Vaccinia virus-like early transcriptional control sequences flank an early gene in orf virus

The parapoxvirus Orf virus inhibits dsDNA-mediated type I IFN expression via STING-dependent and STING-independent signalling pathways

Type I interferons (IFNs) are critical in the host defence against viruses. They induce hundreds of interferon-stimulated genes (ISGs) many of which have an antiviral role. Poxviruses induce IFNs via their pathogen-associated molecular patterns, in particular, their genomic DNA. In a majority of cell types, dsDNA is detected by a range of cytoplasmic DNA sensors that mediate type I IFN expression via stimulator of interferon genes (STING). Orf virus (ORFV) induces cutaneous pustular skin lesions and is the type species of the Parapoxvirus genus within the Poxviridae family. The aim of this study was to investigate whether ORFV modulates dsDNA-induced type I IFN expression via STING-dependent signalling pathways in human dermal fibroblasts (hNDF) and THP-1 cells. We showed that ORFV infection of these cell types treated with poly(dA:dT) resulted in strong inhibition of expression of IFN-β. In hNDFs, we showed using siRNA knock-down that STING was essential for type I IFN induction. IFN-β expression was further reduced when both STING and RIG-I were knocked down. In addition, HEK293 cells that do not express STING or Toll-like receptors also produce IFN-β following stimulation with poly(dA:dT). The 5' triphosphate dsRNA produced by RNA polymerase III specifically results in the induction of type I IFNs through the RIG-I receptor. We showed that ORFV infection resulted in strong inhibition of IFN-β expression in HEK293 cells stimulated with poly(dA:dT). Overall, this study shows that ORFV potently counteracts the STING-dependent and STING-independent IFN response by antagonizing dsDNA-activated IFN signalling pathways.

The parapoxvirus Orf virus inhibits IFN-β expression induced by dsRNA

Orf virus (ORFV) is the type species of the Parapoxvirus genus that belongs to the Poxviridae family. Type I interferons (IFN) are critical in the host defence against viruses. They induce hundreds of interferon stimulated genes (ISGs) many of which have an antiviral role. The ability of ORFV to modulate type I IFN production was undertaken to investigate whether ORFV could inhibit IFN-β expression via dsRNA dependant signalling pathways. HEK293 cells are known to lack DNA pattern-recognition receptors and Toll-like receptors however, they do express the cytosolic dsRNA receptors RIG-I and MDA5. HEK293 cells were shown to produce high levels of IFN-β when cells were stimulated with poly(I:C) and this was shown to be predominantly via RIG-I-dependant signalling as confirmed by siRNA knock-down of RIG-I. Further we showed that HEK293 cells are permissive for ORFV and caused potent inhibition of IFN-β transcription when cells were stimulated with poly(I:C) post-viral infection. Studies using heat inactivated ORFV suggested that de novo synthesis of early genes was required. In addition our findings showed that the ORFV encoded factor ORF020, that is known to bind dsRNA, is involved in antagonising IFN expression. Overall, this study has shown for first time the ability of ORFV to counteract type I IFN expression by antagonising dsRNA-activated RIG-I signalling.

The parapoxvirus Orf virus ORF116 gene encodes an antagonist of the interferon response

Orf virus (ORFV) is the type species of the Parapoxvirus genus of the Poxviridae family. Genetic and functional studies have revealed ORFV has multiple immunomodulatory genes that manipulate innate immune responses, during the early stage of infection. ORF116 is a novel gene of ORFV with hitherto unknown function. Characterization of an ORF116 deletion mutant showed that it replicated in primary lamb testis cells with reduced levels compared to the wild-type and produced a smaller plaque phenotype. ORF116 was shown to be expressed prior to DNA replication. The potential function of ORF116 was investigated by gene-expression microarray analysis in HeLa cells infected with wild-type ORFV or the ORF116 deletion mutant. The analysis of differential cellular gene expression revealed a number of interferon-stimulated genes (ISGs) differentially expressed at either 4 or 6 h post infection. IFI44 showed the greatest differential expression (4.17-fold) between wild-type and knockout virus. Other ISGs that were upregulated in the knockout included RIG-I, IFIT2, MDA5, OAS1, OASL, DDX60, ISG20 and IFIT1 and in addition the inflammatory cytokine IL-8. These findings were validated by infecting HeLa cells with an ORF116 revertant recombinant virus and analysis of transcript expression by quantitative real time-PCR (qRT-PCR). These observations suggested a role for the ORFV gene ORF116 in modulating the IFN response and inflammatory cytokines. This study represents the first functional analysis of ORF116.

