The aminoacyl-tRNA synthetases (also known as aminoacyl-tRNA ligases) catalyse the attachment of an amino acid to its cognate transfer RNA molecule in a highly specific two-step reaction [ (PUBMED:10704480) (PUBMED:12458790) ]. These proteins differ widely in size and oligomeric state, and have limited sequence homology [ (PUBMED:2203971) ]. The 20 aminoacyl-tRNA synthetases are divided into two classes, I and II. Class I aminoacyl-tRNA synthetases contain a characteristic Rossman fold catalytic domain and are mostly monomeric [ (PUBMED:10673435) ]. Class II aminoacyl-tRNA synthetases share an anti-parallel beta-sheet fold flanked by alpha-helices [ (PUBMED:8364025) ], and are mostly dimeric or multimeric, containing at least three conserved regions [ (PUBMED:8274143) (PUBMED:2053131) (PUBMED:1852601) ]. However, tRNA binding involves an alpha-helical structure that is conserved between class I and class II synthetases. In reactions catalysed by the class I aminoacyl-tRNA synthetases, the aminoacyl group is coupled to the 2'-hydroxyl of the tRNA, while, in class II reactions, the 3'-hydroxyl site is preferred. The synthetases specific for arginine, cysteine, glutamic acid, glutamine, isoleucine, leucine, methionine, tyrosine, tryptophan, valine, and some lysine synthetases (non-eukaryotic group) belong to class I synthetases. The synthetases specific for alanine, asparagine, aspartic acid, glycine, histidine, phenylalanine, proline, serine, threonine, and some lysine synthetases (non-archaeal group), belong to class-II synthetases. Based on their mode of binding to the tRNA acceptor stem, both classes of tRNA synthetases have been subdivided into three subclasses, designated 1a, 1b, 1c and 2a, 2b, 2c [ (PUBMED:10447505) ].

This domain is found at the N terminus of Arginyl tRNA synthetase, also called additional domain 1 (Add-1). It is about 140 residues long and it has been suggested to be involved in tRNA recognition [ (PUBMED:9736621) ].