Mapping of a gene coding for a major late structural polypeptide on the vaccinia virus genome

Generation of Vaccinia Virus Gene Deletion Mutants Using Complementing Cell Lines

This protocol describes how to couple two techniques, the generation of complementing cells lines and production of viral deletion mutants, to rapidly construct novel tools for poxvirus analysis. Specifically, the production and utilization of a complementing cell line expressing a poxvirus gene of interest are critical for the generation of poxvirus mutants in which essential genes are disrupted. Complementing cells are also valuable for the characterization of vaccinia genes in the absence of infection. Here, we detail the process of isolating vaccinia virus deletion mutants. Deletion mutant generation involves homologous recombination between replicating viral DNA and transfected DNA followed by selection and screening on a complementing cell line that provides the deleted gene in trans. Finally, deletion is confirmed by polymerase chain reaction, sequencing, and functional assays if available.

Poxviral promoters for improving the immunogenicity of MVA delivered vaccines

Modified vaccinia virus Ankara (MVA) is a replication-deficient poxvirus, attenuated in chick embryo fibroblast primary cells. It has been utilised as a viral vector to develop many vaccines against cancer and infectious diseases such as malaria, HIV/AIDS, influenza, and tuberculosis, MERS-CoV, and Ebola virus infection. There is accumulating data from many preclinical and clinical studies that highlights the excellent safety and immunogenicity of MVA. However, due to the complex nature of many pathogens and their pathogenicity, MVA vectored vaccine candidates need to be optimised to improve their immunogenicity. One of the main approaches to improve MVA immunogenicity focuses on optimising poxviral promoters that drive recombinant vaccine antigens, encoded within recombinant MVA vector genome. A number of promoters were described or optimised to improve the development of MVA based vaccines such as p7.5, pF11, and mH5 promoters. This review focuses on poxviral promoters, their optimisation, genetic stability, and clinical use.

Gamma interferon-induced interferon regulatory factor 1-dependent antiviral response inhibits vaccinia virus replication in mouse but not human fibroblasts

Vaccinia virus (VACV) replicates in mouse and human fibroblasts with comparable kinetics and efficiency, yielding similar titers of infectious progeny. Here we demonstrate that gamma interferon (IFN-gamma) but not IFN-alpha or IFN-beta pretreatment of mouse fibroblasts prior to VACV infection induces a long-lasting antiviral state blocking VACV replication. In contrast, high doses of IFN-gamma failed to establish an antiviral state in human fibroblasts. In mouse fibroblasts, IFN-gamma impeded the viral replication cycle at the level of late gene transcription and blocked the multiplication of VACV genomes. The IFN-gamma-induced antiviral state invariably prevented the growth of different VACV strains but was not effective against the replication of ectromelia virus. The IFN-gamma effect required intact IFN-gamma receptor signaling prior to VACV infection through Janus kinase 2 (Jak2) and signal transducer and activator of transcription 1 (STAT1). The permissive state of IFN-gamma-treated human cells was unrelated to the VACV-encoded IFN decoy receptors B8 and B18 and associated with a complete disruption of STAT1 homodimer formation and DNA binding. Unlike human fibroblasts, mouse cells responded with long-lasting STAT1 activation which was preserved after VACV infection. The deletion of the IFN regulatory factor 1 (IRF-1) gene from mouse cells rescued efficient VACV replication, demonstrating that IRF-1 target genes have a critical role in VACV control. These data have implications for the understanding of VACV pathogenesis and identify an incongruent IFN-gamma response between the human host and the mouse model.

Protein composition of the vaccinia virus mature virion

The protein content of vaccinia virus mature virions, purified by rate zonal and isopycnic centrifugations and solubilized by SDS or a solution of urea and thiourea, was determined by the accurate mass and time tag technology which uses both tandem mass spectrometry and Fourier transform-ion cyclotron resonance mass spectrometry to detect tryptic peptides separated by high-resolution liquid chromatography. Eighty vaccinia virus-encoded proteins representing 37% of the 218 genes annotated in the complete genome sequence were detected in at least three analyses. Ten proteins accounted for approximately 80% of the virion mass. Thirteen identified proteins were not previously reported as components of virions. On the other hand, 8 previously described virion proteins were not detected here, presumably due to technical reasons including small size and hydrophobicity. In addition to vaccinia virus-encoded proteins, 24 host proteins omitting isoforms were detected. The most abundant of these were cytoskeletal proteins, heat shock proteins and proteins involved in translation.

Vaccinia virus proteome: identification of proteins in vaccinia virus intracellular mature virion particles

Vaccinia virus is a large enveloped poxvirus with more than 200 genes in its genome. Although many poxvirus genomes have been sequenced, knowledge of the host and viral protein components of the virions remains incomplete. In this study, we used gel-free liquid chromatography and tandem mass spectroscopy to identify the viral and host proteins in purified vaccinia intracellular mature virions (IMV). Analysis of the proteins in the IMV showed that it contains 75 viral proteins, including structural proteins, enzymes, transcription factors, and predicted viral proteins not known to be expressed or present in the IMV. We also determined the relative abundances of the individual protein components in the IMV. Finally, 23 IMV-associated host proteins were also identified. This study provides the first comprehensive structural analysis of the infectious vaccinia virus IMV.

