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

Human mpox: Biology, epidemiology, therapeutic options, and development of small molecule inhibitors

Although monkeypox (mpox) has been endemic in Western and Central Africa for 50 years, it has not received sufficient prophylactic and therapeutical attention to avoid evolving into an epidemic. From January 2022 to January 2023, more than 84,000 of mpox cases were reported from 110 countries worldwide. Case numbers appear to be rising every day, making mpox an increasing global public health threat for the foreseeable future. In this perspective, we review the known biology and epidemiology of mpox virus, together with the latest therapeutic options available for mpox treatment. Further, small molecule inhibitors against mpox virus and the future directions in this field are discussed as well.

Monkeypox: epidemiology, pathogenesis, treatment and prevention

Monkeypox is a zoonotic disease that was once endemic in west and central Africa caused by monkeypox virus. However, cases recently have been confirmed in many nonendemic countries outside of Africa. WHO declared the ongoing monkeypox outbreak to be a public health emergency of international concern on July 23, 2022, in the context of the COVID-19 pandemic. The rapidly increasing number of confirmed cases could pose a threat to the international community. Here, we review the epidemiology of monkeypox, monkeypox virus reservoirs, novel transmission patterns, mutations and mechanisms of viral infection, clinical characteristics, laboratory diagnosis and treatment measures. In addition, strategies for the prevention, such as vaccination of smallpox vaccine, is also included. Current epidemiological data indicate that high frequency of human-to-human transmission could lead to further outbreaks, especially among men who have sex with men. The development of antiviral drugs and vaccines against monkeypox virus is urgently needed, despite some therapeutic effects of currently used drugs in the clinic. We provide useful information to improve the understanding of monkeypox virus and give guidance for the government and relative agency to prevent and control the further spread of monkeypox virus.

Comparative Analysis of Transcriptional Regulation Patterns: Understanding the Gene Expression Profile in Nucleocytoviricota

The nucleocytoplasmic large DNA viruses (NCLDV) possess unique characteristics that have drawn the attention of the scientific community, and they are now classified in the phylum Nucleocytoviricota. They are characterized by sharing many genes and have their own transcriptional apparatus, which provides certain independence from their host's machinery. Thus, the presence of a robust transcriptional apparatus has raised much discussion about the evolutionary aspects of these viruses and their genomes. Understanding the transcriptional process in NCLDV would provide information regarding their evolutionary history and a better comprehension of the biology of these viruses and their interaction with hosts. In this work, we reviewed NCLDV transcription and performed a comparative functional analysis of the groups of genes expressed at different times of infection of representatives of six different viral families of giant viruses. With this analysis, it was possible to observe a temporal profile of their gene expression and set of genes activated in specific phases throughout the multiplication cycle as a common characteristic of this group. Due to the lack of information regarding the transcriptional regulation process of this group of pathogens, we sought to provide information that contributes to and opens up the field for transcriptional studies of other viruses belonging to Nucleocytoviricota.

Modulation of Early Host Innate Immune Response by an Avipox Vaccine Virus' Lateral Body Protein

The avian pathogen fowlpox virus (FWPV) has been successfully used as a vaccine vector in poultry and humans, but relatively little is known about its ability to modulate host antiviral immune responses in these hosts, which are replication-permissive and nonpermissive, respectively. FWPV is highly resistant to avian type I interferon (IFN) and able to completely block the host IFN-response. Microarray screening of host IFN-regulated gene expression in cells infected with 59 different, nonessential FWPV gene knockout mutants revealed that FPV184 confers immunomodulatory capacity. We report that the FPV184-knockout virus (FWPVΔ184) induces the cellular IFN response as early as 2 h postinfection. The wild-type, uninduced phenotype can be rescued by transient expression of FPV184 in FWPVΔ184-infected cells. Ectopic expression of FPV184 inhibited polyI:C activation of the chicken IFN-β promoter and IFN-α activation of the chicken Mx1 promoter. Confocal and correlative super-resolution light and electron microscopy demonstrated that FPV184 has a functional nuclear localisation signal domain and is packaged in the lateral bodies of the virions. Taken together, these results provide a paradigm for a late poxvirus structural protein packaged in the lateral bodies, capable of suppressing IFN induction early during the next round of infection.

Emerging translation strategies during virus-host interaction

Translation control is crucial during virus-host interaction. On one hand, viruses completely rely on the protein synthesis machinery of host cells to propagate and have evolved various mechanisms to redirect the host's ribosomes toward their viral mRNAs. On the other hand, the host rewires its translation program in an attempt to contain and suppress the virus early on during infection; the antiviral program includes specific control on protein synthesis to translate several antiviral mRNAs involved in quenching the infection. As the infection progresses, host translation is in turn inhibited in order to limit viral propagation. We have learnt of very diverse strategies that both parties utilize to gain or retain control over the protein synthesis machinery. Yet novel strategies continue to be discovered, attesting for the importance of mRNA translation in virus-host interaction. This review focuses on recently described translation strategies employed by both hosts and viruses. These discoveries provide additional pieces in the understanding of the complex virus-host translation landscape. This article is categorized under: Translation > Translation Mechanisms Translation > Translation Regulation.

Cascade regulation of vaccinia virus gene expression is modulated by multistage promoters

Vaccinia virus contains ~200 genes classified temporally as early, intermediate or late. We analyzed 53 intermediate promoters to determine whether any have dual late promoter activity. Our strategy involved (i) construction of a cell line that stably expressed the three late transcription factors, (ii) infection with a vaccinia virus mutant that expresses RNA polymerase but neither intermediate nor late transcription factors, and (iii) transfection with plasmids containing a luciferase reporter regulated by an intermediate promoter. After confirming the specificity of the system for late promoters, we found that many intermediate promoters had late promoter activity, the strength of which correlated with a TAAAT at the initiator site and T-content from positions -12 to -8 of the coding strand. In contrast, intermediate promoter activity correlated with the A-content from positions -22 to -14. The sequence correlations were confirmed by altering the specificities of strict intermediate and late promoters.

