The TAFs in the HAT

The TAFs in the HAT Minireview whether TFTC contains a TBP-related factor (or a modified form of TBP) or whether the subset of TAFs (perhaps and Molecular Pharmacology with additional proteins) are sufficient for promoter rec-Harvard Medical School ognition and assembly of the pol II machinery. Third, Boston, Massachusetts 02115 the Gcn4 activation domain interacts with a subset of TAFs, suggesting the existence of a TAF-containing complex distinct from TFIID (Drysdale et al., 1998). TFIID, a multiprotein complex comprising the TATA-In this issue of Cell, two papers dramatically alter the binding protein (TBP) and approximately 10 associated textbook view by demonstrating that a subset of the factors (TAFs), is a key component of the RNA polymer-TAFs in TFIID are also integral components of histone ase II (pol II) transcription machinery (Burley and Roeder, acetylase complexes (Grant et al., 1998; Ogryzko et al., 1996). The TBP subunit specifically recognizes TATA 1998). Though completely unexpected, this finding is elements, which are found in most eukaryotic promot-provocative and highly significant especially because of ers, and TFIIB, the protein primarily responsible for con-the striking parallels between the TAFs that are present necting TFIID to the pol II holoenzyme. Thus, TBP con-in the human PCAF and yeast SAGA histone acetylase tributes significantly to the overall level of transcription, complexes. At one level, the presence of TAFs in histone and it is important for positioning the pol II machinery acetylase complexes provides yet another example of at the correct location with respect to the mRNA initia-the mechanistic connection between the pol II machin-tion site. Certain TAF subunits of TFIID directly interact ery and chromatin modifying activities (Struhl, 1998). At with initiator or downstream promoter elements, and another level, these new results significantly complicate these interactions are particularly important for tran-our understanding of the physiological functions of scription from promoters lacking conventional TATA ele-TAFs, given that any particular function could be exerted ments. In addition, TAFs play some role in the response either in the context of the TFIID or the histone acetylase of the basal machinery to transcriptional activator pro-complex. Finally, the fact that several of the TAFs in the teins, although their specific functions in this regard are histone acetylase complexes interact to form a histone-not well understood. like octamer provokes evolutionary and mechanistic Considerable work over the last decade has led to a speculations. Before considering the above implications simple and clarifying view that …