Subunit of the carbamoyl-phosphate synthetase (CPS) composed of a small chain CARA/VEN6 and a large chain CARB/VEN3. Involved in arginine biosynthesis. Required for mesophyll development. 2 ATP + H2O + hydrogencarbonate + L-glutamine = 2 ADP + carbamoyl phosphate + 2 H(+) + L-glutamate + phosphate Binds 4 Mg(2+) or Mn(2+) ions per subunit. Amino-acid biosynthesis; L-arginine biosynthesis; carbamoyl phosphate from bicarbonate: step 1/1. Pyrimidine metabolism; UMP biosynthesis via de novo pathway; (S)-dihydroorotate from bicarbonate: step 1/3. Composed of two chains; the small (or glutamine) chain promotes the hydrolysis of glutamine to ammonia, which is used by the large (or ammonia) chain to synthesize carbamoyl phosphate. Expressed in roots and leaves. By phosphate starvation in shoot. The ven3-1, ven3-2, ven3-3 and ven3-4 phenotypes are rescued by exogenous application of citrulline, an arginine precursor. Belongs to the CarB family.