The sequence and crystal structure at 2.75 A resolution of the homodimeric glycyl‐tRNA synthetase from Thermus thermophilus, the first representative of the last unknown class II synthetase subgroup, have been determined. The three class II synthetase sequence motifs are present but the structure was essential for identification of motif 1, which does not possess the proline previously believed to be an essential 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 monomer consists of an active site strongly resembling that of the aspartyl and seryl enzymes, a C‐terminal anticodon recognition domain of 100 residues and a third domain unusually inserted between motifs 1 and 2 almost certainly interacting with the acceptor arm of tRNA(Gly). The C‐terminal domain has a novel five‐stranded parallel‐antiparallel beta‐sheet structure with three surrounding helices. The active site residues most probably responsible for substrate recognition, in particular in the Gly binding pocket, can be identified by inference from aspartyl‐tRNA synthetase due to the conserved nature of the class II active site.

The degenerate base at position 34 of the tRNA anticodon is the target of numerous modification enzymes. In Saccharomyces cerevisiae, five tRNAs exhibit a complex modification of uridine 34 ( . An mtq2-0 trm9-0 strain exhibits a synthetic growth defect, thus revealing the existence of an unexpected link between tRNA anticodon modification and termination of translation. Trm112p is associated with other partners involved in ribosome biogenesis and chromatin remodeling, suggesting that it has additional roles in the cell.