African swine fever virus transcription

African swine fever virus (ASFV), a large, enveloped, icosahedral dsDNA virus, is currently the only known DNA-containing arbovirus and the only recognized member of the family Asfarviridae. Its genome encodes more than 150 open reading frames that are densely distributed, separated by short intergenic regions. ASFV gene expression follows a complex temporal programming. Four classes of mRNAs have been identified by its distinctive accumulation kinetics. Gene transcription is coordinated with DNA replication that acts as the main switch on ASFV gene expression. Immediate early and early genes are expressed before the onset of DNA replication, whereas intermediate and late genes are expressed afterwards. ASFV mRNAs have a cap 1 structure at its 5'-end and a short poly(A) tail on its 3'-end. Transcription initiation and termination occurs at very precise positions within the genome, producing transcripts of definite length throughout the expression program. ASFV devotes approximately 20% of its genome to encode the 20 genes currently considered to be involved in the transcription and modification of its mRNAs. This transcriptional machinery gives to ASFV a remarkable independence from its host and an accurate positional and temporal control of its gene expression. Here, we review the components of the ASFV transcriptional apparatus, its expression strategies and the relevant data about the transcriptional cis-acting control sequences.

Binding of the heterogeneous ribonucleoprotein K (hnRNP K) to the Epstein-Barr virus nuclear antigen 2 (EBNA2) enhances viral LMP2A expression

The Epstein-Barr Virus (EBV) -encoded EBNA2 protein, which is essential for the in vitro transformation of B-lymphocytes, interferes with cellular processes by binding to proteins via conserved sequence motifs. Its Arginine-Glycine (RG) repeat element contains either symmetrically or asymmetrically di-methylated arginine residues (SDMA and ADMA, respectively). EBNA2 binds via its SDMA-modified RG-repeat to the survival motor neurons protein (SMN) and via the ADMA-RG-repeat to the NP9 protein of the human endogenous retrovirus K (HERV-K (HML-2) Type 1). The hypothesis of this work was that the methylated RG-repeat mimics an epitope shared with cellular proteins that is used for interaction with target structures. With monoclonal antibodies against the modified RG-repeat, we indeed identified cellular homologues that apparently have the same surface structure as methylated EBNA2. With the SDMA-specific antibodies, we precipitated the Sm protein D3 (SmD3) which, like EBNA2, binds via its SDMA-modified RG-repeat to SMN. With the ADMA-specific antibodies, we precipitated the heterogeneous ribonucleoprotein K (hnRNP K). Specific binding of the ADMA- antibody to hnRNP K was demonstrated using E. coli expressed/ADMA-methylated hnRNP K. In addition, we show that EBNA2 and hnRNP K form a complex in EBV- infected B-cells. Finally, hnRNP K, when co-expressed with EBNA2, strongly enhances viral latent membrane protein 2A (LMP2A) expression by an unknown mechanism as we did not detect a direct association of hnRNP K with DNA-bound EBNA2 in gel shift experiments. Our data support the notion that the methylated surface of EBNA2 mimics the surface structure of cellular proteins to interfere with or co-opt their functional properties.

An overview of the vaccinia virus infectome: a survey of the proteins of the poxvirus-infected cell

We have quantitatively profiled the proteins of vaccinia virus-infected HEK293T cells early and late during vaccinia virus infection. Proteins corresponding to 4,326 accessions were identified, the products of 3,798 genes. One hundred thirty-six of the proteins were vaccinia virus-encoded (∼64% of the known vaccinia virus proteome). The remaining accessions were from the host cell. A total of 3,403 of the 4,326 accessions could be confidently quantitated at the precursor peptide level. Although vaccinia virus gene products spanned the entire abundance dynamic range of the cellular proteome, nearly all of the proteome dynamics observed as a result of infection were manifest in the virus gene products with very little plasticity in the host cell proteome. The vaccinia virus gene products could be grouped into four kinetic classes (i.e., four combinations of pre- and postreplicative expression). These protein kinetic classes reflected, almost entirely, the corresponding gene classes within the recently characterized vaccinia virus transcriptome map. The few cellular gene products that showed notable changes in abundance upon vaccinia virus infection were concentrated largely in just a few functional groups. After all of the quantitated cellular gene products were assigned to Gene Ontology (GO)-specific groups, quantitation values for a number of these GO-specific groups were significantly skewed toward over- or underabundance with respect to the global distribution of quantitation values. Quantitative analysis of host cell functions reflected several known facets of virus infection, along with some novel observations.

Interaction of orthopoxviruses with the cellular ubiquitin-ligase system

Protein modification by ubiquitin or ubiquitin-like polypeptides is important for the fate and functions of the majority of proteins in the eukaryotic cell and can be involved in regulation of various biological processes, including protein metabolism (degradation), protein transport to several cellular compartments, rearrangement of cytoskeleton, and transcription of cytoprotective genes. The accumulated experimental data suggest that the ankyrin-F-box-like and BTB-kelch-like proteins of orthopoxviruses, represented by the largest viral multigene families, interact with the cellular Cullin-1- and Cullin-3-containing ubiquitin-protein ligases, respectively. In addition, orthopoxviruses code for their own RING-domain-containing ubiquitin ligase. In this review, this author discusses the differences between variola (smallpox), monkeypox, cowpox, vaccinia, and ectromelia (mousepox) viruses in the organization of ankyrin-F-box and BTB-kelch protein families and their likely functions.

Downregulation of vaccinia virus intermediate and late promoters by host transcription factor YY1

Approximately half of the intermediate and late gene transcriptional promoters of vaccinia virus have a binding site for the cellular transcription factor YY1 that overlaps the initiator elements. Depletion of YY1 using RNA interference enhanced the activity of these promoters, while overexpression of YY1 repressed their activity. Viral promoter nucleotide replacements that specifically impair the binding of YY1 mostly alleviated the transcriptional repression and correlated with the ability of YY1 to stably interact with the initiator DNAs in vitro. The transcriptional repression activity was localized to the C-terminal DNA-binding domain of the protein. These results indicate that YY1 functions to negatively regulate these vaccinia virus promoters by binding to their initiator elements.

Kinetic analysis of a complete poxvirus transcriptome reveals an immediate-early class of genes

Vaccinia virus is the prototypic orthopoxvirus and was the vaccine used to eradicate smallpox, yet the expression profiles of many of its genes remain unknown. Using a genome tiling array approach, we simultaneously measured the expression levels of all 223 annotated vaccinia virus genes during infection and determined their kinetics. For 95% of these genes, significant transcript levels were detected. Most remarkably, classification of the genes by their expression profiles revealed 35 genes exhibiting immediate-early expression. Although a similar kinetic class has been described for other virus families, to our knowledge, this is the first demonstration of its existence in orthopoxviruses. Despite expression levels higher than for genes in the other three kinetic classes, the functions of more than half of these remain unknown. Additionally, genes within each kinetic class were spatially grouped together in the genome. This genome-wide picture of transcription alters our understanding of how orthopoxviruses regulate gene expression.

