Ribosomal precursor particles in ascites tumor cell nucleoli

Abstract

Ribonucleoprotein particles have been prepared from isolated nucleoli of ascites tumour cells. The release of nucleolar particles is obtained through the action of heparin. When analysed on sucrose gradients, most of the particles are assignable to the region of 80 S. When the same particles are processed in gradients containing EDTA, the maximum of absorbance at 260 nm is uniformly in the 55 S region. Detailed structural analysis of nucleolar particles was carried out under negative or positive staining in the electron microscope and compared with the finished cytoplasmic ribosomes. The 80 S nucleolar particle measures 230 × 180 Å and displays three striking features: 1. 1. They are often grouped by three or four particles linked by a fine strand of 10 Å. 2. 2. Isolated particles under positive staining are associated with a similar fine strand. 3. 3. Under negative contrast, they appear as tightly coiled irregular structures. On the other hand, the 55 S particle is a loose structure composed of subparticular elements: the average size of this particle is 200 × 170 Å. This 55 S particle is believed to be a dissociation of the 80 S by the action of EDTA. These result are discussed in view of the possible regulatory functions of the nucleolus in the biogenesis of ribosomal RNA and ribosomal peptides.