Structure of defective DNA molecules in Epstein-Barr virus preparations from P3HR-1 cells

Epstein-Barr Virus Lytic Cycle Reactivation

Epstein-Barr virus, which mainly infects B cells and epithelial cells, has two modes of infection: latent and lytic. Epstein-Barr virus infection is predominantly latent; however, lytic infection is detected in healthy seropositive individuals and becomes more prominent in certain pathological conditions. Lytic infection is divided into several stages: early gene expression, DNA replication, late gene expression, assembly, and egress. This chapter summarizes the most recent progress made toward understanding the molecular mechanisms that regulate the different lytic stages leading to production of viral progeny. In addition, the chapter highlights the potential role of lytic infection in disease development and current attempts to purposely induce lytic infection as a therapeutic approach.

Essential role of Rta in lytic DNA replication of Epstein-Barr virus

Two transcription factors, ZEBRA and Rta, switch Epstein-Barr virus (EBV) from the latent to the lytic state. While ZEBRA also plays an obligatory role as an activator of replication, it is not known whether Rta is directly required for replication. Rta is dispensable for amplification of an oriLyt-containing plasmid in a transient-replication assay. Here, we assessed the requirement for Rta in activation of viral DNA synthesis from the endogenous viral genome, a function that has not been established. Initially, we searched for a ZEBRA mutant that supports viral replication but not transcription. We found that Z(S186A), a mutant of ZEBRA unable to activate transcription of Rta or viral genes encoding replication proteins, is competent to bind to oriLyt and to function as an origin recognition protein. Ectopic expression of the six components of the EBV lytic replication machinery failed to rescue replication by Z(S186A). However, addition of Rta to Z(S186A) and the mixture of replication factors activated viral replication and late gene expression. Deletion mutagenesis of Rta indicated that the C-terminal 10 amino acids (aa) were essential for the function of Rta in replication. In vivo DNA binding studies revealed that Rta interacted with the enhancer region of oriLyt. In addition, expression of Rta and Z(S186A) together, but not individually, activated synthesis of the BHLF1 transcript, a lytic transcript required for the process of viral DNA replication. Our findings demonstrate that Rta plays an indispensable role in the process of lytic DNA replication.

Epithelial cell retention of transcriptionally active, P3HR-1-derived heterogeneous Epstein-Barr virus DNA with concurrent loss of parental virus

Deleted, rearranged, heterogeneous (het) Epstein-Barr virus (EBV) DNA with the distinctive capability of disrupting EBV latency has been reported in biopsy samples of EBV-associated tumors whose onset in immunocompetent hosts is characteristically preceded by an antibody response indicative of EBV reactivation. Using the EBV P3HR-1 strain, we have reproduced in long-term culture of SVK epithelial cells an unusual pattern of infection previously observed in a subset of tumor biopsy samples: the persistence of het DNA in the absence of the parental helper virus. Fluorescence in situ hybridization (FISH) of infected cell subclones indicated the retention of het DNA in an integrated form. Incorporation of an intact het DNA molecule was confirmed by PCR, using primers that framed junctions of the four rearranged EBV DNA segments comprising P3HR-1-derived het DNA. Structural analysis of EBV terminal repeats revealed a banding pattern consistent with the integration of het DNA as a concatemer. Linkage of concatemeric monomers was defined at a nucleotide level, and that junctional sequence was detected in cell-free P3HR-1 virion DNA, confirming that subgenomic het DNA was packaged into infectious particles in a concatemeric configuration. Stable integration into cells having lost the standard viral genome allowed the unambiguous designation of het DNA as the source for viral gene products potentially encoded by both. Continuous expression of the latency-to-lytic switch protein Zta and detection of the BALF4 gene product gB, known to expand the target cell range of standard virus when incorporated at augmented levels into infectious progeny, add to a presumption of het DNA-enhanced pathogenesis in diseases of EBV reactivation.

Epstein-Barr virus WZhet DNA can induce lytic replication in epithelial cells in vitro, although WZhet is not detectable in many human tissues in vivo

OBJECTIVE

WZhet is a rearranged and partially deleted form of the Epstein-Barr virus (EBV) genome in which the BamH1W region becomes juxtaposed with and activates BZLF1, resulting in constitutive viral replication. We tested whether WZhet induces viral replication in epithelial cells, and we studied its prevalence in a wide range of lesional tissues arising in vivo.

METHODS

A quantitative real-time PCR assay targeting EBV WZhet DNA was developed to measure this recombinant form of the EBV genome.

RESULTS

WZhet DNA was undetectable in any of 324 plasma or paraffin-embedded tissue samples from patients with EBV-associated and EBV-negative disorders. These included specimens from patients with Hodgkin or non-Hodgkin lymphoma, post-transplant lymphoproliferation, nasopharyngeal or gastric adenocarcinoma, and infectious mononucleosis. However, WZhet DNA was detected in vitro in EBV-infected AGS gastric cancer cells. Additionally, transient transfection of infected AGS gastric cancer cells showed that viral replication could be induced by a WZhet plasmid.

CONCLUSION

This is the first evidence that WZhet induces the EBV lytic cycle in an epithelial cell line. Our negative findings in natural settings suggest that WZhet is a defective viral product that thrives in the absence of a host immune system but is rarely present in vivo.

Points of recombination in Epstein-Barr virus (EBV) strain P3HR-1-derived heterogeneous DNA as indexes to EBV DNA recombinogenic events in vivo

Deletions and rearrangements in the genome of Epstein-Barr virus (EBV) strain P3HR-1 generate subgenomic infectious particles that, unlike defective interfering particles in other viral systems, enhance rather than restrict EBV replication in vitro. Reports of comparable heterogeneous (het) DNA in EBV-linked human diseases, based on detection of an abnormal juxtaposition of EBV DNA fragments BamHI W and BamHI Z that disrupts viral latency, prompted us to determine at the nucleotide level all remaining recombination joints formed by the four constituent segments of P3HR-1-derived het DNA. Guided by endonuclease restriction maps, we chose PCR primer pairs that approximated and framed junctions creating the unique BamHI M/B1 and E/S fusion fragments. Sequencing of PCR products revealed points of recombination that lacked regions of extensive homology between constituent fragments. Identical recombination junctions were detected by PCR in EBV-positive salivary samples from human immunodeficiency virus-infected donors, although the W/Z rearrangement that induces EBV reactivation was frequently found in the absence of the other two. In vitro infection of lymphoid cells similarly indicated that not all three het DNA rearrangements need to reside on a composite molecule. These results connote a precision in the recombination process that dictates both composition and regulation of gene segments altered by genomic rearrangement. Moreover, the apparent frequency of het DNA at sites of EBV replication in vivo is consistent with a likely contribution to the pathogenesis of EBV reactivation.

