The conserved core domain of the human TATA binding protein is sufficient to assemble the multisubunit RNA polymerase I-specific transcription factor SL1.

The human ribosomal RNA polymerase (Pol) I promoter selectivity factor SL1 is a complex consisting of the TATA binding protein (TBP) and three TBP-associated factors (TAFs). We have investigated which elements of TBP are involved in the assembly of Pol I-specific TBP-TAF complexes by comparing SL1 isolated from two human cell lines, one expressing epitope-tagged full-length TBP and another expressing a deletion of nearly the entire N-terminal domain (e delta NTBP). We have immunopurified epitope-tagged full-length TBP- and e delta NTBP-TAF complexes and show that e delta NTBP reconstitutes SL1 activity almost as well as full-length TBP. Moreover, e delta NTBP is shown to be associated with all three Pol I-specific TAFs. Thus, the core of TBP alone is sufficient for the correct assembly of the Pol I-specific TBP-TAF complex, and the variable N-terminal region of human TBP is not required for transcriptional activity. We also demonstrate by an in vitro protein-protein interaction assay that TBP directly interacts with the smallest TAF, TAFI48.