Deoxyribonuclease activity found in Epstein--Barr virus producing lymphoblastoid cells

Site-directed mutagenesis in a conserved motif of Epstein-Barr virus DNase that is homologous to the catalytic centre of type II restriction endonucleases

Sequence alignment of human herpesvirus DNases revealed that they share several conserved regions. One of these, the conserved motif D203...E225XK227 (D.EXK) in the sequence of Epstein-Barr virus (EBV) DNase, has a striking similarity to the catalytic sites of some other nucleases, including type II restriction endonucleases, lambda exonuclease and MutH. The predicted secondary structures of these three residues were shown to resemble the three catalytic residues of type II restriction endonucleases. Site-directed mutagenesis was carried out to replace each of the acidic residues near the motif by residues with different properties. All substitutions of D203, E225 and K227 were shown to cause significant reductions in nuclease activity. Six other acidic residues, within the conserved regions, were also replaced by Asn or Gln. Five of these six variants retained nuclease activity and mutant D195N alone lost nuclease activity. The four charged residues, D195, D203, E225 and K227, of EBV DNase were found to be important for nuclease activity. Biochemical analysis indicated that the preference for divalent cations was altered from Mg2+ to Mn2+ for mutant E225D. The DNA-binding abilities of D203E, E225D and E225Q were shown to be similar to that of wild-type. However, K227 mutants were found to have variable DNA-binding abilities: K227G and K227N mutants retained, K227E and K227D had reduced and K227R lost DNA-binding ability. Comparison of the biochemical properties of the corresponding substitutions among EBV DNase and type II restriction enzymes indicated that the D...EXK motif is most likely the putative catalytic centre of EBV DNase.

Epstein-Barr virus DNase contains two nuclear localization signals, which are different in sensitivity to the hydrophobic regions

The DNase of Epstein-Barr virus (EBV) is a 470-amino-acid protein which possesses both endonuclease and exonuclease activities and accepts both double-stranded DNA and single-stranded DNA as substrates. It has been reported that this protein may be found in the nucleus and/or cytoplasm of infected cells. In this study, using cell fractionation and immunoblotting to determine the distribution of EBV DNase in Akata cells stimulated with anti-human immunoglobulin G antibody (anti-IgG), the DNase was found to be located predominantly in the nucleus. To map the signals in DNase which mediate its nuclear localization, we monitored the nuclear transport of fusion proteins consisting of various fragments of EBV DNase linked to a cytoplasmic protein, beta-galactosidase (beta-Gal). The results demonstrated that two regions of the DNase with nuclear localization signal (NLS) activity, designated NLS-A (amino acids 239-266) and NLS-B (amino acids 291-306), were able independently to localize the beta-Gal to the nuclei of HEp-2 and HeLa cells. Five basic residues (R or K) were found in each NLS and distributed differently in primary structure. The basic domains and flanking residues of NLS-A and NLS-B are 250YKRPCKRSFIRFI262 and 294LKDVRKRKLGPGH306, respectively. Further examination of these sequences revealed that NLS-A contains bulky aromatic amino acids (Y and F) which may diminish its capacity to act as a strong NLS and lacks the typical proline and glycine helix-breakers. However, NLS-B contains typical proline and glycine helix-breakers and the histidine residue at amino acid 306 is required for NLS activity. In addition, two hydrophobic regions within the DNase were found to inhibit the function of NLS-A but not NLS-B, suggesting that these two domains are different types of NLSs and differ in their sensitivity to hydrophobic regions in the context of protein structure.