A guide to viral inclusions, membrane rearrangements, factories, and viroplasm produced during virus replication

Virus replication can cause extensive rearrangement of host cell cytoskeletal and membrane compartments leading to the “cytopathic effect” that has been the hallmark of virus infection in tissue culture for many years. Recent studies are beginning to redefine these signs of viral infection in terms of specific effects of viruses on cellular processes. In this chapter, these concepts have been illustrated by describing the replication sites produced by many different viruses. In many cases, the cellular rearrangements caused during virus infection lead to the construction of sophisticated platforms in the cell that concentrate replicase proteins, virus genomes, and host proteins required for replication, and thereby increase the efficiency of replication. Interestingly, these same structures, called virus factories, virus inclusions, or virosomes, can recruit host components that are associated with cellular defences against infection and cell stress. It is possible that cellular defence pathways can be subverted by viruses to generate sites of replication. The recruitment of cellular membranes and cytoskeleton to generate virus replication sites can also benefit viruses in other ways. Disruption of cellular membranes can, for example, slow the transport of immunomodulatory proteins to the surface of infected cells and protect against innate and acquired immune responses, and rearrangements to cytoskeleton can facilitate virus release.

Myxoma virus M063R is a host range gene essential for virus replication in rabbit cells

The myxoma virus M063R gene product exhibits some sequence similarity to the poxvirus host range gene, C7L, of vaccinia virus. To address the potential host range function of the M063R gene product in rabbits, a deletion mutant of myxoma virus (vMyx63KO) was generated and characterized. vMyx63KO replicated to normal titre levels and produced foci that were indistinguishable from those produced by MV in vitro in a monkey kidney cell line (BGMK) that are permissive for wild type MV. However, vMyx63KO failed to replicate in all rabbit cell lines tested, including both primary and established cells lines, as well as cells derived from a variety of tissues. M063R expression was not required for myxoma virus binding, entry or early gene expression, whereas DNA replication was aborted and late genes were not expressed in vMyx63KO infected rabbit cells. Thus, the replication block for vMyx63KO in rabbit cells preceded the stage of late gene expression and DNA replication. Finally, an in vivo pathogenesis study indicated that vMyx63KO failed to cause any signs of classic myxomatosis in infected rabbits, but functioned as a non-replicating vaccine and provided protection for subsequent challenge by wild type myxoma virus. Altogether, these observations demonstrate that M063R plays a critical role in determining the host specificity of myxoma virus in rabbit cells.

Vaccinia virus intermediate and late promoter elements are targeted by the TATA-binding protein

Vaccinia virus replicates in the cytoplasm of the host cell and encodes its own RNA polymerase and transcription factors. The proteins that target the poxvirus RNA polymerase to intermediate- and late-class promoters have not been identified. In this study, representatives of the intermediate and late promoters were characterized at the nucleotide level to identify essential motifs. Both intermediate and late viral promoters are shown to have an essential element suggestive of TATA boxes, which are potential targets for the TATA-binding protein (TBP). Several approaches were used to test for TBP requirement in vaccinia virus transcription, including overexpression of TBP, expression of a dominant negative mutant of TBP, RNA interference, and expression of adenovirus E1A protein, which inactivates TBP. In each case, the results support an essential role for TBP in vaccinia virus intermediate- and late-gene transcription. These findings indicate that poxviruses have integrated TBP as a central feature into an otherwise heterologous transcription system. A model for transcriptional switching, in which both intermediate and late promoter elements are targeted by TBP that recruits viral transcription factors to assemble a functional complex on their cognate promoters and a dysfunctional, repressed complex on the other class, is proposed.

Differential role played by the MEK/ERK/EGR-1 pathway in orthopoxviruses vaccinia and cowpox biology

Appropriation of signalling pathways facilitates poxvirus replication. Poxviruses, as do most viruses, try to modify the host cell environment to achieve favourable replication conditions. In the present study, we show that the early growth response 1 gene (egr-1) is one of the host cell factors intensely modulated by the orthopoxviruses VV (vaccinia virus) and CPV (cowpox virus). These viruses stimulated the generation of both egr-1 mRNA and its gene product, throughout their entire replication cycles, via the requirement of MEK [mitogen-activated protein kinase/ERK (extracellular-signal-regulated kinase) kinase]/ERK pathway. We showed that, upon VV infection, EGR-1 translocates into the nucleus where it binds to the EBS (egr-1-binding site) positioned at the 5' region of EGR-1-regulated genes. In spite of both viruses belonging to the same genus, several lines of evidence, however, revealed a remarkable contrast between them as far as the roles played by the MEK/ERK/EGR-1 pathway in their biological cycles are concerned. Hence (i) the knocking-down of egr-1 by siRNA (small interfering RNA) proved that this transcription factor is of critical relevance for VV biology, since a decrease of about one log cycle in virus yield was verified, along with a small virus plaque phenotype, whereas the gene silencing did not have a detrimental effect on either CPV multiplication or viral plaque size; (ii) while both pharmacological and genetic inhibition of MEK/ERK resulted in a significant decrease in VV yield, both approaches had no impact on CPV multiplication; and (iii) CPV DNA replication was unaffected by pharmacological inhibition of MEK/ERK, but phosphorylation of MEK/ERK was dependent on CPV DNA replication, contrasting with a significant VV DNA inhibition and VV DNA replication-independence to maintain ERK1/2 phosphorylation, observed under the same conditions.

Vaccinia virus K1L protein mediates host-range function in RK-13 cells via ankyrin repeat and may interact with a cellular GTPase-activating protein

The K1L protein of vaccinia virus is required for its growth in certain cell lines (RK-13 and human). The cowpox host-range protein CP77 has been shown to complement K1L function in RK-13 cells, despite a lack of homology between the two proteins except for ankyrin repeats. We investigated the role of ankyrin repeats of K1L protein in RK-13 cells. The growth of a recombinant vaccinia virus, with K1L gene mutated in the most conserved ankyrin repeat, was severely impaired. Infection with the mutant virus caused shutdown of cellular and viral protein synthesis early in infection. We also investigated the interaction of K1L protein with cellular proteins and found that K1L interacts with the rabbit homologue of human ACAP2, a GTPase-activating protein with ankyrin repeats. Our result suggests the importance of ankyrin repeat for host-range function of K1L in RK-13 cells and identifies ACAP2 as a cellular protein, which may be interacting with K1L.

