Over Expression of a tRNALeu Isoacceptor Changes Charging Pattern of Leucine tRNAs and Reveals New Codon Reading

During mRNA translation, synonymous codons for one amino acid are often read by different isoaccepting tRNAs. The theory of selective tRNA charging predicts greatly varying percentages of aminoacylation among isoacceptors in cells starved for their common amino acid. It also predicts major changes in tRNA charging patterns upon concentration changes of single isoacceptors, which suggests a novel type of translational control of gene expression. We therefore tested the theory by measuring with Northern blots the charging of Leu-tRNAs in Escherichia coli under Leu limitation in response to over expression of image. As predicted, the charged level of image increased and the charged levels of four other Leu isoacceptors decreased or remained unchanged, but the charged level of image increased unexpectedly. To remove this apparent inconsistency between theory and experiment we postulated a previously unknown common codon for image and image. Subsequently, we demonstrated that the image codon CUU is, in fact, read also by image, due to a uridine-5-oxyacetic acid modification.