A cis-acting element from the Epstein-Barr viral genome that permits stable replication of recombinant plasmids in latently infected cells

The Epstein-Barr viral (EBV) genome of approximately equal to 170 kilobase pairs (kbp) is maintained as a plasmid in human B lymphoblasts transformed by the virus. We have identified a cis-acting element within 1.8 kbp of the viral genome that allows recombinant plasmids carrying it to be selected at high frequency and maintained as plasmids in cells latently infected by EBV. This functional element(s) requires a segment of DNA at least 800 bp and at most 1800 bp long, which contains a family of 30-bp tandem repeats at one end. Since this region confers efficient stable replication only to plasmids transfected into cells containing EBV genomes, its function probably requires trans-acting products encoded elsewhere in the viral genome.

Genetic analysis of immortalizing functions of Epstein-Barr virus in human B lymphocytes

Epstein-Barr virus (EBV), a herpes virus, infects human B lymphocytes in vitro and efficiently immortalizes them. About 10 of the approximately 100 genes of EBV are expressed in recently immortalized B cells and although there is circumstantial evidence that at least three of these may contribute to the process of immortalization, there is no direct evidence that any particular gene is required. We have developed a genetic analysis of EBV that uses a transformation-defective strain of the virus as a helper virus in conjunction with DNA that contains all of the viral cis-acting elements required for replication, cleavage and packaging during the lytic phase of the viral life cycle. This DNA can include viral genes required for immortalization that complement the transformation-defective virus strain. The DNA can be amplified and packaged by the products of the helper virus and the packaged DNA is infectious. We have analysed two viral genes expressed in immortalized cells and find that the gene encoding EBV nuclear antigen-2 is required for immortalization, whereas the gene for the EBV nuclear antigen leader protein is not.

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

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

Stimulation of NF-kappa B-mediated transcription by mutant derivatives of the latent membrane protein of Epstein-Barr virus

The latent membrane protein (LMP) of Epstein-Barr virus contributes to the immortalizing activity of the virus in primary, human B lymphocytes, but its mechanism of function is unknown. LMP is expressed at the plasma membrane and may act by influencing the signalling pathways of infected cells. LMP increases transcription of reporter plasmids that are responsive to members of the NF-kappa B/Rel family of transcription factors (M.-L. Hammarskjold and M. C. Simurda, J. Virol. 66:6496-6501, 1992, and A. Krikos, C. D. Laherty, and V. M. Dixit, J. Biol. Chem. 267:17971-17976, 1992). We measured the stimulation of the activity of a reporter plasmid by LMP in Jurkat and 293 cells in transfection experiments. Expression of LMP stimulated plasmids that contained kappa B enhancer elements but not plasmids that lacked the elements. In 293 cells, expression of the NF-kappa B inhibitor, I kappa B-alpha, reduced the stimulatory activity of LMP. We used deletional analysis to map the domains of LMP that are required for its activity in 293 cells. Wild-type LMP stimulated NF-kappa B by a factor of 20 to 30, while mutant derivatives of LMP that lack oncogenic activity stimulated NF-kappa B by a factor of 3. The multiple membrane-spanning segments together with the carboxy-terminal 55 amino acid residues of LMP were required for its maximal stimulatory function. Residues within its cytoplasmic amino terminus were not required for LMP's stimulation of NF-kappa B. We tested also for stimulation of NF-kappa B activity in cell lines known to support phenotypic changes mediated by expression of LMP. LMP stimulated little NF-kappa B activity in HEp2 cells and no detectable NF-kappa B activity in BALB/3T3 cells. The LMP stimulation of NF-kappa B factors that occurs in some cell lines provides a useful and biochemically tractable assay for determining the function of LMP.

A promoter of Epstein-Barr virus that can function during latent infection can be transactivated by EBNA-1, a viral protein required for viral DNA replication during latent infection

A viral promoter that functions on recombinant plasmids in cells immortalized by Epstein-Barr virus was identified and characterized. It is identical to that mapped on the viral genome by Bodescot et al. (M. Bodescot, M. Perricaudet, and P.J. Farrell, J. Virol. 61:3424-3430, 1987) which functions during the latent phase of the viral life cycle in some but not all cells to encode several latent viral gene products. Experiments with these plasmids indicated that this promoter requires the enhancer within oriP of Epstein-Barr virus in cis to function efficiently. They also indicated that it requires the EBNA-1 gene in trans to function efficiently. The EBNA-1 gene therefore positively affects both viral DNA replication (J.L. Yates, N. Warren, and B. Sugden, Nature [London] 313:812-815, 1985) and viral transcription.

