Overexpression, purification, and late transcription factor activity of the 17-kilodalton protein encoded by the vaccinia virus A1L gene

Closely related viruses of the marine picoeukaryotic alga Ostreococcus lucimarinus exhibit different ecological strategies

In marine ecosystems, viruses are major disrupters of the direct flow of carbon and nutrients to higher trophic levels. Although the genetic diversity of several eukaryotic phytoplankton virus groups has been characterized, their infection dynamics are less understood, such that the physiological and ecological implications of their diversity remain unclear. We compared genomes and infection phenotypes of the two most closely related cultured phycodnaviruses infecting the widespread picoprasinophyte Ostreococcus lucimarinus under standard- (1.3 divisions per day) and limited-light (0.41 divisions per day) nutrient replete conditions. OlV7 infection caused early arrest of the host cell cycle, coinciding with a significantly higher proportion of infected cells than OlV1-amended treatments, regardless of host growth rate. OlV7 treatments showed a near-50-fold increase of progeny virions at the higher host growth rate, contrasting with OlV1's 16-fold increase. However, production of OlV7 virions was more sensitive than OlV1 production to reduced host growth rate, suggesting fitness trade-offs between infection efficiency and resilience to host physiology. Moreover, although organic matter released from OlV1- and OlV7-infected hosts had broadly similar chemical composition, some distinct molecular signatures were observed. Collectively, these results suggest that current views on viral relatedness through marker and core gene analyses underplay operational divergence and consequences for host ecology.

The orthopoxvirus 68-kilodalton ankyrin-like protein is essential for DNA replication and complete gene expression of modified vaccinia virus Ankara in nonpermissive human and murine cells

Modified vaccinia virus Ankara (MVA) is a highly attenuated and replication-deficient vaccinia virus (VACV) that is being evaluated as replacement smallpox vaccine and candidate viral vector. MVA lacks many genes associated with virulence and/or regulation of virus tropism. The 68-kDa ankyrin-like protein (68k-ank) is the only ankyrin repeat-containing protein that is encoded by the MVA genome and is highly conserved throughout the Orthopoxvirus genus. We showed previously that 68k-ank is composed of ankyrin repeats and an F-box-like domain and forms an SCF ubiquitin ligase complex together with the cellular proteins Skp1a and Cullin-1. We now report that 68k-ank (MVA open reading frame 186R) is an essential factor for completion of the MVA intracellular life cycle in nonpermissive human and murine cells. Infection of mouse NIH 3T3 and human HaCaT cells with MVA with a deletion of the 68k-ank gene (MVA-Delta68k-ank) was characterized by an extensive reduction of viral intermediate RNA and protein, as well as late transcripts and drastically impaired late protein synthesis. Furthermore, infections with MVA-Delta68k-ank failed to induce the host protein shutoff that is characteristic of VACV infections. Although we demonstrated that proteasome function in general is essential for the completion of the MVA molecular life cycle, we found that a mutant 68k-ank protein with a deletion of the F-box-like domain was able to fully complement the deficiency of MVA-Delta68k-ank to express all classes of viral genes. Thus, our data demonstrate that the 68k-ank protein contains another critical domain that may function independently of SCF ubiquitin ligase complex formation, suggesting multiple activities of this interesting regulatory protein.

Double-stranded RNA-binding protein E3 controls translation of viral intermediate RNA, marking an essential step in the life cycle of modified vaccinia virus Ankara

