Arginyl tRNA synthetase (ArgRS) catalytic core domain. This class I enzyme is a monomer which aminoacylates the 2'-OH of the nucleotide at the 3' of the appropriate tRNA. The core domain is based on the Rossman fold and is responsible for the ATP-dependent formation of the enzyme bound aminoacyl-adenylate. There are at least three subgroups of ArgRS. One type contains both characteristic class I HIGH and KMSKS motifs, which are involved in ATP binding. The second subtype lacks the KMSKS motif; however, it has a lysine N-terminal to the HIGH motif, which serves as the functional counterpart to the second lysine of the KMSKS motif. A third group, which is found primarily in archaea and a few bacteria, lacks both the KMSKS motif and the HIGH loop lysine.