The Epstein-Barr virus R transactivator (Rta) contains a complex, potent activation domain with properties different from those of VP16

Abstract

Rta, encoded by Epstein-Barr virus (EBV), is a potent activator of transcription via enhancer sequences located upstream of several viral genes. To identify the domains of Rta that facilitate transcription by interacting with cellular transcription factors, different segments of Rta were linked to the DNA binding domain of yeast transactivator GAL4 (residues 1 to 147). These GAL4-Rta fusion proteins were tested in transfected cells for their ability to activate the adeno E1b promoter with an upstream GAL4 DNA binding site. The acidic C-terminal domain of Rta (amino acids 520 to 605) was a potent activator but behaved differently from VP16 in dose-response and competition experiments. A subterminal domain of Rta (amino acids 416 to 519) linked to GAL4 had weak activation activity. Deletion of these domains from native Rta showed that the C-terminal domain was required for transactivation, but the subterminal domain was required only in B cells. The C-terminal activation domain of Rta contains a pattern of positionally conserved hydrophobic residues shared with VP16 and other transactivators. Substitution of several conserved hydrophobic amino acids in Rta severely impaired transactivation. The improtance of hydrophobic residues was further substantiated by comparing EBV Rta with that of herpesvirus saimiri, which revealed little sequence similarity except for a few acidic residues and the positionally conserved hydrophobic amino acids. The C-terminal domain of EBV Rta contains three partially overlapping copies of this hydrophobic motif. Mutational analysis indicated that all three copies were required for full activity. However, two of the three copies appeared to be sufficient to produce full activity on a target promoter with multiple binding sites, suggesting that these motifs are functional subdomains that can synergize.