rDNA transcription machinery rDNAs are found in multiple, tandem, head-to-tail arrayed copies in the nucleoli of eukaryotic cells (Hadjiolov 1985) . In mitotic human cells, rDNA clusters are localized on the short arm of the Wve pairs of chromosomes 13, 14, 15, 21 and 22 and are termed NORs. Each rDNA unit consists of a transcribed sequence and an external non-transcribed spacer (Hadjiolov 1985; Liau and Perry 1969) in which all the sequences necessary for proper RNA pol I transcription such as proximal promoters, spacer promoters and terminators are located (Hadjiolov 1985) . In the rDNA promoter two important elements have been described, a CORE element and an upstream control element (UCE) (Haltiner et al. 1986; Windle and Sollner-Webb 1986; Xie et al. 1992 ) that function synergistically to recruit a transcriptionally competent RNA pol I complex. This complex contains in addition to RNA pol I, the upstream binding factor (UBF) (Pikaard et al. 1989; Voit et al. 1992) , the selectivity factor protein complex SL1 (Learned et al. 1985 ) also called TIF-1B in mouse cells (Clos et al. 1986 ), consisting of the TATA-binding protein (TBP) and four transcription activating factors [TAF I s110, 63, 48 and 41 (Comai et al. 1994; Gorski et al. 2007; Zomerdijk et al. 1994) ], the transcription initiation factor TIF-IA, the mouse homolog of Rrn3p (Bodem et al. 2000; MooreWeld et al. 2000) and the transcription termination factor TTF-1 (Bartsch et al. 1988 ). The UBF containing HMG boxes (Bachvarov and Moss 1991; Jantzen et al. 1990 ) that confer a high aYnity for DNA structures plays an architectural role on the rDNA promoter (Mais et al. 2005) . It was proposed that UBF activates rDNA transcription because it stabilizes binding of SL1 required to recruit the initiation-competent subfraction of RNA pol I. This recruitment is achieved by interaction of UBF with the RNA pol I-associated factor PAF53 (Schnapp et al. 1994) , and by interaction of SL1 with TIF-1A/ Rrn3p (Miller et al. 2001) . TIF-1A/Rrn3p interacts also with the RPA43 subunit of RNA pol I and thus facilitates linking between RNA pol I and SL1 complexes (Peyroche et al. 2000; Yuan et al. 2002) . Following initiation, TIF-1A/ Rrn3p is released and can associate with another preinitiation complex. Recycling of TIF-1A/Rrn3p requires a posttranslational phosphorylation event that appears to play a role in its initiation activity (Cavanaugh et al. 2002; Zhao et al. 2003) . Moreover, it was proposed that TTF-1 is not only involved in termination of transcription in cooperation with the release factor PTRF (Jansa and Grummt 1999), but also in the remodeling of ribosomal chromatin by recruiting ATP-dependent remodeling factors to the rDNA promoter (Längst et al. 1997 ). The nucleolar remodeling complex (NoRC) (Strohner et al. 2001) , which acts in repression at the rDNA promoter level (Li et al. 2006; Santoro et al. 2002) , and the transcription activator CSB (Cockayne syndrome group B protein), a DNA-dependent ATPase, interact with TTF-1 (Yuan et al. 2007 ). The Wnding that TTF-1 interacts with both CSB and NoRC suggests that competitive recruitment of CSB and NoRC may determine the epigenetic state of the rDNA.