Host cell nuclear proteins are recruited to cytoplasmic vaccinia virus replication complexes

The initiation and termination of vaccinia virus postreplicative transcription have been reported to require cellular proteins, some of which are believed to be nuclear proteins. Vaccinia virus replicates in the cytoplasmic compartment of the cell, raising questions as to whether vaccinia virus has access to nuclear proteins. This was addressed here by following the fate of several nuclear proteins after infection of cells with vaccinia virus. The nuclear transcription factors YY1, SP1, and TATA binding protein were found to colocalize with virus replication complexes in the cytoplasm of infected cells. In addition, the nuclear proteins RNA polymerase II, TAFIIp32, and histone deacetylase 8, but not the structural protein lamin B, also were found in the cytoplasm of the cell. The association of YY1 with replication complexes was dependent on DNA replication and required only the DNA binding domain of the protein, indicating that DNA binding alone may be responsible for the association of nuclear transcription factors with viral replication complexes in the cytoplasm. The cytoplasmic localization of YY1 was resistant to the nuclear export inhibitor leptomycin B. Evidence is presented indicating that nuclear import and export pathways were not adversely affected by vaccinia virus infection. These observations indicate that vaccinia virus replication complexes have ready access to nuclear proteins by allowing leakage from the nucleus.

Poxvirus tropism

Despite the success of the WHO-led smallpox eradication programme a quarter of a century ago, there remains considerable fear that variola virus, or other related pathogenic poxviruses such as monkeypox, could re-emerge and spread disease in the human population. Even today, we are still mostly ignorant about why most poxvirus infections of vertebrate hosts show strict species specificity, or how zoonotic poxvirus infections occur when poxviruses occasionally leap into novel host species. Poxvirus tropism at the cellular level seems to be regulated by intracellular events downstream of virus binding and entry, rather than at the level of specific host receptors as is the case for many other viruses. This review summarizes our current understanding of poxvirus tropism and host range, and discusses the prospects of exploiting host-restricted poxvirus vectors for vaccines, gene therapy or tissue-targeted oncolytic viral therapies for the treatment of human cancers.

Poxvirus host range varies markedly ? some viruses, such as variola and molluscum contagiosum virus (both of which are human-specific), exhibit strict species tropism, whereas others such as cowpox virus are able to infect multiple host species.

Members of four of the eight genera of chordopoxviruses can zoonotically infect man. For example, monkeypox virus can cause severe smallpox-like disease in humans that clinically resembles variola virus.

The species tropism that is exhibited by many poxviruses in terms of causing disease is frequently quite different from the range of cultured cells that can be infected by these viruses.

Specific host-cell receptors do not mediate the distinction between cells that are permissive as opposed to non-permissive for poxvirus infection. Rather, restrictive host cells fail to support the full replication cycle of the infecting poxvirus at a point downstream of binding and entry.

A variety of poxviral host-range genes have been identified that contribute to the control of permissive versus non-permissive infection of cultured mammalian cells. The gene products of these host-range genes regulate the ability of the virus to complete its cytoplasmic replication cycle.

The development of host-restricted vaccines, like modified vaccinia Ankara (MVA), that do not replicate in humans but that retain potent immunogenicity, will provide safer platforms for recombinant vaccines.

Another advance has been the development of poxvirus-based oncolytic vectors that replicate preferentially in human tumour 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.

UUUUUNU oligonucleotide inhibition of RNA synthesis in vaccinia virus cores

Recent results from this laboratory demonstrated the ability of U5NU-containing oligonucleotides to stimulate premature termination of early gene transcription in vitro. Further studies on the oligonucleotide sequence and structural requirements for stimulating premature termination demonstrated that only oligonucleotides possessing ribouracil U9 with a phosphodiester linkage are active. Because an oligonucleotide as short as 9 bases serves as an effective stimulator of premature transcription termination, we reasoned that short U5NU-containing oligonucleotides might serve as efficacious anti-poxvirus agents because they would prevent the synthesis of full-sized early mRNA. To be useful in vivo, the oligonucleotides must not only be taken up by the infected cells, but also be able to enter the virus core, the site of early gene transcription, and retain their ability to stimulate premature termination. The ability of U9-containing oligonucleotides to inhibit virus core RNA synthesis was evaluated. The U5NU oligonucleotides exhibited a dramatic sequence-specific inhibition of core RNA synthesis, consistent with their ability to stimulate premature termination of early gene transcription. Moreover, the concentration of U5NU oligonucleotide required to exhibit half maximal inhibition of RNA synthesis was found to be less for a 9 mer RNA than it was for a 17 or 22 mer RNA. This suggests the possibility that the smaller oligonucleotides may have easier access to the core. This observation lends support to the notion that such oligonucleotides might serve as effective anti-poxvirus therapeutic agents.

YY1 is recruited to the cytoplasm of vaccinia virus-infected human macrophages by the Crm1 system

The influence of vaccinia virus infection on activity and subcellular localization of cellular transcription factors YY1 and C/EBPbeta in human blood monocytes derived macrophages was examined. YY1 activity, shown by electrophoretic mobility shift assay, decreased upon infection in the nuclear extracts but remained unchanged in whole cell extracts until 48h post-infection (p.i.). Immunohistochemical staining of the fixed cells showed translocation of the factor to the cytoplasm of the infected macrophages. The nuclear export of YY1 was blocked by leptomycin B, an inhibitor of the Crm1-dependent export system. C/EBP DNA binding activity was transiently increased during viral infection. Cytoplasmic translocation of the C/EBPbeta has also been observed and was found to be blocked by leptomycin B treatment of the cells. It appears that the Crm1 system is not impaired by the virus infection in blood-derived macrophages and that it remains operative for redirection of subcellular localization of transcription factors from the nucleus to the cytoplasm. Moreover, blockage of the nuclear export by leptomycin B significantly affected the yield of infectious virus. The results might help to better understand the mechanism of protein transport during viral infection of monocyte-derived cells.

