Carbamyl phosphate synthetase (from Escherichia coli) consists of a 7.3S protomeric unit that contains one heavy polypeptide chain (molecular weight about 130,000) and one light chain (molecular weight about 42,000). The heavy and light chains were separated by gel filtration in the presence of 1 M potassium thiocyanate. In contrast to the native enzyme and the reconstituted enzyme (prepared by mixing the separated heavy and light chains), the heavy chain does not catalyze glutamine-dependent carbamyl phosphate synthesis, although it does catalyze the synthesis of carbamyl phosphate from ammonia. The heavy chain also catalyzes two of the partial reactions catalyzed by the intact enzyme; i.e., the bicarbonate-dependent cleavage of ATP and the synthesis of ATP from ADP and carbamyl phosphate. Both positive (ammonia, ornithine, IMP) and negative (UMP) allosteric regulatory sites are located on the heavy chain. The only catalytic activity exhibited by the light chain is the hydrolysis of glutamine. A model is presented according to which glutamine binds to the light chain, which is followed by release of nitrogen from the amide group for use by the heavy chain. The findings suggest that glutamine-dependent carbamyl phosphate synthetase (and perhaps other glutamine amidotransferases) arose in the course of evolution by a combination of a primitive ammonia-dependent synthetic enzyme and a glutaminase; this combination may have been associated with a change from ammonia to glutamine as the principal source of nitrogen.