De novo protein synthesis is required for lytic cycle reactivation of Epstein-Barr virus, but not Kaposi's sarcoma-associated herpesvirus, in response to histone deacetylase inhibitors and protein kinase C agonists

The oncogenic human gammaherpesviruses, Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV), are latent in cultured lymphoma cells. We asked whether reactivation from latency of either virus requires de novo protein synthesis. Using Northern blotting and quantitative reverse transcriptase PCR, we measured the kinetics of expression of the lytic cycle activator genes and determined whether abundance of mRNAs encoding these genes from either virus was reduced by treatment with cycloheximide (CHX), an inhibitor of protein synthesis. CHX blocked expression of mRNAs of EBV BZLF1 and BRLF1, the two EBV lytic cycle activator genes, when HH514-16 Burkitt lymphoma cells were treated with histone deacetylase (HDAC) inhibitors, sodium butyrate or trichostatin A, or a DNA methyltransferase inhibitor, 5-Aza-2'-deoxycytidine. CHX also inhibited EBV lytic cycle activation in B95-8 marmoset lymphoblastoid cells by phorbol ester phorbol-12-myristate-13-acetate (TPA). EBV lytic cycle induction became resistant to CHX between 4 and 6 h after application of the inducing stimulus. KSHV lytic cycle activation, as assessed by ORF50 mRNA expression, was rapidly induced by the HDAC inhibitors, sodium butyrate and trichostatin A, in HH-B2 primary effusion lymphoma cells. In HH-B2 cells, CHX did not inhibit, but enhanced, expression of the KSHV lytic cycle activator gene, ORF50. In BC-1, a primary effusion lymphoma cell line that is dually infected with EBV and KSHV, CHX blocked EBV BRLF1 lytic gene expression induced by TPA and sodium butyrate; KSHV ORF50 mRNA induced simultaneously in the same cells by the same inducing stimuli was resistant to CHX. The experiments show, for the cell lines and inducing agents studied, that the EBV BZLF1 and BRLF1 genes do not behave with "immediate-early" kinetics upon reactivation from latency. KSHV ORF50 is a true "immediate-early" gene. Our results indicate that the mechanism by which HDAC inhibitors and TPA induce lytic cycle gene expression of the two viruses differs and suggest that EBV but not KSHV requires one or more proteins to be newly synthesized between 4 and 6 h after application of an inducing stimulus.

Genome re-arrangements associated with loss of pathogenicity of the gamma-herpesvirus alcelaphine herpesvirus-1

The alcelaphine herpesvirus 1 (AlHV-1) causes malignant catarrhal fever in ruminants. Previous work had shown that serial passage of AlHV-1 in culture resulted in genome alterations that are associated with a loss in pathogenicity. Here we have analysed the re-arrangements that occur in more detail. None of the observed re-arrangements was entirely consistent. However, they did all involve translocation of a similar region of DNA from around the centre of the genome to areas either next to or in between terminal repeat elements at either end of the genome. There was also a concomitant loss of the wild-type locus. These re-arrangements appeared to be associated with the loss of virulence and the appearance of cell-free virus.

A rearranged form of Epstein-Barr virus DNA is associated with idiopathic pulmonary fibrosis

An association between idiopathic pulmonary fibrosis (IPF) and productive Epstein-Barr virus (EBV) infection has been found previously. Productive EBV replication can be associated with a rearrangement in EBV genomes termed WZhet. We hypothesized that WZhet genomes might be present in patients with IPF. Thirty-nine patients with IPF, 26 lung transplant recipients, and 24 normal subjects were studied. When EBV DNA-positive lung tissue biopsies from IPF patients were analyzed, 11 of 18 (61%) were positive for WZhet. Buffy coat DNA analysis showed that 75-85% were EBV DNA-positive in both IPF and control groups. Buffy coat analysis for WZhet was positive in 16 of 27 (59%) IPF patients, compared with none of 32 lung transplant recipients and 1 of 24 (4%) normal blood donors (p < or = 0.001). There was thus a good correlation between the presence of WZhet in lung tissue and peripheral blood. However, there was no significant association between the presence of WZhet and immunosuppressive therapy. These data further confirm the association between active EBV infection and IPF and provide a potential marker in the peripheral blood for the tracking of EBV in this disease.

A defective, rearranged Epstein-Barr virus genome in EBER-negative and EBER-positive Hodgkin's disease

A ubiquitous herpesvirus that establishes life-long infection, the Epstein-Barr virus (EBV) has yielded little insight into how a single agent in general accord with its host can produce diverse pathologies ranging from oral hairy leukoplakia to nasopharyngeal carcinoma, from infectious mononucleosis to Hodgkin's disease (HD) and Burkitt's lymphoma. Its pathogenesis is further confounded by the less than total association of virus with histologically similar tumors. In other viral systems, defective (interfering) viral genomes are known to modulate outcome of infection, with either ameliorating or intensifying effects on disease processes initiated by prototype strains. To ascertain whether defective EBV genomes are present in HD, we examined paraffin-embedded tissue from 56 HD cases whose EBV status was first determined by cytohybridization for nonpolyadenylated EBV RNAs (EBERs). Using both standard polymerase chain reaction (PCR) and PCR in situ hybridization, we successfully amplified sequences that span abnormally juxtaposed BamHI W and Z fragments characteristic of defective heterogeneous (het) EBV DNA from 10 of 32 (31%) EBER-positive tumors. Of 24 EBER-negative HD, 8 yielded PCR products indicating presence of het EBV DNA. Two of these contained defective EBV in the apparent absence of the prototype virus. Of the 42 tumors analyzed for defective EBV by both PCR techniques, there was concordance of results in 38 (90%). Detection of defective EBV genomes with the potential to disrupt viral gene regulation suggests one mechanism for pathogenic diversity that may also account for loss of prototypic EBV from individual tumor cells.

