Anomalous sedimentation behaviour of the E.coli ribosomal system: 50S-30S forms 50S PLUS 30S

An experimenal and theoretical study has been made into the effect of association-dissociation reactions on the sedimentation of the E.coli ribosomal 50S-30S forms 50S plus 30S. It has been found that (a) the sedimentation pattern is strongly dependent on the rotor speed: (b) the ratio of components as measured using high-speed ultracentrifugation (30000-40000 r.p.m.) is independent of rotor speed; and (c) the speed of ultracentrifugation has a strong effect on the sedimentation coefficeint of the ribosomal system as determined by the mean square second moment. The results of this paper demonstrate that ribosome sedimentation at low-speed ultracentrifugation is affected by some artefactual processes. A theoretical analysis of the experimental findings has shown that the observed effects cannot be attributed to the effect of the association-dissociation reaction nor to the pressure dependence of the equilibrium constant of that reaction. On the other hand, at high-speed ultracentrifugation the ribosomal system sediments as a heterogeneous mixture of non-interacting components. Consequently, the shape of the boundary in this case will reflect the equilibrium composition of the ribosomal system.