Initiation of the Epstein-Barr virus (EBV) lytic cycle is dependent on expression of the viral transactivator Zta, which is encoded by the BZLF1 gene. Described here is an initial mapping of the regions of Zta involved in activating transcription. The data indicate that the amino-terminal 153 amino acids of Zta are important for activity, and in particular the region from residues 28 to 78 appears to be critical for Zta function. However, other features of Zta may be important for activity since a Gal4-Zta chimeric protein, generated by fusing the amino-terminal 167 residues of Zta to the DNA binding domain of the yeast transactivator Gal4, transactivated a minimal promoter containing one upstream Gal4 binding site but was unable to exhibit synergistic transactivation when assayed with a reporter containing five upstream Gal4 binding sites.

The Epstein-Barr virus BRLF1 and BZLF1 genes are the first viral genes transcribed upon induction of the viral lytic cycle. The protein products of both genes (referred to here as Rta and Zta, respectively) activate expression of other viral genes, thereby initiating the lytic cascade. Among the viral antigens expressed upon induction of the lytic cycle, however, Zta is unique in its ability to Epstein-Barr virus (EBV) is a lymphotropic human herpesvirus that latently infects B lymphocytes, resulting in a concomitant growth transformation of the infected cell. Infection is closely associated with several human cancers, including nasopharyngeal carcinoma and African Burkitt's lymphoma, and also plays a role in several lymphoproliferative diseases in immunocompromised individuals (for a review, see reference 29). In vitro the transforming potential of EBV is evidenced by its ability to immortalize B lymphocytes to grow indefinitely in culture. Immortalization is achieved through the expression of a relatively small subset of EBV-encoded genes that serve to establish and maintain cellular transformation (for a review, see reference 45).Propagation of EBV from host to host is dependent upon the activation of an estimated 100 or more viral genes, culminating in the production of infectious virions (2, 4, 9, 27). While these genes remain quiescent during latency, a switch in the genetic program leading to the expression of viral replication associated genes can be accomplished in vitro by treatment of latently infected B lymphocytes with various reagents, including phorbol esters, butyrate, ionophore, and anti-immunoglobulin (2,28,37,46,48,51). Activation of the lytic cascade by treatment with anti-immunoglobulin results initially in the expression of two viral genes, BZLF1 and BRLF1, which exhibit similar induction kinetics (maximal mRNA levels are reached between 2 and 4 h postinduction) (14, 46). The protein products of both the BZLF1 (referred to here as Zta, but also called ZEBRA and EB1) and BRLF1 (Rta) genes have been shown to be transcriptional activators (8,10,12,25,26 speck@visar.wustl.edu.Expression of Zta and Rta leads to the activation of early genes and ultimately viral replication. Of all the viral transactivators examined, Zta is unique in that its expression alone can initiate the entire lytic cascade (10,11,40), and regulation of Zta expression appears to be central to regulating entry into the lytic cycle.We have previously identified multiple elements within the BZLF1 promoter (Zp) that play a role in regulating BZLF1 expression (15,16). Several of these elements bind cellular transcription factors that are responsive to phorbol ester stimulation (15). Zp also contains two Zta binding sites (ZIIIA and ZIIIB) that are important for autoactivation (16,33,49).Zta shares several structural similarities with other transcription factors. As described by Farrell et al. (12), Zta contains a basic region with homology to the DNA-binding domains of the AP-1 family of transcription factors (Fig. 1). In addi...

The Epstein-Barr virus BRLF1 and BZLF1 genes are the first viral genes transcribed upon induction of the viral lytic cycle. The protein products of both genes (referred to here as Rta and Zta, respectively) activate expression of other viral genes, thereby initiating the lytic cascade. Among the viral antigens expressed upon induction of the lytic cycle, however, Zta is unique in its ability to disrupt viral latency; expression of the BZLF1 gene is both necessary and sufficient for triggering the viral lytic cascade. We have previously shown that Zta can activate its own promoter (Zp), through binding to two Zta recognition sequences (ZIIIA and ZIIIB). Here we describe mutant Zta proteins that do not bind DNA (referred to as Zta DNA-binding mutants [Zdbm]) but retain the ability to transactivate Zp. Consistent with the inability of these mutants to bind DNA, transactivation of Zp by Zdbm is not dependent on the Zta recognition sequences. Instead, transactivation by Zdbm is dependent upon promoter elements that bind cellular factors. An examination of other viral and cellular promoters identified promoters that are weakly responsive or unresponsive to Zdbm. An analysis of a panel of artificial promoters containing one copy of various promoter elements demonstrated a specificity for Zdbm activation that is distinct from that of Zta. These results suggest that non-DNA-binding forms of some transactivators retain the ability to transactivate specific target promoters without direct binding to DNA.