Distinct regions of EBV DNase are required for nuclease and DNA binding activities

Epstein-Barr virus (EBV) DNase possesses both endonuclease and exonuclease activities and accepts both double-stranded DNA (dsDNA) and single-stranded DNA (ssDNA) as substrates. To map regions of EBV DNase responsible for nuclease and DNA binding activities, a series of mutant DNase polypeptides was expressed using a bacterial system for the nuclease assay and in an in vitro transcription/translation system to assay binding activity to dsDNA or ssDNA cellulose. The results indicated that the C-terminus of EBV DNase, residues 450-460, is essential for nuclease activity but dispensable for DNA binding. However, deletion of residues 441-470 resulted in the loss of both nuclease and DNA binding activities. Substitution of Phe452 and Val458 led to inactive enzymes. In the N-terminus, deletion of residues 23-28 and residues 7-61 resulted in the loss of nuclease activity but the DNA binding activities of the deleted enzymes were intermediate and low, respectively. Mutation of Leu23 to Gly showed drastically reduced nuclease activity but its DNA binding ability was not affected. Based on the amino acid sequence alignment of various herpesvirus DNases, we chose four highly conserved and two less well conserved regions as controls for mutagenesis studies. These six internal deletion (ID) mutants were prepared using a recombinant PCR method. Each of the polypeptides was expressed in a bacterial system for the nuclease assay and using an in vitro transcription/translation system for the DNA binding assay. DNA binding and nuclease activities of all six internal deletion mutants were abolished, except that mutant ID2, with deletion of residues 138-152, retained an intermediate ability to bind DNA. These data indicate that since mutations at distinct regions within EBV DNase resulted in the loss of nuclease and/or DNA binding activities, it is suggested that these distinct regions are required for maintenance of an intact and highly ordered structure(s) for both activities.

Use of bacterially-expressed antigen for detection of antibodies to the EBV-specific deoxyribonuclease in sera from patients with nasopharyngeal carcinoma

A cDNA clone, BG9, corresponding to the open reading frame BGLF5 of Epstein-Barr virus (EBV) DNase was inserted into an E. coli expression vector, pET3a, to generate a recombinant plasmid, pDNase 5. High level of expression of a DNase activity was detected in the E. coli transformed with pDNase 5 following induction with IPTG. The enzyme activity was purified using DEAE-cellulose, phosphocellulose and DNA-cellulose column chromatography. The purified protein appeared to be nearly homogeneous in SDS-PAGE using Coomassie blue staining. The requirement for divalent cations and optimum pH as well as inhibitory concentrations of ionic strength and polyamines for the purified enzyme activity were determined and seemed to be very similar to those of the enzyme activity purified from an EBV producing lymphoblastoid cell line. Using the purified enzyme as an antigen and anti-IgA as the secondary antibody, 82% (64/78) and 91% (71/78) of sera from patients with nasopharyngeal carcinoma (NPC) were shown to be positive by dot immunobinding assay and ELISA, respectively. The results suggest that purified E. coli expressed EBV DNase may be useful for preparing specific test for large scale screening of patients with NPC.

Cloning and expression of a cDNA encoding the Epstein-Barr virus thymidine kinase gene

A clone of the Epstein-Barr virus (EBV) thymidine kinase (TK) gene was derived from a cDNA library of P3HR1 cells. The gene product was expressed as a fusion protein in a procaryotic system by using T7 RNA polymerase. The recombinant TK showed a molecular mass of 67 kDa and was biologically active. Antiserum raised in mice immunized with partially purified TK recognized an antigen present in EBV-superinfected Raji cells using an indirect immunofluorescence assay.

Structure-activity relationship between (E)-5-(2-bromovinyl)- and 5-vinyl-1-beta-D-arabinofuranosyluracil (BV-araU, V-araU) in inhibition of Epstein-Barr virus replication

The structure-activity relationship between (E)-5-(2-bromovinyl)- and 5-vinyl-1-beta-D-arabinofuranosyluracil (BV-araU and V-araU) in inhibition of Epstein-Barr virus (EBV) was evaluated. Both V-araU and BV-araU effectively inhibited EBV replication in virus-producer P3HR-1(LS) cells, as determined by DNA-DNA hybridization. The 50% effective doses (ED50) for viral DNA replication were 0.005 and 0.3 microM for V-araU and BV-araU, respectively. The in vitro therapeutic index was 4000 for V-araU and 1300 for BV-araU. Synthesis of EBV-induced polypeptides with molecular weights of 145,000 (145, 140, 130, and 110 kDa) was significantly inhibited by both drugs. Only V-araU inhibited the synthesis of 85-, 55-, and 32-kDa polypeptides by approx. 50%. Kinetic analysis of inhibition and reversibility of EBV DNA replication after removal of the drugs indicated that BV-araU has a more prolonged inhibitory effect than V-araU. These results indicate that the substitution of H by Br in the 5-vinyl group results in marked reduction in anti-EBV activity while prolonging the drug effect and diminishing cytotoxicity.

