Electron microscopic and sedimentation studies on rat-liver ribosomal subunits.

Abstract

1 Rotary shadow-casting of isolated rat liver large ribosomal subunits revealed that 50–70% of these subunits carried one RNase-labile tail with a length of about 50–60 nm. 2 The tail is visible in two types of large ribosomal subunit preparations. (A) In isolated large ribosomal subunits obtained by incubating 77-S run-off ribosomes with 0.5 M KCl. Under these conditions the large subunits have a sedimentation value of 57-S. (B) In preparations of run-off ribosomes which have been converted into 59-S particles after lowering the Mg2+ concentration to 0.5 mM. 3 The 59-S material prepared by simply lowering the Mg2+ concentration represents a mixture of large subunits and dimers of small subunits. 4 In both types of ribosomal subunit preparations large subunits easily change into 48-S or 53-S particles. A reversible conversion between these particles exists depending on the Mg2+ concentration. Moreover, 48-S and 53-S particles dimerize at lower Mg2+ concentration than the 57-S large ribosomal subunits. 5 A tail has been observed on large subunits of mouse brain, rabbit reticulocyte, HeLa cell and chick liver ribosomes. They were not present on large ribosomal subunits of Saccharomyces carlsbergensis, Bacillus licheniformis and Escherichia coli MRE 600. 6 It is postulated that the discrete tail visible in rotary-shadowed rat liver large ribosomal subunits represents the stretched conformation of the protrusion visible in negatively stained particles.