Figure 2.

Reversible acetylation of PAF53 and U3–55k regulates pre-rRNA synthesis and processing. In normal growth conditions, SIRT7 keeps the Pol I-associated factor PAF53 hypoacetylated, which is required for rDNA transcription. Deacetylation of the U3–55k protein by SIRT7 facilitates the interaction of U3–55k with U3 snoRNA, thus promoting pre-rRNA cleavage. Nucleolar release of SIRT7 in response to environmental or metabolic stress enhances acetylation of PAF53 and U3–55k, which impairs Pol I transcription and pre-rRNA processing and attenuates ribosome biogenesis.