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

YY1 binds to and regulates cis-acting negative elements in the Epstein-Barr virus BZLF1 promoter

A 48-bp cis-acting negative element in the Epstein-Barr virus BZLF1 gene P1 promoter has been described previously. By DNase I footprinting experiments, two regions were identified as the protein-binding sites (previously designated site I and site II). In this report, the cellular transcription factor YY1 has been identified as a protein which binds to both of these elements, now designated ZIVA and ZIVB. Both ZIVA and ZIVB conferred cis-acting negative regulation on an enhancerless simian virus 40 promoter. In cotransfection experiments, overexpression of YY1 caused further repression of the enhancerless simian virus 40 promoter containing either the ZIVA or ZIVB element. Cotransfection of a plasmid expressing antisense to YY1 increased the expression of the heterologous promoter containing ZIVA but not ZIVB. In similar experiments carried out with the P1 promoter, overexpression of YY1 caused downregulation of P1 whereas antisense RNA to YY1 caused a slight increase in expression. Analyses of various P1 mutant constructions revealed additional YY1 sites downstream of ZIVB. Overexpression of YY1 also caused downregulation of a P1 mutant with no apparent YY1-binding sites. TPA treatment of Raji cells caused a temporal loss of YY1-binding activity but had no effect on the intracellular levels of YY1 protein. Serum induction of quiescent B cells also caused loss of YY1 binding to the ZIVB site, which was found to be a weak serum response element. In contrast, anti-immunoglobulin G treatment of Akata cells had no effect on either the YY1-binding activity or protein levels. The binding of YY1 to the cis-acting negative elements in infected B cells may play a pivotal role in the maintenance of Epstein-Barr virus latency.

The Zif268 cellular transcription factor activates expression of the Epstein-Barr virus immediate-early BRLF1 promoter

The Epstein-Barr virus immediate-early protein BZLF1 mediates the switch from latent to lytic infection. BZLF1 transcription can be derived from either the BZLF1 promoter or the BRLF1 promoter (Rp). Productive viral infection of EBV-infected B cells can be induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment, as well as cross-linking of surface immunoglobulin with antiimmunoglobulin antibody. Both TPA and antiimmunoglobulin antibody are known to activate expression of the cellular transcription factor Zif268 in B cells. In this study, we have examined the regulation of BZLF1 transcription by Zif268. We show that Rp (but not the BZLF1 promoter) is activated by Zif268. Bacterially synthesized Zif268 binds strongly to an upstream sequence in the Rp promoter (located from -131 to -123 relative to the start site) and more weakly to a proximal sequence (-49 to -40). Zif268 activation of Rp requires these two Zif268 binding sites. TPA treatment of B cells induces the expression of Zif268 protein, which binds to Rp. Furthermore, TPA activation of Rp requires the upstream Zif268 site. These findings indicate that Zif268 can activate a critical Epstein-Barr virus immediate-early promoter and, therefore, may play a key role in the regulation of viral latency.

Epstein-Barr virus lytic replication is controlled by posttranscriptional negative regulation of BZLF1

Regulation of the immediate-early gene BZLF1 is assumed to play a key role in triggering the lytic replication of Epstein-Barr virus (EBV). The expression of BZLF1 is regulated on multiple levels, including control of transcription by several positive and negative cis-acting elements as well as posttranslational modifications and protein-protein interactions. Localization of BZLF1 on one strand of the genome and the latent EBNA1 transcription unit on the complementary strand suggests a regulatory mechanism via hybridization of antisense RNA. With a plasmid encoding a defective BZLF1 RNA, which could not be translated, we were able to induce expression of endogenous BZLF1 gene product Zta and other proteins of the lytic cycle. Our data show for the first time that latent replication is stabilized by negative regulation of an immediate-early gene of the lytic cycle by a posttranscriptional mechanism. This might be a common theme of herpes simplex virus and EBV latency.

Differential Epstein-Barr virus gene expression in B-cell subsets recovered from lymphomas in SCID mice after transplantation of human peripheral blood lymphocytes

We have analyzed the human B-cell tumors that arise spontaneously in SCID mice who have been given transplants of peripheral blood lymphocytes from Epstein-Barr virus (EBV)-seropositive donors to determine if patterns of EBV gene expression are correlated with phenotypic changes in the tumor B cells. Tumor cells were separated into two B-cell subsets by cell sorting on the basis of differential coexpression of membrane CD23 and CD38. One subset showed intermediate levels of CD23 and CD38 expression (CD23intCD38int), while a second subset had low-level CD23 but high-level CD38 expression (CD23loCD38hi). The CD23intCD38int cells had a high proliferative index and secreted little immunoglobulin in vitro; the CD23loCD38hi cells had a low proliferative index and high-level immunoglobulin secretion. We next analyzed the sorted cells for viral transcripts associated with latency (EBNA-1, EBNA-2, and LMP-1) or lytic cycle replication (ZEBRA and gp350 envelope protein). Only latent cycle transcripts were found in CD23intCD38int cells, whereas lytic cycle transcripts and transforming virus were present in the CD23loCD38hi cells. Finally, we generated short-term cell lines from the sorted CD23intCD38int cells and transferred these cells to SCID recipients. The resulting secondary tumors were predominantly CD23loCD38hi, suggesting that the CD23intCD38int lymphoblastoid cells are precursors to the well-differentiated, plasmacytoid CD23loCD38hi cells. These observations are discussed in the context of a three-step model for EBV-associated lymphomagenesis in humans.

Epstein-Barr virus latency in blood mononuclear cells: analysis of viral gene transcription during primary infection and in the carrier state

Epstein-Barr virus (EBV) can display different forms of latent infection in B-cell lines in vitro; however, the types of infection normally established by the virus in vivo remain largely unexplored. Here we have approached this question by analyzing the types of viral RNAs present in mononuclear cells freshly isolated from the blood of 14 infectious mononucleosis patients undergoing primary EBV infection and 6 long-term virus carriers. Reverse transcription-PCR amplifications were carried out with a panel of oligonucleotide primers and probes which specifically detect (i) the EBER1 RNA common to all forms of latency, (ii) transcripts either from the Cp and Wp promoters generating all six nuclear antigen (EBNA1, -2, -3A, -3B, -3C, -LP) mRNAs or from the Fp promoter generating a uniquely spliced EBNA1 mRNA, (iii) the latent membrane protein (LMP1 and 2A) mRNAs, and (iv) the BZLF1 mRNA, an immediate-early marker of lytic cycle. Viral transcription in infectious mononucleosis mononuclear cells (and in the B-cell-enriched fraction) regularly included the full spectrum of latent RNAs seen during EBV-induced B-cell growth transformation in vitro, i.e., EBER1, Cp/Wp-initiated EBNA mRNAs, and LMP1/LMP2 mRNAs, in the absence of lytic BZLF1 transcripts. In addition, transcripts with the splice pattern of Fp-initiated EBNA1 mRNA, hitherto seen only in vivo in certain EBV-positive tumors, were frequently detected. In long-term virus carriers, the mononuclear cells were again positive for latent (EBER1) and negative for lytic (BZLF1) markers; Cp/Wp-initiated RNAs were not detected in these samples, but in several individuals it was possible to amplify both Fp-initiated EBNA1 mRNA and LMP2A mRNA signals. We suggest (i) that primary infection is associated with a transient virus-driven expansion of the infected B-cell pool through a program of virus gene expression like that seen in in vitro-transformed cells and (ii) that long-term virus carriage is associated with a switch from Cp/Wp to Fp usage and thus to a more restricted form of latent protein expression that may render the infected cells less susceptible to recognition by the virus-specific cytotoxic T-cell response.