A natural vaccinia virus promoter with exceptional capacity to direct protein synthesis

A survey of vaccinia virus promoters, through a reporter gene approach, has identified the viral I1L promoter as having exceptional activity. The I1L promoter exhibited over 10 times the activity of other vaccinia promoters and even rivaled the activity of the bacteriophage T7 promoter in the hybrid vaccinia/T7 expression system. The I1L promoter had high activity in both transient transfection experiments and in the context of recombinant viruses. The I1L promoter should be useful for high-level protein synthesis and poxvirus studies in general.

Post-transcription cleavage generates the 3' end of F17R transcripts in vaccinia virus

Most vaccinia virus intermediate and late mRNAs possess 3' ends that are extremely heterogeneous in sequence. However, late mRNAs encoding the cowpox A-type inclusion protein (ATI), the second largest subunit of the RNA polymerase, and the late telomeric transcripts possess homogeneous 3' ends. In the case of the ATI mRNA, it has been shown that the homogeneous 3' end is generated by a post-transcriptional endoribonucleolytic cleavage event. We have determined that the F17R gene also produces homogeneous transcripts generated by a post-transcriptional cleavage event. Mapping of in vivo mRNA shows that the major 3' end of the F17R transcript maps 1262 nt downstream of the F17R translational start site. In vitro transcripts spanning the in vivo 3' end are cleaved in an in vitro reaction using extracts from virus infected cells, and the site of cleavage is the same both in vivo and in vitro. Cleavage is not observed using extract from cells infected in the presence of hydroxyurea; therefore, the cleavage factor is either virus-coded or virus-induced during the post-replicative phase of virus replication. The cis-acting sequence responsible for cleavage is orientation specific and the factor responsible for cleavage activity has biochemical properties similar to the factor required for cleavage of ATI transcripts. Partially purified cleavage factor generates cleavage products of expected size when either the ATI or F17R substrates are used in vitro, strongly suggesting that cleavage of both transcripts is mediated by the same factor.

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

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

An unconventional role for cytoplasmic disulfide bonds in vaccinia virus proteins

Previous data have shown that reducing agents disrupt the structure of vaccinia virus (vv). Here, we have analyzed the disulfide bonding of vv proteins in detail. In vv-infected cells cytoplasmically synthesized vv core proteins became disulfide bonded in the newly assembled intracellular mature viruses (IMVs). vv membrane proteins also assembled disulfide bonds, but independent of IMV formation and to a large extent on their cytoplasmic domains. If disulfide bonding was prevented, virus assembly was only partially impaired as shown by electron microscopy as well as a biochemical assay of IMV formation. Under these conditions, however, the membranes around the isolated particles appeared less stable and detached from the underlying core. During the viral infection process the membrane proteins remained disulfide bonded, whereas the core proteins were reduced, concomitant with delivery of the cores into the cytoplasm. Our data show that vv has evolved an unique system for the assembly of cytoplasmic disulfide bonds that are localized both on the exterior and interior parts of the IMV.

Identification by mass spectroscopy of three major early proteins associated with virosomes in vaccinia virus-infected cells

Virosomes are cytoplasmic sites of replication of vaccinia virus DNA and were prepared from virus-infected HeLa cells. The early virosomal proteins were 35S-labelled and SDS polyacrylamide gel electrophoresis revealed the presence of three major early 35S-labelled proteins of 34, 24 and 45 kDa. The masses of molecules present in the 34 and 24 kDa proteins were measured by the convenient and sensitive MALDI TOF mass spectroscopy technique. Identification of the three virosomal proteins was carried out by MALDI mass spectroscopy of corresponding tryptic digests. For each protein at least 13 measured masses matched, within less than 0.1 Da, calculated tryptic peptides of the vaccinia virus proteins H5R (34 kDa), E3L (24 kDa) and E5R (45 kDa). In addition, virosomes contained several structural proteins from the infecting virus and a 45 kDa keratin-related protein. This work demonstrates directly that the abundant early vaccinia virus proteins H5R, E3L and E5R are associated with the virosomes.

Down regulation of gene expression by the vaccinia virus D10 protein

Vaccinia virus genes are expressed in a sequential fashion, suggesting a role for negative as well as positive regulatory mechanisms. A potential down regulator of gene expression was mapped by transfection assays to vaccinia virus open reading frame D10, which encodes a protein with no previously known function. Inhibition was independent of the promoter type used for the reporter gene, indicating that the mechanism did not involve promoter sequence recognition. The inhibition was overcome, however, when the open reading frame of the reporter gene was preceded by the encephalomyocarditis virus internal ribosome entry site, which excludes the possibility of nonspecific metabolic or other antiviral effects and suggests that capped mRNAs or cap-dependent translation might be the target of the D10 product. The inducible overexpression of the D10 gene by a recombinant vaccinia virus severely inhibited viral protein synthesis, decreased the steady-state level of viral late mRNA, and blocked the formation of infectious virus.

Interaction of vaccinia virus with the actin cytoskeleton

Vaccinia virus infection results in large rearrangements of the host actin cytoskeleton including the formation of actin tails that are strikingly similar to those seen in Listeria, Shigella and Rickettsia infections. Using actin polymerization as the driving force the intracellular enveloped form of the vaccinia virus (IEV) is propelled on the tip of actin tails at a speed of 2.8 microns/min, both intra- and intercellularly. The similarities between the actin-based motility of the vaccinia virus, Listeria, Shigella and Rickettsia suggest that intracellular pathogens have developed a common strategy to exploit the actin cytoskeleton of the host to facilitate their intercellular spread. This review focuses on our current understanding of the interactions between the vaccinia virus and the actin cytoskeleton.

