EPSTEIN-BARR-VIRUS EBNA3C REPRESSES CP, THE MAJOR PROMOTER FOR EBNA EXPRESSION, BUT HAS NO EFFECT ON THE PROMOTER OF THE CELL GENE CD21

EBNA3C is a potent repressor of transcription when bound to DNA as a f usion with the DNA binding domain (DBD) of GAL4. A survey of promoters has revealed that the wild-type, unfused EBNA3C can specifically repr ess expression from reporter plasmids containing the Epstein-Barr viru s Cp latency-associated promoter, Repression of Cp activity required a mino acids 207 to 368, which encompasses a region resembling a basic D BD adjacent to a leucine zipper DNA binding motif and a site which bin ds to the cellular factor CBF1/RBP-J kappa. However, amino acids 207 t o 368 are dispensable when the protein is bound to DNA as a fusion,vit h the GAL4 DBD, thus implicating this region in DNA binding, Mutation of the CBF1/RBP-J kappa binding site in EBNA3C abrogated repression, s trongly suggesting that CBF1/RBP-J kappa is necessary for targeting th e viral protein to Cp, Consistent with this result, mutation of the EB NA2 response element (a CBF1/RBP-J kappa binding site) in Cp also prev ented significant repression, In addition, amino acids 346 to 543, whi ch were previously defined as important for the repressor activity of the GAL4-EBNA3C fusion proteins, also appear to be necessary for the r epression of Cp, Since repression by these fusions was not observed in all cell types, it seems likely that EBNA3C either depends on a corep ressor which may interact with amino acids 346 to 543 or is modified i n a cell-specific manner in order to repress, These data are consisten t with EBNA3C contributing to the regulation of EBNA expression in lat ently infected B cells through CBF1/RBP-J kappa and another factor, bu t this need not directly involve EBNA2, Finally, although it has been reported that EBNA3C can upregulate CD21 in some B cells, we were unab le to demonstrate any effect of EBNA3C on reporter plasmids which cont ain the CD21 promoter.