UUUUUNU stimulation of vaccinia virus early gene transcription termination. Oligonucleotide sequence and structural requirements for stimulation of premature termination in vitro

Vaccinia virus early genes are unique in that transcription terminates in a signal- and factor-dependent manner. Recent results from this laboratory demonstrated that a 22-mer RNA oligonucleotide containing a central U9 sequence exhibited sequence- and concentration-dependent stimulation of premature transcription termination and transcript release in trans. In an effort to better understand the different aspects of the U5NU stimulation of premature termination, we evaluated the activity of various oligonucleotides in vitro. Neither RNA containing a mutant U5NU signal nor single-stranded DNA containing T5NT was able to stimulate premature termination, demonstrating both sequence specificity and a requirement for ribose. Furthermore, neither oligonucleotide was able to compete with U5NU, demonstrating that each failed to bind to the U5NU recognition factor. Substitution of the U9 signal with either BrU9 or BrdU9 inhibited normal termination but did not stimulate premature termination. The addition of BrdU5NdU inhibited U5NU stimulation of premature termination, demonstrating that both oligonucleotides bind to the same site on the U5NU recognition factor. Finally, U5NU containing RNA as short as nine bases served as an effective stimulator of premature termination. These observations impact directly on the development of oligonucleotide based anti-poxvirus therapeutic agents.

Vaccinia virus transcription

Vaccinia virus replication takes place in the cytoplasm of the host cell. The nearly 200 kbp genome owes part of its complexity to encoding most of the proteins involved in genome and mRNA synthesis. The multisubunit vaccinia virus RNA polymerase requires a separate set of virus-encoded proteins for the transcription of the early, intermediate and late classes of genes. Cell fractionation studies have provided evidence for a role for host cell proteins in the initiation and termination of vaccinia virus intermediate and late gene transcription. Vaccinia virus resembles nuclear DNA viruses in the integration of viral and host proteins for viral mRNA synthesis, yet is markedly less reliant on host proteins than its nuclear counterparts.

Redistribution of cyclophilin A to viral factories during vaccinia virus infection and its incorporation into mature particles

Cyclophilins are peptidyl-prolyl cis-trans isomerases involved in catalyzing conformational changes and accelerating the rate of protein folding and refolding in several cellular systems. In the present study, we analyzed the expression pattern and intracellular distribution of the cellular isomerase cyclophilin A (CypA) during vaccinia virus (VV) infection. An impressive increase in CypA stability was observed, leading to a practically unchanged accumulation of CypA during infection, although its synthesis was completely inhibited at late times. By confocal microscopy, we observed that CypA went through an intense reorganization in the cell cytoplasm and colocalized with the virosomes late in infection. CypA relocation to viral factories required the synthesis of viral postreplicative proteins, and treatment of infected cells with cyclosporine (CsA) prevented CypA relocation, clearly excluding the virosomes from CypA staining. Immunoelectron microscopy of VV-infected cells showed that CypA was incorporated into VV particles during morphogenesis. Biochemical and electron microscopic assays with purified virions confirmed that CypA was encapsidated within the virus particle and localized specifically in the core. This work suggests that CypA may develop an important role in VV replication.

UUUUUNU oligonucleotide stimulation of vaccinia virus early gene transcription termination, in trans

Vaccinia virus early gene transcription termination requires the vaccinia termination factor (VTF), NPH I, a single stranded DNA-dependent ATPase, the virion form of RNA polymerase containing the Rap 94 subunit, and the signal UUUUUNU, which resides in the nascent mRNA, located 30 to 50 bases upstream from the poly(A) addition site. Evidence indicates that a required termination factor acts through binding to the UUUUUNU signal. To further investigate the function of UUUUUNU, the ability of UUUUUNU containing oligonucleotides to inhibit transcription termination was tested. A 22-mer RNA oligonucleotide containing a central U9 sequence exhibited sequence and concentration-dependent stimulation of premature transcription termination and transcript release, in trans. Activation of premature termination required VTF, NPH I, Rap 94, and ATP, demonstrating that the normal termination machinery was employed. Premature termination was not stimulated by RNA harboring a mutant UUUUUNU, demonstrating specificity. These data are consistent with a model in which a required termination factor is converted from an inactive to an active form by binding to a UUUUUNU containing oligonucleotide. The active termination factor then interacts with the ternary complex stimulating transcription termination through the normal mechanism, independent of the nascent mRNA sequence.

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.

Antibodies directed against an epitope in the N-terminal region of the H4L subunit of the vaccinia virus RNA polymerase inhibit both transcription initiation and transcription termination, in vitro

The vaccinia virus virion RNA polymerase that is active in early gene transcription contains a unique subunit encoded by the H4L gene. Prior studies demonstrated that this protein is required both for early gene transcription initiation and for transcription termination. Polyclonal antibodies raised against H4L amino acids 1 to 256 prevent both initiation and termination of transcription, in vitro. Pretreatment of the anti-H4L antibody with a H4L fragment containing amino acids 1 to 99 prevents antibody inhibition of both steps, mapping the inhibitory antibody-binding site to this region. A combination of immunoprecipitation and competition studies of antibody binding to wild-type and site-specific mutations of H4L(1-195) mapped the strong epitope to a site that includes Y18. H4L fragments containing an Y18A mutation exhibit diminished ability to block antibody inhibition of transcription initiation and termination. Antibodies inhibit preinitiation complex (PIC) formation but not the activity of preformed PICs, indicating that this region of H4L interacts with one or more factors during active PIC formation. Furthermore, isolated H4L(1-195) directly inhibits PIC activity, supporting this model. Anti-H4L antibody inhibition of transcription termination is only observed in the absence of the essential termination cofactor NPH I. In contrast, antibody inhibition of PIC formation is unaffected by NPH I, demonstrating that the inhibitory antibody and NPH I can bind to H4L at the same time.

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.

The viral RNA polymerase H4L subunit is required for Vaccinia virus early gene transcription termination

Vaccinia virus early gene transcription is catalyzed by a multisubunit virion form of RNA polymerase that possesses a unique subunit, H4L. Prior studies from this laboratory showed that the NH(2)-terminal domain of H4L, containing amino acids 1-195, interacts with the COOH-terminal end of nucleoside triphosphate phosphohydrolase I (NPH I), an ATPase that is employed in early gene transcription termination. Carboxyl-terminal deletion mutations of NPH I lose both the ability to mediate transcription termination and binding to H4L, providing evidence that the interaction between NPH I and H4L is required for termination. In order to test this model further, antibodies raised against segments of H4L were tested for their ability to inhibit transcription termination in vitro. A bead-bound template was employed in these studies, which permitted us to separate transcription initiation from elongation and termination. Antibodies raised against H4L amino acids 1-256 inhibited termination in an in vitro assay using virus-infected cell extracts lacking NPH I, but antibodies raised against H4L amino acids 568-795 did not. Preincubation of anti-H4L(1-256) antibodies with H4L fragments 1-256 or 1-195 prevented antibody inhibition of termination, demonstrating that inhibition was mediated by antibody binding to one or more epitopes in the NH(2)-terminal end of H4L. Antibody inhibition of termination is reduced in wild type virus-infected cell extracts containing NPH I. Furthermore, preincubation of a NPH I minus cell extract with NPH I prior to antibody addition, or readdition of NPH I to isolated ternary complexes prepared in the absence of NPH I, prevented antibody inhibition of transcription termination. These data show that NPH I and the inhibitory antibodies compete for a binding site(s) on H4L, providing further evidence that the H4L subunit of the vaccinia virus RNA polymerase plays a direct role in transcription termination.