Infection of human cells with modified vaccinia virus Ankara (MVA) activates the typical cascade-like pattern of viral early-, intermediate- and late-gene expression. In contrast, infection of human HeLa cells with MVA deleted of the E3L gene (MVA-DeltaE3L) results in high-level synthesis of intermediate RNA, but lacks viral late transcription. The viral E3 protein is thought to bind double-stranded RNA (dsRNA) and to act as an inhibitor of dsRNA-activated 2'-5'-oligoadenylate synthetase (2'-5'OA synthetase)/RNase L and protein kinase (PKR). Here, it is demonstrated that viral intermediate RNA can form RNase A/T1-resistant dsRNA, suggestive of activating both the 2'-5'OA synthetase/RNase L pathway and PKR in various human cell lines. Western blot analysis revealed that failure of late transcription in the absence of E3L function resulted from the deficiency to produce essential viral intermediate proteins, as demonstrated for vaccinia late transcription factor 2 (VLTF 2). Substantial host cell-specific differences were found in the level of activation of either RNase L or PKR. However, both rRNA degradation and phosphorylation of eukaryotic translation initiation factor-2alpha (eIF2alpha) inhibited the synthesis of VLTF 2 in human cells. Moreover, intermediate VLTF 2 and late-protein production were restored in MVA-DeltaE3L-infected mouse embryonic fibroblasts from Pkr(0/0) mice. Thus, both host-response pathways may be involved, but activity of PKR is sufficient to block the MVA molecular life cycle. These data imply that an essential function of vaccinia virus E3L is to secure translation of intermediate RNA and, thereby, expression of other viral genes.

Vaccinia virus proteolysis--a review

It is well known that viruses, as obligate intracellular parasites, must use their hosts' metabolic machinery in order to replicate their genomes and form infectious progeny virions. What is less well known are the details of how viruses make sure that once all the necessary proteins are made, that they assume the correct configuration at the proper time in order to catalyse the efficient assembly of infectious virions. One of the methods employed by viruses to regulate this process is the proteolytic cleavage of viral proteins. Over the past several decades, studies in numerous laboratories have demonstrated that morphogenic proteolysis plays a major and essential role during the assembly and maturation of infectious poxvirus virions. In this review we describe the history of vaccinia virus proteolysis as a prototypic viral system.

Protein interactions among the vaccinia virus late transcription factors

The viral proteins A1L, A2L, G8R, and H5R positively modulate vaccinia virus late gene expression. Host-encoded proteins hnRNP A2 and RBM3 may also interact with these viral factors to influence late gene expression. In these studies, a yeast two-hybrid screen and in vitro pulldown and crosslinking experiments were used to investigate protein--protein interactions among these factors. These studies confirmed a previous observation that G8R interacts with itself and A1L. However, self-interactions of A1L and H5R, and interactions between A2L and G8R, A2L and H5R, and H5R and G8R were also observed. In addition, the proteins hnRNP A2 and RBM3 both showed some interaction with A2L. Illustration of these interactions is a step toward understanding the architecture of the late gene transcription complex as it occurs in poxviruses.

The cowpox virus host range gene, CP77, affects phosphorylation of eIF2 alpha and vaccinia viral translation in apoptotic HeLa cells

Host restriction of vaccinia virus has been previously described in CHO and RK13 cells in which a cowpox virus CP77 gene rescues vaccinia virus growth at the viral protein translation level. Here we investigate the restrictive stage of vaccinia virus in HeLa cells using a vaccinia mutant virus (VV-hr) that contains a deletion of 18-kb genome sequences resulting in no growth in HeLa cells. Insertion of CP77 gene into VV-hr generated a recombinant virus (VV-36hr) that multiplied well in HeLa cells. Both viruses could enter cells, initiate viral DNA replication and intermediate gene transcription. However, translation of viral intermediate gene was only detected in cells infected with VV-36hr, indicating that CP77 relieves host restriction at the intermediate gene translation stage in HeLa cells. Caspase-2 and -3 activation was observed in HeLa cells infected with VV-hr coupled with dramatic morphological alterations and cleavage of the translation initiation factor eIF4G. Caspase activation was reduced in HeLa cells infected with VV-36hr, indicating that CP77 acts upstream of caspase activation. Enhanced phosphorylation of PKR and eIF2alpha was also observed in cells infected with VV-hr and was suppressed by CP77. Suppression of eIF4G cleavage with the caspase inhibitor ZVAD did not rescue virus translation, whereas expression of a mutant eIF2alpha protein with an alanine substitution of serine at amino acid position 51 (eIF2alphaS51A) partially restored viral translation and moderately increased virus growth in HeLa cells.

