GLUTAMINYL-TRANSFER-RNA SYNTHETASE

Among the twenty aminoacyl-tRNA synthetases glutaminyl-tRNA synthetase occupies a special position: it is one of only two enzymes of this fa mily which is not found in all organisms, being mainly absent from gra m positive eubacteria, archaebacteria and organelles. The E. coli GlnR S 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 l arger and can be parts of multienzyme complexes. Crystal structures we re solved of E. coli GlnRS complexed with tRNA(Gln) and ATP, of this c omplex containing tRNA(Gln) replaced by unmodified tRNA(Gln), and of t hree complexes with mutated GlnRS enzymes, The GlnRS molecule consists of four domains, the catalytic site is located in the Rossman fold, t ypical for class I synthetases, and the reaction mechanism follows the normal adenylate pathway, The enzyme shows many similarities with glu tamyl-tRNA synthetase; a common ancestor of both molecules is well est ablished. In the E. coli system recognition of the cognate tRNA has be en studied in many details using both natural and artificial mutants o f tRNA(Gln) and of the enzyme: GlnRS recognizes mainly conventional pa rts of the tRNA molecule, namely some bases of the anticodon loop and parts of the acceptor stem.