QRSL1 belongs to the amidase family, similar to glutaminyl-tRNA synthetase. Glutaminyl-tRNA synthetase is a class Ic synthetase and shows several similarities with glutamyl-tRNA synthetase concerning structure and catalytic properties. It is an alpha2 dimer. Glutaminyl-tRNA synthetase is a relatively rare synthetase, found in the cytosolic compartment of eukaryotes, in Escherichia coli and a number of other Gram-negative bacteria, and in Deinococcus radiodurans. In contrast, the pathway to Gln-tRNA in mitochondria, Archaea, Gram-positive bacteria, and a number of other lineages is by misacylation with Glu followed by transamidation to correct the aminoacylation to Gln. A stable glutaminly-adenylate analog, which inhibits GlnRS with a Ki of 1.32 microM, was synthesized and cocrystallized with GlnRS and tRNA2Gln.