Regulation of the Epstein-Barr virus DNA polymerase gene

Evaluation of chimeric antigen receptor of humanized rabbit-derived T cell receptor-like antibody

T-cell receptor (TCR)-like antibodies that specifically recognize antigenic peptides presented on major histocompatibility complex (MHC) molecules have been developed for next-generation cancer immunotherapy. Recently, we reported a rapid and efficient method to generate TCR-like antibodies using a rabbit system. We humanized previously generated rabbit-derived TCR-like antibodies reacting Epstein-Barr virus peptide (BRLF1p, TYPVLEEMF) in the context of HLA-A24 molecules, produced CAR-T cells, and evaluated their anti-tumor effects using in vitro and in vivo tumor models. Humanization of the rabbit-derived TCR-like antibodies using the complementarity-determining region grafting technology maintained their specificity and affinity. We prepared a second-generation chimeric antigen receptor (CAR) using scFv of the humanized TCR-like antibodies and then transduced them into human T-cells. The CAR-T cells specifically recognized BRLF1p/MHC molecules and lysed the target cells in an antigen-specific manner in vitro. They also demonstrated anti-tumor activity in a mouse xenograft model. We report the generation of CAR-T cells using humanized rabbit-derived TCR-like antibodies. Together with our established and efficient generation procedure for TCR-like antibodies using rabbits, our platform for the clinical application of humanized rabbit-derived TCR-like antibodies to CAR-T cells will help improve next-generation cancer immunotherapy.

Replication Compartments-The Great Survival Strategy for Epstein-Barr Virus Lytic Replication

During Epstein–Barr virus (EBV) lytic replication, viral DNA synthesis is carried out in viral replication factories called replication compartments (RCs), which are located at discrete sites in the nucleus. Viral proteins constituting the viral replication machinery are accumulated in the RCs to amplify viral genomes. Newly synthesized viral DNA is stored in a subdomain of the RC termed the BMRF1-core, matured by host factors, and finally packed into assembled viral capsids. Late (L) genes are transcribed from DNA stored in the BMRF1-core through a process that is mainly dependent on the viral pre-initiation complex (vPIC). RC formation is a well-regulated system and strongly advantageous for EBV survival because of the following aspects: (1) RCs enable the spatial separation of newly synthesized viral DNA from the cellular chromosome for protection and maturation of viral DNA; (2) EBV-coded proteins and their interaction partners are recruited to RCs, which enhances the interactions among viral proteins, cellular proteins, and viral DNA; (3) the formation of RCs benefits continuous replication, leading to L gene transcription; and (4) DNA storage and maturation leads to efficient progeny viral production. Here, we review the state of knowledge of this important viral structure and discuss its roles in EBV survival.

[Dynamic changes of cellular environment during Epstein-Barr virus productive replication]

Productive (lytic) replication of DNA viruses elicits host cell DNA damage responses, which cause both beneficial and detrimental effects on viral replication. Viruses utilize them and selectively cancel the 'noisy' downstream signaling pathways, leading to maintain high S-phase CDK activities required for viral replication. To achieve this fine tuning of cellular environment, herpesviruses encode many (>70) genes in their genome, which are expressed in a strictly regulated temporal cascade (immediate-early, early, and late). Here, I introduce and discuss how Epstein-Barr virus, an oncogenic herpesvirus, hijacks the cellular environment and adapt it for the progeny production.

S-Like-Phase Cyclin-Dependent Kinases Stabilize the Epstein-Barr Virus BDLF4 Protein To Temporally Control Late Gene Transcription

Temporally controlled gene expression is necessary for the propagation of herpesviruses. To achieve this, herpesviruses encode several transcriptional regulators. In Epstein-Barr virus, BcRF1 associates with five viral proteins (BDLF4, BGLF3, BFRF2, BVLF1, and BDLF3.5) to form the viral late (L) gene regulatory complex, which is called the viral preinitiation complex (vPIC), on TATT-containing promoters. However, regulation of the vPIC has been largely unexplored. In this study, we performed two screens using a kinase inhibitor library and identified a series of cyclin-dependent kinase (CDK) inhibitors that downregulated the expression of L genes without any impact on viral DNA replication through destabilization of the BDLF4 protein. Knockdown of CDK2 by short hairpin RNA (shRNA) and proteasome inhibitor treatment showed that phosphorylation of the BDLF4 protein prevented ubiquitin-mediated degradation. Moreover, we demonstrated that cyclin A- and E-associated CDK2 complexes phosphorylated BDLF4 in vitro, and we identified several serine/threonine phosphorylation sites in BDLF4. Phosphoinactive and phosphomimic mutants revealed that phosphorylation at threonine 91 plays a role in stabilizing BDLF4. Therefore, our findings indicate that S-like-phase CDKs mediate the regulation of L gene expression through stabilization of the BDLF4 protein, which makes the temporal L gene expression system more robust.IMPORTANCE Late (L) genes represent more than one-third of the herpesvirus genome, suggesting that many of these genes are indispensable for the life cycle of the virus. With the exception of BCRF1, BDLF2, and BDLF3, Epstein-Barr virus L genes are transcribed by viral regulators, which are known as the viral preinitiation complex (vPIC) and the host RNA polymerase II complex. Because the vPIC is conserved in beta- and gammaherpesviruses, studying the control of viral L gene expression by the vPIC contributes to the development of drugs that specifically inhibit these processes in beta- and gammaherpesvirus infections/diseases. In this study, we demonstrated that CDK inhibitors induced destabilization of the vPIC component BDLF4, leading to a reduction in L gene expression and subsequent progeny production. Our findings suggest that CDK inhibitors may be a therapeutic option against beta- and gammaherpesviruses in combination with existing inhibitors of herpesvirus lytic replication, such as ganciclovir.

Trichloromethane fraction of Incarvillea compacta induces lytic cytotoxicity and apoptosis in Epstein-Barr virus-positive gastric cancer AGS cells

Background

Incarvillea compacta Maxim. has been used to treat stomach disease in Tibet for many years. The objectives of this study were to explore the anti-cancer ability of trichloromethane fraction of I. compacta Maxim. roots (IC-TCL, R2) in EBV positive AGS cancer cells and its effects on cell cycle arrest, apoptosis and lytic induction.

Methods

MTT and trypan blue assays were to detect the inhibitory effects of different fraction in different cell lines. Hoechst 33342 staining, Annexin V-PE/7-AAD staining and DIOC₆ staining were used to detect the apoptosis induction effects of R2. Western blot experiments were used to detect the expression of apoptosis related proteins BAX and Bcl-2, EBV lytic related proteins BZLF1 and BMRF1, cell cycle regulation related proteins Cyclin D1 and RB after R2 treatment. Cell cycle arrest was analyzed by flow cytometry.