Genomic Characterization of Orf Virus Strain D1701-V (Parapoxvirus) and Development of Novel Sites for Multiple Transgene Expression

The Orf virus (ORFV; Parapoxvirus) strain D1701 with an attenuated phenotype and excellent immunogenic capacity is successfully used for the generation of recombinant vaccines against different viral infections. Adaption for growth in Vero cells was accompanied by additional major genomic changes resulting in ORFV strain variant D1701-V. In this study, restriction enzyme mapping, blot hybridization and DNA sequencing of the deleted region s (A, AT and D) in comparison to the predecessor strain D1701-B revealed the loss of 7 open reading frames (ORF008, ORF101, ORF102, ORF114, ORF115, ORF116, ORF117). The suitability of deletion site D for expression of foreign genes is demonstrated using novel synthetic early promoter eP1 and eP2. Comparison of promoter strength showed that the original vegf-e promoter Pv as well as promoter eP2 display an up to 11-fold stronger expression than promoter eP1, irrespective of the insertion site. Successful integration and expression of the fluorescent marker genes is demonstrated by gene- and insertion-site specific PCR assays, fluorescence microscopy and flow cytometry. For the first time ORFV recombinants are generated simultaneously expressing transgenes in two different insertion loci. That allows production of polyvalent vaccines containing several antigens against one or different pathogens in a single vectored ORFV vaccine.

Deletion of the Chemokine Binding Protein Gene from the Parapoxvirus Orf Virus Reduces Virulence and Pathogenesis in Sheep

Orf virus (ORFV) is the type species of the Parapoxvirus genus of the family Poxviridae and infects sheep and goats, often around the mouth, resulting in acute pustular skin lesions. ORFV encodes several secreted immunomodulators including a broad-spectrum chemokine binding protein (CBP). Chemokines are a large family of secreted chemotactic proteins that activate and regulate inflammation induced leukocyte recruitment to sites of infection. In this study we investigated the role of CBP in vivo in the context of ORFV infection of sheep. The CBP gene was deleted from ORFV strain NZ7 and infections of sheep used to investigate the effect of CBP on pathogenesis. Animals were either infected with the wild type (wt) virus, CBP-knockout virus or revertant strains. Sheep were infected by scarification on the wool-less area of the hind legs at various doses of virus. The deletion of the CBP gene severely attenuated the virus, as only few papules formed when animals were infected with the CBP-knock-out virus at the highest dose (10⁷ p.f.u). In contrast, large pustular lesions formed on almost all animals infected with the wt and revertant strains at 10⁷ p.f.u. The lesions for the CBP-knock-out virus resolved approximately 5–6 days p.i, at a dose of 10⁷ pfu whereas in animals infected with the wt and revertants at this dose, lesions began to resolve at approximately 10 days p.i. Few pustules developed at the lowest dose of 10³ p.f.u. for all viruses. Immunohistochemistry of biopsy skin-tissue from pustules showed that the CBP-knockout virus replicated in all animals at the highest dose and was localized to the skin epithelium while haematoxylin and eosin staining showed histological features of the CBP-knockout virus typical of the parent virus with acanthosis, elongated rete ridges and orthokeratotic hyperkeratosis. MHC-II immunohistochemistry analysis for monocytes and dendritic cells showed greater staining within the papillary dermis of the CBP-knock-out virus compared with the revertant viruses, however this was not the case with the wt where staining was similar. Our results show that the CBP gene encodes a secreted immunodulator that has a critical role in virulence and pathogenesis.