Molecular virology of Epstein-Barr virus

Epstein-Barr virus (EBV) interacts with its host in three distinct ways in a highly regulated fashion: (i) EBV infects human B lymphocytes and induces proliferation of the infected cells, (ii) it enters into a latent phase in vivo that follows the proliferative phase, and (iii) it can be reactivated giving rise to the production of infectious progeny for reinfection of cells of the same type or transmission of the virus to another individual. In healthy people, these processes take place simultaneously in different anatomical and functional compartments and are linked to each other in a highly dynamic steady-state equilibrium. The development of a genetic system has paved the way for the dissection of those processes at a molecular level that can be studied in vitro, i.e. B-cell immortalization and the lytic cycle leading to production of infectious progeny. Polymerase chain reaction analyses coupled to fluorescent-activated cell sorting has on the other hand allowed a descriptive analysis of the virus-host interaction in peripheral blood cells as well as in tonsillar B cells in vivo. This paper is aimed at compiling our present knowledge on the process of B-cell immortalization in vitro as well as in vivo latency, and attempts to integrate this knowledge into the framework of the viral life cycle in vivo.

The Epstein-Barr virus EBNA-2 gene in oral hairy leukoplakia: strain variation, genetic recombination, and transcriptional expression

Oral hairy leukoplakia (HLP) lesions frequently contain defective Epstein-Barr virus (EBV) genomes with deletions in the EBNA-2 gene that abundantly replicate and persist within the lesion. To characterize these viral strains and recombinant variants, the EBNA-2 gene in EBV DNA from several different HLP biopsy specimens was analyzed. Amplification of EBNA-2 coding sequences by PCR demonstrated the presence in HLP of intact EBNA-2 genes as well as a variety of internally deleted variants of both EBNA-2A and EBNA-2B. Some of the deletion variants evolved within the HLP lesion from intact EBNA-2 genes, while other variants appeared to be transmissible strains that directly infected the lesion. Intrastrain recombination within the HLP lesion also generated variation within the EBNA-2 polyproline region. Cloning and sequencing of HLP cDNA demonstrated transcription from the internally deleted EBNA-2 open reading frame, indicating that these variant genes are expressed in HLP. Comparative analysis of the HLP EBNA-2 sequences confirmed previous findings of EBV coinfection with multiple types and strains. Sequence variation of these wild-type genes demonstrated that EBNA-2A sequences distinguish at least two separate strains and a variety of substrains of EBV type 1. Two of the HLP EBNA-2A sequences contained amino acid changes in a cytotoxic T-cell epitope within an otherwise highly conserved region of the gene. These data indicate that EBV coinfection, strain variation, and recombination within the EBNA-2 gene are common features of HLP and suggest that the expression of internally deleted EBNA-2 variants could contribute to EBV pathogenesis in permissive infection.

Epstein-Barr virus intrastrain recombination in oral hairy leukoplakia

In laboratory lymphoblastoid cell lines and in natural human infections, Epstein-Barr virus (EBV) strains have been identified by DNA restriction fragment length polymorphisms of the BamHI H fragment. Multiple, heterogeneous BamHI H fragments have been detected in oral hairy leukoplakia (HLP), raising the question of EBV coinfection with multiple strains. To investigate whether the heterogeneous BamHI H fragments represent different EBV strains or recombinant variants of the same strain, EBV DNA from HLP lesions was analyzed to characterize the viral strains and determine the source of possible recombinant variants. Clones of heterogeneous BamHI H fragments from a single HLP lesion were determined to have strain identity on the basis of sequence identity of the EBNA-2 genes. Intrastrain homologous recombination within the IR2 internal repeat region and nonhomologous recombination of other sequences accounted for the heterogeneity of the BamHI H fragments. PCR amplification from additional HLP specimens detected similar recombinant variants. A possible example of site-specific recombination joining the BamHI Y portion of the EBNA-2 gene to sequences within the BamHI S fragment was also detected in multiple HLP specimens. These data indicate that intrastrain recombination during productive replication confounds the use of restriction fragment length polymorphism analysis of the BamHI Y and H fragments to identify EBV strains in HLP. In patients with permissive epithelial EBV infections, EBV strains could be more accurately distinguished by sequence identity or divergence within known regions of genetic strain variation.

Marked variation in the size of genomic plasmids among members of a family of related Epstein-Barr viruses

Epstein-Barr virus (EBV) genomes in the P3J-HR-1 (HR-1) Burkitt lymphoma cell line rearrange at a high rate. Previously described deletions and rearrangements in HR-1 cells have been found at sites of EBV replication in vivo, suggesting that DNA rearrangement may be an integral aspect of EBV biology and pathogenesis. We examined the structure of linear EBV genomes in subcultures of HR-1 cells using contour-clamped homogenous electric field gel electrophoresis. We developed a second pulsed electrophoretic technique to separate intracellular circular EBV plasmids. The standard, linear HR-1 EBV genome was approximately 155 kilobases in length. Linear molecules of less than unit length, presumably defective genomes, were seen in numerous subcultures. Linear intracellular genomes greater than 155 kilobases were also detected, but only linear genomes of 155 kilobases or less were packaged into virions. The size of circular EBV plasmids also varied greatly among HR-1 subcultures, some of which contained two plasmids of different size. The progeny of the unusual circular plasmids could be either standard or nonstandard linear genomes. No aberrant linear or circular form was detected in a subculture carrying the previously described het fragments. Pulsed-gel electrophoresis has provided two additional characteristics of mutant EBVs: abnormal linear and circular genome configurations.

Efficient transcription of the Epstein-Barr virus immediate-early BZLF1 and BRLF1 genes requires protein synthesis

The Epstein-Barr virus BRLF1 and BZLF1 genes appear to be the first viral genes transcribed upon induction of the Epstein-Barr virus lytic cycle. Both gene products activate transcription of other viral genes, thereby initiating the lytic cascade. Among the viral antigens expressed upon induction of the lytic cycle, the product of the BZLF1 gene is unique in its ability to disrupt viral latency; thus, expression of this gene is both necessary and sufficient for triggering the viral lytic cascade. Moreover, transcription initiation from both the BRLF1 and BZLF1 promoters can be activated by the BZLF1 gene product. The latter results suggest a two-step model for induction of the viral lytic cycle in which the initial signal leads to low-level transcription of the BZLF1 gene, followed by upregulation of transcription by the BZLF1 gene product. In this report we demonstrate that efficient transcription from the BRLF1 and BZLF1 promoters after anti-immunoglobulin induction of the lytic cycle, in a synchronous induction system, is dependent on de novo protein synthesis. These data support the two-step induction model in which synthesis of BZLF1 protein is required to activate expression of the BRLF1 and BZLF1 genes.

