Ribosome precursor particles in the nucleolus of rat liver. Easily extractable nucleolar 60-S ribonucleoprotein particles and their relation to cytoplasmic large ribosomal subunits.

Abstract

Ribonucleoprotein (RNA · protein) particles extracted from isolated nucleoli of rat liver with the aid of DNAase and detergents sediment at approximately 60 S (termed nucleolar 60-S RNA · protein). These particles contained an extra RNA species besides 28-S RNA. The former RNA, when extracted with dodecylsulfate and phenol at room temperature, migrates in polyacrylamide-agarose gel a little slower than the 28-S RNA of cytoplasmic large ribosomal subunit (termed 30-S RNA). The nucleolar 60-S RNA · protein contained 5-S RNA as a low-molecular-weight counterpart. When RNA was extracted at high temperature (65°C), the 30-S RNA was converted to 28-S RNA with a concomitant release of low-molecular-weight RNA's. These findings suggest that these low-molecular-weight RNA's are hydrogen-bonded to 28-S RNA. The protein moiety of the nucleolar 60-S RNA · protein was compared with that of large ribosomal subunits on polyacrylamide gel with a split-gel technique. The proteins of nucleolar 60-S RNA · protein gave approximately 20 bands, many of which were identical in the mobility to the proteins of large ribosomal subunits. However, at least two bands (bands 7 and 15) were not detected in the gel pattern of cytoplasmic large ribosomal subunits. On the other hand, there were protein bands (bands a ∼ k) of large ribosomal subunits which were not detectable in the proteins of nucleolar 60-S particles. Nucleolar 60-S particles were composed of three RNA · protein species having different buoyant densities. The · values of nucleolar 60-S RNA · protein were significantly lower than that of the cytoplasmic large ribosomal subunits, showing that the nucleolar 60-S RNA · protein has a larger protein content than cytoplasmic large ribosomal subunits. It is concluded from these results that when the nucleolar 60-S RNA · protein is transferred from the nucleolus into cytoplasm to form a mature large ribosomal subunit, (a) 30-S RNA is converted to 28-S RNA with a concomitant release of low-molecular-weight RNA's, (b) at least two protein species are removed from the nucleolar 60-S RNA · protein and at least several other proteins are added to it, but (c) the overall protein content decreases during the course of these protein exchanges.