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Abstract
Most DNA viruses selfishly exploit the cellular transcription machinery of infected cells. Poxviruses are unique among DNA viruses in that they encode the majority of the enzymes required for RNA synthesis. Poxviruses are large DNA viruses that replicate entirely within the cytoplasmic compartment of the cell, and they encode their own multisubunit RNA polymerase and gene-specific transcription and termination factors. The virus-encoded RNA polymerase has sequence and structural homology to eukaryotic RNA polymerases. Virus-encoded and cellular proteins regulate promoter specificity by recruiting the viral RNA polymerase to one of three different classes of genes. Functional interplay between viral and cellular transcription factors in viral gene regulation represents a new frontier in poxvirus biology. Targeting these transcription systems may serve as an undeveloped and potent antiviral strategy to combat poxvirus infections.
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Affiliation(s)
- Steven S Broyles
- Department of Biochemistry, Purdue University, West Lafayette, IN 47907, USA
| | - Bruce A Knutson
- Fred Hutchinson Cancer Research Center, 1100 Fairview Ave. N., Seattle, WA 98109–1024, USA
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2
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Dellis S, Strickland KC, McCrary WJ, Patel A, Stocum E, Wright CF. Protein interactions among the vaccinia virus late transcription factors. Virology 2005; 329:328-36. [PMID: 15518812 DOI: 10.1016/j.virol.2004.08.017] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2004] [Revised: 07/23/2004] [Accepted: 08/17/2004] [Indexed: 11/18/2022]
Abstract
The viral proteins A1L, A2L, G8R, and H5R positively modulate vaccinia virus late gene expression. Host-encoded proteins hnRNP A2 and RBM3 may also interact with these viral factors to influence late gene expression. In these studies, a yeast two-hybrid screen and in vitro pulldown and crosslinking experiments were used to investigate protein--protein interactions among these factors. These studies confirmed a previous observation that G8R interacts with itself and A1L. However, self-interactions of A1L and H5R, and interactions between A2L and G8R, A2L and H5R, and H5R and G8R were also observed. In addition, the proteins hnRNP A2 and RBM3 both showed some interaction with A2L. Illustration of these interactions is a step toward understanding the architecture of the late gene transcription complex as it occurs in poxviruses.
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Affiliation(s)
- Stephanie Dellis
- Biology Department, College of Charleston, Charleston, SC 29401, USA
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3
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Katsafanas GC, Moss B. 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. J Biol Chem 2004; 279:52210-7. [PMID: 15471883 DOI: 10.1074/jbc.m411033200] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
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.
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Affiliation(s)
- George C Katsafanas
- Laboratory of Viral Diseases, NIAID, National Institutes of Health, Bethesda, Maryland 20892, USA
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4
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Shchelkunov SN, Totmenin AV, Safronov PF, Mikheev MV, Gutorov VV, Ryazankina OI, Petrov NA, Babkin IV, Uvarova EA, Sandakhchiev LS, Sisler JR, Esposito JJ, Damon IK, Jahrling PB, Moss B. Analysis of the monkeypox virus genome. Virology 2002; 297:172-94. [PMID: 12083817 PMCID: PMC9534300 DOI: 10.1006/viro.2002.1446] [Citation(s) in RCA: 195] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
Abstract
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.
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Affiliation(s)
- S N Shchelkunov
- State Research Center of Virology and Biotechnology Vector, Koltsovo, Novosibirsk Region, Russia
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5
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Wright CF, Oswald BW, Dellis S. Vaccinia virus late transcription is activated in vitro by cellular heterogeneous nuclear ribonucleoproteins. J Biol Chem 2001; 276:40680-6. [PMID: 11546759 DOI: 10.1074/jbc.m102399200] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Vaccinia virus gene expression is temporally regulated, and three gene classes have been identified: early, intermediate, and late. Several virus-encoded proteins and an activity designated VLTF-X are required for maximum transcription in vitro of a template containing a late promoter. VLTF-X is present in both cytoplasmic and nuclear extracts prepared from uninfected mammalian cells and co-purifies with a late promoter DNA-binding activity. Here, extensive purification of VLTF-X has revealed that heterogeneous nuclear ribonucleoproteins A2/B1 and RBM3 co-purified with in vitro late transcription stimulation. Overexpression and purification of these proteins from Escherichia coli demonstrated that they both complemented for VLTF-X activity in in vitro transcription reactions. These studies identify two host cell factors potentially contributing to poxvirus replication in vivo.
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Affiliation(s)
- C F Wright
- Department of Pathology and Laboratory Medicine, Medical University of South Carolina, Charleston, South Carolina 29425, USA.
