No abstract

Determination of the crystal structure of the human TBP-associated factor (hTAF(II))28/hTAF(II)18 heterodimer shows that these TAF(II)s form a novel histone-like pair in the TFIID complex. The histone folds in hTAF(II)28 and hTAF(II)18 were not predicted from their primary sequence, indicating that these TAF(II)s define a novel family of atypical histone fold sequences. The TAF(II)18 and TAF(II)28 histone fold motifs are also present in the N- and C-terminal regions of the SPT3 proteins, suggesting that the histone fold in SPT3 may be reconstituted by intramolecular rather than classical intermolecular interactions. The existence of additional histone-like pairs in both the TFIID and SAGA complexes shows that the histone fold is a more commonly used motif for mediating TAF-TAF interactions than previously believed.

TFIID is the DNA binding component of the RNA polymerase II transcriptional machinery and is composed of the TATA binding protein (TBP) and TBP‐associated factors (TAFIIs). Here we report the characterization of a new human TAF, hTAFII100, which is the human homologue of Drosophila TAFII80 and yeast TAFII90. hTAFII100 interacts strongly with hTAFII250, hTAFII55 and hTAFII28, less with hTAFII20 and hTAFII18, weakly with TBP and not at all with delta NTAFII135 and hTAFII30. Deletion analysis revealed that the C‐terminal half of hTAFII100, which contains six WD‐40 repeats, is not required for incorporation into the TFIID complex. Our results suggest that hTAFII100 can be divided into two domains, the N‐terminal region responsible for interactions within the TFIID complex and the C‐terminal WD repeat‐containing half responsible for interactions between hTAFII100 and other factors. An anti‐hTAFII100 antibody, raised against a C‐terminal epitope, selectively inhibited basal TFIID‐dependent in vitro transcription and the specific interaction between hTAFII100 and the 30 kDa subunit of TFIIF (RAP30). We demonstrate that the hTAFII100‐TFIIF interaction supports pre‐initiation complex formation in the presence of TFIID. Thus, this is the first demonstration that a TAFII functionally interacts with a basal transcription factor in vitro.