AP-1 homolog BZLF1 of Epstein–Barr virus has two essential functions dependent on the epigenetic state of the viral genome

Abstract

EBV, a member of the herpes virus family, is a paradigm for human tumor viruses and a model of viral latency amenable for study in vitro. It induces resting human B lymphocytes to proliferate indefinitely in vitro and initially establishes a strictly latent infection in these cells. BZLF1, related to the cellular activating protein 1 (AP-1) family of transcription factors, is the viral master gene essential and sufficient to mediate the switch to induce the EBV lytic phase in latently infected B cells. Enigmatically, after infection BZLF1 is expressed very early in the majority of primary B cells, but its early expression fails to induce the EBV lytic phase. We show that the early expression of BZLF1 has a critical role in driving the proliferation of quiescent naïve and memory B cells but not of activated germinal center B cells. BZLF1's initial failure to induce the EBV lytic phase relies on the viral DNA at first being unmethylated. We have found that the eventual and inevitable methylation of viral DNA is a prerequisite for productive infection in stably, latently infected B cells which then yield progeny virus lacking cytosine-phosphatidylguanosine (CpG) methylation. This progeny virus then can repeat EBV's epigenetically regulated, biphasic life cycle. Our data indicate that the viral BZLF1 protein is crucial both to establish latency and to escape from it. Our data also indicate that EBV has evolved to appropriate its host's mode of methylating DNA for its own epigenetic regulation.cytosine-phosphatidyl-guanosine methylation | latency | transcription | herpesvirus | reactivation

RNAs in Epstein–Barr virions control early steps of infection

“…Genetic data suggested that the early expression of BZLF1 has a critical role in driving the proliferation of resting B cells (8). However, tvRNAs were previously not considered as a source of BZLF1 protein immediately after infection and before de novo transcription.…”

“…Previous studies have already described transcripts of EBV in primary B cells shortly after infection, but the origin of these RNAs was not further analyzed (8,26). We set out to determine the time when viral transcripts appear after infection.…”

“…It has recently been described that, in newly infected cells, before establishment of latency, EBV expresses a small set of viral genes that had previously been classified as immediate-early or early genes of the lytic cycle. At this prelatent stage of infection, the immediate expression of these genes activates resting B cells (8) while protecting them from immediate activation-induced apoptosis (9) and other endogenous stress response signals (10). The prelatent phase is followed by a stable latent phase, characterized by the expression of viral latent genes that support cellular proliferation and sustain the outgrowth of lymphoblastoid cells lines in vitro (11).…”

Epstein-Barr virus interactions with the Bcl-2 protein family and apoptosis in human tumor cells

“…After primary infection, BZLF1 is expressed very early in B cells. However, its early expression does not immediately initiate the EBV lytic cycle but promotes the proliferation of resting memory and naive B cells (Kalla et al, 2010). It is also very important for maintaining cell survival during the lytic phase.…”

Repression of CIITA by the Epstein–Barr virus transcription factor Zta is independent of its dimerization and DNA binding

“…Routes by which Zta activates gene expression have been documented for both viral and host promoters. Many promoters are targeted by the interaction of the sequence-specific DNA-binding domain of Zta with sequence-specific 7 nt DNA elements, termed ZREs (Adamson & Kenney, 1999;Bergbauer et al, 2010;Bhende et al, 2004Bhende et al, , 2005Broderick et al, 2009;Dickerson et al, 2009;Flower et al, 2011;Holley-Guthrie et al, 1990;Kalla et al, 2012Kalla et al, , 2010Karlsson et al, 2008;Kenney et al, 1989;Ramasubramanyan et al, 2012a, b;Sinclair, 2003;Sinclair et al, 1991;Woellmer et al, 2012). At least 32 distinct ZRE sequence variants are specifically recognized by Zta (Flower et al, 2011).…”