NoRC--a novel member of mammalian ISWI-containing chromatin remodeling machines

Transcription by RNA polymerase I on nucleosomal templates requires binding of the transcription termination factor TTF-I to a cognate site 160 bp upstream of the transcription start site. Binding of TTF-I is accompanied by changes in the chromatin architecture which suggests that TTF-I recruits a remodeling activity to the rDNA promoter. We have cloned a cDNA that encodes TIP5 (TTF-I-interacting protein 5), a 205 kDa protein that shares a number of important protein domains with WSTF (Williams syndrome transcription factor) and hAcf1/WCRF180, the largest subunits of human chromatin remodeling complexes hCHRAC and WCRF. TIP5 co-localizes with the basal RNA polymerase I transcription factor UBF in the nucleolus and is associated with SNF2h. The cellular TIP5-SNF2h complex, termed NoRC (nucleolar remodeling complex), induces nucleosome sliding in an ATP- and histone H4 tail-dependent fashion. The results suggest that NoRC is a novel nucleolar chromatin remodeling machine that may serve a role in the regulation of the rDNA locus.