Molecular genetic analysis of orf virus: a poxvirus that has adapted to skin

Orf virus is the type species of the Parapoxvirus genus of the family Poxviridae. It induces acute pustular skin lesions in sheep and goats and is transmissible to humans. The genome is G+C rich, 138 kbp and encodes 132 genes. It shares many essential genes with vaccinia virus that are required for survival but encodes a number of unique factors that allow it to replicate in the highly specific immune environment of skin. Phylogenetic analysis suggests that both viral interleukin-10 and vascular endothelial growth factor genes have been "captured" from their host during the evolution of the parapoxviruses. Genes such as a chemokine binding protein and a protein that binds granulocyte-macrophage colony-stimulating factor and interleukin-2 appear to have evolved from a common poxvirus ancestral gene while three parapoxvirus nuclear factor (NF)-κB signalling pathway inhibitors have no homology to other known NF-κB inhibitors. A homologue of an anaphase-promoting complex subunit that is believed to manipulate the cell cycle and enhance viral DNA synthesis appears to be a specific adaptation for viral-replication in keratinocytes. The review focuses on the unique genes of orf virus, discusses their evolutionary origins and their role in allowing viral-replication in the skin epidermis.

Genus Parapoxvirus

Highly contagious pustular skin infections of sheep, goats and cattle that were unwittingly transmitted to humans from close contact with infected animals, have been the scourge of shepherds, herdsmen and dairy farmers for centuries. In more recent times we recognise that these proliferative pustular lesions are likely to be caused by a group of zoonotic viruses that are classified as parapoxviruses. In addition to infecting the above ungulates, parapoxviruses have more recently been isolated from seals, camels, red deer and reindeer and most have been shown to infect man. The parapoxviruses have one of the smallest genomes of the poxvirus family (140 kb) yet share over 70% of their genes with the most virulent members. Like other poxviruses, the central core of the genomes encode factors for virus transcription and replication, and structural proteins, whereas the terminal regions encode accessory factors that give the parapoxvirus group many of its unique features. Several genes of parapoxviruses are unique to this genus and encode factors that target inflammation, the innate immune responses and the development of acquired immunity. These factors include a homologue of mammalian interleukin (IL)-10, a chemokine binding protein and a granulocyte-macrophage colony stimulating factor /IL-2 binding protein. The ability of this group to reinfect their hosts, even though a cell-mediated memory response is induced during primary infection, may be related to their epitheliotropic niche and the immunomodulators they produce. In this highly localised environment, the secreted immunomodulators only interfere with the local immune response and thus do not compromise the host’s immune system. The discovery of a vascular endothelial growth factor-like gene may explain the highly vascular nature of parapoxvirus lesions. There are many genes of parapoxviruses which do not encode polypeptides with significant matches with protein sequences in public databases, separating this genus from most other mammalian poxviruses. These genes appear to be involved in inhibiting apoptosis, manipulating cell cycle progression and degradation of cellular proteins that may be involved in the stress response, thus allowing the virus to subvert intracellular antiviral mechanisms and enhance the availability of cellular molecules required for replication. Parapoxviruses in common with Molluscum contagiosum virus lack a number of genes that are highly conserved in other poxviruses, including factors for nucleotide metabolism, serine protease inhibitors and kelch-like proteins. It is apparent that parapoxviruses have evolved a unique repertoire of genes that have allowed adaptation to the highly specialised environment of the epidermis.

Parapoxvirus of red deer in New Zealand encodes a variant of viral vascular endothelial growth factor

Parapoxvirus of red deer in New Zealand (PVNZ), a species of the Parapoxvirus genus, causes scabby lesions on the skin and the velvet of red deer. The three other species of the genus have each been shown to encode homologs of vascular endothelial growth factor (VEGF). We report here that PVNZ strain RD86 also encodes a VEGF and that the predicted PVNZ protein shows only 37-54% amino acid identity to VEGFs encoded by the other species of the genus. Despite this extensive sequence divergence, assays of purified PVNZ VEGF (PVNZ(RD86)VEGF) demonstrated that it shares the unique VEGF receptor (VEGFR) binding profile of the other parapoxvirus VEGFs, in that it binds VEGFR-2 and induces VEGFR-2-mediated proliferation of Ba/F3-derived cells, but does not bind VEGFR-1 or VEGFR-3. In contrast to some other viral VEGFs, it does not bind neuropilin-1. Our results indicate that PVNZ(RD86)VEGF is a biologically active member of the VEGF family and is likely to contribute to the proliferative and highly vascularized nature of PVNZ lesions. Our data also reveal that all members of the genus encode a VEGF and that an extraordinary degree of inter-species sequence variation is a general feature of the parapoxvirus VEGFs.

