Late rDNA condensation ensures timely Cdc14 release and coordination of mitotic exit signaling with nucleolar segregation

Abstract

The nucleolus plays a pivotal role in multiple key cellular processes. An illustrative example is the regulation of mitotic exit in Saccharomyces cerevisiae through the nucleolar sequestration of the Cdc14 phosphatase. The peculiar structure of the nucleolus, however, has also its drawbacks. The repetitive nature of the ribosomal DNA (rDNA) originates cohesion-independent linkages whose resolution in budding yeast requires the Cdc14-dependent inhibition of ribosomal RNA transcription, which facilitates condensin accessibility to this locus. Thus, the rDNA condenses and segregates later than most other yeast genomic regions. Our results show that defective function of a small nucleolar ribonucleoprotein (snoRNP) assembly factor facilitates condensin accessibility to the rDNA and induces premature nucleolar hyper-condensation. Excitingly, this accelerated compaction of the nucleolus interferes with the proper release of Cdc14 from this organelle. This observation provides an explanation for the delayed rDNA condensation in budding yeast, which is necessary to efficiently coordinate timely Cdc14 release and mitotic exit with nucleolar compaction and segregation.