TTF-I DETERMINES THE CHROMATIN ARCHITECTURE OF THE ACTIVE RDNA PROMOTER

Transcription of ribosomal genes assembled into chromatin requires bin ding of the transcription termination factor TTF-I to the promoter-pro ximal terminator To, To analyze the mechanism of TTF-I-mediated transc riptional activation, we have used mutant templates with altered seque nce, polarity and distance of To with respect to the transcription sta rt site, Transcription activation by TTF-I is chromatin specific and r equires the precise positioning of the terminator relative to the prom oter, Whereas termination by TTF-I depends on the correct orientation of a terminator, TTF-I-mediated transcriptional activation is orientat ion independent. TTF-I can bind to nucleosomal DNA in the absence of e nzymatic activities that destabilize nucleosome structure. Chromatin-b ound TTF-I synergizes with ATP-dependent cofactors present in extracts of Drosophila embryos and mouse cells to position a nucleosome over t he rDNA promoter and the transcription start site. Nucleosome position ing correlates tightly with the activation of rDNA transcription. We s uggest that transcriptional activation by TTF-I is a stepwise process involving the creation of a defined promoter architecture and that the positioning of a nucleosome is compatible with, if not a prerequisite for, transcription initiation from rDNA chromatin.