A putative origin of replication of plasmids derived from Epstein-Barr virus is composed of two cis-acting components

A genetic element of Epstein-Barr virus, oriP, when present on recombinant plasmids allows those plasmids to replicate and to be maintained in cells that express the Epstein-Barr virus-encoded nuclear antigen EBNA-1. Here we define the DNA sequences required for oriP activity. Two noncontiguous regions of oriP are required in cis for activity. One consists of approximately 20 tandem, imperfect copies of a 30-base-pair (bp) sequence. The other required region, approximately 1,000 bp away, is at most 114 bp in length and contains a 65-bp region of dyad symmetry. When present together on a plasmid, these two components supported plasmid replication even when the distance between them was varied or their relative orientation was altered, or both. When present alone on a plasmid that expresses a selectable marker, the family of 30-bp repeats efficiently conferred a transient drug-resistant phenotype in human 143 cells that is dependent on the presence of EBNA-1. This result leads us to suggest that EBNA-1 interacts with the 30-bp repeated sequence to activate oriP. To test whether the 30-bp repeats might cause the increased transient expression of drug resistance by enhancing transcription, the family of 30-bp repeats was tested for the ability to activate the simian virus 40 early promoter present in plasmid pA10CAT2 (Laimins, et al., Proc. Natl. Acad. Sci. U.S.A. 79:6453-6457). In this assay, the 30-bp repeats could activate the simian virus 40 early promoter in Raji cells, an EBNA-positive Burkitt's lymphoma cell line, but not detectably an EBNA-positive 143 cells in which oriP also functions.

Clonal transformation of adult human leukocytes by Epstein-Barr virus

We have developed a clonal transformation assay for Epstein-Barr virus which uses adult human leukocytes as target cells. The target cells were isolated from Epstein-Barr seronegative donors, and the same donor's cells could be studied repeatedly over long periods of time. When these cells were transformed by Epstein-Barr virus and had proliferated sufficiently to be studied, they had an average cloning efficiency of 3%. Assuming this average cloning efficiency obtains at the onset of transformation, we calculate that transformation by Epstein-Barr virus leads to immortalization maximally of about 1 in 30 of the adult peripheral leukocytes exposed to the virus. Studying the number of colonies transformed as a function of the amount of virus to which the cells are exposed indicates that a single DNA-containing virus particle is sufficient to transform a cell. All of the transformed clones studied harbored viral DNA. This technique will now permit, for the first time, our studying clonal variations in adult peripheral leukocytes transformed by Epstein-Barr virus as a function of input multiplicity of the virus and of the donor's immune status.

trans activation of an Epstein-Barr viral transcriptional enhancer by the Epstein-Barr viral nuclear antigen 1

Two regions of the Epstein-Barr virus (EBV) genome together make up an element, oriP, which acts in cis to support plasmid replication in cells that express the EBV nuclear antigen 1 (EBNA-1). The two components of oriP are a region containing a 65-base-pair (bp) dyad symmetry and a region containing 20 copies of a 30-bp direct repeat. Here we show that the 30-bp family of repeats of oriP can function as a transcriptional enhancer that is activated in trans by the EBNA-1 gene product. In either EBV-genome-positive cells or in cells that express EBNA-1, the 30-bp family of repeats, when positioned in either orientation upstream or downstream, enhances expression of the chloramphenicol acetyltransferase (CAT) gene expressed from either the simian virus 40 early promoter or the herpes simplex virus type 1 thymidine kinase promoter. The extent of transcriptional enhancement varies with the promoter and cell type. This enhanced CAT expression reflects an increased level of CAT mRNA and does not result from amplification of the plasmids expressing CAT. In addition, plasmids carrying the gene for resistance to hygromycin B and the 30-bp family of repeats yielded 10 to 100 times more hygromycin B-resistant colonies than the vector lacking the 30-bp family of repeats in both EBV-genome-positive cells and cells that express EBNA-1. EBNA-1 is known to bind to sequences within the 30-bp family of repeats (D. R. Rawlins, G. Milman, S. D. Hayward, and G. S. Hayward, Cell 42:859-868, 1985), and these trans- and cis-acting elements together have at least two functional roles: (i) they are required for DNA replication dependent upon oriP, and (ii) they can enhance expression of genes linked to the 30-bp family of repeats of oriP.