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6
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Tortorici MA, Ghiringhelli PD, Lozano ME, Albariño CG, Romanowski V. Zinc-binding properties of Junín virus nucleocapsid protein. J Gen Virol 2001; 82:121-128. [PMID: 11125165 DOI: 10.1099/0022-1317-82-1-121] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The arenavirus nucleocapsid protein (N) is a highly basic 63 kDa protein with a dual function during the virus life-cycle. First, it is involved in essential steps of genome replication, promoting the synthesis of the full-length antigenomic copy of S RNA, and second it associates with the genomic RNA to form the nucleocapsid. We have expressed the N protein of Junín virus in E. coli and shown that it binds zinc in vitro. This property is in agreement with the presence in the carboxy-terminal region of the N protein of the CX(2)HX(23)CX(4)C sequence, which resembles a classical zinc-finger motif. The specificity for zinc binding was demonstrated by competition with other divalent metal ions. The ability of the predicted motif to bind zinc was established by analysis of a series of N mutants, including truncated variants and amino acid substitutions. In addition, alternative zinc-binding sites were found.
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Affiliation(s)
- M Alejandra Tortorici
- Instituto de Bioquímica y Biología Molecular, Depto de Ciencias Biológicas, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Calles 47 y 115, 1900 La Plata, Buenos Aires, Argentina1
| | - P Daniel Ghiringhelli
- Laboratorio de Ingeniería Genética y Biología Celular y Molecular, Departamento de Ciencia y Tecnología, Centro de Estudios e Investigaciones, Universidad Nacional de Quilmes, Roque Saenz Peña 180, 1876 Bernal, Buenos Aires, Argentina2
| | - Mario E Lozano
- Laboratorio de Ingeniería Genética y Biología Celular y Molecular, Departamento de Ciencia y Tecnología, Centro de Estudios e Investigaciones, Universidad Nacional de Quilmes, Roque Saenz Peña 180, 1876 Bernal, Buenos Aires, Argentina2
| | - César G Albariño
- Instituto de Bioquímica y Biología Molecular, Depto de Ciencias Biológicas, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Calles 47 y 115, 1900 La Plata, Buenos Aires, Argentina1
| | - Víctor Romanowski
- Laboratorio de Ingeniería Genética y Biología Celular y Molecular, Departamento de Ciencia y Tecnología, Centro de Estudios e Investigaciones, Universidad Nacional de Quilmes, Roque Saenz Peña 180, 1876 Bernal, Buenos Aires, Argentina2
- Instituto de Bioquímica y Biología Molecular, Depto de Ciencias Biológicas, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Calles 47 y 115, 1900 La Plata, Buenos Aires, Argentina1
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Liu X, Yang J, Ghazi AM, Frey TK. Characterization of the zinc binding activity of the rubella virus nonstructural protease. J Virol 2000; 74:5949-56. [PMID: 10846076 PMCID: PMC112091 DOI: 10.1128/jvi.74.13.5949-5956.2000] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/22/1999] [Accepted: 04/13/2000] [Indexed: 11/20/2022] Open
Abstract
The rubella virus (RUB) nonstructural (NS) protein (NSP) ORF encodes a protease that cleaves the NSP precursor (240 kDa) at a single site to produce two products. A cleavage site mutation was introduced into a RUB infectious cDNA clone and found to be lethal, demonstrating that cleavage of the NSP precursor is necessary for RUB replication. Based on computer alignments, the RUB NS protease was predicted to be a papain-like cysteine protease (PCP) with the residues Cys1152 and His1273 as the catalytic dyad; however, the RUB NS protease was recently found to require divalent cations such as Zn, Co, and Cd for activity (X. Liu, S. L. Ropp, R. J. Jackson, and T. K. Frey, J. Virol. 72:4463-4466, 1998). To analyze the function of metal cation binding in protease activity, Zn binding studies were performed using the minimal NS protease domain within the NSP ORF. When expressed as a maltose binding protein (MBP) fusion protein by bacteria, the NS protease exhibited activity both in the bacteria and in vitro following purification when denatured and refolded in the presence of Zn. Atomic absorption analysis detected 1.6 mol of Zn bound per mol of protein refolded in this manner. Expression of individual domains within the protease as MBP fusions and analysis by a Zn(65) binding assay revealed two Zn binding domains: one located at a predicted metal binding motif beginning at Cys1175 and the other one close to the cleavage site. Mutagenesis studies showed that Cys1175 and Cys1178 in the first domain and Cys1227 and His1273, the His in the predicted catalytic site, in the second domain are essential for zinc binding. All of these residues are also necessary for the protease activity, as were several other Cys residues not involved in Zn binding. Far-UV circular dichroism (CD) analysis of the MBP-NS protease fusion protein showed that the protease domain contained a large amount of alpha-helical structure, which is consistent with the results of secondary-structural prediction. Both far-UV-CD and fluorescence studies suggested that Zn did not exert a major effect on the overall structure of the fusion protein. Finally, protease inhibitor assays found that the protease activity can be blocked by both metal ion chelators and the metalloprotease inhibitor captopril. In conjunction with the finding that the previously predicted catalytic site, His1273, is essential for zinc binding, this suggests that the RUB NS protease is actually a novel virus metalloprotease rather than a PCP.