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.

Different activation of Epstein-Barr virus immediate-early and early genes in Burkitt lymphoma cells and lymphoblastoid cell lines

Specific expression of the Epstein-Barr virus (EBV) immediate-early and early gene products Zta, Rta, I'ta, and MSta by a recombinant vaccinia virus system allowed us to analyze the first steps in the induction of the lytic cycle in EBV-infected Burkitt lymphoma (BL) cells and lymphoblastoid cell lines (LCLs). Significant differences in the induction of early genes were found between these cell types: whereas in BL cells the trans activator Zta was found to induce key steps of the early lytic cycle, only minor activities of Zta were noted in LCLs. Contrary to Zta, the trans activator Rta was found to be highly effective in LCLs. These observations suggest that Rta may play an important role in the activation of the early lytic cycle in LCLs, although it cannot be activated by Zta. The latter may be a reason for the lower tendency of LCLs to switch into the lytic cycle compared with BL cells or differentiated epithelial cells.

Epstein-Barr virus induces fragmentation of chromosomal DNA during lytic infection

Pulsed-field agarose gel electrophoresis showed that fragmentation of chromosomal DNA in Raji cells was induced by infection with the P3HR-1 strain of Epstein-Barr virus (EBV). S1 nuclease treatment of the agarose plugs containing cells suggested that the majority of DNA fragments did not contain single-strand gaps. Chromosomal DNA fragmentation was inhibited by cycloheximide, indicating that protein synthesis was required for DNA fragmentation. Phosphonoacetic acid, an inhibitor of EBV DNA polymerase, did not inhibit fragmentation of chromosomal DNA. These findings suggest that EBV-specific early proteins participate in fragmentation of chromosomal DNA. Chromosomal DNA of P3HR-1 cells was also fragmented by treatment with n-butyrate plus 12-O-tetradecanoylphorbol-13-acetate (TPA), which induced activation of latent EBV genome following viral replication. In addition, fragmentation of DNA preceded cell death during lytic infection. These results suggest that fragmentation of chromosomal DNA is generally induced during EBV replication and probably contributes to the cytopathic effect of EBV. The role of DNA fragmentation in death of infected cells is discussed in relation to apoptosis.

Activation of the Epstein-Barr virus replicative cycle by human herpesvirus 6

One common attribute of herpesviruses is the ability to establish latent, life-long infections. The role of virus-virus interaction in viral reactivation between or among herpesviruses has not been studied. Preliminary experiments in our laboratory had indicated that infection of Epstein-Barr virus (EBV) genome-positive human lymphoid cell lines with human herpesvirus 6 (HHV-6) results in EBV reactivation in these cells. To further our knowledge of this complex phenomenon, we investigated the effect of HHV-6 infection on expression of the viral lytic cycle proteins of EBV. Our results indicate that HHV-6 upregulates, by up to 10-fold, expression of the immediate-early Zebra antigen and the diffuse and restricted (85 kDa) early antigens (EA-D and EA-R, respectively) in both EBV producer and nonproducer cell lines (i.e., P3HR1, Akata, and Raji). Maximal EA-D induction was observed at 72 h post-HHV-6 infection. Furthermore, expression of late EBV gene products, namely, the viral capsid antigen (125 kDa) and viral membrane glycoprotein gp350, was also increased in EBV producer cells (P3HR1 and Akata) following infection by HHV-6. By using dual-color membrane immunofluorescence, it was found that most of the cells expressing viral membrane glycoprotein gp350 were also positive for HHV-6 antigens, suggesting a direct effect of HHV-6 replication on induction of the EBV replicative cycle. No expression of late EBV antigens was observed in Raji cells following infection by HHV-6, implying a lack of functional complementation between the deleted form of EBV found in Raji cells and the superinfecting HHV-6. The susceptibility of the cell lines to infection by HHV-6 correlated with increased expression of various EBV proteins in that B95-8 cells, which are not susceptible to HHV-6 infection, did not show an increase in expression of EBV antigens following treatment with HHV-6. Moreover, UV light-irradiated or heat-inactivated HHV-6 had no upregulating effect on the Zebra antigen or EA-D in Raji cells, indicating that infectious virus is required for the observed effects of HHV-6 on these EBV products. These results show that HHV-6, another lymphotropic human herpesvirus, can activate EBV replication and may thus contribute to the pathogenesis of EBV-associated diseases.

Analysis of the BZLF1 promoter of Epstein-Barr virus: identification of an anti-immunoglobulin response sequence

The induction of the viral lytic cycle in latently Epstein-Barr virus (EBV)-infected B cells is initiated by activation of the BZLF1 gene, whose expression is sufficient to disrupt EBV latency, suggesting that BZLF1 acts as the switch to change from a latent to a lytic replicative cycle. In the present studies, a series of deletion plasmids encompassing positions bp -552 to +12 of the BZLF1 promoter were constructed and tested for their response to anti-immunoglobulin (anti-Ig), an inducer of the viral lytic cycle, upon transfection into EBV-negative and -positive lymphoid cells. The promoter consisted of three functionally distinct regions. Region I (bp -552 to -221) had a negative influence on promoter activity; its deletion made the promoter highly responsive to anti-Ig. Region II (bp -203 to -177) was important for conferring responsiveness to anti-Ig. The response to anti-Ig did not require the presence of the EBV genome or EBV gene products. This sequence also enhanced expression of the chloramphenicol acetyltransferase (cat) gene from the simian virus 40 promoter in response to anti-Ig, even when inserted downstream of the cat gene. Region III (-134 to -116) was a positive element that was transactivated by the BZLF1 gene product.