Inhibition of vaccinia virus replication by N-(phosphonoacetyl)-L-aspartate: differential effects on viral gene expression result from a reduced pyrimidine nucleotide pool

The replication of vaccinia virus was reduced by 3 logs in cells that had been treated before and during infection with a concentration of N-(phosphonoacetyl)-L-aspartate (PALA) which lowered the UTP and CTP to 5 and 20% of controls, respectively, without affecting cell viability. The antiviral activity of PALA was reversed with uridine, indicating that it was entirely due to the diminution in pyrimidine nucleotides. Analysis of viral proteins revealed prolonged synthesis of some early stage species but a drastic reduction in late stage species, even though the nucleotide concentrations remained relatively constant throughout the infection. Although the gene expression pattern resembled that caused by a potent inhibitor of DNA synthesis, viral DNA accumulation was reduced by only 60%. Very little of the DNA made in the presence of PALA was converted to genome length molecules. The effect of PALA on transcription of early genes was complex: there was a twofold increase in the amount of a relatively short mRNA of 500 nucleotides but a two- to threefold decrease in the amount of a 4300-nucleotide mRNA encoding the largest subunit of RNA polymerase. In contrast, PALA severely inhibited the accumulation of viral intermediate and late stage mRNAs. The extreme sensitivity of vaccinia virus to PALA and the differential effects of the drug on viral gene expression result from the cascade mechanism of viral gene regulation.

The vaccinia virus F17R protein interacts with actin

We have examined the possible role of the F17R protein in vaccinia virus-induced rearrangements of the host actin cytoskeleton. F17R is localized to vaccinia-induced actin tails late during infection. The recombinant vaccinia strain vRO11k is able to induce actin tails that are indistinguishable from controls in the absence of F17R expression. The association of vaccinia and myxoma virus F17R with the actin cytoskeleton in the absence of additional viral factors suggests a basic region in the N-terminal half of the protein is important for this interaction. A peptide corresponding to this region efficiently bundles actin filaments in vitro, confirming that the protein interacts directly with actin. Our results show F17R is not required for actin tail formation and highlight the difficulty in discriminating functional actin-binding proteins from those that associate by virtue of their basic nature.

cis- and trans-acting elements involved in reactivation of vaccinia virus early transcription

We have previously shown that transcription from the vaccinia virus 7.5K early promoter is reactivated late in infection (J. Garcés, K. Masternak, B. Kunz, and R. Wittek, J. Virol. 67:5394-5401, 1993). To identify the sequence elements mediating reactivation, we constructed recombinant viruses harboring deletions, substitutions, or insertions in the 7.5K promoter or its flanking regions. The analysis of these viruses showed that sequences both upstream as well as downstream of the transcription initiation site contribute to reactivation of the 7.5K promoter. We tested whether reactivation could be explained by a high affinity of vaccinia virus early transcription factor to reactivated promoters. Bandshift experiments using purified protein showed that promoters which bind the factor with high affinity in general also have high early transcriptional activity. However, no correlation was found between affinity of the factor and reactivation. Interestingly, overexpression of recombinant early transcription factor in vaccinia virus-infected cells resulted in a shutdown of late transcription and in reactivation of promoters, which are normally not reactivated.

Identification of the major membrane and core proteins of vaccinia virus by two-dimensional electrophoresis

Vaccinia virus assembly has been well studied at the ultrastructural level, but little is known about the molecular events that occur during that process. Towards this goal, we have identified the major membrane and core proteins of the intracellular mature virus (IMV). Pure IMV preparations were subjected to Nonidet P-40 (NP-40) and dithiothreitol (DTT) treatment to separate the core proteins from the membrane proteins. These proteins were subsequently separated by two-dimensional (2D) gel electrophoresis, and the major polypeptide spots, as detected by silver staining and 35S labeling, were identified by either matrix-assisted laser desorption/ionization mass spectrometry, N-terminal amino acid sequencing, or immunoprecipitation with defined antibodies. Sixteen major spots that partitioned into the NP-40-DTT-soluble fraction were identified; 11 of these were previously described virally encoded proteins and 5 were cellular proteins, mostly of mitochondrial origin. The core fraction revealed four major spots of previously described core proteins, two of which were also detected in the membrane fraction. Subsequently, the NP-40-DTT-soluble and -insoluble fractions from purified virus preparations, separated by 2D gels, were compared with postnuclear supernatants of infected cells that had been metabolically labeled at late times (6 to 8 h) postinfection. This relatively short labeling period as well as the apparent shutoff of host protein synthesis allowed the selective detection in such postnuclear supernatants of virus-encoded proteins. These postnuclear supernatants were subsequently treated with Triton X-114 or with sodium carbonate to distinguish the membrane proteins from the soluble proteins. We have identified the major late membrane and nonmembrane proteins of the IMV as they occur in the virus as well as in infected cells. This 2D gel map should provide an important reference for future molecular studies of vaccinia virus morphogenesis.

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.

Electron spectroscopic imaging of encapsidated DNA in vaccinia virus

We have used electron spectroscopic imaging to locate the phosphorus in vaccinia DNA in situ in unstained, ultrathin sections of virions. The phosphorus of the DNA backbone appeared to form a halo on the core periphery surrounding a phosphorus-impoverished central element. These results constrain models for how DNA could be packaged into mature vaccinia particles.