Vaccinia virus intermediate stage transcription is complemented by Ras-GTPase-activating protein SH3 domain-binding protein (G3BP) and cytoplasmic activation/proliferation-associated protein (p137) individually or as a heterodimer

Transcription of the DNA genome of vaccinia virus occurs in the cytoplasm and is temporally programmed by early, intermediate, and late stage-specific transcription factors in conjunction with a viral multisubunit RNA polymerase. The RNA polymerase, capping enzyme, and three factors (VITF-1, VITF-2, and VITF-3) are sufficient for in vitro transcription of a DNA template containing an intermediate stage promoter. Vaccinia virus intermediate transcription factor (VITF)-1 and -3 are virus-encoded, whereas VITF-2 was partially purified from extracts of uninfected HeLa cells. Using purified and recombinant viral proteins, we showed that the HeLa cell factor was required for transcription of linear or nicked circular templates but not of super coiled DNA. HeLa cell polypeptides of approximately 110 and 66 kDa copurified with VITF-2 activity through multiple chromatographic steps. The polypeptides were separated by SDS-polyacrylamide gel electrophoresis and identified by mass spectrometry as Ras-GTPase-activating protein SH3 domain-binding protein (G3BP) and p137, recently named cytoplasmic activation/proliferation-associated protein-1. The co-purification of the two polypeptides with transcription-complementing activity was confirmed with specific antibodies, and their association with each other was demonstrated by affinity chromatography of tagged recombinant forms. Furthermore, recombinant G3BP and p137 expressed individually or together in mammalian or bacterial cells complemented the activity of the viral RNA polymerase and transcription factors. The involvement of cellular proteins in transcription of intermediate stage genes may regulate the transition between early and late phases of vaccinia virus replication.

Regulation of viral transcription elongation and termination during vaccinia virus infection

Vaccinia virus provides a useful genetic and biochemical tool for studies of the basic mechanisms of eukaryotic transcription. Vaccinia genes are transcribed in three successive gene classes during infection, early, intermediate, and late. Vaccinia transcription is regulated primarily by virus gene products not only during initiation, but also during elongation and termination. The factors and mechanisms regulating early elongation and termination differ from those regulating intermediate and late gene expression. Control of transcription elongation and termination in vaccinia virus bears some similarity to the same process in other prokaryotic and eukaryotic systems, yet features some novel mechanisms as well.

Analysis of the monkeypox virus genome

Monkeypox virus (MPV) belongs to the orthopoxvirus genus of the family Poxviridae, is endemic in parts of Africa, and causes a human disease that resembles smallpox. The 196,858-bp MPV genome was analyzed with regard to structural features and open reading frames. Each end of the genome contains an identical but oppositely oriented 6379-bp terminal inverted repetition, which similar to that of other orthopoxviruses, includes a putative telomere resolution sequence and short tandem repeats. Computer-assisted analysis was used to identify 190 open reading frames containing >/=60 amino acid residues. Of these, four were present within the inverted terminal repetition. MPV contained the known essential orthopoxvirus genes but only a subset of the putative immunomodulatory and host range genes. Sequence comparisons confirmed the assignment of MPV as a distinct species of orthopoxvirus that is not a direct ancestor or a direct descendent of variola virus, the causative agent of smallpox.

