TAF7: A possible transcription initiation check-point regulator

Abstract

Transcription consists of a series of highly regulated steps: assembly of a preinitiation complex (PIC) at the promoter nucleated by TFIID, followed by initiation, elongation, and termination. The present study has focused on the role of the TFIID component, TAF7, in regulating transcription initiation. In TFIID, TAF7 binds to TAF1 and inhibits its intrinsic acetyl transferase activity. We now report that although TAF7 remains bound to TAF1 and associated with TFIID during the formation of the PIC, TAF7 dissociates from the PIC upon transcription initiation. Entry of polymerase II into the assembling PIC is associated with TAF1 and TAF7 phosphorylation, coincident with TAF7 release. We propose that the TFIID composition is dynamic and that TAF7 functions as a check-point regulator suppressing premature transcription initiation until PIC assembly is complete.MHC class I ͉ regulation ͉ preinitiation complex ͉ TFIID I n eukaryotic cells, expression of most protein-encoding genes depends on the RNA polymerase II (Pol II)-dependent transcription machinery. The RNA Pol II machinery assembled on the promoter is composed of distinct complexes that are responsible for effecting the sequential steps of transcription initiation and elongation (1-5). The first step in transcription is the recognition of the core promoter by the TFIID complex and the assembly of a preinitiation complex (PIC) through an ordered recruitment of the general transcription factors (GTFs) TFIIB and TFIIA, followed by the RNA Pol II holoenzyme, the mediator and the remaining GTFs, TFIIF, TFIIE, and TFIIH. Once the PIC has been assembled, transcription initiation ensues: local melting of the promoter DNA, formation of the first phosphodiester bond, followed by the synthesis of a short nascent RNA at which point the polymerase pauses. The initiation of transcription is accompanied by the phosphorylation of serine 5 in the C-terminal domain (CTD) heptad repeat of RNA Pol II (CTD) by the kinase subunit of TFIIH, CDK7 (6-9).Despite the extensive understanding of the general requirements of transcription, many details remain unresolved and there is considerable variation among different promoters and cell types. Promoter recognition is mediated by members of either the TFIID complex family (TFIID, TFIID-like) or the SAGA complex family (e.g., TFTC, PCAF, SAGA) (10-12). In yeast, 90% of gene expression is TFIID-dependent transcription; the remaining 10% is largely dependent on the SAGA complex (13). The TFIID complexes are composed of either the TATAbinding protein (TBP) or a TBP related protein (TRF1, TRF2) and several TBP-associated factors (TAFs) (14-17). The SAGA family complexes do not contain TBP or TBP-related proteins; rather, they contain a GCN5-related histone acetyl transferase (AT) subunit, several adapter and Spt proteins and a subset of TAFs. The composition of the TAFs present in these different complexes varies depending on the structure of the promoter and the cell cycle and tissue-specific gene expression requirement. For example, in yeas...

TFIID component TAF7 functionally interacts with both TFIIH and P-TEFb

“…S7) as follows. As a component of TFIID, TAF7 inhibits the TAF1 AT activity until PIC assembly is complete, thereby preventing premature initiation (27). Upon completion of PIC assembly, TAF7 is released from TAF1, allowing transcription to initiate with the first phosphodiester bond formation.…”

“…Importantly, MHC class I transcription is inhibited both in vitro and in vivo by the viral transactivator, HIV Tat, which binds to the TAF1 AT domain, inhibiting its enzymatic activity (22,23). TAF7, a cellular 55-kDa TFIID component (24,25), also binds to TAF1 inhibiting its AT activity and repressing MHC class I transcription (26) Significantly, we have demonstrated that TAF7 remains bound to TAF1/TFIID until PIC assembly is complete, whereupon it is released enabling transcription initiation and elongation (27). Thus, TAF7 is an intrinsic regulator of transcription.…”

“…In vitro transcription assays, immunoprecipitations, and Western blot anlyses were performed as described in ref. 27. Column fractionation of C8166 T whole cells extracts was on a Superose 6 column in 150 mM Hepes (pH 7.9), 1.5 mM MgCl2, 1 mM EGTA (pH 8), 10% glycerol, 150 mM NaCl, 10 mM B-glycerophosphate, 1 mM DTT, 50 mM NaF, 0.2 mM sodium vanadate, and protease inhibitors.…”

“…We demonstrated that TAF7 is released from the TFIID complex upon transcription initiation (27). The present studies were designed to determine whether TAF7 associates with other components of the transcription complex after its dissociation from TFIID.…”

“…The present studies were designed to determine whether TAF7 associates with other components of the transcription complex after its dissociation from TFIID. Because TAF7 was released from a partially assembled preinitiation complex with TFIIB, TFIIF, and Pol II (27), none of these factors is likely to be a TAF7 interacting partner. Therefore, we considered the possibility that TAF7 functionally interacts with TFIIH and/or P-TEFb (7).…”