Genomes of the parapoxviruses ORF virus and bovine papular stomatitis virus

Bovine papular stomatitis virus (BPSV) and orf virus (ORFV), members of the genus Parapoxvirus of the Poxviridae, are etiologic agents of worldwide diseases affecting cattle and small ruminants, respectively. Here we report the genomic sequences and comparative analysis of BPSV strain BV-AR02 and ORFV strains OV-SA00, isolated from a goat, and OV-IA82, isolated from a sheep. Parapoxvirus (PPV) BV-AR02, OV-SA00, and OV-IA82 genomes range in size from 134 to 139 kbp, with an average nucleotide composition of 64% G+C. BPSV and ORFV genomes contain 131 and 130 putative genes, respectively, and share colinearity over 127 genes, 88 of which are conserved in all characterized chordopoxviruses. BPSV and ORFV contain 15 and 16 open reading frames (ORFs), respectively, which lack similarity to other poxvirus or cellular proteins. All genes with putative roles in pathogenesis, including a vascular endothelial growth factor (VEGF)-like gene, are present in both viruses; however, BPSV contains two extra ankyrin repeat genes absent in ORFV. Interspecies sequence variability is observed in all functional classes of genes but is highest in putative virulence/host range genes, including genes unique to PPV. At the amino acid level, OV-SA00 is 94% identical to OV-IA82 and 71% identical to BV-AR02. Notably, ORFV 006/132, 103, 109, 110, and 116 genes (VEGF, homologues of vaccinia virus A26L, A33R, and A34R, and a novel PPV ORF) show an unusual degree of intraspecies variability. These genomic differences are consistent with the classification of BPSV and ORFV as two PPV species. Compared to other mammalian chordopoxviruses, PPV shares unique genomic features with molluscum contagiosum virus, including a G+C-rich nucleotide composition, three orthologous genes, and a paucity of nucleotide metabolism genes. Together, these data provide a comparative view of PPV genomics.

Transcript mapping of the ‘early’ genes of Orf virus

The full complement of genes encoded by Orf virus (ORFV) is not yet known. A cDNA library was constructed using mRNA isolated 5 h post-infection from cells infected with ORFV in vitro and grown in the presence of cytosine arabinoside. Using 12 non-overlapping probes representing the entire genome of the Orf-11 strain of the virus, cDNA clones representing individual genes expressed early in infection were isolated. Thirty-eight early genes were identified, either via isolation of their cDNA from the library or via Northern blotting. Twenty-nine of the isolated cDNAs represented orthologues of other poxvirus genes or had been identified previously as genes of ORFV, whilst seven appeared unrelated to any known poxvirus gene or indeed to any known gene in the DNA databases. The sequences described in this paper constitute approximately 30 kb of the ORFV genome and contain the complete or partial sequence of 47 genes.

Relatedness and heterogeneity at the near-terminal end of the genome of a parapoxvirus bovis 1 strain (B177) compared with parapoxvirus ovis (Orf virus)

The present study provides for the first time an extended investigation of individual genes located at the near-terminal right end of the genome of parapoxvirus bovis 1, Bovine papular stomatitis virus (BPSV) strain B177 and Orf virus (ORFV). Comparison of the respective DNA sequences of ORFV strain D1701 (9.9 kbp) and BPSV B177 (7.7 kbp) revealed a very similar organization of closely related genes transcribed in a rightward orientation. The most salient findings of this study were: (i) the absence of the ORFV-specific vascular endothelial growth factor (VEGF-E) gene in the BPSV isolate; (ii) the presence of an interleukin-10 (IL-10) orthologue; and (iii) the detection of three new genes encoding ankyrin-repeat-containing polypeptides. These results not only contribute to potential improvements of future molecular differentiation between the parapoxvirus species, but also shed new light on different pathobiologies among parapoxviruses.