The zta transactivator involved in induction of lytic cycle gene expression in Epstein-Barr virus-infected lymphocytes binds to both AP-1 and ZRE sites in target promoter and enhancer regions

The BZLF1 or zta immediate-early gene of Epstein-Barr virus (EBV) encodes a 33-kilodalton phosphorylated nuclear protein that is a specific transcriptional activator of the EBV lytic cycle when introduced into latently infected B lymphocytes. We have shown previously that the divergent EBV DSL target promoter contains two zta-response regions, one within the minimal promoter and the other in an upstream lymphocyte-dependent enhancer region. In this study, we used footprinting and gel mobility retardation assays to reveal that bacterially synthesized Zta fusion proteins bound directly to six TGTGCAA-like motifs within DSL. Four of the Zta-binding sites lay adjacent to cellular TATA and CAAT factor-binding sites within the minimal promoter, and two mapped within the enhancer region. Single-copy oligonucleotides containing these Zta-binding sites conferred Zta responsiveness to heterologous promoters. In addition, the Zta protein, which possesses a similar basic domain to the conserved DNA-binding region of the c-Fos, c-Jun, GCN4, and CREB protein family, proved to bind directly to the consensus AP-1 site in the collagenase 12-O-tetradecanoylphorbol-13-acetate response element. Cotransfection with zta also trans activated a target reporter gene containing inserted wild-type 12-O-tetradecanoylphorbol-13-acetate response element oligonucleotides. Cellular AP-1 binding activity proved to be low in latently EBV-infected Raji cells but was induced (together with the Zta protein) after activation of the lytic cycle with 12-O-tetradecanoylphorbol-13-acetate. We conclude that EBV may have captured and modified a cellular gene encoding a c-jun-like DNA-binding protein during its evolutionary divergence from other herpesviruses and that this protein is used to specifically redirect transcriptional activity toward expression of EBV lytic-cycle genes in infected cells.

An enhancer within the divergent promoter of Epstein-Barr virus responds synergistically to the R and Z transactivators

The EA-R and NotI repeat genes of Epstein-Barr virus (EBV) are oriented head to head and separated by a 1,000-base-pair (bp) divergent promoter region. We have identified functional domains within this divergent promoter which are important for regulation of the rightward EA-R gene. Both the R transactivator (Rta) and the Z transactivator (Zta) increase the abundance of correctly initiated EA-R transcripts. A 258-bp fragment (-114 to -372 from the EA-R cap site) contained the primary Rta and Zta response elements and was capable of transferring Rta and Zta activity to a heterologous promoter in an orientation- and position-independent manner. Rta activated this 258-bp enhancer region in both EBV-positive and EBV-negative cells. However, Zta activity appeared to be dependent on another EBV gene product, since Zta activated the enhancer efficiently (500- to 2,000-fold) in EBV-positive cells but had little or no activity in EBV-negative cells. The combination of Rta and Zta produced a striking synergistic effect on the enhancer in the absence of any additional EBV components, suggesting that the interaction between Zta and Rta accounts for the Zta response observed in EBV-positive cells. An Rta response element was mapped to a domain located 60 bp away from a Zta-binding site within the enhancer. Although Rta activated the enhancer and other early promoters without additional EBV- or B-cell-specific factors, it did not activate the lytic cycle of EBV, in contrast to Zta. Immunofluorescence patterns of Rta and Zta with antipeptide antisera indicated that they have overlapping but different subcellular localizations. Both transactivators were found in the nucleus, but Rta was also found in the cytoplasm.

Defective viral DNA in Epstein-Barr virus-associated oral hairy leukoplakia

Defective Epstein-Barr virus (EBV) has a deleted and rearranged genome (termed het DNA) that disrupts latency and induces standard EBV to replicate in vitro. We used the polymerase chain reaction to detect, in 2 of 10 patient samples, the junction of abnormally juxtaposed EBV DNA fragments BamHI W and Z, a genomic rearrangement responsible for the biologic activity of het DNA. By sequence analysis, the junction in wild-type defective DNA appears to be similar but not identical to the recombination in the DNA of laboratory strain P3HR-1. The presence of this marker for het DNA in the epithelial lesions of two patients suggests a role for defective EBV in a human pathologic process.

Expression of the BZLF1 latency-disrupting gene differs in standard and defective Epstein-Barr viruses

Previous experiments using gene transfer of plasmids with heterologous promoters identified an Epstein-Barr virus (EBV) gene (BZLF1) whose product (ZEBRA) switches the virus from a latent to a replicative state. We have now studied expression of ZEBRA in lymphoid cells harboring either standard virus or a mixture of standard and defective (heterogeneous [het]) viruses. A high-titer rabbit antiserum to a TrpE-BZLF1 fusion protein was used to identify ZEBRA expressed from standard and het EBV DNA. These ZEBRA proteins could be distinguished from each other on the basis of their electrophoretic mobilities. ZEBRA could not be detected in cells latently infected with standard EBV. However, within 6 h after induction of replication by sodium butyrate, ZEBRA appeared and persisted long thereafter. Synthesis of ZEBRA was insensitive to phosphonoacetic acid or acycloguanosine, behavior characteristic of an early replicative protein. ZEBRA was constitutively expressed in cells containing both defective and standard EBV genomes. ZEBRA was made predominantly from the het genome but also from the standard genome. Control of BZLF1 expression appears to occur at the transcriptional level. No BZLF1-specific transcript was detected in cells containing only standard latent EBV. BZLF1 transcripts could be detected in these cells if virus replication was induced by treatment with butyrate. Cells bearing both standard and het genomes did not require addition of an exogenous inducing agent to transcribe the BZLF1 gene. The experiments suggest that regulation of transcription of the BZLF1 gene is a pivotal event in the control of EBV replication.

The Epstein-Barr virus (EBV) DR enhancer contains two functionally different domains: domain A is constitutive and cell specific, domain B is transactivated by the EBV early protein R

The Epstein-Barr virus (EBV) DR promoter is located upstream of the PstI repeats, and besides the TATA box, it contains two cis-acting regulatory elements. One of them has enhancer properties. To define more precisely the functional region(s) in the DR enhancer, we generated 5' and 3' deletion mutants. These deletion mutants, which were transfected into various recipient cells of different origins, allowed us to identify two functionally distinct domains, A and B. Domain A was constitutively active in all cell lines tested, except in lymphoid B cells. Domain B was active in lymphoid B cells, and its activity required both EB1 (the BZLF1-encoded EBV trans-acting factor) and the presence of the EBV genome. This suggested that an EBV-encoded, EB1-inducible factor was activating the enhancer B domain. In effect, the B domain was trans-activated by R, an EBV early product encoded by the open reading frame BRLF1, and the activation by R occurred in epithelial, fibroblastic, and lymphoid cells. The R-responsive element has been reduced to 28 base pairs containing the double palindromic sequence TTGTCCCGTGGACAATGTCC. Both domains A and B act by increasing the initiation of specific RNAs.