Results

MTT and trypan blue assays revealed that R2 could significantly reduce cell viability in a dose-dependent manner in EBV positive AGS cells compared with non-EBV infected AGS and other cancer cell lines, whereas n-BuOH and H₂O fractions showed non-inhibitory effects in tested cancer cells. R2 could decrease mitochondrial membrane potential and the expression of Bcl-2, while increase the expression of BAX. R2 could also induce EBV lytic replication by activating mRNA levels of BZLF1, BRLF1 and BMRF1. Protein expressions of BZLF1 and BMRF1 were also increased after R2 treatment. Cell cycle analysis showed that R2 treatment could induce G0/G1 phase arrest. The expression of Cyclin D1 decreased, while Rb increased.

Conclusions

These results demonstrated that R2 could inhibit the proliferation of AGS-EBV cancer cells by inducing EBV lytic replication, apoptosis and G0/G1 arrest, through the regulation of related proteins. Therefore, R2 could be used as a potential treatment in AGS-EBV cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12906-016-1331-6) contains supplementary material, which is available to authorized users.

Epstein-Barr virus infection in chronically inflamed periapical granulomas

Periapical granulomas are lesions around the apex of a tooth caused by a polymicrobial infection. Treatment with antibacterial agents is normally performed to eliminate bacteria from root canals; however, loss of the supporting alveolar bone is typically observed, and tooth extraction is often selected if root canal treatment does not work well. Therefore, bacteria and other microorganisms could be involved in this disease. To understand the pathogenesis of periapical granulomas more precisely, we focused on the association with Epstein-Barr virus (EBV) using surgically removed periapical granulomas (n = 32). EBV DNA was detected in 25 of 32 periapical granulomas (78.1%) by real-time PCR, and the median number of EBV DNA copies was approximately 8,688.01/μg total DNA. In contrast, EBV DNA was not detected in healthy gingival tissues (n = 10); the difference was statistically significant according to the Mann-Whitney U test (p = 0.0001). Paraffin sections were also analyzed by in situ hybridization to detect EBV-encoded small RNA (EBER)-expressing cells. EBER was detected in the cytoplasm and nuclei of B cells and plasma cells in six of nine periapical granulomas, but not in healthy gingival tissues. In addition, immunohistochemical analysis for latent membrane protein 1 (LMP-1) of EBV using serial tissue sections showed that LMP-1-expressing cells were localized to the same areas as EBER-expressing cells. These data suggest that B cells and plasma cells in inflamed granulomas are a major source of EBV infection, and that EBV could play a pivotal role in controlling immune cell responses in periapical granulomas.

Different distributions of Epstein-Barr virus early and late gene transcripts within viral replication compartments

Productive replication of the Epstein-Barr virus (EBV) occurs in discrete sites in nuclei, called replication compartments, where viral genome DNA synthesis and transcription take place. The replication compartments include subnuclear domains, designated BMRF1 cores, which are highly enriched in the BMRF1 protein. During viral lytic replication, newly synthesized viral DNA genomes are organized around and then stored inside BMRF1 cores. Here, we examined spatial distribution of viral early and late gene mRNAs within replication compartments using confocal laser scanning microscopy and three-dimensional surface reconstruction imaging. EBV early mRNAs were mainly located outside the BMRF1 cores, while viral late mRNAs were identified inside, corresponding well with the fact that late gene transcription is dependent on viral DNA replication. From these results, we speculate that sites for viral early and late gene transcription are separated with reference to BMRF1 cores.

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.

Genome-wide analysis of Epstein-Barr virus Rta DNA binding

The Epstein-Barr virus (EBV) lytic transactivator Rta activates promoters through direct binding to cognate DNA sites termed Rta response elements (RREs). Rta also activates promoters that apparently lack Rta binding sites, notably Zp and Rp. Chromatin immunoprecipitation (ChIP) of endogenous Rta expressed during early replication in B95-8 cells was performed to identify Rta binding sites in the EBV genome. Quantitative PCR (qPCR) analysis showed strong enrichment for known RREs but little or no enrichment for Rp or Zp, suggesting that the Rta ChIP approach enriches for direct Rta binding sites. Rta ChIP combined with deep sequencing (ChIP-seq) identified most known RREs and several novel Rta binding sites. Rta ChIP-seq peaks were frequently upstream of Rta-responsive genes, indicating that these Rta binding sites are likely functioning as RREs. Unexpectedly, the BALF5 promoter contained an Rta binding peak. To assess whether BALF5 might be activated by an RRE-dependent mechanism, an Rta mutant (Rta K156A), deficient for DNA binding and RRE activation but competent for Zp/Rp activation, was used. Rta K156A failed to activate BALF5p, suggesting this promoter can be activated by an RRE-dependent mechanism. Rta binding to late gene promoters was not seen at early time points but was specifically detected at later times within the Rta-responsive BLRF2 and BFRF3 promoters, even when DNA replication was inhibited. Our results represent the first characterization of Rta binding to the EBV genome during replication, identify previously unknown RREs, such as one in BALF5p, and highlight the complexity of EBV late gene promoter activation by Rta.

Epstein-Barr virus LF2 protein regulates viral replication by altering Rta subcellular localization

The switch from Epstein-Barr virus (EBV) latent infection to lytic replication is governed by two viral transactivators, Zta and Rta. We previously reported that the EBV protein LF2 binds Rta, inhibits Rta promoter activation, and blocks EBV replication in cells. In addition, LF2 induces SUMO2/3 modification of Rta. We now show that this modification occurs at four lysines within the Rta activation domain (426, 446, 517, and 530) and that sumoylation of Rta is not essential for its repression. Coexpression studies demonstrated that Rta is sequestered to the extranuclear cytoskeleton in the presence of LF2. We mapped the LF2 binding site to Rta amino acids (aa) 476 to 519 and showed that LF2 binding is critical for Rta relocalization and repression. The core of this binding site, Rta aa 500 to 526, confers LF2-mediated relocalization and repression onto the artificial transcription factor GAL4-VP16. Mutational analysis of LF2 provided further evidence that Rta redistribution is essential for repression. Rta localization changes during replication of the LF2-positive P3HR1 genome, but not during replication of the LF2-negative B95-8 genome. BLRF2 protein expression was decreased and delayed in P3HR1 cells compared with B95-8 cells, consistent with reduced Rta activity. By contrast, BMRF1 expression, regulated primarily by Zta, did not differ significantly between the two cell lines. Our results support a model in which LF2 regulates EBV replication by binding to Rta and redistributing it out of the nucleus.

