Ten novel mutations were introduced into the Zp promoter to test the role of sequences outside the established transcription factor-binding sites in Epstein-Barr virus (EBV) reactivation. Most of these had only small effects, but mutations in the ZID site were shown to reduce Zp activity strongly at early times after induction by anti-immunoglobulin (anti-Ig). The binding of MEF2 transcription factor to ZID was characterized in detail and linked functionally to Zp promoter activity. The presence of XBP-1s, the active form of XBP-1, after administration of anti-Ig to Akata Burkitt's lymphoma cells is consistent with a role for this factor in reactivation of the EBV lytic cycle, although signalling through MEF2D was quantitatively much more significant in activation of Zp. Silencing of Zp during latency is thought to be primarily a consequence of a repressive chromatin structure on Zp, and this aspect of Zp regulation can be observed in the Akata genome through protection of Zp from activation by BZLF1 in the absence of signalling from the B-cell receptor.