Structure and expression in E. coli of the gene coding for protein p10 of African swine fever virus

Deleting the C84L Gene from the Virulent African Swine Fever Virus SY18 Does Not Affect Its Replication in Porcine Primary Macrophages but Reduces Its Virulence in Swine

(1) Background: African swine fever (ASF) is a highly contagious disease that causes high pig mortality. Due to the absence of vaccines, prevention and control are relatively challenging. The pathogenic African swine fever virus (ASFV) has a complex structure and encodes over 160 proteins, many of which still need to be studied and verified for their functions. In this study, we identified one of the unknown functional genes, C84L. (2) Methods: A gene deficient strain was obtained through homologous recombination and several rounds of purification, and its replication characteristics and virulence were studied through in vitro and in vivo experiments, respectively. (3) Results: Deleting this gene from the wild-type virulent strain SY18 did not affect its replication in porcine primary macrophages but reduced its virulence in pigs. In animal experiments, we injected pigs with a 102 TCID50, 105 TCID50 deletion virus, and a 102 TCID50 wild-type strain SY18 intramuscularly. The control group pigs reached the humane endpoint on the ninth day (0/5) and were euthanized. Two pigs in the 102 TCID50(2/5) deletion virus group survived on the twenty-first day, and one in the 105 TCID50(1/5) deletion virus group survived. On the twenty-first day, the surviving pigs were euthanized, which was the end of the experiment. The necropsies of the survival group and control groups' necropsies showed that the surviving pigs' liver, spleen, lungs, kidneys, and submaxillary lymph nodes did not show significant lesions associated with the ASFV. ASFV-specific antibodies were first detected on the seventh day after immunization; (4) Conclusions: This is the first study to complete the replication and virulence functional exploration of the C84L gene of SY18. In this study, C84L gene was preliminarily found not a necessary gene for replication, gene deletion strain SY18ΔC84L has similar growth characteristics to SY18 in porcine primary alveolar macrophages. The C84L gene affects the virulence of the SY18 strain.

In Silico Characterization of African Swine Fever Virus Nucleoprotein p10 Interaction with DNA

African swine fever virus (ASFV) is the etiological agent of a highly contagious, hemorrhagic infectious swine disease, with a tremendous sanitary and economic impact on a global scale. Currently, there are no globally available vaccines or treatments. The p10 protein, a structural nucleoprotein encoded by ASFV, has been previously described as capable of binding double-stranded DNA (dsDNA), which may have implications for viral replication. However, the molecular mechanism that governs this interaction is still unknown, mostly due to the lack of a structural model for this protein. In this work, we have generated an ab initio model of the p10 protein and performed extensive structural characterization, using molecular dynamics simulations to identify the motifs and residues regulating DNA recognition. The helix-turn-helix motif identified at the C-terminal region of the protein was shown to be crucial to the dsDNA-binding efficiency. As with other DNA-binding proteins, two distinct serine and lysine-rich regions found in the two helices were identified as key players in the binding to DNA, whose importance was later validated using experimental binding assays. Altogether, these findings may contribute to a better understanding of the p10 function in ASFV replication.

Proteome Analysis of Swine Macrophages after Infection with Two Genotype II African Swine Fever Isolates of Different Pathogenicity

Since the introduction of a highly pathogenic genotype II isolate of the African swine fever virus (ASFV) into Georgia in 2007, African swine fever (ASF) has gone panzootic. Outbreaks have been reported in Europe, Asia and, more recently, Latin America. Thus, ASFV has become a major threat to the pig industry worldwide, as broadly applicable vaccines are not available. While the majority of ASFV strains show high virulence in domestic pigs and wild boar, variations within the ASFV genome have resulted in the emergence of attenuated strains with low or moderate virulence. However, the molecular basis of the differences in virulence has not yet been discovered. To reveal virulence-associated protein expression patterns, we analysed the proteomes of the natural target cells of ASFV, primary porcine macrophages, after infection with two genotype II ASFV strains displaying high (Armenia 2008) and moderate (Estonia 2014) virulence using quantitative mass spectrometry. Very similar expression patterns were observed for the viral genes, and any differences were limited to the deletions within the Estonia 2014 genome. In addition to the canonical ASFV proteins, twelve novel protein products from recently described transcripts were confirmed in both isolates. Pathway analyses showed that both isolates evoked a similar host proteome response, despite their difference in virulence. However, subtle differences in the manipulation of the proteins involved in the proinflammatory response mediated by the MAPK14/p38 signalling cascade were observed.

