Interactions between UGA-suppressor tRNATrp and the Ribosome: Mechanisms of tRNA Selection

Abstract

The notion that codon–anticodon interactions are the sole determinants for the selection of aminoacyl-tRNAs on the ribosome has long been central to our ideas about the mechanism of protein synthesis. There is, however, at least one nonsense suppressor tRNA that seems to contradict this generalization. The anomalous tRNA species is the UGA suppressor form of tRNA^Trp. This tRNA was shown by Hirsh (1971) to contain a normal tryptophan anticodon (CCA) and a single-base change in the D stem, where G24 is replaced by an A. Thus, we are obliged to explain how a base change far from the anticodon can transform this tRNA from one that reads the codon UGG to one that can read both the UGG codon and the nonsense codon UGA.

One simple way to explain the behavior of the UGA suppressor is to assume that the D-stem alteration changes the structure of the tRNA so that its affinity for the UGA codon is enhanced. Two experiments have been performed to detect such an effect. First, Högenauer (1974) measured the binding of the oligonucleotide UpGpA to wild-type and mutant tRNAs. No significant differences between the affinities of these two tRNA species for the nonsense codon analog were detected by equilibrium dialysis. Similarly, Buckingham (1976) compared the affinities of the wild-type and suppressor forms of tRNA^Trp for tRNA^Pro, which possesses an anticodon complementary to that of the tRNA^Trp species. As shown in Figure 1, both wild-type and suppressor tRNA^Trp are retarded to the same extent by tRNA^Pro...