Two phenylalanines in the C-terminus of Epstein-Barr virus Rta protein reciprocally modulate its DNA binding and transactivation function

The Rta (R transactivator) protein plays an essential role in the Epstein-Barr viral (EBV) lytic cascade. Rta activates viral gene expression by several mechanisms including direct and indirect binding to target viral promoters, synergy with EBV ZEBRA protein, and stimulation of cellular signaling pathways. We previously found that Rta proteins with C-terminal truncations of 30 aa were markedly enhanced in their capacity to bind DNA (Chen, L.W., Chang, P.J., Delecluse, H.J., and Miller, G., (2005). Marked variation in response of consensus binding elements for the Rta protein of Epstein-Barr virus. J. Virol. 79(15), 9635-9650.). Here we show that two phenylalanines (F600 and F605) in the C-terminus of Rta play a crucial role in mediating this DNA binding inhibitory function. Amino acids 555 to 605 of Rta constitute a functional DNA binding inhibitory sequence (DBIS) that markedly decreased DNA binding when transferred to a minimal DNA binding domain of Rta (aa 1-350). Alanine substitution mutants, F600A/F605A, abolished activity of the DBIS. F600 and F605 are located in the transcriptional activation domain of Rta. Alanine substitutions, F600A/F605A, decreased transcriptional activation by Rta protein, whereas aromatic substitutions, such as F600Y/F605Y or F600W/F605W, partially restored transcriptional activation. Full-length Rta protein with F600A/F605A mutations were enhanced in DNA binding compared to wild-type, whereas Rta proteins with F600Y/F605Y or F600W/F605W substitutions were, like wild-type Rta, relatively poor DNA binders. GAL4 (1-147)/Rta (416-605) fusion proteins with F600A/F605A mutations were diminished in transcriptional activation, relative to GAL4/Rta chimeras without such mutations. The results suggest that, in the context of a larger DBIS, F600 and F605 play a role in the reciprocal regulation of DNA binding and transcriptional activation by Rta. Regulation of DNA binding by Rta is likely to be important in controlling its different modes of action.

The Epstein-Barr virus LF2 protein inhibits viral replication

The switch from Epstein-Barr virus (EBV) latent infection to lytic replication is governed by two transcriptional regulators, Zta and Rta. We previously reported that the EBV protein encoded by the LF2 gene binds to Rta and can inhibit Rta activity in reporter gene assays. We now report that LF2 associates with Rta in the context of EBV-infected cells induced for lytic replication. LF2 inhibition of Rta occurs in both epithelial and B cells, and this downregulation is promoter specific: LF2 decreases Rta activation of the BALF2, BMLF1, and BMRF1 promoters by 60 to 90% but does not significantly decrease Rta activation of its own promoter (Rp). LF2 decreases Rta activation by at least two mechanisms: decreased DNA binding and interference with transcriptional activation by the Rta acidic activation domain. Coexpression of LF2 also specifically induces modification of Rta by the small ubiquitin-like modifiers SUMO2 and SUMO3. We further demonstrate that LF2 overexpression blocks lytic activation in EBV-infected cells induced with Rta or Zta. Our results demonstrate that LF2, a gene deleted from the EBV reference strain B95-8, encodes a potent inhibitor of EBV replication, and they suggest that future studies of EBV replication need to account for the potential effects of LF2 on Rta activity.

Epstein-Barr virus-encoded microRNA miR-BART2 down-regulates the viral DNA polymerase BALF5

MicroRNAs (miRNAs) have been implicated in sequence-specific cleavage, translational repression or deadenylation of specific target mRNAs resulting in post-transcriptional gene silencing. Epstein–Barr virus (EBV) encodes 23 miRNAs of unknown function. Here we show that the EBV-encoded miRNA miR-BART2 down-regulates the viral DNA polymerase BALF5. MiR-BART2 guides cleavage within the 3′-untranslated region (3′UTR) of BALF5 by virtue of its complete complementarity to its target. Induction of the lytic viral replication cycle results in a reduction of the level of miR-BART2 with a strong concomitant decrease of cleavage of the BALF5 3′UTR. Expression of miR-BART2 down-regulates the activity of a luciferase reporter gene containing the BALF5 3′UTR. Forced expression of miR-BART2 during lytic replication resulted in a 40–50% reduction of the level of BALF5 protein and a 20% reduction of the amount of virus released from EBV-infected cells. Our results are compatible with the notion that EBV-miR-BART2 inhibits transition from latent to lytic viral replication.

Lytic Cycle Switches of Oncogenic Human Gammaherpesviruses1

The seminal experiments of George and Eva Klein helped to define the two life cycles of Epstein-Barr Virus (EBV), namely latency and lytic or productive infection. Their laboratories described latent nuclear antigens expressed during latency and discovered several chemicals that activated the viral lytic cycle. The mechanism of the switch between latency and the lytic cycle of EBV and Kaposi's sarcoma-associated herpesvirus (KSHV) can be studied in cultured B cell lines. Lytic cycle activation of EBV is controlled by two viral transcription factors, ZEBRA and Rta. The homologue of Rta encoded in ORF50 is the lytic cycle activator of KSHV. Control of the lytic cycle can be divided into two distinct phases. Upstream events control expression of the virally encoded lytic cycle activator genes. Downstream events represent tasks carried out by the viral proteins in driving expression of lytic cycle genes and lytic viral DNA replication. In this chapter, we report three recent groups of experiments relating to upstream and downstream events. Azacytidine (AzaC) is a DNA methyltransferase inhibitor whose lytic cycle activation capacity was discovered by G. Klein and coworkers. We find that AzaC rapidly activates the EBV lytic cycle but does not detectably alter DNA methylation or histone acetylation on the promoters of the EBV lytic cycle activator genes. AzaC probably acts via a novel, yet to be elucidated, mechanism. The lytic cycle of both EBV and KSHV can be activated by sodium butyrate (NaB), a histone deacetylase inhibitor whose activity in disrupting latency was also discovered by G. Klein and coworkers. Activation of EBV by NaB requires protein synthesis; activation of KSHV is independent of protein synthesis. Thus, NaB works by a different pathway on the two closely related viruses. ZEBRA, the major downstream mediator of EBV lytic cycle activation is both a transcription activator and an essential replication protein. We show that phosphorylation of ZEBRA at its casein kinase 2 (CK2) site separates these two functions. Phosphorylation by CK2 is required for ZEBRA to activate lytic replication but not to induce expression of early lytic cycle genes. We discuss a number of unsolved mysteries about lytic cycle activation which should provide fertile territory for future research.

