Leu-tRNA(Leu)-dependent protein synthesis has been examined in extracts of Escherichia coli cells which have a temperature-sensitive mutation in the leucyl-tRNA synthetase in order to assess the usefulness and fidelity of tRNA-dependent systems for studying codon-anticodon interaction. The results suggest that in MS2 RNA-directed reactions, there is a significant amount of misreading in the absence of added aminoacylated tRNA(Leu). By the incorporation of [35S]methionine or [14C]tyrosine, we estimate this level of misreading to be 15 to 30% of the amount of protein synthesized in samples reconstituted with several Leu-tRNA(Leu) isoaccepting species under these conditions. Examination of the synthesis of specific, known peptides spaced throughout the MS2 coat protein further suggests that in the absence of Leu-tRNA(Leu) the ribosomes manage to read through the CUC-encoded leucine at position 9 of the MS2 coat protein, and that in the presence of only Leu-tRNA3(Leu) or both (1 and 3)Leu-tRNA(Leu) (whose sequenced anticodons correspond to CUG and CUC codons, respectively), the ribosomes manage to read through the entire coat to the COOH terminus, including reading of the UUA-encoded leucine at position 86 of the MS2 coat. Beyond the endogenous misreading level, we have also examined the ability of purified Leu-tRNA(Leu) isoaccepting species to permit readthrough past the first leucine in an 'elongation' assay. These results argue that tRNA(4 and 5)(Leu) cannot read the CUC-encoded leucine at position 9, while tRNA(1 and 3)(Leu) can read this codon in vitro. Further, the unsequenced codon for the leucine residue at position 9 of the f2 coat protein would also seem to be of the CUX type. We conclude that tRNA-dependent systems should be regarded with caution for precise examination of codon-anticodon interaction, but such systems appear to be useful for certain kinds of experiments such as the elongation data reported here.
|Original language||English (US)|
|Number of pages||5|
|Journal||Journal of Biological Chemistry|
|State||Published - Dec 1 1978|
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Cell Biology