Expression of the Epstein-Barr virus nuclear antigen-1 (EBNA-1) protein is mediated by the virus Fp promoter in Burkitt lymphoma and nasopharyngeal carcinoma. This promoter is silent in latently infected B lymphoblastoid and most Burkitt lymphoma-derived cell lines in vitro, which utilize separate promoters approximately 50 kb upstream of Fp to express EBNA proteins. Fp-mediated activation of EBNA-1 expression is also activated upon induction of the virus replication cycle. We previously demonstrated that activation of Fp in Burkitt cells requires cis-regulatory elements downstream of the site of transcription initiation. We have now mapped two positive regulatory elements within the Fp promoter. One element contains two potential binding sites for the cellular transcription factor LBP-1 between +138 and +150. A second regulatory element was mapped between +177 and +192 and can be specifically bound in vitro by protein from nuclear extracts of Burkitt cells. Although this element overlaps two partial E2F binding sites and Fp reporter plasmids could be activated in trans by the adenovirus ElA protein in cotransfection experiments, mutational analysis and DNA binding studies suggest that these are unlikely to be functional E2F response elements within Fp. We also demonstrate that Fp-directed transcription initiates at multiple sites within both the genome and the Fp reporter plasmids. However, the principal site of transcription initiation within the genome is not utilized within reporter plasmids, in which the majority of transcripts initiate at multiple sites between +150 and +200. This finding suggests that additional elements may be necessary for Fp to function normally in these assays or that the context of Fp within the viral genome is critical to its regulation.
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