In Lactobacillus plantarum, carbamoyl phosphate is synthesized by two carbamoyl-phosphate synthetases (CPS): Carbon dioxide differentiates the arginine-repressed from the pyrimidine-regulated CPS

Carbamoyl phosphate (CP) is an intermediate in pyrimidine and arginine bios ynthesis. Carbamoyl-phosphate synthetase (CPS) contains a small amidotransf erase subunit (GLN) that hydrolyzes glutamine and transfers ammonia to the large synthetase subunit (SYN), where CP biosynthesis occurs in the presenc e of ATP and CO2. Lactobacillus plantarum, a lactic acid bacterium, harbors a pyrimidine-inhibited CPS (CPS-P; Elagoz et al., Gene 182:37-43, 1996) an d an arginine-repressed CPS (CPS-A). Sequencing has shown that CPS-A is enc oded by carA (GLN) and carB (SYN). Transcriptional studies have demonstrate d that carB is transcribed both monocistronically and in the carAB arginine -repressed operon. CP biosynthesis in L. plantarum was studied with three m utants (Delta CPS-P, Delta CPS-A, and double deletion). In the absence of b oth CPSs, auxotrophy for pyrimidines and arginine was observed. CPS-P produ ced enough CP for both pathways. In CO2-enriched air but not in ordinary ai r, CPS-A provided CP only for arginine biosynthesis. Therefore, the uracil sensitivity observed in prototrophic wild-type L. plantarum without CO2 enr ichment may be due to the low affinity of CPS-A for its substrate CO2 or to regulation of the CP pool by the cellular CO2/bicarbonate level.