The Role of TAFs in RNA Polymerase II Transcription

Roeder, 1996; Oelgeschlager et al., 1998). In vitro tranAt RNA polymerase II (Pol II) promoters, a unique proscription using purified yeast holoenzyme showed actitein–DNA complex is formed by the TATA-binding provation by VP-16 using only TBP. In another system, tein (TBP) binding to the TATA element, which serves depletion of most (but not all) TAFs from human extracts as a platform for the assembly of the remaining trancaused no defect in activation by VP-16 and CTF-1. scription machinery. TBP exists primarily as a subunit of These findings suggest that activators can target holoseveral larger complexes. The Pol II–specific complex, enzyme or other factors that are missing from highly termed TFIID, consists of TBP and 10–12 TAFs (TBPpurified transcription systems. Recently, the role of TAFs associated factors) most of which have been highly conhas been further complicated by the finding that TAFs served from yeast to humans (Burley and Roeder, 1996; are components of at least one additional complex beLee and Young, 1998). An ongoing debate in the transides TFIID (Struhl and Moqtaderi, 1998). scription field the past several years has been waged Function of TAFs In Vivo over the role of TAFs in transcription by Pol II. This controThe conflicting in vitro data underscores the need for versy is focused on two issues: (1) how universal is in vivo experiments to identify which genes require TFIID the requirement for TAFs in gene regulation, and (2) at function and what role the TAFs play at these genes. promoters dependent on TAFs for normal expression, Until this new crop of papers, in vivo evidence for a is their role to mediate stimulation of transcription by general function of TAFs in transcription has been missactivators, specific recognition of promoter sequences, ing. While the genes encoding TAFs are essential for or some other function? This debate was fueled by exgrowth in almost all cases tested, this does not prove periments in yeast which suggested that some TAFs do a general role in transcription. Previously, only genenot play a general role in Pol II transcription (Moqtaderi specific transcription defects were observed upon TAF et al., 1996; Walker et al., 1996). These findings were inactivation in vivo. First, a temperature-sensitive (ts) in surprising contrast to early biochemical experiments mutation in the largest mammalian TAF (TAF250; prowhich showed that TAFs were necessary to reconstitute posed to be the central scaffold for assembly of the activated transcription in purified human and Drosophila TAFs), caused gene-specific transcription defects at the systems (Burley and Roeder, 1996; Verrijzer and Tjian, nonpermissive temperature. This dependence on TAF25