An RNA polymerase III-defective mutation in TATA-binding protein disrupts its interaction with a transcription factor IIIB subunit in Drosophila cells

Abstract

A subunit of the Drosophila RNA polymerase III transcription factor IIIB (TFIIIB) complex has been identified using antibodies directed against the analogous human protein, hIIIB90. This protein has an apparent molecular mass of 105 kDa and has been designated dTAF(III)105. Drosophila S-2 cell extracts that were immunodepleted of dTAF(III)105 were substantially reduced in their capacity to support tRNA gene transcription. A protein (far Western) blot analysis revealed that dTAF(III)105, present in e TFIIIB fraction, directly interacts with TATA-binding protein (TBP). Coimmunoprecipitation assays demonstrated that this protein associates with TBP in S-2 cell extracts. Our previous studies have identified a mutation at position 332 within Drosophila TBP that changes a highly conserved arginine residue to a histidine residue, which renders it specifically defective in its ability to support RNA polymerase III transcription in S-2 cells (Trivedi, A., Vilalta, A., Gopalan, S., and Johnson, D. L. (1996) Mol. Cell Biol, 16, 6909-6916). We further demonstrate that extracts prepared from a stable cell line expressing epitope-tagged wild-type TBP exhibit an increase in tRNA gene transcription, whereas extracts derived from cells expressing the mutant TBP protein do not. Coimmunoprecipitation assays and far Western blot analysis demonstrate that this mutation in TBP abolishes its ability to stably interact with dTAF(III)105. Thus, we have identified both a Drosophila protein that is directly associated with TBP in the TFIIIB complex, dTAF(III)105, and an amino acid residue within the highly conserved carboxyl-terminal region of TBP that is critical for dTAF(III)105-TBP interactions.