Functional organization of variola major and vaccinia virus genomes

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

Analysis of the nucleotide sequence of a 43 kbp segment of the genome of variola virus India-1967 strain

Sequencing and computer analysis of the nucleotide sequence of the variola virus strain India-1967 (VAR) genome segment (43069 bp) from the region of HindIII C, E, R, Q, K, H DNA fragments has been carried out. Forty-three potential open reading frames (ORFs) have been identified, and the polypeptides encoded by them have been compared with the analogous proteins of vaccinia virus strain Copenhagen (COP). ORF E7R of VAR is much shorter than the COP analog. The other polypeptides coded by the potential ORFs of VAR are highly conserved in comparison with COP. Possible functions of the predicted viral polypeptides are discussed.

Comparative analysis of the conserved region of the orthopoxvirus genome encoding the 36K and 12K proteins

Genes encoding virus-specific proteins with molecular masses of 36 kDa and 12 kDa were mapped in HindIII-P and HindIII-U DNA fragments of vaccinia strain LIVP and ectromelia strain K-1 viruses, respectively, by hybrid selection of RNA to cloned DNA fragments followed by in vitro translation. The 36K translation initiation codon was detected in the HindIII-J fragment. The nucleotide sequences of corresponding genes from vaccinia, ectromelia, cowpox and variola virus genomes were determined. The 12K protein has similarity to mammalian glutaredoxins. The derived amino acid sequence of the 36K polypeptide was compared with the protein bank PIR. No homology was found between the 36K protein and known structures of proteins. The 36K protein genes of vaccinia and ectromelia viruses were cloned in pUR290, which led to the production of E. coli chimeric proteins, consisting of the sequence of beta-galactosidase and the viral protein on their C-ends. The chimeric proteins were shown to possess viral antigenic specificity. To identify the protein product of the 36K gene monospecific antisera to chimeric proteins were obtained. The late 36K protein is associated with virosomes but is not incorporated into the virions of orthopoxviruses.

Transcription of viral late genes is dependent on expression of the viral intermediate gene G8R in cells infected with an inducible conditional-lethal mutant vaccinia virus

There are three temporal classes of vaccinia virus genes: early, intermediate, and late. The object of this study was to determine the effects on virus replication of regulating the expression of G8R, an intermediate gene that encodes a late transcription factor. We inserted the lac operator adjacent to the RNA start site of the G8R gene in a recombinant vaccinia virus that constitutively expresses the Escherichia coli lac repressor to make expression of the G8R gene dependent on the inducer isopropyl-beta-D-thiogalactopyranoside (IPTG). In case repression would not be complete, we also weakened the promoter of the G8R gene by making a single-nucleotide substitution designed to reduce its basal level of transcription. The mutant virus replicated well in the presence of the inducer, although synthesis of the G8R-encoded 30,000-M(r) protein was only 10% of that of the wild-type virus. In the absence of IPTG, (i) synthesis of the G8R protein was inhibited by more than 99% relative to that of the wild-type virus, (ii) synthesis of early and intermediate mRNAs appeared to be unaffected, (iii) intermediate proteins accumulated to higher than normal levels, (iv) synthesis of late mRNA and protein was reduced by about 90%, (v) viral DNA was replicated but incompletely resolved concatemeric molecules accumulated, (vi) not even the earliest stages of virion assembly were detectable by transmission electron microscopy, and (vii) virus yield under one-step growth conditions and plaque formation were 10(-3) and 10(-4) times the wild-type values, respectively. The defect in late gene expression could be overcome by transfection of a G8R gene that was not under lac operator control, as well as by addition of IPTG, further demonstrating the specificity of the repression. The correlation between decreased expression of the G8R intermediate gene and inhibition of late mRNA synthesis is consistent with the notion that the G8R product serves as an essential late transcription factor and supports a cascade mechanism of vaccinia virus gene regulation. In addition, the inducer-dependent vaccinia virus mutant provided a tool for selective inhibition of late gene expression while allowing synthesis of early and intermediate mRNAs and proteins.

Introduction of foreign DNA into the vaccinia virus genome by in vitro ligation: recombination-independent selectable cloning vectors

Homologous recombination has been the exclusive means of introducing foreign DNA into the genomes of large DNA viruses. Here we demonstrate that direct in vitro ligation can be used to efficiently insert DNA fragments of up to 26,000 bp into the genome of vaccinia virus modified to contain a single NotI site either in the Escherichia coli lacZ gene or in the vaccinia virus thymidine kinase gene. Viruses containing chimeric genomes can be identified by chromogenic screening or thymidine-kinase-negative selection.

