Description:
<jats:p>The sedimentation properties of ribosomes and ribosomal subunits from rat liver have been examined in media containing Mg<jats:sup>2+</jats:sup> ions at different concentrations.</jats:p><jats:p>Purified ribosomes contain 51.4% RNA and 48.6% protein; the phospholipid content is less than 0.1%.</jats:p><jats:p>At 10<jats:sup>−3</jats:sup> M Mg<jats:sup>2+</jats:sup> the S value of monomeric ribosomes is 80 S. At 10<jats:sup>−4</jats:sup> M Mg<jats:sup>2+</jats:sup> and in Mg<jats:sup>2+</jats:sup>‐free media there are present in addition many particles having a S value of 55 S or 49 S respectively. We have attempted to determine the nature of these particles. All the proteins which we regard as characteristic of 80 S ribosomes, can be extracted from the 55 S particles. The particles also contain two components of RNA sedimenting at a mass ratio of 2:1 at 33 and 18 S, respectively. By raising the concentration of Mg<jats:sup>2+</jats:sup> to 10<jats:sup>−3</jats:sup> M the 55 S particles revert to 80 S particles. In the presence of EDTA the 55 S particles are dissociated into particles with values of 48 and 28 S.</jats:p><jats:p>The separation of the ribosomes into their subunits can be achieved only when all the ribosomal Mg<jats:sup>2+</jats:sup> is removed by equimolar amounts of EDTA. The results of short and long time sucrose gradient centrifugation of ribosomes, which were treated before with EDTA clearly show, that the 80 S ribosomes are in an equilibrium with their subunits which is largely on the side of the dissociation products. Complete separation of the subunits with gradient centrifugation is possible only by continuous displacement of the equilibrium during prolonged centrifugation. The subunits have S values of 49 and 29 S in the presence of EDTA, and contain 45.6% RNA.</jats:p><jats:p>Studies on the sedimentation characteristics of mixtures of <jats:sup>32</jats:sup>P‐labelled small subunits with the unlabelled large subunits or with 80 S particles exclude the possibility that the 55 S particle fraction consists of mixture of the 49 S subunit with dimers of the 29 S subunit. Thus our evidence supports the idea that the 55 S and 80 S particles are interconvertible forms of an intact ribosomal particle.</jats:p><jats:p>The results are interpreted as indicating that two processes result from a progressive reduction in the amount of Mg<jats:sup>2+</jats:sup> bound to the 80 S ribosomes. Firstly, the structure of the subunits is changed and secondly, at the same time, the association of the subunits is loosened due to a stepwise cleavage of the Mg<jats:sup>2+</jats:sup> dependent linkages between the subunits. Both processes change the structure of the ribosome such that its S value is reduced from 80 S to lower values. The 49 S and 29 S subunits become separated when the ribosomal Mg<jats:sup>2+</jats:sup> is removed quantitatively.</jats:p>