Epstein-Barr Virus DNA Replication Research Articles

Latent and lytic Epstein-Barr virus replication strategies.

The Epstein-Barr virus (EBV) can choose between two alternative lifestyles; latent or lytic replication. In the latent state, the EBV genomic DNA, which exists as a closed circular plasmid, appears to behave just like host chromosomal DNA and it has been demonstrated recently that replication of OriP-containing plasmids is indeed dependent on the chromosomal initiation factors, ORC2 and Cdt1. On the other hand, in the viral productive cycle, the EBV genome is amplified 100- to 1000-fold by the viral replication machinery. EBV productive DNA replication occurs at discrete sites in nuclei, called replication compartments and the lytic programme arrests cell cycle progression and changes the cellular environment greatly. It has been revealed recently that the EBV lytic programme promotes an S-phase like cellular condition, which most favours viral lytic replication. This review describes recent advances regarding the molecular basis of EBV DNA replication during latent and lytic infections and then refers to cellular circumstances after induction of the lytic replication of EBV. Based on the molecular mechanism for the EBV lifestyle, purposeful induction of the lytic form of EBV infection is now advocated as one of the strategies for specific destruction of Epstein-Barr virus (EBV)-associated malignancies where the virus is latently infected.

The papillomavirus E8-E2C protein represses DNA replication from extrachromosomal origins.

Carcinogenic DNA viruses such as high-risk human papillomaviruses (HPV) and Epstein-Barr-Virus (EBV) replicate during persistent infections as low-copy-number plasmids. EBV DNA replication is restricted by host cell replication licensing mechanisms. In contrast, copy number control of HPV genomes is not under cellular control but involves the viral sequence-specific DNA-binding E2 activator and E8-E2C repressor proteins. Analysis of HPV31 mutant genomes revealed that residues outside of the DNA-binding/dimerization domain of E8-E2C limit viral DNA replication, indicating that binding site competition or heterodimerization among E2 and E8-E2C proteins does not contribute to copy number control. Domain swap experiments demonstrated that the amino-terminal 21 amino acids of E8-E2C represent a novel, transferable DNA replication repressor domain, whose activity requires conserved lysine and tryptophan residues. Furthermore, E8-E2C (1-21)-GAL4 fusion proteins inhibited the replication of the plasmid origin of replication of EBV, suggesting that E8-E2C functions as a general replication repressor of extrachromosomal origins. This finding could be important for the development of novel therapies against persistent DNA tumor virus infections.

Complex protein-DNA dynamics at the latent origin of DNA replication of Epstein-Barr virus.

The sequential binding of the origin recognition complex (ORC), Cdc6p and the minichromosome maintenance proteins (MCM2-7) mediates replication competence at eukaryotic origins of DNA replication. The latent origin of Epstein-Barr virus, oriP, is a viral origin known to recruit ORC. OriP also binds EBNA1, a virally encoded protein that lacks any activity predicted to be required for replication initiation. Here, we used chromatin immunoprecipitation and chromatin binding to compare the cell-cycle-dependent binding of pre-RC components and EBNA1 to oriP and to global cellular chromatin. Prereplicative-complex components such as the Mcm2p-Mcm7p proteins and HsOrc1p are regulated in a cell-cycle-dependent fashion, whereas other HsOrc subunits and EBNA1 remain constantly bound. In addition, HsOrc1p becomes sensitive to the 26S proteasome after release from DNA during S phase. These results show that the complex protein-DNA dynamics at the viral oriP are synchronized with the cell division cycle. Chromatin-binding and chromatin-immunoprecipitation experiments on G0 arrested cells indicated that the ORC core complex (ORC2-5) and EBNA1 remain bound to chromatin and oriP. HsOrc6p and the MCM2-7 complex are released in resting cells. HsOrc1p is partly liberated from chromatin. Our data suggest that origins remain marked in resting cells by the ORC core complex to ensure a rapid and regulated reentry into the cell cycle. These findings indicate that HsOrc is a dynamic complex and that its DNA binding activity is regulated differently in the various stages of the cell cycle.