Quantitative assessment of poxvirus promoters in fowlpox and vaccinia virus recombinants

A comparison was undertaken of poxvirus promoters in vaccinia and fowlpox virus (FPV) recombinants using the level of beta-galactosidase expressed from the LacZ gene as a measure of promoter function. In this study a comparison was made of the vaccinia virus promoters, P 7.5 and P L11, the major late promoter of cowpox virus, P CPX (expressing the abundant inclusion body protein), and the FPV promoters, P E/L and P L. In vaccinia virus recombinants the FPV P E/L promoter expressed one-third to one-half the level of beta-galactosidase expressed by the P L11 promoter. In comparison with the P 7.5 promoter, the FPV P E/L promoter expressed four to five times the level of beta-galactosidase. In FPV recombinants beta-galactosidase activity expressed was equal for the P E/L and P CPX promoters. Levels expressed by P L11 and P L were one-half and one-fifth that level, respectively. The temporal regulation of the promoters was maintained in both vaccinia virus and FPV recombinants. The P E/L promoter of FPV has the TAAATG sequence characteristic of late poxvirus promoters at the transcription initiation site. In an attempt to enhance the utility of this promoter for the expression of foreign genes in FPV and vaccinia virus recombinants, the effect upon promoter function of changing the G of the ATG to A, T, or C was determined using transient expression assays with vaccinia virus. Substitution of A, T, or C for the G abolished promoter function. Because of its early/late function, the level of expression and the presence of the oppositely oriented late P L promoter, the FPV P E/L promoter will be valuable for the expression of foreign genes in poxvirus recombinants.

Vaccinia virus morphogenesis is interrupted when expression of the gene encoding an 11-kilodalton phosphorylated protein is prevented by the Escherichia coli lac repressor

A conditional lethal vaccinia virus mutant, which constitutively expresses the Escherichia coli lac repressor and has the lac operator controlling the F18R gene (the 18th open reading frame of the HindIII F fragment of the vaccinia virus strain WR genome) encoding an 11-kDa protein, was previously shown to be dependent on the inducer isopropyl-beta-D-thiogalactoside (IPTG) for replication (Y. Zhang and B. Moss, Proc. Natl. Acad. Sci. USA 88:1511-1515, 1991). Further studies indicated that the yield of infectious virus could be regulated by titration with IPTG and that virus production was arrested by IPTG removal at appropriate times. Under nonpermissive conditions, an 11-kDa protein reactive with antiserum raised to a previously described DNA-binding phosphoprotein (S. Y. Kao and W. R. Bauer, Virology 159:399-407, 1987) was not synthesized, indicating that the latter is the product of the F18R gene. In the absence of IPTG, replication of viral DNA and the subsequent resolution of concatemeric DNA molecules appeared normal. Omission of IPTG did not alter the kinetics of early and late viral protein synthesis, although the absence of the 11-kDa polypeptide was noted by labeling infected cells with [35S]methionine or [32P]phosphate. Pulse-chase experiments revealed that proteolytic processing of the major viral structural proteins, P4a and P4b, was inhibited under nonpermissive conditions, suggesting a block in virus maturation. Without addition of IPTG, the failure of virus particle formation was indicated by sucrose gradient centrifugation of infected cell lysates and by the absence of vaccinia virus-mediated pH-dependent cell fusion. Electron microscopic examination of infected cells revealed that immature virus particles, with aberrant internal structures, accumulated when synthesis of the 11-kDa DNA-binding protein was prevented.

Inducer-dependent conditional-lethal mutant animal viruses

Regulatory elements of the Escherichia coli lac operon were used to construct an inducer-dependent conditional-lethal mutant animal virus. The gene encoding the repressor protein of the lac operon was integrated into the vaccinia virus genome so that it was expressed constitutively, and the lac operator was inserted next to the promoter of a gene that encodes an 11-kDa virion-associated protein of unknown function. The addition of inducer to the cell culture medium provided permissive conditions for isolation of a conditional-lethal mutant virus. Under nonpermissive conditions, the isolated virus did not form plaques, and the yield was decreased by at least 1000-fold under one-step growth conditions. Transcription of the operator-controlled gene was inducer-dependent and necessary for synthesis of the 11-kDa protein. Application of this mutagenesis strategy to other viruses is discussed.

Mutation of conserved N-glycosylation sites around the CD4-binding site of human immunodeficiency virus type 1 GP120 affects viral infectivity

Infection by the human immunodeficiency virus type 1 (HIV-1) is initiated through interaction of its exterior envelope glycoprotein gp120 with the CD4 receptor on target cells. To address the possible role of N-glycosylation of HIV-1 gp120 in binding CD4, we mutated different conserved N-glycosylation site Asn-residues in the vicinity of the putative CD4 binding site, as single mutations or in combinations. Authentic and mutant gp120 proteins were produced using the baculovirus expression system. All mutant proteins were produced and secreted at similar levels and could be immunoprecipitated with an HIV(+)-serum. Furthermore, all glycosylation mutants retained the full capacity to bind CD4 except for a triple mutant which showed reduced binding. Different gp120 mutant genes were then introduced in an infectious proviral DNA clone. Upon transfection of MT-2 cells, the authentic HIV-1 clone induced maximal virus production after 6 days. In the case of the triple glycosylation mutant, comparable virus production was first reached after a delay of about 12 days. Moreover, in contrast to native HIV, the mutant virus induced no typical multinucleated giant cells. These results suggest that the attached carbohydrates around the CD4-binding site of gp120, may contribute to the generation of this protein domain required for high affinity receptor interaction.

