RIBOSOMAL PRESCURSOR PARTICLES IN HELA CELL NUCLEUS

Extracts of purified HeLa cell nuclei were found to contain small amounts of two types of ribo-nucleoprotein particles. These are identical with the cytoplasmic subribosomal particles by two criteria the nuclear and cytoplasmic particles sediment together and contain RNA species of the same sedimentation coefficient and base composition. Data from kinetic analyses provide strong support for the notion that the nuclear particles are precursors to the free cytoplasmic subribosomal particles. It was found that newly synthesized subribosomal particles constitute a minority of such particles in the cytoplasm, the bulk being pre-existing particles in equilibrium with the majority of the cell ribosomes. A summary of our present understanding of the stages of ribosome synthesis is presented.