Role of The Zebra Protein in the Switch Between Epstein-Barr Virus Latency and Replication

Abstract

Experiments will be summarized which address two general questions about the BZLF1 gene. The first is, “what factors regulate expression of the gene?” The second is “which other EB viral genes are activated as the result of ZEBRA expression?”

At least three variables have been identified which affect ZEBRA expression; these are genome rearrangements such as occur in defective virus, cell background, and the response to different inducing agents. In the standard EBV genome ZEBRA is repressed; in certain defective genomes ZEBRA is constitutively expressed. EBV converted BL cells tightly regulate ZEBRA expression, whereas more permissive cells, such as EBV immortalized marmoset cell lines permit spontaneous ZEBRA expression. The inducing agents TPA and butyrate seem to act by releasing transcription of the BZLF1 gene, but their effects vary with genome configuration and cell background.

Cell lines have been created which stably express ZEBRA from BZLF1 maintained on oriP Plasmids. While some of these cell lines express EBV late genes and produce virions, others express only a group of early genes. Among the early genes activated are BZLF1 itself, suggesting an autostimulatory loop. Other activated genes include products recognized by Pearson’s R3 (BMRF1), 5B11 (BHRF1) and K8 (BORF2) monoclonal antibodies. Whether ZEBRA activates expression of these four genes directly or through a cascade remains unknown. Preliminary experiments indicate that ZEBRA is a DNA binding protein. Therefore its mechanism is likely to be that of a transcriptional activator of specific EBV early genes.