Common origin of four diverse families of large eukaryotic DNA viruses

Comparative analysis of the protein sequences encoded in the genomes of three families of large DNA viruses that replicate, completely or partly, in the cytoplasm of eukaryotic cells (poxviruses, asfarviruses, and iridoviruses) and phycodnaviruses that replicate in the nucleus reveals 9 genes that are shared by all of these viruses and 22 more genes that are present in at least three of the four compared viral families. Although orthologous proteins from different viral families typically show weak sequence similarity, because of which some of them have not been identified previously, at least five of the conserved genes appear to be synapomorphies (shared derived characters) that unite these four viral families, to the exclusion of all other known viruses and cellular life forms. Cladistic analysis with the genes shared by at least two viral families as evolutionary characters supports the monophyly of poxviruses, asfarviruses, iridoviruses, and phycodnaviruses. The results of genome comparison allow a tentative reconstruction of the ancestral viral genome and suggest that the common ancestor of all of these viral families was a nucleocytoplasmic virus with an icosahedral capsid, which encoded complex systems for DNA replication and transcription, a redox protein involved in disulfide bond formation in virion membrane proteins, and probably inhibitors of apoptosis. The conservation of the disulfide-oxidoreductase, a major capsid protein, and two virion membrane proteins indicates that the odd-shaped virions of poxviruses have evolved from the more common icosahedral virion seen in asfarviruses, iridoviruses, and phycodnaviruses.

Vaccinia virus late transcription is activated in vitro by cellular heterogeneous nuclear ribonucleoproteins

Vaccinia virus gene expression is temporally regulated, and three gene classes have been identified: early, intermediate, and late. Several virus-encoded proteins and an activity designated VLTF-X are required for maximum transcription in vitro of a template containing a late promoter. VLTF-X is present in both cytoplasmic and nuclear extracts prepared from uninfected mammalian cells and co-purifies with a late promoter DNA-binding activity. Here, extensive purification of VLTF-X has revealed that heterogeneous nuclear ribonucleoproteins A2/B1 and RBM3 co-purified with in vitro late transcription stimulation. Overexpression and purification of these proteins from Escherichia coli demonstrated that they both complemented for VLTF-X activity in in vitro transcription reactions. These studies identify two host cell factors potentially contributing to poxvirus replication in vivo.

The genome sequence of Yaba-like disease virus, a yatapoxvirus

The genome sequence of Yaba-like disease virus (YLDV), an unclassified member of the yatapoxvirus genus, has been determined. Excluding the terminal hairpin loops, the YLDV genome is 144,575 bp in length and contains inverted terminal repeats (ITRs) of 1883 bp. Within 20 nucleotides of the termini, there is a sequence that is conserved in other poxviruses and is required for the resolution of concatemeric replicative DNA intermediates. The nucleotide composition of the genome is 73% A+T, but the ITRs are only 63% A+T. The genome contains 151 tightly packed open reading frames (ORFs) that either are > or =180 nucleotides in length or are conserved in other poxviruses. ORFs within 23 kb of each end are transcribed toward the termini, whereas ORFs within the central region of the genome are encoded on either DNA strand. In the central region ORFs have a conserved position, orientation, and sequence compared with vaccinia virus ORFs and encode many enzymes, transcription factors, or structural proteins. In contrast, ORFs near the termini are more divergent and in seven cases are without counterparts in other poxviruses. The YLDV genome encodes several predicted immunomodulators; examples include two proteins with similarity to CC chemokine receptors and predicted secreted proteins with similarity to MHC class I antigen, OX-2, interleukin-10/mda-7, poxvirus growth factor, serpins, and a type I interferon-binding protein. Phylogenic analyses indicated that YLDV is very closely related to yaba monkey tumor virus, but outside the yatapoxvirus genus YLDV is more closely related to swinepox virus and leporipoxviruses than to other chordopoxvirus genera.

