Sharing the mitotic pre-ribosomal particles between daughter cells

Abstract

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 prenucleolar 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 was investigated. We demonstrate that the recently synthesized 45S precursor ribosomal RNAs (pre-rRNAs) but also 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 may 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 that only occurs when functional nucleoli reform. Strikingly, during their maturation process, mitotic pre-rRNAs localize in reforming nucleoli.