Evidence for a novel cytoplasmic processingevent in ribosome maturation in the sea urchinParacentrotus lividus

In this work, we demonstrate the existence of a cytoplasmic processing step, never before described, involving both the pre-ribosomal subunits in the sea urchin Paracentrotus lividus.Northern-blot hybridization, primer extension, S1 mapping experiments and in situ hybridizations allowed us to demonstrate that cytoplasmic processed particles are successively re-importedinto the nucleus, where maturation of their RNAs is completed prior to being exported to the cytoplasm. Our findings lead to the proposal of a new model of ribosome maturation and shuttling. Moreover, preliminary data from our laboratorysuggest that the maturation pathway we propose in P. lividus may not be unique to the sea urchin, but a common feature in eukaryotes.