Transcriptional activation is weakened when Taf1p N-terminal domain 1 is substituted with its Drosophila counterpart in yeast TFIID.

Transcription factor II D (TFIID), a multiprotein complex consisting of TATA-binding protein (TBP) and 13-14 TBP-associated factors (Tafs), plays a central role in transcription and regulates nearly all class II genes. The N-terminal domain of Taf1p (TAND) can be divided into two subdomains, TAND1 and TAND2, which bind to the concave and convex surfaces of TBP, respectively. The interaction between TAND and TBP is thought to be regulated by TFIIA, activators and/or DNA during transcriptional activation, as the TAND1-bound form of TBP cannot bind to the TATA box. We previously demonstrated that Drosophila TAND1 binds to TBP with a much stronger affinity than yeast TAND1 and that the expression levels of full-length chimeric Taf1p, whose TAND1 is replaced with the Drosophila counterpart, can be varied in vivo by substituting several methionine residues downstream of TAND2 with alanine residues in various combinations. In this study, we examined the transcriptional activation of the GAL1-lacZ reporter or endogenous genes such as RNR3 or GAL1 in yeast cells expressing various levels of full-length chimeric Taf1p. The results showed that the substitution of TAND1 with the Drosophila counterpart in yeast TFIID weakened the transcriptional activation of GAL1-lacZ and RNR3 but not that of GAL1. These findings strongly support a model in which TBP must be released efficiently from TAND1 within TFIID upon transcriptional activation.