eight of the 9 nonteratogens had no effect on virus proliferation at dosages as high as 600 times the lowest reported teratogenic dosage. The number of new virions could be directly related to the concentration of the teratogen in vitro, thus allowing each compound to be characterized by an RD50. The RD50 dosage in milligrams per liter was 98% correlated with the lowest reported teratogenic dose in vivo in milligrams per kilogram. In sum, vaccinia-infected cells have an easily identifiable endpoint, plaque-forming units, which may be an accurate prognosticator of teratogenesis.

The mechanism of cytoplasmic orthopoxvirus DNA replication

Orthopoxvirus DNA replication occurs in the cytoplasm of infected cells within discrete foci designated as virosomes. We show that newly synthesized rabbit poxvirus (RPV) virosomal DNA consists predominantly of concatamers wherein unit length molecules are joined by fusion of two left (LL) or right (RR) ends, resulting in genomes aligned in alternating head-to-head and tail-to tail mirror image arrays. These concatameric molecules serve as the substrates from which unit length DNA molecules are excised during morphogenesis. We propose a mechanism by which internal deletions within these concatameric arrays prior to genome excision and packaging could create inverted terminal repeats and generate gene duplications.

Control of expression of the vaccinia virus thymidine kinase gene

mRNA extracted from vaccinia virus-infected cells early after infection directs cell-free synthesis of enzymatically active viral thymidine kinase (Hruby and Ball, Virology, in press). We used this assay for a specific vaccinia virus mRNA to study the induction and repression of the viral thymidine kinase gene during infection of thymidine kinase-deficient L-cells. As observed previously by other workers, the synthesis of thymidine kinase occurred immediately after infection but was switched off after 4 h later. We observed similar kinetics of accumulation and shutoff under conditions where viral DNA synthesis and late gene expression were inhibited. Cell-free translation of mRNA from infected cells showed that the concentration of functional message for viral thymidine kinase reached a peak 3 to 4 h after infection and then decreased with a half-life of about 1 h. These kinetics indicated that significant levels of thymidine kinase mRNA persisted in cells which had stopped synthesizing the enzyme. Under conditions where late gene expression was inhibited, high concentrations of functional mRNA could be isolated from cells at late times after infection. On the basis of these results, we conclude that the repression of thymidine kinase expression is mediated at the translational level by one or more early or delayed early viral genes. Repression is accompanied by, but does not depend on, the inactivation or degradation of thymidine kinase mRNA, which is a late gene function.

The coronavirus avian infectious bronchitis virus requires the cell nucleus and host transcriptional factors

Replication of avian infectious bronchitis virus in permissive BHK-21 cells is blocked when these cells are enucleated or irradiated with ultraviolet light prior to infection, or if cells are treated with α-amanitin during the virus growth cycle. This coronavirus, like influenza virus, can replicate normally in the presence of α-amanitin in Chinese hamster ovary cells which possess a drug-resistant RNA polymerase II. These findings indicate that avian infectious bronchitis virus requires the intact cell nucleus and one or more host transcriptional functions for productive infections. Preliminary data suggest that these cellular functions involve some aspect of virus-directed RNA synthesis.

Splicing of the late mRNAs of polyoma virus does not occur in the cytoplasm of the infected cell

The three mRNAs that encode the capsid proteins of polyoma virus are produced by the excision of different sequences from continuous transcripts of the L strand of viral DNA. All three of the mRNAs have long half lives, and the larger species are not converted to the smaller ones to any measurable extent within the cytoplasm. Therefore the cytoplasmic proportions of late polyoma mRNAs are predetermined by splicing that is confined to the nucleus of the infected cell and which is complete by the time that mRNA is transported to the cytoplasm.

Biogenesis of poxviruses: role for the DNA-dependent RNA polymerase II of the host during expression of late functions

The participation of host RNA polymerase II in the vaccinia life cycle was examined by comparing efficiency of multiplication after treating the Ama+ sensitive and Ama 102 drug resistant lines with alpha-amanitin. In the latter, resistance is due to a mutation in RNA polymerase II. The toxin profoundly reduces synthesis of virus-specified polypeptides and morphopoeisis in Ama+ but not in Ama 102 rat myoblasts without appreciably altering vaccinia DNA replication in either cell type. This implicates RNA polymerase II in the expression of late virus functions. Circumstantial evidence from a model system indicates that gamma irradiation of the host prior to infection might disrupt transcription into functional mRNA from the nucleus. Irradiation does not, however, alter the capability of the host to support vaccinia multiplication fully. Therefore, ongoing host nuclear transcription may not be required by this virus. The above results are consistent with the ability of cytoplasts to produce small quantities of mature progeny. Our studies lead us to hypothesize that RNA polymerase II or a subunit of the host enzyme may participate directly in late transcription of the vaccinia genome.

Vaccinia virus replication requires active participation of the host cell transcriptional apparatus

The ability of vaccinia virus to replicate in BSC-40 monkey cells whose nuclei have been functionally inactivated was examined. Exposure of cell monolayers to ultraviolet radiation at doses that did not alter the cells' capacity to support a subsequent infection by a cytoplasmic virus (vesicular stomatitis virus) caused a reduction to less than 10% in the observed yield of infectious progeny from vaccinia virus and herpes simplex virus (type 1) infections. Similarly, replication of vaccinia virus was reduced to 5% by treatment of BCS-40 cells with alpha-amanitin (10 microgram/ml), a potent inhibitor of nuclear mRNA synthesis. In both situations, ultraviolet irradiation and alpha-amanitin treatment, early and late vaccinia viral genes were expressed at high levels, but the newly synthesized virion components were not assembled into mature infectious particles. Taken together, these data suggest that the active involvement of the host cell nuclear transcriptive system is obligatory in the vaccinia virus replicative cycle.