Guardians of the rDNA: Control of chromatin dynamics during rDNA damage by PTMs

CAPELLA, MATIAS

Munich

Conferencia; 1st Munich Yeast Meeting; 2019

Maintaining genome integrity is essential for cellular viability. Due to its repetitive nature, the rDNA locus often undergoes homologous recombination (HR), resulting in gain or loss of individual repeats. Uncontrolled recombination is suppressed by restricting the rDNA repeats to the nuclear periphery within the nucleolus, preventing access to the HR machinery. Following double-strand breaks, the damaged rDNA repeat thus needs to be released from the nucleolus, a process that is conserved from yeast to humans. However, the molecular mechanism controlling rDNA relocalization remains unknown. We discovered that permanent rDNA tethering in S. cerevisiae causes lethality, highlighting the importance of rDNA dynamics for cell survival. In addition, we unveiled the molecular events in rDNA dynamics control, which consist of a series of posttranslational modifications of the main players at the nuclear periphery. In particular, we showed that phosphorylation of Nur1, a member of the CLIP-cohibin complex involved in rDNA tethering, is required for the release of repeats outside the nucleolus. A phosphomimic mutant of Nur1 disrupts the CLIP-cohibin interaction, thus reducing perinuclear positioning and destabilizing the rDNA repeats. Nur1 phosphorylation is counteracted by the phosphatase Cdc14, which itself is regulated by SUMO. SUMOylation removes Cdc14 from rDNA repeats through recognition by the segregase Cdc48, which shows specificity for SUMO-modified substrates. Finally, during rDNA damage, a Cdc48 mutant blocking the CLIP-cohibin disruption impairs the relocalization of the damaged repeats. Our findings reveal that the interplay between phosphorylation and SUMOylation regulates perinuclear tethering of the rDNA repeats to maintain genome stability.