Epstein-Barr virus mRNA export factor EB2 is essential for production of infectious virus.

The splicing machinery which positions a protein export complex near the exon-exon junction mediates nuclear export of mRNAs generated from intron-containing genes. Many Epstein-Barr virus (EBV) early and late genes are intronless, and an alternative pathway, independent of splicing, must export the corresponding mRNAs. Since the EBV EB2 protein induces the cytoplasmic accumulation of intronless mRNA, it is tempting to speculate that EB2 is a viral adapter involved in the export of intronless viral mRNA. If this is true, then the EB2 protein is essential for the production of EBV infectious virions. To test this hypothesis, we generated an EBV mutant in which the BMLF1 gene, encoding the EB2 protein, has been deleted (EBV(BMLF1-KO)). Our studies show that EB2 is necessary for the production of infectious EBV and that its function cannot be transcomplemented by a cellular factor. In the EBV(BMLF1-KO) 293 cells, oriLyt-dependent DNA replication was greatly enhanced by EB2. Accordingly, EB2 induced the cytoplasmic accumulation of a subset of EBV early mRNAs coding for essential proteins implicated in EBV DNA replication during the productive cycle. Two herpesvirus homologs of the EB2 protein, the herpes simplex virus type 1 protein ICP27 and, the human cytomegalovirus protein UL69, only partly rescued the phenotype of the EBV(BMLF1-KO) mutant, indicating that some EB2 functions in virus production cannot be transcomplemented by ICP27 and UL69.

Human origin recognition complex binds to the region of the latent origin of DNA replication of Epstein-Barr virus.

Epstein-Barr virus (EBV) replicates in its latent phase once per cell cycle in proliferating B cells. The latent origin of DNA replication, oriP, supports replication and stable maintenance of the EBV genome. OriP comprises two essential elements: the dyad symmetry (DS) and the family of repeats (FR), both containing clusters of binding sites for the transactivator EBNA1. The DS element appears to be the functional replicator. It is not yet understood how oriP-dependent replication is integrated into the cell cycle and how EBNA1 acts at the molecular level. Using chromatin immunoprecipitation experiments, we show that the human origin recognition complex (hsORC) binds at or near the DS element. The association of hsORC with oriP depends on the DS element. Deletion of this element not only abolishes hsORC binding but also reduces replication initiation at oriP to background level. Co-immunoprecipitation experiments indicate that EBNA1 is associated with hsORC in vivo. These results indicate that oriP might use the same cellular initiation factors that regulate chromosomal replication, and that EBNA1 may be involved in recruiting hsORC to oriP.

Epstein-Barr Virus Nuclear Antigen-1-Dependent and -Independent oriP-Binding Cellular Proteins

Objective: Epstein-Barr virus (EBV) nuclear antigen-1 (EBNA-1) and the replication origin, oriP, are essential for the replication and maintenance of latent EBV DNA in cells, but no enzymatic activity has been associated with EBNA-1 protein alone. In this study, we have searched for host cellular proteins that interact with EBNA-1 protein in various B cell lines latently infected with EBV, including a recently EBV growth-transformed cell line. Methods: By using gel shift analysis, we investigated the interactions of an oligonucleotide containing a single EBNA-1 recognition site, derived from the family of repeats (FR) element of oriP, with protein from cell extracts. Results: The FR oligonucleotide bound a (72-kD) cellular protein in the absence of EBNA-1 and without induction of the previously reported ‘anti-EBNA-1 proteins’. The FR oligonucleotide formed complexes with additional proteins from EBNA-1-synthesizing cell lines; these complexes were abolished or supershifted by anti-EBNA-1 monoclonal antibodies. SDS-PAGE analyses of ³⁵S-Met-labeled proteins that bound to a biotin- conjugated FR oligonucleotide, fractionated by a glycerol gradient centrifugation and affinity-purified with streptavidin, showed three major bands, a 72-kD protein, the FR binding of which seemed to be independent of EBNA-1, a 64-kD protein in both EBNA-1-transfected and latently EBV-infected cell lines, and a 45-kD protein in EBV-infected cell lines, which was most prominent in a recently EBV growth-transformed cell line. Conclusions: The FR element forms complexes with cellular proteins in the absence and presence of EBNA-1. These 72-, 64- and 45-kD cellular proteins might be involved in the function of the oriP and EBNA-1 system.