Genome rearrangements activate the Epstein-Barr virus gene whose product disrupts latency

A defective Epstein-Barr virus (EBV) containing a deleted and rearranged genome (het DNA) causes latent EBV to replicate. This activity maps to the 2.7-kilobase-pair WZhet fragment. The BZLF1 open reading frame, present within WZhet as well as in the standard viral BamHI Z fragment, encodes the protein ZEBRA, which induces viral replication. Using gene transfers into Burkitt lymphoma cells, we now demonstrate that rearranged sequences juxtaposed to BZLF1 in het DNA facilitate expression of ZEBRA protein. Two stretches of EBV sequences within a palindromic region of het DNA contain positive regulatory elements. One set, derived from the viral large internal repeat, is newly positioned upstream of BZLF1; the second set is downstream of BZLF1 in het DNA. The capacity of defective HR-1 viruses to disrupt latency of the standard EBV genome is due to abnormal regulation of the BZLF1 gene as a result of genomic rearrangements.

Identification and characterization of oriLyt, a lytic origin of DNA replication of Epstein-Barr virus

We have identified a cis-acting element of Epstein-Barr virus (EBV) that mediates viral DNA replication during the lytic phase of this virus's life cycle. This lytic origin of DNA replication, termed oriLyt, is complex in structure in that it contains multiple regions that are required for replication and additional DNA sequences that increase replication. One of the required regions of oriLyt can be functionally substituted by a transcriptional enhancing element. DNA replication mediated by oriLyt depends on EBV DNA polymerase and yields a concatemeric molecule. A vector, which contains both oriP (the EBV plasmid origin of replication) and oriLyt, can be maintained as a plasmid in latently EBV-infected cells and can be amplified 100- to 1000-fold in cells in which the lytic phase of the viral life cycle is induced.

Amplification of Epstein-Barr virus (EBV) DNA by superinfection with a strain of EBV derived from nasopharyngeal carcinoma

Epstein-Barr virus (EBV) from a nasopharyngeal carcinoma (NPC) hybrid cell line (NPC-KT) lacking defective viral DNA molecules superinfected Raji cells and induced EBV early antigens (EA), as did virus from P3HR-1 cells, which contained defective molecules. The EBV polypeptides induced by NPC-KT appeared to be identical to those induced by P3HR-1 virus. The ability of NPC-KT virus to induce EA was enhanced more than 10-fold by treatment of superinfected cells with dimethyl sulfoxide; however, dimethyl sulfoxide treatment did not enhance superinfection by P3HR-1 virus. After infection, DNA synthesis of both the superinfecting NPC-KT virus and the resident Raji viral genome was induced. In addition to amplified Raji EBV episomal DNA, a fused terminal fragment of NPC-KT viral DNA was detected. The detection of fused terminal DNA fragments suggests that the superinfecting virion DNA either circularizes or polymerizes after superinfection and is possibly amplified through circular or concatenated replicative intermediates.

Polymorphisms of the region of the Epstein-Barr virus genome which disrupts latency

The nucleotide sequence of the portion of naturally occurring defective EBV (HR-1) DNA (designated het DNA) which is responsible for disruption of EBV latency has been determined and compared with the regions of the standard HR-1 viral genome from which it was derived. This rearranged 2.7-kbp DNA fragment represented an apparently nonhomologous recombination between sequences found in the BamHI W and BamHI Z fragments of the standard HR-1 genome. Only one intact open reading frame, comparable to standard HR-1 BZLF1, was present within this 2.7 kbp. No new open reading frames were created by the recombination. The BZLF1 sequence and predicted polypeptide products of standard HR-1 and het DNA were compared to B95-8 EBV. If an unspliced version of BZLF1 is used, the carboxy end of the BZLF1 polypeptides would differ considerably, principally due to an identical 28-bp insertion in both standard HR-1 and het BZLF1 relative to B95-8. However, if both virus strains use the same mRNA splicing strategy, the BZLF1 products from HR-1 and B95-8 would be similar, though distinguished by seventeen 1-bp differences which would result in nine amino acid changes. Both unspliced and spliced versions of het BZLF1 had five amino acid changes by comparison to standard HR-1 BZLF1. Differences in predicted secondary structure were found, consistent with dissimilar electrophoretic mobility of the polypeptide products. The amino acid differences between the BZLF1 polypeptide products of HR-1, het DNA, and B95-8 virus are all compatible with the creation of a protein which would function similarly to disrupt EBV latency. The differences in these polypeptides may account for some of the variation in the level of biologic activity of the BZLF1 products of different EBV strains. However, the major difference in activity between standard HR-1 and HR-1 het virus to disrupt EBV latency appears to be due to up-regulation of expression of het BZLF1 due to juxtaposition of BamHI W sequences upstream of het BZLF1.

Epstein-Barr virus gene expression in P3HR1-superinfected Raji cells

The pattern of Epstein-Barr virus (EBV) RNAs expressed in Raji cells superinfected with P3HR1 EBV was examined. RNAs whose expression was of an immediate-early type (resistant to treatment of the cells with anisomycin) were identified. These RNAs, encoding the EBV reading frames BZLF1 and BRLF1, were probably expressed from defective virus within the P3HR1 preparation, and some of them were responsible for the induction of the EBV productive cycle in the Raji cells. The structures of the B95-8 RNAs equivalent to the anisomycin-resistant RNAs were determined. The RNA encoding the BZLF1 reading frame contained two splices which extended and modified the reading frame from that previously described.

Biomolecular analysis of a defective nontransforming Epstein-Barr virus (EBV) from a patient with chronic active EBV infection

A virus recovered from the saliva of a child with chronic active Epstein-Barr virus (EBV) infection for 8 years was shown to induce EBV early antigen (EBV-EA) in Raji cells and to be expressed into EBV-EA in fresh EBV-negative peripheral blood leukocytes. However, it did not replicate its DNA. Oropharyngeal epithelial cells scraped from recurrent mouth lesions were similarly positive for EBV-EA. DNA extracted from these cells and digested with BamHI contained a 6-kilobase-pair fragment homologous to BamHI fragment V and B1 EBV DNA probes. Furthermore, Southern blots of the BamHI and EcoRI digests of the DNA extracted from the cell lines of the patient (transformed with EBV strain B95-8) and of her mother (spontaneous) revealed, in addition to the expected BamHI G, H, H2, and B1 fragments used as probes, additional shorter ones of a presumably endogenous defective virus.

