ACTIVATION OF THE EPSTEIN-BARR-VIRUS TRANSCRIPTION FACTOR BZLF1 BY 12-O-TETRADECANOYLPHORBOL-13-ACETATE-INDUCED PHOSPHORYLATION

BZLF1 is a member of the extended AP-1 family of transcription factors which binds to specific BZLF1 sequence motifs within early Epstein-Ba rr virus (EBV) promoters and to closely related AP-I motifs. BZLF1's a ctivity is regulated at the transcriptional level as well as through p rotein interactions and posttranslational modifications. Phorbol ester s or immunoglobulin cross-linking both reactivate EBV from latently in fected B cells via transactivation of BZLF1. We report here that the p horbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) is capable of inducing BZLF1's activity even further. The induction occurs at the po sttranscriptional level and depends on a single serine residue located in the DNA binding domain of BZLF1. This serine residue (S186) is pho sphorylated by protein kinase C in vitro and in vivo after stimulation with TPA. Phosphorylation of S186 per se interferes with the DNA bind ing affinity of BZLF1 in vitro but is mandatory for TPA-induced increa se in DNA binding of BZLF1, as shown in gel retardation assays and rec onstruction experiments with cellular extracts. In transcriptional rep orter assays, S186 is essential for the activation of BZLF1 by TPA. Pr esumably, a yet-to-be-identified cellular factor restores the DNA bind ing affinity and enhances the transcriptional activity of S186-phospho rylated BZLF1, which is required to induce the lytic phase of EBV's li fe cycle.