Structure and role of TFIID and other transcription factors (TBP, TAFs)

Induction of DNA bending by TFIID, to facilitate enhanced interaction between regulatory factors and the basic transcription factors
Fig. 32.17. Induction of DNA bending by TFIID, to facilitate enhanced interaction between regulatory factors and the basic transcription factors.
Of the above transcription factors, TFIID is the most important and has become available in purified form as a multiprotein complex. A subunit of TFIID is 'TATA binding proteins (TBP)', which is actually required by all the three eukaryotic RNA polymerases (even for promoters lacking a TATA box). However, there are other 'TBP associated factors (TAFs)', which differ in different RNA polymersses. Therfore, it is suggested that each of the three RNA polymerases requires an essential transcription factor, which is a complex containing TBP and TAFs, the cmposition of TAFs differing in the three cases.
Another abundant complex is B-TFIID, which is involved in tanscription by RNA polymerase I or RNA polymerase III. B-TFIID contains TBP and TAFs, and can perform the TBP function of TFIID. the TBP-TAFs arrangement is dynamic and TAFs can be displaced from TBP to accommodate contacts by other transcription factors and activators.
Induction of DNA bending by TFIID, to facilitate enhanced interaction between regulatory factors and the basic transcription factors
Fig. 32.17. Induction of DNA bending by TFIID, to facilitate enhanced interaction between regulatory factors and the basic transcription factors.

Binding of TFIID at the TATA box also leads to bending of DNA. This bending perhaps facilitates contact between regulatory factors bound at upstream sites and one or more components of the general transcriptional machinery (Fig. 32.17). This DNA bending due to TFIID can also be compared with CAP induced DNA bending described for prokaryotic promoters (Regulation of Gene Expression 1.  Operon Circuits in Bacteria and other Prokaryotes). Several other transcription factors including TFIIB are being purified and their sturcture will be known in detail during 1990s.

In October, 1993, two important papers were published, reporting a major advance in our understanding of DNA-protein interaction, between TATA-box and TBP. It was shown that the binding of saddle-shaped protein (TBP) induces TATA-box to curve (at about 80°) and partially unwind (by about 110°), thus presenting a widened minor groove to the concave surface of TBP. The dimensions of molecular saddle of TBP are -such that its concave surface could sit nicely on a DNA-double helix, at right angle to the DNA axis (as was earlier assumed). However, the results reported in October, 1993 suggest that long axis of the saddle of TBP is actually parallel to DNA axis, wrapping around the DNA in the minor groove. This bending of TATA-box, brings other initiation factors together.and its unwinding allows interaction of DNA with other factors leading to further unwinding towards the start site. Eventually this leads to separation of two DNA strands (at the start site) required for initiation of transcription. It is believed that, in due course of time, the three dimensional structure of the complete pre-initiation complex (to be appropriately designated as 'transcriptosome'), will be determined so that the mechanistic and regulatory aspects of transcription will be fully understood.