Nopp 140 involvement in nucleologenesis of mouse preimplantation embryos

As it was shown earlier, resumption of rRNA transcription in early mouse em bryo is localized in the peripheral region of nucleolus precursor body/NPB/ during the two-cell stage. Recently, nucleolar phosphoprotein Nopp140 was p resented to shuttle between the nucleolus and cytoplasm as chaperone of sno RNPs. Nopp140 interacts with RNA polymerase I in nucleolus and also accumul ates in CBs, suggesting a pathway between the two organelles. The aim of th e study was to describe the changing location of Nopp140 during the first c leavage stages of mouse embryos and its re-location after inhibition of rRN A synthesis with actinomycin D. Light microscope immunocytochemical stainin g showed Nopp140 in the periphery of NPBs before activation of rDNA transcr iption and in addition confirmed its localization in CBs. Immunolabelling w ith antibodies against RNA Pol I and UBF gave co-localization of these prot eins, implicating that Nopp140 may actively participate to rDNA transcripti on. We suggest that fundamental differences in molecular organization of rD NA synthesis and postranscriptional processes between cycling somatic and p re-implantation embryonic cells may be in selective transport of transcript ion and/or processing-complexes of proteins to the nucleolar organizer regi ons (NOR). (C) 2001 Wiley-Liss, Inc.