EPSTEIN-BARR-VIRUS NUCLEAR ANTIGEN 3C IS A POWERFUL REPRESSOR OF TRANSCRIPTION WHEN TETHERED TO DNA

The expression of Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA3C) is essential for the activation and immortalization of human B lympho cytes by EBV. EBNA3C consists of 992 amino acids and includes a potent ial bZIP motif and regions rich in acidic, proline, and glutamine resi dues. Thus, EBNA3C resembles several trans regulators of gene expressi on. It has recently been shown that a fragment of EBNA3C can activate reporter gene expression when fused to the DNA-binding domain of GAL4 (D. Marshall and C. Sample, J. Virol. 69:3624-3630, 1995). Although EB NA3C binds DNA, a specific site for EBNA3C binding has not been identi fied; to test the ability of full-length EBNA3C to regulate transcript ion, EBNA3C (amino acids 11 to 992) was fused to the DNA-binding domai n of GAL4. We show that this fusion protein does not transactivate but rather is a potent repressor of reporter gene expression. Repression is dependent on the dose of GAL4-EBNA3C and on the presence of GAL4-bi nding sites within reporter plasmids. Repression is not restricted to B cells nor is it species or promoter specific. Repression is independ ent of the location of the GAL4-binding sites relative to the transcri ption start site. A fragment of EBNA3C (amino acids 280 to 525) which represses expression in a manner which is nearly identical to that of the full-length protein has been identified; this fragment is rich in acidic and proline residues. A second, less potent repressor region lo cated C terminal to amino acids 280 to 525 has also been identified; t his domain is rich in proline and glutamine residues. We also show bin ding of EBNA3C, in vitro, to the TATA-binding protein component of TFI ID, and this suggests a mechanism by which EBNA3C may communicate with the basal transcription complex.