An EBNA-1-dependent enhancer acts from a distance of 10 kilobase pairs to increase expression of the Epstein-Barr virus LMP gene

Upon infection of human B lymphocytes, the 172-kbp Epstein-Barr virus genome forms a covalently closed circle via its terminal repeats. This event brings all of the promoters that control expression of the latent gene products, and the viral origin of plasmid replication, oriP, within a 20-kbp stretch of contiguous DNA. We have found that the EBNA-1-dependent transcriptional enhancer FR, located in oriP, increased the expression of a tagged viral oncogene encoding the latent membrane protein (LMP) up to 200-fold in normal Epstein-Barr virus-positive cells. The effect of FR was exerted across 10 kbp of viral DNA that spans the circularized ends of the viral genome. Enhancement of the tagged LMP gene by FR/EBNA-1 did not require the EBNA-2-responsive element.

Characterization of LMP-1's association with TRAF1, TRAF2, and TRAF3

The latent membrane protein 1 (LMP-1) of Epstein-Barr virus (EBV) contributes to the immortalizing activity of EBV in primary, human B lymphocytes. LMP-1 is targeted to the plasma membrane, where it influences signaling pathways of infected cells. LMP-1 has been found to associate with members of the tumor necrosis factor receptor-associated factor (TRAF) family of proteins. As with LMP-1, the TRAF molecules have been shown to participate in cell signaling pathways. We have characterized and mapped in detail a region of LMP-1 that associates with TRAF1, TRAF2, and TRAF3. TRAF3 alone associates with LMP-1 in a yeast two-hybrid assay, whereas all three TRAF molecules associate with LMP-1 under various conditions when they are assayed in extracts of human cells. TRAF1, TRAF2, and TRAF3 appear to associate independently with LMP-1 but bind an overlapping target site. TRAF3 associates with LMP-1 most avidly and can compete with TRAF1 and TRAF2 for binding to LMP-1. TRAF2 associates with truncated derivatives of the carboxy terminus of LMP-1 more efficiently than with the intact terminus, indicating that LMP-1's conformation may regulate its association with TRAF2. Finally, point mutations that decrease LMP-1's association with the three TRAF molecules to 3 to 20% of wild-type levels do not detectably affect otherwise intact LMP-1's induction of NF-kappaB activity. Therefore, these associations are not necessary for the majority of intact LMP-1's induction of this signaling pathway.

EBNA-1: a protein pivotal to latent infection by Epstein-Barr virus

Epstein-Barr nuclear antigen 1, or EBNA-1, is required for the replication of the EBV genome as an extra-chromosomal element and is a key transcriptional regulator of this virus's latent gene expression. In this review we will describe the salient features of EBNA-1 and oriP, the latent origin of EBV to which EBNA-1 binds site-specifically. EBNA-1's association with host cellular factors, its association with metaphase chromosomes, and its ability to link DNAs to which it binds will be discussed in relation to its roles in replication and transcriptional activation. Although the mechanisms by which EBNA-1 facilitates replication and transcription largely remain enigmatic, EBV's viral replicon has been exploited successfully for applications in gene therapy and in the design of eukaryotic vectors for use in cell culture.

The transforming domain alone of the latent membrane protein of Epstein-Barr virus is toxic to cells when expressed at high levels

A previously unrecognized activity has been associated with the product of the BNLF-1 gene of Epstein-Barr virus. This gene encodes the latent membrane protein of Epstein-Barr virus. When the gene was expressed at high levels, it was toxic to all cell lines tested, which included six human B-lymphoid lines as well as BALB/3T3, 143/EBNA-1, and HEp-2 cells. The BNLF-1 gene was previously shown to induce anchorage-independent and tumorigenic growth in Rat-1 and BALB/3T3 cells. We demonstrate here that only those mutations in the BNLF-1 gene that score positively in the anchorage-independent growth assay were cytotoxic when expressed at high levels. It is therefore possible that the same activities of the latent membrane protein that are necessary to induce anchorage-independent growth of some rodent cell lines also confer toxicity to many cell lines when expressed at high levels.