Pseudocowpox virus encodes a homolog of vascular endothelial growth factor

We have identified a gene encoding a homolog of vascular endothelial growth factor (VEGF) in the Pseudocowpox virus (PCPV) genome. The predicted protein shows 27% amino acid identity to human VEGF-A. It also shows 41 and 61% amino acid identity to VEGFs encoded by orf virus (ORFV) strains NZ2 and NZ7, respectively. Assays of the expressed VEGF-like protein of PCPV (PCPV(VR634)VEGF) demonstrated that PCPV(VR634)VEGF is mitogenic for endothelial cells and is capable of inducing vascular permeability. PCPV(VR634)VEGF bound VEGF receptor-2 (VEGFR-2) but did not bind VEGFR-1 or VEGFR-3. These results indicate that PCPV(VR634)VEGF is a biologically active member of the VEGF family which shares with the ORFV-encoded VEGFs a receptor binding profile that differs from those of all cellular members of the VEGF family. It seems likely that the biological activities of PCPV(VR634)VEGF contribute to the proliferative and highly vascularized nature of PCPV lesions.

Viral vascular endothelial growth factor plays a critical role in orf virus infection

Infection by the parapoxvirus orf virus causes proliferative skin lesions in which extensive capillary proliferation and dilation are prominent histological features. This infective phenotype may be linked to a unique virus-encoded factor, a distinctive new member of the vascular endothelial growth factor (VEGF) family of molecules. We constructed a recombinant orf virus in which the VEGF-like gene was disrupted and show that inactivation of this gene resulted in the loss of three VEGF activities expressed by the parent virus: mitogenesis of vascular endothelial cells, induction of vascular permeability, and activation of VEGF receptor 2. We used the recombinant orf virus to assess the contribution of the viral VEGF to the vascular response seen during orf virus infection of skin. Our results demonstrate that the viral VEGF, while recognizing a unique profile of the known VEGF receptors (receptor 2 and neuropilin 1), is able to stimulate a striking proliferation of blood vessels in the dermis underlying the site of infection. Furthermore, the data demonstrate that the viral VEGF participates in promoting a distinctive pattern of epidermal proliferation. Loss of a functional viral VEGF resulted in lesions with markedly reduced clinical indications of infection. However, viral replication in the early stages of infection was not impaired, and only at later times did it appear that replication of the recombinant virus might be reduced.

Analysis of genomic rearrangement and subsequent gene deletion of the attenuated Orf virus strain D1701

The orf virus (OV) strain D1701 belongs to the genetically heterogenous parapoxvirus (PPV) genus of the family Poxviridae. The attenuated OV D1701 has been licensed as a live vaccine against contagious ecthyma in sheep. Detailed knowledge on the genetic structure and organization of this PPV vaccine strain is an important prerequisite to reveal possible genetic mechanisms of PPV attenuation. The present study demonstrates a genomic map of the approximately 158 kbp DNA of OV D1701 established by hybridization studies of cloned restriction fragments covering the complete viral genome. The results show an enlargement of the inverted terminal repeats (ITR) to up to 18 kbp due to recombination between nonhomologous sequences during cell culture adaptation. DNA sequencing of the region adjacent to the ITR junction revealed the absence of one open reading frame designated E2L. In contrast to a transposition-deletion variant of the New Zealand OV strain NZ2 (Fleming et al., 1995) the two genes E3L (a homologue of dUTPase) and G1L neighbouring E2L are retained in OV D1701. DNA and RNA analyses proved the presence of E2L gene in wild-type OV isolated directly from scab material. The data presented indicate that the E2L gene is nonessential for virus replication in vitro and in vivo, and may represent one important viral gene in determining virulence and pathogenesis of OV.

A homolog of interleukin-10 is encoded by the poxvirus orf virus

A gene encoding a polypeptide with homology to interleukin-10 (IL-10) has been discovered in the genome of orf virus (OV) strain NZ2, a parapoxvirus that infects sheep, goats, and humans. The predicted polypeptide sequence shows high levels of amino acid identity to IL-10 of sheep (80%), cattle (75%), humans (67%), and mice (64%), as well as IL-10-like proteins of Epstein-Barr virus (63%) and equine herpesvirus (67%). The C-terminal region, comprising two-thirds of the OV protein, is identical to ovine IL-10, which suggests that this gene has been captured from its host sheep during the evolution of OV. The IL-10-like gene is transcribed early. Conditioned medium from COS cells transfected with a eukaryotic expression vector containing the OV IL-10-like gene showed the same biological activity as ovine IL-10 in a murine thymocyte proliferation assay. OV IL-10 is likely to be important in immune evasion by OV, since IL-10 is a multifunctional cytokine that has inhibitory effects on nonspecific immunity and Th1 effector function.