The lytic transition of Epstein-Barr virus is imitated by recombinant B-cells

Lytic transition of Epstein-Barr virus (EBV) is initiated by distinct immediate early regulators of the viral cycle, in synchronization to temporary, permissive conditions during host cell differentiation. We developed eukaryotic vectors suitable to imitate the processes involved in lytic transition in cell culture systems. Two stable B cell lines were established: R59Z activator cells were used to induce lytic EBV expression in a constitutive manner by the production of the BZLF 1 trans-activator (Zta). R7-57 reporter cells, on the other hand, signaled induced activity of the lytic origin of EBV replication (ori Lyt). Different modes, like chemical induction, lytic superinfection with EBV and single gene trans-activation converted the recombinant ori Lyt element in R7-57 reporter cells. BZLF 1, transiently expressed in R7-57 reporter cells, was the only EBV trans-activator found, sufficient in inducing the viral lytic cycle. Basing on these experiments, trans-cellular activation of EBV was tested by cocultivation of BZLF 1-expressing R59Z activator cells with the R7-57 reporter line. No lytic effect on the reporter cells could be measured, neither by cocultivation of activator cells nor by coincubation of BZLF 1-containing cell lysates. Latency breaking activity, however, was transferred from activator to reporter cells when active, exogenous virus was added. The cell system described in these experiments provides a tool for the detection of EBV reactivation and demonstrates the potential of the lytic regulatory gene BZLF 1.

Characterization of a bovine herpesvirus 4 immediate-early RNA encoding a homolog of the Epstein-Barr virus R transactivator

Immediate-early (IE) RNA 2, the less abundant of two bovine herpesvirus 4 (BHV-4) RNAs detected in Madin-Darby bovine kidney cells infected in the presence of cycloheximide, is a 1.8-kb cytoplasmic polyadenylated RNA transcribed from the 8.3-kb HindIII fragment F. The structure of IE RNA 2 has been determined by S1 nuclease and exonuclease VII mapping, primer extension analysis, and sequencing of a partial cDNA. IE RNA 2 consists of a short, approximately 60-nucleotide 5' exon spliced to a 1.8-kb 3' exon. DNA sequence analysis revealed an open reading frame encoding 551 amino acids with sequence homology to the Epstein-Barr virus (EBV) R transactivator and its homolog in herpesvirus saimiri, HVS.R.IE 2 and HVS.R show higher homology to each other than to the EBV R transactivator. The homology is highest in the approximately 320 amino-terminal amino acids. All three proteins have acidic carboxyl termini but have little amino acid sequence homology in this region. In transient expression cotransfection assays, IE 2 activated expression from the BHV-4 early promoter-regulatory region of the major DNA-binding protein homolog over 100-fold in bovine turbinate cells. IE 1 was not necessary for this transactivation and did not augment it. However, IE 2 did not transactivate EBV or herpesvirus saimiri early promoter-regulatory regions that are transactivated by the EBV R transactivator or HVS.R.

The DNA-binding domain of two bZIP transcription factors, the Epstein-Barr virus switch gene product EB1 and Jun, is a bipartite nuclear targeting sequence

The Epstein-Barr virus BZLF1 gene product EB1 (also called ZEBRA and Zta), is a transcription factor belonging to the bZIP (basic domain leucine zipper) family of nuclear proteins. Translocation to the nucleus of EB1 (J. Becker, U. Leser, M. Marschall, A. Langford, W. Jilg, H. Gelderblom, P. Reichart, and H. Wolf, Proc. Natl. Acad. Sci. USA 88:8332-8336, 1991) and of two other bZIP proteins, c-Jun and c-Fos (P. Roux, J.-M. Blanchard, A. Fernandez, N. Lamb, P. Jeanteur, and M. Piechaczyk, Cell 63:341-351, 1990), has been shown to be subject to regulation. We show here that for both EB1 and Jun the nuclear targeting signals (NTS) in the proteins' primary sequences are two clusters of positively charged amino acids. These clusters, called BRA and BRB, are necessary and sufficient to direct beta-galactosidase to the nuclear compartment and act as a bipartite NTS. They are conserved among all the bZIP proteins, and although they are not identical, they probably share the same function. Site-directed mutagenesis studies made on these basic clusters suggest that they also act as a bipartite NTS in the EB1 protein. Our results also demonstrate that in EB1 and Jun, these bipartite NTS are superimposed with bipartite DNA-binding domains, since BRA and BRB are required in vitro for direct and specific contact between these proteins and their DNA-binding sites.

The Epstein-Barr virus immediate-early promoter BRLF1 can be activated by the cellular Sp1 transcription factor

Disruption of viral latency in Epstein-Barr virus-infected cells is mediated through the activation of the BZLF1 (Z) immediate-early gene product. The Z protein can be derived from either of two promoters: the BZLF1 promoter, which directs transcription of a 1.0-kb mRNA encoding the Z gene product alone, or the upstream BRLF1 promoter, which directs transcription of a 2.8-kb bicistronic mRNA encoding the BRLF1 and BZLF1 immediate-early proteins. In this study we have examined the regulation of the BRLF1 promoter by viral and cellular factors. We found that the BRLF1 promoter is autoregulated by the BRLF1 transactivator through a nonbinding mechanism. We show that the BRLF1 (but not the BZLF1) promoter is highly responsive to the Sp1 transcription factor. Sp1 activation of the BRLF1 promoter is mediated through a consensus Sp1-binding site located from -39 to -44 (relative to the mRNA start site). We demonstrate that the BRLF1 promoter has high constitutive activity in C-33 cells (an epithelial cell line) and that the proximal Sp1-binding site is required for this activity. Despite the ubiquitous presence of Sp1 in many cell types, we found that the BRLF1 promoter has essentially no activity in lymphoid cell lines, suggesting that factors other than Sp1 may negatively regulate the BRLF1 promoter in these cells. Our findings demonstrate that the two potential promoters directing BZLF1 transcription are differentially regulated and that Sp1 can activate the BRLF1 promoter but not the BZLF1 promoter.

The Epstein-Barr virus (EBV) nuclear antigen 1 BamHI F promoter is activated on entry of EBV-transformed B cells into the lytic cycle

In Epstein-Barr virus (EBV)-positive Burkitt's lymphoma cell lines exhibiting the latency I form of infection (i.e., EBV nuclear antigen 1 [EBNA1] positive in the absence of other latent proteins), the EBNA1 mRNA has a unique BamHI Q/U/K splice structure and is expressed from a novel promoter, Fp, located near the BamHI FQ boundary. This contrasts with the situation in EBV-transformed lymphoblastoid cell lines (LCLs) exhibiting the latency III form of infection (i.e., positive for all latent proteins), in which transcription from the upstream Cp or Wp promoters is the principal source of EBNA mRNAs. We carried out cDNA amplifications with oligonucleotide primer-probe combinations to determine whether Fp is ever active in an LCL environment. The results clearly showed that some LCLs express a Q/U/K-spliced EBNA1 mRNA in addition to the expected Cp/Wp-initiated transcripts; this seemed inconsistent with the concept of Cp/Wp and Fp as mutually exclusive promoters. Here we show that Fp is indeed silent in latency III cells but is activated at an early stage following the switch from latency III into the virus lytic cycle. Four pieces of evidence support this conclusion: (i) examples of coincident Cp/Wp and Fp usage in LCLs are restricted to those lines in which a small subpopulation of cells have spontaneously entered the lytic cycle; (ii) transcripts initiating from Fp can readily be demonstrated in spontaneously productive lines by S1 nuclease protection; (iii) the presence of Fp-initiated transcripts is not affected by acyclovir blockade of the late lytic cycle; and (iv) infection of latently infected LCLs with a recombinant vaccinia virus encoding the EBV immediate-early protein BZLF1, a transcriptional transactivator which normally initiates the lytic cycle, results in the appearance of the diagnostic Q/U/K-spliced transcripts.