Molecular cloning, characterization, and expression of the Tipula iridescent virus capsid gene

The capsid protein is the major structural component of the icosahedral Tipula iridescent virus (TIV) that replicates in cytoplasmic inclusion bodies of insect cells. TIV capsid protein purified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was digested with trypsin and fractionated by reverse-phase high-pressure liquid chromatography. A mixed oligonucleotide constructed from the amino acid sequence of a capsid tryptic peptide was used for the identification and cloning of the corresponding gene. The single-copy capsid gene, located on a 2.47-kilobase-pair HindIII TIV genomic fragment, codes for a 464-amino-acid protein (50,831 daltons) with a predicted pI of 6.34. Analysis of total RNA from infected Estigmene acrea cells indicated that the 1.8-kilobase capsid transcript was maximally produced between 14 and 24 h after infection. Transcript mapping by primer extension indicated that the RNA start site was in the A+T-rich TGCTACTAAT sequence, 19 nucleotides upstream from the first ATG codon of the capsid open reading frame. Expression of the TIV capsid protein in infected E. acrea cells was demonstrated by in vivo labeling of total proteins with [35S]methionine, using anti-capsid antiserum as the probe. Capsid protein was also expressed in Escherichia coli cells by using a pUC19 plasmid containing a lacZ-capsid gene fusion.

Genetic evidence for involvement of vaccinia virus DNA-dependent ATPase I in intermediate and late gene expression

To delineate the role of the vaccinia virus-encapsidated DNA-dependent ATPase I in the life cycle of the virus, we performed a detailed study of two temperature-sensitive mutants with lesions in the gene encoding the enzyme. Profiles of viral DNA and protein accumulation during infection showed the mutants to be competent for DNA synthesis but deficient in late protein synthesis, confirming their defective late phenotype (R. C. Condit and A. Motyczka, Virology 113:224-241, 1981: R. C. Condit, A. Motyczka, and G. Spizz, Virology 128:429-443, 1983). In vitro translation of viral RNA and S1 nuclease mapping of selected mRNAs demonstrated that the deficit in late protein synthesis stemmed from a defect in the transcriptional machinery. Intermediate and late gene expression appeared to be most affected. The transcriptional defect was of unequal severity in the two mutants. However, their phenotypes were indistinguishable and their respective lesions were mapped to the same 300 nucleotides at the 5' end of the gene. DNA sequence analysis assigned a single nucleotide and amino acid change to one of the mutants.

Analysis of the fowlpoxvirus gene encoding the 4b core polypeptide and demonstration that it possesses efficient promoter sequences

The gene encoding the fowlpox virus 4b core polypeptide has been identified by analogy with the vaccinia 4b gene. It has been cloned, and its nucleotide sequence determined. The gene, which is 1971 nucleotides long, can encode a protein of 75,200 Da (75.2K polypeptide), slightly longer than its vaccinia counterpart with which it shares 52% identity. Sequences upstream of the fowlpox virus 4b gene correspond to the consensus sequence determined for vaccinia late promoters, suggesting that late promoter signals may be shared by the different genera of poxviruses. Upstream sequences have been cloned into a beta-galactosidase translational fusion vector and shown to promote the efficient expression of beta-galactosidase in a transient assay system. This expression was abolished in the presence of araC, an inhibitor of DNA replication which blocks late gene expression in poxviruses. The fowlpoxvirus 4b promoter should be a useful component of genetically engineered fowlpox virus vaccines.

Concentration dependence of transcriptional transactivation in inducible E1A-containing human cells

The adenovirus E1A proteins are essential for the normal temporal activation of transcription from every other adenoviral early promoter. High-level E1A expression in the absence of viral infection would facilitate biochemical studies of E1A-mediated transactivation. Toward this end, we introduced the adenovirus type 2 E1A gene under the control of the murine mammary tumor virus promoter into HeLa cells. Uninduced cells expressed little or no detectable E1A mRNA. Upon induction, mRNA levels accumulated to about 50% of the level observed in 293 cells. The level of E1A expression in these cells could be controlled by varying the concentration of the inducing glucocorticoid. Under these conditions of varying E1A concentrations, it was observed that activation of the E2, E3, and E4 promoters of H5dl312 initiated at the same E1A concentration and that transcription from each promoter increased as the E1A concentration increased. These results indicate that E1A-mediated transactivation is proportional to the concentration of E1A protein. E1A-dependent transcriptional stimulation of the E4 promoter was reproduced in an in vitro transcription system, demonstrating that expression of only the E1A proteins was sufficient to increase the transcriptional activity of nuclear extracts.

Antigenic specificity of antibody-dependent cell-mediated cytotoxicity directed against human immunodeficiency virus in antibody-positive sera

Antibody-dependent cell-mediated cytotoxicity (ADCC) specific for human immunodeficiency virus (HIV) has been described for HIV-infected individuals. To determine the antigenic specificity of this immune response and to define its relationship to the disease state, an ADCC assay was developed using Epstein-Barr virus-transformed lymphoblastoid cell line targets infected with vaccinia virus vectors expressing HIV proteins. The vaccinia virus vectors induced appropriate HIV proteins (envelope glycoproteins gp160, gp120, and gp41 or gag proteins p55, p40, p24, and p17) in infected lymphoblastoid cell lines as demonstrated by radioimmunoprecipitation and syncytia formation with c8166 cells. Killer cell-mediated, HIV-specific ADCC was found in sera from HIV-seropositive but not HIV-seronegative hemophiliacs. This HIV-specific response was directed against envelope glycoprotein but was completely absent against target cells expressing the HIV gag proteins. The ADCC directed against gp160 was present at serum dilutions up to 1/316,000. There was no correlation between serum ADCC titer and the stage of HIV-related illness as determined by T-helper-cell numbers. These experiments clearly implicated gp160 as the target antigen of HIV-specific ADCC activity following natural infection. Vaccines which stimulate antibodies directed against gp160, which are capable of mediating ADCC against infected cells, could be important for protection against infection by cell-associated virus.