Role of the DNA-Binding Protein pA104R in ASFV Genome Packaging and as a Novel Target for Vaccine and Drug Development

The recent incursions of African swine fever (ASF), a severe, highly contagious, transboundary viral disease that affects members of the Suidae family, in Europe and China have had a catastrophic impact on trade and pig production, with serious implications for global food security. Despite efforts made over past decades, there is no vaccine or treatment available for preventing and controlling the ASF virus (ASFV) infection, and there is an urgent need to develop novel strategies. Genome condensation and packaging are essential processes in the life cycle of viruses. The involvement of viral DNA-binding proteins in the regulation of virulence genes, transcription, DNA replication, and repair make them significant targets. pA104R is a highly conserved HU/IHF-like DNA-packaging protein identified in the ASFV nucleoid that appears to be profoundly involved in the spatial organization and packaging of the ASFV genome. Here, we briefly review the components of the ASFV packaging machinery, the structure, function, and phylogeny of pA104R, and its potential as a target for vaccine and drug development.

A Proteomic Atlas of the African Swine Fever Virus Particle

African swine fever virus (ASFV) is a large and complex DNA virus that causes a highly lethal swine disease for which there is no vaccine available. The ASFV particle, with an icosahedral multilayered structure, contains multiple polypeptides whose identity is largely unknown. Here, we analyzed by mass spectroscopy the protein composition of highly purified extracellular ASFV particles and performed immunoelectron microscopy to localize several of the detected proteins. The proteomic analysis identified 68 viral proteins, which account for 39% of the genome coding capacity. The ASFV proteome includes essentially all the previously described virion proteins and, interestingly, 44 newly identified virus-packaged polypeptides, half of which have an unknown function. A great proportion of the virion proteins are committed to the virus architecture, including two newly identified structural proteins, p5 and p8, which are derived from the core polyproteins pp220 and pp62, respectively. In addition, the virion contains a full complement of enzymes and factors involved in viral transcription, various enzymes implicated in DNA repair and protein modification, and some proteins concerned with virus entry and host defense evasion. Finally, 21 host proteins, many of them localized at the cell surface and related to the cortical actin cytoskeleton, were reproducibly detected in the ASFV particle. Immunoelectron microscopy strongly supports the suggestion that these host membrane-associated proteins are recruited during virus budding at actin-dependent membrane protrusions. Altogether, the results of this study provide a comprehensive model of the ASFV architecture that integrates both compositional and structural information.IMPORTANCE African swine fever virus causes a highly contagious and lethal disease of swine that currently affects many countries of sub-Saharan Africa, the Caucasus, the Russian Federation, and Eastern Europe and has very recently spread to China. Despite extensive research, effective vaccines or antiviral strategies are still lacking, and many basic questions on the molecular mechanisms underlying the infective cycle remain. One such gap regards the composition and structure of the infectious virus particle. In the study described in this report, we identified the set of viral and host proteins that compose the virion and determined or inferred the localization of many of them. This information significantly increases our understanding of the biological and structural features of an infectious African swine fever virus particle and will help direct future research efforts.

