CRYSTAL-STRUCTURE OF GLYCYL-TRANSFER-RNA SYNTHETASE FROM THERMUS-THERMOPHILUS

The sequence and crystal structure at 2.75 Angstrom resolution of the homodimeric glycyl-tRNA synthetase from Thermus thermophilus, the firs t representative of the last unknown class II synthetase subgroup, hav e been determined, The three class II synthetase sequence motifs are p resent but the structure was essential for identification of motif 1, which does not possess the proline previously believed to be an essent ial class II invariant. Nevertheless, crucial contacts with the active site of the other monomer involving motif 1 are conserved and a more comprehensive description of class II now becomes possible, Each monom er consists of an active site strongly resembling that of the aspartyl and seryl enzymes, a C-terminal anticodon recognition domain of 100 r esidues and a third domain unusually inserted between motifs 1 and 2 a lmost certainly interacting with the acceptor arm of tRNA(Gly). The C- terminal domain has a novel five-stranded parallel-antiparallel beta-s heet structure with three surrounding helices, The active site residue s most probably responsible for substrate recognition, in particular i n the Gly binding pocket, can be identified by inference from aspartyl -tRNA synthetase due to the conserved nature of the class II active si te.