On the Heterogeneity of Eukaryotic Ribosomal Subunits in Vitro

抄録

The circumstances in which monomers and dimers of eukaryotic small and large ribosomal subunits occur were examined by measuring the sedimentation profiles of each subunit with glutaraldehyde fixation. When small ribosomal subunits were prepared from rat liver ribosomes in the medium containing 50mM Tris-HCl (pH 7.6), 880mM KCl, 12.5mM MgCl₂, 10mM mercaptoethanol and 0.1mM puromycin, about 41.4% of the preparations sedimented as a monomer and the other as a dimer in T₁₀K₂₀M₅ (10mM Tris-HCl, pH 7.6 ; 80mM KCl ; 5mM MgCl₂) buffer. There were no difference in the monomer content of small ribosomal subunits from free and membrane bound ribosomes from rat liver. Moreover, small ribosomal subunits from the muscle of normal and diabetic rats contained just the same proportion of the monomer and dimer particles when assayed under the same ionic conditions. The proportion of the monomer and the dimer of the small ribosomal subunits changed with change in the concentration of potassium and magnesium at which ribosomal subunits were prepared ; the content of the monomer decreased with lowering the ratio of magnesium concentration and was only 10.1% when the preparation was conducted in the medium containing 880 mM KCl and 3mM MgCl₂. The addition of poly U to the small subunit preparations caused the conversion of the dimer into the monomer. Large ribosomal subunits dimerized gradually at low temperature even in high potassium concentration but the dimers of the large subunits dissociated easily into their monomers by incubation at 30°.