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Affiliation(s)
- X Liu
- Department of Biology, Georgia State University, Atlanta 30303, USA
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Wright CF, Hubbs AE, Gunasinghe SK, Oswald BW. 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. J Virol 1998; 72:1446-51. [PMID: 9445047 PMCID: PMC124625 DOI: 10.1128/jvi.72.2.1446-1451.1998] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
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.
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Affiliation(s)
- C F Wright
- Department of Pathology and Laboratory Medicine, Medical University of South Carolina, Charleston 29425, USA.
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9
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Passarelli AL, Kovacs GR, Moss B. Transcription of a vaccinia virus late promoter template: requirement for the product of the A2L intermediate-stage gene. J Virol 1996; 70:4444-50. [PMID: 8676468 PMCID: PMC190378 DOI: 10.1128/jvi.70.7.4444-4450.1996] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
Evidence is presented that a 26-kDa protein encoded by the vaccinia virus A2L open reading frame, originally shown to be one of three intermediate-stage genes that together can transactivate late-stage gene expression in transfection assays (J. G. Keck, C. J. Baldick, and B. Moss, Cell 61:801-809, 1990), is required for in vitro transcription of a template with a late promoter. The critical step in this analysis was the preparation of an extract containing all the required factors except for the A2L protein. This extract was prepared from cells infected with a recombinant vaccinia virus expressing the bacteriophage T7 RNA polymerase in the presence of the DNA synthesis inhibitor cytosine arabinoside and transfected with plasmids containing the two other known transactivator genes, A1L and G8R, under T7 promoter control. Reaction mixtures made with extracts of these cells had background levels of late transcription activity, unless they were supplemented with extracts of cells transfected with the A2L gene. Active transcription mixtures were also made by mixing extracts from three sets of cells, each transfected with a gene (A1L, A2L, or G8R) encoding a separate factor, indicating the absence of any requirement for their coexpression. To minimize the possibility that the A2L protein functions indirectly by activating another viral or cellular protein, this gene was expressed in insect cells by using a baculovirus vector. The partially purified recombinant protein complemented the activity of A2L-deficient cell extracts. Recombinant A1L, A2L, and G8R proteins, all produced in insect cells, together complemented extracts from mammalian cells containing only viral early proteins, concordant with previous in vivo transfection data.
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Affiliation(s)
- A L Passarelli
- Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland 20892, USA
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Chen HH, Kong WP, Roos RP. The leader peptide of Theiler's murine encephalomyelitis virus is a zinc-binding protein. J Virol 1995; 69:8076-8. [PMID: 7494325 PMCID: PMC189757 DOI: 10.1128/jvi.69.12.8076-8078.1995] [Citation(s) in RCA: 51] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
The leader (L) peptide is located in the amino-terminal part of the polyprotein of members of the Cardiovirus (which includes Theiler's murine encephalomyelitis virus) and Aphthovirus genera of picornaviruses. Although the function of L is unknown, strain DA of Theiler's murine encephalomyelitis virus with a mutation of L produces a cell-specific restricted infection. We now report that the DA L peptide is a metalloprotein and that zinc binds to a Cys-His motif that is conserved among cardioviruses.
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Affiliation(s)
- H H Chen
- Department of Neurology, University of Chicago Medical Center, Illinois 60637, USA
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11
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Abstract
Comparison of the genomic organization of variola and vaccinia viruses has been carried out. Molecular factors of virulence of these viruses is the focus of this review. Possible roles of the genes of soluble cytokine receptors, complement control proteins, factors of virus replication, and dissemination in vivo for variola virus pathogenesis are discussed. The existence of "buffer" genes in the vaccinia virus genome is proposed.
