It is now established that the presence of a fully active nucleolus depends on cell cycle regulators. rDNA transcription is maximum in the S and G2 phases, silent in mitosis, and slowly recovers in G1. Post-translational mod-iWcations of the RNA pol I machinery are required for the formation of a productive preinitiation complex. The phosphorylation status of several components of the RNA pol I machinery can modify the activity and interactions of these proteins and thus can modulate rDNA transcription during the cell cycle. Concerning silencing of rDNA transcription during mitosis, it is well established that some components of the rDNA transcription machinery such as SL1 (Heix et al. 1998 ) and TTF-1 (Sirri et al. 1999) , are mitotically phosphorylated by CDK1-cyclin B. As shown in vitro, CDK1-cyclin B-mediated phosphorylation of SL1 abrogates its transcriptional activity (Heix et al. 1998 ). Moreover CDK1-cyclin B is necessary not only to establish repression but also to maintain it from prophase to telophase. Indeed, in vivo inhibition of CDK1-cyclin B leads to dephosphorylation of the mitotically phosphorylated forms of components of the RNA pol I machinery and restores rDNA transcription in mitotic cells (Sirri et al. 2000) . On the other hand, rDNA transcription also appears regulated by CDK(s) during interphase: the increase of rDNA transcription during G1 progression depends on phosphorylation of UBF by G1-speciWc CDK-cyclin complexes (Voit et al. 1999) , and CDK inhibitor treatments partially inhibit rDNA transcription in interphase cells (Sirri et al. 2000) . ModiWcations of the phosphorylation status of UBF and/or TAF I 110 aVect the interactions between UBF and SL1 necessary for recruitment of RNA pol I (Zhai and Comai 1999) . In addition to phosphorylation, it has been speculated that acetyltransferases might also regulate the activity of RNA pol I transcription factors. Indeed, two studies have demonstrated that UBF and one of the SL1 subunits are acetylated in vivo (Muth et al. 2001; Pelletier et al. 2000) . Functional studies indicated that acetylated UBF is transcriptionally more active than deacetylated UBF. However, acetylation of UBF does not aVect its DNA binding activity as shown for other transcription factors, and it is unclear how this post-transcriptional modiWcation modulates UBF activity. The TAF I 68 subunit of SL1 is speciWcally acetylated by recruitment of PCAF (p300/CBP associated factor) to the rDNA promoter. In vitro analyses indicate that acetylation of TAF I 68 is likely to increase the activity of SL1 facilitating interaction of the complex with DNA. Sirtuins, the human homologues of the yeast Sir2 (silent information regulator) with NAD-dependent deacetylase and ADP-ribosyltranferase activity, have recently been implicated in the regulation of the RNA pol I machinery. In particular, nuclear sirtuin1 deacetylates TAF I 68 and represses RNA pol I transcription in vitro (Muth et al. 2001) . Conversely, the nucleolar sir-tuin7 is described as activator of rDNA transcription by increasing RNA pol I recruitment to the rDNA, but no substrates of such activity have as yet been identiWed (Ford et al. 2006) . Additional in vivo approaches are necessary to better understand the role of sirtuins in the regulation of rDNA transcription.