Regulation of viral intermediate gene expression by the vaccinia virus B1 protein kinase

The B1 gene of vaccinia virus encodes a serine/threonine protein kinase that is expressed early after infection. Under nonpermissive conditions, temperature-sensitive mutants (ts2 and ts25) that map to B1 fail to efficiently replicate viral DNA. Our goal was to extend studies on the function of B1 by determining if the kinase is required for intermediate or late gene expression, two events that ordinarily depend on viral DNA replication. First, we established that early viral gene expression occurred at the nonpermissive temperature. By using a transfection procedure that circumvents the viral DNA replication requirement, we found that reporter genes regulated by an intermediate promoter were transcribed only under conditions permissive for expression of active B1. To assay late gene expression, the T7 RNA polymerase gene was inserted into the genome of ts25 to form ts25/T7. A DNA replication-independent late gene transcription system was established by cotransfecting plasmids containing T7 promoter-driven late gene transcription factors and a late promoter reporter gene into ts25/T7-infected cells. Late genes, unlike intermediate genes, were expressed at the nonpermissive temperature. Last, we showed that overexpression of B1 stimulated intermediate but inhibited late gene expression in cells infected with wild-type virus.

Interaction between the J3R subunit of vaccinia virus poly(A) polymerase and the H4L subunit of the viral RNA polymerase

J3R, the 39-kDa subunit of vaccinia virus poly(A) polymerase, is a multifunctional protein that catalyzes (nucleoside-2'-O-)-methyltransferase activity, serves as a poly(A) polymerase stimulatory factor, and acts as a postreplicative positive transcription elongation factor. Prior results support an association between poly(A) polymerase and the virion RNA polymerase. A possible direct interaction between J3R and H4L subunit of virion RNA polymerase was evaluated. J3R was shown to specifically bind to H4L amino acids 235-256, C terminal to NPH I binding site on H4L. H4L binds to the C-terminal region of J3R between amino acids 169 and 333. The presence of a J3R binding site near to the NPH I binding region on H4L led us to evaluate a physical interaction between NPH I and J3R. The NPH I binding site was located on J3R between amino acids 169 and 249, and J3R was shown to bind to NPH I between amino acids 457 and 524. To evaluate a role for J3R in early gene mRNA synthesis, transcription termination, and/or release, a transcription-competent extract prepared from cells infected with mutant virus lacking J3R, J3-7. Analysis of transcription activity demonstrated that J3R is not required for early mRNA synthesis and is not an essential factor in early gene transcription termination or transcript release in vitro. J3R interaction with NPH I and H4L may serve as a docking site for J3R on the virion RNA polymerase, linking transcription to mRNA cap formation and poly(A) addition.

Interaction between nucleoside triphosphate phosphohydrolase I and the H4L subunit of the viral RNA polymerase is required for vaccinia virus early gene transcript release

Signal-dependent termination is restricted to early poxvirus genes whose transcription is catalyzed by the virion form of RNA polymerase. Two termination factors have been identified. Vaccinia termination factor/capping enzyme is a multifunctional heterodimer that also catalyzes the first three steps of mRNA cap formation and is an essential intermediate gene transcription initiation factor. Nucleoside triphosphate phosphohydrolase I (NPH I) is a single-stranded DNA-dependent ATPase. COOH-terminal deletion mutations of NPH I retain both ATPase and DNA binding activities but are unable either to terminate transcription or to act as dominant negative mutants in vitro. One appealing model posits that the COOH-terminal region of NPH I binds to one or more components in the termination complex. In an attempt to identify NPH I-related protein/protein interactions involved in transcription termination, a series of pull-down experiments were done. Among several vaccinia virus proteins tested, the H4L subunit, unique to the virion form of RNA polymerase, was shown to bind glutathione S-transferase (GST)-NPH I. To further confirm this interaction in virus-infected cells, we constructed recombinant vaccinia virus, vNPHINGST, that expresses GST-tagged NPH I. The H4L subunit of virion RNA polymerase specifically co-purified with GST-NPH I, consistent with a physical interaction. Through the analysis of a series of NH(2)- and COOH-terminal truncation mutations of H4L, the NPH I interaction site was localized to the NH(2)-terminal 195 amino acids of the H4L protein. The H4L binding site on NPH I was mapped to the COOH-terminal region between 457 and 631. Furthermore, COOH-terminal deletion mutations of NPH I failed to bind the NH(2)-terminal region of H4L, explaining their inability to support transcription termination. The COOH-terminal end of NPH I was also shown to be required for transcript release activity and for dominant negative inhibition of release. The requirement for an essential interaction between NPH I and H4L provides an explanation for the observed restriction of transcription termination to early viral genes.

Vaccinia virus gene A18R DNA helicase is a transcript release factor

Prior phenotypic analysis of a vaccinia virus gene A18R mutant, Cts23, showed the synthesis of longer than wild type (Wt) length viral transcripts during the intermediate stage of infection, indicating that the A18R protein may act as a negative transcription elongation factor. The purpose of the work described here was to determine a biochemical activity for the A18R protein. Pulse-labeled transcription complexes established from intermediate virus promoters on bead-bound DNA templates were assayed for transcript release during an elongation step that contained nucleotides and various proteins. Pulse-labeled transcription complexes elongated in the presence of only nucleotides were unable to release nascent RNA. The addition of Wt extract during the elongation phase resulted in release of the nascent transcript, indicating that additional factors present in the Wt extract are capable of inducing transcript release. Extract from Cts23 or mock-infected cells was unable to induce release. The lack of release upon addition of Cts23 extract suggests that A18R is involved in release of nascent RNA. By itself, purified polyhistidine-tagged A18R protein (His-A18R) was unable to induce release; however, release did occur in the presence of purified His-A18R protein plus extract from either Cts23 or mock-infected cells. These data taken together indicate that A18R is necessary but not sufficient for release of nascent transcripts. We have also demonstrated that the combination of A18R protein and mock extract induces transcript release in an ATP-dependent manner, consistent with the fact that the A18R protein is an ATP-dependent helicase. Further analysis revealed that the release activity is not restricted to a vaccinia intermediate promoter but is observed using pulse-labeled transcription complexes initiated from all three viral gene class promoters. Therefore, we conclude that A18R and an as yet unidentified cellular factor(s) are required for the in vitro release of nascent RNA from a vaccinia virus transcription elongation complex.