African Swine Fever Virus Biology and Vaccine Approaches

African swine fever (ASF) is an acute and often fatal disease affecting domestic pigs and wild boar, with severe economic consequences for affected countries. ASF is endemic in sub-Saharan Africa and the island of Sardinia, Italy. Since 2007, the virus emerged in the republic of Georgia, and since then spread throughout the Caucasus region and Russia. Outbreaks have also been reported in Belarus, Ukraine, Lithuania, Latvia, Estonia, Romania, Moldova, Czech Republic, and Poland, threatening neighboring West European countries. The causative agent, the African swine fever virus (ASFV), is a large, enveloped, double-stranded DNA virus that enters the cell by macropinocytosis and a clathrin-dependent mechanism. African Swine Fever Virus is able to interfere with various cellular signaling pathways resulting in immunomodulation, thus making the development of an efficacious vaccine very challenging. Inactivated preparations of African Swine Fever Virus do not confer protection, and the role of antibodies in protection remains unclear. The use of live-attenuated vaccines, although rendering suitable levels of protection, presents difficulties due to safety and side effects in the vaccinated animals. Several African Swine Fever Virus proteins have been reported to induce neutralizing antibodies in immunized pigs, and vaccination strategies based on DNA vaccines and recombinant proteins have also been explored, however, without being very successful. The complexity of the virus particle and the ability of the virus to modulate host immune responses are most likely the reason for this failure. Furthermore, no permanent cell lines able to sustain productive virus infection by both virulent and naturally attenuated African Swine Fever Virus strains exist so far, thus impairing basic research and the commercial production of attenuated vaccine candidates.

African swine fever virus replication and genomics

African swine fever virus (ASFV) is a large icosahedral DNA virus which replicates predominantly in the cytoplasm of infected cells. The ASFV double-stranded DNA genome varies in length from about 170 to 193 kbp depending on the isolate and contains between 150 and 167 open reading frames. These are closely spaced and read from both DNA strands. The virus genome termini are covalently closed by imperfectly base-paired hairpin loops that are present in two forms that are complimentary and inverted with respect to each other. Adjacent to the termini are inverted arrays of different tandem repeats. Head to head concatemeric genome replication intermediates have been described. A similar mechanism of replication to Poxviruses has been proposed for ASFV. Virus genome transcription occurs independently of the host RNA polymerase II and virus particles contain all of the enzymes and factors required for early gene transcription. DNA replication begins in perinuclear factory areas about 6h post-infection although an earlier stage of nuclear DNA synthesis has been reported. The virus genome encodes enzymes required for transcription and replication of the virus genome and virion structural proteins. Enzymes that are involved in a base excision repair pathway may be an adaptation to enable virus replication in the oxidative environment of the macrophage cytoplasm. Other ASFV genes encode factors involved in evading host defence systems and modulating host cell function. Variation between the genomes of different ASFV isolates is most commonly due to gain or loss of members of multigene families, MGFs 100, 110, 300, 360, 505/530 and family p22. These are located within the left terminal 40kbp and right terminal 20kbp. ASFV is the only member of the Asfarviridae, which is one of the families within the nucleocytoplasmic large DNA virus superfamily.

African swine fever virus

African swine fever virus (ASFV) is a large, intracytoplasmically-replicating DNA arbovirus and the sole member of the family Asfarviridae. It is the etiologic agent of a highly lethal hemorrhagic disease of domestic swine and therefore extensively studied to elucidate the structures, genes, and mechanisms affecting viral replication in the host, virus-host interactions, and viral virulence. Increasingly apparent is the complexity with which ASFV replicates and interacts with the host cell during infection. ASFV encodes novel genes involved in host immune response modulation, viral virulence for domestic swine, and in the ability of ASFV to replicate and spread in its tick vector. The unique nature of ASFV has contributed to a broader understanding of DNA virus/host interactions.