Immunological characterization of the Epstein-Barr virus phosphoprotein PP58 and deoxyribonuclease expressed in the baculovirus expression system

The open reading frames of the phosphoprotein pp58 (BMRFI) and the deoxyribonuclease (BGLF5) of the Epstein-Barr-virus (EBV) strain M-ABA were cloned in the baculovirus expression vectors pAc373 and pAc360 and expressed in the Spodoptera frugiperda (SF158) insect cells. The recombinant phosphoprotein pp58 expressed in SF158 cells was recognized by the anti-pp58 rabbit anti-sera which were generated by immunizing rabbits with a TrpE-BMRFI fusion protein expressed in E. coli. DNA-cellulose chromatography showed that the recombinant pp58 exhibited DNA-binding activities. Immunofluorescence, immunoblot and ELISA analysis indicated that sera from patients with nasopharyngeal carcinoma (NPC) contained antibodies against pp58. The recombinant EBV DNase expressed in SF158 cells was recognized by the anti-EBV DNase rabbit anti-sera which were generated by immunizing rabbits with a TrpE-C-terminal part of BGLF5 fusion protein expressed in E. coli. The anti-EBV DNase rabbit anti-sera recognized also a protein of about 52 kDa in the EBV-harboring human B-cell lines Raji, Jijoye, B95-8, M-ABA and BL74 induced by TPA and n-butyrate. The recombinant EBV DNase exhibited exonuclease and endonuclease activities, a requirement for magnesium, and a high pH optimum (8.0). Its enzyme activities could be inhibited by sera from NPC patients and anti-EBV DNase rabbit anti-sera. Comparable studies of Raji EBV-DNase and recombinant EBV-DNase implied that recombinant EBV-DNase could also be used in the enzyme activity assay for the detection of NPC. In contrast to the enzyme inhibition test, immunofluorescence and immunoblot analysis demonstrated that the recombinant EBV DNase exhibited only a weak immunological reaction with NPC sera.

Molecular characterization of a cDNA clone encoding the Epstein-Barr virus (EBV) DNase

RNA from IdUrd-treated P3HR1 cells was used for the construction of a cDNA library and screened with B95-8 EBV DNA BamHI fragment B and G probes. One clone, BG9, containing a 1.7 kb cDNA insert was further studied. Complete DNA sequence analysis revealed that BG9 encompassed the B95-8 EBV DNA sequences from nucleotide 120,747 to nucleotide 122,412 and corresponded to the BGLF5 open reading frame of the EBV DNase gene. Comparison of the sequences of BG9 with that of published B95-8 EBV DNA indicated that there were 14 different bases which results in 7 amino acid residue changes. The product of in vitro transcription/translation of a subclone, pGEM-BG9, contained the EBV DNase activity and a 52 kDa protein was immunoprecipitated from the in vitro translation products using serum from a patient with nasopharyngeal carcinoma which contained a high level of anti-DNase activity. Northern hybridization of P3HR1 RNA with the BG9 probe revealed a complex pattern of transcription in this region. Subgenomic DNA fragments were then used to map these RNA species to the B95-8 EBV DNA sequence. The result of S1 nuclease analysis indicated that a DNase ORF containing transcript sized 2.0 kb is initiated at nucleotide 122,435 +/- 1 and terminated at nucleotide 120,741 of the EBV genome.