A novel strategy for determining protective antigens of the parapoxvirus, orf virus

We investigated the feasibility of using vaccinia virus (VAC) recombinants containing large multigene fragments of orf virus DNA to identify protective antigens of orf virus (OV). Sixteen OV strain NZ2 DNA fragments with an average size of 11.4 kb were recombined into VAC strain Lister. Each fragment was mapped relative to OV restriction endonuclease maps but was otherwise uncharacterized. Together the recombinants represent 95% of the OV genome in an overlapping manner. Immunofluorescence showed all 16 constructs expressed products recognized by OV antiserum and radioimmune precipitation with the same antiserum allowed the localization of the major antigens of OV to specific recombinants. These data indicated the approximate genomic locations of the genes encoding the OV major antigens and showed that their expression was authentic rather than resulting from read through from VAC sequences adjacent to the site of recombination. Vaccination of OV-naive sheep with the recombinant library provided protection against a subsequent challenge with virulent OV. These data confirm the feasibility of the proposed strategy.

Strategy for identifying the gene encoding the DNA polymerase of molluscum contagiosum virus type 1

Molluscum contagiosum virus (MCV) is a member of the family Poxviridae and pathogenic to humans. MCV causes benign epidermal tumors mainly in children and young adults and is a common pathogen in immunecompromised individuals. The viral DNA polymerase is the essential enzyme involved in the replication of the genome of DNA viruses. The identification and characterization of the gene encoding the DNA polymerase of molluscum contagiosum virus type 1 (MCV-1) was carried out by PCR technology and nucleotide sequence analysis. Computer-aided analysis of known amino acid sequences of DNA polymerases from two members of the poxvirus family revealed a high amino acid sequence homology of about 49.7% as detected between the DNA polymerases of vaccinia virus (genus Orthopoxvirus) and fowlpoxvirus (genus Avipoxvirus). Specific oligonucleotide primers were designed and synthesized according to the distinct conserved regions of amino acid sequences of the DNA polymerases in which the codon usage of the MCV-1 genome was considered. Using this technology a 228 bp DNA fragment was amplified and used as hybridization probe for identifying the corresponding gene of the MCV-1 genome. It was found that the PCR product was able to hybridize to the BamHI MCV-1 DNA fragment G (9.2 kbp, 0.284 to 0.332 map units). The nucleotide sequence of this particular region of the MCV-1 genome (7267 bp) between map coordinates 0.284 and 0.315 was determined. The analysis of the DNA sequences revealed the presence of 22 open reading frames (ORFs-1 to -22). ORF-13 (3012 bp; nucleotide positions 6624 to 3612) codes for a putative protein of a predicted size of 115 kDa (1004 aa) which shows 40.1% identity and 35% similarity to the amino acid sequences of the DNA polymerases of vaccinia, variola, and fowlpoxvirus. In addition significant homologies (30% to 55%) were found between the amino acid sequences of the ORFs 3, -5, -9, and -14 and the amino acid sequences of the E6R, E8R, E10R, and a 7.3 kDa protein of vaccinia and variola virus, respectively. Comparative analysis of the genomic positions of the loci of the detected viral genes including the DNA polymerases of MCV-1, vaccinia, and variola virus revealed a similar gene organization and arrangement.

Sequence and transcriptional analysis of a near-terminal region of the orf virus genome

A 3605 bp region located approximately 6.6 kb from the left end of the orf virus genome (strain NZ2) was sequenced. The sequence revealed two open reading frames, which we have designated G2L and B1L. The predicted amino acid sequences of G2L and B1L were found to be homologous to the vaccinia virus (VAC) F11L and F12L gene products, respectively, and were found to be arranged on the genome in the same orientation and relative position as their VAC counterparts. Transcriptional analysis of both G2L and B1L showed they were transcribed toward the genome terminus during the early phase of infection. S1 nuclease and primer-extension analyses showed that the transcriptional start sites of both genes were located a short distance downstream from A+T-rich sequences, similar to vac virus early promoters.

