Activation of the Epstein-Barr virus BMRF1 and BZLF1 promoters by ZEBRA in Saccharomyces cerevisiae

ZEBRA has been shown to activate model reporter genes consisting of synthetic oligomerized ZEBRA response elements upstream of a minimal CYC1 promoter fused to beta-galactosidase in the yeast Saccharomyces cerevisiae. Here it is shown that in S. cerevisiae ZEBRA activates transcription of natural Epstein-Barr virus promoters. Two Epstein-Barr virus promoters were shown to be activated by ZEBRA in S. cerevisiae: Zp, the promoter that regulates expression of BZLF1, which encodes ZEBRA; and EAp, the promoter controlling expression of BMRF1, which encodes diffuse early antigen, EA-D. These observations indicate that neither mammalian-specific nor virally encoded coactivators are obligatory for ZEBRA to stimulate expression from these two promoters. Zp was also strongly activated by endogenous yeast factors. EAp was not activated by yeast factors. The results show that in S. cerevisiae and in B cells, ZEBRA dominates the response of EAp; ZEBRA plus endogenous cell factors activate Zp.

Comparing regions of the Epstein-Barr virus ZEBRA protein which function as transcriptional activating sequences in Saccharomyces cerevisiae and in B cells

The ZEBRA protein activates expression of Epstein-Barr virus early-lytic-cycle genes in human B lymphocytes. Here it is shown that ZEBRA also behaves as a sequence-specific transcriptional activator in Saccharomyces cerevisiae. Deletional mutagenesis defined three regions of ZEBRA that participate in activation in S. cerevisiae. These regions are designated YI (amino acids [aa] 1 to 25), YII (aa 51 to 102), and YIII (aa 228 to 245). Two of the three regions of the native ZEBRA protein act together to mediate activation when assayed on ZEBRA binding sites. However, when fused to the DNA binding domain of GAL4 and assayed on GAL4 binding sites, regions YII and YIII were each sufficient to confer activation in S. cerevisiae. Regions of ZEBRA which affected activation in S. cerevisiae were also required in human B lymphocytes. The amino-terminal region of ZEBRA (aa 1 to 98) was required for activation both in S. cerevisiae and in human B cells; deletion of the carboxy-terminal 18 aa also significantly reduced activation in both cell types. Thus, the behavior of ZEBRA in human B cells and S. cerevisiae suggests that the protein contains universal activation motifs that interact with conserved components of the transcription machinery. However, certain deletion mutants of ZEBRA containing mutations in the N-terminal region exhibited discordant behaviors in S. cerevisiae and in B cells. For example, deletion of ZEBRA aa 26 to 51 impaired activation to a great extent in B cells but had little or no effect in S. cerevisiae. The discordant mutants may reflect interactions with a variable domain of a conserved component or unique interactions with specialized components of the basal transcription apparatus in different cells.