Capped poly(A) leaders of variable lengths at the 5' ends of vaccinia virus late mRNAs

Evidence for capped poly(A) leaders of variable lengths located immediately upstream of the translation initiation codon was obtained by direct analyses of a major late mRNA species. A decapping-recapping method was used to specifically substitute a radioactively labeled phosphate for an unlabeled one within the cap structure. RNase H-susceptible sites were made by hybridizing synthetic oligodeoxyribonucleotides to the mRNA encoding a late major structural protein of 11 kilodaltons. Sequences of the type m7G(5')pppAmp (Ap)nUpG. . ., where n varies from a few to more than 40 nucleotides, were deduced by analysis of the length and sequence of RNase H, RNase T1, and RNase U2 digestion products.

Concentration dependence of transcriptional transactivation in inducible E1A-containing human cells

The adenovirus E1A proteins are essential for the normal temporal activation of transcription from every other adenoviral early promoter. High-level E1A expression in the absence of viral infection would facilitate biochemical studies of E1A-mediated transactivation. Toward this end, we introduced the adenovirus type 2 E1A gene under the control of the murine mammary tumor virus promoter into HeLa cells. Uninduced cells expressed little or no detectable E1A mRNA. Upon induction, mRNA levels accumulated to about 50% of the level observed in 293 cells. The level of E1A expression in these cells could be controlled by varying the concentration of the inducing glucocorticoid. Under these conditions of varying E1A concentrations, it was observed that activation of the E2, E3, and E4 promoters of H5dl312 initiated at the same E1A concentration and that transcription from each promoter increased as the E1A concentration increased. These results indicate that E1A-mediated transactivation is proportional to the concentration of E1A protein. E1A-dependent transcriptional stimulation of the E4 promoter was reproduced in an in vitro transcription system, demonstrating that expression of only the E1A proteins was sufficient to increase the transcriptional activity of nuclear extracts.

Vaccinia virus recombinants expressing an 11-kilodalton beta-galactosidase fusion protein incorporate active beta-galactosidase in virus particles

Recombinant plasmids in which vaccinia virus transcriptional regulatory sequences were fused to the Escherichia coli lacZ gene were constructed for insertion of the lacZ gene into the vaccinia virus genome. beta-Galactosidase (beta-gal) was found in some purified recombinant vaccinia virions. By enzyme activity, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and microscopic techniques, the evidence suggested that beta-gal accounted for 5% of the total protein in the virion. These recombinant viruses were constructed so that a portion of the coding sequences of a late vaccinia virus structural polypeptide was fused to the amino terminus of beta-gal to produce the fusion protein. Removal of the coding sequences resulted in the complete loss of beta-gal activity. This demonstrated that a vaccinia virus DNA segment from a late structural gene is responsible for the incorporation of beta-gal into the virion.

Synthesis of virus-specific RNA in permeabilized murine coronavirus-infected cells

We have developed a permeabilized cell system for assaying mouse hepatitis virus-specific RNA polymerase activity. This activity was characterized as to its requirements for mono- and divalent cations, requirements for an exogenous energy source, and pH optimum. This system faithfully reflects MHV-specific RNA synthesis in the intact cell, with regard to both its time of appearance during the course of infection and the products synthesized. The system is efficient and the RNA products were identical to those observed in intact MHV-infected cells as judged by agarose gel electrophoresis and hybridization. Permeabilized cells appear to be an ideal system for studying coronavirus RNA synthesis since they closely mimic in vivo conditions while allowing much of the experimental flexibility of truly cell-free systems.

Escherichia coli gpt gene provides dominant selection for vaccinia virus open reading frame expression vectors

Mycophenolic acid, an inhibitor of purine metabolism, was shown to block the replication of vaccinia virus in normal cell lines. This observation led to the development of a dominant one-step plaque selection system, based on expression of the Escherichia coli gpt gene, for the isolation of recombinant vaccinia viruses. Synthesis of xanthine-guanine phosphoribosyltransferase enabled only the recombinant viruses to form large plaques in a selective medium containing mycophenolic acid, xanthine, and hypoxanthine. To utilize the selection system efficiently, we constructed a series of plasmids that contain the E. coli gpt gene and allow insertion of foreign genes into multiple unique restriction endonuclease sites in all three reading frames between the translation initiation codon of a strong late promoter and synthetic translation termination sequences. The selection-expression cassette is flanked by vaccinia virus DNA that directs homologous recombination into the virus genome. The new vectors allow high-level expression of complete or partial open reading frames and rapid construction of recombinant viruses by facilitating the cloning steps and by simplifying their isolation. The system was tested by cloning the E. coli beta-galactosidase gene; in 24 h, this enzyme accounted for approximately 3.5% of the total infected-cell protein.