Differential splicing of Epstein-Barr virus immediate-early RNA

A variant form of splicing of RNA crossing the Epstein-Barr virus (EBV) BZLF1 gene was observed in the late productive cycle of EBV. This splice omits the middle exon of BZLF1 and joins the outer two exons of BZLF1 in frame, but the shortened form of BZLF1 protein (Z delta) could not be detected in natural EBV infections.

The Epstein-Barr virus R transactivator (Rta) contains a complex, potent activation domain with properties different from those of VP16

Rta, encoded by Epstein-Barr virus (EBV), is a potent activator of transcription via enhancer sequences located upstream of several viral genes. To identify the domains of Rta that facilitate transcription by interacting with cellular transcription factors, different segments of Rta were linked to the DNA binding domain of yeast transactivator GAL4 (residues 1 to 147). These GAL4-Rta fusion proteins were tested in transfected cells for their ability to activate the adeno E1b promoter with an upstream GAL4 DNA binding site. The acidic C-terminal domain of Rta (amino acids 520 to 605) was a potent activator but behaved differently from VP16 in dose-response and competition experiments. A subterminal domain of Rta (amino acids 416 to 519) linked to GAL4 had weak activation activity. Deletion of these domains from native Rta showed that the C-terminal domain was required for transactivation, but the subterminal domain was required only in B cells. The C-terminal activation domain of Rta contains a pattern of positionally conserved hydrophobic residues shared with VP16 and other transactivators. Substitution of several conserved hydrophobic amino acids in Rta severely impaired transactivation. The improtance of hydrophobic residues was further substantiated by comparing EBV Rta with that of herpesvirus saimiri, which revealed little sequence similarity except for a few acidic residues and the positionally conserved hydrophobic amino acids. The C-terminal domain of EBV Rta contains three partially overlapping copies of this hydrophobic motif. Mutational analysis indicated that all three copies were required for full activity. However, two of the three copies appeared to be sufficient to produce full activity on a target promoter with multiple binding sites, suggesting that these motifs are functional subdomains that can synergize.

Epstein-Barr virus BZLF1 transactivator is a negative regulator of Jun

The Epstein-Barr virus BZLF1 protein that can induce the lytic cycle in latently infected cells is a transcription factor partially homologous to Fos and binds not only the canonical TPA (tetradecanoyl phorbol acetate)-responsive element (TRE) site but also sequences deviating from the TRE consensus sequence. Thus, expression of cellular genes regulated by AP-1, including the autoregulated AP-1 family, should be affected by BZLF1. However, induction of only Fos by BZLF1 was observed in a gel mobility shift assay using an oligonucleotide probe containing the TRE sequence and the antibody against Fos protein. The c-jun promoter, which contains a binding site for Jun and BZLF1, was stimulated by Jun but not by BZLF1. Furthermore, BZLF1 inhibited stimulation of the c-jun promoter by Jun. Jun together with Fos effectively activated the collagenase promoter that contains a single TRE site. However, not only was BZLF1 unable to stimulate the collagenase promoter, but it also inhibited activation by Jun and Fos. On the other hand, BZLF1 stimulated constructs containing multimeric binding sites. These results and those of previous studies of Epstein-Barr virus promoters regulated by BZLF1 indicate that BZLF1 requires adjacent multiple DNA-binding sites for cooperative interaction to function as a transactivator and to repress the activation by Jun of promoters containing a single TRE site. This suggests that BZLF1 evolved to confer distinct regulatory patterns upon viral target genes and cellular AP-1-responsive genes.

Expression cloning of a cDNA encoding a retinoblastoma-binding protein with E2F-like properties

An expression vector was modified to permit the rapid synthesis of purified, 32P-labeled, glutathione S-transferase (GST)-retinoblastoma (RB) fusion proteins. The products were used to screen lambda gt11 expression libraries, from which we cloned a cDNA encoding a polypeptide (RBAP-1) capable of binding directly to a putative functional domain (the pocket) of the retinoblastoma gene product (RB). The RB "pocket" is known to bind, directly or indirectly, to the cellular transcription factor, E2F, implicated in cell growth control. We have found that RBAP-1 copurifies with E2F, interacts specifically with the adenovirus E4 ORF 6/7 protein, binds specifically and directly to a known E2F DNA recognition sequence, and contains a functional tranasactivation domain. Therefore, RBAP-1 is a species of E2F and can bind specifically to the RB pocket.

Characterization of the Epstein-Barr virus BZLF1 protein transactivation domain

Initiation of the Epstein-Barr virus (EBV) lytic cycle is dependent on expression of the viral transactivator Zta, which is encoded by the BZLF1 gene. Described here is an initial mapping of the regions of Zta involved in activating transcription. The data indicate that the amino-terminal 153 amino acids of Zta are important for activity, and in particular the region from residues 28 to 78 appears to be critical for Zta function. However, other features of Zta may be important for activity since a Gal4-Zta chimeric protein, generated by fusing the amino-terminal 167 residues of Zta to the DNA binding domain of the yeast transactivator Gal4, transactivated a minimal promoter containing one upstream Gal4 binding site but was unable to exhibit synergistic transactivation when assayed with a reporter containing five upstream Gal4 binding sites.

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.

Early events in Epstein-Barr virus genome expression after activation: regulation by second messengers of B cell activation

RNA transcription from the BamHI Z and BamHI R and HindIII G regions of the Epstein-Barr virus (EBV) genome was studied after treatment of Akata cells with anti-immunoglobulin G (IgG), with second messenger agonists or antagonists to determine how latent EBV activation is regulated by B cell second messengers. Northern gel analysis demonstrated that BZLF1, BZLF1 + BRLF1, and BMLF1 + BSLF2 transcripts were induced at 2 hr and increased in concentration at 4 hr after induction with anti-IgG; transcripts from BRRF1, BaRF1, BMLF1, and BMRF1 were initiated at 4 hr; a transcript from BRRF2 appeared at 6 hr. The patterns of transcription from these genes after repeated stimulations with calcium ionophore A23187 + dioctanoylglycerol paralleled those with anti-IgG except that times of initiation were delayed by about 2 hr. Nuclear run-off assay of BZLF1 gene showed rapid increases in their transcriptions from 30 to 60 min after anti-IgG treatment. The protein kinase C antagonist, staurosporine, completely blocked the appearance of these transcripts, while 8-bromo cAMP + theophylline suppressed the transcription by about 40%. The regulation of EBV activation in Akata cells with anti-IgG or with second messenger agonists or antagonists can be explained by regulation at the level of transcription of immediate-early genes of EBV.

