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Preparation and properties of ribosomal subunits from mouse plasmocytoma tumors.

Mouse myeloma ribosomes were prepared without exposure to detergent and their behavior with either decreasing concentrations of Mg2+ or increasing concentrations of KC1 examined. The minimal conditions for dissociation into subunits were determined and the properties of the subunits obtained under different conditions compared. Ribosomes which are first freed of nascent chains dissociate into 40 S and 60 S subunits at physiological Mg2+ concentrations (5 mM) and at only 0.3 M KC1 (K3 subunits). Although they lack one distinct protein present in low Mg2+ subunits, these K3 subunits reassociate spontaneously to form 80 S particles at lower ionic strength (0.05 M KC1) and are highly active in poly(U)-directed polyphenylalanine synthesis. This activity is maintained after storage at - 70 "C for several months. Subunits obtained at a higher concentration of KC1 (0.88 M) are less active and less stable, and they lack at least one protein which is present in K3 subunits. Low Mg2+ subunits are unable to reassociate spontaneously, are inactive in the poly(U) system, have lower sedimentation coefficients, and have lost the 5 S RNA.