Retention of plasmid DNA in mammalian cells is enhanced by binding of the Epstein-Barr virus replication protein EBNA1

The capacity to bind the Epstein-Barr viral protein EBNA1 increases the retention of the plasmid in dividing cells. This retention requires binding of multiple EBNA1 molecules for function, although significant retention activity is seen with fewer EBNA1 binding sites than are required to activate replication or transcription. The regions of EBNA1 that are required for increased plasmid retention overlap with those required for activation of transcription and replication. The similarities in traits of EBNA1 that are required for support of DNA replication and retention of plasmid DNA indicate that both may be mediated by interactions with an overlapping set of cellular proteins.

Antibodies to recoverin induce apoptosis of photoreceptor and bipolar cells in vivo

Autoantibodies against recoverin are found in the sera of patients with cancer-associated retinopathy (CAR) syndrome. In these studies we examined the effect of anti-recoverin antibodies from the sera of patients with CAR and rat monoclonal antibody on the retinas of Lewis rats. Anti-recoverin autoanti-bodies penetrated into the photoreceptor and bipolar cell layers following intravitreal injection. Their presence in the retina could be detected by immunofluorescence 24 h after injection. At the same time, individual cells undergoing apoptosis were identified throughout photoreceptor and bipolar cell layers using terminal transferase-mediated dUTP nick-end labeling (TUNEL) and electron microscopy. Normal antibodies used in control experiments did not produce TUNEL labeling. At 24 h, DNA fragmentation was confirmed by DNA ladder electrophoresis. At the electron microscopic level, there was clear evidence of cells undergoing apoptotic cell death in the retinas treated with anti-recoverin antibodies. At 24 and 96 h, nuclear chromatin condensation and increased vacuolization of photoreceptor outer segments were observed. An examination of retinas from animals receiving anti-retinal antibodies revealed a loss of 1-2 rows of nuclei in the outer and inner nuclear layers whereas all controls (sham, normal IgG, phosphate buffered saline) showed an unchanged number of nuclei rows. In addition, there was an increase in spacing between the rows of nuclei of the outer nuclear layer in retinas treated with anti- recoverin antibodies, indicating additional cell loss. These studies provide clear evidence that anti-recoverin antibodies are capable of penetrating photoreceptor and bipolar cells, the normal site of recoverin expression in the retina, and that anti-recoverin antibodies produce apoptotic cell death. A similar mechanism may occur in patients with CAR, which may lead to visual loss and blindness.

The plasmid replicon of EBV consists of multiple cis-acting elements that facilitate DNA synthesis by the cell and a viral maintenance element

Plasmids containing oriP, the plasmid origin of Epstein-Barr virus (EBV), are replicated stably in human cells that express a single viral trans-acting factor, EBNA-1. Unlike plasmids of other viruses, but akin to human chromosomes, oriP plasmids are synthesized once per cell cycle, and are partitioned faithfully to daughter cells during mitosis. Although EBNA-1 binds multiple sites within oriP, its role in DNA synthesis and partitioning has been obscure. EBNA-1 lacks enzymatic activities that are present in the origin-binding proteins of other mammalian viruses, and does not interact with human cellular proteins that provide equivalent enzymatic functions. We demonstrate that plasmids with oriP or its constituent elements are synthesized efficiently in human cells in the absence of EBNA-1. Further, we show that human cells rapidly eliminate or destroy newly synthesized plasmids, and that both EBNA-1 and the family of repeats of oriP are required for oriP plasmids to escape this catastrophic loss. These findings indicate that EBV's plasmid replicon consists of genetic elements with distinct functions, multiple cis-acting elements that facilitate DNA synthesis and viral cis/trans elements that permit retention of replicated DNA in daughter cells. They also explain historical failures to identify mammalian origins of DNA synthesis as autonomously replicating sequences.

