FUNCTIONAL AND PHYSICAL INTERACTIONS BETWEEN THE EPSTEIN-BARR-VIRUS (EBV) PROTEINS BZLF1 AND BMRF1 - EFFECTS ON EBV TRANSCRIPTION AND LYTICREPLICATION

The Epstein-Barr virus (EBV) proteins BZLF1 and BMRF1 are both essenti al for lytic EBV replication. BZLF1 is a transcriptional activator whi ch binds directly to the lytic origin of replication (oriLyt) and play s a critical role in the disruption of viral latency. The BMRF1 protei n is required for viral polymerase processivity. Here we demonstrate t hat the BMRF1 gene product functions as a transcriptional activator an d has direct (as well as indirect) interactions with the BZLF1 gene pr oduct. The BMRF1 gene product activates an essential oriLyt promoter, BHLF1, but does not activate two other early EBV promoters (BMRF1 and BHRF1). Direct interaction between the BMRF1 and BZLF1 gene products r equires the first 45 amino acids of BMRF1 and the bZip domain of BZLF1 . The effect of the BZLF1-BMRF1 interaction on early EBV transcription is complex and is promoter specific. The oriLyt BHLF1 promoter is act ivated by either the BZLF1 or BMRF1 gene product alone and is further activated by the combination of the BZLF1 and BMRF1 gene products. Enh anced activation of BHLF1 transcription by the BMRF1-BZLF1 combination does not require direct interaction between these proteins. In contra st, BZLF1-induced activation of the BMRF1 promoter is inhibited in the presence of the BMRF1 gene product. A point mutation in the BZLF1 pro tein (amino acid 200), which prevents in vitro interaction with the BM RF1 protein but which does not reduce BZLF1 transactivator function, a llows the BZLF1 protein to activate the BMRF1 promoter equally well in the presence or absence of the BMRF1 gene product. Therefore, direct interaction between the BZLF1 and BMRF1 proteins may inhibit BZLF1-ind uced transcription of the BMRF1 promoter. BZLF1 mutated at amino acid 200 is as efficient as wild-type BZLF1 in promoting replication of an oriLyt plasmid. However, this mutation reduces the ability of BZLF1 to induce lytic replication of the endogenous viral genome in D98/HE-R-1 cells. Our results indicate that functional and physical interactions between the BMRF1 and BZLF1 proteins may modulate the efficiency of l ytic EBV infection. The BMRF1 gene product clearly has a transcription al, as well as replicative, role during lytic EBV infection.