Evidence against involvement of host transcription in the replication of vaccinia and herpes simplex viruses

Mechanisms of replication-deficient vaccinia virus/T7 RNA polymerase hybrid expression: effect of T7 RNA polymerase levels and alpha-amanitin

Components of the eukaryotic vaccinia virus/T7 RNA polymerase hybrid expression system were assessed using recombinant and nonrecombinant forms of modified vaccinia Ankara (MVA), a replication-deficient vaccinia virus strain. Recombinant MVA virus expressing T7 RNA polymerase (Wyatt, L. S., Moss, B., and Rozenblatt, S. (1995). Virology 210, 202-205) stimulated high levels of expression from a T7 promoter-chloramphenicol acetyltransferase (CAT) reporter. Most, but not all, of the virally induced expression was T7 RNA polymerase and T7 promoter dependent, with no viral enhancement of translation of T7 transcripts. The efficacy of supplying T7 RNA polymerase expression from nonviral sources was evaluated using a self-amplifying T7 RNA polymerase autogene or an inducible T7 RNA polymerase expression vector. The latter modes yielded CAT activity dependent on T7 RNA polymerase expression; however, expression required viral factors independent of T7 RNA polymerase and did not reach that attained using the recombinant virus. In further experiments, MVA-induced T7 RNA polymerase expression was upregulated by alpha-amanitin, an inhibitor of eukaryotic polymerases. This indicates that MVA/T7 RNA polymerase hybrid expression may be rendered still more efficient by ameliorating transcriptional interference due to an alpha-amanitin-sensitive eukaryotic factor(s).

Anchoring a vaccinia virus promoter in the nucleus prevents its trans-activation by viral infection

The vaccinia virus 7.5 kDa constitutive promoter, when fused to a reporter gene and recombined into the genome of L cells, is not activatable upon subsequent infection with vaccinia virus. However, the same promoter is actively transcribed during transient cytoplasmic transfection procedures or within the context of the viral genome. This suggests that the intact vaccinia transcriptional machinery either does not enter the nucleus or, if it does, is unable to interact with cellular chromatin.

Relationship between RNA polymerase II and efficiency of vaccinia virus replication

It is clear from previous studies that host transcriptase or RNA polymerase II (pol II) has a role in poxvirus replication. To elucidate the participation of this enzyme further, in this study we examined several parameters related to pol II during the cycle of vaccinia virus infection in L-strain fibroblasts, HeLa cells, and L6H9 rat myoblasts. Nucleocytoplasmic transposition of pol II into virus factories and virions was assessed by immunofluorescence and immunoblotting by using anti-pol II immunoglobulin G. RNA polymerase activities were compared in nuclear extracts containing crude enzyme preparations. Rates of translation into cellular or viral polypeptides were ascertained by labeling with [35S]methionine. In L and HeLa cells, which produced vaccinia virus more abundantly, the rates of RNA polymerase and translation in controls and following infection were higher than in myoblasts. The data on synthesis and virus formation could be correlated with observations on transmigration of pol II, which was more efficient and complete in L and HeLa cells. The stimulus for pol II to leave the nucleus required the expression of both early and late viral functions. On the basis of current and past information, we suggest that mobilization of pol II depends on the efficiency of vaccina virus replication and furthermore that control over vaccinia virus production by the host is related to the content or availability (or both) of pol II in different cell types.

Nucleotide sequence and molecular genetic analysis of the vaccinia virus HindIII N/M region encoding the genes responsible for resistance to alpha-amanitin

The genomic location of the gene(s) which provides vaccinia virus (VV) alpha-amanitin-resistant mutants with a drug-resistant phenotype have been mapped to the HindIII N/M region of the genome by the use of marker rescue techniques [E. C. Villarreal and D. E. Hruby (1986) J. Virol. 57, 65-70]. Nucleotide sequencing of a 2356-bp HindIII-Sau3A fragment of the vaccinia virus genome encompassing this region reveals the presence of two complete leftward-reading open reading frames (ORFs, N2 and M1) and two incomplete ORFs (N1 and M2). By computer analysis the N2 and M1 ORFs would be predicted to encode soluble VV polypeptides with molecular weights of approximately 20 and 48 kDa, respectively. The N2 and M1 ORFs have extremely A-T-rich 5'-proximal sequences, consistent with previous data regarding the location and A-T-richness of viral early promoters. Likewise, the consensus signal believed to be involved in terminating VV early gene transcription, TTTTTNT, was evident at the 3'-boundary of both the N2 and M1 ORFs suggesting that these genes may be VV early genes. The in vivo transcriptional activity, orientation, and limits of these putative transcriptional units were investigated by Northern blot, nuclease S1, and primer extension analysis. Both N2- and M1-specific transcripts were detected in the cytoplasm of VV-infected cells, suggesting that these loci are bonafide viral genes. Time-course nuclease S1 experiments revealed that the N2 gene was transcribed exclusively prior to VV DNA replication. In contrast, the M1 gene was transcribed throughout infection, although different start sites were used at early versus late times postinfection. These results are discussed in relation to the drug-resistant phenotype and future experiments to identify the viral gene product responsible.

Effect of inhibitors of the host cell RNA polymerase II on African swine fever virus multiplication

The role of the host cell RNA polymerase II in African swine fever (ASF) virus growth has been examined using inhibitors of this enzyme. The adenosine analog 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), an inhibitor of mRNA precursor synthesis in mammalian cells, strongly inhibits the production of infectious progeny virus in Vero cells, but does not significantly affect the synthesis of virus-specific macromolecules. On the other hand, virion assembly seems to proceed normally in the presence of DRB, as virus particles can be seen in electron micrographs with a morphology indistinguishable from that observed in the absence of the inhibitor. However, taking into account the inhibition of the infectivity caused by the drug, most of these particles must be defective. In contrast with this effect of DRB on ASF virus replication, the toxin alpha-amanitin does not inhibit the production of infectious ASF virus in Vero cells or porcine alveolar macrophages. This result indicates that the host RNA polymerase II does not transcribe viral genes and that active transcription of the cell genome is not needed for ASF virus replication.

