The expression of the gene for threonyl-tRNA synthetase (thrS) has previously been shown as being negatively autoregulated at the translational level. The region of the thrS leader mRNA responsible for that control is located immediately upstream of the ribosomal binding site, and was proposed to fold in a tRNA(Thr) anticodon arm-like structure. The present paper reviews experiments using enzymatic and chemical probes that prove the existence of a tRNA(Thr) anticodon-like structure in the thrS mRNA. These structural studies have also shown the presence of another arm upstream in the leader mRNA that has striking similarities with the acceptor arm of the tRNA(Thr) isoacceptors. This second arm was shown, by mutational analysis, to also be involved in thrS regulation. Footprinting experiments have shown that both the anticodon-like and the acceptor-like arms interact with the synthetase. Finally, the similarity of the interaction of the synthetase with its 2 RNA ligands (mRNA and tRNA) has been investigated by selecting and studying mutants of the synthetase itself. The observed correlation between regulatory and aminoacylation defects in these mutants strongly suggests that the synthetase recognizes similar regions of its 2 RNA ligands in an analogous manner.