[Nature of the heterogeneity of 30S ribosomal subparticles in vitro. I. Effect of large centrifugal fields during 30S subparticle isolation on their capacity for codon-dependent tRNA binding].

Measurements of association constants (Ka) of specific [14C]Phe-tRNAPhe with a 30S..poly(U) complex revealed that values of these constants vary from 0.5.10(7) up to 1.5.10(8) M--1 when different 30S subunit preparations were used at the same medium conditions (20 mM Mg2+, 200 mM NH4, 0 degrees C). Analysis of these data showed that the higher the rotor speeds were used during separation of 70S ribosomes into subunits, the less Ka values were measured. In special experiments on sedimentation of pure 30S subunits at different rotor speeds it was found that the decrease of Ka values was caused due to the additional reversible dissocation of ribosomal proteins from 30S subunits at high (the order of 100 000.g) centrifugal fields. As a possible mechanism of such dissociation we suggest the influence of high hydrostatic pressure on the association constants of S-proteins with 30S subunits. Data presented in this paper demonstrate that at least one of the reasons for the physical and functional heterogeneity of 30S subunits in vitro derives from the application of high centrifugal fields during isolation of ribosomal subunits.