tRNA-tRNA interactions within cellular ribosomes.

We describe an assay that converts the effects of tRNA-tRNA contacts at two particular codons into a quantitative effect on beta-galactosidase level. The assay measures the separate and combined efficiency of suppression at adjacent nonsense codons in vivo using a set of specially created homologous messages. In a survey of distal anticodon arm substitutions, we find that particular mutant tRNAs occupying the P-site reduce the apparent efficiency of the suppressor tRNA reading the A-site codon by factors of 2-170. By using measured tRNA-tRNA distances and the crystallographic tRNA structure, we propose a model of the tRNA-tRNA-mRNA complex. In the model, the anticodon loops of the P-site and A-site tRNAs contact one another in a way that is consistent with our combined tRNA efficiency data. These results suggest that tRNA-tRNA interactions that modulate tRNA action are an inevitable feature of translation.