The dissociation of Yoshida hepatoma ribosomes into active particles.

1. The sedimentation pattern of Yoshida hepatoma ribosomes shows mainly a high dimer peak and an intermediate peak between those of monomer and dimer. 2. The treatment of the postmitochondrial supernatant with EDTA or potassium chloride leads to the dissociation of ribosomes into subunits, through a decrease ofthe ribosomal Mg(2+)/phosphorus ratio. Provided that the deprivation of endogenous Mg(2+) is not complete, the subunits reassociate into active polyribosomes after addition of magnesium chloride to the medium. If the Mg(2+)/phosphorus ratio is lowered below 0.01 (mumol/mumol), structural changes occur, that become evident by a loss of protein and by a decreased sedimentation rate, which render the subribosomal particles unable to reassociate. 3. In the re-formation of polyribosomes from subunits, during the progressive increase of the concentration of magnesium chloride in the medium, the formation of monomeric ribosomes seems to be intermediate. 4. A dimerization of monomers and subunits occurs at concentrations of magnesium chloride greater than those required for the re-formation of polyribosomes and a preincubation for 1h at 0 degrees C is necessary for the maximum dimerization, whereas the complete reconstitution of polyribosomes is immediate.