TRANSITION-STATE STABILIZATION BY THE HIGH MOTIF OF CLASS-I AMINOACYL-TRANSFER-RNA SYNTHETASES - THE CASE OF ESCHERICHIA-COLI METHIONYL-TRANSFER-RNA SYNTHETASE

Methionyl-tRNA synthetase belongs to the class I aminoacyl-tRNA synthe tase family characterized both by a catalytic center built around a Ro ssmann Fold and by the presence of the two peptidic marker sequences H IGH and KMSKS. In this study, the role of the (21)HLGH(24) motif of Es cherichia coli methionyl-tRNA synthetase was studied in a systematic f ashion by site-directed mutagenesis. It is shown that the two histidin e residues play a crucial role in the catalysis of the methionyl adeny late formation by participating in the stabilisation of the ATP phosph ate chain during the transition state. Moreover, the results suggest t he involvement of the epsilon-imino group of histidine 21 and of the d elta-imino group of histidine 24. Notably, the substitution of either the leucine or the glycine residue of the HLGH motif by alanine had no effect on the catalysis. From the data and from other studies with cl ass I aminoacyl-tRNA synthetases, concomitant positive contributions o f the HIGH and KMSKS sequences to reach the transition state of aminoa cyl adenylate formation can be envisaged.