The Effect of the Mg2+ Concentration on the Sedimentation Coefficient of Ribosomes from Rat Liver

Abstract

1 Polyribosomes were broken down into 77 S monomers either by RNAase action (RNAase ribosomes) or by incubation under conditions of amino acid incorporation at 5 mM Mg2+- (“run-off ribosomes”). Various properties of these two types of monomers were compared. 2 “Run-off ribosomes” showed a high degree of poly(U)-directed phenylalanine incorporation. At 0.5 mM Mg2+ they contained a large number of particles with a sedimentation coefficient of 59 S. 3 When “run-off ribosomes” were incubated under conditions favourable for poly(U)-directed [14C]phenylalanine incorporation and then examined at 0.5 mM Mg2+, radioactivity was associated with 77 S monomers and not with 59 S particles. 4 A preparation of “run-off ribosomes” was isolated which contained a substantial amount of 77 S particles at 0.5 mM Mg2+ after the phenylalanine incorporation directed by poly(U). Before incubation these “run-off ribosomes” contained a negligible amount of 77 S ribosomes at 0.5 mM Mg2+. 5 It was concluded that before “run-off ribosomes” had reacted with poly(U), the sedimentation coefficient could be converted from 77 S into 59 S at 0.5 mM Mg2+. After [14C]phenylalanine incorporation directed by poly(U), however, the sedimentation coefficient remained 77 S at 0.5 mM Mg2+.