Chromatographic purification and characterization of EBV DNase from chemically induced lymphoid cells

Epstein-Barr virus-associated deoxyribonuclease (EBV-DNase) was purified to homogeneity, as determined by silver staining, sequential column chromatography, and FPLC from Raji and P3HR-1 cells treated with 12-O-tetradecanoyl-phorbol-13-acetate and sodium butyrate. This viral protein was immunogenic and elicited high neutralization titer sera in rabbits. By silver staining of SDS-PAGE, Western immunoblot, and radioimmunoprecipitation using NPC patient sera and both polyclonal and monoclonal antibodies, the EBV DNase was identified as a 58K protein. The potential presence of two EBV DNases was also discussed.

Purification and properties of Epstein-Barr virus DNase expressed in Escherichia coli

A cDNA corresponding to the BGLF5 open reading frame of the Epstein-Barr virus (EBV) genome and coding for an early DNase was inserted into the procaryotic expression vector pKK223-3. One bacterial clone producing the expected 52-kilodalton DNase was used as a source of EBV DNase. The 52-kilodalton Dnase was purified in the active form to near homogeneity by ammonium sulfate precipitation and successive chromatographies on phosphocellulose, DNA-cellulose, and gel filtration columns. The purified enzyme exhibited both exonuclease and endonuclease activities, an absolute requirement for divalent cations, an alkaline pH preference, and a typical residual activity in presence of 300 mM KCl. Moreover, the enzyme was specifically inhibited by human sera with high antibody titers to EBV early antigens. These properties are similar to those observed for EBV-induced DNase from lymphoblastoid cell extracts. In addition, the enzyme was recognized by both immunoglobulin G and A serum fractions from patients with nasopharyngeal carcinoma (NPC). From these results and previous studies which demonstrated the value of antibody titers to this viral DNase as an NPC marker, it appears that EBV-encoded DNase produced in a heterologous expression system could be used in the development of a specific and early NPC diagnosis test.

The characterization of the EBV alkaline deoxyribonuclease cloned and expressed in E. coli

Studies of nucleic acid homology suggest the BGLF5 open reading frame of Epstein-Barr virus (EBV) encodes an alkaline deoxyribonuclease (DNase) sharing some homology with that of herpes simplex virus. We report here the expression of the BGLF5 open reading frame in E. coli and the expression of high levels of a novel alkaline DNase activity in induced cells. This alkaline DNase has been purified to apparent homogeneity as a single protein species. This is the first report of the expression of a herpesvirus coded DNase in a prokaryotic system and of the purification of the EBV DNase to demonstrable purity. It has the biochemical characteristics of a typical herpesvirus alkaline exonuclease showing a high pH optimum, an absolute requirement for Mg2+ for activity and sensitivity to high salt concentrations and polyamines. The enzyme activity was neutralized by sera from patients with nasopharyngeal carcinoma and was reactive with these sera in Western blot analysis. Thus the prokaryotic expression system described here provides an economical and efficient source of the EBV DNase for biochemical and seroepidemiological analysis.

Antibody to Epstein-Barr virus-specific DNase as a marker for field survey of patients with nasopharyngeal carcinoma in Taiwan

A serological survey using antibody to Epstein-Barr virus (EBV)-specific DNase activity as a marker for the identification of patients with nasopharyngeal carcinoma (NPC) has been carried out on healthy subjects who visited Government Employees' Clinic Center (GECC) for routine health examination and on individuals residing in NPC high-risk areas (HRA) in Taiwan. During a 3-year prospective study, 22,596 and 9,869 sera were collected from the GECC and HRA groups, respectively. Taking neutralization of 2 or more units of EBV DNase activity as a positive response, the positivity rates in the GECC and HRA groups were 5.4% and 11.92%, respectively. Among the antibody-positive individuals, three cases of NPC were found in the GECC group (detection rate 0.63%) and 11 in the HRA group (detection rate 1.32%). A further patient at stage III of the disease was found in the first year of following up of 1,005 antibody-positive individuals. Among the 12 NPC patients in the HRA, five were newly diagnosed as having stage II (three patients) and stage III (two patients) NPC. These results support the hypothesis that antibody against EBV-specific DNase activity may be a useful marker for detection of patients with NPC, and they imply that individuals having high levels of antibody to EBV DNase activity may have an increased risk of development of NPC.