Transcription factor YY1 is a vaccinia virus late promoter activator

Vaccinia virus has a DNA genome, yet replicates in the cytoplasmic compartment of the cell. We previously described the identification of a cellular protein having high affinity for vaccinia virus late promoter DNA. Sequence substitutions in the vaccinia I1L promoter were used to define a 5-nucleotide block at the transcription initiation site as essential for interaction with the protein. Within this sequence is the recognition motif for the nuclear transcription factor YY1. This factor regulates a multitude of cellular promoters, as an activator of transcription, as a repressor, or as an initiator element-binding protein. Antibodies directed against YY1 were used to show that YY1 copurified with the vaccinia late promoter-binding protein and was present in late promoter-protein complexes in gel supershift assays. Bacterially expressed YY1 also bound specifically to late promoter DNA. A dinucleotide replacement within the YY1 recognition motif directly adjacent to the transcription start site severely reduced the affinity of YY1 for the I1L promoter in vitro and impaired I1L promoter-dependent transcription in vivo. The intracellular localization of YY1 was shown by immunofluorescence microscopy to shift from primarily nuclear to the cytoplasm after vaccinia infection. These results indicate that YY1 has a positive role in the regulation of vaccinia virus late gene transcription and suggest that poxviruses have adapted cellular initiator elements as a means of regulating viral gene expression. This is the first identifiable cellular protein implicated in poxvirus transcription.

Vaccinia virus serpin-1 deletion mutant exhibits a host range defect characterized by low levels of intermediate and late mRNAs

Orthopoxviruses encode three serpin homologs-SPI-1, SPI-2 and SPI-3-of which SPI-2 has been well characterized as an inhibitor of ICE-like proteases. A rabbitpox virus SPI-1 deletion mutant exhibited a host range restriction in human lung A549 and pig kidney 15 cell lines that was attributed to apoptosis. Here we report that replication of a vaccinia virus SPI-1 deletion mutant (DeltaSPI-1) was restricted in primary human keratinocytes as well as A549 cells. Although chromatin condensation was detected in some A549 cells, other morphological or biochemical signs of apoptosis including DNA fragmentation, cleavage of poly(ADP-ribose)polymerase or nuclear mitotic apparatus protein, or caspase 3 activation were not found. Moreover, DeltaSPI-1 protected A549 cells from apoptosis induced by tumor necrosis factor, whereas the corresponding DeltaSPI-2 mutant did not. Further studies indicated undiminished amounts of vaccinia virus early mRNA and replicated DNA in the absence of the SPI-1 product. However, there were reduced amounts of viral intermediate and late mRNAs, viral late proteins, cleaved core proteins, and virus particles. These data suggested that apoptosis is not the determining factor in the host range restriction of DeltaSPI-1 and that the SPI-1 gene product is needed to allow efficient expression of intermediate and late genes in A549 cells.

Identification of a transcription factor, encoded by two vaccinia virus early genes, that regulates the intermediate stage of viral gene expression

Vaccinia virus early, intermediate, and late stage genes are sequentially transcribed by the viral RNA polymerase within the cytoplasm of infected cells. We found that the 34- and 45-kDa polypeptides encoded by vaccinia virus ORFs A8R and A23R, respectively, were necessary to reconstitute transcription of a template with an intermediate stage promoter. Coexpression of the A8R and A23R genes in Escherichia coli was required for in vitro activity. In addition, the two polypeptides copurified, indicating their association as protein subunits of a vaccinia virus intermediate transcription factor. This factor, which we named VITF-3, complemented three viral proteins-namely, the RNA polymerase, capping enzyme, and a 30-kDa protein called VITF-1 that is also a subunit of the RNA polymerase-and an unidentified cell factor called VITF-2. Expression of the A8R and A23R genes occurred between 1 and 5 h after vaccinia virus infection and was not prevented by an inhibitor of DNA replication, consistent with a role for VITF-3 in specifically regulating intermediate transcription in vivo. The vaccinia virus A8R and A23R genes are highly conserved among vertebrate poxviruses, but no other viral or cellular homologs were identified.

Vaccinia virus-induced apoptosis in immature B lymphocytes: role of cellular Bcl-2

Apoptosis is a form of physiological cell death which can be initiated in response to various stimuli including virus infections. We show that vaccinia virus (VV) infection induces apoptosis in an immature B lymphocyte line, WEHI-231. In these cells, several VV-specific proteins were synthesized during the infection, but neither virus production nor viral DNA synthesis were detected. The intracellular levels of the proto-oncogene Bcl-2, which effectively protects cells from programmed cell death, were found to be down-regulated by the VV infection, suggesting that this down-regulation might be involved in the viral induction of apoptosis in WEHI-231 cells. Stable transfectants overexpressing human Bcl-2 were shown to be resistant to the apoptosis produced by the infection, a finding consistent with the proposed role for the down-regulation of endogenous Bcl-2 in VV-induced apoptotic death.

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 vaccinia virus A18R DNA helicase is a postreplicative negative transcription elongation factor

Loss of vaccinia virus A18R gene function results in an aberrant transcription profile termed promiscuous transcription, defined as transcription within regions of the genome which are normally transcriptionally silent late during infection. Promiscuous transcription results in an increase in the intracellular concentration of double-stranded RNA, which in turn results in activation of the cellular 2-5A pathway and subsequent RNase L-catalyzed degradation of viral and cellular RNAs. One of three hypotheses could account for promiscuous transcription: (i) reactivation of early promoters late during infection, (ii) random transcription initiation, (iii) readthrough transcription from upstream promoters. Transcriptional analysis of several viral genes, presented here, argues strongly against the first two hypotheses. We have tested the readthrough hypothesis by conducting a detailed transcriptional analysis of a region of the vaccinia virus genome which contains three early genes (M1L, M2L, and K1L) positioned directly downstream of the intermediate gene, K2L. The results show that mutation of the A18R gene results in increased readthrough transcription of the M1L gene originating from the K2L intermediate promoter. A18R mutant infection of RNase L knockout mouse fibroblast (KO3) cells does not result in 2-5A pathway activation, yet the virus mutant is defective in late viral gene expression and remains temperature sensitive. These results demonstrate that the A18R gene product is a negative transcription elongation factor for postreplicative viral genes.

