SELECTION OF AMINOACYL-TRANSFER-RNAS AT SENSE CODONS - THE SIZE OF THE TRANSFER-RNA VARIABLE LOOP DETERMINES WHETHER THE IMMEDIATE 3'-NUCLEOTIDE TO THE CODON HAS A CONTEXT EFFECT

Codon context can affect translational efficiency by several molecular mechanisms, The base stacking interactions between a codon-anticodon complex and the neighboring nucleotide immediately 3' can facilitate t ranslation by amber suppressors and the tRNA structure is also known t o modulate the sensitivity to context. In this study the relative rate s of aminoacyl-tRNA selection were measured at four sense codons (UGG, CUC, UUC and UCA), in all four 3' nucleotide contexts, through direct competition with a programed frameshift at a site derived from the re lease factor 2 gene. Two codons (UGG and UUC) are read by tRNAs with s malt variable regions and their rates of aminoacyl-tRNA selection corr elated with the potential base stacking strength of the 3' neighboring nucleotide. The other two codons (CUC and UCA) are read by tRNAs with large variable regions and the rate of selection of the aminoacyl-tRN As in these cases varied little among the four contexts. Re-examinatio n of published data on amber suppression also revealed an inverse corr elation between context sensitivity and the size of the variable regio n. Collectively the data suggest that a large variable loop in a tRNA decreases the influence of the 3' context on tRNA selection, probably by strengthening tRNA-ribosomal interactions.