Expression of the Epstein-Barr virus BamHI A fragment in nasopharyngeal carcinoma: evidence for a viral protein expressed in vivo

A family of mRNAs that are transcribed rightward through the BamHI A fragment have been detected in C15, a nasopharyngeal carcinoma (NPC) which has been passaged in nude mice. Northern (RNA) blot hybridizations indicate that these RNAs are also expressed in three other NPCs which have been established in nude mice and in an NPC obtained at biopsy. Moreover, hybridization in situ detected transcription from BamHI A in 12 NPCs and 1 Epstein-Barr virus (EBV)-containing carcinoma of the parotid gland. In each case, transcription was detected in all of the malignant epithelial cells. Transcription was not detected in two cases of EBV-positive lymphoma biopsies by in situ hybridization nor in latently infected EBV-positive lymphoblastoid cell lines by Northern blot hybridization. The consistent transcription of these sequences in latently infected epithelial malignancy but not in lymphoid cells suggests that this viral function is associated with latent EBV infection of epithelial cells. Sequence analysis of a cDNA synthesized from the C15 tumor, representing the 3' end of BamHI A messenger RNA, revealed an open reading frame (ORF). Translation of this ORF in vitro produced several peptides that were immunoprecipitated with antisera from patients with NPC. The detection of antibodies to the protein encoded by the ORF present in the BamHI A cDNA indicates that BamHI A encodes a protein which is expressed in vivo and is antigenic.

ZEBRA and a Fos-GCN4 chimeric protein differ in their DNA-binding specificities for sites in the Epstein-Barr virus BZLF1 promoter

Epstein-Barr virus (EBV) encodes a protein, ZEBRA, which enables the virus to switch from a latent to a lytic life cycle. The basic domain of ZEBRA is homologous to the Fos/Jun oncogene family, and both proteins bind the canonical AP-1 site (TGAGTCA). However, ZEBRA does not contain a leucine zipper dimerization domain which has been shown to be necessary for DNA binding of Fos/Jun proteins. Additionally, ZEBRA binds to sites which deviate from the AP-1 consensus sequence. Thus, it was of interest to define the domain of the ZEBRA protein required for DNA binding. We have determined by mutagenesis that ZEBRA residues 172 to 227, representing the basic domain and a putative dimerization domain, are required for specific binding to AP-1 and divergent sites. Mutagenesis of the basic amino acids 178 to 180 or 187 to 189 abrogates ZEBRA binding to all DNA target sequences. These residues are conserved in Fos and are also necessary for Fos DNA-binding activity. We have found that a Fos-GCN4 chimera and ZEBRA have different cognate binding specificities. The autoregulated BZLF1 promoter contains three divergent AP-1 sequences, ZIIIA (TGAGCCA), ZIIIB (TTAGCAA), and Z-AP-1-octamer (TGACATCA). ZEBRA binds with high specificity to ZIIIA and ZIIIB but weakly to the Z-AP-1 octamer. Conversely, the Fos-GCN4 chimera recognizes only the Z-AP-1 octamer. ZEBRA binds the ZIIIA and ZIIIB sites together in a noncooperative fashion, while Fos-GCN4 binds these sites as a higher-order complex. Additionally, we have found that flanking sequences influence binding of Fos-GCN4 to a degenerate AP-1 site (TGAGCAA). The characteristic binding specificities of ZEBRA and cellular AP-1 proteins suggest that they differentially affect viral and cellular transcription.

Pathways of activation of the Epstein-Barr virus productive cycle

The promoter for the 2.8-kb RNA of Epstein-Barr virus encoding BZLF1 and BRLF1 was identified and shown to be activated by both BZLF1 and BRLF1 but not by 12-O-tetradecanoylphorbol-13-acetate. Site-directed mutagenesis suggests that two binding sites for BZLF1 within the promoter contribute to the transactivation by BZLF1. The early kinetics of induction of the 2.8- and 1.0-kb RNAs encoding BZLF1 and BRLF1 in Akata cells treated with anti-immunoglobulin indicate that both RNAs appear within 60 min. The results indicate some likely pathways of activation of Epstein-Barr virus productive cycle gene expression.

Regulation of the herpesvirus saimiri (HVS) delayed-early 110-kilodalton promoter by HVS immediate-early gene products and a homolog of the Epstein-Barr virus R trans activator

We have reported previously the detection of two stable immediate-early (IE) transcripts that accumulate in cycloheximide-treated cells infected with herpesvirus saimiri (HVS). These are the 1.6-kb mRNA from the 52-kDa gene (which is homologous to the BSLF2-BMLF1 gene of Epstein-Barr virus) and the 1.3-kb mRNA from the HindIII-G fragment of virus DNA. In order to study the roles of the HVS IE gene products in the progression of a lytic infection, the promoter region of the delayed-early 110-kDa gene of HVS was sequenced, the transcription initiation site was mapped by RNase protection, and the promoter sequences were cloned upstream of the chloramphenicol acetyltransferase (CAT) gene. Sequences between -447 and +37 (relative to the 110-kDa transcription initiation site) were sufficient for response to HVS superinfection of transfected cells, but the 110-kDa promoter was activated only poorly by the 52-kDa and HindIII-G IE (IE-G) proteins in cotransfection experiments. However, a distinct region of the genome, EcoRI-D (15 kbp), was able to activate 110-kDa-CAT expression relatively efficiently in similar experiments. A 4.7-kbp PstI fragment encoding this function was isolated and sequenced, and further subcloning identified the gene encoding the EcoRI-D trans activator. This gene, which we now designate HVS.R, is homologous to the BRLF1-encoded transcriptional effector of Epstein-Barr virus.

The BI'LF4 trans-activator of Epstein-Barr virus is modulated by type and differentiation of the host cell

We have analyzed the activity and regulated expression of a new Epstein-Barr virus (EBV) trans-activator (I'ta) encoded by left reading frame 4 (BI'LF4) of the BamHI I'fragment. The gene was detected in all genomes of established EBV strains and individual isolates, with the exception of B95-8, where the type-specific deletion of this open reading frame is tolerated in vitro. Specific trans-activation of two EBV promoters (early MS and I'ta promoter) could be shown in cotransfection assays. The I'ta product affected autoactivation but had no influence on heterologous target promoters. The I'ta promoter segment was shown to be costimulated in the process of host cell differentiation in the absence of other EBV gene products. Expression of the reading frame in bacteria identified a 48-kDa protein as a stable gene product. I'ta-specific antibodies were detected in sera from EBV-positive persons (nasopharyngeal carcinoma). When expressed with suitable eucaryotic vectors, a nuclear protein could be immunostained in transfected cells. Our experiments suggest a cell type-specific requirement for I'ta in the lytic cycle of EBV at a determined differentiation stage of the host cell.