EBV replication enzymes.

The Epstein-Barr virus (EBV) can choose between two alternative life styles. It infects B lymphocytes, transforming them into lymphoblastoid lines and, in contrast to neurotropic herpesviruses such as herpes simplex virus type I that establish latency in nondividing neurons, must maintain its latent genomes in cells that have the potential to divide. In B lymphoblastoid cell lines established by EBV infection, the viral genome is maintained as covalently closed circular plasmids forming nucleosomal structures with histone proteins. The number of copies is maintained at 10–50 per cell to be duplicated once during each cell division cycle by the host cellular DNA replication machinery. When production of virus is induced, the circular genome becomes a ready template for amplification, generating thousands of copies per cell during lytic infection. Replication intermediates are head-to-tail concatamers, perhaps through a rolling-circle DNA replication, which are cleaved and packaged into infectious viral particles. The lytic phase of EBV DNA replication is dependent on seven viral replication proteins: BZLF1, BALF5, BMRF1, BALF2, BBLF4, BSLF1, and BBLF2/3 gene products. The BZLF1 protein is an oriLyt-binding protein and also acts as the lytic transactivator. The BALF5 gene encodes the DNA Pol catalytic subunit and the BMRF1 gene encodes the DNA Pol accessory subunit. They form a complex to act as the Pol holoenzyme. A single-stranded (ss)DNA-binding protein is encoded by the BALF2 gene.

De novo DNA methylation at nonrandom founder sites 5' from an unmethylated minimal origin of DNA replication in latent Epstein-Barr virus genomes.

Latent episomal genomes of Epstein-Barr virus, a human gammaherpesvirus, represent a suitable model system for studying replication and methylation of chromosomal DNA in mammals. We analyzed the methylation patterns of CpG dinucleotides in the latent origin of DNA replication of Epstein-Barr virus using automated fluorescent genomic sequencing of bisulfite-modified DNA samples. We observed that the minimal origin of DNA replication was unmethylated in 8 well-characterized human cell lines or clones carrying latent Epstein-Barr virus genomes as well as in a prototype virus producer marmoset cell line. This observation suggests that unmethylated DNA domains can function as initiation sites or zones of DNA replication in human cells. Furthermore, 5' from this unmethylated region we observed focal points of de novo DNA methylation in nonrandom positions in the majority of Burkitt's lymphoma cell lines and clones studied while the corresponding CpG dinucleotides in viral genomes carried by lymphoblastoid cell lines and marmoset cells were completely unmethylated. Clustering of highly methylated CpG dinucleotides suggests that de novo methylation of unmethylated double-stranded episomal viral genomes starts at discrete founder sites in vivo. This is the first comparative high-resolution methylation analysis of a latent viral origin of DNA replication in human cells.

Cellular transcription factors recruit viral replication proteins to activate the Epstein-Barr virus origin of lytic DNA replication, oriLyt.

DNA replication of Epstein-Barr virus (EBV) during the productive phase of the life cycle of this herpesvirus depends on the cis-acting element oriLyt. It consists of two essential domains, the upstream and the downstream component. Whereas the upstream component contains several DNA-binding motifs for the viral activator protein BZLF1, the downstream component is known to be the binding site of several cellular proteins. We identified cellular transcription factors that bind synergistically to a functionally relevant subsequence of the downstream component, the TD element. Two of these transcription factors, ZBP-89 and Sp1, stimulate replication as shown by protein fusions with the GAL4 DNA-binding domain and a single GAL4 DNA-binding motif inserted into the TD element. In protein binding assays, we observed an interaction of Sp1 and ZBP-89 with the viral DNA polymerase and its processivity factor. Our data indicate that cellular transcriptional activators tether viral replication proteins to the lytic origin via direct protein-protein interactions to assemble the viral replication complex at oriLyt.

