The dTAF_II80 subunit of Drosophila TFIID contains β-transducin repeats

Abstract

A KEY component of the RNA polymerase II transcriptional apparatus, TFIID, is a multi-protein complex containing the TATA box-binding protein (TBP) and at least seven tightly associated factors (TAFs)^1,2. Although the functions of most TFIID subunits are unknown, it is clear that TAFs are not necessary for basal activity but that one or more are required for regulated transcription, and so behave as coactivators^1–4. The presence of multiple subunits indicates that there is an intricate assembly process and that TAFs may be responsible for other activities. We have described the properties of the subunit dTAF_II110, which can interact directly with the transcriptional activator Sp1 (ref. 5). In addition, the largest subunit, dTAF_II250, binds directly to TBP and links other TAFs to the complex⁶. Here we describe the cloning, expression and partial characterization of the Drosophila TAF of M_r 80,000, dTAF_II80. Sequence analysis reveals that dTAFII80 contains several copies of the WD40 (β-transducin) repeat⁷. Moreover, dTAF_II80 shares extended sequence similarity with an Arabidopsis gene, COP1, which encodes a putative transcription factor that is thought to regulate development⁸. We have expressed recombinant dTAF_II80 and begun to characterize its interaction with other members of the TFIID complex. Purified recombinant dTAF_II80 is unable to bind TBP directly or to interact strongly with the C-terminal domain of dTAF_II250 (Δ250). Instead, dTAF_II80 is only able to recognize and interact with a higher-order complex containing TBP, Δ250, 110 and 60. These findings suggest the formation of TFIID may require an ordered assembly of the TAFs, some of which bind directly to TBP and others that are tethered to the complex as a result of specific TAF/TAF interactions.