Cerebrospinal fluid CD4+ T cells from a multiple sclerosis patient cross-recognize Epstein-Barr virus and myelin basic protein

Epstein-Barr virus-specific CD4+ T cells could be involved in the pathogenesis of multiple sclerosis, provided they can gain entry to the intrathecal compartment. The authors have previously demonstrated that cerebrospinal fluid T cells from multiple sclerosis patients recognize autologous Epstein-Barr virus-transformed B cells. They now report that CD4+ T cells specific for the Epstein-Barr virus DNA polymerase peptide EBV 627-641 were present in the cerebrospinal fluid from one of two multiple sclerosis patients, and that a high proportion of these CD4+ T cells cross-recognized an immunodominant myelin basic protein peptide, MBP 85-99. In the observed patient, the proportion of EBV 627-641-specific CD4+ T cells seemed to exceed 1/10,000 in cerebrospinal fluid, compared to approximately 1/100,000 in blood. These findings prove that Epstein-Barr-virus specific CD4+ T cells can gain access to the intrathecal compartment, and suggest that Epstein-Barr virus-specific CD4+ T cells could target myelin basic protein in the central nervous system.

Intracellular signaling molecules activated by Epstein-Barr virus for induction of interferon regulatory factor 7

Epstein-Barr virus (EBV) latent membrane protein 1 (LMP-1) is the principal oncogenic protein in the EBV transformation process. LMP-1 induces the expression of interferon regulatory factor 7 (IRF-7) and activates IRF-7 protein by phosphorylation and nuclear translocation. LMP-1 is an integral membrane protein with two regions in its C terminus that initiate signaling processes, the C-terminal activator regions 1 (CTAR-1) and CTAR-2. Here, genetic analysis of LMP-1 has determined that the PXQXT motif that governs the interaction between LMP-1 CTAR-1 and tumor necrosis factor receptor-associated factors (TRAFs) is needed to induce the expression of IRF-7. Mutations in the PXQXT motif in CTAR-1 that disrupt the interaction between LMP-1 and TRAFs abolished the induction of IRF-7. Also, dominant-negative mutants of TRAFs inhibited the induction of IRF-7 by CTAR-1. The last three amino acids (YYD) of CTAR-2 are also important for the induction of IRF-7. When both PXQXT and YYD were mutated (LMP-DM), the LMP-1 mutant failed to induce IRF-7. Also, LMP-DM blocked the induction of IRF-7 by wild-type LMP-1. These data strongly suggest that both CTAR-1 and CTAR-2 of LMP-1 independently induce the expression of IRF-7. In addition, NF-kappaB is involved in the induction of IRF-7. A superrepressor of IkappaB (sr-IkappaB) could block the induction of IRF-7 by LMP-1, and overexpression of NF-kappaB (p65 plus p50) could induce the expression of IRF-7. In addition, we have found that human IRF-7 is a stable protein, and sodium butyrate, a modifier of chromatin structure, induces IRF-7.

Interferon regulatory factor 7 mediates activation of Tap-2 by Epstein-Barr virus latent membrane protein 1

Transporter associated with antigen processing 2 (Tap-2) is responsible for ATP-dependent transport of peptides from the cytosol to the endoplasmic reticulum, where peptides bind to newly synthesized human leukocyte antigen (HLA) class I molecules, which are essential for cellular immune responses. Epstein-Barr virus (EBV) latent membrane protein 1 (LMP-1) has been shown to induce the expression of Tap-2. In this study, the induction of endogenous Tap-2 by LMP-1 is shown to be associated with and requires the expression of interferon regulatory factor 7 (IRF-7). In DG75 Burkitt's lymphoma (BL) cells, in which LMP-1 induces the expression of IRF-7, LMP-1 induced endogenous Tap-2, and ectopic expression of IRF-7 could enhance the induction. In Akata BL cells, in which LMP-1 could not induce IRF-7, LMP-1 could not induce Tap-2. Addition of IRF-7, which complements the defect in Akata cells, could stimulate the expression of Tap-2. Furthermore, LMP-1 and IRF-7A but not other IRF-7 splicing variants could activate endogenous Tap-2. A Tap-2 promoter reporter construct could be activated by the overexpression of IRF-7A. The activation could be specifically enhanced by LMP-1 and was dependent on an intact interferon-stimulated response element (ISRE) present in the Tap-2 promoter. Also, IRF-7 can bind to the Tap-2 promoter under physiological conditions in vivo, as shown by formaldehyde cross-linking, as well as to the Tap-2 ISRE in vitro, as shown by gel mobility shift assays. Furthermore, LMP-1 facilitates the phosphorylation and nuclear translocation of IRF-7. These data point to the role of IRF-7 as a secondary mediator of LMP-1-activated signal transduction for Tap-2 as follows: LMP-1 stimulates the expression of IRF-7 and facilitates its phosphorylation and nuclear translocation, and then the activated IRF-7 mediates the activation of the cellular Tap-2 gene. The induction of Tap-2 by IRF-7 and LMP-1 may have an important implication for the immune response to EBV and its persistence in vivo.

Interferon regulatory factor 7 is induced by Epstein-Barr virus latent membrane protein 1

Infection by Epstein-Barr virus (EBV) generates several types of latency with different profiles of gene expression but with expression of Epstein-Barr nuclear antigen 1 (EBNA-1) in common. The BamHI Q promoter (Qp) is used for the transcription of EBNA-1 mRNA in type I latency, which is an EBV infection state exemplified by Burkitt's lymphoma (BL). However, Qp is inactive in type III latency, and other promoters (C/Wp) are used for transcription of EBNA-1, which raises the question of how usage of these promoters is governed. Interferon (IFN) regulatory factor 7 (IRF-7) was identified first as a negative regulator of Qp. Expression of IRF-7 is associated with EBV type III latency, where Qp is inactive, but not with type I latency, raising the possibility that a viral gene product(s) expressed in type III latency might induce IRF-7 and repress Qp. Here, detailed analysis of the expression of IRF-7 revealed that it is associated with the expression of EBV latent membrane protein 1 (LMP-1) and that LMP-1 stimulates the expression of IRF-7 in type III latency in which Qp is inactive. In contrast, LMP-1 is not expressed in type I latency cells in which Qp is active. LMP-1 represses the constitutive activity of Qp reporter constructs. Mutational analysis of Qp reporter constructs revealed that the Qp IFN-stimulated response element (ISRE) is essential for the repression by LMP-1. Furthermore, LMP-1 reduced EBNA-1 mRNA derived from Qp only in type I cells in which IRF-7 could be induced. Finally, IFN-alpha, but not IFN-gamma, repressed endogenous Qp activity, which is consistent with the ability of IFN-alpha to induce IRF-7. Thus, IRF-7 may mediate repression of Qp by LMP-1. The induction of IRF-7 by LMP-1 may be relevant to the silencing of Qp in EBV type III latency.