Two strains of Epstein-Barr virus (B95-8 and a P3HR-1 subclone) that lack defective genomes induce early antigen and cause abortive infection of Raji cells

The heterogeneity of Epstein-Barr virus (EBV) obtained from P3HR-1 cells has permitted derivation of a distinct subclone of P3HR-1 (L. Heston, M. Rabson, N. Brown, and G. Miller, Nature (London) 295:160-163, 1982). We have analyzed the biologic properties and genomic structure of this subclonal virus (clone 13) compared with those of parental P3HR-1 and B95-8 viruses. Synthesis of EBV compared with those of parental P3HR-1 and B95-8 viruses. Synthesis of EBV proteins in Raji cells superinfected with virus derived from P3HR-1, clone 13, and B95-8 was analyzed both by fluorography of radiolabeled proteins and by immunoblotting. Highly concentrated preparations of clone 13 and B95-8 virus induced most of the spectrum of EBV proteins in Raji cells with the exception of the 145,000-, 140,000-, and 110,000-molecular-weight proteins, which were either undetectable or reduced. Moreover, both clone 13 and B95-8 viruses also induced the same patterns of early antigen diffuse components as the parental P3HR-1 virus did. However, only P3HR-1 virus could induce EBV DNA synthesis in superinfected Raji cells, as determined both by buoyant density centrifugation and by in situ cytohybridization with biotinylated recombinant EBV DNA probes. Defective heterogeneous molecules present in P3HR-1 virus have been implicated in early antigen induction after superinfection of Raji cells. Therefore, Southern blots of clone 13, P3HR-1, and B95-8 viruses were hybridized to recombinant EBV fragments representing the sequences contained within the defective molecules in P3HR-1. The parental P3HR-1 contained the previously described defective molecules. No evidence for defective molecules was found in clone 13 or B95-8 viruses. These data indicate that concentrated preparations of both clone 13 and B95-8 viruses can induce abortive infection in Raji cells, but while the defective molecules are not needed for induction of early antigen diffuse components, they may be required for the induction of viral DNA synthesis.

Sequences of the Epstein-Barr Virus (EBV) large internal repeat form the center of a 16-kilobase-pair palindrome of EBV (P3HR-1) heterogeneous DNA

We have previously characterized several genomic rearrangements of Epstein-Barr virus (EBV) DNA contained in one of the defective EBV genomes harbored by the P3HR-1 (HR-1) line (H. B. Jenson, M. S. Rabson, and G. Miller, J. Virol. 58:475-486, 1986). One recombinant clone of heterogeneous DNA (het DNA) from this defective genome is an EcoRI fragment of 16 kilobase pairs (kbp) which is a palindrome. DNA digestion fragments specific for the center of this palindrome were present in cells which contained het DNA but not in cells which lacked het DNA. Thus, the palindrome was not an artifact of DNA cloning. The organization of the center of this palindrome was studied by DNA sequencing. The comparable region of the parental HR-1 genome was also studied by DNA sequencing. The central 3,495 base pairs (bp) of the palindrome were composed of sequences derived exclusively from internal repeat 1 of EBV, represented by BamHI W fragment. At each end of the central 3,495 hp was a symmetrical recombination with sequences of BamHI-Z, located more than 50 kbp away on the standard EBV genome. The central 3,495 bp were composed of an unduplicated 341 bp flanked by two perfect palindromic repeats of 1,577 bp. The 341-bp unique region was a portion of a 387-bp region of standard HR-1 BamHI-W which was identical to the central 387 bp of the palindrome. This central 387-bp region contained numerous stretches of dyad symmetry capable of forming a large stem-and-loop structure. The palindromic rearrangement had created two novel open reading frames in het DNA derived from standard HR-1 BamHI-W sequences. These two het DNA open reading frames had different amino termini but identical carboxy termini derived from the large open reading frame in standard HR-1 BamHI-W (HR-1 BWRF1). The BamHI-W sequences found in het DNA did not include either the TATA box of standard HR-1 BamHI-W or the exons which are present in the potentially polycistronic latent mRNAs encoding EBV nuclear antigens. These marked alterations in genomic structure may relate to the unique biologic properties of virus stocks containing het DNA by creation of new polypeptides or by formation or deletion of regulatory or functional signals.

Strain-specific transcription and translation of the BamHI Z area of Epstein-Barr Virus

The expression of the 1,800-base-pair BamHI Z region of Epstein-Barr virus DNA was analyzed by hybrid-selected translation with several DNA subclones and RNA from different cell lines. Furthermore, large segments of the three reading frames extending in this area were expressed as fusion proteins into Escherichia coli. The fusion proteins were partially purified and used to immunize rabbits. These sera were used to confirm our mapping assignments and to identify the respective posttranslationally modified proteins in in vivo labeling experiments. The reading frame BRLF1 (the first reading frame starting in the BamHI R fragment in leftward orientation) encoded a 93- to 96-kilodalton (kDa) protein depending on the cell line. The molecular weight of in vivo-labeled proteins was increased relative to that of in vitro-translated proteins, indicating that a posttranslational modification had occurred. The BZLF1 reading frame encoded a 35-kDa protein. It was posttranslationally cleaved from a 38-kDa precursor in induced B95-8 and induced Raji cells and from a 40-kDa precursor in induced P3HR1 cells. In Raji cells superinfected with virus derived from P3HR1 cells, the protein seemed to be expressed both from endogenous Raji genomes and from infecting genomes. The transcripts for the 93- to 96-kDa and the 35-kDa protein overlapped partially. The serum against the expressed third reading frame BZLF2 specifically precipitated a 140-kDa protein. This reading frame contains only 650 nucleotides, and therefore further coding sequences were presumably spliced to BZLF2. The latter is deleted in the Raji cell line; therefore, the observed 140kDa protein in superinfected Raji cells was expressed from infecting P3HR1 genomes.