Evidence for coiled-coil dimer formation by an Epstein-Barr virus transactivator that lacks a heptad repeat of leucine residues

Two regions of the Epstein-Barr virus (EBV) BZLF1 gene product, ZEBRA, share sequence homology with c-Fos, one of which corresponds to the DNA binding domain of c-Fos. ZEBRA does not, however, contain the heptad repeat of leucines present in the dimerization domains of leucine zipper proteins. Here it is shown that ZEBRA binds its recognition sites as a homodimer and that the region adjacent to the basic DNA binding domain is essential for dimerization. This region contains a 4-3 repeat of predominantly hydrophobic residues, which is precisely in register with the hydrophobic heptad repeat present in the leucine zipper proteins with respect to the basic DNA binding domain. A mutational analysis of ZEBRA supports a model for dimerization involving a coiled-coil interaction. These results indicate that a heptad repeat of leucines is not a structural requirement for formation of coiled-coil dimers by transcription factors.

Sequence and transcription of Raji Epstein-Barr virus DNA spanning the B95-8 deletion region

The DNA sequence of Raji DNA spanning the deletion found in B95-8 cells has been determined. Three open reading frames and a region of homology with the BamHI-H fragment are found within the deletion. The deletion contains a region of 102-bp repeats which is transcribed into an mRNA. The Raji sequence reported here varies slightly from a smaller M-ABA sequence reported previously. This paper completes the sequence of all parts of the wild-type Epstein-Barr virus genome.

Epstein-Barr virus small nuclear RNAs are not expressed in permissively infected cells in AIDS-associated leukoplakia

Epstein-Barr virus (EBV) DNA structure and gene expression were analyzed in tissue specimens from oral hairy leukoplakia (HLP), a mucocutaneous lesion that develops in patients infected with human immunodeficiency virus (HIV). The structure of the terminal restriction enzyme fragments of EBV revealed that HLP is a permissive infection without a predominant, detectable population of EBV episomal DNA. In RNA preparations from this uniquely permissive infection, EBV replicative mRNAs could be identified by Northern analysis; however, the virally encoded small nuclear RNAs, the EBERs, were not detected in most HLP RNA preparations. In situ hybridization detected EBER expression in very rare cells. These data indicate that unlike other viral small nuclear RNAs, the EBERs are not expressed during viral replication and must participate in the complex maintenance of latent EBV infection.

Transfection and gene expression in normal and malignant primary B lymphocytes

Although many different protocols for transfection of lymphoid cell lines exist, successful DNA transfer into primary B lymphocytes has, to date, not been demonstrated. We now describe a simple method for gene transfer into highly purified normal and malignant B lymphocytes by electroporation. Using a powerful expression vector containing two copies of the cytomegalovirus (CMV) immediate early enhancer linked to the human T cell lymphotropic virus I (HTLV I) promoter, we could demonstrate transfected gene expression in both high density small 'resting' B cells and in low density 'activated' B cells. Successful transfection was detected by expression of chloramphenicol acetyl transferase and by immunofluorescence. The neoplastic cells of B cell chronic lymphocytic leukemia could also be transfected with an efficiency of 5-10%, but only after preactivation. This method of transfection will permit analysis of the contribution of individual genes and their products to normal and malignant B cell growth and differentiation.

The Epstein-Barr virus Zta transactivator: a member of the bZIP family with unique DNA-binding specificity and a dimerization domain that lacks the characteristic heptad leucine zipper motif

Introduction of the zta gene of Epstein-Barr virus into latently infected B cells leads to induction of the entire lytic cycle program of the virus. The Zta gene product is a sequence-specific DNA-binding protein of 35 kilodaltons that behaves as a specific transcriptional transactivator in transient cotransfection assays. All known Zta-responsive target promoters contain one or more members of a family of consensus-binding sites known as ZREs. On the basis of the presence of limited amino acid similarity within a basic carboxy-terminal domain, Zta has been proposed to be a highly divergent member of the c-Jun/c-Fos/GCN4 family of AP-1-binding proteins. We show here that in vitro-translated Zta and the Jun:Fos proteins have overlapping but distinct target DNA-binding specificies; both recognize canonical AP-1 sites, but only Zta recognizes ZRE sites and only Jun:Fos recognizes CRE sites. The relative binding affinity of Zta for oligonucleotides containing the 7-base-pair c-Fos AP-1 site TGAGTCA was twofold greater than that for the ZRE core motifs TGAGCAA, TG TGCAA, and TGAGTAA, but 10-fold greater than that for TGTGTCA, as measured by gel mobility retardation and competition DNA-binding assays. Cross-linking and cotranslational heterodimerization assays showed that like GCN4, Zta forms a stable homodimer in both its DNA-bound and unbound forms. Furthermore, we show that a potential coiled-coil helical domain adjacent to the basic domain of Zta can substitute for the leucine zipper of c-Fos to produce a DNA-binding protein that has a very stringent target DNA specificity and can only recognize symmetric 9-base-pair AP-1 sites (ATGAGTCAT). Therefore, despite the absence of the repeated heptad leucine zipper motifs, the Zta protein retains the characteristic features of a juxtaposed basic region and an exactly aligned coiled-coil alpha-helical dimerization domain of the bZIP class of transcriptional regulatory factors.

In vitro transcriptional activation, dimerization, and DNA-binding specificity of the Epstein-Barr virus Zta protein

The Epstein-Barr virus BZLF1 immediate-early gene encodes a transcriptional activator protein, Zta, which acts as a key regulatory switch in the transition between the latent and lytic viral life cycle. In this work, full-length Zta was expressed at high levels in Escherichia coli and purified to homogeneity by DNA affinity chromatography. The bacterial protein bound to specific target sequences (Zta response elements) and activated transcription in vitro from an Epstein-Barr virus early target promoter (BHLF1). Zta bound to DNA as a dimer. The formation of a heterodimer with a Zta deletion mutant was detected by gel electrophoresis mobility shift assays. Footprinting analysis on the BHLF1, BZLF1, and simian virus 40 control regions revealed multiple binding sites with no simple consensus sequence. Zta bound upstream from its own promoter at low concentrations, while at high concentrations it bound at the transcription start site, suggesting that it may activate and then autoregulate its own expression. These results demonstrate that Zta is a sequence-specific DNA-binding transcription factor.

Structure and function of the Epstein-Barr virus BZLF1 protein

Five DNA-binding sites for the Epstein-Barr virus BZLF1 protein have been identified within three of the early viral promoters, and four of these binding sites contain a consensus AP-1 site. The part of the BZLF1 protein required for sequence-specific DNA binding to one of these AP-1-like sites was identified by deletion mapping. Site-directed mutagenesis of this DNA target suggests that BZLF1 may work partly by overcoming a cellular repressor of viral transcription.