Antigenic variation in alkaline deoxyribonuclease induced by three different strains of Epstein-Barr virus

To determine whether biological and/or biochemical variants exist between strains of Epstein-Barr virus (EBV), we superinfected Raji cells with the nontransforming lytic strain of EBV (HR-1), and two isolates that both transform B-lymphocytes and superinfect Raji cells, B95-8, and NPC-EBV. The superinfected cells were assayed for EBV specific DNase. A new electrophoretic form of DNase was observed in cells superinfected with B95-8 EBV as compared to the enzymes induced by the HR-1 and NPC-EBV isolates. There were antigenic differences in the DNase induced by the EBV strains. Since antibody to EBV DNase is a marker for nasopharyngeal carcinoma (NPC), these data may have implications for EBV-associated disease.

Comparison of two bromovinyl nucleoside analogs, 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil and E-5-(2-bromovinyl)-2'-deoxyuridine, with acyclovir in inhibition of Epstein-Barr virus replication

The effect of 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-araU), a new antiviral drug, on Epstein-Barr virus (EBV) was studied and compared with those of E-5-(2-bromovinyl)-2'-deoxyuridine (BVdU) and acyclovir (ACV). BV-araU effectively inhibited EBV replication both in superinfected Raji cells and in virus producer P3HR-1(LS) cells, as determined by density gradient centrifugation, in situ cytohybridization with an EBV DNA probe, and cRNA-DNA hybridization. The 50% effective doses for viral DNA replication were 0.26, 0.06, and 0.3 microM for BV-araU, BVdU, and ACV, respectively. The relative efficacy on the basis of the in vitro therapeutic index was BVdU (6,500) greater than BV-araU (1,500) greater than ACV (850). Synthesis of EBV-induced polypeptides with molecular weights of 145,000 and 140,000 was inhibited by these drugs. Kinetic analysis of reversibility of inhibition of EBV DNA replication after removal of the drugs indicated that BV-araU, like BVdU, has a more prolonged inhibitory effect than ACV. These results indicate that the 2' OH group in the arabinosyl configuration of BV-araU results in marked reduction in anti-EBV activity while slightly diminishing cytotoxicity.

Specific immune serum to the Epstein-Barr virus DNA polymerase

Epstein-Barr virus (EBV) DNA polymerase was released from phorbol ester-treated tamarin (Saguinus oedipus) cells (B95-8) and prepared for use as an antigen by sequential column chromatography with DEAE-Sephadex A-25, DEAE-cellulose, phosphocellulose, and single-stranded DNA cellulose. Proteins from single-stranded DNA cellulose with DNA polymerase activity in 100 mM ammonium sulfate were mixed with complete Freund adjuvant and injected intradermally into rats and rabbits. Immune sera that were screened for specific antibody by indirect immunofluorescence procedures reacted with approximately 3% of the cells in EBV-producer cultures (B95-8 and P3HR-1) but not with EBV genome-negative cells (BJAB). In functional enzyme assays, immune sera or the immunoglobulin fraction inhibited the activity of purified EBV DNA polymerase 90%. Inhibition of enzyme activity was not affected by absorption of immune sera with insoluble matrices of proteins prepared with tamarin and human cells which lacked the EBV genome. Cellular DNA polymerase alpha was not inhibited by immune sera to the EBV enzyme.

Identification of an Epstein-Barr virus-specific desoxyribonuclease gene using complementary DNA

We have recently obtained 18 distinct cDNA clones representing different genes expressed in the early phase of EBV infection. One of them, c37, which is situated at the position 12907-122451 in the B95-8 viral genome, is shown here to code for a viral desoxyribonuclease [DNase]. Cell free translation of c37-selected messenger RNA yielded a protein of about 52 KDa which was immunoprecipitated by a high EA titer serum from nasopharyngeal carcinoma patient. This protein showed a DNase activity which was resistant to high salt concentrations (150 to 300 mM KCl) and was specifically neutralized by EA positive serum. These properties are typical of the EBV-specific DNase activity that we recently described in chemically induced EBV-transformed lymphoid cells. The same results were obtained on cell-free translation of the native RNA synthesized in vitro from pGEM-37 plasmid containing the entire c37 cDNA sequence (1.53 Kb). These data indicate that the BGLF5 open reading frame contained in c37 encodes for the EBV-specific DNase.