The non-permissive infection of insect (gypsy moth) LD-652 cells by Vaccinia virus

The members of Poxviridae family are among the most complex of animal viruses and subfamily members infect both vertebrate (Chordopoxvirinae) and invertebrate (Entomopoxvirinae) hosts, respectively. Vaccinia virus (VV) is the most commonly studied vertebrate virus and the entomopoxvirus of Amsacta moorei (AmEPV) is the prototypic insect virus. AmEPV, while not able to productively infect vertebrate cells, does enter vertebrate cells and expresses early genes after which the infection aborts although the cells survive (Y. Li, R. L. Hall, and R. W. Moyer. J.Virol. 71(12), 95579562, 1997). We show here that a recombinant VV, containing the lacZ gene regulated by the cowpox virus A-type inclusion (ATI) late promoter, likewise does not productively infect insect cells. Our results suggest that the recombinant VV enters insect cells, host protein synthesis is inhibited, early gene expression is normal, and viral DNA replication occurs as does late protein synthesis. However, little if any proteolytic processing of late viral proteins, typical of morphogenesis, is observed. Electron micrographs of infected cells suggest that while cytoplasmic virosomes (factories) are formed, there is little indication of further morphogenesis or any formation of mature virions. Therefore, while both orthopoxviruses and entomopoxviruses fail to replicate in heterologous hosts, the nature of abortive infections is quite different.

A new vaccinia virus intermediate transcription factor

Transcription of the vaccinia virus genome is mediated by a virus-encoded multisubunit DNA-dependent RNA polymerase in conjunction with early-, intermediate-, and late-stage-specific factors. Previous studies indicated that two virus-encoded proteins (capping enzyme and VITF-1) and one unidentified cellular protein (VITF-2) are required for specific transcription of an intermediate promoter template in vitro. We have now extensively purified an additional virus-induced intermediate transcription factor with a native mass of approximately 100 kDa.

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.

The genomic sequence analysis of the left and right species-specific terminal region of a cowpox virus strain reveals unique sequences and a cluster of intact ORFs for immunomodulatory and host range proteins

Sequencing and computer analysis of the left (52,283 bp) and right (49,649 bp) variable DNA regions of the cowpox virus strain GRI-90 (CPV-GRI) has revealed 51 and 37 potential open reading frames (ORFs), respectively. Comparison of the structure-function organization of these DNA regions of CPV-GRI with those previously published for corresponding regions of genomes of vaccinia virus, strains Copenhagen (VAC-COP) and Western Reserve (VAC-WR); and variola major virus, strains India-1967 (VAR-IND), Bangladesh-1975 (VAR-BSH); and alastrim variola minor virus, strain Garcia-1966 (VAR-GAR), was performed. Within the left terminal region under study, an extended DNA sequence (14,171 bp), unique to CPV, has been found. Within the right region of the CPV-GRI genome two segments, which are unique to CPV DNA (1579 and 3585 bp) have been found. Numerous differences have been revealed in the genetic structure of CPV-GRI DNA regions, homologous to fragments of the genomes of the above-mentioned orthopoxvirus strains. A cluster of ORFs with structural similarity ot immunomodulatory and host range function of other poxviruses have also been detected. A comparison of the sequences of ORF B, crmA, crmB, crmC, IMP, and CHO hr genes of CPV Brighton strain (CPV-BRI) with the corresponding genes in strain GRI-90 have revealed an identity at the amino acid level ranging from 82 to 96% between the two strains. The findings are significant in light of the recent demonstration of CPV as an important poxvirus model system to probe the precise in vivo role(s) of the unique virally encoded immunomodulatory proteins. Also, the presence of a complete and intact repertoire of immunomodulatory proteins, ring canal proteins family, and host range genes indicates that CPV may have been the most ancient of all studied orthopoxviruses.

Changes in alpha-1-antichymotrypsin expression in vaccinia virus infected HepG2 cells

Human hepatoma cells (HepG2) synthesize and secrete several plasma proteins that are inhibited in a time- and dose-dependent manner after vaccinia virus infection. However, infection of the HepG2 cells with a low dose of the virus (up to 1 plaque forming unit/cell) stimulated the expression of alpha-1-antichymotrypsin, which was demonstrated by means of electroimmunoassay and Northern blot analysis. This stimulation appeared to be on the level of transcription as shown in transient transfection experiments using various alpha-1-antichymotrypsin gene promoter constructs. In contrast to interleukin-6, virus-induced activation of the alpha-1-antichymotrypsin gene transcription does not require the STAT (signal transducers and activators of transcription) binding elements present in the alpha-1-antichymotrypsin gene promoter. Furthermore, alpha-amanitin, which inhibits eukaryotic RNA polymerase II and III, did not affect alpha-1-antichymotrypsin stimulation by the virus, indicating involvement of the viral transcriptional apparatus in transient activation of alpha-1-antichymotrypsin gene expression.

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.

Vaccinia virion protein VP8, the 25 kDa product of the L4R gene, binds single-stranded DNA and RNA with similar affinity

Vaccinia virus protein VP8 is a 25 kDa product of the L4R gene and is an abundant virion protein that binds single-stranded (ss) and double-stranded (ds) DNA. Binding of ssDNA is preferred at high salt concentrations. Using a recombinant 25 kDa L4R (rL4R) protein and a gel mobility shift assay with radiolabelled oligonucleotides, the Kd for a 45mer oligonucleotide was determined to be 2 nM. The Kd was unaltered by 50 mM KCl but was reduced 35-fold by 100 mM KCl. Multiple rL4R molecules bound to a single 45mer oligonucleotide, and using oligonucleotides of different lengths it was calculated that one rL4R molecule bound every 17 nt. Binding to ssDNA was competed by both deoxyribo- and ribo-polynucleotides. RNA binding was observed for both rL4R and native VP8, purified from virions, using a gel mobility shift with a radiolabelled ssRNA of 130 nt. The Kd of rL4R for this ssRNA substrate was 3 nM in the absence of salt and binding was positively cooperative. The potential roles of L4R protein in vaccinia virus early transcription are discussed.

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.