Figure 2.

SIRT7 has a dual function in the unstressed nucleolus. SIRT7 stimulates rDNA transcription by facilitating recruitment of Pol I both by interacting with UBF1 and through direct deacetylation of the Pol I subunit PAF53. In addition, SIRT7 deacetylates fibrillarin (FBL) and thereby favors FBL-mediated chromatin remodeling required for stimulation of rDNA transcription. SIRT7 also promotes pre-rRNA processing by deacetylating U3–55k, a core component of the U3 snoRNP complex. SIRT7 is a key player for maintaining rDNA stability at inactive rDNA genes by promoting heterochromatin formation through recruitment of SIRT1, DNMT1, and the chromatin remodeling complex (NORC). Moreover, SIRT7 might maintain rDNA stability by facilitating resolution of R-loops.