Epstein-Barr virus-specific DNA polymerase in virus-nonproducer Raji cells

Virus-nonproducer Raji cells, when induced to early antigen synthesis by 12-O-tetradecanoyl-phorbol-13-acetate and sodium butyrate, showed an increase in DNA polymerase activity. This enzyme has the characteristics of a typical Epstein-Barr virus DNA polymerase with regard to chromatographical pattern and biological properties: it is eluted from DEAE-cellulose at 0.08 M NaCl, has a high salt resistance, is sensitive to phosphonoacetic acid and phosphonoformate, and shows a substrate preference for poly(dC)-oligo(dG12-18). The resistance of Epstein-Barr virus polymerase activity to aphidicolin is a property distinct from that of HSV DNA polymerase. Viral DNA polymerase activity increases in the absence of Epstein-Barr virus DNA replication, indicating that this enzyme is an early viral protein.

Demonstration of a stimulatory protein for virus-specified DNA polymerase in phorbol ester-treated Epstein-Barr virus-carrying cells

A heat-labile Epstein-Barr virus-specific DNA polymerase stimulatory protein having a molecular mass of 45 kDa was purified from phorbol 12-myristate 13-acetate-treated P3HR-1 cells by column chromatography. The virus DNA polymerase stimulatory protein was precipitated by sera from patients with nasopharyngeal carcinoma but not by sera from healthy donors. The interaction of the stimulatory protein with DNA polymerase was stoichiometric. Furthermore, this protein stimulated Epstein-Barr virus DNA polymerase but not herpes simplex virus type 1 or type 2 or human DNA polymerase alpha. The stimulatory protein did not alter the Km value of dTTP or DNA but did increase the Vmax of DNA polymerase. Salt concentrations between 100 mM and 150 mM KCl were optimal for this protein-induced stimulation of Epstein-Barr virus DNA polymerase activity. The presence of the stimulatory protein in the reaction mixture enhanced the sensitivity of virus DNA polymerase to phosphonoformate.

Antibody to Epstein-Barr virus-specific DNase as a marker for the early detection of nasopharyngeal carcinoma

We have previously shown that high levels of antibody to Epstein-Barr virus (EBV)-specific DNase may be a useful marker for the early diagnosis of nasopharyngeal carcinoma (NPC). Sera from 3,368 males in an area with a high risk for the development of NPC were examined for the presence of antibody to EBV-specific DNase activity. Significant levels of the antibody were found in 430 of these sera. As a result, 32 individuals have attended the out-patient department of otolaryngology for clinical examination. Among them, one individual was found to have a stage II nasopharyngeal carcinoma. This result supports the value of determination of antibody to EBV-specific DNase as a marker for the early detection of NPC.

Antibody response to Epstein-Barr-virus-specific DNase in 13 patients with nasopharyngeal carcinoma in Taiwan: a retrospective study

Serum samples obtained from 13 individuals who were found to have a previous history of or to be suffering from nasopharyngeal carcinoma (NPC) were examined for the presence of antibody to Epstein-Barr virus (EBV) DNase activity. Significant to high levels of antibody to EBV DNase activity were detected in most serum samples obtained from four patients prior to the diagnosis of NPC. The samples from the other nine patients showed variable levels of antibody. The majority of the samples collected at the remission stage of the disease, especially those from long-term survivors, contained little or no antibody to the DNase activity. Data presented here suggest that antibody to EBV DNase activity may be a useful marker for the early diagnosis as well as the prognosis of NPC.

