Mammalian aspartate transcarbamylase (ATCase; carbamoyl-phosphate:L-aspartate carbamoyltransferase, EC 126.96.36.199) is part of a 240-kDa multifunctional polypeptide called CAD, which also has carbamoyl-phosphate synthetase and dihydroorotase activities. We have sequenced selected restriction fragments of a Syrian hamster CAD cDNA that are clearly homologous to three prokaryotic ATCases. These studies, combined with previous sequence data, showed that the ATCase domain of CAD is encoded by 924 base pairs and has a mass of 34,323 Da and a pI of 9.8. While the bacterial pyrimidine biosynthetic enzymes are separate proteins, in mammals the ATCase domain is fused to the carboxyl end of the CAD chimera via a 133-amino acid (14-kDa) linker with an unusual amino acid composition, a pI of 10.2, and pronounced hydrophilic character. The fully active domain isolated from proteolytic digests was characterized by partial amino acid sequencing and amino acid analysis. Trypsin cleavage produced the ATCase domain with a 20-residue amino-terminal extension. Hydrodynamic studies showed that the isolated domain is a 110-kDa trimer with a Stokes radius of 41 A. The mammalian ATCase domain and the prokaryotic enzymes have virtually identical active-site residues and are likely to have the same tertiary fold.