Epstein-Barr virus (EBV) replicates in a latent or a lyticway in the infected organism, depending on the type and level of differentiation of the host cell. The switch between latency and lytic replication was previously shown, for Burkitt's lymphoma cell lines, to depend on the viral BZLF1 gene product. Protein-DNA assays were used to identify the cis-acting elements that represent the link between regulating signal transduction pathways and the viral cascade of gene expression. confirmed that these sites act as specific protein recognition sites. Using a set of reporter plasmids we were able to demonstrate a negative regulatory effect of the HI motif in some B lymphoid cell lines, in contrast to epithelial HeLa cells. The HI silencer elements are different from other silencer elements described so far in respect of their sequence and protein-binding pattern during the activation of BZLF1.

Summary. Lytic transition of Epstein-Barr virus (EBV) is initiated by distinctimmediate early regulators of the viral cycle, in synchronization to temporary, permissive conditions during host cell differentiation. We developed eukaryotic vectors suitable to imitate the processes involved in lyric transition in cell culture systems. Two stable B cell lines were established: R59Z activator cells were used to induce lytic EBV expression in a constitutive manner by the production of the BZLF 1 trans-activator (Zta). R7-57 reporter cells, on the other hand, signaled induced activity of the lyric origin of EBV replication (ori Lyt). Different modes, like chemical induction, lyric superinfection with EBV and single gene trans-activation converted the recombinant ori Lyt element in R7-57 reporter cells. BZLF 1, transiently expressed in R7-57 reporter cells, was the only EBV trans-activator found, sufficient in inducing the viral lytic cycle. Basing on these experiments, trans-cellular activation of EBV was tested by cocultivation of BZLF 1-expressing R59Z activator cells with the R7-57 reporter line. No lyric effect on the reporter cells could be measured, neither by cocultivation of activator cells nor by coincubation of BZLF 1-containing cell lysates. Latency breaking activity, however, was transferred from activator to reporter cells when active, exogenous virus was added. The cell system described in these experiments provides a tool for the detection of EBV reactivation and demonstrates the potential of the lytic regulatory gene BZLF 1.