The complete genome sequence of shope (rabbit) fibroma virus

We have determined the complete DNA sequence of the Leporipoxvirus Shope fibroma virus (SFV). The SFV genome spans 159.8 kb and encodes 165 putative genes of which 13 are duplicated in the 12.4-kb terminal inverted repeats. Although most SFV genes have homologs encoded by other Chordopoxvirinae, the SFV genome lacks a key gene required for the production of extracellular enveloped virus. SFV also encodes only the smaller ribonucleotide reductase subunit and has a limited nucleotide biosynthetic capacity. SFV preserves the Chordopoxvirinae gene order from S012L near the left end of the chromosome through to S142R (homologs of vaccinia F2L and B1R, respectively). The unique right end of SFV appears to be genetically unstable because when the sequence is compared with that of myxoma virus, five myxoma homologs have been deleted (C. Cameron, S. Hota-Mitchell, L. Chen, J. Barrett, J.-X. Cao, C. Macaulay, D. Willer, D. Evans, and G. McFadden, 1999, Virology 264, 298-318). Most other differences between these two Leporipoxviruses are located in the telomeres. Leporipoxviruses encode several genes not found in other poxviruses including four small hydrophobic proteins of unknown function (S023R, S119L, S125R, and S132L), an alpha 2, 3-sialyltransferase (S143R), a protein belonging to the Ig-like protein superfamily (S141R), and a protein resembling the DNA-binding domain of proteins belonging to the HIN-200 protein family S013L). SFV also encodes a type II DNA photolyase (S127L). Melanoplus sanguinipes entomopoxvirus encodes a similar protein, but SFV is the first mammalian virus potentially capable of photoreactivating ultraviolet DNA damage.

A vaccinia virus late transcription factor with biochemical and molecular identity to a human cellular protein

A factor designated VLTF-X is required to support vaccinia virus late transcription in vitro. It has been found that a late promoter DNA binding activity cochromatographs and cosediments with VLTF-X activity. Current experiments show that VLTF-X activity is present in a variety of uninfected mammalian cell types and is indistinguishable from that recovered from infected cells based upon several criteria. VLTF-X activity from both sources displays the same purification profile over phosphocellulose and DNA affinity resins and has the same sedimentation coefficient. In addition, the factors purified from both infected and uninfected cells form protein-DNA complexes of identical electrophoretic mobility in the presence of vaccinia virus late promoter-containing DNA. The affinity of these factors for the late promoter probes is identical and late promoter-specific based on competition experiments. Moreover, VLTF-X purified from both sources bound to late promoter-containing DNA in the presence or absence of MgCl2 and ATP and formed complexes resistant to heat inactivation. These experiments offer proof that vaccinia virus factor VLTF-X is a host cell protein that supports transcription of the viral late genes.

The complete genomic sequence of the modified vaccinia Ankara strain: comparison with other orthopoxviruses

The complete genomic DNA sequence of the highly attenuated vaccinia strain modified vaccinia Ankara (MVA) was determined. The genome of MVA is 178 kb in length, significantly smaller than that of the vaccinia Copenhagen genome, which is 192 kb. The 193 open reading frames (ORFs) mapped in the MVA genome probably correspond to 177 genes, 25 of which are split and/or have suffered mutations resulting in truncated proteins. The left terminal genomic region of MVA contains four large deletions and one large insertion relative to the Copenhagen strain. In addition, many ORFs in this region are fragmented, leaving only eight genes structurally intact and therefore presumably functional. The inserted DNA codes for a cluster of genes that is also found in the vaccinia WR strain and in cowpox virus and includes a highly fragmented gene homologous to the cowpox virus host range gene, providing further evidence that a cowpox-like virus was the ancestor of vaccinia. Surprisingly, the central conserved region of the genome also contains some fragmented genes, including ORF F5L, encoding a major membrane protein, and ORFs F11L and O1L, encoding proteins of 39.7 and 77.6 kDa, respectively. The right terminal genomic region carries three large deletions all classical poxviral immune evasion genes and all ankyrin-like genes located in this region are fragmented except for those encoding the interleukin-1 beta receptor and the 68-kDa ankyrin-like protein B18R. Thus, the attenuated phenotype of MVA is the result of numerous mutations, particularly affecting the host interactive proteins, including the ankyrin-like genes, but also involving some structural proteins.

A cellular protein binds vaccinia virus late promoters and activates transcription in vitro

Available evidence indicates that the transcription of the late class of vaccinia virus genes requires the participation of several virus-encoded proteins in addition to the viral RNA polymerase. In this report we describe the identification of a protein present in extracts of uninfected HeLa cells that binds avidly to viral late promoter DNA. The protein bound specifically to several different vaccinia virus late promoters but not an early nor an intermediate promoter. DNase I footprinting localized the protein's binding site to nucleotides surrounding the transcriptional start site of the I1L promoter. Optimal promoter binding required sequences in the highly conserved TAAAT motif at the transcriptional start site as well as sequences immediately upstream; however, one variation on the motif's sequence did not affect promoter binding by the protein. Partially purified late promoter binding protein (LPBP) was capable of stimulating the transcription activity of extracts depleted of LPBP on a late promoter-driven template, establishing LPBP as a transcription activator in vitro. These results suggest that a cellular protein is responsible for targeting vaccinia virus late promoters for initiation of transcription.