Isolation and characterization of two novel rat ovarian lactogen receptor cDNA species

Two novel lactogen receptor cDNA clones (2.1 and 1.2 kb) were isolated from a rat ovarian cDNA library. Nucleotide sequence of the 2.1 kb clone codes for a 610 aa receptor (nonglycosylated mol. wgt. 66,000 D) with an extracellular domain, a transmembrane region and an intracellular domain, and exhibited significant overall similarity with the rat liver receptor (310 aa) and both rabbit mammary and human hepatoma receptors (616 and 622 aa). However, the ovarian lactogen receptor sequence contains a unique cytoplasmic domain of 110 aa and consensus sequences for both a tyrosine phosphorylation site and an ATP/GTP type A binding site, and thus has potential for signal transduction and mitogenic activity. The 1.2 kb clone codes for a truncated binding form of 150 aa that is identical with the ovarian long form over only the first 130 residues, and lacks the transmembrane region. Differences between long and short forms of the ovarian lactogen receptors and the truncated liver species may result from alternative splicing. The prolactin holoreceptor gene(s) has the potential for producing several receptor subtypes that differ in tissue-specific expression, size, compartmentalization and mode of signal transduction, and may subserve the divergent functions of prolactin in its several target cells.

Identification of phorbol ester response elements in the promoter of Epstein-Barr virus putative lytic switch gene BZLF1

The product of the Epstein-Barr virus BZLF1 gene encodes a protein which is related to c-fos, it has been shown to bind specifically to a consensus AP-1 site, and its expression in latently Epstein-Barr virus-infected lymphocytes is sufficient to trigger the viral lytic cycle. We identified several elements within the BZLF1 promoter (Zp) which are responsive to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), an inducer of the viral lytic cycle. These elements fall into two classes based on the factors which bind to these sequences and their resulting functional behavior. Four of the elements are homologous (ZI elements) and share homology to a protein-binding domain in the promoter region of the coordinately expressed BRLF1 gene. When cloned upstream of heterologous promoters, the ZI elements function as silencers which exhibit TPA-inducible enhancer activity. A distinct TPA-responsive element (ZII) is located near the TATA box and shares homology with the AP-1-binding site in the c-jun promoter. A synthetic oligonucleotide with a sequence corresponding to the ZII element effectively competes for binding of nuclear factors to the c-jun AP-1 site. Furthermore, we found that a complex of c-jun and c-fos bound to the ZII domain.

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.

Autoregulation of Epstein-Barr virus putative lytic switch gene BZLF1

Expression of the Epstein-Barr virus (EBV) BZLF1 gene in latently infected lymphocytes is sufficient to trigger the viral lytic cycle. As shown in the accompanying report (E. Flemington and S.H. Speck, J. Virol. 64:1217-1226, 1990), the promoter for the BZLF1 gene (Zp) contains two distinct types of elements (ZI and ZII [an AP-1-like domain]) which are responsive to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), an inducer of the viral lytic cycle. Although Zp can be activated with TPA in an EBV-negative Burkitt's lymphoma cell line (Ramos), its activity is considerably lower than in EBV-positive cell lines which can be induced with TPA. Here we show that the protein product of the BZLF1 gene (ZEBRA) can transactivate its own promoter by a mechanism which involves direct binding to a region distinct from the ZI and ZII element. Moreover, we show that this region is composed of two distinct ZEBRA-binding-transactivation domains. Interestingly, these two domains are not homologous, and while one domain (ZIIIA) is similar to previously described ZEBRA-binding domains, the second (ZIIIB) is a higher-affinity site which bears no detectable homology to the consensus ZEBRA recognition sequence. We also show that transactivation is independent of the otherwise essential ZII domain, suggesting that ZEBRA binding may functionally replace or supercede the need for a functional ZII domain. This observation supports a model for activation of the lytic cycle whereby synthesis of a critical level of ZEBRA signals commitment to BZLF1 transcription and initiation of the lytic cascade.

Levels of Epstein-Barr virus DNA in lymphoblastoid cell lines are correlated with frequencies of spontaneous lytic growth but not with levels of expression of EBNA-1, EBNA-2, or latent membrane protein

The process of Epstein-Barr virus (EBV)-induced transformation of human B lymphocytes results in a cell line that is a mixture of latently and lytically infected cells, with the lytic cells composing roughly 5% to less than 0.0001% of the overall population. A set of nine normal lymphoblastoid cell lines that span a 100- to 200-fold range in average EBV DNA content were studied, and the frequency with which these cells entered a lytic phase of viral growth correlated with their EBV DNA copy number (as a population average). However, neither factor correlated with the levels of expression of transcript for the viral genes EBNA-1, EBNA-2, and latent membrane protein, nor did they correlate with the levels of EBNA-2 protein and latent membrane protein. The rate at which a cell line enters into lytic growth spontaneously is therefore not dependent on the overall steady-state levels of expression of these latent-phase genes.

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.

Relative rates of RNA synthesis across the genome of Epstein-Barr virus are highest near oriP and oriLyt

The rates of Epstein-Barr virus transcription were measured in isolated nuclei from marmoset and human lymphoblasts transformed in vitro. In B95-8, a marmoset B-lymphoid cell line, the most frequently transcribed viral genes are the EBERs (small nuclear RNAs) and BHLF-1 (encoding a lytic-phase gene product). The EBERs and BHLF-1 genes are separated by nearly 50 kilobase pairs on the Epstein-Barr virus genome and lie adjacent to (less than 300 base pairs from) oriP and oriLyt, respectively. oriP and oriLyt are putative origins of viral DNA replication, and each is associated with a transcriptional enhancer element. Among the human B-lymphoblastoid cell lines tested, only the transcription of EBERs predominates.

Hepatitis B virus surface antigen as a reporter of promoter activity

The coding sequence for the hepatitis B virus surface antigen (HBsAg) was used as a new reporter gene for studies on eukaryotic promoter activity and upstream regulatory sequences. The data observed in transfection assays were comparable to results obtained with conventional chloramphenicol acetyltransferase (CAT) assays, as was demonstrated using various transcriptional regulation sequences. The expression of HBsAg as a reporter protein offered some advantages: (i) In transient expression assays, a time course of promoter activity depending on variable culture conditions could be monitored over a period of time, since the HBsAg was secreted into the culture supernatant. (ii) Evaluation of HBsAg from supernatant aliquots and quantification of the corresponding promoter activities could be performed easily, using the very sensitive and readily available diagnostic HBsAg kits. (iii) In contrast to the conventional CAT assay, the cells remained available for further tests, e.g., Western blot, immunofluorescence or transcript analysis. Characteristics of several Epstein-Barr virus (EBV) promoters, depending on the virus state of EBV-positive B-cells (latency, chemical induction, lytic superinfection, trans-activation), were assayed using the HBsAg reporter system.

