Abstract
1. Ribosomes and the tetramer arrangement peculiar to the tissues of chick embryos exposed to low temperatures were separated by sucrose-density-gradient centrifugation, and the effects of variation of the concentrations of Mg2+, Ca2+ and K+ studied. 2. Lowering of the Mg2+ concentration from standard buffer conditions caused a reversible dissociation of tetramers into monomers and of these into subunits. 3. Ca2+ replaced Mg2+ in causing the re-formation of tetramers and monomers from subunits after dissociation in low Mg2+ concentrations. 4. Ca2+ also caused an almost complete conversion of monomers into dimers in the presence of Mg2+. 5. The effect of Ca2+ on the formation of dimers was abolished by pretreatment of the ribosomes with ribonuclease, but the re-formation of tetramers was unaffected. 6. Increase of the K+ concentration from that of the standard buffer caused dissociation of monomers and dimers into subunits. 7. Raised K+ concentration also caused a stepwise alteration of the tetramer from a particle with a sedimentation coefficient of 197S, which constitutes the bulk of the tetramer at low K+ concentrations, first to a 184S peak and finally to material with a sedimentation coefficient of about 155S. 8. The implications of these results on hypotheses of the arrangement of the individual monomers in the tetramer are discussed and a new model for the structure is proposed.
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