Fine structure of the vaccinia virus gene encoding the precursor of the major core protein 4 a

A 6008 base pair fragment of the vaccinia virus DNA containing the gene for the precursor of the major core protein 4 a, which has been designated P4 a, was sequenced. A long open reading frame (ORF) encoding a protein of molecular weight 102,157 started close to the position where the P4 a mRNA had been mapped. Analysis of the mRNA by S1 nuclease mapping and primer extension indicated that the 5' end defined by the former method is not the true 5' end. This suggests that the P4 a coding region is preceded by leader sequences that are not derived from the immediate vicinity of the gene, similar to what has been reported for another late vaccinia virus mRNA. The sequenced DNA contained several further ORFs on the same, or opposite DNA strand, providing further evidence for the close spacing of protein-coding sequences in the viral genome.

Vaccinia virus recombinants expressing rabiesvirus glycoprotein protect against rabies

Six recombinants of New York Board of Health (NYBH) vaccinia virus containing cDNA for Challenge Virus Standard (CVS) rabiesvirus glycoprotein (G) were produced by directing gene insertion into the vaccinia thymidine kinase (TK) locus. To regulate expression of G the promoter P7.5 (functions at early and late times postinfection) from the gene for the vaccinia 7.5 kilodalton (kD) protein was used in two of the recombinants; late promoter P11 of the vaccinia 11 kD protein was used in four recombinants. The six differed in nucleotide sequences flanking the translation start codon; in two constructs the encoded signal peptide of G was fused to several additional amino acids. Cells infected with each recombinant made G that reacted with G-specific antibodies, comigrated with authentic G, and was transported to the plasma membrane. The highest amounts of G were made with fusion or standard versions of G with P11 provided that the mRNA leader sequences were identical to the natural gene. Each recombinant in mice and one in dogs induced rabiesvirus neutralizing antibodies and protection against lethal rabiesvirus challenge.

The role of the host cell nucleus in vaccinia virus morphogenesis

Despite the fact that cells infected with wild type vaccinia virus synthesize viral DNA and assemble progeny virus particles within the cytoplasm, the host cell nucleus is required for a productive infection. Recent evidence suggests that vaccinia virus selectively recruits components from the host cell nucleus into the cytoplasm for use by the developing virus. One of these components is the largest subunit of the cellular RNA polymerase II (Pol II).

Oligonucleotide sequence signaling transcriptional termination of vaccinia virus early genes

In an in vitro system containing enzymes extracted from vaccinia virions, transcription of the vaccinia growth factor gene terminated approximately 50 base pairs downstream of a thymidine-rich sequence. Deletion mutagenesis suggested the presence of two tandem termination signals. The signal was identified by replacing the 3' end of the gene with the oligonucleotide AATTTTTAT that induced downstream termination. Further analysis of the transcripts formed with a series of templates containing 16 related synthetic oligonucleotides established the minimum functional termination signal as TTTTTNT, in which N represents any nucleotide. Termination efficiency may be increased, however, by the presence of an adenosine preceding the thymidine cluster. The general use of this signal at early times in infection but not at late times is supported by a survey of vaccinia virus gene sequences.

Vaccinia virus produces late mRNAs by discontinuous synthesis

We describe the unusual structure of a vaccinia virus late mRNA. In these molecules, the protein-coding sequences of a major late structural polypeptide are preceded by long leader RNAs, which in some cases are thousands of nucleotides long. These sequences map to different regions of the viral genome and in one instance are separated from the late gene by more than 100 kb of DNA. Moreover, the leader sequences map either upstream or downstream of the late gene, are transcribed from either DNA strand, and are fused to the late gene coding sequence via a poly(A) stretch. This demonstrates that vaccinia virus produces late mRNAs by tagging the protein-coding sequences onto the 3' end of other RNAs.

Discontinuous transcription or RNA processing of vaccinia virus late messengers results in a 5' poly(A) leader

We have demonstrated by primer elongation and cap analysis that mature vaccinia virus late transcripts are discontinuously synthesized. We have shown that RNA transcripts from a translocated 11K and from the authentic 11K and 4b late promoters are extended by approximately 35 nucleotides beyond the "start site" determined by S1 mapping using vaccinia genomic DNA as a probe. Sequencing of the RNA and of the first strand cDNA reveal that a homopolymeric poly(A) sequence is linked to the 5' terminus of the RNA transcripts. S1 mapping of RNA transcripts with a DNA probe containing an A-stretch, replacing promoter sequences upstream of position -1, confirms the existence of a poly(A) leader of approximately 35 A-residues.

Transient expression system to measure the efficiency of vaccinia promoter regions

A transient expression system has been developed to compare the relative efficiency of expression of various vaccinia virus DNA sequences containing transcriptional regulatory elements. A plasmid vector was constructed containing both the Escherichia coli galactokinase gene (galK) and the guanine phosphoribosyltransferase gene (gpt). To direct the expression of gpt within this vector, a vaccinia virus promoter region was isolated from the HindIII-F fragment of the genome and inserted 5' to gpt coding sequence. Four unique cloning sites in front of galK allow simple and precise fusion of various vaccinia virus DNA fragments that contain the regulatory site of interest to galK. Sequences containing promoter regions were ligated to the coding segment of the galK to create four recombinant plasmids, which were introduced into vaccinia virus-infected cells by transfection. Both galK and gpt were thus expressed under the control of vaccinia virus transcriptional units, and the enzymatic activities were measured in the same cell extract with a filter-binding assay. The major advantage of this transient expression system is that the variations in galK expression are always measured relative to the internal gpt standard. Changes in the galK/gpt ratio resulting from different vaccinia promoters of galK are thus a quantitative measurement of promoter strength.