A study of codon-dependent binding of aminoacyl-tRNA with the ribosomal 30-S subparticle of Escherichia coli. Determination of the active-particle fraction and binding constants in different media.

Titration of isolated Escherichia coli ribosomal 30-S particles with [14C]phenylalanyl-tRNA in the presence of poly(uridylic acid) was used for a quantitative assay of codon-dependent binding of aminoacyl-tRNA with the small ribosomal subparticle. The technique has allowed the estimation both of the fraction of "active" 30-S subparticles capable of forming the 30-S - poly(U) - phenylalanyl-tRNA complexes and the equilibrium constants of phenylalanyl-tRNA binding in different media. Heterogeneity of the ternary complexes formed has been revealed: at least two classes of complexes differing in stability have been observed. The stability of the 30-S - poly(U) - phenylalanyl-tRNA complexes has been shown to decrease with the lowering of the Mg2+ concentration, the increase of K+ concentration and the addition of urea. The stability of the complexes increases with the increase of Mg2+ concentration, with the addition of ethanol and decrease of temperature. It is demonstrated that the fraction of actively binding 30-S particles also varies in different medium conditions; it decreases with the increase of ionic strength (K+) and with the addition of urea, and increases with the increase of Mg2+ concentration and addition of ethanol.