Characterization of the Epstein-Barr virus BALF2 promoter

BALF2, which encodes the major DNA-binding protein of Epstein-Barr virus (EBV), is expressed during the early stage of the lytic cycle. The location of the BALF2 promoter was identified by primer extension, which indicated that the transcription start is located at nucleotide 164,782 of the EBV genome. Transfection analyses revealed that, similar to other EBV early promoters, the BALF2 promoter is activated by the EBV-encoded transcription factors Rta and Zta. The promoter is also synergistically activated if both transcription factors are present in B lymphocytes and in epithelial cells. Deletion analysis and electrophoretic mobility-shift assay revealed that the region between nucleotides -134 and -64 contains Zta-response elements and the region between nucleotides -287 and -254 contains Rta-response elements. This study demonstrates the importance of Rta and Zta in regulating the transcription of EBV early genes.

Interferon regulatory factor 2 represses the Epstein-Barr virus BamHI Q latency promoter in type III latency

Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA-1) is the essential protein for maintenance of the EBV episome and establishment of latency. The BamHI Q promoter (Qp) is used for the transcription of EBNA-1 mRNA in type I and type II latency, which are EBV infection states exemplified by Burkitt's lymphoma and nasopharyngeal carcinoma. However, Qp is inactive in type III latency, and other promoters (the BamHI C promoter and/or the BamHI W promoter) are used for EBNA-1. The involvement of interferon regulatory factors (IRFs) in the regulation of Qp is suggested by the presence of an essential interferon-stimulated response element (ISRE) in the promoter. In this work, expression of IRF-2 is shown to be inversely associated with Qp status, i.e., IRF-2 levels are high in type III latency (when Qp is inactive) and low in type I latency (when Qp is active). Also, IRF-2 is identified by electrophoretic mobility shift assay as the major protein binding to the Qp ISRE in type III latency. In transient transfection assays, IRF-2 represses the activity of Qp-reporter constructs. Overexpression of IRF-2 in a type I latency cell line did not activate the endogenous Qp but marginally reduced the EBNA-1 mRNA level. Switching from type III latency (Qp inactive) to type II latency (Qp active), as produced by cell fusion, is directly associated with greatly reduced expression of IRF-2. These data strongly suggest that IRF-2 is a negative regulator of Qp and may contribute to the silencing of Qp in type III latency.

Rescue of the Epstein-Barr virus BZLF1 mutant, Z(S186A), early gene activation defect by the BRLF1 gene product

Expression of the Epstein-Barr virus (EBV) immediate-early protein, BZLF1 (Z), is sufficient to disrupt viral latency. Z transcriptionally activates the EBV early genes by binding to upstream Z-responsive elements (ZREs). Recently, a serine-to-alanine mutation of Z residue 186 (within the basic DNA binding domain) was shown to inhibit the ability of Z to induce lytic infection in latently infected cells, although the Z(S186A) mutant could still bind several known ZREs and activated an early EBV promoter (BMRF1) in transient reporter gene assays (Francis, A. L., Gradoville, L., and Miller, G. (1997). J. Virol. 71, 3054-3061). We now show that a specific deficiency in the ability to bind to ZRE elements in the immediate-early BRLF1 promoter may account for the inability of Z(S186A) to activate BRLF1 expression. Furthermore, we demonstrate that the ability of Z(S186A) to induce early BMRF1 and BHRF1 gene expression is rescued by cotransfection with a BRLF1 expression vector. However, the Z(S186A)/BRLF1 (R) combination cannot induce full lytic replication, suggesting that Z(S186A) may also be deficient in a replication-specific function. These results suggest that in the context of the intact viral genome, both Z and R expression are required for activation of early gene transcription in latently infected cells.

The Epstein-Barr virus (EBV) SM protein enhances pre-mRNA processing of the EBV DNA polymerase transcript

The Epstein-Barr virus (EBV) DNA polymerase (pol) mRNA, which contains a noncanonical polyadenylation signal, UAUAAA, is cleaved and polyadenylated inefficiently (S. C. S. Key and J. S. Pagano, Virology 234:147-159, 1997). We postulated that the EBV early proteins SM and M, which appear to act posttranscriptionally and are homologs of herpes simplex virus (HSV) ICP27, might compensate for the inefficient processing of pol pre-mRNA. Here we show that the SM and M proteins interact with each other in vitro. In addition, glutathione S-transferase-SM/M fusion proteins precipitate the heterogeneous ribonucleoprotein (hnRNP) C1 splicing protein. Further, the SM protein is coimmunoprecipitated from SM-expressing cell extracts with an antibody to the hnRNP A1/A2 proteins, which are splicing and nuclear shuttling proteins. Finally, the amount of processed EBV DNA polymerase mRNA was increased three- to fourfold in a HeLa cell line expressing SM; this increase was not due to enhanced transcription. Thus, inefficient processing of EBV pol RNA by cellular cleavage and polyadenylation factors appears to be compensated for and may be regulated by the early EBV protein, SM, perhaps via RNA 3'-end formation.

Activation of human herpesvirus 8 (HHV-8) thymidine kinase (TK) TATAA-less promoter by HHV-8 ORF50 gene product is SP1 dependent

Human herpesvirus 8 (HHV-8) is a newly discovered virus closely associated with Kaposi's sarcoma and primary effusion lymphomas. When they occur in patients with AIDS, these B-cell lymphomas frequently harbor another human herpesvirus, Epstein-Barr virus (EBV). To determine the molecular mechanisms of the regulation of early gene expression by the immediate-early gene products of HHV-8 and to assess possible molecular interactions between HHV-8 and EBV, we studied the regulation of the HHV-8 thymidine kinase (TK) promoter in cell lines harboring either or both viruses. The constitutive chloramphenicol acetyltransferase (CAT) activity of the TK promoter was low in all six cell lines tested. A putative immediate-early gene product of HHV-8 ORF50, which is a homolog of EBV BRLF1, was cloned into an expression vector and tested for its transactivating capacity. In the presence of 12-O-tetradecanoyl-phorbol-13-acetate (TPA), the CAT activity of the TK promoter was increased 7- to 720-fold by cotransfection with the ORF50 clone in EBV-producing cell lines (Ramos/AW, P3HR-1, and BC-1) but not in EBV-negative cell lines (BCBL-1 and Ramos), nor in the latently EBV-infected cell line Raji. The TK promoter contains three consensus SP1- and two AP1-binding sites. In electrophoretic mobility shift assays, the cellular factor SP1, but not AP1, was found to bind specifically to the TK promoter. To determine whether the increased CAT activity resulted from the interaction of SP1 with the ORF50 gene product, we introduced mutations into two SP1-binding sites. Both mutated SP1 sites had reduced SP1-binding activity and greatly decreased TK promoter responsiveness to ORF50 transactivation, suggesting that upregulation of TK promoter by ORF50 is SP1 dependent.

