We have isolated fragments of a novel nuclear structure exhibiting the morphological and biochemical characteristics of a ribonucleoprotein network. Under transmission electron microscopy it is visualized as irregularly interconnected branches assembled by tightly packed particles with sizes between 100A and 300A. RNA extracted from this structure shows a complex pattern: as well as ribosomal 28 S and 18 S RNA, small amounts of heterogeneous nuclear RNA and small nuclear RNA are also present. The protein composition of the network indicates a strong domination of ribosomal polypeptides. This fact, considered together with the sedimentation characteristics of the prevailing type of particles isolated directly from the network, supports the conclusion that ribosomal particles are the representative particulate elements. Further electrophoretic analysis of the protein has pointed out that it also contains a significant number of acidic polypeptides. Control experiments have suggested that the site of origin of the network fragments studied is not nucleolar: the fragments are released during extraction, most probably from the nuclear periphery. It is not yet clear whether this structure is localized only at the nuclear periphery or represents an extensive structure, occupying the entire volume of the nucleus. It is speculated that the network is involved in the extranucleolar transport and maturation of ribosomes.

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