Formylglycinamide ribonucleotide amidotransferase (FGAR-AT) catalyzes the ATP-dependent conversion of formylglycinamide ribonucleotide (FGAR) and glutamine to formylglycinamidine ribonucleotide (FGAM), ADP, phosphate, and glutamate in the fourth step of the purine biosynthetic pathway. In eukaryotes and Gram-negative bacteria, FGAR-AT is encoded by the purL gene as a multidomain protein with a molecular mass of about 140 kDa. In Gram-positive bacteria and archaea FGAR-AT is a complex of three proteins: PurS, PurL, and PurQ. PurL itself contains two tandem N- and C-terminal domains (four domains altogether). The N-terminal domains bind ATP and are related to the ATP-binding domains of HypE, ThiL, SelD and PurM.