Cooperative action in eukaryotic gene regulation: physical properties of a viral example

The Epstein-Barr virus (EBV) infects more than 90% of the human population, and causes glandular fever as well as several more serious diseases. It is a tumor virus, and has been widely studied as a model system for cell transformation in humans. A central feature of the EBV life cycle is its ability to persist in human B cells in different latency states, denoted latency I, II, and III. In latency III the host cell is driven to cell proliferation and hence expansion of the viral population without entering the lytic pathway, while the latency I state is almost completely dormant. We here study the effective cooperativity of the viral C promoter, active in latency III EBV cell lines. We show that the unusually large number of binding sites of two competing transcription factors, one viral and one from the host, serves to make the switch sharper (higher Hill coefficient), either by cooperative binding between molecules of the same species when they bind, or by competition between the two species if there is sufficient steric hindrance.