Promiscuous trans activation of gene expression by an Epstein-Barr virus-encoded early nuclear protein

We identified an Epstein-Barr virus (EBV) gene product which functions in transient-expression assays as a nonspecific trans activator. In Vero cells, cotransfection of the BglII J DNA fragment of EBV together with recombinant constructs containing the bacterial chloramphenicol acetyltransferase (CAT) gene gave up to a 100-fold increased expression of CAT activity over that in cells transfected with the recombinant CAT constructs alone. The BglII J fragment acted promiscuously, in that increased CAT synthesis was observed regardless of whether the promoter sequences driving the CAT gene were of EBV, simian virus 40, adenovirus, or herpes simplex virus origin. Cleavage of cloned BglII-J plasmid DNA before transfection revealed that activation was dependent upon the presence of an intact BMLF1 open reading frame. This was confirmed with subclones of BglII-J and with hybrid promoter-open reading frame constructs. This region of the genome is also present in the rearranged P3HR-1-defective DNA species, and defective DNA clones containing these sequences produced a similar activation of CAT expression in cotransfection experiments. The heterogeneous 45-60-kilodalton polypeptide product of BMLF1 may play an important regulatory role in expression of lytic-cycle proteins in EBV-infected lymphocytes.

Palindromic structure and polypeptide expression of 36 kilobase pairs of heterogeneous Epstein-Barr virus (P3HR-1) DNA

Among the Epstein-Barr virions (EBV) produced by the P3HR-1 (HR-1) cell line are a defective subpopulation with rearranged viral DNA designated heterogeneous DNA (het DNA). These defective virions are responsible for the capacity of HR-1 virus to induce early antigen in Raji c cells and for trans activation of latent EBV in X50-7 cells. Virions with het DNA are independent replicons which pass horizontally from cell to cell rather than being partitioned vertically. We analyzed the structure and defined several polypeptide products of het DNA to understand these remarkable biologic properties. A 36-kilobase-pair (kbp) stretch of het DNA was cloned (as two EcoRI fragments of 20 and 16 kbp) from virions released from a cellular subclone of HR-1 cells. The unusual aspect of the 20-kbp fragment was the linkage of sequences of BamHI-M and BamHI-B', which are not adjacent on the standard EBV genome. The 16-kbp fragment was a palindrome in which at least two additional recombinations on each side of the palindrome had linked regions of the standard EBV genome which are not normally contiguous. The 20-kbp het DNA fragment was attached to at least one and possibly both ends of the 16-kbp het DNA fragment. We identified antigenic polypeptides produced in COS-1 cells after gene transfer of various cloned het DNA fragments. The 20-kbp fragment encoded a cytoplasmic antigen of about 95 kilodaltons (kDa). The 16-kbp fragment encoded antigens located in the nucleus, nuclear membrane, and cytoplasm. These were represented by several polypeptides, the most prominent of which were about 55, 52, and 36 kDa. The 36-kDa polypeptide was localized to a 2.7-kbp BamHI fragment which had homology to standard BamHI-W and BamHI-Z. Another polypeptide of 50 kDa found in the nucleus was mapped to the 7.1-kbp BamHI het DNA fragment which spans the EcoRI site linking the 20- and 16-kbp fragments of het DNA. Thus, HR-1 het DNA encodes several discrete polypeptide products, one or more of which could be responsible for the unusual biologic properties of the virus. The composition, regulation, and ultimately the expression of some of these products relative to standard EBV is probably altered by the genomic rearrangements of het DNA.

Mapping of genes in BamHI fragment M of Epstein-Barr virus DNA that may determine the fate of viral infection

We used nuclease digestion to map RNA transcripts encoded in the BamHI M fragment of the Epstein-Barr virus (EBV) genome (strain B95-8). Of the five RNAs, three are rightwardly transcribed, have different cap sites but common 3' termini, and are unspliced. The two remaining RNAs are leftwardly transcribed and are 5' and 3' coterminal. One of these transcripts is spliced, resulting in the removal of a small intron from the 5' region of this RNA. We have previously published data which indicated that the BamHI M region is the first actively transcribed region of the viral genome during the replicative cycle, suggesting that one or more genes in this region is important in the initiation of EBV replication. We have now mapped two large EcoRI restriction fragments which span approximately 75% of the P3HR-1 defective genome and which contain DNA from the BamHI M region of the standard genome. The data indicate that only the coding and 5' flanking sequences for the leftwardly transcribed RNAs are intact within the defective genome. Fewer than 500 bases coding for the 3'-most regions of the rightwardly transcribed RNAs are intact, and it is unlikely that these encode functional native polypeptides. Therefore, it seems that transcriptional activation of the BamHI M-region genes is not mediated directly by the rearrangement of M genes in defective P3HR-1 EBV.

Localization of the coding region for an Epstein-Barr virus early antigen and inducible expression of this 60-kilodalton nuclear protein in transfected fibroblast cell lines

Expression of a component of the Epstein-Barr virus early antigen (EA) complex has been studied in fibroblast cells transfected with both wild-type and P3HR-1 defective DNA fragments covering the BamHI-M-S region of the Epstein-Barr virus genome. Baby hamster kidney (BHK) cells transfected with the BglII-J fragment and stained with human serum that was positive for the diffuse component of EA [EA(D)] in an indirect immunofluorescence assay exhibited positive nuclear staining in 5% of the cell population. Cleavage of BglII-J before transfection with the restriction enzyme BglII, StuI, HindIII, or PvuII did not affect EA expression, whereas prior cleavage with BamHI or EcoRI reduced or eliminated synthesis of EA. These observations were confirmed by using individual cloned subfragments. A Bal 31 deletion clone (pTS1) in which the HindIII and StuI sites were eliminated retained activity, whereas a clone (pTS5) in which the deletion extended closer to the EcoRI site had greatly reduced activity. Transfection of the individual BamHI-M or BamHI-S fragments, which span BglII-J, also resulted in little or no EA expression. The 2.1-kilobase biologically active region defined by these experiments corresponds precisely to the BMLF1 open reading frame. Immunoblot analyses of BHK cells transfected with either P3HR-1 defective DNA clones or the BglII-J wild-type fragment identified the product of this EA(D) coding region as a family of polypeptides consisting of a major 60-kilodalton product and minor 45- and 50-kilodalton species. In latently Epstein-Barr virus-infected lymphocytes these early antigens are not expressed, but can be induced by treatment of the cultures with sodium butyrate or phorbol esters. Using the BglII-J and pTS6 clones that were positive in transient assays, we also established Neor coselected BHK and Vero cell lines which showed similar regulated expression of the 60-kilodalton EA(D) protein. In these cell lines constitutive expression of EA(D) was limited (0.1% positive by indirect immunofluorescence and undetectable by immunoblot analysis). However, expression of EA(D) could be induced by treatment with sodium butyrate. In the induced cultures, up to 30% of the cells were EA(D) positive by immunofluorescence, and there was a concomitant appearance of the 60-kilodalton EA(D) polypeptide.

