Exclusive expression of Epstein-Barr virus nuclear antigen 1 in Burkitt lymphoma arises from a third promoter, distinct from the promoters used in latently infected lymphocytes.

Epstein-Barr virus transformation of human B lymphocytes in vitro results in the expression of six viral nuclear antigens (EBNAs) and three viral membrane proteins. However, examination of viral gene expression in fresh Burkitt lymphoma isolates has revealed expression of only one of the nuclear antigens, EBNA-1. Previous transcriptional analyses of the EBNA-encoding genes demonstrated that all these genes are driven from one of two distal promoters located near the left end of the viral genome, raising the question of how exclusive expression of EBNA-1 occurs in Burkitt lymphoma tumors. Although most established Burkitt lymphoma cell lines (group 3) exhibit the full-expression pattern of viral antigens seen in lymphoblastoid cell lines, a few cell lines have been established that retain the restricted pattern of viral gene expression (group 1). In this paper we characterize transcription of the EBNA-1 gene in a group 1 Burkitt lymphoma cell line and show that (i) neither Cp nor Wp, the promoters involved in driving EBNA gene expression in lymphoblastoid cell lines, are active in this cell line; (ii) treatment of this cell line with 5-azacytidine, previously shown to induce expression of all EBNA genes, induced Cp and Wp activity; (iii) sizes of the EBNA-1 transcripts detected in two group 1 Burkitt lymphoma cell lines correlated with each other and were distinct from the size of the EBNA-1 transcript seen in lymphoblastoid cell lines; (iv) the EBNA-1 transcripts in the group 1 Burkitt lymphoma cell lines do not hybridize to a probe containing the common 5' exons present in all the EBNA transcripts from lymphoblastoid cell lines; and (v) anchored-PCR cloning the 5' region of the EBNA-1 transcript from one of the group 1 cell lines identified two exons, FQ and U, upstream of the EBNA-1 coding exon. The FQ exon lies just downstream of a TATAA box, which may represent the promoter for transcription of EBNA-1 in these cells. It is particularly noteworthy that an incomplete EBNA-1 cDNA clone from a nasopharyngeal carcinoma tumor line that expresses EBNA-1, but not the other EBNAs, has been characterized; this EBNA-1 transcript also contains the FQ/U splice junction, suggesting that the organization of exons upstream of the EBNA-1 coding exon is the same and that this organization may reflect a viral program for exclusive EBNA-1 expression.