Translational control inE. coli: The case of threonyl-tRNA synthetase

Genetic studies have shown that expression of theE. coli threonyl-tRNA synthetase (thrS) gene is negatively auto-regulated at the translational level. A region called the operator, located 110 nucleotides downstream of the 5 end of the mRNA and between 10 and 50bp upstream of the translational initiation codon in thethrS gene, is directly involved in that control. The conformation of anin vitro RNA fragment extending over thethrS regulatory region has been investigated with chemical and enzymatic probes. The operator locus displays structural similarities to the anti-codon arm of threonyl tRNA. The conformation of 3 constitutent mutants containing single base changes in the operator region shows that replacement of a base in the anti-codon-like loop does not induce any conformational change, suggesting that the residue concerned is directly involved in regulation. However mutation in or close to the anti-codon-like stem results in a partial or complete rearrangement of the structure of the operator region. Further experiments indicate that there is a clear correlation between the way the synthetase recognises each operator, causing translational repression, and threonyl-tRNA.