Cytosolic tRNA synthetases catalyze the reactions of tRNAs encoded in the nuclear genome, their cognate amino acids, and ATP to form aminoacyl-tRNAs, AMP, and pyrophosphate. Eight of the tRNA synthetases, those specific for arginine, aspartate, glutamate and proline, glutamine, isoleucine, leucine, lysine, and methionine, associate to form a complex with three accessory proteins. Each of the component synthetases is active in vitro as a purified protein; complex formation is thought to channel aminoacylated tRNAs more efficiently to the site of protein synthesis in mRNA:ribosome complexes (Quevillon et al. 1999; Wolfe et al. 2003, 2005).