EXPERIMENTAL INDUCTION OF PRENUCLEOLAR BODIES (PNBS) IN INTERPHASE CELLS - INTERPHASE PNBS SHOW SIMILAR CHARACTERISTICS AS THOSE TYPICALLY OBSERVED AT TELOPHASE OF MITOSIS IN UNTREATED CELLS

Recently, it was shown that a short exposure of living mammalian cells to low ionic strength buffers (hypotonic shock) caused partial or alm ost complete unraveling of interphase nucleoli. However, when the cell s were released from the hypotonic shock and transferred to normal iso tonic medium, functionally active and structurally integral nucleoli w ere reassembled at their initial positions within interphase nuclei. H ere, we show further that this process is accompanied by the appearanc e of numerous discrete extranucleolar bodies, which have striking simi larities to the prenucleolar bodies (PNBs) observed in untreated cells at telophase of mitosis. (1) Like PNBs at mitosis, hypotonically indu ced interphase PNBs are composed of RNA-positive granules and fibrils, contain the major nucleolar protein B23 and silver-binding proteins, but lack DNA and RNA polymerase I transcription factor UBF. (2) As for mitotic PNBs, disappearance of the interphase PNB counterparts coinci des with the increase in size of reconstructed nucleoli. (3) Addition of actinomycin D does not prevent assembly of interphase PNBs, but doe s arrest their coalescence with the chromosomal nucleolus-organizing r egions and blocks the complete reformation of nucleoli. It is conclude d that the assembly of PNBs generally observed at telophase of mitosis can be induced experimentally in nuclei of interphase mammalian cells in vivo. At interphase, this process is probably initiated by changes in the intracellular ionic environment.