[Selectivity of A-site on Escherichia coli ribosomes in relation to deacylation of transfer RNA].

To evaluate the equilibrium affinity of several deacylated tRNA from E. coli and yeast tRNA(Phe) for the A site of poly(U)- and poly(A)-programmed E. coli ribosomes, we studied their inhibitory effect on the subsequent binding of the template-cognate ternary complex EFTu*GTP* [14C]aminoacyl-tRNA. At 0 degree C and 10 mM Mg2+, affinities of template-cognate tRNAs were characterized by association constants Ka approximately 1-7.10(7) M-1. At least for the poly(U)-programmed system, this value practically coincided with the one determined earlier from direct nonenzymatic binding. Splitting off two 3'-terminal nucleotides did not appreciably change the tRNA affinity, that is, these nucleotides do not form direct contacts with the A site. Noncognate tRNAs were 120-1800-fold weaker as inhibitors; there was no considerable correlation between tRNA affinity for the A site and the extent of noncomplementarity of its anticodon to the template. The only except was tRNA(Tyr) in the system with poly(U), which displayed an affinity only 9-fold lower than that of template-cognate tRNA(Phe). The results obtained here show that the mRNA-programmed ribosomal A site reveals selectivity not only for energy-dependent enzymatic binding of aminoacyl-tRNA, but also for nonenzymatically bound nonacylated tRNA. Even in this simplified model, the selectivity is much higher than that observed when codons interact with anticodons in solution without ribosomes.