Epstein-Barr virus (EBV) establishes a latent infection in the human host. In
order to produce infectious virus particles EBV must reactivate from latency and
enter its lytic cycle. BZLF1 is the immediate early gene in EBV that mediates
the switch between latency and the lytic cycle. The BZLF1 gene is under the
control of the Zp promoter.
Reactivation from latency is studied in EBV positive Akata cell lines
where EBV can be reactivated by crosslinking the B-cell receptor (BCR) using
antibodies to mimic antigen binding. This system uses stably transfected
reporter plasmids to study Zp regulation. Mutagenesis identified additional
regions of the promoter that contribute to regulation and the ZID MEF2 binding
site was demonstrated to be functionally important during the initial stages of Zp
activation. XBP-1 splicing, previously implicated in Zp reactivation, was found
to occur rapidly in this system in response to BCR crosslinking and parallels the
transient induction of Zp. Chromatin remodelling also plays an important role in
Zp regulation. An inducible BZLF1 expression system, independent of BCR
signalling, was developed in Akata cells that accurately mimics BZLF1 activity
and provides a novel approach to study repression at Zp.
The BZLF1 protein is related to the bZIP family of transcription factors.
BZLF1 contains a bZIP motif in which C-terminal residues fold back against a
zipper region that forms an a-helical coiled-coil. The 208SSENDRLR215
sequence in the zipper region is conserved between BZLF1 and C/EBP. Point
mutagenesis in this sequence revealed the importance of individual residues for
transactivation and progression to DNA replication. The restoration of BZLF1
DNA replication activity by complementation of two deleterious mutations
(S208E and D236K) indicated that the interaction of the C-terminal tail and the
core zipper region is required for DNA replication, identifying a functional role
for this structural feature unique to BZLF1.