The 2.2 Å structure of a permanganate-sensitive DNA site bound by the epstein-barr virus origin binding protein, EBNA1

Epstein-Barr nuclear antigen 1 (EBNA1) binds to four recognition sites in the minimal origin of latent DNA replication of Epstein-Barr virus and activates latent-phase replication of the viral genomes. Two of these EBNA1 binding sites become sensitive to permanganate oxidation when bound by the DNA binding and dimerization domains of EBNA1. We have previously solved the co-crystal structure of this EBNA1 fragment bound to a consensus recognition site that is not sensitive to permanganate oxidation (CS). To understand the structural difference that underlies the permanganate sensitivity of EBNA1 binding sites, we have now solved the crystal structure of the EBNA1 DNA-binding and dimerization domains bound to a permanganate-sensitive site (CSA/T). Comparisons of permanganate-sensitive and insensitive EBNA1-DNA complexes have revealed only minor differences in protein and DNA structures. In the EBNA1-CSA/T structure, interstrand H-bonds for three consecutive base-pairs centered over the permanganate-sensitive thymine base are lengthened relative to the corresponding bonds in the EBNA1-CS complex, and three potential intrastrand H-bonds were observed between adjacent bases. We also observed that both the CS and CSA/T sequences are overwound by EBNA1 in the vicinity of the permanganate-sensitive thymine base. Finally, we show that the permanganate-sensitive thymine base in the CSA/T-EBNA1 complex is more accessible to solvent than the corresponding T in the EBNA-CS complex.

Overexpression, purification and helix-destabilizing properties of Epstein-Barr virus ssDNA-binding protein.

The Epstein-Barr virus (EBV) ssDNA-binding protein (SSB) encoded by the BALF2 gene is one of the essential replication proteins in the lytic phase of EBV DNA replication. In order to obtain the amount of EBV SSB required for characterization, a recombinant baculovirus containing the complete sequence of the BALF2 open reading frame under the control of the baculovirus polyhedrin promoter was constructed. Insect cells infected with the recombinant virus produced a protein of 130 kDa, recognized by anti-BALF2 protein-specific polyclonal antibody. The overexpressed EBV SSB was purified homogeneously from the cytosolic fraction of the recombinant virus-infected cells. The purified protein displaced short DNA strands from their complementary sequences in the single-stranded form of M13. The helix-destabilizing activity was neutralized by the anti-BALF2 protein-specific antibody. Maximum unwinding occurred at EBV SSB concentrations exceeding saturation level of the DNA substrate. The DNA unwinding reaction mediated by the EBV SSB was highly cooperative and extremely rapid. The reaction displayed no directionality and required neither ATP nor MgCl2, two essential cofactors for DNA helicase activity. The helix-destabilizing property of the EBV SSB may function to melt out secondary structures on the ssDNA template, thereby facilitating the movement of the EBV DNA polymerase.

Late gene expression from the Epstein-Barr virus BcLF1 and BFRF3 promoters does not require DNA replication in cis.

Late gene expression follows and is dependent upon lytic replication of the viral genome. Although experimental evidence is lacking, lytic viral DNA replication is believed to remove modifications or binding factors from the genome which serve to repress late gene expression during latency or the early lytic cycle. We have developed a reporter assay to begin characterizing the mechanisms that regulate late gene expression in Epstein-Barr virus (EBV). In this model system, the activities of late promoter-reporter fusions are measured following transient transfection into tissue culture cells expressing EBV during different stages of the lytic cycle. This system faithfully recapitulates late expression patterns from the endogenous virus, implicating specific cis-active sequences in the control of late gene expression. In addition, these promoters respond only indirectly to the viral immediate-early transactivator, ZEBRA. This indirect response is mediated by other viral or virally induced activities downstream of ZEBRA in the lytic cascade. In this system, late gene expression is sensitive to inhibitors of the viral DNA polymerase such as phosphonoacetic acid, although the reporters lack a eukaryotic origin of replication and are not replicated under the assay conditions. Thus, replication of the transcriptional template is not a prerequisite for expression with late kinetics, a finding inconsistent with the current models which posit a cis-active relationship between lytic EBV DNA replication and late gene expression. Rather, analysis of this system has revealed a trans relationship between late gene expression and viral DNA replication and highlights the indirect and complex link between these two events.

