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 tRNALeu

GAG. As predicted, the

charged level of tRNALeu

GAG increased and the charged levels of four other

Leu isoacceptors decreased or remained unchanged, but the charged level

of tRNALeu

UAG increased unexpectedly. To remove this apparent inconsistency

between theory and experiment we postulated a previously

unknown common codon for tRNALeu

GAG and tRNALeu

UAG. Subsequently, we

demonstrated that the tRNALeu

GAG codon CUU is, in fact, read also by

tRNALeu

UAG, due to a uridine-5-oxyacetic acid modification.