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Affiliation(s)
- S N Shchelkunov
- Institute of Molecular Biology, State Research Center of Virology and Biotechnology, Vector, Koltsovo, Russia
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Whitehead SS, Hruby DE. A transcriptionally controlled trans-processing assay: putative identification of a vaccinia virus-encoded proteinase which cleaves precursor protein P25K. J Virol 1994; 68:7603-8. [PMID: 7933150 PMCID: PMC237210 DOI: 10.1128/jvi.68.11.7603-7608.1994] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
Vaccinia virus maturation into infectious particles appears to be dependent on the proteolytic processing of at least five viral proteins, each containing a conserved AG*X cleavage motif and each requiring proper association with the previrion particle. To identify the responsible proteinase, a transcriptionally controlled trans-processing assay was developed to monitor cleavage at the permissive AG*S site of the P25K core protein precursor. This assay led to the putative identification of a VV proteinase encoded by open reading frame G1L. The predicted protein contains an HXXEH sequence which is a direct inversion of the active site consensus sequence present in thermolysin and other metalloendopeptidases. Site-directed mutation of this consensus sequence suggests that the G1L protein may be a novel, virus-encoded metalloendoproteinase, although confirmation of this activity must await the development of a suitable cell-free processing assay.
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Affiliation(s)
- S S Whitehead
- Department of Microbiology, Oregon State University, Corvallis 97331-3804
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Upton C, Schiff L, Rice SA, Dowdeswell T, Yang X, McFadden G. A poxvirus protein with a RING finger motif binds zinc and localizes in virus factories. J Virol 1994; 68:4186-95. [PMID: 8207794 PMCID: PMC236341 DOI: 10.1128/jvi.68.7.4186-4195.1994] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
Abstract
Shope fibroma virus (SFV) is a Leporipoxvirus closely related to the highly virulent myxoma virus. The DNA sequence of the BamHI N fragment of the SFV DNA genome was determined, and the single complete open reading frame (N1R) was characterized. The protein encoded by the N1R gene was found to contain a C3HC4 RING finger motif at the C terminus. This C3HC4 motif is the hallmark of a growing family of proteins, many of which are involved in regulation of gene expression, DNA repair, or DNA recombination. Complete homologs of the SFV N1R gene were also detected in variola virus, myxoma virus, and vaccinia virus strain IHD-W. In contrast, the gene is completely absent from vaccinia virus strain Copenhagen, and in vaccinia virus strain WR, the open reading frame is truncated prior to the zinc binding domain because of an 11-bp deletion, thus producing a frameshift and premature stop codon. Recombinant N1R protein from SFV was expressed in Escherichia coli and shown to bind zinc in a specific manner. Using fluorescence microscopy to visualize a peptide epitope tag (derived from ICP27 of herpes simplex virus) fused to the N terminus of the poxvirus proteins, we observed that the N1R protein of SFV and its homologs in myxoma virus and vaccinia virus IHD-W were localized primarily to the virus factories in the cytoplasm of infected cells and, to a lesser degree, the host cell nucleus. The truncated protein of vaccinia virus strain WR failed to localize in this manner but instead was observed throughout the cytoplasm.
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Affiliation(s)
- C Upton
- Department of Biochemistry, University of Alberta, Edmonton, Canada
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Keck JG, Kovacs GR, Moss B. Overexpression, purification, and late transcription factor activity of the 17-kilodalton protein encoded by the vaccinia virus A1L gene. J Virol 1993; 67:5740-8. [PMID: 8371339 PMCID: PMC237991 DOI: 10.1128/jvi.67.10.5740-5748.1993] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023] Open
Abstract
The A1L, A2L, and G8R open reading frames (ORFs) were previously shown by transfection assays to encode transactivators of late gene expression. We now present evidence that the 17-kDa protein product of the A1L gene can function in vitro as a transcription factor. Simultaneous overexpression of the transactivators was achieved by coinfecting HeLa cells with one recombinant vaccinia virus that encodes the bacteriophage T7 RNA polymerase and three recombinant vaccinia viruses that contain copies of A1L, A2L, and G8R ORFs regulated by T7 promoters. Extracts from the recombinant virus-infected cells exhibited greatly enhanced late in vitro transcription activity and served as a source of factors. The 17-kDa product of the A1L ORF represented approximately 8% of the ammonium sulfate-precipitated cell protein and copurified with a late transcription factor activity. The transcription factor activity could be specifically immunodepleted with immobilized antibody to the bacterially expressed A1L-encoded protein, providing additional evidence for its identity and role. A sequence encoding six consecutive histidines was added to the A1L ORF, which was then incorporated into the genome of a baculovirus expression vector. The 17-kDa protein, synthesized in insect cells and purified by binding to an Ni(2+)-chelating affinity column, could replace the vaccinia virus-overexpressed 17-kDa protein in transcription assays. In addition to the 17-kDa product of the A1L gene, which was named vaccinia virus late transcription factor 2, the proteins that stimulate specific transcription of late promoter-regulated templates included the viral multisubunit RNA polymerase, vaccinia virus late transcription factor 1 (the product of the G8R ORF), and at least one other partially purified protein.
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Affiliation(s)
- J G Keck
- Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland 20892
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