A vaccinia virus late transcription factor copurifies with a factor that binds to a viral late promoter and is complemented by extracts from uninfected HeLa cells

We have previously described a vaccinia virus late transcription factor, VLTF-X, which we found to be present in cells at early and late times in infection. In this study, transcription complementation assays were used to demonstrate that VLTF-X activity is also present in virion extracts and in the cytoplasm of uninfected HeLa cells. Mobility shift assays performed on various VLTF-X preparations revealed that a late promoter DNA-binding activity cochromatographed and cosedimented with VLTF-X activity. Competition experiments demonstrated that this binding was specific for the late promoter region of the probe and that late transcription was dramatically reduced by an oligonucleotide that blocked factor-DNA complex formation but was only minimally affected by an oligonucleotide that did not inhibit complex formation. These results suggest that a cellular factor may participate in vaccinia virus late transcription. These findings also confirm the requirement for VLTF-X and distinguish it from any of the previously described vaccinia virus late transcription factors, which have all been mapped to the viral genome. Finally, these studies also suggest that the biochemical role for VLTF-X may be in late promoter recognition.

Adenovirus E1a interferes with expression of vaccinia viral genes

The 12S and 13S cDNAs of the oncogene E1a encoded by the early region of adenovirus 12 (Ad12) were overexpressed using the T7/encephalomyocarditis (EMC)/vaccinia hybrid expression system. The E1a proteins were stable for at least 12 h in monkey epithelial BSC1 cells. The E1a proteins were recognized by a rabbit polyclonal antibody and displayed phosphorylation patterns similar to those displayed by the E1a proteins expressed in Ad12-transformed cells. Expression of E1a proteins by recombinant vaccinia virus led to inhibition of vaccinia viral protein synthesis which was observed as soon as 6 h after infection. This suppression was mediated by both the 12S and the 13S products of Ad12E1a and to a somewhat lesser extent by the 13S product of Ad2E1a. The inhibition of vaccinia virus gene expression resulted in enhanced survival of vaccinia virus-infected cells. These results suggest that the proteins encoded by the E1a sequester a viral or a cellular product(s) that is essential for the expression of vaccinia virus-encoded genes.

De novo synthesis of the early transcription factor 70-kilodalton subunit is required for morphogenesis of vaccinia virions

Vaccinia virus early transcription factor (VETF) is a heterodimeric protein that is packaged in virus particles for expression of early genes during the next round of infection. To investigate additional roles of VETF, we constructed a conditionally lethal recombinant vaccinia virus in which the D6R gene, encoding the 70-kDa subunit of VETF, is under stringent Escherichia coli lac operator control. When cells were infected with the recombinant virus in the absence of an inducer, synthesis of the 70-kDa protein was undetectable and the yield of infectious virus was severely reduced. Under these nonpermissive conditions, DNA replication and synthesis of viral proteins other than the one encoded by D6R occurred, suggesting that de novo synthesis of VETF is not required for expression of early or late genes during the virus growth cycle. Electron microscopy, however, revealed that immature virus particles and masses of electron-dense material accumulated in the absence of an inducer. We concluded that VETF has a direct role in virion morphogenesis or is required for expression of a novel subset of genes that have such a role.

