Lytic cycle switches of oncogenic human gammaherpesviruses

The seminal experiments of George and Eva Klein helped to define the two life cycles of Epstein-Barr Virus (EBV), namely latency and lyric or productive infection. Their laboratories described latent nuclear antigens expressed during latency and discovered several chemicals that activated the viral lyric cycle. The mechanism of the switch between latency and the lyric cycle of EBV and Kaposi's sarcoma-associated herpesvirus (KSHV) can be studied in cultured B cell lines. Lytic cycle activation of EBV is controlled by two viral transcription factors, ZEBRA and Rta. The homologue of Rta encoded in ORF50 is the lyric cycle activator of KSHV. Control of the lyric cycle can be divided into two distinct phases. Upstream events control expression of the vitally encoded lytic cycle activator genes. Downstream events represent tasks carried out by the viral proteins in driving expression of lyric cycle genes and lyric viral DNA replication. In this chapter, we report three recent groups of experiments relating to upstream and downstream events. Azacytidine (AzaC) is a DNA methyltransferase inhibitor whose lytic cycle activation capacity was discovered by G. Klein and coworkers. We find that AzaC rapidly activates the EBV lytic cycle but does not detectably alter DNA methylation or histone acetylation on the promoters of the EBV lyric cycle activator genes. AzaC probably acts via a novel, yet to be elucidated, mechanism. The lyric cycle of both EBV and KSHV can be activated by sodium butyrate (NaB), a histone deacetylase inhibitor whose activity in disrupting latency was also discovered by G. Klein and coworkers. Activation of EBV by NaB requires protein synthesis; activation of KSHV is independent of protein synthesis. Thus, NaB works by a different pathway on the two closely related viruses. ZEBRA, the major downstream mediator of EBV lyric cycle activation is both a transcription activator and an essential replication protein. We show that phosphorylation of ZEBRA at its casein kinase 2 (CK2) site separates these two functions. Phosphorylation by CK2 is required for ZEBRA to activate lyric replication but not to induce expression of early lytic cycle genes. We discuss a number of unsolved mysteries about lyric cycle activation which should provide fertile territory for future research. (c) 2007 Elsevier Inc.