The Epstein-Barr virus immediate-early gene product, BMLF1, acts in trans by a posttranscriptional mechanism which is reporter gene dependent

In DNA cotransfection experiments, the Epstein-Barr virus immediate-early gene product, BMLF1, stimulated the chloramphenicol acetyltransferase (CAT) activity of both latent and productive EBV promoters linked to CAT. This BMLF1-induced increase in CAT activity was out of proportion to the effect on CAT mRNA, suggesting a posttranscriptional mechanism. Furthermore, when growth hormone was used as a reporter gene instead of CAT, BMLF1 no longer functioned. Thus, the BMLF1 effect was reporter-gene dependent. The effect of the BMLF1 gene product does not then appear to be directed at promoter activation, but instead may function to increase the level of an as yet unidentified protein(s) required for Epstein-Barr virus infection.

Responsiveness of the Epstein-Barr virus NotI repeat promoter to the Z transactivator is mediated in a cell-type-specific manner by two independent signal regions

Cells latently infected with Epstein-Barr virus (EBV) can be activated to express lytic-cycle polypeptides by the introduction of the EBV-encoded Z transactivator, indicating that this protein has a pivotal role in virus reactivation. We examined the target specificity of the Z transactivator in short-term contransfection assays and found that the most responsive target to Z transactivation was the divergent NotI repeat promoter, located within the EBV BamHI H fragment. In contrast, target plasmids containing the cat gene linked to heterologous viral promoters were not activated by cotransfection with the Z gene. S1 nuclease analysis of RNA from chemically induced B95-8 cells and from Vero cells cotransfected with NotI repeat promoter-CAT and Z showed that Z transactivation increased the level of correctly initiated, stable RNA transcripts. The NotI repeat gene (ntr) gives rise to a highly abundant mRNA species after chemical induction of lytic virus replication, but no protein product had been previously identified. Using monospecific antiserum raised against a synthetic peptide from the BHLF1 open reading frame, we demonstrated that the ntr gene encodes a protein product that is found in nuclear patches colocalizing with nucleoli. A series of deletions introduced into the upstream sequences of the NotI-repeat-promoter revealed two separate Z-response regions. The minimal promoter region between -7 and -155 of the leftward RNA cap site and an upstream region between -644 and -902 were both independently capable of conferring Z responsiveness. However, the minimal region, which was activated by Z cotransfection in Vero cells, was poorly responsive in lymphocytes, whereas the response of the far-upstream region to Z cotransfection was lymphocyte specific. In its human host, EBV infects both epithelial and lymphocyte populations. This dual lifestyle may have led to the evolution of multiple Z-response signals that enable the Z transactivator to interact with both cell-specific promoter and enhancer factors.

The spliced BZLF1 gene of Epstein-Barr virus (EBV) transactivates an early EBV promoter and induces the virus productive cycle

A spliced cDNA spanning the Epstein-Barr virus BZLF1 gene expresses the BZLF1 protein and is active in inducing the virus productive cycle. A deletion mutant which lacks the N-terminal half of the protein is inactive. Cotransfection experiments in EBV-negative B-lymphocyte cell lines demonstrated that the BZLF1 gene activates the promoter for the BSLF2 + BMLF1 gene in the absence of any other EBV gene product. These results confirmed that the spliced BZLF1 gene is the transactivating gene structure in BamHI-Z.

Cycloheximide-resistant gene of Epstein-Barr virus in freshly infected B lymphocytes

The transcription of latent Epstein-Barr viral (EBV) genes, i.e., genes encoding EBNA-1, EBNA-2, EBNA-3A, -3B, -3C, and LMP, were detected in human tonsillar lymphocytes early after infection with EBV. Transcription of the BHLF-1 open reading frame was also detected at this initial phase of immortalization. Cycloheximide treatment inhibited the transcription of all the latent EBV genes but not BHLF-1. These results suggest that BHLF-1 might be considered an immediate-early gene of EBV. Cycloheximide treatment of EBV-infected cells reduced not only the degree of the transcription but also the size of the transcript for EBNA-2. We hypothesize that the immediate-early expression of BHLF-1 may be required for the enhanced transcription of the viral genes in lymphocytes early after infection with EBV.

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) BZLF1 immediate-early gene product differentially affects latent versus productive EBV promoters

The Epstein-Barr virus (EBV) BZLF1 gene product is thought to mediate the disruption of latent EBV infection. We have examined the regulatory effects of BZLF1 by studying its transactivating effects on seven different EBV promoters. We find that whereas the BZLF1 gene product increases the activity of the two early promoters, BMLF1 and BMRF1, it decreases the activity of three latent promoters (the BamHI-C and BamHI-W Epstein-Barr nuclear antigen promoters and the latent membrane protein promoter). The BZLF1-induced changes in promoter-directed chloramphenicol acetyltransferase activity occur in EBV-negative as well as EBV-positive cell lines and are accompanied by a similar change in chloramphenicol acetyltransferase mRNA. Deletion analysis of the BamHI Z fragment indicates that in a portion of the amino-terminal half of the BZLF1 gene product (amino acids 24 to 86) is not essential for positive transactivating effects but is required for down-regulating effects. Thus, different domains of the same EBV immediate-early gene product can either increase the function of EBV promoters active in productive infection or decrease the function of key promoters active in latent infection.

Identification of proteins encoded by Epstein-Barr virus trans-activator genes

Specific antisera were generated to characterize Epstein-Barr virus proteins reported to have trans-activating properties. Open reading frame BRLF1 was found to be expressed in two modifications in vivo, with molecular sizes ranging from 94 to 98 kilodaltons (kDa) depending on the cell line, whereas only one protein (Raji cells, 96 kDa) was detected by in vitro translation. Open reading frame BZLF1 encoded polypeptides of 38 and 35 kDa and additional smaller forms. A BZLF1-encoded 30-kDa protein could be detected under conditions in which expression was restricted to immediate early genes. Nuclear localization could be detected under conditions in which expression was restricted to immediate early genes. Nuclear localization could be shown for the proteins derived from reading frames BZLF1 and BMLF1. BMLF1 expression gave a heterogeneous protein pattern, with molecular sizes between 45 and 70 kDa, including a predominant 60-kDa protein detected in different B-cell lines.

Synchronous and sequential activation of latently infected Epstein-Barr virus genomes

A lymphoid cell system was established that can induce the prompt and synchronous activation of latent Epstein-Barr virus (EBV) genomes and thus allows the identification of viral genes that are activated sequentially depending on their functions. With this system, we proved that disruption of EBV latency is initiated by activation of four EBV genes and that protein synthesis is not required prior to activation of latent EBV. The system should be an in vitro model for studying the mechanism of herpesvirus latency.