CELL-CYCLE REDISTRIBUTION OF U3 SNRNA AND FIBRILLARIN - PRESENCE IN THE CYTOPLASMIC NUCLEOLUS REMNANT AND IN THE PRENUCLEOLAR BODIES AT TELOPHASE

The distribution of the U3 small nuclear RNA during the cell cycle of the CHO cell line was studied by in situ hybridization using digoxigen in-labelled oligonucleotide probes. The location of the hybrids by imm unofluorescence microscopy and at the ultrastructural level was correl ated with the distribution of two nucleolar proteins, nucleolin and fi brillarin. The U3 snRNA molecules persist throughout mitosis in close association with the nucleolar remnant. U3 snRNA is present in the pre nucleolar bodies (PNBs) and could participate in nucleologenesis in as sociation with several nucleolar proteins such as nucleolin and fibril larin. The interaction of U3 snRNP with the 5' external spacer of pre- RNA newly synthesized by active NORs is proposed to be the promoting e vent of nucleologenesis.