The smaller subparticle of rabbit reticulocyte ribosomes was shown to yield core-particle and split-protein fractions on treatment with 2.5 M-NH4Cl/61 mM-MgCl2. The core-particle fraction was inactive in poly(U)-directed polyphenylalanine synthesis, but activity was restored after recombination with the split-protein fraction. Optimum ionic conditions for the reconstruction of active subparticles were found to be 0.75 M-NH4Cl/19 mM-MgCl2 at 0 degrees C. Improved extents of reconstruction were obtained when the core-particles were isolated by methods that avoided pelleting. Core-particles isolated from subparticles pretreated with either proteinases or ribonucleases had diminished capacity to become re-activated.