Strand Displacement Associated DNA Synthesis Catalyzed by the Epstein-Barr Virus DNA Polymerase

The Epstein-Barr virus (EBV) DNA polymerase (Pol) holoenzyme is an essential enzyme required for ori-Lyt dependent EBV DNA replication. Using singly primed M13ssDNA circles as template, the EBV DNA Pol holoenzyme synthesized DNA chains greater than the unit length of M13 ssDNA in addition to full length products even at a low ratio of polymerase molecule per templates. The long replication products consisted of circular double-stranded DNA with single-stranded tails that were sensitive to mung bean nuclease. Reconstitution of the EBV Pol holoenzyme by preincubation of BALF5 Pol catalytic subunit and BMRF1 Pol accessory subunit in vitro resulted in reproduction of the strand displcement DNA synthesis. Thus, the EBV DNA Pol holoenzyme by itself is able to produce strand displacement coupled to the polymerization process in a highly processive way in the absence of any other protein.

Dissociation of EBV Genome Replication and Host Cell Proliferation in Anti-IgG-Stimulated Akata Cells

The Epstein–Barr virus (EBV)-positive Burkitt's lymphoma cell line, Akata, was treated with dimethyl sulfoxide (DMSO) for 96 hr in order to reversibly arrest cell cycle progression in G1 phase. Stimulation of the cells with anti-IgG antibody induced a marked and synchronous replication of EBV DNA within 12 hr, before the cells entered into S-phase after release from DMSO-induced arrest. Furthermore, a reduced efficiency of productive replication was demonstrated if anti-IgG stimulation was delayed after release. The results indicate that entry into S-phase of host cells is not only unnecessary for, but also may have negative consequences for the productive phase of EBV infection. Also, it was shown that addition of acyclovir, an inhibitor of the EBV-encoded DNA polymerase, to anti-IgG-stimulated Akata cells inhibited the productive replication of EBV DNA, but had no effect on the expression of early genes of the virus, including BZLF1, BRLF1, BMRF1, and BHRF1.

Epstein–Barr Virus Single-Stranded DNA-Binding Protein: Purification, Characterization, and Action on DNA Synthesis by the Viral DNA Polymerase

Epstein–Barr virus (EBV) BALF2 gene product is one of the essential components in the lytic phase of the EBV DNA replication. The BALF2 protein was purified to near homogeneity from the nuclear extract of B95-8 cells with virus productive cycle induced by chemical agents. SDS–polyacrylamide gel electrophoresis showed the presence of a single polypeptide with a molecular weight of 130 K, which was identified as BALF2 protein by Western immunoblot analysis. On Superose 6 HR 10/30 gel filtration the BALF2 protein eluted at a position corresponding to an apparent molecular mass of approximately 128 K, indicating that the BALF2 protein behaves as a monomer in solution. The purified BALF2 protein bound to single-stranded DNA preferentially over double-stranded DNA or single-stranded RNA. Replication of singly primed M13 single-stranded DNA by the EBV DNA polymerase complex in the absence of the BALF2 protein exhibited a highly processive mode of replication and generated full length products in addition to some bands of pausing sites. Although the addition of the BALF2 protein did not affect the replication rate, the average chain length of the replication products was slightly increased with eliminating bands of pausing sites. Similar effects were observed with the reconstituted polymerase complex composed of the BALF5 and BMRF1 Pol subunits. On the other hand, in the absence of the BALF2 protein, the BALF5 Pol catalytic subunit alone extended the primer slightly and paused at specific sites on M13 ssDNA template where stable secondary structure is predicted. However, addition of the BALF2 protein, in contrast to the case of herpes simplex virus ICP8 which does not affect the overall distribution of length of the replication products synthesized by the HSV Pol catalytic subunit (Gottliebet al.,1990,J. Virol.64, 5976–5987), stimulated DNA synthesis and yielded a distribution of replication products with long lengths in addition to full length products. Although the BALF2 protein behaved as if it converts a low processive enzyme of the EBV Pol catalytic subunit to a highly processive form like the BMRF1 Pol accessory subunit, challenger DNA experiments revealed that the EBV Pol catalytic subunit is transferred to challenger DNA even in the presence of the BALF2 protein. It is therefore likely that the EBV BALF2 protein functions to melt out the regions of secondary structure on the single-stranded DNA template, thereby reducing and eliminating pausing of the EBV DNA polymerase at specific sites. These properties indicate that the EBV BALF2 protein acts as a single-stranded DNA-binding protein during lytic phase of EBV DNA replication.

