Yeast Taf1p is an integral component of the multiprotein transcription factor TFIID. By using coimmunoprecipitation assays, coupled with a comprehensive set of deletion mutants encompassing the entire open reading frame of TAF1, we have discovered an essential role of a small portion of yeast Taf1p. This domain of Taf1p, termed region 4, consisting of amino acids 200 to 303, contributes critically to the assembly and stability of the 15-subunit TFIID holocomplex. Region 4 of Taf1p is mutationally sensitive, can assemble several Tafps into a partial TFIID complex, and interacts directly with Taf4p and Taf6p. Mutations in Taf1p-region 4 induce temperature-conditional growth of yeast cells. At the nonpermissive temperature these mutations have drastic effects on both TFIID integrity and mRNA synthesis. These data are consistent with the hypothesis that Taf1p subserves a critical scaffold function within the TFIID complex. The significance of these data with regard to TFIID structure and function is discussed.TFIID, one of several multiprotein general transcription factors required for RNA polymerase II (PolII)-catalyzed mRNA gene transcription, is comprised of the TATA-binding protein (TBP) and 14 distinct TBP-associated factors (TAFs or Tafps) that range in M r from 17,000 to 150,000 in Saccharomyces cerevisiae (62,63,65,75). These TFIID subunits share significant sequence conservation with their metazoan orthologs (1,22,26). In vitro, PolII needs six general transcription factors, TFIIA, -B, -D, -E, -F, and -H, to form functional preinitiation complexes (PICs) (12, 51, 54); TFIID plays several critical roles in this process. First, TFIID functions during promoter recognition by binding directly to the promoter (67,76,77). On promoters containing a TATA box, Tafps help stabilize TBP binding to the TATA element via interaction with TATA-proximal initiator DNA sequences (69). Tafp-DNA interactions may be particularly important for TFIID binding to promoters that lack a canonical TATA element and contain TATA-proximal initiator and distal promoter element sequences (13,15,42,69,76). Second, Tafps can function as coactivators during transcriptional activation by making direct contacts with specific activators leading to an increase in PolII PIC formation (1, 16). Finally, Taf1p contains several distinct enzymatic activities, those of histone acetyltransferase (HAT) (46), protein kinase (19), and ubiquitin ligase (55). These enzymatic activities presumably modify proteins that stimulate PIC formation and/or function, leading to PolII transcription initiation.In most contexts, the Tafp subunits of TFIID are essential for survival, as Tafp inactivation or depletion results in cessation of specific mRNA synthesis and loss of cell viability (3,4,33,34,56,59,64,65,78). Because of these critical roles, the composition, organization, assembly, structure, and function of the TFIID complex have been topics of great interest. TBP appears to be incorporated into the TFIID complex primarily through its interaction with the biparti...