Inhibition of a large double-stranded DNA virus by MxA protein

Increasing evidence points to the importance of the interferon (IFN) response in determining the host range and virulence of African swine fever virus (ASFV). Infection with attenuated strains of ASFV leads to the upregulation of genes controlled by IFN pathways, including myxovirus resistance (Mx) genes that are potent effectors of the antiviral state. Mx gene products are known to inhibit the replication of many negative-sense single-stranded RNA viruses, as well as double-stranded RNA viruses, positive-sense single-stranded RNA viruses, and the reverse-transcribing DNA virus hepatitis B virus. Here, we provide data that extend the known range of viruses inhibited by Mx to include the large double-stranded DNA viruses. Stably transfected Vero cells expressing human MxA protein did not support ASFV plaque formation, and virus replication in these cells was reduced 100-fold compared with that in control cells. In contrast, ASFV replication in cells expressing MxB protein or a mutant MxA protein was similar to that in control Vero cells. There was a drastic reduction in ASFV late protein synthesis in MxA-expressing cells, correlating with the results of previous work on the effect of IFN on viral replication. Strikingly, the inhibition of ASFV replication was linked to the recruitment of MxA protein to perinuclear viral assembly sites, where the protein surrounded the virus factories. Interactions between ASFV and MxA were similar to those seen between MxA and different RNA viruses, suggesting a common inhibitory mechanism.

African swine fever virus p10 protein exhibits nuclear import capacity and accumulates in the nucleus during viral infection

African swine fever virus (ASFV), a large enveloped DNA-containing virus, infects domestic and wild pigs, and multiplies in soft ticks, causing an economically relevant hemorrhagic disease. Evaluation of the nuclear import ability of ASFV p10 protein was the major purpose of the present work. Two approaches were used to determine if p10 protein is imported into the nucleus by an active process: a yeast-based nuclear import assay and the determination of the subcellular localization of p10 protein in mammalian cells by fluorescence microscopy. The results obtained clearly demonstrate that p10 protein is actively imported into the nucleus, both in yeast and mammalian cells. Experiments aiming at identifying the critical residues responsible for the nuclear import of ASFV p10 protein indicate that the amino acids comprised between the positions 71 and 77 are important, although not sufficient, for the protein active nuclear import. In ASFV-infected cells, the p10 protein strongly accumulates in the nucleus at late times post-infection, indicating that p10 protein may accomplish an important function inside the nucleus during the late phase of the viral replication cycle.

Repression of African swine fever virus polyprotein pp220-encoding gene leads to the assembly of icosahedral core-less particles

African swine fever virus (ASFV) polyprotein pp220, encoded by the CP2475L gene, is an N-myristoylated precursor polypeptide that, after proteolytic processing, gives rise to the major structural proteins p150, p37, p34, and p14. These proteins localize at the core shell, a matrix-like virus domain placed between the DNA-containing nucleoid and the inner envelope. In this study, we have examined the role of polyprotein pp220 in virus morphogenesis by means of an ASFV recombinant, v220i, containing an inducible copy of the CP2475L gene regulated by the Escherichia coli repressor-operator system. Under conditions that repress pp220 expression, the virus yield of v220i was about 2.6 log units lower than that of the parental virus or of the recombinant grown under permissive conditions. Electron microscopy revealed that pp220 repression leads to the assembly of icosahedral particles virtually devoid of the core structure. Analysis of recombinant v220i by immunoelectron microscopy, immunoblotting, and DNA hybridization showed that mutant particles essentially lack, besides the pp220-derived products, a number of major core proteins as well as the viral DNA. On the other hand, transient expression of the CP2475L gene in COS cells showed that polyprotein pp220 assembles into electron-dense membrane-bound coats, whereas a mutant nonmyristoylated version of pp220 does not associate with cellular membranes but forms large cytoplasmic aggregates. Together, these findings indicate that polyprotein pp220 is essential for the core assembly and suggest that its myristoyl moiety may function as a membrane-anchoring signal to bind the developing core shell to the inner viral envelope.