Some nucleoside analogs with anti-human immunodeficiency virus activity inhibit replication of Epstein-Barr virus

The effects of (+)-β- d-dioxolane-cytosine ((+)- d-β-DOC), (−)-β- l-dioxolane-cytosine ((−)- l-β-DOC), (+)-β- d-oxathiolane-cytosine ((+)- d-β-OTC), (−)-β- l-oxathiolane-cytosine ((−)- l-β-OTC, or 3TC), 3′-azido-2′,3′-dideoxy-5-methyl-cytidine (5-Me-AZDC), and 3′-azido-2′,3′-dideoxyuridine (AZDU) on Epstein-Barr virus (EBV) DNA replication in vitro were tested in P3HR-1 cells. Two anti-EBV drugs, 3′-azido-3′-deoxythymidine (AZT) and 9-(1,3-dihydroxy-2-propoxymethyl)guanine (DHPG, or ganciclovir), were used as positive controls. The inhibitory effects on EBV DNA synthesis were quantified by membrane filter and Southern blot hybridizations with an EBV-specific probe BamHI-W fragment. The 50% effective doses (ED 50) for EBV DNA replication were 0.15, 0.83, 1.5, 8.3, 14, and 7.7 μM for DHPG, (−)- l-β-DOC, (+)- d-β-DOC, (+)- d-β-OTC, (−)- l-β-OTC, and AZT, respectively. In contrast, 5-Me-AZDC and AZDU were not effective at concentrations as high as 30 μM. These results indicated that both (−)- l-β-DOC and (+)- d-β-DOC were more potent than AZT, which has previously been shown to have anti-EBV activity. (−)- l-β-DOC and (+)- d-β-DOC have also been previously demonstrated to suppress the infectivity of human immunodeficiency virus type 1 (HIV-1). Thus, (−)- l-β-DOC represents the first nucleoside analog with l-configuration exhibiting significant antiviral activities against both EBV and HIV.

Cellular proteins bind to the downstream component of the lytic origin of DNA replication of Epstein-Barr virus

The lytic origin of DNA replication of Epstein-Barr virus, oriLyt, is a complex eukaryotic origin which is activated during the lytic phase of the viral life cycle. It consists of at least two independent cis-acting components, one of which plays a dual role in transcription and DNA replication. The binding of the viral factor BZLF1, a member of the AP1 family of transcription factors, to this upstream component is crucial for oriLyt function (A. Schepers, D. Pich, and W. Hammerschmidt, EMBO J. 12:3921-3929, 1993). The second cis-acting element, the downstream component of oriLyt, is equally indispensable; however, its function is unknown. In this study, the downstream component was found to be the binding target of several cellular proteins. One could be identified as Sp1 or as a related protein which binds twice to the downstream component of oriLyt. Mutational analysis indicated that Sp1 alone is not directly involved in mediating DNA replication; however, other factors which share the same binding sequence or bind closely to one of the Sp1 binding sites are likely candidates to contribute to a replication protein complex at the downstream component of oriLyt. The sequence requirements for the downstream component are remarkably stringent, indicating that at least one of the putative factors is a sequence-specific DNA-binding protein which is required for the activation of oriLyt.

Nitric oxide inhibits Epstein-Barr virus DNA replication and activation of latent EBV.

