Fine structural ribonucleoprotein components of the cell nucleus visualized after spreading and high resolution...

The fine structure of the nuclear components was studied following mild lysis of mouse or Drosophila tissue culture cells and spreading of nuclear material. Particular attention was paid to nuclear ribonucleoprotein (RNP) constituents, which were analysed by high resolution autoradiography after [3H]uridine pulse labelling of cells. Comparison with the labelling kinetics of various in situ nuclear RNP constituents described previously suggests strong similarities between in situ constituents and structures observed within spread nuclear components. The present observations suggest that the nucleolar dense fibrillar component, shown previously in ultrathin sections of [3H]uridine-labelled intact cells as carrying rapidly labelled pre-rRNA, in fact consists of highly compacted transcribing ribosomal genes. The growing RNP fibrils appearing in transcription complexes of extranucleolar active genes and the in situ observed perichromatin fibrils also show the same labelling properties. This confirms that the two structures indeed represent the same nucleoplasmic constituents. As for the nuclear structures involved in post-transcriptional events, our observations demonstrate the occurrence of a rapidly labelled RNP fibro-granular network. Its granular elements correspond, in size and perichromatin location, to the perichromatin granules seen in the situ preparations and suggest similarities between the two constituents. The results are discussed in the light of other data providing information on the role of various nuclear structural constituents.