African swine fever virus structural protein pE120R is essential for virus transport from assembly sites to plasma membrane but not for infectivity

This report examines the role of African swine fever virus (ASFV) structural protein pE120R in virus replication. Immunoelectron microscopy revealed that protein pE120R localizes at the surface of the intracellular virions. Consistent with this, coimmunoprecipitation assays showed that protein pE120R binds to the major capsid protein p72. Moreover, it was found that, in cells infected with an ASFV recombinant that inducibly expresses protein p72, the incorporation of pE120R into the virus particle is dependent on p72 expression. Protein pE120R was also studied using an ASFV recombinant in which E120R gene expression is regulated by the Escherichia coli lac repressor-operator system. In the absence of inducer, pE120R expression was reduced about 100-fold compared to that obtained with the parental virus or the recombinant virus grown under permissive conditions. One-step virus growth curves showed that, under conditions that repress pE120R expression, the titer of intracellular progeny was similar to the total virus yield obtained under permissive conditions, whereas the extracellular virus yield was about 100-fold lower than in control infections. Immunofluorescence and electron microscopy demonstrated that, under restrictive conditions, intracellular mature virions are properly assembled but remain confined to the replication areas. Altogether, these results indicate that pE120R is necessary for virus dissemination but not for virus infectivity. The data also suggest that protein pE120R might be involved in the microtubule-mediated transport of ASFV particles from the viral factories to the plasma membrane.

Intracellular virus DNA distribution and the acquisition of the nucleoprotein core during African swine fever virus particle assembly: ultrastructural in situ hybridisation and DNase-gold labelling

African swine fever virus (ASFV) is a large complex icosahedral double-stranded DNA virus that replicates in the cytoplasm of susceptible cells. Assembly of new virus particles occurs within the perinuclear viroplasm bodies known as virus factories. Two types of virus particle are routinely observed: "fulls," which are particles with an electron-dense DNA-containing nucleoid, and "empties," which consist of the virus protein and membrane icosahedral shell but are without the incorporation of the virus genome. The objective of this study was to understand ASFV morphogenesis by determining the distribution of intracellular viral DNA in the virus factory and during virus particle assembly. The ultrastructural localisation of DNA within ASFV-infected cells was achieved using two complementary methods: with an ASFV-specific DNA probe to the major capsid protein (p73) gene (B646L) hybridised in situ or through detection of all forms of DNA (viral and cellular) with gold-labelled DNase. Conditions for in situ hybridisation at the electron microscopic level were optimised for infected cells in two Lowicryl resins (K4M and HM20) and using two nonradioactive probe labels (digoxygenin and biotin). The morphological data indicate that the viral DNA, perhaps from specialised storage sites within the factory, begins to condense into a pronucleoid and is then inserted, at a single vertex, into an "empty" particle. Further maturation of the viral particle, including closure of the narrow opening in the icosahedron, gives rise to "intermediate" particles, where the nucleoprotein core undergoes additional consolidation to produce the characteristic mature or "full" virions. The site of particle closure may represent a "weak point" at one vertex, but the mechanisms and structures involved in the packaging and release of the virus genome via such a port are yet to be determined.

The major structural protein of African swine fever virus, p73, is packaged into large structures, indicative of viral capsid or matrix precursors, on the endoplasmic reticulum

