Epression of BRLF1-BZLF1 mRNA of Epstein-Barr Virus

BRLF1 and BZLF1 are the two immediate-early genes of Epstein-Barr virus (EBV) expressed at the onset stage of lytic cycle. BZLF1 encodes a transcriptional factor called Zta which activates the expression of its own gene (BZLF1) and the early genes that are required for EBV lytic replication. The Zta rotein also binds to the lytic origin of replication; this binding is a prerequisite for EBV lytic replication. Therefore, expression of BZLF1 is necessary for activating EBV lytic cycle. The second immediate-early gene of EBV, BRLF1, also encodes a transcriptional factor called Rta. The Rta protein frequently acts synergistically with Zta to activate the EBV early genes. During the lytic cycle, BZLF1 is transcribed from a promoter (PZ) situated immediately upstream from the gene. The mRNA transcribed from PZ is monocistronic containing only BZLF1. BZLF1 is also transcribed from the BRLF1 promoter (PR). The mRNA transcribed from PR is bicistronic, containing both BRLF1 and BZLF1. In this bicistronic mRNA, BZLF1 is preceded by BRLF1 which has a length of 1.8 kb. Under normal circumstances, an upstream open reading frame of such a length would hinder the translation of the downstream gene in a bicistronic mRNA. However, in the case of BRLF1-BZLF1 bicistronic mRNA, both BRLF1 and BZLF1 are efficiently translated; BRLF1 does not appear to affect the translation of BZLF1. Futhermore, translation of BZLF1 from the bicistronic mRNA apparently involves the functions of Rta and EBV-induced cellular factors. Therefore, this research will elucidate how the bicistronic mRNA is translated and how this translation affects the EBV lytic cycle. Of particular focus are the cis and the trans-acting elements involved and the role in which Rta plays in the translation of BRLF1-BZLF1 bicistronic mRNA. As generally accepted, after lytic induction by 12-tetradecanoyl porbol-13-acetate (TPA) and sodium butyrate, the monocistronic BZLF1 is immediately transcribed and this transcription activates EBV lytic cycle. However, EBV in B lymphocytes can also enter lytic cycle spontaneously without the TPA treatment. Until now, the mechanism, which triggers this spontaneous activation, remains unknown. A possibility also arises that translation of Zta from the BRLF1-BZLF1 bicistronic mRNA may play an essential role in the lytic activation. If this hypothesis is accurate, translation of BRLF1-BZLF1 bicistronic mRNA may profoundly influence the lytic development of EBV. Therefore, studies on the PR promoter and on the factors affecting the transcription of BRLF1-BZLF1 bicistronic mRNA may shed further insight into the mechanism that activates the EBV lytic cycle.

Project ID:PG9203-0469
External Project ID:NHRI-EX92-8901SL