tRNA RECOGNITION BY PROCARYOTIC ASPARTYL tRNA-SYNTHETASES. D. Moras, L. Moulinier, C. Briand, S. Eiler, A. Poterzmann and J.C. Thierry. UPR de Biologie Structurale, IGBMC, 1 rue Laurent Fries, BP 163, 67404 Illkirch Cedex, France.

Aspartyl tRNA-synthetase (AspRS) like all members of class II synthetases attaches its aminoacid substrate to the 3'OH group of the ribose of the terminal adenosine of its cognate tRNA. The structural analysis done with the yeast system, corroborated by mutagenesis experiments, established the details of the mechanism of the two steps reaction.

The crystal structure of the homologous complex of E.coli reveals a different relative position of the terminal adenosine in the catalytic pocket. On the other hand the crystal structure of the inactive heterologous complex between the E.coli enzyme and yeast tRNAasp shows two different conformations of the bound tRNA. One is non productive with the CCA end away from the active site. The other one is very reminiscent of that observed in the yeast homologous complex. The main differences between eucaryotic and procaryotic aspRSs are associated with an insertion domain of more than hundred aminoacids involved in the correct positionning of the acceptor stem. Two crystal structures of the complex between ttAspRS and (i) its cognate tRNA as well as (ii) E. coli tRNA^Asp have been solved. In both cases, the tRNA terminal acceptor end is not located in the active site pocket. Taken together, these observations shed new light on the reaction mechanism and the procaryotic-eucaryotic systems separation.