TBP, a universal eukaryotic transcription factor?

In eukaryotes, transcription is carried out by three different RNA polymerases, RNA polymerases I, II, and III, each of which is dedicated to the transcription of different sets of genes. The genes in each class contain characteristic promoters, which often consist of two types of elements: the basal promoter elements and the modulator promoter elements. The basal promoter elements are sufficient to determine RNA polymerase specificity and direct low levels of transcription, whereas the modulator elements enhance or reduce the basal levels of transcription. None of the RNA polymerases can recognize its target promoters directly. Instead, basal promoter elements are first recognized by specific transcription factors, which then recruit the correct RNA polymerase. Because the RNA polymerase I, II, and III promoters contain distinctive elements, it had long been assumed that they achieve recruitment of the correct RNA polymerase by binding very different transcription factors. Consistent with this view, the transcription factors required for RNA polymerase I, II, and III transcription can be separated from each other chromatographically, as shown in Figure 1. This scheme separates a cellular extract into four fractions, termed A, B, C, and D (Matsui et al. 1980; Segall et al. 1980; Samuels et al. 1982). RNA polymerase II transcription requires the A, C, and D fractions; the A and C fractions are now known to contain several factors. The D fraction contains TFIID, a factor that binds to the TATA box present in a large number of RNA polymerase II promoters (for reviews, see Sawadogo and Sentenac 1990; Buratowski and Sharp 1992; Zawel and Reinberg 1993). TFIID proved very difficult to purify and was not characterized in detail until 1988, when an activity capable of functionally replacing mammalian TFIID in a reconstituted in vitro transcription system was identified in Saccharomyces cerevisiae extracts (Buratowski et al. 1988; Cavallini et al. 1988). This was a major breakthrough because, unlike mammalian TFIID, the yeast activity could be purified to homogeneity and turned out to correspond to a single polypetide of 27 kD (Cavallini et al. 1989; Hahn et al. 1989; Horikoshi et al. 1989a; Schmidt et al. 1989). The cDNA encoding the yeast TATA box-binding protein (TBP) was then cloned by several groups (Cavallini et al. 1989; Hahn et al. 1989; Horikoshi et al. 1989b; Schmidt et al. 1989), and the protein sequence information was used to isolate TBP-encoding 11742