Sequence and transcriptional analysis of an orf virus gene encoding ankyrin-like repeat sequences

A 1608 bp region located approximately 5.0 kb from the left end of the orf virus (OV) genome (strain NZ2) was sequenced. The sequence revealed a single open reading frame designated G1L. The predicted amino acid sequence of G1L contained eight ankyrinlike repeat sequences. Transcriptional analysis of G1L showed it was transcribed towards the genome terminus during the early phase of infection. S1 nuclease and primer extension analyses showed that the transcriptional start site of the gene was located a short distance downstream from an A + T-rich sequence similar to a vaccinia virus early promoter.

Homologs of vascular endothelial growth factor are encoded by the poxvirus orf virus

A gene encoding a polypeptide with homology to mammalian vascular endothelial growth factors (VEGFs) has been discovered in the genome of orf virus (OV), a parapoxvirus that affects sheep and goats and, occasionally, humans. The gene is transcribed abundantly early in infection and is found immediately outside the inverted terminal repeat at the right end of the genome. In the NZ2 strain of OV (OV NZ2), the gene encodes a polypeptide with a molecular size of 14.7 kDa, while in another strain, OV NZ7, there is a variant gene that encodes a polypeptide of 16 kDa. The OV NZ2 and OV NZ7 polypeptides show 22 to 27% and 16 to 23% identity, respectively, to the mammalian VEGFs. The viral polypeptides are only 41.1% identical to each other, and there is little homology between the two genes at the nucleotide level. Another unusual feature of these genes is their G+C content, particularly that of OV NZ7. In a genome that is otherwise 63% G+C, the OV NZ2 gene is 57.2% G+C and the OV NZ7 gene is 39.7% G+C. The OV NZ2 gene, but not the OV NZ7 gene, is homologous to the mammalian VEGF genes at the DNA level, suggesting that the gene has been acquired from a mammalian host and is undergoing genetic drift. The lesions induced in sheep and humans after infection with OV show extensive dermal vascular endothelial proliferation and dilatation, and it is likely that this is a direct effect of the expression of the VEGF-like gene.

In vivo recognition of orf virus early transcriptional promoters in a vaccinia virus recombinant

The 4.4-kb BamHI-E fragment of the orf virus (OV) genome contains three discrete open reading frames designated ORF-pp, ORF-1, and ORF-3, all of which are flanked by vaccinia virus-like early transcriptional control sequences. To determine whether the vaccinia transcriptional machinery would recognize these promoters and faithfully transcribe OV genes in vivo the BamHI-E fragment was inserted into the thymidine kinase (TK) locus of vaccinia virus and the recombinant used in transcription studies. Northern blotting analysis of early RNA isolated from 143B-TK- cells infected with the recombinant virus showed that OV genes were transcribed and that the three transcripts of 0.70-(ORF-pp), 0.48- (ORF1), and 0.75-kb (ORF-3) were the same size as their counterparts in OV-infected cells. Analysis of the 5' end of transcripts by S1 nuclease and primer extension showed that the transcriptional start points (tsp) of ORF-pp, ORF-1, and ORF-3 in the recombinant were identical or within four nucleotides of the tsps of the same ORFs in OV. However, there were quantitative differences. ORF-1 was transcribed more efficiently in recombinant virus-infected cells than in those infected with OV and analysis of the putative promoter, 5'-AAAATTGTAAATGTA, showed that it was similar to the 7.5-kDa early promoter of vaccinia virus. This demonstrates that the transcriptional control sequences of OV genes are recognized by vaccinia virus transcriptional factors but that quantitative differences exist suggesting that the generically different transcriptional factors have different promoter sequence requirements for maximal transcription.

In vitro recognition of an orf virus early promoter in a vaccinia virus extract

DNA fragments containing varying lengths of the 5' end of an orf virus early gene (ORF3) and its associated promoter were introduced into sodium deoxycholate-solubilized vaccinia virus extracts capable of initiating transcription in vitro from vaccinia virus early promoters. After separation of the radiolabelled products of the reactions on a 5% polyacrylamide/7 M urea gel, discrete transcripts were detected the sizes of which were consistent with initiation of transcription from the orf virus early promoter. This is the first demonstration in a functional assay of the conservation of early transcriptional promoters between an orthopoxvirus and a parapoxvirus.