African swine fever virus (ASFV) is a large enveloped DNA virus that shares the striking icosahedral symmetry of iridoviruses. To understand the mechanism of assembly of ASFV, we have been studying the biosynthesis and subcellular distribution of p73, the major structural protein of ASFV. Sucrose density sedimentation of lysates prepared from infected cells showed that newly synthesized p73 was incorporated into a complex with a size of 150 to 250 kDa. p73 synthesized by in vitro translation migrated at 70 kDa, suggesting that cellular and/or viral proteins are required for the formation of the 150- to 250-kDa complex. During a 2-h chase, approximately 50% of the newly synthesized pool of p73 bound to the endoplasmic reticulum (ER). During this period, the membrane-bound pool of p73, but not the cytosolic pool, formed large complexes of approximately 50,000 kDa. The complexes were formed via assembly intermediates, and the entire membrane-associated pool of p73 was incorporated into the 50,000-kDa complex within 2 h. The 50,000-kDa complexes containing p73 were also detected in virions secreted from cells. Immunoprecipitation of sucrose gradients with sera taken from hyperimmune pigs suggested that p73 was the major component of the 50,000-kDa complex. It is possible, therefore, that the complex contains between 600 and 700 copies of p73. The kinetics of complex formation and envelopment of p73 were similar, and complex formation and envelopment were both reversibly inhibited by cycloheximide, suggesting a functional link between complex assembly and ASFV envelopment. A protease protection assay detected 50,000-kDa complexes on the inside and outside of the membranes forming the viral envelope. The identification of a complex containing p73 beneath the envelope of ASFV suggests that p73 may be a component of the inner core shell or matrix of ASFV. The outer pool may represent p73 within the outer capsid layer of the virus. In summary, the data suggest that the assembly of the inner core matrix and outer capsid of ASFV takes place on the ER membrane during envelopment and that these structures are not preassembled in the cytosol.

African swine fever virus is wrapped by the endoplasmic reticulum

African swine fever (ASF) virus is a large DNA virus that shares the striking icosahedral symmetry of iridoviruses and the genomic organization of poxviruses. Both groups of viruses have a complex envelope structure. In this study, the mechanism of formation of the inner envelope of ASF virus was investigated. Examination of thin cryosections by electron microscopy showed two internal membranes in mature intracellular virions and all structural intermediates. These membranes were in continuity with intracellular membrane compartments, suggesting that the virus gained two membranes from intracellular membrane cisternae. Immunogold electron microscopy showed the viral structural protein p17 and resident membrane proteins of the endoplasmic reticulum (ER) within virus assembly sites, virus assembly intermediates, and mature virions. Resident ER proteins were also detected by Western blotting of isolated virions. The data suggested the ASF virus was wrapped by the ER. Analysis of the published sequence of ASF virus (R. J. Yanez et al., Virology 208:249-278, 1995) revealed a reading frame, XP124L, that encoded a protein predicted to translocate into the lumen of the ER. Pulse-chase immunoprecipitation and glycosylation analysis of pXP124L, the product of the XP124L gene, showed that pXP124L was retained in the ER lumen after synthesis. When analyzed by immunogold electron microscopy, pXP124L localized to virus assembly intermediates and fully assembled virions. Western blot analysis detected pXP124L in virions isolated from Percoll gradients. The packaging of pXP124L from the lumen of the ER into the virion is consistent with ASF virus being wrapped by ER cisternae: a mechanism which explains the presence of two membranes in the viral envelope.

Characterization of the African swine fever virus structural protein p14.5: a DNA binding protein

The gene encoding the structural protein p14.5 of African swine fever virus (ASFV) has been mapped and sequenced. This gene, designated E120R, is located in the Sa/l H/EcoRl E restriction fragment of the ASFV genome and is predicted to encode a protein of 120 amino acids with a molecular weight of 13.4 kDa. Northern-blot analysis showed that E120R is transcribed at late times during the viral replication cycle. The E120R gene product has been expressed in Escherichia coli, purified, and used as an antigen for antibody production. The antiserum anti-pE120R recognized a protein in infected cell extracts with an apparent molecular mass of 14.5 kDa, named p14.5. This antiserum also detected protein p14.5 in purified virus particles. Protein p14.5 is synthesized late in infection and is located in viral factories. Immunoprecipitation analysis and binding-assay experiments have shown that protein p14.5 interacts with a protein that could correspond to the major structural protein p72. Purified protein p14.5 interacts with DNA in a sequence-independent manner. It binds to both single-stranded and double-stranded DNA. A possible role of protein p14.5 in the encapsidation of ASFV DNA is suggested.