Viral RNA polymerases

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

Molecular dissection of cis-acting regulatory elements from 5'-proximal regions of a vaccinia virus late gene cluster

Promoter elements responsible for directing the transcription of six tightly clustered vaccinia virus (VV) late genes (open reading frames [ORFs] D11, D12, D13, A1, A2, and A3) from the HindIII D/A region of the viral genome were identified within the upstream sequences proximal to each individual locus. These regions were identified as promoters by excising them from the VV genome, abutting them to the bacterial chloramphenicol acetyl transferase gene, and demonstrating their ability to drive expression of the reporter gene in transient-expression assays in an orientation-specific manner. To delineate the 5' boundary of the upstream elements, two of the VV late gene (A1 and D13) promoter: CAT constructs were subjected to deletion mutagenesis procedures. A series of 5' deletions of the ORF A1 promoter from -114 to -24 showed no reduction in promoter activity, whereas additional deletion of the sequences from -24 to +2 resulted in the complete loss of activity. Deletion of the ORF A1 fragment from -114 to -104 resulted in a 24% increase in activity, suggesting the presence of a negative regulatory region. In marked contrast to previous 5' deletion analyses which have identified VV late promoters as 20- to 30-base-pair cap-proximal sequences, 5' deletions to define the upstream boundary of the ORF D13 promoter identified two positive regulatory regions, the first between -235 and -170 and the second between -123 and -106. Background levels of chloramphenicol acetyltransferase expression were obtained with deletions past -88. Significantly, this places the ORF D13 regulatory regions within the upstream coding sequences of the ORF A1. A high-stringency computer search for homologies between VV late promoters that have been thus far characterized was carried out. Several potential consensus sequences were found just upstream from RNA start sites of temporally related promoter elements. Three major conclusions are drawn from these experiments. (i) The presence of promoters preceding each late ORF supports the hypothesis that each is expressed as an individual transcriptional unit. (ii) Promoter elements can be located within the coding portion of the upstream gene. (iii) Sequence homologies between temporally related promoter elements support the notion of kinetic subclasses of late genes.

The role of the host cell nucleus in vaccinia virus morphogenesis

Despite the fact that cells infected with wild type vaccinia virus synthesize viral DNA and assemble progeny virus particles within the cytoplasm, the host cell nucleus is required for a productive infection. Recent evidence suggests that vaccinia virus selectively recruits components from the host cell nucleus into the cytoplasm for use by the developing virus. One of these components is the largest subunit of the cellular RNA polymerase II (Pol II).

Influence of RNA polymerase II upon vaccinia virus-related translation examined by means of alpha-amanitin

Our previous studies employing alpha-amanitin-sensitive H-9 and resistant Ama 102 mutant host cells demonstrated that polymerase II (Pol II), or a drug-sensitive component of the enzyme, is required for replication of vaccinia virus. Evidence was also obtained indicating that transcription from the host genome does not appear to be involved (Silver et al., 1979; Silver and Dales, 1982), suggesting a possible role for Pol II in transcription from the viral genome. This idea is consistent with the present findings, based on immunofluorescence analysis, which revealed that upon infection Pol II antigen is mobilized out of the nucleus into discrete cytoplasmic foci. Effects of treating H-9 rat myoblasts with alpha-amanitin upon vaccinia-specific protein synthesis were also examined. Under the experimental conditions employed, the toxin drastically curtailed in vivo translation into early, late and late-late proteins without altering the spectrum of polypeptides produced. By contrast, treatment with the drug affected, only minimally, the rate of transcription into viral RNA, whether in vivo or from isolated vaccinia factories. The mRNA isolated from infected and treated or untreated cells was translated in a reticulocyte lysate with equal efficiency and general fidelity. This finding suggests that Pol II may be involved in transcription into RNAs related to factors controlling the in vivo translation process. The possible mechanisms for exercising such controls are discussed in relation to factors regulating transcription by host RNA polymerases from a viral DNA genome.

Vaccinia virus DNA sequences in the nucleus of persistently infected Friend erythroleukemia cells

FL vac cell lines are Friend erythroleukemia cells persistently infected with vaccinia virus. These cells produce attenuated leukemia virus, virulent poxvirus, resist superinfection with vaccinia, and show high levels of spontaneous erythrodifferentiation and decreased tumorigenicity in syngeneic hosts (Pogo, G.T. and Friend, C. (1982) Proc. Natl. Acad. Sci. USA 79, 4805-4809). To determine whether resistance to superinfection was associated with the presence of vaccinia DNA in the nucleus, DNA from cells at different passage levels was hybridized to a vaccinia DNA probe. Vaccinia DNA sequences were detected in the nucleus of cells of lines that were productively infected with vaccinia. No such sequences were detected in productively infected with vaccinia. No such sequences were detected in productively infected L cells nor in persistently infected cell lines that no longer produced infectious particles but were resistant to superinfection. Although no evidence of integration of vaccinia DNA was observed, differences in the restriction patterns were detected at some passage levels. The presence of vaccinia virus DNA sequences in the nucleus apparently did not affect the size of the provirus, the integration pattern or the expression of the leukemia virus.

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.

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.

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.