Immortalization of human B lymphocytes by a plasmid containing 71 kilobase pairs of Epstein-Barr virus DNA

We have assembled derivatives of Epstein-Barr Virus (EBV) that include 71 kbp of noncontiguous DNA sequences cloned into a prokaryotic F-factor plasmid. These mini-EBVs, when introduced into an EBV-containing lymphoblastoid cell, can be packaged by the endogenous helper virus. One such mini-EBV was found to have a single C residue deleted from its EBNA3a open reading frame. When packaged, this mini-EBV initiates proliferation of infected primary human B lymphocytes only in conjunction with a complementing helper virus. Proliferation of the infected cells, however, was maintained either alone by the mini-EBV containing the mutated EBNA3a open reading frame or alone by its derivative in which the EBNA3a open reading frame had been healed of its lesion by recombination with the helper virus. The mini-EBV with a wild-type EBNA3a open reading frame when packaged alone can both initiate and maintain proliferation upon infection of primary human B lymphocytes. These findings identify 41% of EBV DNA which is sufficient to immortalize primary human B lymphocytes and provide an assay to distinguish virus contributions to initiation or maintenance of cell proliferation or both. They also identify EBNA3a as a transforming gene, which contributes primarily to the initiation of cell proliferation.

DNA looping between the origin of replication of Epstein-Barr virus and its enhancer site: stabilization of an origin complex with Epstein-Barr nuclear antigen 1

Epstein-Barr nuclear antigen 1 (EBNA-1) is the only viral protein required to support replication of Epstein-Barr virus during the latent phase of its life cycle. The DNA segment required for latent replication, oriP, contains two essential binding regions for EBNA-1, termed FR and DS, that are separated by 1 kilobase pair. The FR site appears to function as a replicational enhancer providing for the start of replication at the DS site. We have used electron microscopy to visualize the interaction of EBNA-1 with its binding sites and to study the mechanism for communication between the FR and DS sites. We have found that DNA-bound EBNA-1 forms a DNA loop between the FR and DS sites. From these results, we suggest that EBNA-1 bound to the replicational enhancer acts by a DNA-looping mechanism to facilitate the initiation of DNA replication. Occupancy of the DS site alone is highly sensitive to competition with nonspecific DNA. In contrast, occupancy of the DS site by looping from FR is largely resistant to the competitor DNA. These experiments support the concept that enhancers act in cis from nearby sites to provide a high local concentration of regulatory proteins at their target sites and to stabilize regulatory interactions.

The linking regions of EBNA1 are essential for its support of replication and transcription

The ability of distant cis-acting DNA elements to interact functionally has been proposed to be mediated by the interaction of proteins associated site specifically with those cis-acting elements. We have found that the DNA-linking regions of EBNA1 are essential for its contribution to both replication and transcription. The synthesis of plasmids containing the Epstein-Barr virus (EBV) origin of plasmid replication (oriP) can be mediated entirely by the cellular machinery; however, the replicated molecules are lost rapidly from proliferating cells. When EBNA1 is provided in trans, plasmids containing oriP (oriP plasmids) are synthesized during repeated S phases, and the newly formed daughter molecules are precisely segregated to the daughter cells. The contribution(s) of EBNA1 to the stable replication of oriP plasmids is therefore likely to be postsynthetic. In latently infected cells, EBNA1 also regulates the expression of multiple EBV promoters located as many as 10 kbp away. EBNA1 supports replication and transcription through binding to oriP; both the ability of EBNA1 to bind to DNA and the integrity of its binding sites in oriP are required. However, DNA binding by EBNA1 is not sufficient to support replication or transcription, indicating that an additional activity (or activities) is required. EBNA1 links DNAs to which it binds and can form a loop between the two subelements of oriP, the family of repeats and the region of dyad symmetry, each of which contains multiple binding sites for EBNA1. We have constructed a set of derivatives of EBNA1 which contain both, one, or neither of its linking regions in various contexts. Analyses of these derivatives demonstrate that the linking regions of EBNA1 are essential for its support of replication and transcription and that the ability of derivatives of EBNA1 to link DNAs correlates strongly with their support of these activities in cells. These findings indicate that protein-protein associations of the linking regions of EBNA1 underlie its long-range contributions to replication and transcription.

Dominant-negative inhibitors of EBNA-1 of Epstein-Barr virus

Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA-1) is required in trans to support replication of the EBV genome once per cell cycle via the latent origin of replication, oriP. EBNA-1 can also activate transcription on binding to the family of repeats of oriP to enhance some heterologous as well as native EBV promoters. We have made and screened derivatives of EBNA-1 for the ability to act as inhibitors of wild-type EBNA-1. These derivatives lack the linking or the retention functions of EBNA-1 and were analyzed for the residual ability to activate transcription and replication. We have identified derivatives of EBNA-1 that can inhibit up to 98% of wild-type EBNA-1's activities. We have also identified one derivative of EBNA-1 with only two of EBNA-1's three linking domains which can support transcription and replication inefficiently.