The vaccinia virus H5R gene encodes late gene transcription factor 4: purification, cloning, and overexpression

The vaccinia virus late stage-specific transcription factor P3 was purified to homogeneity from HeLa cells that were infected in the presence of an inhibitor of viral DNA replication. The purified 36-kDa protein was digested with trypsin, and the peptides were analyzed by mass spectroscopy and amino-terminal sequencing. The purified factor was identified as the product of the vaccinia virus H5R open reading frame by both methods. A recombinant baculovirus was engineered to express the H5R open reading frame. The partially purified recombinant protein could replace the vaccinia virus P3 factor in transcription assays. On the basis of these findings, we assigned the H5R gene product the name viral late gene transcription factor 4 (VLTF-4). Unlike VLTF-1, -2, and -3, which are synthesized exclusively after viral DNA replication, VLTF-4 is synthesized before and after viral DNA synthesis. Indirect immunofluorescence of infected cells with anti-H5R protein antiserum demonstrated that VLTF-4 is diffusely distributed in the cytoplasm when DNA replication is blocked but is localized to discrete viral DNA-containing factories during a productive infection. Its expression pattern and subcellular distribution suggest that the H5R gene product may have multiple roles in the viral life cycle.

Transcription of a vaccinia virus late promoter template: requirement for the product of the A2L intermediate-stage gene

Evidence is presented that a 26-kDa protein encoded by the vaccinia virus A2L open reading frame, originally shown to be one of three intermediate-stage genes that together can transactivate late-stage gene expression in transfection assays (J. G. Keck, C. J. Baldick, and B. Moss, Cell 61:801-809, 1990), is required for in vitro transcription of a template with a late promoter. The critical step in this analysis was the preparation of an extract containing all the required factors except for the A2L protein. This extract was prepared from cells infected with a recombinant vaccinia virus expressing the bacteriophage T7 RNA polymerase in the presence of the DNA synthesis inhibitor cytosine arabinoside and transfected with plasmids containing the two other known transactivator genes, A1L and G8R, under T7 promoter control. Reaction mixtures made with extracts of these cells had background levels of late transcription activity, unless they were supplemented with extracts of cells transfected with the A2L gene. Active transcription mixtures were also made by mixing extracts from three sets of cells, each transfected with a gene (A1L, A2L, or G8R) encoding a separate factor, indicating the absence of any requirement for their coexpression. To minimize the possibility that the A2L protein functions indirectly by activating another viral or cellular protein, this gene was expressed in insect cells by using a baculovirus vector. The partially purified recombinant protein complemented the activity of A2L-deficient cell extracts. Recombinant A1L, A2L, and G8R proteins, all produced in insect cells, together complemented extracts from mammalian cells containing only viral early proteins, concordant with previous in vivo transfection data.

The A2L intermediate gene product is required for in vitro transcription from a vaccinia virus late promoter

Previously, the in vitro late transcription system of vaccinia virus was resolved into four components: the 17- and 30-kDa products of the A1L and G8R intermediate genes, respectively, the viral DNA-dependent RNA polymerase, and an unmapped factor sedimenting at 32 to 38 kDa. Another protein, the 26-kDa product of the A2L open reading frame was predicted to be a late transcription factor on the basis of a transient-expression assay but was not recognized as being necessary for transcriptional activity in vitro. We now report that both the unmapped factor and the 26-kDa protein are required for transcription from a vaccinia virus late promoter in vitro. Since the 26-kDa protein has now been shown to be a trans-activator of late transcription and it is the product of a known gene, we suggest that it be designated VLTF-3.

The H4 subunit of vaccinia virus RNA polymerase is not required for transcription initiation at a viral late promoter

Chromatography of RNA polymerase purified from vaccinia virions and from vaccinia virus-infected HeLa cells resulted in the separation of populations active for early and late transcription. An RNA polymerase population immunodepleted for the vaccinia virus H4 gene peptide could support late transcription.

Functional organization of variola major and vaccinia virus genomes

Comparison of the genomic organization of variola and vaccinia viruses has been carried out. Molecular factors of virulence of these viruses is the focus of this review. Possible roles of the genes of soluble cytokine receptors, complement control proteins, factors of virus replication, and dissemination in vivo for variola virus pathogenesis are discussed. The existence of "buffer" genes in the vaccinia virus genome is proposed.

