Studies on the biosynthesis of the large ribosomal subunit in rat liver.

Abstract

In order to elucidate biosynthetic process of large ribosomal subunits, we have isolated precursor particles from rat liver nucleoli by stepwise extraction with 50mM Na acetate buffer(pH5.5)-25 mM KC1-50pg DNase followed by 50 mM Tris-HCI buffer(pH7.4)-25 mM KCI both in the presence of 0.1% PVS and 1% Brij 98. These extracts were further fractionated by sucrose density gradient centrifugation into four snbfractions: a, b, c and d, which cover the two broad peaks in 60S and 80S region. RNA species of each fraction were characterized by poly-acrylamide gel electrophoresis and sucrose density gradient centrifugation and the particles in each fraction were observed by electron microscopy using shadow casting technique. Comparison of RNA species present with shape distribution of the particles in each fraction suggests that the maturation process of the ribosomal large subparticles could be schematized as follows: Filamentous molecules(contain predominantly 45S RNA)rod like molecules(probably contain 35S RNA) →spherical particles(contain →2 8S RNA) → Mature ribosomal large subparticles. Maturation process of ribosomal particles may involve, in addition to the so called sequential cleavage of RNA, folding of loose ribonucleoprotein filaments into compact spherical large subparticles.