TATA-binding protein-interacting protein 120, TIP120, stimulates three classes of eukaryotic transcription via a unique mechanism

We previously identified a novel TATA-binding protein (TBP)-interacting pro tein (TIP120) from the rat liver. Here, in an RNA polymerase II (RNAP II)-r econstituted transcription system, we demonstrate that recombinant TIP120 a ctivates the basal level of transcription from various kinds of promoters r egardless of the template DNA topology and the presence of TFIIE/TFIIH and TBP-associated factors. Deletion analysis demonstrated that a 412-residue N -terminal domain, which includes an acidic region and the TBP-binding domai n, is required for TIP120 function. Kinetic studies suggest that TIP120 fun ctions during preinitiation complex (PIC) formation at the step of RNAP II/ TFIIF recruitment to the promoter but not after the completion of PIC forma tion. Electrophoretic mobility shift assays showed that TIP120 enhanced PTC formation, and TIP120 also stimulated the nonspecific transcription and DN A-binding activity of RNAP II. These lines of evidence suggest that TIP120 is able to activate basal transcription by overcoming a kinetic impediment to RNAP II/TFIIF integration into the TBP (TFIID)-TFIIB-DNA-complex. Intere stingly, TIP120 also stimulates RNAP I- and III-driven transcription and bi nds to RPB5, one of the common subunits of the eukaryotic RNA polymerases, in vitro. Furthermore, in mouse cells, ectopically expressed TIP120 enhance s transcription from all three classes (I, II, and III) of promoters. We pr opose that TIP120 globally regulates transcription through interaction with basal transcription mechanisms common to all three transcription systems.