Growth suppression of glioma cells by PTEN requires a functional phosphatase catalytic domain

Deletions of all or part of chromosome 10 are the most common genetic alterations in high-grade gliomas. The PTEN gene (also called MMAC1 and TEP1) maps to chromosome region 10q23 and has been implicated as a target of alteration in gliomas and also in other cancers such as those of the breast, prostate, and kidney. Here we sought to provide a functional test of its candidacy as a growth suppressor in glioma cells. We used a combination of Northern blot analysis, protein truncation assays, and sequence analysis to determine the types and frequency of PTEN mutations in glioma cell lines so that we could define appropriate recipients to assess the growth suppressive function of PTEN by gene transfer. Introduction of wild-type PTEN into glioma cells containing endogenous mutant alleles caused growth suppression, but was without effect in cells containing endogenous wild-type PTEN. The ectopic expression of PTEN alleles, which carried mutations found in primary tumors and have been shown or are expected to inactivate its phosphatase activity, caused little growth suppression. These data strongly suggest that PTEN is a protein phosphatase that exhibits functional and specific growth-suppressing activity.

Varicella-zoster virus open reading frame 4 encodes an immediate-early protein with posttranscriptional regulatory properties

Varicella-zoster virus (VZV) encodes four putative immediate-early proteins based on sequence homology with herpes simplex virus type 1: the products of ORF4, -61, -62, and -63. Until now, only two VZV proteins have been described as being truly expressed with immediate-early kinetics (IE62 and IE63). The ORF4-encoded protein can stimulate gene expression either alone or in synergy with the major regulatory protein IE62. Making use of a sequential combination of transcription and protein synthesis inhibitors (actinomycin D and cycloheximide, respectively), we demonstrated the immediate-early nature of the ORF4 gene product, which can thus be named IE4. The fact that IE4 is expressed with kinetics similar to that of IE62 further underlines the possible cooperation between these two VZV proteins in gene expression. Analysis of the IE4-mediated autologous or heterologous viral gene expression at the mRNA levels clearly indicated that IE4 may have several mechanisms of action. Activation of the two VZV genes tested could occur partly by a posttranscriptional mechanism, since increases in chloramphenicol acetyltransferase (CAT) mRNA levels do not account for the stimulation of CAT activity. On the other hand, stimulation of the human immunodeficiency virus type 1 long terminal repeat- or the cytomegalovirus promoter-associated CAT activity is correlated with an increase in the corresponding CAT mRNA.

A noncanonical poly(A) signal, UAUAAA, and flanking elements in Epstein-Barr virus DNA polymerase mRNA function in cleavage and polyadenylation assays

Two forms of the Epstein-Barr virus DNA polymerase (pol) mRNA (3.7 and 5.1 kb) have been detected, neither of which contains a canonical poly(A) signal. The 5.1-kb pol mRNA, which contains a rare poly(A) signal, UAUAAA, studied only in transcripts of Hepadnaviridae and a plant pararetrovirus, was analyzed in cleavage and polyadenylation assays. Incubation of the pol transcript in cell extracts produced relatively low efficiency of cleavage (12 to 14%), which was improved by conversion of the poly(A) signal to AAUAAA. Deletion of the UAUAAA signal abolished cleavage and polyadenylation. An auxiliary element, UUUGUA, 3-8 nt upstream of the poly(A) signal and two downstream core elements, a GU-rich sequence 36-46 nt, and an AUUUGUGU sequence 47-53 nt downstream of the signal (8-19 nt and 20-28 nt downstream of cleavage site) facilitated processing of pol mRNA. Replacement of sequences near the cleavage/poly(A) site affected cleavage accuracy. Binding of the 64-kDa cleavage stimulatory factor to the U-rich as well as the GU-rich elements correlated with cleavage efficiency. Thus the UAUAAA hexanucleotide plus the other cis-acting elements are clearly functional in the native pol mRNA, but are relatively inefficient. Implications of the use of an anomalous poly(A) signal and its elements are discussed.

Inhibition of the Synthesis of Proteins Needed for Epstein-Barr Virus Replication by Antisense RNA against the Zta Gene

Antisense RNA complementary to the Epstein-Barr virus (EBV) Zta gene, an immediate-early gene encoding a transactivator, was applied to inhibit EBV protein synthesis during its lytic cycle. A DNA fragment containing the Zta gene sequence was inserted into an expression vector, pMAMneo, in a sense and antisense direction under a dexamethasone-inducible murine mammary tumor virus LTR promoter, resulting in the construction of plasmids pZ(+) and pZ(-), respectively. Synthesis of Zta protein was reduced in pZ(-)-transfected cells upon dexamethasone induction. Because D-form early antigen and DNA polymerase are essential for viral DNA replication, the contents of these two viral proteins were examined. Amounts of the two lytic proteins were observed to be significantly repressed in pZ(-)-transfected cells. In contrast, both proteins were normally expressed in the sense plasmid pZ(+) or cells transfected with vector alone. Above results demonstrate that Zta antisense RNA can reduce the production of Zta protein and the other lytic proteins, possibly resulting in the inhibition of EBV replication. Copyright 1997 S. Karger AG, Basel

Activation of the Epstein-Barr virus DNA polymerase promoter by the BRLF1 immediate-early protein is mediated through USF and E2F

The Epstein-Barr virus (EBV) DNA polymerase (pol) is essential for the replication of viral genomes during productive EBV infection. We have previously reported that the EBV DNA pol promoter, which is TATA-less and constitutively inactive, is activated by a genomic clone expressing both immediate-early viral transactivators, BZLF1Z and BRLF1 (R), in EBV-infected lymphoid cells. Here we demonstrate that R alone is sufficient to activate the pol promoter in EBV-negative B cells. Unlike other early promoters to which the R protein binds directly, its effect on the pol promoter does not appear to involve a direct DNA-binding mechanism. Instead, we found that two cellular transcription factors, an upstream stimulatory factor USF, and a member of the E2F family of proteins, bind directly to the pol promoter at positions -795 to -786 and -186 to -170, respectively, regions previously identified as important for activation of the pol promoter. These two sites contribute to or are essential for transactivation of the pol promoter by R in EBV-noninfected B cells. These data suggest that the R immediate-early protein may activate a key early EBV promoter (pol) through both USF and E2F.