Purification of Epstein-Barr virus DNA polymerase from P3HR-1 cells

The Epstein-Barr virus DNA polymerase was purified from extracts of P3HR-1 cells treated with n-butyrate for induction of the viral cycle. Sequential chromatography on DNA cellulose, phosphocellulose, and blue Sepharose yielded an enzyme preparation purified more than 1,300-fold. The purified enzyme was distinct from cellular enzymes but resembled the viral DNA polymerase in cells infected with herpes simplex virus type 1 or 2. The active enzyme had an apparent molecular weight of 185,000 as estimated by gel filtration on Sephacryl S-300. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a major polypeptide corresponding to a molecular weight of ca. 110,000. This polypeptide correlated with the catalytic function of the purified enzyme, whereas the other, less abundant polypeptides did not. By immunoblotting, the 110,000-molecular-weight polypeptide could be identified as a viral polypeptide. It could not be determined whether the native enzyme was composed of more than one polypeptide.

Epstein-Barr virus induces a unique pyrimidine deoxynucleoside kinase activity in superinfected and virus-producer B cell lines

Epstein-Barr (EB) virus induces a new pyrimidine deoxynucleoside kinase [thymidine kinase (dTk)] activity in Raji B lymphocyte cells after superinfection. This dTk activity is also present in small amounts in the HR-1 virus-producer cell line and in larger amounts in the B95-8 virus-producer line. The dTk activity induced by EB virus coelutes from DEAE-cellulose columns with deoxycytidine kinase (dCk) activity and elutes as a broad peak well separated from the large peaks of cellular dTk and dCk activities. This EB virus-induced pyrimidine deoxynucleoside kinase activity from HR-1 cells differs from cellular kinases in most basic biochemical properties but shares certain properties with the herpes simplex virus dTk.

Epstein-Barr virus-specific DNase activity in nonproducer Raji cells after treatment with 12-o-tetradecanoylphorbol-13-acetate and sodium butyrate

An Epstein-Barr virus (EBV)-specific DNase was induced in EBV nonproducer Raji cells after treatment with 12-O-tetradecanoylphorbol-13-acetate and sodium butyrate. The increase in EBV DNase activity was related to the appearance of early antigen-positive cells. The enzyme had a sedimentation coefficient of 4S and was resistant to 300 mM KCl, and its induction did not depend on viral DNA synthesis. The EBV-specific DNase activity was specifically inhibited by sera from patients who had nasopharyngeal carcinoma with high early antigen activities but not by sera from normal, healthy individuals. There was a correlation between the degree of anti-EBV DNase activity and the titers of early antigen antibody.

Effect of arabinofuranosylthymine on the replication of Epstein-Barr virus and relationship with a new induced thymidine kinase activity

1-beta-D-Arabinofuranosylthymine (araT) is a selective inhibitor of Epstein-Barr virus replication induced in both thymidine kinase (TK)-negative (TK-) and TK+ variants of the lymphoid cell line P3HR-I. This analog has no effect on the growth of noninduced cells (T. Ooka and A. Calender, Virology 104:219-223, 1980). The synthesis of early antigens is not affected by the analog, whereas that of late viral capsid antigens is completely inhibited, as demonstrated by the indirect immunofluorescence technique; kinetic reassociation experiments have also shown that araT strongly inhibits replication of viral DNA. Phosphorylation of the tritiated form of the analog ([3H]araT) was analyzed by thin-layer chromatography in cultures of control and induced cells, and the results demonstrated that only induced cells can convert the analog to the triphosphate form. These results indicate that the selective effect of araT in induced cells is probably related to a new virally induced TK activity. Preliminary characterization of this new activity has shown that it is able to phosphorylate the analog specifically, whereas cellular TKs cannot. araTTP, a final phosphorylation product of araT, is a potent inhibitor of Epstein-Barr virus-specific DNA polymerase, suggesting a possible inhibitory action of this product on Epstein-Barr virus replication.