Plasmid maintenance of derivatives of oriP of Epstein-Barr virus

oriP is the origin of plasmid replication of Epstein-Barr virus. Replication from oriP requires both the cis-acting elements (the family of repeats and the dyad symmetry element) and the viral origin-binding protein, EBNA-1. The ability of plasmids containing oriP to be maintained stably in EBNA-1-positive cells reflects the efficiency both of their replication and of their segregation each cell cycle. The efficiency of plasmid maintenance was determined for plasmids containing derivatives of oriP with one copy of the dyad symmetry element and two copies of the family of repeats by measuring the rate at which they were lost from cells in the absence of selection. These measurements demonstrated that plasmids with derivatives of oriP with two copies of the family of repeats in one orientation are maintained only slightly less efficiently than is wild-type oriP. To determine whether plasmid maintenance could be affected by reinitiation at the dyad symmetry element (T. A. Gahn and C. L. Schildkraut, Cell 58:527-535, 1989), plasmids containing derivatives of oriP with two copies of the dyad symmetry element and one copy of the family of repeats were compared with plasmids containing wild-type oriP in EBNA-1-positive cells. These measurements showed that plasmids containing a derivative of oriP with two copies of the dyad symmetry element are maintained as efficiently as is wild-type oriP and are not amplified relative to wild-type oriP. These observations indicate that the trans-acting factors that regulate DNA to replicate once per S phase are insensitive to multiple cis-acting regulatory sites within a replicon.

The structure of the termini of the DNA of Epstein-Barr virus

We have studied the DNA of Epstein-Barr virus (EBV) isolated from the B95-8 strain of that virus (Miller and Lipman, 1973). When EBV DNA is partially digested with lambda-exonuclease and allowed to reanneal, up to 50% of the full-length molecules circularize. The arrangements of nucleotide sequences containing the terminal repeats identified in this circularization experiment have been determined. Those fragments of viral DNA generated by digestion with restriction endonucleases which are terminal and contain the terminal repeats have been identified by their sensitivity to digestion of full-length DNA by lambda-exonuclease and by virtue of their being partially homologous to one another. The population of DNA molecules in the B95-8 strain of EBV was found to be nonuniform. The nonuniformity results from different molecules having different numbers of a 0.37 megadalton terminal repeat at each end. About 70% of molecules have four terminal repeats at one end, while four equal classes, each comprising approximately 25% of the population, have one, two, three or four repeats at the other end. The arrangements of nucleotide sequences identified as being terminal in virion DNA were studied in the intracellular circular viral DNA of cells transformed by a single particle on EBV. All fragments produced by digestion with endonucleases and scored as being terminal in virion DNA were absent from intracellular circular DNA. An additional fragment was identified in the digests of intracellular DNA of each transformed clone. The molecular weights of the new fragments equal the sum of the molecular weights of two terminal fragments which are joined upon intracellular circularization of viral DNA.

EBNA1 can link the enhancer element to the initiator element of the Epstein-Barr virus plasmid origin of DNA replication

The plasmid origin of DNA replication of Epstein-Barr virus, oriP, is replicated once per cell division, employing cellular replication machinery and only one viral protein. To understand how replication from this origin is initiated and regulated, we purified this viral protein, EBNA1. EBNA1 was expressed in CV-1p cells by using an infectious simian virus 40 vector containing the EBNA1 gene. It was purified in two chromatographic steps to apparent homogeneity. The purified protein is capable of supporting transcription of the luciferase gene from a reporter plasmid carrying the FR enhancer element to which EBNA1 binds. EBNA1 does not have oriP-dependent ATPase activity, indicating that it does not carry out an energy-dependent step in the initiation of DNA replication. However, EBNA1 does mediate an association between the two elements of oriP. We measured this association by binding one of the elements, the enhancer element, to a solid matrix and measuring retention by this element of the other one, the initiator element, in the presence of EBNA1. This retention is specific for DNA fragments containing EBNA1-binding sites. EBNA1 thus can link the two elements of the origin, providing a locally high concentration of EBNA1 at the site of initiation of DNA replication. We propose that this association is important either (i) to affect DNA structure to allow a cellular helicase to initiate DNA strand separation or (ii) to bind replication proteins to bring them to the origin of replication.