THE DTAF(II)80 SUBUNIT OF DROSOPHILA TFIID CONTAINS BETA-TRANSDUCIN REPEATS

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 coactivators1-4. 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(II)110, which can interact directly with the transcriptional activator Sp1 (ref. 5). In addition, the largest subunit, dTAF(II)250, 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(II)80. Sequence analysis reveals that dTAF(II)80 contains several copies of the WD40 (beta-transducin) repeat7. Moreover, dTAF(II)80 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(II)80 and begun to characterize its interaction with other members of the TFIID complex. Purified recombinant dTAF(II)80 is unable to bind TBP directly or to interact strongly with the C-terminal domain of dTAF(II)250 (DELTAN250). Instead, dTAF(II)80 is only able to recognize and interact with a higher-order complex containing TBP, DELTAN250, 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.