Complex forms of mitochondrial DNA in human B cells transformed by Epstein-Barr virus (EBV)

Human lymphocytes and lymphoid cell lines were analyzed for the presence of complex forms of mitochondrial DNA (mtDNA) by electron microscopy. A high frequency (9%-14.5%) of catenated dimers, circular dimers, or oligomers were found in samples from Epstein-Barr-virus-(EBV) transformed lymphoblastoid cell lines. These complex forms of mtDNA were present in much lower frequencies in lymphocytes isolated from donor blood (1.3%-4.6%). Similar low frequencies were found with primary fibroblasts (1.1%) or freshly isolated monkey liver cells (2.1%). Samples from cultures of Burkitt lymphoma (BL) cell lines of EBV-positive or -negative origin contained intermediate (5%-7%) frequencies of complex forms of mtDNA.

Identification and characterization of a DNase induced by Epstein-Barr virus

The diterpene ester promoter of mouse skin tumors, 12-O-tetradecanoyl-phorbol-13-acetate, induced a DNase activity in the Epstein-Barr virus-producer cell line P3HR-1. The elution patterns of the enzyme from DEAE-cellulose, phosphocellulose, and DNA-cellulose columns were different from virus-associated DNA polymerase activity. The partially purified activity could be neutralized to the extent of 90% by sera of patients with nasopharyngeal carcinoma. Purified immunoglobulin G from sera of nasopharyngeal carcinoma patients inhibited this enzyme and that obtained from superinfected Raji cells to the same extent. The partially purified enzyme preferred native DNA as a substrate over denatured DNA and 3'-terminally labeled activated calf thymus DNA. The activity was inhibited by high ionic strength. Phosphonoformic acid did not have any effect on this enzyme activity.

Characterization of the Epstein-Barr virion-associated DNA polymerase as isolated from superinfected and drug-stimulated cells

We reported previously that Epstein-Barr (EB) virions and detergent-treated nucleocapsids co-purified with significant amounts of DNA polymerase activity that did not resemble other known host or viral polymerases. We report here that this species of DNA polymerase activity is present at early times after infection in lymphocytes abortively lytically infected (superinfected) with EB virus. However, studies with [35S]methionine labeling suggest de novo synthesis of enzyme has not occurred. Conversely, drug-stimulated lymphocytes that synthesize EB viral late proteins and virions contain this species of polymerase to the virtual exclusion of all others. This EB viral polymerase shows a marked preference for nicked and gapped double-stranded rather than primed single-stranded DNA templates. Its processiveness as measured on primed theta X174 phage DNA template is lower than that of lymphocyte beta polymerase. The data reported here are consistent with the hypothesis that the EB virion-associated DNA polymerase is synthesized at late times in the viral life cycle as are other structural proteins but it plays an important role early after viral infection. It is known that mature herpes virion DNA (including that of EB virus) is nicked and gapped and we propose that virion polymerase repairs the viral DNA at an early stage in infection before viral DNA replication begins.

An endonuclease isolated from Epstein-Barr virus-producing human lymphoblastoid cells

An endonuclease has been isolated from human B lymphoblastoid cells that copurifies with an exonucleolytic activity and has been shown to produce double-strand breaks and a high proportion of single-strandedness in phage lambda DNA in vitro. The data are consistent with a model in which single-strand cuts are made by the endonucleolytic activity, possibly in A+T-rich regions of the DNA, followed by creation of single-stranded regions (gaps) precessing from the site of a cut. Generation of overlapping gaps on opposite strands or of a gap opposite a nick would lead to the creation of the banding patterns that we have seen on electrophoretic gels. This endonucleolytic activity copurifies with other enzymes induced by Epstein-Barr virus that relate to the process of viral DNA replication in productively infected cells. However, a more general role is proposed for this class of eukaryotic endonuclease activities. A marked degree of single-strandedness has been found in the replicating DNAs of many eukaryotes, ad these gaps could be generated by endonucleases with associated exonucleolytic activity such as that reported here. This Epstein-Barr virus-induced nuclease activity has been shown to resemble the recBC nuclease isolated from the prokaryote Escherichia coli and also the endonuclease isolated from the eukaryote Chlamydomonas.