A second Epstein-Barr virus early antigen gene in BamHI fragment M encodes a 48- to 50-kilodalton nuclear protein

We used antiserum raised against the bacterially synthesized product of one of the open reading frames in Epstein-Barr virus (EBV) BamHI fragment M to demonstrate that this reading frame (BMRF1) codes for a nuclear protein of the diffuse early antigen (EA) class. In indirect immunofluorescence assays, the rabbit anti-BMRF1 antiserum gave nuclear staining in approximately 5% of Raji cells which had been treated with sodium butyrate, and positive fluorescence was observed in both acetone- and methanol-fixed cells. Uninduced Raji cultures contained less than 0.1% positive cells regardless of whether indirect immunofluorescence or anti-complement immunofluorescence was used. In immunoblot analyses, the rabbit serum identified a family of polypeptides of 46 to 55 kilodaltons (kDa) in total protein extracts from B95-8 cells or from butyrate-induced Raji cells. In both cell types, the dominant polypeptides were the 48- and 50-kDa species. This same family of polypeptides was identified when the immunoblots were reacted with the R3 monoclonal antibody, and we concluded that this antibody also recognized the product of the BMRF1 open reading frame. Fibroblast cell lines containing EBV BamHI fragment M were established by cotransfection of baby hamster kidney cells with BamHI-M and the gene for neomycin resistance. Aminoglycoside G418-resistant colonies which showed evidence for EBV antigen expression in immunofluorescence assays were selected, and clonal cell lines were established. After 3 to 4 months of passaging, constitutive synthesis of EA was no longer detectable in these cell lines either by immunofluorescence or by immunoblot analysis. However, in the one cell line examined, synthesis of the 48- to 50-kDa EA was induced by treatment of the culture with sodium butyrate. Thus, the regulation of expression of this EA in transfected fibroblasts is analogous to that seen in Raji lymphoblasts. We showed previously that BamHI fragment M also contains the coding sequences for a 60-kDa nuclear EA, and hence BamHI-M encodes two separate components of the diffuse EA complex.

Errant processing and structural alterations of genomes present in a varicella-zoster virus vaccine

Five minority populations of aberrant, varicella-zoster virus (VZV)-derived genomes were identified among the encapsidated DNAs obtained from the nuclear and cytoplasmic fractions of an in vitro infection initiated with a lyophilized sample of the BIKEN VZV vaccine (strain Oka). These were (i) VZV genomes, present within nuclear but not cytoplasmic viral capsids, which had been cleaved at a specific site within the short segment and which were, therefore, 3.15 megadaltons (approximately 4% of the VZV genome length) short of full length; (ii) highly deleted, repetitive VZV genomes which contained the errant cleavage site but not the usual VZV genome terminal sequences; (iii) VZV genomes into which multiples of 1 through 5 defective genome repeat units had been inserted into a homologous site; (iv) VZV genomes with additions of 0.1 or 0.18 megadaltons of DNA at both the terminal and internal ends of the short segment; and (v) VZV DNA which had lost the HindIII restriction site at map position 0.11.

Activation of expression of latent Epstein-Barr herpesvirus after gene transfer with a small cloned subfragment of heterogeneous viral DNA

We previously found that a form of Epstein-Barr virus with rearranged DNA induces replication of latent Epstein-Barr virus. We now have found that one of three fragments of this rearranged DNA, when cloned in recombinant plasmids and used to transfect cells, can activate expression of several polypeptides from a latent viral genome. The 33-kDa protein that is the product of the active fragment is likely to be responsible for disruption of latency.

P3HR-1 Epstein-Barr virus with heterogeneous DNA is an independent replicon maintained by cell-to-cell spread

We present results of biological experiments which indicate that the subpopulation of Epstein-Barr virus strain P3HR-1 with heterogeneous (het) DNA consists of self-contained replicons which multiply alongside, but independently of, Epstein-Barr virus strain HR-1 containing standard DNA. When a population of HR-1 virions containing het DNA was introduced into X50-7 cells, the input heterogeneous DNA increased in abundance, as did the DNA of the endogenous virus of X50-7 cells. The input standard HR-1 viral DNA, however, was not amplified. When parental HR-1 cells or a cellular subclone containing het DNA were grown for several weeks in the presence of human serum with neutralizing antibody, the het DNA was lost from the culture; standard HR-1 DNA, however, was not affected by antiserum. Furthermore, virions containing het DNA could be serially propagated through cellular subclones of HR-1 cells which lack het DNA. After each serial passage, cells which acquired het DNA released virions with the ability to induce early antigens in Raji cells. These experiments define a novel in vitro life cycle of an Epstein-Barr virus variant which is maintained, not vertically by partitioning to daughter cells in cell division, but horizontally by cell-to-cell spread.

Epstein-Barr virus (P3HR-1) defective DNA codes for components of both the early antigen and viral capsid antigen complexes

A set of lambda phages containing overlapping fragments of Epstein-Barr virus (EBV) defective DNA has been cloned from P3HR-1-superinfected Raji cells. Mapping data obtained using these cloned DNA fragments confirmed the structure of P3HR-1 defective DNA previously deduced directly from virion DNA (M.-S. Cho, G. W. Bornkamm, and H. zur Hausen, 1984, J. Virol., in press). The ability of the cloned defective DNA fragments to induce EBV antigens in transfected baby hamster kidney (BHK) cells was tested using indirect immunofluorescence assays. Up to 5% of those cells receiving a defective DNA fragment BamHI-W'C' transiently expressed a de novo nuclear antigen which was identified as being a component of the EAD complex by its reactivity with characterized EBV-positive human sera. A 20-kb clone of P3HR-1 defective DNA (EcoRI-C1) was found to induce the synthesis of a component of the VCA complex. One percent of cells transfected with this clone showed cytoplasmic fluorescence when tested with either VCA+ human sera or EBV anti-VCA monoclonal antibody. Subcloning of the EcoRI-C1 fragment localized the VCA gene to a 4.1-kb segment which maps within the BamHI-A fragment of the standard genome. This segment contains a single large open reading frame of 2.6 kb (B. Barrell, A. Bankier, R. Baer, P. Biggin, P. Deininger, P. Farrell, T. Gibson, G. Hatfull, G. Hudson, S. Stachwell, and C. Sequin, 1984, Nature (London), in press). None of the defective DNA clones were capable of inducing EBV-specific nuclear antigens (EBNAs) which is consistent with the absence of the known EBNA coding regions from the defective genome.