Abstract
Ribonucleoprotein particles have been extracted in high yield from isolated nucleoli of rat livers and characterized biochemically and by electron microscopy. Release of ribonucleoprotein particles is only observed when nucleoli were prepared from magnesium nuclei; by contrast the release is poor when nucleoli were derived from calcium nuclei. When analyzed on sucrose density gradients the majority of the rapidly sedimenting particles are assignable to the region of 55–80 S, with a peak at 60 S, by comparison with the position of cytoplasmic 50-S and 30-S derived subunits analyzed under identical conditions. Under the experimental conditions employed, the rate of sedimentation is not drastically altered by the presence or absence of magnesium ions. The sedimentation patterns of nucleolar ribonucleoprotein particles from thioacetamide-treated rats, in which the nucleolar ribonuclease levels are reduced, shows the same heterogeneous sedimentation pattern but with a marked increase in the yield of ribonucleoprotein particles. The radioactivity profiles of the nucleolar ribonucleoprotein extracts on sucrose gradients were studied at various times after intraperitoneal injection of [6- 14C]-orotic acid. The nucleolar ribonucleoprotein quickly attains high specific activities while the cytoplasmic, native 60-S subunit does not label extensively at early times. Incorporation is mainly into a nucleolar “31-S” RNA containing “60-S” ribonucleoprotein particles and no labelled precursoral ribonucleoprotein containing 45-S RNA is detected in the extracts. The detailed structure of the rapidly sedimenting nucleolar ribonucleoprotein particles was studied in the electron microscope by negative and positive staining techniques and compared to cytoplasmic ribosomes and 50-S derived ribosomal subunits. The morphology of the nucleolar 60-S particle is similar to that of the 50-S cytoplasmic subunit and both appear to be made up from fine fibrils. However, the nucleolar particle is more porous and larger in size than the cytoplasmic component. The fine structure and width of the 60-S nucleolar ribonucleoprotein is remarkably close to that of the ribosome-like particles seen in nucleoli in situ.
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