Vaccinia virions lacking core protein VP8 are deficient in early transcription

When synthesis of the 25-kDa vaccinia virus core protein VP8 is repressed, mature virus particles of normal appearance are produced to approximately 80% of wild-type levels but these particles are over 100-fold less infectious than wild-type particles (D. Wilcock and G. L. Smith, Virology 202:294-304, 1994). Here we show that virions which lack VP8 can bind to and enter cells but the levels of steady-state RNA are greatly reduced in comparison with those for wild-type infections. In vitro assays using permeabilized virions demonstrated that VP8-deficient virions had drastically reduced rates of transcription (RNA synthesis was decreased by 80 to 96%) and that the extrusion of RNA transcripts from these virions was also decreased. Low concentrations of sodium deoxycholate extracted proteins more efficiently from VP8-deficient virions than from wild-type virions. The increased fragility of VP8-deficient virions and their slower RNA extrusion rates suggest that VP8 may be required for the correct formation of the core. Virions which lack VP8 were shown to contain a full complement of transcription enzymes, and soluble extracts from these virions were active in transcription assays using either single-stranded M13 DNA or exogenous plasmid template containing a vaccinia virus early promoter. Thus, the defect in transcription is due not to a lack of specific transcriptional enzymes within virions but rather to the inability of these enzymes to efficiently transcribe the DNA genome packaged within VP8-deficient virions. These results suggest that VP8 is required for the correct packaging of the viral DNA genome and/or for the efficient transcription of packaged virion DNA, which has a higher degree of structural complexity than plasmid templates. Possible roles for VP8 in these processes are discussed.

A structural DNA binding protein of African swine fever virus with similarity to bacterial histone-like proteins

Here we describe an African swine fever virus (ASFV) protein encoded by the open reading frame 5-AR that shares structural and functional similarities with the family of bacterial histone-like proteins which include histone-like DNA binding proteins, integration host factor, and Bacillus phage SPO1 transcription factor, TF1. The ASFV 5-AR gene was cloned by PCR and expressed in E. coli. Monospecific antiserum prepared to the 5-AR bacterial expression product specifically immunoprecipitated a protein of approximately 11.6 kDa from ASFV infected swine macrophages at late times post infection. Additionally, the 5-AR expression product was strongly recognized by ASFV convalescent pig serum, indicating its antigenicity during natural infection. Cloned p11.6 bound both double and single stranded DNA-cellulose columns. Consistent with a DNA binding function, immunoelectronmicroscopy localized p11.6 to the virion nucleoid, To our knowledge, p11.6 is the first bacterial histone-like DNA-binding protein found in an animal virus or eukaryotic cell system.

Gene homology between orf virus and smallpox variola virus

About 47% identity was observed between the deduced amino acid sequences of a protein encoded by a gene of the parapoxvirus orf virus (OV) strain NZ2 and a 6 kDa protein of unknown function reported to be produced by an open reading frame expressed early after infection by the orthopoxvirus Western Reserve vaccinia virus (VAC); the open reading frame is absent from VAC strain Copenhagen. Examination of sequences reported for variola virus (VAR) strains Bangladesh, India, Congo- 1970, Somalia- 1977 and Garcia- 1966 revealed each encoded a correlate 58 amino acid protein. The open reading frame was not reported in the original analyses of these sequences because a lower limit of 60 amino acids was used to identify potential encoded proteins. Inspection of partial reading frames reported for cowpox virus (CWV) and ectromelia virus (EMV) suggested that these viruses might also code for a correlate of the VAC WR protein. DNA sequencing of cloned fragments of CWV and EMV confirmed that both these orthopoxviruses encode closely related, full length variants of the VAC and VAR open reading frames. The OV homologue is coded in the OV strain NZ2 BamHI-E fragment E2L open reading frame, which we reported is transcribed early postinfection; moreover, analysis of an NZ2 variant showed E2L was absent, indicating that E2L, like the VAC cognate, is nonessential for virus replication in cell culture. The parapoxvirus and orthopoxvirus correlates have about 20% amino acid sequence resemblance to African swine fever virus DNA binding protein p10, suggesting an ancestral relation of genes.