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Open reading frame (ORF) 50 protein is capable of activating the entire lytic cycle of Kaposi's sarcomaassociated herpesvirus (KSHV), but its mechanism of action is not well characterized. Here we demonstrate that ORF 50 protein activates two KSHV lytic cycle genes, PAN (polyadenylated nuclear RNA) and K12, by binding to closely related response elements located approximately 60 to 100 nucleotides (

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The BC-1 cell line, derived from a body cavity-based, B-cell lymphoma, is dually infected with Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV). In these studies, the relationships between these two gammaherpesviruses and BC-1 cells were characterized and compared. Single-cell cloning experiments suggested that all BC-1 cells contain both genomes. In more than 98% of cells, both viruses were latent. The two viruses could be differentially induced into their lytic cycles by chemicals. EBV was activated into DNA replication and late-gene expression by the phorbol ester tetradecanoyl phorbol acetate (TPA). KSHV was induced into DNA replication and late-gene expression by n-butyrate. Amplification of both EBV and KSHV DNAs was inhibited by phosphonoacetic acid. Induction of the KSHV lytic cycle by n-butyrate was accompanied by the disappearance of host-cell ␤-actin mRNA. Induction of EBV by TPA was not accompanied by such an effect on host-cell gene expression. Induction of the KSHV lytic cycle by n-butyrate was associated with the expression of several novel polypeptides. Recognition of one of these, p40, served as the basis of development of an assay for antibodies to KSHV in the sera of infected patients. BC-1 cells released infectious EBV; however, there was no evidence for the release of encapsidated KSHV genomes by BC-1 cells, even though n-butyratetreated cells contained numerous intranuclear nucleocapsids. The differential inducibility of these two herpesviruses in the same cell line points to the importance of viral factors in the switch from latency to lytic cycle.

Al cells that harbor the Epstein-Barr virus (EBV) genome contain a neoantigen in the nucleus (EBNA). By transfection we located a segment of the genome that encodes or induces an antigen serologically related to EBNA. The responsible genes are found in the 3.4-megadalton BamHI fragment K of EBV DNA, specifically in the left 1.9 megadaltons represented by HindIu fragment I. Mouse LTK-cells were cotransformed with recombinant plasmids containing the herpes simplex virus thymidine kinase gene and either EcoRP fragment B or BamHI fragment K ofEBV DNA. The TKV cells surviving in selective medium were cloned. About 50% of the clones expressed the neoantigen in every nucleus. These mouse cells were used as antigens in immunofluorescence tests. Antibody to the nuclear antigen was found in 30 human sera known to contain antibody to EBNA; it was not detected in 18 sera that did not have antibody to EBNA. Mouse cells expressing EBNA as the result of acquisition of cloned EBV DNA fragments should prove useful in the characterization of the structure of this antigen and as reagents for the diagnosis of EBV infections.Genetic study of the human lymphotropic herpes virus Epstein-Barr virus (EBV) is hindered by the lack of mutants and by the absence ofa fully permissive host cell that allows genetic recombination between viruses. In view of these limitations, a suitable approach for defining the products ofsome EBV genes is gene transfer with defined segments of viral DNA. This approach is feasible because DNA prepared from virions is infectious by either microinjection or transfection (1-3). Human placental fibroblasts exposed to intact virion DNA produce EBV which is able to immortalize lymphocytes (ref. 2; unpublished data). Several different types of tissue culture cells exposed to EBV DNA display various morphologic forms ofantigens in the cytoplasm, nuclear membrane, and nucleus; these antigens are detectable by certain human sera with antibody to EBV (3,4).If a mixture of virion DNA fragments produced by cleavage with one of several restriction endonucleases is introduced, antigens also appear in the fibroblasts. IfEcoRI and Sal I are used, antigens appear in both nucleus and cytoplasm but if BamHI and HindIII, are used, the antigen seems to be limited to the nucleus (4). The need to identify this nuclear antigen and its relationship to "EB nuclear antigen" (EBNA) underlies the present experiments.The finding that a mixture of virion DNA fragments led to nuclear antigen expression suggested the possibility that individual cloned subfragments of viral DNA would also be able to induce the antigen. We found that a single large cloned fragment, EcoRI fragment B of EBV (FF41) DNA, approximately 19 megadaltons, was competent to cause expression ofa nuclear antigen in human fibroblasts (4). We were unable to study this nuclear antigen in human cells by anticomplement immunofluorescence, the usual immunologic assay for EBNA, because of nonspecific binding of complement to the cytoskeleton of the placental fibroblasts. ...