The vaccinia virus A18R protein plays a role in viral transcription during both the early and the late phases of infection

The vaccinia virus gene A18R is essential for virus infection. The loss of A18R protein function results in unregulated transcription late during virus infection from regions of the viral genome which are normally transcriptionally quiescent. We have characterized A18R protein expression in cells infected with wild-type virus and the A18R temperature-sensitive mutant Cts23. The A18R protein is expressed during early and late phases of infection. The A18R protein expressed by Cts23 virus at permissive and nonpermissive temperatures is significantly less stable than the wild-type A18R protein. The A18R protein was identified as a virion component and localized by detergent extraction to the virion core. Virions purified from cells infected with the A18R temperature-sensitive mutants Cts4, Cts22, and Cts23 are defective in early viral transcription in vitro. The mutant transcription defect is not attributable to gross defects in virion structure or virion DNA-dependent RNA polymerase activity. We conclude that the A18R protein plays a role in viral transcription during the early phase of infection as well as during the late phase.

Modification of the cascade model for regulation of vaccinia virus gene expression: purification of a prereplicative, late-stage-specific transcription factor

In vivo and in vitro studies have provided evidence that vaccinia virus late gene transcription factors are intermediate gene products synthesized exclusively after DNA replication. Here, we describe an additional transcription factor (P3 factor) that stimulates late gene transcription between 10- and 40-fold but is made in the absence of viral DNA replication. P3 factor activity was not detected either in uninfected cells or in purified virions. A > 1,500-fold purification of P3 factor was achieved by column chromatography of cytoplasmic extracts prepared from cells infected with vaccinia virus in the presence of a DNA replication inhibitor. P3 factor was stage specific, since it could not substitute for early or intermediate transcription factors. Evidence that late stage-specific transcription factors are made both before and after DNA replication necessitates a modification of the cascade model for vaccinia virus gene regulation.

A cellular factor is required for transcription of vaccinia viral intermediate-stage genes

The cytoplasmic location of vaccinia virus replication and evidence that the multisubunit DNA-dependent RNA polymerase, early and late stage transcription factors, capping and methylating enzymes, and poly(A) polymerase are virus encoded raised the possibility that all of the proteins needed for viral mRNA synthesis are of viral origin. Previous studies showed that four components from infected cells, the viral RNA polymerase and capping enzyme and two factors called vaccinia virus intermediate transcription factors (VITFs) 1 and 2, can reconstitute transcription of vaccinia virus intermediate-stage genes in vitro. Here, we demonstrate that VITF-2 can be isolated from the nuclei of uninfected HeLa cells as well as from the cytoplasm of infected cells. The proteins with VITF-2 activity from uninfected and infected cells cochromatographed and cosedimented, suggesting that they are identical. VITF-2 activity was found in extracts of other uninfected human and monkey cells but not in nonpermissive Trichoplusia ni insect cells or in conditionally permissive rabbit kidney 13 cells. VITF-2 activity was present, however, in a permissive line of rabbit kidney 13 cells that had been stably transfected with the vaccinia virus K1L host range gene. We suggest that the VITF-2 level acts as a gauge of the permissive state of the cell and thereby regulates the length of the early prereplicative phase of the infection.

Mutational analysis of a predicted zinc-binding motif in the 26-kilodalton protein encoded by the vaccinia virus A2L gene: correlation of zinc binding with late transcriptional transactivation activity

Transient transfection assays indicated that A2L is one of three vaccinia virus intermediate genes that are required for the transcriptional transactivation of viral late genes. We have expressed the A2L open reading frame in Escherichia coli and shown by blotting experiments that the 26-kDa protein binds zinc, a property predicted by the presence of a CX2CX13CX2C zinc finger motif. The specificity for zinc binding was demonstrated by competition with other metals. The role of the sequence motif in zinc binding was established by analysis of a series of mutations, including truncations and conservative single amino acid substitutions. Mutations that reduced zinc binding in vitro prevented the ability of A2L to transactivate late genes in vivo.