TFIID subunit structure

Fig. 1. Possible subunit structure of TFIID. On the basis of immuno-EM studies (Leurent et al., 2002), yeast two-hybrid data (Andel et al., 1999 Brand et al., 1999 Yatherajam et al., 2003), and in vitro protein interaction data (see text), a model of TFIID interactions was constructed. TAFs that appear in both TFIID and the SAGA HAT complex are shaded. BD1 and BD2 signify the bromodomains found in higher eukaryotic TAF1 or in yeast Bdfl. The exact stoichiometry of histone-like TAFs within each lobe is unclear some data indicate an octamer-like structure, whereas others support two tetramers.

TFIID and its individual subunits have been subjected to intense study. As will be discussed below, complementary information from biochemical, genetic, and structural studies has shown, and will continue to give, a clearer picture of the forms and functions of this crucial component of the transcription machinery. 

The purification of TFIID was soon followed by the identification of its subunits (reviewed in Albright and Tjian, 2000 Burley and Roeder, 1996). The amino acid sequences of many TAFs revealed a high level of conservation. TFIID structure has changed little through evolution, with all... 

TFIID is the only GTF with intrinsic DNA sequence specificity and is responsible for nucleating the assembly of Pol II preinitiation complex at core promoters. The interaction between TFIID and core promoter is mediated by the affinity of several specific subunits for distinct core promoter elements (Verrijzer and Tjian, 1996). Because TBP functions in transcription by all three nuclear RNA polymerases, these specific interactions help TFIID to distinguish different promoter structures and properly direct Pol II transcription machinery to its target genes. 

Although great progress has been made in recent years, there is still a great deal to be discovered about TFIID. The functions of all the individual subunits need to be discovered, not only in the context of TFIID but also in other TAF-containing complexes. This task will be gready aided when a high-resolution structure of TFIID becomes available. There is still debate about which transcription events are supported by TFIID and which are TAF independent. Only when we understand how TFIID interacts with activators, chromatin, and the rest of the transcription machinery will all these questions be answered. 

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