Nucleotide sequence and transcriptional analysis of the DNA polymerase gene of Bombyx mori nuclear polyhedrosis virus

A gene encoding a putative DNA polymerase (pol) of Bombyx mori nuclear polyhedrosis virus (BmSNPV) was cloned and sequenced. The gene included an open reading frame (ORF) encoding a polypeptide of 988 amino acids with a predicted molecular mass of 114.65 kDa. The deduced amino acid sequence of the BmSNPV pol ORF showed an overall identity of 96 and 45% to those of the Autographa californica NPV (AcMNPV) pol ORF and the Lymantria dispar NPV pol ORF, respectively, and contained sequences conserved in a variety of eukaryotic and viral replicative DNA polymerases. The BmSNPV pol lacked a canonical TATAA element but contained a G+C-rich sequence in the transcriptional initiation region. Analyses by Northern blot hybridization, RNase protection assay, primer extension, and 3' and 5' RACE (rapid amplification of cDNA ends) showed that at least seven different transcripts of approximately 3.1 kb that shared a common 3' end were expressed from the BmSNPV pol. The expression of these transcripts from BmSNPV pol was regulated differentially during virus infection. Transcription of five of the seven species initiated in the close vicinity of and within the motif 5'-GCGTGCT-3'. One transcript placed its initiation site within the motif 5'-AGAGCGT-3' and the remaining one within the motif 5'-GGCGGTGG-3'. The motifs 5'-GCGTGCT-3' and 5'-AGAGCGT-3' have been identified in pol and other genes of AcMNPV as conserved sequences containing transcriptional initiation sites, whereas the motif 5'-GGCGGTGG-3', which is arranged as a direct repeat in BmSNPV pol but not in AcMNPV pol, has not been defined as the sequence responsible for transcriptional initiation sites. The BmSNPV pol transcripts were detectable at 2 hr postinfection (p.i.), peaked at 10 hr p.i., and declined to a low level by 18 hr p.i. The expression of BmSNPV pol was not inhibited but delayed dramatically by the protein synthesis inhibitor cycloheximide upon treatment of infected cells, whereas aphidicolin, an inhibitor of DNA polymerase, inhibited BmSNPV pol transcription. These results suggest a complicated and unique mechanism for the regulation of BmSNPV pol expression.

Reciprocal regulation of the Epstein-Barr virus BamHI-F promoter by EBNA-1 and an E2F transcription factor

The Epstein-Barr virus BamHI-F promoter (Fp) is one of three used to transcribe the EBNA latency proteins, in particular, EBNA-1, the only viral gene product needed for episomal replication. Fp is distinguished by possession of the only EBNA-1 binding sites (the Q locus) in the Epstein-Barr virus genome outside oriP. Activity of Fp is negatively autoregulated by interaction of EBNA-1 at two sites in the Q locus, which is situated downstream of the RNA start site. We demonstrate in transient assays that this EBNA-1-mediated repression of Fp can be overcome by an E2F transcription factor which interacts with the DNA at a site centered between the two EBNA-1 binding sites within the Q locus. An E2F-1 fusion protein protects the sequence 5'-GGATGGCGGGTAATA-3' from DNase I digestion, and a DNA probe containing this sequence binds an E2F-specific protein complex from cell extracts, although this region is only loosely homologous with known consensus binding sites for E2F transcription factors. In mobility shift assays, E2F can displace the binding of EBNA-1 from the Q locus but not from oriP, where the E2F binding site is not present. E2F also activates expression of Fp in epithelial cells. These findings identify a potentially new binding site for members of the E2F family of transcription factors and suggest that such a factor is important for expression of EBNA-1 in lymphoid and epithelial cells by displacing EBNA-1 from the Q locus. In addition, the possibility that Fp activity is under cell cycle control is raised. Since the supply of functional E2F varies during the cell cycle and since in these assays overexpression of E2F can overcome repression of Fp by EBNA-1, control of transcription of EBNA-1 mRNA by cell cycle regulatory factors may help to bring about ordered replication of episomes.

Reciprocal regulation of the Epstein-Barr virus BamHI-F promoter by EBNA-1 and an E2F transcription factor

The Epstein-Barr virus BamHI-F promoter (Fp) is one of three used to transcribe the EBNA latency proteins, in particular, EBNA-1, the only viral gene product needed for episomal replication. Fp is distinguished by possession of the only EBNA-1 binding sites (the Q locus) in the Epstein-Barr virus genome outside oriP. Activity of Fp is negatively autoregulated by interaction of EBNA-1 at two sites in the Q locus, which is situated downstream of the RNA start site. We demonstrate in transient assays that this EBNA-1-mediated repression of Fp can be overcome by an E2F transcription factor which interacts with the DNA at a site centered between the two EBNA-1 binding sites within the Q locus. An E2F-1 fusion protein protects the sequence 5'-GGATGGCGGGTAATA-3' from DNase I digestion, and a DNA probe containing this sequence binds an E2F-specific protein complex from cell extracts, although this region is only loosely homologous with known consensus binding sites for E2F transcription factors. In mobility shift assays, E2F can displace the binding of EBNA-1 from the Q locus but not from oriP, where the E2F binding site is not present. E2F also activates expression of Fp in epithelial cells. These findings identify a potentially new binding site for members of the E2F family of transcription factors and suggest that such a factor is important for expression of EBNA-1 in lymphoid and epithelial cells by displacing EBNA-1 from the Q locus. In addition, the possibility that Fp activity is under cell cycle control is raised. Since the supply of functional E2F varies during the cell cycle and since in these assays overexpression of E2F can overcome repression of Fp by EBNA-1, control of transcription of EBNA-1 mRNA by cell cycle regulatory factors may help to bring about ordered replication of episomes.

