Upstream binding factor functional domains

Fig. 2. Functional domains of human upstream binding factor.

The formation of the PIC described above is based on the sequential addition of purified components in in vitro experiments. An essential feature of this model is that the assembly takes place on the DNA template. Accordingly, transcription activators, which have autonomous DNA binding and activation domains (see Chapter 39), are thought to function by stimulating either PIC formation or PIC function. The TAF coactivators are viewed as bridging factors that communicate between the upstream activators, the proteins associated with pol II, or the many other components of TFIID. This view, which assumes that there is stepwise assembly of the PIC—promoted by various interactions between activators, coactivators, and PIC components— is illustrated in panel A of Figure 37-10. This model was supported by observations that many of these proteins could indeed bind to one another in vitro. 

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