Transcription Termination Factor-I (TTF-I) is a sequence-specific binding protein that binds sites 5' (Tsp and T0 sites) and 3' (T1-10 site) of rRNA genes. As inferred from mouse, when TTF-I is bound to the promoter-proximal T0 site TTF-I either recruits ERCC6 (also known as Cockayne Syndrome Protein, CSB), EHMT2 (also known as histone methyltransferase G9a), and NuRD to activate expression (Shimono et al. 2005, Lebedev et al. 2008) or recruits the Nucleolar Remodeling Complex (NoRC) to repress expression. How one is selected over the other is unknown.CHD4 and presumably the rest of the NuRD complex is associated with bivalent domains containing H3K4me3 (active chromatin mark) and H3K27me3 (inactive chromatin mark). ERCC6 and EHMT2 appear to cooperate to regulate activation of rRNA expression with ERCC6 mediating the transition to permissive chromatin (Lebedev et al. 2008) and EHMT2 mediating the transition to active chromatin, which involves the positional shift of one nucleosome at the promoter.
Proietti de Santis, L,