Nitric oxide (NO), a mediator of biological functions, has antimicrobial activity against a variety of pathogens including viruses. Effects of NO donors on EBV replication in two EBV lytic systems, Raji cells infected with P3HR-1 virus and P3HR-1 cells activated with TPA plus n-butyrate, were studied. S-nitroso-N-acetylpenicillamine (SNAP), which generates NO when placed in an aqueous solution, and 3-morpholinosydnonimine (SIN-1), which liberates NO and O2-, resulting in the formation of peroxynitrite, were used as NO donors. Immunoprecipitation analysis showed that in superinfected Raji cells, SNAP inhibited EBV late protein synthesis but not EBV early protein expression. Analysis of the structure of EBV DNA termini demonstrated that SNAP suppressed the amplification of EBV DNA in superinfected Raji cells at a dose which did not affect synthesis of EBV early proteins required for EBV DNA replication. In TPA plus n-butyrate-treated P3HR-1 cells, SNAP inhibited synthesis of both early and late proteins of EBV. Northern blot analysis of RNA expressed in TPA plus n-butyrate-treated P3HR-1 cells demonstrated that expression of EBV immediate-early mRNAs coded from BZLF1 and BRLF1 genes was inhibited by SNAP. SIN-1 showed no or little effect on EBV replication in both cell systems. Cell viability and cellular protein synthesis were not affected by either NO donor under the conditions used. These findings suggest that NO prevents EBV replication by inhibiting EBV DNA amplification during the lytic phase of the life cycle as well as by blocking activation of the latent EBV genome. The mechanism for inhibiting of EBV replication by NO was discussed in relation to the role of NO in EBV latency in vivo.

A transcription factor with homology to the AP-1 family links RNA transcription and DNA replication in the lytic cycle of Epstein-Barr virus.

oriLyt, the lytic origin of DNA replication of Epstein-Barr virus (EBV), ensures viral DNA amplification during the productive or lytic phase of the virus' life cycle. To understand the contribution of cis- and transacting elements involved in DNA replication of oriLyt, a detailed mutational analysis was undertaken which defined BZLF1, a viral transcriptional activator, as an essential replication factor. The BZLF1 protein belongs to the extended fos/jun family of transcription factors and binds to specific BZLF1 binding motifs within oriLyt, as well as to consensus AP-1 sites. Recombinant, chimeric transcription factors identified the transcriptional activation domain of BZLF1 as being necessary to mediate DNA replication, a function which could not be substituted by any other transcription factor tested, including jun, E2, myc or VP16.

Epstein-Barr Virus (EBV) Replication and Expressions of EA-D (BMRF1 Gene Product), Virus-Specific Deoxyribonuclease, and DNA Polymerase in EBV-Activated Akata Cells

The replication of Epstein-Barr virus (EBV) and the expression of EBV early proteins were studied in the Burkitt's lymphoma cell line Akata stimulated with anti-human immunoglobulin G antibody (anti-IgG). Akata cells contained approximately 20 copies of EBV genome per cell as covalently closed, circular DNA. EBV DNA replication was observed at 6 hr and reached a maximal level at 24 hr after treatment with anti-IgG. Virion DNA was found in the culture medium at 12 hr. The kinetics of expression of BMRF1 gene product (early antigen diffuse component; EA-D) paralleled that of EBV deoxyribonuclease (DNase) and of DNA polymerase. Immunoblotting analysis showed that three polypeptides with molecular masses of 54, 52, and 49 kilodaltons (kDa) were recognized as EA-D components. The EBV DNase polypeptide was detected by immunoblotting at 53 kDa. The anti-EBV DNA polymerase antibody recognized 120- and 54-kDa polypeptides in the Akata cells. Immunoprecipitation followed by immunoblotting showed that EA-D and EBV DNase polypeptides were coimmunoprecipitated with anti-EBV DNase antibody and with anti-EA-D monoclonal antibody. These findings indicate that EA-D forms a complex with EBV DNase polypeptide. The molecules of EA-D, EBV DNase, and DNA polymerase appear to be closely associated together on the EBV replication.