Sharing of mitotic pre-ribosomal particles between daughter cells.

Ribosome biogenesis is a fundamental multistep process initiated by the synthesis of 90S pre-ribosomal particles in the nucleoli of higher eukaryotes. Even though synthesis of ribosomes stops during mitosis while nucleoli disappear, mitotic pre-ribosomal particles persist as observed in pre-nucleolar bodies (PNBs) during telophase. To further understand the relationship between the nucleolus and the PNBs, the presence and the fate of the mitotic pre-ribosomal particles during cell division were investigated. We demonstrate that the recently synthesized 45S precursor ribosomal RNAs (pre-rRNAs) as well as the 32S and 30S pre-rRNAs are maintained during mitosis and associated with the chromosome periphery together with pre-rRNA processing factors. Maturation of the mitotic pre-ribosomal particles, as assessed by the stability of the mitotic pre-rRNAs, is transiently arrested during mitosis by a cyclin-dependent kinase (CDK)1-cyclin-B-dependent mechanism and can be restored by CDK inhibitor treatments. At the M-G1 transition, the resumption of mitotic pre-rRNA processing in PNBs does not induce the disappearance of PNBs; this only occurs when functional nucleoli reform. Strikingly, during their maturation process, mitotic pre-rRNAs localize in reforming nucleoli.