Functional characterization of partially purified Epstein-Barr virus DNA polymerase expressed in the baculovirus system

The DNA polymerase gene of Epstein-Barr virus (EBV) was cloned into baculovirus transfer vector (pBlueBac). The recombinant baculovirus (AcEBP-15) was obtained by cotransfection of Spodoptera frugiperda (Sf9) cells with infectious DNA from Autographa californica multiple nuclear polyhedrin virus (AcMNPV) and pBlueBac plasmid carrying EBV polymerase gene. Infection of Sf9 cells with the recombinant virus produced substantial quantities of the EBV DNA polymerase protein of the expected size (110 kD). The identity of the EBV polymerase 110-kD polypeptide was determined by (a) immunoprecipitation and Western blot analyses with rabbit polyclonal antiserum specific for a synthetic peptide derived from the coding sequence of the polymerase gene; (b) identification of a polypeptide of identical size (110 kD) from EBV-infected cells; (c) measurement of DNA polymerase activity similar to that of the enzyme induced in EBV-infected cells; and (d) neutralization of the enzymatic activity by the rabbit antiserum and inhibition by phosphonoacetic acid. Our results indicate that the baculovirus expression system provides large quantities of functional polymerase suitable for biochemical and structural analyses, thereby furthering our understanding of the mechanism of viral DNA replication and its inhibition by antiviral drugs.

An ATF/CREB site is the major regulatory element in the human herpesvirus 6 DNA polymerase promoter

Human herpesvirus 6 (HHV-6) is a recently described T-cell pathogen whose medical relevance and molecular biology are just beginning to be addressed. As a first look at the regulation of viral genes, control of the HHV-6 DNA polymerase promoter was examined. Polymerase gene transcription in HHV-6-infected cells was found to initiate from a single site located 115 bases upstream of the translation start codon. A polymerase promoter-chloramphenicol acetyltransferase reporter gene construct failed to be expressed in uninfected T cells but was highly active in HHV-6-infected cells. Mutational data indicated that the polymerase promoter is TATA-less. Mutational analysis also revealed that the major upstream promoter regulatory element required for transcriptional activity in HHV-6-infected cells is a palindromic ATF/CREB transcription factor binding site. The significance of this site for promoter induction was further demonstrated by the fact that the polymerase ATF/CREB element, when appended to a heterologous basal promoter, is highly responsive to HHV-6 infection. Two protein complexes were found to bind in a specific manner to the ATF/CREB motif in both uninfected and HHV-6-infected T-cell nuclear extracts. Site-specific mutation of the ATF/CREB site resulted in loss of protein binding as well as loss of promoter activity in HHV-6-infected cells.

Characterization of the DNA-binding site repertoire for the Epstein-Barr virus transcription factor R

The Epstein-Barr virus gene BRLF1 encodes the transcription factor R, which is a sequence-specific DNA-binding protein important for the switch from latency to a productive cycle. We have defined a repertoire of specific R-binding sites using a GST-R fusion protein and a pool of 23 bp random DNA sequences. The R-bound sequences were selected by several rounds of Electrophoretic Mobility Shift Assay (EMSA) and amplification by PCR. Among the 45 sites selected, some positions in the sequences were highly conserved, i.e., 5'-GTGCC N7GTGGTG-3'. The guanine methylation assay revealed that R simultaneously contacts guanines in the two conserved cores, defining the consensus binding site 5'-GNCC N9 GGNG-3', and 30 sites among the 45 selected have this sequence. This last result also suggests that R binds two adjacent major grooves of the DNA. As shown by EMSA assay, R binds to all the sites tested with a comparable affinity, and they all mediate R-induced transcriptional activation in a transient expression assay.

RAZ, an Epstein-Barr virus transdominant repressor that modulates the viral reactivation mechanism

Epstein-Barr virus (EBV) is associated with the development of several types of human cancers and is an important cause of lymphomas in immunocompromised hosts. Expression of the EBV BZLF1 immediate-early gene product (Z) triggers disruption of latency in EBV-infected cells. Z is a member of the b-Zip family of proteins and binds to AP-1-like sites in early viral promoters. Here we show that a viral RNA related to Z, in which there is replacement of the transactivation domain of Z by fusion through alternate splicing with a portion of another EBV transactivator, BRLF1 (R), can repress Z function. This differentially spliced mRNA is predicted to express a novel chimeric protein which we call RAZ for R and Z. RAZ retains the dimerization and DNA-binding domains of Z but loses its transactivation domain. We show that in vitro the RAZ protein acts transdominantly to repress transactivation of early promoters by Z. Repression is produced by dimerization of RAZ with Z resulting in RAZ:Z heterodimers that can no longer bind to Z-binding sites despite retention of the DNA-binding domains in both proteins. Deletion of the R domain of RAZ restores the ability of the truncated RAZ homodimers and RAZ:Z heterodimers to bind to DNA. A biologic effect of RAZ was shown by cotransfection of latently infected Raji cells with Z and RAZ expression clones; RAZ diminished viral reactivation induced by Z, as indicated by amount of early replicative antigens (EA-D) detected. The RAZ protein presents a model for transcriptional control unique among the herpesvirus and distinct from analogous viral and cellular repressors. RAZ, by limiting the availability of Z protein, is likely to modulate EBV reactivation.

Unconventional processing of the 3' termini of the Epstein-Barr virus DNA polymerase mRNA

Northern blot analysis of the Epstein-Barr virus DNA polymerase mRNA identified two discrete sizes of virally encoded polymerase transcripts, 5.08 kb detected in strains P3HR1, Raji, W-91, and FF-41 and 3.7 kb detected solely in the prototype B95-8 strain. 3' S1-nuclease mapping and analysis of cDNA sequence generated by RNA-based PCR demonstrated that the 3.7-kb polymerase mRNA from B95-8 terminates 484 base pairs downstream of the open reading frame in a region of the genome remarkable for its lack of an apparent polyadenylylation signal. Moreover, between the cleavage point and the poly(A) tract of the cDNAs are a series of inserted nucleotides, mostly adenosine and uridine residues of unknown origin. A similar analysis of the 3' terminus of the 5.0-kb mRNA from the other cell lines revealed that polyadenylylation occurs 1.4 kb downstream of the B95-8 terminus. This region is deleted in B95-8, which accounts for the alternate upstream terminus used in B95-8. Like the 3.7-kb terminus, the 5.0-kb terminus lacks a canonical polyadenylylation signal, but contains a rarely used UAUAAA sequence 32 bp upstream of the poly(A) tail. These results indicate that the mRNA encoded by the Epstein-Barr virus DNA polymerase gene is polyadenylylated at two different termini without the use of canonical signals, raising the possibility of involvement of a virus-encoded factor in 3' processing of this message.