title: The arrangement of ribosomes in ribosome tetramers from hypothermic chick embryos.
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 Mg(2+), Ca(2+) and K(+) studied. 2. Lowering of the Mg(2+) concentration from standard buffer conditions caused a reversible dissociation of tetramers into monomers and of these into subunits. 3. Ca(2+) replaced Mg(2+) in causing the re-formation of tetramers and monomers from subunits after dissociation in low Mg(2+) concentrations. 4. Ca(2+) also caused an almost complete conversion of monomers into dimers in the presence of Mg(2+). 5. The effect of Ca(2+) 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.
mesh_terms: D000818:Animals; D002118:Calcium; D002499:Centrifugation, Density Gradient; D002642:Chick Embryo; D007035:Hypothermia; D008274:Magnesium; D008854:Microscopy, Electron; D008967:Molecular Biology; D011108:Polymers; D011188:Potassium; D012270:Ribosomes; D013395:Sucrose
chemical_list: D011108:Polymers; D013395:Sucrose; D008274:Magnesium; D011188:Potassium; D002118:Calcium