The chromatin remodeling complex NoRC and TTF-I cooperate in the regulation of the mammalian rRNA genes in vivo.

The transcription termination factor (TTF)-I is a multifunctional nucleolar protein that terminates ribosomal gene transcription, mediates replication fork arrest and regulates RNA polymerase I transcription on chromatin. TTF-I plays a dual role in rDNA regulation, being involved in both activation and silencing of rDNA transcription. The N-terminal part of TTF-I contains a negative regulatory domain (NRD) that inhibits DNA binding. Here we show that interactions between the NRD and the C-terminal part of TTF-I mask the DNA-binding domain of TTF-I. However, interaction with TIP5, a subunit of the nucleolar chromatin remodeling complex, NoRC, recovers DNA-binding activity. We have mapped the protein domains that mediate the interaction between TTF-I and TIP5. The association of TIP5 with the NRD facilitates DNA binding of TTF-I and leads to the recruitment of NoRC to the rDNA promoter. Thus, TTF-I and NoRC act in concert to silence rDNA transcription.