Glutaminyl-tRNA Synthetase

Among the twenty aminoacyl-tRNA synthetases glutaminyl-tRNA synthetase occupies a special position: it is one of only two enzymes of this family which is not found in all organisms, being mainly absent from gram positive eubacteria, archaebacteria and organelles. The E.coliGlnRS is relatively small with 553 amino acids and a molecular mass of 64.4 kDa and functions as a monomer. The mammalian enzymes are somewhat larger and can be parts of multienzyme complexes. Crystal structures were solved of E.coliGlnRS complexed with tRNA^Gln and ATP, of this complex containing tRNA^Gln replaced by unmodified tRNA^Gln, and of three complexes with mutated GlnRS enzymes. The GlnRS molecule consists of four domains, the catalytic site is located in the Rossman fold, typical for class I synthetases, and the reaction mechanism follows the normal adenylate pathway. The enzyme shows many similarities with glutamyl-tRNA synthetase; a common ancestor of both molecules is well established. In the E.colisystem recognition of the cognate tRNA has been studied in many details using both natural and artificial mutants of tRNA^Gln and of the enzyme: GlnRS recognizes mainly conventional parts of the tRNA molecule, namely some bases of the anticodon loop and parts of the acceptor stem.